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Economic Geology, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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To the Memory of Walther E. Petrascheck, (1906–1991), Inspiring Geologist and Academic Teacher, , COMPANION WEBSITE, This book has a companion website:, www.wiley.com/go/pohl/geology, with Figures and Tables from the book for downloading
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Economic Geology, Principles and Practice, Metals, Minerals, Coal and, Hydrocarbons – Introduction, to Formation and Sustainable, Exploitation of Mineral, Deposits, Walter L. Pohl
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This edition first published 2011, Ó 2011 by Walter L. Pohl, Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing, program has been merged with Wiley’s global Scientific, Technical and Medical business to form, Wiley-Blackwell., Registered office: John Wiley & Sons Ltd, The Atrium, Southern Gate, Chichester, West Sussex,, PO19 8SQ, UK, Editorial offices: 9600 Garsington Road, Oxford, OX4 2DQ, UK, The Atrium, Southern Gate, Chichester, West Sussex, PO19 8SQ, UK, 111 River Street, Hoboken, NJ 07030-5774, USA, For details of our global editorial offices, for customer services and for information about how to, apply for permission to reuse the copyright material in this book please see our website at, www.wiley.com/wiley-blackwell, The right of the author to be identified as the author of this work has been asserted in accordance, with the Copyright, Designs and Patents Act 1988., All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted,, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as, permitted by the UK Copyright, Designs and Patents Act 1988, without the prior permission of the publisher., Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may, not be available in electronic books., Designations used by companies to distinguish their products are often claimed as trademarks., All brand names and product names used in this book are trade names, service marks, trademarks, or registered trademarks of their respective owners. The publisher is not associated with any product or, vendor mentioned in this book. This publication is designed to provide accurate and authoritative information, in regard to the subject matter covered. It is sold on the understanding that the publisher is not engaged in, rendering professional services. If professional advice or other expert assistance is required, the services of, a competent professional should be sought., Library of Congress Cataloguing-in-Publication Data, Pohl, Walter, 1941Economic geology : principles and practice : metals, minerals, coal and hydrocarbons introduction to, formation and sustainable exploitation of mineral deposits / Walter L. Pohl., p. cm., Includes bibliographical references and index., ISBN 978-1-4443-3662-7 (hardback) – ISBN 978-1-4443-3663-4 (pbk.) 1. Geology, Economic. I. Title., TN260.P64 2011, 553—dc22, 2010047192, A catalogue record for this book is available from the British Library., This book is published in the following electronic formats: ePDF 9781444394863;, Wiley Online Library 9781444394870; ePub 9781444394856, Set in 9/11.5pt, TrumpMediaeval by Thomson Digital, Noida, India, , 1 2011
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Contents, Preface, , xiii, , Introduction, What are ore deposits?, Mining in the stress field between society and environment, The mineral resources conundrum, , Part I, 1, , Metalliferous Ore Deposits, , 1, 1, 2, 4, , 5, , Geological ore formation process systems (metallogenesis), , 7, , Synopsis, , 7, , 1.1 Magmatic Ore Formation Systems, 1.1.1 Orthomagmatic ore formation, 1.1.2 Ore deposits at mid-ocean ridges and in ophiolites, 1.1.3 Ore formation related to alkaline magmatic rocks, carbonatites, and kimberlites, 1.1.4 Granitoids and ore formation processes, 1.1.5 Ore deposits in pegmatites, 1.1.6 Hydrothermal ore formation, 1.1.7 Skarn- and contact-metasomatic ore deposits, 1.1.8 Porphyry copper (Mo-Au-Sn-W) deposits, 1.1.9 Hydrothermal-metasomatic ore deposits, 1.1.10 Hydrothermal vein deposits, 1.1.11 Volcanogenic ore deposits, , 23, 25, 32, 35, 54, 56, 59, 62, 68, , 1.2 Supergene Ore Formation Systems, 1.2.1 Residual (eluvial) ore deposits, 1.2.2 Supergene enrichment by descending (vadose) solutions, 1.2.3 Infiltration as an agent of ore formation, , 76, 80, 82, 88, , 1.3 Sedimentary Ore Formation Systems, 1.3.1 Black shales in metallogenesis, 1.3.2 Placer deposits, 1.3.3 Autochthonous iron and manganese deposits, 1.3.4 Sediment-hosted, submarine-exhalative (sedex) base metal deposits, , 8, 11, 18, , 92, 93, 94, 100, 107
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vi, , 2, , CONTENTS, , 1.4 Diagenetic Ore Formation Systems, 1.4.1 The European Copper Shale, 1.4.2 Diagenetic-hydrothermal carbonate-hosted Pb-Zn (F-Ba) deposits, 1.4.3 Diagenetic-hydrothermal ore formation related to salt diapirs, , 110, 114, 116, 119, , 1.5 Metamorphic and Metamorphosed Ore Deposits, , 121, , 1.6 Metamorphogenic Ore Formation Systems, , 125, , 1.7 Metallogeny – Ore Deposit Formation in Space and Time, 1.7.1 Metallogenetic epochs and provinces, 1.7.2 Metallogeny and plate tectonics, , 132, 133, 134, , 1.8 Genetic Classification of Ore and Mineral Deposits, , 139, , 1.9 Summary and Further Reading, , 146, , Economic geology of metals, , 149, , Synopsis, , 149, , 2.1 The Iron and Steel Metals, 2.1.1 Iron, 2.1.2 Manganese, 2.1.3 Chromium, 2.1.4 Nickel, 2.1.5 Cobalt, 2.1.6 Molybdenum, 2.1.7 Tungsten (Wolfram), 2.1.8 Vanadium, , 149, 149, 159, 163, 168, 173, 175, 179, 183, , 2.2 Base Metals, 2.2.1 Copper, 2.2.2 Lead and zinc, 2.2.3 Tin, , 185, 185, 195, 202, , 2.3 Precious Metals, 2.3.1 Gold, 2.3.2 Silver, 2.3.3 Platinum and Platinum Group Metals, , 207, 207, 221, 228, , 2.4 Light Metals, 2.4.1 Aluminium, 2.4.2 Magnesium, , 233, 233, 238, , 2.5 Minor and Speciality Metals, 2.5.1 Mercury, 2.5.2 Antimony, , 239, 239, 243
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CONTENTS, , 2.5.3, 2.5.4, 2.5.5, 2.5.6, 2.5.7, 2.5.8, 2.5.9, 2.5.10, 2.5.11, 2.5.12, , Arsenic, By-product electronic metals (selenium, tellurium, gallium,, germanium, indium, cadmium) and silicon, Bismuth, Zirconium and hafnium, Titanium, Rare earth elements (REE, lanthanides), Niobium and tantalum, Lithium, Beryllium, Uranium (and thorium), , 2.6 Summary and Further Reading, , Part II, 3, , Non-Metallic Minerals and Rocks, , vii, 245, 247, 250, 251, 254, 257, 261, 265, 268, 270, 283, , 285, , Industrial minerals, earths and rocks, , 287, , Synopsis, , 287, , 3.1 Andalusite, Kyanite and Sillimanite, 3.1.1 Andalusite, 3.1.2 Kyanite, 3.1.3 Sillimanite, , 288, 289, 290, 291, , 3.2 Asbestos, 3.2.1 Asbestos mineralization types, , 291, 292, , 3.3 Barite and Celestite, 3.3.1 Geochemistry, 3.3.2 Barite deposit types, , 293, 294, 296, , 3.4 Bentonite (Smectite Rocks), 3.4.1 Bentonite deposit types, , 299, 301, , 3.5 Boron, 3.5.1, 3.5.2, , 302, 303, 303, , Geochemistry, Boron deposit types, , 3.6 Carbonate Rocks: Limestone, Calcite Marble, Marlstone, Dolomite, 3.6.1 Limestone, 3.6.2 Metamorphic calcite (and occasionally dolomite) marbles, 3.6.3 Marlstone, 3.6.4 Dolomite, , 305, 306, 306, 306, 307, , 3.7 Clay and Clay Rocks, 3.7.1 Clay deposit types, , 308, 308
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viii, , CONTENTS, , 3.8 Diamond, 3.8.1 Source and formation of diamonds, 3.8.2 Diamond deposit types, , 310, 311, 312, , 3.9 Diatomite and Tripoli, 3.9.1 Diatomite deposit types, 3.9.2 Tripoli, , 317, 318, 319, , 3.10, , Feldspar, , 319, , 3.11, , Fluorite, 3.11.1 Geochemistry, 3.11.2 Fluorite deposit types, , 320, 321, 322, , 3.12, , Graphite, 3.12.1 Graphite deposit types, , 325, 326, , 3.13, , Gypsum and Anhydrite, 3.13.1 Deposits of gypsum and anhydrite, , 327, 328, , 3.14, , Kaolin, 3.14.1 Kaolin deposit types, , 330, 331, , 3.15, , Magnesite, 3.15.1 Magnesite deposit types, , 333, 334, , 3.16, , Mica (Muscovite, Phlogopite, Vermiculite), 3.16.1 Muscovite and phlogopite, 3.16.2 Vermiculite, , 339, 339, 340, , 3.17, , Olivine, 3.17.1 Olivine deposits, , 342, 342, , 3.18, , Phosphates, 3.18.1 Geochemistry, 3.18.2 Phosphate deposit types, , 342, 345, 345, , 3.19, , Quartz and Silicon, 3.19.1 Quartz deposit types, , 347, 348, , 3.20, , Quartzite, 3.20.1 Metamorphic quartzite deposits, 3.20.2 Sedimentary quartzite, , 349, 350, 350, , 3.21, , Quartz Sand and Gravel, 3.21.1 Industrial sand and gravel, 3.21.2 Building sand and gravel, , 350, 351, 352
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CONTENTS, , 4, , ix, , 3.22, , Sodium Carbonate, Sulfate and Alum, 3.22.1 Sodium sulphate, 3.22.2 Alum salts, , 354, 355, 355, , 3.23, , Sulphur, 3.23.1 Geochemistry, 3.23.2 Deposit types of elementary sulphur, , 355, 356, 356, , 3.24, , Talc and Pyrophyllite, 3.24.1 Talc deposit types, 3.24.2 Pyrophyllite, , 358, 359, 361, , 3.25, , Volcaniclastic Rocks, 3.25.1 Pumice, 3.25.2 Perlite, 3.25.3 Trass, , 362, 362, 363, 363, , 3.26, , Wollastonite, 3.26.1 Wollastonite deposit formation, , 364, 364, , 3.27, , Zeolites, 3.27.1 Zeolite deposit types, , 365, 366, , 3.28, , Summary and Further Reading, , 367, , Salt deposits (evaporites), , 369, , Synopsis, , 369, , 4.1 Salt Minerals and Salt Rocks, 4.1.1 Salt minerals, 4.1.2 Salt rocks, , 371, 371, 371, , 4.2 The Formation of Salt Deposits, 4.2.1 Salt formation today, 4.2.2 Salt formation in the geological past, , 376, 376, 384, , 4.3 Post-Depositional Fate of Salt Rocks, 4.3.1 Diagenesis and metamorphism of evaporites, 4.3.2 Deformation of salt rocks, 4.3.3 Forms and structures of salt deposits, 4.3.4 Supergene alteration of salt deposits, , 394, 394, 397, 398, 403, , 4.4 From Exploration to Salt Mining, 4.4.1 Exploration and development of salt deposits, 4.4.2 Geological practice of salt mining, , 405, 405, 406, , 4.5 Summary and Further Reading, , 409
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x, , CONTENTS, , Part III, 5, , 411, , Geological concepts and methods in the mining cycle: exploration,, exploitation and closure of mines, , 413, , Synopsis, , 413, , 5.1 Economic Considerations, , 414, , 5.2 The Search for Mineral Deposits (Exploration), 5.2.1 The pre-exploration stage, 5.2.2 Geological exploration, 5.2.3 Geological remote sensing, 5.2.4 Geochemical exploration, 5.2.5 Geophysical exploration, 5.2.6 Trenching and drilling, , 416, 416, 417, 420, 422, 428, 432, , 5.3 Development and Valuation of Mineral Deposits, 5.3.1 Geological mapping and sampling, 5.3.2 Ore reserve estimation and determination of grade, 5.3.3 Valuation of mineral deposits, , 437, 439, 440, 447, , 5.4 Mining and the Environment, 5.4.1 Potential environmental problems related to mining and mine-site, processing plants, , 448, , 5.5 Deep Geological Disposal of Dangerous Waste, , 458, , 5.6 Summary and Further Reading, , 462, , Part IV, 6, , The Practice of Economic Geology, , Fossil Energy Raw Materials – Coal, Oil and Gas, , 450, , 465, , Coal, , 467, , Synopsis, , 467, , 6.1 The Substance of Coal, 6.1.1 Coal types, 6.1.2 Petrography of coal, 6.1.3 The chemical composition of coal, , 471, 471, 474, 479, , 6.2 Peat Formation and Coal Deposits, 6.2.1 Types and dimensions of coal seams, 6.2.2 Concordant and discordant clastic sediments in coal seams, 6.2.3 Peat formation environments, 6.2.4 Host rocks of coal, 6.2.5 Marker beds in coal formations, 6.2.6 Coal formation in geological space and time, , 487, 487, 488, 491, 495, 497, 498
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CONTENTS, , 7, , xi, , 6.3 The Coalification Process, 6.3.1 Biochemical peatification, 6.3.2 Geochemical coalification, 6.3.3 Measuring the degree of coalification, 6.3.4 Causes of coalification, 6.3.5 Coal maturity and diagenesis of country rocks, , 499, 499, 500, 500, 501, 505, , 6.4 Post-Depositional Changes of Coal Seams, 6.4.1 Tectonic deformation, 6.4.2 Epigenetic mineralization of coal seams, 6.4.3 Exogenic alteration of coal, , 505, 505, 506, 506, , 6.5 Applications of Coal Geology, 6.5.1 Exploration, 6.5.2 Reserve estimation, 6.5.3 Coal mining geology, 6.5.4 Environmental aspects of coal mining, , 507, 507, 510, 512, 513, , 6.6 Summary and Further Reading, , 518, , Petroleum and natural gas deposits, , 521, , Synopsis, , 521, , 7.1 Species, 7.1.1, 7.1.2, 7.1.3, 7.1.4, 7.1.5, 7.1.6, 7.1.7, 7.1.8, , of Natural Bitumens, Gas and Kerogen, and their Properties, Crude oil, or petroleum, Natural gas, Natural gas hydrates (clathrates), Tar, Earth wax (ozocerite), Pyrobitumens, Natural asphalt, Kerogen, , 524, 524, 527, 530, 530, 531, 531, 531, 531, , 7.2 The Origin of Petroleum and Natural Gas, 7.2.1 Petroleum source rocks, 7.2.2 Dry gas source rocks, 7.2.3 Eogenesis and catagenesis of kerogen, 7.2.4 The oil window, , 533, 533, 536, 537, 539, , 7.3 Formation of Petroleum and Natural Gas Deposits, 7.3.1 Migration, 7.3.2 Reservoir rocks, 7.3.3 Petroleum and gas traps, 7.3.4 Formation and reservoir waters, 7.3.5 Alteration of petroleum in reservoirs (degradation), 7.3.6 Tectonic environments and age of hydrocarbon provinces, , 540, 540, 543, 545, 551, 551, 552
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xii, , CONTENTS, , 7.4 Exploring for Petroleum and Natural Gas Deposits, 7.4.1 Geophysical methods, 7.4.2 Geochemical methods of hydrocarbon exploration, 7.4.3 Exploration drilling, 7.4.4 Geophysical borehole measurements, , 553, 555, 556, 557, 558, , 7.5 The Exploitation of Petroleum and Natural Gas Deposits, 7.5.1 Reservoir conditions, 7.5.2 Oil or gasfield development, 7.5.3 Oil and gas production, 7.5.4 Petroleum mining, 7.5.5 Reserve and resource estimation, 7.5.6 Post-production uses of oil and gas fields, , 561, 561, 563, 565, 568, 568, 570, , 7.6 Tar Sand, Asphalt, Pyrobitumen and Shungite, 7.6.1 Tar sand, 7.6.2 Asphalt, 7.6.3 Pyrobitumens, 7.6.4 Shungite, , 570, 570, 572, 572, 573, , 7.7 Oil Shales, , 573, , 7.8 Environmental Aspects of Oil and Gas Production, 7.8.1 Water resources protection, 7.8.2 Induced seismic activity, 7.8.3 Tar sand mining, 7.8.4 Hydrocarbons and climate, , 575, 577, 577, 577, 578, , 7.9 Summary and Further Reading, , 579, , Epilogue, , 583, , References, , 585, , Index, , 655, This book has a companion website: www.wiley.com/go/pohl/geology
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Preface, Wisely used, mineral resources create wealth,, employment, a vital social and natural environment and peace. If the reverse of these conditions, occurs only too often, illustrating the so-called, “resource curse”, this should be attributed to the, true perpetrators, namely irresponsible, weak or, selfish leaders. This book, however, does not, intend to provide rules for good governance. I, wrote it as a broad overview on geoscientific aspects of mineral deposits, including their origin, and geological characteristics, the principles of the, search for ores and minerals, and the investigation, of newly found deposits. In addition, practical and, environmental aspects are addressed that arise, during the life-cycle of a mine and after its closure., I am convinced that in our time, economic geology, cannot be taught, studied or practised without an, understanding of environmental issues. The scientific core of the book is the attempt to present, the extraordinary genetic variability of mineral, deposits in the frame of fundamental geological, process systems. The comprehensive approach –, covering materials from metal ores to minerals, and hydrocarbons – is both an advantage and a loss., The second concerns the sacrifice of much detail, but I chose the first for its benefit of a panoramic, view over the whole field of economic geology., Being aware that the specialist level of subjects, presented in this book fills whole libraries, I do, hope that even experienced practitioners, academic teachers and advanced students of particular subjects will find the synopsis useful., Over more than 50 years, five editions of this, book were published in German. Since the first, edition (Wilhelm & Walther E. Petrascheck, 1950), the book was intended to provide a concise, introduction to the geology of mineral deposits,, including its applications to exploration and mining. The target audience has changed, however., Originally, it was written for students of mining, engineering. Today, it is mainly directed to aspir-, , ing and practising geologists. Each of the seven, chapters of the book was developed with my own, students as a university course and should be, useful to fellow academic teachers. After initially, working in industry I never lost contact with, applications of economic geology, which is my, motive for the constant interweaving of practical, aspects in the text and for dedicating one of the, chapters to the practice of economic geology. For, professional reference purposes, practitioners in, geology and mining should appreciate this melange of science and application. Frequent explanations and references to environmental and, health aspects of extraction and processing of, ores and minerals should assist users involved in, environmental work. To those with no background in geology, I recommend they acquire an, introductory geoscience text for looking up terms, that are employed but cannot be explained in the, available space., Compared with the last German edition (Pohl, 2005), this book has been rewritten for an international public. Although it retains a moderate European penchant by referring to examples from this, region, important deposits worldwide are preferentially chosen to explain genetic types and practical aspects. I trust that this will be useful to both, scholars and practitioners, wherever they work., Generally, it was my ambition to present the state, of the art in economic geology, by referring to and, citing recent publications as well as earlier fundamental concepts. This should assist and motivate, students to pursue topics to greater depth., Many people have supported me in my life-long, pursuit of theory and practice of economic geology, and helped with this book, especially by, donating photographs. I cannot name them all, but in captions, donors are acknowledged. Here,, just let me say thank you., Walter L. Pohl
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Plate 1.1 Bauxite extraction at Huntley mine, southwestern Australia. On the Darling Plateau, bauxite is part of a, mature soil profile developed over Archaean gneiss and granite. The area is covered by woodland (the jarrah, or, Eucalyptus marginata forest). Mining depends on the availability of land and its social acceptance on rapid, re-establishment of the native ecosystem. Reproduced by permission of Alcoa Inc., , Plate 1.2 Rehabilitated jarrah forest covers former extraction panels of Huntley bauxite mine in front of the lake., Reproduced by permission of Alcoa Inc., , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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Plate 1.11 Black smoker in the Mid-Atlantic Ridge graben, 4 48 S, 12 37 W, at a water depth of ca. 3000 m. Courtesy, P.M. Herzig, IFM-GEOMAR (ROV Kiel 6000, 2009)., , Plate 1.13 Mid-oceanic copper mineralization in chimney fragments from inactive Sonne Field in the Central Indian, Ocean. Courtesy P. Halbach (ÓFU Berlin). The polished section shows pyrite (white) as the earliest sulphide phase,, followed by chalcopyrite (yellow), and increasing hydrothermal depletion of iron in bornite (Cu5FeS4, blue-brown) and a, thin rim of digenite (Cu9S5, pale blue). The long side of the image corresponds to 0.6 mm. Pore space filled by casting, resin is variably dark.
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Plate 1.21 Geothermal hot springs and siliceous sinter mound at Sempaya in northwestern Uganda.The convective, system is related to the large border fault of the Ruwenzori Mountains, with a vertical displacement of more than 10 km, between the petroliferous Tertiary Albert Rift in the west and the Palaeoproterozoic crystalline horst in the east., , Plate 1.29 The essence of hydrothermal, alteration visible at outcrop-scale, depicted by, the halo centred on a small fissure branching off, from formerly exploited wolframite-quartz, veins in Panafrican granite at Gash Emir, Red, Sea Hills, Sudan. Note the enhancement by later, supergene oxidation.
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Plate 1.31 Chuquicamata open pit in Chile, one of the world’s largest porphyry copper mines. Courtesy Bernd, Lehmann, Clausthal.The pit measures 2 3 km and approaches a depth of 900 m. Total pre-mining resources were, nearly 3000 Mt at 1% Cu and by-product Mo. Annual production is 1.2 Mt copper and 20,000 t molybdenum plus, rhenium., , Plate 1.33 Typical copper porphyry ore from Chuquicamata, displaying brecciation and quartz veining, pervasive, sericitic (greenish) and argillic alteration. Courtesy Bernd Lehmann, Clausthal.Grey ore in veinlets is largely chalcocite, due to supergene enrichment, which reaches 800 m below surface.
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Plate 1.50 Brilliantly white, supergene kaolin below red, laterite in the rainforest of, northern Burundi illustrates, lateritic soil profiles which, are shaped by tropical, weathering., , Plate 1.56 For over 100 years, the supergene,, high-grade chalcocite ore of the black shalehosted copper deposit at Mt Oxide in the Mt Isa, district, Australia, was the symbolic example of, fortune and destitution for investors and, miners.Gossan (red) covered an accumulation, zone of 55 m thickness which graded into, subeconomic primary sulphides. A sizeable ore, shoot of 15.9% Cu was extracted by, underground methods. Later, the pit was, excavated and overall, the deposit yielded, 23,000 t of copper (J.H. Brooks in Glasson &, Rattigan 1990).
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Plate 1.61 Aeolian lag enrichment of, magnetite (dark sand patches) at An Kor, Red Sea, Hills, Sudan. Note the exploration trench testing, the Neoproterozoic primary mineralization in, the foreground., , Plate 1.62 Panning, cassiterite-columbite ore, from Ngara pegmatite,, eastern Rwanda. Note the, small mass of black ore, mineral sand which, remains from washing the, pan filled with ore.
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Plate 1.64 Alluvial placer mining near Ruhanga in the tin-tantalum district of Gatumba, Rwanda. After extraction,, the devastated valley must be restored. Courtesy B. Lehmann, Clausthal., , Plate 1.67 Folded and metamorphosed Superior type banded iron formation near Mt Tom Price mine in the, Hamersley Gorge (Karijini National Park, Western Australia) with marine scientists Aivo Lepland and Mark van Zuilen, kindly posing for scale. Iron-rich beds black, silica (jasper) red. Photograph by Aivo Lepland, courtesy Geological Survey, of Western Australia.
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Plate 1.68 Haematitic iron oolite ore formed in a Late Cretaceous marine embayment at Aswan, Upper Egypt., , Plate 1.72 Undeformed shale-banded copper-zinc dominated sulphide ore from Rammelsberg sedex deposit (Germany), displays ductile soft-sediment deformation and cross-lamination. Width of image 20 cm. Courtesy B. Lehmann,, Clausthal.
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Plate 1.76 Bandedand brecciacaveoreofbrownsphaleriteinthehistoricLafatschmine,Karwendel,Tyrol.Thisisoneof, the outliers of the Triassic Alpine type carbonate-hosted Pb-Zn deposits. Courtesy B. Lehmann, Clausthal., , Plate 1.81 Ductilely folded sedimentary bedding in very-low-grade metamorphic shale-banded Fe-Cu-Zn sulphide, ore with wispy white dolomite laminae. Note the diffuse axial plane cleavage. Sample from the closure of the orebody, synform, Rammelsberg, Germany. Width of image 20 cm. Courtesy B. Lehmann, Clausthal.
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Plate 1.89 Metallogenetic overview of Southwestern Europe and adjacent Africa (clipping from Juve & Storseth, 1997). With kind permission of NGU, Trondheim. Text and symbols (cf. Plate 1.87) in four classes that indicate, relative size of deposits. The geological background is simplified to Hercynian (purple, mainly Palaeozoic); Alpine, (yellow, mainly Mesozoic) and cover sediments (light grey, mainly Tertiary). The distance between 5 latitude, parallels is 550 km.
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Plate 2.4 Birds-eye view of the high-grade haematite Mt Tom Price mine in the Hamersley Basin, looking to the, northwest (cf. Plate 1.52). Southwest dipping Precambrian banded iron formations build the hills that rise above the, plains. Courtesy of A.E. Harding, Rio Tinto Iron Ore, Perth, Western Australia., , Plate 2.9 Dunite with nodular chromite in the Ingessana Hills, southern Sudan. The hills expose a large, Neoproterozoic ophiolite and host several former chromite mines.
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Plate 2.12a Folded scheelite-quartz veinlet cutting across foliation of host greenschist at Felbertal mine, Austria., Length of specimen 50 cm., , Plate 2.12b UV illumination reveals the distribution of scheelite (white)., , Plate 2.18 Among dark Neoproterozoic volcanic rocks, the white peak of Abu Dabbab leucogranite in the Eastern, Desert, Egypt, is striking (although marred by desert varnish). The cupola is investigated for production of tantalum,, cassiterite and ceramic-grade feldspar. An exploration adit produced the white waste rock dump on the left slope.
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Plate 2.25 The Golden Mile Superpit at Kalgoorlie, Western Australia (2006). Note supergene oxidation in the, foreground, the city of Kalgoorlie-Boulder on the left and tailings (white) in the right-hand background. Photo provided, courtesy Kalgoorlie Consolidated Gold Mines (KCGM).
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Plate 2.30 Cerro Rico de Potosı, Central Cordillera, Bolivia, contained the world’s largest pre-mining silver, concentration. Courtesy B. Lehmann, Clausthal. Visible are ubiquitous traces of vein mining on the slopes and the, oxidized cap of the Miocene volcano. During the 17th century, Potosı was the principle source of fabulous wealth for, Spain and of silver inflation in Europe.
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Plate 2.32 Merensky Reef at Impala mine, S.A., with, footwall anorthosite (lower part), a thin chromitite band, in the middle and the pegmatoid reef on top, (orthopyroxene, plagioclase and the sulphides, pentlandite-pyrrhotite- chalcopyrite). Height of sample, is 15 cm. Courtesy B. Lehmann, Clausthal., , Plate 2.39 Bondi East heavy mineral deposit near the southeastern margin of the Murray Basin in Victoria, Australia,, looking north at an active mine face, which has been cleaned up for channel sampling and detailed mapping. Courtesy, David Whitworth (Iluka Resources). Barren overburden is removed; upper low-grade ore (ca. 10% HM) is white sand with, 30% clay. Note the near-vertical incision of the high grade ilmenite-zircon-rutile sand (dark, 50–70% HM) at the right of, the image. The footwall consists of barren massive silty sand (white). Patchy oxidation is ubiquitous.
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Plate 2.43 Giant Manono pegmatite in D.R. Congo is a sub-horizontal sheet and asymmetrically zoned. An upper, marginal zone is made up of near-vertical palisades of spodumene (with microcline) and patches of stanniferous albitite, (centre)., , Plate 2.44 Artisanal salt production at Salar de Uyuni, at 3500 m altitude in the Altiplano of Bolivia: One of the largest, salt lakes on Earth, the salar contains giant resources of lithium, potassium, boron and magnesium in brines below the, surface. The background mountains are part of the Central Cordillera, which hosts Cerro Rico de Potosi. Courtesy B., Lehmann, Clausthal.
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Plate 3.3 Celestite crystals in roughly bedding-parallel solution cavities of Neogene gypsum at Wadi Essel, Red Sea, Coast, Egypt., , Plate 3.8 Bentonite sample from Moosburg mining district, Germany. Copyright Ó S€, ud-Chemie AG 2009.
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, , Plate 3.27 Aheim dunite in thin section (crossed nicols). Olivine grain diameter is 1–2 mm. Note the fractures with weak, , net-textured serpentinization. Courtesy of Havard Gautneb, Geological Survey of Norway, Trondheim., , Plate 3.30 High-grade industrial sands at Uhry in northern Germany were deposited in a shallow bay of the Late, Cretaceous sea north of the Harz Mountains and chemically upgraded when the sea retreated westwards and tropical, forests covered the area.
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Plate 3.34 Eastward bird’s eye view of the talc quarry at, Luzenac in the French Pyrenees. White talc marks the, working face. Ó Philippe Psaila/SPL/PictureDesk.com., , Plate 4.3 Haselgebirge sample from Bad Aussee mine, near Salzburg, Austria. This variety displays a red halite, matrix with dispersed angular fragments of black, claystone., , Plate 4.6 Solar seawater evaporation pans for industrial, salt production on the West Australian coast. Courtesy, Dampier Salt Ltd and Rio Tinto Minerals. Seawater is, first concentrated to specific gravity 1.21 in order to, precipitate carbonate and gypsum. Different grades of salt, are crystallized between brine gravity 1.21 and 1.275., Harvesting is visible in the foreground. The remaining, K-Mg brine (“bittern”) may be processed or pumped back, into the sea., , Plate 4.12a Salt rafts floating on brine at the shore of, Lake Katwe, western Uganda. Katwe is a large maar lake, due to Pleistocene volcanism. It draws seepage water, from close-by fresh Lake Edward. The dark red colour of, the brine is caused by teeming micro-organisms. Upper, left corner is organic-rich mud.
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Plate 4.30 Historic salt exploitation by borehole solution at Haraucourt (Meurthe-et-Moselle, France) caused these, flooded collapse craters. The deposits are subhorizontal Late Triassic (Keuper) salt beds at shallow depth. Courtesy, Christian Wolkersdorfer, CBU, Sydney, Canada., , Plate 5.6 Environmental stream sediment and water, sampling (including in-situ determination of pH, T and, Eh) in the Gatumba tin-tantalum mining district,, Rwanda., , Plate 5.18 Lignite pit Sch€, oningen in northern Germany, with its captive power station. Coal extraction takes place, at the pit bottom. Overburden and combustion residues, are used to refill nearby exhausted pits. Flue gas cleaning, yields by-product elementary sulphur. Note white sands, on bench to the right. Strata are limnic and marine due to, marine transgression during the Palaeocene-Eocene, thermal maximum.
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Plate 5.19 Recultivation of the lignite pit Geiseltal in, northern Germany during the flooding operation in 2005., Photograph by Christian Bedeschinski 2005. Ó LMBV, (Lausitzer und Mitteldeutsche BergbauVerwaltungsgesellschaft mbH). The sunny slope on, footwall limestone in the foreground was planted with, grapevines. The Eocene lignite seam attained a thickness, of 100 m in a large salt subrosion depression within, Triassic limestone measuring 5 15 km. Because of the, induced alkalinity, the coal was famous for exceptional, preservation of vertebrate fossils and of chlorophyll in, green leaves. Exploited through nearly 300 years, original, lignite resources were 1600 Mt., , Plate 5.24 Cascades for aerating mine water from the, abandoned Dominion number 25 coal mine on Cape, Breton Island, Canada. Oxidation initiates precipitation, of colloidal red oxyferrohydrate, which gradually matures, into minerals such as goethite, ferrihydrite and jarosite., Courtesy Christian Wolkersdorfer, CBU, Sydney,, Canada., , Plate 5.25 Polishing reed bed as the last element in a, passive treatment system consisting of a combined, reducing and alkalinity-producing (RAPS) wetland, system for acid mine drainage from an abandoned coal, mine (Bowden Close near Durham, County Durham,, UK). Courtesy Christian Wolkersdorfer, CBU, Sydney,, Canada.
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Plate 5.29 Gorleben mine in northern Germany is destined (although not yet licensed) to function as a repository for, heat-producing radioactive waste in salt rock. The aerial view shows surface installations, the two shaft buildings,, waste disposal and Elbe River in the far distance to the left. Ó Bundesamt f€, ur Strahlenschutz, Germany., , Plate 6.2 Sleipner platform in the North Sea offshore Norway is the world’s first large-scale geological CO2, sequestration operation. On the platform, carbon dioxide is separated from natural gas and pumped into a sub-seafloor, aquifer. Ó Øyvind Hagen, Statoil.
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Plate 6.7 Microphotograph of the coal maceral textinite (a huminite; ungelified woody tissue with intact botanical, cell structures) in Tertiary lignite, Poland. Reflected light, oil immersion; long side of image 0.5 mm. Courtesy of Maria, Mastalerz and Indiana Geological Survey., , Plate 6.8 Microphotograph of the coal maceral ulminite (a huminite; more or less gelified woody tissue) in Tertiary, lignite, Poland. Reflected light, oil immersion; long side of image 0.5 mm. Courtesy of Maria Mastalerz and Indiana, Geological Survey.
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Plate 6.9 Microphotograph of the coal maceral sporinite (liptinite; wax-coated fossil spores and pollen) in, Pennsylvanian bituminous coal, USA. Fluorescent light; long side of image 1.0 mm. Courtesy of Maria Mastalerz and, Indiana Geological Survey., , Plate 6.20 Outcrop of Permian Great Northern coal seam below fluvial conglomerate on the Pacific shore, Sydney, basin, New South Wales, Australia. Note vertical joints and subdivision of seam into plies. Courtesy of Keith Bartlett,, Minarco-Mineconsult, Tuggerah, NSW.
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Plate 6.30 Post-mining lignite open pit lakes in the Lausitz region, Germany, in the last stages of filling and, rehabilitation. Courtesy of P. Radke, Ó LMBV, Lausitzer und Mitteldeutsche Bergbau-Verwaltungsgesellschaft mbH., , Plate 7.6 Early Mesozoic bituminous rocks in the upper Kali Gandaki valley of the Annapurna-Dhaulagiri zone,, western Nepal. This is part of the unmetamorphosed sediments of the Tethyan zone above the crystalline Greater, Himalayan Sequence. Courtesy Krishna Karki, Ó Travel-to-Nature Asia. The image serves as a paradigm for, hydrocarbon source rocks. Further up in the mountains, natural methane seepage feeds eternal flames in Jwala Mai, temple.
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Plate 7.29 Oil sand mining at Muskeg mine, Athabasca, Canada (2009). Copyright Shell plc., , Plate 7.32 Floating production storage and loading vessel in the Bonga field offshore Nigeria. The field lies 120 km, from the River Niger mouth in water more than 1000 m deep. Copyright Shell plc.
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Introduction, , Human societies need sufficient water, productive, soil, energy in different forms, and organic and, mineral raw materials as a base for their physical, existence. An additional important requirement is, a healthy natural and socio-economic environment., Economic Geology is a subdiscipline of the, geosciences. It devotes itself to the scientific study, of the Earth’s sources of mineral raw materials and, to the practical application of the acquired knowledge. Considering the life-cycle of a mine, economic geology leads in the search for new mineral, deposits and in their detailed investigation. It contributes to economic and technical evaluations,, which confirm the feasibility of a project and end, with the physical establishment of a mine. While, mining goes on, economic geology provides many, services that assist rational exploitation, foremost, by continuously renewing mineable reserves and, by limiting effects on the mine’s environment to, a minimum. Possible negative impacts of mining, include surface subsidence, lowering of the water, table, various emissions and mechanically unstable or environmentally doubtful waste rock, dumps. In the phase of mine closure, economic, geology helps to avoid insufficient or outright, wrong measures of physical and chemical stabilization, recultivation and renaturalization., In recent years, the economic evolution of, industrial and of rapidly developing countries, caused incisive changes in supply and consumption of mineral raw materials. China, rather than, , Europe or North America, provides world markets, with essential metals and minerals, although at, the same time importing large quantities of, needed feedstock for its expanding population and, industry. The future supply of petroleum appears, to be unreliable, but its role as the main source, of liquid fuels for transport is hardly dented by, biofuels and other developments. Wind and geothermal energy are increasingly contributing to, electricity production, yet without coal, nuclear, power and natural gas, industrial economies, would soon break down and developing nations, would be locked in poverty. Ours is a time of, transition but we cannot yet discern the outcome., Whatever it will be, metals, minerals and energy, raw materials will remain a precondition of, human welfare., , WHAT ARE ORE DEPOSITS?, Ore and mineral deposits are natural concentrations of useful metals, minerals or rocks, which, can be economically exploited. Concentrations, that are too small or too low-grade for mining, are called occurrences or mineralizations. It is, very important to understand the economic, implications of the difference between these, terms. Unfortunately, their wrong application, is common and leads to fundamentally misleading deductions. Therefore, the denomination, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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2, , INTRODUCTION, , “economic ore deposit” may be used when a clear, attribution to this class is to be emphasized. Note, that not all ores are strictly natural – it is very, common that waste of a former miners’ generation, is today’s profitable ore, such as tailings of earlier, gold, copper and diamond mining., Mineral deposits are basically just valuable, rocks. Their formation is compared with processes, that have produced ordinary rocks and is investigated with petrological methods (Robb 2005)., Mineral deposits can also be thought of as a geochemical enrichment of elements or compounds, in the Earth’s crust, which is determined by their, chemical properties (Railsback 2003; Lehmann, et al. 2000b). The ratio between the content of, a valued element in an ore deposit and its crustal, average (Clarke values, Wedepohl 1995) is called, the “concentration factor”. Formation of iron, ore, with today’s typical grade of 60% Fe relative, to an average crustal iron concentration of 5%,, requires 12-fold concentration. Copper ore that, has 1% Cu compared to the crustal average of, 0.007% Cu in the crust exhibits a 140-fold enrichment. Gold ore with 10 grams/tonne “distilled”, from ordinary rocks with 0.002 g/t Au attests to a, 5000-fold concentration., Manifold are the processes and factors leading, to the concentration of elements and minerals,, including the formation of mineral deposits (Robb, 2005). Final causes are the dynamic interactions, between the Earth’s core, mantle and crust, and, of the hydro-, bio- and atmosphere. Cooling and, devolatilization of the Earth and unmixing of the, system in the geological-geochemical cycle and, during the transfer of elements have important, roles (Lehmann et al. 2000b). With reference to the, origin, endogenous and exogenous process systems are distinguished. The first are those resulting from the dynamics of the Earth’s interior that, are ultimately driven by the Earth’s heat flow., At present, the total heat flow at the Earth’s, surface is 46 3 Terawatts (1012 J/s), resulting, from heat entering the mantle from the core, of, mantle cooling, radiogenic heating of mantle and, crust by the decay of radioactive elements and of, various minor processes (Lay et al. 2008). Exogenous processes take place at the Earth’s surface, and are mainly due to the flow of energy from the, , sun (12.1018 J/s). In rare cases, extraterrestrial, processes have contributed to the formation of, mineral deposits by impact of meteorites and, asteroids., The origin of mineral deposits is often due to a, complex combination of several processes, boundary conditions and modifying factors, collectively, making up the metallogenetic, or minerogenetic, system. Evidence for such systems that operated, in the geological past is always fragmentary. Some, questions can possibly be answered by studying, presently active ore-forming systems (e.g. black, smokers in the deep oceans), but this method, (“actualism”) has limitations. Because of the, unknown factors, there is often room for different, interpretations (hypotheses) of the scientific, facts. Economic geology strives to improve continuously the genetic models of ore formation,, i.e. complete schemes of these systems. This effort, is assisted by progress in many other sciences, (from biology to physics), but the reverse is also, true. Economic geology provides a fascinating, insight into geological systems that are extremely, rare and can only be illuminated by studying, mineral deposits. The practical mission of economic geology is the provision of metals and, minerals that society requires. Of course, this, implies cooperation with other scientific, technical and financial professionals., MINING IN THE STRESS FIELD BETWEEN, SOCIETY AND ENVIRONMENT, Cum semper fuerit inter homines de metallis dissensio, quod alii eis praeconium tribuerent, alii ea, graviter vituperarent (the original text in Latin by, Georgius Agricola 1556)., In English: “People were always divided in their, opinion about mining, as some praised it highly while, others condemned it fiercely.”, , Agricola reports that enemies of mining in his time, deplored not only harmful effects on the immediate environs but even moral aspects – they accused, mining of advancing greed. Today, this remains, one motive of opposition to the industry, but, fundamental rejection of any extraction of minerals is more common. The main reasons given are
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INTRODUCTION, , that mining visibly uses the land and often leaves a, profound and enduring change., Certainly there are often sound arguments, against mining at a specific location. Compromises should be sought, however, because mineral, deposits cannot be installed at arbitrary places., Their locations are predetermined by nature., Examples are sand and gravel deposits in river, plains. Today, these raw materials are so scarce, in many regions that they have to be protected, against other claims (e.g. housing developments)., Yet, everyone consumes minerals and mineralderived products for homes, heating, transport,, computers, medicinal use and numerous articles, of daily life. Mining provides these minerals., Recycling replaces only part of primary production., As a percentage of total area, land use by mining, is very small and only locally visible. Biofuel, agriculture, solar and wind energy plants require, much more land. Indeed, they create additional, demand for minerals (e.g. fertilizer, metals for, machines and processing plants, transport). Toxic, elements, such as arsenic and cadmium, are essential for sustainable energy production, for example, in photovoltaics. In many cases, even low footprint technologies such as geothermal power, plants have serious problems with waste, such as, brines, salt, toxic and heavy metals (most notably, arsenic, mercury and radionuclides). This demonstrates that there are no simple solutions for a, sustainable economy without mining. On the, contrary, it is undeniable that conservation of our, quality of life and development for the major part, of humans who still lack the most basic necessities for a life of dignity, require both mineral raw, materials and an intact environment., Mining without an impact on the environment is impossible (Figure 1.1), but the industry, strives to minimize negative effects (Figure 1.2), and to improve the welfare of affected communities (“green mining”). Some mining operations, create an enriched landscape of constructed ecosystems, which provide humans with a variety of, services (e.g. food, flood and erosion control, areas, for recreation and aesthetics, and clean water)., Examples include lignite and clay pits, which, bequeath beautiful new lakes. Hard rock mines, and quarries may grow into rare islands of nature, , 3, , in a sea of human occupation. Many of these sites, support rare and threatened species from archaea, and bacteria to plants and animals, helping to, preserve biodiversity (Batty 2005)., Reversing mineral extraction, mines also have, an extremely important role as deep disposal sites, for the safe storage of society’s unavoidable toxic, and radioactive waste. Chemically dangerous, waste is usually stored in worked sections of, suitable underground mines. For highly toxic and, radioactive waste, the construction of dedicated, underground disposal mines is the best solution, for protecting the biosphere. Underground disposal takes lessons from nature that has preserved, high concentrations of hazardous solid and gaseous substances in the form of mineral deposits, over many millions of years (e.g. sulphide metal, ore, natural gas, uranium and even the remains of, natural nuclear reactors)., The World Commission on Environment and, Development (“Brundtland-Report”, Brundtland, 1987) extended the concept of sustainable development to non-renewable resources. Clearly, few, mineral resources fit into the concept of sustainability, as it was formulated 300 years ago for, the management of forests, “that the amount of, wood cut should not exceed the growth rate”, (Carlowitz 1713). Such exceptions may be salt,, magnesium and potassium harvested from seawater. Most metals and minerals are non-renewable, and their use should be managed according to, the following rules: i) Consume as little as possible; ii) optimize the recycling rate; and iii) increase, the efficiency of using natural resources, especially of energy. The original concept of sustainability considered mainly the interests of later, generations. In the Rio Declaration (UN Conference on Environment and Development 1992) the, concept of intrageneration fairness was added, to, allow for the interests of the living generation of, mankind., In fact, the world population’s rapid growth, and demands for a better life enforce a continuing expansion of raw materials production. Yet,, every individual extractive operation must have, the acceptance of public opinion. To reach that, aim, all stakeholders must profit and the mine’s, social as well as the natural environment need to
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4, , INTRODUCTION, , be improved. The radical call that sustainability, requires immediate termination of mineral exploitation is, of course, social and economic nonsense, (Gilpin 2000). Let us use needed resources in the, interest of living humans, and let us trust in, technical and economic inventiveness to provide, for later generations., , THE MINERAL RESOURCES, CONUNDRUM, But is there a sufficient mass of minerals for an, ever-increasing consumption? Because of the limited size of our planet it is true that geological, resources are principally finite, although very, large indeed. The search for most minerals has, hardly gone deeper than a few hundred metres, below the surface, and only land, shallow seas and, the margins of the vast oceans are fully explored, for petroleum and gas deposits. Yet, even in the, well-known Gulf of Mexico, the new giant Tiber, oilfield was recently (2009) discovered. Metal mining on the sea floor has a large future potential., Giant unconventional gas resources promise to, fundamentally alter geopolitics of global energy, supply. In contrast to resources, reserves that can, be exploited at present economic and technological conditions are only a small part of the total, geological endowment, because searching and, defining reserves is a capital investment that must, be paid back with interests. Due to the rules of, depreciation of a future income, reserves are typically defined for the next 10 to 30 years. The result, is that at any time a division of total reserves by the, yearly consumption (the R/C ratio or “life-index”), will predict that in 10 (or 20, or 30) year’s time,, , “the world will run out of” the respective minerals. This fundamental error was famously made, by the Club of Rome when it predicted this dire, fate for the years 1990–2000 (Meadows et al., 1974). However, predictions of impending catastrophes are always popular and this gave the, Club of Rome’s hypothesis a sweeping impact., Actually, the imminent scarcity of important, minerals was announced many times in the past, but never arrived. The term “life-index” is misleading, and the figure is rather an indication of, specific conditions that dictate financing, production and marketing of individual metals and, minerals. With few exceptions, individual R/C, ratios change little over time-scales of several, decades. In the future, just as in the past, science, and technology will continue to provide the mineral raw materials needed by society, both by, finding new deposits and by providing natural or, synthetic replacement (Wellmer 2008). Temporary scarceness of certain critical raw materials is, only possible if political constraints distort markets (European Commission 2010). Furthermore,, exploiting lower-grade ores, producing functional replacements for certain minerals and, metals, and recycling of materials, all need, energy. Accordingly, energy is the most important natural resource of all., It is undeniable that there are physical limits, to the availability of certain quality classes of raw, materials. Severe problems arising from this fact, are not expected as long as the unlimited resource, of human creativity is given the freedom to search, for solutions. The continuously expanding reserve, base for practically all minerals, roughly in parallel, to increasing consumption, is the best proof of, this principle in the mining industry.
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PART I, Metalliferous Ore, Deposits, Economic geology defines ore as a natural material (ore rock) from which metals or minerals can, be profitably extracted. Mining professionals use, the word in an identical connotation. Note that, metals and minerals can also be recovered from, quite exotic materials that are not ore sensu, stricto, for example saline brines (lithium, magnesium), geothermal waters (zinc), metal-accumulating plants (nickel; “phytomining”), acid, mine water (copper) and of course, recycled scrap, (iron). The term ore is also applied to metalliferous minerals in a rock, for example chromite in, dunite, or magnetite in gabbro (“ore minerals”)., Ore rock, commonly just called ore, is typically, an intergrowth of useless minerals (“gangue”), with ore minerals. Massive ore consists of ore, minerals only, with little gangue, for example, high-grade haematite iron ore., , Ore deposits form by geological process systems, that can be viewed as a large cycle of constructive, (e.g. magmatism) and destructive sectors (e.g., weathering; Figure 1.3). Within this cycling of, earth materials, individual metals have specific, enrichment sites that depend on chemical and, physical properties of the metal or its compounds., It is very important to remember that biogeochemical fluxes mediated by life (Falkowski et al. 2008), control many ore forming processes., Part I of this book is divided into two chapters. In, Chapter 1, general observations, characteristics, and interpretations of ore deposit formation processes, process systems and associated outstanding deposit types are presented. In Chapter 2, the, economic geology of metals is systematically presented and illustrated by reference to specific, mining districts and deposits., , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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CHAPTER 1, Geological ore formation, process systems, (metallogenesis), Synopsis, Energy flow from the Earth’s interior and from the sun drives geological process systems. The, concentration of ore and minerals is part of these systems, which comprise intrusive and extrusive, magmatism, weathering, erosion, transport and sedimentation, followed by diagenesis and metamorphism. In this chapter, we aim to acquire an overview of these systems in respect of the, principles which govern the generation of ore deposits. Finally, the inspection of the different major, systems is brought together in a synthetic view of global dynamics and metallogeny (i.e. the science, of ore deposit formation). This chapter lays the ground for the rest of the book., For a long time in the past, processes associated, with differentiation and cooling of magmatic, bodies were thought to be the main agents of, ore deposit formation. Starting with mafic melt,, ore minerals can form upon cooling or metal-rich, melts can segregate from the silicate liquid., Because mafic silicate minerals crystallize at, higher temperature, intermediate and felsic residual melts are formed with their own suite of ore, deposits. Late-stage magmatic fluids collect metals and produce hydrothermal mineralization., Lindgren (1933), Niggli (1948), Schneiderh€, ohn, 1932, 1962), Stanton (1972), Guilbert & Park, (1986) and many others developed this, concept of igneous ore formation. In addition,, the role of weathering, erosion and sedimentation, in concentrating metals was recognized. Metamorphic processes were seen to transform previ-, , ously existing ore but without appreciable mass, transfer., More recently, these earlier views (here very simplified) on ore deposit formation were fundamentally expanded (Robb 2005, Evans 1998). First, the, discovery of plate tectonics caused a revolution in, understanding the dynamic interaction of the, Earth’s crust and mantle. Plate tectonics determine, the origin and distribution of many ore deposits., Present ore-forming processes were investigated., Outstanding impulses brought the exploration of, ocean floor hydrothermal venting that produces, metal concentrations, which closely resemble longknown ore deposits (e.g. copper on Cyprus Island)., The application of new technologies of the geosciences (e.g. trace element analysis, microprobe,, isotope geochemistry, fluid inclusions investigations, mathematical modelling and simulation), , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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8, , PART I METALLIFEROUS ORE DEPOSITS, , guided by old and new hypotheses, led to changes, in metallogenetic thinking and to the recognition, of additional ore formation systems. One example, is the dehydration of sedimentary basins during, diagenesis: Expelled fluids cause appreciable geochemical mass-transport and formation of numerous metallic and mineral concentrations, without, involvement of igneous processes. Furthermore,, the role of dissolved salt, hydrocarbons, reefs and, karst cavities in diagenetic ore formation was illuminated. Long after the first hypothetical considerations, metamorphism was finally proved to, cause migration of aqueous fluids that transport, and precipitate metals., The classification of ore deposits by major earth, process systems is in principle quite simple. Complications arise mainly because of the extreme variability of individual deposits due to manifold, combinations of different processes and factors., Therefore, some authors prefer to arrange deposits, into associations and types, which are related by, geological setting, paragenesis and form, but not, necessarily by the same genetic process (Routhier, , 1963, Laznicka 1985, 1993). Other authors dispense, with geological environs and concentrate mainly on, processes (Robb 2005). In this book, fundamental, geological cycles (Figure 1.3) and ore-forming systems are to guide the reader through metallogeny., The genetic terms of Table 1.1 provide the basic, vocabulary of metallogeny. The non-genetic descriptors stratiform (layer-shaped) and stratabound (restricted to certain strata) only denote, shape and position of an orebody in relation to, sedimentary features, not its origin. Comprehensive explanations of geological and mining terms, can be found in the Dictionary of Mining (AGI, 1999) and the Glossary of Geology by Neuendorf, et al. (2005). Geological time nomenclature in this, book follows Walker & Geissmann (2009)., , 1.1 MAGMATIC ORE FORMATION, , SYSTEMS, , A very large and diverse group of ore deposits, originates by various processes during formation,, , Figure 1.1 (Plate 1.1) Bauxite extraction at Huntley mine, southwestern Australia. On the Darling Plateau, bauxite is, part of a mature soil profile developed over Archaean gneiss and granite. The area is covered by woodland (the jarrah, or, Eucalyptus marginata forest). Mining depends on the availability of land and its social acceptance on rapid reestablishment of the native ecosystem. Reproduced by permission of Alcoa Inc.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 9, , Figure 1.2 (Plate 1.2) Rehabilitated jarrah forest covers former extraction panels of Huntley bauxite mine in front, of the lake. Reproduced by permission of Alcoa Inc., , evolution, emplacement and crystallization of silicate melts (magmas) in the upper mantle and in, the Earth’s crust., Most post-Archaean magmatic rocks can be, classed according to their plate-tectonic environment. Rocks of the ophiolite association (basalt,, , gabbro, ultramafic rocks) are remnants of former, mid-ocean ridges, back arc basins, and of early and, primitive parts of immature oceanic island arcs., Mature island arcs and active continental margins, are distinguished by profuse amounts of orogenic, andesites and equivalent intrusive magmatic, Weathering, Sedimentation, , ns, , s, , r, , G, , sa, , Lat, eri, te, , os, , Epithermal veins, sinte, , Extrusive, , Gold, Deposit, , Magmatism, y, Au porph, , Intrusive, , Placers, , ries, , ska, Veins,, , Diagenesis, , rn, , Metamorphism, Figure 1.3 The origin of gold deposits in, relation to major geological process, systems within the Earth’s crust,, demonstrating the variety of ore-forming, systems., , Anatexis, Magmas,, fluids and volatiles, from the Earth’s mantle, , Subduction
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10, , PART I METALLIFEROUS ORE DEPOSITS, , Table 1.1 Common metallogenetic terms, ., , Syngenetic – denotes ores and minerals that formed at the same time as their host rocks (most often applied to sedimentary rocks, and ore), . Epigenetic – ores were emplaced into pre-existing rocks of any origin (e.g. veins, metasomatic ore), . Hypogene – ores that were formed by ascending solutions (e.g. Mississippi Valley type lead-zinc), . Supergene – ore formation by descending solutions (meteoric water interacting with rocks during surficial weathering, processes), . Lateral secretion – concentration of metals by abstraction from surrounding rock, . Endogenetic – concentration caused by processes in the Earth’s interior (magmatism or metamorphism), . Exogenetic – concentration caused by processes at the Earth’s surface (sedimentation, weathering)., , rocks. Continental collision causes melting of, sialic crust and voluminous granitic magmatism., Continental rifts are associated with bimodal, alkaline volcanism (basalt and rhyolite). Extensional deformation of continents and mantle melting result in emplacement of layered mafic, intrusions, flood basalts and alkaline magmatic, provinces. Most notable are subvolcanic ring complexes and kimberlite diatremes that transport, diamond from 200 km depth to the surface., The association of certain igneous rocks, with specific metal ores was established long ago., Ultramafic rocks host ores of nickel, chromium, and platinum, gabbro and norite copper,, cobalt, nickel, iron, titanium and vanadium,, andesite and intermediate intrusive rocks induce, copper and gold ore, and granites are related to, beryllium, lithium, tin and tungsten concentrations. Essentially, this distribution was understood as a result of the geochemical fate of, different metals during fractional crystallization, (solid-liquid fractionation) of silicate melt, bodies (Goldschmidt 1958). Meanwhile, magmatic rocks can be further differentiated, according to plate-tectonic setting, source rocks,, degree of partial melting, role of volatiles and, many other genetic variables. Examples are the, various basalt types (N- and E-MORB, intraplate,, island arc: Pearce et al. 1984; Pearce 1982;, Winchester & Floyd 1977), or the S-, I- and Agranitoids. We shall see later in this chapter that, some of these rock classes are related to specific, ore deposits., Impact magmas result from heat and high pressure caused by collision of extraterrestrial bodies, with the Earth. Melting affects part of the crust, and in rare cases even the upper mantle. Impact, , magmas differ chemically from other melts, because whole volumes of crust are liquefied,, whereas normally partial melting is the rule. In, addition, the impacting body may induce geochemical anomalies, especially regarding siderophile elements (e.g. platinum, iridium, cobalt and, nickel). Post-impact cooling can induce hydrothermal systems that are able to redistribute matter and provoke ore deposit formation., In conclusion, the geodynamic environment, controls the formation of ores from silicate, melts in several ways. At the scale of ore-forming, processes caused by single magmatic bodies,, the following major genetic stages are, differentiated:, . Orthomagmatic ore deposits are formed before, the melt cools to complete solidification, or in, other terms, in the liquid stage before solidus., . Pegmatitic ore deposits are the result of segregation of small residual melt batches from a large, crystallizing magma body approaching the solid, state; fertile pegmatite melt is characterized by, high amounts of volatiles and of incompatible and, rare elements., . Magmatic-hydrothermal ore deposits are produced by super- or sub-critical fluids, solutions, and gases that are segregated by all magmas,, which had more dissolved volatiles (H2O, CO2,, S, B, F, Cl, etc.) than the amount that could be, accommodated in silicates during crystallization;, because of this connection, the time of fluid, phase expulsion is commonly coeval with the, formation of solid phases (minerals) from the melt;, a significant part of the geochemical signature of, magmatic-hydrothermal ore deposits is determined by processes at the magmatic stage, (Aud, etat et al. 2008).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 1.1.1 Orthomagmatic ore formation, Oxide (magnetite, ilmenite, chromite), base metal, sulphide (Ni, Cu), and ore of precious metals (Pt,, Pd, Au) is often found in ultramafic and mafic, igneous rocks. More rarely, magnetite occurs in, intermediate and felsic silicate melts. Textural, (and many other) observations show that these, ores were formed at magmatic temperatures,, while the melt was essentially liquid and before, total solidification (Naldrett 2004). Therefore, this, class of ore deposits is called “orthomagmatic”., Numerous observations suggest that enrichment processes concentrate (“segregate”) low, metal traces from a large mass of silicate melt into, small volumes. A common evolution is that the, parent melt evolves towards saturation so that, either a solid (e.g. chromite) ora liquid (e.g. sulphide, melt) accumulates the metal in question. At some, stage, residual fluids may intervene. Many parameters influence these processes, including the, depth of intrusion, tectonic activities, the temperature gradient in space and time, fractional crystallization, dynamics of the melt body (e.g. convective, flow), repeated injection of fresh melt, assimilation, of country rocks, sulphur or external fluids, liquid, immiscibility of ore and silicate melts and mixing, or redissolution (Kerr & Leitch 2005). Because of, their higher density compared to silicate liquids,, ore melt droplets or solid ore phases typically accumulate above or within floor rocks, which may be, cumulates below still liquid magma (gravitational, accumulation; Sparks et al. 1993). Consolidation of, cumulate minerals can lead to expulsion of intercumulus liquid (“filter pressing”). As the system, cools, ore melts themselves may then separate into, cumulates (e.g. Fe-sulphides) and residual liquids, (Cu-rich sulphide melt)., Various mathematical models have been proposed that describe the orthomagmatic enrichment, process. Concentration of metals such as PGM, (platinum group metals), Au, Ni and Cu in sulphide, meltiscontrolledbytheNernstpartitioncoefficient, (D) between sulphide and silicate liquids, and by, kinetic factors. Disequilibrium is exposed by calculating the silicate/sulphide liquid mass ratio, (“R-factor”; Robb 2005, Campbell & Naldrett, 1979). A zone refining model is appropriate when,, , 11, , for example, sulphide droplets sink through a, magma chamber and collect chalcophile metals., Limited base metal content but very high PGM, enrichment can be explained by resorption of iron, sulphide liquid in undersaturated magma (Kerr &, Leitch 2005). In this process, the residual sulphide, retains the precious metals, which it may bequeath, to a later batch of undersaturated mafic melt., Most orthomagmatic ore deposits are found in, intrusive rocks. Eruptive equivalents are also, notable, such as the Ni-Cu-Fe sulphides in komatiitic lava flows of Archaean greenstone belts, or, the magnetite and haematite lavas and tuffs in, andesitic-rhyolitic volcanoes in Chile, Mexico, and Pakistan., Basic shapes of orthomagmatic orebodies are, layers in stratified magmatic rocks (often formed, as cumulates), lenses or cross-cutting dykes and, veins. This depends on the morphology of the, segregation (sedimentation) surface and on, dynamic factors. Massive ore is the product of, highly efficient unmixing of ore particles or melt, droplets and silicates, whereas disseminated mineralization reflects lower efficiency. Highly complex orebody shapes can be found in flow channels, and pipes of mafic lavas and intrusions, for example when widening or curvature of flow tubes, induce lower flow velocity of silicate melt that, carries chromite crystals or sulphide melt droplets, (Naldrett 1200, 1199; e.g. Voisey’s Bay, Canada)., Textbook examples of orthomagmatic deposits, are sulphide Fe-Ni-(Cu-PGE) ores hosted by, Archaean komatiitic lavas of the Yilgarn Craton, in Western Australia (Box 1.1; Barnes et al. 2007,, Barnes 2004, Hoatson et al. 2006)., Gravitational settling can also explain many, features of ore formation in layered mafic intrusions (Naldrett 2004, Cawthorn 1996, Irvine 1982)., Other contributing processes include flowage differentiation and convective scavenging (Rice &, Von Gruenewaldt 1994), in-situ crystallization on, the floor of the melt body, mixing of two different, melts, and uptake of material from outside (e.g. by, melting siliceous or sulphur-rich host rocks)., Often, the formation and segregation of a sulphide, melt is the key to enrichment of exploitable metals (Barnes et al. 2009). Layered melt bodies (in, respect of composition, temperature and density)
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12, , PART I METALLIFEROUS ORE DEPOSITS, , BOX 1.1, , Orthomagmatic nickel sulphide ore in komatiites, , Komatiites are ultramafic volcanic rocks with melting temperatures of 1700 C containing >18 wt.% MgO (Arndt et al., 2008). Their formation is the consequence of mantle temperatures 100 C higher than today (Naldrett 2010). Komatiitic, melts originated by 30–50% partial melting of mantle. Normally, these melts remained sulphide-undersaturated from, extraction through ascent and eruption to emplacement. Liquid komatiite lavas had a very high temperature and very low, viscosity (similar to water), resulting in high flow velocities. Komatiite lavas are capable of eroding and melting most, volcanic and sedimentary footwall rocks (“ground melting” or “thermochemical erosion”). The flows followed troughs on, the seafloor and formed flow tubes similar to submarine basalts. Komatiites were typically charged with suspended olivine, crystals. During cooling, cumulates and vesicular textures formed. Because of the extreme temperature difference, between ocean water and Mg-rich melts, flow tops are characterized by skeletal growth of olivine and pyroxene (“spinifex, textures”; Shore & Fowler 1999)., Elongate massive sulphide bodies, which were clearly formed from liquid sulphide melt, occur at the floor of flow tubes, and grade upwards into disseminated (matrix or net-textured) ore (Figure 1.4). Ore textures reflect gravitational settling in, the liquid phase. Sulphide melts can only form upon sulphur saturation, implying high sulphur content. The common, association of sulphide mineralization with the presence of sulphur-rich interflow sediments and isotopic compositions of, sulphur (Bekker et al. 2009) are strong arguments that admixture of crustal sulphur is the main difference between fertile and, barren komatiites. The availability of nickel (and elements such as platinum) for partitioning into iron sulphide melt is, favoured by low redox conditions. In that case, nickel is dissolved in the silicate liquid as uncharged Ni0 and is not, available for incorporation into olivine. Apart from the redox constraints, nickel content of magmatic sulphide liquids is, largely controlled by partitioning equilibria and the mass balance between silicate and sulphide melt (the “R-factor”;, Campbell & Naldrett 1979), including dynamic factors such as mixing and redissolution. The ore minerals of komatiite, nickel deposits comprise pyrrhotite, pentlandite, chalcopyrite and pyrite, with nickel concentrations in ore reaching 20%., Prendergast (2003) describes the nickel-bearing komatiites of Zimbabwe as proximal and basal parts of submarine, volcanoes that were quite flat and extended over hundreds of kilometres. The Neoarchaean komatiites of Western Australia, (2.7–2.9 Ga) host the majority of the world’s komatiite-associated Ni-Cu-PGE deposits. Palaeoarchaean komatiites are, depleted in platinum group elements (PGE) because the metal source of the younger, metalliferous komatiites, i.e. cosmic, matter bombarding the Earth during the period from 4.5 to 3.8 Ga, was only gradually mixed into the mantle (Maier et al., 2009). In contrast to Western Australia, the closely comparable Abitibi Greenstone Belt in Canada is richly endowed with, volcanogenic massive sulphide deposits (e.g. Kidd Creek, Noranda). Both regions display abundant orogenic gold deposits., It is assumed that the disparity is due to a different lithospheric structure (Barnes et al. 2007)., , Random spinifex texture, , Platy spinifex texture, , ~20 m, Komatiite lava flow, , Aphyric zone, , Thermal erosion trough, , Interpillow, sulfides, , Porphyritic komatiite, , Massive Fe-Ni-Cu sulfides, , Pillow basalt, , Cumulate komatiite, , Interflow sediment, , Basalt flow top, , Matrix/disseminated sulfides, , Footwall basalt, , Figure 1.4 Massive and disseminated nickeliferous pyrrhotite orebodies (Lunnon shoot, Kambalda,, Western Australia) at the base of an Archaean komatiite lava flow (adapted after Groves et al. 1986). By, permission from Macmillan Publishers Ltd. Nature Ó 1986.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , BOX 1.2, , Orthomagmatic ore formation in the Bushveld Complex, , The Bushveld Intrusive Complex comprises the Rustenburg Layered Suite and the Lebowa Granites. The first term, designates the layered mafic-ultramafic intrusion, which was formed in the Palaeoproterozoic at 2054 Ma, and which, contains enormous metal resources. The granites have nearly the same age and host less important fluorite and tin, deposits. The roof of the Bushveld Complex and the overlying granites consists of thick precursor basaltic andesites to, rhyolites (the “Rooiberg Group” volcanics). Rooiberg volcanics and the intruding granites are the products of crustal, melting caused by the giant mass of hot mafic melt. Country rocks of the Bushveld intrusion are Palaeoproterozoic, sediments and volcanics of the Transvaal Supergroup and Archaean basement. The Rustenburg Layered Suite reaches a, thickness of 9000 m. It is strongly layered at all scales. The major units from bottom to top comprise (Figure 1.5 and, Figure 1.6):, . the Lower Zone with dunite, bronzitite, and harzburgite;, . the conspicuously banded Critical Zone with a lower part of orthopyroxenite, chromitite bands and some harzburgite,, and a higher part marked by the first cumulus plagioclase and by cyclic layering of economically significant platiniferous, chromitite, harzburgite, bronzitite, norite and anorthosite in this order (cyclic units); its upper boundary is marked by the, Merensky Reef (Pt, Ni, Cu);, . the Main Zone with gabbronorite and minor layering;, . the Upper Zone with magnetite (ferro) gabbro and ferrodiorite, which contains numerous magnetite (V-Ti) layers., , Figure 1.5 Simplified lithostratigraphic column, of the mafic Rustenburg Layered Suite in the, Eastern Bushveld, South Africa, with major, ore horizons., , 13
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14, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.6 Bushveld Upper Group (UG1) chromite seam in anorthosite at Dwars River, S.A. Thin bands, at the bottom display bifurcation, illustrating the complexity of cumulate ore formation in layered mafic, intrusions. Courtesy Bernd Lehmann, Clausthal., An extremely detailed lithostratigraphic scheme has been established for most parts (“lobes”) of the intrusion (Vermaak &, Gruenewaldt 1986, Eales & Cawthorn 1996). Strontium isotope stratigraphy reveals that the intrusion formed by numerous, influxes of magma of contrasting isotopic composition with concomitant mixing, crystallization and deposition of cumulates., Locally, in all five lobes of the Complex, remarkable differences in thickness and facies of layers have been noted that point to, proximity of magma inflow (Maier & Eales 1994). The cyclic units of the Critical Zone were formed by mixing or mingling of, two different magmas, a resident magma of Main Zone type (or T-type) precipitating plagioclase, and fresh inflows of Critical, Zone type (U-type) contributing orthopyroxene (Naldrett et al. 2009). Apart from chemical processes, pressure fluctuations, are thought to have controlled rhythmic layering and ore deposition in the Bushveld melt chamber (Cawthorn 2005b)., The Bushveld contains the world’s largest exploitable resources of chromium, platinum metals and vanadium. For the future,, large amounts of titaniferous magnetite and apatite are available that have at present no economic value., , undergo thermal and chemical diffusion that can, concentrate ore metals. Although much less obvious than in many felsic intrusions, mafic melt, bodies may also experience unmixing and expulsion of magmatic fluids that can form ore. The, largest preserved layered intrusion in the world is, the Bushveld Complex of South Africa, hosting an, exceptional variety and mass of high-grade metal, ores (Box 1.2; Vermaak & Von Gruenewaldt 1986,, Eales & Cawthorn 1996)., Layered mafic intrusions occur in several geodynamic settings:, , Archaean greenstone belts;, intracratonic regions (the Bushveld);, . at passive margins of continents; and, . in active orogenic belts., Intracratonic regions that experienced tensional, tectonics can also exhibit unstratified, very complex mafic-ultramafic intrusions with Cu-NiPGM ores. The most important district of this, kind is Noril’sk-Talnakh in Siberia, which originated at the Permo-Triassic boundary as a feeder to, the giant Siberian trap basalt province (Yakubchuk, & Nikishin 2004, Li et al. 2009)., ., .
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Impact magma bodies with orthomagmatic, ore deposits, Mineralized impact structures are very rare. A, giant example is the Sudbury Igneous Complex, (SIC) of Ontario, Canada, the second-largest, source of nickel (plus much copper and platinum, metals) in the world, after Noril’sk in Russia. The, SIC is the remnant of a voluminous melt body that, has been produced by the impact of a meteorite, into continental crust 1.85 Ga ago (Dietz 1964)., The surrounding rocks comprise Archaean granites and gneisses, and metamorphic Palaeoproterozoic volcano-sedimentary suites. The elliptic, outline of the intrusion is thought to be due to, later orogenic deformation. Hydrothermally, altered (Ames et al. 1998) suevitic breccias, shales, and turbidites (Whitewater Group) cover part of, the intrusive complex, forming a central basin., Ore deposits occur mainly in embayments of the, footwall contact of the intrusion, in radiating, dykes (“offsets”, Figure 1.7) and within intensely, , 15, , brecciated footwall rocks up to 2 km from the, contact. There is a lithologic zonation from, the footwall upwards and towards the centre of, the intrusion: Marginal norite, gabbro and quartzdiorite with dunite inclusions and the Ni-Cu-sulphides form the “Sublayer Norite” and the offsets,, followed by norite of the “Lower Zone”, quartzgabbro of the transitional “Middle Zone” and, granophyre of the “Upper Zone”. The rocks are, clearly the product of crustal melting (Therriault, et al. 2002), but are very different from typical, layered intrusions (e.g. there is no rhythmic banding). At Sudbury, lithologic zonation is interpreted, to be due to gravity separation of mafic and felsic, liquids that formed an emulsion immediately, after the impact, and subsequent vigorous thermal, convection within the norite and granophyre, layers (Zieg & Marsh 2005). The ore-bearing sublayer displays typical features of mafic cumulates, and gravity segregation of sulphide liquids. Offset, dykes and footwall deposits host an important part, , Tyrnoe, , N, , Parkin, , Foy, Whistle, , Hess, , MacLennan, Ministic, , Whitewater Group, Suevitic breccias,, shales and turbidites, , Kirkwood, Manchester, & McConnell, Frood, Figure 1.7 Overview map of the, Sudbury impact structure,, Canada, one of the giant nickelcopper mining districts of the, world. Of close to 90 single, deposits known (Ames et al., 2008), a selection is shown here., , Creighton, Copper Cliff, , 10 km, , Worthington, Main mass of, Sudbury Igneous Complex, , Sublayer and, offset dikes, , Ni-Cu-PGM, deposits
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16, , PART I METALLIFEROUS ORE DEPOSITS, , of metal resources. Total past production and, current reserves of the Sudbury District are estimated at >1700 Mt of Ni, Cu, Co, Pt, Pd, Au and, Ag ore (Ames et al. 2008). Among approximately, 90 known Ni-Cu-PGE deposits, 14 are currently, worked., Anorthosite-ferrodiorite complexes, Anorthosite-ferrodiorite complexes of Mesoproterozoic age have an outstanding role as sources, of titanium (Ashwal 1993). Orebodies consist of, ilmenite and/or rutile, magnetite or haematite,, and a gangue of apatite and some graphite. Similar, rocks of lesser economic importance occur as, strata in the upper parts of layered mafic intrusions. The anorthosites are commonly coarsely, crystalline, rather massive than layered and, consist of >90% andesine to labradorite. Anorthosite plutons may be associated with coeval intrusions of troctolite, charnockite, rapakivi granite,, ferrogabbro and ferrodiorite. The latter are often, remnant melts after plagioclase crystallized, to form anorthosite and are parental to the immiscible Ti-Fe-P melts. Because of their high density,, the ore melts accumulate near the base of the, magma chamber. Resulting orebodies are stratiform and either massive or disseminated (as at Lac, Tio in Quebec, Canada). Some occur as thick dykelike bodies in deeper parts of the intrusive suite or, in country rocks. The origin of anorthosite-ferrodiorite complexes is not fully understood; one, hypothesis presumes late to post-orogenic partial, melting of tongues of lower crust in the mantle, (Duchesne et al. 1999). From these rocks, 50% of, the world’s titanium supply is derived; they also, contain about half of the total titanium resources., Since the high-grade Ni-Cu sulphides at Voisey’s, Bay in Canada were found in troctolites belonging, to this family, these rare rocks have acquired a new, prospective role., Fe-rich melts segregated from intermediate, to felsic magmas, The metallogenetic role of these melts is a more, contentious case of orthomagmatic ore formation., While there is no doubt that under conditions of, , high fO2 an immiscible FeOx liquid is in equilibrium with melt of felsic composition (Naslund, 1983), there is no general consensus that this is, a path to the formation of large ore deposits. One, objection is that the high viscosity of SiO2-rich, liquids should physically inhibit segregation by, gravity. Arguments brought forward to support the, concept include: i) shearing by slow convection of, the melt so that low-viscosity FeOx liquid may be, concentrated; and ii) high content of sodium and, phosphorous that act as fluxing agents for iron, melt. The process should produce ore of magnetite, and apatite in the proportion of about 2 : 1, as, exploited in the Kiruna-Malmberget District, (Sweden). High fluorine and chlorine content of, the apatites, and the presence of minerals such as, amphibole and scapolite, imply an eminent role of, magmatic volatiles (H2O, Cl, F, CO2, etc.). Volatiles promote segregation and mobility of ore melt., High fluid and salt content of melt batches segregating from felsic magmas evoke a likeness to, pegmatites and, similar to pegmatite systems, the, transition to hydrothermal ore formation may be, gradual and indistinctive (Borrok et al. 1998). The, unusual chemical composition of parental magmas and fluids may be due to assimilation of, evaporitic country rocks or of migrating saline, brines (Barton & Johnson 1996). After the discovery and investigation, at the giant Olympic Dam in, southern Australia, numerous iron oxide-rich deposits were subsumed in a new class, the iron, oxide-copper-gold (U-REE) deposits (IOCG), (Groves et al. 2010, Cox & Singer 2007, Pollard, 2006, Hitzman et al. 1992). Although a hydrothermal origin of IOCG mineralization is generally, accepted, the broader group displays a considerable variety of geological setting, the ratio, of orthomagmatic and magmatic-hydrothermal, mineralization, the nature of hydrothermal alteration systems and mineralizing fluid compositions. In some cases, metamorphic fluids may, have formed Cu-Au deposits resembling IOCG, (Baker et al. 2008):, Kiruna in northern Sweden (Figure 1.8), the largest, iron ore district in Europe, is traditionally considered, as the type-deposit of orthomagmatic iron ore formation in felsic intrusions (Harlov et al. 2002), because
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18, , PART I METALLIFEROUS ORE DEPOSITS, , originate by mixing and mingling of (ultra)mafic, and silicic melt (Clark & Kontak 2004)., Lower sections of ophiolites also contain orthomagmatic ore deposits. This includes diapiric, dunite bodies with streaky or lenticular disseminated and massive chromitite. The dunites occur, mainly within deformed refractory harzburgite of, tectonized mantle. Tabular chromitite seams may, occur in the lowermost ultramafic cumulates of, the mid-ocean gabbroic magma chamber. Both, cases are considered to be a consequence of chromite segregation from the melts that rise from the, mantle beneath mid-ocean spreading ridges. The, metallogeny of ophiolites is considered in more, detail below., Ultramafic complexes of the Urals-Alaska, type are concentrically zoned intrusions in orogenic and platform settings (Taylor 1967). In the, central dunite of the ring complexes, important, chromium and platinum ores can be concentrated (Urals: Garuti et al. 2003). Orthomagmatic concentrations of minerals and metals, are also part of the economic significance of, carbonatites., 1.1.2 Ore deposits at mid-ocean ridges, and in ophiolites, Exploration of ocean floors resulted not only in the, recognition of plate tectonics but also in the discovery of conspicuous signs of active ore forming, systems – the “black smokers”. Black smokers are, points of discharge of hot metalliferous solutions, from the ocean floor. Black smoker fields build, accumulations of metal sulphides on the ocean, floor, some of which may soon be economically, exploitable. Comparative investigations revealed, that ophiolites, an association of mafic and ultramafic rocks common in orogenic belts, are remnants of oceanic spreading processes that took, place at mid-ocean ridges, in marginal basins in, front of or behind island arcs (Dilek et al. 2001), or, within intra-ocean primitive island arcs. Ophiolites host ore deposits that display features reminiscent of black smoker fields and sulphide, mounds. However, fluid venting on the seafloor, may also occur in other tectonic settings, including magmatic arcs above subduction zones, , (Stoffers et al. 2006), hotspot ocean island volcanoes and dewatering sediments of active and passive continental margins., Ophiolites, Ophiolites are fragments of oceanic crust and, mantle that have been transported (obducted) as, thrust sheets (nappes) or schuppen towards continental masses. The tectonic emplacement was, normally associated with dismemberment of the, original succession. Yet there exist some exceptionally well preserved ophiolites (e.g. Cyprus,, Oman, Dongwanzi, China). A complete ophiolite, sequence comprises (Anon 1972):, . Extrusive basalts of typical chemical (MORB), characteristics at the top, often in the shape of, pillow lavas; ocean floor metamorphism of basalt, increases from the zeolite facies at the top to, greenschist facies at the bottom; however, many, ophiolites display volcanic rocks with supra-subduction zone signatures., . The sheeted dyke complex, consisting of vertical, basalt dykes that strike parallel to the former, oceanic graben; greenschist facies metamorphism, is dominant but may grade into amphibolite facies, near the footwall; contact metamorphism resulting from underlying melt bodies may occur; many, ophiolites, however, lack sheeted dykes., . The plutonic complex, comprising higher, intrusive homogeneous gabbro, diorite, tonalite, and trondhjemite (“plagiogranite”), and deeper, layered gabbro and peridotites, that display, properties of cumulate rocks (the “cumulate, sequence”); the magmatic rocks are normally not, metamorphosed., . The tectonized and depleted mantle, dominated, by large masses of serpentinite (after harzburgite), and characteristic pods of dunite., Various marine sediments may cover the igneous rocks, but most frequent are biogenic cherts, and pelagic limestones. It is interesting to note, that abiogenic exhalite cherts and jasper have, only formed before the Middle Cretaceous; since, that time, seawater is highly undersaturated, with respect to silica because of the emergence, of silica-consuming diatoms (Grenne & Slack, 2005).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Formation of the ophiolite sequence can be, modelled by partial melting of primitive mantle, under mid-ocean ridges, due to mantle heat flow, and the decompression caused by extension. The, liquid rises in numerous melt batches and accumulates to fill large shallow magma chambers., These evolve by fractional crystallization and episodic volcanic processes. Mid-ocean ridges with, low spreading rates are composed of sections with, high magmatic activity that alternate with sections of tectonic rifting only. The first produces, the typically layered ophiolite sequence (“Penrose, Crust”); the second may lead to exposure of mantle rocks (serpentinites and peridotites: “Hess, Crust”). The latter is quite inconspicuous because, black smokers are very rare, and has received little, attention. Superfast spreading rates produce thin, crust and shallow melt bodies., Economic interest focuses on deep ultramafic, rock suites of the Penrose sequence (for chromite, and platinum) and the near-surface volcanic, hydrothermal system (for base metals). Tectonized (foliated) harzburgite and the lower cumulates host dunite bodies that may contain massive, and disseminated chromite ore. Dunite in harzburgite can be understood as lag segregation from, rising basaltic melt diapirs. Chromitites originate, from dunite by liquid-liquid immiscibility., Because of ductile shearing in the oceanic mantle,, both dunites and chromite orebodies are strongly, deformed, resulting in lenticular pod-like shapes., , 19, , Chromites in cumulates occur in seams that lack, early deformation., From the beginning of geological history, newlyformed ocean crust was cooled by convecting seawater. This is at the origin of ocean floor metamorphism and possibly, of earliest life on Earth, (Russell et al. 2005). Processes taking place along, the mid-ocean ridges are interconnected with the, whole Earth System (Halbach et al. 2003). Rising, branches of convection cells transport metals and, reduced sulphur leached from mafic and ultramafic rocks towards the ocean floor (Figure 1.10)., Some of the volatiles and metals may be derived, from magmatic fluids. More than 100 active, submarine venting sites have been discovered, over the last 30 years along the 60,000 km-long, worldwide network of mid-ocean ridges. Most, vents are basalt-hosted and of the black smoker, type (Herzig & Hannington 1995)., Black smokers, Submarine black smoker vents are hydrothermal, cones or chimneys that may reach a height, of about 20 m, built on outcrops of bare basalt, (Figure 1.11). From an opening at the top, a highspeed jet of hot clear fluid is ejected. The vents are, tubes with zoned walls, from pyrite and chalcopyrite inside through sphalerite, marcasite, barite,, anhydrite and amorphous SiO2 to the exterior, (Figure 1.12 and Figure 1.13). Oxidation of, , Metal-rich brine pool sediments or black smokers, , Seawater, Pillow lavas, , z, g, , z, g, Sheeted dike complex, , g, a, Figure 1.10 Seawater convection, ocean, floor metamorphism and focusing of rising, hot fluids by apical parts of a mid-ocean, magma chamber and by faulting (modified, from Gass 1980). By kind permission of, Geological Survey Department Cyprus., , g, a, , Gabbro, Magma, , z, g, g, a, , Magmatic fluids?, , Zeolite-greenschist facies boundary, Greenschist-amphibolite facies boundary, , 1 km
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20, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.11 (Plate 1.11) Black smoker in the Mid-Atlantic Ridge graben, 4 48 S, 12 37 W, at a water depth of ca., 3000 m. Courtesy P.M. Herzig, IFM-GEOMAR (ROV Kiel 6000, 2009)., , Figure 1.12 Hydrothermal, vents on pillow basalt lava, in a mid-ocean rift. Adapted, from Haymon (1983). With, permission from, Macmillan Publishers Ltd., Nature Ó 1983. In the, foreground “black smoker”, vents, white smoker on the, right. Section from interior, to exterior: Dotted ¼ Cu-Fesulphides;, hatched ¼ weathered, sulphides; white ¼ talc and, sulphides;, black ¼ anhydrite and, sulphides.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 21, , Figure 1.13 (Plate 1.13) Mid-oceanic copper mineralization in chimney fragments from inactive Sonne Field in the, Central Indian Ocean. Courtesy P. Halbach (ÓFU Berlin). The polished section shows pyrite (white) as the earliest, sulphide phase, followed by chalcopyrite (yellow), and increasing hydrothermal depletion of iron in bornite (Cu5FeS4,, blue-brown) and a thin rim of digenite (Cu9S5, pale blue). The long side of the image corresponds to 0.6 mm. Pore space, filled by casting resin is variably dark., , sulphides by seawater (“seafloor weathering”) produces varicoloured ochreous alteration fragments,, which mainly consist of iron oxy-hydroxides, that assemble on the floor around the vents and, build gossan-like mounds. Because the leaky vent, tubes draw in cold seawater, an amazing macrofauna of large mussels, bright-red tube-worms,, crabs and shrimp finds perfect ecological conditions (Van Dover 2000, Fisher & Girguis 2007)., The base of the higher life forms are thermoacidophilic and hyperthermophilic chemotrophic, microbes (bacteria and archaea, e.g. Pyrolobus, fumarii) that inhabit the vent walls at temperatures up to 121 C (read more about archaea, the, third domain of life, in Friend 2007). Macrofossils, similar to some of the fauna living on active vents, today were described from the Cretaceous sulphide ore deposits of the Troodos ophiolites on, the Mediterranean island of Cyprus (Little et al., 1999). The expulsion temperature of the metalliferous solutions is most often at 350 C. With, 407 C (fluid) and 464 C (vapour), the highest, , temperatures were measured in the equatorial, Atlantic (Koschinsky et al. 2008). Alkaline, “white smokers” and diffuse discharges of warm, water with little dissolved matter have lower, temperatures., The hot Na-Ca-Cl fluids of the black smokers, are reducing and have pH from 2 to 5.5 (mostly, 4–5), salinities from 0.1 to 3 times seawater, elevated iron, copper, zinc, barium and SiO2, and, traces of As, Cd, Li, Be, Cs, Mn, B, Cl, HCl,, H2S, and CH4. Different solutes are derived from, various protoliths, possibly from magma, and, reflect also different conditions of water-rock reactions. For example, copper is enriched relative to, iron under moderately oxidizing conditions,, whereas a low fO2 results in a high Fe/Cu ratio, (Seyfried & Ding 1993). Deep unboiled fluids display higher metal concentrations than vent fluids, (e.g. Cu 14–17 ppm, Zn 5–27 ppm: Hardardóttir, et al. 2009). Fluid properties change by phase, separation, boiling, alteration and mineral precipitation during rise to the seafloor. Manganese, iron
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22, , PART I METALLIFEROUS ORE DEPOSITS, , and 3He of the venting fluids disperse in ocean, water and form a wide geochemical halo that is a, guide to the search for submarine hydrothermal, zones. Indications for supercritical unmixing of, fluids before discharge into a depleted gas phase, and a metal-rich brine were found at several, spreading ridges. Supercritical unmixing and normal boiling followed by condensation of vapour, and mixing of products are thought to explain the, spread of salinities and unusual compositions of, vent fluids (Cathles 1993). Only one site is known, were fluids discharge at supercritical conditions, (the critical point of seawater is at 298 bar and, 407 C: Koschinsky et al. 2008)., Upon discharge at the ocean floor, hot acidic, fluids mix with cold alkalic seawater, which results in immediate precipitation of solutes. If iron, prevails, black or grey smoke-like plumes of amorphous iron sulphide and iron-manganese oxy-hydroxides rise several hundred metres upwards and, disperse over a distance of many kilometres. Zinc, forms bluish, SiO2, barite and anhydrite white, clouds (white smokers). So-called “snow-blower, vents” emit dense clouds of white filaments of, native sulphur that is produced from H2S by sulphur-oxidizing bacteria (Taylor & Wirsen 1997)., Particle chemistry and 3He, CH4 and H2S content, of seawater near vents allow predictions of temperature and composition of vent fluids (Feely, et al. 1994)., White smokers, White smoker vents discharge fluids between, 100 and 300 C (Halbach et al. 2003). They form, mainly: i) in the early stage of a newly established, hydrothermal system; and ii) by subseafloor mixing of hot black smoker fluid with cooler waters., The second probably leads to precipitation of sulphides at depth. Therefore, white smokers may, indicate the presence of hidden stockwork and, vein deposits of copper and zinc. In Figure 1.12,, the third vent from the left in the background is a, “white smoker”, consisting of barite, anhydrite, and amorphous silica., The activity of vents seems to be constant for a, long time. Sudden large hydrothermal plumes are, related to seismic seafloor spreading events, rapid, , injection of dykes and seafloor extrusion of basalt, lavas (event plume, or megaplume; Palmer &, Ernst 1998). In common vents, seismic strain, causes changing temperature and discharge rate., The origin of mid-ocean submarine hydrothermal systems is mainly seawater convection in hot, young oceanic crust, on top or above the flanks, (Cathles 1993) of shallow magma bodies 1 to 3 km, below the seafloor (Figure 1.10). This setting explains the steep geothermal gradient (up to 500 C/, km), that is deduced from several observations., The required high permeabilities are provided by, intensive tensional fracturing in mid-ocean rifts, and around calderas. In the periphery, seawater, flows downwards to more than 3 km depth., Because of the retrograde solubility of calcium, sulphate, anhydrite is precipitated at moderate, temperature during down-flow. At higher temperature and deeper levels, seawater reacts with basalts causing ocean floor greenschist facies, metamorphism. Oxygen is rapidly consumed by, reaction with Fe(II). Newly formed hydrated, minerals (e.g. chlorite, amphibole) incorporate, OH, which increases Hþ concentration in the, fluid. The reduced state and high acidity favour, dissolution of metals and of sulphur., Alteration of basalt at low water/rock mass, ratios leads to a paragenesis of albite, epidote,, chlorite and actinolite. The product is greenstone, (note the absence of penetrative deformation, such as schistosity). Extreme alteration produces, epidosites (equigranular epidote-quartz-titanite, rocks) and chlorite-quartz rocks (e.g. beneath the, Bent Hill deposit: Teagle & Alt 2004). Similar, alteration products are known from ophiolitehosted sulphide deposits. Magmatic fluids may, mix with altered seawater and do have a role in, mineralization (Yang & Scott 1996, 2005), as indicated by high 3 He and CO2 content, although, stable isotopes imply mainly seawater. Part of the, fluids could have a multi-stage history, for example dehydration of hydrated basalts by prograde, thermal metamorphism induced by a new magma, intrusion. Near transform faults, rocks of the deeper ophiolite sequence may be hydrothermally, altered (e.g. formation of chlorine serpentinite, from peridotite). Ultramafic-hosted hydrothermal, systems produce highly reducing and high-pH
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , vent fluids, such as the Lost City hydrothermal, field at 30 N on the mid-Atlantic ridge (Seyfried, et al. 2007). In these systems, the coexistence of, dissolved CO2 and H2 favours the formation of, methane (toukos & Seyfried 2004). High-energy,, fast-spreading mid-ocean ridges are dotted by submarine volcanoes and intrusions extending to an, off-axis distance of 25 km from the graben margin., Hydrothermal fields and large ocean floor orebodies display a similar range (Fouquet et al. 1996)., Although most of the emitted metals are diluted, in ocean water and sediments, approximately 250, metalliferous bodies of economic mass and grade, have meanwhile been discovered. One example is, a low sulphide mound on the mid-Atlantic rise at, 26 N with a diameter of 250 m and a height of, 50 m, that contains about 10 Mt of metalliferous, sediments, sulphide debris and anhydrite, covered, by oxidized sulphides and active as well as inactive, chimneys. It is underlain by a sub-seafloor stockwork (Petersen et al. 2000). This is comparable to, ancient volcanic-hosted massive sulphide (VMS), deposits of obducted ophiolites (the Cyprus type)., The Bent Hill deposit on the San Juan de Fuca ridge, is remarkably copper-rich, very much like the ores, on Cyprus (Ziehrenberg et al. 1998). In the shallow, crust beneath vent fields, large Cu, Zn and Au, accumulations are probably formed by precipitation because of boiling and vapour loss during, depressurization (Hardardóttir et al. 2009). Metalliferous mud in several depressions of the Red Sea, represents the largest known submarine base, metal mineralization. Sulphides in the Atlantis II, basin, for example, cover 60 km2 and reach a thickness of 30 m. The deposit comprises nearly 100 Mt, (dry) of sulphides (Halbach et al. 2003). In these, locations, the causative hydrothermal activity is, not spectacular because metal-bearing solutions, do not vent but discharge into stratified brines, (Blanc & Anschutz 1995)., Ore deposits in ophiolites include two major, groups: i) Chromite of the “Alpine” type (so-called, because of the numerous former mines in the, Alpine chains of southeastern Europe), in rare, cases with co-precipitated exploitable platinum, (Lord & Prichard 1997); and ii) exhalative volcanic, massive sulphide (VMS) deposits of iron, copper, and zinc sulphides ( Ag and Au, but note the, , 23, , virtual absence of Pb), including possible underlying stockwork ore. Because the ophiolite-hosted, VMS deposits have been historically of great significance on the Mediterranean island of Cyprus,, they are also termed “Cyprus type sulphide deposits”. However, most ophiolitic VMS sulphides are, pyrite-rich and have low base metal grades. In the, future, recently formed concentrations of this type, that occur on the floors of the world’s oceans may, become viable sources of metals. Ophiolites host, other important mineralizations that they have, “acquired” during obduction, nappe transport,, deformation, metamorphism and finally weathering. These include asbestos, magnesite, gold (in, listvaenite), talc, and lateritic Ni-(Cr-Co-Fe) ore in, deeply weathered soil profiles., 1.1.3 Ore formation related to alkaline magmatic, rocks, carbonatites and kimberlites, Rocks of these families generally have low SiO2, and high alkali element content, especially of, sodium and potassium. They occur commonly in, cratonic, consolidated portions of continents,, rarely within oceanic plates. An anorogenic setting is affirmed by their clustering near continental rifts, in lithospheric distension zones and over, heat anomalies of the mantle (hot spots, plumes,, superplumes). Occurrences of the typically, “continental” rocks, nephelinite and carbonatite, in association with oceanic intraplate volcanism,, are very rare. Most magmas of this group originate, by a low degree of partial melting of enriched, mantle material, but for some melts the reverse, is true. The enrichment may stem from subducted, oceanic crust, or more probably, from metasomatized lithospheric mantle (Pilet et al. 2008). Nephelinite magma is the most common mafic alkaline, liquid that crystallizes to give a range of igneous, rocks (termed the ijolite suite). They are typically, associated with the much rarer carbonatites that, have a more prominent metallogenetic role. However, complex intrusions displaying felsic rocks, (e.g. A-type granite) in addition to the mafic ones, are not rare. This is due to secondary melting and, assimilation of deep continental crust., Alkaline melts form in the subcontinental, mantle lithosphere by rising temperature, falling
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24, , PART I METALLIFEROUS ORE DEPOSITS, , pressure, or under the influence of volatile and, incompatible substances (mantle metasomatism),, the ultimate origin of which may be mantle, plumes or subducted crust. “Shallow” carbonatitic and deep kimberlitic melts with high CO2 and, low H2O content originate in lithospheric mantle, at 120–260 km depth. The high gas content facilitates rapid rise of magma diapirs to the surface, where eruption takes place. The present outcrop, pattern depends on the depth of erosion. By erosion, to subvolcanic levels, ring complexes or diatremes, are exposed, whereas recent uneroded examples, are simple stratovolcanoes (such as the active, alkali-carbonatite volcano Oldoinyo Lengai in the, East African rift, Tanzania: Bell & Keller 1995;, Figure 1.14). Examples of erosion to deeper levels, are (B) Napak, Uganda and (C) Chilwa Island,, Malawi. Kimberlite volcanoes build maar-like circular depressions or maar lakes., Carbonatites, Carbonatites are igneous rocks with more than, 50% of carbonate minerals. They are further subdivided depending on the nature of the carbonates, (calcite, dolomite, and ankerite) and the silicate, phases (biotite, pyroxene, amphibole, etc.). The, formation of carbonatite-alkali complexes is probably controlled by i) fractional crystallization and, ii) unmixing of carbonate and silicate melts (ijolite, pyroxenite and nepheline syenite) in the, crust. Experimental and petrologic investigations, of African carbonatites have shown that primary, , carbonatite melt can also originate iii) by a very, low degree of melting in the mantle at elevated, pCO2, temperatures of 930–1080 C and pressures, of 21–30 kbar (Bailey 1993). Anomalous amounts, of rare earth elements (REE) are remarkable features of carbonatites, especially of the light REE, elements. Half of all known carbonatites occur in, Africa, and of those, most are related to the East, African Rift System., Metals exploited from complex intrusions of, carbonatite, alkali-pyroxenites and nepheline syenites include copper, rare earth elements, irontitanium-vanadium, uranium-thorium and zirconium; non-metallic resources are the industrial, minerals vermiculite, apatite, fluorite and barite,, and limestone (Notholt et al. 1990). In southern, Siberia, one mine produces nepheline syenite for, Russia’s aluminium industry. Three characteristic ore associations occur with carbonatites:, 1 Magnetite-apatite carbonatite and nephelinite, (e.g. Khibiny, Kola Peninsula) are sources of pyrochlore (a niobium ore), copper (Palabora, South, Africa) and zirconium hafnium; at Bayan Obo,, Mongolia, iron, rare earth elements and niobium, are exploited. worldwide, apatite is the prevalent, mineral extracted from carbonatites;, 2 Rare earths, fluorine and uranium; characteristic ore minerals are bastnaesite (as at Mountain, Pass, California), monazite, and strontianite., 3 A hydrothermal paragenesis of barite, fluorite,, manganese and sulphides., The first group is orthomagmatic, often concentrated in aplitic and pegmatitic segregations. The, A, volcanic, , B, subvolcanic, Country rocks, , C, , intrusive, , Fenitized, country rocks, , Carbonatite, core, , 1, , 6, , 2, , 7, , 3, , 8, , 4, 5, , Figure 1.14 Hypothetical section of a, carbonatite complex (modified from, Kinnaird & Bowden 1991). With kind, permission from Springer, Science þ Business Media. 1: Phonolitic, and nephelinitic lava and tuff; 2:, Natrocarbonatite; 3: Breccia; 4:, Carbonatite ring dykes; 5: Carbonatite, cone dykes; 6: Syenitic fenite; 7:, Nepheline syenite; 8: Ijolite.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , second is transitional, and the third occurs in, hydrothermal-metasomatic or hydrothermal vein, orebodies. Intruded silicate country rocks are fenitized. The term “fenitization” denotes alteration, into a greenish rock consisting of alkali feldspar,, aegirine and alkali hornblende. This alteration is a, consequence of the high alkali element and volatiles content of carbonatite melt that are released, during solidification (B€, uhn & Rankin 1999)., Kimberlites, Kimberlites are derived from the Earth’s mantle at, more than 140 km depth. They are petrographically variable rocks and almost always comprise, strongly altered breccias and tuffs. Basically,, they are porphyric, SiO2-undersaturated, K-rich, (1–3 wt.% K2O) peridotites with xenoliths, and, xenocrysts of diamond, olivine, Mg-ilmenite and, chrome-rich pyrope in a carbonated and serpentinized groundmass with accessory phlogopite,, perovskite, Cr-spinel and magnetite (Mitchell, 1991, 1986; cf. Chapter 3 “Diamond”). Extremely, rare are unaltered kimberlites, which in Siberia, have been found to contain at least 8 wt.% of, water-soluble alkali chlorides, alkali carbonates, and sulphates (Kamenetsky et al. 2004). Close, relatives of kimberlites are the K-Mg rich, ultramafic lamproites that are characterized by leucite,, titanium-phlogopite, clinopyroxene, amphibole, olivine and sanidine. Lamproites received, more attention when the great diamond deposit, Argyle in Western Australia was found to be, hosted in this rock. Kimberlites occur most frequently in subvolcanic pipes and occasionally in, sills and dykes, whereas lamproite magmas with, lower CO2 content also form shallow intrusions., Kimberlites in old cratons are more often diamondiferous than others, possibly because of the, prolonged and complex evolution of the subcontinental lithosphere. Age dating revealed that kimberlites and their diamonds may have same or, widely differing ages. This implies that diamond, formation is not directly connected with the phase, of melting that produces kimberlite liquid. Diamonds are rather exotic xenocrystals that have, been passively extracted from the mantle, in several cases long after their crystallization. It is, , 25, , important to remember that most kimberlites and, lamproites contain no diamonds., 1.1.4 Granitoids and ore formation processes, Granitoids are felsic plutonic rocks with more, than 20 mode % of quartz (Streckeisen 1976). The, term is sometimes used in a wider sense to include, more mafic rocks of an igneous suite., Granitic rocks are commonly used as construction aggregates, building material and for the production of feldspar concentrates. Weathered, granites are exploited for kaolin and quartz sand., Deep granite bodies are a potential source of geothermal energy extracted with the hot dry rock, technology. In Sweden and in Finland, granites, were chosen for the underground storage of, heat-producing radioactive waste. In this chapter,, however, the formation of magmatic and, magmatic-hydrothermal ore deposits that are, spatially and genetically related to granitoids will, be discussed. Members of this diverse group, include disseminated, stockwork and breccia, ores of copper, molybdenum and gold (the porphyry deposits), highly differentiated small intrusions hosting tin and tantalum (“tin granites”),, rare element pegmatites with ores of lithium, tin,, tantalum and beryllium, skarn deposits of, copper, lead and zinc, and hydrothermal vein and, stockwork deposits of tin, tungsten and gold that, occur in the roof of granite intrusions (Figure 1.15, and Figure 1.16). Apart from metals, many industrial minerals (e.g. fluorite, talc and wollastonite), can be enriched to exploitable grades in the magmatic and hydrothermal aureole of felsic, intrusions., The ore formation potential depends on origin, and evolution of the parental granitoid. Important, controls are the plate tectonic setting, the nature, of source rocks, P/T-parameters of melting, content of water and other volatiles, the depth of, intrusion, coeval tectonic deformation, partial, pressure of oxygen (redox state) of the melt, assimilation of country rocks and the evolution of the, magma by fractionation, cooling and crystallization including fluid segregation. Favourably coinciding factors account for the fertility of a granite, intrusion. Understanding these controls and
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27, , GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , icant melting is attained (Clemens 2003). Products of these partial melts are mainly, leucocratic, SiO2 rich rocks of a monzogranitic, nature, often with muscovite besides biotite., Accessory minerals include cordierite, garnet,, kyanite and ilmenite as the prevalent opaque, mineral. This is why S- (and A-) granites are, part of the ilmenite-series magmatic rocks, (Ishihara 1981, Figure 1.17). The degree of oxidation of these magmas is low, due to organic, carbon in the source sediments. The water of, the melts is derived by dehydration of muscovite in the metasediments. The resulting melts, are rather “wet” and crystallize at greater, depth. Therefore, volcanic rocks are rare in this, group. In the Palaeozoic Lachlan Fold Belt, (Australia), both I- and S-type granites are common. Highly fractionated intrusions (“tin granites”) of both suites are clearly associated with, tin, tungsten and tantalum ore deposits (Blevin, & Chappell 1995)., Ad 4: A-type granitoids are the product of, repeated melt-extraction from the same source, rocks (Eby 1992). Note that with every cycle of, melting the source rocks acquire a more pronounced restite composition, marked by enrichment of less mobile substances. The “A” in the, term may be translated into “abnormal, anhydrous, alkali rich, aluminous and anorogenic”., Another possible source of A-type magma is, lithospheric mantle (Turner et al. 1992) and not, all A-granites are anorogenic (Whalen et al., , abundance of hornblende and higher concentrations of Ca, Na and Sr compared with granites, derived from sediments. Typically, large plutons consist of tonalites and granodiorites, but, are often intimately associated with more basic, rocks from gabbro to diorite. Some of these, melts were undersaturated with water, which, enabled them to rise to the surface, forming, volcanic rocks (e.g. andesite and dacite). It is, commonly accepted that I-type granitoids originated by melting of pre-existing infracrustal, igneous rocks (Blevin & Chappell 1995). However, geochemical and isotopic I-type characteristics can also be explained by mixing of rising, mantle magmas with anatectic lower as well as, upper crustal melts (Kemp et al. 2007). Accessory minerals of I-type rocks are often, magnetite and titanite, causing their attribution, to the class of magnetite-series magmatic, rocks (Ishihara 1981, Figure 1.17). This is due, to a commonly higher oxidation degree of, these magmas, although reduced I-type granitoids are known. Characteristic ore deposits, related to oxidized granitoids are the iron, oxide-copper-gold (U-REE) deposits (IOCG),, copper-molybdenum porphyries, Mo-W-Cu, skarn, hydrothermal lead-zinc and certain gold, and silver ores., Ad 3: S-type granitoids originate by continental, collision and deep subduction of sediments to, great depths and high temperatures, where the, transition between metamorphism and signif-, , 10, , Tungsten, , Oxidized, , Figure 1.17 Typical fields of granites, which are genetically associated with, tungsten, tin and gold-bismuth deposits,, in a plot of redox-state (vertical axis) versus, increasing specialization (horizontal axis)., Modified from Baker, T., Pollard, P. J.,, Mustard, R., Mark, G. and Graham, J. L., 2005, Society of Economic Geologists,, SEG Newsletter, Figure 4, p. 12., , Fe 2O3 /FeO, , Gold-Bismuth, 1, Magnetite series, Ilmenite series, , Tin, , 0.1, , Reduced, , 0.01, 0.001, , 0.01, , 0.1, , 1, , Rb/Sr, , 10, , 100, , 1000
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28, , PART I METALLIFEROUS ORE DEPOSITS, , 1987). Some granites that have A-type characteristics may be derived by extreme fractionation of I- and S-type magmas (Creaser et al., 1991; Taylor & Fallick 1997). Typical A-type, granites are the alkali granites of continental, rifts, for example the Jurassic tin-mineralized, Younger Granites of Nigeria and the Mesoproterozoic Rapakivi granites in Finland, with their, large red perthitic alkali feldspars mantled by, green plagioclase. Volcanic equivalents include, tin-rich topaz rhyolites in fields of crustal distension (Tertiary volcanics in Mexico). Two, different ore associations occur with A-type, granitoids: i) Sodium-rich granites, striking, because of attached albitite bodies, contain concentrations of niobium, uranium, thorium, rare, earth elements and some tin, whereas; ii) potassium-rich granites with profuse hydrothermal, silicification, tourmalinization and acidity produce deposits of tin, tungsten, lead, zinc and, fluorspar. The second association may occur, within the granite body (endogranitic greisen,, pegmatite, and porphyry stockwork ore) or in, vein fields within intruded rocks (exogranitic)., HHP (high heat producing) granites, These granites are U-Th rich granitoids that, attract attention because they seem parental to, hydrothermal deposits, which formed much later, than the granite:, The Early Devonian Weardale granite in Northern, England, for example, seems to have produced the, hydrothermal Cu-Pb-Zn-fluorite-barite ore deposits, of the North Pennines in the Permian. The explanation is its long-lasting heat production due to elevated, amounts of K, U and Th (Plant et al. 1985), which, produce heat by radioactive decay. Latent heat is one, element of the HHP ore-forming systems. The second, is sufficient permeability that allows deep circulation, of down-flowing, usually meteoric water. Crustal, permeability is commonly created by tensional tectonic fracturing. Heated water at depth can dissolve, substances that are precipitated during ascent. Note, that HHP-stocks must be quite large with a minimal, diameter of 15 km and a considerable depth extension, because the heat of small granites is rapidly, dissipated. Typical HHP-granites include the, , post-orogenic Variscan tin granites of Cornwall,, which are related to important polymetallic mineralization and are now explored for the production of, geothermal energy., , Not all granites can be assigned to one of the, source categories because of several reasons, including complex mixtures of source rocks and, extreme fractionation, which leads to increasing, convergence of magma chemistry (Taylor &, Fallick 1997). This is why the youngest granites, in orogenic provinces geochemically approach, those that occur in anorogenic settings such as, continental rifts, zones of crustal extension and, continental “hot spots” (Turner et al. 1992). These, observations motivate classification of Phanerozoic (and older) granitoids according to tectonic, setting: Island arc, active continental margin,, post-orogenic uplift, continental collision and, continental rifting granitoids (Pitcher 1997)., A time-dependent chemical evolution of intrusions has been noted in many granite-related ore, provinces of the world: Early batholithic intrusions (i) are geochemically ordinary, whereas later, and smaller precursor granites(ii) are transitional, to small, geochemically specialized (iii) and to, usually very small, mineralized granites (iv),, which are intimately related to ore formation., Compared with ordinary granites, precursor, granites display higher content of K, SiO2 and, granophile trace elements, and less Fe, Ti, Ca, Sr, and Mg. Precursor intrusions always predate specialized granites, although they are genetically, related. Specialized and mineralized (parental), granites are distinguished by geochemically elevated content of metals, such as Sn, W, Nb, Ta,, Mo, U, Th, REE, Rb, Cs, Li, Be, often F (the latter, include “topaz granites”) and P (Figure 1.17 and, Figure 1.18). Petrographically, specialized and, mineralized granites are mostly aplitic muscovite-biotite rocks, alaskites and leucogranites that, intruded at shallow crustal levels. In specialized, granites, rare elements are enriched in silicates, and accessory minerals. Mineralized or parental, granites, in contrast, stand out by their close relations to ore-grade concentration of rare elements., The geochemical changes from ordinary to mineralized granitoids are mainly caused by a process
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 29, , 1000, , Ta-ore grade, , Beauvoir, , Ta (ppm), , 100, , 10, , Erzgebirge, Figure 1.18 Ta/TiO2 variation of granites, from the northern French Massif Central, (Beauvoir) and the Saxo-Bohemian Erzgebirge, (modified from Lehmann 1990). With kind, permission from Springer ScienceþBusiness, Media., , Crustal average, , 1, 0.001, , system, which is generally termed “magmatic, fractionation” (see below). The degree of fractionation may be measured by various indices. In, metallogenetic research dealing with granitoids,, increasing Cs concentration in melt, minerals and, fluids is a particularly useful tracer of fractionation (Audetat et al. 2008)., Granites and related pegmatites with extreme, chemical fractionation are the source (and often, the hosts) of deposits of rare elements including, Sn, Li and Be. They are enriched in large ion, lithophile elements (LILE) such as Rb and Cs, and, high field strength elements (HFSE) such as P, Y,, Zr, Hf, Nb, Ta, Th and U (Linnen & Samson 2005)., If ordinary and precursor granites are present,, the enrichment may be traced back to common,, geochemically not anomalous crustal rocks, (Lehmann et al. 2000b, Cerny 1991). Figure 1.18, illustrates this principle and shows how tantalum, (an incompatible element) is continuously enriched by increasing differentiation of successively more fractionated (and younger) granite, melt batches and finally reaches exploitable, grades. Concurrently, concentrations of the compatible element titanium decrease. Derivation of, , 0.01, , 0.1, , TiO2 (wt. %), , 1, , tantalum from geochemically ordinary crust appears possible., The increasing differentiation of magmas is, caused by fractional crystallization, early crystal, settling and/or removal of liquid melt from a solid, framework of phenocrysts (Weinberg 2006). Other, explanations include a very low degree of partial, melting (Robb 2005), or unmixing of melts because, of fluid saturation. In some cases, melting of geochemically anomalous source rocks is considered, to account for metal enrichment. An example are, magmas with a high content of the chalcophile, elements Au, Ag, Bi, Sb, Hg and Tl, which are, supposedly inherited from a pre-enriched melt, region (Tomkins & Mavrogenes 2003). Giant, molybdenum porphyry deposits of the southwestern USA are thought to be extracted from an inherent large-scale Mo enrichment of the crust or, lithospheric mantle (Klemm et al. 2008)., The petrological evolution of mineralized granites can be schematically subdivided into two, phases: The first phase comprises the fractionation of elements between melt and early solid, phases that crystallize during cooling, and is followed by a second phase of partitioning of trace
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30, , PART I METALLIFEROUS ORE DEPOSITS, , elements between the small remaining melt fraction and exsolving fluids, which concentrate volatiles and metals:, Elements that partition preferentially into the solid, phase are referred to as compatible because they are, included in nascent rock-forming silicate minerals,, for example europium in plagioclase. Incompatible, elements concentrate in the liquid (melt) phase., Lithophile or oxyphile elements are common in, crustal silicates but are incompatible with minerals, that have an important role in the formation of mantle magmas (e.g. olivine, pyroxene, spinel, garnet)., Lithophile elements include Al, Si, O, alkalis, earth, alkalis, rare earth elements and actinides, as well as, metals such as Ti, Ta, Nb and W (Goldschmidt 1958)., LIL elements (large ion lithophile) such as Rb, Sr, K,, Ba, Zr, Th, U and light REE are preferentially enriched, in late, highly differentiated melt derived from restites, because these elements are less prone to partition, into early water-rich liquids. Cations with a high, charge (þ3 to þ6) such as Mo, Nb, Zr, Sn, W, Ta, U,, Th, Y and REE are normally abstracted from the melt, by incorporation in crystallizing biotite, amphibole,, apatite, zircon, monazite and magnetite. This process, is inhibited by high activity of complexing volatile, compounds, which cause these HFS (high field, strength) elements to collect in late liquid and fluid, phases. Because this is uncommon, elements such as, Ti, Zr, Y, Ta, Nb, Hf, Th and REE are generally, immobile and useful petrogenetic indicators (Pearce, 1982, Pearce et al. 1984)., , The fertility of granitoids is closely related to, differentiation, fractionation and the formation of, exsolved magmatic volatile phases. The composition of magmatic volatile phases is investigated by, sampling volcanic exhalations, fluid inclusions in, minerals (especially from miarolitic cavities) and, volatiles included in volcanic glass. Miaroles are, crystal-lined cavities in granitoids that are thought, to have formed from fluid pockets during the solidification of magma. Fluid and melt inclusions preserved in miarolitic minerals reveal details about, segregation, composition and evolution of mineralizing fluids (Audetat et al. 2008)., Water is the most common substance in magmatic, volatiles. In silicate melts, dissolved water reaches a, maximum of 8 wt.% or 25 mole % (Ochs & Lange, , 1999). Arc magmas are more hydrous (and more oxidized) than those of other tectonic settings. Water is, followed in decreasing order by CO2, H2S or SO2, HCl, and HF, and small amounts of N2, H2, CO, P, B, Br,, CH4 and O2. Trace elements in volcanic emanations, include the metals Pb, Bi, Cd, Cu, Zn, Hg, Sb, Te and, As, and major rock-forming elements such as Al, Mg,, Na and K. The annual exhalative emissions of Etna, volcano, Sicily, include 2.5 t lead and 250 t potassium. Metal fluxes in vapour from White Island volcano, New Zealand, are estimated to >106 t Cu and, 100 t Au in 10,000 years of volcanic activity (Hedenquist et al. 1993). Different volcanoes have ratios of, SO2/HCl and SO2/HF between 1 and >100. Ratios, from 1 to 10 are typical for volcanoes above convergent plate boundaries, because they discharge more, HCl. Gas released in explosive mode by Stromboli, volcano (Italy), for example, consists of 64% H2O,, 33% CO2, 1.8% SO2 and 0.33% HCl (Burton et al., 2007). Note, however, that molecular HCl is only, stable at very low pressure or in the absence of water., Above 200 bar, chlorine occurs as an ion or is dissolved in magmatic water as a complex with Na, K, and Fe. In epizonal magmatic water, the average, chlorine concentration in solution is 3.5%, or, 6 wt.% (range 2–10) NaCl equivalent. The segregation, of saturated or hypersaline solutions is only possible, at pressures >1.3 kbar. In that case, a hypersaline, solution (or should it better be called a hydrous saline, melt?) at 700 C contains 84% NaCl., , Fertile granitoid magmas are distinguished by, high content of volatiles. Volatiles collect the rare, elements that form ore, and also lower density,, viscosity and solidus temperature of a melt, increasing its mobility (Baker & Vaillancourt, 1995). Low magmatic temperatures and high salt, concentrations favour the fractionation of metals, into the fluid phase (Aud, etat & Pettke 2003)., Oxygen fugacity in the melt is an important control (Figure 1.17). High oxygen in granitic magma, causes depletion of tin and tungsten in the liquid, and in late fluids, because the metals are, abstracted in dispersed accessory minerals already, during main-stage crystallization. Behaviour of, copper and uranium is quite the reverse. Oxidized, magmas (of the magnetite series) dissolve more, sulphur (as an “anhydrite component”, compare, Streck & Dilles 1998) and derived fluids may, produce large Cu-Au deposits. In “reduced”
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , granitic magmas (ilmenite series), early sulphur, saturation causes formation of dispersed sulphide, droplets that collect copper and gold. Late fluids, will be barren., Formation of a volatile phase (degassing) during, the ascent of silicate magmas through the crust is, initiated by decreasing pressure, or in the case of, roof fracturing, by sudden decompression (“first, boiling”). The solubility of CO2 in basaltic magmas can decrease rapidly at a depth of 70 km, but, most basalts reach this stage at 10 to 15 km below, the surface (Lowenstern 2001). Supersaturation, leads first to formation of bubbles of liquid CO2, that expand with further ascent until the density, reaches that of gas. Water has a similar behaviour,, although changes occur at shallower depths. As a, function of the total water content in melt (commonly 3–5%, maximum 6–8 wt.%) and chlorine, concentration, felsic magmas start to segregate an, aqueous fluid phase at a pressure of 260 MPa (ca., 9 km, but more often around 4 km below the surface). Commonly at this stage, the melt body will, freeze. Kinetic factors may, however, allow further, ascent of the magma. In that case, expansion of, fluid bubbles will produce a frothy liquid that, is able to convect vigorously. Rare elements, are effectively scavenged from the melt, and as, the fluids move upwards, ore formation can take, place either in the apex of the intrusion or in its, roof. The remaining melt solidifies rapidly., After final emplacement, when ascent is stopped, and first boiling is finished, the intensive formation of solid phases from melt causes renewed concentration of volatiles. If the amount of volatiles, is higher than the mass that can be included in, minerals (e.g. OH in mica), a free volatile phase, will form once more (“second boiling”). In the, case of relatively shallow intrusions, fluid pressures may cause fracturing of the roof and sudden, injection of mineralizing fluids. In this case, fluidinduced stress is larger than the sum of rock, mass strength and the minimum natural principle, stress (Brady and Brown 2004). Unmixing of a fluid, phase from mesozonal intrusions (at pressures >2, kbar) is restricted to a narrow temperature interval., Often in this environment, volatiles form small, batches of fluid-rich silicate melt that solidify to, coarsely crystalline pegmatites. Shallow epizonal, , 31, , intrusions degas over a wide temperature interval, (Fournier 1999), producing miarolitic and hydrothermally altered granites, but foremost the many, variegated magmatic-hydrothermal ore deposits., In Japan, granites related to magmatichydrothermal mineralization display a correlation, between the main ore metals and the solidification, pressure of the parent pluton. Total Al-content in, biotite, the sphalerite and hornblende geobarometers and the petrology of surrounding rocks all, suggest that Pb-Zn and Mo deposits formed at, pressures of less than 1 kbar, Cu-Fe and Sn at 1 to, 2 kbar and W deposits at 2 to 3 kbar (Uchida et al., 2007). Deeper intrusions are barren. The principle,, although not the precise figures, may be of a wider, significance., In the past, a “pneumatogenic phase” of ore, formation was distinguished between pegmatitic, and hydrothermal conditions. The assumption, was that certain ores were formed by precipitation, from a magmatic gas phase of extreme mobility, and with high concentrations of rare metals. Greisen orebodies of tin and many skarn ores were thus, explained. More recently, the term is rarely used, and often explicitly dismissed. Arguments include, that water vapour (with a density below the critical density of 0.32 g/cm3) cannot carry more than, traces of dissolved matter, and supercritical fluids, with densities that reach 2 g/cm3 can hardly be, called a gas (Roedder 1984; remember that pneuma, in classical Greek is “air” or “breath”). Furthermore, the distinction is thought to be difficult,, because supercritical fluids may pass into subcritical solutions or gases without any discontinuity, (condensation). The limit between pneumatogenic and hydrothermal conditions had been, assumed at the critical temperature of water,, which occurs at 374 C and 225 bar, but rises as a, function of solute concentration:, Even if there is no need for the old term, attention, must be drawn to the fact that supercritical fluids do, have properties that differ from solutions with the, same composition: The density of supercritical fluids, varies widely with changing pressure and temperature; they have higher pH (Ding & Seyfried 1996),, are able to dissolve many organic substances, and, exhibit extreme dissociation of water and diffusion
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32, , PART I METALLIFEROUS ORE DEPOSITS, , coefficients. Geologically most important are probably the variable density (from dense fluid to gas),, reaction rates and equilibrium constants that control, transport and precipitation., , Of course, there is no doubt that magmatic gases, and fluids of low density can transport metals (e.g., Etna volcano in Sicily) and precipitate ore. Cassiterite-quartz veins in the Mole granite, New South, Wales (Australia), contain two types of inclusions:, a saline brine (a solution) with elevated trace, content of Mn, Fe, Zn, Pb and Sn, whereas the, other is “gas” with 1% Cu. Observations suggest, that both are cogenetic and unmixed from magma., Copper was concentrated into the gas phase (Heinrich et al. 1992). A broader investigation of magmatic-hydrothermal ore deposits revealed that, boiling of magmatic fluids concentrates Cu, As,, Au and B preferentially into the vapour phase,, whereas Na, K, Fe, Mn, Zn, Rb, Cs, Ag, W, Sn,, Pb and Tl are enriched in the residual saline, brine (Heinrich et al. 1999, Williams-Jones &, Heinrich 2005). The most important controls, on this behaviour appear to be fluid density, (Pokrovski et al. 2005) and sulphur content, (Nagaseki & Hayashi 2008)., Exsolution of magmatic fluids and hydrothermal ore formation are an integral part of ascent and, crystallization of an intrusion. Cooling down to, environmental temperatures drives meteoric,, convective hydrothermal systems that are often, called “geothermal”. In these systems, magmatic, fluids and volatiles typically mix with meteoric, water. Age determinations, model calculations, and observation in nature reveal that an isolated, single upper crustal intrusion may sustain a hydrothermal system for 100,000 years (Cathles et al., 1997). Magmatic-hydrothermal processes can be, prolonged to millions of years in the case of, incremental growth of a pluton by multiple intrusions, regional deep magmatism and autonomous, heating, due to radioactive decay that retard, cooling., , 1.1.5 Ore deposits in pegmatites, Pegmatites crystallize from highly fractionated, hydrous residual melt batches of felsic magma, , bodies that are enriched in volatiles and incompatible trace elements. Pegmatites are characterized by coarsely crystalline textures, occasionally, by giant crystals, miarolitic cavities and by minerals of rare elements. Most pegmatites are related to, granites and have a paragenesis of orthoclase, (perthite), microcline, albite, mica and quartz., Common minor minerals include tourmaline,, topaz, beryl, cassiterite and lithium minerals., Felsic pegmatite melts intruding ultramafic, rocks suffer desilication resulting in plumasites, that are characterized by corundum, kyanite and, anorthite. Gabbro pegmatites are derived from, mafic magmas and are composed of anorthite,, pyroxene, amphibole, biotite and titanomagnetite, occasionally including carbonates and sulphides (similar to the Merensky Reef, “pegmatoid” of the Bushveld Complex). Iron-rich, ultramafic pegmatites composed of olivine, intrude the cumulates of the upper Critical Zone, of the Bushveld Complex (Figure 1.5). Rare syenite, pegmatites with microcline, nepheline, apatite,, niobium and rare earth element minerals are, related to alkaline intrusions. Certain diamonds, are thought to have crystallized from kimberlitic, pegmatite melt deep in the mantle (Moore 2009)., Anatectic pegmatites (metamorphic segregations), that are formed in the upper amphibolite facies, are rarely mineralized. Yet, some mineralized, pegmatites may have originated by partial anatexis at great depth and not from large magma, bodies (e.g. the uranium-molybdenum-rare earth, pegmatites of the Grenville orogen: Lentz 1996)., Most pegmatites crystallized at intermediate, crustal levels, at fluid pressures of 200 MPa, (2 kbar). Therefore, they are commonly found in, metamorphic country rocks. Volcanic equivalents, of highly differentiated pegmatitic melts are, extremely rare., Classifying granitic pegmatites by their, emplacement depth leads to differentiation of the, following types:, . Abyssal pegmatites are anatectic stringers in, migmatites of amphibolite and granulite facies, metamorphic zones., . Muscovite pegmatites occur in amphibolite, facies kyanite-mica schists and are commonly, related to granites, but exhibit little fractionation.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , compositions of the field classification by Varlamoff 1972)., The internal zonation of complex pegmatites, (Figure 1.20) reflects crystallization from the walls, to the centre of a pegmatite body where the last, melt solidified. The following zones are, distinguished:, . Border zone: often fine-grained, aplitic, and, very thin;, . Wall zone: coarsely crystalline, in some deposits with exploitable muscovite and beryl;, . Intermediate zones: which contain most of the, valuable minerals (cassiterite, columbite, spodumene, beryl, etc.); the outer intermediate zones are, often sugary albitites, followed by coarse albitite, (cleavelandite) or microcline towards the core;, . Core: which is commonly a solid mass of barren, grey or white quartz, but may also contain feldspar, tourmaline and spodumen., Subhorizontal pegmatite sheets often have, asymmetric zones, in contrast to the concentric, zones that characterize most steeply-dipping, bodies. Flat-lying pegmatites typically display bottom-to-top differentiation (e.g. Kenticha in Ethiopia: K€, uster 2009). Some cases of internal zonation, are so complicated as to appear chaotic. Post-solidus cross-cutting fracture fillings, irregular metasomatic masses and hydrothermal quartz veins, can further disarrange the internal structure., Because of crystallization from the wall to the, centre, exsolving aqueous fluids will act from, , . Highly fractionated rare element pegmatites are, derived from strongly differentiated fertile granites; host rocks typically contain cordierite and, andalusite., . Miarolitic pegmatites form at low pressure, (1.5–2 kbar) and are proximal to granites. They, may contain quartz of optical quality, various, gemstones and valuable crystals of many rare, minerals., Granitic pegmatites occur in the form of dykes,, oval and lenticular bodies. Most pegmatite bodies, are relatively small with a thickness that rarely, surpasses tens of metres and a length of a few, hundred metres. Some pegmatites occur at the, roof of granite cupolas and form a thin shell, between the intrusion and the roof rock (stockscheider, i.e. “border pegmatite” in the German, Erzgebirge). Giant pegmatites of this type are rare, but economically significant (e.g. the tin-tantalum and spodumene pegmatite at Manono, D.R., Congo, which extends over 12 km2). Granitic, pegmatites may be homogeneous (without a, change of mineralogy or texture from wall to wall), and isotropic, or strikingly inhomogeneous and, anisotropic (“zoned” or “complex” pegmatites). In, contrast to the zonation of the interior of a pegmatite body, an external zonation can also be, observed. This term designates the occurrence of, different pegmatite types in one district, with a, map-scale zonation, ideally around a central, parental granite (Figure 1.19 provides pegmatite, , 8), , ein, zv, , ar t, -qu, , Sn, , Figure 1.19 External, zonation of rare element, pegmatites and cassiteritequartz veins near fertile, granites in Central Africa, (adapted from Varlamoff, 1972; cf. Figure 1.16). With, kind permission from, Springer ScienceþBusiness, Media., , 33, , (, lds, fie, , e, , bit, -al, Be (7), i, -L s, -Ta tite, Sn gma, pe, , a), e, n,T, S, lin, (, -Be 5), e- ), ma (4), e, B, r, t, i, (, u, 6, v s, lex s (, -to s, sco tite, ite ite, mp tite, Mu gma, ov gmat, Co gma, c, s, pe, pe, Mu pe, , Highly differentiated fertile, granite and older, precursor granites, , ~ 1000 m
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34, , PART I METALLIFEROUS ORE DEPOSITS, , White to grey massive, quartz core, , SW, , NE, , Coarse beryl and, simpsonite, Tantalite zone, , Aplite with disseminated tantalite, Weathered, pegmatite, , Microcline ± quartzpegmatite, (± muscovite), Aplitic albitite, (quartz-albite-muscovite, without Ta-minerals), , the centre outwards. Therefore, post-solidification hydrothermal alteration of magmatic minerals is not rare (e.g. sericitization, kaolinization)., In contrast to granites, pegmatites frequently, exhibit striking anisotropic textures. Most obvious, examples are the giant crystals of amblygonitemontebrasite, beryl, topaz, tourmaline and other, minerals that grow inwards from the cooling surface, (“unidirectional solidification textures”: Shannon, et al. 1982). Related features are bands in sugary, albitites, and graphic, skeletal or radial crystal, growth., Alteration of host rocks at the contact with, pegmatite is often observed. However, the nature, of this alteration is not of high-temperature contact-metamorphic type but hydrothermal, for, example tourmalinization, silicification and propylitization (cf. “Hydrothermal Host Rock Alteration”). The alteration is due to expulsion of water, and other volatiles from the pegmatite. A chemical, exchange directed from enclosing rocks to the pegmatite is also possible, as shown by garnet appearing in the wall zone, the plumasite formation, mentioned above, or the occurrence of a tourmaline, border zone that is due to reaction of iron and, magnesium mobilized from the host rocks with, boron from the volatile phase of the pegmatite., Cooling and crystallization of pegmatite melt, bodies induces the segregation of immiscible liquid phases, which may include a peraluminous, melt containing less water, and a water-rich peralkaline melt, as at Ehrenfriedersdorf in Germany,, , 5m, , Figure 1.20 Section of the, Mesoarchaean tantalum-tinberyllium pegmatite at Tabba, Tabba near Port Hedland,, northwestern Australia, with a, well-developed internal, zonation. After Sweetapple,, M.T. & Collins, P.L.F. 2002,, Society of Economic Geologists,, Economic Geology 97, Figure 5,, p. 882. Simpsonite is Al4(Ta,, Nb)3(O,OH,F)14 and indicates, extreme fractionation., , co-existing from 720–490 C (Rickers et al. 2006)., Other phases, such as hydrosaline melt (a highly, concentrated aqueous fluid), saline fluids and a, vapour phase may appear at certain evolution, stages. Inclusions of co-existing melts and hydrothermal fluids in quartz confirm this model, (Rickers et al. 2006, Thomas & Webster 2000,, Thomas et al. 2000). Ordinary unmineralized pegmatites crystallize at temperatures between 690, and 540 C, whereas fractionated melts with elevated content of boron, fluorine, phosphorous,, chlorine, etc. finally solidify at 450 C. Below the, solidus, aqueous fluids dominate the system. Undercooling of the melt bodies injected into cooler, host rocks may play an important role in the, formation of zoned pegmatites (London 2008). The, development of giant crystals seems to contradict, the concept of undercooling, but a low density of, nucleation sites, water-like viscosity of the peralkaline and hydrosaline melts, and high diffusion, rates explain its feasibility., The internal zonation in complex pegmatites, might have two causes: i) fractional crystallization, in a closed system; or ii) repeated injection of new, melt batches in an open system. The worldwide, similarity of zoned pegmatites (and many other, data) argues for a closed system. Undercooling and, back-reaction between remaining melt and earlier, solids are the main factors supporting the, “disequilibrium fractional crystallization through, liquidus undercooling hypothesis” (London 2008)., The unusual occurrence of nearly monomineralic
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , rocks such as albitites and part of the enrichment, of metals can probably be explained by zone refining. This concept assumes that pore liquids in, outer zones, consisting of semi-solid crystal mush,, take up solutes and move inwards., The external zonation of different pegmatites, around parent intrusions is explained by the, increasing mobility of more fractionated melts,, because increasing fractionation lowers solidus, temperature, density and viscosity of melts (Cerny, 1991). This explains why ordinary pegmatites, occur near the parent intrusion, whereas specialized, complex pegmatites and hydrothermal ore, deposits are found at a greater distance (Figure 1.16, and Figure 1.19). The degree of fractionation of, pegmatites can be determined by trace element, analyses of whole rock samples (Li, Rb, Cs, Ta),, mica (Cs, Ta, Nb, Zn, Li, U, Be, Ba) and potassium, feldspar (Cs, Rb, Na, Ba) (Wise 1995, Morteani, et al. 2000). Some pegmatites display fractionation, to less than 0.05% of the parental melt volume, (Evensen & London 2002)., Little is known about details of the physical, derivation of pegmatite melts from parent intrusions. The common implication is that pulses of, increasingly specialized melt are ejected while the, main magma body crystallizes. Rickers et al. (2006), report that melt inclusions in parent granites show, a gradual transition to embryonic pegmatite melts,, along with enrichment of water, B, F, P, Sn, Rb and, other incompatible elements. Yet it is not impossible that small pegmatitic melt batches rise, directly from the source region of the “parent”, granite (Shearer et al. 1992)., Pegmatites may host many useful raw materials, (Sweetapple & Collins 2002, Morteani et al. 2000,, Martin & Cerny 1992). These include ores of Be,, Li, Rb, Cs, Ta > Nb, U, Th, REE, Mo, Bi, Sn and W,, the industrial minerals muscovite, feldspar, kaolin, quartz, spodumene, petalite and fluorite, and, gemstones as well as rare mineral specimens, (emerald, topaz, tourmaline, ruby, etc.). The derivation of pegmatites from I-, S- and A-type granites, is probably the main control of the availability of, specific elements for enrichment. The feasibility, of pegmatite mining is often limited by a small, tonnage and a heterogeneous distribution of economic minerals. The determination of mining, , 35, , reserves is notoriously difficult. Therefore, pegmatite mining is more common in countries with, low labour costs., 1.1.6 Hydrothermal ore formation, Nearly each of the preceding sections contained a, reference to the important role of aqueous fluids,, although relations between igneous rocks and ore, deposits were the main subject. In the following,, important aspects of ore formation by hot aqueous, fluids are presented., The term “hydrothermal water” applies to subsurface water with a temperature that makes it an, agent of geological processes, including hydrothermal ore formation. “Geothermal waters” are a, subgroup of hydrothermal solutions that occur, near the Earth’s surface and are mainly used as, an energy source, but also for balneology., Thermal springs (Figure 1.21) are common indicators of geothermal reservoirs at depth. Many, hot springs and geysers currently display precipitation of minerals and ore. However, most hydrothermal ore deposits were formed at depth, at, temperatures of 700–50 C and pressures of a few, hundred to >3000 bar. In the past, the term hydrothermal was mainly understood to imply condensed magmatic vapours below ca. 400 C,, based on observations in volcanic geothermal districts. Meanwhile, isotopic investigations revealed that many geothermal and hydrothermal, waters are not of magmatic but of meteoric derivation (i.e. from local precipitation). Similarly,, hot water in mud volcanoes of oilfields is, not magmatic but formation or connate water, (diagenetically altered seawater enclosed in sediments). Many other observations confirm that, “hydrothermal water” has no unique but many, possible sources., Much has been learnt by studying natural thermal springs and geothermal water tapped by drilling. Most hot waters are dilute solutions of, chloride, carbonate and sulphate, but dissolved, silica, boron and sulphide are also common, (Gallup 1998). Examples of thoroughly investigated hydrothermal systems include the mid-oceanic black smokers, gold-rich hot waters of deep, geothermal boreholes in New Zealand (Simmons
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36, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.21 (Plate 1.21) Geothermal hot springs and siliceous sinter mound at Sempaya in northwestern Uganda., The convective system is related to the large border fault of the Ruwenzori Mountains, with a vertical displacement of, more than 10 km between the petroliferous Tertiary Albert Rift in the west and the Palaeoproterozoic crystalline, horst in the east., , & Brown 2000, 2007) and base metal-bearing, brines of the geothermal field at Salton Sea in, Southern California:, Salton Sea is a playa lake in the Colorado River delta, of the Imperial Valley graben. It occurs in a transtensional pull-apart basin formed atop the Pacific-North, America plate boundary near the transition of the, spreading Gulf of California to the San Andreas transform fault zone. The basin fill comprises Tertiary and, Quaternary sediments and basaltic sills. Seeps of hot, water and gas (CO2, methane) are widespread (Svensen et al. 2007). Very high heat flow caused by shallow, intrusions supports geothermal electricity production. Brine is produced from 1.4 km depth with a, temperature exceeding 350 C and content of 506 ppm, Zn, 95 ppm Pb and 6 ppm Cu, as well as Na, K, Ca, Cl,, S and many other elements. Total salinity reaches, 27 wt.%. Zinc is precipitated from the brine after its, , passage through the turbines; production attains, 30,000 t/year. Isotope data reveal that the geothermal water is of meteoric origin and closely resembles, Colorado River water. Apparently, river water infiltrates along faults down into the basin sediments,, where it is heated and acquires solutes from the rocks., Metal-rich scales in the drill pipes substantiate the, hypothesis that focused upflow of such waters is a, way of ore deposit formation., Fluids in the Rotokawa and Mokai geothermal, fields of the Taupo Volcanic Zone in New Zealand, have an exceptionally high content of precious, metals (gold and silver) and of arsenic, antimony and, mercury (Simmons & Brown 2006, 2007). Ore deposits, however, have not been found until recently, and the systems are only used for the production, of electric energy. More than 50 drillholes have, been sunk to explore and develop the fields, some of
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , them to >2600 m depth. Very few surface signs of, the vast geothermal zone had existed before. One, famous site is the Ohaaki pool with siliceous sinter, and ochreous muds that contain 85 ppm gold,, 500 ppm silver and 10% antimony as well as high, trace content of As, Hg and Tl. This material would be, gold ore if the total mass were not so tiny. The, geothermal zone is an excellent natural laboratory, for investigating hydrothermal ore formation of the, epithermal low sulphidation type (Simmons & Brown, 2000). Note that high content of solutes is rather a, nuisance for geothermal energy production. The precious metals, for example, precipitate from the, ascending fluids because of boiling and gas loss, so, that tubes are clogged., , Hydrothermal convection, Hydrothermal convection cells, such as the ones at, Salton Sea, are established where heat sources below, the surface coincide with permeable flow paths,, often provided by extensional tectonic deformation, (Figure 1.10). Cold infiltrating surface and groundwater is drawn to the “heat exchanger” at depth., The lower density of hot compared to cold water, causes ascent of hydrothermal solutions and establishes hydrothermal convection., , Chemical composition, The chemical composition of hydrothermal solutions is extremely variable. Generally, chlorine, and sulphur are the most important anions. Salinity ranges from very low to more than 50% and the, source of salinity (e.g. halite dissolution, evaporation of seawater, etc.) is detectable by determination of halogens and electrolytes (Botrell et al., 1988). Metals are to some extent dissolved as, simple ions or ion pairs, but more commonly in, the form of complex ions, which combine chlorine, dissociated OH groups and bisulphides, as, well as NH3, H2S and CO32-. The fraction of dissolved matter in hydrothermal solutions varies, from less than 1 to over 50 wt.%. Metal concentrations range from less than 1 to several 1000 ppm, (parts per million, equal to gram/tonne). Even, higher concentrations in solution are possible, when metals are part of complex ions. This reduces the mass of water needed to produce an, , 37, , ore deposit compared to transport in the form of, simple ions. Hydrothermal solutions carry metals, not only in dissolved form but also as colloidal, particles. Colloids are tiny particles (1–1000 nm),, which are quite common in many natural, waters, usually at low concentrations (Ranville, & Schmiermund 1999). High concentrations of, dispersed colloids in water are called hydrosols., In many cases, hydrosols are the precursors of gels., Hydrosols and gels may form by local supersaturation of a substance, for example because of a, sudden change of pH, T, P or Eh., Essentially, the chemistry of hydrothermal solutions is the result of interaction between rocks, and hot water, in space and time (Barnes 1998)., Most important variables controlling these interactions are the initial state of rock and water, the, water/rock mass ratio, temperature, chloride concentration, pressure and redox state (Yardley, 2005). Geochemical thermodynamic modelling of, hydrothermal systems is a tool that provides both, a deeper scientific understanding (Moore et al., 2000) and solutions to very practical problems,, especially in geothermal reservoir and production, management., Possible phase states of hydrothermal waters are, liquid, gaseous (vapour) and fluid (supercritical, “gas” or “liquid”). In fact, many hydrothermal, deposits were formed by supercritical fluids., Water reaches its supercritical state at T > 374 C, and P > 225 bar (the critical point of pure water)., Increasing salinity moves the critical point to, higher T and P (Haas 1971), for example 298 bar, and 407 C for seawater. Similar to gas, supercritical fluids are more compressible than subcritical, aqueous solutions and have a smaller kinematic, viscosity; they are highly mobile (Eckert et al., 1996). A fluid comprising CO2 or CH4 in addition, to water has a high carrying capacity that depends, on pressure and density variations. Very small, variations cause either dissolution or precipitation, of solids. Highly concentrated supercritical fluids, have a texture similar to molten salt with polynuclear ion clusters such as CuNa2Cl2þ or, FeNa2Cl3þ. These clusters reach the size of megacomplexes with over six constituent ions that can, transport metals efficiently at very low concentrations (Oelkers & Helgeson 1993). In contrast to
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38, , PART I METALLIFEROUS ORE DEPOSITS, , subcritical solutions, which form a gas-rich, vapour with low content of dissolved matter when, pressure is released (true boiling), supercritical, fluids segregate into a brine and a dense vapour,, which is capable of effective metal transport (Pokrovski et al. 2005)., Hydrogen ion activity (pH) of hydrothermal solutions varies from moderately acidic to moderately, alkalic. Exceptions occur, of course, and can be, recognized by formation of indicative alteration, minerals. Acidic conditions, for example, cause, formation of kaolinite, alunite or topaz from feldspar. Deep hydrothermal water is normally reduced;, oxygen content may increase near the surface by, mixing with fresh meteoric water. Bituminous substances are a common accessory in hydrothermal, deposits, for example pyrobitumen in the silver, veins at Kongsberg, Norway, and oil in the Pb-Zn, deposit at Pine Point, Canada. This can be a sign that, the hydrothermal solutions were sourced in basinal, sediments (e.g. diagenetic formation water mixed, with hydrocarbon fluids). Present-day examples are, submarine seeps of hot solutions in the Guaymas, Basin (Gulf of California) that contain large drops of, petroleum (1–2 cm diameter); the vents are mainly, built of barite. Organic-chemical investigations (of, biomarkers, etc.) allow a very detailed reconstruction of causative processes, covering both hydrotherms and hydrocarbon genesis (Jochum 2000,, Svenson et al. 2007)., Biological processes, Biological processes at depth influence many, hydrothermal, solutions., Hyperthermophilic, bacteria and archaea are known from hot, springs at the Earth’s surface to black smoker vents, on the ocean floor. With descending branches of, convection systems, microbes can be swept deep, into crustal rocks. At 100 C and in the presence, of sulphate (from seawater) and organic matter (oil, or gas, kerogen), the anaerobes reduce the SO4 ion, to H2S. This has been observed in oil reservoirs, where flooding with seawater was employed in, order to boost production. High H2S content favours dissolution and transport of a number of, metals. At falling temperature, however, H2S contributes to precipitation of metals., , Precipitation of ore and gangue minerals, Many causes may induce the precipitation of ore, and gangue minerals from solutions. Their understanding is an important element of the search for, ore. In the first place, decreasing temperature and, pressure reduce solubility. Precipitation is a function of the relative stability of metal complexes, and decreasing temperature often results in the, common sulphide precipitation sequence from, early Cu to Zn, Pb, Ag and finally Hg. Pressure, drops may cause fluid immiscibility, such as the, formation of two fluids (e.g. aqueous and carbonic), from an originally homogeneous fluid (aqueouscarbonic). This can drastically change pH, fO2 and, temperature, thus inducing mineral deposition., Note that rapid pressure fluctuations are typically, caused by tectonic events (“seismic pumping”, Sibson 1990). Falling pressure is especially effective if common boiling takes place in the twophase field of liquid plus vapour. Boiling suddenly, changes several chemical properties of a hydrothermal solution (concentration, pH, Eh, stability, of complex ions), which reduce the solubility of, dissolved matter. The term “effervescence” is, preferably used in place of “boiling”, when gas, bubbles form that are not vapour of the host liquid, (e.g. carbon dioxide gas bubbles in water). Yet like, boiling, effervescence may also induce rapid precipitation of minerals., Relatively simple systems of this type can be, investigated in geothermal power plants where, production wells are often affected by mineral, precipitation on pipe walls. With time, this process diminishes the open diameter of the tubes, (“scaling”). Corrosion, and the effects of re-injecting cooled and depressurized solutions into the, reservoir, are also instructive (CzernichowskiLauriol & Fouillac 1991)., Often, mixing of chemically different waters, induces deposition of ores and minerals. A common example is the formation of barite. Barite, (BaSO4) is precipitated when ascending chloride, solutions with dissolved barium ions encounter, sulphate-ion bearing water (e.g. seawater). Furthermore, the reaction of hydrothermal solutions with, host rocks or with previously deposited ore minerals is a very efficient means of immobilizing
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , dissolved elements. When metal-bearing solutions, encounter sulphide minerals, more noble metals, are precipitated, whereas less noble elements pass, into solution (eq. 1.1). This is a function of properties such as electronegativity, ionization potential,, electron affinity, redox potential and the energy of, chemical bond formation., Selective precipitation of more noble metals from, solution by exchange with less valuable, elements:, CuFeS2 þCu2þsolution ! 2CuSþFe2þsolution, , ð1:1Þ, , Gold (electronegativity 2.4 Pauling’s) is more, noble than silver (1.9), which is followed by Cu, (1.9) and Fe (1.8), explaining common replacement, relations. Note that in physical terms, only copper, silver and gold are noble metals. In chemistry,, the electric ionization potential of elements is, used to define relative nobility. Host rocks exert, a strong control on noble metal enrichment. Silver, ore veins at Kongsberg (Norway) were fabulously, rich in native silver and argentite (Figure 1.22),, where they crossed pyrite-rich layers (“fahlband”), in host rock gneisses. Gold ore veins of the Mother, , Figure 1.22 Hydrothermal, native silver crystals, (isometric, malformed) from, Kongsberg in Norway. Gangue, includes calcite, barite, zeolite,, fluorite and quartz. Courtesy, Wolfhart Pohl, Washington., , 39, , Lode system, USA are of higher grade where they, intersect pyrite-bearing amphibolite. At the, giant Golden Mile deposit, Kalgoorlie, Western, Australia, deposition of gold is explained by, reaction of sulphide solutions with reduced iron, of doleritic host rocks, forming pyrite, (“sulphidation”). This triggered a radical decrease, of reduced sulphur in the hydrothermal solutions, causing gold precipitation., Organic substances (coal, kerogen, oil, gas) also, provoke immobilization of many metals by, adsorption or reduction. Gold ore veins at Ballarat,, Australia and the metasomatic gold orebodies of, Carlin, USA are enriched where host rocks contain, kerogen-rich layers. Sulphide precipitation in Mississippi Valley deposits is often caused by reaction, between solutions and the organic substance of, host rock carbonates (Spirakis & Heyl 1995)., Incompletely oxidized sulphur (e.g. thiosulphate,, S2O32, polysulphides, SnS2, or colloidal sulphur,, S0) supports high metal content in solution. These, compounds, however, are easily reduced by contact with organic matter so that metals are, instantly immobilized as sulphides. An indirect, consequence is the precipitation of gangue, such, as barite and fluorite.
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40, , PART I METALLIFEROUS ORE DEPOSITS, , Although reduction is a frequent means of ore, mineral deposition, oxidation can have a similar, role, most often concerning iron and manganese., Hydrothermal solutions transport these metals in, reduced form (Fe2þ, Mn2þ) and precipitation of, haematite, magnetite or pyrolusite requires oxidation to Fe3þ or Mn4þ., Contact of metal-bearing solutions with carbonate rocks is a frequent factor of precipitation., Individual agents include the “pH-shock”, upon contact with alkaline rocks and formation, fluids, a larger permeability compared with, pelitic country rocks, a higher solubility of, carbonates in acidic or CO2-rich solutions (which, may result in the formation of “hydrothermal, karst”), and mixing with formation water in carbonate rocks. Orebodies in carbonates take the, form of veins, breccia and karst pipes. They can, also occur as stratiform orebodies with irregular, outlines (“mantos”) and as cross-cutting cloudy, masses. When the replacing masses consist of, sulphides, it is obvious that dissolution of the, original carbonate rock and replacement, (“metasomatism”) by ore must have taken place., The same process term is used for cases where only, cations are exchanged (e.g. siderite in limestone)., Systems of delicately balanced dissolution of one, component and precipitation of another are best, investigated by geochemical modelling based on, thermodynamic principles (Anderson 1996;, Bethke 1996)., Source and origin of hydrothermal, fluids and solutions, Source and origin of hydrothermal fluids and solutions may be related to quite different geological, process systems:, . magmatism (exsolution of an aqueous fluid, phase from silicate magma);, . heating of meteoric, oceanic or formation, water by convection within or near cooling intrusions, in HHP granites and other heat anomalies,, including large faults or uplifted hot metamorphic, complexes;, . diagenesis (mainly physical dehydration of sediments by increasing pressure and temperature, because of increasing overburden, thrust sheet, , superposition, or accretion on active continental, margins);, . metamorphism (mainly chemical dehydration, of minerals that include OH-groups in their, crystal lattice, caused by prograde metamorphic, reactions);, . mixing of two or more of the mentioned source, systems., “Juvenile water” is more a concept than reality., The term refers to water that originates from, degassing of the mantle, and that has never before, been at the Earth’s surface. The second stipulation, cannot be proved with present scientific methods., Casual usage of the word wrongly implies any, magmatic water. Note that “geothermal water”, does not refer to a specific origin but to any hot, water that occurs near the surface. Usually the, term is employed in the context with production, of geothermal energy., Differentiation of the various genetic, possibilities is often difficult, as is shown by, the persistent discussion concerning the precise, origin of many mineral deposits. Geological, and geochemical arguments are most decisive,, especially based on isotope systems including, noble gases that characterize major earth domains, (e.g. 40 Ar the crust, 3 He the mantle: Kendrick et al., 2001, 2002). Halogens, such as Br and Cl, conserve, source ratios and isotope compositions through, subduction and magmatism (Nahnybida et al., 2009)., In the Earth’s crust, aqueous solutions and, fluids are ubiquitous (Yardley 2005, Fyfe et al., 1978) and therefore, hydrothermal ore deposits, occur in a fascinating diversity. This includes, veins (e.g. “orogenic gold”), metasomatic, bodies in carbonates, breccia ores in magmatic, rocks (“porphyry deposits”), ore stockworks and, pipes, volcanogenic terrestrial and submarine exhalations, stratiform base metal ore beds in marine, sediments (sedimentary-exhalative ore) and stratabound diagenetic Pb-Zn-Ba-F deposits in marine, carbonates. More detailed information on these, deposit types and their formation is provided in, later sections. Here, it is essential to introduce, first the most important methods, which provide, data that constitute essential building blocks of, genetic models.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Methods of genetic investigations (1), Isotope Geochemistry – The Origin of Water and, Minerals, and the Age Determination of Hydrothermal Mineral Deposits, The application of isotope geochemical methods provides a new dimension of metallogenetic, research (Hoefs 2009, Allegre 2008, Faure &, Mensing 2004, Valley & Cole 2001, Dickin, 1995). Isotope systems indicate the source of, water, gas, metals and other compounds in hydrothermal solutions, illuminate reactions between, solutions and host rocks, reveal formation temperatures and expose mixing processes between, solutions of different origin. In addition, isotopes, are invaluable means of age determination of, hydrothermal, magmatic and supergene ore, deposits. Isotope geochemistry is an essential element in the quest for a full understanding of, genetic processes. Too often, however, complex, relations still limit reconstruction of past processes (Hoefs 2009)., Unstable (radioactive) isotopes are distinguished from stable (non-radioactive) isotopes., Radiogenic isotopes or daughter elements (nuclides) originate by radioactive decay of unstable, parent elements. Isotopes of an element have an, identical electron configuration but different, mass. In physical, chemical and biological processes, this causes thermodynamic and kinetic, effects. Isotopes of light elements that have a high, mass difference display isotope fractionation. A, simple example is evaporation: Lighter isotopes, preferentially enter the vapour phase because of, their higher vibration energy. Mass difference also, affects reaction rate and bond strength, inducing, fractionation between syngenetic minerals, and, between a mineral and its parent solution. Because, the fractionation is a function of temperature,, several isotope systems (e.g. sulphur, oxygen, silicon) are very sensitive geothermometers,, although only if full equilibrium was attained, (Onasch & Vennemann 1995)., Isotopic compositions are measured relative to a, standard, using mass spectrometers. Therefore,, the fractionation of stable isotopes is commonly, expressed as the deviation (d, in ‰, per mil, of, mass) from a standard (eq. 1.2)., , 41, , Calculation of isotope fractionation as a deviation, from a standard:, dð‰Þ ¼ 103 ½ðRSample RStandard Þ=RStandard , , ð1:2Þ, , R is the ratio of an isotope pair, for example 18 O=16 O, D/, H, etc. Negative d values indicate an enrichment of the, light isotope relative to the standard, whereas the heavy, isotope is enriched if the sign is positive., , Isotope age dating of minerals that originated as, a consequence of ore formation processes is based, on the time dependence of the genesis of a stable, daughter nuclide (D) by radioactive decay of an, unstable parent element (P). The age expressed in, unit “annum” (a, ka, Ma) is calculated with the, following equation 1.3., Determination of geological ages by radioactive, decay:, t ¼ 1=l lnð1þD=PÞ, , ð1:3Þ, , Time (t) is the date before present, when radioactive, decay started, D is the number of atoms, of the daughter element, P the number of atoms of the, parent element, and lambda (l) the decay constant of the, parent element (l ¼ 0.693 divided by its half-life). Resulting age data or model ages can only be considered as, real ages if the analysed system was closed for both, parent and daughter element during the whole time span, (Dickin 1995)., , Commonly, gangue silicates or alteration minerals, including K-feldspar, white mica (muscovite,, sericite), biotite, apatite, monazite, rutile, titanite,, xenotime and zircon, are used for the, age determination of hydrothermal ore deposits., Several isotope systems can be studied, for example, U-He, U-Pb, Pb-Pb, Rb-Sr, Sm-Nd, K-Ar and, 40, Ar=39 Ar. Also, ore minerals such as cassiterite,, columbite, sphalerite, several sulphides and scheelite can be dated with these systems. U-He, U-Pb,, Th-Pb and K-Ar systems, however, are subject to, complications by a-recoil that will damage the, “container” with time, allowing partial loss of, parent and daughter. Moreover, very careful investigations revealed that many isotope systems are, impaired by later thermal events or the passage, of migrating fluids, even at relatively low temperature (Kerrich & Cassidy 1994, Selby et al. 2002)., Clearly, the geological significance of age data and, model ages must always be critically examined.
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42, , PART I METALLIFEROUS ORE DEPOSITS, , dard Mean Ocean Water). Evaporation of ocean, water is the starting point of precipitation/evaporation cycles that produce isotopically “lighter”, water vapour (and derived precipitation), as the, distance to the sea and to the equator increases., The isotopic composition of precipitation,, surface and shallow groundwater forms a band, that is termed the meteoric water line (Figure 1.23)., “Meteoric water” is water that has been part of the, meteorological cycle evaporation – condensation –, precipitation (excluding seawater). The meteoric, water line is in reality a band because of local, deviations. Note that “formation water” is not a, genetic term but simply designates water of, unknown origin and age in sediments; formation, waters generally show an increase of d18 O with, increasing burial depth. Brines that form by evaporation in semi-closed marine lagoons are isotopically heavy. Isotope exchange between rocks, and ocean or meteoric water produces fluids that, retain largely the original hydrogen isotopic composition. Oxygen isotope ratios, however, are considerably altered by partial exchange with oxygen, contained in rocks. An equivalent effect is commonly observed in geothermal fields; while the, fluids are increasingly enriched in heavy oxygen,, the hydrothermally altered rocks are depleted in, 18, O. The depletion zones allow mapping of fluid, passage and can be vectors to ore (Hoefs 2009, Holk, et al. 2008)., In the dD=d18 O diagram, waters of magmatic,, metamorphic and sedimentary origin occupy, partly overlapping fields (Figure 1.23) so that they, cannot always be clearly discerned. Water, , The age of most ore minerals (e.g. sulphides,, oxides) cannot be determined precisely with the, above mentioned methods, because they rarely, host lithophile elements in their crystal lattice., One alternative is direct dating of sulphides, oxides and even gold using the chalcophile-siderophile element rhenium. The rhenium-osmium, method is based on the b-decay of 187 Re to 187 Os, with a half-life of 41.6 Ga. Thus, the age of molybdenite, arsenopyrite, sulphides and oxides can, easily be established. This is especially useful for, investigating gold deposits (Morelli et al. 2007)., Similar to lead isotopes, the Re-Os system allows, conclusions concerning the source of metals apart, from dating (Stein et al. 2001)., Improvements of the precision of dating techniques allow increasing resolution of ages and, with that, reliable measurement of the duration, of ore deposit formation. Results show that many, hydrothermal ore deposits have been formed in, geologically very short time (several thousands to, a few ten thousands of years), which is just about, the error margin of present methods. Some data, imply that other hydrothermal systems may have, been surprisingly long-lived (hundred thousands, to millions of years)., Stables isotopes of water, Stable isotopes of water, including 1 H (hydrogen), and 2 H (D, deuterium) as well as 16 O and 18 O, are, exceptionally revealing keys for the comprehension of hydrothermal processes. Average ocean, water is employed as a standard (SMOW ¼ StanSaline, brines, , g), Metamorphic, water 300-600°C, , PMW, Sediments, , Poles,, inland, -10, , 18, δ OPDB at 0.0‰, , r in, B, , -60, , -80, , (w, ea, the, , VV, FMW, , Me, teo, , -40, , Equator,, coastal, , ric, , -20, , Formation waters, , A, , Ka, oli, nit, e, , 0, , Seawater, (SMOW), wa, ter, , δ D (‰), , +20, , -5, , 0, , 5, , δ OSMOW (‰), 18, , 10, , 15, , 20, , 25, , 30, , Figure 1.23 Isotopic composition of, waters that participate in hydrothermal, ore formation (adapted from Hoefs 2009)., With kind permission from Springer, ScienceþBusiness Media. PMW is primary, magmatic water; FMW felsic magmatic, water; VV is volcanic vapour. Path A, shows evolution of ocean water to, formation water. Path B suggests mixing of, magmatic with meteoric water.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , involved in fossil hydrothermal systems can be, sampled from fluid inclusions in minerals, but, this is fraught with various limitations (Faure, 2003). The use of gangue minerals with hydroxyl, (OH) groups is generally less error-prone. In, that case, however, the mineral/fluid fractionation factor must be known. Later isotope, exchange with passing fluids (e.g. the common, flooding of cooling magmatic bodies and of hydrothermal systems by meteoric and formation, waters) needs to be investigated. Ore formation, systems involving high pressure fluctuations (e.g., Sibson’s “seismic pumping”) are characterized by, wide variations in D/H fractionation between, water and minerals (Horita et al. 1999). This is, intensified at conditions above the critical temperature of water and generally with fluids of very, low “gaseous” density (Driesner 1997)., , Carbon isotopes, Carbon consists of the stable isotopes 12 C and 13 C., Because of its short half-life (5730 years), age, dating with cosmogenic 14 C is limited to very, recent geological processes. The origin of carbonate minerals and rocks is investigated by stable, carbon and oxygen isotope fractionation relative, to the PDB standard (belemnites found in the, Cretaceous Peedee Formation, Carolina, USA)., Consequently, marine carbonates have dPDB, values about zero, mantle carbon about 7‰ and, kerogen between 20 and 30‰. Hydrothermal, carbonate carbon varies from 6 to 9‰ indicating a deep origin of the CO2. However, the, source signal may be veiled by inorganic isotope, fractionation of carbon, which is a function of, oxygen partial pressure, temperature, pH, ionic, strength and carbon concentration, complicating, genetic interpretations. Comparative investigations of the large travertine deposits near, Florence, Italy and warm (29–34 C) springs in the, same area that deposit travertine (Guo et al. 1996),, provide an instructive example. Recent calcite, shows an increase of 13 C with higher distance to, the spring orifice, due to two factors: i) simple, degassing (12 CO2 evaporates preferentially,, , 43, , whereas 13 CO2 is retained in the solution and, collects in the travertine); and ii) bacterial fractionation. Bacteria predominantly abstract 12 C, from the solution, so that d13CCalcite is increased, compared to abiotic processes., , Sulphur isotopes, Sulphur isotopes provide genetic information on, sulphides and sulphates of ore deposits. Standard, is troilite (FeS) from the Cañon Diablo meteorite, that has about the same 34 S=32 S ratio as the Earth’s, mantle. Therefore, basalts and other mantle, magmas and sulphide ore deposits derived from, them have d34 S near zero. Clearly different is, sulphide sulphur produced by organic fractionation with d34 S from 20 to 30‰ (the so-called, “bacteriogenic sulphur”; Canfield & Thamdrup, 1994) and marine sulphates with þ10 to þ30‰., This is why the origin of sedimentary sulphur can, often be determined with high precision. Magmatic and hydrothermal ore deposits may contain, sulphur of mixed origin, veiled by inorganic, fractionation. The isotope fractionation between, cogenetic sulphide minerals, or sulphides and, sulphates can be a useful geothermometer, (Zheng 1991)., , Strontium isotopes, Strontium isotopes also provide evidence of the, derivation of ore solutions. The system comprises, radiogenic 87 Sr, a decay nuclide from 87 Rb, and the, stable 86 Sr. It is the base of a widely used method of, age determination. In hydrothermal ore deposits,, the ratio 87 Sr=86 Sr is a means of characterizing, fluids that formed carbonates (Ca, Mg, Fe), sulphates (Ca, Ba, Sr), fluorite and apatite. Samples, used should have extremely small rubidium content. The 87 Sr=86 Sr ratio of large source reservoirs, such as the Earth’s mantle, continental crust and, seawater are quite different. Seawater displays, systematic variations in geological time. On that, basis, hydrothermal and marine-sedimentary, carbonates can be distinguished. Analysis of
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44, , PART I METALLIFEROUS ORE DEPOSITS, , time-dependent diagenetic processes is possible, (Schreiber & Tabakh 2000)., , Lead isotopes, Lead isotopes are an especially powerful tool to, trace metal sources in mineral deposits, because, their geochemical behaviour in aqueous fluids, resembles that of many associated metals (e.g. Zn,, Cu, and Ag). Also, traces of lead are ubiquitous in, most rocks and in many ore minerals. Terrestrial, lead consists of four stable isotopes:, . 1.0–1.6 wt.% 204 Pb, primordial (not radiogenic),, used as a reference base;, . 20.8–27.4% 206 Pb, radiogenic decay product, of 238 U;, . 17.6–23.6% 207 Pb, radiogenic decay product, of 235 U;, . 51.2–56.2% 208 Pb, radiogenic decay product, of 232 Th., Because of the minimal mass difference, between lead isotopes, fractionation by geological processes is very small. Their inherent useful, information rests in formation and mixing of, the radiogenic isotopes. Both are controlled by, properties of the source rocks, including age,, lithology (Chiaradia & Fontbote 2003) and, geochemistry (uranium and thorium content)., Uranium concentrations in the source are, described by m-values (the ratio 238 U=204 Pb)., Because uranium and thorium are markedly, lithophile elements, they are enriched in the, continental crust (with high m) relative to the, mantle. The evolution of present-day lead started, with the primordial or meteoritic composition, 4550 Ma BP (before present), which is derived, from troilite in the Cañon Diablo meteorite (like, sulphur isotopes). Since then, radiogenic lead, increased because of uranium and thorium decay, in the major geological reservoirs (mantle, lower, and upper crust). Without any disturbance, ordinary lead is the result. When ordinary lead is, separated from uranium and thorium, and concentrated in ore deposits (e.g. as galena), its isotopic composition reveals both the age of, mineralization and the source reservoir of the, , lead (“plumbotectonics”, Zartman & Doe, 1981). “Single stage lead” is derived by singlephase extraction from a very large reservoir, probably continental lower crust or uppermost mantle and is present in a number of large ore deposits, (e.g. Mt Isa, Broken Hill, Australia). Other ore, deposits have lead of an anomalous composition,, which originated by multi-stage processes. Typically, anomalous lead provides model ages that, are clearly different from the geological age. Two, extremes shall serve as an example:, An unusually high radiogenic fraction was found in, lead of the Tri-State mining district (USA). Its source, is thought to lie in uraniferous Cambrian sandstones, and Precambrian basement rocks that had a very low, content of ordinary lead. Therefore, the migrating, hydrothermal brines took up the easily soluble radiogenic lead from decayed U-Th phases. Lead model, ages of the resulting galena plot in the geological, future, whereas the mineralization took place in the, Late Palaeozoic (Symons et al. 2005). In the mining, district of Bleiberg, Austria, the lead model age is, Mesopalaeozoic, but the ore deposits clearly formed, much later, in Triassic time. It is assumed that the, lead of Bleiberg is derived by leaching of feldspars in, Palaeozoic gneisses that contain up to 50 ppm Pb, but, very little uranium (<1 ppm U þ Th). Therefore,, hardly any radiogenic lead was available for extraction by the hydrothermal solutions., , Accordingly, lead model ages are very often not, “dates”, but nevertheless provide important information on ore genesis. Lead isotopes may indicate, the source of the metal and this is an essential, element of genetic modelling (cf. Everett et al. 2003)., Many other isotope systems provide deeper, insight into ore forming processes. Examples, include chlorine (Nahnybida et al. 2009, Banks, et al. 2000), nitrogen (Jia et al. 2003), iron (Johnson, et al. 2009, 2008, Staubwasser et al. 2006,, Beard et al. 2003) and boron (Perez Xavier et al., 2008). Subrecent geological processes (younger, than 20 Ma), such as the age of supergene ore, deposits, are investigated with cosmogenic nuclides (e.g. 10 Be, 14 C, 26 Al, 36 Cl and 129 I; Siame, et al. 2006, Muzikar et al. 2003) and with the, uranium-series disequilibrium method (Reich
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , et al. 2009). Typically, results reveal extraordinarily long durations of supergene processes,, although hardly at constant rates., Methods of genetic investigations (2), Temperature and pressure of hydrothermal ore, formation, Apart from geochemical parameters, temperature, and pressure of a hydrothermal system are, essential determinants of solubility, transport, and precipitation of elements and compounds., In order to understand such systems, formation, temperature and pressure have to be measured., Unfortunately, there are relatively few methods, that allow the precise determination of pressure,, but the reverse can be said concerning temperature. Conventional petrologic geothermometers, and geobarometers can rarely be applied to hydrothermal mineralizations., Frequently employed “geothermometers” of, economic geology include:, . microthermometry of fluid inclusions in, minerals;, . cation-exchange geothermometers, indicating, the temperature of the last equilibration of fluids, with host rocks (e.g. silica: Fournier & Potter 1982;, magnesium-lithium: Kharaka & Mariner 1990);, . trace element geothermometers (e.g. Ti-inzircon for very high temperatures: Watson et al., 2006);, . different modifications of minerals (e.g. a- vs. bquartz at 573–600 C);, . exsolution or unmixing of phases during cooling, (e.g. cubanite lamellae in chalcopyrite that formed, at >205 C);, . oxygen, silicon and sulphur isotope fractionation in cogenetic minerals (stable isotope, geothermometry);, . the distribution of certain elements in cogenetic, minerals (e.g. Fe-Zn-S or Au-Ag-S systems; Ascontents, e.g. in arsenopyrite);, . chlorite, and, muscovite, mineral, geothermometers., All of the above-mentioned geological thermometers are inaccurate due to factors that control, the results but cannot be precisely determined. It, is best practice in economic geology to combine, , 45, , several methods, for example microthermometry, of fluid inclusions (Roedder 1984, Samson, et al. 2003), the arsenopyrite geothermometer, (Kretschmar & Scott 1976) and the fractionation, of stable isotopes., Fluid inclusions, During formation of minerals from fluids or solutions, irregularities on growth planes often cause, inclusion of tiny droplets of the parent liquid, (primary inclusions). Quite common are also inclusions that are hosted by microfractures in a, mineral (secondary inclusions). Fluid inclusions, are usually very small (below 100 mm), so that, investigations are carried out on polished thin or, thick sections using a microscope. At room temperature, many inclusions contain an aqueous, liquid with a gas bubble that occupies 10–40%, of the volume (type 1 of Figure 1.24). Upon, heating of the sample, the bubble disappears at, a determinate temperature that is described as, the homogenization temperature Th. In the simplest case, Th is equal to the formation, or trapping, temperature Tt, because the bubble results, from cooling and shrinking of the hot inclusion, within a volume fixed by the surrounding host, mineral. However, Th is only the minimum temperature of trapping since pressure at the time of, formation controls size and density of the bubble., An accurate determination of the formation temperature is only possible if the formation pressure, is known and the so-called pressure correction can, be calculated (actually, this is a correction of, temperature as a function of pressure: Roedder, 1984). The density of the inclusion, its degree of, fill (liquid vs. total volume), the salinity of the, liquid (mostly due to dissolved NaCl, but Mg, Ca,, K, etc. may be involved), the presence of daughter, crystals (common are halite cubes: type 3 in Figure 1.24), of fluid hydrocarbons and contents of, non-aqueous gases (frequently CO2, followed by, CH4, N2, etc.) are all needed for a full understanding. Investigations must also address the identification of groups of associated inclusions that were, trapped from a fluid of the same composition at the, same time, temperature and pressure (a “fluid, inclusion assemblage” or FIA). Natural minerals
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46, , PART I METALLIFEROUS ORE DEPOSITS, , Type 1, , Type 2, V, , V, , L, L, , Type 3, , Type 4, Anhydrite, , V, , L, Vco2, , Halite, , Lco2, L, , 10 - 50 µm, , typically contain many different FIAs, which, reveal the evolution of a hydrothermal system., Most fluid inclusions observed in hydrothermal, ore deposits belong to one of four groups, (Figure 1.24):, 1 Type 1: Liquid (L) aqueous inclusions of low to, moderate salinity (<26.5 wt.% NaCl, which, marks saturation at room temperature), a water, vapour bubble (V) and a density of 1 g/cm3;, 2 Type 2: Water vapour occupies >60% of the, inclusion volume, the salinity is similar to type, 1; the density is clearly <1 g/cm3; if types 1 and 2, (or 2 and 3) are cogenetic in the same sample,, , Figure 1.24 The four most common types of fluid, inclusions as they appear under the microscope at room, temperature., , true boiling (as opposed to effervescence of a, dissolved gas like carbon dioxide or methane, in water) of the solution below the critical point, or at boiling curve conditions is indicated;, this gives rise to the term “boiling assemblage”, (Figure 1.25);, 3 Type 3: Highly saline (26.4 to >50 wt.% salinity) aqueous inclusions with halite daughter crystals and with a high density; fluids of this nature, can result from dissolution of evaporites, seawater, evaporation, segregation of brines from melt at, magmatic temperatures (Aud, etat et al. 2008), by, formation of residual brines because of vigorous, , Figure 1.25 Fluid, inclusion assemblage (FIA), formed by subcritical, boiling in quartz from, miarolic cavities in the, barren Torres del Paine, granite (Patagonia, Chile)., Vapour (dark), concentrated, Cu and As, whereas the, brine inclusions marked by, halite crystals are enriched, in metals such as Mn, Fe, and Zn (L€, uders et al. 2005)., Courtesy Volker L€, uders,, GFZ Potsdam.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 47, , Temperature(°C), 200, , 400, , d, ui, liq l, + aC, as N, G +, , Liquid, , 1000, , A, Gas + liquid, , 1 km, , 400, 2 km, , C, , 10 wt.%, in vapour, , 800, , subcritical boiling (abstraction of vapour causes, concentration of salt in the liquid phase and by, supercritical phase separation;, 4 Type 4: CO2-rich inclusions, with little water;, at room temperature carbon dioxide in the inclusions occurs in both liquid (LCO2 ) and gaseous, phase (VCO2 )., In Figure 1.26, path I is entered by very hot,, hypersaline magmatic liquids (vapour). Above, the phase boundary A, cogenetic type 2 and type, 3 fluid inclusions segregate. Path II illustrates the, fate of late, less saline and much cooler fluids. Part, of these may be derived from vapour that because, of cooling contracts at higher pressure into the, liquid phase. Inclusions trapping these fluids are, type 1 with low to intermediate salinity (3–20 wt., % NaCl). Phase boundaries between liquid, and gasþliquid are shown for different salt, contents. Boiling assemblages must have formed, at a point along such a phase boundary. C is the, critical point for an aqueous solution with 10 wt., % NaCl., The salinity of 26.4 wt.% separating type 1 and, type 3 inclusions is the solubility of NaCl in water, at room temperature (20 C). In practice, the presence of other salts, such as CaCl2 lowers the NaCl, saturation to 23 wt.%. The salinity of fluid inclusions is determined by freezing with liquid, nitrogen and remelting, i.e. the temperature is, allowed to rise back to ambient conditions., Because dissolved salts depress the freezing point, (Bodnar 1993, Potter et al. 1978), the melting, temperature of ice (Tm ice) provides a precise, , Brittle, , 600, Ductile, , Pressure (bar), , 200, , Figure 1.26 A crystallizing shallow, intrusion may exsolve (I) early and very, hot brines, or (II) later saline fluids of, intermediate temperature, resulting in, characteristic fluid inclusions (after, Brathwaite et al. 2001). With kind, permission from Springer, Science þ Business Media., , 800, , 600, , Crystallizing intrusion, , Gas + liquid, , 3 km, 50 wt.%, 70 wt.%, , Approximate depth (lithostatic pressure), , 0, , 0, , 30 wt.%, in brine, , measure of salinity. Results are expressed in the, form of equivalent weight percent of NaCl (wt.%, NaCl equiv.). The chemical determination of the, composition of inclusions is complicated by the, tiny mass of individuals. Yet, a number of methods, are available; most advanced is the approach with, laser ablation (LA) connected to ICP-MS (Sylvester, 2008, Aud, etat et al. 1998). Merging the data, with results from high-resolution investigations, of the hydrothermal paragenesis (e.g. SEM-CL,, Scanning Electron Microscope-Cathodoluminescence: Rusk & Reed 2002) allows a detailed unravelling of the chemical and physical evolution of, a hydrothermal system., The investigation of fluid inclusions is somewhat restricted by the need for samples that are, transparent to visible light, thus excluding most, ore minerals. Of the opaque oxides, sulphides and, sulphosalts only few, such as wolframite, ironrich sphalerite, and antimonite are transparent for, infrared light and can be examined with the methods described above (Figure 1.27). The result is that, most fluid inclusion investigations are based on, gangue and not on ore minerals. This may introduce severe errors (Wilkinson et al. 2009), for, example if ore and gangue formed from differing, fluids (L€, uders 1996). Other problems are connected with post-formation changes of the inclusions, for example by renewed equilibration at, different P/T-conditions. Metamorphic shearing, and recrystallization often destroy fluid inclusions. Even synmetamorphic inclusions in mobilizates are usually decrepitated because of pressure
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48, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.27 Aqueous fluid, inclusions in wolframite of, Panasqueira mine,, Portugal, studied by nearinfrared microscopy. The, fluids homogenize between, 320 and 325 C and have a, salinity of 12% NaCl, (L€, uders 1996). Note the, large vapour bubbles in, inclusions which resemble, type 2 of Figure 1.24., Courtesy Volker L€, uders,, GFZ Potsdam., , release during uplift (e.g. Joma Mine, Norway:, Giles & Marshall 1994). However, in spite of many, obstacles, petrographic and microthermometric, work on fluid inclusions brought great advances, of understanding hydrothermal ore deposit formation. Even eminently practical questions can be, solved, such as the differentiation between mineralized and barren veins., The broad application of microthermometric, methods suppressed the older decrepitation analysis. Its principle was to count acoustic emissions,, while a mineral was submitted to stepped heating., Because each micro-acoustic signal is due to a, bursting inclusion, maxima were taken to indicate, roughly the temperature of formation. Of course,, precise genetic data cannot be obtained with this, method. But as decrepitation analysis is relatively, simple and gives quick results, it may be a valuable, tool in exploration. In the Cowra Gold District,, New South Wales, Australia, auriferous veins, can easily be distinguished from sterile quartz, veins, because the first contain much more CO2, and display a unique decrepitation pattern, (Mavrogenes et al. 1995)., Traditionally, hydrothermal ore deposits were, grouped according to assumed formation temperatures into hypo- or katathermal (500–300 C),, mesothermal (300–200 C) and epithermal (below, , 200 C). This classification was quietly abandoned,, while a wealth of data has been acquired on real, temperatures of hydrothermal processes and resulting deposits. Temperatures vary widely, even, during the lifetime of one single hydrothermal, system. Accordingly, temperature is a poor criterion for classification, although, of course, an, integral part of the description of the formation, of mineral deposits. In many scientific reports, the, terms named above are still used in a very wide, sense, indicating rather depth than temperature., In this usage, epithermal deposits are those formed, in the uppermost part of the crust (<1 km from the, surface) and below 200 C (but note that the most, important member of this class, the “epithermal, gold deposits”, typically originated at 300 C)., “Meso-” and “hypothermal” describe increasing, depth and temperature. However, depth and temperature are not always correlated. Fluids in shallow porphyry copper deposits, for example, may, originally have had a temperature approaching, 800 C (Figure 1.26). Clearly, depth (or pressure) is, a much more useful criterion to describe related, groups of hydrothermal deposits. Therefore,, Gebre-Mariam et al. (1995) suggested to adopt the, terms “epi-, meso- and hypozonal” (Table 1.2),, similar to the notations referring to metamorphism or the intrusion depths of granites. The
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Table 1.2 Depth-zone classification of hydrothermal, mineral deposits, Epizonal, Mesozonal, Hypozonal, , 150–300 C, 300–475 C, 475–700 C, , 0.5–1.5 kbar, 1.5–3.5 kbar, 3–6 kbar, , <6 km depth, 6–12 km, >12 km, , Temperature is not a classification criterion. T-values given, here apply to epigenetic Archaean gold deposits only (GebreMariam et al. 1995)., , proposal was aimed at epigenetic Archaean gold, deposits, but could be advantageously used for all, hydrothermal mineral deposits., Formation pressures, Rarely, formation pressures of hydrothermal ore, deposits can be precisely measured. Usually, geological estimates are derived from the assumed, depth of formation and lithostatic or hydrostatic, boundary conditions. Other approaches include, modern methods of metamorphic petrology in, combination with fluid inclusions data. If the, minerals are present, methods such as the sphalerite-pyrite-pyrrhotite geobarometer can be, applied (Lusk et al. 1993): Between 300 and, 600 C, the FeS-content in sphalerite in contact, with pyrite and pyrrhotite is only a function of, pressure. Isochores of fluid inclusions (lines of, constant volume and density in P-T space) can be, intersected with independent data, such as the, solidus temperature of the parental granite, (Audetat et al. 2008). Some fluid systems allow, pressure analysis based on experimental data of, thermodynamic properties of synthetic fluids similar to natural inclusions; simple systems, such as, H2O-NaCl provide a good approximation, but high, vapour and gas contents cause large error margins., Unambiguous evidence of boiling still provides, the most accurate geobarometry data available, (Roedder 1984)., Methods of genetic investigations (3), Mineral succession, textures and structures of, hydrothermal mineral deposits, The precipitation of minerals from hydrothermal, solution is controlled by various boundary conditions, some of which have been discussed earlier., , 49, , For a reconstruction of the conditions at a point in, time and for the whole duration of hydrothermal, activity, minerals that formed simultaneously, (the paragenesis) must be strictly discerned from, the sequential order of mineral assemblages (the, paragenetic sequence). Macroscopic observations, of mine exposures and hand specimen, ore microscopy and other mineralogical techniques are combined to resolve the evolution of hydrothermal, systems., Because most ore minerals are opaque, microscopic examination is usually based on polished, specimen and reflected light. This allows resolution of the intergrowth of different minerals. The, position of minerals in one paragenesis or in the, whole paragenetic sequence is revealed by observations that include exsolution, replacement, along grain boundaries or microfractures and, many other textures and structures. Ore microscopy has attained a level of very high perfection, (Craig & Vaughan 1994, Ramdohr 1980, Stanton, 1972). Methods, such as electron probe microanalysis (EPMA), transmission electron microscopy, (TEM), scanning electron microscopy – cathodoluminescence (SEM-CL) and other methods of, mineralogical analysis (e.g. QEMSCAN, CSIRO,, Australia), confirm and complement optical determination. Even greater resolving power is available with 100-nm-resolution secondary ionizing, mass spectrometry (nanoSIMS; Barker et al. 2009)., Macroscopic features that assist in establishing, the mineral succession include overgrowths and, crusts, symmetric vein fillings and cross-cutting, relations of veins. This is supplemented by colour,, texture type and grain form or crystal habit of, minerals, and by examination of fluorescence and, luminescence. Because quartz is an important part, of most hydrothermal parageneses, recording its, structures and textures is most instructive (Dong, et al. 1995). Combined with results of microthermometry, trace and isotope geochemistry and, geological investigations, the evolution of an individual hydrothermal ore deposit can be resolved., Open space filling is a characteristic feature of, many hydrothermal ore deposits. The term implies that precipitation of minerals occurred in, open fissures, pipes or caves, which acted as flow, channels for hydrothermal solutions. Open space
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50, , PART I METALLIFEROUS ORE DEPOSITS, , Host rocks, , Barite - sulfide vein fill, , Vein, structure, Gangue, Quartz, Siderite, Barite, Strontianite, Carbonates, , Ore minerals, Galena, Ag-tetrahedrite, Cataclasis, Alteration, Silicification, , Early, , Main phase, , with, Sr-rich barite (1) and sulfides, , Late phase, with, barite (2) and sulfides, , filling starts with nucleation of minerals on a solid, surface of the flow channel walls. This explains, the often observed younging of the mineral succession towards the centre. In Figure 1.28, different parageneses of a specific phase of vein, formation can be read vertically, whereas the horizontal axis reflects evolution in time (from left, to right; crystals in the open space on the right, represent the youngest hydrothermal minerals, and the vein’s symmetry axis). Banding of hydrothermal precipitates results from variations of, physical and chemical conditions in time. Tectonic movements during hydrothermal activity, cause fracturing of earlier minerals and cementation by younger precipitates. Fibrous texture of, minerals, such as quartz aligned vertically or, obliquely to vein margins, indicates synchronized, opening and mineral growth. In near-surface, hydrothermal channels, clastic sediments have, been observed that indicate high flow velocity of, hydrothermal solutions. Hydrothermal sediments, formed in this way include bedded rock flour, clay,, sand, solid hydrothermal precipitates and even, gravel (pebble dykes). Mainly epizonal features, include hydrothermal breccias that may host fabulous orebodies. Many breccias were formed by, self-sealing of flow paths and consequent phreatic, explosions., Sedimentary textures of material filling open, spaces of hydrothermal veins are not rare (e.g. in, the fluorite vein at Rossignol, France). This occurs, when solid material is eroded, or when minerals, and colloidal particles nucleate in suspension, from ascending fluid. Sedimentation of these par-, , Figure 1.28 Parageneses and paragenetic, sequence of Tertiary rift-related leadsilver-barite veins in the Caroline mine,, near Freiburg, Germany (modified from, Germann et al. 1994). With kind, permission of R. Lang and LGRB Freiburg., , ticles depends on the ratio of the upflow speed of, the fluid to gravitational settling of the particles., Stoke’s law can be used to calculate the settling, velocity (V) of the particles, allowing an estimate, of the fluids’ upflow velocity (eq. 1.4). Suspended, minerals nucleating and growing in fluids settle to, produce “blocky” vein textures (Okamoto & Tsuchiya 2009)., Settling velocity of particles in a fluid:, V¼, , 1 ðsrÞg:D2, , m, 18, , ð1:4Þ, , V ¼ free-falling velocity (m/s), s ¼ density of the particle,, r ¼ density of the fluid, D ¼ particle diameter (m), g ¼, acceleration due to gravity (m/s2), m ¼ viscosity of the, fluid (m2/s)., , Hydrothermal open space deposits are usually, coarsely crystalline. In certain deposits, minerals, may occur in dense, reniform-botryoidal and, banded crusts, often with fissures due to shrinkage. Such textures are described as “colloform”, and are thought to imply formation from a gel (a, hydrosol with a mechanical resistance). Such precipitates are amorphous and may gradually mature, into crystalline phases (i.e. opal, wood tin and, garnierite) or into proper minerals including chalcedony, reniform sphalerite and malachite. Often,, but not necessarily always, the origin of colloform, substances may be traced to colloids forming in, the hydrothermal fluids. It is also possible that, coagulated colloids are transported with the, hydrothermal flow and deposited higher up as, a gel. Colloidal particles in water typically
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , have a free negative charge, which leads to electrostatic repulsion and dissipation (Ranville &, Schmiermund 1999), similar to heat energy. While, the formation of aggregates may be due to van der, Waals forces, neutralization of the charge (by mixing with an electrolyte, pH-change, cooling, etc.), frequently effects aggregation and precipitation as, a gel. Some hydrosols form “colloidal crystals”., Apart from colloids, hydrosols and gels, botryoidal, textures may be inherited from microbial, mats (Labrenz et al. 2000). Obviously, different, ways may lead to formation of a colloform, mineralization., The characteristic banding of many colloform, precipitates may be due to external factors, but, also to self-organization. Heany & Davis (1995), propose that the banding of agate, which is, a rhythmic succession of chalcedony and, quartz, is the product of a changing degree of, polymerization of the solution on the tips of the, growing chalcedony fibres. Polymerization decreases when chalcedony is rapidly deposited,, so that the system flips to slow quartz precipitation from a monomeric solution. In this phase,, diffusion slowly raises the concentration of the, solution until the state of a concentrated polymeric solution is again reached. Colloform, SiO2 can also form from “silicothermal fluids”, (Wilkinson et al. 1996). These are liquids with, 90% SiO2 that co-exist with an aqueous supercritical fluid in a wide temperature field (400 to, >750 C). Silicothermal fluids might be the explanation of the high frequency of amorphous silica, phases in epizonal and especially in epithermal ore, deposits., Replacement of earlier solid phases is very common in hydrothermal ore deposits. As one mineral, is dissolved, another forms in the same place, often, without a change of volume and with conservation, of very fine textures (pseudomorphism). Characteristic examples include cassiterite replacing, orthoclase (Cornwall, England) or even crinoids, (New South Wales, Australia), scheelite after, wolframite, and from low-temperature solutions,, the silicification and pyritization of whole tree, trunks in coal seams. The term “replacement,, or metasomatic ore deposit” is only used if the, process produced most of the actual ore. Carbo-, , 51, , nates are frequently the subject of replacement, because of their disposition for chemical reactions. Advection and evacuation of matter is usually enacted by physical flow but diffusion can also, play a role., The waning stages of hydrothermal systems, typically display falling temperature, lower mass, flow, pervasive fracturing and cataclasis, and often, infiltration of oxidizing meteoric water. This, leaves fractures and thin fissures of the hydrothermal vein mass covered by films of goethite, haematite or manganese oxides., , Methods of genetic investigations (4), Hydrothermal host rock alteration, Host rocks influence hydrothermal solutions to, the point of controlling the site of ore precipitation, but at the same time they are affected by, alterations that emanate from the solutions. The, resulting changes (“hydrothermal alteration”), may extend from only centimetres off the flow, channel (Figure 1.29) to very wide halos in the case, of pervasive flow. Alteration zones are important, clues for ore deposit exploration and for the prospectivity of an area (Lentz 1994). Some alteration, halos can be mapped by remote sensing from, space. Hydrothermally altered rocks are often so, fine-grained that even the microscope may not, suffice in revealing the constituent minerals. Portable SWIR (short-wave infrared) spectrometers, are routinely used to determine alteration, parageneses., The changes concern colour, texture, mineralogical and chemical composition including stable, isotope ratios in different combinations. The end, product is a function of the nature of both the, solutions (pH, Eh, T, pressure, dissolved matter), and of the affected rocks (mineralogy, permeability, porosity). Ion exchange is ubiquitous, implicating an open system with import and export of, matter (Anthony & Titley 1994). Dissociated, water plays an eminent role, because of reactions, with silicate minerals that include incorporation, of OH groups and exchange of cations with Hþ, (hydrolysis). Frequently, altered rocks display a, zebra-like pattern of rhythmic banding, which is
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52, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.29 (Plate 1.29) The essence of, hydrothermal alteration visible at outcropscale, depicted by the halo centred on a small, fissure branching off from formerly exploited, wolframite-quartz veins in Panafrican granite at, Gash Emir, Red Sea Hills, Sudan. Note the, enhancement by later supergene oxidation., , caused by coupling of diffusion with precipitation, reactions (Kapral & Showater 1995). Hydrothermal alterations also affect mass and volume of the, altered body, with the possibility of a positive or, negative balance. Calculation of the balance relies, on assumed immobility of specific elements (e.g., zirconium, titanium). By comparison of concentrations in unaltered rock and in its altered equivalent, enrichment or dilution can be calculated, (Herrmann et al. 2009)., In many cases, hydrothermal solutions are, weakly acidic because of fluorine or CO2 contents, and dissociation of water. In that case, carbonates,, zeolites, feldspathoids and calcium-rich plagioclase are especially prone to alteration. Pyroxene,, , amphibole and biotite are somewhat more stable,, whereas albite, K-feldspar and muscovite are relatively resistant. Quartz is rarely affected., Alteration of pervaded rocks before the solutions reach the site of ore formation may extract, trace metals that are then concentrated in the ore., It is quite possible that many ore deposits are a, product of this process (e.g. cassiterite ore produced from aqueous solutions that have passed, through tin granite bodies: Lehmann 1990). Examples of trace elements that are incorporated in, minerals include copper and zinc in biotite,, amphibole, pyroxene and magnetite, lead in, K-feldspar and accessory uranium minerals, tin in, mica and ilmenite, tungsten in biotite, fluorine in
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , amphibole and mica, and barium in mica and, feldspar. Based on appropriate data, genetic ore, formation models incorporate such relations., Not only ascending hypogene hydrothermal solutions may cause host rock alteration. Some alterations are the product of downward-percolating, waters. Examples include solutions formed from, magmatic-derived HCl, HF, H2S and SO2 steam, condensating into, and heating shallow groundwater. Resulting strong acids cause acid-sulphate, alteration, characterized by alunite (eq. 1.5), typically in near-surface epithermal gold ore-forming, systems (Mutlu et al. 2005)., Alunite formation under extremely acidic hydrothermal conditions:, þ3, , KAlSi3 O8 þ 2Al þ2H2 SO4 þ4H2 O, K-feldspar, , ! KAl3 ðSO4 Þ2 ðOHÞ6 þ 3SiO2 þ6Hþ, , ð1:5Þ, , Alunite, , Hydrothermal host rock alterations include, the less visible change of increased maturation, of dispersed organic matter (kerogen) under, the influence of heat imported by the solutions, (Heroux et al. 1996). Along flow paths and, around orebodies, the induced thermal anomaly, can be mapped by appropriate methods, for example increased reflectance of organic particles, (cf. Chapter “The Coalification Process”)., Spatial zonation of alteration types, Spatial zonation of alteration types has an, outstanding role in ore deposit investigations., Alteration zoning illustrates changing chemistry, of the solutions by reactions with wall rocks, and, different physico-chemical boundary conditions, (temperature, pressure, etc.). Zoning is an, extremely useful guide and spatial vector pointing, to ore, as this is always connected with a certain, type of alteration. Zonation is best illustrated by, the classical alteration model of porphyry copper, ore deposits., There are many different hydrothermal alteration types (Thompson & Thompson 1997). Some, characterize certain ore deposits, while others are, linked to the presence of specific rocks. Propyli-, , 53, , tization, for example, typically affects andesite, and diorite, listwaenitization ultramafic rocks and, dolomitization limestone. The names given to, alteration types are mainly derived from the most, noticeable newly formed mineral, but some have, their own traditional rock names (e.g. greisen,, propylite). Important examples of hydrothermal, wall rock alteration include:, . Silicification is the permeation of host rocks, with dissolved silica, resulting in increased contents of opal, chalcedony or quartz. The rocks, often obtain the aspect of quartzites. Silicification, is very frequent around epithermal gold deposits., . Albitization (or sodic alteration) often affects, magmatic rocks by replacing more calcic with, sodic plagioclase, but albite ( chlorite, epidote,, etc.) may be introduced into most rock types. The, source of the sodium may be seawater, evaporative, brines, or dissolution of salt bodies and transport, by basinal or metamorphic brines. Also, sodium is, enriched in liquids and fluids segregating at high, temperatures from crystallizing magmas., . Argillic alteration is the conversion of rockforming silicates into clay minerals. Two variants, are distinguished, advanced and intermediate argillization. In the first case, the core of the altered, rock body typically consists of alunite and quartz,, and is surrounded by argillic zone minerals nacrite, dickite, kaolinite and pyrophyllite with variable contents of sericite, pyrite, tourmaline and, topaz. The agents are strong acids, which impose a, nearly total loss of alkali elements and aluminium, enrichment. Advanced argillization is often found, with gold or tin mineralization. Intermediate argillization is marked by conversion of feldspars to, kaolinite (proximal to main flow channels) or, montmorillonite (in a distal position). Extending, into the distance, a propylitization halo is often, developed., . Propylitization may affect large rock masses in, the wider vicinity of ore deposits, by imparting a, greenish tinge. It is a complex alteration with, neoformation of chlorite, epidote, albite and carbonates (calcite, dolomite, ankerite), which was, first observed in diorite and andesite near gold, deposits. If one of those minerals prevails, terms, such chloritization can be applied and mapped as, subzones.
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54, , PART I METALLIFEROUS ORE DEPOSITS, , Sericitization is a common alteration of aluminium-rich felsic rocks (granite, gneiss, schists). The, typical paragenesis comprises sericite and quartz, ( pyrite). The involved solutions were acidic and, leached part of alkali element contents (Ca, Na, and part of K; eq. 1.6)., ., , Sericitization of K-feldspar:, 3KAlSi3 O8 þ2Hþ ), , K-feldspar, , KAl2 ðAlSi3 O10 ÞðOHÞ2 þ2Kþ þ6SiO2, , ð1:6Þ, , Sericite, , . Greisenization: Acidic, fluorine-rich solutions, reacting with felsic rocks convert feldspar to, topaz. Medieval tin miners in the German Erzgebirge called the resulting grey rock greisen. Greisen is a massive or vuggy rock that consists of, quartz, topaz and light mica (muscovite or lepidolite). Accessory tourmaline, fluorite, rutile, cassiterite and wolframite are common. Tin and, tungsten enriched greisen bodies can be profitable, ore (Halter et al. 1996)., . Dolomitization is the prevalent alteration of, limestone adjacent to lead-zinc ore deposits. It is, due to import and exchange of magnesium for part, of calcium (e.g. Navan, Ireland: Braithwaite &, Rizzi 1997)., . Silicate rocks react with CO2-rich solutions, by carbonatization. Calcite, dolomite and, ankerite (more rarely siderite and magnesite) are, common alteration products adjacent to gold, quartz veins in Archaean and Palaeoproterozoic, greenstones (Eilu et al. 2001). In practice, the distinction of different carbonate minerals is easiest, with colouring techniques (Hitzman 1999). Listwaenitization describes carbonated and silicified, ultramafic rocks that may be associated with, hydrothermal epigenetic gold mineralization. The, altered rocks consist of magnesium-calcium-iron, carbonates, chromian muscovite and quartz, with, various accessory minerals (Halls and Zhao 1995)., . Tourmalinization, is, common, in, tin, (Lehmann et al. 2000a), tungsten and gold deposits. If it is very intensive, tough black rocks, (“tourmalinites”) result that resist both surficial, alteration and the geologist’s hammer. Tourmalinites mark the proximity of orebodies. In more, , distal position, pervasive growth of dispersed tiny, needles of tourmaline in country rock is only, revealed by the microscope., The study of hydrothermally altered wall rocks, is an important subject of economic geology., These rocks reveal the nature of the hydrothermal, solutions, they assist in the search for ore because, they present a much larger and hence more visible, target compared to the actual orebodies, and they, allow dating of ore formation because of neogenesis of minerals such as K-feldspar, alunite and, mica. Zonation mapping provides a vector pointing to the site of possible ore. In addition, hydrothermal alteration is always accompanied by, the dispersion of trace metals around orebodies., The resulting trace metal halos are excellent lithogeochemical prospecting guides. And last not, least, several alteration products are valuable, industrial minerals (e.g. alunite, kaolin, sericite, and vermiculite)., 1.1.7 Skarn- and contact-metasomatic, ore deposits, Many ore deposits are formed close to intrusive, igneous rock bodies. The location of the ore, may be at the immediate contact between the, intrusion and its host rocks, or at a certain distance., In the first case, the host rocks will be affected by, contact metamorphism due to heating (e.g. the, formation of andalusite in slates and schists). If, carbonate rocks are present, skarn (a Ca-Mg silicate, rock) is frequently formed by decarbonation and, addition of silica. This process releases large quantities of CO2 that may pass into the magma inducing profound changes, for example of fO2. Massive, orebodies may occur in proximity to the skarn, (proximal contact-metasomatic ore). The ore replaces carbonate rocks (or skarn) by a process called, metasomatism. The replacement is the result, of the passage of hot aqueous fluids that are, given off by the cooling magmatic body or by, dehydrating country rocks. If the metasomatic, ore formation takes place at a distance from the, intrusion, the ore will less likely be associated, with skarn rock. Resulting mineralizations are, either distal skarn, or distal contact-metasomatic, orebodies.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Skarn is an old Swedish mining term for a tough, calc-silicate gangue that is associated with certain, Palaeoproterozoic iron and sulphide ores. Swedish, skarn consists of coarsely crystalline andradite,, diopside, various amphiboles, scapolite, quartz, and carbonate. Skarn bodies are stratabound and, pass laterally into fine-grained banded exhalites, and iron formations that are hosted in a suite of, felsic and mafic volcanic rocks, carbonates and, clastic metasediments. Compaction structures, around skarn garnets indicate very early formation, of these skarns already during diagenesis of the, rocks (Oen & Lustenhouwer 1992). This is overprinted by later metamorphic reactions. Similar, diagenetic skarn rocks occur occasionally with, submarine sediment-hosted sulphide ore and, represent exhalative precipitates of Ca, Fe, Al, and SiO2 in a reducing, carbonate-poor environment. High heat-flow that induced syndiagenetic, skarn mineral formation during ongoing sedimentation was probably caused by synsedimentary, intrusion of subvolcanic magma (e.g. mafic sills)., Calc-silicate skarn rocks are also common in, high-grade metasedimentary rocks, for example, at the contact between carbonate and silicate, strata. This “reaction skarn” expresses local, exchange of matter during essentially isochemical, metamorphism., “Skarn” in the context of economic geology, “Skarn” (in North America also called tactite), commonly describes iron-rich rock bodies of, Ca-Mg silicates formed from limestone or dolostone by abstraction of CO2 and hydrothermal, addition of SiO2, Al, Fe and Mg in the contact, aureole of intrusions. Most skarn ore deposits are, related to intermediate igneous rocks, but association with granites and gabbros is known (Lentz, 1998). Often, fertile plutons show evidence of, magma mixing that enhances the generation of, hydrothermal fluids (Grammatikopoulos & Clark, 2006). Emplacement of a hot magma body in cool, country rocks causes the build-up of a thermal halo, with outward migrating isotherms, driving off, water and other volatiles. During this prograde, phase, mainly anhydrous minerals are formed that, include grossular-andradite, diopside, forsterite, , 55, , and periclase (MgO, if dolomite was present), and, part of the ore. Outward from the intrusion,, skarn is followed by a narrow zone of wollastonite, and a shell of isochemical recrystallization of, the precursor carbonate rock to carbonate marble., The export of matter from the cooling magma into, the country rocks is due to hot (maximum >700 C), hypersaline melt, saline fluids and gas (Baker et al., 2004). The fluid flow in aureole rocks may be, quite complex in T-XCOfluid, -t space. Commonly,, 2, initial heating will produce high XCO2 followed by, subsequent passage of magmatic and even meteoric water (Nabelek 2007). Magmatic waters, i.e., hydrothermal fluids continue to exsolve from the, intrusion during further cooling and declining pressure. The aqueous fluids represent a retrograde, phase, which is characterized by formation of brucite (Mg(OH)2) after periclase and water-rich silicates (amphibole, epidote, clinochlore, talc,, chlorite) replacing earlier anhydrous ones, concurrently with the main mass of the ore. Finally, a, system of convective cooling by external, nonmagmatic water may be established that imprints, low-T hydrothermal alterations., Limestone and dolostone skarn ores are common. Features that resemble skarn in some aspects are also found at intrusive contacts with, intermediate, mafic and ultramafic host rocks,, because of their relatively high Ca-Mg contents., Iron formations are generally prone to conversion, into skarn as in the Swedish prototype. Archaean, iron formations of the Southern Cross greenstone, belt in Western Australia host 20 mines and, numerous smaller gold skarn deposits, with a total, production of >260 t gold (1887–2001). The parent, intrusion is a peraluminous two-mica granite, (M€, uller et al. 2004). Most skarn bodies occur in, host rocks adjacent to an intrusion (exoskarn) but, occasionally marginal parts of the intrusions are, transformed into skarn because of inward mass, transport (endoskarn)., Skarn ore deposits can be related to specific, magmas and geodynamic positions: Diorites of, primitive island arcs produce mainly magnetite, orebodies that may have recoverable contents of, Cu, Co and Au. In North America, intermediate, to felsic, differentiated within-plate intrusions, of active continental margins generated some of
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56, , PART I METALLIFEROUS ORE DEPOSITS, , SE, , NW, Calcite marble, , Opencast, , Upper Schists, Dolomite, marble, , Lower, , Schists, (hornfels), , East Zone, Calcsilicate hornfels, , Biotite granite, , 500 m, , the most important tungsten ore deposits, Figure 1.30). At CanTung mine, two tungsten orebodies occur in contact metamorphic Cambrian, carbonate sediments above a granite stock: In the, opencut, the ore consists of scheelite, garnet, diopside and epidote, whereas ore in the underground, East Zone is characterized by pyrrhotite and tremolite. In British Columbia, intrusions of Jurassic, island arc terranes are related to polymetallic skarn, ore of Cu, Fe, Zn-Pb-Ag, Mo, Bi and Au (Ray et al., 1995). Mesozoic, post-orogenic and rift-related, ilmenite series granites produced skarn deposits, of tin with As, Pb-Zn, W, Mo and some Fe-sulphides (China). These consistent relations are a, weighty argument for a predominantly magmatic-hydrothermal derivation of the ore elements, which is confirmed by geochemical data, on fluid inclusions (Samson et al. 2008)., Skarn orebodies display characteristically irregular outlines that can be explained by the two main, factors, lithology and structures of the replaced, host rocks, which impose chemical and physical, controls on permeability and reactivity. Orebodies, are often zoned, for example with copper in a, proximal and lead-zinc in a more distal position., Distal contact-metasomatic ore deposits are, closely related to skarn ore formation, although, skarn rocks may be absent. Ore formation by, replacement of carbonate rocks is solely a hydrothermal process. In the Tertiary Pb-Zn-Ag province, in Mexico, for example, the connection between, proximal skarn to distal stratiform ore (“mantos”), is provided by pipe and chimney-shaped orebodies., Skarn orebodies are a major source of many, metals but also of industrial minerals including, , Figure 1.30 Geological section, of the scheelite skarn deposit, CanTung, North West, Territories, Canada (modified, from Middelaar & Keith 1990)., , wollastonite, graphite, asbestos, magnesite, talc,, boron and fluorite., 1.1.8 Porphyry copper (Mo-Au-Sn-W) deposits, Porphyry ore deposits are a product of magmatic-hydrothermal activity at shallow crustal, levels. Primarily copper, but also molybdenum,, tin, tungsten and gold occur closely related to, epizonal intrusions of porphyric magmatic, rocks (Figure 1.26). Porphyries contain phenocrysts of hornblende, biotite, feldspar or quartz, in aplitic groundmass, which solidifies by pressure decay, volatile loss and sudden freezing, during rapid ascent. This is the only connotation of the term porphyry ore deposits, not the, arrangement or distribution of ore minerals., Because of the large diversity of the group,, porphyry deposits of Mo, Sn and W are presented, in Chapter 2, whereas the following discussion, centres on Cu-Au-Mo porphyry ore deposits,, which presently supply 75% of the world’s, Cu, 50% of Mo, nearly all of Re and 20% of, gold. The most significant characteristics of, copper porphyries include:, . plug-like, multiple, porphyric, intrusions, below comagmatic volcanoes, formed before, mineralization;, . an extraordinary tonnage of magmatic-hydrothermal ore;, . the ore occurs mainly in stockwork vein systems within the intrusion;, . metal contents in ore are low to moderate, and, supergene enrichment is often the key to, exploitability;
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , BOX 1.3, , Porphyry copper deposits, , The average porphyry copper deposit comprises 1000 million tonnes (Mt) of ore. Common hypogene ore grades are, 0.5–1.5% Cu, 0.01–0.04% Mo and <1.5 g/t Au (Sillitoe 2010). Giant deposits enclose >2 Mt, supergiants such as, Chuquicamata (Figure 1.31), >24 Mt of copper metal. The largest is El Teniente in central Chile, with >94 Mt of contained, copper (Cannell et al. 2005). Large copper porphyry open cut mines expose orebodies that extend over several km2., Alteration halos surrounding ore can be 10 to 20 times this size. Metal contents are generally low but the large mass allows, profitable mining. Apart from copper, by-product metals include significant tonnages of Ag, Au, Mo, Re, Pb, Zn, Mn and, minor amounts of As, Bi, Sn, W, U and Pt. Clusters of giant deposits that far exceed average copper porphyry size occur in, central and northern Chile, and southwestern Arizona to northern Mexico (Cooke et al. 2005)., , Figure 1.31 (Plate 1.31) Chuquicamata open pit in Chile, one of the world’s largest porphyry copper mines., Courtesy Bernd Lehmann, Clausthal.The pit measures 2 3 km and approaches a depth of 900 m. Total premining resources were nearly 3000 Mt at 1% Cu and by-product Mo. Annual production is 1.2 Mt copper and, 20,000 t molybdenum plus rhenium., Porphyry copper systems develop above large parental magma chambers at 5 to 15 km palaeodepth, which establish, the heat and flux regime of both the porphyry intrusions and the mineralizing fluids. In a non-telescoped system, the, potassic hydraulic fracturing-mineralization-alteration is surrounded by propylitization due to moderate-temperature, hydration reactions. Above the porphyry level, advanced argillic (quartz-pyrophyllite-kaolinite-alunite) and vuggy quartz, lithocaps may be formed by extreme base leaching. The vuggy quartz level is often the site of epithermal high sulphidation, gold mineralization (Sillitoe 2010). Intermediate sulphidation mineralization with higher contents of Pb, Ag, Zn and Mn, may be formed at a greater distance from the porphyry centre., Porphyry copper ore deposits display a hydrothermal alteration zoning that is best characterized by the Lowell-Guilbert, (1970) model (Figure 1.32a). Note that this sketch is time-integrated and displayed features originate at different times., Typically, sericitization and the main ore precipitation overprint earlier potassic (mafic minerals replaced by secondary, biotite) and propylitic alteration. In practice, there is much variance because of complexities such as repeated intrusive, activity and previous alteration of affected rocks (Sillitoe 2010). Hydrothermal systems that produce porphyry ores at, depth can extend to the surface where shallow veins and hot springs ore deposits may be formed (Heinrich et al. 2004,, Heinrich 2005). Skarn ore may be generated at contacts of the porphyric intrusion with carbonate rocks., , 57
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 59, , fluid inclusions. In most porphyries, main-stage potassic alteration mineralization is connected with co-existing, immiscible brine and vapour. Brine is dense (>1.3 g/cm3) and saline (35 to >70 wt.% NaCl equivalent), with variable, contents of K, Na, Ca, Fe, Mo and Cu chlorides (Klemm et al.875, 874; Rusk et al. 2008). The same elements are found in, the multitude of daughter minerals in fluid inclusions (halite, sylvite, anhydrite, chalcopyrite, haematite, Fe-chloride, etc.), of minerals precipitated at this stage. Early fluids have metal ratios that correlate with those calculated for the whole, deposit, which is a further argument for a magmatic derivation of the metals (Ulrich et al. 1999). Another confirmation is, the observation that melt and fluid inclusions co-exist in early hydrothermal quartz (Harris et al. 2003). Cogenetic, supercritical liquid brine and low-density vapour phase inclusions (D <0.1 g/cm3) document boiling and unmixing of the, fluids. Vapour collects acidic volatile species (SO2, H2S, CO2, HCl, HF) and most Cu and Au, plus much of As, Ag, Sb, Te, and B. Vapour flow dominates transport and precipitation. Cooling and contraction of magmatic vapour to a, liquid appears to dominate copper mineralization (Klemm et al. 2007). Reduction can be induced by magnetite, crystallization that triggers sulphate to sulphide conversion. Consequent sulphide precipitation promotes ore formation, (Liang et al. 2009)., , . extensive hydrothermal alteration and metals, are vertically and cylindrically zoned in relation, to the axis of the intrusion., Host rocks of copper (Au-Mo) porphyry deposits, are shallow (<4000 m), subvolcanic and mostly, cylindric intrusions. The parent rocks are frequently calc-alkaline diorites (or the volcanic, equivalent andesite-dacite), monzonites (latite) or, granites (rhyolite) of I-type that occur in volcanoplutonic arcs above subduction zones, either on, active continental margins or in island arcs. Subduction of topographic and thermal anomalies, appears to favour copper porphyry genesis (Cooke, 2005). Less frequent are Cu-Au porphyry and, related epithermal gold deposits in continental, collision zones and post-subduction settings (Richards 2009). The parent intrusives are geochemically not peculiar, apart from high oxidation, (Mungall 2002). Their hydrous nature, elevated, oxidation and SO2 contents (including anhydrite, as a magmatic phase) are the main differentiating, characteristics in comparison to the numerous, barren intrusions of convergent plate margins., Porphyries on active continental margins are, marked by elevated Sn and Mo concentrations, apart from copper, those of island arcs more often, contain gold as the second metal. Porphyry deposits older than Early Tertiary are rare (Cooke et al., 2005) but the oldest date from the Archaean., Several porphyry copper ore deposits display a, strong epithermal signature (cf. “Volcanogenic, Ore Deposits”) and overprinting of earlier hydrothermal alteration, for example clays replacing, potassic zone minerals, as at the gold deposit, , Ladolam on Lihir Island, Papua New Guinea. Such, extreme telescoping is best explained by rapid, erosion or a sudden collapse of the original volcano, (Sillitoe 1994). Moderate telescoping is common,, however, because of contraction of the hydrothermal system with progressive cooling., Sources of the metals concentrated in porphyry, copper ore deposits are probably deeper mafic, magmas (Hattori & Keith 2001) but ultimately a, fertile mantle. Melts and supercritical fluids that, originate in the subducting oceanic crust are oxidizing and dissolve chalcophile metals. Absence of, reduced sulphur in the source region is a precondition because otherwise sulphide melts would, form, which must lag behind rising silicate liquid, (Mungall 2002). In epizonal staging chambers,, however, an intermediate step of sulphide melt, formation may intervene (Halter et al. 2002, 2005)., Transfer of the metals from mafic melt into the, subvolcanic felsic magma may be enacted by, highly metal-charged ore brines (Figure 1.34). The, shallow felsic magma is the apparent source of ore, fluids that cause brecciation, alteration and mineralization in the solid roof, concomitant with the, dynamics of a rising stratovolcano. The activity of, individual porphyry systems may last for, 100,000 to several million years; districts are, active for 10–20 My (Sillitoe 2010)., 1.1.9 Hydrothermal-metasomatic ore deposits, The term “metasomatic ore deposit” implies that, the mass of ore was formed by hydrothermal, chemical conversion and replacement of a pre-
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60, , PART I METALLIFEROUS ORE DEPOSITS, , Meteoric, water, Solidified cupola, of intrusion, , Ore formation, Unmixed fluid, with high Cu-Cl, , Felsic magma, , SO2 - CO2 - H2 O, Cl, Cu, , Primitive, mafic, melt, , existing rock. Local metasomatic processes, for, example in ore veins, are ubiquitous co-products, of hydrothermal activities, but do not justify attribution to this genetic group. Metasomatic ore, formation near igneous intrusions is extremely, frequent (cf. Skarn- and Contact-Metasomatic Ore, Deposits). In order to avoid repetition, we focus, here the discussion on metasomatic ore deposits, that have no recognizable genetic relation to magmas. Non-magmatic hydrothermal metasomatic, ore formation is usually due to the passage of, diagenetic and metamorphic fluids, and especially, of evaporative and salt-solution brines (cf., Sections 1.4 and 1.6)., Typically, the metasomatized rocks are marine, limestones. This preference can be demonstrated, , Figure 1.34 Genetic concept of deep magmatic, processes preparing the formation of a porphyry, copper deposit (modified from Hattori & Keith, 2001). With kind permission from Springer, ScienceþBusiness Media., , with numerous examples (e.g. many lead-zinc, orebodies, gold as at Carlin, USA, magnesite and, siderite: Pohl 1988). Main controls of the replacement process include the reactive surface and, permeability of the precursor rock, pH and Eh of, the mineralizing solutions, and the relative solubility of the participating minerals. A simplified, equation 1.7 describes the metasomatic formation, of siderite rock (an iron ore) from limestone., Metasomatic formation of siderite from limestone:, CaCO3 þFeCl2 ðaqÞ ) FeCO3 þCaCl2 ðaqÞ ð1:7Þ, In this case, cation exchange is the dominant, mechanism, replacing each molecule of calcite
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 61, , Figure 1.35 The metasomatic siderite deposit Erzberg (“ore mountain” in German) in Austria. The active quarry, faces away from the camera. The view shows disposal of barren carbonate host rock., , with one of siderite. Because of the smaller molecular volume of siderite, the resulting ore rock, should have a reduced volume. This can be, observed in the form of drusy ore at a number of, siderite deposits (Erzberg, Austria, Figure 1.35;, Ouenza, Algeria), but in many other cases metasomatism exchanges equal volumes. Indicators for, this process include the conservation of intricate, textures and structures of the replaced rocks (e.g., layering, banding, fossils, stylolites, etc.). Generally, however, metasomatism induces a dramatic, change in both chemistry and fabric of the precursor. Some metasomatic ores appear to replace, newly deposited soft sediment (e.g. in some magnesite deposits: Pohl & Siegl 1986), but well consolidated precursor rocks are more common., Hydrothermal-metasomatic ore deposits are, often stratabound and occur in the same stratigraphic level across large regions. For example,, siderite deposits in North Africa and in northern, Spain occur in Early Cretaceous micritic limestone, and lead-zinc ores of the Viburnum MVT, district of mid-western USA are confined to stromatolite reefs of the Late Cambrian Bonneterre, , Formation. Zinc-rich MVT ores in the Tri-State, district occur in Mississippian carbonates. In the, past, observations like this were believed to prove, synsedimentary mineralization. Today, a hydrothermal and epigenetic emplacement is undisputed, but the causes of the preference for certain, strata are not always clear. Possibilities include, chemical, hydraulic and mechanical parameters., The emplacement of metasomatic ore is favoured by low-permeability rock horizons (e.g., shales) that form a physical barrier to upward flow, (similar to petroleum traps). Focused hydrothermal solutions react more intensively with the, carbonate host. Model calculations show that, metasomatism is promoted by: i) large temperature differences in flow direction; ii) large focused, quantities of fluids; and iii) anomalous phase equilibria (Ferry & Dipple 1991)., The form of many metasomatic deposits is, characterized by irregular reaction fronts, (“reaction fingering”) between ore and host, rocks. Higher permeability tongues may be caused, by the commonly smaller volume of the product, (“induced, permeability”)., Orebodies, are
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62, S, , PART I METALLIFEROUS ORE DEPOSITS, , Alquife Hill, , N, , Quarry, , 1000 m, , Pliocene-Quaternary detrital material, , Micaschists (Palaeozoic), , Calcite and dolomite marbles (Triassic?), , Graphite-rich micaschists (Palaeozoic), , Micaschists (Permo-Triassic?), , Siderite marble, , Meta-evaporites (Permian?), , Secondary iron oxides after siderite, , stratabound and cloudy masses with irregular outlines, but some borders mimic structures of the, precursor rock (joints, faults, bedding planes)., Other ores may take the shape of extensive stratiform bodies (mantos). Both are demonstrated at, Alquife in southern Spain (Figure 1.36). In this, region, metasomatic siderite was formed, during the Triassic by acidic and reducing solutions that leached iron from graphite-rich micaschists in the Palaeozoic basement and ascended, along synsedimentary extensional faults into Permo-Triassic cover carbonates. Host rocks and ore, were later deformed and metamorphosed by a, polyphase Alpine orogeny. In the Tertiary, most, of the siderite bodies were affected by supergene, alteration and now consist of goethite and, haematite., The flow direction of hydrothermal solutions, generates two boundary types of very different, nature:, 1 In the flow direction, various reactions of mineral neogenesis and isotope exchange migrate with, different speed in the form of a chromatographic, model (Korzhinskii 1970); assuming kinetically, fast reactions, several spatially separate reaction, fronts should be the result., 2 Boundaries that laterally limit flow channels, (e.g. massive impermeable limestone constricting, flow in jointed limestone) can display direct contacts between totally unaltered host rock and, high-grade metasomatites (Yardley & Lloyd 1995)., Metasomatism can be accompanied or closely, followed in time by mineralization with the character of simple open space filling (veins, fissures,, druses, karst cavities). In the North African siderite deposits, this role is played by quartz-barite-, , Figure 1.36 Geological crosssection of the Alquife, metasomatic iron ore district,, Betic Cordillera, southern, Spain. Modified from TorresRuiz, J. 2006, Society of, Economic Geologists, Inc.,, Economic Geology Vol. 101,, Figure 3, p. 670. For location, refer to Figure/Plate 1.89., , fluorite veins, in Bleiberg, Austria by cave-filling, ore. Cave ores may include bedded sediments and, are a rare, fascinating aspect of hydrothermal systems (cf. “Karst”)., 1.1.10 Hydrothermal vein deposits, For a long time in the past, ore veins were the most, important deposit type. Practice and theory of, mining and geosciences grew with the challenges, of vein mining, as shown by fundamental books, from Agricola (1556) to Lindgren (1933). More, recently, the economic relevance of vein mining, decreased compared to large-tonnage low-grade, operations such as those based on copper porphyries. However, several high-grade base metal vein, deposits successfully compete with the mechanized giants., Veins are tabular bodies of hydrothermal precipitates that typically occupy fissures. Less often,, veins originate by metasomatic replacement of, rock (replacement veins), propagating from a joint, or shear plane. Vein walls range from clean parting, planes to “frozen” contacts. Many veins develop, upwards into a fan of thinner veins and veinlets, (Figure 1.15), which resemble a branching tree. At, district scale, veins tend to occur in groups that, form vein systems., Thickness, vertical extent and horizontal length, of veins vary widely. Less than 0.5 m thickness, may allow profitable mining of high-grade gold, and silver ore veins, whereas tin and tungsten, require a width of 1 m, barite and fluorite a minimum of 2 m. The world’s longest veins may be, those of the Mother Lode system of California,, with 120 km strike length. Most veins, however,
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Metapelites, , 63, , Quartzite, , N, , Cassiterite, quartz veins, (steeply dipping), , Figure 1.37 Geological map of one level in, Rutongo tin mine, Rwanda, showing, closely spaced parallel quartz-cassiterite, veins (for location, compare Figure 1.16)., , have lengths between a few tens to several thousand metres. The disposition of veins in space, ranges from horizontal to vertical, but because, hydrothermal solutions have a general tendency, to flow upwards (more precisely towards lower, hydraulic potential) steeply dipping veins are in, the majority., Mechanical properties of host rocks are the most, important controls of vein formation, in contrast, to metasomatic ore deposits that depend first on, chemical properties. Fractures form more readily, in competent rocks than in ductile material., Therefore, the cassiterite–quartz (muscovite, arsenopyrite, tourmaline) veins at Rutongo, Rwanda, occur preferentially in vitreous quartzites and few, cut across low-grade metamorphic schists, (Figure 1.37). Very brittle rocks such as dolomite,, rhyolite and quartzite are prone to form a network, , 50 m, , of short fractures instead of spatially separated, longer ones (Figure 1.38). In that case, hydrothermal activity may result in stockwork ore., “Stockwork orebodies” consist of numerous short, veins of three-dimensional orientation, which are, so closely spaced (e.g. 10–30 veins/m in the tin, deposit of Tongkeng-Changpo, South China) that, the whole rock mass can be mined., Many vein deposits are spatially and genetically, associated with brecciated rock bodies that may, host rich ore (Jebrak 1997). As a rule, fissures and, breccias have a much higher permeability than, most consolidated rocks, which are commonly, aquitards. Therefore, these structures focus, fluid flow at all scales, from single veins to large, structures such as rift faults and crustal shear, zones (Weinberg et al. 2004). Many fissures,, for example the tin ore veins of Cornwall,
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64, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.38 Breccia ore at Mammoth copper mine near Mt Isa, Queensland. Supergene secondary chalcocite (grey near, hammer) replaces primary chalcopyrite and pyrite in veinlets within quartzite (white)., , England, are surrounded by broad zones of, intensive micro-fracturing that provide access to, fluids from the main flow channel (Dominy et al., 1996). It is well-known in the oil industry that in, contrast to common permeable faults, clay and, shale gouge can transform faults into very effective barriers to fluid flow (Egholm et al. 2008)., Unconsolidated rocks (clay, sand) are unlikely to, fracture but deform ductilely. Typically, this reduces their permeability so that such faults, restrict flow., Veins are channels of former fluid flow. Principally, hydraulic mass flow is a function of hydraulic conductivity and velocity. Flow in a single, fissure is controlled by its aperture and secondary, properties such as morphology and roughness of, the walls. Note that the hydraulic aperture of, a vein was hardly ever equal to its present thickness, because most veins were filled while gradually opening. For cases of sheeted vein systems, (Figure 1.37), the total cross-sectional permeability can be estimated by incorporating the distance, between veins (Lee & Farmer 1993; eq. 1.8). Only, in favourable cases, the flow velocity can be, measured, for example when upflowing water, deposited suspended sediment or hydrothermal, , precipitates (cf. eq. 1.4 in section “Mineral, Succession“)., Hydraulic permeability (k) of a single vein and of, a sheeted vein system:, Single fissure, Sheeted fissures, , kvein ¼ ðr g=mÞ ða2 =12Þ, ksum ¼ ðr g=mÞ ða3 =12dÞ, ð1:8Þ, , r ¼ density of the fluid (g/cm3), g ¼ gravitational acceleration (m/s2), m ¼ dynamic viscosity (m2/s), a ¼ aperture (m), and d ¼ distance between veins (m)., , Methods of fractal analysis (Mandelbrot 1982), were conceived for studying structures built of, many elements. Examples of vein-related topics, include the statistical distribution of vein thickness and the spacing of veins, and extrapolation, of results into adjacent blocks or to different, scales (Roberts et al. 1998, Marrett et al. 1999). In, some cases, the fractal distribution of tectonic, features and of mineralization, that is their similarity from microscopic to regional scale, can be, used for the definition of new exploration targets, (Weinberg et al. 2004).
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65, , GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , & Mikleswaithe 2007): Main faults that experience a high displacement seismic event enter a, healing regime that rapidly reduces permeability., The rock mass at some distance from the fault,, however, undergoes a period of weakening that, may result in seismic aftershocks. Because, this causes elevated permeability for weeks or, months after an earthquake, the time-integrated, fluid flow will be larger than in the main fault,, favouring the formation of ore., , Rock mechanic and tectonic interpretation of, vein systems play a central role in vein mining,, from creating exploration targets to predicting, the shifted position of a vein behind a fault. Mining districts display individual controls that, must be identified by careful mapping and structural analysis at all scales. Neogenesis and opening of fissures, shear planes and faults reflect, spatial orientation and the ratios of principle tectonic stresses during vein formation (Figure 1.39)., The opening of fissures is only possible if, normal stress is negative (simple tension) or if, fluid pressure (u) is high enough to counteract, normal stress (effective stress seff ¼ s u). This, condition is often realized by injection of highpressured hydrothermal fluids. Many veins display pronounced banding (Figure 1.28) that indicates a correlation between pressure increase of, the fluids, movement of the fissure walls and, precipitation of hydrothermal fill. This is called, “seismic pumping” or “fault valve cycling”, (Sibson 1990)., Very large faults and shear zones are rarely the, site of hydrothermal mineralization. Orebodies, occur rather in clusters of fractured rock near, jogs or bends in the large structures. This can be, explained by the time-integrated evolution of, rock mass permeability after faulting (Sheldon, , The tectonic control of ore veins may be neogenesis of new fractures or opening of older structures., Both allow an examination of the tectonic processes that were active during vein formation. Not, unexpectedly, many veins are associated with, large-scale tensional tectonics including rifting, (e.g. silver-lead-barite ore veins near the Tertiary, Upper Rhine Rift, Figure 1.28; silver veins at, Kongsberg near the Permian Oslo Graben; fluorite, in the Tertiary East African Rift near Naivasha,, Kenya) and late-orogenic relaxation of orogens (e.g., silver veins near Freiberg, Saxony, lead-zinc veins, in the Harz Mountains, Germany). However,, veins may also originate during convergent tectonics, synchronous with shearing, folding,, , ing, , 4, , re, ptu, Ru, , ord, cc, it a, m, F1, li, , σ1, , 3, , σ3, , ctu, , σ, , Fra, , τ=f(σ), F3, -T, , α, , re, , F2, , τ, , ne, , 5, , r, oh, -M tan φ, b, m, +σ, ulo, Co τ = c, to, , pla, , 2, Shear stress (kN/m ), , Figure 1.39 The formation of shear, fractures (F1), shear fissures (F2) and, tensile fissures (F3) with the, corresponding angle of fracturing (a), and, tectonic stresses s1 > s2 > s3 in the Mohr, diagram.The inset triangle on the right, depicts a section of the upper half of an, originally cylindrical sample specimen, with the stress geometry of fracture, case F1. Fracturing is often assisted by, high fluid pressure (u) which reduces, strength according to seffective ¼ s u., , Tectronic control of ore veins, , 0, , 1, , 2α, , 2α, , 140°, , 120°, , 2, , 3, , 4, , Normal stress, , (kN/m2)
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66, , PART I METALLIFEROUS ORE DEPOSITS, , Gold-Sb-As ore, , Migrating sulfide, melt, Figure 1.40 Schematic concept of mobilization, of disseminated ore melt from high-strain host, rock into dilatational sites in folds and boudin, necks (Hemlo gold mine, Canada). Modified from, Tomkins, A.G., Pattison, D.R.M. & Zaleski, E., 2004, Society of Economic Geologists, Inc.,, Economic Geology Vol. 99, Figure 3 p. 1066., , ~ 0.5 m, , thrusting and nappe advance (gold quartz veins in, Western Australia: Vearncombe 1998; tin and, tungsten veins in Central Africa: Figure 1.39;, the gold-rich “saddle reefs” in folded shales of, Victoria, Australia: Windh 1995). At transitional brittle/ductile conditions, boudinage, often controls ore deposition, from the regional, to exposure scale (Findlay 1998, Tomkins et al., 2004: Figure 1.40). At Hemlo, pre-enriched, auriferous sulphides and sulphosalts were partially remelted during deformation and metamorphism. The resulting sulphide liquid, migrated to low pressure spaces. Residual ore, in the host rocks (baritic metasediments and, metavolcanics) includes pyrite and molybdenite, (Au), whereas stibnite, realgar, sulphosalts and, gold characterize the solidified and recrystallized sulphide melt., Vein systems may consist of cross-cutting vein, sets that reveal changes in time, including both, the stress field and the nature of hydrothermal, solutions. At Gifurwe in Rwanda, an early vein, generation was injected into anticlinal fold, hinges and cut across by later subvertical veins,, but both systems have the same paragenesis of, wolframite, quartz, muscovite, tourmaline and, arsenopyrite (Figure 1.41 and Figure 1.42). Vein, formation is late orogenic compressional. Initially, high fluid pressure counteracted vertical, rock mass stress, followed by near-vertical fracturing and pressure release., , The distribution of ore in veins is inhomogeneous and only a small part of the total vein fill, may be exploitable. Related to either surface or, volume, the ratio of economic to uneconomic, , NE, , SW, , Wolframite, quartz veins, Dark banded, schists and slates, 50 cm, , Figure 1.41 Cross-cutting wolframite-quartz veins in, folded black shales of the tungsten mine at Gifurwe in, Rwanda.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 67, , Figure 1.42 Granite-related wolframite veins, at Bugarama in Rwanda display a gangue of, coarsely crystalline muscovite (white,, glittering), dark grey quartz and black shale, fragments, which fill steeply dipping beddingparallel deformation and flow channels between, thick competent quartzite beds., , parts of a vein is called the “coefficient of, workability” (frequent values are 0.2–0.3)., High-grade ore zones (“ore shoots”) display distribution patterns that are important guides to, exploration (Figure 1.43). During ore deposit, formation, ore shoots were preferential channels of fluid flow and ore precipitation. Intersections of veins are often enriched, as are vein, contacts with agents of metal precipitation in, host rocks mentioned earlier, such as sulphides, organic matter and carbonates. The, morphology of a vein surface is never a perfect, plane but includes depressions, furrows and, highs that are depicted in so-called “Conolly, diagrams” (a map of the vein surface similar to, , Figure 1.43). Replete with data on vein thickness, grade of ore, different host rocks and, intersecting structures, these diagrams are, extremely instructive., Metal zonation, Most veins display a change of ore and gangue, minerals with increasing depth until a barren, zone is reached. The vector of these changes, points generally downwards, but may have a, strong horizontal component. In the Harz Mountains, Germany, lead and silver predominate at, upper levels, whereas zinc appears below. In the, same area, barite veins end at depth in a barren
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68, , PART I METALLIFEROUS ORE DEPOSITS, , SE, , St, .J, N oha, eu n, er nis, Al Tu r, m, te, rT, R, os, ur, en, m, ho, Br, R, os, an, f, en, d, ho, -S, f, ch, D, ac, re, ht, iB, rü, de, r, , Al, te, rs, eg, en, , Si, lb, er, se, ge, n, , O, , tti, , lia, , e, , NW, , Medieval mine workings, , Pl, , un, , gi, , ng, , or, , e, , sh, , oo, , t, , Uneconomic mineralization, , 500 m, , quartz zone. Such a primary zonation is thought, to be due to pressure, temperature and chemical, gradients. The term secondary metal zonation,, in contrast, describes the re-arrangement of, elements imprinted by supergene weathering., Spatial zoning models can be used to define, hydrothermal centres and exploration targets., In the case of complicated and long-lived hydrothermal systems, metal zone reconstruction, should be based on specific activity periods, or parageneses of the same age. Thermal and, geochemical halos of hydrothermal systems, may wax and wane in time, which causes overlapping of zones (“telescoping”, Audetat et al., 2000)., Vein districts are characterized by recurring, parageneses of ore and gangue minerals. In fact,, many of these mineral and chemical associations occur worldwide. In the German Erzgebirge, with numerous cross-cutting vein, systems, this observation was of high practical, relevance to miners and a formal classification, of “vein formations” was developed (e.g. “the, Pb-Zn-Ag Formation” in the Freiberg District;, , Figure 1.43 Exploited ore, shoots (black) of the steeply, dipping Rosenh€, ofer vein, in, the lead-silver-zinc mining, district west of Clausthal,, Harz Mountains, Germany,, illustrate the distribution of, mineable and subeconomic, areas of the vein. With, permission from http://www, .schweizerbart.de., , cf. Stemprok & Seltmann 1994). For descriptive, and practical purposes, notations like this are, certainly useful. The word “formation”, however, is ambiguous., 1.1.11 Volcanogenic ore deposits, The formation of a large number of important ore, deposits is closely related to terrestrial and submarine volcanism. We have met two members of, this group in earlier chapters; the subvolcanic, porphyry ore deposits and the mineralization, related to the volcanic section of ophiolites, (Cu-Zn-Au ore of Cyprus type). Mid-oceanic deposits, however, are commonly subducted and, rarely preserved. Here, other economically significant classes of volcanogenic ore deposits, are introduced: i) the submarine volcanogenic, massive sulphide (VMS) deposits; and ii) the terrestrial epithermal gold-silver-base metal deposits. Banded iron formations of the Algoma, type are also a member of this group (cf., Section 1.3 “Autochthonous Iron and Manganese, Deposits”).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , BOX 1.4, , Polymetallic vein fields of Cornwall, , Already 3200–2900 BCE (before the Common Era), the tin and copper veins of Cornwall were the destination of, adventurous maritime expeditions by the Phoenicians, famous traders in the Mediterranean and beyond. The Cornubian, batholith is built of several S-type granite pulses that intruded after the peak of Variscan orogeny (Carboniferous/Permian;, from 300–270 Ma: Chesley et al. 1993). The youngest phases of single plutons are fractionated and geochemically, specialized, with abundant lithium, boron, caesium and uranium (Chappell & Hine 2006). All granites contain enough, uranium and thorium (an average of 11, respectively 19 ppm) to be classified as HHP granites. The relative age of, mineralization varies between different plutons. Some veins are synchronous with granites, others apparently up to 40, million years younger. Similar age spreads are displayed by felsic porphyry dykes that cut granites and country rocks, and, are controlled by tectonic structures that parallel ore veins., , Figure 1.44 Schematic section of primary (hypogene) zonation in polymetallic vein mineralization, Cornwall,, England., Hydrothermal ore veins of Cornwall are zoned about “emanation centres” located in granites, rarely in Palaeozoic, country rock schists (Scrivener & Shepherd 1998). Sixty emanation centres are known, with an elliptical shape and a long, axis of 4 km length. The mineralization is not monophase but the product of several activity periods. The oldest episode, comprises mineralized pegmatites, sheeted veins with greisen selvages and breccias, all with tin, tungsten and much, tourmaline. Highly saline aqueous fluid inclusions with tin, and cogenetic gaseous inclusions with CO2 and W Sn,, suggest an origin by unmixing from high-temperature magmatic fluids. The economically prominent main phase of the, Cornubian mineralization produced polymetallic (Sn, Cu, Pb, Zn, Ag, Fe, As) veins with a gangue of quartz, tourmaline,, chlorite and fluorite, which are very distinctly zoned (Figure 1.44). TH of inclusions is between 200 and 380 C. The, similarity in age of the main polymetallic mineralization indicates formation independent from host granites of varying, ages. Relations to younger unexposed pulses of magma are one suggested explanation (Chesley et al. 1993). The youngest, and last episode of mineralization is represented by low-temperature quartz-fluorite-barite veins with Pb, Zn, Ag and U, (the “cross-courses”). Cross-course forming fluids were saline (Na-Ca-Cl), similar to deep formation waters and, homogenize at 105–180 C. Until closure of the South Crofty Mine in 1998, the mine production of Cornwall reached, a total of 1.75 Mt tin, 1 Mt copper and 0.13 Mt zinc., Apart from metal ores, Cornubian granites host giant kaolin deposits that occur in the form of pipes, alteration envelopes, of veins and horizontal blankets. Many observations suggest kaolin formation by hot, acidic hydrothermal solutions, (Dominy & Camm 1998), but water isotopes indicate formation from meteoric water. In fact, many vein mineralizations in, Cornwall were precipitated from deeply-circulating meteoric water, with the exception of the early stages of the Permian, main phase of Cornubian mineralization, which originated from regional extension, deep heat-flow and crustal melting,, followed by fractionation and intrusion of the Cornubian batholith (Chappell & Hine 2006, Shail et al. 2003). Later, , 69
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70, , PART I METALLIFEROUS ORE DEPOSITS, , episodes of mineralization were made possible by radiogenic heat production of the granites. Plate tectonic events, such, as the opening of the Atlantic, may have favoured deep convection of meteoric and basinal fluids, which leached metals at, depth and precipitated ores in the present vein deposits., Clearly, hydrothermal metal zoning in veins centred on granites is not sufficient evidence for a magmatic source. The, example of Cornwall demonstrates coincidence of both the magmatic-hydrothermal and the post-solidus (retrogrademetamorphogenic) mineralization potential of high heat production (HHP) granites., , Important terms, Generally, ores that were formed by hydrothermal, solutions venting on the ocean floor are called, “submarine exhalative” or “submarine hydrothermal”. The second term is preferred by some, authors, because the word “exhale” sensu stricto, implies a gas, not an aqueous liquid. “Submarine, volcanic-exhalative deposits” are discerned from, “sedimentary-exhalative type” (commonly shortened to “sedex”). Whereas the first are clearly, localized in volcanic centres, the second occur in, sedimentation-dominated basins. In both cases,, the rocks consisting of ore and non-ore minerals, (gangue) that result from these processes are called, “exhalites” or “hydrothermal sediments”. The, term “massive ore” designates a body that consists, mainly of ore minerals such as base metal sulphides or iron oxides., Submarine volcanogenic massive sulphide, (VMS) deposits, The concept of ore formation caused by submarine volcanic activity, or more precisely, by, associated sub-seafloor hydrothermal activity, resulting in massive ore mounds on the seafloor,, is confirmed by many observations. Their origin, by exhalation (outpouring) of metalliferous, hydrothermal fluids on the seafloor was recognized by Oftedahl (1958). The concept was further developed by Stanton (1994 and earlier, papers), Ohmoto (1996) and Allen et al. (2001)., Recent exploration of the world’s oceans added, a wealth of observations from divergent and, convergent tectono-magmatic environments,, including submarine arc volcanoes (Stoffers, et al. 2006, Halbach et al. 2003). The VMS class, encompasses a wide variety of geodynamic and, more local genetic settings. Relations to volca-, , noes range from proximity to quite tenuous, connections with volcanism, as in parts of the, Southern Iberian Pyrite Belt (cf. Chapter 2.2, “Copper”). VMS-deposits with copper and zinc, are hosted by sequences dominated by mafic, volcanic rocks, and are particularly abundant in, Archaean and Palaeoproterozoic greenstone, belts. VMS-deposits with zinc-lead-copper occur, in sequences dominated by felsic volcanic rocks, sourced from continental crust and are best, exemplified by the Kuroko deposits of the, Miocene Green Tuff Belt of Japan., Submarine, volcanic-hosted massive sulphide, deposits (VHMS, a non-genetic term) may be characterized in more detail, as follows (Stanton 1994,, and cited sources):, The volcanic rocks are commonly andesites, dacites, and rhyolites, basalts are less frequently hosts to, significant mineralization (e.g. in Cyprus-type deposits). Throughout ore districts, the deposits occur, characteristically in certain volcanostratigraphic, horizons and are related to short and singular geological events such as caldera formation (Stix et al. 2003),, subvolcanic intrusions and generally, enhanced heat, flow. Fertile volcanic rocks are petrochemically distinct (Hart et al. 2004). The most important metals in, these deposits are Fe-Cu-Pb-Zn, with elevated trace, contents of Cd-As-Sb-Bi, more rarely including gold, and silver. Polymetallic ore deposits are commonly, zoned with Fe-Cu at the base, followed upwards by Zn, and Pb, and capped by barite, anhydrite or dense SiO2, exhalites (chert, jasper: Grenne & Slack 2005). This, chemical stratification can in some cases be explained by changes in the composition of the hydrothermal solutions. In other cases, a secondary, mobilization of more easily soluble components, (e.g. zinc) of early precipitates results in this pattern,, caused by continuous flow of hydrothermal solutions, upward through the earlier metalliferous hydrothermal sediments (“zone refining”). It is thought that in
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , the precipitates, colloidal textures and framboidal, (microcrystalline) pyrite prevailed initially, before, zone refining, diagenesis and metamorphism caused, coarsening and recrystallization. In the very lowgrade metamorphic Archaean ore deposit Sulphur, Banks in the Pilbara craton, Australia, pyritic filaments were found in colloidal-textured sulphides,, which may represent fossil bacterial colonies. Banding, synsedimentary soft-sediment deformation and, graded bedding of sulphides are often observed in, VHMS deposits. Ooids and pisoids form where outpouring solutions caused both ore precipitation, and agitation of hydrothermal seafloor sediment, (Heikoop et al. 1996). Breccias are produced by, hydrothermal eruptions. Most probably, breccias, were first cemented with anhydrite, which is often, later removed by low-temperature solutions, because of its retrograde solubility. A wide halo, of anomalous manganese contents surrounds sites, of submarine hydrothermal activity and these horizons survive into high-grade metamorphism, for, example as spessartine bands (Stumpfl 1979)., Other deposits occur at distinct exhalative centres, in regional metalliferous horizons as in the Ordovician Algoma-Type iron formation of northern, New Brunswick, Canada (McClenaghan et al., 2009). Exhalative systems with low contents of, sulphur and dissolved carbonate ions form stratiform skarn, because silica-iron-calcium precipitates develop into metamorphic assemblages of, hedenbergite, grossular, Ca-amphibole, wollastonite and diopside., , Iron-rich sulphides occur predominantly with, basalts, whereas Fe-Cu-Zn appear in volcanic, terranes of andesite-dacites and Fe-Pb-Zn with, rhyolites. Water isotope data suggest that the, hydrothermal solutions are mainly altered, seawater. Therefore, VHMS ore deposits are, thought to result from seawater convection and, hydrothermal dissolution of metals from pervaded volcanic rocks (Figure 1.10). This hypothesis, however, does not fully explain the ties, between specific metals and certain volcanic, rocks. Stanton (1994) proposed that a link with, the magmatic evolution and degassing of the, volcanic rocks may complement the convection, hypothesis. The scarcity of Ti, V, Cr, Co and Ni, in VHMS deposits, for example, may be due to, early precipitation of magnetite from mafic melt, , 71, , which extracts these metals. Also, magnetite, crystallization triggers sulphate to sulphide, conversion (Sun et al. 2004), which tends to, abstract chalcophile metals. Stanton’s hypothesis is supported by recent research. In the Manus, back-arc basin near Papua New Guinea, metalrich glass inclusions in submarine andesites, were found close to a presently active submarine hydrothermal system (Yang & Scott 1996,, 2005). Sulphur isotope data from the Lau basin, indicate magmatic input (Herzig et al. 1998)., Mass balance calculations by Beaudoin & Scott, (2009) confirmed that a very small fraction, (1%) of metalliferous magmatic fluid mixed, with convecting seawater accounts best for Pbenriched sulphides and volcanic rocks in the, Pacmanus vent field., Boiling P/T of submarine hydrothermal systems, depends on the salinity of the fluids, and depth of, boiling relative to the seafloor controls both form, and location of ore precipitation (Finlow-Bates &, Large 1978, Figure 1.45):, 1 If boiling takes place below the seafloor, metals, preferentially precipitate within the upflow channels, giving rise to stockwork, veins and disseminated ore., 2 Where phase separation and venting coincide at, the seafloor, massive orebodies may accumulate., 3 Ore with a sedimentary (e.g. bedded) character results from single-phase fluids that vent, below the boiling curve and collect in brine, pools., Accordingly, orebodies of VMS deposits show a, great variety of forms, including lenses and blankets (the most common), but also mounds, pipes, and stringer deposits (Large 1992)., Volcanogenic massive sulphide deposits occur, throughout the geological past from as early as, the Archaean. They cluster in periods of supercontinent assembly (Huston et al. 2010). Many, of the older deposits resemble the Kuroko, type (e.g. the Cambro-Ordovician deposits, in the Charters Towers District, Queensland,, Australia: Monecke et al. 2006). Generally, they, occur in convergent plate boundary settings., However, the prevailing bimodal nature of the, volcanic rocks and geochemical indices imply, that most VMS were generated during phases of
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72, , PART I METALLIFEROUS ORE DEPOSITS, , (a), , Sea surface, , 0, , 0% NaCl, 600, , 20% NaCl, , 1000, Ca. 250 oC, oC = K - 273.15, 400, , 500, , 600, , 700, , Temperature (K), Sea surface, , (b), , Depth of marine basin, , Water depth (m), , 200, , Assumed, level of boiling, (vapour phase separation), , B, S, , (1) Stockwork, , BOX 1.5, , (2) Sulfide, mound, , (3) Brine pool (B), and sulfide mud (S), , Figure 1.45 Boiling (formation of aqueous vapour), of submarine hydrothermal solutions at subcritical, conditions (a) and resulting form and location of, submarine ore deposits (b) (adapted from FinlowBates & Large 1978). With permission from http://, www.schweizerbart.de., , Submarine volcanogenic Kuroko type sulphide deposits, , Among the submarine exhalative VMS deposits, Kuroko ore deposits are an economically significant subtype (Ohmoto, 1996, Morozumi et al. 2007). The term “kuroko” is derived from the black lead-zinc ore that was exploited in Japan for, centuries. Many mines had also stockwork orebodies of yellow ore (“oko”) consisting of pyrite and chalcopyrite (gold)., The deposits occur in a belt 800 km long, built of Tertiary submarine volcanic rocks. The horizon hosting the deposits is, easily recognized because it separates monotonous Miocene “Green Tuffs” (dacitic pumice) in the hanging wall from a, bimodal volcanic series below, which is characterized by calderas, rhyolitic and dacitic pyroclastic breccias, and, resurgent domes of white rhyolite-dacite. The breccias are the product of submarine neptunian (Allen & McPhie 2009), volcanic eruptions. Explosive pyroclastics occur down to at least 1700 m below sea level at several mid-ocean ridges, (Fouquet et al. 1999). Geodynamically, the Kuroko province was undergoing strong extension and rifting within a back, arc basin of the Tertiary suprasubduction volcanic arcs of Japan., Early, fine-grained ores of ZnS, PbS, pyrite and barite were formed during extrusion of rhyolite domes by hydrothermal, exhalation through submarine vents and mound-building on the seafloor (Figure 1.46). In this figure, white spaces are, zones of hydrothermal-sedimentary sulphide ore of copper, lead and zinc, some of which was syngenetically altered (e.g., by zone refining) and recrystallized. The pyrite-rich stockwork of copper ( gold) veinlets is centred below the former. The, early ore muds were altered by reaction with upward percolating hot fluids (zone refining) that produced stockwork
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 73, , Figure 1.46 Generalized section, of a VMS Kuroko type ore deposit, (adapted from Finlow-Bates 1980)., With permission from http://www, .schweizerbart.de., , veinlets with coarse pyrite, chalcopyrite and quartz. Some elements such as Au, Sb, As, Ag and Sn were “distilled”, towards the hanging wall of the orebody. Both the stockwork and the ubiquitous ore breccias imply repetitive, hydrothermal explosions. Stockworks below exhalative ore beds mark the ascent of the hydrothermal fluids; they are, often independent economic orebodies. Distal sedimentary orebodies originate by slumping or mass flow from proximal, sulphide mounds. Because of rhyolite dome extrusion, the temperature of the hydrothermal fluids rose during ore, formation from 150 to 350 C. Updoming and steepening of ore beds caused complex syngenetic deformation. Due to, the considerable hydrostatic pressure, boiling of fluids was rare. Stable water isotopes indicate a seawater origin of the, solutions with a low salinity of <5% NaCl (equivalent). Seawater contributed sulphur, but part of sulphide sulphur was, leached from the magmatic rocks. Lead isotope characteristics are those of underlying volcanics and sediments. The, hydrothermal alteration in the footwall of the massive sulphide ores is concentrically zoned around the stockwork, from, innermost K-feldspar and illite to distal montmorillonite and zeolites. Concurrently with alteration, large masses of, calcium and barium were leached from the percolated rocks and reprecipitated on the seafloor forming barite, anhydrite, and gypsum (Ogawa et al. 2005). The hydrothermal fluids were mostly neutral, reduced and had low sulphur contents,, probably because in the downward branch of convection SO42 is generally removed from solution. Occasionally, clues, have been found for the presence of acidic, oxidized and sulphur-rich magmatic fluids of the “high sulphidation type”, (Sillitoe et al. 1996b; see “Epithermal Ore Deposits” below)., , major crustal extension, which may be caused, by slab rollback (e.g. in back arcs), resulting in, rifting, subsidence and deep marine conditions, (Lentz 1998, Allen et al. 2001). Present equivalents of VMS-forming processes, such as metalliferous submarine hydrothermal vents in, extensional provinces of island arcs, have been, discovered in the Okinawa trough and the, Manus basin near Papua New Guinea (Yang &, Scott 1996). Iizasa et al. (1999) describe a submarine gold-base metal deposit off the eastern, coast of Japan, with 9 Mt of ore. The deposit, occurs in a large rhyolitic caldera with numerous black smokers that coalesce above faults, and form walls. Recently, mining of a gold-rich, volcanic massive sulphide deposit at 1600 m, , water depth is seriously investigated offshore, Papua New Guinea. The Solwara orebody contains indicated and inferred resources of 2 Mt, at 6.8% Cu, 4.8 g/t Au and 23 g/t Ag., Many VMS-deposits have undergone metamorphic overprinting and strong deformation (cf., Section 1.5 “Metamorphosed Ore Deposits”). This, may destroy original spatial relations and change, rocks to a point where identification of their premetamorphic nature is very difficult:, Notable examples are Besshi type sulphide, deposits. Besshi is one of numerous comparable, mines on the Japanese island of Shikoku. Besshi, ore consists mainly of pyrite, chalcopyrite and pyrrhotite, with some sphalerite, gold and magnetite., Deposits occur within the Sangabawa high-P/T
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74, , PART I METALLIFEROUS ORE DEPOSITS, , metamorphic belt that represents the deepest exposed, part of the Mesozoic accretionary complexes in the, Japanese island arc. Host rocks include Carboniferous, to Jurassic schistose metapelites, metacherts and metabasalts of epi- to mesozonal metamorphic facies that, were deformed in the Cretaceous. In the Besshi region,, the schists wrap around a large eclogitic ophiolite, nappe (Moure & Enami 2008). The orebodies appear, to be bound to a regional horizon and are stratiform, lenses, either banded or massive, and deformed into, spindle-like shapes. Syngenetic formation with host, rocks is highly probable. After long disputes, the, geodynamic setting was recently affirmed to have, been mid-oceanic (Nozaki et al. 2008, Watanabe, et al. 1998). Besshi sulphides are highly deformed and, metamorphosed Cyprus Type deposits., , Worldwide, many pyrite-rich beds in metamorphic volcano-sedimentary sequences are known,, for example, the “fahlband” (pyritic) gneiss layers, in southern Norway. Of course, this does not, imply the same genetic setting as that of Besshi, sulphides., Terrestrial epithermal gold, silver and base, metal ore deposits, Many important ore deposits are connected with, terrestrial volcanism. In previous sections, iron, oxide lavas and tuffs, and the subvolcanic porphyry, copper ore deposits were mentioned. Here, we, , BOX 1.6, , focus on a deposit class that is commonly called, “epithermal”. These deposits formed by nearsurface hydrothermal processes (they occur at shallow epizonal depth). Epithermal orebodies take the, shape of veins, breccias, metasomatic masses and, impregnations within hydrothermally altered host, rocks. They are closely related to metalliferous, sinter mounds that occur on the land surface (“hot, springs deposits”). Epithermal ore formation takes, place in the aureole of near-surface subvolcanic, magma bodies that induce a high heat flow and a, steep geothermal gradient. This is the reason for a, commonly restricted vertical downward extension, of epithermal orebodies. Primary metal zonation, occurs in narrow intervals, or the zones overlap and, are “telescoped”. Causes for telescoping may be, erosion and collapse of volcanoes (Sillitoe 1994) or, repeated subvolcanic intrusions that radically, change the p/T-field. Epithermal ore deposits are, remarkable as a major source of gold, silver, zinc,, lead, bismuth and the minerals alunite, barite and, fluorite., Active epithermal systems of today’s volcanic, regions are mainly investigated in view of their, role as an energy source. Energy can be exploited, from geothermal systems (Figure 1.47) where heat, is transported close to the Earth’s surface, usually, by a combined action of rising magma, magmatic, volatiles and convecting non-magmatic waters., , Terrestrial volcanogenic epithermal ore deposits, , Epithermal ore deposits are remarkable as a major source of gold, silver, zinc, lead, bismuth and the minerals alunite,, barite and fluorite. The descriptor “epithermal” was originally given because ore deposition was thought to have occurred, near the surface from low-temperature fluids between 50 and 200 C (Lindgren 1933). Today, the temperatures are better, known, but Lindgren’s summary of characteristic features is still a valuable guide (freely adapted):, . depth of formation between 0 and 1500 m below the palaeosurface;, . temperature of formation 150–300 C (modern data);, . host rocks may be volcanic or sedimentary rocks, but always close to volcanic centres; ore controls are often faults;, . orebodies occur as veins, breccia pipes and stockworks; stratiform and replacement ore (e.g. in carbonates) is less, common;, . main ore metals are Pb, Ag, Zn, Au, Cu, Hg, Sb, Se, Te and Bi; metals occur in native form and as sulphides, sulphosalts,, selenides and tellurides;, . gangue includes chert, chalcedony and quartz (often crystalline, e.g. amethyst), chlorite, epidote, carbonates, fluorite,, barite, adularia, alunite, dickite, rhodochrosite and zeolite;, . hydrothermal alteration is characterized by moderate to intense silicification, kaolinization, pyritization, dolomitization and chloritization;
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , . ore textures typically indicate mineral growth in open spaces, in some cases including open space sedimentation;, colloform banding is common;, . most deposits have a restricted depth extent; vertical zonation is very pronounced and includes telescoping., The fluids responsible for epithermal ore formation include magmatic water and vapour, heated meteoric water and, in some cases seawater. Magmatic heat and fluids are essential components of the entire hydrothermal system, (Figure 1.47). In the centre-right of the figure, oxidized magmatic volatiles and fluids released from subvolcanic, porphyry intrusions produce a high-sulphidation Au-Cu deposit (“volcanic-hydrothermal system”). On the left, hydrothermal convection is dominated by meteoric water (“geothermal system”). The heated water dissolves volcanic gas and, vapour that are neutralized by reaction with the host rocks. At or after boiling, the solutions precipitate Au-Ag ore of, intermediate to low sulphidation type. Solfataras (sulphurous vents) can be associated with the precipitation of native, sulphur (S)., , Geothermal system, , Volcanic-hydrothermal system, , 500° - 900°C, SO2 , HCI , CO 2, 100°C, CO2 , H 2 S, , 200° - 300°C, CO2 , HCI , S, , S, , Au - Ag, , Boiling, , 20, , 300°, , S, , Hot springs, , 0°, , 30, , 0°, , Intermediate, sulfidation, , High sulfidation, Au - Cu, , Primary, neutralization, , Volcanic rocks, , 600°, , Porphyry intrusions, , Older rocks, Aqueous fluids, , 5 km, Gas and vapour, , Saline magmatic fluids, , Figure 1.47 Formation of epithermal ore deposits related to an andesite volcano. Modified from John, D.A. 2001,, Society of Economic Geologists, Inc., Economic Geology 96, Figure 13, p. 1847., , Salinity of fluids is commonly low (<3%) but may be higher in liquids that lost much vapour. Occasionally, the, presence of brines has been noted. Ore precipitation is typically by boiling as the hot solutions move upwards and, approach the surface (Robb 2005). Self-sealing of flow channels by precipitates (e.g. silica) is common and causes, pressure increase until hydrothermal (phreatic) fracturing or phreatomagmatic eruption releases built-up stresses. Large, highly permeable breccia bodies and pipes may result that attract the flow of hydrothermal fluids (Davies et al. 2008) and, may consequently become bonanza ore. Cyclic repetition of sealing and rupture is reflected in banded ore veins. Mixing, of rising hot magmatic gas or boiling fluid with cold groundwater is a possible factor of ore deposition. Where epithermal, solutions reach the surface, hot springs establish siliceous sinter deposits. Current formation of hot springs precipitates, with high contents of Au, Ag, Hg, Sb and As was described from the Californian Coast Ranges (Peters 1991)., Hyperthermophilic microbes (e.g. Sulpholobus acidocaldarius) are important actors in these environments. Freshly, precipitated siliceous sinters are often filamentous and microbe-rich. They age through a range of silica phases from opalA, opal-A/CT, opal-CT and opal-C to quartz, with surprisingly rapid transformation rates (Lynne et al. 2006)., , 75
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76, , PART I METALLIFEROUS ORE DEPOSITS, , Several scientific studies illuminate the role of, geothermal systems in ore formation (e.g. Taupo, Volcanic Zone, New Zealand: Giggenbach 1995,, Simmons and Brown 2000, 2007; cf. Ladolam,, Lihir Island, Papua New Guinea: Simmons &, Brown 2006)., Sulphur is an important component of magmatic volatiles. Near the surface, oxidation of, sulphur produces strong acids that react with host, rocks. The resulting hydrothermal alteration is, very conspicuous because of bleaching and, extreme argillization (e.g. alunite, equation 1.5)., Alunite can also be formed when oxic magmatic, exhalations that are enriched in HCl and SO2, dissolve in groundwater and react with host rocks, (Figure 1.47). This is the environment of the formation of epithermal gold deposits of the alunite,, or high sulphidation type (Hedenquist et al. 1993,, Simmons et al. 2005). High sulphidation (high fS2, in oxic conditions) is indicated by ore minerals, such as enargite, tennantite and covellite, and by, advanced argillic alteration gangue minerals, including alunite, pyrophyllite, dickite and kaolinite. In contrast to the former, epithermal deposits of the adularia-sericite, or low sulphidation, type are produced by reduced and neutral to mildly, alkaline fluids that carry H2S and other reduced, sulphur species (Hedenquist et al. 1993, Simmons, et al. 2005). In these deposits, silver and base, metals dominate and typical gangue minerals are, calcite, adularia and illite (Figure 1.47, White &, Hedenquist 1995). Members of the group with, exploitable base and precious metals are called, “epithermal polymetallic” (Cu-Zn-Pb-Ag-Au-Bi), or “Cordilleran” base metal deposits (Baumgartner, et al. 2008). High sulphidation epithermal deposits are often in close spatial association, with copper-gold porphyries. In active volcanoes, solfatara fields mark this environment., Earlier speculations that there are physical and, chemical links between the two deposit classes, are confirmed by modelling studies (Heinrich, et al. 2004, Heinrich 2005) and field observations (Sillitoe 2010). The described sulphidation, types of epithermal deposits are end members of, a continuum in nature., Submarine and terrestrial volcanogenic ore deposits are quite different. However, a number of, , transitional, or hybrid ore forming systems, have, been found (Hannington 1997, Naden et al. 2005),, with a setting at volcanoes emerging from the sea, (e.g. Milos, Greece). By definition, all volcanogenic deposits were formed near active volcanoes, or subvolcanic centres, and occur in a “proximal”, position. A number of submarine base metal deposits display several properties that are similar to, bedded volcanogenic ore, but are located at a considerable distance from contemporaneous volcanic activity. Volcanism may be indicated only by, thin tuff bands in sediments or known as distant, volcanic centres (in parts of the Iberian Pyrite Belt,, also at Mt Isa, Australia). This corresponds to a, “distal” position. In this case, a genetic connection of mineralization and volcanism is tenuous or, even unlikely (cf. Section 1.3 “Sedimentary Ore, Formation Systems”)., Studies aiming at a full understanding of volcanogenic ore deposits, regardless of scientific or, practical objectives, always include investigations, of structure, petrogenesis and evolution of the, volcanoes, the petrology of synvolcanic intrusions, concerning cosanguinity, hydrothermal alteration, (Gaboury 2006) and the precise plate tectonic situation (Sawkins 1990a,b). Generally, terrestrial, volcanogenic deposits occur in convergent plate, boundary and subduction settings, for example in, the circum-Pacific “ring of fire”. However, they, are not necessarily products of subduction and, compressional deformation (orogenesis). Many of, these systems were triggered by tensional events., Volcanic graben and horst terranes are especially, favourable locations (e.g. the Basin and Range, Province, USA), as are subvolcanic domes and, resurgent calderas of andesitic to rhyolitic, composition., , 1.2 SUPERGENE ORE FORMATION, , SYSTEMS, , Weathering of rocks and soil formation are the, preconditions of most higher life on Earth. They, are also at the origin of many important ore deposits. Raw materials that are predominantly produced from supergene mineral deposits include, a diverse range of metals and minerals such as
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 77, , consequent warm and humid climate and a profuse, growth of land plants as, for example, in the Early, Eocene climatic optimum when CO2 levels reached, 1125 ppm (Smith et al. 2008)., , iron, manganese, aluminium, gallium, niobium, and kaolin:, The term weathering integrates a number of processes that are caused by the interaction of Earth, materials with the atmosphere and the energy flow, from the sun. Physical and chemical process groups, can be distinguished, frequently with a strong biological component (Banfield & Nealson 1997, Southam, & Saunders 2005). Chemical weathering is dominated by reactions between minerals and rocks with, meteoric water containing dissolved oxygen and carbon dioxide. The first causes oxidation, for example, Fe(II) ! Fe(III), the second induces moderate acidity, (Figure 1.48, eq. 1.14), especially after passage through, a humic soil horizon that multiplies CO2 concentration in seepage water. Generally, chemical weathering removes mobile cations (e.g. Ca2þ, Na þ, Kþ), relative to stable residual components (Al3þ, Ti 4þ)., Site-specific conditions of pH and Eh are the main, controls on reaction products. Weathering rates, are a function of climate. Highest rates occurred, during periods of high CO2 in the atmosphere with, , All forms of life (Figure 1.49) participate in, supergene processes. Very direct is the action of land, plants, which are the main drivers of rock weathering,, not least because they are an important source of, oxygen in water and air. Organic acids derived from, partial decay of plants intensify rock disintegration., In the Late Archaean and Early Proterozoic, Cyanobacteria (blue-green “algae”) proliferated to a degree, that led to the first oxygenation of oceans and atmosphere (“the Great Oxidation Event”). Archaea were, probably always rather specialists for extreme environments, such as very hot, acidic and alkaline settings. In the Mesozoic, diatoms (algae) started to, dominate the marine photosynthetic productivity., Remember that for a geologist, the Tree of Life is, also a symbol of the time vector of life’s evolution on, our planet, as first explained by Charles Darwin, (1859)., , +1.0, , +0.8, , En, , mi, , ne, , +0.6, , ox, va idizi, de n adi n g, po um, sit, , vir, on, , wa, , ter, , s, , pe, , r li, , mi, , to, , me, , nts, rai, n, , in, , co, nta, , str, , +0.4, , ea, ms, , ct, , wi, , th, , fw, ate, , rs, , tab, ility, ox, id i, z, de ing l, po ea, sit d, , the, no, atm, oc rma, Tr a, wa e a n l a, o, ter, era sph, ns, sa te, itio, bo, ere, d, l, i, gw, br i n e, En, na, ne, ate, l, vir, s, r, s, on, gr, m, w a o und, ter, wa ents, ter, iso, - lo, so, l, ils gged ate, d, , +0.2, , Eh (Volt), , Up, , 0.0, , -0.2, , f, , Lo, we, r, , -0.4, , -0.6, , ro, eu, m, ma xinic, the, r in, atm, e, lim, or, os, it o, sa ganic, ph, fw, lin, ate, ere, e b - ric, rs, r in h, tab, es, ility, , -0.8, , Figure 1.48 Typical Eh/pH range of surface waters, (modified from Garrels & Christ 1965). Note that today,, the concept of a determinate Eh (pe) is doubted., , -1.0, , 0, , 2, , 4, , 6, , 8, , 10, , 12, , pH
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78, , PART I METALLIFEROUS ORE DEPOSITS, , Animalia, , Fungi, , Plantae, , Eukaryotes, Archaea, , Bacteria, , Prokaryotes, Figure 1.49 The Tree of Life in a very simplified version, showing what is poetically called the Kingdoms of Life, (consult the complex background in Crandall & Buhay, 2004)., , The principle of supergene ore deposit formation is the concentration of some dilute but valuable component of the primary rock. Two, basically different process types may lead to, concentration:, 1 The valued component is enriched in a residuum, while much of the rock mass is dissolved and, carried away., 2 The valued component is dissolved, transported, and concentrated on reprecipitation., In the latter case, the transport distance is, commonly very short (metres to tens of metres)., Some ore deposits, however, originate after, long-distance transport dissolved in surface and, groundwater. Examples include uranium in calcrete and in sandstone, iron in Tertiary river, valleys of Western Australia and manganese in, limestone karst caves. An important special, case of ore formation by weathering with metal, transfer by meteoric seepage water is the supergene enrichment of unexploitable low-grade primary mineralizations (“protore”). Many copper,, iron, manganese and silver deposits owe an, economic ore grade to supergene enrichment, processes., , Supergene ore deposits form in regions where, weathering is favoured by a humid and hot climate, that promotes the profuse growth of vegetation., Vegetation and organic matter affect supergene, alteration by two mechanisms. The first is, the direct influence of plants on the soil water (e., g. retention). The second is the abundance of, organic acids and of microbial activity that, promote dissolution of primary minerals. In this, environment, iron and aluminium have a very low, solubility compared to alkalis and SiO2. Therefore,, they are enriched in the red, clayey-sandy soils of the, tropics and subtropics that are generally called laterites (in soil science variously termed “ferralsol”,, “oxisol”, etc.). Ordinary laterites have little value, except for making bricks and building roads (the, Latin word later translates into air-dried brick)., Lateritic ore, Lateritic ore (of Al, Ni, Co, Fe, Mn, Cr, Au, etc.) is, formed when exposure of suitable rocks coincides, with favourable morphological, hydrogeological, and geochemical conditions that enhance dissolution, transport and precipitation (Valeton 1999)., Time also has a role – long-lasting weathering under, stable conditions may be expected to result in more, mature and higher-grade ores, although peaks of, lateritization last less than 100,000 years and occur, during global greenhouse events (Retallack 2010)., This is why supergene ore deposits are preferentially found in tablelands of stable cratons, for, example in wide regions of former Gondwana and, less likely in young orogenic belts (one exception, are bauxites in the European Alps)., Laterite sensu stricto designates the upper leached and oxidized part of certain soil profiles with, a well-developed vertical zonation (Figure 1.50)., Lateritic regolith reaches more than 100 m in, thickness. Principally, the lateritic regolith profile, comprises an uppermost eluvial horizon (A),, underlain by an illuvial zone (B) and the altered, but still recognizable precursor rock in situ (C), that rests on unweathered fresh rock (R). Depending on the groundwater table, zone (B) is often, divided into an upper kaolinitic and haematite, mottled layer and a greenish-grey smectitic material below. In (A) or in (B), many laterites have hard
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 79, , Figure 1.50 (Plate 1.50) Brilliantly white supergene kaolin below red laterite in the rainforest of northern Burundi, illustrates lateritic soil profiles which originate by tropical weathering., , crusts (duricrust) that are composed of SiO2 (silcrete – the geological home of precious opal), iron, (ferricrete) or manganese (mangcrete) oxides and, oxy-hydroxides. Zone C can be crumbled by hand, but often displays an upper clay horizon (feldspars, are argillized) and a lower sandy horizon with, intact feldspars but vermiculized biotite. The first, is called “saprolite”, the second “saprock”. Colours vary from red and brown to white or grey. Of, course, bleaching indicates reduced conditions, with dissolution and abstraction of iron, which is, caused by organic acids derived from decaying, vegetation., Laterite zonation is due to the effect of, seasonal changes between dry and wet periods:, During dry seasons, soil water moves towards, the surface and evaporates, inducing alkalic conditions that promote silica solubility. In wet seasons, acidic rain water seeps downward, dissolves, and reprecipitates matter or takes it out of the, system. Textures of lateritic soils include banded, crusts, shrinking fissures, concretions, pisolites, and oolites, small vertical tubular structures, Lie-, , segang banding and many more. Newly formed, solids are often amorphous and colloform, but, diagenetic aging into minerals is omnipresent. An, example is the transformation of iron hydroxide, gel into goethite and haematite (eq. 1.9)., Aging of Fe(III) hydroxide into goethite and haematite:, FeðOHÞ3 ! FeOOHþH2 O, 2FeOOH ! Fe2 O3 þH2 O, , ð1:9Þ, , The crucial factor for effective leaching is water,, modulated by hydraulic properties of soil and, bedrock that influence the contact time between, water and minerals (Gabet et al. 2006). Of course,, lateritic ore deposits are a variety of soil and, consequently their investigation should always, include methods and concepts of soil science:, Relative and absolute dating of supergene ore formation remains difficult. Cosmogenic nuclides such as, 10, Be are usually employed for dating soil formation, (Siame et al. 2006). If supergene alteration causes the
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80, , PART I METALLIFEROUS ORE DEPOSITS, , precipitation of new minerals, age determination, may be possible. One useful mineral is cryptomelane,, a K-bearing manganese ore that can be dated with the, 40, Ar=39 Ar method. Based on this approach, continuous formation of a supergene manganese deposit in, South China from 40 to <2 Ma is revealed (Li et al., 2007). Supergene alunite in Chile’s giant copper deposits was formed between 45 and 5 Ma, followed by, atacamite (dated by 36 Cl) during the hyperarid Pleistocene (Reich et al. 2009). If haematite is present,, palaeomagnetic methods may be useful that also, provide a measure of palaeolatitude. Oxygen isotopes, of hydrous minerals precipitated from weathering, solutions (e.g. goethite, clay, gypsum) are effective, archives of the past d18 O of meteoric water, an important proxy of palaeoclimate (cf. “Isotope, Geochemistry”)., , 1.2.1 Residual (eluvial) ore deposits, In residual ore deposits, the economically interesting component is concentrated in situ, while, weathering removes diluting parts of the rock., Examples are residual and eluvial placers, bauxite,, , Original land surface, (1) Start of weathering, , lateritic gold, platinum, iron (Ni, Co) and nickel, ores, residual enrichment of subeconomic protore, iron and manganese, and industrial minerals such, as phosphate, magnesite and kaolin., The fundamental geochemical principle of the, enrichment is the steady activity of a reaction, front in soil, while the land surface is lowered by, weathering and erosion. At the reaction front, the, valuable component is immobilized. Note that, this implies that the oldest parts of the profile, are near the top of the regolith, and the youngest, at the reaction front. The enrichment is due to, retention and accumulation of the component of, interest contained in the removed rock and soil, volume (Figure 1.51). Theoretically, the mass of, metal concentrated in stage (3) equals accumulation from the rock volume (R1 þ R2). However, the, system is not closed, as soil and solutions may, move laterally out of it. An illuminating case is, eluvial enrichment of phosphate from carbonatites by leaching of carbonate whereas apatite, remains in place., , A, B, C, , R1, , A, B, C, , (2) Intermediate stage, R2, , A, B, C, , (3) Present land surface, Ore, , R3, , Figure 1.51 Idealized scheme of residual, ore deposit formation by enrichment of a, valuable component at a reaction front, within the regolith (e.g. nickel in Chorizon saprolite).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Residual placers, Residual placers are concentrations of weatheringresistant ore minerals that are due to supergene, abstraction of non-ore material. The term eluvial, (outwash) describes the same result, although, with more emphasis on physical removal of, barren gangue. Many ore deposits contain heavy, minerals that resist dissolution, such as native, gold, cassiterite and wolframite. In this case,, physical and chemical properties determine, enrichment. Residual placers are only stable in, flat morphologies, because steeper slopes induce, soil creep and with it a down-slope displacement, of ore fragments (resulting in colluvial placers)., Residual placers often occur at the “stone-line”, level of a soil profile, where quartz and durable ore, minerals are concentrated., Bauxite ore deposits, Bauxite ore deposits originate either by in-situ, weathering of aluminium silicate rocks, or by, allochthonous sedimentation after erosion of, bauxite soil blankets. Autochthonous upland, bauxites (e.g. covering the Dekkan trap basalts in, India) are discerned from lowland bauxites that, include detrital (sedimentary) deposits. Residual, bauxite horizons often extend over large areas and, mark regional unconformities that are related to, favourable tectonic and climatic conditions, (D’Argenio & Mindszenty 1995). Most deposits, date from the Mesozoic and the Cenozoic, because, the probability of conservation of soil decreases, rapidly with increasing geological age. Many aluminium silicate rocks contain an average of 15%, Al2O3 that must be upgraded by weathering to at, least 35% Al2O3 to be economically exploitable., Alumina enrichment is caused by leaching of SiO2, and iron, two substances that are not easily dissolved. The key are the repetitive cycles of favourable pH/Eh conditions in soil water as described, above., Lateritic gold deposits, Lateritic gold deposits as a class are a relatively, recent discovery. One of the largest representa-, , 81, , tives of this group was the Boddington bauxite, mine in Western Australia, which until closure, in 2001 was the biggest gold mine in Australia, with an annual gold production of 2500 kg. Premining resources amounted to 60 Mt of ore at, 1.6 ppm Au, apart from bauxite with gold contents, of less than 1 ppm. Exploitable gold was located in, near-surface, iron-alumina hard crusts that, reached a thickness of 5 m and in additional 8 m, thick lumpy Fe-Al laterite of the B-horizon., Sources of the gold in soil at Boddington are, quartz veins and hydrothermally altered, bodies of Archaean greenstone bedrock. Since, 2007, resources of 400 Mt of this primary ore, with a grade of 0.9 g/t Au and 0.12% Cu are, exploited in a new mine. Worldwide, numerous, lateritic gold deposits are worked. They are attractive because exploration, extraction and processing of soil is less costly compared with hard, rock mining., Lateritic iron ore deposits, Lateritic iron ore deposits are not an important, source of iron, because both deposit size and iron, grades are rather low. The ore consists of oolitic,, red, yellow or brown haematite and goethite with, elevated contents of H2O, SiO2 and Al2O3. Most, deposits take the form of autochthonous or locally, transported hard crusts (ferricrete) that reach a, thickness of only a few metres. Nevertheless,, lateritic iron ore derived from ultramafic source, rocks is exploited in several countries (Albania,, Greece, Moa Bay, Cuba and in the Philippines) as a, high-iron limonite nickel ore (also termed, “oxide nickel ore”) for pig-iron blast furnaces. In, oxide nickel ore, nickel is absorbed in amorphous, iron-hydroxides or occurs as inclusions in goethite. The new technology of high-pressure acidleaching (PAL) appears to alleviate the former, economic disadvantage of these ores as a source, of nickel., Residual supergene enrichment of iron preconcentrations is the last upgrading event in the, multistage evolution of high-grade haematite ore, deposits with 60–68 wt.% Fe, which are derived, from Precambrian BIF (cf. Section 1.3). Primary BIF, are altered, silica-depleted and iron-enriched, in
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82, , PART I METALLIFEROUS ORE DEPOSITS, , Depth of weathering, , 1000 m above sea level, , S, , N, , HG, , ale, Sh, ae, ale, cR, Sh, M, d, ., n, a, Mt, t, er, Ch, ia, ylv, ite, .S, t, M, lom, Do, om, o, n, tte, Wi, , HemP, , MagP, , 500 m, , Brockman Iron Formation, Joffre Member, , Whaleback Shale Member, , Dales Gorge Member BIF, , Figure 1.52 Profile of the high-grade haematite orebody at Mt Tom Price, Western Australia. Modified from Taylor, D.,, Dalstra, H.J., Harding, A.E., Broadbent, G.C. & Barley, M.E. 2001, Society of Economic Geologists, Inc., Economic, Geology Vol. 96, Figure 5, p. 845., , Western Australia by the passage of diagenetic, brines (Thorne et al. 2009, Thorne et al. 2004). In, the vicinity of a normal fault at Mt Tom Price, (Figure 1.52), the Brockman Iron Formation of the, Palaeoproterozoic Hamersley Group was progressively altered to massive P-rich magnetite bodies, (MagP) and microplaty P-rich haematite (HemP)., Leaching in the supergene alteration zone is, responsible for the final upgrading to economic, ore of low-phosphorous high-grade haematite, (HG). Note that phosphorous and carbonate, abstraction at Mt Tom Price hardly changes iron, concentration but makes the ore commercially, more favourable., , exhalative-sedimentary pre-enriched horizons. In, India, metamorphosed Precambrian protore rocks, were called gondite. Gondites consist of quartz,, spessartine, rhodonite and rhodochrosite with, minor amounts of pyrite, alabandite (MnS) and, other minerals. Manganese contents in gondites, remain below 30%. Weathering removes carbonate and SiO2, enriching manganese oxides and, hydroxides (at Morro da Mina, Brazil to a depth, of >200 m). Carbonates and sulphides in gondite, promote weathering and ore formation, whereas, purely silicate-based gondites only bear thin ore, blankets., , Residual manganese ore deposits, , 1.2.2 Supergene enrichment by descending, (vadose) solutions, , Residual manganese ore deposits are derived from, rocks with above-average manganese contents., Laterites developed from such rocks include hard, crusts (mangcrete) or earthy manganese ore that, may blanket considerable areas. In contrast to iron, that tends to concentrate in the upper soil horizons (ferricrete), the slightly more mobile manganese is typically enriched in lower parts of the, laterite profile. Although smaller than sedimentary manganese deposits, lateritic manganese ores, are often high-grade and of superior quality., Sources (protore) are manganese quartzites, carbonates and volcano-sedimentary rocks that contain, , Lateritic nickel ore deposits are of major economic, importance (Gleeson et al. 2003). They form by, intensive and long-lived tropical weathering of, ordinary ultramafic rocks, which enriches nickel, (and cobalt). Two types of nickel laterite are distinguished: i) the oxide type in the upper, oxidized, iron-rich part of the laterite; and ii) the silicate type, in the lower, reduced saprolitic section of the, regolith., Laterite regolith zonation and the redistribution, of nickel (Table 1.3) are the result of meteoric, water percolating through the soil. Olivine, pyroxene and serpentine are rapidly decomposed by
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , BOX 1.7, , Lateritic nickel deposits, , Nickel laterites were discovered by Jules Garnier in 1863 on the island of New Caledonia in the Southwest Pacific Ocean., Exploitation started in 1876. Meanwhile, numerous similar deposits have been discovered worldwide. New Caledonia, remains, however, the largest ore province of this type, with a past cumulate nickel production of 3.5 Mt (metal) and, considerable remaining resources. Today, 4 large mines are in operation, based on reserves of 1 Mt nickel in ore at, >2.7 wt.% Ni, and 1 Mt of nickel in ore at 2.5–2.7%., The ultramafic rocks of New Caledonia are part of a giant ophiolite nappe that was obducted during the Eocene and, forms most of the island (Paquette & Cluzel 2007; Figure 1.53). Serpentinized and unaltered harzburgite-dunite displays, an average 0.25% Ni (mainly in olivine) and 0.02% Co. Lateritization of the exposed ophiolite probably started in the, Miocene, resulting in a mature profile (Figure 1.54):, . Top: Massive goethitic iron crusts (ferricrete; French cuirasse); low residual nickel concentration;, . Limonite zone: with residual manganese, chromium and aluminium; mostly bright red-coloured because of haematite, traces; earthy or concretionary texture; nickel, cobalt, magnesium, calcium and silica are leached and strongly depleted;, in rare cases, residual nickel is exploitable;, . Nontronite zone: Ferrallitic, earthy, red and yellow clays are the norm, but in some profiles massive or network, silicification is prominent (opal, chalcedony, jasper, etc.); cobalt may occur in exploitable pockets of asbolane (“earthy, , Miocene peneplain, Barrier reef, , Ni-laterite regolith, Allochthonous laterite, debris, , Partially serpentinized, peridotite, Basalts and, parautochthonous rocks, , Figure 1.53 Schematic profile of laterite blanket with nickel ore (black, garnierite-saprolite horizon) above, peridotitic rocks such as the New Caledonian ophiolite (not to scale)., , Ferricrete, Limonite, and, nontronite, , Saprolite, nickel, ore, , Peridotite, (little altered,, garnierite in, fractures), , Figure 1.54 Regolith profile of exploitable nickel laterite in New Caledonia. Note the control of garnierite by, joints and fractures of peridotite. Modified from Gleeson, S.A., Butt, C.R.M. & Elias, M. 2003, Society of, Economic Geologists, SEG Newsletter, Figure 3, p. 13., , 83
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84, , PART I METALLIFEROUS ORE DEPOSITS, , cobalt”, a variety of wad); occasionally, nickel and manganese are enriched to exploitable grades; the depletion of, magnesium is minor, SiO2 contents are equal to those in unaltered parent rock;, . Saprolite zone: This is altered parent rock with clearly recognizable structures and textures of the ultramafics;, alteration is controlled by joints; in lower parts of the profile, altered rock includes increasingly fresh cores of, parent rock until the base of surficial weathering is reached; the saprolite zone is the main nickel ore horizon, with, veinlets, pockets and irregular masses of green garnierite (chalcedony and magnesite); in the saprolite, olivine and, pyroxene are altered to colloform magnesium silicates that age into the minerals antigorite (serpentine), talc and smectitic, clays; nickel is taken up by these minerals in their lattice by cation exchange of Mg2þ and may reach several percent in the, garnieritic ore; along permeable fractures, garnierite can penetrate far into the unaltered rock below the regular base of the, regolith;, . Bottom: Unaltered hard rock with rare garnierite in joints and fractures., , acidic rain and soil water. From a surface value of, 6, the pH of deeper soil water turns alkaline (pH, 8.5). Nickel is dissolved and flushed downward,, where it is immobilized by ion exchange with Mg, in newly forming magnesium hydrosilicates., A schematic example of this reaction (neogenesis, of the Ni-serpentine nepouite from serpentine) is, given in equation (1.10)., Neogenesis of Ni-silicate in the saprolite zone:, Mg3 Si2 O5 ðOHÞ4 þxNi2þ aq, ! ðMg; NiÞ3 Si2 O5 ðOHÞ4 þxMg2þ aq, , ð1:10Þ, , Normally, groundwater removes the dissolved, magnesium from the system. Some is precipitated, as a carbonate (magnesite) in joints. Whether, this process is able to form stockwork ore deposits, of magnesite remains doubtful (cf. Chapter 3, “Magnesite”)., Supergene enrichment of pre-existing, mineralization, Supergene, descendent alteration may be exceptionally beneficial where pre-existing uneconomic mineralization or low-grade orebodies are, Table 1.3 Typical chemical characteristics (wt.%) of the, iron oxide and silicate sections of nickel laterite deposits, in New Caledonia (Gleeson et al. 2003), , Ferricrete, Limonite and nontronite, Saprolite, Peridotite (little altered), , MgO, , Fe2O3, , SiO2, , NiO, , 0.5, 1.0, 12.5, 42.7, , 84, 70, 11.5, 9.1, , 2, 2.2, 63.2, 41.3, , 1.2, 1.8, 5.3, 0.3, , affected. Primary ore (protore) in deeper parts of, the weathering profile is upgraded by seepage solutions. For sulphide copper and silver ores, iron, oxides and certain uranium ore deposits, this process is of economic significance. Non-sulphide, zinc ore produced by supergene enrichment is a, less common economically attractive target., The supergene enrichment of sulphide ore is a, consequence of near-surface oxidation of sulphides caused by meteoric water seeping downwards through an unsaturated zone. Other agents, include dissolved oxygen and microbes that have a, role as “self-replicating catalysts” (Sato 1992)., These microbes include chemolithotrophic mesophile bacteria such as (Acidi-) Thiobacillus or, Leptospirillum ferrooxidans and extremophile, archaea such as Ferroplasma acidarmanus, (Newman 2010, Edwards et al. 2000). In modern, metallurgical processing, sulphide-oxidizing microbes are cultivated in leach pads or large vats in, order to decompose sulphide ores of, for example,, gold, copper, nickel and cobalt., Because it is kinetically slow, purely inorganic, oxidation of sulphides has a minor share in the, supergene enrichment process system (e.g. pyrite:, Rimstidt & Vaughan 2003, Balci et al. 2007)., The role of microbes in the oxidation of sulphides, is essentially understood but its biochemistry is, very complex (Newman 2010, Nordstrom &, Southam 1997). L. ferrooxidans, for example,, attacks the surface of pyrite at T > 40 C and pH, 0.7–1.0 by catalytic oxidation of Fe(II) to Fe(III). As, an electron acceptor, Fe3þaq abiotically oxidizes, pyrite. T. ferrooxidans has a similar function but, prefers cooler and less acidic conditions. Contact, of Fe3þaq with dissolved oxygen causes
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86, , PART I METALLIFEROUS ORE DEPOSITS, , paragenesis. In some locations, this zone is enriched to an exploitable volume and grade (Melchiorre & Enders 2003). Prominent examples are, several Chilean copper porphyry deposits that, have important “exotic oxide” outliers (infiltration deposits, see below). Exotic ores were formed, in drainage channels to a distance of about 7 km, from the leached portion of the primary deposit, (Chuquicamata, El Salvador, El Abra). In the, hyperarid Atacama Desert, the copper salt atacamite Cu2Cl(OH)3 is an economically important, ore mineral of oxide orebodies (Reich et al. 2008,, 2009)., Where percolating water meets the groundwater table, the active reaction front is reached., Here, exothermic oxidation of primary sulphides, takes place, as well as the secondary enrichment, (Figure 1.55). Microscopy of replacement textures illustrates these processes. This zone is the, preferred habitat of the thermophilic microbes, mentioned above. Vestiges of the microbes were, demonstrated by electron microscopy of contacts, between chalcocite and primary sulphides, (Sillitoe et al. 1996a). Isotope data indicate heating to nearly 100 C (Melchiorre & Williams, 2001). The efficiency of secondary enrichment, depends mainly on conservation of the acidic, environment, but also on favourable hydraulic, conditions (e.g. permeability). Carbonates and, other basic host rocks that buffer acidity inhibit, both organic and inorganic processes, resulting in, very little secondary enrichment., The chemical reactions produce galvanic mineral interactions (Sikka et al. 1991) and an electrochemical field near the orebody, which is a very, clear target for geophysical exploration methods., Precipitation of enriched ore can be schematized, as a cation-exchange reaction that results in formation of chalcocite, covellite or bornite, (equation 1.12)., Formation of enriched secondary copper ore:, 5FeS2 þ14CuSO4 aqþ12H2 O, ! 7Cu2 S ðchalcociteÞþ5FeSO4 aqþ12H2 SO4, CuFeS2 þCuSO4 aq ! 2CuS ðcovelliteÞ, þFeSO4 aq, , ð1:12Þ, , The primary ore gains more of the valuable metal, (here copper), often resulting in spectacularly rich, orebodies. Dissolved iron and sulphate are transported away out of the system; part of the sulphate, may be reduced by microbes so that additional, sulphide is available. It is a frequent observation, that many ore deposits are only economically, exploitable because of supergene enrichment, based, on primary sulphides of poor grade. This is true for a, number of porphyry copper deposits, whereas others, extract both enriched and primary ore., Comparative investigation of replacement, order showed that more “noble” metals (copper, in eq. 1.12) replace more “common” metals (iron, in eq. 1.12) that are dispersed. Earlier in this chapter, electrochemical reasons for this behaviour, were cited. More noble metals (copper, silver, gold), also exhibit the tendency to occur in the native, form, which may be explained by a redox reaction, (eq. 1.13):, Formation of native metals by electron exchange:, 2Agþ þ2Fe2þ ! 2Ag0 þ2Fe3þ, ðin water with dissolved SO2, 4 Þ, , ð1:13Þ, , Overall, weathering of a sulphide orebody comprises a complex system of physical, chemical and, biological processes. Often, the observed end products do not match results of thermodynamic, calculations. Unexpected parageneses can partly, be explained by the argument that at the low, temperatures of weathering, the activation energy, needed for many reactions cannot be provided., This causes enlargement of thermodynamic stabilities of minerals into what Sato (1992) called, “persistence fields” that are in good accordance, with geological observations., Prolonged weathering of a sulphide ore deposit, results in a vertical zonation (Figure 1.55 and, Figure 1.56) that comprises a leached (oxidation), zone, and a metal-rich cementation zone (supergene blanket), grading into unaltered mineralized, rock of the primary zone (protore). In contrast to, the primary depth zonation of hydrothermal mineral deposits, the supergene pattern is called, “secondary vertical zoning”. In stable cratons,, oxidation reaches a depth of over 100 m, mainly, as a function of the groundwater table that roughly
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 87, , Figure 1.56 (Plate 1.56) For over 100 years,, the supergene, high-grade chalcocite ore of the, black shale-hosted copper deposit at Mt Oxide, in the Mt Isa district, Australia, was the, symbolic example of fortune and destitution for, investors and miners.Gossan (red) covered an, accumulation zone of 55 m thickness which, graded into subeconomic primary sulphides. A, sizeable ore shoot of 15.9% Cu was extracted by, underground methods. Later, the pit was, excavated and overall, the deposit yielded, 23,000 t of copper (J.H. Brooks in Glasson &, Rattigan 1990)., , limits the penetration of oxygen dissolved in soil, water. In special cases, oxidation occurs to much, greater depths (Tsumeb Mine, Namibia to 1160 m:, Lombaard et al. 1986) or far beneath the groundwater table (e.g. Bougainville, where oxidation, “fingers” down along permeable faults). Obviously, the full understanding of a cementation or, secondary enrichment deposit requires a thorough, reconstruction of the morphological and hydrological evolution., Iron ore deposits as a product of supergene, enrichment, Iron ore deposits as a product of supergene, enrichment provide an important part of world, , iron ore supplies. Protores are usually Palaeoproterozoic BIF (banded iron ore formations), with, initial contents of 25–45 wt.% Fe that are upgraded to 60–63% Fe. Note that there is a sharp, divide between: i) medium-grade martite-goethite, deposits; and ii) high-grade haematite deposits,, which reach a maximum of 68% Fe. Haematite, orebodies differ from supergene enrichment systems by reaching a great depth beneath the, surface (1500 m) and huge tonnages (3000 Mt,, although most are in the range between, 200 and 500 Mt). Based on various observations,, current genetic interpretations of high-grade, haematite deposits involve mainly epigenetic, hydrothermal models (Thorne et al. 2009,, Clout 2006).
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88, , PART I METALLIFEROUS ORE DEPOSITS, , Supergene enrichment is at the origin of, common martite-goethite ore (Morris 1985,, 1993). However, genetic models are very different, from the simple laterite blanket type. It is assumed, that near the surface, ferrous iron is leached from, BIF by reduced seepage water rich in dissolved, carbon and is carried downwards along flow channels such as faults crossing BIF beds. This establishes an electrochemical system that allows flow, of electrons. The surface region functions as a, cathode, the BIF at depth as an anode. In this, system, conversion (“oxidation”) of ferrous to ferric iron takes place without the presence of free, oxygen. Magnetite is replaced by martite (haematite pseudomorphous after magnetite) or by maghemite. The reaction also multiplies the, solubility of silica and other gangue minerals that, are leached and replaced volume for volume by, goethite (FeOOH). By-product acidity causes, transformation of BIF shale bands into kaolinite, marker horizons in ore. Orebody growth is upward, from depth. If exposed by erosion, lateritic hard, crusts may form from this ore, underlain by a, friable zone of haematite. Let me add that it is, quite possible that martite-goethite protore is, hydrothermal just like high-grade haematite preconcentrations; research will soon provide, clarification., 1.2.3 Infiltration as an agent of ore formation, Infiltration is a term borrowed from hydrology,, where it is used to describe the movement of, surface water into soil, porous rock, or karst., Infiltration ore deposits are formed when meteoric, waters take up a substance that is dissolved by, weathering, and concentrate it after considerable, , BOX 1.8, , transport by infiltration in a different geological, setting. Uranium is an illuminating example,, because during surficial alteration it is easily, dissolved from ordinary rocks such as granite,, gneiss and felsic tuff, and is transported by creeks, and rivers for tens to hundreds of kilometres, until, infiltrating into an aquifer where reduced conditions cause precipitation and concentration. Certain deposits of copper, iron, vanadium, silver and, Pb-Zn-Ba-F are suggested by some scientists to, have a similar origin. In the preceding text, we, have mentioned Chile’s exotic copper deposits as, part of the infiltration class. Generally, the metals, may be derived from older preconcentrations or, from rocks with common trace element contents., Selective weathering of different minerals may, produce a pattern of spatial or temporal separation,, for example by first leaching traces of uranium,, copper and zinc from plagioclase and Fe-Mn oxyhydroxides, followed by barium, lead and SiO2, when the more stable K-feldspar is decomposed., The concentration of solutes in surface and, groundwater is generally very low. Enrichment to, ore-grade and an exploitable volume is only possible where a large mass flow is focused into a highly, efficient filter. For dissolved matter, geochemical, barriers are most effective, commonly in the form, of a rapid change of pH and Eh. Geological actors, include carbonates, H2S or SO4 in the pore waters,, as well as live, decaying and fossil organic matter, (biomats, peat, coal, bitumen), including by-product methane., Because precipitation takes place in pre-existing, rock, infiltration mineralization is clearly epigenetic, not syngenetic. Investigations of infiltration, ore genesis always include (palaeo-) hydrogeological methods., , Infiltration ore deposits of uranium, , Infiltration deposits of uranium are very common and economically important. Oxidative chemical weathering transforms uranium (IV) in rocks to uranium (VI), which forms complex ions with free SO42, CO32, OH, alkalis and, humates. Under oxic conditions in surface and groundwater, these complexes are stable and allow long-distance, transport. Geochemical barriers for uranium are phosphates, arsenates, vanadates and carbonates in percolated rocks,, strong reduction (e.g. methane) and sorption by colloids of iron hydroxides, silica and organic matter (Min et al. 2005)., Correspondingly, uranium infiltration mineralization and ore deposits occur in permeable sandstone and conglomerate, (Colorado Plateau, or “sandstone type”), in volcanic ash beds, in faults and breccia bodies, in peat, lignite and coal seams,
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 89, , in asphalt, and in terrestrial calcite crusts of semiarid lands (the “calcrete type”). The genesis of the last includes a, pronounced component of evaporation., The Colorado Plateau is a Precambrian block covered by some 3000–4000 m of little deformed Permian to Eocene, sediments. Numerous terrestrial sandstone and conglomerate strata occur in this package. Uranium deposits and, occurrences were found in all stratigraphic positions, with the largest endowment in Triassic and Jurassic rocks (Sanford, 1990). The time span of mineralization, however, is from 80–50 Ma (Late Cretaceous to Palaeogene), independent of host, rock age. The shape of orebodies varies widely, including tabular lenses, sometimes stacked on top of each other, pockets, and the characteristic “roll fronts” (Figure 1.57), often in connection with buried river courses. Roll fronts mark the redox, boundary in space and time where the infiltrating meteoric water lost its capacity to oxidize the percolated sandstone and to, retain uranium in solution. Tabular deposits appear to have been controlled by the interface between two different meteoric, water bodies, a stagnant saline brine and freely flowing fresh groundwater that carried uranium and vanadium. In the tabular, type, the main ore mineral is coffinite, in contrast to the uraninite of the roll front type. Oxidation/reduction fronts in the, aquifers are clearly visible in the field, by red and grey colours of the rocks. Spotty reduction by coal fragments in sandstone, caused formation of high-grade ore pockets, but also the complete replacement of large tree trunks by uraninite. It is thought, that sources of uranium were the many tuff bands in the sediments, but other possibilities include former basement islands or, detrital uraniferous minerals in sandstone. Ore minerals include coffinite, uraninite, vanadium-rich clays and minor, sulphides (of Fe, Mo, Cu, Pb, Zn, Se, etc.). Neogene uplift of the plateau caused a late phase of renewed weathering that, affected many deposits and produced highly conspicuous varicoloured secondary uranium minerals., , Low-permeability rocks, , Sandstone, aquifer, Flow direction, , Figure 1.57 Roll front, uranium orebodies develop,, where infiltrating uraniferous, meteoric water passes through, a redox boundary., , "Roll front", , Redox boundary, , Low-permeability rocks, , Hematitic, core, , Alteration, halo, , Uranium, ore, , Pyritization, , Unaltered, sandstone, , Hematite, Magnetite, , Siderite, Sulfur, Ferroselite, (FeSe2), , Uraninite, Pyrite, FeS, Se, , Pyrite, FeS, , Pyrite, , The example of the Colorado Plateau demonstrates that infiltration ore deposits are formed by, meteoric waters that descend into basins – the, reverse from diagenetic-hydrothermal ore deposits, which are also found in marginal areas of, basins but are formed from ascending basinal solutions (cf. Section 1.4 “Diagenetic Ore Formation, Systems”). Copper, lead and silver ores embedded, in haematitic sandstone suites (“red bed deposits”), may have originated in a way similar to infiltration, deposits of uranium (e.g. the Transfiguration, deposit in Quebec: Cabral et al. 2009). There are,, however, viable alternative modes of their formation by basin dewatering, so that each case, must be carefully investigated. Resemblance and, , Jordisite (MoS2), Calcite, , differences are illustrated by the large sandstonehosted but diagenetic-hydrothermal Zn-Pb, deposit at Jinding, Yunnan, China, with 220 Mt, at 7.4 wt.% combined grade (Chi et al. 2007, Kyle, & Ning 2002). Sandstone ore of base metals and, silver is generally low-grade and therefore at present economically not very attractive., Formation of karst systems in carbonate rock, bodies and the contemporaneous deposition of, mineralization in the caves may also be related to, infiltration processes. However, karst infiltration, deposits are more a suspected than a proved, deposit class. In the Maghreb Region of northern, Africa, numerous small deposits of Pb and Zn (, barite, fluorite) in solution caves were formerly
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90, , PART I METALLIFEROUS ORE DEPOSITS, , exploited. Following earlier authors who studied, lead-zinc karst ores in Europe, many scientists, proposed that the North African deposits have, formed by concentration of dissolved trace metals, originally hosted in the karstified carbonates (Bernard 1973, Zuffardi 1976). Meanwhile, the application of modern research technologies suggests, an epigenetic hydrothermal origin, probably, related to migrating saline basinal brines. However, the karst infiltration model retains its theoretical validity: Examples may be supergene, manganese ores in solution caves of Devonian, carbonate rocks in South China (Li et al. 2007), and in Cenomanian-Turonian dolostone at Imini,, Morocco (Gutzmer et al. 2006), or vanadium, karst ore in Neoproterozoic dolomites of Namibia, (Boni et al. 2007)., Karst formation is caused by infiltration of, meteoric water, which is enriched in CO2, for, example by percolation through the organic soil, horizon. Dissociation of carbonic acid produces, acidity (Hþ) that is essential in limestone dissolution. Because of a much slower reaction rate,, dolomite is less affected by karstification. The, reactions of limestone dissolution are described, in equation 1.14:, , calcite dissolution and sulphide precipitation, (Figure 1.58); note the possible connection with, metasomatic ore formation., 2 Common meteoric karst, formed by downward, percolating water with dissolved carbon dioxide,, which was later filled by ascending hydrothermal, fluids with ore and gangue, independent of, karstification., 3 Common meteoric karst, and synchronous, supergene infiltration of ore elements., Only the last case answers the definition of a, supergene ore deposit. Bernard (1973) discussed, the possibility of ore formation in the setting of, a common karst system. The hydrogeological, zonation is the key (Figure 1.59):, Underneath the karst infiltration surface is the zone, of vertical percolation (A). Rain or melt water seeps, down rapidly, mechanical erosion and leaching of, calcite along flow-tubes are very intensive. Any sediments will be quite coarse-grained. The environment, is strongly oxidizing. The following zone of horizontal, highly variable water flow (B) is reached where, the vertical seepage is collected by underground rivers. Sediments are mainly of the sand and silt grain, size. The environment is still oxic and the only, mineralization may be barite. In the lower parts of, a mature karst system, a zone that is permanently, submerged under water (C) can have sections with a, “micro-euxinic” environment. Microbes feeding on, imported organic matter reduce sulphate to H2S., Sediments are either very fine-grained or chemical, precipitates. Host rocks are silicified and dolomitized; cave walls are encrusted with limonite and, clay. Mineralization grows inward and includes sulphides (mainly Pb, Zn, Sb and Cu), fluorite, barite,, calcite, dolomite and occasional vanadium ores. Bernard (1973) thought that the metals were derived from, the dissolved limestone (carbonates have contents in, the ppm-range), but more often infiltration of weathering solutions from surrounding non-carbonatic, rocks is assumed., , Carbonic acid-induced limestone dissolution:, þ, H2 OþCO2 ðgÞ $ H2 CO3 $ HCO, 3 þH, , CaCO3 ðsÞþHþ ! Caþ, 2 þHCO3, , ð1:14Þ, , There are three possible modes of ore deposit, formation in karst:, 1 Hydrothermal karst originating from hot, ascending fluids (e.g. the famous Carlsbad cave in, New Mexico, which was formed by hypogene, sulphate solutions: Polyak et al. 1998); structural, relations such as cementation of host rock, karst breccia by sulphides reveal simultaneous, , Limestone, , Dolomite, , 10 m, , Figure 1.58 Lead-zinc ore (black) in a, hydrothermal karst tunnel in Zechstein, carbonate, Upper Silesia, Poland. Modified, from Sass-Gustkiewicz, M. 1996, Society, of Economic Geologists, Inc., Economic, Geology Special Publication 4, Figure 2,, p. 174.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 91, , Karst surface, , A, Wet season groundwater table, , B, Figure 1.59 Hydrogeological crosssection of a mature karst system (modified, from Bernard 1973). With kind permission, from Springer Science þ Business Media., , Dry season groundwater table, , C, , Discrimination of supergene karst-infiltration, from hypogene mineralization is only possible, with modern analytical methods (e.g. a study of, fluid inclusions). Observation alone is unable to, decide. This is vividly demonstrated by the scientific controversies concerning the origin of the, Mississippi Valley type deposits, which lasted, more than half a century. Consider the sedimentary geopetal (gravity-induced) structures of ore,, gangue and internal sediments in a cave in an, Alpine lead and zinc deposit (Figure 1.60). The, bedding planes of host and cave fill are nearly, parallel. Both were tilted, as shown here by later, tectonic movements. Similar to Figure 1.58,, repeated caving of the roof is visible. Obviously,, the pattern may be due to both ascending or, descending solutions. Complicating the situation, even more, in a near-surface setting (in this case, an emerged reef) both super- and hypogene processes may interact at the same time., Karst has several properties that may occur, alike in hydrothermal and common meteoric, , water systems (Ford & Williams 2007). These, have to be fully understood for rationally exploring and mining karst orebodies. Most important, is the observation that thick carbonate strata, always have horizons that are more prone to the, development of solution caves and tunnels. This, is, of course, a first-order guide for finding orebodies. Causes for easier karstification may have, been higher initial porosity or brittleness and high, purity of the carbonate rock. Clay beds often, enhance mineralization by channelling or constricting the flow of water or of hydrothermal, fluids. Faults and joints are additional elements, that, together with stratigraphic ones, control the, three-dimensional structure of a karst system. It, is also important to elucidate the fourth dimension, that is the evolution of the system in time., Very practical aspects of mining a karst system, concern hazards such as sudden inrushes of, water, mud and debris, unstable surroundings of, mine openings and damage to water rights owned, by third parties., , Limestone, , 0.5 m, , Cave fill:, Figure 1.60 Section of a Late Triassic, hydrothermal karst cave at Bleiberg,, Austria, that shows sedimentary layering, involving fluorite, zinc ore and clastic cave, sediment., , Crust of calcite crystals, Massive pyrite, Banded sphalerite, , Limestone, , Sediment with fluorite, Micritic limestone
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92, , PART I METALLIFEROUS ORE DEPOSITS, , 1.3 SEDIMENTARY ORE FORMATION SYSTEMS, The processes of sedimentation include physical,, chemical and biological components. The prevailing regime lends its name to the major subgroups, of sediments and sedimentary rocks: mechanical,, chemical and biogenic sediments. Sediments are, also classified according to the provenance of the, material (allochthonous/autochthonous), and the, manifold variations of locality and environment of, deposition (Nichols 2009). Classifications of ore, deposits that originate by sedimentary processes, apply analogous criteria., Allochthonous, terrigenous materials of prime, economic importance are gravel, sand and certain, clays. The source of the particles composing these, rocks is distant from the deposits, the material was, transported. Metallic ore deposits of this genetic, group are called placers. Placers are mechanical, enrichments of heavy and chemically resistant, native metals and minerals by flowing or otherwise agitated water., Autochthonous raw material deposits were, formed at the site of the deposit. This group includes chemical precipitates and partially biogenic substances such as carbonates (limestone,, dolomite, magnesite), evaporites (rock salt, potassium salt, gypsum), some massive and oolitic, ironstones, banded iron formations, marine phosphates, sedimentary sulphide ores and manganese, ore beds. Organic formation is the prevailing, component for coal, oil shale, diatomite and for, part of the limestones (e.g. chalk). All forms of life, (Figure 1.49) participate in sedimentary minerogenesis. In sedimentology, soils are also considered as autochthonous sediments. However, ore, deposits related to soil formation processes were, presented in the preceding section., Many metalliferous sulphide, oxide, carbonate, and sulphate deposits originate by autochthonous, sedimentation following hydrothermal exhalation, precipitation of solutes and deposition of ore, particles on the seafloor. Sensu stricto, all ores, with this origin are “sedimentary”. However, the, different process systems that may cause subaqueous exhalation of ore fluids and the prevailing, geological setting must be part of the classification. This is why Cyprus type sulphides are, , grouped with “Ore Deposits at Mid-Ocean Ridges, and in Ophiolites” and Kuroko and Besshi types, with “Volcanogenic Ore Deposits”. In this section, only those deposits of exhalative-sedimentary character are discussed, which were formed in, a sedimentation-dominated environment and distal from contemporaneous volcanism. This is the, class of sedimentary-exhalative (sedex) deposits., Sedimentation is limited to the surface of the, Earth, which is also the realm of life and its, biogeochemical cycles (Falkowski et al. 2008)., Therefore, sedimentary ore formation will almost, always have a biogenic component (Southam &, Saunders 2005). In some cases this is very obvious,, for example in phosphate deposits made of bones, and coprolites, or a lignite seam composed of fallen, trees. In few ore deposits, however, the biogenic, component is immediately visible. This is illustrated by sedimentary sulphide deposits that display a very light, biogenic sulphur isotopic, composition, although the metals were introduced by hydrothermal-exhalative systems from, below the seafloor. The cause for the light sulphur, lies in oxygen depletion of seawater around hydrothermal centres and consequently, massive proliferation of anaerobic sulphate-reducing bacteria, such as Desulphobacter or Desulphomaculum, (Boschker et al. 1998). In the photic zone, green, sulphur bacteria (Chlorobiaceae) thrive by anoxigenic photosynthesis. Sulphate-reducing bacteria, often form consortia with archaea, whose role is, not yet fully understood. These microbe communities reduce seawater SO4 to H2S, using organic, matter as an electron donor (eq. 1.15). Simultaneously, carbon is “mineralized” to CO2 or carbonate. As the microbially mediated reduction of, sulphur involves a considerable negative isotope, effect, metal sulphides formed from this H2S contain light, “biogenic sulphur”. However, a minimal sulphate content in seawater of more than, 1 mM is a precondition. Low contents of dissolved, sulphate in seawater may have several reasons (e., g. the precipitation of evaporite gypsum: Wortmann & Chernyavsky 2007; cf. Section 4.2.2, “Seawater in the Geological Past”). The maximum isotopic fractionation by bacterial sulphate, reduction (BSR) is 46‰, but most natural samples, have a much smaller spread. Some rare microbial
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , BSR-communities produce intermediate-valence, sulphur species (like S0 or S2O32) by bacterial, sulphur disproportionation (BSD) that results in, d34 S fractionation between sulphate and sulphide, approaching 70‰ (Canfield & Thamdrup 1994;, Canfield & Teske 1996). Quite often, the involved, microbe colonies are fossilized in the form of, framboids (microscopic aggregates of pyrite cubes, in shale). Earliest evidence for biogenic sulphate, reduction dates from 2.7 Ga. Earlier in the, Archaean, bacterial sulphur disproportionation, seems to have been the main microbial metabolism (Philippot et al. 2007)., Anaerobic microbial (bacterial) sulphate reduction:, 106ðCH2 OÞ16ðNH3 ÞðH3 PO4 Þ þ53SO2, 4, ðorganic matterÞ, , , , ) 106CO2 þ16NH3 þH3 PO4 þ106H2 Oþ53 S2, ð1:15Þ, Another frequent form of microbial sulphate, reduction occurs at diffuse (“pockmark fields”) or, vent-like locations (“cold seeps”) of methane degassing from sediments or from gas hydrate layers, beneath the ocean floor (eq. 1.16). The process is, very similar to the one described above., Anaerobic microbial sulphate reduction and concurrent methane oxidation:, , , SO2, 4 þCH4 ) HS þHCO3 þH2 O, , ð1:16Þ, , The precipitation of metals and other elements, from exhalations and seawater depends on the, presence of suitable partner ions (e.g. HS) and, pH/Eh conditions. One mode of precipitation, is anion exchange, for example mixing of, dissolved chloride-metal complexes with H2S-rich, seawater, resulting in sulphide formation. Note, that sulphide precipitation results in lowering the, pH (equation 1.17). Where redox-sensitive metals, are transported across zones of different Eh, for, example from the coast into a euxinic marine, basin, the precipitates reflect this by lateral zoning., Zones may comprise oxides near the coast followed, by carbonates, silicates and finally sulphides in the, most reduced environment. Zoning is most noticeable if the same metal is present through all zones, , 93, , as in some iron ore districts. In polymetallic ore, deposits, several subzones may be distinguished, within the major facies zones., Production of acidity by sulphide precipitation:, Cuþ þFe2þ þ 2H2 S, metal ions, , ), , hydrogen sulphide, , CuFeS2, chalcopyrite, , þ0:5H2 ðaqÞþ 3Hþ, acidity, , ð1:17Þ, , 1.3.1 Black shales in metallogenesis, Carbonaceous shales are important host rocks of, sedimentary ore deposits. They are fine-grained, laminated clastic sediments consisting variably of, quartz, carbonate and clay with more than 1% of, organic matter. Global black shale units may, reflect major perturbations of global carbon and, climate cycles, which are controlled by endogenous (tectonics, mantle plume break-out) and, exogenous processes (Emeis & Weissert 2009)., Their sedimentary environment is characterized, by low energy, benthic anoxia and often, by euxinic, conditions (associated with stable reduced sulphur, species in water above the seafloor). As shown, earlier, metal cations dissolved in seawater are precipitated by H2S and HS. The mass of metals, dissolved in modern seawater is very large:, 13,300 Mt molybdenum, 4400 Mt uranium,, 2690 Mt vanadium, 690 Mt zinc and 620,000 t lead., Marine anoxic periods in the geological past may, have temporarily reduced the concentrations to less, than half of these figures (Algeo 2004)., Organic-rich pelites are generally characterized by, elevated contents of redox-sensitive trace elements, (including U, V, Mo, Ni, Co, Cr, Cu, Pb, Zn, Cd, Au,, Ag, As: Morford & Emerson 1999) and phosphate., Many of the enriched elements are essential for life, and their changing concentration in seawater, through geological history influenced the evolution, of life on our planet (Anbar 2008). Some may have, been sourced from continents and entered the sea, adsorbed on detrital organic particles. The import of a, large mass of plant remains into marine black shale, basins is often questioned. Yet, many marine petroleum source rocks produce oil with a considerable, fraction of paraffins, whose main precursors are, waxes and lipids of plants (Hunt 1996). Apart from, continental sources, mid-ocean hydrothermal
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94, , PART I METALLIFEROUS ORE DEPOSITS, , activity, submarine volcanism and extraterrestrial, matter provide parts of the dissolved metal stock., Explanations for preferential metal concentration in, marine black shales include:, . Significant metal enrichment may be acquired, by the special faculty of marine organic matter to, bind trace metals dissolved in seawater (especially, Ni, Co, Cu and Zn);, . in contrast to the first group, V, U, Mo and Cr are, not fixed by organic species but by reduction;, . elements like Ba, P and Cd are imported from, and indicators of a high biological productivity in, the upper, oxic layer of the stratified water, column., , Manganese (and sometimes even iron) is typically, impoverished in black shales relative to ordinary, pelites, because it dissolves in the highly reducing, bottom sediment pore waters and with increasing, consolidation, is flushed out back into the sea. This, reduction is in part due to bacterial activity (Kim et al., 2004). Low Fe-contents (expressed by low Fe/Ti, ratios) inhibit the formation of synsedimentary to, early-diagenetic pyrite, favouring the production of, significant amounts of H2S in anoxic pore fluids. This, may be an important factor controlling the amount of, reduced sulphur available to form ore deposits by, capturing metals from migrating fluids., , Accordingly, most black shales have anomalous metal contents, but these are not economically exploitable. The only current profitable use, of ordinary black shales is the recovery of the, energy inherent in organic matter, either by, direct burning or by the production of synthetic, oil (cf. Chapter 7.7 “Oil Shale”). The most important economic significance of black shales is their, role as the main source of petroleum and natural, gas. Similar to the expulsion of hydrocarbons,, metals mobilized from black shales during diagenesis or metamorphism can form ore deposits., Some former black shales have been transformed, into metamorphic graphite deposits., Yet, there are many large sulphide ore deposits, hosted in black shales (with metals such as Cu, Sb,, Zn, Pb, Ag and Au), which were typically formed, from submarine hydrothermal-exhalative fluids,, but precipitation from average seawater without, , increased hydrothermal input is a plausible alternative. A current example of this scientific controversy is the Chinese Mo-Ni sulphide ore shales, (Shao-Yong Jiang et al. 2006, Lehmann et al. 2007,, Wille et al. 2008; cf. Chapter 2 “Molybdenum”)., Similarly, the European copper shale and the Central African copper-cobalt shale deposits were for a, long time considered to be purely sedimentary, formations. Meanwhile, the scientific consensus, is that both were formed epigenetically by migrating diagenetic, hot fluids (cf. Section 1.4, “Diagenetic Ore Formation Systems”)., The distinction between sedimentary and diagenetic ore deposits is sometimes ambiguous,, both in theoretical classification and in practical, application. The critical difference is the time of, concentration of the considered element: Was it, effected during sedimentation of the immediate, host rocks (by syngenesis) or during later diagenesis (by epigenesis, for example concentration by, migrating formation waters)? Isochemical recrystallization during diagenesis or later metamorphism does not justify a different genetic class,, except that the sedimentary ore may be called, consolidated, lithified or metamorphosed., Sedimentary ore deposits are increasingly, explored and investigated by methods of, dynamic basin analysis, sequence stratigraphy, and palaeogeographic reconstructions, that were, first developed for oil and natural gas exploration, (Einsele 2000). These methods encompass concepts such as distinction between sedexdeposits formed during early rifting of a basin and, others that originated during later thermal contraction. If the stratigraphic, facies and tectonic, evolution of a basin is well understood, it, appears possible to predict prospective localities, and stratigraphic positions of potential ore (Ruffell, et al. 1998)., 1.3.2 Placer deposits, Sedimentary placer deposits are mechanically, formed concentrations of heavy, durable minerals, that may occur in transported soil or regolith, (colluvial), in fluviatile and in coastal sediments., Aeolian placers (Figure 1.61) and placers in glacial, sediments are also known, but rarely of economic
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 95, , Figure 1.61 (Plate 1.61) Aeolian lag, enrichment of magnetite (dark sand patches) at, An Kor, Red Sea Hills, Sudan. Note the, exploration trench testing the Neoproterozoic, primary mineralization in the foreground., , significance. Placer deposits of economic importance are classified as residual, eluvial, colluvial,, fluviatile and coastal (for the latter, terms also, used include marine and beach placer). Geologically young, usually Quaternary placers (DukRodkin et al. 2001) are discerned from fossil placers. Residual and eluvial placers were briefly, introduced earlier., Placers are important sources of gold, platinum, metals, tin, titanium (rutile, ilmenite), zircon, rare, earth elements (monazite) and gemstones (diamond, garnet, ruby). Precondition of the concentration of placer minerals is their mechanical and, chemical durability during weathering and trans-, , port, and their elevated density compared with the, ordinary rock forming minerals (Table 1.4)., Simple washing in a gold pan (Figure 1.62) easily, separates light quartz and feldspar from dark mafic, silicates, garnet, ilmenite and magnetite. This, dark fraction of “black sand” contains the valuable, minerals such as flitters of gold. Note, however,, that the same term black sand is often used to, describe Canada’s giant oil sand resources, which, are definitely not placer deposits., Colluvial placers originate by downslope, creep of soil from weathered primary deposits, (Figure 1.63). Heavy minerals move to the base of, the regolith, whereas lighter and fine-grained
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96, , PART I METALLIFEROUS ORE DEPOSITS, , Table 1.4 Density of common placer and gangue minerals (g/cm3), Feldspar, Quartz, Mafic silicates, Diamond, Topaz, Garnet, Corundum (ruby), Rutile, Ilmenite, Zircon, , 2.5–2.8, 2.65, ca. 3–4, 3.5, 3.5–3.6, 3.6–4.3, 3.9–4.1, 4.2–4.3, 4.5–5.0, 4.7, , Monazite, Magnetite, Columbite, Scheelite, Cassiterite, Wolframite, Cinnabarite, Uraninite, Platinum metals, Gold, , 5.2–5.4, 5.2, 5.2–7.9, 5.9–6.1, 6.8–7.1, 7.0–7.5, 8.1, 7.5–9.7, 12–21.5, 15–19.3, , The density of many minerals varies considerably because of chemical variation. Alloys of native metals vary in composition., , fractions are displaced upwards. Orebodies are, sheet- or channel-like bodies consisting of ore, (e.g. cassiterite) and gangue (quartz) minerals, and, of rock fragments. Colluvial ore minerals at the, foot of a slope can be eroded by rivers and reconcentrated by alluvial processes., Fluviatile, or alluvial placers, Fluviatile, or alluvial placers occur in active, stream channels and in older river terraces, , (Figure 1.63). Heavy mineral concentrations form, at morphologically well-defined sites, mainly, characterized by changes of flow velocity. Such, sites include large boulders, rock bars, gravel beds, on the inside bank of river bends, the downstream, end of gravel and sand islands (point bars), but also, reed patches. Another important trapping mechanism is infiltration of heavy minerals into open, pore space of sand-free pebble banks (Hattingh &, Rust 1993). This mechanism explains exceptionally high gold grades in the ancient Witwatersrand, , Figure 1.62 (Plate 1.62) Panning cassiterite-columbite ore from Ngara pegmatite, eastern Rwanda. Note the small, mass of black ore mineral sand which remains from washing the pan initially filled with ore to the brim.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 97, , Eluvial (residual) placer, Colluvial placer, Fluviatile placers, Figure 1.63 Cross-section of a river, valley near an outcropping primary ore, deposit, which is the source of residual, (eluvial), colluvial and fluviatile placers, (black dots)., , Upper terrace, River, , conglomerates (see below). Saxton et al. (2008), confirmed experimentally that high-grade longitudinal pebble to cobble bar systems trap gold, during their formation from fast-moving suspension flows. Many alluvial placers in mountain, stream valleys comprise the whole volume of the, valley fill (Figure 1.64). Downstream, trapping, sites, such as those mentioned above, control, exploitable grades. Alluvial placers rarely extend, for more than several kilometres along a valley,, because both concentration and grain size of ore, , Lower terrace, , Primary, ore deposit, , minerals diminish with increasing transport, distance., Transport, deposition and density sorting of, minerals in flowing water are controlled by factors, of “hydraulic equivalence”. Originally, this term, was used in the sense of “settling equivalence”,, which refers to the physical principle that the, settling velocity of suspended particles in water, depends on diameter and density (Stoke’s Law,, eq. 1.4). Hydraulic equivalence sensu stricto results in concurrent deposition of small high density, , Figure 1.64 (Plate 1.64) Alluvial placer mining near Ruhanga in the tin-tantalum district of Gatumba, Rwanda. After, extraction, the devastated valley must be restored. Courtesy B. Lehmann, Clausthal.
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98, , PART I METALLIFEROUS ORE DEPOSITS, , grains with larger and lighter ones (e.g. fine-grained, gold in coarse alluvial gravel). Meanwhile it has, been recognized that concentration of heavy, minerals is due to several dynamic processes,, which include lag mechanisms: When river sediment is entrained, high density particles tend to, remain essentially in situ, whereas light particles, are removed downstream (this is called “selective, entrainment” or, linguistically not quite correct,, entrainment equivalence; e.g. James & Minter, 1999). Today, the term hydraulic equivalence is, usually understood to comprise both settling and, entrainment equivalence, and other hydrodynamic factors such as shape, size and surface properties of particles (Carling & Breakspear 2006)., Investigations of fluvial placer formation must, include studies of the particular sedimentary system (Fielding et al. 2009). Physical models of, turbulent mass flow in fast-moving rivers support, genetic understanding (“granular physics”, Frey &, Church 2009). Extensive pebble to cobble longitudinal bar systems of braided rivers at Witwatersrand, South Africa (gold) and Elliot Lake, Canada, (uraninite) are believed to have originated in such, an environment. Enrichment of the heavy minerals is caused by their tendency to accumulate near, the base of a flow where they are easily trapped, (Saxton et al. 2008)., Often, the highest concentration is located at, the base of the valley fill directly on bed rock, but, ore is also found above beds of fine-grained or, cohesive ground between pebble beds (“false bed, rock”). This may be caused by two different, mechanisms:, 1 The heavy minerals gradually work themselves, to the bottom of a pebble bed (this is the hypothesis, of an “active bed”, Bilibin 1938); or, 2 a perpetual and complete reworking of the pebble bed is assumed so that selective entrainment, concentrates heavy minerals at the base (“lag, deposit”)., The second explanation appears to be more, likely, because floods provide the main input of, energy into erosion and transport processes of river, valleys. When exploring for placer deposits, or, sampling river sediments for other purposes, it is, always advisable to invoke practical aspects of, , hydraulic models. In addition to hydraulics, chemical and biochemical processes in valley sediments mobilize and reprecipitate elements,, including native gold (Reith et al. 2010). Chemical, mobility of gold in placers is suggested by both, the occurrence of very heavy nuggets that could, hardly be moved by flowing water and by the, occasional observation of idiomorphic, undeformed gold crystals in river sediments:, Today, geologically young alluvial placers are mainly, exploited for gold. Their economic role is feeble,, however, compared to the time of the great gold, rushes in California, Alaska and Australia in the 19th, century. Among fossil alluvial placers, the Archaean, Witwatersrand Basin in South Africa hosts by a wide, margin the largest gold resources of the world., Because of the costs of increasingly deep mining,, however, its share of world gold production is steadily, decreasing. It is one of the world’s most intensively, investigated placer districts (for more details refer to, Chapter 2.3 “Gold”). One remarkable result of the, research is the reconstruction of the palaeohydrologic, situation (Figure 1.65a) showing braided streams,, pebble diameters and flow vectors of one of the gold, conglomerate beds. Figure 1.65b demonstrates that, higher gold contents occur in the middle section of, the fan, whereas both marginal (entry front) and, basinal areas contain less gold. Geological mapping, and precise determination of fluvial facies regimes are, essential contributions to a rational exploitation of, these superdeep deposits., , The source of ore minerals in placers is an, exciting question, both for the practician posing, questions such as “Can we locate the primary, source?” and for scientists. One way is to estimate, the mass balance between erosion and uplift in the, source region in comparison to the depositional, system. Application of this approach reveals that, the mass of gold in the Witwatersrand District can, be explained without invoking exceptionally high, gold contents in the former source region (Loen, 1992)., Coastal placers, Coastal placers are mainly formed in surf zones. In, contrast to typical alluvial placers, both light and
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , a, , 99, , b, , N, , N, , Basal, Steyn, , Basal, , y, , en t, , fron, , ry, , 30 mm, , entr, , n, Stey, , 20 mm, , t, , f ro, nt, , 40 mm, , Low gold, , Pebble size, 5 km, , Mean flow vector, , 5 km, , High gold, , Figure 1.65a,b Facies control of gold deposition in the braided alluvial fan complex of the Basal and Steyn Reefs, in the Steyn Mine, West Rand Goldfields, South Africa (after Minter et al. 1986). Courtesy Geological Society of, South Africa., , heavy minerals occur roughly in the same grain, size that is almost exclusively sand. Obviously, the, hydraulic equivalence sensu stricto has no role in, this environment and the critical factor is mainly, entrainment equivalence. The incoming surf imposes a turbulent regime that transports suspended, sand including heavy minerals towards the beach., When a wave runs out, sand grains settle for a, moment. Return flow to the sea is laminar and, only light minerals can be entrained. Heavy minerals are enriched in a narrow linear strip along the, beach. Other coastal processes may enhance placer, formation including tides, lateral currents, wind, and especially storms that induce higher waves, (Roy et al. 2000, Hoefel & Elgar 2003)., Coastal placers are more frequent on stable, coasts, because neither strong erosion nor rapid, , sedimentation is propitious. Single orebodies are, elongated narrow lenses or strips of dark sand that, may extend over hundreds of kilometres. In some, cases, the connection of placers with a river mouth, delivering sediment is obvious, but erosion of older coastal strata may also be a source. Sea level, changes produced either submerged placers (e.g., during Quaternary glacial conditions, the sea was, >100 m below the present level) or deposits above, today’s sea level associated with the raised beaches of Pleistocene interglacial and older climate, epochs warmer than today. Recently, a major new, province of marine placers was discovered in the, Murray Basin of interior Australia. The placers, occur in 400 km long barrier complexes as beds, and narrow “shoestrings”, which formed in a very, shallow transgressive Pliocene inland sea. The
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100, , PART I METALLIFEROUS ORE DEPOSITS, , barriers developed across more than half of the, basin. Many promising coastal placer deposits of, coarse-grained (90–300 mm) quartz and the heavy, minerals rutile, zircon, ilmenite and altered, ilmenite (leucoxene) have been outlined. In, lower-energy environments, only low-grade and, fine-grained (40–80 mm) heavy mineral sands were, deposited. The concentration is attributed to large, storms and consequent ground swells, which may, have eroded pre-enriched underlying Miocene, sands (Roy et al. 2000)., Coastal placers can consist of up to 80 wt.%, heavy minerals. Their contribution of rutile and, zircon to world markets is of high economic importance. They are lesser sources of diamond and, cassiterite. Gold and platinum are rare in coastal, placers. Iron ore breccias (“detrital iron ore”), hosted by Early Cretaceous transgressional sediments in Northern Germany constitute a rarely, mentioned type of coastal placers. In this region,, oxidized pyrite nodules of emerged Jurassic shales, were swept by the surf into depressions that developed due to salt diapirism and subrosion. The ores, were low-grade and mining ceased with increasing, competition by overseas imports., 1.3.3 Autochthonous iron and manganese, deposits, Autochthonous iron and manganese ores are, chemical, partly biogenic marine sediments., Today, terrestrial iron ores (including recent bog, iron ore, or after diagenesis siderite in black coal), are economically insignificant. In the future, manganese nodules and crusts of the deep oceans, may become an essential source of metals. For, now, the most important raw materials of this, group are enriched parts of marine banded iron, , Fe-oxid, es, , Sedim, ents fine-grained, , coars, , e-gra, , She, , ined, , rgin, , 3000 m, 10 km, , Banded iron formations (BIF, including the varieties itabirite, jaspilite, cherty iron formations), are layered, banded (0.5–3 cm) and laminated (<1, mm) rocks containing >15% iron. Essentially,, BIF consist of quartz and magnetite layers that, form sedimentary units reaching lateral extensions of thousands of kilometres and a thickness, of hundreds of metres. Geological setting and, associated rocks allow a subdivision of BIF into, three types:, 1 Algoma type in submarine volcanic settings;, 2 Superior type in marine shelf sediments; and, 3 Rapitan type, which is closely related to, glaciogenic marine sediments (James & Trendall, 1992)., Banded iron ores of Algoma type are especially, common in Archaean greenstone belts, but occasionally, geologically similar deposits formed in, younger periods (Figure 1.61). The oldest examples, are known in the Isua Belt on Greenland (Whitehouse & Fedo 2007; 3800 Ma). Host rocks of Algoma ore are volcanogenic sediments such as, greywackes, tuffaceous and magmatic volcanic, rocks. Typical ore beds are less than 50 m thick and, may display a transition from oxide through carbonate and silicate to sulphide facies. Oxide facies iron, occurs as magnetite, haematite is rare (Figure 1.66)., Algoma type iron formations extend to several, thousand square kilometres, which is quite small, compared with Superior type basins. In several districts, numerous exhalative centres with sulphides, , nates, , e-gra, , Mafic, , Banded iron formations, , Mafic, , Fe-carb, o, , coars, , lf m, a, , and manganese formations (predominantly formed, in the Palaeoproterozoic), and ooidal or massive, iron and manganese ore beds that are of much, younger, Phanerozoic age., , ined, , volc, anic, s, , Iron formation, , Felsic, pyroclasti, cs, , volcanics, Fe-sulfides, fine-grained, , Basin, , Figure 1.66 Stratigraphical, context and facies zones of an, Algoma Type volcanogenic iron, formation in the Archaean, Michipicoten Basin, Canada., Modified from Goodwin, A. M., 1973, Society of Economic, Geologists, Inc., Economic, Geology Vol. 68, Figure 2 p. 919.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , and some larger VHMS deposits punctuate Algoma, BIF horizons (McClenaghan et al. 2009). This observation, and their close association with volcanism,, suggest formation on the seafloor by exhalative,, hydrothermal-sedimentary processes driven by, heat anomalies. Magmatic fluids may also participate in ore formation. Consequently, Algoma type, ore is essentially volcanogenic (cf. Section 1.1, “Volcanogenic Ore Deposits”)., Banded iron ores of Superior type (Figure 1.67) are, hosted in sedimentary sequences that include, black, organic-rich and silicic shale, quartzite and, dolomite, and transgress older basement. Bimodal, volcanic rocks (rhyolite and basalt) may occasionally form part of the country rocks. This indicates a, sedimentary environment of stable continental, shelves covered by relatively shallow seas with, some extensional tectonic strain and associated, volcanism. Shallow, coastal and deep basinal facies, can be recognized (James & Trendall 1992)., , 101, , Iron formations of the Superior type are marine, sediments of global extension. They are preserved, in remnants of marine basins that reach tens of, thousands of square kilometres. Banding and lamination are remarkably persistent but not all BIF, are strictly autochthonous. Chert-free, massive, iron sediments locally replacing bedded BIF are, interpreted to reflect synsedimentary sifting and, deposition by density currents (Lascelles 2006)., Truncated beds, flow ripples and synsedimentary, deformation structures are recorded. Commonly,, BIF contain iron in oxide, carbonate and silicate, phases; sulphides are rare. Iron and SiO2 were, deposited as fine-grained ooze (or as a gel), but, ooids, concretions and biogenic textures have been, observed. Primary precipitates were probably, amorphous SiO2, ferric hydroxide Fe(OH)3 and, nontronite (near landmasses, oxic conditions), or, precursors of magnetite, siderite and greenalite, [(Fe)2-3Si2O5(OH)4] (far from land; in deeper,, , Figure 1.67 (Plate 1.67) Folded and metamorphosed Superior type banded iron formation near Mt Tom Price, mine in the Hamersley Gorge (Karijini National Park, Western Australia) with marine scientists Aivo, Lepland and Mark van Zuilen kindly posing for scale. Iron-rich beds black, silica (jasper) red. Photograph, by Aivo Lepland, courtesy Geological Survey of Western Australia.
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102, , PART I METALLIFEROUS ORE DEPOSITS, , dysoxic-anoxic basins). Diagenesis and common, low-grade metamorphism produced the ordinary, paragenesis of BIF ore, comprising quartz, haematite, magnetite, siderite, minnesotaite [(Fe,Mg), 3Si4O10(OH)2], stilpnomelane, asbestiform amphiboles and many other Fe-rich silicates. Primary, ores have an Fe2O3/SiO2 ratio of 0.98–1.26, typically 25–45 wt.% Fe, <3% each of Al2O3, MgO, and CaO, and small contents of Mn, Ti, P and S., This discriminates Superior type BIF from other, marine sedimentary iron ores. At sufficiently high, , BOX 1.9, , magnetite grades, the primary ores are exploited, as “taconite”. Most BIF-based iron ore mines,, however, extract parts of iron formations that, were enriched either: i) by supergene processes, typically resulting in “martite-goethite ore”, with 60–63 wt.% Fe; or ii) by hypogene hydrothermal processes forming “high-grade haematite ore”, with 60–68 wt.% Fe (cf. Section 1.2, “Supergene Enrichment”). Martite is a term that, denotes haematite pseudomorphs replacing, magnetite., , The origin of Superior type iron formations – still, disputed, , Superior type BIF occur exclusively in the Late Archaean and Early Palaeoproterozoic (2.6–1.8 Ga). Apart from the, formation of giant iron and manganese deposits, the Archaean-Palaeoproterozoic transition includes other remarkable, events of Earth history:, . The Lomagundi-Jatuli carbon isotope excursion (Melezhik et al. 2004);, . the Earth’s earliest (Huronian) glaciation;, . the first global appearance of terrestrial “red beds”;, . the earliest abundant calcium sulphate sediments; and, . the Shunga event, a giant accumulation of Corg-rich sediments., The common cause of these changes is probably the rapid expansion of photosynthetic eukaryotes, which was triggered, by enhanced nutrient supply and the first formation of an ozone shield in the atmosphere. Most scientists relate the, precipitation of the giant mass of iron contained in BIF to the transition of oceans and atmosphere from a reduced to an, oxidized state (the “Great Oxidation Event” between 2.45 and 2.2 Ga: Holland 2002, 2005; Anbar et al. 2007, Anbar, 2008, Kump 2008, Qingjun Guo et al. 2009). In the Archaean before oxidation, PO2 of the atmosphere was <10–5 PAL, (present atmospheric level). Other scientists, however, suggest that oxidized seawater existed much earlier (Ohmoto et al., 2006, Kesler & Ohmoto 2006)., Sources of iron in BIF may have been:, . submarine-exhalative systems (Kimberley 1994, Morris 1993) similar to today’s mid-ocean ridges;, . submarine-exhalative systems associated with intraplate tensional tectonic structures and volcanoes (Bekker et al., 2010, Barley et al. 1997); and, . continental weathering., Probably, all these sources contributed iron at different times and locations (Alibert & McCulloch 1993). However,, worldwide trace metal contents of Superior type BIF are very low (Morey 1992), arguing against local hydrothermal, sources and for homogenized ocean water. Sources of SiO2 in the chert bands were most probably weathering landmasses, (Hamade et al. 2003). It is assumed that the ancient oceans were saturated with dissolved silica (as Si(OH)4), resulting in a, steady rain of siliceous matter to the ocean floor. The modern oceans are markedly undersaturated in silica and this may be, the main reason for the changing character of iron ores, from ancient BIF to more recent ironstones. The banding in BIF can, be explained by diurnal to seasonal cycles of biological activity, to temperature fluctuations (Posth et al. 2008), or to, episodic depletion of ferrous iron in the water column., Essential parameters of the formation of Superior type BIF are still discussed, apart from the basic agreement that the ores, are chemical or biogenic precipitates from seawater (eq. 18)., Oxidative precipitation of dissolved reduced iron forming banded iron formations:, 2Fe2þ þ0:5O2 þ5H2 O ! 2FeðOHÞ3 þ4Hþ, , ð1:18Þ, , Many scientists support the following hypothesis: “The atmosphere may have been nearly free of oxygen, while the, oxygen in the oceans started to increase. Seasonal blooms of the earliest photosynthetic micro-organisms (cyanobacteria), increased oxygen concentration in seawater that oxidized and precipitated dissolved Fe2þ.”
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 103, , Recently, this was both confirmed (Qingjun Guo et al. 2009) and vividly contradicted (Ohmoto 2003a, Ohmoto et al., 2006). The latter cast doubt on both a low-oxygen atmosphere and the late appearance of cyanobacteria, and propose a, euxinic model, with precipitation of iron at the contact between reduced deep water and O2-rich surface water., Widespread euxinia is confirmed by sulphur isotope and iron speciation data (Reinhard et al. 2009). Under anoxic, conditions, BIF deposition may have been catalysed by anoxygenic photoautotrophic bacteria (Kappler et al. 2005). A, third possible precipitation mechanism is abiogenic UV-induced oxidation (Bekker et al. 2010). Iron isotope investigations support the concept of a redox-stratified ocean (Rouxel et al. 2005) and of an important role of early diagenetic, dissimilatory iron reduction (Johnson et al. 2008). The worldwide termination of Superior type BIF deposition at 1.85 Ga, may have been caused by the Sudbury impact: Slack & Cannon (2009) suggest that the bolide’s oceanic impact destroyed, redox layering in the oceans, which impeded dispersion of dissolved Fe(II) from ocean-floor hydrothermal vents. In their, thorough review, Bekker et al. (2010) conclude that no single parameter explains BIF formation and suggest a control by, multiple environmental changes of earth systems., , Manganese formations, In the Transvaal Basin of South Africa, marine, manganese oxide strata (manganese formations,, MnF) of the ca. 2.22 Ma Hotazel Formation occur, interlayered with Superior type BIF, which hosts, important iron ore deposits. The outstanding feature is, however, that this province (the Kalahari, manganese field: Maynard 2010) comprises about, half of the world’s manganese resources. The concentration of Mn relative to Fe is believed to have, been effected by earlier abstraction of iron from, seawater and precipitation of BIF. In consequence,, the more soluble manganese was relatively enriched and only when PO2 continued to rise, Mn2þ, was oxidized to Mn3þ and manganese-rich beds, were deposited. Note that similar to Superior type, BIF, manganese formations are only exploitable if, enrichment processes acted on protore (cf. Chapter, “Manganese”)., Banded iron formations of Rapitan type were, deposited immediately following the Sturtian, (730 Ma) and the Marinoan (630 Ma) glaciations,, during the Cryogenian System of the Neoproterozoic. The name Rapitan is derived from a Sturtian, locality in the Mackenzie Mountains, Northwest, Canada. Similar occurrences were found in Brazil, (Urucum: Walde & Steffen 2007) and in Namibia, (Otjosondu). During these glacial events, land and, even oceans were widely covered with ice, (“Snowball Earth” hypothesis: Hoffman et al., 1998), causing a nearly global anoxia and the presence of dissolved ferrous iron in seawater. Interand post-glacial melting of the ice caps resulted in, , formation of glacial sediments, re-oxidation of the, oceans, and precipitation of ferric iron and manganese oxides. As marine sediments, Rapitan type, iron formations are similar to Superior type BIF,, but have little economic significance., Oolitic iron ore, Oolitic iron ore was exploited from approximately, 500 deposits worldwide. Formation of this ore type, is restricted to Phanerozoic time. Economically, outstanding historic mining districts in the Jurassic of western Europe, and those in the Palaeozoic, of the northern USA led to the distinction of, “Minette type” (limonitic) and “Clinton type”, (haematitic). Always, these ores are sediments of, shallow epicontinental seas or of shelf-regions and, are interbedded with clastic sediments. Ore beds, reach a thickness of about 30 m and lateral extensions up to 150 km. This suggests that eustatic, highstands are favourable conditions for their formation, as shown by Young (1992) for the Ordovician deposits of the West-Mediterranean province., Oolitic iron ore (Figure 1.68) is not banded and, contains no primary colloidal silica. Textural varieties include mainly the namesake round accretionary bodies called ooids (earlier termed, “ooliths”) and pisoids, but also fine-grained iron, ore particles forming massive “ironstone” sensu, stricto. Precipitation of ooids in agitated water is, indicated by many observations, including intraclasts of oolitic rock in ooidal matrix, fractured, and freshly mantled ooids, cores of ooids formed by, quartz, phosphorite and fragments of fossils,
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104, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.68 (Plate 1.68), Haematitic iron oolite ore, formed in a Late Cretaceous, marine embayment at Aswan,, Upper Egypt., , oblique and cross-bedding. Iron oolite rock of ore, grade has closely packed ooids, but host rock, bituminous marls of the minette enclose widely, disseminated ooids. The ore beds are often connected with rich fossil communities. Fossils are, typically replaced and cemented by iron ore minerals. A lateral differentiation of iron facies (e.g. oxic,, carbonate, silicate) in oolite beds is rarely clear., Single ooids consist of goethite, haematite, chamosite and siderite (with very little magnetite and, pyrite) that alternate in growth zones or in crosscutting metasomatic zones. Compared to the, crustal average, iron and phosphorus are enriched, by a factor of 6 (Fe to 34%, P to 0.6%), similar to, Mn, As, V and other elements. An apparent paradoxon is the high percentage of reduced iron in, ooids that were formed in oxygen-rich seawater, as, proven by the associated fossils., The coincidence of both sedimentary and diagenetic textures caused very different genetic interpretations. Sorby (1857) thought that originally, aragonitic oolitic rock had been replaced by iron, ore after lithification. However, many observations favour a synsedimentary supply of iron followed by chemical and biochemical precipitation, (Dahanayake & Krumbein 1986). This is supported by many modern submarine studies. Sturesson et al. (2000) observed the formation of iron, , ooids on the submerged flank of an island volcano, in the Philippines, where local seawater is enriched with dissolved Fe-Al-Si and oxic ooids are, formed. The source of the iron could not be determined; the authors think that submarine hydrothermal springs or ash falling into the sea, or, leaching of recent lava flows, are equally probable, alternatives. In recent marine sediments of the, Niger delta, Porrenga (1967) described the presence of goethite pellets in the uppermost 10 m of, sediment, underlain by 50 m of chamositic, sediment., The unlithified sediments at the origin of oolitic, iron ore consisted of ooids, pisoids and peloids that, originated by rolling on the seafloor. The matrix, between these larger grains consists of detrital, minerals, fossils, organic material and finegrained iron-bearing kaolinitic clay (M€, ucke &, Farshad 2005). Subsequent diagenesis, mostly, under reducing conditions, caused by bacterial, decomposition of organic matter, typically leads, to transformation of kaolinite into chamosite (an, iron-rich chlorite) and neoformation of pyrite and, siderite. Finally, many deposits were upgraded by, oxidation caused by meteoric seepage waters., The source of iron in marine oolitic iron ores, were probably lowlands experiencing intensive, weathering in tropical climates, where humidity
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , and profuse plant growth produced wetlands discharging river water rich in dissolved organic matter. The Amazon River, for example, annually, exports 70 Mt carbon to the sea. Such waters are, acidic (pH <5) and dissolve reduced iron (Fe2þ)., Oxidized iron (Fe3þ) may be adsorbed to organic, matter and clay (either in colloidal or particle size), suspended in the water. Transport of siliciclastic, sediment in these rivers must have been very, limited, because this would have diluted the iron., Mixing of river water with seawater (pH >7) provokes immediate precipitation of iron oxy-hydroxides. Other possible iron sources that have been, considered are coastal submarine groundwater, springs (James 1966) and seawater flowing up from, a deep CO2 zone, where iron is soluble, into coastal, waters where it will precipitate (Borchert 1960)., Even hydrothermal sources rising from great depth, have been invoked (Kimberley 1994)., Because of the availability of higher quality and, inexpensive BIF ore, few oolitic iron ore deposits, are exploited today. In contrast, genetically equivalent manganese ores occupy a central role in the, world economy. Nearly one half of the primary, manganese supply is produced from this type of, deposit, mainly located in the Paratethys seaway, of Eastern Europe and in northern Australia., Oolitic manganese ore, Oolitic manganese ore occurs in seams within, sediments of epicontinental seas, mainly associ-, , 105, , ated with clay, marl and sand (rarely carbonates)., The largest province of these ores is the South, Ukrainian basin of Eocene-Oligocene age, with, many mines around Nikopol. In this region, the, sedimentary succession is transgressive, starting, with palaeosoil developed on Precambrian crystalline rocks, covered by coal-bearing limnic sediments and finally glauconite-rich sands that, announce the sea. The manganese horizon occurs, within clastic marine sediments (Varentsov and, Muzylev 2001, Figure 1.69):, The manganese seam of Nikopol reaches a thickness, of 4.5 m and an extension along strike of 250 km. A, marginal facies of ooids, concretions and earthy, masses of manganese oxy-hydroxides (pyrolusite and, psilomelane) gives way to a carbonate facies of rhodochrosite and manganocalcite at deeper basinal levels and finally to bluish-green clay with Mn, concretions. Sedimentary manganese deposits elsewhere display a similar zonation, for example those of, Cretaceous age on Groote Eylant, Australia (Bolton, et al. 1990). Note that the Australian ores have a, complicated history, with sedimentary features overprinted by diagenesis and later supergene, lateritic, alteration., , Similar to ironstone and oolitic iron ore, the, metal source for oolitic manganese deposits may, have been continental weathering (Frakes & Bolton 1992) or ocean-floor hydrothermal venting, (Maynard 2010). In principle, manganiferous particles may flocculate and form an enriched bed,, , S, , N, Sand, , Silt, sand, and clay, , Clay and, marlstone, , 10 km, Oxide manganese ore, Carbonate-oxide, manganese ore, , Clay, marl, and siltstone, , Sand, and clay, , Carbonate, manganese ore, , Coal, , Palaeosoil, , Precambrian, basement, , Figure 1.69 Generalized section of facies zones in the Eocene-Oligocene marine-sedimentary manganese horizon in, southern Ukraine (adapted from Varentsov & Muzylev 2001).
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106, , PART I METALLIFEROUS ORE DEPOSITS, , but a more convoluted path to enrichment is, assumed: When the particles settle into the, reduced lower section of a restricted sea, they are, dissolved, either abiotically or supposedly more, often, by microbial anaerobic methane oxidation, (Beal et al. 2009; eq. 1.18)., , (and P: Maynard 2010). Plate tectonics, basin configuration and climate cycles control ocean redox, conditions favourable for manganese precipitation, (Frakes & Bolton 1992)., , Reduction and dissolution of manganese by, microbial methane oxidation:, , Manganese nodules (Figure 1.70) occur over wide, expanses of deep seafloor below the carbonate, compensation depth (CCD). In these areas, pelagic, sediment (mainly radiolarian ooze) accumulates, at a very small rate. In shape and size, the Mn, nodules are compared to potatoes but, of course,, smaller and larger specimens do occur. Cobaltiferous manganese crusts cover submarine rock outcrops of mid-ocean ridges and of oceanic intraplate, volcanoes. Proximity to submarine hydrothermal, fields is not essential, although these are surrounded by Mn-rich sediments (e.g. manganese, mud mounds of low-temperature exhalations near, the Galapagos Islands, the manganese umbers, (brown earthy pigments) and Mn-SiO2 exhalites, of many ophiolites)., Economically prospective manganese nodule, fields include the Clarion-Clipperton-Zone in the, Pacific, the Peru Basin and the central part of the, Indian Ocean. Nodules contain 29 wt.% Mn, 5%, iron, 1.2% copper, 1.37% nickel, 1.2% cobalt and, 15% SiO2 (Lenoble 1996). Minor contents of other, , 2þ, CH4 þ4MnO2 þ7Hþ ! HCO, 3 þ4Mn þ5H2 O, , ð1:19Þ, This process leads to enrichment of anoxic deep, water with dissolved manganese (Mn2þ) while, iron and other easily reduced metals are precipitated by hydrogen sulphide. Interaction of the Mnenriched deep water with oxygen-rich marginal, zones induces precipitation of Mn oxides, especially during transgressive phases (Roy 1992). This, situation is demonstrated by the recent Black Sea, with its high Mn2þ/Fe2þ ratio of deep, highly, reduced euxinic water and MnO2 precipitation in, shallow mixing zones with oxic surface water (Force, & Maynard 1991). A second viable path to Mnenrichment is the oxygen minimum zone (OMZ), model; an OMZ develops below high productivity, seas but deep ocean water underneath is oxic, not, euxinic. Upwelling OMZ water precipitates Mn, , Manganese nodules, , Figure 1.70 Seafloor, aspect of polymetallic, manganese nodules in the, Clarion-Clipperton-Zone, (CCZ) of the Central, Pacific. The density is here, about 20 kg/m2. Individual, nodules have diameters of 3, to 5 cm, the matrix is, radiolarian ooze. Courtesy, P. Halbach (FU Berlin).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , metals including platinum are potentially recoverable. Mn nodules have a botryoidal or smooth, surface and consist of concentric shells of amorphous and crystalline manganese and iron oxihydroxides. Formation of the nodules is partly, by in-situ precipitation from pore fluids, (“diagenetic”) and partly by attachment of colloidal particles or dissolved matter from seawater, (“hydrogenetic”; Hein et al. 2005). In both cases,, microbial intervention may be essential. Manganese crusts grow mainly by hydrogenetic processes. Nodules and crusts have remarkable, contents of valuable metals. The source of these, metals are most likely hydrothermal exhalations, at mid-ocean ridges, but submarine weathering of, Hess Crust serpentinites, extraterrestrial dust and, input from land to the oceans may contribute part, of the endowment., , 1.3.4 Sediment-hosted, submarine-exhalative, (sedex) base metal deposits, Sediment-hosted sulphide deposits formed by, hydrothermal outpouring on the seafloor compete, with porphyry deposits for the role of the largest, base metal accumulations on Earth. Clearly, systems forming these ores must be both of large scale, and geologically common. Earlier, we have noted, that hydrothermal-sedimentary sulphide ores, , BOX 1.10, , 107, , occur in a continuum from: i) a proximal position, to submarine volcanoes; to ii) more distal positions where only sparse ash layers in the sedimentary column point to synchronous volcanism; and, iii) to purely sedimentary settings. Only the cases, ii) and iii) are referred to as sedimentary-exhalative, or short, sedex type ore deposits. The first are, volcanogenic (cf. Section 1.1)., Distal to volcanism are the locations of Early, Proterozoic HYC-McArthur River, Australia and, of the Devonian Rammelsberg deposit in Germany (Large & Walcher 1999). Some of the giant, Mississippian Zn-Pb-Ag deposits of the Red Dog, district, Alaska, are close to igneous rocks but, genetically unrelated (see below). Mesoproterozoic, Sullivan, BC, Canada, was earlier thought to be, purely sedimentary without any magmatic influence (“Sullivan type sulphide deposits”). The choice, was not opportune, however, because deep magmatic bodies may have controlled ore formation at, Sullivan and the discussion appears to be still open, (Slack et al. 1998). Hardly any traces of volcanism, occur near the Pb-Zn-Ag orebodies within Palaeoproterozoic carbonatic-evaporitic black shales of, Mt Isa, Australia, and in the Early Carboniferous, carbonate host rocks of Irish deposits., Sedimentological investigations indicate that, the sulphides were deposited in a geologically very, short time. The clastic sedimentation rate in the, host basins was certainly not exceptionally low,, , Submarine-exhalative (sedex) base metal deposits, , The term sedex implies syngenetic origin of ore and host rocks. Because this cannot always be clearly established, “clasticdominated lead-zinc ore” was instead proposed by Leach et al. (2010). The characteristic setting of these deposits is, submarine rifting within epicontinental seas. Orebodies are typically related to faults, synsedimentary tectonic activity, and the formation of local subbasins. Pluriphase extensional strain, reefs, sediment-starved restricted basins and distal, volcanic centres may be part of the environment (Figure 1.71). Many hydrothermal-sedimentary sulphide ore deposits are, close to or enclosed in black shales indicating a state of partial (dysoxia) or severe oxygen depletion (hypoxia to anoxia), of bottom waters. The correlation may be caused by incidental outpouring into a euxinic environment (i.e. anoxic, highly, reduced, H2S stable), or by “poisoning” of a marine sub-basin by profuse amounts of hydrothermal metalliferous fluids., Some sedex ore deposits are hosted by carbonatic silt- and claystones. Cherts are often part of the host sediments., Sedex orebodies are stratiform and stratabound, and consist of massive, laminated or banded sulphides (Figure 1.72),, with varying admixture of clastic matter, barite and other exhalites (SiO2, haematite, etc.). In appearance they are very, similar to volcanic-hydrothermal exhalative (VMS) orebodies; the decisive difference is the sedimentary as opposed to, the volcanic genetic setting. Whereas the first are principally controlled by volcanic point heat sources, sedex ore is, commonly localized by extensional faults (e.g. basinal growth faults). Single ore beds are either monomineralic or simple
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108, , PART I METALLIFEROUS ORE DEPOSITS, , Terrigenous material, , Eu, xi, ni, c, , m, for ents, t, a, Pl dim, se, ef, Re, , Sulfides, Black, shale, , Older, basement, , Post-rift basin, 1, , 2-5 km, , Syn-rift sediments, 2, , Figure 1.71 Schematic sketch of the geological setting of an exhalative-sedimentary (sedex) sulphide (-barite), ore deposit in a black shale basin. Note distal rift-related basaltic and felsic volcanic centres., , Figure 1.72 (Plate 1.72), Undeformed shale-banded, copper-zinc dominated sulphide, ore from Rammelsberg sedex, deposit (Germany) displays, ductile soft-sediment, deformation and crosslamination. Width of image, 20 cm. Courtesy B. Lehmann,, Clausthal.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 109, , mixtures of galena, sphalerite, pyrite and pyrrhotite, rarely including more than traces of arsenopyrite and copper, sulphides. Beds display sharp boundaries with each other, whereas in the same bed, lateral facies changes include, decreasing metal grades and often, a metal zonation (e.g. copper in the centre, surrounded by annular zones of Pb, Zn, Ba, and Mn). Elevated pyrite contents and geochemical manganese enrichment are found in a wide perimeter of the deposit., Initial ore precipitates were extremely fine-grained (<5 mm), often with colloidal organo-metallic phases. Coarser grain, size required for economic ore processing is a consequence of diagenesis, orogenic deformation and metamorphism. The, unmetamorphosed Palaeoproterozoic HYC deposit, McArthur River district in northern Australia, provides an interesting, case history. First reported in 1880, the high-grade but very fine-grained mineralization was technically not exploitable., Serious exploration was only done 80 years later (Logan et al. 1990). Total resources of 125 Mt of ore with 13% Zn and, 6% Pb were defined. Again, 30 years later in 1992, a mine was established. New milling and metallurgical methods allow, profitable exploitation of reserves comprising 27 Mt at 14% Zn, 6.2% Pb and 63g/t Ag., Structures and textures of sedex ores confirm the synsedimentary formation by fluids emanating from the seafloor. They, include fine lamination, graded bedding, the frequent presence of framboidal pyrite, colloform banding, soft sediment, deformation and mass transfer by submarine mudflows (as at HYC: Ireland et al. 2004). Fossil hydrothermal vents, resembling those of black smokers were found at Silvermines and Tynagh in Ireland providing further proof for exhalative, processes. Below the soft sediment surface, early diagenetic replacement and zone refining may take place simultaneously, as at HYC (Ireland et al. 2004) and the Red Dog district (Kelley & Jennings 2004). This leaves features of, epigenesis that must be weighed against arguments for syngenesis and interpretations may differ as at HYC (Symons 2007)., Hydrothermal alteration is only visible where footwall rocks with advection flow paths are exposed (e.g. the “Kniest” at, Rammelsberg, Germany, or vein breccias below the Red Dog orebodies, Figure 1.73). If subjacent flow channels cannot, be found, heavy hot metalliferous brines may have flowed from elevated outflow points into nearby depressions on the, seafloor (Sangster 2002)., Superimposed barite, and Sulphide, , Sulphide and barite, separate, , Sulphide, only, , Siksikpuk Formation, (red and green shales), , Carbonate platform, Sulphide, , Barite, , Kuna Formation, (black shale/, mudstone), , Barite, Veins, , Volcanics, , Sulphide, Carbonate turbidites, , Endicott Group, (red bed silt- and, sandstone, conglomerate), , 100 m, , oxic, anoxic or, dysoxic, , 2 km, , Figure 1.73 Horst-and-graben architecture of the Late Mississippian Brooks Range Basin, Alaska, hosting the, giant Red Dog Zn-Pb-Ge-Ag massive sulphide (and barite) sedex deposits. Modified from Kelley, K.D. & Jennings,, S. 2004, Society of Economic Geologists, Inc., Economic Geology Vol. 99, Figure 4 p. 1270., , but the sulphide deposition rate was very, high. Only this allowed the formation of massive, sulphide rocks that are common in this, environment., The source of metals in sedex ore deposits is, sought in either the sedimentary basinal rocks, (diagenetic) or in the basement underneath, (retrograde-metamorphic). Different ratios of, , Pb-Zn-Cu in mining districts indicate varying, lithochemical source material, but also different, processes or conditions of metal leaching. Lead, isotope data may help to identify the source, rocks of metals. Fluids involved in ore formation, include:, . deeply convecting seawater and evaporitic brine;, . occluded formation fluids (Slack et al. 1998);
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110, , PART I METALLIFEROUS ORE DEPOSITS, , diagenetic water of basinal sediments; and, fluids mobilized from a deep source (e.g. elevated, heatflow from the mantle)., Rising mantle fluids were invoked for giant Broken Hill, Australia by Plimer (1985). Most sedex, deposits were generated by migrating formation, fluids and seawater or brine fed into convection, cells. Convection cells may reach many kilometres, downward below the floor of a basin. The metalliferous fluids are either oxidized (SO4-rich) or, reduced (H2S-rich), depending on the nature of the, basin fill. Oxidized, neutral or weakly acidic fluids, originate in sediments that are dominated by carbonates, evaporites and haematitic sandstones (e.g., McArthur River, Red Dog), whereas dark shale and, sandstone produce reduced and acidic fluids (Rammelsberg, Cooke et al. 2000)., The geodynamic environment of submarine,, hydrothermal sediment-hosted sulphide deposits, is different from Kuroko and Cyprus types, which, form near plate boundaries. Sedex deposits occur, typically at within-plate locations, such as sedimentary epicontinental shelf regions grading into, passive continental margins. This environment, combines tensional tectonics and crustal thinning, with high heat flow and thermal basin subsidence, that all favour fluid convection (Betts et al. 2003)., The giant Alaskan Red Dog sedex deposits with, approximately 40 Mt of contained lead and zinc, illustrate these observations (Figure 1.73). The, Kuna formation hosting the orebodies is intruded, by igneous rocks including mafic sills and rhyolite, plugs. Yet, the source of the hydrothermal oreforming fluids was not magmatic, but brine from, evaporating seawater. Sources of the metals were, probably fluvial-deltaic red beds of the subjacent, Endicott group (Kelley & Jennings 2004)., Because sedex ore deposits are geologically common, both in space and time, numerous members, of the group were metamorphosed. Low-grade, metamorphism is the most frequent state found, (Red Dog, Mt. Isa). Very high-grade metamorphic, deposits such as Gamsberg, South Africa (amphibolite facies) and Broken Hill, Australia (granulite, facies) are of special scientific interest (cf. Section 1.5 “Metamorphic and Metamorphosed Ore, Deposits”). Often, metamorphosed ore displays, signs of local mobilization (e.g. cross-cutting, ., ., , coarsely crystalline sulphide veins). In the past,, observations of this kind have occasionally caused, erroneous epigenetic interpretations of sedex ore, formation., , 1.4 DIAGENETIC ORE FORMATION, , SYSTEMS, , Diagenesis is “the sum of all chemical, physical, and biologic changes undergone by sediment after, its initial deposition, and during and after its, lithification, exclusive of surficial alteration, (weathering) and metamorphism” (Neuendorf, et al. 2005). The role of diagenesis in the formation, of petroleum and natural gas deposits has been, known for a long time. Metallogenesis by diagenetic processes was suspected, but decisive evidence could only be assembled in more recent, times. Examples of ore deposit types that illustrate, the diagenetic realm include the European Copper, Shale, many deposits of the Central African, Copper-Cobalt Belt, Mississippi Valley type, lead-zinc (barite-fluorite-celestite) deposits, and, orebodies that originate from brines derived by, interaction with salt rocks (e.g. lead-zinc and siderite in Northern Africa). Note that brine sensu, stricto is defined by a salt content of more than, 10% (Table 1.5). Saturation of NaCl in water is, reached at a concentration of 26.4% (25 C)., The salinity, or more precisely, the total dissolved solids concentration (TDS) of brines is, measured and expressed in two different forms,, ppm (g/t) or mg/L (eq. 1.20). In oceanography,, “practical salinity units” (psu) are used, which, , Table 1.5 What exactly is a brine? Terms for water with, different salinities (Davies & DeWiest 1966), Term, , Concentration of total dissolved, solids (TDS) in ppm (parts per, million) and weight percent, , Fresh water, Brackish water, Seawater, Saline, or salty water, Brine, , 0–1000, 1000–10,000, 31,000–38,000, 10,000–100,000, >100,000, , <0.1%, <1%, 3.1–3.8%, <10%, >10%
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , represent the conductivity ratio of a seawater, sample relative to a standard KCl solution., Contents of total dissolved solids in water and, brine:, ppm ðparts TDS per million TDSþH2 OÞ, mg=L ðmilligrams per litreÞ, ¼ ppm=density ðmg=cm3 Þ, , ð1:20Þ, , In order to determine the origin of salinity in basinal, brines, formation water and hydrothermal fluids,, mole ratios of halogens and electrolytes are measured, (Botrell et al. 1988). Because of its conservative behaviour and small coefficient of partitioning from brine, into halite, bromine (Br) is an ideal discriminator, between two main solute sources: i) subaerial evaporation of seawater; or ii) dissolution of evaporite salts, (mainly halite). Seawater has a characteristic Cl/Br, molar ratio of 655. When seawater evaporates and, deposits halite, Br content relative to Cl and Na of the, residual brine dramatically increases along the, “seawater evaporation path”. Consequently, precipitated halite contains little Br (Cl/Br >655, Hermann, 1980, Holser 1979). Thus, the congruent dissolution, of halite results in high Cl/Br and Na/Br molar ratios, and Na/Cl of 1, very different from the seawater, evaporation array. Brines resulting from seawater, evaporation have Cl/Br molar ratios distinctly, <655. Although halogen ratios appear to be quite, conservative (Nahnybida et al. 2009), several processes do induce changes, for example leaching of, bromine from organic matter (Collins 1975). As a, highly organophilic element, iodine (I) is strongly, enriched by organisms and an elevated I/Cl ratio is, a useful indicator of the passage of solutions through, organic-rich sediments (Kendrick et al. 2001)., , The core of a diagenetic ore formation model is, the argument that most sediment includes a large, mass of water at the time of deposition (“connate, water”). During diagenesis, most of this initial, water is expelled by mechanical compaction, with, a minor contribution by chemical liberation., When newly deposited, sand may contain 40 vol., % of water, clay 90%, and carbonates 50%. By, diagenesis, the pore space occupied by water is, reduced to <1 vol.% at pressures and temperatures, approaching the transition to metamorphism., Obviously, all this water must leave the system, , 111, , by flowing upwards and to the margin of sedimentary basins. Drilling for hydrocarbons revealed, that water in sediments frequently attains temperatures of more than 100 C and reaches a maximum of about 300 C. By reacting with rocks,, the “formation water” acquires a high content of, dissolved matter. Where large masses of such, fluids pass through geochemical or physical, “traps”, minerals may be precipitated which concentrate useful elements. Ore deposits that are, formed as a result of these processes are hydrothermal and epigenetic., Stable isotope data prove that part of the water in, sediments can have a meteoric origin (precipitation infiltrating basin aquifers), but most is initially occluded seawater. Both the rocks and the, pore water chemistry change as diagenesis progresses (“diagenesis of formation water”). Typical, formation waters are brines (Table 1.5) dominated, by Na-Ca-Cl and with elevated traces of HCO3, K, I, and Br. Compared with seawater, iron, manganese, copper, zinc and many other elements are, enriched. Magnesium and sulphate are depleted;, the first by neogenesis of dolomite and chlorite,, the second by bacterial (eqs 1.15 and 1.16) or, thermochemical sulphate reduction to H2S. In the, geological past, however, seawater composition, was not constant (cf. Chapter 4.2.2 “Seawater in, the Geological Past”). In the Early Palaeozoic, the, Jurassic and the Cretaceous, seawater had low, concentrations of magnesium and sulphate., Therefore, some basinal brines were dominated, by Na-Ca-Cl already at the time of occlusion, (Lowenstein et al. 2003)., Recently, the presence of live microbes was, substantiated to a depth of several kilometres, below the surface and to temperatures of 140 C, (Schippers et al. 2005). Microbes convert organic, matter to CH4 (and CO2) and produce the methane, of biogenic gas deposits. In the near-surface realm, of early diagenesis, methanogenesis and sediment, dewatering combine to cause methane-rich cold, seeps that are sites of submarine barite deposit, formation (Torres et al. 2003). Many microbes, have a role in diagenesis. Geobacter metallireducens, for example, catalyses Fe(III) reduction and, decomposition of organic substance to CO2, (Childers et al. 2002). In the absence of microbes,
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112, , PART I METALLIFEROUS ORE DEPOSITS, , for example at higher temperature (>80 C: Thom, & Anderson 2008) and greater depth, abiotic sulphate reduction based on destruction of organic, matter (here represented by CH4 and CH2) is, possible (eq. 1.21)., Thermochemical sulphate reduction (TSR):, 2Hþ þSO2, 4 þCH4 ) H2 SþCO2 þH2 O, 0, , SO2, 4 þ3H2 S ) 4S þ2H2 Oþ2OH, , 4S0 þ1:33ðCH2 Þþ2:66H2 O ) 4H2 Sþ1:33CO2, ð1:21Þ, By disintegrating anhydrite, this process can, cause a relative enrichment of CaCl2 in the, fluids. NaCl may be retained and concentrated by, clay beds acting as a semi-permeable membrane., Probably more often, NaCl brines originate, by evaporation at the surface or by dissolution, of salt rocks in the basinal sediments. Mature,, commonly deep formation waters are mostly concentrated NaCl brines. Maturation and decomposition of organic matter in sediments results in, dissolved CH4, CO2, H2, N2 and higher hydrocarbons (e.g. petroleum). Oxygen induced by infiltrating seawater and meteoric water, and oxygen, produced by disproportionation of formation, waters is quickly consumed by oxidation of Fe(II), or of organic matter. Therefore, most formation, waters display a low redox state, favouring high, solubility of many metals. In solution, some metals may occur in the form of organo-metallic, complexes (Ni, V, part of Zn and Cu), while others, such as Pb, Fe and most Cu and Zn are dissolved as, simple ions. The important metallogenetic role of, basinal NaCl-brines extends into the realms of, metamorphism and even to magmatic ore formation (e.g. iron oxide Cu-Au deposits: Barton &, Johnson 1996)., Not all basinal fluids are reduced, and oxidized, formation waters are commonly enriched in SO4., This is possible when the source rocks include, predominantly haematitic sandstone and mudrock, limestone, anhydrite and halite, for example in the Rotliegend (Permian) desert and salt lake, sediments of Northern Germany. Oxidized fluids, are able to dissolve Cu, Pb and Zn, to transport the, metals over wide distances and to precipitate ore at, , suitable traps such as redox fronts established by, organic-rich shale (e.g. the European Copper, Shale). Barium and iron, in contrast, are insoluble, in oxidized fluids and are not part of the resulting, ore paragenesis., Metal uptake in the source region (reaction, zone) or along flow paths is a function of kinetic, and thermodynamic factors, including rock and, water chemistry, T, P, Eh and pH. In contact with, formation waters, several common minerals are, unstable, including amphibole, pyroxene, olivine, and epidote. Their alteration releases trace metals, into the aqueous phase. Other mechanisms of, liberating metals include ion exchange with clay, minerals and simple diagenetic transformation of, clays. An example is lead that is often adsorbed on, kaolinite, but is released into solution when illite, replaces kaolinite., The flow paths of expelled deep fluids are determined by hydrogeological parameters such as permeable rock units, tectonic disaggregation and the, general pressure gradient. Petroleum and gas deposits in crystalline and magmatic rocks clearly, demonstrate that diagenetic flow may encompass, basement underlying basins. The norm is probably, a slowly moving, diffuse compaction flow that, loses its dissolved matter by forming pore-cement, in sediments or by discharge into seawater. Ore, formation will only occur where a large mass of, hot brine is focused into preferential flow paths,, which may be envisaged as streams within permeable rocks or tectonic structures. In these channels, a rapid drop of temperature or pressure,, reaction with host rocks, and mixing with chemically different water may precipitate solutes in a, small rock volume. Because of their high permeability, near-surface faults, reefs or karst cavities, at the margins of sedimentary basins or basement, islands often locate ore deposit formation. Note, that traps of diagenetic ore deposits are quite the, reverse of petroleum and gas deposits; the first are, open flow systems with hydrostatic pressure,, whereas the second are closed and often overpressured. The origin of secondary porosity and of, caves in carbonate rocks may be caused by metal, sulphide precipitation, which produces hydrogen, ions (eq. 1.22) that maintain limestone dissolution, (eq. 1.23).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , Precipitation of metal sulphides by reaction with, H2S:, Zn2þ þH2 S ðaqÞ ) ZnS ðsÞþ2Hþ, , ð1:22Þ, , Dissolution of calcite:, CaCO3 ðsÞþ2Hþ ) Ca2þ þCO2 ðaqÞþH2 O ð1:23Þ, Intracratonic basinal hydrothermal flow systems, There are many variations to the theme of intracratonic basinal hydrothermal flow systems., Some of the more frequent geological driving, agents of basin dewatering and fluid migration are, the following:, . standard diagenetic dewatering by compaction, of pore space (consolidation: Terzaghi et al. 1996,, Noble 1963), in a slowly progressive process,, mainly caused by continuing sedimentation and, subsidence in the basin;, . episodic expulsion of brine by increasing overpressure in sand lenses that are enveloped by clay, beds of low permeability (Fowler & Anderson, 1991); this can be observed as mud-eruptions in, the Mississippi delta, although without leading to, mineral deposit formation;, . displacement of deep, hot and therefore relatively light brines by cold, heavy groundwater, infiltrating from topographically elevated basin, margins; this hypothesis assumes that permeable, rocks or active extensional faults provide linkedup flow paths (Daubree 1887);, . tectonic expulsion of formation fluids by a, moving nappe pile, causing flow of diagenetic, and metamorphic fluids towards the foreland,, where transported heat and dissolved matter, result in mineral deposit formation (Oliver, , 1986, Figure 1.74); a present-day analogue are, methane-rich hot fluids ejected from the accretion wedge at many subduction fronts;, . fluid overpressures can also be induced by pulses, of hydrocarbon generation caused by a rapid thickness increase of cover sediments and consequently, of rock temperature; or by a rapid fall of sea levels, (Eisenlohr et al. 1994);, . Expulsion of formation waters by deep heatflow, (e.g. cryptic intrusions, underplating) imposing a, wide halo of higher-grade diagenesis; this may, trigger submarine venting of fluids resulting in, synsedimentary sedex type deposits (Figure 1.71), or of epigenetic ore., Several of these hypotheses have been, tested by Oliver et al. (2006), who showed that, tectonic extension alone is insufficient for inducing fluid upflow and the main drive is a favourable, thermal field. The precise causes, timing and, mode of fluid generation and flow in sedimentary, basins are a long-standing subject of applied, research in the oil and gas industry. Concepts, and approaches similar to modelling petroleum, systems (Magoon & Dow 1994) considerably, advance investigations concerning diagenetic ore, formation., Many diagenetic mineral deposits display strikingly banded textures of newly-formed precipitates that have been called “diagenetic, crystallization rhythmites” (DCR) or “zebra, textures”. The thickness of the bands varies, between millimetres and decimetres. They occur, in dolomite, evaporite, in sparry magnesite, in, siderite and most typically, in stratabound PbZn-Ba-F deposits hosted by platform carbonates., Their formation is attributed to coupling of, , W, Bituminous coal, Pb- Zn, Petroleum, , E, Anthracite, , Erosion, , Natural gas, , Thrust, Continental crust, Tectonic brines, , 113, , Moho, , sheets, , Oceanic crust, , Figure 1.74 Expulsion of fluids (“tectonic brines”) towards the foreland from underneath the west-moving nappe pile, of the Appalachian Mountain Belt (modified from Oliver 1986).
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114, , PART I METALLIFEROUS ORE DEPOSITS, , diffusion with precipitation reactions (Kapral &, Showater 1995)., 1.4.1 The European Copper Shale, Diagenetic mineral deposits may display properties that resemble synsedimentary features and in, a number of cases, only persistent investigations, revealed an epigenetic origin from diagenetic solutions. The ambivalence is clearly illuminated by, the research history of the European Copper Shale, (in German: Kupferschiefer)., In the Central European Permian rift basin, the, Copper Shale is a thin stratum that forms the base, of the post-Variscan transgressive marine sedi-, , ments (consult Chapter 4.2.2, “The Zechstein Salt, Formation in Northern Europe”). Copper shale is, known over an area of more than 600,000 km2., Basinal Copper Shale intersected in hydrocarbon, drillholes displays very low base metal values, (Pašava et al. 2010). Exploitable metal grades are, restricted to southern basin margins (in Germany, the Harz District with Mansfeld and Sangerhausen;, Silesia in Poland). The Copper Shale of the mining, districts is a euxinic sediment deposited in a shallow sea of only a few hundred metres water depth., Ubiquitous fossils of fish and brachiopods indicate, that the upper water column was well aerated,, whereas deep water was poisoned with H2S. Continent-derived clastic sediment import was very, , BOX 1.11 Copper Shale mineralization, Petrographically, the Copper Shale is a black laminated marly claystone formed from illite, montmorillonite, dolomite,, less than 30% organic matter (kerogens include bituminite, vitrinite, fusinite and liptinite: Koch 1997), sulphides,, anhydrite or gypsum, phosphates and little quartz. Its footwall is mostly Rotliegend (literally “red footwall” in German), sand or conglomerate, but in some marginal exposures Copper Shale rests directly on folded Variscan basement. The time, when Rotliegend sediments experienced reddening (Fe2þ oxidized to haematite) is a critical point for a detailed, understanding of copper ore introduction (Brown 2009). One obvious possibility is that iron oxide (desert varnish), originally coated the sand grains; palaeomagnetic and illite ages seem to support this assumption (Nawrocki 2000)., Hanging wall rocks are lowermost Zechstein carbonates overlain by anhydrite and salt rock. In the North Sea and central, Polish basins, the Copper Shale is downwarped to more than 7000 metres below surface. The Copper Shale is not folded, but dissected by faults. Fissures and fault intersections are notably mineralized. The bulk of the extracted ore, however, is, from stratiform bodies., As a result of basin palaeomorphology, the Copper Shale horizon occurs in different sedimentary facies: In marginal, sub-basins it is quite thin (0.3–0.4 m), rich in Fe, Al2O3 and organic substance. Contents of carbonates and quartz as well, as thickness increase where shale approaches sills and sand bars. Sills are not mineralized and strikingly red (because of, haematite). Miners used to call this barren rock “Rote F€aule” (German for “red rot”). Rich chalcocite orebodies occur at, reduction fronts adjacent to Rote F€aule (Figure 1.75; Rentzsch et al. 1997)., , Zechstein, anhydrite, Limestone, unaltered, , Cu, min, era, liz, , Ro, , te, , Fä, , Pb, -Z, n, , Dolomitized, , m, , ine, , atio, n, , ra, li, , Zechstein, limestone, , za, , tio, , n, , Cu, Pb-Zn ore, ore, ul, , e, , Copper, Shale, , White, bleached, by CH4?, , 5m, , Basinal fluids, 5000 m, , Reductio, n, Oxidatio, n, , Rotliegend, sandstone, , Figure 1.75 Schematic section of a, Copper Shale deposit in Northern, Germany and Poland (modified from, Jowett 1992 with permission from, Elsevier).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 115, , The sulphides in Copper Shale are of major interest, because they contain the valuable metals. Textural variations from, early to late parageneses include (Wodzicki & Piestrzynski 1994):, . Very fine (0.02–0.06 mm), disseminated grains and aggregates, framboids (“mineralized bacteria”, Schneiderh€, ohn, 1962) and crusts on organic particles; sulphides consist mainly of pyrite, with occasional traces of chalcopyrite,, bornite, chalcocite and marcasite; copper concentrations of a few 100 ppm are considered to have a purely sedimentary, origin., . Spindles, lenses, concretions and thin lenticular bands of sulphides that are parallel to bedding planes; these features, reflect diagenetic remobilization within the rock; copper concentrations reach 2000 ppm., . The main mass of the sulphide ore paragenesis (with base metal contents reaching 3%) replaces rock forming minerals;, ore samples may contain 90% chalcocite; microscopic veinlets of different spatial orientation appear in the ore; ore, minerals include mainly silver-rich chalcocite, with some digenite, covellite, bornite, chalcopyrite, sphalerite, galena,, pyrite and haematite., . Where no shale is developed on sills, zones of anhydrite pore cementation finger down into bleached footwall, sandstone; these zones are mantled by rich chalcocite ore replacing carbonate and clay minerals., . Thin and cross-cutting veinlets of coarsely crystalline sulphides (bornite, chalcopyrite, tennantite, galena, sphalerite,, Fe-Ni-Co sulphides and arsenides) with a gangue of barite, calcite and anhydrite are the latest phase of ore formation; they, represent less than 1% of ore produced., The metals Cu-Pb-Zn display a distinct zonation, both horizontally around oxidized sills and vertically from copper in, sandstone and shale to lead and zinc in hanging wall carbonates that are dolomitized (Figure 1.75). However, some, copper orebodies extend from bleached footwall sandstone through shale to hanging wall carbonate. Metal contents in, exploited Copper Shale deposits in Poland are reported at 1.2–2.8% Cu and 30–80 g/t Ag. Thickness of stratiform, orebodies varies from 1.2 to >20 m. Exploitation takes place underground between 600 and 1200 m below the surface., Metals produced include copper, lead, silver, nickel, gold (Piestrzynski et al. 2002), palladium, platinum, rhenium,, selenium and sulphuric acid. Until 1970, when mining in the Mansfeld District in Germany was terminated, 1.5 Mt, copper had been produced. The nearby Sangerhausen mines were closed in 1990. Economic reserves in Poland are, reported as 2.4 Gt of ore; 50% of this occurs in footwall sandstone, 20% in shale and 30% in limestone. The figures, underline that a large mass of copper is concentrated within a very small area of the Copper Shale basin., , small and the fine lamination indicates extremely, tranquil conditions in the bottom waters., Genetic interpretations of Copper Shale ore formation have always been dominated by the controversy between syn- and epigenetic models. The, difficulty was essentially to determine the time of, metal concentration, during sedimentation, or, later? Of course, diagenetic ore textures and veinlets were observed, but were they only caused by, local remobilization of a metal stock imported, earlier? Meanwhile, modern technologies have, refined data to a level where epigenetic ore formation during late diagenesis is strongly supported., One of the few mappable characteristics of Copper, Shale orebodies supporting epigenesis is their, cross-cutting relations with the Copper Shale stratum. In Figure 1.75, this is emphasized by drastically different vertical and horizontal scales, revealing the cross-cutting geometry and metal, zoning. In underground exposures, Copper Shale, orebodies appear to be perfectly stratiform. Epi-, , genesis is also suggested by rings of high-grade ore, surrounding red sandstone hills that protrude, through the shale. This pattern can hardly be of, synsedimentary origin., Elements of today’s diagenetic model of, Copper Shale ore formation include hot, oxidized, (e.g. sulphate-bearing) Ca-Mg-K-Cl formation, waters flowing up from deeply buried Permian, molasse red bed sediments and volcanics, where they acquired their metal contents (Oszczepalski 2000). Increasing compaction and the low, permeability of Zechstein evaporites acting as a, seal focused the flow towards basin margins, where the metals were precipitated. Possible, agents of precipitation include decreasing T and, P, mixing with cool, alkaline, meteoric (or sea-), water and most importantly, reduction of fluids, by contact with organic matter and sulphides of, the shale (Wodzicki & Piestrzynski 1994), and, methane in the sandstone (Jowett 1992). The, metal zonation of Cu-Pb-Zn is consistent with
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116, , PART I METALLIFEROUS ORE DEPOSITS, , this model and conforms to the relative solubility, of the sulphides (Figure 1.75). To a certain extent,, the Copper Shale bed may have acted as a semipermeable, reactive membrane. The question, when exactly the epigenetic Copper Shale mineralization took place is still not satisfactorily, answered (Pašava 2010). Evidence has been offered, for early diagenetic, Permian (Wedepohl &, Rentzsch 2006), to late diagenetic metal introduction during the Triassic (Nawrocki 2000). It appears, unlikely that steady-state consolidation of sediments in the Zechstein basin was the driving force., More probable agents are large-scale events, such, as the giant mantle plume (Marzoli et al. 1999) at, 200 Ma and Triassic-Jurassic rifting phases that, punctuated the initial break-up of Pangaea., Isotope geochemistry supports the diagenetic model, of Copper Shale ore formation. Generally, mineralized Copper Shale is isotopically different both from, ordinary shale and from red rot. 13C in organic substance is clearly enriched, probably because light, carbon was preferentially oxidized and expulsed from, the system. Sulphide sulphur with d34 S between 40, and 25‰ implies bacterial reduction of seawater, sulphate. Early pyrite contains the lightest, later, sulphides heavier sulphur. This suggests: i) a closed, system with a restricted mass of available sulphate: ii), later fluids with higher 34S-contents (Jowett et al., 1991); or iii) late thermochemical sulphate reduction, (Sun & P€, uttmann 1998). Oxygen and hydrogen investigations of clays resulted in confining the mineralizing fluids to d18 O between 3 and 6‰, and dD from, 10 to þ3‰, at probable temperatures of 130 C, (Bechtel & Hoernes 1993). These characteristics conform to properties of diagenetic formation waters, (Figure 1.23)., , The copper province of White Pine, Michigan,, USA and the African Copper Belt of the Democratic Republic Congo and Zambia are comparable, but not identical to the European Copper Shale (cf., Chapter 2.2 “Copper”). All three are included in, the non-genetic term of “stratabound and/or stratiform sediment-hosted Cu deposits”., 1.4.2 Diagenetic-hydrothermal carbonatehosted Pb-Zn (F-Ba) deposits, Lead and zinc ore deposits hosted by marine carbonate rocks are a large and heterogeneous group., , Single deposits and districts display end member, characteristics of syngenetic as opposed to epigenetic hydrothermal ore emplacement. The first are, sedimentary-exhalative or volcanogenic (cf., Sections 1.1 and 1.3), whereas the second may, have been formed by igneous, metamorphic and, diagenetic process systems. In the following discussion, the subject is epigenetic deposits formed, by essentially diagenetic fluids. Let us note here, that passage of the fluids through igneous intercalations and deeper crystalline basement is not, rare. Host rocks are not restricted to carbonates,, but world-class deposits in siliciclastic sediments, are rare compared to carbonate-hosted deposits, (Chapter 2 “Lead and Zinc”)., Several provinces or districts with a more homogeneous subset of traits are commonly designated, as “types” for discussion. Well-studied types, include the “Alpine” (Middle to Late Triassic in, Austria-Italy-Slovenia, Figure 1.76), “Silesian”, (Mid-Triassic in Poland), “Irish” (Early Carboniferous in Ireland) and “Mississippi Valley” (Pennsylvanian-Permian of the mid-continental, USA). Similar deposits occur worldwide, for example in Australia, in several provinces of China and, in Mexico. Here, we shall mainly refer to the, Mississippi Valley type (MVT) province, because, it hosts the most intensively investigated, deposits., The paragenesis of diagenetic-hydrothermal, carbonate-hosted deposits is usually very simple,, comprising galena and sphalerite, with a gangue of, barite, fluorite, pyrite, marcasite, calcite, dolomite, aragonite, ankerite, siderite, quartz, colloform silica (chert) and occasionally, bitumen., Sphalerite appears mostly in colloform-banded, texture (sphalerite-wurtzite banding). Low iron, and elevated trace contents of cadmium are characteristic. Galena contains little silver. Copper, contents are negligible and there are no copper ore, deposits of this type. Often, a wide halo of dolomitization due to the passage of brines envelops, the ore and many deposits occur at dolomitization, fronts (Harper & Borrok 2007). Lead isotopic compositions of districts vary widely and include ordinary (Silesia, Pine Point, Canada) or anomalous, lead (southeast Missouri: Goldhaber et al. 1995)., Sulphur is in part quite heavy and in that case not
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , BOX 1.12, , 117, , Mississippi Valley type Pb-Zn deposit characteristics, , Large to giant MVT deposits contain 1–20 Mt combined zinc and lead, with an average Zn:Pb ratio of 3. Even within the, type region, a considerable variance of ore-forming processes and modifying factors is observed. This results in variable, proportions of metals and minerals in different districts of the Mississippi province. Generally, orebodies are stratiform or, at least stratabound, and include tectonic or karst cavity breccias that were cemented by ore and gangue minerals;, replacement bodies and cross-cutting veins are not rare (Leach et al. 2010). Some ores fill open cavities in carbonate and, DCR textures are very common., , Figure 1.76 (Plate 1.76) Banded and breccia cave ore of brown sphalerite in the historic Lafatsch mine,, Karwendel, Tyrol. This is one of the outliers of the Triassic Alpine type carbonate-hosted Pb-Zn deposits., Courtesy B. Lehmann, Clausthal., World-scale lead deposits were discovered in 1955 in the Viburnum Trend, Missouri, USA. Together with the “Old Lead, Belt” nearby, the Viburnum Trend forms a horseshoe-like belt around the Precambrian St Francois Mountains, which are, surrounded by Palaeozoic sedimentary basins. During marine transgression in the Cambrian, the mountains were islands, with surrounding algal reefs and lagoons, partly of evaporitic facies. The basement is overlain by quartzitic sandstone, (Lamotte Sandstone), followed by the carbonatic Bonneterre Formation and dolomites with shale beds., Most orebodies occur in the Bonneterre Formation within an offshore stromatolitic reef that separates a micritic-clayey, basinal facies from coastal oolites. Oolites also fill channels in the reef, form its hanging wall and detrital cones outside of, the reef. In the oolitic calcite arenites, long tubular collapse channels are aligned parallel to the coast. Stratabound, orebodies occur in these karst-like collapse structures and along faults (Figure 1.77). Both were channels of higher, permeability that focused the solutions flowing up from the Lamotte aquifer where it thins out towards the coast., Apart from galena, the Viburnum ore contains sphalerite, marcasite and rather untypically for MVT deposits, more than, traces of chalcopyrite, bravoite and siegenite. Main gangue minerals are quartz and dolomite. Mineralization took place, in cyclic pulses that were controlled by mixing of at least two fluids. A weak zoning of Cu-Pb-Zn is compatible with the, assumed hydrothermal flow vector.
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118, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 1.77 Simplified genetic, model of the Pb-dominated, deposits of Mississippi Valley Type, (MVT) in the Viburnum Trend,, Missouri. After Plumlee, G.S.,, Leach, D.L., Hofstra, A.H., Landis,, G.P., Rowan, E.L. and Viets, J.G., 1994, Society of Economic, Geologists, Inc., Economic Geology, Vol. 89, Figure 18 (C) p. 1379., Another important mining area in the MVT province was the Tri-State district, which straddles adjoining parts of the, states of Oklahoma, Kansas and Missouri and covers an area exceeding 1200 km2. This region hosts some MVT deposits,, which are characterized by anomalous and highly radiogenic lead (cf Section 1.1 “Isotope Geochemistry”). From 1850 to, 1965, Tri-State mines processed an estimated 500 million tonnes (Mt) of ore at a grade of 3% Zn and 0.5% Pb. Today, the, MVT province is past peak production but paradigmatic for environmental remediation of the mining heritage., In the mid-western USA, different fluids were involved in MVT ore formation. Viburnum District fluids are marked by, reactions in the basal sandstone aquifer, whereas Tri-State district fluids display properties of equilibration with marine, clay and carbonate (Viets et al. 1992). From a southern source, magmatic fluids rich in fluorine were mixed into the, diagenetic fluids (Rowan et al. 2002). Metal-bearing brines of Mississippi Valley type deposits display the following, general characteristics (Roedder 1984, Kendrick et al. 2002):, . Salt concentrations above 15 and often >20 % and accordingly, the fluids are brines (Table 1.5); Br/Cl ratios indicate, variable fractions of evaporated seawater and salinity derived from halite dissolution., . General composition is Cl > Na > Ca K > Mg; dissolved salts are predominantly NaCl and CaCl2; sulphur concentrations are low, metal contents reach several thousand ppm (Wilkinson et al. 2009)., . Brines are usually reduced and moderately acidic., . Density is always >1 and often >1.1 g/cm3., . Temperatures vary from ca. 80–200 C., . Formation pressures were generally low but above vapour pressure; fluid inclusions evidence of subcritical boiling is, hardly ever observed., . CH4 in gas bubbles and oil in unmixed droplets are both common, and dissolved hydrocarbons occur in the liquid part of, fluid inclusions; often, the fluids are compared to oil field brines., Precipitation of ore and gangue was probably induced by mixing with near-surface waters that contained reduced, sulphur. Some of the latter originated by bacterial sulphate reduction and consumption of organic matter of the carbonate, rocks (H2S-fluids: Plumlee et al. 1994, 1995; Figure 1.77). Note the characteristic focusing of ascending fluids into, permeable reef carbonates. Reduction may also have been induced by methane and petroleum generated in the heated, wallrocks (Anderson 2008)., Rb/Sr ages of Upper Mississippi Valley district sphalerites fall to 270 Ma (Mid-Permian), the time of Alleghenian/, Ouachita orogenesis in the Appalachian Mountain Belt (Brannon et al. 1992). Crustal shortening in the east and westward, nappe movement are thought to have intensified diagenesis in foreland basins, causing fluid liberation and migration, (Figure 1.74)., , biogenic but probably due to thermochemical sulphate reduction (TSR, eq. 1.21). As a function of, time, strontium isotope ratios of gangue carbonates display higher concentrations of radiogenic, 87, Sr. This is probably due to an increasing share of, strontium derived from clays and other silicates of, basinal sediments while diagenesis progresses, (Brannon et al. 1991)., , Diagenetic-hydrothermal carbonate-hosted PbZn (F-Ba) deposits are common in most of the, world’s marine carbonate platform sequences of, Phanerozoic age. This is evidence that the specific, metallogenetic process systems leading to this, mineralization class are integral to basinal evolution. Although orogenic foreland basins, such as, the MVT province, are favourable geodynamic
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , settings, relations to orogeny are not general., Other type regions are related to submarine shelf, rifting, to large-scale lithospheric flexure, or to, enhanced heat flow and elevated permeability of, the crust induced by major plate reorganizations, (such as the break-up of Pangaea in Western, Europe)., 1.4.3 Diagenetic-hydrothermal ore formation, related to salt diapirs, In the preceding discussion of diagenetic ore formation, the role of oxidized saline fluids in Copper, Shale mineralization and of reduced brines in MVT, lead-zinc deposits, were discussed. Also, giant, high-grade haematite orebodies derived from BIF, were preconcentrated by basinal brines, although, the last transformation to economic ore is due to, supergene enrichment (Figure 1.52, Thorne et al., 2004, 2009). Basinal brines and salt rocks, which are, assimilated by magmatic intrusions and consequently leach metals from pervaded rocks, may be, the key for understanding the iron oxide-coppergold class of deposits (Cox & Singer 2007). Of, course, the essentially magmatic-hydrothermal, IOCG class can hardly be considered as part of the, diagenetic process system. In this chapter, brines, formed by dissolution and/or dehydration of saltrich evaporites within basinal sediments shall be, singled out for their specific origin., Most sedimentary basins contain common, evaporitic rocks such as anhydrite and rock salt., Dehydration water from evaporites and formation, water in contact with these rocks must acquire the, character of a saline brine (Table 1.5, eq. 1.20)., Other sources of brines in sedimentary basins, include evaporated seawater, terrestrial saline, lakes and mature formation water. The source of, salinity is discerned by determination of halogen, and electrolyte ratios (Botrell et al. 1988)., Metallogenetic action of brines is especially, obvious when salt and epigenetic hydrothermal, ore deposits are closely related, in space and time., The best sites for studying this relationship are, salt diapir related ore deposits. Salt structures, induce peculiar geochemical, hydraulic and thermal conditions in their host sediments (Rouvier, et al. 1985, Kyle & Price 1986, Pohl et al. 1986)., , 119, , Deposits of petroleum and native sulphur in salt, diapir cap rocks resulting from the passage of, basinal hydrocarbon fluids are known for more, than 100 years. However, in addition to oil, some, salt diapirs are intimately related to ore deposits, (Pb, Ag, Zn and Fe in North Africa), and to nonmetallic mineral deposits (barite, fluorite, strontianite; emerald in Colombia)., The southern foreland of the Atlas Orogen in, Tunisia and Algeria is underlain by a basin with, thick Mesozoic sediments. The package starts, with 1000 m of Triassic evaporites that resemble, the Haselgebirge of the Eastern Alps (a melange, of salt, anhydrite, clay, dolomite and basalt: cf., Chapter 4.2). Very high subsidence in the Cretaceous reached a total of 8000 m and initiated synsedimentary diapirism that continued into the, Eocene, when orogenic folding encompassed the, basin. Several large metasomatic siderite and, numerous small lead-silver-zinc deposits are, found in Triassic cap rocks and in Early Cretaceous limestones where these are in contact with, apical parts of the diapirs:, The largest diapir-related ore deposits in the region, are the massive siderite bodies at Ouenza and Jerissa., The siderite replaces Aptian limestone that is in, direct contact with salt rock. The limestones are, extremely fine-grained (lithographic) and contain, numerous rudist (Hippuritoida) fossils, typical for the, “Urgonian” facies of Western Europe. Orebodies are, stratabound and ore boundaries show marvellous, examples of metasomatic fronts. Siderite near the, boundaries faithfully preserves bedding, stylolites, and fossils of metasomatized limestone and contains, large druses of coarse calcite, quartz and tetrahedrite., Figure 1.78 illustrates the margin of a large orebody., Epigenesis of ore is clearly visible as sedimentary, textures of host limestones are erased. Antimony, fahlore (tetrahedrite), quartz, calcite and aragonite, only occur in marginal parts of the ore and are controlled by joints and faults. Within the siderite mass, the ore is a nearly monomineralic crystalline rock,, with occasional small druses of calcite and dolomite., Ore and host rocks are traversed by rare veins of, barite, fluorite and carbonates., The origin of these deposits is not fully elucidated., It is believed to represent a complex interplay of, halokinesis (cf. Chapter 4.3.3) with fluid generation
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120, , PART I METALLIFEROUS ORE DEPOSITS, , SE, , NW, , m, , Early, Cretaceous, biomicritic, limestone, Ara, go, nit, e, , 605, , Calcite, , Tetrahedrite, + quartz, , Figure 1.78 Metasomatic front of massive siderite ore, replacing bedded limestone at Douamis mine, Ouenza,, Algeria (For location, refer to Figure/Plate 1.89.), , Massive siderite ore, 600, , by diagenesis of evaporites and host rocks. Mass, estimates imply that most of the water of the fluids, must have been derived from basinal sediments (Bouzenoune & Lecolle 1997). Partial admixture of meteoric or seawater and dehydration brines sourced from, the diapir are all possible. At elevated temperature,, iron is extremely soluble in reduced, acidic and highly, concentrated brines. Precipitation of siderite is, induced by drastic pH-change in contact with the, carbonates and by sudden lowering of pressure and, , temperature. In this model, the diapirs and their, solution breccia envelope are preferred channels for, upflowing deep diagenetic brines (Figure 1.79). In, active halokinetic phases diapir salt is permeable., Many diapirs grow in short spurts of active halokinesis (or halotectonics) that interrupt steady-state passive quietude. During the active phase, major fluid, flow may take place. Mineral deposits related to, diapirs include petroleum, natural gas, iron, lead,, , Halokinetic/tectonic ± hydrothermally, active phase, , Halokinetic/tectonic, ± hydrothermally, inactive state, , Meteoric or, marine water, , Diagenetic, saline brines, Formation water, ± oil/gas, , Fa, , Basement, , ult, , Salt, diapir, , Figure 1.79 Diagenetic metallogenesis related, to brines released from salt diapirs, based on, observations in the Maghreb region in North, Africa (adapted from Pohl et al. 1986).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , zinc, sulphur, barite and fluorite (apart from salts, cf., Chapter 4). Arsenic, antimony, mercury, silver, copper, manganese and strontium mineralization is, rarely economically exploitable., , Occasionally, a pre-enrichment of metals in, evaporites is invoked. Of course, many terrestrial, salt lake brines do contain anomalous and in part, exploitable metal concentrations, for example of, lithium, tungsten and magnesium. Sabkhas are, believed by some to trap metals and elements such, as fluorine and barium that are delivered by inflowing groundwater. Later diagenesis may remobilize the pre-enriched elements and produce ore, deposits. As yet, the metallogenetic significance, of these hypotheses remains untested., , 1.5 METAMORPHIC AND, , METAMORPHOSED ORE, , DEPOSITS, , Ore deposits in metamorphosed terranes may have, been formed before, during or after metamorphic, processes. This chapter deals mainly with the first, category, which are of premetamorphic origin, independent from later metamorphic overprinting. This is the class of “metamorphosed” ore, deposits., It is possible to distinguish another class of, mineral deposits from metamorphosed ores and, minerals, a class that owes its economic interest to, largely isochemical metamorphic re-equilibration, and recrystallization of pre-existing material, which had no use in its original state. Examples, provide alumina-rich claystones that were transformed into kyanite or sillimanite deposits, or, graphite flakes formed from dispersed bitumen., Among ore deposits, gold may be recrystallized, from unrecoverable dilute traces and so turn into, recoverable economic ore. In allusion to the term, metamorphic rocks, this class may be called, “metamorphic” ore deposits (Pohl 1992)., Orogenic (regional) and contact (local) types of, metamorphism are most common (Bucher & Frey, 2002). In the context of this chapter, ocean floor and, dynamometamorphism (due to shearing or cataclasis), extraterrestrial impacts and metasomatic, changes to rocks will not be further discussed. Ore, , 121, , deposits that have originated in collisional plate, tectonic settings (e.g. island arcs) are most likely, affected by later orogenesis and metamorphism., The reverse applies to anorogenic orthomagmatic, deposits (e.g. layered mafic intrusions, carbonatites), post-orogenic granitic deposits (tin, tantalum) and superficial alteration deposits (bauxite), which are rarely metamorphosed., Contact metamorphism of ore in the heated, zone around magmatic bodies is usually static, (i.e. in the absence of dynamic deformation). Exposure to high temperatures (with a maximum of, 750 C) affects fabric, mineralogy and mineral, chemistry (e.g. by driving off water and other, volatiles). Fabric changes are confined to a general, increase in grain size with rising temperature., Monomineralic ores recrystallize by annealing to, “foam” textures (Stanton 1972) that are characterized by triple grain boundary junctions at angles of, 120 . Instructive observations can be made at, magmatic dyke contacts, where changes affect, very narrow zones. The contact zone around the, Bushveld Complex reaches hundreds of kilometres. Sulphur release (e.g. from pyrite, eq. 1.24), may induce formation of metamorphic pyrrhotite, or even magnetite. Because pyrrhotite (like, most sulphide minerals) has a pronounced nonrefractory behaviour, retrograde cooling will, always bring about equilibration at lower temperatures (Vokes 2000). This is the reason why, sulphides have a limited role as geological, geothermometers or geobarometers. Iron oxide ore, at contacts may recrystallize to a different oxidation state (e.g. haematite to magnetite, eq. 1.24), controlled by the oxygen activity imposed by, magma or by heated country rocks., Oxidation/reduction and sulphidation/desulphidation reactions during metamorphism:, 6Fe2 O3 ðhaematiteÞ $, 4Fe3 O4 ðmagnetiteÞþO2ðFluidÞ, FeS2 ðpyriteÞþH2 $, FeS ðpyrrhotiteÞþH2 SðFluidÞ, , ð1:24Þ, , Orogenic metamorphism of ore deposits is common. Temperatures may reach 1100 C and
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122, , PART I METALLIFEROUS ORE DEPOSITS, , pressures 30 kbar for crustal rocks. Ore metamorphosed in the zeolite, greenschist and amphibolite, facies is quite frequent, whereas granulite facies, ore is rare (e.g. Broken Hill, Australia). Surprisingly, subducted and high-pressure metamorphic, oceanic crust seems to lose mid-oceanic sulphide, concentrations: Orebodies hosted in exhumed, eclogites and glaucophane schists of subduction, zones are very rare (Laznicka 1985). This observation supports assumptions that a part of suprasubduction zone metal concentrations may be, mobilized oceanic mineralization., Orogenic metamorphism is the result of penetrative deformation, while minerals re-equilibrate, to new assemblages at geologically elevated temperature and pressure. Volatiles (water, etc.) are, partly to wholly (at very high metamorphic grades), removed from the system. Metamorphic rocks, exhibit grain coarsening, preferred orientation of, minerals and a penetrative fabric (e.g. schistosity,, foliation). Ordinary metamorphic rocks display a, , SW, , number of typical deformation styles, including, folding, thinning and rupture of fold limbs, thickening of certain rock types in fold hinges and, stretching fabrics that produce elongate shapes., Metamorphosed massive sulphide orebodies, (and other metallic and mineral deposits) have, been shown to react in the same way as common, rocks. Generally, sulphides tend to be more, ductile than most host rocks, so that fold hinge, thickening and thinning of limbs is very characteristic (Figure 1.80 and Figure 1.81). Of course,, sulphide layers in ductile rock, such as black shale, or migmatite, may exhibit brittle response, for, example boudinage. Extreme elongation leads to, orebodies shaped like rods, pencils or spindles., Original spatial relations between ore and alteration zones are often severely disturbed and cannot be reconstructed. This is a serious impediment, to geological studies and to exploration of these, deposits. Also, it explains why a satisfactory, understanding of metamorphosed volcanogenic, , NE, , Drilling fan, , 580, , Metabasalt, (greenstone), 540, , 500, Greenstone with, Cu-Zn impregnation, Massive pyrite, (0.2-0.4%Cu,, 1-3% Zn), , Chlorite schist (2-5%Cu), 50 m, , Massive pyrrhotite (>5%Cu), , Figure 1.80 Section of Joma Cu-Zn mine in metabasalt northeast of Trondheim, Norway (Bowie et al. 1978)., Reproduced with permission of The Institution of Mining and Metallurgy, the Mineralogical Society of Great Britain, & Ireland and Maney Publishing www.maney.co.uk. This is an instructive example of orogenic, synmetamorphic, deformation of massive sulphide orebodies. Note the drill pattern collared from the surface and from underground, mine adits.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 123, , Figure 1.81 (Plate 1.81) Ductilely folded sedimentary bedding in very-low-grade metamorphic shale-banded, Fe-Cu-Zn sulphide ore with wispy white dolomite laminae. Note the diffuse axial plane cleavage. Sample from the, closure of the orebody synform, Rammelsberg, Germany. Width of image 20 cm. Courtesy B. Lehmann, Clausthal., , and sedex ore deposits was only recently achieved, (Vokes 2000)., An important aspect in studies of metamorphosed ore deposits is the identification of rocks, with unusual mineral composition as premetamorphic exhalites or hydrothermally altered rocks, (Spry et al. 2000b). Quartzites, with or without, iron-manganese oxides, are easily identified as, former chert, jasper and siliceous sinter. Sillimanite-corundum rock may be metamorphosed alunite alteration; kyanite-andalusite rock might, have been advanced argillic alteration and cordierite-anthophyllite schist originally propylitized, andesite or basalt (Roberts et al. 2003). A premetamorphic potassic alteration and sulphidation, halo at the Hemlo gold deposit was transformed by, upper amphibolite facies metamorphism into, microcline and muscovite-quartz schist, apparently of lower metamorphic grade than the kyanite-bearing host formations (Heiligmann et al., 2008)., Oxide ore minerals, especially of iron and manganese, react readily with carbonate and silicate, , minerals. This caused, for example, formation of, the diagenetic-metamorphic skarn rocks in Sweden. Precambrian banded iron formations exhibit, many interesting metamorphic features (Stanton, 1972). For instance, if alternating magnetite and, haematite laminae remain stable, oxygen (and, fluid) mobility must have been very restricted., During deformation, haematite typically recrystallizes to micaceous specularite so that the result, is a rock similar to biotite schist. Iron silicates and, siderite are quite reactive, too. At higher metamorphic grades, siderite loses CO2 and converts, into magnetite or, in the presence of silica, to, fayalite (Fe2SiO4)., Like iron, manganese is a redox-sensitive and, reactive element. During prograde reactions, manganese enters a number of metamorphic minerals,, most often spessartine, Mn-rich almandine, rhodonite and rhodochrosite. Rocks of this composition are favourable material for the formation, of supergene-residual manganese ore deposits., Elevated Mn-contents of metamorphic silicates, (garnet, pyroxene, stilpnomelane, etc.) conserve
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124, , PART I METALLIFEROUS ORE DEPOSITS, , the geochemical halos of sedex ore deposits and are, useful prospecting tools., Sulphide ore is chemically less reactive, but, allows important deductions concerning deformation and heating history. Increase of grain size by, metamorphism is important in practice, because, processing of coarse ore is less energy-intensive., Controls on recrystallization and deformation of, sulphides include pressure, temperature, synchronous penetrative deformation and the presence of, fluids. Ductile deformation by creep along certain, lattice planes characterizes galena, stibnite, (>250 C) and chalcopyrite (>300 C), whereas, pyrite (Craig et al. 1998), magnetite and arsenopyrite display nearly always brittle deformation. The, brittle textures, however, may conceal plastic, strain during peak metamorphic conditions (Barrie et al. 2010). Banded ores typically display folds, and injections formed by galena, while pyrite exhibits boudinage culminating in “augen” (eye), structures. Sulphides may be foliated so that ore, textures resemble gneiss. In many cases this is due, to pressure solution according to Riecke’s principle. If the temperature peak follows well after, strain, traces of deformation may be erased by, recrystallization to foam textures. The presence, of fluids favours recrystallization and local mobilization of sulphides, resulting in the formation of, pegmatite-like ore veins in fractures within the, orebody or its immediate host rocks., In contrast to most silicate minerals, sulphides, are stable over wide metamorphic P-T conditions., Mineralogical and chemical changes are inconspicuous. Typically, metamorphic recrystallization causes little more than homogenization of, pre-existing sulphide minerals and as a consequence, mobilization of trace elements or neogenesis of mineral phases of these elements (Craig &, Vokes 1992). Stable isotope systems of the sulphides may be reset by metamorphism. Crowe, (1994) reports that sulphur isotopes in chalcopyrite-pyrrhotite assemblages were equilibrated by, metamorphism, whereas grains isolated in quartz, retained premetamorphic ratios. Orogenic metamorphism may cause a loss of sulphur and formation of pyrrhotite or magnetite from pyrite, (“desulphidation”). Sulphur is abstracted by dissolution in prograde dehydration fluids (e.g. water, , from breakdown of chlorite), mainly in the form of, H2S. These fluids may mobilize metals such as, gold and silver that were hosted in the original, pyrite/arsenopyrite because they complex with, H2S. Precipitation commonly takes place in the, immediate wall rocks, where the fluids encounter, minerals with reduced iron. In this way, metamorphic pyrrhotite, gold and silver ores can be formed, (Tomkins 2007), for example, in the Bergslagen, district, Sweden (Wagner et al. 2005, 2007). In, the Brunswick No. 12 volcanic-hosted massive, Fe-Cu-Zn-Pb sulphide deposit in Canada, however, gold was not upgraded and mobilized, in spite, of upper greenschist metamorphism and intensive, deformation (McClenaghan et al. 2009)., Beginning at low amphibolite facies metamorphism, the formation of sulphide melts from preexisting ore is possible (Frost et al. 2002). Metals, prone to melting include Au, Ag, As, Sb, Bi, Hg, Te, and Tl, some of which are low-melting-point chalcophile elements. Until now, this has only been, confirmed at a small number of localities. At, Broken Hill, Australia, for example, argentiferous, galena fills veins in the sillimanite-facies host, rocks near stratiform orebodies (Mavrogenes, et al. 2001). At the large lode gold deposit of, Hemlo, Ontario, Canada, premetamorphic disseminated ore was mobilized as a sulphosalt melt, within amphibolite facies host rocks. The melt, concentrated into dilatational domains such as, boudin necks and fractures (Figure 1.40). Sulphide, melts have low viscosities close to those of water, and therefore, are highly mobile. During metamorphism and deformation, they migrate like, other fluids into structurally favourable traps,, although in a limited perimeter., Note that the observations reported in the last, two paragraphs demonstrate the passage from, metamorphosed ( in situ) to metamorphogenic, (metals are dissolved, transported and reprecipitated) ore deposits, which are described in the next, chapter., Generally, most studies of metamorphosed ore, deposits confirm that the concept of an essentially, isochemical nature of metamorphism is appropriate. Mobilization of matter affects almost exclusively volatile elements and compounds (H2O,, CO2, O2, H2S). As mentioned above, only local
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , remobilization of very small parts of a deposit’s, total metal stock is observed (Marshall et al. 2000,, Vokes 2000, Wagner et al. 2005, 2007). Therefore,, earlier hypotheses are refuted, that metamorphic, “regeneration” of pre-existing ore deposits by dissolution-transport-reprecipitation is both ubiquitous and of more than occasional economic, significance. It is yet uncertain whether anatectic, partial melting induced by metamorphism may be, a path to the formation of ore deposits, although, selective enrichment of certain metals in the melt, fraction is possible (Tomkins et al. 2009). In, fact, these authors suggest that pre-existing gold, mineralization may be recycled through partial, melting to form intrusion-related (magmatichydrothermal) gold systems. Proven examples for, this connection have yet to be found, because the, source of fertile igneous rocks is deep crust,, whereas the deposits occur rather in the middle, and upper crust. The Challenger gold prospect in, South Australia seems to represent such a source, of felsic auriferous melt. There, exploration drill, core displays leucosomes in migmatite, which, contain 8 g/t Au. Round droplets consisting of, native gold and sulphides are thought to be frozen, ore melt (Tomkins & Mavrogenes 2002)., , 1.6 METAMORPHOGENIC ORE FORMATION, SYSTEMS, , Clearly, skarn and contact-metasomatic ore deposits are intimately related to thermal aureoles of, magmatic intrusions. They may be said to be, products of contact metamorphism, but the causal, agent is the interaction with magmatic fluids and, not simple change by heating. Therefore, we discussed this group in the magmatic domain (Section 1.1). The formation of ore deposits by regional, (most often orogenic) metamorphism, although, suspected earlier (Schneiderh€, ohn 1932), is, only now generally accepted. Consequently,, “metamorphosed” and “metamorphogenic” ore, deposits must be distinguished (Pohl 1992, Spry, et al. 2000a). Examples of mineral deposits that are, members of the metamorphogenic class include, orogenic gold (Groves et al. 2003), the graphite, veins of Sri Lanka, ruby in the Himalayas, several, , 125, , large talc deposits (Rabenwald, Luzenac), certain, metasomatic siderite deposits (Erzberg, Austria, and Bakal, Russia) and metallic ores (e.g. silver, and cobalt in Bou Azzer, Maroc; silver, lead and, zinc in western Canada: Beaudoin & Therrien, 1999; iron oxide-copper-gold in Australia: Fisher, & Kendrick 2008). Generally, deposits of this, genetic class were formed from passing hot aqueous fluids and are therefore hydrothermal and, epigenetic. Rarely, metamorphic sulphide melting, may have been the means of metal migration and, concentration, as possibly at Hemlo (Ontario, Canada: Tomkins et al. 2004)., In the preceding section, it was argued that apart, from devolatilization, regional metamorphism is, commonly an isochemical process. Here, this, statement must be qualified, in recognition of the, mass transfer caused by metamorphic fluids. Orogenic metamorphism induces a large-scale outward flow of heat and fluids (Masters & Ague, 2005) that may continue over geological timescales (e.g. 70 million years in the case of the, Palaeoproterozoic Ophtalmia orogen, Pilbara,, Australia: Rasmussen et al. 2005). The metamorphic fluids can be considered as solutions that are, in equilibrium with host rocks, and although they, are dilute, their sheer mass allows significant, transfer of dissolved matter. Even simple field, observations of metamorphic rocks provide clues,, such as the ubiquitous quartz or carbonate veinlets of epizonal rocks, the unidirectional dissolution of fossils and detrital grains, and the solution, schistosity that forms parallel to axial planes., Using these criteria, a loss of 20% of the original, rock mass may not be rare, even at low metamorphic grades. The mass loss of high-grade rocks is, probably even higher, but its determination is, rarely possible. In cases where identical rocks can, be sampled at different metamorphic grades, the, mobility of major, minor and trace elements can, be quantified. In New Zealand, ore-forming elements (Au, Ag, As, Sb, Hg, Mo and W) are depleted, in metamorphic rocks relative to unmetamorphosed protolith samples; the same elements are, enriched in the orogenic gold deposits of the region, (Pitcairn et al. 2006). Some data suggest that even, “immobile” elements (e.g. Ti, Zr, Y, Ta) are not, reliably conserved, although they are commonly
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126, , PART I METALLIFEROUS ORE DEPOSITS, , used for petrogenetic and geodynamic investigation of magmatic rocks (Pearce et al. 1984,, Winchester & Floyd 1977)., Lateral secretion, Lateral secretion is different from the metamorphogenic model. It is best exemplified by the, extremely common and ubiquitous generation of, quartz and carbonate veinlets from host rocks at, low metamorphic grades and of pegmatitic mobilizates at high grades. This interpretation is confirmed by the observation that the paragenesis of, these veins depends on the chemistry of the enclosing rocks. Quartz mobilizates, for example,, occur in siliceous metasediments, whereas calcite, veinlets form in limestone marble and in mafic, metavolcanic rocks. In the latter case, veinlets, include epidote, chlorite and sulphides. Lateral, secretion veins are mostly tensile structures that, originate in syn- to late-metamorphic stages. They, , may contain large crystals of both common and, rare minerals, such as the famous Alpine fissure, veins (Figure 1.82; Mullis 1996) and many gemstone deposits. Lateral secretion should result in a, balanced mass exchange between host rocks and, veins. Mobilized elements in the first decrease, with proximity to the veins. Because of the chemical equilibrium between fluids and rocks, lateral, secretion imprints no hydrothermal alteration on, host rocks and stable isotopes reflect equilibrium:, The fluids are of a local derivation. A transient, pressure gradient induces movement of fluids,, which were originally dispersed in the rocks, to, the opening fissure. Flow may be along grain, boundaries or by diffusion. This system is spatially, limited and essentially closed. In its pure form, there should be no inflow from beyond its boundary and little outflow. In tectonically quiescent, times, temperature and pressure in the fissure and, the country rocks are equal, but pressure drops, when active extension takes place (Figure 1.83)., , Figure 1.82 Alpine fissure with large quartz crystals encountered in 1974, in Aar granite gneiss (Switzerland) during, excavation of an access tunnel to the Grimsel II power station. Courtesy Nagra, Comet, Weisslingen.The crystal cave is, now a national geological monument, accessible from Nagra’s underground laboratory Grimsel. The fissure formed at, 16 Ma from fluids >400 C. Width of image 1 m.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 127, , m boundary, Syste, , P fluid, = P lithostatic, , Fissure, Dehydration, Fluids, , Figure 1.83 Principles of lateral secretion.Changes in the, orogenic stress field lead to opening of a fissure. The, resulting pressure gradient (Pfissure < Plithostatic) enforces, flow of metamorphic fluids into the fissure where, minerals precipitate. The system is closed within narrow, limits (“halo of dissolution”). The fluids are “dilute, solutions” of the surrounding host rocks., , Generally, fluid pressure is lithostatic. Because of, the limitations concerning the mass of participating fluids and dissolved matter, a lateral secretion, system is not expected to form economic metallic, or mineral deposits (apart from pockets of rare, mineral specimen). Lateral secretion veinlets, demonstrate the retention of metamorphic fluids, in a dehydrating rock body. Synchronous shearing, is the key to outflow of metamorphic fluids and, eventual formation of metamorphogenic ore, deposits., Fluid systems that produce sizeable mineral, deposits are open, in contrast to the closed or low, flow systems of lateral secretion. In shallower, parts of metamorphic complexes, the passage of, large masses of deep metamorphic fluids may, leave conspicuous clues in the form of regional, import of matter (e.g. potassium in the Caledonides: Mark et al. 2007). Less visible, oxygen isotope systematics of rocks are changed by the, passing fluids (Beaudoin & Therrien 1999). Flow, channels of the fluids (e.g. veins) are marked by, , P fissure, < P lithostatic, , "Halo of dissolution", , hydrothermal alteration of the wall rocks. Also,, veins are surrounded by trace-metal halos with, contents increasing as the vein is approached., Changes of this kind can only be explained by the, influence of migrating fluids that have passed, through the system. They originated in deeper, parts of the orogenic body, where metamorphism, was active. Prograde metamorphism liberates, large amounts of water by endothermic reactions., This can be substantiated both by simple calculations and by comparative analyses of metamorphic rocks (Hanson 1997)., Metamorphic fluids, Metamorphic fluids originate primarily by chemical release (devolatilization), different from diagenetic fluids. Increasing metamorphism, from, sub-greenschist facies to anatexis, produces a steady flow of metamorphic dehydration fluids and a, decrease of volatiles in the respective metamorphic rocks. The dehydration reactions (eq. 1.25) are
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 129, , metamorphic origin, but a contribution from other, sources including the mantle, magmatic and, meteoric domains is possible. Although the gold, concentration in metamorphic fluids is very low,, focused mass flow through chemical and physical, traps formed many large gold deposits., Time and rate of fluid flow from rock bodies, undergoing metamorphism is difficult to determine. Two end-member modes are possible: i), Either fluids formed in the rock mass are conserved, and later expulsed all at once (“batch volatilization”); or ii) fluids flow out as they are set free, (“Rayleigh distillation”). In geological reality, it, may be assumed that both processes act alternately, and in various combinations, mainly due to physical, boundary conditions (e.g. stress and strain, pore, pressures, pressure gradient, permeability, temperature gradient, etc.) imposed by concomitant tectonics and the geological framework. The mineral, specimen-rich Alpine fissures, for example, testify, to retention of metamorphic fluids until after the, , Eoalpine Orogeny (e.g. siderite, copper, barite, talc), display similar inclusion fluids. This data was, interpreted as evidence of a Synorogenic EastAlpine Fluid Province (Pohl & Belocky 1994,, 1999). Note that preliminary data on fluid halogen, mol ratios and Sm/Nd ages of ore and host rock, suggest generation of siderites by deeply circulating evaporative brines in Late Triassic time (W., Prochaska, unpublished)., Many gold ore deposits are interpreted to have a, metamorphogenic origin. Common examples are, gold quartz veins with sulphides of arsenic, antimony and iron, which occur far from igneous, intrusions in black schist, metaturbidites or in, greenschist-facies metamorphic rocks. Commonly, they were formed in orogenic belts and at, the time of waning orogeny, correlated with latemetamorphic uplift, lateral spreading, deep shear, zones and intrusive activity. This is the rationale, for the term “orogenic gold deposits” (Figure 1.84)., Most of the fluids and the solutes are probably of, , Age of orogenic gold provinces, Korea & N. China, , Silurian, , Bu, rm, a, , Devonian, , New, Guin, ea, , Carboniferous, Permian, Tien Shan, , Middle and Late, Palaeozoic, (undifferentiated), , Indo china, , Australia, Ta, rim, , S. China, , Tasmania, , Ti b, et, , W. Iran, , W. New, Zealand, , Lut, , India, Arabia, , Turkey, , Antarctica, , E. Europe, Madagascar, , C. Europe, , France, , Africa, Spain, , Figure 1.84 Large gold ore provinces, formed in the Middle to Late Palaeozoic, within Gondwana and adjacent plates, (adapted from Haeberlin et al. 2003 with, permission from Elsevier). Active, continental plate margins controlled the, location of gold ore formation, illustrating, the term “orogenic gold deposits”., , E. Avalonia, , Mauritanides, , W. Avalonia, , a, , eric, , Piedmont, Yucatan, , So, , u, , m, th A, , E. New, Zealand
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130, , PART I METALLIFEROUS ORE DEPOSITS, , BOX 1.13, , Two end member models of metamorphogenic ore, formation, , 1. Prograde metamorphogenic ore formation. Normally, metamorphic fluids are expulsed in the form of a wide and, diffuse flow into regions of lower pressure (Hanson 1997, Jamveit & Yardley 1997). Large regional tectonic structures, (shear zones, extensional faults and thrust faults) focus the diffuse flow, because they can be channels of higher, permeability (Figure 1.85). The permeability of the lower ductile crust (10–15 km beneath the surface, depending on the, geothermal gradient) undergoing prograde metamorphism is very low with a flow of only 0.25 m/year (Beaudoin &, Therrien 1999) and the pressure regime is lithostatic. Note, however, that even in the middle and lower crust, an interplay, between brittle and ductile deformation may occur (Mancktelow 2006). In the brittle upper crust, permeability is much, higher and flow in faults reaches 100–1000 m/year. When rising fluids enter this regime, pressure is released and, approaches hydrostatic conditions. Descending (e.g. meteoric) water can penetrate as far as the brittle/ductile boundary, (Ingebritsen & Manning 1999). Because of these particular conditions, the brittle/ductile transition at ca. 425–375 C is a, very frequent location of metamorphogenic ore deposit formation., , Figure 1.85 Ore deposit formation by prograde, metamorphism.A shear zone focuses upflow of, metamorphic fluids because of higher, permeability. Its crustal-scale vertical extent, facilitates transfer of fluids from lithostatic to, hydrostatic pressure domains. Ore formation is a, consequence of chemical or physical traps. The, system is open, the mass flow is unidirectional., 2. Retrograde metamorphogenic ore formation. Many geological observations (e.g. concerning the structural control of, orebodies) indicate that ore formation took place long after peak metamorphic conditions (or even totally unconnected to, orogenic metamorphism). Petrographic investigations show that cooling and uplift of metamorphic complexes is, accompanied by retrograde exothermic reactions of rocks with infiltrating fluids (mainly hydration; Haack & Zimmermann 1996, Yardley et al. 2000). Usually, these fluids will be derived from a near-surface reservoir (e.g. meteoric, marine, or basinal water). The water descends along structural conduits and in contact with suitable rocks forms hydrous minerals., Oxygen is consumed in oxidation reactions. Tectonic preparation of a large rock surface (e.g. breccias) is favourable. In, the reaction zone, rocks and surplus water are heated and country rocks sustain retrograde metamorphism. Heated fluids, are charged with dissolved matter. Hydrothermal convection cells are established with hot fluids of low density rising, back to the surface. Along the upflow channels, precipitation of dissolved matter from the fluids can form retrogrademetamorphogenic ore deposits (Pohl 1992, Figure 1.86). Convection cells may reach the brittle/ductile boundary, where, descending waters mix with deeper prograde-metamorphic fluids, as shown by Templeton et al. (1997) in the Alps of
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 131, , New Zealand. In the European Eastern Alps, many ore deposits are correlated with the second Eocene metamorphic, phase. Gold quartz veins in the metamorphic core region of the Penninic unit (Tauern Mountains) were formed by ponded, prograde fluids, whereas iron ore in higher tectonic units is the product of mixing of prograde fluids with descending, meteoric water (Pohl & Belocky 1999)., , Surface, , 0, Faults, Pfluid = Phydrostatic, , Pressure (kbar), , 1, Ore, precipitation, , 2, Reaction, zone, , 3, 400°C, , Figure 1.86 Ore deposit formation by, retrograde metamorphism.Tectonic, extension opens flow paths for descending, cool waters that react at depth with hot, rocks (“reaction zone”), take up solutes, and rise back to the surface as, hydrothermal fluids. The pressure regime, is usually hydrostatic and the convection, system is essentially open., , Brittleductile, transition, , 4, Descending, cool water, , 5, , Ascending, hydrothermal fluid, , An instructive concept of ore formation during late-metamorphic cooling processes was described from the Scottish, Dalradian belt (Craw & Chamberlain 1996). Infiltrating meteoric water induced an oxidation front that migrated through, the orogenic body and facilitated leaching of trace gold. The gold was precipitated in reducing environments of the, hydrothermal upflow limb (e.g. in the Tyndrum deposit)., , temperature and pressure maximum of metamorphism. Ore deposit formation is more probable, when large masses of stored fluid flow off in a, geologically short time. This may often be the, period of uplift, shearing and distension of metamorphic complexes. Synchronous metamorphogenic ore deposit formation in large areas supports, the concept of tectonically activated “fluid pulses”., Many ore deposits, which were formed in stable, cratons during geologically short events of major, plate reorganizations (Whittaker et al. 2007), may, be regarded as members of the retrograde-metamorphogenic class. Prominent examples are the, unconformity uranium ore deposits in Canada and, Australia (cf. Chapter 2.5 “Uranium”). Ore depos-, , its formed during phases of extensional plate, deformation, such as Alpine and Irish type PbZn-Ag, often display a retrograde-metamorphogenic component when convection systems reach, down into older basement. This is easily revealed, by Pb-isotope investigations., Both models of metamorphogenic-hydrothermal ore deposit formation systems, prograde and, retrograde, appear scientifically well founded, but, the genetic attribution of specific deposits remains, difficult. One reason is that orogenic metamorphism is widespread and operates in settings as, different as active continental margins (Andes), in, island arcs (New Zealand) and in zones of continental collision (Alps). Regions where mantle
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132, , PART I METALLIFEROUS ORE DEPOSITS, , magmas heat the crust from below (underplating,, deep intrusions, continental extension, rifting), experience mobilization of volatiles and the establishment of deep convection systems. In all these, settings, synchronous crustal melting and igneous, activity are common, too. This causes difficulties, in distinguishing metamorphogenic from magmatic ore deposits (Bucci et al. 2004). A severe, problem is also that in nature, mixed variants of, the theoretical end member models described, seem to be prevalent., 1.7 METALLOGENY – ORE DEPOSIT FORMATION IN, SPACE AND TIME, , “Deciphering the predisposition of orogen segments, for exceptional mineral endowment is one of the, most critical as well as intractable metallogenetic, research topics”, R.H. Sillitoe 2008, , Metallogeny is the science of origin and distribution of ore deposits in geological space and time, (de Launay 1913). The term includes both metallic, and non-metallic mineral deposits, as does, “minerogeny”. Note that the adjective form of the, word “metallogeny” is metallogenetic, although, the abbreviated “metallogenic” is also used. As we, have seen in the preceding sections, metallogenesis (ore deposit formation) is a function of the, Earth’s process systems. Mineral deposits are a byproduct of dynamic processes in mantle and crust, of the Earth, and of their interaction with hydrosphere and atmosphere. In geological time, these, processes occur in geodynamic cycles that produce, defined geological entities, such as the Alpine,, Variscan and Caledonian orogenic belts, each with, characteristic types of mineral deposits., Important targets of metallogenetic studies are:, i) the quest for the source of a valued element, ii), understanding mobilization and transport systems; and iii) the nature of the trap that caused, the enrichment resulting in a mineral deposit., Some regions have similar ore deposits dating, from more than one geodynamic cycle, for example Palaeozoic to Tertiary carbonate-hosted leadzinc in the Mediterranean realm and gold in the, North American Cordillera (Sillitoe 2008). This is, , described but not explained by the term, “metallogenetic heredity”. The word implies that, the crust contains a geochemically distinct trace, metal reservoir that is the source of repeated, mobilization and mineralization. An opposing, hypothesis dismisses this notion and claims that, ordinary crust or mantle can be the source for most, ore deposits. The formation of fertile tin granites,, for example, can be traced to average crustal concentrations of tin; the critical factor is highly, efficient magmatic differentiation (Lehmann, 1990, Figure 1.18). There is no doubt, however,, that both crust and mantle are geochemically, highly heterogeneous. Anomalous metal contents, are possible, and the heredity need not be based on, an anomaly of a specific ore metal such as tin, but, on other parameters such as redox state or elevated, F, Cl, B and Li contents of source rocks that are, conducive to mobilization and concentration of, tin. This remains unproven, but with a more, positive note, Plant et al. (1997) demonstrate that, regional geochemical maps do allow important, deductions about metallogenesis and metal, endowment of an area., The unidirectional evolution of the Earth in, time is of superordinate rank compared to geodynamic cycles (Holland 2005, Sleep 2001, Windley, 1995). In the 4500 million years (Ma) of geological, history, Earth systems experienced severe changes, reaching from the atmosphere, the biosphere and, the oceans down into the mantle. Of course, metallogenetic evolution reflects these changes, (Goldfarb et al. 2010). Several deposit-types occur, only in certain periods of geological history. Examples include the komatiite-hosted nickel sulphides in Archaean greenstone belts (common, from 3.8–2.5, rare until 2 Ga), banded iron formations of the Superior type (2.6–1.8 Ga), graniterelated tin deposits in the Late Palaeozoic and, Mesozoic, and uranium in sandstones of Cretaceous and Tertiary age. Another important factor, controlling the distribution of ore deposits in geological time is the preservation potential. Just as, some rocks are more prone to erosion than others, in long-term geological processes (Hawkesworth, et al. 2009), so are ore concentrations. Preferential, exhumation of near-surface deposits and their, destruction by erosion must have occurred
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , throughout geological time. This is probably the, reason why epithermal and porphyry copper deposits are much more frequent in Phanerozoic, compared to Precambrian time (Wilkinson & Kesler 2009, Kesler & Wilkinson 2006)., Impacts of extraterrestrial bodies may have influenced metallogenesis more profoundly than we, realize today. The impact origin of the giant structure of Sudbury, Canada and its ore deposits (Dietz, 1964) is, of course, generally acknowledged. The, bombardment of Earth from space was most intensive during early geological time (ca . 4500–3800 Ma)., The imported matter must have been mixed into the, mantle. Impact structures on the surface were, erased by Archaean crust building and by plate, tectonics. Today, about 400 younger impact structures are exposed but apart from Sudbury, there are, hardly ever convincing relations with economic ore, deposits. Yet, we may ask, for example, if the supergiant ore concentrations in southern Africa (e.g., Bushveld, Great Dyke) could be inherited from, extraterrestrial material mixed into the mantle. In, contrast, little doubt exists that part of the giant oil, and gas resources in the Gulf of Mexico are intimately related to the Cretaceous-Palaeogene boundary Chicxulub impact (Grajales-Nishimura et al., 2000)., 1.7.1 Metallogenetic epochs and provinces, Useful metallogenetic concepts and terms include, (Petrascheck 1965):, . Metallogenetic Domains implicate crustal sectors with a comparable geological evolution that, include a number of metallogenetic provinces of, different ages; examples are the European Alps, the, South American Cordillera, or the Archaean Pilbara Nucleus of the West-Australian Shield., . Metallogenetic, or Ore Province combines all, deposits that were formed in a major geotectonic, unit during one geodynamic cycle, and that are, closely related by chemical affinity, form and, metal endowment; for example, the tin province, in the Late Palaeozoic Erzgebirge of Germany and, Czechia; metallogenetic provinces extend over, large areas, in contrast to districts., . Metallogenetic District, or Zone describes parts, of an ore province; Sillitoe (2008) uses the term, , 133, , “metallogenetic belt” with the same meaning;, deposits within such a district are very closely, related, single deposit types predominate, and, metallogenetic activity was typically restricted, to a short geological time Marcoux & Jebrak, (1999) distinguish migrating and stationary districts; the latter are thought to be more productive because an energy source (e.g. a mantle, plume) remained stationary for a geologically, longer time (>10 Ma)., . Metal Province describes the distribution of all, deposits of one metal (or a group of metals such as, Pb, Ag, Zn) irrespective of their age; this illuminates regions of possible metallogenetic heredity., . Metallotects are geological features that have, caused formation or localization of mineral deposits; they include major crustal structures (Hough, et al. 2007), metamorphic, volcanic and plutonic, centres, regional geochemical barriers (the European Copper Shale) and discordances (uranium)., Utilization of metallogenetic terms tends to be, vague, which is caused by problems of practical, application. The main difficulty is often the age, attribution of deposits, enforcing a loose definition, of metallogenetic provinces and zones in respect of, time (Routhier 1980). A statistical method to, identify metallogenetic provinces and epochs was, presented by Wilkinson & Kesler (2009), built, from a large database on porphyry copper deposits., The authors propose to determine regions with a, special endowment (metallogenetic provinces), and periods of enhanced deposit formation (metallogenetic epochs), after correcting the agefrequency and deposit-density distributions for loss, by uplift and erosion as well as subsidence and, burial. Although Wilkinson & Kesler (2009) reveal, the Late Eocene and the Middle Miocene as epochs, of enhanced porphyry copper mineralization in, South America, they state that spatial distribution, remains unpredictable. The key to “deciphering the, predisposition of orogen segments for exceptional, mineral endowment” (Sillitoe 2008) remains, improvement of geological understanding., Metallogenetic maps are produced in order to, allow a synopsis of metallogenetic, geological (metallotects) and basic economic data (e.g. the size of, deposits). A generalized geological background is, used to display information on size and nature of
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134, , PART I METALLIFEROUS ORE DEPOSITS, , ore deposits. Symbols aim at depicting generalized, genetic and morphological information, and the, nature of the main metal or mineral (Figure 1.87)., Metals are indicated by colour, for example yellow, for gold and silver, blue for lead and zinc, red for tin, and tungsten, etc. The relative size of deposits is, expressed by varying the size of symbols. Usually,, boundaries of metallogenetic provinces and districts are shown. Many countries have published, national metallogenetic maps. Europe is covered, by several sheets of the Metallogenetic Map of, Europe and Neighbouring Countries (scale 1:1, 250 000; Emberger 1984) and by one sheet of the, Mineral Atlas of the World (scale 1:10 000 000;, published by the Commission de la Carte, Geologique du Monde, CGMW, Paris, and the, Geological Survey of Norway, NGU, Trondhejm, 1997 (Juve & Storseth 1977)). Metallogenetic maps, and the supporting mineral deposit data banks, serve scientific interests, but their main use is, practical; they are indispensable for estimates of, undiscovered mineral resources and for planning, strategic exploration., , Stratiform deposit, layers, beds, Stratabound deposit, lens, Vein or shear-zone filling, Skarn, contact-metamorphic deposit, Stockwork, including porphyry, deposit, Stock, massive, magmatic deposit, or salt dome, Magmatic stratiform deposit, , 1.7.2 Metallogeny and plate tectonics, About 60 years ago, the understanding of global, tectonics experienced a revolutionary advance, (Kearey et al. 2009). Until then, the Earth’s crust, was considered to move either up or down, but, rarely in a horizontal direction. The new concept, of plate tectonics recognized that the lithosphere, is divided into a number of rigid plates, which, display considerable lateral movement. The, engine of plate tectonics is convective cooling of, the mantle. The resulting lithosphere is in part, recycled back into the mantle. Extensional and, compressional interactions at plate boundaries are, the cause of profusely fertile metallogenetic, systems., The Theory of Plate Tectonics was worked out, only recently, but its foundations are much older., The similarity of the coastal geometry of South, America, Africa and India, and their sharing Permian sediments with the striking Glossopteris flora, made already Eduard Suess (1831–1914) speculate, that continents were not fixed in time and that in, the geological past, the three formed a large supercontinent that he called Gondwana (Suess 1885)., Building on this, Alfred Wegener developed the, hypothesis of continental drift (Wegener 1924), that is in large parts still valid. Modern understanding of plate tectonics led to great progress in, many fields of the earth sciences, including metallogeny (Robb 2005, Sawkins 1990b). Several, lines of evidence indicate that plate tectonics may, have started to operate as early as 4.4 Ga when a, stiff lithosphere had been established (Moyen et al., 2006, Furnes et al. 2007). Already in the Archaean,, ore deposits are known that suggest a suprasubduction zone setting., Main elements of plate tectonic process systems, that are “metallogenetic factories” include:, , Pipe (e.g. kimberlite), Lateritic deposit, Placer, Figure 1.87 Common morphological and genetic, symbols for different deposit types on metallogenetic, maps., , The formation of intracontinental rifts,, aulacogens and large sedimentary basins, (incipient divergent plate boundaries), Rifts originate by extensional deformation of lithospheric plates and may or may not evolve into a, new plate boundary. Very often, rifting causes, thinning of the crust, upflow of hot mantle and
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , updoming of rift shoulders (Tackley 2000). Volcanic activity within the rifts is a frequent consequence, often organized into large volcanic centres, (“hot spots”: Foulger & Natland 2003). Hot spots, can be the origin of three diverging rifts (triple, junction). Two of the three rift arms may widen to, form a new ocean, whereas the third remains, inactive and is called a failed rift arm. Several, failed rift arms display thick sediments with, bimodal volcanic rock suites, which were later, folded by horizontal shortening. Considerable, intrusive activity may occur. Settings like, this have been called aulacogens (Eriksson &, Chuck 1985)., Sediments of continental rifts include early,, mainly terrestrial, alluvial clastic infill that can, contain uranium, placers and coal deposits. In, many cases, a freshwater, saline or marine-influenced lake stage succeeds with beds of salt, gypsum, magnesite, phosphate, valuable clays or oil, shale. Full marine ingression into the widening, rift and inception of oceanic spreading can induce, submarine metalliferous exhalation of the black, smoker or brine pool type (Red Sea) and the deposition of thick marine sedimentary sequences., Later, as diagenesis is enforced by rising temperature and pressure, oil and natural gas deposits are, generated., Hot spot-related ore-forming systems include, the Bushveld in South Africa, tin-fertile A-granites, in Nigeria and worldwide, many alkali-carbonatite igneous complexes. When rifting reaches the, stage of a deep graben with vertical displacement, at marginal faults approaching kilometres (Scholz, & Contreras 1998), hydrothermal convection systems may form, based on the permeable tensional, structures, the heat contrast and the hydraulic, head imposed by rift shoulder mountains. The, ascending branch of these hydrothermal systems, typically results in deposits of lead, zinc, silver,, manganese, fluorine and barite, which take the, form of veins and metasomatic replacement bodies in rift margin rocks, or of ore beds in the graben, sediments. Good examples are many Pb-Zn and, Mn occurrences in Tertiary sediments on both, sides of the Red Sea, and part of the Ag-Pb-Zn-FBa veins along the Rhine graben in France and, Germany (Figure 1.28). Carbonatites and alkali, , 135, , intrusions with apatite, fluorine, niobium and rare, earth element ores characterize the CretaceousTertiary rifts in Eastern and Central Africa. Submarine, epicontinental rifts and half-grabens are, related to base metal deposits of the sedex type., Sullivan in British Columbia, Canada (base metals in the Neoproterozoic Alberta Rift), Mt Isa in, Queensland, Australia (Pb-Zn-Cu in the early Mesoproterozoic) and the large deposits of native, copper in basalts and of chalcocite in fine sands, of the Nonesuch Shale in the Keweenawan Rift, (USA, Late Mesoproterozoic) were proposed as, remarkable examples of mineralization in aulacogens. The Panafrican Damara Orogen in southern, Africa has also been interpreted as an aulacogen,, although with exceptionally strong tectonic shortening. Its main mineralizations are late to posttectonic, including the giant hydrothermal karst, pipe Tsumeb with polymetallic ores of Pb, Zn, Cu,, Cd and Ge (Chetti & Frimmel 2000), and the, uranium-deposit R€, ossing in alaskitic granite., Intracontinental basins with prominent ore provinces include the European Copper Shale (Mesozoic), Witwatersrand gold (Late Archaean) and, Mississippi Valley type lead-zinc-barite-fluorite, deposits (Palaeozoic) in North America., Major plate reorganizations affect both continental and oceanic systems intensely (Whittaker, et al. 2007). Within short periods of a few million, years, new subduction zones are installed, vectors, of plate drift change (wander paths form “loops”), and the plates are subjected to new stress fields., Oceanic and continental crust is stretched or, sheared, new mantle regions experience partial, melting, resulting in magma underplating, the, formation of hotspots and the rise of mantle volatiles. Flood basalt volcanism may be a consequence, producing giant Cu-Ni-PGE deposits such, as Noril’sk, as well as climate change and global, extinction of life due to huge emissions of sulphur, and chlorine such as those of the Dekkan traps at, the end of the Cretaceous (Self et al. 2008)., Enhanced heat flow and elevated permeability of, the crust are favourable factors for the formation of, deep convective hydrothermal systems and of, mineral deposits. Mantle volatiles (mainly water, and CO2, with solutes like fluorine, arsenic, etc.), may rise, leach metals, mix with crustal fluids and
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136, , PART I METALLIFEROUS ORE DEPOSITS, , form ore deposits. Examples of mineralization, caused by plate reorganization include the hydrothermal “Saxonian Mineralization” of Europe, north of the Alps (Walther 1983), several kimberlite provinces and unconformity uranium ore deposits in Canada and Australia., , Seafloor spreading and the production of new, lithosphere at mid-ocean ridges (oceanicdivergent, or “constructive” plate boundaries), This is the domain of ore formation at mid-ocean, ridges that was presented earlier in more detail, (Section 1.1 “Ore Deposits at Mid-Ocean Ridges, and in Ophiolites”). After obduction, the products, of these processes are ophiolite-hosted deposits., Many ophiolites, however, were not formed at, mid-ocean rifts but in tensional supra-subduction, settings including back-arc spreading systems, or, rifts of primitive island arcs (e.g. the Cyprus, ophiolite). Yet, there is no doubt that all midocean rifts display segments of hydrothermal, activity, including black smokers. Related ores, are sulphide mounds or mud-pools in a proximate, position, iron-manganese oxides (ochres and, numbers) and distal manganese crusts and nodules, with important contents of Cu, Ni and Co. Oceanic transform faults that offset ridges are, apparently not metallotects for mid-ocean, metallogenesis., , The evolution of passive continental margins, and the disruption of older ore provinces, (divergent plate boundaries), The opening of new oceans passes from a high, heat-flow rift stage into a marine transgression, and thermal contraction phase. Relatively shallow, epicontinental seas may form. As the young, ocean widens, passive continental margins, develop. Sediments include salt, phosphate, and hydrocarbon source rocks. Manganese ore, beds of the Tertiary Black Sea province, Quaternary metalliferous marine placers and Palaeoproterozoic banded iron ores of the Superior type, represent typical marine epicontinental shelf ore, deposits., The separation of continents by rifting and seafloor spreading may cut across older orogenic, belts, cratons and other crustal-scale structures., With them, older ore provinces are ruptured and, the fragments can be found on remote coasts, across an ocean (e.g. the Atlantic borderlands, of Africa and South America). In these cases,, metallogenetic knowledge acquired on one, coast is a valuable tool for work in its twin across, the seas., Mid-Ocean Ridge, , Cr (obducted ophiolites), , Hydro, us, Ductile flow, , ocea, nic, , Porphyry, Cu-Mo-Au, IOCG, Fe-P, (Cu,, Au), , cru, st, , Sn-Ag, , Partial melting (crust), Hig, h, , Oceanic mantle, , Subduction recycles oceanic lithosphere back into, the mantle (Figure 1.88). The trace of subduction, on the seafloor is marked by deep oceanic, trenches. Volcanic arcs develop on the overriding, plate. Between trench and arc, four structural, zones are typically developed: Nearest to the, South America, , Nazca Plate, , Sulfides, , Subduction of lithospheric plates at convergent, (“destructive”) plate boundaries, , P-, , low, , T, , O, , Partial melting, (mantle, wedge), , xi, , d, ze, di, , s, id, flu, , M, et, am, or, ph, is, m, , d, , an, m, el, ts, , Figure 1.88 Metallogeny of active, continental margins with the typical, zonation, here illustrated by a schematic, profile from the subducting Pacific Nazca, Plate through the Central Andes (South, America). Adapted from Sillitoe (1972,, 2008).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , trench (1) an accretionary complex of low-grade, metamorphic sediments is followed by (2) a wedge, of mainly continental crust with minor oceanic, and hydrated mantle material of medium to highpressure metamorphic grade. This is overlain by, (3) a mega-scale melange composed of high-pressure and ultrahigh-pressure oceanic and continental crust fragments that are extruded from the, subduction channel. Finally follows (4) the frontal, part of the upper plate that carries the volcanic arc., Volcanic arcs in dominantly oceanic settings form, island arcs, whereas active continental margins, display continental or Cordilleran arcs. Recent, primitive island arcs are geologically young, (Tonga, Scotia), because maturation sets in, quickly and produces arcs with a partially continental character (Japan, Kurile Islands). Other, island arcs pass into continental collision belts, (Sumatra-Malaysia-Himalaya). Andean volcanic, arcs build upon older, strong upper crust that is, largely of Precambrian age in both Americas., Behind the magmatic arcs appear back-arc spreading systems that include the back arc basins of, island arcs, the continental “molasse” basins and, broad distended regions such as the Basin and, Range Province of North America., There is a great diversity of subduction, zone configurations, due to many variables including slab density, thickness and length (Schellart, et al. 2007). Subduction zones show variously, high or low trenchward plate velocities, trench, retreat (or more rarely trench advance) velocities,, slab dip angles and so forth. Trench retreat, (“subduction rollback”) is caused by the negative, buoyancy of the cold, dense descending slab., This places the overriding lithosphere into a state, of tension as the subduction zone moves oceanwards and facilitates movement of magmas and, fluids (Hamilton 1995). Slab rollback, slab, breakoff and delamination of mantle lithosphere, allow asthenospheric upwelling that can, provide the heat pulses required for ore forming, processes, including magmatism and regional, hydrothermal fluid systems. Extensive intracontinental compressional deformation migrating, cratonwards is explained by flat-slab subduction., This is probably caused by the subduction of oceanic plateaus and plume tracks (Livaccari et al., , 137, , 1981), which typically ends in delamination, (foundering) of the slab from the continental, lithosphere. This is the environment of Basinand-Range type tectonic and magmatic provinces, (e.g. in Mesozoic South China: Li & Li 2007)., Fertile anorogenic magmatism including alkaline, basalts, bimodal volcanic rocks and I- and A-type, granitoids are characteristic for this setting., It is important to stress that most of the Earth’s, richest ore provinces are found above subduction, zones. This is conspicuously so along the margins, of the Pacific Ocean, which are largely formed by, long-lived destructive plate boundaries. Associated are numerous active volcanoes, accounting, for the term “ring of fire”. Reconstruction of similar settings for stages in the geological past is, crucial for strategic exploration planning (Haeberlin et al. 2003, Figure 1.84)., Island arc ore deposits may be either allochthonous, which implies tectonic transport, for example of slivers of oceanic lithosphere, or, autochthonous, formed within the arc. Allochthonous are first of all the ophiolite-related ores,, including chromite (Cuba, Luzon) and platinum, placers; Cyprus type sulphide deposits are infrequent. Lateritic nickel ore deposits (New Caledonia) are autochthonous formations. Major, autochthonous deposits are associated with the, large mass of calc-alkaline to potassic intrusive, and volcanic rocks. Of outstanding economic, prominence are porphyry and skarn copper-gold, deposits, epithermal gold deposits and volcanogenic massive sulphides. Similar to continental, margin arcs, sources of the metals may be subducted oceanic crust or the mantle wedge above, the subduction zone. The latter was confirmed for, Lihir, Papua New Guinea (McInnes et al. 1999),, which is a giant epithermal gold deposit of very, recent geological age (ca. 690 ka). It is significant, that some metals such as tin and mercury appear, only in older, more complex island arcs with a, partially continental character., Active continental margin ore deposits are often, more clearly zoned compared with island arcs, as a, function of increasing distance from the subduction zone. In the apparently simple geotectonic, setting of Central South America, the results are, long and narrow ore provinces (Sillitoe 1972).
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138, , PART I METALLIFEROUS ORE DEPOSITS, , Prominent along the western coast within Precambrian basement and Cretaceous plutons are, iron-apatite and iron oxide-copper-gold (IOCG), deposits associated with hydrous intermediate, magmatism (Sillitoe 2003, Oyarzun et al. 2003)., A belt of giant porphyry Cu-Mo-Au deposits follows towards the east, roughly along the NeogeneRecent volcanic arc. These mines currently dominate world copper production. Near the eastern, margin of the Cordillera, a Sn-Ag belt is developed, (Figure 1.88). However, neither this zonation nor, all deposits in single belts are synchronous, but are, the product of several regional metallogenetic, episodes that range from Late Triassic (earliest tin, deposits) to Cretaceous (iron and part of copper), and Tertiary ages (most of the copper and tinsilver). Subduction configurations during this, time changed considerably (James & Sacks 1999)., The structure of the North American Cordillera is even more complex. One example is the, subducting East Pacific Ridge, which is a factor, that enhances metallogenetic processes. Partial, melting of young, hot subducting oceanic plates, favours the formation of oxidized adakitic magmas and of important gold and copper-gold deposits (Cooke et al. 2005, Mungall 2002). South, Alaskan gold deposits are thought to be related, to ridge subduction (Haeussler et al. 1995)., Another difference is the collage-like nature of, the North American Cordillera that consists, of many “suspect” or “exotic” terranes, which, preserved distinct but interrelated geological records (Colpron & Nelson 2006). This complicates, metallogenetic interpretation., Ore deposit formation above subduction zones is, causally tied to the fate of the subducting lithospheric slab of oceanic crust and mantle. At midocean ridges, the crust is largely hydrated and, oxidized. When the oceanic slab bends before, entering the subduction zone, additional hydration, appears to take place (Faccenda et al. 2009). Altered, basalts, gabbros and depleted mantle peridotites, enter the subduction zone as a “cold” slab at, geothermal gradients of 15 C/km or less. Along, the subduction plane, continental material can be, scraped off (“subduction erosion”) and taken down, to the zone of dehydration and melting. The highpressure/low-temperature metamorphism of sub-, , duction zones converts the rocks to the typical, blueschist and eclogite lithologies. Mantle rocks,, oceanic crust and its sedimentary cover incur devolatilization and possibly, partial anatexis. Dehydration processes control the structure of slabs, from ca. 40 to 150 km depth (Rondenay et al., 2008). As a function of T and P, hydrous fluids,, anatectic melts or supercritical liquids may be set, free (Kessel et al. 2005). The latter are characterized, by high trace element solubilities and consist of, H2O, Cl, S, CO2, etc., including large ion lithophile, elements (LILE: Ba, K, Rb, Cs, Ca, Sr) and other, incompatible elements (U and Pb). This transfer, “metasomatizes” the mantle wedge above the subduction zone and triggers widespread melting., Because of relatively high fO2 (roughly from fayalite-magnetite-quartz [FMQ] to FMQ þ 2) sulphide, (S2 ) and sulphate (S6þ ) coexist and combine to high, total sulphur contents in melts, which favours, sulphur saturation and mineralization in the upper, crust (Jugo 2009). Extensive formation of sulphide, melt during metasomatism and partial melting of, the hot mantle wedge would be detrimental,, because sulphide melts scavenge chalcophile and, siderophile elements such as copper and gold from, silicate melt and being heavy, tend to remain, trapped in the deep crust (Mungall 2002)., Magma batches rising through the crust continue to change by complex assimilation and contamination processes, until they reach the surface, as calc-alkaline melts of andesitic-dioritic nature., These magmas have only 50% material from the, mantle, the other half is derived from the crust., Intrusive and extrusive activity of continental arcs, is concentrated in short pulses of 10–15 My (“flareups”) that occur during and after tectonic shortening (Ducea & Barton 2007). Porphyry coppermolybdenum-gold deposits are direct products of, these processes within and above the subduction, zone (Richards 2003, 2009). Among many other, arguments, this can be substantiated by the observation that localization and metal contents of, porphyries are largely independent of their specific, setting (e.g. primitive or evolved island arcs,, diverse types of active margins). The precise, source of the chalcophile metals and gold – oceanic, crust or mantle wedge – remains obscure (Dreher, et al. 2005). A continental source, however, is
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , implied for the metals tin, tungsten and tantalum,, because deposits appear only in regions with thick, and old crust., Continental collision, Oceans that were consumed by subduction leave a, suture in the newly welded continent, which is, marked by ophiolites. One of the most remarkable, and metal-endowed sutures worldwide is the Palaeozoic accretionary orogenic collage of the Altaids in Central Asia, with a length of 3000 km, (Xiao et al. 2009). Usually, continental collision, results in the subduction of continental crust, (Ampferer or A-subduction), although this is limited by the buoyancy of crustal rocks. The process, results in thickened crust below collisional belts, and the formation of anatectic S-type granitoid, melts. Less frequent are post-subduction Cu-Au, porphyries and related epithermal gold deposits,, which are formed where former magmatic arcs are, involved in the collision (Richards 2009). Continental crust of the lower plate can be subducted to, depths of more than 100 km and exhumed after, ultrahigh-pressure metamorphism. Also, collision, causes giant systems of hydrothermal fluid flow, involving metamorphic, basinal and meteoric, fluids (Mark et al. 2007, Craw et al. 2002, Oliver, 1986). Similar features are reported from intracontinental mountain belts involving very narrow, oceans (Alps, European Variscan Belt) and from, purely intracontinental orogens (Kibarides in Central Africa: Pohl 1994). Typically, collisional orogens exhibit: i) granitoid-related deposits of tin,, tungsten, gold and rare metals; and ii) deposits, formed by migrating metamorphic fluids. Gold is, especially common in this setting (orogenic gold, deposits: Groves et al. 2003). Mineralization in, orogenic belts is favoured by phases of extension,, because melts and fluids can more easily rise to, shallow depths. Extension may be related to orogenic collapse and other post-collisional processes., Assemblage and break-up of supercontinents, The plate-tectonic evolution of the Earth’s crust, follows not only the relatively short Wilson cycles, (opening and closure of oceans) but also a, , 139, , trend of large-scale cycles of amalgamation of all, continental plates into supercontinents and the, following break-up. The Phanerozoic supercontinent Pangaea is well-known, existing from, the Permian into the Jurassic (300–175 Ma). Mesoproterozoic Rodinia (1100–800 Ma) is generally, accepted but its assemblage is more contentious, because data are insufficient for a unique solution, (Torsvik 2003). Older supercontinents are even less, well-defined. Supercontinents can be related to, specific characteristics of the metallogenetic evolution, including the incidence of anorogenic ore, formation (e.g. titaniferous anorthosite-ferrodiorite complexes) and the prevalence of continental,, sediment-hosted deposits (Kupferschiefer: Robb, 2005). When Gondwana and Laurasia finally fused, into the supercontinent Pangaea, the Variscan belt, in Europe experienced a short-lived metallogenetic, peak of unique fertility. Deep processes inducing, initial crustal distension and break-up of Pangaea,, at about the Triassic/Jurassic boundary, again produced an ore-forming heat and fluid pulse across, much of Europe (Box 1.14)., Apart from the relatively simple plate tectonic, model situations described above, many quite, complex interaction fields are known today. One, recent example is the Gulf of California, where a, subducted oceanic ridge passes along strike into a, continental rift and ultimately into an intra-continental transform structure (San Andreas Fault)., Only rarely, connections such as these can be, reconstructed for the geological past so that the, precise plate-tectonic setting of some ore deposits, may never be fully understood. Yet, the quest for, solving a given plate-tectonic puzzle is always, scientifically fascinating and results benefit applications of economic geology., , 1.8 GENETIC CLASSIFICATION OF ORE AND, MINERAL DEPOSITS, , We can get so wrapped up in debating, terms that we forget entirely about the, subjects of our original interest: minerals, rocks, geology, and such. . ., Stephen A. Langford in GSA Today,, February 2002
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140, , PART I METALLIFEROUS ORE DEPOSITS, , BOX 1.14, , The metallogenetic evolution of Europe, , The following sketch of Europe’s metallogenetic evolution is intended to serve as an example that may have implications, for other continents. Although the second smallest of all continents (after Australia), Europe is made up of crustal rocks of, all ages, starting with an Archaean nucleus in the Arctic North. In the Mediterranean South, subduction and mountainbuilding are still active. A detailed description of single deposits and districts with their geological frame and with, references is available in Mineral Deposits of Europe (Volumes 1–5, Bowie et al. 1979; Dunning et al. 1982, 1989;, Dunning & Evans 1986; Ridge 1990). Emberger (1984) provides a succinct report on metallogenetic mapping in Europe., The regional geology of Europe is accessibly presented in the encyclopaedia by Moores & Fairbridge (1997). A, geodynamic analysis of metallogeny provide Blundell et al. (2005)., Northern Europe (Scandinavia and adjoining Russia) and Northwestern Europe (The British Isles) are largely built of, Precambrian and Early Palaeozoic rocks. Mountain belts formed in this time are deeply eroded, so that many preserved, ore deposits are metamorphosed or metamorphogenic. The region comprises the Precambrian Baltic Shield and the, Lower Palaeozoic Caledonides. In Northern Finland and Sweden, the Baltic Shield contains an Archaean nucleus with, attached Proterozoic orogenic belts younging to the southwest, namely the Svecokarelides (ca. 2.0–1.54 Ga) and the, Sveconorwegian Province (1200–850 Ma). The Archaean nucleus hosts greenstone belts with little notable mineralization. More prospective seems to be a belt of Ni-Cu-PGE mineralized layered mafic intrusions (Andersen et al. 2006),, which appear to record a Palaeoproterozoic (2.44 Ga) rifting event. In contrast, the Svecokarelian (also called, Svecofennian) belt is well endowed with important mines. Northern Sweden is renowned for “Kiruna-type” magmatic, iron oxide-apatite deposits within bimodal metavolcanic rocks and also contains a number of recently discovered ironoxide-copper-gold deposits (Weihed & Williams 2005). Further south, the Svecokarelides host a broad belt of, volcanogenic sulphide ore deposits (Cu, Zn, Pb, Au, Ag, Ni, Co) that extend from Boliden in the Skellefte volcanic, arc (Sweden) to Outokumpu-Pyh€asalmi (Finland) and further east to Lake Ladoga (Russia). The geological environment of, ore formation included island arcs and oceanic rifting (Nironen 1997; Sundblatt & Parr 1994; Gaal 1990). Recently,, exploration for diamonds (in the Archaean), for orogenic gold deposits (both in Archaean and Proterozoic units: Sundblatt, 2003) and for platinum in mafic intrusions, resulted in a number of new mines and many promising prospects. Meso- to, Neoproterozoic units are moderately mineralized. Only the orthomagmatic titanium ore of the Egersund District in, southwest Norway (920 Ma) is of more than local significance. This region was part of the collision zone between, Fennoscandia and Laurentia when the Supercontinent Rodinia was assembled., The Caledonides are the largest metallogenetic domain of the region, with a length of 2700 km and a maximum width of, 250 km. The belt resulted from convergence of Baltica and Laurentia, and final closure and suturing of Iapetus Ocean in, the Silurian. The Caledonian orogen comprises metasediments and magmatic rocks of Eocambrian to Silurian age, and a, large number of significant ore deposits (Grenne et al. 1999). During the early rift phase of its evolution (at 550 Ma), the, famous alkali-carbonatite complex at Fen in southern Norway was emplaced. This is the location where terms such as, fenite and søvite were coined. Periodically, niobium, iron, thorium and cerium were produced from the Fen carbonatite., Volcanic-hosted massive sulphide deposits resembling the Cyprus type formed in back-arc rifts or in primitive island arcs., In Norway, they were important sources of copper and zinc (Figure 1.80). More recently, exploitable metamorphogenic, gold deposits were found in Ireland, Wales and Scotland. Along the thrust front of the Caledonides in Sweden, a number of, stratiform lead ore deposits occur in Early Cambrian platform quartzites. These galena impregnation deposits of the, “Laisvall type” are thought to be derived from metamorphic fluids, which were mobilized from autochthonous and parautochthonous sediments under the eastward advancing Caledonian nappe sheets. In the Middle and Late Cambrian in, Sweden, the platform sediments include oil shales (“alum shales”) that represent giant potential resources of synthetic oil,, uranium, vanadium, phosphorous, molybdenum and nickel. A “Late Caledonian” phase (350 Ma) of alkaline intrusions, is expressed by the Sokli carbonatite in Finland, exploited for apatite (with by-products niobium and rare earth elements)., The Oslo Graben is an important magmatic, tectonic and metallogenetic rift province in southwest Norway. It is, considered here because of its location, although its active phase dates from the Carboniferous and the Permian. The Oslo, Graben Province displays remarkable silver and base metals veins (Kongsberg, Figure 1.22), molybdenum impregnations, in granite and felsic volcanics, and iron skarn ore. Of about the same age is the alkali ring intrusion of Khibiny on Kola, Peninsula (Russia) with its important apatite and nepheline resources., Central Europe and large parts of Southwestern Europe are underlain by rocks of the Late Palaeozoic Variscan orogen,, although large areas are covered by younger platform sediments. Regions near the Mediterranean Sea are shaped by the, later (Mesozoic-Cenozoic) Alpine orogeny., The European Variscides are a complex mosaic of Precambrian microcontinents and of Palaeozoic orogenic belts, that were finally welded together and deformed by the Variscan orogeny in the Late Carboniferous (Tait et al. 1997).
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 141, , The Variscan chain in Europe is part of the 10,000 km collisional zone suturing Gondwana and Laurasia when the Rheic, Ocean closed to form Supercontinent Pangaea. The development of the Variscan orogen involved major crustal, shortening and subduction of substantial amounts of supracrustal rocks, continental and oceanic crust, and mantle, lithosphere. Yet, ophiolites derived from Palaeozoic oceanic lithosphere are rare and contain little economic mineralization. Marginal to the region considered in this chapter, however, large ophiolites are present in the Ural Mountains,, hosting deposits of chromite, platinum and base metal sulphides (Herrington et al. 2003). Silurian submarine hot-spot, volcanism is probably the crucial metallotect of the unique and giant mercury deposit Almaden in Spain (Figure 1.89)., Large synsedimentary ore deposits were formed during extensional tectonics in Devonian marine strata of the, Rhenohercynian shallow sea (volcanogenic iron ore and sedex type zinc-copper-lead sulphides with barite, e.g., Rammelsberg, Germany). Still in a tensional setting during the Early Carboniferous of southern Spain and Portugal,, volcanic massive sulphide (VMS) and brine pool sedex deposits in shale constitute the giant massive sulphide ore province, of the Iberian Pyrite Belt with pyrite, copper, tin, lead, zinc and gold (Figure 1.89). At about the same time, the syngeneticdiagenetic Irish zinc-lead-silver deposits formed in a shallow sea. Peak synkinematic and synmetamorphic processes, during the Variscan orogeny were not fertile. However, giant coal deposits formed from forest mires at tropical latitudes, during the Westphalian (Late Carboniferous), located in foreland and intramontane basins of the Variscan orogen. The, coal belt in Europe has an east-west extension of 3000 km and a north-south width of 800 km (Ziegler 1982)., Following the orogenic peak, regional exhumation and post-collisional collapse of the Variscides was strongly affected, by transtensional and transpressional wrench-deformation, detachment of subducted lithospheric slabs, asthenospheric, upwelling and thermal thinning of the mantle-lithosphere. At 300 Ma (Stephanian-Early Permian), when Gondwana, and Laurasia finally sutured to form Pangaea, the deep-reaching disturbance of the Variscan domain reached its climax, with widespread melting of the uppermost asthenosphere and the lithospheric boundary layer. Mantle-derived basic, melts underplated the crust, inducing anatexis and the intrusion of granites in the upper crust throughout the Variscides., The most evolved, and youngest phase of these granites was highly fertile: Tin and copper deposits were formed in, Cornwall, tin, tungsten, bismuth and uranium ore in the Erzgebirge (Baumann et al. 2000, Breiter et al. 1999, Stemprok &, Seltmann 1994), tungsten, tin, uranium, arsenic, lithium and fluorine in the French Massif Central, tungsten in the, Pyrenees, and tungsten (Panasqueira) and tin in Portugal (Figure 1.89). At the same time, C-O-H-N fluids were released, from the deep crust that deposited orogenic gold-arsenic-antimony ores in the French Massif Central. It is thought that the, reason for this short-lived late-orogenic “metallogenetic peak” was devolatilization of the lower crust by granulite, metamorphism (Marignac & Cuney 1999)., In the late Early Permian, magmatism ceased and increasingly larger areas subsided below the erosional base level, forming a new system of intracratonic basins. Terrestrial and coastal sediments of the Rotliegend desert sediments contain, several deposits and numerous occurrences of uranium. In the Late Permian (Zechstein), the giant salt deposits of, Germany, Poland and England formed in a restricted sea, which invaded the desert basins of Pangaea from the north., Potassium salt resources of this basin are second only to those of Canada. The Copper Shale stratum at the base of the, transgressive marine Zechstein evaporites is evidence of an early euxinic stage; along the southern basin margins, it, trapped epigenetic copper concentrations emplaced in the Triassic. The end of the Permian brought the biggest mass, extinction of life in geological history, with the loss of 95% of all species. Its cause is generally believed to be the eruption, of the Siberian and South Chinese Emeishan trap basalts (Kamo et al. 2003)., Announcing the break-up of Pangaea, a new Wilson cycle started with crustal distension in the Triassic, which, culminated near the Triassic/Jurassic boundary with the formation of a giant magmatic province resembling trap basalts in, the later Central Atlantic Ocean (Olsen 1999). This initiated opening of the Tethys and the Atlantic Ocean, and induced a, heat and fluid pulse across much of Europe. Copper Shale ore deposits were emplaced as well as numerous karst and vein, deposits of Pb-Zn-F-Ba (e.g. Poland: Sass-Gustkiewicz 1996; Germany: Walther 1983). The deposits are located both in, Variscan basement (e.g. Schwarzwald, Massif Central) and in the Mesozoic cover from Poland to Spain. Mesozoic, mainly, marine sediments of the platform rocks contain oil and natural gas deposits, oil shale and sedimentary oolitic iron ore, (Lorraine district in France)., In the Mesozoic and Cenozoic, Alpine and Mediterranean Europe was part of the giant Tethyan-Eurasian metallogenetic domain, which is the product of convergence, subduction and collision of Eurasia with fragments of Gondwana., European Alpine chains include the Pyrenees, the Alps, the Appenines, the Carpathians, the Balkan Mountains,, Dinarides, Helennides and Caucasus. Along the convoluted trace of the orogen, the geodynamic character changes, from segments with major calc-alkaline magmatism to segments with extensive regional metamorphism. This is attributed, to the complex geometry of the collision interface, with numerous microplates and transient oceans (Heinrich &, Neubauer 2002). Orogenic compression including formation of the great nappes (thrust sheets of thick rock bodies that, moved many kilometres over each other) culminated in the Late Cretaceous and the Early Tertiary. Palaeozoic, and even, Precambrian rocks are either part of the nappes or form the basement of microplates. They contain some mineral deposits
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Figure 1.89 (Plate 1.89) Metallogenetic overview of Southwestern Europe and adjacent Africa (clipping from Juve & Storseth 1997). With kind, permission of NGU, Trondheim.Text and symbols (cf. Figure 1.86) in four classes that indicate relative size of deposits. The geological background is, simplified to Hercynian (purple, mainly Palaeozoic); Alpine (yellow, mainly Mesozoic) and cover sediments (light grey, mainly Tertiary). The, distance between 5 latitude parallels is 550 km.
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , 143, , that are clearly older than the Alpine cycle. Other deposits in pre-Alpine rock units originated during the Alpine, metallogenetic cycle (e.g. the large metasomatic siderite deposit Erzberg in Austria and orogenic gold-quartz veins in, gneisses of the Eastern and Western Alps)., Pre-Alpine basement rocks that are exposed within the Alpine belts allow the reconstruction of a protracted plate, tectonic evolution throughout the Palaeozoic, including ocean subduction and the obduction of ophiolites. Some, important pre-Alpine mineral deposits in the basement are: i) granite-related tungsten stockwork orebodies at Mittersill, (Tauern, Eastern Alps); and ii) sparry metasomatic magnesite lenses in the Eastern Alps and in the Carpathians. The second, is clearly different from the Variscan north of the Alps, but the first recalls the rare metal metallogeny of the German, Erzgebirge., In Alpine and Mediterranean Europe, widespread extensional tectonic strain and heating in Mid-Triassic to Early, Jurassic time are related to the break-up of Pangaea and caused a major metallogenetic pulse (Jankovic 1997). In the, Balkans, associated volcanism is implied in formation of the exhalative iron ore strata at Vareš (Bosnia), the rift-bound, mercury deposit of Idria (Slovenia) and a number of lead-zinc deposits. The large lead-zinc deposits in the Southern Alps, (e.g. Bleiberg, Me, zica) formed in a shallow marine rift zone distal to volcanic activity. Larger oceanic basins existed, mainly in the Jurassic and were closed in the Cretaceous. Obducted ophiolites in the Balkans host important chromite, mineralization. Cyprus type sulphides are rare and have little economic relevance. Alpine ophiolites are largely barren,, from the Eastern Alps to the Pyrenees., With the inception of the Cretaceous, the tectonic regime in the Alpine realm changed to subduction and convergence., From Romania through Serbia to Bulgaria, arc magmatism is represented by a Late Cretaceous (90–60 Ma) ore-bearing, igneous belt, which is referred to as the Banatitic Metallogenetic Belt (or, geographically, the Apuseni-Banat-TimokSrednogorie belt: Heinrich & Neubauer 2002). Banatites are calc-alkaline intermediate intrusive and extrusive igneous, rocks of I-type and magnetite series character. The belt extends for 750 km with a width of 30–70 km. Copper-gold, porphyry and high-sulphidation epithermal gold-copper deposits occur from the Timok district in Serbia (e.g. Bor and, Majdanpek) in the west to Bulgaria (Panagyurishte district) in the east. Iron-lead-zinc skarn ore deposits (including Ocna, de Fier in Romania: Nicolescu et al. 1999) and smaller deposits of iron, molybdenite and bismuth are also considered as, legacies of this magmatic metallogenetic belt., In the western Carpathians (Slovakia) and in the Eastern Alps (Austria), Cretaceous magmatic activity is practically, absent. Instead, the onset of continental collision and subduction induced a pervasive flow of metamorphic fluids that, produced metamorphogenic deposits of siderite (Erzberg), talc (Rabenwald), and smaller deposits of copper and barite, (Pohl & Belocky 1999, Figure 1.90). Coal seams in Late Carboniferous sediments were metamorphosed to exploitable, graphite., , Figure 1.90 Metamorphogenic ore formation in the European Eastern Alps.During peak conditions of, metamorphism in the Turonian (Late Cretaceous), dehydration affected deeper parts of the nappe pile including, Penninic autochthonous cover, oceanic crust and marine sediments with evaporite horizons. ta – Talc at, Rabenwald, Pe – Penninic nappes, L-M-U – Lower, Middle and Upper East Alpine thrust sheets., Intermittent emersion of wide regions of the European and African plates allowed the formation of supergene alteration, deposits, most famously bauxite in southern France (Les Baux is the name-giving district), Hungary, Italy and Greece, and, nickel laterites in Albania and Greece. Cretaceous convergence between African and European plates caused a wave of, compressional deformation in the northern foreland of the Alps (“inversion tectonics”), which can be demonstrated even, in North Sea oilfields over 1000 km distant.
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144, , PART I METALLIFEROUS ORE DEPOSITS, , In the Early to Mid-Tertiary, major continental blocks collided in the Eastern and Western Alps (Neubauer et al. 2000)., This led to extensive Barrovian-type regional metamorphism, commonly preceded by an eclogite-facies stage. During the, following orogenic collapse and rise of high-grade metamorphic domes, numerous small orogenic gold vein deposits, were formed (Monte Rosa, Tauern). Giant lignite basins developed in Germany and Poland, mainly in the EoceneOligocene., Also in the Early to Mid-Tertiary, oceanic sub-basins in the Inner Carpathians and the Apuseni Mountains closed,, continental microplates indented into the evolving orogen, and slab break-off and mantle delamination caused, asthenospheric incursions (Heinrich & Neubauer 2002). De Boorder et al. (1998) suggested that orogenic collapse and, slab rupture caused the heat and fluid flow responsible for the subsequent (mainly Miocene) mineralization. In the, Apuseni Mountains, gold and base metal ores are related to localized centres and short belts of Tertiary andesitic and, rhyolitic volcanism (Neubauer et al. 2005). For thousands of years until today, this region was the source of much of the, gold produced in Europe. In the Inner Dinarides and the Rhodope Mountains, lead, zinc and antimony deposits occur in, similar settings. This magmatic and ore formation style extends into Greece and the Greek islands, where it lasts longer, (into the Pliocene and locally, into the Holocene) and where manganese, barite and gold gain a more central role. In, Western Europe, Tertiary mineralization includes skarn deposits of magnetite and haematite on the island of Elba, above a, granite cupola that intruded Mesozoic sediments some 6 Ma ago. Across the sea in Italy, volcanogenic deposits in the, Toscana include epithermal mercury at Monte Amiata. In southern Spain near Almeria, andesitic and rhyolitic volcanoes, host epithermal deposits of gold, silver, lead and zinc (Rodalquilar, Figure 1.89). Messinian evaporites along the, Mediterranean shores are exploited for gypsum, sulphur, rock salt and potassium salts. The giant strontium deposit at, Montevives in the Sub-Betic zone of southern Spain is especially remarkable., In the Late Tertiary and the Holocene north and west of the Alps, a broad mantle plume caused basaltic volcanism (Goes, et al. 1999) of little metallogenetic significance. The plume is related to the large crustal break that traverses Europe from, the mouth of the River Rhone to the Oslo Graben in Norway. The Upper Rhine Rift is a section of this structure and is, endowed with historically important hydrothermal lead-zinc-silver ore veins in the rift shoulders (Figure 1.28), and with, oil and gas deposits as well as potassium salt beds in the Tertiary graben fill., , In spite of this reminder that terms and classes, are not the first target of science, efforts towards, classification are both necessary and useful. Classifications are needed because they clarify terms, and provide a common reference frame, and this, makes them useful for scientific communication, and practical application. Various geological aspects are employed to classify ore deposits, including the presence of certain metals or minerals (e.g., silver, haematite), the form of the orebody (vein,, bed, etc.), the local geological environment (submarine or terrestrial volcanism), the plate tectonic, setting (island arc, continental margin) and other, genetic characteristics such as formation temperatures and fluid chemistry. The thoughts of Lindgren (1933), Niggli (1948) and Schneiderh€, ohn (1932,, 1962) represent important stages of metallogenetic classification. Because ores and useful, minerals are basically just rocks, although usually, rare ones, a petrogenetic approach is obviously, practicable. Main petrogenetic process systems, are magmatism, sedimentation, diagenesis, meta-, , morphism and surficial weathering (Figure 1.1)., Parallel to other classification systems in science,, these five main classes are the root for a branching, order of subclasses. Apart from petrogenesis, criteria for ordering often include the local, regional, and plate tectonic situation., However, a stringent genetic classification of, mineral deposits is very difficult. One reason for, this is that many ore deposits represent a position, in a complex multi-dimensional space of welldefined end members. The formation of Kuroko, ore deposits, for example, is an interplay of volcanic, intrusive, sedimentary and diagenetic processes. The origin of high-grade BIF-haematite, ore seems to comprise sedimentation induced by, proliferating marine life, later passage of saline, basinal brines and supergene components. May I, remind the reader that already Charles Darwin, (1859) described lucidly how impossible it is for, the naturalist to define species, families and genera of plants and animals only by structural differences. Darwin states “All true classification is
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , genealogical”. Ore deposit classification is certainly not easier than establishing biological systems and should also aim for a genetic logic., Scientific progress continuously modifies and, improves genetic models. An ever more detailed, understanding of ore-forming processes is the, result (Robb 2005). Some practicians of exploration and mining think little of genetic models and, , 145, , prefer a pragmatic and empirical classification., Many scientists employ non-genetic terms of classification, such as “granite-related” or “sedimenthosted stratiform” deposits. Economic geologists, use terms such as “deposit styles” (Hough et al., 2007) or “deposit types” (Cox & Singer 1986; e.g., copper porphyry type, orogenic gold, lateritic, nickel). Attribution to certain styles and types is, , Table 1.6 A simple genetic classification of ore deposits, I. Magmatogenic Ore Deposits, 1. Orthomagmatic Deposits:, Sulphide Fe-Ni-(Cu-PGE) ore hosted by Archaean komatiites and subvolcanic ultramafic intrusions; Alpine type Cr-PGM, in ophiolites, and seams in layered mafic intrusions; Cu-Ni-PGM “reefs” in layered mafic intrusions; complex, mafic-ultramafic intrusions with, for example, conduit-hosted Cu-Ni-PGM; impact magma bodies with Ni-Cu-PGM;, Ural-Alaska type ultramafic ring intrusions with Cr-PGM; Ti-Fe in Mesoproterozoic anorthosite-ferrodiorite complexes;, orthomagmatic iron ore deposits of iron oxides and apatite in intermediate to felsic igneous rocks (Kiruna type);, apatite-Fe-Nb-Zr, or REE-U-F in carbonatite plugs and nephelinite intrusions, 2. Pegmatites with ore of Be, Li, Rb, Cs, Ta (Nb), U, Th, REE, Mo, Bi, Sn and W, industrial minerals, gemstones, 3. Magmatic-Hydrothermal Deposits:, Skarn ore, with magnetite-Cu-Co-Au, W, Zn-Pb-Ag, Mo-Bi-Au and Sn-As-Pb-Zn-W-Mo; contact-metasomatic ore (Pb, Ag,, Zn); Fe-oxide-Cu-Au (U-REE) deposits (IOCG); porphyry deposits (Cu-Mo-Au, Sn-W); submarine volcanogenic (Kuroko), and volcanic-hosted massive sulphide deposits (VMS); vein deposits (Sn, W, Cu); terrestrial epithermal Ag-base metal, deposits; epithermal Au-Ag deposits, II. Supergene Ore Deposits, 1. Residual (Eluvial) Deposits:, Residual placers (e.g. W, Sn); bauxite; lateritic Au, Fe and Mn ore deposits, 2. Supergene Enrichment Deposits:, Enriched sulphide ore (Cu, Ag); lateritic Ni, 3. Infiltration Deposits, U in sandstone; Pb-Zn-F-Ba and Mn in karst cave systems, III. Sedimentary Ore Deposits, 1. Allochthonous:, Colluvial, alluvial (gold, columbite, cassiterite, wolframite, platinum) and coastal (rutile, ilmenite, zircon, monazite), placers, 2. Autochthonous:, Sulphide deposits, mainly in black shales; polymetallic deposits of Cu-Sb-Zn-Pb-Ag (-Au), mostly of sedex type;, Palaeoproterozoic banded Fe ore (BIF) of Algoma and Superior type, and banded Mn ore; oolitic Fe and Mn ore; deep sea, manganese nodules and crusts (Mn-Cu-Ni-Co-PGM), IV. Diagenetic-Hydrothermal Ore Deposits, 1. Stratabound and/or stratiform sediment-hosted Cu deposits:, European Copper Shale (Cu); Central African Copper Belt (Cu, Co, Pb, Zn, U), 2. Mississippi Valley type (MVT) Pb-Zn-F-Ba deposits (hosted in marine carbonates), 3. Saline brine-related deposits, Pb-Zn-F-Ba, metasomatic siderite, preconcentration of high-grade hematite, V. Metamorphosed and Metamorphic Ore Deposits, Metamorphism of pre-existing ore generally improves processing characteristics of ore, but is rarely a factor of metal, accumulation and ore formation; metamorphic examples include in situ redistribution, concentration and recrystallization, of gold, VI. Metamorphogenic-Hydrothermal Ore Deposits, Prograde and retrograde metamorphogenic-hydrothermal ore deposits (e.g. orogenic Au deposits in accretion-subductioncollision complexes; part of the Central African Cu-Co ore deposits; Cu in Mt Isa, Australia).
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146, , PART I METALLIFEROUS ORE DEPOSITS, , determined by descriptive attributes and relations, to certain rock associations (e.g. “turbidite-hosted, gold deposits”, “alkaline igneous association”–, Laznicka 1985, 1993). In this book also, descriptive, terms are frequently used. The advantage is that, short denominations facilitate communication, and that changes of genetic understanding do not, enforce new terms. Also, this solves the problem, of classifying deposits of intermediate position, between genetic end members. Yet, genetic concepts are a strong element in finding new ore, deposits (Kreuzer et al. 2008), guide rational, exploitation and mine closure. Therefore, genesis, must be reflected in ore deposit classification., Guilbert & Park (1986) admirably describe the, problems of ore deposit classification but nevertheless argue for the exercise, in spite of shortcomings. In this spirit, I endeavour to provide a, simple genetic classification that should help the, reader to understand the logic of this chapter’s, arrangement. As a general rule, the geological, setting and the last concentrating process (e.g., sedimentation) determine placement in a certain, class, for example formation of metalliferous, sediments by hydrothermal fluids venting from, the seafloor resulting in sedimentary exhalative, (sedex) deposits. In Table 1.6, ore deposit groups, and types presented in Section 1.1 are listed with, the intention to summarize the genetic panorama. In Chapter 3, a similar overview is provided for selected non-metallic minerals and, rocks., , 1.9 SUMMARY AND FURTHER READING, Ore formation is an integral component of the, Earth’s dynamics and of petrogenetic process systems. Ore deposits, however, typically result from, the interaction of several processes and modifying, factors. Reducing the complexity to simple end, member concepts leads to the following shortlist, of genetic variety., Orthomagmatic ore deposits of metal oxides, (magnetite, ilmenite, chromite), sulphides (Ni,, Cu) and of precious metals (Pt, Pd, Au) are formed, by the segregation of solid ore minerals or of ore, , melt from a large liquid silicate magma body., Gravitational settling of segregates is the basic, concentrating factor (Boxes 1.1 and 1.2)., Metalliferous pegmatites originate by fractionation of volatiles (water, boron, fluorine) and rare, metals (Be, Li, Rb, Cs, Ta, Sn) into the very last, silicate melt batches of crystallizing parental, granites., Magmatic-hydrothermal deposits are formed, from metal-enriched magmatic volatile phases, which are released by solidifying magma bodies., Copper porphyry systems (Box 1.3) provide economically outstanding examples. Note the great, variety within this class, including skarn (e.g. Cu,, Zn-Pb-Ag, Mo, Bi, Au) and contact-metasomatic, deposits, Fe-oxide-Cu-Au (U-REE) deposits, (IOCG), and tin, copper and tungsten veins in the, roof of parent intrusions (Box 1.4). Whereas the, former are related to intrusive and subvolcanic, magma bodies, volcanic-hosted massive sulphides, (VMS, Box 1.5) and epithermal gold and silver, deposits (Box 1.6) originate near the surface in, volcanic centres, either beneath the sea or on land., Supergene ore deposits result from weathering,, which describes the interaction of Earth materials, with air, water, life and the energy flow from the, sun. During decomposition of rocks, metals are, concentrated either in situ as an insoluble residuum (e.g. bauxite) or by precipitation after some, movement in soil and ground water (e.g. lateritic, nickel deposits: Box 1.7). Lateral transport distances of 1 to 100 km characterize infiltration, deposits of some metals such as copper and uranium (Box 1.8)., Sedimentary ore deposit formation is foremost, the product of exogenous process systems with, physical and chemical transport in water. Part of, the metals moved is concentrated in rivers (alluvial, placers, e.g. Au) and along coasts (coastal placers,, e.g. Ti). Most enter the marine realm in aqueous, solution from the land or from hydrothermal, sources underneath the seafloor. The world’s largest metal concentrations formed in the Early Proterozoic when photosynthetic cyanobacteria, expanded rapidly, oxidizing the seawater and precipitating iron dissolved in the oceans (banded iron, formations; Box 1.9). Many giant base metal deposits (sedimentary-exhalative, “sedex” Cu, Pb, Zn)
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GEOLOGICAL ORE FORMING PROCESS SYSTEMS (METALLOGENESIS) CHAPTER 1, , are the product of hot metalliferous fluids venting, on the seafloor. In contrast to mid-ocean black, smoker activity, sedex deposits are formed by rifting in shallow epicontinental seas (Box 1.10)., Diagenetic ore deposits are a consequence of, physical and chemical changes affecting basinal, sediments. With rising temperature and pressure,, pore fluids are charged with dissolved matter and, eventually are expulsed towards lower pressure, regions, commonly basin margins where metals, are precipitated. Traps are often low-permeability, barriers, which is evident for the most valued, products of diagenesis, oil and natural gas. Low, permeability and reducing power of the European, Copper Shale acted together in concentrating metals transported by oxidized brines (Box 1.11)., Worldwide, reduced brines, which resemble reservoir water in oil pools, generated numerous diagenetic carbonate-hosted Pb and Zn deposits, (Box 1.12)., Metamorphism affects previously established, metal concentrations in two different ways. One, group of minerals such as sulphides of Cu, Pb and, Zn hardly changes and we call this class metamorphosed ore deposits. Other materials that are only, valuable because of transformation into an, exploitable ore (e.g. originally dilute traces of gold, concentrated by in-situ redistribution) may be, called metamorphic ore deposits., Prograde metamorphic recrystallization of large, rock volumes liberates a giant mass of dehydration, fluids, which transport trace metals into shallow, crust where chemical and physical traps precipitate ore (e.g. orogenic gold deposits); this is the, prograde metamorphogenic class (Box 1.13). Cooling metamorphic massifs are flooded by surface, water, which induces hydration and re-equilibration of high-grade metamorphic rocks. Where, , 147, , tectonic fragmentation provides sufficient, permeability, hydrothermal convection systems, are established that produce retrograde metamorphogenic deposits (Box 1.13)., Metallogeny is the synthesis of scientific endeavours to understand ore formation. Plate tectonics provide the unifying frame. The world’s, most prolific metallogenetic domains include, supra-subduction island arcs and continental margins (e.g. the Pacific “ring of fire”). The Bushveld in, southern Africa, however, with its giant resources, of Cr, Pt and V, and many other large ore districts, of the world are not related to subduction., Europe, briefly described as an example of metallogenetic evolution (Box 1.14), is one of the, smallest continents and yet provides a fascinating, diversity of metallogenetic activity throughout, geological history. It can be seen as an assemblage, of orogenic belts younging southwards from an, Archaean nucleus in the North. Along its Mediterranean margins, subduction, volcanism and, ore-forming processes are still active., For readers who wish to pursue the theme of oreforming processes to greater detail, Robb (2005) is, highly recommended. The present state of the, science of economic geology is illustrated and, described in the impressive Economic Geology, 100th Anniversary Volume (Hedenquist et al., 2005); be aware, however, that the treatment is, selective and at a highly demanding scientific, level. An admirable insight into the complexity, of ore-forming systems gives Sillitoe (2010) in his, paper on “Porphyry copper systems”. For details, on single deposits and districts, search the extensive and systematic descriptions of thousands of, ore deposits worldwide by Laznicka (1985, 1993)., Global tectonics is comprehensively presented by, Kearey, Klepeis & Vine (2009).
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CHAPTER 2, Economic geology of metals, Synopsis, Because of their unique fracture toughness, metals are the most important engineering materials of, civilization (Lu 2010) and have many other essential applications. With few exceptions (e.g. only, 1–2% of ordinary lead is primordial), metallic elements were part of the solar nebula material that, accreted to form the Earth. Within the nascent planet, siderophile metals fractionated into the core, and lithophile metals into the crust. In the aeons since then, geological cycling caused concentration of ore deposits. Individual types and the variability of each metal’s ore deposits are mainly a, function of geochemical properties. In this chapter, the economic geology of more than 40 important, metals is systematically presented and illustrated by reference to specific mining districts and, deposits. Information for each metal includes its main ore minerals, exploitable ore grade, use of, the metal, geochemical behaviour, environmental aspects, a short list of significant deposit types, followed by more detailed descriptions, and brief lines on production and resources. Practical notes, are added where appropriate., Many formulae, numbers and various data have been assembled from Mining Journal, Mining, Magazine, Mining Annual Review, Berkman (2001), Klein & Hurlbut (1999), Neuendorf et al., (2005), United States Geological Survey (USGS Minerals Information Webpages) and Walker &, Cohen (2007). Discrepancies between different sources are not rare. Therefore, in case of the need, for highest accuracy, I advise the reader to verify critical data., 2.1 THE, , IRON AND STEEL METALS, , 2.1.1 Iron, Common Ore Minerals:, , Magnetite, Haematite, Maghemite, Goethite, Lepidocrocite, Siderite, , FeO.Fe2O3, a-Fe2O3, g-Fe2O3, a-FeO(OH), g-FeO(OH), FeCO3, , Max. wt.% Fe, , Density, (g/cm3), , 72.4, 70, 70, 62, 62, 48.2, , 5.2, 5.2, 4.8, 3.8–4.3, 4.0, 3.8, , Depending on the genetic setting, magnetite displays different concentrations of Ti, Mn, Mg, Al, and V. In siderite, Mn and Ca content reaches the, percent range. “Martite” is haematitized magnetite, and coarsely micaceous haematite is also, known as “specularite”. Iron silicates such as, chamosite and thuringite, with a maximum of, 42% Fe, are rarely of economic interest but are, always part of the ore paragenesis in oolitic deposits. “Limonite” is a field term; it may contain, variable parts of amorphous and microcrystalline, iron oxy-hydroxides., The cut-off grade for iron ore is 30 wt.% Fe,, although with the exception of the internal, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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150, , PART I METALLIFEROUS ORE DEPOSITS, , Chinese market, most large producers deliver, lumpy ore, concentrate, pelletized and sintered, fines with 55–65% Fe. An exception is magnetite, ore that can be exploited at relatively low grades,, because low-cost magnetic processing is very, effective. Iron content of more than 67.5% Fe (after, calcining) is required as feed for direct reduction, iron (DRI). If carbon dioxide charges are applied, to siderite calcining, its role as an iron ore may, soon end., Specifications for marketable iron ore (and concentrate) concern first of all the chemical composition, but also grain size and behaviour in the, blast furnace. Ore destined for the blast furnace, should resist abrasion and have a good permeability for gas flow. Because large masses of iron ore, are transported across the oceans (second only to, crude petroleum), mechanical stability during, handling and transport is very important. Requirements for metallurgical processes and shipment, are attained by sintering or pelletizing fine ore near, the mine. Only lumpy ore (6–32 mm diameter) is, shipped without upgrading. Sintering may also be, applied in order to reduce high water content, before transport., Desirable chemical and mineralogical properties of iron ore are imposed by its main purpose,, production of pig iron in the blast furnace. Only, 10% of the world’s iron ore is processed to iron, sponge by direct reduction technologies. Pig iron, or iron sponge, combined with coke, are the main, raw materials for the production of crude steel, the, toughest of all construction materials (Lu 2010)., High iron concentration in ore, low content of, SiO2 and alumina, and coarse grain size are favourable properties. Further desirables are useful content of CaO þ MgO (“basic” ore), low phosphorous, (<0.1%) and other impurities (Mn < 2%, Cr < 1%,, Ni < 0.5%, S < 0.2%, and As, Cu, Zn and, Pb < 0.1%). Phosphorus of more than 0.1% is not, acceptable, because it adversely affects steel quality. Ti and V interfere with common iron and steel, production, but can be valuable by-products of, specialized plants. Generally, long-term constant, composition of ore is an advantage, because, this allows compensation for undesirable properties. Slags from iron and steel production are, valuable by-products, not waste. Slags are applied, , as fertilizers in forestry and agriculture, as part, of the cement formula and as aggregates in the, construction industry. Some slags, however, may, concentrate toxic or environmentally hazardous, trace elements., Non-metallurgical uses of iron ore consume a, very small share of total production. They include, chemical applications (the sulphate is a fungicide),, pigments and abrasives. In the future, iron sulphate compounds might be used for fertilizing the, high oceans, because Fe is the main limiting factor, of phytoplankton production. Its application is, one of the potential geoengineering methods of, carbon sequestration from the atmosphere (Boyd, 2007). Many specialized mines produce iron oxide, pigments (200,000 t/y worldwide) for colouring, construction materials (e.g. bricks), paints (e.g., ochre, specular haematite), animal feed and as a, filler in plastics. Typically, these fields demand, extremely high quality, both of the ore and its, processing., Geochemistry, The geochemical behaviour of iron is indicated by, its namesake position (iron in ancient Greek is, sideros) in Goldschmidt’s siderophile elements, (associated with the Earth’s iron-nickel core and, comprising Fe, Co, Ni, Mo, PGE, C, P, Ge, Sn and, Au: Goldschmidt 1958), as a transition metal in, the periodic table and its high contents in nearly, all rocks and in the continental crust (5%, range, 3–6.5%). Iron’s variable oxidation states in nature, are of major significance: native iron Fe0, ferrous, (Fe2þ) and ferric iron (Fe3þ). Reduced iron is commonly mobile and oxidized iron immobile. This, causes retention of huge masses of iron in the, exogenous cycle of earth processes. Redox states, of iron mark certain geological environments,, usually controlled by contact with the atmosphere, or with organic substances. Of course, surficial, alteration induces oxidation, whereas reduced, iron occurs in coal seams (siderite), and native, iron is found at contacts of basalt with bituminous, schists or coal (e.g. Diskø, Greenland)., A large percentage of iron ore was formed or, upgraded by weathering and/or sedimentation., Therefore, the geochemical behaviour of iron in
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , the near-surface environment is of the highest, interest. Weathering of ferriferous rocks may lead, to residual enrichment, as in lateritic iron ore, deposits. Some oxidized iron can be mobilized, in the form of colloidal or solid oxy-hydroxide, particles, adsorbed on suspended clay flakes and, chelated with organic matter, but rarely in ionic, solution. Stability and formation of ore minerals, are well defined in pH/Eh space (Figure 2.1)., A lateral disposition of pH/Eh zones in a sedimentary basin induces iron facies zoning (Figure 1.66)., Biological activity may contribute to precipitation, indirectly by consumption or production of oxygen and CO2, and rarely in a direct way (iron, , +1.0, , III, , Fe aq, , +0.8, , +0.6, , +0.4, , FeII aq, , Eh (Volt), , +0.2, , Haematite Fe2O3, , Pyrite FeS2, , 0.0, , H, H, , 2O, , 2, , -0.2, , -0.4, , Ma, , Siderite, FeCO3, , -0.6, , gn, Fe etite, 3O, 4, , -0.8, , -1.0, 0, , 2, , 4, , 6, , 8, , 10, , 12, , pH, , Figure 2.1 Stability fields of oxide, sulphide and, carbonate iron phases in water at 25 C, a total pressure of, 1 atm and molarities of Fe, S and CO3 at 106, 106 and 1,, respectively (modified from Garrels & Christ 1965)., An increase of the activities of total carbonate or sulphur, in solution would expand the fields of siderite or pyrite., The magnetite field would extend into the range of, neutral pH if the activities of total carbonate or sulphur, in solution were lowered. Compare this diagram with, Figure 1.48., , 151, , bacteria: Konhauser 1998; certain fungi). Biological processes (e.g. dissimilatory iron reduction by, bacteria) may cause fractionation of the isotopes, 56, Fe and 54 Fe up to d56 Fe of 3‰ (related to bulk, Earth: Johnson & Beard 2005). Abiotic redox, cycling of iron and mineral fluid reactions also, produce measurable changes, in spite of the small, mass difference of the four natural Fe-isotopes (54,, 56, 57 and 58; Staubwasser et al. 2006; Whitehouse, & Fedo 2007). Black smoker fluids, for example,, have isotopically lighter iron compared with sediments and magmatic rocks (Beard et al. 2003)., Iron is one of the essential trace elements in, supporting biological functions, and only some, chemicals are toxic. A risk of fatal iron poisoning, by accidental ingestion of iron tablets concerns, children below the age of six years. Essential trace, elements in humans include iron, copper, zinc,, manganese, magnesium, chromium, molybdenum, tungsten, cobalt, selenium and iodine (Lindh, 2005). Non-metallic elements essential to human, health are calcium, chlorine, potassium, phosphorous and sodium (all with recommended daily, intake in the gram range) and fluorine. Ocean, water was iron-rich in the first half of the Earth’s, history, but became highly depleted by formation, of Palaeoproterozoic banded iron ores (the “Great, Oxidation Event”). In the following period,, between 1.8 Ga and 800 Ma, iron was probably, abstracted from the oceans into sediments by, formation of insoluble sulphides. Since complete, ocean oxygenation at 700 Ma, iron entering the, oceans is oxidized to Feþ3 and reacts with OH to, form insoluble oxy-hydroxides (Anbar 2008,, eq. 1.18). Because of its biological functions, iron, is at present a limiting nutrient in large parts of the, world’s oceans., Hydrothermal solubility of iron is positively, correlated with fluid salinity and temperature, (Fein et al. 1992). Elevated chlorine content of, hydrothermal solutions is usually caused by dissolution of halite or by convection of evaporated, sea water (Barton & Johnson 1996). In acidic,, reduced and chlorine-rich fluids, high iron content, occurs in the form of FeClþ and FeCl20. Rising, pressure, fO2 and pH are negative controls of solubility. Precipitation is induced by falling P or T,, by mixing with water of low salinity (e.g. meteoric
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152, , PART I METALLIFEROUS ORE DEPOSITS, , water), by oxygen access, or by rising pH, the latter, commonly being a consequence of contact with, carbonate rocks., Iron ore deposit types, Iron ore deposits occur in a large genetic variety., Two factors of iron concentration are especially, prominent – redox changes and the metal’s high, solubility in reduced saline fluids. The first controls the giant mass of iron in sedimentary deposits, such as Precambrian banded iron formations., Elevated salinity is a common theme in iron concentrations as diverse as iron oxide-copper-gold,, skarn, hydrothermal-metasomatic siderite and, high-grade haematite ore deposits. Scientific, and economic significance suggests the following, genetic tabulation:, . orthomagmatic iron ore deposits (intrusive and, extrusive; mafic and felsic, alkaline, carbonatitic);, . contact-metasomatic, and iron skarn ore, deposits;, . magmatic-hydrothermal, epigenetic massive, and vein deposits, including an apatite-iron oxide, subclass;, . hydrothermal-metasomatic siderite stocks in, carbonates (diagenetic or metamorphogenic);, . volcanogenic-exhalative, and sedimentaryhydrothermal (sedex) deposits;, . residual, lateritic deposits;, . supergene infiltration “channel” ore deposits;, . terrestrial-sedimentary iron deposits (bog ore,, karst iron ore, alluvial magnetite placers);, . marine-sedimentary and associated supergeneenrichment iron ore deposits (magnetite BIF,, BIF-derived martite-goethite ore, ironstones,, oolitic iron ore, detrital iron ore, coastal placers);, . high-grade haematite ore deposits replacing, banded iron formations (pre-enrichment by diagenetic-hydrothermal processes, final supergene, enrichment)., Orthomagmatic-intrusive iron ores, Orthomagmatic-intrusive iron ores include the, magnetite seams of the Upper Zone of the Bushveld, South Africa (Figure 1.5). Economically most, significant is the Main Magnetite Layer with an, , average thickness of 1.8 m and 1.6% V2O5, containing opencast reserves of over 1000 million, tonnes (Mt) of ore. Worldwide, numerous small, and middle-sized iron ore deposits are associated, with gabbro and norite intrusions. Orebodies of, this “gabbro type” occur as layers, streaks or veins., Elevated titanium traces in magnetite may interfere with economic use, but some mines thrive, on titaniferous magnetite. Orthomagmatic iron, ores of the “anorthosite type” often have a paragenesis of haematite and ilmenite, allowing more, economic processing. At high ilmenite concentrations, these ores are sources of titanium and iron is, the by-product., For a long period, magnetite-apatite ores at Kiruna, and Malmberget in northern Sweden were thought to, be orthomagmatic segregations, either subvolcanic or, intrusive. More recently, a magmatic-hydrothermal, origin similar to Olympic Dam (cf. “Copper”) was, considered (Hitzman et al. 1992). Host rocks are, within-plate Palaeoproterozoic felsic volcanic and, intrusive rocks (Weihed & Williams 2005). Orebodies, are roughly stratabound, tabular and at Kiruna, reach, a thickness of 200 m and a strike length of 4 km. This, orebody dips at 60 (Figure 1.8) and drilling to a depth, of 1500 m confirmed downward continuity. Until, 2012, production haulage will be based at the 1045 m, level. The large mine voids left after ore extraction, are causing significant subsidence of Kiruna town., The ores are massive or banded with apparent crossstratification. Columnar and dendritic magnetite is, suggested to indicate rapid cooling of an ore melt., Vesicular portions may indicate lava flows; and finely, banded ore might have originated by ash falls and, brecciated ore from pyroclastic breccias (Nystr€, om &, Henriquez 1994). Thin apophyses of the ore extend, into both hanging wall and footwall rocks. This was, earlier thought to prove intrusion of an ore melt. Age, determinations demonstrate that ore and volcanic, host rocks have the same age (Romer et al. 1994)., The contrary opinions need not exclude each other, and it seems possible that both melts and fluids, existed at subvolcanic and extrusive levels. The interaction of ore melt, and later of the solidified massive, ore with fluids was substantiated by Harlov et al., (2002). The ore paragenesis comprises magnetite,, haematite, fluorapatite, actinolite, tremolite and, clinopyroxene, with 55–65% Fe and 1–2% phosphorous. Reserves of the district to 800 m below the, surface are estimated at 3000 Mt; annual production
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , is 30 Mt. This makes Kiruna the most important, iron ore producer of Western Europe., , Orthomagmatic-extrusive iron ores, Orthomagmatic-extrusive iron ores are infrequent, but several deposits are of an impressive, size. Some iron oxide melt reached the surface and, solidified as lava flows, volcanic agglomerate and, tuff; others filled veins and massive bodies within, subvolcanic intrusions. Host rocks are felsic and, intermediate volcanics that build calderas or, shield volcanoes. The FeOx liquid may result from, subvolcanic mingling of mafic and felsic melt, (Clark & Kontak 2004). Large deposits of this, genetic group, with ore made up of magnetite,, haematite, apatite and dendritic pyroxene, are, exploited in Chile (El Laco: Nystr€, om & Henriquez, 1994; note, however, the objection by Sillitoe &, Burrows 2003) and Mexico (Cerro de Mercado:, Figure 1.9, Swanson et al. 1978, Lyons 1988), both, containing 1000 Mt of ore., Skarn- and contact-metasomatic iron, ore deposits, Skarn- and contact-metasomatic iron ore deposits, form in the contact zone of certain gabbros, diorites, syenites and granites that intrude carbonate, rocks. Scientific enquiry concerns mainly the derivation of: i) the ore elements; and ii) the fluids that, transported these elements. There is no uniform, answer, as both ore elements and fluids may originate in the magmatic body, or in nearby host, rocks, or in wide regions around these. In the latter, case, saline fluids are often prominent,causing, regional sodic alteration (e.g. albite, scapolite), and thus forming a link to the IOCG-group of ore, deposits., In the Ural Mountains, economically outstanding skarn iron ore deposits formed in a collisional, Late Palaeozoic setting (Herrington et al. 2003):, Orebodies at Gora Magnitnaja occur in Early Carboniferous carbonates and minor volcanogenic sediments, which were intruded by Late Carboniferous, porphyric granite. Orebodies are grossly stratiform, within an envelope of skarn rock and calcite marble., An early anhydrous paragenesis of mineralization, , 153, , comprises magnetite, garnet and pyroxene, followed, by magnetite, haematite, pyrite, chalcopyrite, quartz,, epidote and calcite. Total ore in the deposit before, mining was 500 Mt, with an average 56% Fe, 0.8%, Mn, 14.85% SiO2, 1.82% CaO, 0.04% P and 1.94% S., Surficial ore was enriched by martitization. A comparable deposit is Sarbai in Kazakhstan, with a size of, 1500 Mt, which was found in 1948 by aeromagnetics., At Sarbai, gangue minerals include Na-Cl-scapolite,, clear evidence for the important role of saline fluids, in skarn metallogenesis., , Magmatic-hydrothermal, epigenetic, massive, and vein iron ore deposits, Magmatic-hydrothermal, epigenetic, massive and, vein iron ore deposits occur near many mafic to, intermediate calcalkaline plutons of recent or, ancient convergent plate margins (Chile, Peru,, Central Americas, Sweden and Japan). Some of, Phanerozoic age seem to be transitional to iron, oxide-copper-gold (IOCG) deposits sensu stricto, (Groves et al. 2010). Preliminary Sm-Nd and, Re-Os isotope datas suggest that iron oxide-apatite, ore is sourced by the crust, whereas systems with, economically predominating copper or gold display a mantle component (Skirrow et al. 2007):, El Romeral in Chile is a magnetite-apatite ore deposit, hosted in a Cretaceous andesite porphyry and metasediments near a diorite stock. The largest orebody, has dimensions of 850 m by 250 m. The ore consists of, sieve-like magnetite with inclusions of actinolite,, clinozoisite, titanite, Cl-apatite, andesine, scapolite,, chlorite and quartz. This gangue also fills stockwork, veinlets surrounding massive ore. The ore clearly, replaces hydrothermally altered rock bodies that are, limited by faults. The deposit is enveloped by the halo, of thermal amphibolite facies metamorphism, at, 475–550 C and 2 kbar. The iron seems to be derived, from strongly leached diorite (Menard 1995). Deposits like this can be considered as contact-metasomatic formations in non-carbonate country rocks., , Hydrothermal-metasomatic massive siderite, Hydrothermal-metasomatic massive siderite orebodies are of low grade (40% Fe) compared with, BIF-derived ores (60%). Districts of some renown, include Bakal, Ural Mountains, Russia; Bilbao,
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154, , PART I METALLIFEROUS ORE DEPOSITS, , Spain; Erzberg, Austria; Jerissa, Tunesia; and, Ouenza, Algeria. Typical resources of major deposits comprise several hundred Mt of ore; smaller, deposits and occurrences are quite frequent (e.g., the Alquife in southern Spain, Figure 1.36, Figure/, Plate 1.89). The heavy-media iron carbonate concentrate can be improved by oxidation in a hot, airstream and subsequent magnetic processing,, the magnetic fraction attaining nearly 60% Fe:, Epigenetic siderite occurs in marine epicontinental, platform carbonate rocks of Proterozoic to Mesozoic, age. The sedimentary association includes clastic, sediments and evaporites, and occasionally mafic, volcanics. Mineralized carbonates are not part of a, specific sedimentary facies, although within single, districts certain stratigraphical and lithologic levels, are preferred, such as Early Cretaceous lithographic, rudist limestones in Northern Africa and Spain., Orebodies are irregular stocks or stratiform lenses,, often controlled by pre-existing faults. In detail, ore, limits are commonly cross-cutting and cloud-like, (Figure 1.78). Apart from the massive siderite ore,, cross-cutting veins with siderite, quartz, barite, fluorite and sulphides of lead, zinc and copper (Ouenza,, Bilbao) may be present. Contacts between siderite, and unaltered carbonate are clearly metasomatic. In, Jerissa, within the first several metres inside the, siderite body rudists, bedding planes and stylolites, are preserved. Further towards the centre of the siderite stock, all remains of the precursor features, are effaced and only massive, sugar-grained monomineralic siderite rock is present. Parts of it are, drusy and display coarse calcite crystals. At Erzberg, (Figure 1.35), the siderite bodies are enveloped by, ankerite-dolomite alteration. Footwall and hanging, wall of orebodies are frequently formed by low-permeability and incompetent rocks such as shales., None of these deposits exhibit any traces of a, redox-zonation (oxide-carbonate-sulphide or silicate, facies), which is common in synsedimentary iron ore, deposits, both horizontally and vertically. Surrounding carbonates are typically rather pure and have low, geochemical iron contents. Nothing indicates that, iron was pre-enriched in these rocks., , In conclusion, masses of siderite replacing, carbonates are the product of epigenetic hydrothermal processes. The metasomatic replacement, may have happened at any time after deposition, of the carbonates, from early diagenesis to, , metamorphism. The giant deposits at Bakal,, Russia are hosted by Mesoproterozoic limestones, (1400 Ma), whereas Pb-Pb dates of siderite concur with the Grenvillean orogeny (1000 Ma:, Kuznetsov et al. 2005). Possible sources of oreforming fluids include deeply circulating seawater, or evaporitic brines (Strmi, c Palinkaš et al. 2009),, formation waters (Torrez-Ruiz 2006), migrating, basinal or salt-solution brines (Pohl et al. 1986;, Bouzenoune & Lecolle 1997) and metamorphic (, saline) fluids (Erzberg: Pohl & Belocky 1994; Bakal, Russia: Kholodov & Butuzova 2004). Only, magmatic sources have not been invoked., Volcanogenic-exhalative and, sedimentary-hydrothermal (sedex) deposits, Volcanogenic-exhalative and sedimentary-hydrothermal (sedex) deposits are not significant producers of iron ore but some are co-producers of iron, and metals such as Cu, Au, As, Tl, Pb and Zn., Pyrite bodies of the Kuroko, Cyprus and sedex, type, which are exploited for the production of, sulphuric acid, yield an iron oxide sponge that can, be suitable blast furnace feed, provided the chemical composition is acceptable. Volcanogenic and, sedex type iron ores may also occur in carbonate or, oxide facies. Algoma Type iron ores are part of, this group., Residual, or lateritic iron ore deposits, Residual, or lateritic iron ore deposits, occur above, ferriferous mafic or ultramafic bedrocks (mostly, ophiolites). From these precursor rocks, they often, inherit Cr-Ni-Co that impedes regular metallurgical processing. In addition, these ores contain up to, 20% Al2O3, have relatively low iron grades and, high moisture contents reaching 30%. All this, dampens interest in such ores. If concentrations, of chromium or nickel reach higher grades, mining, may be profitable (Albania, Cuba, Greece, Guinea,, Philippines, Western Australia). The material is, currently extracted as a high-iron limonite, nickel ore (“oxide nickel ore”) for Chinese pig-iron, blast furnaces. In oxide nickel ore, nickel is absorbed on amorphous iron-hydroxides or occurs, as inclusions in goethite. The new technology of
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , high-pressure acid-leaching (PAL) appears to alleviate the former economic disadvantage of processing these ores:, Conakry, Guinea is a characteristic example of this, group. In the deposit, 6 m of hard ferricrete cover, dunite with 10% iron. Run-of-mine ore contains 52%, Fe, 12% Al2O3, 1.8% SiO2, 0.25% P, 1.8% Cr, 0.15%, Ni, 0.5% TiO2 and 11% water., , Obviously, residual iron ores are rarely attractive. In contrast, supergene alteration has a prime, role in upgrading low-grade protore by supergene, enrichment. Of course, this process may in principle act on any genetic variety of iron ore and, nearly all near-surface deposits display some, enrichment (Figure 1.36). However, Proterozoic, banded iron formations of the Superior Type are, by far the most important precursor rock, because, of their wide distribution and the giant initial, mass of iron involved (Figure 1.52)., Supergene infiltration “channel” ore deposits, Around 1990, a new type of giant iron ore deposits, began to be developed in the Hamersley Basin of, Western Australia, although they had been recognized much earlier in about 1955 (Glasson &, Rattigan 1990). Several river courses that drain, the Hamersley BIF province towards the east and, north are shadowed for many kilometres by table, mountains. These are built of iron ore that resisted, the erosion that lowered the surrounding land, surface. The mountains represent the fill of Early, Tertiary broad valleys (“limonite rivers”) formed, by precursors of recent rivers, giving rise to the, term “channel iron ore deposits” (Figure 2.2). In, the lower reaches of rivers draining the Hamersley, Ranges, part of these ores are buried beneath, alluvial valley fill., W, , 155, , Several of the first mines were sited along Marillana, Creek, based on resources of >3500 Mt over a, length of 80 km. The ore grades 58% Fe, 0.04% P,, 4.9% SiO2, 1.3% Al2O3 and 10% H2O. It consists of, densely packed pisolitic concretions (not ore pebbles), of goethite and haematite. Concretions are partly, concentrically banded but also massive. The matrix, contains the same iron minerals, and some clay,, calcite, chalcedony, and rarely limonitized plant remains. Single orebodies reach a thickness of 100 m., These ores formed by precipitation from ferriferous, alluvial groundwater, either directly or by replacement of clastic particles. The source of iron was, weathering of upstream outcrops of banded iron formations (Stone et al. 2002)., , Terrestrial-sedimentary iron ore deposits, Terrestrial-sedimentary iron ore deposits were the, first source of the metal for humans, but with the, onset of industrialization they were quickly abandoned because the mass of mineralization is commonly very small. Inflow of acidic water produces, bog ore in swamps and corresponding siderite ore, in coal sequences (Kholodov & Butuzova 2004)., Sluggish ferriferous drainage fills karst cavities., Alluvial placers (black sands) are ubiquitous, but tiny., Marine-sedimentary and associated, supergene-enrichment iron ore deposits, Marine-sedimentary and associated supergeneenrichment iron ore deposits include BIF, ironstones, oolitic iron ore, detrital iron ore, and, coastal placers. Among these, the banded iron, formations of the Superior Type dwarf all other, sources. However, banded iron formations with, their relatively low primary iron grades (25–45 wt.%, Fe) are rarely exploited. The bulk of the BIF-iron, worldwide is derived from locally enriched, , Marillana Creek, , E, Water table, , Weathered ore, Semi-weathered ore, High grade ore, , Basal zone, Basal gravels & conglomerate, , Basement (BIF, shale, dolerite), 100 m, , Figure 2.2 Typical cross-section of an Early Tertiary fluviatile (“channel”) infiltration iron ore deposit near Marillana, Creek in the Hamersley Basin, Western Australia (Stone et al. 2002). Note the relief inversion from former valley fill to, present table mountain.
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156, , PART I METALLIFEROUS ORE DEPOSITS, , the Witwatersrand Reefs), basalt, tuff and black, shale of the Fortescue Group (1), which are overlain by the Hamersley Group. The Hamersley, Group (2) displays five important BIF-horizons, (among them the oldest known 2.6 Ma Superior-type BIF, the Marra Mamba Formation, and the, 2.5 Ma Brockman Iron Formation of Figure 1.52,, Figure/Plate 1.67) and intercalated shales, dolomite, and rare bimodal volcanic rocks. The overlying greywackes and basalts of the Wyloo Group, (3) are separated from (2) by an unconformity,, which is marked by haematite conglomerates., The sediments attest to periods of quiet epicontinental marine conditions, whereas volcanic rocks, imply tectonic extension and mantle-derived, magmatism. The latter might have contributed, to hydrothermal import of iron and silica into the, shallow sea (Bekker et al. 2010, Barley et al. 1997,, 1999). In the north, metamorphism and deformation of the rocks is restricted, whereas both, increase to the south, reaching greenschist facies, and vergent folds approaching the collisional, Ophtalmia (Capricorn) Orogen (Rasmussen et al., 2005). Several granites are known in the southern, part of the basin, with intrusion ages of 1700 Ma., , sections of banded iron formations (Chapters 1.2, “Supergene Ore Formation Systems”, Chapter 1.3, Sedimentary Ore Formation Systems” and Chapter 1.4 “Diagenetic Ore Formation Systems”)., These are the “BIF-hosted iron ore deposits” that, range from a few million to over 3000 million, tonnes at >64 wt.% Fe, although most are between, 200 and 500 Mt. Common BIF varieties are, described as taconite (a low-grade BIF with, 30% Fe in Minnesota, which can be exploited, because magnetic enrichment of magnetite and, haematite is feasible), itabirite (a metamorphosed,, coarse-grained banded iron formation, Figure 2.3),, canga (scree ore deposits) and jaspilite (very lowgrade metamorphic, fine-grained iron formation)., Important iron ore mining provinces based on, Palaeoproterozoic banded iron formations occur, in Australia (Hamersley), South America (Minas, Gerais and Caraj, as in Brazil), North America (Lake, Superior, Labrador Trough), Europe (Ukraine, with Kryvyi Rih, or in Russian Krivoj Rog, and, Kursk Magnetic Anomaly in Russia), Asia (Orissa,, Goa, India) and in Africa (Liberia, Angola, Transvaal S.A.)., One of the largest iron ore provinces in the world, is the Hamersley Basin in the Pilbara region of, Western Australia. The basin covers a surface, of 100,000 km2 and is built from Neoarchaean, to Palaeoproterozoic (2.77–2.41 Ga) sediments and, volcanics of the 15-km thick Mt Bruce Supergroup. In the north, these rocks transgress older, Archaean rocks discordantly, whereas in the south, they are in turn overlain by younger Proterozoic, sediments. The Mt Bruce Supergroup starts with, basal conglomerates (partly auriferous similar to, , Exploitable iron ore resources of the Hamersley Basin, are estimated at 22,000 Mt, additional marginal ores, at 18,000 Mt. High-grade haematite deposits with, 60–68% Fe represent only 0.1% of the total iron, endowment. The bulk of the iron rests in the primary, BIF with 30% Fe and 50% SiO2, which cannot be, profitably exploited. Profitable ore types include, high-grade haematite, martite-goethite, and pisolitic, channel as well as detrital scree ore. Orebodies of the, first two types replace BIF in situ (Figure 1.52). Mass, , 1600, , Pico de Itabira, , 1500, Hard haematite ore, 1400, Soft haematite ore, 1300, Medium itabirite, , Phyllites, 1200, Dolomite, , Siliceous itabirite, , Canga, , 100 m, , m a.s.l., , Figure 2.3 Geological profile of the, Palaeoproterozoic high-grade haematite, deposit of Pico de Itabira, Brazil. After, Klein, C. & Ladeira, E.A. 2000, Society of, Economic Geologists, Inc., Economic, Geology Vol. 95, Figure 9, p. 419. ProtoreBIF is siliceous itabirite (40% Fe)., Medium itabirite is moderately enriched, to 58% Fe. Both hard ore and soft ore are, high-grade with about 67% Fe. Host, rocks are metamorphosed in the, greenschist facies.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , calculations prove that iron must have been added, while silica was removed. Limits to ore are often ironpoor host rocks or faults, and orebodies mirror strike, and orogenic structures of BIF packages. Orebodies, may reach 70 m thickness and a length of many kilometres so that some of them contain >1000 Mt (Mt, Whaleback). All enriched ore occurs near a Tertiary, land surface (the “Hamersley surface”). None has, been found deeper than 400 m below this level, only, primary unaltered BIF., Banded iron formations of the Hamersley Basin are, metamorphosed and consist of quartz and magnetite,, with some stilpnomelane (a mineral resembling, biotite), carbonate and apatite. Ore derived from this, paragenesis by supergene alteration only contains, martite and goethite in different proportions, and a, gangue of quartz, opal and kaolin. Goethite replaces, former quartz bands but may be redissolved. These, processes result in various ore classes (as a function of, Fe and P content, texture, porosity, strength and, mineralogical composition: Clout 2006) that are differentiated for economic reasons. Phosphorous is, almost exclusively associated with goethite, so that, high-grade haematite is nearly free of it. Goethitebearing ores contain up to 0.12% P, which is economically problematic. Giant additional reserves of iron, ore will be available if technological developments, allow cost-effective removal of phosphorous. Average, run-of-mine ore in the province is characterized by a, mineralogical composition of 70% haematite with, 23% goethite, 5% kaolin and 2% quartz, resulting in, a chemical composition of 63.5% Fe, 4.3% SiO2,, 2% Al2O3 and 3% loss on ignition (mainly water)., , High-grade haematite ore deposits associated, with Superior type BIF, Generally, martite-goethite ores are thought to, result essentially from supergene alteration of BIF, (Lascelles 2006), whereas microplaty, porous highgrade haematite ore (Figure 1.52 and Figure 2.3), was explained by metamorphism of BIF that experienced previous supergene enrichment (Morris, 1985, 1993, 2003). Recently it was recognized that, deep hydrothermal magnetite, haematite and siderite bodies were later affected by supergene leaching and/or oxidation (Thorne et al. 2004, Dalstra &, Guedes 2004, Taylor et al. 2001, Barley et al. 1999)., Several stages of hydrothermal alteration are indicated, starting with NaCl-CaCl2 rich brine,, , 157, , followed by leaching of carbonate and apatite in, the supergene alteration domain. Oxygen isotopes, confirm this model (Thorne et al. 2009). The first, fluid event is clearly post-metamorphic and dated, to 2000 Ma, which was a period of continental, extension (M€, uller et al. 2005). In the hydrothermal, models, haematite can be formed from magnetite, by hydrothermal leaching of Fe(II) and of silica,, without the need of extraneous oxygen (eq. 2.1,, Ohmoto 2003b)., Formation of haematite from magnetite by hydrothermal leaching of Fe(II):, Fe3 O4ðMagÞ þ2Hþ ) Fe2 O3ðHemÞ þFe2þaq þH2 O, ð2:1Þ, Deposits of high-grade haematite reach 3000 Mt, of nearly pure iron oxide (Caraj, as, Brazil) and are by, far the largest metal concentrations on Earth., Note, however, that the ore genesis in the Caraj, as, province is also not fully understood (Rosi, ere et al., 2006):, The high-grade haematite orebody at Mt Tom Price,, Western Australia (Figure 1.52, Figure/Plate 2.4) contained an original resource of 900 Mt at 63.9% Fe and, <0.05% P. Situated in the southern greenschistmetamorphic and folded part of the Hamersley Basin,, the orebody occurs as a SW-dipping sheet on the flank, of an anticline. The deposit extends for 7 km and is up, to 1.6 km wide, with a maximum depth of 250 m, below the original topographic surface (Taylor et al., 2001). Extensional faults channelled basinal brines,, which enriched metamorphic BIF in iron and, abstracted silica., , Oolitic and detrital iron ore, Large concentrations of oolitic and detrital iron, ore occur in the environs of Salzgitter in the Mesozoic foreland of the Palaeozoic Harz Mountains, in Northern Germany. Several stratigraphic horizons within the marine platform sediments have, been exploited. Mining was terminated in 1976, because of low ore grade and resulting higher costs, compared with concentrates from overseas. Overall, 340 Mt of ore had been produced. Remaining, potential resources are estimated at 4000 Mt., The average composition of the ore was 29% Fe,, 26% SiO2, 5% CaO, 0.1% Mn and 0.4% P:
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158, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 2.4 (Plate 2.4) Birds-eye view of the high-grade haematite Mt Tom Price mine in the Hamersley Basin, looking, to the northwest (cf. Figure 1.52). Southwest dipping Precambrian banded iron formations build the hills that rise above, the plains. Courtesy of A.E. Harding, Rio Tinto Iron Ore, Perth, Western Australia., These iron ores were formed when a shallow epicontinental sea covered parts of Northern and Central, Europe, with several large islands and wide landmasses (Figure 2.5). Throughout the Mesozoic, the, distribution of land and sea changed continuously., The ore deposits of the Salzgitter district occur near, the coast of a southern landmass and smaller islands, that originated (and foundered again) by a complicated interaction of the rise of salt diapirs, tectonics, and sea level changes. Oolitic iron ores were deposited at different sites in the Liassic, Malm, Early and, Late Cretaceous. The last iron ore mine in the district, (Schacht Konrad) was not closed, but given a new, use as a final storage site for disposal of nuclear waste, of low to medium-level activity. A few seams (in the, Jurassic) extend across very large areas. Some oolite, ore developed above coarse detrital ore that is, restricted to the Cretaceous. Detrital iron ore accumulated in localized, very deep troughs that formed, , by faulting on the flanks of salt diapirs. Sources of the, detrital iron ore are siderite nodules and pyritized, fossils in Early and Middle Jurassic shales, which, were oxidized during phases of emersion. Renewed, transgression of the sea imposed a high-energy coastal, environment with strong tides, breakers and currents, that swept coarse iron oxide detritus into depressions., Some detrital orebodies reached a thickness of 120 m, and a length of over 1 km., , Important oolitic ironstone seams were, exploited in Lothringia (the limonitic “minette”, of Early Dogger age), in the Prague basin (Ordovician), in Algeria (Gara Djebilet, Early Devonian), and in the Clinton District (Alabama, USA, Late, Silurian, haematitic). With the exception of, China, few mines now exploit oolitic ironstones., This is due to the rather low iron grade and
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Figure 2.5 Palaeogeographic, environment of oolitic and detrital iron, ore deposition in the Salzgitter district in, Northern Germany. Distribution of land, and sea (adapted from Ziegler 1982)., Reproduced with permission Shell &, Elsevier Ó (1982). Dashed lines indicate, today’s coasts. Cretaceous sediments in, the island belt of the inland sea include, deposits of oolitic and detrital iron ore, (black dot, Fe), glass sand, clay and coal., , Early Cretaceous, inland sea, , Fe, Harz Mts., , London-, , Brabant, Massif, , unwanted high content of phosphorus, aluminium and SiO2. BIF-ores are both cheaper and of, higher quality., Main producers of iron ore are China, Brazil,, Australia, India and Russia. Together, the five, countries delivered 80% of the world total of, 2200 Mt (2008; 2300 Mt in 2009: USGS 2010)., World crude steel production in 2008 was 1360 Mt, (1100 Mt in 2009, after the financial crisis). Iron, ore resources are immense, exceeding 800,000 Mt., Exploration for iron ore is directed at large (>100, Mt) or very large (>1000 Mt) deposits of high grade,, which are amenable to open pit mining. Geological methods such as mapping are mainly employed. Surprisingly, the usefulness of magnetic, methods may be quite limited; in Western Australia, for example, ordinary BIF can be mapped,, but not the economically interesting enriched, orebodies (Dentith et al. 1994). Of course, geochemical exploration methods are hardly useful., , 2.1.2 Manganese, Common Ore Minerals:, Max. wt.% Density, Mn, (g/cm3), Pyrolusite, Braunite, Hausmannite, Rhodochrosite, Cryptomelane, Manganite, , b-MnO2, 3Mn2O3. MnSiO3, Mn3O4, MnCO3, K[(Mn4þ,Mn2þ)8O16], g-MnO(OH), , 159, , 63, 64, 72, 49, 57, 62, , 4.75, 4.8, 4.8, 3.6, 4.3, 4.3, , Rhenish Massif, , Bohemian Massif, , This list is far from complete, as there are more, than 100 manganese-bearing minerals. Predominant are rhodochrosite and braunite. Currently, exploited manganese ore is often a mixture of, ore minerals, usually of various oxy-hydroxides., Therefore, in mining practice, hard ores are called, psilomelane or manganomelane, soft ores are, called pyrolusite, and earthy impure masses are, called wad. Psilomelane, for example, is commonly a mixture of botryoidal roman, echite, BaMn2þMn84þO16(OH)4,, cryptomelane, and, todorokite (Mn,Ca,Mg)Mn3O7.H2O. Rhodochrosite is chemically variable and may contain Fe,, Mg, Ca and Zn. According to their industrial, destination, manganese ores are subdivided as, metallurgical, chemical and battery grade. Metallurgical grade ore (chiefly for alloying steel and, aluminium) must contain a minimum of 46% Mn., Manganiferous iron ore with 10–25% Mn is alternatively used in iron and steel production. Chemical and battery grade ores should have between 70, and 85% MnO2. Most processors of manganese, refuse concentrates with significant contents of, non-ferrous metals, such as Ni, Pb, Cu, Co, As, Sb,, etc., and phosphorous, earth alkalis, SiO2 and, Al2O3. In such cases, the value of the ore depends, on possible improvement by processing., About 90% of the world output of manganese, is used in the steel industry, mainly for de-sulphurization and oxygen-control of the melt, but, also for alloying. Manganese is added in the, form of silico-manganese, high-carbon (HC), ferromanganese and refined ferro-manganese., Silico-manganese is the predominant manganese
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160, , PART I METALLIFEROUS ORE DEPOSITS, , alloy used in the production of long steel products. Most ordinary steel contains 0.5–1.6%, Mn. Special steel for rails, mining machinery, and crushers has 11–15% Mn, which raises resistance to wear and fatigue failures. Some stainless, steel varieties contain manganese in addition to, Cr and Ni. Dry cell batteries are the second, important market sector. Zinc-carbon batteries, are made directly with good battery ore, whereas, synthetic (electrolytic) MnO2 is prepared for, alkali and lithium batteries. Chemicals (e.g., KMnO4, which is used for purification of drinking water) and ferrites (components of many, electronic devices) consume only a small part of, total production., For humans and animals, manganese is one of, the essential trace elements, because it supports, certain biological functions (Lindh 2005). Adverse, effects can arise from both deficiency and overexposure, but the latter is very rare except in, occupational settings (WHO 2006)., Geochemistry, The geochemical behaviour of manganese is very, similar to iron, although it is classified as, a lithophile element (Goldschmidt 1958). Manganese, however, displays considerably higher, mobility. Its crustal average is 900 ppm; the, Mn/Fe ratio of the crust 0.02. Mafic magmatic, rocks have the highest manganese content (ca., 0.2%), which is negatively correlated with SiO2,, again comparable to iron. Exploitable concentrations of manganese in ore require geochemical, separation from iron. Geochemical possibilities, of separation are indicated in the Eh/pH diagram,, where stable solid Mn-phases occupy a smaller, space compared to iron minerals (Figure 2.1). Precipitation of Mn-minerals requires higher oxidation potential and/or alkalinity. This is physically, illustrated by the distal manganese halo of submarine hydrothermal sedex ore deposits, where iron, concentrates in the proximal facies. Principally,, iron and manganese may be separated in the, source region, during transport, or by differential, precipitation. In the sedimentary realm, redox, facies zones similar to iron are less often seen in, manganese-rich sediments. In some important, , mining districts, a carbonate-silicate-oxide succession is observed (Figure 1.69), but never massive sulphide, as with iron because Mn-sulphides, are rare., Like iron, manganese is a redox-sensitive element. Mn(II) is common in minerals of magmatic, rocks such as Fe-Mg silicates and carbonates. Its, ionic radius is similar to Fe(II), Mg(II) and Ca(II), favouring substitution in rock-forming minerals., In supergene alteration settings and possibly, by, sedimentation in the oxic zone, minerals with, Mn(IV) are formed. Pyrolusite is the dominant, oxide phase at high oxidation potential (Eh) and, over a range of pH. The intermediate oxidation, state Mn(III) (braunite, hausmannite) originates by, partial reduction or oxidation, often related to, metamorphism and hydrothermal processes. The, Mn(IV) ion is so small that substitution by other, elements is restricted, which is one reason why, manganese ores of Mn(IV) are practically free of, iron. Manganese oxides have a high sorption, capacity for cations, and certain trace element, associations characterize specific genetic settings, (Maynard 2010, Cabral et al. 2002, Nicholson, 1992)., Manganese ore deposit types, Manganese ore deposit formation and protore concentrations are due to the high solubility of manganese in moderately reduced aqueous fluids, from, seawater to hydrothermal systems. Separation of, Mn and the geochemically similar and prevalent, Fe is due to incidental small shifts in the pH/Eh, field. Two genetic modes prevail, hydrothermalexhalative and marine-sedimentary (Frakes &, Bolton 1992, Roy 1992). In many instances, uneconomic primary carbonatic manganese formations, were enriched and transformed by supergene, processes to valuable oxide ore. In contrast to, iron, magmatic Mn-deposits are unknown. Epigenetic hydrothermal manganese ore veins and, replacement orebodies are of minor economic significance. Two exotic karst infiltration deposits, have recently been described (Li et al. 2007,, Gutzmer et al. 2006)., . geologically young marine-sedimentary manganese deposits (Groote Eylandt, Australia);
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , . enriched sections of marine-sedimentary Palaeoproterozoic manganese ( iron) formations, (MnF), e.g. in the Kalahari Field, South Africa;, . sedimentary-exhalative, manganese deposits, (Jurassic Molango, Mexico);, . volcanogenic hydrothermal-exhalative manganese deposits similar to Algoma type iron ore, (e.g. Nsuta primary ore);, . supergene manganese oxide ore deposits derived, from protore (e.g. Nsuta supergene ore)., , Hydrothermal vein and replacement, origin, An example of hydrothermal vein and replacement origin is Umm Bogma on the Sinai Peninsula,, where stratabound orebodies replace Carboniferous dolomite, probably in genetic relation to Tertiary rifting of the Gulf of Suez (note, however, that, marine-sedimentary formation was also proposed:, Elagami et al. 2000). Numerous small epigenetic, manganese mineralizations are known along, the faulted margins of the Red Sea. Submarine, exhalation from hydrothermal systems at rift, faults may result in considerable concentrations, of manganese (Polgari et al. 1991). Ore from hydrothermal deposits can be burdened with undesirable base metal contents., Hydrothermal-exhalative manganese, deposits, Submarine hydrothermal-exhalative manganese, deposits are quite common (Polgari et al. 1991)., Some occur in close proximity to volcanic centres, (volcanogenic), others distal to volcanism and, within a sedimentary environment (sedex). The, first include the manganese-rich metalliferous, muds of the Red Sea, the manganese earths, (umber) and Mn-cherts of many ophiolites (e.g., Cyprus, Franciscan, California) and manganese, beds near submarine volcanoes (Iberian Pyrite, Belt). The second group is represented by the, giant Molango District in Mexico, by carbonatic, manganese ore in black shale basins and Mn-rich, metamorphic rocks such as the Proterozoic, “gondites” in India, that are an important protore, for supergene deposits:, , 161, , Mineralization in the Molango District occurs as, primary to early diagenetic manganese carbonate, (rhodochrosite, kutnahorite and Mn-calcite) and, supergene battery-grade Mn-oxide (pyrolusite, ramsdellite MnO2 and manganese oxy-hydroxide). The, Late Jurassic Mn-carbonate ore member extends over, an area of 50 by 25 km and contains 1500 Mt manganese. At this time, a complex pattern of marine, basins and ridges originated by rifting and deepening, of the Gulf of Mexico Basin. This was part of the, break-up and distension of Pangaea. The ore bed is, underlain by a black, finely laminated, pyritiferous, and calcareous shale, which may imply a restricted, basin (euxinic) model of formation. The immediate, footwall of the ore bed with abundant wood fragments, and oyster beds indicates precipitation in shallow, oxidizing water. Its hanging wall is formed by alternating limestone and shale. No traces of syngenetic, volcanism are known. Exploitable stratiform ore at, the Tetzintla mine displayed a thickness of 10 m and, up to 30% of manganese in rhodochrosite (Okita, 1992). Subeconomic lower grade Mn-carbonates continue upwards in the section for tens of metres. The, finely laminated ore consists of rhodochrosite, silty, shale and iron oxides (magnetite, maghemite) and is, strikingly deficient in sulphur. Mn-carbonate probably formed during diagenesis by Mn oxy-hydroxide, reduction coupled with organic matter oxidation., Possible sources for this giant metal accumulation, include: 1) seawater, 2) hydrothermal activity and 3), alluvial import from land in the west, with 2 and 3, both equally possible. Overall at present, a sedex, origin in combination with submarine rifting is, assumed (Okita 1992)., , Marine-sedimentary manganese deposits, Marine-sedimentary manganese deposits (Frakes, & Bolton 1992) were introduced in Chapter 1.3, “Autochthonous Iron and Manganese Deposits”., They occur as oolitic or massive seams in sediments of epicontinental seas. Host sediments, comprise mainly clay, marl and sand (rarely carbonates). Similar to ironstones and oolitic iron, ores, the metal source for oolitic manganese deposits is usually supposed to be continental weathering and alluvial transport into the sea, but a, higher activity of suboceanic hydrothermal, sources is also considered. The first hypothesis, fits very well with the observation that the interaction of anoxic deep water with manganese
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162, , PART I METALLIFEROUS ORE DEPOSITS, , dissolved by anaerobic methane oxidation (Beal, et al. 2009) and oxygen-rich seawater of marginal, zones causes manganese oxide precipitation (Roy, 1992). However, formation according to the oxygen minimum zone model is equally possible, (Maynard 2010). Similar to iron, sedimentary manganese strata reflect climate and redox cycles,, affecting oceans worldwide or at local scales:, The largest metallogenic provinces of this deposit, type include the Oligocene Black Sea province, (Nikopol, Ukraine: Figure 1.69; Chiatura, Georgia), and the Cretaceous province in Northern Australia,, with Groote Eylandt as an important district, (Figure 2.6). In the Ukraine, formerly rich oxide ores, are depleted and remaining resources are carbonatic, with high phosphorous contents. Run-of-mine ore at, Groote Eylandt is high-grade with 51% Mn, 3.1% Fe,, 3.9% SiO2, 4% Al2O3, 1.9% BaO and 0.16% P2O5., Resources are 160 Mt (USGS 2009)., , Precambrian manganese ( iron), formations (MnF), Similar to BIF, Precambrian manganese ( iron), formations (MnF) are of high economic importance, although comparable to iron, only enriched, parts are exploitable. The large Palaeoproterozoic, deposits in the Transvaal Sequence of western, Griqualand, South Africa are exploited in a number of mines. The sequence comprises sediments, and volcanic rocks of a post-Archaean platform, , 80, , W, , Highest Cretaceous sediment, Highest pisolitic Mn-ore (~+40 m), , 40, , m, , basin that experienced very mild later folding, and metamorphism. The Transvaal Sequence is, remarkably mineralized, with manganese and, Superior Type banded iron formations, fluorspar,, Pb/Zn, asbestos, andalusite, and dolomite. In the, Kalahari Field, manganiferous material >20% Mn, amounts to 13,000 Mt and of this, resources of, 4000 Mt are considered to be economic (constituting 77% of total identified terrestrial Mnresources). The Mn-rich rocks are believed to have, been formed as distal, volcanogenic, hydrothermal-exhalative deposits on the seafloor (Cornell &, Sch€, utte 1995). Trace element characteristics are, similar to Superior Type BIF, so that a purely, sedimentary origin is also feasible, possibly with, a very small hydrothermal component (Tsikos &, Moore 1997). The concentration of Mn relative to, Fe is believed to have been amplified by earlier, deposition of thick iron formations, possibly in, a restricted sea. A small area (ca. 3%) in the, northwestern part of the Kalahari Field displays, high-grade ore adjacent to faults. This area contains 20% of high-grade manganese reserves of the, world. Agents of enrichment were hydrothermal, solutions that leached SiO2 and CaCO3 from protore and transferred some of the Mn. Opinions, differ as to whether this process took place during, a synsedimentary hydrothermal phase (Cornell &, Sch€, utte 1995, Tsikos et al. 2003), or during later, deformation and metamorphism (Gutzmer &, Beukes 1995)., , E, +75m, bench, , Sea level, 1 km, , 0, -40, , Proterozoic quartzite, Sand, , -80, Post-ore clay, soil, , Mn-oxide ore, , Clay, , Figure 2.6 Cross-section of, the marine-sedimentary, manganese deposit at Groote, Eylandt, Northern Australia, (Bolton et al. 1990).The, manganese horizon is part of, an Early Cretaceous, transgressive series., Enrichment by supergene, alteration upgraded protore, to exploitable grades which, occur across an area of, >150 km2.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , The largest mine in the high-grade zone is Mawatwan., Manganese seams occur within an iron formation that, overlies basalts. Mining is based on the main manganese seam with an average thickness of 15 m. Far from, faults, the seam displays a fine-grained, laminated, paragenesis of braunite, kutnahorite Ca(Mn,Mg,Fe), (CO3)2 and hausmannite, with minor haematite and, calcite (Mawatwan type ore). Enriched ore near faults, contains 38–50% Mn, is coarsely crystalline, drusy,, free of carbonate, and consists of hausmannite and, braunite (Wessels type ore). Lenses of coarse sparry, Mn-calcite and dolomite are often observed. This, deposit is practically free of Mn(IV) minerals., , Supergene manganese ore deposits, Supergene manganese ore deposits are derived from, rocks with high but unexploitable manganese concentrations of more than 30%. Above this protore,, the laterite blanket often includes hard Mn crusts, and earthy Mn ore. In contrast to iron that tends to, concentrate in the upper soil horizons (as ferricrete), the more mobile manganese is typically, enriched in lower parts of the weathering profile., Although smaller than sedimentary manganese, deposits, supergene lateritic Mn ores are often, high-grade and, because of effective leaching, of, superior quality. Important mining districts exist, in Gondwanan continents (South Africa, Minas, Gerais, Brazil, Orissa, India, Moanda, Gabon, Nsuta, Ghana) and in China. Source rocks (protore), include spessartine quartzites (Roy 1992: gondite, or coticule rock, the second meaning “wetstone”),, manganese phyllites, manganese carbonates or any, volcano-sedimentary rocks that contain pre-enriched horizons of exhalative or sedimentary origin. Weathering removes or depletes carbonate and, SiO2, enriching manganese oxides and hydroxides., Carbonates and sulphides in protore promote, weathering and ore formation, whereas purely silicate-based rocks form thin ore blankets., 40, Ar=39 Ar-dating of cryptomelane revealed that the, formation of residual Mn-deposits is an extremely, slow and long-lasting process (Li et al. 2007):, Oxide ore at Nsuta in Ghana developed from Palaeoproterozoic volcano-sedimentary rocks that were, folded and metamorphosed at 2100 Ma. For a long, time, the deposit was a major source of high-grade, , 163, , supergene ores (the total production since 1917, amounts to 13 Mt). Today, lower grade unweathered primary rhodochrosite ore is exploited from a, horizon with a thickness of 50 m. Resources are, estimated at >50 Mt. The ore horizon is banded and, is hosted by fine-grained marine metasediments., Its origin is probably similar to Algoma type iron ore, (“carbonatic manganese formation”: M€, ucke et al., 1999). A new, significant resource of high-grade manganese was found in extensive supergene deposits on, the plateaus around Moanda in Gabon., , The leading manganese-producing countries in, 2008 were South Africa, Australia, China, Gabon, and Brazil. World mine production in 2008 was, 13.2 Mt ore, dropping to 9.6 Mt in 2009 (USGS, 2010). Manganese production and consumption, closely follow the trade cycles of world steel production. Manganese resources are very large, sufficient for hundreds of years. Eighty percent of, identified resources occur in South Africa (Kalahari), and 10% in the Black Sea region (USGS 2010). Into, the far future, giant potential resources of manganese and other metals exist on the seafloor, in the, form of manganese nodules and crusts (cf. Chapter, 1.3 “Autochthonous Iron and Manganese Deposits”). At present, economic exploitation of subsea, manganese appears hardly conceivable, because, land-based reserves are abundant. Geological methods are mainly employed in exploration, eventually, assisted by gravity surveys, which rely on the density contrast between ore and host rocks., 2.1.3 Chromium, Ore Mineral:, Chromite, , (Fe,Mg)O(Cr,Al,Fe)2O3, , Density, 4.3–4.6 (g/cm3), Typically 45 to 55,, maximal, 68% Cr2O3, , By definition, the mineral chromite contains, >25% Cr2O3; at lower contents chromiferous, spinel is the correct designation. Chromite composition varies within 6–18% FeO, 0–22% MgO,, 0–62% Al2O3 and 0–30% Fe2O3. Some chromites, display minor contents of Mn, Ti, V or Zn. The
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164, , PART I METALLIFEROUS ORE DEPOSITS, , chemistry of chromite is closely controlled by, the genetic setting (Page & Barnes 2009, Irvine, 1965, 1967). In some cases, traces of nickel,, copper and platinum metals in chromite ore can, be recovered (e.g. from the UG2 chromite seam in, the Bushveld; cf. Section 2.3.3 “Platinum and, Platinum Group Metals”)., Chromium ore may be exploitable at minimum, contents of 20% chromite, provided that a concentrate >44% Cr2O3 can be produced at reasonable cost by methods such as gravity separation,, magnetic separation and flotation. Very often,, however, massive chromitite is exploited. Gangue, minerals are typically olivine or serpentine. Producers strive not to market concentrate but the, intermediate industrial product ferrochromium., About 70% of chromium is used for manufacturing stainless steel (Cr 13–25%, 1–10% Ni, 0–5%, Mn, 0–5% Mo and 0–1.5% Cu), and of steel alloys, with exceptional hardness, durability and tempering properties, for example for aerospace components. Suitable ore for this metallurgical market, segment has a high chromium content (>46%), allowing fabrication of high-quality ferrochromium. Zimbabwe, Kazakhstan, Albania, South, Africa and Turkey provide metallurgical concentrates. Chromite of intermediate Cr-contents is, used in the chemical industry (chemical grade),, but also for metallurgical applications. Examples, of chemical products include chrome-plated metal, and plastics commodities, pigments, salts for wood, preservation and tannery, and pure chromium, metal (D ¼ 7.19 g/cm3, melting point 1907 C),, which is a component of special alloys in spacecraft, and aircraft. Low-Cr ore with >20% Al2O3 and, >60% Al2O3 þ Cr2O3 (refractory ore) is increasingly sought. It is processed into refractory basic, chromite-magnesite (actually periclase ¼ MgO), bricks and mortars, which are used as an internal, lining in blast furnaces. Refractory chromite is, mainly provided by certain ophiolite-hosted deposits (e.g. Philippines). Chromite of suitable size and, durability is also used as a replacement of the more, expensive zircon foundry sand., Both for humans and for animals, chromium is, an essential trace element (Lindh 2005). In higher, concentrations, however, it is toxic and carcinogenic, especially in the hexavalent ionic state., , Because chromic acid and its salts were widely, used by small industry such as galvanization and, tannery, chromium is a common groundwater, pollutant. Another frequent source of pollution, is chrome-plated waste in refuse dumps. Crþ6, polluted waters can be cleaned by passage through, reducing, permeable barriers that are installed in, the aquifer. Isotope analysis assists efficiency controls, because light Cr-isotopes are preferentially, reduced (Blowes 2002, Ellis et al. 2002). Reduction, leads to precipitation of insoluble Cr(III) hydroxide, Cr(OH)3. It is an environmental curiosity that, due to neoformation of carbonates in tailings,, mines that process ultramafic-hosted chromium, ore sequester enough carbon dioxide from the, atmosphere to more than offset greenhouse gas, emissions from operations (Wilson et al. 2009)., Geochemistry, The geochemical behaviour of chromium is lithophile in Goldschmidt’s (1958) classification,, because it is most often associated with (mafic), silicates. Nevertheless, the element is enriched to, 0.9 wt.% in the Earth’s core, 0.3% in the mantle,, and strongly depleted in the crust (100 ppm)., Primary ore deposits are exclusively formed by, mantle-derived mafic and ultramafic magmas., The distribution coefficient of chromium between, residual mantle and ultramafic melt is near unity., Both reservoirs display equal chromium concentrations (2000–4000 ppm). Peridotite and komatiite contain an average of 2700 ppm Cr, boninite, 1000–1500 ppm, gabbro and basalt 200–500 ppm,, and granite only 20 ppm., Chromium traces in rock and soil occur com, monly as Cr(III) with an ionic radius of 0.64 A., Weathering of chromiferous rocks results in oxidation to Cr(VI), which occurs in surface and groundwater as the highly mobile chromate anion, (CrO4)2. Organic matter and other reductants, but, also certain microbes, immobilize hexavalent, chromium (Nriagou & Nieboer 1988, Richards &, Bourg 1991). Cr3þ is geochemically similar to Fe3þ, and Al3þ, and therefore Cr is preferentially enriched in organic, ferriferous and aluminous soil,, typically in the B-horizon. Black shales and, phosphorites concentrate chromium dissolved in
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Chromium ore deposit types, Ultramafic and mafic magmas can be oversaturated in respect of chromite by several petrogenetic processes such as: i) magma mixing; ii), assimilation of country rocks; and iii) pressure, decrease. Exsolved solid chromite or chromite, melt is concentrated by orthomagmatic segregation processes, which reach from quiet gravitational settling to dynamic flow channels. Ore, deposits occur either in layered mafic intrusions, where tabular seams prevail (stratiform chromite, or Bushveld Type), or in the mantle section of, ophiolites (podiform chromite or Alpine Type)., Chromites of these two main metallogenic settings display a different composition and chemical evolution controlled by the pseudostratigraphic position (Figure 2.7). Trace elements provide illuminating constraints on chromite petrogenesis (Page & Barnes 2009). A third, setting of chromite deposits are concentrically, zoned ultramafic ring intrusions of the UralAlaska Type (Auge et al. 2005). In the Ural, , 100, Ural-Alaska type complexes, , 80, , 60, Cr / (Cr + Al), , seawater by reduction and adsorption. Petroleum, inherits chromium from its source rocks. Chromite resists surficial alteration better than most, silicates so that eluvial and residual placer deposits, are common. Proximal alluvial deposits are also, developed, but coastal placers are extremely rare., Although chromite is known in sedimentology as a, very resistant heavy mineral tracer, river transport, reduces it quickly to very small grain sizes., Fluids associated with serpentinization and, metamorphism provoke mobilization of Cr from, disseminated chromite. This is easily recognized, in polished sections under the microscope by lighter, iron-rich rims of chromite grains, while cores, retain the magmatic composition (Prichard et al., 2008). Extreme depletion results in chromiferous, iron ore, or relic magnetite in serpentinite. The, dissolved chromium is eventually immobilized in, newly formed minerals such as chromian chlorite,, k€, ammererite (a chromium clinochlore), fuchsite, (a chromium-rich muscovite), uwarovite, emerald, and ruby. However, metamorphic-hydrothermal, processes only dissipate and never concentrate, chromium., , 165, , Layered, mafic, intrusions, , 40, , 20, , Ophiolites, , 0, , 0, , 20, , 40, 2+, , 60, , 80, , 100, , 2+, , Fe / (Mg + Fe ), Figure 2.7 Variation of chromium, magnesium, iron, and aluminium in chromites of layered mafic intrusions,, Ural-Alaska type ultramafic complexes and ophiolites, (adapted from Irvine 1967). Reproduced with permission, Ó 2008 NRC Canada or its licensors. Chromium number, (Cr #) and magnesium number (Mg #) are the equivalent, of the scale shown divided by one hundred., , Mountains, these intrusions were the source of, historically very significant placers of chromite, and platinum (cf. “Platinum”). Some Archaean, komatiitic intrusions host minor chromite deposits (Rollinson 1997, Prendergast 2008). Note, that all chromite is orthomagmatic. Significant, genetic settings of chromite deposits include:, . stratiform chromite in layered mafic intrusions, (e.g. Bushveld);, . podiform chromite within dunite pods of the, mantle section of ophiolites;, . stratiform chromite in ultramafic cumulates of, igneous intrusions in ophiolites (e.g. Philippines);, . stratiform chromite in Ural-Alaska Type ring, intrusions.
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166, , PART I METALLIFEROUS ORE DEPOSITS, , on physical, not chemical principles. Pressure, changes by injection of new melt batches also, induce precipitation of chromite. This explanation better accounts for the giant area covered by, the seams, simply because pressure must change, instantly everywhere in the melt chamber (Cawthorn 2005b)., , Stratiform chromite, Stratiform chromite occurs in seams within ultramafic/mafic cumulates of mafic intrusions. In the, Bushveld, chromite is concentrated in the Critical, Zone (Figure 1.5 and Figure 1.6). Twenty-nine, chromite layers in three groups (LG, MG and UG), are distinguished, with a thickness between a few, centimetres to 2 m. The seams display a number of, “sedimentary” features, including gradual transition to host rocks, lenticular and wedge shapes,, synsedimentary folds, erosion channels, splits, along strike and eventual convergence. Chromitites consist of fine-grained, rounded and idiomorphic chromite crystals that indicate formation as a, solid cumulate phase. Disseminated chromite is, found in the matrix between cumulus olivine and, pyroxene. Later compaction and sintering may,, however, destroy early crystal settling textures,, for example, as poikilitic grains overgrow cumulus silicates. These observations confirm the, hypothesis that stratiform chromitites are the, product of fractional crystallization and gravitational segregation. Chromites at lower levels in, the intrusion have elevated Mg and Cr, whereas, Fe, Ti and V increase upwards. Two main hypotheses have been proposed to explain the cyclic, repetition of chromite formation in the Bushveld:, The first invokes mixing of resident and fresh, magma injected into the chamber (Naldrett, et al. 2009), or its assimilation of siliceous roof, rocks (Kinnaird et al. 2002). The hybrid melt is, oversaturated in respect to chromite, leading to, extensive chromite (and PGE) crystallization, (Spandler et al. 2005). The second hypothesis relies, , The Bushveld contains giant exploitable resources, of 2300 Mt of mainly metallurgical chromite at, >50% Cr2O3 (Vermaak 1986). Genetically similar, but much smaller deposits occur in the Great Dyke,, Zimbabwe and the Stillwater Complex, Montana. In, the Great Dyke, massive chromitite dykes (offsets), extend 100 m downwards into footwall sandstone, and conglomerate., , Podiform chromite deposits, Podiform chromite deposits (so-called because, they are pouch-shaped like the shell of a fruit), occur in dunite bodies of the mantle section of, ophiolites and more rarely, in dunitic and pyroxenitic cumulates of the ophiolitic magma chamber., Orebodies are irregularly dispersed and relatively, small (mostly between 0.1 and 1 Mt). Shape, structure and texture depend on the precise origin., Dunites with chromitite occur in tectonized mantle harzburgite, displaying ductile deformation, and alignment along flow structures (orebody 4 in, Figure 2.8). Both dunite and associated chromite, are explained as refractory phases of mantle melts, rising beneath mid-oceanic rifts (Thayer 1969)., Chromite saturation and formation may be, induced by metasomatic reaction of upflowing, melt with surrounding harzburgite; assimilation, , Mafic magma chamber, Spreading axis, , ing, , s form, , mulate, , cu, Mafic, , 1, Dunites of transition zone, , 2, 3, 4, , Mantle, (Harzburgite), 500 m, , Figure 2.8 Structural locations of, podiform chromitite orebodies in, ophiolites, deduced from observations at, Maqsad, Oman (modified from Ceuleneer, & Nicolas 1985). With kind permission, from Springer ScienceþBusiness Media., (1) Stratiform chromitite in cumulates;, (2) Discordant nodule and leopard ore;, (3) Disseminated discordant ore, segregated from upwelling mantle, magma; (4) Concordant orebody in, tectonized harzburgite.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , of pyroxenes and pressure decrease drive basaltic, and boninitic liquids into the field of chromite, ( olivine) precipitation (Page & Barnes 2009,, Zhou & Robinson 1997, Arai & Yurimoto 1994)., Chromite and dunite bodies are products of liquid, unmixing (or fractional crystallization) and gravitational segregation. In the feeder zone of the, mid-oceanic magmatic chamber (Figure 2.8), discordant disseminated (3) and massive ore (2) is, formed. Ultramafic cumulates contain concordant lenses and seams of chromitite (1). Podiform, chromite ore can be associated with cross-cutting, dykes of chromitite that represent oxide melt injected into host rocks. Structural types of podiform, chromite ore include the following:, . massive crystalline chromitite, typical for large, orebodies; clearly different are:, T chromitite with foam structures (Stanton, 1972) due to crystallization from an oxide, melt;, , 167, , T chromite cumulate composed of individual, idiomorphic crystals, and, T chromitite displaying ductile deformation, features, for example boudinage;, . discontinuous layers of disseminated chromite, crystals (cumulate) or flattened aggregates (tectonized) in light-green iron-depleted dunite or serpentinite; some cumulate chromite occurs in, net-like structures;, . spotted chromite ore and orbicular (or leopard), chromite ore, elongated in one direction, and typically found in tabular flow structures;, . nodular chromite ore that consists of ovoids, sized between beans and plums, also with a constant orientation of the long axis;, . anti-nodular ore with light dunite nodules dispersed in chromitite;, . breccia chromitite ore., The frequency of rounded structures in podiform chromite (Figure 2.9) is a striking difference, , Figure 2.9 (Plate 2.9) Dunite with nodular chromite in the Ingessana Hills, southern Sudan. The hills expose a large, Neoproterozoic ophiolite and host several former chromite mines.
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168, , PART I METALLIFEROUS ORE DEPOSITS, , to stratiform chromitite, which never displays, such features. The cause is probably liquid unmixing of olivine and chromite melt in the former,, assisted by aqueous fluids (Matveev & Ballhaus, 1998, 2002). The directional structures of chromite ores may be due to viscous liquid flow, or to, high-temperature ductile deformation in the, mantle. Detailed structural investigations provide, fascinating insight into mantle dynamics below, oceanic spreading centres (Ceuleneer & Niccolas, 1985, Ceuleneer et al. 1996). However, the preferred geodynamic setting of podiform chromite, ore deposits may be rather suprasubduction, fore-arc and back-arc rifts and primitive island, arcs than mid-oceanic spreading zones (Zhou &, Robinson 1997)., Podiform chromite is chemically different from stratiform ore, with higher Mg/Fe and Cr/Fe ratios, and, Al2O3 contents up to 62%. Alumina and iron concentrations increase with higher position in the, ophiolite stratigraphy (Figure 2.7). This favours formation of refractive ore (e.g. Philippines, Cuba and, New Caledonia). The composition of chromites is, also linked to the degree of partial melting of the, mantle source. High Cr # chromite is believed to, originate from higher degrees, whereas low Cr # ore, may stem from low degrees of mantle melting (Stowe, 1994). Highest chromium and magnesium contents, are observed in lowest sections of ophiolites. Compared with stratiform and disseminated ore, podiform, chromite orebodies in the mantle section tend to, display higher Cr2O3 and lower Al2O3., Palaeozoic ophiolites of the southern and middle Ural, Mountains display a number of remarkably large, chromite and platinum deposits. The largest orebody,, Moledeshnoje, occurs in the Kempirsai ophiolite, (Kazakhstan). It is 1.4 km long and 140 m thick, and, contains 90 Mt of chromite ore (Melcher et al. 1999), with extractable contents of iridium, ruthenium and, osmium (Distler et al. 2008)., , Stowe (1994) pointed out that chromite deposits, vary through geological time. Chromite deposits, with features similar to the podiform type occur in, Palaeo- to Mesoarchaean (3.5–2.9 Ga) greenstone, belts. Giant stratiform deposits appear as soon as, large cratons were consolidated (2.9–2.0 Ga). Podiform deposits sensu stricto emerge with the first, modern ophiolites, at 800 Ma., , In 2008, world production of chromite was, 23.8 Mt, declining to 23 Mt in 2009. Main producers are South Africa (44%), Kazakhstan (17%), and India (15%). Chromite resources are very large, and sufficient for centuries of consumption., However, only two countries control 90% of the, resources (South Africa and Kazakhstan), implying a certain geopolitical risk., Exploration for podiform chromite is challenging. Outcropping ore has been found and exploited, long ago. Deep orebodies are sought by a combination of detailed geological mapping, structural, geology and geophysical methods for locating, high-density or magnetic material at depth (gravimetric and magnetic methods). Seismic methods, may help to locate massive ore. Stratiform chromite seams are found by geological and petrological investigations. Note that it is always advisable, to examine any chromitite for possible by-product, platinum element contents., 2.1.4 Nickel, Common Ore Minerals:, , Pentlandite, Nickeline, Garnierite, , (Ni, Fe)9S8, NiAs, (Ni, Mg)6(OH)8Si4O10, , Max. wt.%, Ni, , Density, (g/cm3), , 35, 44, 30, , 4.6–5, 7.8, 2.2–2.8, , Pentlandite is a component of pyrrhotite formed, from mafic and ultramafic melt. Usually, its paragenesis includes minor chalcopyrite and cobalt, sulphide, and traces of platinum and gold. Nickeline is a characteristic ore mineral of hydrothermal, deposits, and may contain Sb, Co, Fe and S. Garnierite is not a defined mineral, but describes lateritic nickel ore that consists of amorphous to partly, crystalline substances similar to talc and serpentine. Other minerals of nickel laterites include, Ni-chlorite, Ni-talc, Ni-sepiolite, Ni-montmorillonite and nickeliferous goethite (Gleeson et al., 2003)., Economic grades of nickel-only ores are at least, 2–3% Ni. Kabanga in Western Tanzania, for example, contains an indicated plus inferred resource
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , of 50 Mt grading 2.7% Ni in sulphide, which makes, it one of the most attractive undeveloped deposits, in the world. By-production of other metals (e.g., copper, cobalt, gold, platinum) allows extraction of, lower-grade ore. Critical controls are properties of, the ore related to processing and metallurgy,, including separability of deleterious elements (Pb,, Zn, Bi, As). High sulphur-content of sulphide ore is, favourable because it allows using the energy-saving “flash smelting” process. Lateritic nickel is, extracted either by heap-leaching (suitable for ore, with little goethite) or by high-pressure acid-leaching (HPAL) at >4 MPa and T >250 C. Most lateritic, nickel ores yield the by-product cobalt. Only, recently, nickeliferous limonite ore (that is lateritic, iron ore with <1% nickel) has gained the acceptance of international markets., Use of nickel is favoured by the “seven virtues”, of the metal. At 1455 C, its melting point is high,, it is suitable for galvanization and alloying with, many other metals, it is ductile, at room temperature ferromagnetic, it forms a stable oxide film in, contact with air, is not corroded by alkalis and is a, good catalyst. Nickel is one of the heavy, metals (8.9 g/cm3), but neither nickel metal nor, its chemical compounds are toxic (the latter with a, few exceptions). However, permanent contact, with the metal may induce dermatitis and chronic, inhalation of nickel sulphide can be cancerogenic., This is contrasted by the metal’s role as an essential trace element for humans, animals and plants, (Lindh 2005). Nickel’s properties allow numerous, applications, although >65% of nickel ( Cr, Mn), is used in ductile stainless steel manufacturing., Other major applications are Ni-Cd batteries (in, spite of cadmium’s toxicity, far from abandoned),, the non-toxic rechargeable Ni-hydride (NiMH), batteries, super alloys (e.g. Ni3AlTi) and catalysts., Ni-based super alloys are heat-resistant and highstrength materials that find applications in aircraft, engines, industrial gas turbines, reactors and the, chemical industry., Geochemistry, The siderophile geochemical behaviour of nickel, is illuminated by its average contents in magmatic, rocks; ultramafics 0.1–0.6%, basalt 0.016%, gran-, , 169, , ite 0.006%. The crustal average of nickel is 99 ppm, (Clarke value, Smith & Huyck 1999). Nickel is, retained in the mantle when basaltic melts are, extracted. One cause of this behaviour is that, Mg2þ and Ni2þ have the same charge and a similar, ionic radius, favouring incorporation of the metal, in olivine. The fundamental process in the genesis, of magmatic sulphide deposits is the extraction of, chalcophile metals (e.g. Ni, Cu, Co, and the PGE), from silicate magma into a co-existing Fe-sulphide, liquid. As shown in eq. 2.2, the efficiency of the, process is a function of the Nernst partition coefficient (D) of a metal between silicate melt (sil) and, a sulphide melt (sulph)., Distribution of a metal between co-existing sulphide melt and silicate magma:, Sulphme ¼ D Silme, , ð2:2Þ, , Extraction efficiency depends on intrinsic (D) as, well as kinetic factors (e.g. the R-factor, cf. Chapter 1.1 “Orthomagmatic Ore Formation”). Typical, D-values are 40–200 for Co, 100 to >5000 for Ni,, 250–2000 for Cu, 1000–100,000 for Pt, 1500 for, Pd and 480–19,000 for gold (Peach et al. 1990)., Clearly, this will lead to a high concentration of, nickel in sulphide melt that interacts with silicate, melt. Formation of a sulphide melt depends on, sulphur saturation of the magma, which may be, caused by:, . assimilation of sedimentary sulphides or evaporitic country rocks, as at Noril’sk (d 34 S ¼ þ10, to þ12‰, Li et al. 2009);, . derivation of sulphur from undepleted mantle, (Sudbury: d 34 S ¼ þ1:7‰: Naldrett 1999); and, . assimilation of felsic rocks reducing sulphur, solubility., The base metals lead and zinc exhibit partition, coefficients near one; sulphide liquid is not, enriched., Polymetallic shales are an interesting lowgrade high-tonnage resource of nickel. Examples, include the Mo-Ni ore shales of South China (cf., “Molybdenum”) and amphibolite-facies Ni-CuCo-Zn black schists in Finland. The latter contain, 8% graphite, 9% S, 0.23% Ni, 0.13% Cu, 0.02%, Co and 0.5% Zn. The deposits are not far from, the Cyprus type massive sulphide deposits at
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170, , PART I METALLIFEROUS ORE DEPOSITS, , Outukumpu (cf. Section 2.2.1 “Copper”) and, are probably derived from ocean-floor hydrothermal fluids pervading organic mud (LoukolaRuskeeniemi & Heino 1996). In 2008, a mine was, launched at Talvivaara, comprising open pit, extraction and metal recovery by bacterial, heap leaching., In hydrothermal parageneses, Ni is often present but rarely exploitable. However, the metal’s, discovery setting and the first industrial nickel, source were hydrothermal Bi-Co-Ni veins in, the German Erzgebirge (cf. Section 2.5.12, “Uranium”; Stemprok & Seltmann 1994, Baumann et al. 2000). In the exogenous cycle, the, element is strongly complexed by organic substances, limiting its mobility except under, highly oxidizing and acidic conditions (Smith, & Huyck 1999). Petroleum, its source rocks,, black shales and oil shales all display elevated, traces of nickel. Iron hydroxides readily adsorb, nickel, and hydrogen sulphide effectively immobilizes nickel., , such as Voisey’s Bay, Labrador, Canada: Scoates, & Mitchell 2000);, . magmatic rocks of Archaean and Palaeoproterozoic greenstone belts, with the two types:, 1 tholeiitic intrusions (e.g. Pechenga, Kola, Peninsula);, 2 ultramafic komatiitic lavas and subvolcanic, intrusions (Archaean, cf. Chapter 1.1, “Orthomagmatic Ore Formation” (Figure 1.4)., . synorogenic mafic intrusions in post-Archaean, orogenic belts (Jinchuan, Gansu, China)., The instability of mafic minerals hosting nickel, under surface conditions and the metal’s tendency, for adsorption in limonite and for substitution of, magnesium are the main factors of supergene, lateritic nickel concentration (with Co as a byproduct). Two types of lateritic Ni deposits are, distinguished:, . oxide nickel ore in the upper, oxidized iron-rich, part of the laterite profile;, . silicate nickel ore in the lower, reduced saprolitic section of the regolith., , Nickel ore deposit types, , Orthomagmatic sulphidic nickel deposits, , Economically significant nickel deposits are, exclusively related to mafic and ultramafic melts, originating in the mantle. Nickel concentrations, of silicate melts are founded in conditions of mantle melting (e.g. the degree of partial melting, Robb, 2005). Extraction of nickel from the silicate melt, is initiated by exsolution of a sulphide melt., Efficient partitioning of Ni into the sulphide liquid, is favoured by dynamic interaction between the, two melts. Segregation of the Ni-enriched liquid,, for example by gravity, leads to formation of orthomagmatic sulphidic nickel deposits, which often, display co- or by-production of Cu, Co and PGM., Nickel sulphide deposits occur in volcanic and, intrusive magmatic systems of different petrotectonic environments (Naldrett 1999):, . noritic intrusives in impact structures (Sudbury,, Canada, Figure 1.7);, . intrusive parts of flood basalt systems, related to, mantle plumes and extensional deformation of, continental crust (Noril’sk, Sibiria);, . troctolitic phases of complex Mesoproterozoic, anorthosite plutons (conduit-hosted deposits, , Disputing the title of the world’s largest nickel, mining district, both Noril’sk (see below) and Sudbury, Canada, are giant accumulations of the, metal (Ames et al. 2008). Since the first ore was, found at Sudbury in 1883 when a railway was, constructed, more than 90 deposits have been, developed. Total nickel ore exploited plus remaining reserves amounts to 930 Mt, containing over, 10 Mt of the metal. Present run-of-mine ore grades, are 1.4% Cu and 1.3% Ni and contain small, amounts of ten additional commercialized elements. The search for new orebodies reaches, depths of >3000 m. The last newly developed, mine Nickel Rim South is based on resources of, 13.2 Mt, with 1.7% Ni, 0.04% Co, 3.5% Cu, 2.2 g/t, Pd, 0.8 g/t Au and 1.9 g/t Pt., About 50% of all Sudbury ore occurs in the “sublayer”, at the basal contact of the intrusion, other deposits, include veins within footwall breccias, or dykes injected far into footwall country rocks (“offsets”). Sublayer orebodies lie in dells of the footwall, (“embayments”), which are thought to have originated as settlement terraces of the crater wall during
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 171, , Mafic norite, Leuconorite, , Sublayer norite, Breccia, , Granite gneiss, megabreccia, , Figure 2.10 The nickel-copper orebody, no. 4 at Levack-Mine, Sudbury, Canada,, as an example of the characteristic control, by sagging terraces of the impact crater, (modified from Farrow & Watkinson, 1992). With kind permission from, Springer ScienceþBusiness Media., impact (Figure 2.10). Massive sulphides collected on, the footwall, and with increasing distance, sulphides, are more disseminated. Contact ore is rich in Ni, Cu,, Pd and Au, whereas contents of Co, Rh, Ru, Ir and Os, rise to the interior of the intrusion. This appears to be, the result of fractional crystallization of the sulphide, melt while it settled downward. The observed paragenesis (pyrrhotite, chalcopyrite, pentlandite, etc.) is, the product of secondary unmixing and alteration of a, primary Cu-Ni-Fe-S monosulphide phase that solidified at 700–600 C. In several deposits, magmatichydrothermal fluids played a significant role in transport and deposition of metals (Farrow & Watkinson, 1992), although initial magmatic sulphide melt, emplacement was the dominant process., , The Noril’sk District in western Siberia, Russia,, comprises several distinct deposits. Near the, northwestern margin of the Siberian Shield, a huge, nappe of trap basalts was erupted at the end of, the Permian (at 250 Ma). In the Noril’sk region,, its thickness reaches 4000 m. It is underlain by, Palaeozoic sediments (Carboniferous coal, Devonian evaporites) and Proterozoic crystalline rocks., Whereas the main mass of the basalts is tholeiitic,, the base consists of picritic and alkaline basalts., Mafic and ultramafic sills abound both in the, Palaeozoic basement and in the traps., Noril’sk orebodies are hosted exclusively by differentiated, stratified, gabbroic sills. Outcrops of sulphide, , Massive sulphide, Disseminated, sulphides in breccia, , 100 m, , ores that initiated exploration are due to an Upper, Triassic deformation related to the Taymir Orogen, further north. Massive and disseminated ore is, exploited. Main ore minerals are chalcopyrite, pyrrhotite and pentlandite, often with important contents of palladium and platinum (10–11 ppm)., Average ore grades in the district are 1.7% Ni and, 3.1% Cu. With a Ni-production of 330,000 t (2003),, the Noril’sk District is probably the world’s leading, source of this metal. Not all details of the formation, of these giant deposits are fully understood. Most, authors imply the presence of a mantle plume, deep, magma chambers where differentiation took place,, assimilation of country-rock anhydrite (Li et al. 2009), and, for certain orebodies, further differentiation and, unmixing of sulphide melts within the gabbroic sills, (Naldrett 1999, Arndt et al. 2003). The hypothesis, that mineralized intrusions represent a conduit, system related to overlying flood basalts was contradicted by Latypov (2007) but re-affirmed by, Li et al. (2009)., , In 1993, the discovery of nickel at Voisey’s Bay, in Labrador, Canada first pointed to Proterozoic, anorthosite complexes as a potential parental setting for magmatic Ni-Cu-Co sulphide ore. The, orebodies at Voisey’s Bay occur in dyke-like intrusive bodies comprising ferrodiorite, ferrogabbro,, troctolite, olivine gabbro and even in country rock, gneisses. In contrast to Sudbury, emplacement of
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172, , PART I METALLIFEROUS ORE DEPOSITS, , the ores was not dominated by simple gravitative, separation, but is a product of flow dynamics of, the magma. Traps for the heavy sulphide liquids, were provided by the complex morphology of the, flow channels. The locus of primary segregation, was probably much deeper. Orebodies consist of, massive and disseminated sulphides and of mineralized intrusive breccias. Main ore minerals are, pyrrhotite, pentlandite, chalcopyrite, cubanite, and magnetite. Total resources comprise 137 Mt, at 1.59% Ni, 0.85% Cu and 0.09% Co (Li &, Naldrett 1999, Naldrett & Li 2000). Production at, Voisey’s Bay commenced in 2005 at a rate of, 60,000 t/a nickel (plus by-product Co and Cu), contained in concentrate., The komatiite-hosted nickel sulphide deposits, of Western Australia were discovered in 1966, (Hoatson et al. 2006). They occur in Archaean, greenstone belts of the Eastern Goldfields Province of the Yilgarn Craton. Channels of relatively, thin lava flows host high-grade ore (Kambalda, Type, Figure 1.4), whereas thick dunite sills contain lower-grade disseminated but much larger, orebodies (Mt Keith Type: Fiorentini et al., 2007). Past production and remaining resources, add up to 13 Mt of nickel metal contained. The, province comprises five world-class nickel deposits (>1 Mt nickel contained). By-product amounts, of copper, cobalt, gold and platinum metals are, significant., Komatiite-hosted nickel-cobalt-copper-PGE sulphide ores are thought to have been formed by a, combination of processes: The hot, rapidly flowing, lavas eroded and assimilated iron sulphide-rich, interflow cherts. Sulphur-saturation of the melt led, to unmixing of liquid iron sulphide droplets and, stringers. In this liquid, metals with affinity to sulphide melt (expressed by a high distribution coefficient sulphide/silicate melt, see above) were, enriched. The efficiency of the enrichment is controlled by a number of parameters including the, mass ratio of sulphide/silicate melts, the reactive, surface of the sulphide melt, and the dynamics of, the flow (e.g. turbulence; Lesher & Campbell 1993)., Because of this, sulphide droplets (“blebby ores”), often have the highest chalcophile metal contents,, whereas massive ores are of lower grade. Overall,, , upon eruption, komatiites were neither exceptionally rich in metals nor in sulphur. Ore formation is, enabled by an external source of sulphur (Bekker, et al. 2009)., , In Finland, a number of orthomagmatic nickel, deposits occur within stock- and tube-like synorogenic intrusions of peridotite, pyroxenite and, norite. The deposits are aligned over 400 km, length in a narrow zone within the Svecokarelian, Orogen, which was termed the Kotalahti Nickel, Belt. The first ore at the later Kotalahti mine, was accidentally found by road builders in 1954., Sulphides are disseminated or brecciated in ultramafic matrix. Kotalahti contained 20 Mt of ore, grading 0.7% Ni and 0.27% Cu. Similar to Voisey’s, Bay, these deposits may have been formed in, magma conduits of mafic intrusions., Jinchuan, China seems to be a very special case. This, is essentially a peridotite body intruded into Proterozoic gneiss and marbles. It was first considered as, another magma conduit deposit (Li et al. 2004, Ripley, et al. 2005) intruded at 830 Ma. The deposit is, peculiar because the inducement for sulphide melt, formation may have been assimilation of marble, material. Most mafic intrusions with Ni-Cu-PGM, ore reached sulphur saturation in the magma either, by assimilation of sulphur-rich country rocks or, of felsic rocks, which reduces sulphur solubility, (Naldrett 2004). At Jinchuan, carbonate assimilation, may have caused oxidation of Fe2þ to Fe3þ, thereby, reducing sulphur solubility and enforcing formation, of an immiscible sulphide melt (Lehmann et al. 2007)., Jinchuan is the world’s third-largest magmatic, Ni-Cu-PGM deposit, with 500 Mt of ore grading, 1.2% Ni and 0.7% Cu., , Lateritic nickel ore deposits, The origin of this group by supergene alteration, of ultramafic magmatic rocks was discussed in, Chapter 1.2 “Supergene Enrichment by Descending Solutions” (Figure 1.51, Figure 1.53 and Figure, 1.54). This genetic group contributes 40% of, primary world nickel production. Most of the, deposits are rather young (Miocene to subrecent), and are still found at tropical or subtropical latitudes (Cuba, New Caledonia, Australia, Southeast
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Asia). However, others are older (Mesozoic to, Cenozoic: Urals, Albania, Greece: Eliopoulos and, Economou-Eliopoulos 2000):, A number of large new nickel laterite mines are, currently (2009) developed on the island of Sulawesi,, Indonesia. One of the projects reports installation, costs of US$ 1500–2000 million. Total inferred resources of limonitic and saprolitic ore are estimated, at 162 Mt and a grade of 1.62% Ni and 0.08% Co., Metal recovery will be based on high-pressure acid, leaching., , Land based mineable nickel resources with, >1% Ni are estimated to contain at least 130 Mt, of metal (USGS 2010). Of this total, 60% occur in, lateritic nickel ore. Considering world production, of 1.6 Mt of refined nickel (2008; 1.4 Mt in 2009),, the supply is assured for a long time. Major primary nickel producers are Russia, Canada, Indonesia, Australia and New Caledonia. Giant, but, yet uneconomic resources exist on the deep ocean, floor. In the central Pacific, 800 Mt Ni are contained in manganese nodules (Figure 1.70) that, grade 29 wt.% Mn, 5% iron, 1.2% copper,, 1.37% nickel, 1.2% cobalt and 15% SiO2 (Lenoble, 1996)., Exploration for nickel ore deposits is very effectively guided by geochemical methods (e.g. nickel, concentrations in soil and rocks) and by petrological investigation of magmatic host rocks (keys, include sulphur-saturation and PGE contents). In, 1970, the West Australian deposits were found, by prospecting gossans with peculiar green coatings (annabergite Ni3(AsO4)2.8H2O or “nickel, bloom”). In 1993, it was again a gossan that led, to the discovery of a new important nickel deposit, in Labrador (Canada), Voisey’s Bay. For detailed, investigation of anomalies, magnetic, electromagnetic and induced polarization methods are employed. Drilling and geophysical sections should, be closely spaced ( 30 m). Very deep ore in Sudbury is now sought with new electromagnetic, probes that sample a radius of 300 m around the, drill hole. Lateritic deposits are mainly found by, geological methods, shallow grid drilling and, geochemistry., , 173, , 2.1.5 Cobalt, Common Ore Minerals:, , Cobaltite, Smaltite, Linneite-Siegenite, Carrollite, Asbolane, (cobaltian wad,, amorphous), Erythrite, Heterogenite, , CoAsS, CoAs3, (Co,Ni)3S4, Cu(Co,Ni)2S4, Mn(O,OH)2., (Co,Ni,Ca)x, (OH)2.nH2O, Co3(AsO4)2.8H2O, CoOOH, , Max., mass %, Co, , Density, (g/cm3), , 35, 24, 58, 28.6, 32, , 6.3, 6.4, 4.8–5.8, 4.5–4.8, 3–4, , 37, 61, , 3, 2–4.5, , Cobaltite, smaltite and linneite-siegenite are characteristic for hydrothermal vein parageneses. Carollite is the typical copper belt ore mineral. Pink, erythrite guides prospectors to silver ore veins., Asbolane and heterogenite are Co carriers in oxide, nickel ore., In recent years, cobalt is again produced from, dedicated mines, not only as a by-product as previously. Nearly 60% of world production is, derived from the Central African Copper Belt,, with typical ore grades of 2–5% Cu and, 0.3–0.5% Co. Cobalt recovery with traditional, metallurgy remains low (50–75% of metal content, in ore) and bioleaching with much better, results is increasingly employed. Cobalt metal, (D ¼ 8.92 g/cm3, melting point 1495 C) is mainly, used for alloying steel, although consumption includes many different cobalt chemicals for catalysts, paint dryers, polymerization promoters and, rechargeable batteries, for example in hybrid vehicles. In the near future, cobalt-phosphate catalysts may provide a cheap alternative to platinum, for splitting water in order to produce hydrogen as, a liquid fuel. Cobalt use in the form of Li-Co, dioxides for the production of Li-ion batteries increases rapidly. High-purity metallic cobalt is a, component of superalloys destined for the construction of jet engines (45% of world consumption). For more than 4600 years, cobalt was the, basis of blue dye manufacturing.
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174, , PART I METALLIFEROUS ORE DEPOSITS, , Geochemically, cobalt is similar to nickel, According to Goldschmidt (1958), both elements, are siderophile and cobalt attains highest concentrations in ultramafic (150 ppm) and mafic, (40 ppm) magmatic rocks. The average abundance of cobalt in the crust is 29 ppm (Clarke, value, Smith & Huyck 1999) and the Co:Ni ratio, is 1:3. Because of the similarity of ionic radii,, , nickel (0.69 A) rather associates with magnesium, , , (0.66 A), whereas cobalt (0.72 A) substitutes biva, lent iron (0.74 A). Manganese nodules and especially the Fe-Mn crusts of the deep sea floor are, enriched to 1.2% Co, even far from mid-oceanic, hydrothermal activity. Seawater contains dissolved Co2þ that is fixed in the crusts by oxidation to Co3þ. In proximity to hydrothermal vents,, precipitation of cobalt does take place, but this is, diluted to very low concentrations by the overwhelming mass of iron and manganese. Under, surface conditions cationic cobalt mobility is, moderate except in strongly oxidizing and acidic, environs (Smith & Huyck 1999). Cobalt is not, redox-sensitive. Contact with hydrogen sulphide, precipitates Co. Cobalt is one of the essential, trace elements for humans, animals and plants, (Lindh 2005). In the environment it is a harmless, element; only some industrial water-soluble, compounds are toxic., , Cobalt ore deposit types, Cobalt ore deposits are typically polymetallic., Associated metals include Cu, Ni, Ag, Pb, Zn,, Fe, and rarely, uranium. Whereas nickel ore deposits are predominantly orthomagmatic and lateritic, cobalt is a by-product only in these, settings. Most cobalt ore deposits are formed by, hydrothermal processes. Hydrothermal cobalt, occurs in vein and volcanogenic-exhalative deposits (Outokumpu, Idaho, USA: Bending &, Scales 2001), but the economically most significant source of the metal is copper ore in the, Central African Copper Belt. Significant genetic, settings of cobalt include:, . diagenetic-hydrothermal Cu-Co sulphide ore in, the Central African Copper Belt;, , orthomagmatic sulphidic nickel ore is nearly, always a source of by-product Co (e.g. 0.15–, 0.22% Co in Sudbury ore; cf. “Nickel”);, . lateritic silicate and oxide nickel ores are often, a source of by-product Co (cf. “Nickel”);, . by-product Co in certain volcanogenic-exhalative pyrite-copper orebodies;, . by-product Co in polymetallic hydrothermal, vein deposits of the Bi-Co-Ni type., ., , The Central African Copper-Cobalt Belt, with, numerous ore deposits in Zambia and the D.R., Congo (Shaba, Katanga), is a dominating factor in, world cobalt production. Both stratabound and, cross-cutting copper orebodies contain 0.5%, Co, mainly in the form of linneite and carrollite, (Annels & Simmonds 1984). Cobalt and significant platinum contents indicate leaching of, mafic source rocks. Copper-belt deposits are a, product of basin-dewatering, probably intensified, by deep mafic intrusions with consequent high, heat flow inducing diagenetic or metamorphic, pulses, and tectonic deformation (for details, cf. “Copper”)., Minor sources of cobalt include: i) Lateritic, Ni-Co ore deposits are an economically significant source of by-product cobalt and contain a, high percentage of future resources (Australia,, Philippines, New-Caledonia, Moa Bay/Cuba,, southeastern Cameroon, etc.). An example is, Greenvale near Townsville/Australia where the, ore grades 1.6% Ni and 0.12% Co. ii) Several, volcanogenic-exhalative pyrite-copper orebodies, display recoverable Co-contents, e.g. the Outokumpu district in Finland (0.2% Co) and the, Kilembe copper deposit (Kasese, Uganda). At, Kilembe, an average 1.35% Co in pyrite concentrate is currently extracted by bacterial leaching., iii) Polymetallic quartz-carbonate veins in the, region of Bou Azzer/Morocco are exploited for, Co-Ni-As-Au-Ag. Ore grade is reported at ~1%, Co, 1% Ni and 3 g/t Au. Cobalt and nickel were, mobilized by metamorphic or magmatic fluids, from Neoproterozoic (Panafrican) ophiolitic ultramafics (Ahmed et al. 2009)., Historically, hydrothermal veins of by-product, cobalt associated with Ni-Ag-Fe-(Bi)-S-(U) were
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , the metal’s main source, for example in the German Erzgebirge Bi-Co-Ni type, cf. “Uranium”),, in Norway and, so prospectors first believed, in, Canada:, In the Cobalt District (Ontario, Canada), the nearhorizontal Nipissing diabase sill intruded Archaean, greenstones and Early Proterozoic metamorphic clastic sediments (the Huronian “Cobalt Series”) at, 2219 Ma. All three rock units are cut by joints and, faults, which display arsenian Fe-Co-Ni-Ag ore and in, places, bonanza ore with native silver. In fact, silver, was the target of mining, because at that time, available metallurgical methods were not able to recover, cobalt. The district yielded a total of 15,552 t Ag., At present, cobalt recovery may be possible with a, new hydrometallurgical technology (Cobatec). Gangue minerals are dolomite, calcite and rhodochrosite,, with traces of silicates. The vein orientation is nearly, vertical with a northwesterly strike. Vein infill is thin, (on average 5 cm), and veins are barren beyond a depth, of maximal 100 m below the surface. High-grade, mineralization correlates with a thicker facies of the, Cobalt Series. Hydrothermal fluids were one-phase, liquid and saline; precipitation was induced by mixing with dilute waters at 290–350 C and 480–1350, bar (Marshall et al. 1993). The source of the advected, elements remains vague. Lead isotope model ages, imply formation of the veins at 2200 Ma. The, emplacement of Nipissing diabase was most likely, the driver of regional hydrothermal fluid circulation, (Potter & Taylor 2009), but certainly not the direct, parent of the mineralizing fluids., , A former cobalt source was the famously rich, silver veins of the Permian ore district of Kongsberg, Norway. In the region of nearby Modum,, cobalt mines exploited enriched parts of fahlband, layers in Mesoproterozoic Svecokarelian gneisses, and migmatites. A “fahlband” is a sulphide, impregnated (not massive-sulphide) concordant, band in metamorphic rocks, which acquires a, rusty-brown taint on weathering. In southern, Norway, these rocks are hosted in an assemblage, including quartzite, mica schist and amphibolites. The disseminates sulphide beds are known, to attain a strike length of 11 km and a thickness, of 400 m. Metal grades, however, do not allow, bulk mining. The dispersed sulphides were prob-, , 175, , ably formed by volcanogenic-exhalative processes. Marginally exploitable cobalt and, copper grades (0.1% Co and 0.35% Cu) occur in, graphite-rich zones. This enrichment may be an, effect of metamorphism. An unusual content of, uranium in this ore is striking (Andersen &, Grorud 1998)., About 56% of world cobalt mine production, (2008 76,000 t of metal contained, plunging to, 62,000 t in 2009) are contributed by D.R. Congo, and Zambia, with an increasing share derived by, bioleaching of previous tailings. Congo, Canada,, Zambia, Australia and Russia are the biggest producers. About 40% of cobalt supply are a by-product of nickel mining (both sulphide and laterite, deposits) and consequently, output depends on the, economic cycle of the nickel market. Estimated, world reserves of cobalt amount to 15 Mt. Potential cobalt resources in ocean-floor manganese, crusts and nodules are huge, but economic recovery is far from imminent., 2.1.6 Molybdenum, Common Ore Minerals:, , Molybdenite, Wulfenite, Powellite, , MoS2, PbMoO4, CaMoO4, , Max. wt.%, Mo, , Density, (g/cm3), , 60, 26, 48, , 4.7, 6.5–7.5, 4.2, , Molybdenite is the predominant ore mineral of, molybdenum mining. Amorphous MoS2 is called, jordisite. As a powellite component, scheelite, often carries significant molybdenum. Molybdenite ages can be precisely determined with the, Re-Os method. The system is quite resilient after, closure and remains stable even at high temperatures (Selby & Creaser 2001). Rhenium concentrations in molybdenite are genetic indicators (Stein, 2006): High to very high (>1000 ppm) contents, indicate a source in fertile mantle or juvenile, crust; intermediate values point to evolved crust,, and very low (<10 ppm) levels are characteristic, for metamorphogenic molybdenite:
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176, , PART I METALLIFEROUS ORE DEPOSITS, , Molybdenite is the main primary source for the rare, metal rhenium. ReS2 contents in MoS2 reach 1.9%., Much rhenium is recovered from molybdenum-bearing porphyry copper ore in Chile (2008: 27 t of, worldwide 57 t). Together with the platinum group, metals (PGM), rhenium is one of the most precious, of traded mineral commodities. The metal has high, density (21 g/cm3; melting point 3180 C), high conductivity and mechanical toughness into high temperature ranges. Rhenium is an important catalyst, for gasoline-production from petroleum, but most is, used as a component of superalloys (e.g. turbine, blades in the aerospace industry). Macroscopic rhenium minerals are extremely rare. A unique occurrence was found in sublimates of a 910 C hot, fumarole-jet situated in a volcanic crater of the Kurile, Islands (Korzhinsky et al. 1994). The paragenesis of, the sublimates comprises rhenium sulphide, molybdenite, halite, sylvite and magnetite., , The exploitable minimum grade of molybdenum ores is 0.1% MoS2. Very large deposits can, be mined at even lower grades. In the year 2007, for, example, the proved reserves at Endako, Canada, (Selby & Creaser 2001) were published as 112 Mt at, 0.053% Mo. The Ruby Creek project, also in British Columbia, Canada, is based on resources of, 213 Mt at an average grade of 0.063% Mo, including a “high-grade” open pit resource of 0.084%., However, 75% of molybdenum is a by-product of, porphyry copper ore mining. Because of the large, mass of ore processed at porphyry mines, very low, Mo-grades can be economically recovered. The, iron and steel industry accounts for about twothirds of molybdenum consumption, in the form, of roasted concentrate (“technical oxide”). Alloying steel with up to 16% Mo results in high, strength and resistance to heat and strong acids,, which are common in hydrocarbon well pipelines, and in desulphurizing plants of coal-fired power, stations. Common stainless steel contains 1%, molybdenum. The metal has a high melting point, (2623 C) but relatively low density (10.22 g/cm3)., Further applications are as catalysts in desulphurizing petroleum, for producing chemicals (e.g., orange to yellow pigments), for water treatment,, polymers and airbags. Due to its softness and, layered structure, molybdenite can be directly, used as a lubricant, similar to graphite., , Geochemistry, Molybdenum is a redox-sensitive siderophile element (with a strong chalcophile tendency). Average abundance in basalt is 0.7 ppm, in granite, 1 ppm. In magmatic rocks, trace molybdenum is, hosted by magnetite, titanite and ilmenite (up to, 500 ppm), in biotite and amphibole, and even in, feldspar. In the presence of reduced sulphur in, magmas, most molybdenum is fixed in disseminated MoS2. The majority of endogenous rocks, contain chalcophile Mo4þ that is relatively, immobile. Economic concentrations are almost, exclusively related to high-temperature hydrothermal processes in certain felsic intrusions. Mo, is enriched by a factor of 9 4 in a fluid phase that, segregates from magma (Aud, etat & Pettke 2003)., In moderately hot hydrothermal fluids, molybdenum is mainly transported as molybdate or molybdic acid species (H2MoO4), similar to tungsten., At high temperatures (500–800 C) molybdenum, solubility in KCl solutions reaches 1.5% (Ulrich, & Mavrogenes 2008). Characteristically, molybdenite crystallizes from a vapour phase by reaction of hydrated molybdate species (MoO3 nH2O), with H2S as oxygen fugacity falls with declining, temperature below the SO2-H2S buffer (Hannah, et al. 2007). Seven naturally occurring stable isotopes of molybdenum with a mass between 92, and 100 are known. When molybdenite crystallizes from vapour, the Mo isotope composition, evolves by Rayleigh distillation (Hannah et al., 2007)., The estimated Clarke value (crustal abundance), of Mo is 1.2 ppm. Accordingly, molybdenum is, less abundant than U, Th, Sc and most rare earth, elements. Generally, sediments contain very little, molybdenum (<1 ppm). Because the metal is an, essential micronutrient, it is enriched in organic, matter (coal, petroleum, kerogen, graphite). In, oxic river and seawater, Mo is dissolved as a stable, molybdate anion (Mo(VI)O42). Therefore, with a, concentration of 11 mg/kg, molybdenum is the, most abundant transition element in ocean water., In euxinic basins with H2S in deep water, molybdenum and sulphur are precipitated and enriched, in organic bottom sediments. Consequently,, black shales reach contents of 150 ppm Mo,
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , commonly hosted by pyrite. This process leads to, measurable Mo-isotope fractionation (Arnold, et al. 2004). Modern euxinic sediments, however,, reflect the homogeneous seawater isotopic composition (Neubert et al. 2008)., By supergene alteration, molybdenite alters to, ochreous oxides, including yellow ferrimolybdenite Fe2(MoO4)3.8H2O. These weathering products dissolve easily and the oxidized zone of, molybdenum deposits is commonly depleted. Soil, and plant samples, however, are useful geochemical exploration media. Molybdenum in sediments, is sourced by weathering of ore and rocks, which, liberates highly mobile, lithophile, oxidized, Mo6þ. The dissolved molybdate ion MoO42 may, be immobilized by Fe-Mn-Al hydroxides (e.g. the, deep sea manganese nodules), or by cations including Pb (formation of wulfenite), Cu, Ca, UO2, Bi, and Co, and, of course, by reaction with reduced, sulphur (formation of jordisite). Finally, enzymes, of most life forms are biochemical traps for traces, of molybdenum. Similar to other heavy metals, (Fe, Mn, Cr, Co, Cu, Zn), molybdenum is an, essential trace element (Lindh 2005), although, excessive intake is adverse, especially for cattle., Unlikely sources for a case of molybdenosis, (excess of Mo) in cattle were the brickworks of, Bedfordshire in England, which worked sulphide, and organic-rich Oxford clay., Molybdenum ore deposit types, Present commercial sources of molybdenum are, almost exclusively magmatic-hydrothermal ore, deposits associated with I-type subvolcanic monzonitic and granitic intrusions. Monzonite-related, sources of molybdenum include the Cu-Mo, porphyry deposits of Chile and the USA (cf., Chapter 1.1 “Porphyry Copper Deposits”, and, “Copper”). Granite-related sources of molybdenum are the molybdenum porphyry deposits, (please note the confusing similarity of terms)., Large molybdenum porphyries occur in North, America and in China (Stein et al. 1997). Fertile, magmas intruding carbonate rocks produce, important polymetallic skarn deposits (cf., “Tungsten”). Significant molybdenum deposits, include:, , 177, , magmatic-hydrothermal molybdenum (byproduct tungsten) porphyry deposits related to, granite or rhyolite;, . magmatic-hydrothermal, copper (by-product, molybdenum) porphyry deposits related to monzonite or latite;, . polymetallic skarn deposits (e.g. Mo, W, Sn, Be,, Cu and other metals);, . sandstone type uranium infiltration deposits, with by-product Mo;, . ocean-floor hydrothermal (or sedimentary?), Mo-Ni-Cu-Co-Zn ore shales., ., , Magmatic-hydrothermal molybdenum deposits, Magmatic-hydrothermal molybdenum deposits, can be classified according to shape (stockwork,, impregnations, replacement, veins), depth of formation (plutonic, subvolcanic), the position of ore, related to the intrusion (exo- or endocontact) and,, of course, the geodynamic nature of the parental, granites (oceanic or continental alkaline rift, or, subduction-related calc-alkaline granite)., A unique insight into one possible source, of molybdenum-fertile magmas was obtained in, Rogaland, Norway (Bingen & Stein 2003). Here,, formerly exploited molybdenite and wolframite, occur disseminated in felsic mobilizates, which, were secreted during the Sveconorwegian granulite-facies metamorphism (1030–970 Ma). Typical, molybdenum porphyry deposits (e.g. Questa,, USA: Klemm et al. 2008) probably take the following path of development: The magma originates in a geochemically enriched lower crust,, possibly caused by massive underplating of mafic, mantle melts. At shallow crustal levels, rifting, and intrusion of high-silica (ca. 75%) leucogranite, batholiths and rhyolites characterize ore districts. The parental magma body may be as large, as 500 km3 and is enriched in water, K, Rb, Nb,, Sn, W, Mo and F. At mid-crustal levels, fractionation causes formation of water-saturated melt, batches and their ascent to subvolcanic levels., Loss of pressure leads to segregation of fluids and, frothing of the melt. Bubbles of volatile phases, may reach a volume of >50%. This allows vigorous convection of the melt so that fluid contents, of large magma volumes are brought upwards,
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178, , PART I METALLIFEROUS ORE DEPOSITS, , whereas degassed magma sinks downwards, (Shinohara et al. 1995). Halogen-rich fluids effectively concentrate molybdenum as well as other, trace elements (e.g. copper, silver, tin, tungsten,, rhenium, bismuth) from the melt. Eventually,, single-phase fluids of intermediate density escape, upwards, where boiling starts under hydrostatic, load at 420 C. Molybdenum and other elements concentrate in the resulting brine, which, precipitates molybdenite and quartz (note the, dissenting view of Hannah et al. 2007). The, vapour phase may remove copper and sulphur, from the system. The high K-content of fluids, causes the strong potassium alteration typical, for large molybdenum deposits. Isotope data, (Pb, O, S and Sr) confirm magmatic derivation of, ore fluids. At Endako, Canada, however, early, involvement of meteoric water was detected (Selby et al. 2000)., Until its closure, the world’s largest molybdenum, operation was Climax, Colorado, with a pre-mining, resources of 500 Mt of ore at 0.25% Mo and minor, by-product tin and tungsten. This grade is, of course,, much higher than Mo-contents in Cu-Mo porphyry, deposits (e.g. 0.028% at Butte). At Climax, Precam-, , brian gneisses and granites are faulted along a, NS-structure against Carboniferous sediments., This is one of the deep marginal structures of the, Colorado Plateau. In the Tertiary (33–24 Ma), the, Precambrian of the upper block was intruded near, the fault by at least 11 superimposed rhyolitic, stocks, 2 of which are enveloped by caps of mineralization and alteration. The innermost alteration is, intensive silicification, surrounded by the ore, zone consisting of a stockwork of molybdenitequartz veinlets with a variable gangue of K-feldspar,, biotite, fluorite, topaz, pyrite, magnetite, and distally more huebnerite. Outwards and upwards,, zones of sericitization, argillization and propylitization are developed. The apical parts of intrusions, display marginal pegmatites (“stockscheider”) or, rhythmic unidirectional solidification textures, (UST) of aplites and pegmatitic quartz with Kfeldspar (Figure 2.11)., Climax dominated world molybdenum supply for a, long time. Depletion of ore and costs of underground, mining eventually enforced closure of the mine., Foresighted exploration had, however, secured several similar deposits (Henderson and Mt Emmons,, Redwell, both in Colorado; Mt Hope, Nevada; Questa, New Mexico). Today, the Henderson and Questa, mines are leading molybdenum producers., , Replacement veins, , Molybdenum ore shell, , Open space, vein, P (fluid), > P(litho), , USTs + aplite, , 300 m, , Granite porphyry, (no high-T veins), , Figure 2.11 Schematic section of a molybdenum, porphyry ore deposit of the Climax type, Colorado, (modified from Shinohara et al. 1995). The central, granite porphyry is capped by bands of pegmatite, and aplite with unidirectional solidification, textures (UST) that reflect crystallization from, margins to the centre and the former presence of, exsolved magmatic-hydrothermal fluids., Replacement veins radiate from this zone into the, roof. Concentric veins with open space, mineralization prove fluid overpressure but, are rare.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , An exciting new find is Mesoproterozoic Merlin, in Northwest Queensland, Australia, with an, interim resource of 13 Mt at 0.8% Mo and 14 ppm, Re (Brown et al. 2010). High-grade ore occurs in a, deformed zone near the contact with granite. In the, absence of gangue, molybdenite fills veins and breccias or is disseminated in black metapelite and, phyllite. This may indicate precipitation from, high-T vapour as proposed by Hannah et al. (2007)., , Molybdenum porphyries are closely related to, Cu-Mo porphyries, although clear differences, exist. In the USA, the first are somewhat younger, (Mid-Tertiary), and appear in phases of relaxation, and extensional deformation of the crust instead of, active subduction. The character of parental magmas tends to A-Type compared to mainly I-Type, Cu-Mo porphyries. Magmas contain more alkalis, and silica. A comparison of metal contents reveal, that there are very few transitional deposits, between Cu-Mo porphyries and Mo-W porphyries, of the Climax Type., Because molybdenum and uranium are both, mobile in oxidized surface water and are commonly precipitated in anoxic environs, many infiltration-derived uranium ores of the sandstone, type are a source of by-product molybdenum, (e.g. U-deposits near Akouta, Nigeria, with a, yearly production of 400 t Mo)., Mo-Ni ore shales, Of high scientific interest are the Mo-Ni ore shales, of the Yangtze platform of South China. The, metal-rich horizon is known to extend over 1600, km at the base of an Early Cambrian shale formation. Economically exploitable ores occur locally, and have the form of bedded lenses, with a maximum thickness of 1 to 2 m. The ore consists, mainly of organometallic (Mo,Fe,Ni)[(S,As)2C7, phases with minor sulphides. Grades reach, 4% Mo, 4% Ni and 2% Zn, with traces of Au, Ag,, PGM, redox-sensitive and base metals. Several, important barite deposits occur in the same horizon. Formation by exhalative-hydrothermal fluids, venting on the seafloor is supported by several, lines of evidence (Coveney & Nansheng 1991,, Shao-Yong Jiang et al. 2006). A purely sedimentary, origin by precipitation from seawater, similar to, , 179, , deepsea manganese crusts was considered by Lehmann et al. (2007). Wille et al. (2008) suggest that, upwelling deep sulphidic seawater caused both, molybdenum precipitation and extinction of the, Ediacaran fauna. The analogue amphibolite-facies, Ni-Cu-Co-Zn black schists of Talvivaara in Finland are suggested to be of ocean-floor hydrothermal origin (cf. Section 2.1.4 “Nickel”)., Main producers of molybdenum (2008, 218,000 t; in 2009 200,000 t Mo) are China, USA, and Chile, which also host ample reserves. Exploration for molybdenum employs methods similar, to the search for porphyry copper deposits., 2.1.7 Tungsten (Wolfram), Common Ore Minerals:, , Wolframite, Scheelite, , (Fe,Mn)WO4, CaWO4, , Max. wt.%, WO3, , Density, (g/cm3), , 76.5, 80.5, , 7.0–7.5, 5.9–6.1, , Iron-rich members of the wolframite solid solution, series are called ferberite (0–20 wt.% Mn) and manganese-rich end members h€, ubnerite (80–100%, Mn). Often, wolframite contains traces of Ca, Mg,, Nb, Ta and Zn. Endogranitic wolframite (and, cassiterite) may have very high, economically, recoverable Y and Sc contents (cf. Section 2.5.9, “Niobium and Tantalum”, Kempe & Wolf 2005)., In scheelite, molybdenum substitutes for tungsten, and some scheelites display molybdenum contents, of several percent. This drives the short-wave UV, fluorescence colour from blue for pure scheelite, to yellow. Metasomatic replacement of wolframite, by scheelite and the reverse, pseudomorphic wolframite after scheelite may be relevant for ore, quality and processing characteristics., Exploitable grade of primary tungsten ore is, typically 0.7 wt.% in vein deposits, 0.3% in, high-tonnage orebodies underground, and down, to 0.1% in open pits. Because tungsten prices are, notoriously volatile, these figures reflect longterm averages. Tungsten is a strategic metal, with important military applications. It is heavy,, has the highest melting point of all metals
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180, , PART I METALLIFEROUS ORE DEPOSITS, , (D ¼ 19.3 g/cm3, T ¼ 3410 C) and high mechanical strength. Tungsten is used as carbide or alloyed, with steel in high-speed metal and wood-working,, construction, drilling and mining tools. Light, bulbs and many other household applications, contain tungsten. Chemical applications include, catalysts and pigments. Sodium tungstate is used, for fireproofing cloth., Geochemistry, The geochemical character of tungsten is lithophile. Average contents are 2 ppm in granites and, sediments, 0.5–1 ppm in mafic and 0.1–0.8 ppm in, ultramafic magmatic rocks. Tungsten displays a, siderophile tendency (iron meteorites contain, 1.24 ppm W). Its concentration in crustal rocks, ranges from 0.4–70 ppm, with an average of 1 ppm, (Smith & Huyck 1999). In several aspects, tungsten is chemically very similar to molybdenum:, Both display a range of possible oxidation states, (2 to þ6), an ability to form polynuclear complexes, identical atomic and ionic radii, a similar, electron affinity and are a mixture of several stable, isotopes. However, in contrast to Mo, W(VI) tends, to be fixed as scheelite or wolframite, whereas, W(IV), which occurs in the rare mineral tungstenite WS2 is easily solubilized (eq. 2.3):, Solubilization of tungstenite WS2:, WS2 þ4H2 O ! WO42 þ2H2 Sþ4Hþ þ2e, , ð2:3Þ, , Trace contents of W in granitoids are due to, substitution in Fe-Ti spinel, mica and feldspar., Elevated traces of tungsten are relatively common,, for example in Nb-Ta minerals, hydrothermal manganese ores and oceanic manganese nodules (Kunzendorf & Glasby 1992). Anomalous tungsten, concentrations are reported from geothermal hot, springs and sinters. Brines in sediments of the dry, intramontane Searles Lake, California, which are, exploited for borax, soda ash and salt, contain, 70 ppm of W. The source of tungsten is suspected, in scheelite occurrences of the surrounding hills., The concentration of W in seawater is extremely, low (1 mg/kg), much less than that of Mo (11 mg/kg)., However, like many other heavy metals, tungsten is, enriched in marine sediments with high content of, , organic matter. Bottom sediments of the Okhotsk, Sea reportedly display up to 50 ppm W. These observations indicate that tungsten may be enriched, in playa lake and marine sediments of volcanic, provinces, and that fertile granites may inherit, tungsten by melting pre-enriched sediments., In a fertile crystallizing magma, tungsten is, incompatible and concentrates in the exsolving, fluid phase (1–1000 ppm). Hydrothermal transport, of tungsten at moderately acidic conditions is commonly in the form of tungstic acids (e.g. HWO4,, H2WO4) or tungstates (WO42) (Wood & Samson, 2000). Therefore, at 500–200 C, neutralization by, contact with carbonates, ultramafics and rocks, with calcium-rich plagioclase is often the reason, for precipitation. Gangue commonly comprises, quartz and muscovite (Figure 1.42). Apart from, water, ore forming fluids typically contain moderate NaCl-contents (0–15%, rarely up to 55%) and, some CO2 (with N2, CH4). However, fluids in, wolframite at Panasqueira are free of CO2, which, is present in the quartz gangue (Figure 1.27, L€, uders, 1996). The solubility of tungsten rises with pressure and temperature, with increasing acidity,, with low activities of Ca-Fe-Mn and with fluid, salinity. Inverse relations control precipitation, (Heinrich 1990). Scheelite formation is favoured, by reaction of tungsteniferous solutions with, Ca-rich rocks or solutions. In several mining districts (Erzgebirge, Cornwall), the ferberite component in wolframite seems to increase with, decreasing temperature. This agrees with theoretical phase relations at constant solution composition but the reverse has been found elsewhere., Clearly, tungsteniferous hydrothermal solutions, may have various evolution paths in time, concerning pH, activity of Ca2þ, Fe2þ and Mn2þ, and temperature. Wolframite composition is not a simple, geothermometer., Surficial alteration of wolframite produces yellow ochres that consist of hydrous minerals such as, ferritungstite and tungstite (WO3.H2O). Scheelite, may form earthy white anthoinite WAl(O,OH)4., Well-crystallized scheelite and wolframite are, quite resistant to chemical weathering and occur, in eluvial and colluvial placers. At Fenglin mine, (Jiangxi, China), a hematitic gossan above, a stratiform ferberite-pyrite-Cu orebody contains
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 1.5% W. Scheelite and wolframite are both brittle, and therefore in water courses quickly reduce to, fines. Consequently, alluvial tungsten placers are, rare. Chemical tungsten solubility and mobility, in surface waters at ambient conditions is very, low compared to Mo, due to intensive sorptional, precipitation. In the mining environment, the element is no hazard for living organisms or the, environment in general. Biologically, tungsten is, one of the essential trace elements (Lindh 2005), and a number of microbes employ the element in, building enzymes. Hyperthermophilic archaea, appear to be obligately tungsten-dependent, (Kletzin & Adams 1996)., Tungsten ore deposit types, Tungsten ore deposits are predominantly formed, by magmatic-hydrothermal processes related to, felsic intrusions (cf. Chapter 1.1 “Granitoids and, Ore Formation Processes”). Parental magmas are, moderately to highly specialized and display a, redox spread from reduced to oxidized (Figure, 1.17). The close genetic relations between intrusions and tungsten ores are confirmed by stable, isotope data. Ore-stage fluids clearly contain, magmatic water (e.g. Zaw and Singoyi 2000)., Later flooding by low-T meteoric waters is not, rare, however. Deposits include Sn-W greisen, and polymetallic tungsten porphyry deposits, with Bi, Mo and Sn, which occur in apical, parts of intrusions. Wolframite quartz vein fields, (Figure 1.15 and Figure 1.16 and Figure 1.41),, stockwork orebodies and breccia pipes are preferentially situated in the perimagmatic environment. Scheelite skarn deposits appear in the, contact zone with carbonates (Figure 1.32) or, with mafic-ultramafic magmatic rocks. Epizonal, tungsten ore is rare and of little economic importance. In summary, main deposit types of tungsten include:, . scheelite-only and polymetallic skarn deposits, related to felsic intrusions;, . tungsten-only and polymetallic endogranitic, greisen and porphyry deposits;, . wolframite quartz vein fields and stockwork, deposits within parental granite or in its exocontact country rocks;, , 181, , by-product tungsten from magmatic-hydrothermal molybdenum porphyry deposits related to, granite or rhyolite., Close metallogenetic relations between W and, Sn are common (China, Erzgebirge, Cornwall:, Figure 1.44, Central Africa), but tungsten ore provinces with little tin do exist (western North, America). Both W and Sn are enriched in A- and, S-Type granites. I-Type granites rarely produce tin, deposits, but many important tungsten concentrations. Mo-W porphyry deposits of the Climax Type, (cf. Section 2.1.6 “Molybdenum”) illustrate the, geochemical kinship of the two metals., , ., , The world’s largest tungsten (and tin) resources occur, in eastern Asia, forming a giant W-Sn-F-U-Nb-Ta-SEE, and base metal-Mo province, which is part of the, Circum-Pacific metallogenetic realm. Thousands of, significant tungsten deposits form a broad belt that, reaches from Kamchatka through Korea, Japan, eastern China and Malaysia to Sumatra. The South Chinese provinces of Jiangxi and Hunan are especially, endowed with tungsten ( Sn, Mo, Bi, Pb, Zn, Cu and, Ag), making China the dominating force in the world, tungsten market. Economically most important are, skarn deposits, including the world’s largest scheelite, deposit Shizhuyuan in southern Hunan that contains, >1 Mt tungsten, 500,000 t tin, 300,000 t bismuth,, 130,000 t molybdenum and 200,000 t beryllium, (Huan-Zhang et al. 2003). Tungsten porphyry and, wolframite quartz vein deposits are also common., Most of these deposits are genetically related to postorogenic, subvolcanic and intrusive Yanshanian, granitoids formed between 190 and 150 Ma (mainly, in the Jurassic), which intruded Palaeozoic to Mesozoic sediments and volcanics. Yanshanian granites, are of A-, S- and I-Type, the latter producing ores that, contain more sulphides. In the Jurassic, South China, was a giant Basin-and-Range style magmatic province, which developed after the Indosinian orogeny, (Li & Li 2007). Geochemical studies seem to confirm, that the granites and their metals are sourced in, LIL-enriched mantle and lower crust (Pan & Dong, 1999, Minghai et al. 2007)., , Tungsten deposits in Europe formed during the, Late Palaeozoic. Both the northern branch of, the Variscan orogenic belt (Erzgebirge, Cornwall,, Portugal), and the southern branch (Pyrenees, the, French Massif Central, Eastern Alps) host deposits. All are related to Late Carboniferous to
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182, , PART I METALLIFEROUS ORE DEPOSITS, , Permian (300 Ma) granites, which formed as a, consequence of the final welding of Pangaea., Discovered in 1967, the scheelite deposit Felbertal, near Mittersill (Salzburg) in the Eastern Alps presents, a most interesting case history, as it was initially, considered to have been formed by volcanogenic, seafloor exhalation. For many years, this hypothesis, strongly influenced international metallogenic, research and exploration concepts. Host rocks are, Cambrian ultramafic (ophiolitic) lenses in mafic to, felsic metavolcanics, subvolcanic intrusions and metasediments of a primitive island arc. In the western, main part of the deposit, Variscan I-type leucogranites with I- to A-type characteristics (“K1-3, gneisses”) intruded these rocks. Locally, the intrusive, contact is marked by a pegmatite sheet with large, crystals of scheelite, beryl and molybdenite, similar, to contact pegmatites (“stockscheider”) in the Erzgebirge. Intense orogenic deformation and amphibolite, metamorphism both during the Variscan and the, Alpidic cycle (at 280 and 30 Ma) transformed the, rocks and their spatial relations profoundly. Orebodies in the western part of the deposit are stockworks, of numerous small veinlets of scheelite and quartz, (Figure/Plate 2.12a and b), with rare sulphides (Cu,, Mo, Bi, Zn, Pb, As), cassiterite and traces of Au and, Ag. In an eastern outlier, rich orebodies consisted of, fine-grained quartzite lenses laminated by scheelite, bands. This “scheelite quartzite” ore type was, believed to represent a synsedimentary exhalite but, is now interpreted to have originated as large hydrothermal quartz veins, which were later strongly, sheared. Various dating methods have failed to provide definitive ages of all rocks and thus to elucidate, their role in mineralization (H€, oll & Eichhorn 2000)., Molybdenite Re–Os dating, however, confirmed a, Late Palaeozoic age of mineralization (Raith & Stein, 2006). Overall, the deposit is of a mixed stockwork,, vein and contact-replacement type, produced by magmatic-hydrothermal fluids derived from the leucogranites. The mafic-ultramafic rocks acted as a, geochemical trap, by neutralization and provision of, reactive calcium. The underground mine exploits ore, with 0.4% WO3, producing 4500 t/year of scheelite, concentrate at 31% WO3., , Common scheelite skarn deposits – at the contact of granitoids intruding limestone – were, exploited at Salau (Ariege) in the French Pyrenees, and at King Island in Tasmania, which closed, , Figure 2.12 a (Plate 2.12a) Folded scheelite-quartz, veinlet cutting across foliation of host greenschist at, Felbertal mine, Austria. Length of specimen 50 cm., , Figure 2.12 b (Plate 2.12b) UV illumination reveals, the distribution of scheelite (white)., , in 1990 but is currently revitalized. Several, important skarn (“tactite”) deposits occur in, the Canadian Cordillera (e.g. CanTung mine,, Figure 1.30). Mactung, a large undeveloped tungsten skarn deposit in Yukon, Canada, contains, >33 Mt of indicated resources at an average grade, of 0.88% WO3. Planned development is based, on 8 Mt with 1.09% WO3., More than 100 former wolframite mines based on, quartz veins near Variscan granite cupolas are, known in Portugal. Panasqueira is the largest deposit
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , (Figure/Plate 1.89), characterized by near-horizontal, veins in the apical part of a muscovite-albite leucogranite and in its roof. Intruded rocks include siliceous metapelites and greywackes. The granite, forms a small cupola of quartz-muscovite greisen, topped by a quartz cap. The vein paragenesis comprises quartz and wolframite, some cassiterite and, arsenopyrite, sulphides of Fe, Zn, Cu and Sn, apatite,, siderite, Ca-Mg carbonates and fluorite. The paragenetic succession generally follows this enumeration. Near the veins, country rocks are silicified,, sericitized and tourmalinized. The horizontal, disposition of the veins indicates that fluid pressures were higher than lithostatic stress (Pfluid >, Plithostatic). Vein opening induced phase separation, and ore formation (Figure 1.27). Mineralizing fluids, were moderately saline and strongly influenced by, the organic-rich country rocks (Polya et al. 2000)., Yearly production at Panasqueira is 2500 t wolframite concentrate, 100 t cassiterite and 1000 t, chalcopyrite., , With a moderate annual production, many, quartz vein and tourmaline breccia pipe deposits, with ferberite, scheelite, stibnite and other sulphides occur between La Paz and Oruro in the, more deeply eroded part of the tin province in the, Bolivian Andes. They are genetically related to, Tertiary granitoids., Exploration for tungsten deposits employs geological, petrological and geochemical methods., The conspicuous fluorescence of scheelite in, UV-light was the means for many discoveries, worldwide, by locating outcropping orebodies and, alluvial trails of scheelite., The world’s largest tungsten producers are, China (2009, 80% of the world total of, 58,000 t tungsten contained in concentrate), Russia and Canada. China is also the world’s biggest, consumer of tungsten. Reserves totalling 2.8 Mt, of tungsten metal occur foremost in China,, Russia, USA and Canada (USGS 2010). Tungsten, concentrates are usually traded in metric tonne, units (originally designating one tonne of ore, containing 1% of WO3), today used to measure, WO3 quantities in 10 kg units. One metric ton, unit (mtu) of tungsten (VI) trioxide contains 7.93, kilograms of tungsten., , 183, , 2.1.8 Vanadium, Common Ore Minerals:, Wt. % V2O5 Density, (g/cm3), Coulsonite, FeV2O4, (in vanadomagnetite), Montroseite, (V,Fe)O(OH), Carnotite, K2(UO2)2 (VO4)2., 3H2O, Tyuyamunite, Ca(UO2)2 (VO4)2., 5–8H2O, Vanadinite, Pb5Cl(VO4)3, , variable, , 5.2, , variable, 20, , 4.0, 4.7–5, , 20, , 3.6–4.3, , 19, , 6.9, , The most important vanadium ore mineral is, coulsonite, although in nature this mineral is, extremely rare. In common titaniferous magnetite, however, coulsonite occurs as a minor component, in, solid, solution., Montroseite,, vanadiferous clays, carnotite and tyuyamunite are, minerals of uranium-vanadium ore deposits of, sandstone (e.g. Colorado Plateau), karst and calcrete types. Vanadinite is one of a group of vanadium minerals that occur in oxidized lead, zinc, and copper deposits (typically with descloizite, and mottramite: Boni et al. 2007). Patronite (VS4), is very rare and without economic significance;, previously, it was extracted from asphalt at Minasraga, Peru., Average economically exploitable grades of, vanadium-only ores are 1% V. However, vanadium is predominantly a by-product of iron (and, titanium) mining. Several large deposits of this, type reach 2.8% vanadium, but content as small as, 0.02% V may be utilizable., Vanadium metal is corrosion-resistant, soft, silver-grey and ductile. It melts at 1910 C and has, a density of 6.11 g/cm3. Most vanadium (>90%) is, used as an additive in the steel industry, for products such as flat-rolled steel, rails, tubes, tools, (including medical equipment) and springs. Vanadium stabilizes C and N by the formation of, carbides and nitrides that harden and strengthen, steel. Ti-Al-V alloys are a component of highspeed airframes. Vanadium oxide (V2O5) is used
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184, , PART I METALLIFEROUS ORE DEPOSITS, , as a catalyst in manufacturing sulphuric acid and, for making batteries (e.g. the new generation of, Li3V2(PO4)3 batteries for electric cars)., Geochemistry, Vanadium’s geochemical character is lithophile, (Goldschmidt 1954) with siderophile and biophile, tendencies. In nature, vanadium occurs in the, three oxidation levels V3þ, V4þ and V5þ and is one, of the redox-sensitive elements. Its crustal abundance is 150 (53–200) ppm (Smith & Huyck, 1999). In magmatic rocks, trace vanadium is preferentially included in magnetite, in titanium, minerals and in chromite, compared to mafic silicates. Therefore, average vanadium contents, decrease steeply from mafic rocks (250 ppm) to, granites (20 ppm). As may be expected, pegmatites, and granite-derived hydrothermal solutions contain little vanadium. Rare exceptions are mafic or, emerald-bearing pegmatites. The green colour of, emerald (beryl) is caused by traces of chromium, and vanadium. Note that some emerald deposits, are not granite-related but products of diagenetic, or metamorphic mobilization of black shalehosted trace elements., Weathering of primary minerals liberates vanadium. In a humid climate (where soil water is, moderately acidic) vanadium is hardly soluble and, and is mainly dispersed by mechanical processes., Oxygen-rich, alkaline or strongly acidic seepage, and groundwaters of semi-arid and arid climates, dissolve V5þ or V4þ as oxyanion vanadate (VO3) or, cation vanadyl (VO2þ). In such waters vanadium, reaches concentrations of several 100 ppm and can, be transported over considerable distances (Wanty, & Goldhaber 1992). Similar to uranium, precipitation is induced by reduction (especially by organic, substances), or by combination with cations such, as Pb2þ, Zn2þ, Cu2þ, and UO22þ (resulting in, carnotite formation, e.g. Yeelirrie, Australia) or, by inclusion/adsorption in iron and aluminium, oxy-hydroxides (e.g. bauxite)., Deep seawater has a uniform dissolved vanadate, concentration of 35–37 nmol/kg V (Halbach et al., 2003). Most sediments contain little vanadium., Notable exceptions are bituminous rocks (copper, shale in Germany and Poland, alum shale in, , Sweden), sedimentary iron and manganese ores,, bauxite, phosphorites and deepsea manganese nodules. Samples of the metalliferous mud pools in, the Red Sea contain up to 1.3% V2O3 (dry)., Vanadium is one of the micronutrients for humans (Combs 2005) and is essential for many other, organisms. In the natural and mining environment, vanadium is harmless but some industrial, compounds are toxic. Exceptional vanadium, enrichment is known from a number of marine, organisms, including the ascidiacea (sea squirts), and holothuroidea (sea cucumbers) that attain, >1% of vanadium (in dry mass). However, the, strikingly elevated vanadium trace contents in, petroleum, tar and asphalt are not due to biomass, concentration but to early diagenetic replacement, of magnesium in chlorophyll by the vanadyl ion, (Hunt 1996). Vanadium (and nickel) are complexed, to porphyrins in the petroleum source rocks and, carry the metals on into crude oil. Therefore, petroleum ash may attain 20% vanadium and is an, important source of the metal. Large amounts of, vanadium are hosted in oil shales. One example is, Middle Cretaceous Julia Creek (Australia), with, resources of 4000 Mt of shale in situ, which contain, a recoverable 1700 Mbl of oil and, in oxidized parts, of the deposit to 20 m depth (Lewis et al. 2010),, measured resources of 200 Mt vanadium ore at, 0.4% V2O5 and 300 g/t MoO3. Similar to numerous, other oil shale deposits in the world, Julia Creek, appears to be marginally not profitable at present, market conditions., Vanadium ore deposit types, At present, significant vanadium ore deposits are, almost exclusively orthomagmatic titano-magnetite segregations. Minor or by-product sources of, vanadium are certain deposits of uranium, sedimentary iron and manganese ores, combustion, residues from coal and oil-burning power stations,, steel slags and residues of crude oil processing., Potential sources of vanadium include phosphorite, ocean-floor manganese nodules, iron-rich, bauxite, oil shale and tar sand bitumen. The, short-list comprises:, . orthomagmatic deposits of V-Ti iron ore in layered and non-layered mafic intrusions;
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , . infiltration deposits of uranium with by-product, vanadium;, . diagenetic heavy oil, oil shale and tar sands are, giant potential sources;, . vanadium is one of the potentially recoverable, minor and trace elements of sedimentary, phosphorites., Thirty to fifty percent of world vanadium resources (63 Mt according to an estimate by the, USGS) occur in the magnetite seams of the Bushveld Complex, South Africa (Figure 1.5):, , The uppermost 1750 m (the Upper Zone) of the, Rustenburg Layered Suite contain an average 8% of, disseminated magnetite within gabbronorite and, magnetite gabbroic rocks. More than 20 seams of, massive titanium-magnetite are interbedded with, these rocks. Vanadium-contents in magnetite are, highest in the lower seams (2.1% V2O5) and decrease, upward to nearly nil, whereas titanium (TiO2) increases from 12 to 20%. Apart from magnetite, the, seams contain a few percent of silicates and of ilmenite. Open-cut exploitation takes place at Mapochs, Mine and in the Brits District, based on the Main, Magnetite Layer that has an average thickness of 2 m., The processes contributing to magnetite enrichment, in the seams are not fully understood. Possible factors, may have been:, . unmixing of silicate and oxide melts;, . gravitative settling of magnetite liquid or of, crystals;, . multiple magma injection;, . changes in oxygen fugacity; and, . changes, in, pressure, (cf., Chapter, 1.1, “Orthomagmatic Ore Formation”)., The last interpretation is favoured by Cawthorn &, Molyneux (1986), because chemical gradients across, the seams are very small. It is remarkable that vanadium content in the Upper Zone is concentrated in, magnetite, whereas cogenetic silicates contain very, little vanadium. This underlines the high partitioning coefficient for vanadium into magnetite from, silicate liquid., , Other vanadium deposits of the magnetiteilmenite type are known in Norway (R€, odsand) and, Finland (Mustavaara), in the Urals (GusevogorskKatschkanar), in India (Orissa), China (Lanshan,, Panzhihua) and in Australia (Barrambie, Wind-, , 185, , imurra, Coates Siding and Buddadoo, W.A.)., Exploration for this deposit type may build on, earlier iron ore investigations. In the past, numerous magnetite occurrences in mafic intrusions, had been rejected because of elevated titanium, contents. At that time, Ti and V were not considered as utilizable by-products., Most of vanadium mine production (55,500 t of, vanadium contained in 2008, 54,000 t in 2009), originates from South Africa (40%), China and, Russia. Vanadium extracted from petroleum residues, pig-iron and steel slag, ash from coal-fired, power stations and spent industrial catalysts, makes up the difference to total world, consumption., 2.2 BASE METALS, 2.2.1 Copper, Common Ore Minerals:, , Chalcopyrite, Enargite, Cu-Tetrahedrite, Cu-Tennantite, Chalcocite, Digenite, Covellite, Cuprite, Malachite, Atacamite, , CuFeS2, Cu3AsS4, Cu12Sb4S13, Cu12As4S13, Cu2S, Cu9S5, CuS, Cu2O, Cu2(OH)2CO3, Cu2Cl(OH)3, , Wt. % Cu, , Density, (g/cm3), , 34, 47, max. 45, max. 53, 80, 79, 66, 88, 57, 59, , 4.1–4.3, 4.4, 4.6–5.1, 4.6–5.1, 5.5–5.8, 5.6, 4.7, 6, 4, 3.7, , Apart from these important copper ore minerals,, many others are known that may locally gain, economic importance. Note the distinction, between: i) primary ore minerals (commonly the, first group listed): ii) secondary enrichment sulphides; and iii) supergene “oxide” minerals (third, group). Atacamite associated with gypsum is particularly widespread in oxidized porphyry copper, ores of the Atacama Desert (Reich et al. 2008)., More than 70% of producing copper mines, however, exploit chalcopyrite ore. Copper-tetrahedrite, and copper-tennantite are not sensu stricto ore, minerals but are theoretical end members of
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186, , PART I METALLIFEROUS ORE DEPOSITS, , complex sulphosalt fahlores that contain Cu, Ag,, Fe, Zn, Hg, Cd, Sb, As, Bi, Te, S and Se., Exploitable grades of smaller copper deposits, are 3–5% Cu, whereas very large operations with, orebodies suitable for opencut extraction thrive, on 0.5% Cu, and even at 0.3% in the presence of, by-product metals such as molybdenum and gold., Sulphide ore is commonly enriched by flotation to, saleable concentrates of 30 to >60% Cu. Concentrates (containing enargite or tennantite) with, >5000 ppm As are not accepted by smelters, and, deliveries with >2000 ppm As are penalized. This, is avoided by removing arsenian minerals during, flotation. Other penalty elements include bismuth and antimony. Copper concentrates are processed to cathode sheets by smelting and, electrochemical treatment. Noble metals (Au, Ag,, PGM) are recovered from anode slimes. Traces of, selenium and tellurium can be profitable by-products. High sulphur contents in concentrates, allow the energy-saving “flash smelting” technology and production of native sulphur and sulphuric acid. “Oxide” ore deposits, disused mine shafts,, tailings and waste rock dumps, and even primary, sulphide ores are increasingly treated by microbial, chloride or sulphate leaching in order to, extract copper. Copper is stripped from leach, solutions by cementation on scrap iron, or with, organic chemicals., Uses of copper are primarily defined by the, metal’s high conduction of heat and electricity, (second only to silver). Over 50% of consumption, concerns various products of the electrical and, electronics industries. In some of these fields,, copper competes with other materials (e.g. glass, fibre cables, conducting synthetic materials). Copper metal (melting point 1083 C, density 8.94 g/, cm3) and its alloys (bronze: Cu þ Sn; brass: Cu þ, Zn) are distinguished by excellent malleability,, ductility and resistance against atmospheric, attack. Also, copper has strong antibacterial properties. Therefore, water pipes or work surfaces, made from copper metal may help to prevent, infection. In low concentrations, copper is an, essential element for humans, plants and animals, (Lindh 2005). However, excess copper is toxic and, should be controlled in drinking water (<2 mg/, litre: WHO 2006) and in food. In the western, , United States, soils contain an average of 21 ppm, Cu (range 2–300; Smith & Huyck 1999). Among, grazing animals, sheep are most sensitive to copper, suffering from deficiency at concentrations (in, herbage) <5 mg/g and toxicity at >10 mg/g. Aquatic, life displays a similar variance, with fish, for example, being poisoned at levels of dissolved copper, acceptable to most other life forms., Geochemistry, The geochemical properties of copper are dominated by its great affinity to sulphur, which characterizes the “chalcophile or thiophilic elements”, Cu, Zn, Ag, Cd, In, Hg, Tl, Pb, Bi, As, S, Se, Sb and, Te (Goldschmidt 1958; chalkos in ancient Greek, denotes copper, theion sulphur). Although Cu is, redox-sensitive, both Cuþ and Cu2þ are mobile, cations under oxidizing conditions (Smith &, Huyck 1999). Reduced sulphur and carbonate ions, effectively precipitate and immobilize the element in sulphides and malachite, or at higher, pCO2 azurite. Copper forms stable complexes, with organic substances. Therefore, black shales,, coal and petroleum ashes always have elevated Cu, traces. Copper is adsorbed by clay and Mn-Fe oxyhydroxides (e.g. the manganese nodules of the deep, sea). Its average abundance in the crust is 68, (14–100) ppm, 100 in mafic magmatic rocks and, 10 ppm in felsic rocks (Smith & Huyck 1999)., Among sediments, pelites have highest trace contents with 70 ppm, carbonates with 6 ppm the, lowest. Acidic and sulphur-poor ore-forming, hydrothermal solutions transport copper mainly, in the form of chloride complexes, such as (CuCl)o, >250 C and CuCl32þ or CuCl2 at lower temperatures. At high concentrations of reduced sulphur, effective transport, for example in the form of CuS, (HS)22 is only possible if solutions are alkaline, (Mountain & Seward 1999). Typical copper concentrations in ore-forming hydrothermal solutions are between 100 and 500 ppm. Sulphurrich vapours segregating from magma may contain, up to 1% Cu (Heinrich et al. 1992). HCl-rich, volcanic gas can transport 280 ppm copper (Archibald et al. 2002)., Many copper ore deposits owe exploitable, grades to supergene enrichment processes
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , (Figure 1.55). Leached sections are characterized, by gossans and acidic alteration of silicate rocks., Carbonate host rocks inhibit displacement of, copper and its secondary enrichment. Although, varicoloured copper carbonates may taint every, visible surface of the weathered material, copper, contents remain unchanged. However, because, of the low cost of leaching operations, even, low-grade deposits of oxide copper are attractive, exploration targets (Chavez 2000). The common, abundance of pyrite in copper ore is the cause why, acid rock drainage (ARD) prevention and mitigation is one of the most serious and costly environmental hazards of copper mining (cf. Chapter 5.4, “Mining and the Environment”)., , 187, , hosted massive sulphides (Iberian Pyrite Belt,, Kuroko type, Figure 1.46), and the sedex group, (Figure 1.71), although the latter typically contain, more Zn than Cu;, . diagenetic-hydrothermal, stratabound/stratiform sediment-hosted deposits (European Copper, Shale, Chapter 1.4 and Figure 1.75; White Pine,, Michigan; Central African Copper Belt);, . retrograde-metamorphogenic, hydrothermal, saline brine-related (Copper Belt, Mt Isa);, . secondary copper deposits enriched by supergene processes and oxide ores were briefly, described in Chapter 1.2 “Supergene Ore Formation Systems”., Carbonatite-hosted copper at Palabora, , Copper ore deposit types, Copper ore deposits are formed in all major metallogenetic process systems. A common feature, of the majority is copper transport in oxidized and, acidic fluids, and concentration and immobilization upon encountering reduced sulphur. The, most important primary genetic groups include:, . orthomagmatic, sulphides of Cu-Ni-Co-Fe, ( PGM) hosted in mafic magmatic rocks, (cf. Section 2.1.4 “Nickel”, e.g. Noril’sk and Sudbury, Figure 1.7 and Figure 2.10);, . orthomagmatic to magmatic hydrothermal copper sulphide ore in carbonatite (Palabora);, . skarn and magmatic-hydrothermal replacement, deposits, partly in close association with copper, porphyry deposits;, . magmatic-hydrothermal porphyry Cu (Mo, Au), deposits that host 60% of world copper resources, (Cooke et al. 2005);, . magmatic-hydrothermal,, low-sulphur, iron, oxide-copper-gold (IOCG) deposits, characterized, by large masses of magnetite or haematite (Groves, et al. 2010, Cox & Singer 2007, Pollard 2006,, Hitzman et al. 1992; e.g. Olympic Dam);, . copper ore veins, usually intrusion-related (e.g., Cu-Sn in Cornwall, Figure 1.44; San Rafael in Peru,, cf. Section 2.2.3 “Tin”);, . submarine exhalative massive sulphide deposits, often polymetallic and associated with Au,, Zn Pb, and even Sn; this group includes Cyprus, type deposits (Outukumpu, Finland), volcanic-, , The copper-bearing carbonatite at Palabora (Phalaborwa) in northern Transvaal, South Africa is unique, although in some respects resembling the, alkali complexes of Kola Peninsula (e.g. Kovdor)., Palabora is a Palaeoproterozoic (2030 Ma) complex, alkali intrusion hosted in Archaean granite, gneisses. The complex displays two intrusive, centres: The northern centre consists of olivinevermiculite-pegmatoid and isthe base of the world’s, largest vermiculite mine. The southern centre consists of carbonatite with peripheral magnetite-apatite (“phoscorite”) and copper ore in the middle:, The Palabora copper orebody is an elliptically shaped,, vertical volcanic pipe. Copper grades of 1% are, found in the core and decrease gradually to the periphery with no sharp ore/waste contact. The ore is hosted, in banded and transgressive carbonatites, which both, contain abundant magnetite. Copper grades located, in stringers and veinlets are highest in transgressive, carbonatite. Ore minerals include chalcopyrite, bornite and cubanite replacing magnetite. Mineralization, is the consequence of the injection of iron and fluidrich oxidized carbonatite magma. The sequence, illustrates an intriguing passage from orthomagmatic, (banded carbonatite) to magmatic hydrothermal, breccia pipe ore. Outwards, the carbonatite plug is, surrounded by rings of pyroxene-vermiculite-pegmatoid, pyroxenite (the main mass) and a marginal, fenitic syenite (Verwoerd 1986). Groves & Vielreicher, (2001) have considered genetic relations between, Palabora, Olympic Dam and the IOCG-family of, metal deposits.
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188, , PART I METALLIFEROUS ORE DEPOSITS, , The copper orebody has a diameter of 500 m and, extends to a depth of >1300 m. In 2002, the open pit, reached a depth of 760 m, having yielded 900 Mt of ore, and 4.8 Mt of refined copper. The average copper grade, was always low (0.5%) but recoverable by-products, (titaniferous magnetite, sulphuric acid, zirconium, oxide, hafnium, uranium, silver, gold and platinum, metals) make this a profitable operation. Underneath, the open cut, a new underground mine is exploiting ore, over a depth interval of 500 m that contains proved and, probable reserves of 112 Mt grading 0.56% copper., , Porphyry Cu-(Mo-Au) deposits, Porphyry Cu-(Mo-Au) deposits of great economic, prominence are exploited in the western United, States (Bingham, Utah, Bisbee, Arizona, Ely,, Nevada), in Mexico (La Caridad), in the Chilean, Andes (e.g. Chuquicamata, the world’s largest, copper producer; La Escondida, the latest new, mine), in Southeastern Asia (Bougainville, Ok, Tedi, Papua-New Guinea), in Iran (Sar Cheshmeh), and in southeastern Europe (Recsk, Hungary,, Medet, Bulgaria, Bor, Serbia and Deva, Romania)., Porphyry deposits provide 75%, and oxide ore from, porphyries nearly 20% of current world copper, production (Gerst 2008):, Essential aspects of this deposit type are described in, Chapter 1.1 “Porphyry Copper Deposits” (Figure 1.32, and Figure 1.34) and in Chapter 1.7 “Metallogeny and, Plate Tectonics” (Figure 1.88). Numbers characterizing Bingham Canyon mine illustrate the singular, scale of porphyry deposits. Bingham near Salt Lake, City is the largest North American copper and gold, deposit. It occurs in the eastern boundary zone of the, Basin and Range Province. Selective high-grade mining at Bingham started in 1863 and industrial operations in 1903. Today, the pit is 800 m wide, 1200, metres deep and 4 km long. From this volume 11 Mt, of copper were extracted and 5000 Mt of rock had to be, moved. Today, Bingham produces annually 250,000 t, Cu, 11 t Au, 81 t Ag, 15,000 t Mo and 500,000 t, sulphuric acid. Of the total metal endowment of this, extraordinary deposit, only 50% have been removed, until now. Three papers in Economic Geology, Volume 105, Number 1 (2010) exhaustively describe the, metallogeny of the Bingham porphyry deposit. Chuquicamata mine (Figure/Plate 1.31) is nearly twice, Bingham’s volume. Its past production amounts, to 1500 Mt at 1.5% Cu þ 0.07% Mo; remaining, resources are roughly the same mass., , Magmatic-hydrothermal iron, oxide-copper-gold (IOCG) deposits, Magmatic-hydrothermal iron oxide-copper-gold, (IOCG) deposits were proposed as a new class of, deposits after the discovery of the spectacular, copper-uranium deposit at Olympic Dam in, South Australia in 1976 (Hitzman et al. 1992)., The defining difference to other copper sulphide, deposits (e.g. porphyries) is the large fraction of, iron oxides in the ore, explaining, for example,, the controversial (Groves et al. 2010) inclusion of, Palabora as a magmatic member of this class., Exploration for copper in southern Australia, was based on the expectation of ore deposits of, Keweenawan (Michigan) or Copper Belt type, (Central Africa) in the geologically similar Gawler craton of South Australia. Drilling highamplitude Bouguer gravity and magnetic aerogeophysical anomalies soon located ore of a, wholly unexpected nature.:, The Olympic Dam deposit occurs within an anorogenic oxidized potassic granite (dated to 1590 Ma),, which is set within a Palaeo-/Mesoproterozoic graben, (note parallels to Mt Isa and Broken Hill) and is, covered by 350 m of younger, unmineralized sedimentary rocks. Host rocks are coarse haematite-rich, granite breccias (Figure 2.13) of explosive volcanic, and phreatomagmatic origin. The breccia ore contains copper sulphide and by-product grades of rare, earth elements, uranium, gold and silver. Total resources are estimated to 7700 Mt of ore with 0.9%, Cu, 0.3 kg/t U3O8, 0.3 g/t Au and 1.6 g/t Ag (BHP, Billiton 2007). The mineralization appears to be the, product of mixing of ascending hot magmatic brines, (carrying reduced sulphur species) with shallow, highly oxidized haematite-forming groundwater, leaching uranium and LREE. The source of copper, and gold can hardly have been the host granite, (Groves & Vielreicher 2001). Mingling of mafic and, silicic melt (Clark & Kontak 2004), deep crust (Heinson et al. 2006) and fertile mantle enriched by prior, subduction (Groves et al. 2010, Skirrow et al. 2007), may have contributed to the metal endowment. This, setting is very different from the Central African, Copper Belt (see below). Additional data about, Olympic Dam are provided in Section 2.5.12, “Uranium”. Olympic Dam is a textbook case of, highly successful exploration that was based on the, “wrong” geological model.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Haematite-quartz, breccia, Haematite-granite, breccia: main host, to Cu-U ore, , Figure 2.13 Simplified subsurface, geological map of the iron oxide, copper-gold-uranium deposit, Olympic Dam in South Australia,, illuminating its origin in a giant, complex breccia pipe and diatreme, system (modified after Groves &, Vielreicher 2001). Note the ore shell, surrounding volcanic centres. With, kind permission from Springer, ScienceþBusiness Media., , 189, , Roxby Downs, Granite, , Granite breccia, , Volcanic rocks, in diatremes, , N, 1 km, , Cu-Au-deposits of the Cloncurry Type are a, subgroup of IOCG-deposits. They derive their, name from a district of the Mesoproterozoic, Mt Isa copper province in northern Australia (Williams 1998, Pollard 2006, Fisher & Kendrick 2008)., The Cloncurry deposits are much smaller than, Olympic Dam. Their origin is related to late-tectonic intrusions and saline hydrothermal solutions, of mixed origin, including metamorphic fluids, (Baker et al. 2008). In other IOCG provinces, participation of basinal brines is invoked. Although, rather alkaline to subalkaline, the granitoids, related to IOCG deposits are petrologically similar, to copper porphyry rocks, but hydrothermal alteration is dominantly sodic-calcic. Also characteristic is a strong structural control by major fault, systems. Host rocks of ores vary widely. Confirming Clark & Kontak (2004), preliminary Sm-Nd and, Re-Os isotope data indicate that fertile (copper-gold, rich) IOCG systems display a contribution from, mantle-derived source rocks or magmas, whereas, “barren” iron oxide apatite ore only carries crustal, signatures (Skirrow et al. 2007). Iron oxide-coppergold deposits should be viewed as features of, lithospheric scale (Groves et al. 2010)., Copper ore deposits of the Cyprus type, Copper ore deposits of the Cyprus type occur in, ophiolitic host rocks and have a genetic setting, similar to present black smokers (Figure/Plate, 1.11) and sulphide mounds. Although common,, , Cyprus type sulphides have a restricted economic, role because deposits are rather small. Copper is, named after the island of Cyprus in the Eastern, Mediterranean. The Romans called the metal aes, cyprium and later cuprum (metal from Cyprus)., Copper extraction on the island was already practised in the 4th millenium BCE and ended with the, decline of the Roman Empire at 500 CE. After, rediscovery, industrial operations started in 1921, and eventually, more than 30 cupriferous sulphide, orebodies were exploited:, Pillow lavas of the Troodos ophiolite host all copper, orebodies on Cyprus. Most ore is found in the stratigraphic interval between the Lower, hydrothermally, altered, and the Upper Pillow Lavas, which are nearly, unaltered. Epidote-quartz rocks (epidosites) in the, sheeted dykes are thought to mark upflow channels, of the hydrothermal fluids (Bettison-Varga et al., 1992). From footwall to hanging wall, individual, orebodies are zoned as follows: The lowermost, expression of hydrothermal processes is the stockwork zone (i) with quartz, pyrite and chalcopyrite in, chlorite-quartz rocks. This is overlain by silicified, massive sulphide ore (ii) and by the main massive, sulphide orebody (iii). There are a number of different, ore types, for example “conglomeratic ore” with a, matrix of sandy sulphides and “hard compact ore”., Both consist of chalcopyrite and traces of sphalerite,, but mainly of pyrite, which was formerly used for the, production of sulphur. Copper graded 0.3–4.5% and, orebodies reached a mass of 50,000 t to 20 Mt. The, sulphides are covered by the Ochre Group, goethitic, rocks, which have provided pigments since antiquity.
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190, , PART I METALLIFEROUS ORE DEPOSITS, , Ochres are clearly different from the darker and, browner manganese-iron dominant umber beds of, similar use that follow on top of the Upper, Pillow Lavas. Both are exhalative precipitates (cf., Chapter 1.3 “Sedex Deposits”). Systematic structural, mapping of basaltic dykes and faults suggests genesis, of the large sulphide orebodies in fossil rifts near, transform faults (Bettison-Varga et al. 1992)., , Sulphide ore deposits on Cyprus are now, depleted, but favourable economic circumstances, will certainly result in new finds. The same applies to metamorphic sulphides in the Scandinavian Caledonides, some of which are of the Cyprus, type (e.g. Joma, Figure 1.80):, The mining district of Outokumpu in Finland is both, economically important and scientifically interesting. From 1910 to 1988, three mines of the district, produced a total of ca. 50 Mt of ore averaging 2.8% Cu,, 1% Zn and 0.2% Co, with minor amounts of Ni, Ag, and Au. In this region, large bodies of serpentinites, occur within micaceous gneisses of the Palaeoproterozoic Svekokarelide thrust belt. The serpentinites, are thought to be parts of ophiolites, although uncommon ones because basaltic crust is missing. They, probably originated as depleted ultramafic mantle, rocks exposed on the ocean floor (“Hess Crust”). In, this setting, black smoker-related hydrothermal, activity produced sulphide ores and non-sulphide, exhalites. Typical host rocks of ore (the “Outokumpu, Association”) include talc-carbonate fels (CO2-metasomatites), metamorphic skarn, finely laminated, quartzite (former exhalite) and black shales (cf. Talvivaara, Nickel). High-grade Cu-Co ore occurred preferentially within the quartzites. The massive, sulphide orebody at Outokumpu had a length of, 4000 m, a width of 400 m and an average thickness, of 10 m. The genetic model of these ores is, of course,, not a simple Cyprus type. Its hydrothermal ocean, floor setting, however, places it firmly into this, family, even if some features remain equivocal, (Peltonen et al. 2008)., , Volcanic-hosted massive sulphide (VMS), deposits, Volcanic-hosted massive sulphide (VMS) deposits are a highly diversified group. Although the, attribution is only partially correct (see below),, , the giant sulphide ore province of the Iberian, Pyrite Belt is commonly included in this class., The belt has a length of more than 250 km, from, Sevilla in southern Spain to Portugal. Host rocks, are part of the southern margin of the Variscan, Orogen, comprising a basement of over 2000 m of, siliciclastic sediments (phyllites and quartzites),, which is overlain by 50–1300 m of bimodal volcanic, sedimentary and rarely plutonic rocks of, the Volcano-Sedimentary Complex (Late Devonian to Early Carboniferous) and Vis, ean to Late, Carboniferous flysch (up to 3000 m of greywacke,, shale and conglomerates in Culm deepwater, facies). During collisional welding of Africa with, Iberia in the Westphalian when Pangaea was, finally assembled, the belt was deformed by, southward thrusting and folding. Metamorphism, is of very low to low grade:, More than 80 former and operating mines are known., The deposits form a southern and a northern belt that, have contrasting geological settings. In the south,, shale-hosted stratiform orebodies are characteristic, (e.g. Tharsis, Aznalcollar), whereas in the north, the, orebodies are mainly hosted by dacitic or rhyolitic, domes, sills and pyroclastic rocks (parts of Rio Tinto,, La Zarza, Aljustrel: Tornos 2006, and in the west:, Neves Corvo, Relvas et al. 2006) and ore formation is, clearly synvolcanic., Until today, 280 Mt of ore with an average grade, of 46% S, 0.7% Cu, 4% Pb þ Zn, 30 g/t Ag and 0.8 g/t, Au have been extracted in the Iberian Pyrite belt., Total sulphide resources probably reach 2500 Mt., Only one deposit (Rio Tinto) surpasses 500 Mt. This, is the world’s largest province of massive sulphide ore, and a unique concentration of supergiant VMS deposits. The ore paragenesis comprises mainly pyrite,, minor chalcopyrite, sphalerite and galena, and traces, of arsenopyrite, pyrrhotite, tetrahedrite, Pb-sulphosalts and cassiterite. Gangue minerals are silicates of, host rocks and hydrothermal precipitates such as, chlorite precursors. Barite and gypsum are rare., Orebodies occur commonly near eruptive centres,, either in dark shale/phyllite or in pyroclastic rocks., Shale-hosted orebodies consist mainly of massive and, structureless fine-grained pyrite with low base metal, and gold content, and of banded and brecciated parts, that were formed by debris flows. Several of the, massive sulphide bodies contain a significant
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , proportion of siderite. Hydrothermal alteration and, stockwork ores are confined to footwall rocks. Manganese-rich jasper in “purple shale” above the sulphide deposits was used for exploration and produced, many high-grade supergene Mn ore deposits. Orebodies in volcanic rocks are discordant and replacive,, and may be totally enveloped by hydrothermal alteration (silica, chlorite, sericite). The sulphides are, medium to coarse-grained and include remnants of, altered host rocks. Base metal and gold contents are, clearly higher than in the shale-hosted ores. In addition, oxidized facies as well as barite-rich ores are, known. Hydrothermal refining, metal zonation (CuZn-Pb), footwall stockworks (with Cu, Zn, Ag and, Au, and traces of As, Co, Bi) and breccias are ubiquitous. Sulphur isotope d34 S data ranging from 34 to, þ21‰ indicate diverse sources of sulphide sulphur,, including basement, sedimentary and volcanic host, rocks, and reduced seawater sulphur. Microbial, reduction of sulphate in a confined system is thought, to explain the most negative values (Tornos 2006)., The ore forming fluids were saline brines with 3–12, wt.% NaCl equivalent. In spite of intensive research,, their origin remains ambiguous. Interpretations, range from modified seawater to magmatic derivation, (Solomon et al. 2002)., The shale-hosted orebodies were formed by exhalation into brine pools, whereas the replacive, volcanichosted massive sulphides were precipitated well, below the seafloor, in reactive rocks with high permeability. Considering the size of the orebodies, the, hydrothermal systems must have reached deep into, the basement, possibly driven by deep intrusions., There is a general agreement that extensional tectonics had an essential role (ensialic rifting, pull-apart, basins). The basement may have been the source of, both fluids and metals (Tornos 2006, Blundell et al., 2005)., , A number of near-surface orebodies had welldeveloped gossans with important gold content, and rich zones of secondary enrichment which, are, however, long exhausted., The largest Pyrite Belt deposits are Rio Tinto,, Tharsis, Aznalcollar and the latest large discovery,, Neves Corvo in Portugal. The latter is remarkable, because of higher base metal grades including, favourable tin contents. The deposit comprises, >300 Mt of resources, with 100 Mt of ore at, 3.46% Cu, 3.54% Zn, 0.25% Sn, 0.8% Pb and, , 191, , 55 g/t Ag (Relvas et al. 2006). Direct host rocks to, ore include rhyolitic to rhyodacitic volcanic rocks., An early stage of stringer and massive cassiterite, mineralization, some of which formed by direct, venting onto the sea floor, was followed by the, main stage generation of stratabound massive, sulphide ore. Relvas et al. (2006) propose that, the tin-rich fluids were of magmatic derivation,, possibly from a tin granite intruding at depth. The, discovery of the five orebodies at Neves Corvo,, which are covered by 200–700 m of barren rock,, was a highlight of modern exploration methods,, in this case a combination of geological mapping, and deduction confirmed by regional gravimetric, surveys., Sedimentary-exhalative (sedex) copper, ore deposits, Sedimentary-exhalative (sedex) copper ore deposits are here exemplified by reference to the venerable Rammelsberg mine at Goslar, Harz, Mountains, Germany. The mine was closed in, 1988 after a history of more than 1000 years of, exploitation. The orebody occupied a stratigraphic, level in the Mid-Devonian Wissenbach Formation, a thick black shale unit with rare felsic tuff, bands. Its location is controlled by a synsedimentary fault that separates shale of <150 m thickness, from a basinal facies of >1000 m. Several basalt, sills occur in the basin a few kilometres distant, from the orebody. Variscan orogenic deformation, folded the ore lens into an isoclinal, NW-vergent, synform (Figure/Plate 1.81). The total ore mass, before mining was 27 Mt with a metal content of, 7 Mt (mainly zinc, lead and copper, plus some, silver and gold). Gangue included shale, barite,, dolomite and ankerite. In the last years before, closure, ore graded 19% Zn, 9% Pb, 1% Cu,, 160 ppm Ag and 1.2 ppm Au. Shale-banded and, laminated ore with synsedimentary deformation, structures was prominent (Figure/Plate 1.72). The, diagenetic grade of the rocks is within anthracite, stability, with some excursions into the graphite, field. The ore is strongly recrystallized and locally, mobilized. In the footwall, intense silicification of, the shale and a stockwork of quartz veinlets with
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192, , PART I METALLIFEROUS ORE DEPOSITS, , sulphides (“kniest”) are thought to mark upflow, channels of the submarine hydrothermal system, (Muchez & Stassen 2006):, The stratiform ore bed of the Rammelsberg mine, displayed the characteristic vertical ore stratification, of sedex deposits: Mineralization sets in with pyrite, (i), followed by spotted copper-rich pyrite-chalcopyrite, ore (ii), sphalerite and galena (iii), and barite (iv)., Transitional ore interbedded or mixed with shale, extends laterally and into the hanging wall. Tectonic, deformation at Rammelsberg is very intensive; a, reconstruction of the orebody’s original shape remains conjectural. It may have been elliptical with, a long axis of 1000 m and a thickness of 10–30 m., Observations and data confirm its formation by synsedimentary exhalation of 300 C hydrothermal, solutions (Large & Walcher 1999). Considering the, distal occurrence of synchronous mafic and felsic, volcanism, a magma-related heat anomaly at depth, can neither be excluded nor confirmed. Figure 1.71 is, a tentative illustration of the genetic setting of the, Rammelsberg deposit., , Stratabound and/or stratiform sediment-hosted, copper deposits, Stratabound and/or stratiform sediment-hosted, copper deposits contain large copper resources, second only to copper porphyries. Today, most, deposits of this class are considered to be of diagenetic origin. Examples include the European, Copper Shale (cf. Chapter 1.4 “Diagenetic Ore, Formation Systems”), the White Pine District in, Michigan and the Central African Copper Belt:, The White Pine mining district’s setting is a Mesoproterozoic rift (1.3–1.0 Ga) with a fill of thick tholeiitic olivine basalt, some andesite and rhyolite, and a, transgressive sedimentary cover series. Across a very, large area, mineralization is concentrated at the base, of the marine Nonesuch Shale and underlying conglomerates flanking an older volcanic massif. The ore, bed is siltstone with an average thickness of 5 m and, a grade of 1.1% Cu. Stratabound ore contains chalcocite, different from native copper lodes that are hosted, in flood basalts of the same region. The stratiform ore, is associated with diagenetic dewatering of fluviatilevolcanoclastic rocks of the early graben fill, (Swenson & Person 2000). The native copper lodes, of the Keweenaw Peninsula on Lake Superior formed, , 20–35 Ma later, possibly from hybrid evolved meteoric-metamorphogenic fluids (Brown 2006). Native, copper (and silver) accompanied by calcite, chlorite,, quartz, epidote and zeolites fill open spaces in basalt, flow tops and conglomerates. The district produced, more than 6 Mt of copper., , The world’s largest province of stratabound, copper ore is the Central African Copper Belt of, DR Congo and Zambia. Its historic production, amounts to >1000 Mt of ore with 2.7% Cu and, by-product Co, but also Ni, Au, Ag, U, Pb, Zn and, PGE. It is estimated that more than 190 Mt copper, and 8 Mt cobalt remain to be exploited. Both primary sulphides and spectacularly enriched bodies, of oxide, carbonate and secondary sulphide ore, were extracted:, Ore deposits occur mainly near the base of Neoproterozoic sediments of the Katanga Supergroup. This, succession of dolomitic shales, dolomites, siltstones,, sandstones and arkoses reaches a thickness of 11 km., Its basal part is the Roan Group with the Mines, Subgroup, which hosts most ore. With an age of, <900 Ma, the Roan represents post-tectonic rift and, molasse sedimentation of the Kibara orogen grading, into sediments of a passive continental margin that, developed by extensional break-up of Supercontinent, Rodinia. The fragments were rewelded by the main, Pan-African orogeny peaking between 570 and, 530 Ma, during which the Congo-Tanzania, Kalahari,, East Antarctica and India Cratons assembled to form, Gondwana. In the Mines suite, anhydrite and dolomitic-magnesitic carbonates are evidence of former, extensive evaporite horizons. Similar to the European, Rotliegend and the Zechstein salts (cf. Chapter 1.4, “European Copper Shale”), these sediments illustrate, the transition from terrestrial settings with playa, lakes to coastal sabkhas and shallow-marine conditions. The top of the Roan is capped by the glaciogenic, “Grand Conglomerat”, a tillite that may be the product of one of the “Snowball Earth” events at 760 Ma, (Hoffmann 1998). The Roan is overlain by marine, sandstones, shales and carbonates of the Nguba and, Kundelungu Groups. Orogenic deformation (locally, called the Lufilian orogeny) terminated sedimentation and created a large orogenic belt (Lufilian Arc), that is characterized by outward vergent folds and, nappes. Metamorphic grade increases towards the, centre of the orogen, where amphibolite facies with, sporadic eclogites is attained and basement windows, are ubiquitous.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , The Katanga basin formed by rifting and crustal, extension that created a wide ocean. This favoured, voluminous mafic magmatism; felsic magmatic, rocks are less common. Towards the north-northeast,, a little deformed failed rift branches off extending to, Lake Victoria (the Kundelungu aulacogen). In this, region north of the Lufilian arcs, rare mineral deposits, are epigenetic and fault-bound (Haest et al. 2007)., Copper Belt ore (bornite, chalcopyrite, chalcocite,, carollite) is mostly stratabound and hosted in all, rocks forming the Mines Subgroup (carbonates, evaporites, sandstones). However, the sandy dark “ore, shales” attracted special attention as an easily recognizable lithostratigraphic level. Much of the sulphide, ore replaces anhydrite nodules and lenses, together, with dolomite and authigenic quartz (Muchez et al., 2008). The stratigraphical control of mineralization, in single districts is pronounced but varies with, greater distance. Some deposits follow a structural, control and consequently, have an epigenetic character (e.g. Kipushi). Similar to the European Copper, Shale, basin margins adjacent to palaeogeographical, highs are preferred sites of ore formation. Deep, basinal areas are barren., , The evolution of genetic thinking about Copper, Belt metallogeny was last described by Sweeny, et al. (1991). Syngenetic hypotheses proposed that, metals were derived by weathering from emerged, areas and concentrated in near-coastal marine, sediments. Precipitation would have taken place, by reduction in sabkhas or in marginal basins., However, age determinations and structural stud-, , Figure 2.14 Genetic model of the Nchanga, copper-cobalt deposits in the Zambian, Copperbelt (modified from McGowan et al., 2003, 2006). With kind permission from, Springer ScienceþBusiness Media., Upflowing oxidized, sulphate-rich, basinal, fluids (with Cu and Co) are focused by, permeable beds and tectonic structures., Low-permeability metapelites above, sandstones form traps that probably, contained methane. Methane reacting with, sulphate of the fluids formed H2S, (thermochemical sulphate reduction),, precipitating the metals., , 193, , ies imply more diversity, from pre- to post-orogenic (Torrealday et al. 2000). Therefore at present,, epigenetic hypotheses are preferred: Metals were, transported by oxidized saline formation waters, or metamorphic fluids mobilized by diagenesis, and metamorphism in the deep basin. Orogeny,, or deep mafic intrusions may have provided the, necessary driving force (McGowan et al. 2003,, McGowan et al. 2006). The metals were leached, from deeply buried sediments and magmatic, rocks. This is more plausible than a surficial, source considering that Cu, Co and Pt characterize, mafic source rocks. The near-surface reducing, environment may have been induced by one of, the “Snowball Earth” events, by microbial sulphate reduction based on organic matter, or by, thermochemical sulphate reduction of methane, in suitable traps (Figure 2.14)., , Retrograde-metamorphogenic, saline, brine-related deposits, The Cu-Pb-Zn-Ag district of Mt Isa in Queensland, Australia is both economically outstanding, and scientifically intriguing. Mt Isa is the largest of, four supergiant Pb-Zn Cu deposits in the region., It occurs within a block of 4.5 km length, 250 m, width and 1000 m depth. The estimated total ore, content reaches 200 Mt at 3% Cu and 100 Mt at, 7% Pb, 6% Zn and 160 ppm Ag (production since, 1932 plus remaining reserves). In the year 2000,
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194, , PART I METALLIFEROUS ORE DEPOSITS, , dolomite”). Copper orebodies occur in the form of, vein stockworks, massive lenses, and irregular, replacement bodies, and cluster near the basal fault, (proximal) in extremely silicified rock. The paragenesis includes pyrite, chalcopyrite and pyrrhotite,, and minor sulphides and sulphosalts of Ag, As, Zn,, Pb and Co. The hydrothermally altered and mineralized body has a length of 2.6 km and a width of 530 m., Before mining, it contained 20 Mt of copper. All, copper ore is enveloped by coarsely crystalline silica, dolomite., , the youngest mine in the district, Enterprise,, started operations, based on ore with 4% Cu to a, depth of 1800 m:, Mt Isa district is located in a Palaeo-Mesoproterozoic, back arc system that experienced later orogenic deformation (Williams 1998), similar to McArthur River, and Broken Hill (cf. section “Lead and Zinc”). At Mt, Isa mine, very low-grade metamorphic evaporiticclastic sediments (deposited at 1680–1590 Ma), overly a thick mafic volcanic succession. A narrow, synform of the sediments was downthrown by a, regional fault relative to amphibolite facies basement. Orebodies are stratabound within the Urquhart, Shale, a black banded dolomitic marlstone. At depth,, the shale abuts tectonically against greenschist facies, meta-basalts (Paroo fault, Figure 2.15). Hydrothermal, alteration of the basalts is characterized by epidote,, Mg-chlorite and rutile. Above the fault, hydrothermal, alteration of the Urquhart Shale is pervasive. The, resulting coarsely crystalline and brecciated rock, consists mainly of Fe-dolomite and quartz (“silica, , 1 km, , The origin of this deposit is undoubtedly due to, exceptionally large hydrothermal systems. A plausible syngenetic-synsedimentary model assumed, synchronous formation of copper and lead-zinc, ores in a sedex-like submarine setting (FinlowBates & Stumpfl 1979). This would explain the, separation of the metals, because copper typically, precipitates within the sub-seafloor feeder zone,, whereas lead and zinc remain in solution until, , NaCl-rich SO4-brine, infiltrating from evaporitic, cover sequence, E, , Stratiform Pb-Zn-Ag ore, Silica dolomite, , W, , Low-grade, metasediments, pyritic, High-grade, metasediments, dolomitic, , Cu-ore, , Regional carbonate - Fe oxide, alteration of metabasalts, , Mg-chlorite alteration, , NaCl-rich SO4 brine, ? radiogenic Pb?, , Mount Isa fault zone, Paroo fault ?, , Figure 2.15 Retrograde-metamorphogenic formation of copper ore at Mt Isa, Queensland, Australia. After Heinrich,, C.A., Andrew, A.S. & Knill, M.D. 2000, Society of Economic Geologists, Inc., Reviews in Economic Geology Vol. 11,, Figure 5 p. 105. Convecting oxidized evaporitic brines leached copper from greenschist-metamorphic continental, tholeiites. Precipitation was induced by mixing with reduced, sulphur-rich fluids (not shown).
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , venting on the seafloor, resulting in stratiform, syngenetic ore. Lead isotope model ages of ore at, 1653 Ma, alteration of footwall basalts, and high, Br/Cl-ratios of fluids indicating an evaporitic origin conform to this model. However, fluids in, silica dolomite were trapped at >300 C and at, pressures of 1–2 kbar. This is hardly compatible, with a syngenetic sedex setting. Doubts were confirmed by structural analyses of ore control and by, 40, Ar=39 Ar ages of hydrothermal phyllosilicates., The new data suggest a late orogenic retrogrademetamorphogenic formation of the copper, orebodies independent of the syngenetic leadzinc-silver ores (Figure 2.15; Heinrich et al. 2000;, Betts et al. 2003, Chapman 2004). Oxidized,, sulphur-rich brines extracted copper from the, underlying meta-basalts (Gregory et al. 2008). Davis (2004) provides structural arguments that, imply a late orogenic origin of all ores at Mt Isa,, by demonstrating that the overall geometries of, both copper orebodies and of high-grade shoots in, the Zn-Pb-Ag orebodies are controlled by F4-folds., The implication is that copper and lead-zinc were, formed synchronously by the same processes. It, can hardly be expected, however, that this is the, final word on the formation of the giant metal, concentrations at Mt Isa., Exploration for copper is a multi-technology, undertaking based on the whole spectrum of geological, geochemical and geophysical methods, (Sillitoe 1995). The search is assisted by indicator, minerals in soil, till and stream sediments. Copper, porphyries, for example, shed native gold, rutile,, tourmaline, garnet, jarosite and alunite (McClenaghan, 2005). Land-based targets are preferably, world-class, low-cost potential mines. In the past, but also quite recently, successful exploration for, copper porphyries was based on the recognition, that small high-grade fringe mineralization and, alteration point to porphyry ore (Cadia district,, N.S.W.; Wilson et al. 2003). Exhalative seafloor, copper-gold deposits are seriously investigated for, submarine mining off Papua New Guinea and, New Zealand., In 2008, world mine production of copper contained in concentrate was 15.4 Mt (in 2009, 15.8 Mt), with the major share provided by Chile,, USA, Peru, China and Australia. Large resources, , 195, , are available, both on land (3000 Mt) and on the sea, floor (manganese nodules: 700 Mt). Land-based, resources comprise mainly porphyry (50%) and, sediment-hosted deposits (30%). In recent years,, the combination of volatile prices, low demand, growth, large reserves and surplus mine production capacity combined to make the copper, market highly competitive., , 2.2.2 Lead and zinc, Common Ore Minerals:, , Galena, Cerussite, Anglesite, Sphalerite, Smithsonite, Hemimorphite, Willemite, , PbS, PbCO3, PbSO4, ZnS, ZnCO3, Zn4(OH)2Si2O7.H2O, Zn2SiO4, , Max., wt.%, , Density, (g/cm3), , 86 Pb, 77, 68, 38–67 Zn, 52, 54, 58.5, , 7.5, 6.5, 6.3, 3.9–4.1, 4.4, 3.5, 3.9–4.2, , There are few ore deposits that contain only lead or, zinc. Most mines are sources of both metals., Therefore it appears rational to discuss their metallogeny combined in one chapter., Galena is the most important lead ore mineral., Also, it is a common source of silver, with contents from 0.01 to >1% Ag. Silver substitutes for, lead in the crystal lattice or forms minute inclusions of silver minerals such as acanthite and, tetrahedrite. Microscopic intergrowths are also, responsible for traces of As, Sb, Zn, Cd, Bi, Fe and, Cu in galena. Sphalerite may contain more than, 25% Fe in its lattice, as well as Mn and Cd (up to, 5%) in solid solution, and In, Ga, Ge, Tl, As, Se, and Hg (Labrenz et al. 2000). In fact, sphalerite is, the most important source of cadmium, indium,, gallium, and germanium (cf. “By-Product Electronic Metals”). Cu, Sn, Ag, Pb and Au in sphalerite are commonly sited in micro-inclusions. The, botryoidal finely banded variety of sphalerite that, resembles agate consists of both cubic and hexagonal ZnS (wurtzite). Calamine is a miners’ name, for supergene non-sulphide zinc ore, including
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196, , PART I METALLIFEROUS ORE DEPOSITS, , Table 2.1 Properties of lead and zinc metal, Density, , Melting, Point, , Boiling, Point, , Remarks, , Lead, , 11.3, , 327 C, , 1740 C, , Zinc, , 7.1, , 419 C, , 907 C, , Corrosion resistant,, soft, and ductile, Corrosion resistant, , are needed for life functions. One of them is alcohol dehydrogenase that is man’s primary defence, against intoxication. Zinc is also involved in, genetic stability and gene expression, and for, proper functioning of the immune system. Zinc, deficiency endangers life as much as overdoses of, zinc chemicals., Geochemistry, , smithsonite and hemimorphite. Willemite is the, typical ore mineral of non-sulphide hydrothermal, replacement and vein deposits., Lowest exploitable grades of lead and zinc ore, vary between 6 to 10% of combined metal content., Zinc-rich ores are preferred because the markets, value zinc higher than lead., Whereas lead was already known and used in, prehistoric times, production and use of pure, zinc metal is a discovery of the industrial age, (Table 2.1). Brass, the alloy of copper and zinc, was, common in ancient Rome. Today, the key market, for Pb is the lead-acid battery for vehicles and for, industrial use (85%), followed by paint, glass (e.g., computer screens), various implementations for, radiation and noise protection, and ammunition., For humans, lead is extremely neurotoxic and, children are especially vulnerable (Plant et al., 2005, Troesken 2006). Therefore, its use is tightly, regulated. Lead has been phased out as an additive, to gasoline, paint and water pipes. Tin-lead alloy, solders used in the electronics industry are to be, replaced by Sn-Ag-Cu. Todays’ urban dust contains a dangerous heritage of earlier use in gasoline, (Filipelli et al. 2005)., About 50% of total zinc supply is consumed as, a protective anticorrosion cover for steel in the, construction, automotive and appliances sectors., It is also an ingredient of many alloys including, brass (Cu-Zn), of paints and chemicals. A new, application is the Ni-Zn battery that is expected, to replace Ni-Cd in order to reduce consumption, of extremely toxic cadmium (Fergusson 1990,, Selinus et al. 2005). Cadmium is rapidly becoming, a burden for zinc mines because the market cannot, absorb the coproduced quantities. For humans,, animals and plants, zinc is an essential element, (Lindh 2005). More than 300 zinc metallo-enzymes, , Among the common metals, lead is a curious, exception because most lead is the product of, radioactive decay of lithophile uranium and thorium. Radiogenic lead isotopes are fundamental, for age dating and for tracing the origin of metal, concentrations (cf. Chapter 1.1 “Isotope, Geochemistry”; “Uranium”). Following U and, Th, chalcophile lead is enriched in felsic magmatic rocks: Mid-ocean ridge basalts have 2,, gabbros 5 and granites 20 ppm (mainly sited in, K-feldspar). In rock-forming minerals, Pb2þ substitutes for Kþ, Naþ, Ca2þ, Sr2þ and Ba2þ. Similar, to lead, zinc is chalcophile with a lithophile tendency. Bivalent zinc, however, substitutes, strongly for Fe2þ and Mn2þ resulting in higher, zinc contents in basalts (100 ppm) compared to, felsic magmatic rocks (40 ppm). Crustal abundance range estimates are 12–20 ppm Pb and, 40–200 Zn (Smith & Huyck 1999)., In hydrothermal fluids, lead and zinc rarely, occur as simple ions (Pb2þ, Zn2þ) but are typically, complexed with either chloride or bisulphide ligands. Metal-chloride complexes are favoured by, elevated salinities and oxidation. At low salinity, and <300 C, Zn(OH)þ und PbCO30 are possible, metal carriers, above this temperature Zn(HS)20, and Pb(HS)20. Precipitation mechanisms depend, on the nature of the fluids. Common causes are, pH-change, cooling, dilution or mixing with other, fluids especially those carrying reduced sulphur., Where barite gangue is present, the fluids must, have been reduced and acidic because these conditions determine solubility of barium. If barite is, absent, oxidized, neutral to alkaline solutions may, have formed lead-zinc ore. At low temperatures,, precipitation of dissolved zinc by sulphate-reducing microbes seems to be common, often accompanied by dolomitization (Labrenz et al. 2000,
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Vasconselos & McKenzie 2000). High fO2/fS2 conditions at temperatures exceeding 150o C (e.g., mixing of hypogene with meteoric waters) favour, the precipitation of the non-sulphide zinc ore, willemite (Brugger et al. 2003)., Under conditions of supergene alteration,, galena is more stable than sphalerite, because a, thin film of nearly insoluble lead minerals (cerussite and anglesite) quickly develops on all, exposed surfaces. Therefore, galena embedded in, zinc carbonate is a common observation. In oxidized parts of a sulphide orebody cerussite and, anglesite are retained (e.g. Magellan Mine, Western Australia), whereas zinc is more mobile., Downward and lateral displacement by meteoric, water flow is characteristic (e.g. Skorpion,, Namibia: Borg et al. 2003). Some zinc may, remain in the cap as “red calamine” but most, is dissolved and reprecipitated as “white, calamine”. This process results in secondary orebodies of the “non-sulphide zinc type” that are, economically attractive because the metal can be, extracted by low-cost leaching (Large 2001). The, formation of secondary zinc-enriched sulphide, ores by sulphate-reducing microbes is infrequent, (Bawden et al. 2003). In geochemical exploration,, the higher mobility of zinc in near-surface environment tends to cause wider dispersion halos, compared to lead., Lead and zinc ore deposit types, Lead and zinc ore deposits occur in most genetic, groups, although with the notable exception of the, orthomagmatic environment. Economically more, prominent types include:, . sedimentary-exhalative, (sedex), syngenetichydrothermal, deposits, (Rammelsberg:, cf., “Copper”; Pb-Zn-Ag ore at Mt Isa);, . diagenetic stratabound, epigenetic, low-temperature hydrothermal ore deposits in carbonates, (Mississippi Valley, Irish and Alpine types; saltdiapir-bound and deposits of migrating saline, brines; Kipushi in the Central African Copper, Belt);, . diagenetic stratabound, epigenetic, low-temperature hydrothermal deposits in siliciclastic sediments (Jinding, Laisvall);, , 197, , volcanogenic deposits (Cyprus type Zn only;, Kuroko type, Iberian Pyrite Belt: cf. section, “Copper”);, . contact-metasomatic (high-temperature hydrothermal) and skarn deposits (Bajiazi, China, cf., “Silver”);, . vein deposits in various genetic settings (Harz, Mountains, Schwarzwald);, . surficial alteration produces in situ oxidized, lead ore, and transposed secondary zinc orebodies., ., , Sediment-hosted, submarine-exhalative base, metal (sedex) deposits, Sediment-hosted, submarine-exhalative base metal, (sedex) deposits, also called clastic-dominated, Pb-Zn (Leach et al. 2010) were introduced earlier, (cf. Chapter 1.3 “Sedimentary Ore Formation, Systems”, “Copper”). An interesting aspect is the, variable metal endowment, such as Zn-Pb-Cu-Ag at, Rammelsberg, Germany, but only Zn-Pb-Ag at Mt, Isa and Broken Hill/Australia, and Zn-Pb-Ag with a, giant mass of barite in the Red Dog district, Alaska, (Figure 1.73). Possible explanations of this observation include different availability in source rocks,, chemical variables of rocks and fluids, and precipitation mechanisms. For individual deposits, precise, understanding of these aspects is rather the exception.Hostrocksarecommonlycarbonaceousshales:, Lead-zinc-silver orebodies at Mt Isa in Queensland,, Australia are stratiform and stratabound within, Urquhart shale outside of, but near the huge hydrothermal alteration enveloping copper orebodies. The, Pb-Zn-Ag ore is a banded and laminated rock exhibiting synsedimentary soft-sediment deformation. Ore, bands consist of galena, sphalerite, pyrite and pyrrhotite, with a thickness from 1 mm to 1 m. Stratigraphical dispersion of ore reaches a thickness >1000 m and, a distance of 800 m from the silica dolomite. Lead and, silver grades are highest proximal to silica dolomite,, whereas highest zinc grades occur more distally., For genetic hypotheses and more details refer to, Section 2.2.1 “Copper” (Figure 2.15)., Before mining, the giant zinc-lead-silver deposit at, Broken Hill in New South Wales, Australia held a, total of 200 Mt of ore with the fabulous grade of 25%, Pb þ Zn. Broken Hill occurs within Palaeoproterozoic, crystalline rocks of the Willyama Block, which were
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198, , PART I METALLIFEROUS ORE DEPOSITS, , folded and metamorphosed to granulite facies at, 1600 Ma. Peak metamorphic conditions attained, 800 C and 5 kbar (Page et al. 2005). At Broken Hill,, “Mine Sequence” rocks appear to form an anticline, with hanging-wall gneiss on either flank. The mine, sequence comprises granite gneiss, amphibolite,, Potosi gneiss (probably felsic metavolcanics), banded, iron formations with high trace contents of Pb, Zn,, Mn and P, siliceous exhalites, sillimanite gneiss, (a metapelite, or a hydrothermal alteration rock) and, the lode horizon with sulphide orebodies. Characteristic rocks of the lode horizon include Mn-rich garnetite and quartz garnetite, bands of blue quartz with, gahnite (ZnAl2O4) and stratiform pegmatites with, green feldspars that contain up to 1% Pb. Granulite, facies metamorphism affected all these rocks and the, ore. Therefore, Broken Hill is the most famous object, for studies of the effects of high-grade metamorphism, on sulphide ore. Partial melting of the main base, metal sulphide body resulted in a moderately mobile, sulphide melt, which was relatively enriched in Cu,, Sb, As, Ag, Ni and Au (Mavrogenes et al. 2001,, Sparks & Mavrogenes 2005). Mobilized sulphides, intruded host rocks as far as 100 m from the ore, horizon., Broken Hill ore minerals comprise galena and Fe-rich, sphalerite, silver ore minerals, pyrrhotite, chalcopyrite and arsenopyrite. Gangue includes quartz, calcite,, Ca-Fe-Mn silicates, garnet, fluorite, gahnite and wollastonite. Individual orebodies have distinct parageneses; “zinc lodes”, for example, consist mainly of, sphalerite. Age determinations confirm formation of, the ore at the same time as immediate host rocks (ca., 1690 Ma: Page et al. 2005). Lode horizon rocks and, orebodies are metamorphosed sediments that resulted, from submarine hydrothermal (sedex) processes. The, lithostratigraphic sequence is inverted and the feeder, zone to the system has been located (Groves et al., 2008). Possibly, part of the ores formed by replacement, below the sea floor, while exhalites precipitated above, (Parr et al. 2004). The nature of the sediments and the, bimodal volcanism preceding ore formation suggest, an ensialic rift or back arc setting. Syngenetic heat, pulses may have mobilized fluids. Playa lake evaporites influenced the evolution of sediments and mineralization (Cook & Ashley 1992)., , Low-temperature hydrothermal, carbonate, hosted, diagenetic lead-zinc ore deposits, Low-temperature hydrothermal, carbonate hosted,, diagenetic lead-zinc ore deposits include a number, , of different types that have nevertheless many, common features. The majority of these deposits, are of Palaeozoic-Mesozoic age and occur in littledeformed marine platform carbonates. Ore formation is controlled by palaeogeographic factors, by, certain lithological horizons and by faulting., Nature and shape of orebodies are quite variable, and include stratiform lenses, veins, breccias, cave, fill and metasomatic replacement. The paragenesis is simple, most ores displaying only galena,, sphalerite and a carbonate gangue, occasionally, with other accessory minerals. Host rocks are, dolomitized or silicified (“jasperoid”). Sulphur isotopes indicate derivation of sulphur from seawater, sulphate, usually by microbial reduction (refer, to Chapter 1.3 “Sedimentary Ore Formation Systems”). A common feature is the absence of, igneous activity that might be an agent of mineralization. Positive traits include: i) that weathering may have produced valuable calamine, orebodies; and ii) the absence of acid mine drainage, (AMD), because both host and gangue carbonates, buffer incipient acidity. As a result, the risk of, downstream heavy metal contamination is considerably reduced. Economically important provinces or districts gave rise to terms that, designate different settings and genetic variations:, Mississippi Valley Type (MVT) deposits were introduced in Chapter 1.4 “Diagenetic Ore Formation, Systems” (Figure 1.77). They result from basinal, brines that leach metals from sediments and migrate, to basin margins where ore is precipitated. Permeable, rocks and structures focus fluid flow and exert, detailed control on orebody location. Frequent sites, of metal precipitation are dissolution cavities that, resemble karst pipes. Cave ores in the Triassic limestone of Silesia (Poland) demonstrate synchronous, mineralization and cave formation (Heijlen et al., 2003, Figure 1.58). This may be due to limestone, dissolution caused by sulphide precipitation, (eq. 1.21). Mining in some of the US namesake districts has been terminated and today, environmental, remediation programmes are carried out., Alpine Type deposits occur in the Southern Alps, in, the border region between Austria, Italy and, Slovenia. Orebodies and mineralization are always, found in karstifiable Mid- to Late Triassic marine, limestone, although the precise stratigraphical level, varies. Orebodies occupy veins, hydrothermal karst
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 199, , S, , N, 100 m, , Dolomite, , Figure 2.16 Geological profile of the, Zn-Pb-Ag deposit Silvermines, Ireland,, with cross-cutting fault-controlled and, stratabound orebodies in Early, Carboniferous lime- and marlstone, (modified from Johnston 1999)., Orebodies of sulphides and barite in, black; “Waulsortian” is a local term for, stromatolitic reef limestone., , Argillaceous limestone, Dolomite, Silurian schists, , Limestone+shales, Old Red sandstone (Devonian), , Upper dark limestone, , from Pb and Sr (Wilkinson et al. 2005) isotope data., Sulphur isotopic data indicate microbial seawater, sulphate reduction as the source of sulphide sulphur., Fallick et al. (2001) summarize the situation briefly, by “no bacteria, no giant ore deposit”. Ore formed by, mixing of shallow waters with deeply circulating, seawater-derived evaporative brines with 8–19%, salinity and T 130–240 C (Wilkinson 2010, Wilkinson et al. 2005). Orebodies display variable ratios, of Zn-Pb-Ag-Cu sulphides and barite. Some are exhalative-sedimentary, such as the concordant ore in, Silvermines (Boyce et al. 2003, Figure 2.16), but the, majority are epigenetic vein and breccia ores that, formed hundreds of metres below the seafloor (Reed, & Wallace 2004). Much of the ore lead was leached, from Early Palaeozoic basement, confirming genetic, models of deep brine convection during synsedimentary basin dilation (Wilkinson et al. 2005, Everett, et al. 2003, Figure 2.17). Lead derived from basement, introduces a retrograde-metamorphogenic component into the diagenetic-exhalative setting. Several, Irish deposits are giant concentrations of zinc and, lead. The largest is the operating Navan mine (>70 Mt, Zn þ Pb). Historic Silvermines is much smaller,, with 18 Mt of ore at 9% ZnþPb and 23 ppm Ag., , caves (Figure 1.60, Figure/Plate 1.76) and bedding, planes. Sulphide ores display the same age as ordinary, host sediments (Schroll et al. 2006). An essentially, sub-seafloor epigenetic-hydrothermal origin during, the Triassic with occasional exhalation of solutions, on the seafloor unifies present understanding. There, is a connection between tectonic stretching of a, marine platform, deep convection of seawater along, resulting faults and probably, elevated heat-flow from, the mantle. The latter is confirmed by distal mafic, volcanism and intrusive processes. The overall geodynamic setting is early distension of the Supercontinent Pangaea initiating its break-up. After a long, and profitable mining history, resources of Alpine, deposits appear to be exhausted., Irish Type deposits are so-called because of a cluster of mines and mineralizations in Central Ireland., Deposits are controlled by faults (Figure 2.16) that, distended an Early Carboniferous marine carbonate, platform together with its basement of Devonian, (Old Red) sandstone and Early Palaeozoic metamorphic rocks. The structures facilitated establishment, of hydrothermal convection systems, which gradually reached down into Precambrian basement, (Figure 2.17). Deepening with time can be deduced, , Early Carboniferous shallow saline sea, Sea, , Brine pool and orebodies, , Recharge zones, , Recharge zones, , er m, , wat, , Early stage, , fied, odi, , 150°C, Middle stage, , to a, c, cidi, , 5 km, , e, brin, , Figure 2.17 Schematic profile of, seawater convection favoured by, extensional tectonics in the Irish, Midlands, synchronous with formation, of the large Zn-Pb-Ag deposits in the, Early Carboniferous. After Everett, C.E.,, Rye, D.M. & Ellam, R.M. 2003, Society of, Economic Geologists, Inc., Economic, Geology Vol. 98, Figure 15 B, p. 45., , Chert, , Waulsortian, , 250°C, Late stage, , 250°C
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , but very little lead. This is a consequence of the, low uranium and thorium concentrations of midocean basalts and generally, of oceanic crust and, the Earth’s mantle., Contact-metasomatic and skarn, Pb-Zn-Ag deposits, Contact-metasomatic and skarn Pb-Zn-Ag deposits characterize active continental margin metallogeny. Parentage of I-Type granitoid intrusions is, frequently observed. Most of these deposits are, silver-rich (read more in “Silver” in Section 2.3.2):, In Europe, the Trepca (or Trepc€e) Pb-Zn-Ag mine in, Kosovo is a remarkable example of this group. Here,, Triassic carbonates and schists are penetrated by, the subvertical pipe of a Miocene trachyte volcano, (Heinrich & Neubauer 2002). Carbonates proximal to, the vent are replaced by anhydrous skarn with pyroxene, garnet, epidote and ore minerals. Hydrothermal, ores have been formed outside the skarn rock in large, hydrothermal karst caves that contain marvellous, crystals of many rare minerals. Massive sulphide, ore consists of argentiferous galena, black sphalerite, with over 12% Fe, pyrrhotite and pyrite. Important, by-product metals are gold, silver, bismuth and cadmium. Traces of indium, gallium, germanium, arsenic, copper, selenium, tellurium and antimony are, by-products. Gangue comprises quartz, carbonates, and barite; noticeable are large bodies of metasomatic, manganese and iron carbonates. Total ore mined plus, remaining resources at Trepca are estimated to 60 Mt, at 6% Pb and 4% Zn., , Hydrothermal vein deposits, Hydrothermal vein deposits of lead and zinc are, economically less favourable because extraction, costs are higher than in other deposit types. For, 1000 years, the Harz mining district in northern, Germany was a source of silver and lead. Zinc was, only extracted since it attained industrial value, 150 years ago. In 1994, the last mine closed (Bad, Grund):, Ore veins of the western Harz Mountains occur in, Late Palaeozoic greywacke and shale that were folded, during the Variscan orogeny. The veins are part of a, late to post-orogenic system of NW-SE striking extensional faults. Down-throw of single faults reaches, several hundreds of metres. They can be traced for, , 201, , 20 km and reach a thickness of 70 m. Fault planes, display an undulating topography; only certain parts, are mineralized (Figure 1.43). Nineteen of these structures host exploitable Pb-Ag-Zn ore. Host rocks are, haematitized, silicified, dolomitized and ankeritized,, indicating the passage of very different hydrothermal, fluids. Vein-fill is banded (similar to Figure 1.28) and, displays many druses. The paragenetic sequence is, composed of barite, siderite, quartz, dolomite and, calcite, apart from ample galena, silver minerals,, sphalerite and some chalcopyrite. The Bad Grund, mine produced a total of 1 Mt lead, 2000 t silver, and 500,000 t zinc. Remaining resources are large but, not economically recoverable., , In spite of intensive research, the genesis of the, Harz veins remains enigmatic. The faults formed, in response to late-orogenic relaxation of Variscan, compressional deformation. Lead and sulphur, isotopic composition of ore suggests that both, elements are sourced in the Palaeozoic sediments., Mobilization of fluids may be related to a thermal, event at 300 Ma (Carboniferous/Permian boundary), when gabbro and granite intruded near the, vein district but derivation from the intrusions is, improbable. Fluid inclusions have elevated salt, content and this might indicate that the descending branch of convection systems tapped Zechstein, brines or dissolved salt from Zechstein evaporites, that covered the region during much of the Mesozoic. Some of the vein faults can be traced into, nearby Permian cover where displacement is very, small, suggesting that faulting (and mineralization) occurred mainly before cover sediments were, laid down. Dating of alteration minerals produced, ambiguous results; at least one phase of hydrothermal activity falls into the Late Triassic (Schneider, et al. 2003). This age is shared with many important ore districts in Europe, for example Copper, Shale, Pb-Zn in Silesia and in the Alps, and Early, Jurassic fluorite veins and sinter barite of the Massif Central in France. Break-up of the Supercontinent Pangaea may be the common cause., World mine production of zinc (in 2008 11.6 Mt;, in 2009 11.1 Mt) and lead (in 2008 3.8 Mt; in 2009, 3.9 Mt) is steadily growing. Reserves and resources, of lead are large; Australia, China, USA and, Peru are best endowed. Main producers are China,, Australia, USA and Peru. About 70% of Western
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202, , PART I METALLIFEROUS ORE DEPOSITS, , World consumption of refined lead is recycled, from secondary sources, whereas for zinc this, fraction is small. The primary (mine) production, of lead is fluctuating widely because the metal is, essentially a co-product of silver and zinc mining., Reserves of zinc are considerable, especially in, China, Australia and Peru. Largest producers of, zinc are China, Australia and Peru. Exploration, for lead and zinc is characterized by geological,, geochemical and geophysical methods that are, common to most sulphide ores., 2.2.3 Tin, Common Ore Minerals:, , Cassiterite, Stannite, , SnO2, Cu2FeSnS4, , Max. wt.% Sn, , Density (g/cm3), , 78, 27, , 6.8–7.1, 4.4, , Cassiterite is the main ore of tin. Different shapes, of cassiterite crystals are characteristic for certain, genetic settings. Bipyramidal and short, stocky, prismatic crystals are found in pegmatites, greisen, and high-temperature veins, whereas long prismas, and needle-like crystals appear in lower-temperature hydrothermal deposits. “Colloidal” wood-tin, occurs in low-T veinlets associated with rhyolites., Cassiterite usually contains a small amount of, iron in substitution for tin and minor to trace, concentrations of Ta, Nb, Ge, Sc, Zr, Ga, Be, Hf,, In, W, Mn and V, some of which may be valuable, by-products. Uraniferous cassiterite is not rare and, because of its resistance to alteration is useful for, precise U-Pb dating. Stannite is a frequent trace, mineral in most tin ores but only attains economic, importance in some Bolivian deposits., Exploitable grades in primary hard-rock tin mines, are typically 0.3 to 1.2 wt.% Sn, whereas placers can, be mined at <0.01% Sn. At sufficiently large grain, size, cassiterite is concentrated by simple gravitative processes (jigs and tables). Very fine-grained, cassiterite, stannite and other tin sulphosalts, enforce higher investments for treatment (including flotation) and recovery is often poor (<50%)., Uses of tin are manifold because of a combination of positive qualities. Tin metal is not toxic, is, soft and melts at low temperatures (melting point, , 231.9 C; density 7.29 g/cm3). Its properties are, easily modified by alloying. Examples of common, alloys are bronze (Sn with Cu), which was already, used in ancient Mesopotamia (3500 BCE), and tin, solder (Sn with Pb) of Roman antiquity but still, used in electronic applications. Tin-brass (Sn with, Zn) and alloys with Al, Si, Sb and many other, metals are more recent developments. Alloys are, employed in electrical appliances, vehicles,, machinery and buildings. Tin-zirconium alloys, are utilized in nuclear reactors. The largest part, of production is consumed for tin plating,, especially of food and drink containers, and for, lead-free solders. Tin chemicals include SnF in, toothpaste, amalgam (Ag-Cu-Zn-Sn-Hg) in dental, medicine, fillers in plastics and non-toxic chemicals for wood protection. Of all metals, tin has the, widest field of uses. Although tin is environmentally benign, arsenopyrite is often part of the ore, paragenesis. The fate of the arsenic present in tin, mines must be closely monitored., Geochemistry, The geochemical properties of tin are characterized by its incompatible and lithophile behaviour., Ultramafics contain an average of 0.15 ppm Sn, (Jochum et al. 1993), mafic rocks 1.2 ppm and, felsic magmatic rocks 3.5 ppm. The solubility of, Sn in granitic liquids was described by Taylor &, Wall (1992) and by Linnen et al. (1996, 1998). Trace, contents are variously hosted in accessory magnetite, ilmenite, titanite, garnet and in micas. Sn, may be extracted from melt or these minerals by, reduced, acidic, chloride or fluoride-rich fluids, that form during crystallization of specialized felsic magmas. Reduction is favoured by interaction, with carbon-bearing country rocks. Highly fractionated granites are characterized by tin contents, that may reach 50 times the average for ordinary, granite. Rb (Figure 1.17), Li, F, B and many other, elements are anomalous, too. Expulsion of fluids,, hydrothermal alteration and ore formation cause, erratic diminution of Sn in the granite body., Numerous investigations have centred on the, geochemical, mineralogical and geological characterization of fertile “tin-bearing” or “tin granites” (Breiter et al. 1999, Inverno & Hutchinson
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 2006). It is, however, probably vain to search for, universally valid characteristics of fertility., Most tin-fertile granites occur near active plate, boundaries. This is very obvious for Bolivia and, Peru, and the Southeast Asian tin province (Burma, to Indonesia), which is the source of nearly 50%, of historic world tin production (Schwartz et al., 1995). Other significant habitats include collisional orogens such as the European Variscides,, the Mesoproterozoic Kibara belt in Africa (Pohl, 1994) and the Mesozoic orogen of the South Chinese platform (Figure 1.15). Typically, tin granites, intrude at late to post-orogenic time. A third group, of tin granites appears as members of A-Type, granite suites in anorogenically mobilized cratons, (e.g. the Jurassic granites of the Benue aulacogen in, Nigeria; Mesoproterozoic granites of Rondonia, and the Amazon basin in Brazil). Tin porphyries, in the Eastern Andes of Bolivia and Peru may also, belong to this group (Figure 1.88). Tectonic control, in this province was Tertiary crustal extension., However, most tin granites are S-Type, highly, fractionated and strongly reduced (Figure 1.17)., High initial Sr-isotope ratios confirm a continental crustal source, which was earlier deduced from, geological observations. Good examples are the, tin deposits related to the Lebowa granites of, the Bushveld Intrusive Complex. The granites are, the product of crustal melting induced by the giant, mafic magma body. Tin granites tend to have more, boron in some provinces and more fluorine (“topaz, granites”) in others (Lehmann et al. 2000). The, first typically produce hydrothermal deposits, as, in Bolivia, Cornwall and the Kibara belt, whereas, fluorine tends to induce greisen and disseminated, ores in granite (e.g. in the Erzgebirge and Nigeria)., The volcanic equivalent to tin granites rich in, fluorine are topaz rhyolites (“ongonites”), reported, in Mexico, USA and Mongolia, with economically, insignificant occurrences of wood tin in tuff and, other pyroclastic rocks. Topaz rhyolites may be, genetically connected with playa lake deposits,, such as Li-brines at Searles Lake in California,, uranium in playa tuffites, borate and iodine, (Warren 1999). Solubility of tin (and Fe) is highest, in hot, reduced, saline and acidic solutions., Magmatic high-T fluids carry > 200 ppm reduced, Sn2þ in chloride complexes (Heinrich 1990). Below, , 203, , 350 C and at pH 2–7.5 tin is complexed as Sn(OH)2, or Sn(OH)4. Precipitation of cassiterite (Sn4þ) is, induced by destabilization of the complexes following sudden drops of T or P (boiling), rising, oxygen fugacity or pH due to hydrothermal alteration of host rocks, or mixing with fluids of a, markedly different composition. Obviously, SnO2, cannot be formed in the absence of oxygen, but this, may be derived from water in the fluids (eq. 2.4)., Precipitation of cassiterite from hydrothermal, solutions:, þ, , , , SnCl þ2H2 O ¼ SnO2 þCl þ2Hþ þH2, 4SnðOHÞ2 þCO2 ¼ 4SnO2 þCH4 þ2H2 O, , ð2:4Þ, , The first reaction illustrates why acidic alteration is common in tin deposits, expressed as, pervasive sericitization or at lower T, kaolinization. Typical alteration and gangue minerals, include quartz, tourmaline, white mica, arsenopyrite and chlorite (e.g. at San Rafael). Grade and, mass of tin ore may be controlled by the neutralization capacity of the host rock (Heinrich 1990)., Stable isotope data reveal that the water of tin oreforming fluids is initially of magmatic origin, (“juvenile”), whereas later fluids may contain an, increasing component of meteoric water. A third, possible admixture are water and volatiles distilled from intruded country rocks (e.g. As, S,, CO2, CH4 and N2 from black shales at Rutongo,, Rwanda; Figure 1.16)., Cassiterite is very resistant against superficial, chemical alteration. Eluvial, alluvial and coastal, placers of this mineral are common. The distance, from the primary source to the point where grades, fall below economic limits is controlled by, mechanical diminution during transport and by, dilution with clastic sediment. Cut-off grades of, placers are typically reached after a transport of, hundreds of metres to a few kilometres. In contrast, to cassiterite, stannite weathers rapidly to souxite, (also called varlamoffite, H2SnO3 þ aq), a, “mineral” of earthy or colloidal appearance., Tin ore deposit types, The strongly incompatible and lithophile properties of tin are the foundation of its metallogeny,
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , rare element pegmatites is Greenbushes in Western Australia (cf. Section 2.5.10 “Lithium”)., Magmatic-hydrothermal tin deposits, Classical examples of magmatic-hydrothermal, tin deposits in Europe include Cornwall (cf., Chapter 1.1 “Hydrothermal Vein Deposits”) and, the Erzgebirge (Breiter et al. 1999, Baumann et al., 2000):, , 205, , Famous greisen tin deposits within granite, cupolas of the Erzgebirge are Altenberg, Cinovec, (Zinnwald) and Krasno (Sch€, onfeld-Schlaggenwald). At Ehrenfriedersdorf, Sn-W veins were, exploited above a cupola containing cassiterite in, greisen. The origin of greisen ore within granite, cupolas is due to trapping of specialized melts and, fluorine-rich fluids. Trapping is controlled by fluid, pressure and the stress-state of roof rocks., Magmatic-hydrothermal quartz vein deposits, , The Erzgebirge is part of the Saxothuringian zone of, the mid-European Variscan belt, exposed for a length, of 120 km and a width of 45 km. It is built of, Precambrian and Early Palaeozoic rocks that were, folded at 380 Ma (in the Late Devonian). Between, 325 and 300 Ma (Late Carboniferous to Early Permian), it was intruded by numerous granite bodies of variable composition (F€, orster 1999). Investigations of, this intrusive suite were the base of the concept of, precursor, specialized and mineralized granites, (cf. Chapter 1.1 “Granitoids and Ore Formation”)., The batholiths comprise large masses of early biotitic, granodiorite and monzogranite, followed later by, smaller bodies of topaz-biotite-muscovite syenogranites. The youngest members of the latter are highly, specialized lithium-mica granites (containing zinnwaldite). Mineralization is genetically related to, these lithium granites. Agents of ore formation were, fluids exsolved from cooling melt bodies. Ores occur, in stratiform greisen bodies within lithium granites, (endogranitic), as greisen veins within older granite,, in hydrothermal cassiterite-quartz veins located in, siliceous metamorphic country rocks and as small, metasomatic replacement bodies in carbonates., , Earliest tin mining in Europe took place in, western Iberia and in Cornwall (cf. Chapter 1.1, “Hydrothermal Vein Deposits”) with its remarkable metal zonation (Figure 1.44). A similar zonation with Sn at depth and superjacent Cu occurs at, the giant (>1 Mt of tin) and high-grade (4.7% Sn in, ore) San Rafael deposit, that is genetically related, to a Late Oligocene S-type granite (Kontak & Clark, 2001, Mlynarczyk et al. 2003; Figure 2.19). Early, hot, reducing and hypersaline fluids were followed, by ore-stage fluids, which were more diluted,, slightly oxidized and of lower temperature, (290–380 C). This is thought to be due to mixing, of magmatic with meteoric water (Wagner et al., 2009)., Tin skarn and contact-metasomatic orebodies, Tin skarn and contact-metasomatic orebodies are, economically significant because of large resources at high grades. Their formation depends, , NW, , SE, , 5000, , Figure 2.19 Metal zonation, exploitable, areas, and granite/schist contact in a, longitudinal section of San Rafael vein, and breccia deposit, Peru (modified from, Mlynarczyk et al. 2003). Note the, localization of tin within the granite,, whereas much copper resides in the, Palaeozoic roof schists. The vertical extent, (1200 m) of exploitable mineralization is, quite remarkable., , 4500, , 1000 m, , 4000, , Copper ore, , Oligocene granite, , Tin ore, , Ordovician slate and hornfels, , Metres, above, sea level
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Occasionally, tin is an important by-product of, volcanogenic-exhalative massive sulphide (VMS), deposits. Over many years, the only tin production, in North America was derived from the base metal, deposits at Kidd Creek (Ontario), Sullivan (BC) and, the Bathurst Mining Camp in New Brunswick,, Canada (McClenaghan et al. 2009). An example, from Europe is Neves Corvo in Portugal (cf., Section 2.2.1 “Copper”)., Exploration for tin deposits was traditionally, done by systematic panning alluvial sediments, for cassiterite along streams and rivers. This was, very effective in locating placers, eluvial and outcropping primary deposits. Today, exploration is, increasingly directed towards buried hard rock, deposits and ancient “deep” placers that are covered by younger sediments or by volcanic rocks, (Figure 2.21). Geochemical search for Sn-anomalies in soil and sediments is very efficient., Frequently, rock geochemistry is employed in, order to identify fertile and mineralized acidic, intrusives (Baker et al. 2005). Hidden cupolas of, tin granites can by found by geophysical methods, (e.g. gravimetric surveys) and investigated by, drilling., China, Indonesia, Peru, Bolivia and Brazil are, the world’s leading tin producers. World production was 299,000 t in 2008 (307,000 t in 2009;, USGS 2010). Large resources occur in China,, Brazil and Peru. Because of efforts to reduce the, , use of lead, demand for tin as a substitute for lead, in solder, electrical applications and in shot may, increase., , 2.3 PRECIOUS, , METALS, , 2.3.1 Gold, Common Ore Minerals:, , Native gold, Sylvanite, Calaverite, , Max. wt.%, Au, , Density, (g/cm3), , 70–100, 25, 42, , 15.0–19.3, 8.0–8.3, 9.3, , Au (Ag,Cu,Hg,Pd), (Au,Ag)Te4, AuTe2, , Native gold may be coarse-grained (“free-milling”) or included in sulphide and sulphosalt, minerals (“refractory”), where it occurs either, in small inclusions or as impurity defects in, crystal structures. Native gold contains 2–20%, silver (Morrison et al. 1991), 0.1–0.5% copper, and iron, and trace amounts of bismuth, mercury, lead, tin, zinc and platinum group metals., Gold alloyed with PGE metals is relatively rare, (Cabral et al. 2002). Electrum is gold with, 30–70% silver., , NE, , SW, , GP, , Quaternary, Figure 2.21 Profile of Tertiary, escarpment-type tin placers at Tenkeli in, northern Jakutia, Russia, due to repeated, faulting (modified from Laznicka 1985, with permission from Elsevier)., Quaternary alluvial sand and gravel;, T ¼ Tertiary gravels; Sn ¼ Tin placer;, R ¼ Regolith; GP ¼ Mesozoic granite, porphyry., , R, Sn, , Schists and hornfels, , 50 m, , 207, , Sn-quartz stockwork
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208, , PART I METALLIFEROUS ORE DEPOSITS, , Apart from copper, gold is one of the earliest, metals consciously sought by humans. The oldest, known map of a gold mine was drawn in the 19th, Dynasty of Ancient Egypt (1320–1200 BCE),, whereas the first written reference to gold was, recorded in the 12th Dynasty around 1900 BEC., Total gold recovered by humans is estimated at, 155,000 t, which would fit into a cube of 20 m, edge length. Gold was always a metal valued for, wealth, adornment and strong currency. This is, little different today, only 10% are consumed, by industry (e.g. electronics, architectural glass,, dental applications). For the future, an increasing, role of nano-sized gold particles as catalysts in, chemical production, in pollution control and in, medical applications is predicted., Processing costs are an important control on, cut-off grades in gold mining. Other variables, include extraction expenses, which are very different for underground or open pit mining, dredging of unconsolidated placers and recovery of, former mill tailings. The average gold concentration in ore of important mines is currently at, 3 g/t. Alluvial mega-placers can be exploited at, grades as low as 0.2 g/m3. Note that the weight,, or more exactly the mass of gold (and platinum) in, metal trading is commonly cited in ounces (Troy, ounce, abbreviated oz tr). The conversion factor, to SI-units is 31.1034 g/oz tr or one metric tonne, (1000 kilograms) equal to 32,150.7 troy ounces., Pure gold (24 carat) has a density of 19.3 g/cm3 and, melts at 1063 C:, Native gold dissolves in cyanides (e.g. KCN, NaCN), and in mercury, accounting for the chemical part of, mine-site processing. Larger grains of free-milling, gold are amenable to gravity concentration methods, but fine flakes are not. Earlier, fine-grained ore was, mixed with liquid mercury (amalgamation); the, amalgam paste was separated from gangue and, heated to vaporize mercury leaving crude gold bullion. Today, mercury is rarely used in industry and, the cyanidation process is preferred. In this process,, gold in the ore is dissolved by a very dilute solution, of potassium or sodium cyanide producing gold, cyanide. This is stripped from the solution by activated charcoal, concentrated and purified by melting (Adams 2005). By cyanidation and low-cost heap, leaching, very low-grade ore of 0.2 ppm Au (¼g/t), , can be profitably treated. Cyanidation is, however,, restricted to ore with a low concentration of cyanicides, which bind cyanide. Common cyanicides, include S, Fe, Cu, Ag, As, Cd, Sb, Ni, Co and Zn., Auriferous sulphides, sulphosalts and tellurides, (“refractory” ore) are usually pretreated by flotation, and oxidation (roasting), causing higher processing, costs and consequently, cut-off grades. Humic and, bituminous matter in ore is a similar penalty. Refractory “invisible” gold occurs in the form of tiny inclusions (Au0 <10 mm) or as Au1 in the crystal lattice, of pyrite, arsenopyrite and stibnite. Invisible gold, cannot be liberated by milling. The technological, solution is oxidation at pressures of 2–3 MPa and, temperatures reaching 200 C (autoclaving) that converts the sulphides to porous and fractured oxides, so, that leach solutions can reach the gold. Recent developments include processing such ore with high, concentrations of Thiobacillus ferrooxidans and, Leptospirillum ferrooxidans, which oxidize sulphides and make the gold recoverable by heap leaching (“bio-oxidation”). Cyanide solutions are recycled, and when spent, are detoxified before release into the, environment. Several inorganic and biological processes are available, which decompose cyanide to, CO2 and N2. Because of hazards posed by cyanides,, replacement chemicals are sought, although as yet, with little success. Environmental cyanide geochemistry, sampling and analysis are very tricky and, require thorough knowledge and experience (Smith, & Mudder 1999)., , Geochemistry, The geochemical character of gold is strongly, siderophile, in contrast to silver, which is chalcophile (Goldschmidt 1958). Accordingly, average gold contents in iron meteorites are 1 ppm,, but in the Earth’s crust only 0.004 ppm (range, 0.001–0.005 ppm: Smith & Huyck 1999). Gold, contents in magmatic rocks correlate positively, with Fe and S, yet fractionation of felsic melt, may lead to economic gold concentrations (Mustard et al. 2006). Trace gold of magmatic rocks is, variously hosted in sulphides, magnetite, biotite,, pyroxene and hornblende. Dissolution behaviour, of these minerals controls liberation of gold from, source rocks; in passing fluids, sulphides probably dissolve first. Some sediments concentrate, gold. Sandstones and conglomerates average
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 0.03 ppm, coal ashes reach 0.1 and black shale, 2 ppm Au. Arsenian iron sulphides of mid-oceanic ridges concentrate gold and silver, (28–140 ppm Au, 800–2400 ppm Ag: Halbach, et al. 2003). Cyprus Type deposits approach, grades of 8 ppm Au. The crustal gold/silver ratio, is 1:20 and because of a similar ionic radius and, charge, gold and silver are exchangeable in many, minerals. Different from silver, gold does not, form sulphides but enters a number of rare sulphosalts together with metals such antimony,, selenium and bismuth., In the surface environment, gold is nearly, insoluble. More precisely, of all metals gold is, least reactive to atoms and molecules when in, contact with gases or fluids. This makes it the, most noble of all metals (Hammer & Norskov, 1995). However, many observations imply that, gold in surface systems can be mobile. Examples, include the formation of large nuggets that are, derived from fine-grained gold in unaltered ore, (Youngson & Craw 1993), the common idiomorphic and undamaged octahedral gold crystals of, young placers, and the concentration of gold in, enrichment zones beneath gossans. Migration, may be in the form of dissolved inorganic or, organic gold (I) or (III) complexes and with Fe-Mn, colloids. The most likely and mobile compound, of gold in natural systems at the Earth’s surface is, probably gold (I) thiosulphate Au(S2O)23. Gold, solubility is controlled by the availability of S, Cl,, B, I, natural cyanides and Eh/pH. In the tropics,, humic acids (especially fulvic acids: Bowell et al., 1993) and decomposition products of organic, matter (carbonyl CO, NH3, HCO3) may support, solubility. The precipitation of mobile gold is, thought to be assisted by actinomycetes, archaea,, fungi and bacteria (e.g. sulphate-reducing bacteria: Lengke & Southam 2007; Pedomicrobium, films in Alaskan gold placers: Watterson 1992)., Certain bacteria (including Ralstonia metallidurans) reduce and precipitate gold from soil solution, either within their cells or extracellularly,, in order to detoxify their immediate environment. Bacterial biofilms catalyze formation and, growth of secondary Au, promoting chemical and, nanoparticular mobilization and reprecipitation, as bacterioform Au particles that have been, , 209, , described from placers and weathered parts of, Au deposits (Reith et al. 2010). Due to higher, solubility of Ag, Cu and Hg, secondary Au often, contains fewer impurities compared to the primary gold. Transported gold particles in placers, contain identifiable submicroscopic inclusions, of opaque minerals that can be linked to genetic, types and even to certain ore districts (Leake, et al. 1993). Incidentally, this is one of the arguments brought forward against nugget formation, by dissolved mobile gold., In hydrothermal solutions, gold does not form, simple ions but combines with anions resulting, in Auþ complexes such as AuCl2 and Au(HS)2., At the temperatures of supercritical magmatic, and metamorphic fluids (550–725 C), high gold, concentrations occur based on stable complexes, Au(HS)(H2O) and Au(HS)2 (Xiandong Liu et al., 2011). In fluids with reduced sulfur, iron and base, metals such as Pb, Zn and Cu have a very low, solubility. This explains the ubiquitous formation of gold-only ore deposits, in spite of the, omnipresence of base metals. Most aqueous gold, fluids contain 0.10–0.25 mole CO2, which buffers pH (Phillips & Evans 2004), commonly to, neutral or slightly acidic state. Nearly pure CO2, fluids are very rare and their role is not wholly, understood (Schmidt-Mumm et al. 1997, Chi, et al. 2006). Typical NaCl content is low, with, 1.5–6 wt.%. Oxidation potential Eh is predominantly negative, as demonstrated by Fe2þ-rich, minerals of alteration halos and veins (e.g. pyrite,, ankerite). Precipitation of thiocomplexed gold, is induced by falling temperature, pressure and, sulphur activity, which may be due to fluid unmixing or boiling, to sulphide precipitation, because of reaction with Fe-rich country rocks, (“sulphidation”), to dilution or oxidation. In the, case of low S-contents in fluids that can be, assumed when sulphide and gold in ore are negatively correlated, chlorine complexes may have, dominated gold transport. In this case, precipitation of gold is induced by falling temperature,, pressure and chlorine activity, or by degassing of, CO2 (e.g. effervescence) because this raises the, pH of solutions., A frequent precipitation mechanism is adsorption of gold on sulphide mineral surfaces (e.g.
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210, , PART I METALLIFEROUS ORE DEPOSITS, , pyrite, galena, sphalerite, chalcopyrite, etc.) or, colloids. Reduction/oxidation processes also have, a role when dissolved Auþ or Au3þ is fixed as, native Au0. This is illuminated by gold enrichment in contact with reducing material (e.g. black, shales: Matth€ai et al. 1995, Copper Shale: Piestrzynski et al. 2002). Surface discharge of goldbearing fluids in volcanic settings (geothermal, hot springs) displays co-precipitation of gold, with gels of As-Sb sulphides (Broadlands, New, Zealand)., Gold ore deposit types, Together with platinum and rhenium, gold is one, of the rarest elements. Surprisingly, gold deposits, are quite common, in numbers, geographical, spread and genetic diversity (Figure 1.3, Table 2.2)., Clearly, relatively common crustal fluids are able, to mobilize, transport and concentrate gold. Metamorphic and magmatic fluid systems are prolific, producers of primary gold deposits. This is illustrated by thousands of gold deposits that formed in, orogenic belts from large volumes of crustal rocks, with an ordinary geochemical background. The, group is called “orogenic gold deposits”, which are, understood as products of crustal-scale massive, flow of aqueous-carbonic metamorphic fluids, and of more local magmatic-hydrothermal fluids, during orogeny. The majority of primary gold deposits originated in subduction and collision settings. In the American Cordilleras, nearly all, major gold deposits are related to magmatic activity and single districts formed in short periods of, 5–20 My (Sillitoe 2008). Secondary gold deposits, are generated by weathering and soil formation, (lateritic gold, cf. Chapter 1.3 “Residual/Eluvial, Ore Deposits”) or as placers resulting from erosion, transport and sedimentation. The more common genetic types of gold deposits include:, . orthomagmatic by-product gold in sulphide ores, formed from segregated sulphide liquid;, . magmatic-hydrothermal (e.g. quartz veins,, stockwork orebodies, breccia pipes and breccia, bodies, IOCG deposits, copper-gold porphyries,, polymetallic skarn, part of shale or turbidite, hosted gold deposits; part of orogenic deposits;, Carlin type deposits);, , volcanogenic epithermal and hot springs, deposits;, . by-product gold in submarine exhalative deposits (VMS and sedex);, . metamorphic local redistribution, concentration and recrystallization of gold;, . metamorphogenic-hydrothermal quartz veins,, vein stockworks, replacement and disseminated, orebodies; including orogenic deposits (except the, magmatic-hydrothermal part of the group) and, shale, or turbidite hosted deposits;, . lateritic gold deposits;, . residual placers;, . colluvial, alluvial and coastal placers., ., , Plutonic (intrusion-related) deposits, Plutonic (intrusion-related) deposits (Lang &, Baker 2001) include orthomagmatic and various, magmatic-hydrothermal deposit types, for example the iron oxide-copper-gold deposits (IOCG, for, more detail, cf. Section 2.2.1 “Copper”)., Orthomagmatic origin is illustrated by the discovery of gold in the Triple Group of the Middle, Zone of the Eocene Skaergaard Intrusion in eastern, Greenland (Andersen et al. 1998). Skaergaard is, one of the classical examples of layered mafic, intrusions, with a long history of scientific studies. Platinum had been the target of exploration,, by scanning a large number of Early Tertiary mafic, intrusions and tholeiitic flood basalts. Stream sediment samples exposed striking gold contents, that were traced back to Skaergaard rocks by, lithogeochemical prospecting. The source was, located in a sulphidic layer 2 m thick within, banded gabbroic cumulates. Drilling established, inferred resources of 90 Mt with 1.8 ppm gold, and comparable palladium plus platinum contents, (Andersen et al. 1998)., Magmatic-hydrothermal gold deposits include, quartz veins, stockwork orebodies, breccia pipes,, gold porphyry, skarn and replacement ore. Related, intrusions are mostly of felsic or intermediate, character. The mineralization occurs within the, magmatic bodies (as in Cu-Au porphyries) or in, adjacent country rocks (veins, stockwork and, skarn ore). Apart from gold, the metal assemblage, often includes Bi, W, As, Mo, Te and Sb. Parental
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212, , PART I METALLIFEROUS ORE DEPOSITS, , contained free-milling gold (electrum), pyrite,, minor arsenopyrite, galena and sphalerite, and, quartz with carbonates as a gangue. The breccia, fragments were sericitized and carbonatized. Near, the border of the orebody breccia, higher-grade, sheeted gold quartz veins were developed. Fluids, associated with mineralization included saline, brines, gas and aqueous inclusions with a low salt, content. The latter appear to have controlled gold, transport and precipitation, originating by condensation of magmatic vapour (Baker & Andrew, 1991). The large open-cut mining potential was, only realized when the genetic association of gold, mineralization with the breccia pipe was recognized (Lewis 1990). Resources amounted to 90 Mt, with 2.5 ppm Ag and 1.7 ppm Au. Giant, copper sulphide breccia orebodies with gold as a, by-product are exploited at Olympic Dam in South, Australia, which is the largest of the iron oxidecopper-gold deposits (Groves et al. 2010, cf., “Copper”)., Intrusion-related gold quartz veins may be common, but if there is no visible spatial connection to, a potential igneous parent, recognition and differentiation from metamorphogenic deposits is difficult. In the past, all “mesothermal” gold veins, (formed at intermediate depth and temperature), were thought to have a magmatic-hydrothermal, origin. Mesothermal gold-quartz veins are ubiquitous in plutonic-metamorphic provinces such as, the North American Cordilleras, Brazilian, African, Canadian and Australian greenstone belts,, and the Pan-African belts of Ethiopia, Sudan and, Egypt. Accurate and detailed investigations demonstrate that some of these deposits were in fact, formed from magmatic-hydrothermal fluids, (Baker et al. 2006). A striking common feature is, the association of gold with W, Bi, As and Mo, and, the localization within tin and tungsten ore provinces. A somewhat exotic observation in this, context is that bismuth, or more complex polymetallic low-temperature (200–400 C) melts such, as Bi-Te-S, may scavenge gold from hydrothermal, fluids and give rise to economic gold deposits, (Tooth et al. 2008). Rare minerals such as maldonite (Au2Bi) and a strong positive Au-Bi correlation in gold ore suggest that this process may have, been involved., , Carlin type deposits, The precise origin of “sedimentary rock hosted,, disseminated” gold deposits, which are clearly, epigenetic is still disputed. The economically, most prominent examples occur in Palaeozoic, carbonate rocks near Carlin, Nevada, USA, as, replacement orebodies that were probably formed, in the Eocene (Hofstra & Cline 2000, Hofstra et al., 2003). Gold production, reserves and resources, of the Carlin trend are thought to exceed 3800, tonnes. Hydrothermal “jasperoid” (silicified decarbonated limestone) and anomalous arsenic, characterize alteration. Gold precipitation was, induced by sulphidation when H2S-rich auriferous, fluids reacted with reduced iron in the host rocks., One genetic hypothesis implicates an Eocene, mantle diapir (Oppliger et al. 1997). New dating, suggests that the ores formed while a large plutonic complex was emplaced at depth (40–36 Ma:, Ressel & Henry 2006). Most of the fluids seem to, be meteoric (Henry & Boden 1998) but water and, sulphur (Kesler et al. 2005) in ore-related minerals, have a magmatic (or metamorphic) component., Also, Johnston et al. (2008) provide links between, Carlin deposits and magmatic activity. Pulsed, incursion of magmatic Au-As-Hg-Cu-Te fluids of, high-sulphidation epithermal character is suggested by Barker et al. (2009). Overall, a distal, derivation of the Carlin deposits from calc-alkaline magmatism seems to be the best interpretation (also considered by Sillitoe 2010). Similar, deposits have been discovered in Guizhou, southern China, and were suggested to be of late Yanshanian (Cretaceous) metamorphogenic origin, (Su et al. 2009)., Volcanogenic gold deposits, Volcanogenic gold deposits are located in terrestrial and marine settings. Since Lindgren (1933),, the first are called epithermal gold-silver deposits., This class includes many historic, often extremely, rich ore districts, such as the Thracian gold of, classical Greek and Roman time (essentially in, Bulgaria and Romania). In contrast to the old, workings, today’s epithermal gold mines exploit, rather low-grade but high-tonnage orebodies, predominantly sited in the young Circum-Pacific
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 213, , 2 km, , Hg - Sb, , Depth below surface, , Epizonal Au - Sb, , Intrusion - hosted, orogenic, , 5, , Mesozonal, Au - As - Te, , 10, Figure 2.22 Schematic presentation of, orogenic gold deposits in the “crustal, continuum model”. After Groves, D.I.,, Goldfarb, R.J., Robert, F. & Hart, C.J.R. 2003,, Society of Economic Geologists, Inc., Economic, Geology Vol. 98, Figure 2A, p. 5. The majority of, these deposits (>95%) occur in the mesozonal, regime (Table 1.2) and within greenschistmetamorphic rocks., , mountain belts (e.g. Ladolam, Lihir). Epithermal, gold is infrequently found in Precambrian orogens., One exception is Mahd Ad Dhahab, Saudi Arabia, in Neoproterozoic volcanic arcs. Typically,, epithermal gold occurs in late- to post-orogenic, calc-alkaline andesitic-rhyolitic volcanic fields, with a complex magmatic and structural history, (Simmons et al. 2005). The regional stress field is, marked by substantial crustal extension, as in the, Tertiary Basin and Range Province, USA. Both, stratovolcanoes and calderas with subvolcanic intrusions locate deposits (Milos, Aegaean Sea; Fidji,, Pueblo Viejo: Figure 2.22). Note, however, that, Pueblo Viejo is an unusual high-sulphidation, epithermal deposit insofar as it developed in, a largely submarine, island-arc tholeiitic volcanic, sequence (Kesler et al. 2005)., , Hypozonal, Au - As, , 20 km, , Vein, , Replacement, , Disseminated ore, , General characteristics of epithermal deposits, are provided in Chapter 1.1 “Volcanogenic Ore, Deposits”. Epithermal gold deposits are mainly of, the “alunite” or “high sulphidation type”, but, there are important exceptions (e.g. Lihir)., Epithermal gold is typically associated with the, “volatile” metals and semi-metals Hg, As, Sb, Te, and Tl (Figure 2.23). The world’s largest epithermal gold deposit is said to be Lihir on Niolam (or, Lihir) Island, Papua New Guinea, with resources, of 1300 tonnes of gold in ore >1.5 g/t:, At Pueblo Viejo, the gold-silver ore is characterized by, elevated contents of As and Sb, whereas the more, volatile elements Te and Hg migrated in a gas phase, closer to the palaeosurface. The ore-forming system, consisted of hydrothermal fluids, which interacted
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214, , PART I METALLIFEROUS ORE DEPOSITS, , Palaeo surface, , Hg (Te) vapour halo, Base of, phreatic zone, , Pyrophyllite, Gold ore, (auriferous, pyrite), Massive silica, Kaolinite/dickitequartz-pyrite, , Alunite + quartz + pyrite, , Magmatic, fluids, 500 m, , with episodic plumes of magmatic vapour. Host rocks, are Early Cretaceous sandstones and siltstones with, abundant terrestrial plant and algal bituminous, matter, which fill a maar-like diatreme together with, andesite and quartz porphyry breccias. Oxide ore was, exploited earlier (1975–1996: 166 t Au), whereas refractory sulphide resources totalling 195 Mt at 3.25 g/t Au, wait for reopening of the mine. The deposit was formed, in the Albian (Early Cretaceous) at 109 Ma within a, primitive oceanic island arc (Mueller et al. 2008)., At Lihir 400,000 years BP, a sector of a Pleistocene, stratovolcano comprising lavas and tuffs of trachybasalt, trachyandesite and latite collapsed into the sea., This caused the conversion of a weak Au-porphyry, system into a giant epithermal system hosted by, phreatomagmatic breccias (M€, uller et al. 2002). The, matrix of these breccias contains disseminated auriferous pyrite, which represents most of the gold ore., Two orebodies cover 2 km2 and extend from the, surface down to 400 m below sea level. Rocks are, hydrothermally altered, including argillic (smectite), and advanced argillic facies (alunite), as well as parageneses of quartz, potassium feldspar and pyrite, (illite, anhydrite, calcite). Epithermal veins with, quartz, chalcedony, illite, adularia and pyrite (indicating low sulphidation) contain locally up to 120 g/t of, Au. Fluids had low salinity (5–10 wt.% equivalent, NaCl), temperatures between 150–250 C and were, mainly of magmatic origin, with some admixture of, meteoric and seawater (Gemmell et al. 2004). Until, , Figure 2.23 Schematic reconstruction of, funnel-shaped hydrothermal alterationmineralization at the epithermal high, sulphidation gold-silver deposit at Pueblo, Viejo on Hispaniola Island, Dominican, Republic, hosted in Early Cretaceous, volcanic arc rocks of the Greater Antilles, (modified from Kesler et al. 2003)., Pyrophyllite originated from kaolinite, when the deposit and its host rocks were, later metamorphosed at 300 C and 3 kbar, (Mueller et al. 2008)., , today, fluids similar to those that deposited the ore are, venting in the crater (Simmons & Brown 2006, Figure 2.24). Wells were drilled to >1 km depth in order to, remove hot water before it endangers open pit operations. The inherent geothermal energy is used for, producing all electrical power needed at the mine., , The Inner Carpathian Arc of the Alpine orogenic, belt, mainly in Transylvania (formerly Siebenb€, urgen), Romania, supported several thousand years, of gold mining. The most famous ancient mining, district is the “Golden Quadrilateral” in the, southern Apuseni Mountains with several former, and current mining centres. The plate tectonic, setting was an overall geodynamic regime of, northward subduction, modified by slab detachment and brittle tectonics in the crust (Neubauer, et al. 2005). Orebodies are related to Neogene calcalkaline volcanoes that developed above thrust, sheets of Cretaceous flysch and ophiolites., Epithermal low-sulphidation mineralization is, hosted by volcanic vents, sub-volcanic intrusions, and rarely by sedimentary country rocks. The, volcanic rocks are commonly described as dacites,, but may be better classified as biotite and hornblende bearing “quartz andesites” of the IUGS, classification. Au, Ag, Zn and Pb have mainly, been mined from steeply dipping grey quartz, pyrite veins characterized by open space filling
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216, , PART I METALLIFEROUS ORE DEPOSITS, , Metamorphogenic gold deposits, The formation of metamorphogenic gold deposits, is a consequence of the passage of metamorphic, fluids (cf. Chapter 1.6 “Metamorphogenic Ore, Formation Systems”). Prograde metamorphic, dehydration of volcano-sedimentary rocks takes, place during orogenic metamorphism and deep, magmatic activity. The generalized model, (Kerrich 1999) describes extraction of gold from, rocks as an effect of the exudation of crystal water, from OH-bearing minerals, at 400–500 C, (3.5–5 kbar) for the greenschist-amphibolite facies, transition. In this case, metabasalt yields 5%, H2O, which follows the pressure gradient to lower, P/T domains. The intimate fluid-rock interaction, favours dissolution of trace metals. In the case of, high H2S activity in the fluid, iron and base metals, are nearly insoluble so that gold is relatively enriched. The model is confirmed by studies in New, Zealand, where ore-forming elements (Au, Ag, As,, Sb, Hg, Mo, W) are depleted in higher-grade rocks, relative to unmetamorphosed protolith samples., The same elements are enriched in the island’s, orogenic gold deposits (Pitcairn et al. 2006)., Crustal-scale faults (Willman et al. 2010) and, shear zones (Weinberg et al. 2004) take up the, diffuse fluid flow and discharge into fluid escape, zones which are often tectonic highs. Even at low, gold concentrations, the giant mass of fluids, moves a considerable mass of gold. Falling temperature (300–400 C) and pressure, transition, from ductile to brittle mechanical behaviour of, rocks and chemical reactions with wall rocks, precipitate the gold (Figure 1.85)., Metamorphogenic gold deposits commonly, occur in the form of quartz veins and vein stockworks within greenschist facies rocks, but replacement or disseminated orebodies are known, (Figure 2.22). The hydrothermal-epigenetic origin, is revealed by hydrothermal alteration of host, rocks (Campbell-McCuaig & Kerrich 1998) and by, isotopic anomalies. Induced nitrogen and certain, nitrogen isotope ratios, for example, attest to, a distal metamorphic derivation of fluids (Jia &, Kerrich 1999, Jia et al. 2003)., Many gold deposits in metamorphic terranes, postdate the peak of deformation and meta-, , morphism. This implies i) persisting deep metamorphism while the surface cools, ii) ponding, of metamorphic fluids at depth, or iii) retrograde-metamorphogenic formation with pervasive flooding of hot rock volumes by outside, fluids during uplift, distension and shearing (Figure 1.86; Templeton et al. 1997). Apart from retrograde reactions, an oxidation front caused by, infiltrating meteoric water may mobilize gold and, concentrate the metal in hydraulically favourable, tectonic structures (Craw & Chamberlain 1996)., Although most Archaean gold deposits occur in, greenschist facies rocks, some provinces display a, spread of host rocks from granulite facies to very, low metamorphic grade. In Western Australia,, hydrothermal alteration changes parallel to metamorphic P/T-conditions of host rocks. Groves, (1993) concluded from this observation, that, hydrothermal fluid flow systems affected the, whole crustal cross-section. This is the essence, of the “crustal continuum model”. Because metamorphism and melting affect the crust synchronously, both metamorphic and granitoid-derived, magmatic fluids may produce gold concentrations, which have been subsumed as orogenic gold, deposits (Figure 2.22; Goldfarb et al. 2005, Groves, et al. 2003, Groves 1993, Goldfarb 2002). The term, avoids purposely a process-related genetic classification in order to stress the coherence of the, metallogenetic system., The majority of gold deposits in Archaean and, Palaeoproterozoic greenstone belts are genetically linked to orogenic metamorphism, magmatic activity and major, often crustal-scale, shear zones and are prototypes of orogenic gold, deposits. Greenstone belts are elongate narrow, structures crossing Early Archaean gneiss and, granulite terranes, built from felsic to ultramafic, igneous rocks, volcaniclastic, siliciclastic and, chemical sediments, all intruded by granites. If, a generalization of greenstone belt architecture is, at all possible, thick suites of primitive ultramafic and mafic volcanic rocks form the base,, overlain by intermediate and felsic volcanics, in, their turn covered by flysch and molasse-like, sediments. Metamorphic grade is usually, greenschist to amphibolite facies. Deformation, varies from simple sag-basins to folded and
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , overthrusted assemblages. In spite of lingering, doubts, there is now a broad consensus that, greenstone belts are the product of seafloor, spreading followed by subduction-accretion processes in island arcs (Windley 1995). Greenstone, belts are distinguished by nickel-sulphide deposits in komatiites, volcanogenic banded iron ores, of the Algoma type, massive base metal-sulphide, deposits and economically outstanding gold, deposits:, Many greenstone belts host gold quartz vein and, stockwork deposits. With a historic production of, 1800 t and reserves plus resources of 500 t gold,, the Golden Mile at Kalgoorlie in Western Australia is, among the world’s largest. Orebodies extend over, 4000 m in length and to a depth of >1500 m. Host, rocks are greenschist-metamorphic basalts, which, have been deformed by folding and wrench-faulting., Brittle-ductile shear zones control more than 1000, single veins, which together constitute the Golden, Mile deposit. A wide halo of hydrothermal chloritecalcite alteration is developed. Ore is composed of, vein quartz and hydrothermally altered metadolerites, and pillow lavas. Orebodies consist of breccia and, cavity-fill veins, surrounded by a sericite-ankeritesiderite-quartz-haematite-pyrite-telluride alteration, zone, which contains most of the gold. One-third of, gold is native gold in quartz; nearly one-half forms, tiny rounded inclusions in pyrite (with a diameter in, the mm-range) and 25% occurs in tellurides. Geochemically anomalous trace metals include As, Hg,, W, B, Sb, Pb and Zn. Fluid inclusions in quartz display, CO2 content and low salinity; formation temperatures were 300–400 C at pressures of 1–2 kbar. Gold, was probably transported in sulphide complexes and, co-precipitated with pyrite by reaction with reduced, iron in the basalts. The ultimate source of the gold, remains uncertain. Some data suggest an origin, from deeply buried komatiites and their syngenetic, exhalites. The dominantly vertical structures of the, Golden Mile imply vertical upflow of the hydrothermal fluids. Based on age dating, McNaughton et al., (2005) contemplate a convergent plate tectonic setting of the Golden Mile (and most gold in the Eastern, Yilgarn craton) at an active continental margin. The, hydrothermal systems would have been generated, during the change from subduction to wide-spread, crustal melting. Vielreicher et al. (2010) demonstrate, that gold formation took place at 2.64 Ga, late in, orogenesis and overlapped with waning stages of, , 217, , regional metamorphism. After more than a century, of selective underground mining, the deposit is now, worked as a large open pit (Figure/Plate 2.25)., , The important role of ultramafic volcanic rocks, as a source of gold in greenstone belts was, highlighted by Keays (1995). Until eruption, these, magmas remain undersaturated in respect of sulphur, so that a sulphide melt cannot form. This is, contrasted by basaltic systems that more easily, segregate sulphide melts that concentrate chalcophile and siderophile trace metals such as gold., Because of their high density, sulphide melts,, however, are commonly left behind at depth (cf., Chapters 1.1 “Orthomagmatic Ore Formation”;, Section 2.1.4 “Nickel”). Komatiites, picrites and, boninites exsolve sulphides only when approaching the solidus. The sulphides scavenge traces of, Au, Pd, Pt, Ag, Cu, Ni, Zn, Se, Te and some Pb, Mo,, Sn, W and Bi. In contrast to silicates and oxides,, sulphides are easily dissolved by pervading hot, fluids, so that small fluid volumes (e.g. those, liberated by metamorphic dehydration) may form, relatively concentrated ore fluids., Ultramafic rocks of greenstone belts and ophiolites are also frequent host rocks of gold ore deposits, (Barberton Mountain Land, South Africa, Barramiya, Egypt, Bou Azzer, Morocco). Notable carbon, dioxide and potassium addition characterizes, hydrothermal alteration in gold deposits where, former peridotites are changed into Mg-Fe-Ca carbonate rocks with fuchsite (Cr-muscovite) and, quartz. Accessory minerals include talc, serpentine, chlorite, haematite, magnetite, pyrite and, residual chromite. These alteration rocks are called, listwaenite (Halls & Zhao 1995), and may contain, gold in quartz veins and pyrite halos. In several, locations, they are also sites of Ag, Hg, As, Co and, Ni mineralization. Mineralizing fluids are not, necessarily metamorphic but this is not infrequent., Metamorphogenic shale, or turbidite, hosted gold deposits, Metamorphogenic shale, or turbidite hosted gold, deposits are best illustrated by referring to Bendigo, and Ballarat, once fabulously rich gold districts in, Victoria, Australia (Phillips & Hughes 1996):
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218, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 2.25 (Plate 2.25) The Golden Mile Superpit at Kalgoorlie, Western Australia (2006). Note supergene oxidation, in the foreground, the city of Kalgoorlie-Boulder on the left and tailings (white) in the right-hand background. Photo, provided courtesy Kalgoorlie Consolidated Gold Mines (KCGM)., , Host rocks are Cambrian to Early Devonian turbidites of the Lachlan orogen deposited in deep submarine fans. Shales and greywackes are part of an, accretionary wedge above the west-dipping subduction zone along the Pacific Gondwana margin, (Fig. 1.84; Hough et al. 2007). Deformation progressed, from west to east between 455-390 Ma. Concurrent, metamorphism liberated fluids from deeply buried, Cambrian oceanic and arc-related mafic volcanic, crust. Fluid flow was channeled laterally for considerable distances by crustal-scale faults (Willman et al., 2010). Deposits consist of quartz veins and concentrate in anticlinoria which served as vertical fluid, escape zones., Early veins form an interconnected fracture mesh, controlled by folds, bedding planes, cleavage and, reverse faults and are partly deformed. Most gold, mineralization was synchronous with peak defor-, , mation and metamorphism but extended into, later tensional strain (Jia et al. 2000). Fluids were, CO2-rich, of low salinity and had temperatures of, 135–360 C. The paragenesis comprises free gold,, pyrite, arsenopyrite and pyrrhotite in quartz with, ankerite and albite. Host rock alteration is macroscopically visible because of proximal bleaching, and a wide halo of siderite spots; microscopic features include the introduction of sericite, chlorite,, carbonates and pyrite-arsenopyrite (Bierlein et al., 2000). High-grade ore shoots occur at the contact, with pyritic and graphitic beds (“indicator beds”), and lumps of native gold were found that reached, 18.8 kg. Since 1851, Victoria State produced a total, of 2500 t of gold of which 40% were derived from, quartz veins. The larger part of the total was extracted from placer deposits that included some, famously rich bonanzas. The heaviest nugget, weighed 71 kg.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Most of the rich metamorphogenic and orogenic, gold provinces are restricted to three periods, of Earth history: Neoarchaean (2.8–2.5 Ga),, Palaeoproterozoic (2.1–1.8 Ga) and Neoproterozoic (600 Ma) to Early Tertiary (40 Ma). These, timespans include the formation of the first cratons and supercontinents, and of orogenesis,, accretion and collision (Kerrich & Cassidy, 1994). Among the youngest orogenic gold deposits, are Miocene quartz veins in the lowermost (Penninic) tectonic unit of the Alpine orogen in Europe, (Pettke et al. 1999)., Alluvial gold placers, Recent and fossil alluvial gold placers are, in, roughly equal shares, the source of two-thirds of, cumulative world gold production. Recent placer, districts were the scenes of the epic gold rushes of, the 19th century (California; Alaska: Craig &, Rimstidt 1998; Yukon, Canada: Lowey 2006; Australia), but today this deposit type has a limited, economic significance (Columbia, Mongolia)., Among fossil gold placers worldwide, the, Neoarchaean Witwatersrand Basin in South Africa, is unique: Since its discovery in 1884, 50,000, tonnes of gold were produced. Today, after a long, but slow decline the basin delivers each year, 300 t gold, 3000 t uranium and 1000 kg osmiridium. Mining is exclusively underground (except, for the reworking of tailings) and at depths approaching 4000 m. Average gold content in ore, amounts to 5 g/t. Remaining gold in place is estimated at 800 Moz. The volumes edited by Anh€ausser & Maske (1986) are an important source of, detailed descriptions of the Witwatersrand gold, district and its deposits:, The Witwatersrand Basin is built of three major, volcano-sedimentary successions known as the Witwatersrand Triad that were deposited between 3.1, and 2.6 Ga. The oldest rocks are quartzites, basic and, felsic lavas of the Dominion Group, followed by the, Witwatersrand Supergroup starting with unmineralized sediments, lavas and banded iron formations, (West Rand Group), which are overlain by alluvial, fans (the gold-rich Central Rand Group: Figure 1.65)., Youngest rocks are the bimodal volcanics of the, Ventersdorp Supergroup; in sediments near its base, , 219, , above a tectonic unconformity, several horizons of, gold-bearing sediments are exploited. Ventersdorp, lavas have an age near 2700 Ma and their feeder dykes, cut across the gold reefs (Meier et al. 2009). The, Witwatersrand Basin is a half-graben with marginal, faults in the north that were active during late, Witwatersrand and in early Ventersdorp time. Later, deformation was caused by the collision between the, Transvaal and the Zimbabwe cratons. Several phases, of metamorphism did not surpass lower greenschist, facies conditions (350 C and 2–3 kbar), in spite of a, total thickness of 20 km. Thermal metamorphism, was induced by the emplacement of the anorogenic, Bushveld Complex (2.06 Ga), but not in all gold fields, (Rasmussen et al. 2007). At 2.02 Ga, the whole basin, was affected by the Vredefort extraterrestrial impact, (Hayward et al. 2003) that caused widespread brecciation. Parts of the basin are covered by Karroo (PermoMesozoic) platform sediments., Witwatersrand gold is mainly hosted by quartzitic, conglomerates of a certain sedimentary facies:, Highest contents are found in polymictic bouldery, gravels of intermediate grain size building braided, channels of former delta fans (Figure 1.65). Both landward coarser gravels and basinward finer sediments, contain little gold. The thickness of auriferous beds, (“reefs”) varies from <0.2 to 4 m. Host rocks are quartzites, schists and little mineralized conglomerates., Many of the auriferous quartz-pebble reefs display, visible contents of kerogen and bitumen (“carbon seam, reefs”, Figure/Plate 2.26a). Geological observations, such as truncation by palaeochannels confirm that the, organic substance was formed in situ by microbial, mats (Mossman et al. 2008). The organic substance, trapped uranium and thorium (“thucholite”)., Essentially, gold occurs in the matrix of the conglomerates. Striking are rounded pebbles of pyrite with, inclusions of silicates and traces of Au, Ag, Ni, Cu,, Pb and Mo. There is also epigenetic hydrothermal, pyrite that owes its origin to diagenesis or metamorphism. Native gold predominates in the ore, in the, form of detrital grains (50–100 mm diameter) that contain 10% silver and 2% mercury, and have a rhrenium/osmium age of 3030 Ma (Kirk et al. 2002)., Locally, gold is present in thucholite (Figure/Plate, 2.26a) and in diagenetic or metamorphic mobilizates, (Frimmel & Gartz 1997). Exceptionally, hydrothermal, quartz veinlets cutting Witwatersrand rocks are auriferous. Detrital rounded uraninite grains with a diameter of 75–100 mm contain disseminated inclusions of
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220, , PART I METALLIFEROUS ORE DEPOSITS, , radiogenic galena (Figure/Plate 2.26b). During metamorphism, some of the uraninite reacted with fluids to, form brannerite. Carbon seam reefs may be high-grade, sources of gold and uranium. Other heavy minerals in, Witwatersrand placers include arsenopyrite, chromite, cobaltite, zircon, chloritized garnet and native, osmiridium. The absence of magnetite and ilmenite is, not fully understood., , Figure 2.26 a (Plate 2.26a) Overview of high-grade, gold ore in carbon-rich reef at Elsburg Mine,, Witwatersrand, S.A. (plane-polarized reflected light)., Courtesy B. Lehmann, Clausthal. The matrix between, quartz pebbles (not visible) consists of clastic pyrite, (light grey with fissures), pyritized claystone pebbles, (top right margin) and clastic uraninite (medium grey, with specks of radiogenic lead in galena, white) in, thucholite matrix (pyrobitumen, wavy dark grey and, black). Gold (yellow) in intragranular pore space is, mostly remobilized., , Figure 2.26b (Plate 2.26b) Detailed image of highgrade gold ore in carbon-rich reef at Elsburg Mine,, Witwatersrand (polarized reflected light, oblique, nicols, oil immersion). Courtesy B. Lehmann,, Clausthal. Pyrite (light grey with fissures), gold, (yellow, intergranular), uraninite (medium grey, speckled with galena) in thucholite (pyrobitumen,, wavy dark grey and black)., , The scientific consensus on the origin of the, Witwatersrand gold reefs is the “modified placer, model”, as described above. Although some scientists favour hydrothermal-epigenetic, possibly metamorphogenic, formation, processes, (the, “hydrothermal model”, Law & Phillips 2005), geological observations (Meier et al. 2009) and rhenium-osmium dating (Kirk et al. 2001) refute this, hypothesis. The ultimate source of the giant mass, of gold is not known. Most probably, the Witwatersrand palaeorivers eroded older greenstone belts., Hydrothermally altered granites and felsic volcanics were present in the catchment area (Robb &, Meyer 1995) and may have been a source of gold., Predominant pebbles in the reefs (quartz, quartzite,, black schists and chert) provide no clear hint at a, specific source. Questions have been raised concerning the popular hypothesis that river transport, of pyrite and uraninite would only have been possible under a low-oxygen atmosphere. Falconer, et al. (2005) described many textures that are similar to the Witwatersrand deposits, including sedimentary and diagenetic pyrite in Tertiary quartz, pebble conglomerate placers of New Zealand., Exploration for gold is exceptional because even, today, prospectors and small companies may, with, luck, find a new viable deposit. Programmes are, based on geological concepts (e.g. structural and, lithological controls, metamorphic gradient, etc.), and involve available mineralogical, geochemical, and geophysical methods. Specific problems arise, during sampling and analysis because of three, points:, 1 In ore, gold is nearly always irregularly distributed (the “nugget effect”) and “clustering” may, cause an apparent nugget effect (Dominy & Platten 2007);, 2 because of the metal’s softness and ductility,, homogeneous comminution of gold particles is, quite difficult; and
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 3 gold is much heavier than silicate gangue, resulting in rapid gravity separation, for example, within sample bags during shipping., It is therefore extremely important in every, individual project to experiment and work out a, procedure that minimizes resulting errors (cf., Chapter 5 “Geological Mapping and Sampling”)., Reverse circulation drilling methods provide good, recovery of reliable samples, but geological interpretation still needs diamond-drilled core. All, sampling and analytical procedures must be, checked by well-chosen double and triple samples., For gold, time-tested fire assays and INAA (Instrumental Neutron Activation Analysis) are analytical standard methods. The newly adapted gammaactivation analysis (CSIRO, Australia) provides, equal precision and accuracy, but accepts samples, weighing several hundred grams, which avoids, errors induced by subsampling. Precautions must, be taken to avert criminal manipulation of samples, such as “salting” with gold dust or chloride, solution., Geochemical exploration methods for gold are, founded in the concept of pathfinder elements., This dates from times when gold analyses were, time-consuming and costly, but is still justified, by the three problems mentioned above. Compared to gold, pathfinder elements such as Ag, As,, Bi and Sb have a considerably less erratic distribution, form larger halos and occur in higher, concentrations. They can even be used to evaluate the gold potential of large tectonic-stratigraphic terranes (Plant et al. 1997). In specific, gold districts, different elements may be identified as useful pathfinders by pilot projects. Knight, et al. (1999) demonstrated that Au-Ag-Hg-Cu, contents of alluvial gold allow correlation with, known primary sources of the particles. Particles, from unknown sources may suggest a search for, undiscovered deposits., In 2009, the largest gold-producing countries, were China, Australia, South Africa, USA,, Russia, Peru, Canada and Indonesia. Quite the, opposite of other metals, gold mine production, rose in the crisis from 2260 t (2008) to 2350 t, (2009). The reserves/production ratio of only 20, years characterizes the lively business of gold, mining but is no reason to expect “the end of gold”., , 221, , 2.3.2 Silver, Common Ore Minerals:, Max. wt.%, Ag, Native silver, Acanthite, Proustite, Pyrargyrite, Stephanite, Polybasite, Chlorargyrite, Freibergite, (silver-rich, variety of, tetrahedrite), , Ag(Au,Cu,Hg,, As,Sb,Bi), Ag2S, Ag3AsS3, Ag3SbS3, Ag5SbS4, (Ag,Cu)16Sb2S11, AgCl, Cu6(Ag,Fe)6Sb4S13, , 100, 87, 65, 60, 68, 74, 75, 18, , Density, (g/cm3), 10.5, 7.2, 5.6, 5.8, 6.2, 6.1, 5.5, 4.6–5.2, , Telluride minerals of silver including hessite, (Ag2Te) and empressite (AgTe) are not a relevant, source of the metal, although they are not rare in, epithermal and Precambrian gold deposits. Chlorargyrite is important in supergene silver ore; contrary to other salts, silver halides are highly, insoluble in aqueous solutions. Carbonates and, barite are the typical gangue minerals of silver ore,, not quartz as in gold deposits. Silver-only mines, commonly exploit ore with a grade >450 g/t. However, 80 % of the metal’s total mine production is, silver as a by-product, mainly from copper, lead,, zinc and gold ores. Silver contents in galena, sphalerite, pyrite, chalcopyrite and tetrahedrite may be, sited in the crystal lattice of the host mineral or, form mineral inclusions. Silver as a by-product, can be recovered at grades >50 g/t. The economic, viability of many base metal and gold deposits, relies on by-product silver., Use of silver is mainly in industrial applications, but nearly 40% is consumed in jewellery, coins, and silverware. The traditional photographic, sector (silver halides in film and photographic, paper) continues to decline. Like other native, metals in nature, silver was one of the earliest, treasures of humans. Pure silver metal has a density of 10.49 g/cm3 and melts at 960.8 C. Its properties include strength, malleability, ductility,, electrical and thermal conductivity, sensitivity, to, and high reflectance of light and the ability to
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222, , PART I METALLIFEROUS ORE DEPOSITS, , endure extreme temperature ranges. Silver is part, of fabricated goods such as electronics, plasma, display panels, rechargeable and disposable batteries (the “button” style, which have up to 35%, silver by weight), rechargeable Ag-Zn batteries,, electroplating metal for steel bearings, solders for, water pipes, catalysts for production of ethylene, oxide (the starting-substance for plastics), electrical appliances, computers and solar energy mirrors, for electricity production. Medical applications, are based on the high toxicity of silver ions to, microorganisms., , At the base of the oxidized zone of silver deposits,, downward seepage typically precipitates native, silver (eq. 1.13), whereas silver sulphide enrichment (neoformation of acanthite similar to chalcocite in copper ore deposits) is rare. Chlorargyrite, is more common in arid lands where evaporation, tends to concentrate halogens near the surface, (e.g. in the giant Pb-Zn-Ag deposit at Broken Hill,, South Australia). Organic substances effectively, immobilize silver by chelation, resulting, in enrichment of humic soil or coal near outcropping ore., , Geochemistry, , Silver ore deposit types, , Silver is a chalcophile element (Goldschmidt, 1958). The crustal abundance of silver is estimated, to range from 0.02 to 0.1 ppm (Smith & Huyck, 1999). Among sediments, black shales, phosphorites and oil shales have the highest contents., Magmatic differentiation has no discernable influence on concentrations, although basalts display, higher contents than granites (0.1 compared to, 0.04 ppm). This is explained by differing sulphide, contents. Similar to gold, silver in hydrothermal, solutions rarely occurs as a simple monovalent, Agþ ion. Sulphide and chloride complexes are, more common (Stefansson & Seward 2003). In, magmatic and epithermal systems, aqueous, vapour may transport AgCl in concentrations relevant for ore deposit formation (Migdisov et al., 1999). Geothermal waters in the Salton Sea field, in Southern California contain 0.8–2 ppm silver,, and sulphide scales in the steam production boreholes concentrate silver to a maximum of 7%. Ore, mud in the Atlantis-II-Deep of the Red Sea has an, average of 18 ppm Ag and 0.3 ppm Au. It is believed, that the subduction of ocean-floor metalliferous, sediments has a major role in the metallogeny of, convergent plate boundaries (Figure 1.88)., Silver is not redox-sensitive and displays oxidation states Agþ and Ag0. In the near-surface environment, silver is scarcely mobile except under, very acidic conditions (pH < 3: Smith & Huyck, 1999). Sulphide oxidation mobilizes silver in the, presence of ferric sulphate solutions. Silver is also, mobile in dilute nitrate solutions. Upon encountering chloride ions, chlorargyrite is formed., , Most primary silver ore deposits are hydrothermal, and epigenetic (Graybeal et al. 1986). All of the, hydrothermal realms (magmatic, metamorphic,, sea-floor exhalations, diagenetic) are potential, sources of silver, commonly at the by-product level., However, most large silver deposits are magmatic, and occur in the Cordilleras of North and South, America. This is believed to be the result of the, singular longevity of the convergent plate boundary, of the eastern Pacific rim. Supergene, secondary, silver ore deposits are the result of local enrichment, by descending solutions, or by groundwater in arid, regions that may have migrated over considerable, distances before forming “red bed” or similar infiltration deposits of silver., By-product sources of silver include many of the, vein mining districts in Europe, where silver was, the original target of operations in the Middle, Ages. Examples are the northern Alps (Tyrol),, Kongsberg (Norway), Erzgebirge and Harz, (Germany) with argentiferous veins of lead-zinc, and of the Bi-Co-Ni “formation” (cf. Section 2.1.5, “Cobalt”, Section 2.5.12 “Uranium”). In the 17th, century, many of these vein-based mines were, abandoned when large amounts of silver from, newly discovered America flooded Europe. A common genetic signature of the vein districts north, of the Alps is their formation from hydrothermal, convection systems related to extensional tectonic deformation. Today, by-product silver is, recovered from porphyry copper ore, polymetallic, base metal deposits, the Copper Shale in Poland, and, of course, from nearly all gold mining
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , operations. Many volcanogenic and sedex massive, sulphide deposits yield by-product silver (e.g. Cannington in Australia that may at present be the, largest single source of silver in the world; Walters, & Bailey 1998). Silver as an economically significant ore constituent occurs in the following, genetic classes:, . skarn, contact-metasomatic and replacement, ore deposits;, . volcanogenic polymetallic massive sulphides;, . volcanogenic epithermal veins, stockworks, disseminated ore and breccias;, . sedex polymetallic massive sulphides;, . metamorphogenic veins;, . infiltration deposits (“red bed type”);, . supergene enrichment deposits., Skarn and contact-metasomatic ore deposits, High-temperature skarn and contact-metasomatic ore deposits of silver (typically with lead, and zinc) in carbonate rocks are economically, prominent. In contrast to this group, low-temperature diagenetic-hydrothermal lead-zinc deposits, (e.g. Mississippi Valley and Alpine Type) rarely, have noteworthy silver. Silver-rich deposits occur, in the contact zone of intermediate to felsic intrusions. Large silver skarn ore provinces occur in, the American Cordilleras and in China. In southern North America, Palaeozoic and Mesozoic carbonate rocks are intruded by fertile Tertiary, plutons. Ores contain silver, lead, zinc and pyrite,, with minor amounts of Mn, Cu, As, Sb and Au., Gangue is dominated by calcite, quartz and dolomite. Barite, siderite, fluorite, rhodonite, rhodochrosite and the characteristic “jasperoid” are less, ubiquitous. Jasperoid is hydrothermally silicified, limestone (see also Section 2.3.1 “Gold”) that, occurs distal to parent intrusion contacts, associated with dolomitization. Proximal carbonate, alteration is characterized by skarn paragenesis, and recrystallization. Silver is often concentrated, in tetrahedrite and tennantite [(Cu,Ag,Zn,, Fe)12(As,Sb)4(S,Se)13] and associated with galena., Orebodies are variously veins, stratabound, replacement flats (mantos) and cross-cutting pipes, (chimneys). Main control of ore is elevated palaeopermeability of the carbonate rock bodies, due to, , 223, , lithology, tectonics or hydrothermal karst. Impermeable roof rocks may have focused the aqueous, magmatic-hydrothermal fluids. Fluid inclusions, imply formation temperatures reaching 500 C in, the anhydrous phase of skarn formation (Baker, et al. 2004). Water is of magmatic origin., In China, silver-rich vein, skarn and replacement deposits occur in contact zones of Yanshanian Mesozoic granites that intruded Neoproterozoic and Early Palaeozoic sediments. Silver, content of ore reaches 200 g/t in addition to byproduct amounts of tin, tungsten, copper, lead,, zinc, antimony and bismuth (Wu et al. 1993):, One of the largest skarn deposits in China is Bajiazi., Total reserves are estimated to comprise 220,000 t Zn,, 150,000 t Pb and 1647 t Ag, in ore grading 186 g/t Ag, (Zhao et al. 2003). Orebodies occur near the contact of, a Jurassic quartz-monzodiorite with Proterozoic dolomites, following a NW-striking fault zone that channelled fluids (Figure 2.27). Metasomatic veins and, irregular replacement masses occur along a distance, of 3.4 km (Figure 2.28). Skarn zones from the intrusive, contact outwards include a proximal Mg-facies with, magnetite and molybdenite, followed by Mn-Mgskarn with sulphides of Fe, Cu, Pb, Zn and Ag, and, by distal Mn-skarn with Ag, Pb and Zn. This succession reflects reaction of magmatic fluids with the, dolomite and temperatures falling from >500 to 340, , C. Lead isotope data imply that base metals and, probably silver, too, are derived from deep lower crust., , Epithermal vein silver ore deposits, Volcanogenic veins, stockworks and hydrothermal breccias include mainly epithermal vein silver ore deposits with some of the largest silver, producers known. Like epithermal gold deposits, they are associated with Tertiary volcanic belts,, including subvolcanic intrusions. Prominent examples are Guanajuato in Mexico, and the upper, part of Cerro Rico de Potosı in Bolivia:, Since 1548 when Spaniards arrived, Guanajuato, yielded 35,000 t silver and 175 t gold. Most of this, was extracted from one giant vein (Veta Madre) that, has a mineralized strike length of 16 km and reaches a, thickness of tens of metres and a depth of 2000 m, below surface. The structure occupies the marginal, fault of an Eocene-Oligocene rift that is filled with, coarse clastic sediments and a thick volcanic pile of
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224, , PART I METALLIFEROUS ORE DEPOSITS, , Mesoproterozoic, dolomite, , Ag-Pb-Zn, , N, , Mn-skarn, , Quartzdiorite, , S (Cu), Mn-Mg, , skarn, , Pb-Zn-S, (Ag), , Magnetite, Fe (S), , Ι, , Mg-skarn, , Granite, 1 km, , intermediate and felsic rocks. Subvolcanic intrusions, are assumed at depth. The fill of Veta Madre consists, of quartz, calcite, galena, sphalerite and chalcopyrite,, with native silver, silver sulphosalts and selenides., The hydrothermal alteration of host rocks comprises, K-metasomatism (adularia), sericitization, kaolinization and propylitization, hinting at an affinity to, porphyry copper ore systems. Already the first geological visitors of the Guanajuato mining district, classified it as an epithermal deposit. Today, it is one, of the prominent members of the “low sulphidation”, subclass with formation temperatures of 250–300 C, (Mango et al. 1991)., Famous Cerro Rico de Potosı is an extinct Neogene, volcano built of tuff and explosion breccias that mantle a dacite porphyry core (Cunningham et al. 1996;, Figure 2.29, Figure/Plate 2.30). It is built on Ordovician shales. Hydrothermally altered volcanic and, basement rocks host numerous ore veins. The past, , Figure 2.27 Geological zonation map of, Ag-Pb-Zn skarn in the contact zone of the, Jurassic quartz monzodiorite intrusion at, Bajiazi, China; note alignment controlled by, a fault zone (modified from Zhao et al. with, permission from Elsevier). Dotted lines, delimit different skarn zones: Proximal, magnetite is followed by Fe(S) ¼ magnetitepyrite-pyrrhotite, S(Cu) ¼ sulphides of Cu-FeZn-Pb-Ag, and distal Pb-Zn with silver, contents increasing to the Northwest., , production from the mountain is estimated at, between 30,000 and 60,000 t of silver, and the same, metal mass is thought to exist in remaining resources, grading 100 g/t Ag and 0.10–0.17% Sn. Potosı is, probably the largest silver deposit on Earth. Apart, from silver, considerable tonnages of by-product, metals tin and zinc have been extracted, with minor, copper and lead. A primary hydrothermal zonation is, clearly developed, arranged around a high-temperature centre with cassiterite, wolframite, bismuthinite and arsenopyrite. This passes into a lower, temperature paragenesis of sphalerite, galena and, silver sulphosalts. Primary silver minerals are mainly, freibergite and acanthite. However, oxidation has, affected the uppermost 300 m of the mountain, where, a secondary supergene paragenesis developed. Hydrothermal alteration in the centre is characterized by, quartz and tourmaline, followed upwards by strong, argillization and a vuggy quartz cap. This is characteristic for extremely acidic hydrothermal alteration
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226, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 2.30 (Plate 2.30) Cerro Rico de Potosı, Central Cordillera, Bolivia, contained the world’s largest pre-mining, silver concentration. Courtesy B. Lehmann, Clausthal. Visible are ubiquitous traces of vein mining on the slopes, and the oxidized cap of the Miocene volcano. During the 17th century, Potosı was the principle source of fabulous, wealth for Spain and of silver inflation in Europe., of the high-sulphidation type of epithermal ore formation (Sillitoe et al. 1998). The vertical extent of, mining reaches 1150 m. Conglomerates, breccias,, and slide masses (pallacos) at the foot of the mountain, are large potential silver orebodies (Bartos 2000)., In 2008, the first pallaco mine went into operation., , Cerro Rico de Potosı is a revealing example of, the evolution of a giant epithermal deposit from a, subjacent porphyry. Stable isotope characteristics of water in the mineralizing fluids are predominantly magmatic (Cunningham et al. 1996)., The deposit illustrates the vertical metal zonation that is characteristic for the Bolivian silvertin province. The deepest mineralization carries, tin, tungsten and bismuth, and at intermediate, levels appear copper, zinc and tin (e.g. San Rafael,, Figure 2.19). The uppermost levels are rich in, silver and antimony, but little tin (Sillitoe, et al. 1998)., , Epithermal silver of a different setting occurs, disseminated in felsic volcanic rocks in stratabound and stratiform orebodies within permeable volcanic tuffs and interbedded clastic, sediments. Deposits are controlled by calderas, or subvolcanic intrusions. Ore grade is low but, large-scale open pit mining allows profitable, extraction. Silver is associated with sulphides, of Fe, Zn, Pb and variable Cu, Au, As, Sb and, Mn. Hydrothermal gangue in pore space or, replacing host rock minerals includes pyrite,, quartz, opal and minor amounts of sericite,, carbonates and adularia. Decreasing base metal, contents correlate with increasing gold relative, to silver and with higher formation temperatures. Because of their apparent insignificance,, disseminated silver (and gold) ores are a more, recent discovery in the vicinity of former highgrade mines.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , It is both a scientifically and practically interesting problem to understand why epithermal, silver and gold deposits tend to either gold or, silver-dominant end members, with few cases of, more balanced contents. Tentative explanations, include the characteristic vertical zonation with, gold ore near the surface, passing into silver and, base metal ore beneath. However, most large, epithermal gold deposits do not have a deeper, silver and base metal zone. There must be more, fundamental metallogenic reasons, such as varying gold and silver content in the source rocks, or, different chemical evolution paths. Redox control, may be one critical factor. Bolivia’s silver-tin deposits are related to felsic rocks of the ilmenite, type formed from reduced melt implying H2S-rich, magmatic fluids. By contrast, gold porphyries, and associated epithermal deposits are oxidized,, magnetite type and segregate a volatile phase that, is dominated by SO4., , Metamorphogenic vein deposits, Metamorphogenic vein deposits of silver are surprisingly rare compared to gold deposits of this, class (although, of course, metamorphogenic gold, always contains some silver). However, there is, one significant example that may encourage, search for this type. In the Coeur d’Alene district, (Idaho), several large silver veins are known in, an area of 50 by 20 km:, In addition to silver, Pb, Au, Zn and Cu were also, produced at Coeur d’Alene. Host rocks of exploitable, veins are low-grade metamorphic, clastic sediments, of the Mesoproterozoic Belt Supergroup that were, intruded by Cretaceous monzonites. Furthermore,, major strike-slip faults cut through the region, which, have been related to the origin of the vein structures., Ore formation is controlled by late orogenic reverse, faults formed in greenschist facies conditions (Wavra, et al. 1994). Vein ores appear mainly in competent, quartzites and siltstones (Mauk & White 2004)., Although veins are thin, the high-grade ore allows, exploitation to >2400 m below the surface. The veins, are unusually persistent; one, the Morning Star, was, followed for 1300 m along strike and down-dip for, 2300 m. Main ore minerals are galena, sphalerite,, , 227, , silver-rich tetrahedrite, chalcopyrite, pyrrhotite and, arsenopyrite, in a gangue of siderite and quartz. Vertical and horizontal zonation is obscure. Formation, conditions were 350 C and 1–3 kbar. Even modern, methods have not given a clear understanding of age, relations. Apparently, part of the vein fill (Zn, Pb) is of, Proterozoic age (ca. 1 Ga), whereas silver, antimony, and gold were introduced between the Late Cretaceous and Early Tertiary (Eaton et al. 1995, Leach, et al. 1998, Mauk & White 2004). Both phases of, mineralization are believed to have been metamorphogenic, with the latter possibly influenced by, a thermal gradient induced by Late Cretaceous, (Laramic) intrusions., , Infiltration, or red-bed silver ore deposits, In spite of silver’s low solubility in the surface, environment, infiltration deposits of silver are, not insignificant. Because metalliferous meteoric water is more typical for arid regions, the, bulk of clastic host rocks display the namegiving pronounced red colour. Apart from silver,, uranium and copper are often enriched in this, setting. A famous example for exploitable redbed silver is the historical district of Silver, Reef, Utah, where Triassic sandstone carried, strata-bound ore. Above the groundwater table, silver occurred as chlorargyrite and below in the, saturated zone as acanthite and native silver., Fossil plant remains were the focus of rich ore, pockets, which is characteristic of this deposit, class. At the Silurian Transfiguration Cu-Pb-ZnAg deposit in Quebec, Canada, red-bed seepage, reduction and metal sulphide precipitation was, achieved by a hydrocarbon-bearing fluid rising, from basement slate (Cabral et al. 2009). In the, western USA, several red-bed copper and uranium deposits had profitable silver content., Important silver-producing countries include, Peru, China, Mexico, Chile, Bolivia, Australia,, Russia, USA, Poland and Canada. Mine production in 2008 was 21,300 t of Ag-metal (21,400 t in, 2009). One-third of annual silver consumption is, satisfied by recycling of scrap. Poland, China and, USA host large silver resources in polymetallic, and gold deposits. Geochemical methods combined with geological models are most effective, in exploration for silver ore deposits.
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228, , PART I METALLIFEROUS ORE DEPOSITS, , 2.3.3 Platinum and Platinum Group Metals, Common Ore Minerals:, Max., wt.%, Native platinum (alloyed, with other PGM and/or Fe), Cooperite, (Pt,Pd)S, 82 Pt, Braggite, (Pt,Pd,Ni)S, 59 Pt and 20 Pd, Sperrylite, PtAs2, 54 Pt, Moncheite (Pt,Pd)(Te,Bi)2, , Density, (g/cm3), 14–19, 9, 10, 10.5, 10.24, , The six platinum group metals (PGM) or elements, (PGE) comprise relatively light ruthenium (Ru),, rhodium (Rh) and palladium (Pd) with a density of, 12 g/cm3, and the heavy metals osmium (Os),, iridium (Ir) and platinum (Pt, density 21.45). Natural alloys of these elements with iron are common (ferroplatinum). Concentrates of PGM ore, may be composed of many more minerals (Cabri, 2002). Sulphides and chromite are important carriers of PGM (“invisible” PGM, e.g. in the Merensky Reef). Palladium is preferentially hosted in, sulphide minerals, primarily in pyrrhotite., Cut-off grades of primary platinum deposits fluctuate widely with processing characteristics., Coarse-grained ore can be extracted at 3 g/t Pt, equivalent, less favourable ore requires 5 to even, 10 g/t. A new mine in the Bushveld (Maandakshoek, mine) exploits ore from the 63 cm thick UG2 chromitite reef with an average of 8.16 g/t Pt equivalent., After dilution with host rock, run-of-mine ore, grades 7.5 g/t PGE þ Au. Sulphides at Sudbury contain up to 1 g/t Pt þ Pd in sperrylite. In spite of the, low content, the metals are profitably recovered as a, by-product during copper and nickel electrolysis., Placer platinum is mined at grades as low as 0.5 g/t,, for example in the Russian Far East. Averaging, 2.5 g/t PGM, tailings of past chromite processing, in the Bushveld constitute an attractive ore., Platinum metal has a high melting point, (1769 C) and like gold, dissolves only in aqua regia, (nitro-hydrochloric acid). It is a good electrical, conductor, a useful catalysing agent and is highly, resistant to corrosion and oxidation. Although, other platinum group metals do have differing, properties in detail, the general trend is the same., , Platinum’s main use (60%) is as a catalyst in the, petrochemical and automotive industries, closely, followed by production of “white” precious metal, jewellery. Platinum is also employed in the electronics industry (e.g. LCD-screen glass), fuel cells, in vehicles and in many medical applications., Palladium and rhodium are foremost used as autocatalysts for benzine engines, rhodium being especially effective in reducing NOx to nitrogen. PGM, autocatalysts, however, are increasingly replaced, by less expensive silver., With these applications, platinum metals are, obviously beneficial for the environment. Because, of their low solubility in surface water and body, fluids, they are not a health hazard. PGM ore, processing, however, may involve more problematic elements (e.g. sulphur, arsenic), the fate of, which in the mine environment has to be carefully, investigated and controlled., Geochemistry, Platinum group metals are distinctly siderophile, elements, like Au and Re. This is emphasized by, the frequent occurrence of natural alloys of PGM, with iron (4–21%) and the platiniferous fayalite, (“hortonolite”) dunite pipes of the Bushveld, (Figure 1.5) A chalcophile tendency is expressed, by the common occurrence of PGE minerals that, include elements such as Sb, As, Bi, S, Se and Te., Fe-Ni meteorites containing 1–10 ppm PGM attest, to the siderophile character of PGE. During formation of the Earth, PGE were fractionated into, the core. They are depleted in Palaeoarchaean, komatiites, but relatively enriched in Neoarchaean komatiites. This change is explained by, transfer into the mantle of platiniferous cosmic, matter bombarding the Earth during the period, from 4.5 to 3.8 Ga (Maier et al. 2009)., Crustal abundance of platinum and palladium is, roughly equal to gold (Pt 0.005 ppm, Pd 0.01 ppm,, Au 0.004 ppm: Smith & Huyck 1999). Generally,, ultramafic and mafic igneous rocks have relatively, high and granites low PGE trace content. Geochemically anomalous traces occur predominantly in chromian spinels and sulphides. Aurich copper porphyry ores reach 0.05 g/t Pd þ Pt, (Tarkian & Stribrny 1999). Mid-oceanic sulphide
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , mud at the East Pacific Rise displays up to 1% PGE, in pyrite and marcasite, similar to oceanic manganese nodules and crusts. Black shales trap platinum (e.g. gold and platinum in certain parts of the, Copper Shale in Poland). Fractionation of platinum elements is often investigated using the ratio, Pt þ Pd/Ru þ Os þ Ir or Pt/Pd, and by relating data, to C1-chondrites. PGE in ophiolitic chromites, for, example, have roughly chondritic concentrations, (0.1–1) of Ru þ Os þ Ir (“IPGE”) but only 0.01 Pt, þ Pd. The reverse is found in chromites and the, Merensky Reef of the Bushveld Complex, where, Pt þ Pd reach 4 times chondritic concentrations., Generally, Ru þ Os þ Ir are enriched in early crystalline phases formed from a mafic melt, whereas, Pd, Pt and Au rather segregate into residual, liquids., Mafic and ultramafic igneous rocks host practically all primary PGM ore of the world. Felsic, and intermediate magmatic systems are virtually, free of platinum metals. Mafic layered intrusions,, and among them the Bushveld Complex, outweigh by far all other geological settings of platinum deposits. Three genetic models of PGE, mineralization in layered mafic intrusions are, prominent: i) involvement of a sulphide melt; ii), coprecipitation with chromite; and iii) chromatographic extraction of PGE by fluids (Boudreau & McCallum 1992):, Ad i) Reduced sulphur in the magma is the key to, formation of PGE ore. Sulphur solubility of melts is, controlled by iron content, temperature and the SiO2, concentration (Cawthorn 2005a). Sulphur saturation, by change of one of these parameters leads to formation of dispersed droplets of a sulphide melt that, contains 35–40% S and 50% Fe, with some copper, and nickel. This melt is a very effective collector, of PGM, due to the high distribution coefficient, from silicate to sulphide melt (>105 for platinum; cf., Section 2.1.4 “Nickel”, eq. 2.2). Silicate/sulphide, melt mass ratios (R) of ore-forming systems are commonly moderate to low (Naldrett 2010). Dynamic, factors influence the transfer from silicate to sulphide, melt. Transfer will be more efficient if sulphide droplets remain suspended for a longer period and react, with more silicate magma. Vigorous convection is, probably of greatest importance (convective scavenging). Enrichment may be further enhanced if sulphide, droplets sink through several convection cells (cas-, , 229, , cade enrichment: Rice & von Gruenewaldt 1994)., One common element of models implies mixing of, two or more magma pulses, with freshly injected, sulphur, inducing sulphide melt formation (Naldrett, 2010, 1999). In this case, redissolution of earlier, sulphide melt may increase the concentration of PGE, (Kerr & Leitch 2005, Hinchey & Hattori 2007). With, increasing fS2, Pt þ Pd are enriched relative to Os-Rh, (Fleet & Wu 1993). This may be a clue to exotic PGM, fractionations in platinum deposits., Ad ii) and iii) Platinum ores in igneous rocks, that contain very little sulphur (e.g. Stillwater and, Bushveld, excepting the Merensky Reef, see below), and possibly never developed a sulphide melt obviously demand a different explanation. In that case,, the concentration of PGM may have been caused by, chromite precipitation (see below), or by residual, volatile-rich liquids. Complex fluids, hydrous or hydrosaline melts may form in fertile magma bodies, during crystallization (Stumpfl 1986, Boudreau &, McCallum 1992, Mathez & Mey 2005) or are injected, into the chamber from the footwall. Experiments, indicate that at temperatures 1000 C, such liquids, contain little water but much salt and chlorine and, are capable of concentrating PGM., , Similar to gold and silver, hydrothermal transport of platinum metals occurs commonly in the, form of chloride or sulphide complexes. At, reduced conditions below 300 C, bisulphide, complexes prevail; oxidized, saline fluids carry Pt, and Pd in chlorine complexes (Xiong & Wood, 2000). Precipitation from oxidized fluids is enforced by reduction, as for example at the footwall, of the European Copper Shale, where upwelling, Rotliegend (Early Permian) basinal brines encountered organic matter, methane and diagenetic sulphides. In the 300 C hot brines at the geothermal, energy field of the Salton Sea, Southern California,, Au, Pt, Pd and Rh occur at a concentration of, 1 ppb each. Platinum is dissolved as (PtCl4)2 or, Pt(HS)2. When the brines are pumped to the surface, boiling sets in at a certain depth and gold is, precipitated. In contrast to gold, PGM remain in, solution. Gold scales reach concentrations of, 0.1% (McKibben et al. 1990)., Metamorphic hydrothermal fluids formed, PGM-Au-quartz-sulphide veins in schists of the, Pounamu region (Southern Alps, New Zealand),
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230, , PART I METALLIFEROUS ORE DEPOSITS, , Syenite porphyry, , Gneiss, , Pt, Sho, , Dunite, Pyx, , Carbonate rocks, , Pu, , 1 km, , which are related to serpentinite and talc serpentinite bodies. Still enigmatic are the platiniferous, quartz veins of the Waterberg deposit near the, southern boundary of the northern limb of the, Bushveld Complex. The veins occupy a fault, between Rooiberg felsites and Triassic sandstones, (McDonald & Tredoux 2005, Armitage et al. 2007)., During supergene alteration, platinum elements resist dissolution like gold and are enriched, in placers that range from residual to coastal settings. Also like gold, they are not completely, insoluble, especially in complex ore, and limited, chemical transport does take place. In near-surface, meteoric waters, platinum elements form hydroxi- (e.g. PtOHþ), thiosulphate, organic and PtCl, complexes (due to trace chlorine from sea spray)., Residual PGM reside in gossans (frequently, together with gold) and in lower horizons of laterites. Actually, lateritic platinum concentrations, (“oxide ores”) are not rare but rarely economic. As, the solubility of the different platinum elements is, not identical, weathering of primary PGE ore leads, to relative enrichment of Pt over Pd (Oberth€, ur, et al. 2003) and of the notably immobile metals, Os þ Ir. The ratio Pt/Os þ Ir is a measure of the, maturity of weathered platinum occurrences. In, this connection it is remarkable that Witwaters-, , Figure 2.31 Geological map of the Inagli, ring intrusion of the Alaska, Urals type,, Aldan Shield, Russian Far East, with, alluvial platinum placers (modified from, Tolstykh et al. 2002). Copyright (2002), with permission from Elsevier., Sho – Shoshonite (trachyandesite),, Pyx - Pyroxenite, Pu – Pulaskite (syenite),, Pt - Platinum placer., , rand gold mining produces annually 1000 kg Os, and Ir, but no platinum. Precipitation of dissolved, platinum elements is provoked by reduction (e.g., in contact with Fe2þ) or by changing pH. Large, platinum crystals form in alluvial placers, as for, example in the Aldan Shield of the Russian Far, East. Primary sources are zoned intrusions of the, Alaska, Urals Type (Figure 2.31). Platinum placers, contribute 50% of Russia’s impressive annual, PGM production, whereas those of Colombia, and New Zealand have little international, significance., Platinum group element ore deposit types, The key to economically significant primary PGE, deposits is the extraordinary efficiency of orthomagmatic processes in concentrating traces of the, elements to exploitable grade. This is restricted to, only three petrogenetic settings:, 1 layered mafic intrusions (Bushveld, South, Africa, Great Dyke, Zimbabwe);, 2 mafic intrusions related to flood basalts, (Noril’sk, Sibiria; cf. Section 2.1.4 “Nickel”); and, 3 ultramafic complexes of the Alaska, Urals type, (Urals and Russian Far East, e.g. the ring complex, Inagli, Jakutia, Russia).
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232, , PART I METALLIFEROUS ORE DEPOSITS, , settling down to the floor of the magma. In this, model, aqueous fluids in the Merensky Reef caused, a limited redistribution of platinum elements, during cooling of the system. The genetic discussion, continues, however, and other processes such as, ejection of hydrous melts from lower, consolidating, cumulates, or pressure fluctuations in the magma, body are proposed to explain the formation of, PGM-enriched chromitites (Mathez & Mey 2005,, Cawthorn 2005a,b)., Lately, a new zone of giant Pt deposits has been, discovered in the northern limb of the Bushveld, Complex. In this area, Main Zone gabbronorites are, underlain by a 10–400 m thick package of pyroxenitic, rocks with PGE and base metal sulphide mineralization, designated earlier by Hans Merensky as the, Platreef, that overly sediments of the Transvaal, Group and Archaean basement. Rocks of the Lower, and Critical Zones are absent (Holwell & Jordaan, 2006). Mining started in 1992 at Sandsloot north of, the town of Mokopane., , Figure 2.32 (Plate 2.32) Merensky Reef at Impala, mine, S.A., with footwall anorthosite (lower part), a thin, chromitite band in the middle and the pegmatoid reef on, top (orthopyroxene, plagioclase and the sulphides, pentlandite-pyrrhotite- chalcopyrite). Height of sample, is 15 cm. Courtesy B. Lehmann, Clausthal., , With its dunite core sourcing Pt placers and, marginal olivine-bearing trachyandesite the Inagli, ultramafic ring complex is a textbook example of, Alaska-Urals type intrusions (Figure 2.31). Placer, mining ceased in 2008. Alluvial placers in the, Western Cordillera of Columbia are related to, zoned ultramafic intrusions (Tistl 1994). Economically more spectacular is Nizhny Tagil in the Ural, Mountains, because of both rich eluvial and alluvial platinum placers, and historic underground, mines (Aug, e et al. 2005). The dunite unit at Nizhny, Tagil consists of olivine and minor chromite that, accumulated from mafic magma in an open flowsystem. Chromite ore occurs in schlieren and massive lenses, which reach 100 by 5 m, and were, formed by dynamic concentration of chromite, crystals in magma conduits. Chromitites are enriched in PGE. Aug, e et al. (2005) suggest that the, reason for coprecipitation is a physical affinity, between chromite and PGE-phases., By-product platinum is widespread, especially, in norite and gabbro intrusions of different genetic, lineage (e.g. Sudbury, Lac des Iles and Voisey’s Bay, in Canada; Montchegorsk and Pechenga on Kola, Peninsula). In these deposits, PGM and gold are, associated with Cu-Ni sulphides and pyrrhotite., Typically, palladium is concentrated over platinum (Noril’sk Pt:Pd 1:4; Lac des Iles 1:10;, Hinchey & Hattori 2007). Nickel sulphide concentrates of komatiite deposits are also a source of, by-product platinum. Platiniferous chromitites of, ultramafic sections of ophiolites are relatively, rare but may support smaller mining operations, (Albania, Ural Mountains, Ethiopia, Shetland:, Lord & Prichard 1997). High PGE concentrations, in podiform chromitite are probably controlled by, a favourable degree of mantle melting (Prichard, et al. 2008). Several platinum placer districts, for, example alluvial and coastal placers in New Zealand, are sourced from nearby ophiolite hills., PGM mineralizations in the European Copper, Shale at Lubin and Polkowice mines in Silesia,, Poland are economically of little weight but of, scientific interest. At the redox border between, Rote F€, aule (oxidized) and the Cu-Ag sulphide zone, (reduced), noble metals are enriched to 1460 ppb, Pt, 824 ppb Pd, 13.5 ppm Au and 160 ppm Ag,, although in a very thin boundary layer. Remember, that this redox front experienced a giant mass flow
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 233, , Roof rock (Rooiberg rhyolites), Roof-rock melt (granophyre), Entrainment of felsic melt, , Mixi, , ng, , Diffusive interface, Entrainment of mafic melt, , Resident Bushveld magma, , New, magma, Mixedliquid, , Unconformity due to magmatic erosion & solution, Cumulates, Figure 2.33 Model of chromium and platinum ore formation in the Bushveld Complex, caused by injection of fresh, melt into the magma chamber, silicic contamination and mixing with resident liquids. Precipitated platiniferous, chromite (black) accumulated on the floor of the magma chamber (modified from Kinnaird et al. 2002). By permission, of IMM London & Maney Publishing (www.maney.co.uk/journals/aes)., , of hot, oxidized basinal brines (cf. Chapter 1.4, “European Copper Shale”)., Exploration for platinum deposits requires geological and petrological models, followed up with, geochemical and geophysical surveys (e.g. induced, polarization for sulphides). Prime targets are sulphide-enriched sections, chromitite (Ohnenstetter et al. 1999) and magnetite seams (Maier et al., 2003) of layered mafic-ultramafic intrusions. Sections of ophiolites displaying chromite mineralization are second-order targets. Lithostratigraphic, mapping assisted by geochemical data on silicate, minerals is essential. Large areas are scanned by, heavy mineral and sediment sampling of streams,, and by grid-sampling of soil (Prendergast et al., 1998). Pathfinder elements for sulphide-associated PGM mineralization include Cu, Ni, Ag, As, and Te. More than 100 years ago, Merensky used, the frequent association of Pt with Ni and Cu, (resulting in green staining of outcrops) in his, discovery of the Bushveld platinum ore deposits., Of course, the method fails to discover native, alloys of PGM in the absence of base metal sulphides (Potter 2002)., Leaders of primary platinum production are, South Africa and Russia, followed by Canada, Zimbabwe and USA. World production in 2008 was 189, , tonnes Pt (falling to 178 t in 2009) and 204 t Pd, (195 t in 2009). Russia and South Africa equally, share world supply of palladium. Although South, Africa’s reserves and resources of PGM in the, Bushveld are very large indeed (>90% of the, world’s), exploration elsewhere is quite active., Demand, but also recycling, continue to increase., , 2.4 LIGHT METALS, 2.4.1 Aluminium, Common Ore Minerals:, , Gibbsite, (Hydrargillite), Boehmite, Diaspore, Alunite, Nepheline, , Wt. %, Al2O3, , Density, (g/cm3), , Al(OH)3, , 65, , 2.4, , g-AlO(OH), a-AlO(OH), KAl3[(OH)6(SO4)2], (Na,K)[AlSiO4], , 85, 85, 37, 36, , 3, 3.4, 2.7, 2.6, , Aluminium metal (D ¼ 2.7 g/cm3, melting point, 660 C) is commonly produced from bauxite,
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234, , PART I METALLIFEROUS ORE DEPOSITS, , which is loose soil or a hard rock with 30–65%, Al2O3. Bauxite consists of Al-minerals such as, gibbsite, diaspore and boehmite, and amorphous, alumogel in varying proportions. Gangue minerals, include kaolinite, quartz, haematite, goethite,, ilmenite/rutile/anatase and zircon. Alunite and, nepheline (a co-product of certain apatite mines), with marginal Al2O3 content were at times used, for the production of alumina or Al-metal. Among, the oxy-hydroxides, gibbsite is preferred because it, is easily soluble in the standard Bayer digestion, method, whereas diaspore is least soluble. For a, timely planning of plant technology, the distribution of various Al-minerals (and reactive silica) in, the deposit must be carefully mapped. Reactive, silica (commonly in kaolinite) is undesirable., Fe2O3 and quartz are not soluble but should not, exceed low contents. Exploitable bauxite ores contain >35% Al2O3. Enrichment of alumina by processing bauxite is rarely feasible and selective, mining is the common means to guarantee, targeted run-of-mine grade. One exception is, deposits in the Darling Range (Western Australia),, where quartz can be separated by simple processing. Metamorphosed bauxite with the paragenesis corundum and magnetite is called emery, a raw, material for the production of abrasives., Aluminium is the most important of the noniron metals. About 95% of bauxite produced is, processed into aluminium metal. The remaining, 5%, typically ore of very high quality (“chemical, grade”) with >52% Al2O3, serves as an industrial, raw material for numerous special products (e.g., abrasives, portland cement, calcium-aluminate, cement, technical ceramics, glass, chemicals,, paints, enamels and refractories). Favourable attributes of aluminium metal such as light weight,, strength (Lu 2010) and excellent corrosion resistance allow its use in numerous applications from, building air frames to food packaging (the metal is, not toxic). Alloys of aluminium with other metals, including lithium, scandium and magnesium are, produced for specific uses., In the Bayer digestion technology, bauxite is leached, with NaOH. Alumina Al2O3xH2O is precipitated, from the aluminate solution. After drying, the precipitate is mixed with natural or synthetic cryolite, , Na3AlF6, fluorite, LiCO3 and NaCl, and is reduced by, electrolysis to metallic aluminium in an electric arc, furnace. Insoluble residue of leaching is alkaline, (pH 8–13.5) “red mud”, which contains mainly quartz,, calcite, haematite and goethite. Because bauxite is, extremely leached, elevated trace contents of elements such as Sc, Ga, V, U, Th, REE, P and Ti are, rarely observed. In some cases, Sc (Kempe & Wolf, 2005, Wiesheu et al. 1997), V and Ga are extracted, from red mud. Red mud is not radioactive nor toxic,, apart from its caustic nature. Commonly, it is disposed of in settling ponds. Revegetation of pond sediments after closure is an interesting problem, (Courtney & Timpson 2005). Recently, red mud was, investigated for environmental applications such as, lining waste disposal sites, for neutralizing acid mine, waters, and for immobilization of toxic heavy metals, (Snars & Gilkes 2009). At the Kwinana aluminium, plant, Western Australia, red mud is reacted with CO2, from the power station in order to neutralize negative, environmental effects of both waste products., , Geochemistry, With an abundance of 8% (range 7.4–9) lithophile, aluminium is the third most abundant element in, the Earth’s crust after oxygen (47%) and silicon, (27%). In spite of its abundance, enrichment to, exploitable grades and tonnages is relatively rare., The key to economic concentrations of aluminium is humid-tropical weathering leading to the, formation of laterites (oxisols, ferralsols) and, exploitable bauxite (cf. Chapter 1.2 “Supergene, Ore Deposits”). The solubility of aluminium in, natural waters is very low, except as Alþ3 ion, below about pH 4. The element is not redoxsensitive. During supergene alteration, primary, alumosilicate minerals of rocks (e.g. feldspar) are, first hydrolysed to kaolinite, followed by formation of colloidal aluminium hydroxide and silica., Above pH 4.5, silica is more soluble than alumina,, resulting in residual enrichment of Al. At pH >5.6, (Figure 1.48), gibbsite forms from alumina colloids. Seasonally changing tropical climate favours, these processes, because dry season soil water rises, and evaporates towards the surface. The resulting, alkalic soil moisture dissolves SiO2, which is, flushed out with Na, K, Ca and other soluble, elements when rainy season water infiltrates., Low pH/Eh induced by organic acids favour iron
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , mobility and depletion (Figure 2.1), producing, bauxite with low iron contents. Ageing (soil diagenesis) transforms gibbsite into boehmite and, diaspore, and consolidates the original aluminous, soil into solid bauxite rock., Bauxitization is essentially the enrichment of, alumina by abstraction of rock-forming elements, including silica, alkali and alkali earth elements., Likewise, minor and trace elements are either, depleted or enriched. The resulting distribution, pattern of elements in bauxite deposits are powerful tools that allow investigation of genetic parameters, including identification of source rocks,, physico-chemical conditions of alteration and the, degree of element mobility (Nesbitt 1979, Mameli, et al. 2007). Indicators of specific source rocks are, the more immobile elements such as Ti, Zr, Nb,, Ga and Th., Aluminium ore deposit types, Only supergene geological processes enrich aluminium to exploitable grade based on dissolution, of other rock constituents and the extremely low, solubility of its oxy-hydroxides. Drivers are greenhouse maxima of global climate cycles (Retallack, 2010). The resulting ore, bauxite, is residual aluminous soil. The nature of footwall rocks of bauxites is used to differentiate between: i) laterite, or, silicate bauxite (developed on silicate rocks); and, ii) karst, or limestone bauxite that occurs above, limestone karst surfaces or in karst depressions., In short, significant genetic variants of aluminium, deposits include:, . supergene residual bauxite in autochthonous, soil blankets;, . resedimented detrital allochthonous bauxite., , Laterite bauxites, Laterite bauxites are most often products of, autochthonous weathering, but as in most soils,, local erosion and resedimentation are not rare, (Bardossy & Aleva 1990). The bauxite-bearing laterites form soil profiles of several metres to tens of, metres thickness that blanket old, usually Mesozoic-Tertiary land surfaces. Later erosion causes, interruption of “highland” bauxites by incised, valleys. Bauxite scree develops on the slopes, and, sedimentation of fines in depressions results, in deposits of allochthonous detrital bauxite, (Figure 2.34). Textural varieties of bauxite soils, comprise massive, concretionary, pisolitic,, spongy and cellular types. The quality of bauxite, can be directly related to the source rock chemistry: Both the Dekkan and West Australian bauxites above dolerite have high iron and titanium, content. Generally, the lower part of a laterite, profile is often clayey, and several bauxite mines, co-produce high-grade kaolin or smectite clay, from the same pits (e.g. Weipa, Australia with a, production of 100,000 t/y of paper-grade kaolinite)., Australia hosts several of the largest laterite, bauxite districts of the world. Important mining, districts are Weipa (Cape York Peninsula), Gove, (Northern Territory), Darling Ranges (Figure/Plate, 1.1, 1.2) and Kimberley (Western Australia):, The Weipa bauxites form bright red cliffs on the shore, of the Gulf of Carpentaria that had already been, observed by the earliest Dutch explorers. Their potential was only recognized in 1955 during an oil (!), exploration campaign (Schaap 1990). Extraction, started in 1963. The bauxite laterite occurs over, Palaeogene arkosic sand, silt and clay, and in some, areas on Cretaceous marine sediments. The laterite, , e, , it, ux, Ba, , Figure 2.34 Sketch of morphological, position of bauxite deposits in the Dekkan, Plateau (India)., , 235, , es, lop, , e, cre, , Ferralite, Bauxite, Kaolininized basalt, , s, , Unweathered basalt, , Allochthonous/, detrital bauxite, , 50m, 1 km
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236, , PART I METALLIFEROUS ORE DEPOSITS, , profile is covered by several metres of younger soil., Bauxite extends over an area of 11,000 km2 with an, average thickness of 2 m, thickening in places to, 12 m. It consists of loose and little cemented pisoliths, formed from concentric layers of gibbsite and boehmite, with accessory kaolinite, quartz, anatase and, haematite (Taylor & Eggleton 2004). Average grade, of the bauxite bed in the mine is 52–55% Al2O3, 5–9%, SiO2 and 7–16% Fe2O3 (Abzalov & Bower 2009). The, horizon is underlain by 1–2 m of ferricrete and a, kaolinitic saprolite. Normally, soil pisoliths form in, a substrate. At Weipa, winnowing processes seem to, have eroded the fine matrix, while the pisoliths, were hardly transported as evidenced by geochemical, relations to the bedrock (Taylor & Eggleton 2004)., Reserves and resources of the district are very large., , Large bauxite blankets derived from phyllites,, greenschists and gneisses occur in tropical western Africa (Guinea, Ghana and Sierra Leone)., Across the Atlantic Ocean, important bauxite, provinces include the Guiana Shield (e.g. Surinam), and the Amazon Basin in Brazil. Worldwide,, laterite bauxite is found in humid and warm climate zones that have hardly changed their, latitudinal position since the Mesozoic. PreMesozoic bauxites are rare and have little economic significance. This appears to be mainly due, to the high geological probability of soil erosion, compared with its preservation. Undoubtedly, the, absence of land plants in earlier geological times is, another factor. Bauxites of the Tikhvin Type and, karst bauxites are examples of unusually effective, geological preservation of soil., Allochthonous laterite bauxites of the Tikhvin, Type are found in former morphological depressions, similar to young detrital bauxites. Deposits, in the Tikhvin mining district occur in Palaeozoic, rocks of the Moscow basin (Russia) along a strike, length of 260 km. Bedrocks underneath the bauxite are spotted bluish-grey sandy and micaceous, shales of Devonian age and locally Early Carboniferous (Tournaisian) marlstones, dolomites and, limestones. Bauxites were deposited during the, Visean and are covered by later Visean and Quaternary sediments. During the period of bauxitization, the area was above sea level and was drained, by a number of large valleys descending towards, an ocean in the east (which was soon afterwards, , consumed in the collisional Variscan Ural Mountains). Bauxite deposits fill the upstream sectors, of these valleys where they form long, narrow, (<100 m) and thick (40 m) bodies. The ore is reddish-brown without bedding structures or sizegrading. Gibbsite, boehmite and kaolinite are, main minerals, with some secondary calcite. The, ore contains 35–49% Al2O3 and up to 18% SiO2., Devonian shales are thought to be the original, source rocks., Karst bauxites, or limestone-hosted bauxite, deposits, Karst bauxites, or bauxite deposits formed on carbonate rocks (Bardossy 1982), are quite different, from laterite bauxites. They rarely occur as blankets but fill karst depressions of variable form and, origin. Karst bauxites in Mediterranean Europe are, commonly associated with carbonate rocks of, Mesozoic and Cenozoic age (Combes 1984). Main, ore minerals are boehmite and diaspore. Other, important karst bauxite provinces occur in the, Caribbean Islands, the Ural Mountains, Vietnam, and southeastern China., Both karst and bauxite formation occur in, phases of emersion that may be expressed by, unconformities or periods of non-deposition. In, Mediterranean Europe, this includes several short, time spans between the Middle Triassic (e.g. Montenegro) and the Eocene (Dalmatia, Istria, northwest Hungary). Karstified footwall limestones are, always marine deposits, whereas hanging-wall, sediments developed above bauxite may either be, transgressive marine (Figure 2.35) or terrestrial, lacustrine sediments that can host coal deposits, (Gant in Hungary, Laussa in Austria). Like laterite, bauxites, karst bauxites record warm and humid, climates (“greenhouse conditions”), and favourable tectonic and morphologic configurations, (D’Argenio & Mindszenty 1995):, Form and thickness of karst bauxite deposits are, determined by the relief of the depositional site,, which is a function of the duration of the karstification process, the nature of the limestones, and of the, difference between the altitude of the karst plateau, and the drainage system. Deeper karstification
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , m, , Clayey conglomerates (Late Cretaceous), , Dolomite scree, , Claystone, , 237, , 0, , 10, , Bauxite 2, , Bauxite 1, , 20, , Jurassic dolomite, 30, Figure 2.35 Synthetic section of the sedimentary succession in the karst bauxite deposit Uston (Languedoc, France,, modified from Combes 1984). Reproduced by kind permission of Total. Clearly, there were two periods of bauxite, sediment formation that are separated by erosional unconformities and deposition of clay and dolomite scree. For, location of Les Baux district refer to Figure/Plate 1.89., , results from a large height difference resulting in, orebodies of 80 m thickness at Niksicka Zupa (Montenegro) and 50 m in the Haute Var (Provence). Bauxite orebodies are often longitudinally arranged along, certain structural features of the bedrock (e.g. faults), or along former karst valleys. The ore of Iharkut, (Hungary) filled a deeply incised canyon., , Autochthonous or allochthonous formation of, karst bauxites was long disputed. One of the early, theories proposed derivation of bauxite from silicate minerals that were enclosed in the karstified, limestone. However, karstifiable carbonate rocks, have typically very low silicate contents, so that, this “terra rossa hypothesis” can only explain, a few exceptional situations, as in Sardinia, (MacLean et al. 1997). Mineralogical and geochemical research showed universally that the, precursor rocks of karst bauxites were silicate, rocks and not the footwall carbonates. Actually,, most karst bauxites have an ordinary laterite bauxite origin and are detrital bauxite sediments, after, erosion and transport. Discussions also thrived, on the observation that karst plateaus rarely display surface river flow. So, how can the import of, detrital material into isolated depressions be explained? The solution to this question lies in, a nearly-submerged state of the karst, which, enabled rivers to inundate its surface. In some, , cases, aeolic transport of volcanic ash may have, been the source of silicates (Jamaica)., Note, however, that the nature of the sediment, that is now karst bauxite may have been quite, different at deposition. Erosion may have moved, either bauxite or saprolite clays of the laterite, profile and even rock particles. Bauxitization, may have taken place after sedimentation in the, karst depressions. The bauxite must have been, imported when it is interbedded with clay, (Figure 2.35). Karstification may continue after, deposition resulting in apparent metasomatic, replacement of carbonate by bauxite. Tectonically, reversed bauxite bodies experienced renewed carstification in Provence (France), resulting in bauxite-filled sinkholes in the stratigraphic roof., Apart from bauxite, alunite (a product of acidic hydrothermal alteration, cf. Chapter 1.1) and nepheline are, potential ores of aluminium. At present, only one, mine reports production of alumina from nepheline, rock, Kiya-Shaltyr in southern Siberia, Russia. Ore is, a nepheline syenite body that forms part of an ultramafic alkaline complex. Resources comprise 94 Mt of, rock with 26–27% Al2O3 (average nepheline syenite, contains 21% Al2O3). Desired run-of-mine grades, are reached by selective mining. There is no further, concentration. Processing only comprises pulverizing and sintering with limestone, which is followed
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238, , PART I METALLIFEROUS ORE DEPOSITS, , by alkaline leaching. Yearly production attains 1 Mt, of high-grade alumina., , In the reconnaissance stage, exploration for, bauxite deposits builds mainly on geological, methods, because geochemical and geophysical, tools are hardly applicable. During detailed, follow-up work, electrical, soil radar and magnetometric methods may be useful. Field determination of bauxite is often hampered by the wide, variability of colour, consistency and texture of, the ore. Therefore, mineralogical (e.g. portable, field spectrometers PIMA, etc.) and chemical analyses are indispensable. For large sample numbers, in the field, simple determination of loss on ignition and colorimetric tests may help to assess ore, grade., In 2008, world mine output was 200 Mt bauxite or 39 Mt aluminium, dropping to 36.9 Mt in, 2009. Australia, China, Brazil, Jamaica, Guinea, and India are significant producers. Reserves and, resources are very large (Hostermann et al. 1990),, mainly of laterite bauxite., , 2.4.2 Magnesium, Ore Minerals:, , Forsterite, Dolomite, Brucite, Magnesite, Carnallite, Bischofite, , Mg2SiO4, CaMg(CO3)2, Mg(OH)2, MgCO3, KCl.MgCl2.6H2O, MgCl2.6H2O, , Wt. %, Mg, , Density, (D, g/cm3), , 34, 12, 41, 28, 8, 11, , 3.2, 2.8, 2.4, 3.0, 1.6, 1.6, , Very diverse mineral raw materials, natural and, industrial brines, and seawater are used for the, production of magnesium and of magnesium compounds. The first four minerals listed above occur, in nearly mono-mineralic rocks with potentially, recoverable magnesium content. These ore rocks, include forsteritic dunite, dolomite (cf. Chapter 3, “Carbonate Rocks”), brucitite and magnesite, (cf. Chapter 3 “Magnesite”). A project in Quebec,, Canada, which was to produce magnesium from, , serpentine tailings (24% Mg) of the former asbestos mines, failed for economic reasons., In spite of low Mg-contents, the salt minerals, carnallite and bischofite are used for magnesium, and magnesium compound production (Russia,, Ukraine; cf. Chapter 4.1 “Salt Minerals and Salt, Rocks”). Solution mining (Veendam, Netherlands) and processing of potassium salts yields, co-product MgCl2 brines, which are convenient, for, further, chemical, and, metallurgical, manufacturing., Processing of hard rock magnesium “ore” often, involves leaching with HCl, resulting in intermediate product MgCl2. The salt is reduced by electrolytic melting, yielding magnesium melt and, chlorine gas. In China, the world’s largest, Mg-producer, the Pidgeon process route is preferred, in which calcined dolomite or magnesite, are reduced with ferrosilicon or aluminium. Magnesium metal production requires a high energy, input so that availability of cheap energy is a, precondition for economic success. Environmentally acceptable disposal of discarded processing, water and sludge constitutes another significant, cost. It can be reduced by using raw materials that, are free from contaminants., Applications of the extremely light magnesium, metal (density 1.74 g/cm3, melting point 650 C), employ the pure metal or aluminium alloys. Magnesium-aluminium alloys are mainly consumed, for beverage container making. About 40% of, magnesium is used for die casting in the car industry in order to reduce weight and fuel consumption. Other sectors include the space, aircraft and, chemical industry. Lesser qualities serve as agents, of desulphurization of iron and steel., The largest sector of magnesium compound, consumption is the use of magnesium oxide (magnesia) as a refractory material in furnace linings, for the production of iron and steel, nonferrous, metals, glass and cement. Also, MgO and other, compounds, such as brucite, are used in agriculture, chemical, environmental and construction, products (cf. Chapter 3 “Magnesite”)., With 2.1% in the Earth’s crust, magnesium is, the fourth-abundant metallic element after silicon, (27%), aluminium (8%) and iron (5%). Magnesium, is one of the lithophile earth alkali elements.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Average MgO content in common rocks amounts, to 6.7% in basalt, 3.3% in andesite and 0.4% in, rhyolite; average dunite contains 38% MgO. The, element is not redox-sensitive and is mobile in all, surface environments (Smith & Huyck 1999). In, low concentrations Mg is essential for humans,, animals and plants, and for this reason is commonly added to animal feed and fertilizer (Lindh, 2005)., , Reserves and resources of raw materials for, magnesium metal production are practically inexhaustible and widely available. Leading producers, include China (>80% of the total), Russia, Israel, and Kazakhstan. In 2008, world production of, magnesium metal amounted to 810,000 t, but fell, to 700,000 t in 2009., , 2.5 MINOR AND, , SPECIALITY METALS, , Sources of magnesium for metal and, Mg-compounds production, , 2.5.1 Mercury, , seawater, natural brines and residual brines from, potassium salt processing;, . dunite, serpentine, dolomite, brucite and magnesite rock., Seawater with 0.3% MgCl2 or 0.86 g/L Mg is an, unlimited source of magnesium. Some playa lake, brines (Dead Sea, Great Salt Lake) contain recoverable magnesium as one of several products:, , Common Ore Minerals:, , ., , The Dead Sea, shared by Israel and Jordan, occupies a, transtensional basin along the Dead Sea transform, fault that links the divergent plate boundary of the, Red Sea to the convergent Alpine Taurus Mountains, in southern Turkey (Garfunkel & Ben-Avraham, 1996). The brine surface of the Dead Sea at 415 m, below sea level represents the lowest continental, surface on Earth. The depression is 80 km long and, 20 km wide, and receives drainage from 40,000 km2., Very little rainfall, high temperatures and very high, evaporation characterize the climate. With a water, depth of 300 m, the Dead Sea is an interesting, example of present deep-water halite sedimentation, (Warren 2006). The lake brine displays a salinity of, around 280 ‰ compared to 35‰ of seawater, although, it contains relatively more Ca, Mg, K and Br, and less, sodium, sulphate and carbonate. This brine is, pumped into fields of fractionation ponds in the, Southern Basin of the Dead Sea, which cover an area, of roughly 130 km2. A little gypsum and much halite, are first precipitated followed by carnallitite. The, salts are harvested and processed into potash, sodium, chloride, magnesium chloride, magnesium oxide,, hydrochloric acid, chlorine, caustic soda and magnesium metal. From residual brines of carnallite precipitation ponds, bromine and MgCl2 are extracted, before the waste brine is returned to the northern, Dead Sea., , 239, , Cinnabar, Native mercury, , HgS, Hg, , Wt. % Hg, , Density (g/cm3), , 86, 100, , 8.1, 13.5, , Metacinnabar (Hg,Fe,Zn)(S,Se), the tetrahedrite, schwazite (Cu,Hg)3SbS3–4 and calomel HgCl2 are, common Hg-minerals, although not in ore-forming quantities. Sulphides and sulphosalts of, other metals (e.g. Sb, As, Cu, Fe) may make up, part of the paragenesis; antimony and gold, are occasional by-products of mercury. Typical, gangue minerals are quartz, carbonates and, barite. Cinnabar is a heavy mineral and durable, in surficial alteration settings. Proximal eluvial, and alluvial placers are not rare. Hypogene native, mercury is characterized by elevated concentrations of Cu, Ag, Sb, Fe, As and Zn, whereas, secondary supergene mercury contains only trace, amounts. The same elements are concentrated in, cinnabar, which condensed from a gas phase, in, contrast to hydrothermal cinnabar (Barnes &, Seward 1998)., Worldwide, mercury mining has nearly ceased., Formerly, typical grades of mercury ore were, between 0.6 and 2% Hg. Mercury metal (density, 13.55 g/cm3; melting point –38.87 C, boiling point, 356.9 C) and chemical compounds were used for, the production of chlorine and caustic soda (in the, form of native Hg as a cathode in salt electrolysis),, of paints, batteries and numerous chemicals and, pharmaceuticals (e.g. dental amalgam). Supporting, mercury use, USA and EU are currently phasing out
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240, , PART I METALLIFEROUS ORE DEPOSITS, , general service incandescent light bulbs in favour of, mercury-containing compact fluorescent bulbs in a, drive to save energy. Also, mercury is widely used, in the fluorescent tubes that form the backlight to, liquid crystal displays (LCDs) of television sets and, numerous other electronic devices. In many, parts of the world, application of gold amalgamation in small-scale and artisanal mining is still, widespread., Mercury is a powerful neurotoxin that is especially enriched in aquatic food-webs (Selinus et al., 2005, Plant et al. 2005, Alloway 1990, Fergusson, 1990). Ionic mercury contamination, for example, from coal burning, is rapidly converted by microbes to methyl mercury, which is highly toxic, when accumulating in living tissues. Bioremediation of soil by genetically modified plants seems, to be the best choice (Omichinski 2007). Today,, mercury’s use is strictly controlled, and recycling, is employed where it cannot be replaced by other, metals. Sources of mercury, as well as natural and, anthropogenic cycles including environmental, aspects, are presented in detail by Parsons &, Percival (2005)., , Geochemistry, The geochemical behaviour of mercury is chalcophile. Its average content in the continental crust,, in granite and basalt is 0.08 ppm (range 0.03–0.5,, Smith & Huyck 1999). Mercury occurs in the three, oxidation states Hg0, Hgþ1 and Hgþ2. It has a, strong affinity to organic matter, live or dead, so, that shales, especially black shales are enriched, compared with magmatic rocks. The European, Copper Shale, for example, contains an average of, 0.13 ppm Hg. Petroleum, natural gas, formation, waters and coal all have elevated Hg-contents., From certain reservoirs, the oilfield Cymric,, California produced petroleum with 21 ppm Hg, and water with 0.2 ppm Hg, and the methane outgassing from it was saturated with mercury, which, condensed in the gas pipelines. Worldwide, many, geothermal power stations have problems with, mercury in the extracted vapour. The world’s, largest geothermal power field, The Geysers,, California, “produces” 50 kg Hg0 per year. These, , observations suggest how mercury is transported, in geological systems: Mercury is more mobile in, a gas phase or a hydrocarbon fluid than in aqueous, solution. Its characteristic geochemical association comprises other “volatile” and semi-metals,, including As, Sb, Te and Tl (Figure 2.23)., Traces of mercury occur in many sulphide ores,, because Hg substitutes for Cu, Ag, Zn, Cd, Bi, Pb, and other metals. It is also a frequent companion, of gold, antimony, barite and strontium ore, resulting in considerable by-production of mercury., During orebody formation, mercury vapour forms, wide primary dispersion halos (cf. Chapter 5.2, “Geochemical Exploration”), which makes it useful as a pathfinder element., In reduced, low-sulphur hydrothermal fluids of, >200 C, Hg2þ occurs in complex ions such as HgS, (H2S)02 in the aqueous phase; yet, Hg0aq is probably, more abundant than Hg2þ and behaves like a dissolved gas (Varekamp & Buseck 1984). The solubility of Hg0 in hot fluids is very high (400 ppm at, 300 C), but drops rapidly with decreasing temperature (only 0.5 ppm at 100 C). As the fluid ascends, along open fractures, it cools and begins to boil,, and Hg0aq is partitioned into the vapour phase., Diffusion of Hg between the liquid and vapour, bubbles enriches the light isotopes (Smith et al., 2005). Loss of H2S to the vapour phase during, boiling causes reduction of Hg2þ in dissolved complex ions (eq. 2.6)., Mercury vapour formation by dissociation of, complex Hg-bearing ions:, HgSðH2 SÞ02 þH2 ) Hg0g þ3H2 Sg, , ð2:6Þ, , Petroleum is a common component of hydrothermal solutions transporting mercury. This suggests an important role of Hg-complexing organic, molecules (Fein & William-Jones 1997). Salt concentration seems to have little impact on Hg, solubility. Most mercury deposits were formed at, 60–150 C. In this case, condensation of Hg0 from, the gas phase and formation of HgS from the, aqueous phase may cause precipitation. Sources, of mercury are probably quite often siliciclastic, sediments with elevated organic matter contents., From this pool, both mercury and hydrocarbons, can be distilled by heat pulses during diagenesis
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , or low-grade metamorphism. The only reason for, the connection of mercury mineralization with, volcanism may be localized strong heat flow, not, magmatic degassing. Magmatic extraction of mercury from the Earth’s mantle and crust, however,, remains a viable model., Mercury ore deposit types, Likely examples of mercury deposit formation, (or considering the ban on Hg-mining,, “mineralization”) can be studied at several active, geothermal emanation centres. Discharge rates, that may produce deposits have been described, from Ngawha, New Zealand and Sulphur Bank,, California. Submarine hydrothermal vents with, high mercury delivery have been discovered off, New Zealand (Stoffers et al. 1999). Based on these, observations and on the study of older mercury, deposits, common characteristic include:, . mercury deposits are formed near or at the surface (rarely >200 m below the surface; with 800 m,, the deepest mining took place at New Almaden,, California);, . formation temperatures are mostly <200 C;, . ore is typically mono-metallic (mercury only), and mono-mineralic (cinnabar);, . hydrothermal alteration is dominated by silicification and carbonatization;, . very few mines delivered most of total historic, world production., Overall, volatile mercury is concentrated in shallow epizonal and hydrothermal settings, which are, commonly related to circumscribed heat anomalies, rifting and volcanism. At a number of sites, a, metamorphogenic origin is indicated and mercury, deposits may mark the tops of orogenic gold systems, as supposed in the Californian Coast Ranges, (Figure 2.22; Groves et al. 2003). Significant genetic, mercury deposit types include:, . volcanogenic-hydrothermal;, . metamorphogenic-hydrothermal., Sulphur Bank Mine in the northern Coast, Ranges, California, was one of the major historic, mercury producers of North America and one of, the first locations where volcanic geothermal systems were recognized as modern analogues to, epithermal deposits. The deposits lay in a broad, , 241, , hill of Pleistocene andesitic lavas on the shore of, Clear Lake. Andesites above the groundwater, level are strongly altered. Only boxwork quartz, and elemental sulphur indicated mineralization,, the latter giving the site its name. After discovery, in 1856, borax and sulphur were extracted. Prospecting shafts below the groundwater level soon, exposed mercury ore with highest grades in fault, gouges impregnated with fine-grained cinnabar., Nearby fissures in argillized andesites also contained HgS, with some pyrite, markasite, stibnite,, quartz and calcite. The vertical extent of mineralization was only 100 m, hosted by andesites, lake, sediments and Mesozoic greywacke. The mine, yielded a total production of 5000 t Hg:, 14, C-dating of wood in the oldest Hg-bearing lake, sediments allowed determination of the onset of, mineralization at 34,000 years BP. Even today in the, former open pit, thermal springs deposit HgS, markasite, pyrite and native sulphur. Cl in these waters is, remarkably low, whereas CO2, B, NH3 and iodine are, strongly enriched. Contents of liquid hydrocarbons, and methane indicate derivation of the fluids from, marine sediments by thermal maturation and diagenesis. Compared to local meteoric waters, both deuterium and the heavy O-isotopes are enriched, giving, rise to earlier speculations that the geothermal water, has a deep source (metamorphic?). A regional comparative investigation, however, showed that the, composition of the waters is best explained by meteoric-dominated systems with repeated vapour loss at, depth (Sherlock 2005)., , Sulphur Bank is part of a large mercury district, in California. Single deposits typically lie about, subvolcanic intrusions marked by geothermal, activity. Yet, the fluid source is thought to be, rather metamorphic than magmatic, possibly, related to overthrusting of Mesozoic sediments, by the Coastal Ranges or to subvolcanic heat flow., Most of the deposits are hosted by serpentinites of, the Franciscan Complex, sited within a halo of, hydrothermal silica-carbonate alteration. This, suggests that the fluids were rich in CO2. Mercury, probably was transported as vapour-phase Hg0 at, 200–240 C (Peabody & Einaudi 1992). Until 1973,, the Californian mercury province played a major, role in world production. Today, its interest lies in
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242, , PART I METALLIFEROUS ORE DEPOSITS, , In this setting, cross-cutting bonanza orebodies, were exploited (Jebrak et al. 2002). Typical ore, grades in quartzite were 1%, but in places reached, 20%. Radiogenic and stable isotope studies of the, hydrothermal alteration zones suggest a magmatic, derivation of the fluids. Lead isotopic compositions, point to a mainly sedimentary source of Pb and, probably Hg, although a magmatic contribution, cannot be excluded (Higueras et al. 2005)., 40, Ar=39 Ar illite dating shows that hydrothermal, alteration is contemporaneous with magmatism, (Hall et al. 1997). Sulphur isotopes imply Ordovician, shales as source for both cinnabar and pyrite sulphur. Overall, the origin of this giant deposit with a, total mercury endowment of 320,000 t is insufficiently understood. Mantle metasomatism, rifting,, magmatism, diatreme formation, hydrocarbons, formed by maturation of organic matter and hydrothermal convection in sediments and magmatic rocks, may all have had a role (Hernandez et al. 1999, Higueras et al. 2005)., , close genetic and spatial relations to some hot, springs gold deposits (Sherlock 2005)., Almaden in Spain is the largest mercury concentration in the world (300 km southwest of, Madrid; Figure/Plate 1.89). It is the source of 30%, of total historic world production and encloses, 30% of remaining world resources of mercury,, although the mines are indefinitely closed since, 2002. From prehistoric times, mining took place, in the district, with prosperous peaks during, Roman (200 BCE to 400 CE) and Arab (711–1492, CE) occupation, and again in the Renaissance (16th, century). This bequeathed a heavily contaminated, landscape (Molina et al. 2006):, At Almad, en, steeply dipping Ordovician and Early, Silurian sediments contain quartzite bands with, lenses of cinnabar impregnations and veins. Enclosing black shales display native mercury ore. The, stratabound orebodies extend for hundreds of metres, along strike and dip. Most orebodies occur near, Ordovician-Devonian volcano-sedimentary rocks, that include black shales, sills, lava flows, tuffs and, diatreme breccias (“frailesca”) of heavily altered, alkali-basaltic and tholeiitic volcanics (Figure 2.36)., , Presently, China and Kyrgyzstan operate mercury mines. World production in 2008 was an, estimated 1320 t refined mercury (1280 t in, 2009). Consumption is estimated to 2000 t per, , Pozo, San Teodoro, , W, , Pozo, San Miguel, , Pozo, San Aquilino, , E, , Frailesca, , Silurian, black shales, , 378 m, , Mercury, ore bodies, in quartzite, 100 m, , 522 m, , Figure 2.36 Schematic profile of the giant mercury, deposit of Almaden, Spain (modified after Hernandez, et al. 1999). With permission from Springer, Science þ Business Media. The main orebodies occur, in the Criadero quartzite (Earliest Silurian) near, mafic volcanics and diatreme rocks (frailesca), rarely, in black shales. Highest-grade ore occurred near the, surface. Concentrations decreased with increasing, depth. For location refer to Figure/Plate 1.89.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , year, not least because many gold producers use, amalgamation (e.g. China, Indonesia and South, America). However, many mines and smelters, processing ore with traces of mercury have to pay, for permanent disposal. Small amounts of mercury continue to be sold from the stockpile at, Almaden, from decommissioning of chlor-alkali, plants worldwide and by geothermal power stations. Recycling is an important market factor., Mercury is traded in traditional units that are, called flasks (one flask containing 34.5 kg of, refined mercury)., 2.5.2 Antimony, Common Ore Minerals:, Wt. %, Sb, Stibnite, (antimonite), Tetrahedrite, (Antimony, fahlore), , Sb2S3, Cu(Ag,Fe,Zn)3, Sb(As,Bi)3,25, , 71, variable, , Density, (g/cm3), 4.6, 4.6–5.1, , The most common antimony ore mineral is stibnite, which may host traces of Au, Ag, Fe, Pb and, Cu. Other antimony-bearing minerals are of, minor economic significance. These include, native antimony and a number of sulphides, sulphosalts, oxides, antimonates and antimonites., Weathering of primary antimony minerals produces grey-coloured ochres and greyish-white senarmontite. Antimony ore is often a carrier of, important by-product Au and Ag, but also of undesirable As and Hg. Prevailing gangue minerals are, pyrite, quartz and carbonates., Mining grades in antimony-only deposits are, usually >3% Sb. Many lead, silver and copper ore, concentrates yield by-product antimony during, metallurgical processing. Antimony is traded as, a metal (density 6.62 g/cm3; melting point, 630.5 C, boiling point 1750 C) or oxide. The most, important antimony compound is the trioxide, Sb2O3, which is used as a flame retarder applied, to plastics, textiles and building materials (65%, of mine production). Other applications of anti-, , 243, , mony compounds include fireworks, ceramics and, glass. Antimony metal is part of various alloys,, mainly with lead in storage batteries, sheet and, pipe metal, and ammunition., Geochemistry, Similar to mercury and arsenic, geochemical concentration of antimony in near-surface epizonal, hydrothermal systems is striking. It is a chalcophile element such as As, Hg and Pb, and its, geochemical behaviour is intermediate between, mercury and lead. Igneous rocks contain an average of 0.2 ppm Sb (granite and basalt alike),, whereas pelitic sediments reach 2 ppm Sb. Antimony’s crustal abundance is 0.15–1 ppm (Smith &, Huyck 1999). Antimony occurs in valence states, Sb(V) and Sb(III)., Oxidation of stibnite results in formation of, antimony oxides with colourless senarmontite, III, (Sb2 O3 ) as the most common phase. In oxidized, and near-neutral pH surface or groundwater, senarmontite may yield dissolved Sb at concentrations to 50 mg/L (Craw et al. 2004). Dissolved, antimony is toxic and contents in drinking water, should not exceed 0.003 mg/L (Australian Drinking Water Guidelines). At low intake, antimony, poisoning is very similar to clinical effects of, arsenic. High doses of antimony are lethal but, exposure is very rare. Epizonal hydrothermal, metal deposits (e.g. gold) are most likely sources, (Milham & Craw 2009). Mobile antimony tends, to be immobilized and geochemically enriched in, iron oxy-hydroxides., Antimony in hydrothermal fluids may be, derived from degassing magma or from pelites,, especially those rich in organic matter. Mobilization of trace antimony from pelites is observed, in heated aureoles of igneous bodies or of orogenic, metamorphism. This behaviour relates antimony to other “volatile” metals and semi-metals, such as Hg, As, Te and Tl (Figure 2.23). Antimony, complexes, for example Sb2S2(OH)2 or H2Sb2S4, are vehicles of hydrothermal transport. Comparable to mercury, Sb solubility is mainly controlled by temperature (10,000 ppm Sb at, 300 C and only 1 ppm Sb at 100 C). Neither pH, nor H2S-activity or fO2 exert a noteworthy
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244, , PART I METALLIFEROUS ORE DEPOSITS, , influence. Precipitation is caused by falling temperature, either because of adiabatic cooling with, dropping pressure, or by mixing with cool nearsurface waters. Fluid inclusions in quartz of antimony deposits confirm entrapment at low temperature. Late Variscan (Carboniferous/Permian), antimonite mineralization in the Armorican, Massif (France), for example, was formed at, 140–160 C (Bailly et al. 2000)., Antimony ore deposit types, Deposits of volatile antimony are formed in epizonal low-temperature hydrothermal settings., Mobilization may be due to diagenetic heat and, fluid pulses, or to volcanogenic, magmatic and, metamorphogenic process systems. Important, antimony ore deposit types include:, . hydrothermal-exhalative sedex deposits (e.g. Xikuanshan in China), of considerable economic, importance;, . epizonal hydrothermal replacement ore in, limestone and dolomite, controlled by lowpermeability traps and fracture zones, and characterized by pronounced silicification; antimonite is, commonly the prevailing ore mineral, but deposits, that are transitional to epithermal gold deposits of, the Carlin type are known (e.g. Alshar in Macedonia with arsenic, gold and important reserves of, thallium: Volkov et al. 2006);, . antimony ore veins associated with orogenic gold, systems, base metals, and granite-related tin and, tungsten (Hillgrove, Australia; Bolivia: Dill 1998);, . terrestrial hot springs deposits as parts of metamorphogenic systems caused by extensional (graben) tectonics in volcanic regions (e.g. part of the, ores at Murat Dagi, Turkey: Gokce & Spiro 1994)., Antimony deposits are more numerous in, young orogenic belts than in geologically old terranes, due to deeper erosion of older mountain, belts and consequent loss of near-surface metal, concentrations. Numerous antimony vein and, replacement mineralizations occur in Tertiary, provinces of andesite and rhyolite volcanism., Other antimony provinces may have formed during late-tectonic flooding of the crust with granites. In Variscan Europe, a preponderance of, metamorphogenic mobilization is assumed, , (Wagner & Cook 2000), but antimony deposit, formation coincided with a period of granite intrusion and distension after folding (Munoz et al., 1992). Gold-antimony ore in shear zones of the, Archaean Murchison Greenstone Belt in South, Africa is interpreted as the epizonal expression of, an orogenic gold system (Figure 2.22)., In South China, important sedex deposits are, exploited, of which Xikuanshan is the largest, (Fan et al. 2004b). Two ore beds attaining a thickness of 5 m and an extension of several thousand, square metres occur between grey chert bands, within a Late Devonian suite of black shale, dark, marl and limestone (Figure 2.37). Ore strata consist of antimonite, quartz and calcite and traces of, other sulphides. Stockwork antimony ore with, additional barite and fluorite occurs below the, strata to a distance of 200 m. The deposit is, surrounded by a large halo of anomalous Sb, traces. With d34 S of 6.6‰, sulphur isotopes in, antimonite are thought to reflect biogenic sulphate reduction. During the Devonian, the area, was a shallow marine platform undergoing extensional strain., Many of China’s large and smaller antimony, deposits are probably magmatic-hydrothermal, products of the Mesozoic Yanshanian granites,, which display remarkable metallogenic fertility, (e.g. Dachang, Figure 2.20; cf. Section 2.1.7, “Tungsten”, Section 2.3.2 “Silver”, Section 2.2.3, “Tin” and Section 2.5.9 “Tantalum”):, The Au-Sb-W deposits near Woxi have combined, reserves of 1.8 Mt with an average grade of 13 g/t, Au, 4.5% Sb and 0.5% WO3. Host rocks are purple-red, slates of Mesoproterozoic age. Bedding-parallel, quartz veins (<1.5 m thick) account for most of the, production, although cross-cutting veins are also, exploited. The ore contains scheelite, wolframite,, stibnite, native gold, pyrite and other sulphides., Hydrothermal wallrock alteration is pronounced., Nd-Sr-Pb isotopic characteristics imply a granitic, source of the hydrothermal fluids, although granites, are not known in the district (Peng & Frei 2004)., Together with textures reminiscent of bedding, this, absence of granites has led to genetic interpretations, that invoke a sedex-like origin comparable to, Maucher’s (1965) once popular “volcano-sedimentary Sb-W-Hg formation”.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , m, , 245, , SE, , NE, , 700, Devonian, limestone, 600, Shale and, marlstone, , 500, Sb- F-Ba, stockwork, 400, , Sb-ore, Chert, , 300, , 100 m, , Figure 2.37 Stratiform sedex antimony orebodies in Late Devonian sediments at Xikuanshan, Hunan, China, (after Fan et al. 2004b). Copyright (2004) with permission from Elsevier., , Significant metamorphogenic deposits occur in, the Archaean Murchison Greenstone Belt, South, Africa, along a shear zone that extends over 55 km, (termed the “Antimony Line”) and is marked by, strong hydrothermal alteration. Komatiites,, for example, were transformed into conspicuous, massive talc-carbonate rocks. Host rocks of orebodies include quartz-chlorite and quartz-muscovite schist, quartzite, metabasalt and banded iron, formations. All these rocks display alteration in, greenschist facies and addition of CO2:, Orebodies consist of quartz veins and impregnation, zones with traces of scheelite, magnesite and talc,, which were originally worked for gold, but present, mining aims for antimony with by-product gold. Ore, minerals include antimonite, tetrahedrite and complex sulphosalts. d34 S data imply a magmatic source, of sulphur, probably leached from komatiites. Carbonate d13 C (4.7‰) is too heavy for biogenic carbon, and suggests a deep origin of CO2. The crustal-scale, shear zone (Figure 2.22) may have allowed upflow of, mantle fluids. The proposed genetic model emphasizes deep metamorphic fluids that distilled antimony and gold from metapelites (Pearton & Viljoen, 1986). Orebody characteristics and metamorphogenic hypotheses applied to the orogenic antimony-, , gold deposit Wiluna in Western Australia are very, similar (Hagemann & L€, uders 2003)., , The World’s leading producer of primary antimony is China (2008 >90% of a total of 197,000 t;, in 2009 falling to 187,000 t), followed by Bolivia,, Russia, South Africa and Tajikistan. Recycling of, used car batteries satisfies an important part of, demand., , 2.5.3 Arsenic, Common Minerals:, , Arsenopyrite, (mispickel), Loellingite, , Wt. % As, , Density (g/cm3), , FeAsS, , 46, , 5.9–6.2, , FeAs2, , 73, , 7.4, , The semi-metal arsenic occurs in many more, minerals composed of elements such as S, Se, Te,, Tl, Cu, Fe, Ni, Co and the platinum group elements (PGE). Arsenopyrite is especially common, in gold ore and often contains the gold. “Invisible”
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246, , PART I METALLIFEROUS ORE DEPOSITS, , gold in arsenopyrite may occur in the form of tiny, inclusions or in its crystal lattice (possibly with, Au-As bonds). Gold-rich parts are variably certain, growth zones, or fissures, or the rims of grains., Elevated trace contents of nickel and cobalt in, arsenopyrite are not rare. Formerly, the less common sulphides realgar (red, AsS) and orpiment, (yellow, As2S3) used to be precious pigments for, artists, in spite of their toxicity., Arsenic is a by-product of processing ore of, metals such as Ag, Cu, Zn, Sn and Au. The amount, of arsenic produced by mines and metallurgical, plants worldwide is much higher than consumption. Sulphide ore roasting results in sublimation, of “white arsenic” As2O3 that is known as an, infamous poison and a medical restorative. Today,, use of arsenic is strictly limited, for example to, pest control chemicals in agriculture (banned in, the EU) and wood conservation products such as, Cu-Cr arsenate in aqueous solution, that is nontoxic after drying. Arsenic compounds are used as, pesticides, herbicides, insecticides and in various, alloys. Traces of arsenic are added as a micronutrient to plant fertilizers. High-purity arsenic, metal (density 5.72 g/cm3; melting point 817 C,, boiling point 613 C) is increasingly required for, the production of high-capacity semiconductors, (e.g. gallium arsenide). Yet, the unfavourable market for arsenic forces many mines and metallurgical plants to dispose of all As-bearing material as, hazardous waste. One example is the Giant Mine,, Yellowknife, Canada, where between 1948 and, 1999, a total of 215 tonnes of gold and 237,000 t, of arsenic trioxide dust were produced. The dust, was stored underground in mine voids where it is, immobilized by freezing, because the mine lies in, a permafrost zone (Royle 2007). Former metallurgical plants were often a source of arsenic contamination in their neighbourhood, by roasting and, smelting of base and precious metals concentrates. Arsenic-contaminated soil can be bioremediated by planting and harvesting the fern Pteris, vittata (Gonzaga et al. 2007)., Trivalent arsenic is more toxic than other forms., Doses of more than 100 mg of arsenic will cause, acute poisoning, whereas chronic exposure impairs health and increases the risk of cancer. The, main path of arsenic intake of humans is drinking, , water. Plants cultivated on contaminated soil are, less frequently a source of critical exposure. Therefore, acceptable maximum levels in drinking, water are generally set to 10 mg per litre, the intervention level for soil in residential areas to, 55 mg/kg dry mass (The Netherlands) and the, inspection value for agricultural land to 200 mg/kg, (Germany). In small concentrations, however,, arsenic is an essential element (Oremland & Stolz, 2003). Healthy humans contain 18 mg As, that, must be continually replaced because of the short, biological half-life (10–30 h) of the element in the, body (Lindh 2005)., Geochemistry, Arsenic is one of the more “volatile” metals and, metalloids such as Hg, Sb, Te and Tl (Figure 2.23)., Arsenic exhibits redox-sensitive behaviour with, oxidation levels 3, 0, þ3 and þ5. It is geochemically similar to phosphorous and peculiar because, it forms both anions and cations. Its crustal abundance is estimated at 2 ppm (range 1.7–5) similar, to contents in average granite (1.5) and basalt, (2 ppm; Smith & Huyck 1999). Sediments contain, more arsenic than magmatic rocks, with highest, contents in pelites (13 ppm). Coals have particularly high arsenic trace concentrations. In the USA, arsenic (like Hg and Sb) is specified as a hazardous, air pollutant (Chapter 6.5 “Applications of Coal, Geology”)., With gold and silver, arsenic shares an elevated, solubility in the presence of H2S, which immobilizes other metals (Fe, Pb, Zn, Cu). The prevailing, species of arsenic in hydrothermal solutions is, H3AsO30 (aq), with Asþ3 at pH 1–8 and T up to, 275 C. Geothermal fluids often have very high, arsenic content (50 ppm), with varying ratios of, Asþ3 and Asþ5. In magmatic and hydrothermal, vapour above 400 C, arsenic is enriched in the, form of As(OH)3 compared to the liquid phase, (Pokrovski et al. 2002, 2005)., Weathering of arsenian iron sulphides produces, intermediate realgar and orpiment, and arsenates, such as the greenish scorodite FeAsO4.2H2O., Similar to antimony, Asþ5 is mobile as oxyanion, (AsO43) in oxygen-rich water and Asþ3 in, strongly reduced groundwater. Mobile arsenic is
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , efficiently immobilized and geochemically enriched in iron oxides, sulphides, in siderite concretions hosted by claystone, in phosphorites and, black shales. Mine-derived acid seepage is often, contaminated, even where only traces of arsenic, occur in the ore (Zhu et al. 2003). Under oxidizing, conditions and at pH 5–7, arsenic is hardly mobile, and virtually immobile if iron-rich particles are, abundant (e.g. hydrous ferric oxide precipitated, from mine water); transport is limited to suspended solids (Smith & Huyck 1999). Because, arsenic was not measured in earlier national geochemical surveys, the distribution in the Earth’s, surficial materials is poorly known (Plant et al., 2005)., Unrelated to mining, natural arsenic concentrations in aquifers and/or groundwater of Holocene, lowland valleys throughout the world are a hidden, hazard. In the Ganges delta, this caused, humanity’s biggest mass poisoning (Nordstrom, 2002, Smedley & Kinniburgh 2005). Earlier, millions of people in the region had relied on surface, water. From 1980 to 1990 thousands of drinking, water wells were drilled to provide “safe”, microbially clean water. Of these wells, nearly 50% had, As of >10 micrograms/litre. Sadly, this was only, recognized after numerous people had developed, skin and internal disorders, including cancer., With 2–20 ppm, the As-contents in Ganges river, sediments are quite ordinary. Spots of elevated, dissolved arsenic occur in low-sulphate groundwater in the presence of reactive organic matter,, which supports strong microbial reduction of iron, oxides and oxy-hydroxides releasing adsorbed, arsenic (Fendorf et al. 2010). Well-drillers should, be wary of extracting water from reduced, grey, sediments., Arsenic mineralization types, Arsenic is mobilized from crustal material, transported and precipitated wherever hot aqueous, fluids and vapours are cooled, including magmatic, diagenetic and metamorphic systems. As, a chalcophile element it is concentrated in many, sulphidic ore deposits. Processing these ores, for, example roasting, liberates arsenic (III) oxide,, which can be recovered., , 247, , Arsenopyrite and arsenian pyrite Fe(S,As)2 are, common in pyrite-rich copper ores (e.g. volcanogenic Boliden, Sweden; sedex Rammelsberg/, Germany), in metamorphogenic silver (Kongsberg, Norway) and cobalt-nickel ore veins (Bou, Azzer, Morocco: Ahmed et al. 2009), in numerous, gold deposits (distally magmatic-hydrothermal, Carlin, USA) and in many tin deposits (magmatic-hydrothermal Renison Bell in Tasmania, and Altenberg; Erzgebirge in Germany). Arsenopyrite, realgar and orpiment are often part of the, paragenesis in metamorphogenic, volcanogenic, epithermal and hot springs deposits of gold, antimony and mercury., Main producers of arsenic are China, Chile,, Morocco, Peru, Russia, Kazakhstan and Belgium., Annual world production is 53,000 t (trioxide)., Many mines find arsenic a liability, which requires permanent disposal. Arsenic is hardly ever, the target of exploration but retains its important, role as a useful pathfinder element for geochemical gold exploration., 2.5.4 By-Product Electronic Metals (Selenium,, Tellurium, Gallium, Germanium, Indium,, Cadmium) and Silicon, “Electronic metals” is not a strictly defined term., Several non-ferrous and semi-metals used for, manufacturing electronic equipment are treated, in separate parts of this chapter (e.g. copper,, antimony, arsenic, gold). Here, concise information is provided about elements that occur in, minor or trace amounts in ores of more common, metals. With the exception of silicon, the electronic elements are typically by-products of the, processing of sulphide ore of base and precious, metals. Consumption of single electronic metals, is of the order of tens or hundreds to a few thousand tonnes per year. With a market of 20,000 t/, year for cadmium and 40,000 t/year for silicon,, the two are clearly set apart. In metal trading, the, electronic metals are part of the “minor metals”,, but this sector expands rapidly. For applications, as functional electronic materials, an extremely, high purity is required and refining creates most, added value. Because this is often limited to, specialist companies, raw and intermediate
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248, , PART I METALLIFEROUS ORE DEPOSITS, , materials may be shipped across the oceans so, that their true origin remains uncertain. Also,, recycling (e.g. of electronic scrap) is an important, and often undisclosed source., , of electrolytic copper refining. Output, therefore,, is tied to countries that process copper concentrates (e.g. Japan, USA, Belgium), and to the vagaries of the copper market. World production is, 1500 t/year (USGS 2010)., , Selenium, Selenium (melting point 217 C, density 4.79 g/, cm3; crustal abundance 90 ppb: Smith & Huyck, 1999) is primarily recovered from copper sulphide, ore. Selenium also occurs in uranium ore of the, sandstone and unconformity types (Figure 1.57; cf., Section 2.5.12 “Uranium”). Because of similar, chalcophile geochemical characteristics, Se (and, Te) partially substitute for sulphur in many sulphides. Selenium is also enriched in epithermal, Au-Ag ore, in skarn and various base metal deposit, types. Large selenium-only deposits are known, in China (La’erma and Qiongmo in the western, Qingling mountains, and Yutangba in Hubei province: Xiong 2003). Under oxidizing conditions in, surface water, the element occurs in anionic form, but is immobilized by reduction. This effect concentrates selenium in organic matter-rich sediments such as black shales, phosphorite and, coal. Enrichment of selenium relative to sulphur, is favoured by oxidizing conditions in the haematite stability field (Simon et al. 1997, Xiong 2003)., Selenium minerals (e.g. FeSe2, PbSe) are rare and, economically insignificant. Selenium is unusual, in that it is both photovoltaic and photoconductive. Yet, 65% of selenium is used in glass and, pigments manufacturing (yellow, orange, red). Examples of electronic applications (10%) are certain, photovoltaic cells (e.g. solar panels). In recent, years, it is also used as an additive in the production of electrolytic manganese metal. Selenium,, tellurium and sulphur can perform as vulcanizing, agents in rubber compounding. For humans, animals and plants, selenium in low quantities is an, essential element (Lindh 2005). For this reason it is, a common additive to animal feed and to fertilizer., Humans suffer from dietary deficiency at intakes, <40 mg/day and from toxic effects at >400 mg/day, (Fordyce 2005). Because most rocks contain very, low concentrations, selenium-deficient environments are far more widespread than seleniferous, ones. Selenium is nearly exclusively a by-product, , Tellurium, Tellerium (melting point 449.5 C, density, 6.24 g/cm3; crustal average 4 ppb) is a very rare, chalcophile metalloid. Trace amounts are found, in many gold ores (e.g. sylvanite AgAuTe4 in, Transylvania (Romania) and in gold deposits, of the Western Australian greenstone belts; cf., Section 2.3.1 “Gold”), in silver ore (Bolivia), in, coal, lead ore and in many large copper ore deposits. The latter are the chief source of tellurium, recovered as a by-product of electrolytic copper, refining. The world total refinery production may, be assumed at 500 t/year (metal). In the USA,, 50% of tellurium is used as an alloying agent for, iron and steel, 25% for catalysts and chemical, purposes, and only 8% for electronic applications, such as CdTe solar cells. The latter application is, thought to expand rapidly, because the CdTe, technology produces the lowest-cost solar electricity (Zweibel 2010). The use of cadmium in, photovoltaics is contentious. Tellurium is mildly, toxic, similar to selenium., Gallium, Gallium (melting point 29.78 C, density, 5.907 g/cm3), with an estimated crustal average, of 17 ppm (Smith & Huyck 1999) is as abundant, as Pb (16 ppm). In contrast to galena, Ga-minerals are very rare (e.g. gallite CuGaS2). Chalcophile gallium is relatively enriched in, meteorites, in sedimentary iron ore deposits and, in gossans, in sphalerite and sulphosalts, in coal, (with some production from flue dust), pegmatites and greisen. However, most commercial, gallium is derived from bauxite, because Ga3þ, substitutes for Al3þ and Fe3þ in common rockforming minerals. Due to the low solubility of, a-GaOOH in the weathering environment, between pH 3–8 (Wood & Samson 2005), Ga is, enriched to 30–60 ppm in bauxites. During
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Bayer-Process leaching, Ga is precipitated from, the sodium-aluminate solution, providing >95%, of primary world production (2008: 111 t; 78 t in, 2009). Minor sources of economic gallium (plus, Cd and Ge) are sphalerite concentrates (e.g. in, the Mississippi Valley Province, USA). Ga, Ge, and In in sphalerite usually reflect substitutional coupled solid solution. In some coals and, in petroleum ash, gallium and germanium occur, in recoverable concentrations. Gallium is, mainly used in electronic applications (e.g., GaAs-base integrated circuits, LED-light emitting diodes, solar cells). Consumption of gallium, is expected to increase with the production of, gallium arsenide wafers for integrated electronic, circuits, competing with silicon., Germanium, Germanium (melting point 937.4 C, density, 5.32 g/cm3; crustal average 1.4 ppm) is mainly a, by-product of the smelting of certain sulphidic, copper and zinc ore concentrates, with grades of, several 100 ppm Ge (H€, oll et al. 2007). Examples of, primary (mining) sources of this type are Kipushi, (cf. Section 2.2.2 “Lead and Zinc”; St Salvy,, France), the Tristate District (USA), Petsamo (Russia), Tsumeb (Namibia; where large masses of, germanite Cu13Ge2Fe2S16 have been extracted),, Bor, Serbia and Red Dog (Alaska: Figure 1.73)., Germanium can also be enriched in the weathering environment, for example in the Apex deposit, (Utah), where the element occurs adsorbed in, jarosite and goethite/haematite (Wood & Samson, 2005). Ash from coal-combustion can be an economic source of germanium (Chapter 6.1 “The, Chemical Composition of Coal”). Germanium, minerals are rare. Chemical properties and the, geochemical behaviour of siderophile (Goldschmidt 1958) germanium are largely similar to, silicon. Germanium is applied in the production of, polymerization catalysts for PET-plastics (35%),, of infrared lenses and optical fibres for wiring the, internet (ca. 25%), and electronic and solar cell, applications (12%). The world refinery production, (140 t/a) is dominated by China and USA., Secondary sources and recycling are important, features of the market. Germanium displays the, , 249, , best performance of all semiconductor elements, and accordingly, demand is expected to grow., Indium, Indium (melting point 156.17 C, density, 7.31 g/cm3) has nearly the same relative abundance in the Earth’s crust as silver (50 ppb In, against 70 ppb Ag), but never occurs in sufficient, concentrations to be mined in its own right., It is chalcophile and concentrates in sulphides, (Schwarz-Schampera & Herzig 2002). Most, indium is a by-product of tin, high-temperature, zinc and copper ores at grades of up to 1000 g/t, concentrate. In the form of copper indium diselenide, indium is used as a transparent absorbing, semiconductor in solar cells. The metal is applied, as a surface coating in bearings and in low-temperature fusible alloys (e.g. solders). Its most, prominent application (80%) is in the form of, indium-tin oxide as a transparent conductor, in liquid crystal display (LCD) screens. A large, part of the world’s refinery production (2009: 600, t) comes from China, the Republic of Korea, Japan, and Canada (one important source is granitichydrothermal orebodies at Mt Pleasant,, New Brunswick: Sinclair et al. 2006)., Cadmium, Cadmium (melting point 320.9 C, density 8.65 g/, cm3) also is a by-product of zinc ore processing., Because of NiCd battery recycling, the refinery, production of Cd (2009 19,000 t) provides only, 80% of world consumption. Uses of cadmium, include NiCd batteries ( 80% of total consumption, with Cd as an oxide), pigments, stabilizers,, coatings and in small quantities, specialized alloys, and electronic products. In the future, solar panels, with Cd-S/Cd-Te are expected to win wide acceptance, because cadmium compounds provide a, perfect match to the spectrum of the sun. In spite, of the successful introduction of non-toxic Li-ion, batteries, NiCd batteries experience a rennaissance in the favoured electric vehicles. Cadmium, is extremely toxic and its use and recycling are, strictly controlled (Selinus et al. 2005)., Cadmium’s geochemical character is chalcophile
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250, , PART I METALLIFEROUS ORE DEPOSITS, , and its average crustal content 0.1 ppm. Pelites,, phosphorites and manganese nodules concentrate, cadmium to 25 ppm. Sphalerite carries cadmium, either in solid solution or as tiny inclusions of, CdS (greenockite), to a maximum of 5% (average, contents are 0.3%). Refined cadmium metal, production originates predominantly from Asia, (Korea, China, Japan) and North America (Canada, and Mexico)., Silicon, Silicon (melting point 1410 C, density 2.329 g/, cm3) is currently the most important electronic, material. Nearly all micro-electronic equipment, in electronic and photovoltaic applications is, based on ultra-pure (and doped) silicon. Annual, consumption of semiconductor-grade silicon is, estimated to 40,000 t/year. Read more about, silicon in Chapter 3 “Quartz”., 2.5.5 Bismuth, Common Ore Minerals:, , Native bismuth, Bismuthinite, , Bi, Bi2S3, , Wt. % Bi, , D (g/cm3), , 100, 81, , 9.8, 6.8, , Common impurities in native bismuth include Fe,, Te, As, S and Sb. Oxidation of primary bismuth, minerals produces the yellow or greyish-green, bismuth ochre minerals bismutite (BiO)2CO3 and, bismite a-Bi2O3., Bismuth is a non-toxic heavy metal (9.79 g/cm3)., Because of its low melting point (271.3 C) it is, used in lead-free solders and a variety of other, alloys. It is also a metallurgical additive (steel), a, replacement for lead in plumbing and a raw material for chemicals (yellow pigments, pharmaceuticals, cosmetics). Increasingly, lead shot is, replaced by an alloy of 97% bismuth with 3% tin., Geochemistry, Bismuth is an extraordinary element because its, only natural isotope 209 Bi is unstable, although, , with a half-life of 2 1019 years. Its daughter by, radioactive a-decay is stable 205 Tl (Marcillac et al., 2003). Bismuth’s average crustal abundance is, 0.2 ppm. Its geochemical behaviour is chalcophile and like Sb and As it is one of the “volatile”, metals. Trivalent bismuth is among the elements, that are highly enriched in the continental crust,, compared to primitive mantle. Bismuth is concentrated by magmatic and hydrothermal processes., The element is an essential component in low, melting-point assemblages such as the Au-BiTe-S system, which is common in magmatichydrothermal gold deposits (Tooth et al. 2008)., Under oxidizing conditions, bismuth is moderately mobile, but immobile in all reduced, environments., Bismuth deposit types, Bismuth occurs as a by-product in tin pegmatites,, in tungsten, copper, gold and lead skarn deposits,, and generally in magmatic-hydrothermal mineralization related to granites. It is an essential, component of hydrothermal Bi-Co-Ni (U-Ag), veins, for example at Jachymov, CSR (cf. Section 2.5.12 “Uranium”). Workable deposits with, bismuth as main metal are extremely rare; one is, Tasna in Bolivia., Cerro Tasna mine (or Tazna, presently on, standby) is one of the southern Bolivian tin-silver, belt deposits. Orebodies occur in the roof of a, Tertiary felsic intrusion. Host rocks of the deposit, are metapelites that were transformed into proximal quartz-tourmaline fels, overlain by an argillized zone. Numerous veins occur in the altered, rocks, with a paragenesis of native bismuth, bismuthinite, chalcopyrite and ferberite. Gold occurs, locally. Elsewhere in Bolivia, bismuth is a byproduct of silver and tin ore (e.g. Potosı)., The world’s largest single concentration of bismuth is the scheelite skarn deposit in Shizhuyuan,, China (Huan-Zhang et al. 2003; cf. Section 2.1.7, “Tungsten”). By-product bismuth is derived from, the processing of copper and lead ore, rarely also, from ores of tin, zinc, molybdenum, cobalt, gold, and silver. In some cases, bismuth is a detrimental, part of ore concentrates and lowers its value. Most, of the world bismuth production (2008 7700 t;
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , in 2009 7300 t) originates in China, Mexico, Peru,, Bolivia and Kazakhstan., 2.5.6 Zirconium and Hafnium, Common Ore Minerals:, Max. wt.% ZrO2, Baddeleyite, Zircon, , ZrO2, ZrSiO4, , 94, 67.2, , Density, (g/cm3), 5.5–6, max. 4.68, , Baddeleyite is exploited as a by-product of hardrock mining (its only present source is Kovdor in, Russia; cf. Chapter 3 “Phosphates”). Zircon is an, accessory in many common rocks and very stable, in the surface environment. Therefore, it is enriched in the heavy mineral fraction of clastic, sediments and is a by-product of ilmenite and, rutile placer mining. Zircon and baddeleyite typically contain 1.5 to 4% hafnium and a maximum, of 4000 ppm uranium and 2000 ppm thorium, (Hanchar and Hoskin 2003). This makes the, extremely robust zircon highly suitable for petrological studies and age dating with the U-Pb, method. However, the radioactive decay of, alpha-emitters U and Th causes chemical and, physical changes in zircon (“damage domains”), that may finally reach an amorphous state with, low density. In that case, age determination is, highly problematic. Many placer zircons contain, micro-inclusions of other minerals (e.g. of P, Fe,, Ti, etc.) that decrease their market value. Zircon, concentrates should have grades of >65% ZrO2., Exploitable placers commonly contain 0.2–3%, zircon. Zirconium is not toxic, but zircon’s radioactivity enforces careful management from the, mine to the end user. Therefore, zircon with U þ, Th <500 ppm is preferred., A high percentage of traded zircon is used as a, mineral, for example in abrasives, foundries and, refractories (glass, cement). Milled to very fine, particle size, zircon is applied in ceramic frits, and opacifiers (tiles, tableware and sanitary ware)., Zircon is also added to TV glass and computer, screens to block X-rays. Some zircon is processed, to synthetic zirconia (ZrO2, an important ceramic, , 251, , material) and zirconium metal. Zirconium metal, is an outstanding refractory material due to its, resistance to high temperatures (melting point at, 1852 C, boiling point 3580 C, density 6.5 g/cm3), and chemical attack. This accounts for its use in, the chemical and space industriers, and in the, form of zirconium alloy tubes as containers of fuel, pellets (rods) in nuclear reactors. Zircon is considered as a material in which to immobilize the, plutonium isotope 239 Pu over geological time, scales. 239 Pu and other actinides are waste products from uranium-fuelled nuclear reactors:, Hafnium (density 13.09 g/cm3, melting point, 2222 C) is geochemically very similar to zirconium, but with a crustal average of 4.9 ppm being less abundant. Hafnium is recovered as a by-product of zirconium metallurgy. Its separation from zirconium is, expensive, however. Hafnium uses are in superalloys, for gas turbines, control rods for nuclear reactors and,, although in small quantities, manufacturing microprocessors for the latest generation of computer, chips., , Geochemistry, As a lithophile element (similar to Ti und Hf), zirconium has a relatively high crustal abundance, of 160 (130–400) ppm. Zircon is a common mineral, in igneous rocks, clastic sediments and their metamorphic equivalents. Primary enrichment of zircon has been noted in alkaline magmatic rocks., Subeconomic concentrations are known in pegmatites. Hydrothermal zircon concentrations, are rare. One example is epithermal zircon and, uranium impregnation of felsic volcanic rocks at, McDermitt caldera, Nevada (Castor & Henry, 2000; cf. Section 2.5.12 “Uranium”). In surface, waters, zirconium displays a very low solubility., Zirconium salts are of low toxicity; the element, has no known biological function., Zirconium deposit types, Zircon deposits are almost exclusively coastal, placers that typically display a combined valuable, heavy mineral content of 4–10 weight %. Coproduct minerals are mainly rutile and ilmenite. Exploitable hard-rock concentrations of
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252, , PART I METALLIFEROUS ORE DEPOSITS, , baddeleyite are only known in alkali complexes, (Kovdor, Russia, which in 2007 produced 5 Mt, iron concentrate, 2.5 Mt apatite and 8200 t baddeleyite). At Palabora, South Africa, Zr-production, was terminated in 2002. In the near future, zircon, is expected to become a by-product of large sandmoving industries, such as the Canadian tar sand, operations (cf. Chapter 7.6)., One important region of zircon-rich coastal, placers occurs in Geographer Bay, south of Perth, in Western Australia. In this area, Precambrian, granulite and gneiss are overlain by Mesozoic, clastic sediments. Pleistocene to Recent coastal, cliffs and surf platforms are cut into the Mesozoic, at elevations of 0–76 m above sea level., Heavy minerals are enriched at the base of each, strandline. The oldest and highest group of deposits occurs near Yoganup, in a cross-section, consisting of lenticular orebodies (Figure 2.38), that reach a length of 30 km and a width of, 1500 m. The heavy mineral content of the Yoganup level surpasses 10 Mt:, The highest-grade black sands of Yoganup occur just, above Mesozoic or Pleistocene sediments. Their base, is marked by a coarse coastal conglomerate that, grades into black clayey sands with monazite and, , 15–20% zircon in the heavy mineral fraction. This is, overlain by sandy clay (10% zircon in HM) and, yellowish-brown aeolian sand (6% zircon in HM)., Overall, the HM fraction contains an average of, 65–80% ilmenite, 10% magnetic leucoxene, 10%, non-magnetic leucoxene, 5–12% zircon, <1% rutile,, and 0.2–1.1% monazite. The majority of heavy, minerals occur in the 100–150 mm grain size range, (fine sand). Towards the sea and upwards, both overall, HM content and fractions of monazite and zircon, decrease., A new province of mineral sand deposits was lately, discovered in the Pliocene inland sea sediments of the, Murray Basin, Victoria, Australia. The Douglas, deposit, for example, is based on several strips of, HM sands occurring over an area of 4 8 km. The, subhorizontal HM sand beds occur between finegrained sand and clayey silt horizons 10–15 m below, the surface (Figure/Plate 2.39). Measured and indicated resources comprise 45 Mt at 11.3% HM. Fractions of valuable HM in the total mass of heavy, minerals are 8% rutile, 10% zircon, 41% ilmenite, and 4% non-magnetic leucoxene (Whitworth 2009)., In the Tertiary Eucla Basin of western South, Australia, the Jacinth-Ambrosia mine is based on, resources containing 9.5 Mt HM composed of an, extraordinary 48% zircon, 28% ilmenite and 4%, rutile. Average grade is 6.5% HM., , W, , E, 60, , decreasing, HM contents, , Sandy clay (20-25 % HM), , Pleistocene sediments, , 40-60% HM, , Metres above sea level, , Pleistocene, steep coast, , Sand (5-15 % HM), , 30, Mesozoic sediments, 500 m, , Figure 2.38 Geological section of Pleistocene coastal placer deposits near Yoganup, Western Australia (modified after, Welch et al. 1975). The heavy mineral (HM) fraction consists of max. 80% ilmenite and leucoxene, 12% zircon, and, minor parts of rutile, monazite and xenotime. Near the surface, ilmenite is altered to leucoxene but this decreases with, increasing depth. Black ¼ High-grade black sand.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Coastal placers are explored by grid drilling,, which is carried out in prospective areas defined, by geological modelling. Reverse circulation or, sonic drilling is used for precise sampling. With, increasing refinement of the grid, the geology of, coastal sediments and the distribution of heavy, minerals are better understood. Important factors, of feasibility, apart from HM content, include the, thickness of the overburden, a minimal induration of the ore bed, low content of deleterious, “slime” and clay, the grain size distribution,, surface characteristics of HM minerals, the, mineral composition and the degree of ilmenite, oxidation (Jones 2009). Content of uranium and, thorium traces in zircon may result in the need, to control risks of radioactivity. Many coastal, landscapes constitute a precious environment, and are valued by society. In spite of efforts to, , 253, , preserve biota and to re-establish nature after, mining, the social licence for extraction of heavy, minerals may be unattainable in some areas. One, way of avoiding this conflict of interests is, exploring older inland seas (e.g. the Murray and, Eucla basins, Australia). Rehabilitation of heavy, mineral placer mines is much less complicated, compared to metal sulphide or coal pits (Jones, 2009)., Annual world production of zircon amounted to, 1.3 Mt (2008) and 1.2 Mt in 2009. Australia,, South Africa and China are major sources, producing from placers. Identified zirconium resources, are very large (56 Mt: USGS 2010). Nevertheless,, the availability of zirconium is decreasing and, prices are rising. One reason for this situation is, the dependence of by-product zirconium production on titanium placer mining., , Figure 2.39 (Plate 2.39) Bondi East heavy mineral deposit near the southeastern margin of the Murray Basin in, Victoria, Australia, looking north at an active mine face, which has been cleaned up for channel sampling and detailed, mapping. Courtesy David Whitworth (Iluka Resources). Barren overburden is removed; upper low-grade ore (ca. 10%, HM) is white sand with 30% clay. Note the near-vertical incision of the high grade ilmenite-zircon-rutile sand (dark,, 50–70% HM) at the right of the image. The footwall consists of barren massive silty sand (white). Patchy oxidation is, ubiquitous.
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254, , PART I METALLIFEROUS ORE DEPOSITS, , 2.5.7 Titanium, Common Ore Minerals:, , Rutile, Anatase, Ilmenite, , TiO2, TiO2, FeTiO3, , Wt. % TiO2, , Density (g/cm3), , >95, >95, 52 (35–70), , 4.2, 3.8–3.9, 4.5–5.0, , With an average crustal abundance of 0.5% Ti, (range 0.24–0.96%), or 0.8% TiO2, titanium takes, the ninth place of the most common elements in, the Earth’s crust. It occurs in 50 different minerals, but only the above listed are of technical, importance. Possibly, perovskite CaTiO3 found in, carbonatites and nepheline syenites may one day, be considered as an ore., Ilmenite is characterized by limited substitution, of Cr, V, Mg and Mn, rarely Nb and Ta. Elevated, contents of colouring metals such as iron reduce, its quality, as do traces of uranium and thorium., The wide range of TiO2 content is due to intergrowth of primary ilmenite with haematite,, because at >950 C a complete solid solution exists, between Fe2O3 and FeTiO3. Consequently, ilmenite concentrates of orthomagmatic ore contain only, 35–40% TiO2. Titaniferous magnetite with even, lower TiO2 content is of little use except in some, cases as an ore of vanadium (cf. Section 2.1.8, “Vanadium”). Ilmenites of secondary deposits with, a history of weathering, erosion and sedimentation, reach a maximum of 70% TiO2 by leaching of iron., In-situ weathering of magnetic placer ilmenite, transforms it into non-magnetic leucoxene, a, fine-grained, amorphous substance or a “mineral”, with a structure like rutile or anatase with 70–90%, TiO2 (M€, ucke & Chaudhuri 1991). Primary rutile, is only known as an accessory mineral in rocks;, enriched hard-rock occurrences are still subeconomic. Its high resistance to weathering and transport, however, leads to concentration in placers., Lowest exploitable grades in placers are 1% for, ilmenite and 0.1% for rutile. Extraction from hard, rock ore dictates grades ten times as high (the, Titania mine at Tellnes in Norway, for example,, works with ilmenite ore of 18% TiO2). Ilmenite, and leucoxene satisfy >90% of the world’s, , demand for titanium minerals. Traded concentrates should be free of radioactive trace minerals,, such as monazite (Th) and xenotime (U)., Roughly 95% of titanium production is used as, white and highly opaque TiO2 pigment that substitutes for lead oxides because it is not toxic. The, pigment is mainly used for manufacturing enamels, paint, paper and plastics. Other applications, are in the chemical industry and as a UV-blocker, for skin protection. As there is not enough natural, rutile available to satisfy demand, ilmenite is used, for the production of “synthetic rutile”. Rutile’s, main use is as a fluxing agent in welding rods., Titanium dioxide is produced by dissolving rutile,, ilmenite and leucoxene in sulphuric acid (the, sulphate process). Iron compounds are removed, and the remaining solution is hydrolized, precipitating hydrous titanium dioxide, which is washed, and calcined. The sulphate process accommodates, lower grade ilmenite, but the resulting high Fe and, low pH waste water is problematic. Therefore, low, TiO2 ilmenite is less attractive and the alternative, chloride process is preferred, although it requires, higher grade ilmenite (>60%) and rutile., Titanium metal is produced from titanium tetrachloride (TiCl4), which is prepared by chlorinating natural or synthetic rutile in the presence of, carbon. TiCl4 is reacted with molten magnesium, (or with sodium) at 850 C, resulting in formation of titanium sponge that is further processed to, market-grade metal. Titanium is a non-toxic, soft,, ductile, lightweight metal (density 4.5 g/cm3,, melting point 1668 C, boiling point 3260 C),, with excellent corrosion resistance and a higher, strength and flexibility than steel. However, its, production is expensive, resulting in high prices, (200 times the price of aluminium). Therefore,, use of pure titanium and different alloys concentrates on the aerospace industry, in desalination, and petrochemical plants, cars, cameras, laptops,, medical devices and sporting equipment. In 2008,, world production of titanium sponge amounted, to 170,000 tonnes (USGS 2009)., Geochemistry, In many rocks, titanium is one of the major elements. Highest content occurs in mafic (0.9–2.7%
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , TiO2) and alkalic magmatic rocks (0.1–3.3%), and, in eclogites (1.0–6.0%). Titanium is a tetravalent, lithophile high field strength (HFS) element., Because only oxide minerals of titanium can be, economically processed, geological systems that, favour the formation of oxides are required, which, is realized in certain magmas and by high-grade, metamorphism. Metamorphism does not affect, Ti-concentrations but releases titanium from, titanite CaTiO(SiO4), titaniferous biotite and, hornblende at amphibolite-granulite and blueschist-eclogite transitions. Titanium recrystallizes, as an oxide, mostly rutile (Bucher & Frey 1994)., This is the reason why granulites and eclogites are, optimal source rocks for titanium oxide placer, deposits. Ferrogabbroic eclogites may reach TiO2, content of >6%, almost exclusively in the form of, rutile. Such rock bodies (e.g. Piampaludo, Ligurian, Alps; Sunnfjord, Norway; Shubino, Urals) are, important potential resources of titanium, (Force 1991)., Titanium is geochemically rather immobile and a, reference element for many petrogenetic investigations (Pearce et al. 1984). An illuminating example is, the alteration halos of copper porphyry ore deposits., In unaltered porphyry, titanium occurs in titanite,, biotite and hornblende. All three minerals are, destroyed during hydrothermal alteration and finegrained rutile is present in the new paragenesis., The original TiO2- contents of the rock remain, unchanged., , Similar to zirconium and hafnium, titanium, displays a very moderate to low mobility in surface, environments. It constitutes no environmental or, health hazard. Currently, titanium metal is the, most common material used for medical implants, in the human body., Titanium ore deposit types, Titanium deposits include primary, orthomagmatic concentrations in intrusive rocks of anorthosite-ferrodiorite, tholeiitic and alkaline basalt, association, but foremost secondary alluvial and, coastal placers. Orthomagmatic and titanium, placer concentrations may be upgraded by supergene residual enrichment. High-grade metamor-, , 255, , phic rocks constitute potential hard-rock rutile, sources., Orthomagmatic titanium deposits, Igneous hosts of orthomagmatic titanium deposits, are Proterozoic massif anorthosite complexes and, tholeiitic layered intrusions; nelsonitic melts, occur in association with alkali basalts (Clark &, Kontak 2004). During cooling, titanium and iron, are concentrated in an immiscible oxide melt that, accumulates by gravitation at the base of the, magma chamber where massive layers of ilmenite, with magnetite or haematite accumulate. In some, cases, the melt filled interstitial spaces of cumulates or intruded footwall rocks forming dykes., Massive nelsonitic ore consists of ilmenite, rutile,, magnetite and apatite. Phosphorous reduces the, solidus temperature of the liquid. Examples of, primary titanium ore deposits include the giant, Lac Tio (Allard Lake) and other mines in Canada,, Tahawus (Sanford Lake) in the Adirondack Mountaions, USA, Otanm€, aki in Finland and Tellnes, in Norway (Duchesne 1999; cf. “Iron”):, Tellnes in southwestern Norway produces 1 Mt/a, ilmenite concentrate with a grade of 44% TiO2 from, banded ilmenite norite intruded into the centre of an, anorthosite province (ca. 920 Ma). Reserves amount, to 57 Mt TiO2. The sill crystallized in situ to a, cumulate facies with an average of 40% ilmenite, concentration. The ilmenite displays haematite exsolutions and a Cr-content, which decreases upwards, as magnetite increases (Blundell et al. 2005, Duchesne 1999)., , Similar to Tellnes, not all deposits were formed, by liquid unmixing of an ore melt. At the Koivusaarenneva mine in Finland, solid-phase ilmenite, was gravitatively enriched from silicate liquid, (K€, arkk€, ainen & Bornhorst 2003). In this deposit,, a gabbro sill contains several strata with 15%, ilmenite and 6% vanadium-magnetite, which segregated during lateral magma flow in the sill., Primary titanium concentrations in akaline, complexes occur preferentially with pyroxenite, (e.g. jacupirangite consisting of titanian augite,, magnetite and nepheline). Other common rocks, in these complexes are nepheline monzo-syenites
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256, , PART I METALLIFEROUS ORE DEPOSITS, , and a carbonatite core in the centre. The type, locality for the name “ilmenite” is an alkaline, complex in the Ilmen Mountains near Miask,, Urals, Russia. The ore consists of ilmenite, magnetite and perovskite, with changeable amounts, of rutile, brookite (a rutile polymorph), titanite, CaTiO(SiO4), apatite, carbonates and silicates., High trace contents of Nb, Th and REE in several, of these minerals are frequent but rather detrimental for use as a titanium ore. Lateritic leaching, removes gangue and enriches titanium in the form, of anatase. In Minas Gerais, Brazil hosts a province, of Cretaceous ring complexes, some of which, display supergene residual titanium mineralizations (e.g. Tapira carbonatite), while others have, bauxite, uranium, REE and phosphate deposits., Coastal placer deposits, Coastal placer deposits are the main economic, source of titanium. Placers provide 55% of world, production and contain 45% of known resources., The quality of placer concentrates is superior to, primary hard rock ore, because they consist essentially of ilmenite-leucoxene and rutile. Extraction, and processing of unconsolidated beach sands is, inexpensive and consequently, economically, recoverable grades may be lower than the average, crustal abundance of titanium. The formation of, these placers is the product of a complex interaction of source area, weathering (climate), erosion,, alluvial transport, marine dynamics, wind and, the morphological evolution of the coast. Most, large deposits date from the Quaternary and occur, above and inland of today’s coastlines, attesting, to formation during former sea level high-stands, of interglacial periods. Older deposits are more, affected by supergene alteration. Youngest deposits are sections of Holocene beaches. Mining of, titanium minerals below present sea level, concentrated at low-stands during glacial periods is, uncommon (in contrast to cassiterite and diamond). Typically, coastal placer orebodies are sand, beds with a thickness of 10 m, a width of, 100–1000 m and a length of over 10 km. HM sands, are well-rounded, equigranular and medium- to, fine-grained. Both beach and dune sand may be, mineralized., , Nearly all titanium placers comprise valueless, heavy minerals such as magnetite, tourmaline,, sphene and apatite, and others that may be profitable co- or by-products. The most important of, these is zircon, occasionally monazite and silicates such as kyanite and garnet (Port Gregory,, Western Australia). Combined valuable heavy, mineral (VHM) contents are commonly in the, range of 4–10 wt.%., Australia is endowed with large titanium placers both on its eastern (Queensland, New South, Wales, Victoria) and its western coast, especially, south and north of Perth (Figure 2.38). The high, chromium content of ilmenite in the eastern deposits reduces the value of concentrates considerably and rutile is the main product. In the west,, chromium is at acceptable levels (<0.03%)., Extraction of heavy minerals in Australia is now, moving into interior Australia. In the Murray, Basin, Pliocene (palaeo-) coastal barrier placers are, estimated to contain over 50 Mt of rutile, ilmenite, and zircon (Roy et al. 2000). The Douglas mine, for, example, commenced operations in 2004, based on, a resource of 45 Mt at 11.3% heavy minerals, (Whitworth 2009). The Richards Bay deposit near, Durban in South Africa is a broad belt of aeolian, sand with ilmenite, monazite and some leucoxene. Ilmenite is mainly produced at Quilon in, Kerala, India. Large coastal placer districts are, mined or under investigation in China, Florida,, USA, Sri Lanka, Thailand and Madagascar. In, Canada, work is in progress to produce titanium, minerals and zircon from oil sands in Northern, Alberta. Note that in this case, the heavy minerals, are not enriched, as these sands are not placers., The envisioned profit is constituted by the economy of scale, because of the giant mass of sand, turned over by the oil industry (cf. Chapter 7.6, “Tar Sand”)., Alluvial placers, Economically exploitable alluvial placers of titanium oxides are rare. One reason for this is the, small volume of most alluvial placers, considering, that favourable deposits should contain >1 Mt of, recoverable titanium minerals. A second point is, that alluvial placers more often contain unaltered
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , ilmenite, resulting in low titanium content of, concentrates. Exceptions include Gbangbama in, Sierra Leone, where rutile is derived from a granulite source area and cassiterite placers in Malaysia,, where by-product rutile is sourced from highly, specialized felsic intrusions (“tin granites”)., The world’s titanium resources are very large., World production in 2009 was 5.2 Mt ilmenite and, 530,000 t rutile (TiO2 contained). Leading producers are Australia, South Africa, China and Canada, (the latter sourced from hard rock mining). Worldwide search for new deposits is intensive, mainly, targeting coastal placers (for notes on exploration, refer to Section 2.5.6 “Zirconium and Hafnium”)., Exploration, reserve estimation, extraction and, rehabilitation of coastal placer deposits differ in, several aspects from hard rock deposits (Jones, 2009, Lee 2001). Increasingly, communities prefer, non-mining uses of their coasts, which restricts, accessability. It is assumed that because of this,, primary titanium ore deposits will soon gain economic importance (e.g. the rutile-eclogites in the, Norwegian Sunnfjord)., 2.5.8 Rare Earth Elements (REE, Lanthanides), Common Ore Minerals:, , Monazite, Bastnaesite, Xenotime, Loparite, , (Ce,La,Y,Nd,Sm,Th), PO4, (Ce,La,Y) CO3(F,OH), Y(HREE)PO4, (Ce,Th,Na,Ca)2, (Ti,Nb)2O6, , Max. wt.%, REE-oxide, , Density, (g/cm3), , 65, , 4.6–5.4, , 75, 61, 36, , 4.9, 4.8, 4.6–4.9, , The lanthanide series of the rare earths group, comprises 14 elements from lanthanum to ytterbium. Of the actinide series of the rare earths, group, only uranium and thorium are economically significant (cf. “Uranium”). Valued REE, include cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb) and dysprosium (Dy)., Yttrium (Y), scandium (Sc) and thorium (Th) are, not rare earths sensu stricto, but have physical and, , 257, , chemical properties, and uses that are similar to, REE (“pseudolanthanides”). Based on relative, atomic mass, the elements La to Sm are called, light (cerium, or LREE) and Eu to Lu heavy, (yttrium, or HREE)., There are 250 minerals containing rare earths, but only the four listed above are economically, important. Although natural processes never separate individual REE, monazite and bastnaesite, contain mainly LREE (La, Ce, Pr, Nd), whereas, xenotime and loparite host significant HREE content (M€, oller et al. 1989)., Monazite is a common accessory mineral in, intrusive, and amphibolite to granulite facies, metamorphic rocks. Based on U-Th-Pb isotope, systems, it serves as a useful geochronometer., Sourced from eroding land, the mineral is enriched, in placers where it can be a co- or by-product of, ilmenite, rutile, zircon, cassiterite and gold extraction. Monazite is singled out by its g-radiation due, to daughter nuclides of thorium, which makes, radiation a convenient proxy for estimating REE, content. The sources of xenotime are mainly granites and pegmatites, which host the mineral as a, disseminated accessory. It is a minor component, in alluvial tin placers and coastal mineral sands., Bastnaesite is the major ore mineral of hard rock, REE deposits. Mining grades are 2–6% rare earth, oxide (REO), the range being due to content of the, more valuable REE. Orthomagmatic and magmatic-hydrothermal apatites of carbonatite and, alkali complexes (cf. Chapter 3 “Phosphates”), contain anomalous REE yet the concentrations, are commonly sub-economic. Until 1998, 0.85%, REO was recovered from apatite and loparite concentrate produced from nepheline syenite pegmatites of the Khibiny and Lovozero massifs, Kola, Peninsula, northern Russia (cf. Section 2.5.9, “Niobium and Tantalum”)., Uses of REE started more than 100 years ago, with Welsbach mantles for coal gas lanterns made, of 99% thorium dioxide and 1% cerium dioxide., Today, 98% of consumption concerns what is, called mischmetall (German for “mixed metal”),, which is an alloy of unseparated REE in the, proportion inherited from ore (usually cerium,, lanthanum, neodymium and several minor, REEs). It is used in steel-making, for producing
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258, , PART I METALLIFEROUS ORE DEPOSITS, , petroleum-refining catalysts, for lighter flints and, in magnesium metallurgy. The separation of individual REE is elaborate and costly. Enrichment, of single elements to 60–90% allows more specialized uses such as colouring glass and ceramics,, and the production of high-strength magnets (e.g., Nd-Fe-B, SmCo5). Extremely pure (>99.99%) REE, are required in small amounts only. Eu and Y, for, example, are employed as phosphors in television, and computer screens, neon tubes and X-ray equipment, and for some Laser machines. Other applications include superconductor technology,, hydrogen storage and catalytic converters for automobiles. Toyota’s Prius hybrid car reportedly contains >22 kg of REE in nickel-hydride batteries,, braking systems, exhaust converters and electric, motors. Chinese scientists report that traces of, REE in fertilizer considerably improve agricultural, harvests., Geochemistry, All lanthanides have a similar geochemical behaviour, commonly occur in trivalent state and form, stable compounds. Exceptions are europium that, displays both trivalent and bivalent states, and, cerium with an additional oxidation state of Ce4þ., Consequently, the distribution of europium and, cerium in natural systems is a function of redox, conditions, which control enrichment or depletion (“anomalies”) relative to other lanthanides., Furthermore, geological processes can lead to fractionation between light and heavy REE caused by, the difference in atomic mass and ionic radius., This allows improved understanding of earth processes. Anomalous REE-distribution is compared, with C1 chondrites and shale composites (Gromet, et al. 1984)., The combined average crustal abundance of, lanthanides (including yttrium) is estimated at, nearly 200 ppm (Clarke value), with cerium, (66.4 ppm), lanthanum (34.6 ppm) and yttrium, (31 ppm) being most abundant (Smith & Huyck, 1999). In spite of their name, REE are several orders, more abundant than gold, for example. Lanthanides are lithophile elements. In rocks, REE are, mainly concentrated in accessory minerals (monazite, titanite, zircon, apatite) and occur as trace, , elements in amphibole, pyroxene, feldspar, garnet, and mica. Differing ionic radii cause a tendency, for enrichment of heavy REE in certain minerals, (xenotime, garnet, zircon) and of light REE in, monazite and bastnaesite. In hydrothermal systems light REE are more mobile than heavy REE., The mobility of the small, highly charged REE, Y, and Zr ions, depends on the availability of intermediate and hard ligands such as chloride, fluoride, sulphate and carbonate. As these are, common, considerable hydrothermal mobility is, the result (Lottermoser 1992). Magmatic bodies, that have segregated much fluid or interacted with, passing fluids (e.g. tin granites) display a strong, loss of LREE. High salinity and CO2 content of, fluids are crucial factors of REE mobilization,, transport and REE ore formation., Under oxidizing and acidic conditions on the, Earth’s surface, REE are mobile, similar to uranium (but note the near-immobility of thorium)., At circum-neutral pH mobility is reduced and, the presence of iron-rich particles effectively, immobilizes REE. In reduced environments, REE, are virtually immobile, as are uranium and, thorium., Rare earth ore deposit types, REE deposits occur in a number of genetic classes, and types, but most deliver by-product REE only., Hard rock economic concentrations and pre-enriched REE as protore for supergene enrichment, are primarily related to magmatic systems:, . orthomagmatic to pegmatitic magnetite-apatite, with REE (associated with alkali complexes, e.g., Kola Peninsula; cf. Chapter 3 “Phosphates”);, . pegmatites (REE accessory minerals recoverable, with ore of tantalum and tin);, . skarn deposits, e.g. allanite (Ce,Ca,Y)(Al,Fe)3, (SiO4)3(OH) of the former uranium mine Mary, Kathleen in Queensland, Australia;, . orthomagmatic to magmatic-hydrothermal ore, in carbonatite dykes and intrusions (Mountain, Pass), and in associated veins;, . magmatic-hydrothermal,, low-sulphur, iron, oxide-copper-gold (IOCG) deposits, characterized, by large masses of magnetite or haematite (e.g., Olympic Dam, cf. Section 2.2.1 “Copper”);
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260, , PART I METALLIFEROUS ORE DEPOSITS, , Residual enrichment blankets, Residual enrichment blankets above granites of, mixed REE (mainly yttrium, samarium, europium, and gadolinium) in autochthonous saprolite are, exploited in Jiangxi, South China. Source rocks, are Mesozoic granites with bulk REE contents, between 300 and 400 ppm. Primary carriers of REE, in the granites are zircon, apatite and allanite., However, REE fluorocarbonates were found on, grain boundaries and in microscopic fissures of, silicate minerals, and replacing altered biotite., These observations are thought to present evidence, of deuteric (post-solidus) magmatic-hydrothermal, alteration, which produced the easily soluble REE, fluorocarbonates accessible for supergene leaching, and enrichment (Ishihara et al. 2008)., Residual lateritic regoliths, Residual lateritic regoliths above carbonatite are, at present rather niobium (e.g. Araxa, Brazil) and/, or apatite deposits. Most display enrichment of, REE, but recovery preoccupied metallurgists and, scientists for many decades. Although some production from lateritic ore is reported, a general, breakthrough has not happened:, A new effort is under way at Mt Weld in Western, Australia. Mt Weld reserves are reported as 7.7 Mt at, 12% REO þ Y, including a high grade part of 1.2 Mt, with 16% rare earth oxides. The estimate is calculated using a minimum content (cut-off grade) of 4%, , REO in situ. The high grades are due to supergene, enrichment (Figure 2.40, Lottermoser 1995). The, main carrier of REE content is secondary Th-depleted, monazite. The main product of the mine will be, phosphate with by-products Nb, Ta, Y (3.1%) and, HREE (3.9% of total REE). A first batch of 800,000 t, of ore grading 15.4% REO was mined in 2008., , Coastal placer characteristics are presented in, Chapter 1.3 “Sedimentary Ore Formation Systems” and in Section 2.5.6 “Zirconium and Hafnium”, and Section 2.5.7 “Titanium”. Note that for >15, years, the thorium content of monazite from most, operations precluded its use as a rare earth, feedstock., Main producer countries of REE are China, India, and Brazil. Annual world production amounts to, 124,000 t REE-oxides (USGS 2010). Largest reserves and resources occur in China, which dominates world markets of REE with nearly 97% of, world production. Many potential deposits of, mainly LREE are known elsewhere, however., Demand for, and consumption of different rare, earths vary considerably. Generally, light REE are, in oversupply, whereas demand for Eu and several, scarce REE (Nd, Dy and Tb) increases. Recoverable, contents, price and geopolitics of these latter, explain the curious situation that exploration and, new developments are undertaken in spite of giant, REE resources. Carbonatites are an easy target, for magnetic, gravity and radiometric methods., Testing for ore is mainly based on shallow drilling., , E, , W, , m, 420, , Quaternary, Tertiary (lacustrine clay, and sand), , Te, , 400, 380, 360, , Prz/g, , Prz/g, Archaean, basalts, , Prz/g, Carbonatite, , Dol, , Base of, weathering, 1 km, , 340, 320, 300, , Figure 2.40 Section of the residual phosphate-niobium-tantalum-yttrium-rare earth deposit at Mt Weld in Western, Australia (modified after Duncan & Willet 1990). Metal-rich phosphates (black) occur within a deep lateritic, weathering profile of Proterozoic carbonatite. Near the intrusive contact, basaltic country rocks are altered to, phlogopite (Prz/g). Dol – a dolerite dyke.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 261, , 2.5.9 Niobium and Tantalum, Common Ore Minerals:, Density (g/cm3), Niobite, Tantalite, Pyrochlore, , (Fe,Mn)(Nb>Ta)2O6, (Fe,Mn)(Ta>Nb)2O6, (Na,Ca)2(Nb,Ti,Ta)2O6(O,OH,F), , Microlite, (tantalum-rich end, member of the, pyrochlore series), Wodginite, , min. 5.2, max. 7.9, 4.2–4.5, , (Na,Ca)2Ta2O6(O,OH,F), , 31–79% Nb2O5, 52 to 86% Ta2O5, 56–73% (Nb,Ta)2O5, þ3–6% LREE-oxides, max. 80% Ta2O5, , MnSnTa2O6, , max. 70% Ta2O5, , 7.3, , Note that pure niobite and tantalite are unknown, in nature; they are theoretical end members of a, solid solution series called columbite. Tantalite is, defined by an Nb/Ta atomic ratio of <1., “Columbite” is occasionally used to designate, Nb-rich minerals of the columbite group, because, in America until 1949, the element niobium was, called “columbium”. Before discovery of pyrochlore in carbonatites of Brazil and Canada,, columbite was the main source for both tantalum, and niobium. The chemical composition of, columbites is commonly presented in the, “columbite quadrilateral” that depicts changes in, the Fe/Mn and Nb/Ta ratios (Beurlen et al. 2008)., With increasing fractionation of pegmatites,, columbites are enriched in tantalum and manganese. Columbites contain traces of Mg, Bi, Sn, W,, Ti, Sc, REE, Th and U. In concentrates, the latter, may enforce radiation protection measures. Inclusions of columbite and other Ta-minerals in, cassiterite of pegmatite deposits are frequent., As columbite is initially free of lead and because, of its resistance to later alterations, the mineral is, useful for U-Pb dating of granites and pegmatites, (Romer et al. 2007). Scandium in columbite and, related minerals may reach >6 wt.% (Kempe &, Wolf 2005):, Scandium (density 3.0 g/cm3, melting point 1539 C), is a typical dispersed element. Its Clarke value, (25 ppm) is higher than that of tungsten and lead, (1.2 and 13 ppm, respectively) but concentrations, and minerals of this element are extremely rare., , 6.3, , Together with Y it is one of the “pseudolanthanides”, (cf. Section 2.5.8 “Rare Earth Elements”). In igneous, rocks, Sc3þ is positively correlated with Fe2þ. Because, scandium is preferentially enriched in clinopyroxene,, amphibole and ilmenite, high-silica magmatic rocks, have low contents (3–5 ppm). Scandium may be enriched to economically significant by-product grades, in bauxite, nickel laterite, carbonatites, phosphorites,, Ti-placers, Ta-Nb ore, wolframite, cassiterite, uranium ore and in zircon (Wood & Samson 2005, Kempe, & Wolf 2005, Wiesheu et al. 1997). Among other, applications, scandium is useful as a grain refiner in, aluminium alloys, and in solid oxide fuel cells., , Pyrochlore is chemically variable; for example,, calcium can by substituted by Ba, LREE and U., Therefore, pyrochlore tends to be radioactive and, metamict, i.e. its crystal structure is damaged by, radiation. Often, its paragenesis includes minerals of Ti, U, Th and REE. Bariopyrochlore is the, main ore mineral at Arax, a. Formerly in Russia,, niobium was produced from loparite concentrates derived from nepheline syenites of the Kola, Peninsula. Loparite is a perowskite (CaTiO3) containing Nb and Ce. Compared with pyrochlore,, however, loparite is economically not competitive. Exploitable niobium ores have minimal, grades of 0.3%, although at Arax, a the ore contains 2.5% Nb2O5. Tantalum ore is extracted at, cut-off grades of 0.03% Ta2O5, generally as a coproduct of cassiterite and other minerals. Because, of easy overgrinding, recovery of columbite by, crushing and milling hard-rock ore is often as, low as 50%.
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262, , PART I METALLIFEROUS ORE DEPOSITS, , Table 2.2 Properties of tantalum and niobium metal, , Tantalum, Niobium, , Density, (g/cm3), , Melting, Point, , Boiling, Point, , 16.7, 8.57, , 2996 C, 2477 C, , 5429 C, 4927 C, , Tantalum metal is grey, heavy and very hard,, but malleable and ductile. It has a high melting, point exceeded only by tungsten and rhenium, (Table 2.2). Tantalum alloys with other elements, display great strength, good ductility and high, melting points. Niobium is mainly noted for its, enhancement of strength in alloys., Most niobium is consumed for production of, micro-alloyed steel (with 0.03% Nb) used for, high-pressure pipeline tubes, offshore petroleum, drilling and exploitation platforms, and automobiles. Nickel-free stainless steels contain 0.5 and, superalloys ca. 5% niobium (e.g. heat-resistant, equipment, jet engines). Niobium-titanium (-zirconium) alloys are materials suitable for supraconducting magnets. Some niobium, but 65% of, tantalum supply is destined for the electronic, sector. The remainder of tantalum production, serves in manufacturing cutting tools (tantalum, carbide), high-temperature alloys (e.g. turbines of, gas-fired power stations) and extremely corrosionresistant equipment of the chemical industry., There are also medical applications (e.g. implants), based on tantalum’s total inertness to body fluids., Most electronic-grade tantalum metal is used as, a capacitor core, together with some Ta-pentoxide, as dielectric barrier. Tantalum is essential for, portable electronic devices such as mobile phones,, laptop computers, digital cameras and navigation, systems in cars and aeroplanes., Geochemistry, The geochemical behaviour of niobium and tantalum in crustal processes is nearly identical., Both are incompatible and lithophile highfield-strength elements (HFSE). With a crustal, abundance of 20 ppm, niobium is more common, than tantalum (2 ppm). Both elements are en-, , riched in highly differentiated granites, in alkali, granites and syenites, in carbonatites and in raremetal pegmatites. Niobium outweighs tantalum, in carbonatites and syenites, whereas the latter, prevails in granites and pegmatites (Figure 1.18)., Columbite saturation in F- and Li-rich granitic, melt at 600 C is reached at 2000–4000 ppm, TaþNb. In hydrothermal systems, niobium and, tantalum are extremely immobile (Linnen 1998)., Therefore, columbites crystallize from melts and, not from hydrothermal fluids (Lichtervelde et al., 2007). Columbite is very resistant to surficial, chemical alteration but is rapidly reduced to fines, by alluvial transport. Accordingly, only residual, and proximal eluvial/alluvial placers are preserved., Niobium and tantalum ore deposit types, Nb > Ta: Lateritic regolith (residual supergene, enrichment blankets) above carbonatite intrusions, (Arax, a, Brazil, potentially Mt Weld, Australia);, . Nb > Ta: Hydrous schlieren of hard rock carbonatite or nepheline syenite with magmatic pyrochlore (Arax, a, Brazil; Niobec, Canada);, . Ta > Nb: Rare-element granites (“tantalum, granite”) with tantalum-rich columbite and cassiterite (Yichun, South China: Yin et al. 1995;, Figure 1.15; potential future mines include Ghurayyah in Saudi Arabia and Abu Dabbab in Egypt), . Ta > Nb: Rare-element pegmatites of the Li-CsTa type with tantalum-rich columbite and cassiterite (e.g. Wodgina and Greenbushes in Australia:, cf. Section 2.5.10 “Lithium”, Figure 2.41), . Ta > Nb: Tin placers (Malaysia, Nigeria, Central, Africa)., ., , Alkali complexes, Alkali complexes with carbonatites and nepheline, syenites occur along continental rifts or in zones of, extensional crustal thinning. However, the primary hard and low-grade rocks are rarely mined., More than 90% of the mines extract residual, supergene enrichment blankets. At Arax, a in, Minas Gerais, Brazil, unweathered carbonatite, contains only 1.5% Nb2O5 compared to regolith, ore with 2.5% Nb2O5. The complex has a diameter
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 263, , E, , W, Weathered zone, Archaean, metasediments, and amphibolites, , 1300, , m, , Ore, >2% Li2O, A, , 1200, , Li2O, >0,75%, 1100, , uP, Kfs, 1000, , Mafic, sill, Kfs, A = Archaean, metasediments, and amphibolites, , Sn-Ta-albitite, , 900, , 800, 100 m, , Border zone, , Figure 2.41 Cross-section of the Archaean Greenbushes Sn-Ta-Li pegmatite in Western Australia (after Hatcher &, Clynick 1990). Note the complex and irregular internal zonation. uP is undifferentiated pegmatite. Part of the, weathered zone (regolith) contains high-grade kaolinite. The underground mine was closed in 2006., , of 5 km. Weathering reaches to a depth of 240 m., Reserves and resources amount to 450 Mt., Tantalum contents improve the viability of, such deposits, whereas elevated concentrations, of radioactive elements are detrimental. Because, of commonly high phophorous (magnetite and, phlogopite) content in the protore (the unaltered, carbonatites), the supergene ores are phosphaterich and rather constitute phosphate REE deposits like Mt Weld, with estimated resources of, 273 Mt at 0.9% Nb2O5 containing 145 Mt of Ta2O5, ore at 0.034% (cf. Section 2.5.8 “Rare Earth, Elements”, Figure 2.40). Recovery of Nb and Ta, from the supergene ore at Mt Weld is expected to, comprise gravity and magnetic separation, flotation, caustic and acid leaching (Aral & Bruckard, , 2008). The origin of the giant REE-Nb-Fe deposit, Bayan Obo in Northern China remains disputed, (Fan et al. 2004)., Tantalum granites and pegmatites, Tantalum granites and pegmatites are typically, derived from highly fractionated, hydrous, residual melt batches of felsic magma bodies. Similar to, lithium, tantalum is enriched in extremely fractionated felsic magmas at <0.05% of the original, volume (Evensen & London 2002). The resulting, rocks are marked by magmatic albite, often grading into monomineralic albitite (Figure 2.41)., Post-solidification, metasomatic-hydrothermal, albitization is also possible (Schwartz 1992). In a
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264, , PART I METALLIFEROUS ORE DEPOSITS, , produce mainly Ta2O5, cassiterite and ceramic-grade, feldspar. Nearby prospects at Nuweibi in Egypt are, comparable (Helba et al. 1997). Ghurayyah in Saudi, Arabia is thought to be the world’s largest potential, Ta-(Zr) deposit (385 Mt of ore containing 94,000 t, Ta2O5)., , scientific deep continental drilling programme,, the Late Palaeozoic lithium-tantalum-tin granite, of Beauvoir in the French Massif Central was, intensively investigated. Results showed that, only magmatic processes including fractionation, contributed to metal enrichment (Figure 1.18), and, hydrothermal metasomatism could be excluded, (Raimbault et al. 1995). However, Beauvoir is a, kaolin mine with minor by-production of cassiterite (800 g/t Sn) and columbite (190 g/t Ta plus, 120 g/t Nb)., The Abu Dabbab area in Egypt is part of the, Neoproterozoic basement east of the River Nile., In this province, the youngest (and smallest) intrusive members of a suite of post-tectonic granites, are either mineralized or associated with tin and, tungsten pegmatites, and quartz vein deposits:, , Tantalum pegmatites are not rare, with deposits, known in many countries (Canada, Brazil, Ethiopia, Australia: Figure 1.20 and Figure 2.41). The, Kenticha deposit in Ethiopia is a highly fractionated subhorizontal Li-Cs-Ta pegmatite sheet with, a thickness approaching 100 m, which is related to, Late Neoproterozoic I-type granites. Exploitable, tantalum grades occur in its upper zone characterized by spodumene and high concentrations of Rb,, Cs and Ga (K€, uster et al. 2009). Numerous, relatively small deposits of this type are common in, Rwanda, Burundi and Eastern Congo. Swarms of, pegmatites form districts (e.g. Gatumba in, Rwanda: Figure 2.42). The giant subhorizontal, pegmatite sheet at Manono (D.R. Congo) resembles the Kenticha and Bernic Lake (Canada) pegmatites. Central African tin-tantalum pegmatites, are associated with late-tectonic, Early Neoproterozoic tin granites of the Kibara orogen, which are, related to the final assembly of Supercontinent, Rodinia. Varlamoff (1972) discerned lithium-rich, internally zoned pegmatites with by-product beryl, , Abu Dabbab is a small conical mountain (Figure/, Plate 2.18) built of the apical part of a granite cupola., The exposed rock is fine-grained leucogranite consisting mainly of albite and some microcline, quartz, and muscovite. Ore minerals are disseminated in the, uppermost 130 m of the granite beneath the roof. The, paragenesis comprises Ta-rich columbite, cassiterite,, pyrochlore, monazite, rutile, zircon, magnetite,, galena and sphalerite. Total resources are estimated, at 40 Mt at 252 ppm Ta2O5, 116 ppm Nb2O5 and, 0.01% Sn. A bankable feasibility study proposed to, , arong, Nyab, River, , Kabaya, granite, , N, , 5 km, , o, , 2°00'5 S, , 29°40' E, , Kibaran, (Mesoproterozoic), metasediments, , Gitarama, granite, , Figure 2.42 Sketch map of the Kibaran, (ca. 950 Ma) pegmatite dyke swarm in the, Gatumba tin-tantalum mining district,, Rwanda. Near Nyabarongo River, Kabaya, granite attains the character of a tin, granite. From 1928 to 1985, the district, produced about 18,000 t of cassiterite and, 2000 t of columbite concentrate, from, regolith ore of an average combined grade, at 0.5 kg/m3.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , and tantalum plus tin and unzoned albite-rich tintantalum pegmatites. The latter contain the ore, minerals columbo-tantalite (“coltan”), cassiterite,, spodumen, some lepidolite, beryl and muscovite, (e.g. at Gatumba). In the recent past in Central, Africa, mining was artisanal and geared to highprice periods. Earlier semi-industrial hydraulic, mining was limited to near-surface zones of supergene alteration with soft-rock ore. Unaltered hardrock ore was hardly ever exploited., Tantalum-niobium placers are commonly, derived from pegmatites and occur in their immediate neighbourhood. The main resource is cassiterite with columbite as a by-product. Tantalum of, tin placers in Malaysia and Thailand takes the, form of str€, uverite inclusions in cassiterite., Str€, uverite is a tantalum-bearing rutile (Fe,Mn,, Ti)(Ta,Nb)2O6). Tantalum occurring in this context is stripped from slags of tin smelters., The world’s niobium reserves are large, with, >98% located in Brazil. The world’s primary niobium production of 63,000 t niobium contained in, concentrate (2008; 62,000 in 2009) is dominated by, Brazil (92%, mainly from Araxa mine), followed, by Canada. The world’s annual primary tantalum, production is near 800 t tantalum contained in, concentrate (USGS 2010); Australia (50%) and, Brazil (15%) were the largest producers. Tantalum, reserves in the ground are largest in Brazil, closely, followed by Australia. Supply and consumption, of tantalum are occasionally out of balance, as, demonstrated by Boeing delaying production of, its new 787 Dreamliner aircraft in 2007 because, of a tantalum shortage. Very high tantalum metal, prices in the period 1999–2000 incited a wave of, tantalum exploration that resulted in the discovery of many significant prospects. At the same, time, previously known tantalum granites were, explored in detail. In the future, a number of, new hard-rock mines may produce from these, sources. Exploration for niobium and tantalum is, based on alluvial heavy mineral and geochemical, surveys of rocks, soil and sediments (Sweetapple, 2000). Because of large variations within single, pegmatites, columbite chemistry appears to be of, little use for tantalum prospecting (Beurlen et al., 2008)., , 265, , 2.5.10 Lithium, , Common Ore Minerals:, Wt. % Density, Li2O (g/cm3), Amblygonite, Lepidolite, Petalite, Spodumene, Zinnwaldite, , (Li,Na)Al[(F,OH)|PO4], K2Li4Al2[(F,OH)2|Si4O10]2, LiAlSi4O10, LiAl[Si2O6], K(Li,Fe,Al)3(F,OH)2, (AlSi3O10), , 10, 5, 5, 8, 5, , 3–3.1, 2.9, 2.4, 3.2, 2.9–3.1, , Lithium contents of ore minerals are commonly, smaller than listed because of substitution of Liþ, by other elements. Spodumene is formed at higher, pressures than petalite. Lepidolite and zinnwaldite are potential sources for the rare metal rubidium (crustal abundance 120 ppm, density 1.532 g/, cm3, melting point 38.9 C). The Rb-Sr dating, method is based on the radioactive isotope 87 Rb, (half-life 4.751010 years). Rubidium is used for, manufacturing catalysers, atomic clocks for GPS,, solid-state lasers and luminescent materials., Yearly traded mass is in the kg range:, Lithium minerals may be associated with pollucite, CsAlSi2O6.H2O, which is the main carrier of caesium, (crustal abundance 3 ppm, density 1.892 g/cm3, melting point 28.64 C), as in the Bernic Lake rare metal, pegmatite (Tanco mine, Manitoba), which contains, 300,000 t pollucite with an average Cs2O content of, 24% (USGS 2010). Other sources include caesium, beryl exploited from Li-rich pegmatites (Figure 1.20), and lithium brines. Caesium is part of X-ray tubes,, atomic clocks, scintillometers, magnetometers and, special glasses. Most of it is processed into caesium, formate (CsOOH) brine, which is non-toxic and displays a high density (2.3 g/cm3) making it a useful, ingredient of high-pressure and high-temperature, drilling fluids (e.g. ultra-deep holes in hydrocarbon, exploration)., , Exploitable hard rock lithium ore contains >1%, Li2O (3.5–4.5% at Greenbushes). However, most, mines co-produce metallic ores (tantalum, tin) and, minerals (lithium minerals, feldspar, quartz).
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266, , PART I METALLIFEROUS ORE DEPOSITS, , Accordingly, economic comparison of different, deposits can hardly ever be reduced to lithium, grades (Garrett 2004). Because of high processing, costs, lithium minerals (mainly spodumene and, petalite) are rarely used for the production of, Li-metal and chemicals, but are preferred as fluxing agents for manufacturing heat-resistant glass,, pyroceramics and enamels. In these applications,, very low impurities of Fe, Mn, Ti and Cr are, crucial., Lithium carbonate and chloride extracted from, natural brines are converted to lithium fluoride, for aluminium reduction plants. A smaller share, is consumed for production of lithium metal and, chemicals. With a density of 0.53 g/cm3, lithium, is the lightest metal of all (melting point at, 180.5 C, boiling point 1336 C) but is too reactive, for transport and use. Alloys with aluminium are, employed in the aerospace industry. Chemical, applications include lubricants (grease), pharmaceuticals, rechargeable and non-rechargeable, lithium batteries, which have higher energy density and lighter weight compared to NiCd and, NiMH. Forthcoming electric vehicles mainly, depend on Li-ion batteries. In the nuclear industry, lithium is employed for the production of, tritium, which is required for thermonuclear, fusion reactors and for nuclear weapons. In, nuclear power plants, lithium serves as a neutron, absorber and heat exchanger., , trial saline (playa) lakes. In the western USA, some, volcanic tuffs altered to montmorillonite are enriched in lithium substituting for magnesium, (hectorite). The stable lithium isotopes 6Li and 7Li, are efficient tracers of hydrothermal processes in, the crust (Chan et al. 2002)., Lithium is employed as a pharmaceutical for, the treatment of psychic disorders. In nature, the, element (as Cs and Rb) is harmless in all forms., WHO (2006) does not set a limit for lithium in, drinking water., Lithium ore deposit types, Lithium ore deposits of economic significance are, restricted to only two genetic classes:, . Rare, element pegmatites with lithium, minerals and commonly, with exploitable byproduct contents of Sn, Ta (>Nb) and Be (e.g., Greenbushes, W.A.); the mines produce concentrates of lithium minerals;, . Lithium brines of playa lakes or pumped from, subsurface aquifers (e.g. Salar de Atacama, Northern Chile; Salar de Uyuni, Bolivia); operations, market lithium carbonate or chloride., Large rare element pegmatites are exploited in, Western Australia (Greenbushes), Canada (Tanco, mine, Bernic Lake) and in Zimbabwe (Bikita). The, giant Kibaran (ca. 900 Ma) Manono tin-tantalum, pegmatite in Maniema Province, Congo, has, an untested potential of lithium minerals, (Figure/Plate 2.43):, , Geochemistry, Lithium is an extremly lithophile element with, a small atomic and ionic radius. Its crustal abundance is 30 ppm (range 18–65; Smith & Huyck, 1999), basalts contain an average of 5 and granites, 20 ppm. Lithium is enriched in highly differentiated granites and pegmatites, and forms ore deposits together with tin, tantalum and beryllium. All, three rare alkali metals lithium, rubidium and, caesium are geochemically closely related to the, abundant potassium (2.6% in the crust). Like, nearly all alkali elements, lithium is soluble in, surface waters as a mobile cation. This results in, elevated trace concentrations of lithium in pelites,, formation and oil-field waters (e.g. 100–700 ppm in, Arkansas and Texas), and significantly, in terres-, , The, Greenbushes, pegmatite, in, Western, Australia (Figure 2.41) was worked for tin (and tantalum) since 1888, mainly from weathered and, alluvial sources. More recently, spodumene and, kaolin were extracted. In the 1990s, a deep hardrock open cut and a large underground operation, were established which made Greenbushes one, of the world’s leading tantalum and spodumene, mines. The pegmatite dykes are sourced from highly, fractionated “tin granites”, which intruded greenstone belt rocks at 2800–3000 Ma, and are, controlled by crustal-scale shear zones and more, local structures (Sweetapple & Collins 2002, Sweetapple 2000, Partington et al. 1995). In 2010, Greenbushes’ capacity comprised 600,000 t/y of ore feed, up to 260,000 t/y of spodumene concentrate at 7.5%, Li2O.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Lithium brines can be exploited at much lower, cost compared with hard rock minerals. Brine, deposits occur in the western USA, Argentina,, China (Quaidam salt basin and Tibet) and Chile., Vast potential deposits are known in Afghanistan,, Bolivia and Peru. Bolivia’s Salar de Uyuni, (Figure/Plate 2.44) is thought to contain 50% of, world lithium resources (USGS 2009). Generally,, it is assumed that lithium in continental salt, basins and salt lakes originates by meteoric leaching of young volcanic rocks (in Nevada from rhyolites), or by volcanic degassing and hydrothermal, fluids. The second opinion appears to be supported, where lithium associates with borates. Enrichment by evaporation is evident:, The Salar de Atacama in the Chilean Altiplano contains a significant part of the world’s known lithium, resources. This is a dry depression in 2300 m altitude, , 267, , with an area of 3200 km2. A halite facies occurs in the, centre of the basin. The sediments in this area contain, brines with 1510 to 6400 ppm Li, important contents, of potassium (as well as traces of Rb and Cs) and, boron. The brines are pumped from subsurface lake, beds into shallow constructed ponds, where solutes, are concentrated by natural evaporation. Traces of, sulphate and Mg:Li ratios of 6:1 significantly, increase processing costs. Products are LiCO3, KCl,, K2SO4 and H3BO3., , Lithium resources of the world are very large. In, 2009, an estimated 150,000 t of lithium minerals, were produced worldwide. First was Australia, with nearly 76% of the total, followed by Portugal, Canada and Zimbabwe. In the same year, a, total of 10,000 t of lithium carbonate equivalent, (LCE) was extracted from natural brines in Chile, (70% of world production), China, Argentina, and USA., , Figure 2.43 (Plate 2.43) Giant Manono pegmatite in D.R. Congo is a sub-horizontal sheet and asymmetrically zoned., An upper marginal zone is made up of near-vertical palisades of spodumene (with microcline) and patches of, stanniferous albitite (centre).
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268, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 2.44 (Plate 2.44) Artisanal salt production at Salar de Uyuni, at 3500 m altitude in the Altiplano of Bolivia: One, of the largest salt lakes on Earth, the salar contains giant resources of lithium, potassium, boron and magnesium in, brines below the surface. The background mountains are part of the Central Cordillera, which hosts Cerro Rico de, Potosi. Courtesy B. Lehmann, Clausthal., , 2.5.11 Beryllium, Common Ore Minerals:, , Beryl, Bertrandite, , Al2Be3Si6O18, Be4(OH)2Si2O7, , wt. %, BeO, , Density, (g/cm3), , 14, 15.1, , 2.65–2.8, 2.6, , Although 28 minerals are known, in which beryllium is an essential constituent, only two are, found in sufficient quantity and concentration to, allow commercial extraction. The beryllium content in beryl is commonly lowered by the presence, , of Na, Rb, Li and Cs. H2O, CO2 and fluorine are, lodged in large structural channels of the mineral., Deep green, transparent beryl is an extremely, valuable gemstone (emerald). In emerald, up to, 2% Cr þ V replace Al in the crystal lattice. The, pale, greenish-blue colour of aquamarine is due to, content of 0.1–0.3% Fe2þ. Bertrandite occurs like, beryl in miarolitic granites, greisen and pegmatites, where it is commonly formed by alteration of, beryl. Frequent impurities in bertrandite include, Al, Fe and Ca. Its main commercial source is, tuffites in the USA., Exploitable beryllium ores have minimum concentrations from 0.1 to 2% Be, depending on grain, size, purity, texture and co-products.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Beryllium is a very light (D ¼ 1.85 g/cm3) strong, metal with a high melting point (1280 C, boiling, point 2970 C). It displays good mechanical properties, oxidation resistance, and high thermal and, electrical conductivity. Beryllium is useful in a, number of applications, for example by hardening, alloys (75% of consumption) and in ceramics., Alloys with Cu are most common, mainly in the, telecommunications, computer, aerospace and, vehicle markets. Beryllium is an eminently, important material for nuclear and military, equipment., Geochemistry, Beryllium is one of the lithophile, incompatible, earth-alkali elements with a very low crustal, abundance (2.5 ppm; Grew 2003). Basalts contain, an average 0.5, granites 5 ppm Be. In chemical, properties, beryllium resembles magnesium and, aluminium. Its atomic and ionic radius is smaller, than that of lithium. Beryllium dissolves readily, in acidic and alkalic hydrothermal solutions., Natural beryllium only consists of the stable isotope 9 Be. In the atmosphere, “cosmogenic” 10 Be is, produced by cosmic ray spallation of oxygen and, nitrogen. It is radioactive and decays with a halflife of 1.51 million years to 10 B. Cosmogenic 10 Be, accumulates at the Earth’s surface, where its long, residence time makes it useful for measuring processes such as soil erosion, soil formation from, regolith, the development of lateritic ore deposits, and variations in solar activity (Siame et al. 2006)., In natural oxidized water, beryllium occurs in, trace quantities (<1 mg/L), because its oxides are, nearly insoluble. Be2þ is somewhat mobile in, acidic waters at pH < 3. Nearly all beryllium emitted into the atmosphere by humans results from, oil and coal combustion for electric power generation. The element seems to have no biological, function and is specified as a hazardous air pollutant by the US Clean Air Act (1990). In the past,, toxicity of very fine-grained beryllium metal, dust and several compounds affected industrial, workers who developed occupational diseases., Today, workplaces appear to be adequately controlled. For the general public, beryllium exposure, is toxicologically not relevant., , 269, , Beryllium ore deposit types, Similar to lithium, beryllium is enriched in, extremely fractionated late liquids and fluids of, felsic magmas (<0.05% of the original volume:, Evensen & London 2002). These liquids form, pegmatitic beryllium deposits. Much less common are similar melts in the volcanic environment, which are related to epithermal beryllium, impregnations in tuffite. Several of the world’s, emerald deposits hosted in black shale are possibly, of diagenetic or metamorphogenic-hydrothermal, origin, but doubts remain. Economic sources of, beryllium are only:, . epithermal stratiform volcanogenic beryllium, impregnations in rhyolite tuffite;, . rare element pegmatites of the lithium-caesiumtantalum (LCT) type., Several epithermal stratiform volcanogenic beryllium deposits occur in the area of Spor Mountain, in Utah, USA. The region is part of the Basin and, Range Province. Between 42 and 6 Ma, several, phases of volcanism took place. Early eruptions, produced intermediate lava flows and tuffs but, after 21 Ma, alkali-rhyolite tuffs and ignimbrites, were deposited. Topaz rhyolite tuffite beds (the, “beryllium tuff”) with abundant clasts of subjacent Palaeozoic basement dolomites host the ore., In the basement, upflow channels exhibit uraniferous fluorite that was exploited in the past., Fluids invaded permeable tuffites between less, porous rocks and replaced dolomite with bertrandite, uraniferous fluorite, chalcedonic quartz and, manganese oxide (Lindsey 1977). Hydrothermal, alteration enclosing beryllium ore (>1000 ppm Be), comprises adularia and argillization zones, including lithium-bearing trioctahedral smectite. Mining started in 1968. Production rates at Spor, Mountain amount to 45,000 t/y of bertrandite, ore at 0.35% Be. Similar mineralizations are, known elsewhere. A rarity is the occurrence of, beryllium skarn at the contact of a Tertiary rhyolite laccolith intruding Cretaceous limestone at, Round Top in western Texas., Beryl is a characteristic minor mineral in rare, element pegmatites of the lithium-caesium-tantalum (LCT) type that are mainly exploited for, cassiterite, columbite and lithium minerals; beryl
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270, , PART I METALLIFEROUS ORE DEPOSITS, , ( elevated Cs-contents) is a by-product. Beryl is, more often enriched in zoned pegmatites, preferentially in the wall zone or adjacent to the quartz, core where coarse and even giant crystals can be, found (Figure 1.19 and Figure 1.20; Sweetapple, 2000). Beryl in unzoned pegmatite tends to be very, fine-grained and recovery is more expensive., Worldwide, beryl extraction from pegmatites used, to be quite common when artisanal or semi-industrial mining allowed selective recovery. Today,, only large pegmatite bodies warrant the investment for industrial processing plants., Emeralds are extracted from beryllium pegmatites that intruded chromium-rich rocks (e.g. ultramafics in Pakistan: Arif et al. 1996; talc schists, in Zambia). However, some emerald deposits are, hosted by black shale and are a product of diagenetic or metamorphic hydrothermal processes:, Emerald deposits in Colombia occupy the first place, in the world, both concerning quality of the gemstones and quantity of annual production (estimated, at 2 t/y). Near Muzo, 100 km northwest of Bogota,, emerald occurs in fault-bound veinlets in thinly, banded black shale and limestone. This is part of the, strongly folded Cretaceous marine suite of carbonates, sandstone, shales and evaporites, which builds, the Eastern Cordillera. Mineralization is structurally, controlled by a thrust plane (Pogue 1917). The paragenesis of the emerald-veinlets includes calcite,, dolomite, quartz, fluorite, pyrite and muscovite,, which were precipitated from saline brines (up to, 40% NaCl equiv.) at 290–360 C and P1 kbar., Hydrothermal alteration of black shales includes, bleaching, carbonation and albitization. For the, miners, the resulting light “cenicero” (Spanish for, “ash rock”) indicates the presence of emeralds. Cr, and V of the emeralds are probably derived from the, organic matter-rich host rocks. Hydrothermal fluids, have the character of deep formation or low-grade, metamorphic water. Dissolution of gypsum and halitite provided salinity. Migration of fluids was, induced by Eocene-Oligocene orogenic compression, and salt tectonics. The source of beryllium remains, disputed. Hypotheses include dissolution from black, shale near the deposits (Giuliani et al. 1999) or, abstraction from deep basinal sediments and transport in mobile Be-F complexes (Banks et al. 2000)., Pogue (1917), however, mentioned the occurrence of, a pegmatite in the area, and suggests genetic relations, with a buried intrusion., , Annual world mine production of beryllium in, concentrate is 200 tonnes (but only 140 in 2009;, USGS 2010). More than 80% of the total is from, Spor Mountain (USA), the remainder from China, and Mozambique. The search for beryllium ore, depends on a combination of well-planned sampling, and the availability of chemical and X-ray, determination equipment. In the field, portable, “beryllometers” (gamma-ray neutron generators), are used to measure in-situ Be-contents., , 2.5.12 Uranium (and Thorium), Common and Characteristic Ore Minerals:, Max., Density, (g/cm3), Uranium, Uraninite, (Pitchblende), Brannerite, Coffinite, Autunite, Carnotite, Thorium, Uranothorite, Thorite, Thorianite, Monazite, , UO2, , 9.7, , (U,Ca,Ce)(Ti,Fe)2O6, U(SiO4)1x(OH)4x, Ca(UO2)2(PO4)2.8–12H2O, K2(UO2)2 (VO4)2. 3H2O, , 6.3, 7.2, 3.2, 5.0, , (U,Th)SiO4, ThSiO4, (Th,U)O2, (Ce,Th)PO4, , variable, 5.3, 9.7, 5.3, , There are more than 200 different minerals, containing uranium; those listed above are common in workable deposits. The first three minerals, host uranium in its reduced state (U4þ), whereas, the following are samples of a large number of, minerals containing the uranyl ion UO22þ and, uranium in its oxidized state (U6þ). Most uraninite, is partially oxidized and the composition lies, between UO2 and U3O8. Well crystallized uraninite was probably formed at higher temperatures, and has elevated trace contents of Th, Ce, Y, and, Ca. At lower temperature, the botryoidal, massive, or friable soot-like pitchblende is more common,, which contains less of these impurities. Radioactive decay of uranium results in trace contents, of radioactive daughter nuclides such as radium, and stable elements lead and helium (Table 2.3).
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Table 2.3 End products of radioactive decay of uranium, and thorium, 238, , U ! 206 Pbþ84 He, , 235, , U ! 207 Pbþ74 He, , 232, , Th ! 208 Pbþ64 He, , Pb and 4He are used for age determination of ore,, minerals and rocks (cf. Chapter 1.1 “Isotope, Geochemistry”). Radiogenic lead in uraninite, may form galena inclusions (Figure/Plate 2.26)., During geological times, however, passing fluids, can selectively remove lead or uranium, concealing the true age of mineralization. Depending, on age and geological fate, uraninite contains, 45–85 wt.% U., Thucholite is not a mineral but a complex bituminous material with high uranium and thorium, content, and visible pyrite or galena. It has been, noted in pegmatites, but was only extracted as, uranium ore from Witwatersrand conglomerates, where earlier it was thought to represent algal, mats. An origin from migrating hydrocarbons was, suggested by Parnell (1996)., Economic ore grades of large hard-rock uranium, mines can be as low as 0.2% U3O8. Most mines, produce ore of 1%. Cut-off grades depend on, commercial and technological constraints,, because prices of the metal fluctuate widely and, the ore paragenesis determines processing costs, and recovery. Therefore, uranium reserve tonnages are commonly related to certain market, price levels. Of course, mines with exceptional, grades such as Cigar Lake and McArthur River in, Canada with 14% and even >30% U3O8 are, beyond such considerations. An example of, extremely low grade and yet feasible uranium, mining is the calcrete deposit Langer Heinrich in, Namibia, which since 2006 produced ore of 0.06%, U3O8 from a reserve of 50 Mt., Uranium mining by in-situ leaching (ISL) is suitable, for low grade (e.g. 0.12% U3O8 in the Honeymoon, project, Southern Australia) near-surface ore in aquifers. Nearly all uranium produced by Kazakhstan, is from relatively small ISL mines. Characteristic, examples are permeable mineralized sandstone, beds interbedded between aquitards. The aquifer is, , 271, , grid-drilled and leach fluid is injected. This may be, an aqueous solution of sodium hydrogen carbonate or, simply CO2, alternatively H2SO4 and O2. The resulting pregnant solution is pumped from production, wells and processed for recovering uranium in marketable form. In-situ leaching is a very small-impact, extraction method. The environment is hardly, affected and after the end of operations, the aquifer, is remediated., , In-situ leaching (ISL) provided 36% of world, mine production (2009) and 60% was derived, from open pit and underground mines (www., world-nuclear.org; June 2010). “Acidic” (quartzrich) ore is leached with hot sulphuric acid,, whereas “basic” (e.g. carbonate-bearing) ore is, treated with a solution of soda and sodium bicarbonate. From the liquor, uranium is precipitated, as ammonium diuranate (“yellow cake”) or oxide., During leaching, 226-radium of the ore is not, dissolved. Radium, with geochemical properties, similar to barium, remains in the tailings, which, are commonly neutralized and disposed of in tailings dams. Care must be taken if the waste material contains sulphides: In contact with oxygen, most sulphides produce acid, which mobilizes, radium that may then contaminate groundwater, or the environment. Radium is an emitter of aradiation with a half-life of 1620 years and poses a, considerable risk. Its dispersion can be blocked if, the acid-generation potential of the tailings is, reliably neutralized (e.g. by mixing with red mud, waste from alumina production; cf. Section 2.4.1, “Aluminium”). Another serious hazard is, radium’s daughter element radon, a noble gas and, a-emitter with a half-life of only 3.8 days. In the, UK, indoor inhalation of natural radon and, short-lived decay products of natural radon cause, thousands of cancer deaths per annum (Appleton, 2005). In outdoor air, radon is quickly diluted., The principle health hazards posed by uranium, are its chemical toxicity (notably in drinking, water) and the radiological properties of some, of its decay products, especially 226 Ra and 222 Rn, (Appleton 2005). Radionuclide contamination by, man includes nuclear weapons (mainly testing),, nuclear accidents and, to a lesser extent, nuclear, power generation and re-processing of spent fuel., Uranium is also emitted into the atmosphere by
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272, , PART I METALLIFEROUS ORE DEPOSITS, , oil and coal combustion for electric power generation. It is specified as one of the hazardous air, pollutants by the US Clean Air Act (1990) and is, closely controlled. The average dose from these, sources over most of the Earth’s surface, however,, is small compared to natural radioactivity (Plant, et al. 2005). WHO (2006) proposed a provisional, guideline value of uranium in drinking water at, 0.015 mg/litre but notes that 0.03 mg/l may be, sufficiently protective of kidney toxicity. Let us, not forget that uranium is omnipresent in the, environment and that all living matter contains, traces of uranium and other radioactive nuclides., In civil applications, uranium is almost exclusively used as a fuel for base load electrical power, production in nuclear power stations (Wilson, 1996). For this purpose, no carbon-free alternatives, exist. Yet, nuclear reactors can be quickly controlled to make room in the grid for unpredictable, fluctuations of wind and photovoltaic electricity, production. Worldwide in 2009, 439 power reactors, were operating in 30 countries. Fifty-seven were, under construction, mainly in China (23), Russia,, South Korea, India, Japan and Canada. Natural, uranium is composed of three isotopes (Table 2.4)., In water-cooled nuclear fission reactors, only 235 U is, consumed that was previously enriched to 3–5% of, 238, Uþ235 U. Future breeder reactors are supposed to, make additional use of the energy contained in the, more common 238 U. Major differences between, nuclear and fossil power production are in the, amount of fuel required and in resulting emissions., For a typical power station of 1000 megawatt (MW),, annual fuel requirement is 3 Mt of coal, or if, nuclear, 170 tonnes of natural uranium. Seven million tonnes of CO2 and 300,000 tonnes of ash are, generated annually by a coal-fired plant, compared, with 27 t of spent fuel (or 1 t of processed waste) from, a nuclear station. Also, considering the uncertain, future of oil availability, supply and price, the nuclear, industry is well positioned to produce hydrogen as, a transport fuel., Of all energy sources, the nuclear energy cycle has the, lowest impact on the environment, as it produces no, harmful emissions (CO2, toxic elements, etc.) and, requires very little land. Risks associated with, nuclear power generation, reprocessing of spent fuel,, and with disposal of spent fuel and other toxic and, , Table 2.4 Naturally occurring uranium isotopes, 238, , U, U, 234, U, 235, , 99.27 wt.%, 0.72 wt.%, 0.0057 wt.%, , half-life (a) ¼ 4.468 109, 0.703 109, 2.47 105, , Uranium 234 is a member of the 238 U decay series; its, concentration is a function of the radioactive equilibrium, between both isotopes. The precise 238 U=235 U ratio of natural, uranium depends on genetic conditions (Bopp et al. 2009)., , radioactive nuclear waste are major points discussed, in science and society. Safe waste disposal is one of, the most important demands of society from the, geosciences (cf. Chapter 5.5 “Deep Geological Disposal of Dangerous Waste”)., , Highly enriched uranium (to >90% 235 U) is only, produced for nuclear weapons. Depleted, almost, pure non-fissile 238 U is mainly used for its high, density (19.1 g/cm3, melting point 1132 C). Applications are limited and include bullets and, shells, chemical catalysts, but also sailing boat, keels and parts of Formula 1 racing cars. Because, of increasingly tighter control of radioactive material, civil application of non-fissile uranium will, probably cease., Thorium is composed of only one isotope (apart, from a short-lived daughter nuclide), 232 Th with a, half-life of 1.405 1010 years. It is not fissile and is, rarely used for nuclear power production. However, thorium can be converted to fissile 233 U by, neutron irradiation and the Molten Salt Reactor, Process is considered to potentially satisfy future, energy needs. India built a first small reactor of, this type in 1996 and since then launched a major, programme to use its large thorium reserves. The, Subcontinent has little uranium, but is endowed, with coastal placers containing 400,000 t of Th in, monazite, promising energy for hundreds of years., Non-nuclear uses of thorium (11.7 g/cm3, melting, point 1842 C) include the production of incandescent mantles for gas lamps (50% of consumption; but Th is increasingly replaced by yttrium, compounds), Mg-Th alloys for high-temperature, applications, and various electronic, chemical and, metallurgical applications. However, most nonenergy uses of radioactive thorium are being, phased out.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , Geochemistry, The geochemical character of uranium and thorium is lithophile (Goldschmidt 1958). In common magmatic rocks, uranium reaches its highest, average abundance in granite (4.8 ppm; Th, 17 ppm). Large granite bodies with high U and, Th-contents are heated by radioactive decay (High, Heat Production, or HHP granites). This has geological consequences (e.g. hydrothermal mineralization in Cornwall) and may be a source of, environmentally benign geothermal energy. For, several decades in the past and in current projects,, the European Union supports the development of, technologies for harvesting geothermal heat and, electricity at sites in Cornwall and Alsace. As large, ions with a high charge, U(IV) and Th(IV) are, geochemically similar to the rare earth elements., They are incompatible in relation to common, igneous silicate minerals and are preferentially, concentrated in accessories such as allanite, monazite and xenotime. Traces occur in zircon,, orthite, apatite, titanite and magnetite. In many, granites, however, a large part of uranium and, thorium is held in intergranular films (compare, recent findings concerning REE in S-Chinese granites: Ishihara et al. 2008). These films and radiation-damaged accessory minerals are easily, soluble. As much as 90% of total uranium and, thorium in granites may occur in this form. Felsic, volcanic rocks are also a source of readily soluble, uranium, mainly from glass phases., Uranium and thorium are enriched in late melts, and fluids segregated from cooling felsic magma, bodies. The distribution of U and Th between melt, and fluids is a function of the fluid composition., A purely aqueous fluid dissolves little U and Th,, but both elements are enriched in fluids when, fluorine concentrations rise. Uranium and thorium are separated if the fluid is dominated by, CO2 and Cl, because in this case uranium is soluble but thorium is retained in the melt or in, refractory solids. This explains, for example, the, low concentration of uranium in granulite and its, enrichment in carbonatite. In magmatic-hydrothermal fluids, uranium occurs mainly in the form, of chlorine and carbonate complexes of Uþ4,, whereas both Th and U form fluorine complexes, , 273, , (Keppler & Wyllie 1990). Uranyl-carbonate complexes (in circum-neutral conditions) or uranylchlorine complexes (in acidic conditions) prevail, in hydrothermal solutions of low temperature, (<200 C; Kojima et al. 1994). Changing pH, degassing of CO2 and reduction precipitate UO2 from, these solutions., The geochemical behaviour of uranium and, thorium in the exogenic cycle (Burns & Finch, 1999) is peculiar because of the low solubility of, thorium and REE in near-surface environments,, in sharp contrast to uranium, which is easily, dissolved by oxidation. Primary uranium minerals are first transformed into brown, yellow and, green intermediate oxides, and dissolve finally to, form the mobile uranyl ion UO22þ, which can be, transported over large distances by surface and, groundwater. The uranyl ion is complexed by, carbonate, fluoride, sulphate and phosphate anions. Most common is transport as an anionic, uranyl-carbonate complex. Aqueous colloids, with sorbed uranium (VI), such as amorphous, iron hydroxides, may carry much higher concentrations than simple solutions. Precipitation of, uranium is induced by adsorption (e.g. clay, minerals, organic matter, Fe-Mn-Al oxy-hydroxides) or by reduction. Common reducing factors, in geological settings include H2S, Fe2þ, CH4,, petroleum and organic substances. In the laboratory at room temperature and in limited time,, dissolved uranyl is efficiently immobilized by, humic substances without being reduced (Wood, 1996, Nakashima et al. 1999). Rapid reduction in, natural systems is probably encymatically catalysed by microbes, for example by sulphur-reducing bacteria (Lovley et al. 1991, Min et al. 2005). It, is assumed that the organically-bound uranyl is, reduced during diagenesis. Peat and coal (Figure 2.43), phosphate rocks (40–250 ppm U), black, shales and oil shales always display elevated but, rarely exploitable uranium concentrations. Examples include the Cambrian alum shales of, southern Sweden (300 ppm U; Leventhal, 1990) and the European Copper Shale (180 ppm:, Piestrzynski 1990). The ash from large power, stations burning coal or petroleum may be a, feasible source of uranium. Some petroleum and, associated formation waters carry significant
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274, , PART I METALLIFEROUS ORE DEPOSITS, , E, , W, 410, 400, Groundwater table, , UO2, , Back-filled lignite pit, , 380, m, , Tertiary lignite and, clastic sediments, (below pit bottom), , Uraniferous horizon, Weathered sandstone, , 2+, , Uraniferous "hot", sandstone (Triassic), , 50 m, , Figure 2.43 Uranium infiltration at the margin of the Tertiary lignite basin Wackersdorf, Germany. Groundwater, passing through uraniferous (“hot”) Late Triassic sandstone flowed into marginal basin sediments (impure coal,, carbonaceous clay and sand) that immobilized uranium by adsorption and reduction. Peak contents reach several, thousand ppm of uranium., , uranium contents, for example in the Ukhta field, in the South Russian Pechora Basin., Ordinary surface and groundwater contains, less than 1 ppb (1 mg/L) uranium (Merkel et al., 2002). Several tens of ppm may occur in water, that was in contact with uranium-anomalous, rocks or with ore. Average ocean water concentration of uranium is 3 ppb. Seawater is the, source of uranium in phosphorite and black, shale. Because of the large water volume, the, oceans contain 5000 Mt of uranium (Algeo, 2004). Evaporation of seawater causes enrichment of uranium in brines. Thorium is commonly insoluble and immobile in contact with, oxic surface water, apart from physical transport, in flowing streams. Rarely, complexing elements, such as Cl, Br, F or I in brines may lead to exogenetic chemical mobility of thorium., Natural nuclear reactors, Natural nuclear reactors have been found in uranium deposits of the Oklo District, Gabon (Jensen, & Ewing 2001, Figure 2.44). At Oklo, uranium, ore is extracted from faulted Palaeoproterozoic, siliciclastic sediments transgressing Archaean, granites. During the mineralization process, the, critical mass of massive pitchblende was reached, at 14 sites exposed by mining, where the chain, , reaction of nuclear fission started to operate. Criticality of the reactors was dated to 1950 Ma and, proved by the presence of long-lived fission products. The first of these reactors was discovered in, 1972 when the reason for a strong depletion of 235 U, in certain ore shipments from Oklo was investigated. Today, the natural assembly of a nuclear, reactor is not possible because the concentration, of fissile 235 U in modern uranium is too low. The, preservation of the Oklo reactors is remarkable,, considering the shallow burial and the high mobility of uranium and its daughter and fission nuclides. This makes Oklo one of the demonstration, sites for the long-term safety of engineered underground disposal facilities. Note that the rocks of, Oklo are fractured and permeable, so they would, certainly not be chosen for the construction of a, nuclear waste repository. Coogan & Cullen (2009), suggested a causal connection between the “Great, Oxidation Event” and an assumed abundance of, widespread near-surface uranium deposits that, developed into natural fission reactors. Under a, reduced atmosphere, detrital uranium must have, been common. First islands of oxygenation by, early photosynthesis might have established aqueous cycling and concentration of uranium at redox, boundaries., Thorium mineralizations originate almost, exclusively by magmatic, magmatic-hydrothermal
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 275, , Figure 2.44 Underground mine exposure of the Palaeoproterozoic Rzoke nuclear reactor in the uranium mine, Okelobondo in Gabon (after Jensen & Ewing 2001). Ascending uraniferous hydrothermal fluids precipitated uraninite, at the contact between black shales and sandstone. Locally, critical mass and concentration of 15% UO2 were reached, and nuclear fission started to operate. Water and petroleum (now pyrobitumen) functioned as neutron moderators. Note, the pouch-like sagging form. Abreviations: w – white, leached shale; bs – bitumen-rich, si – silicified sandstone., , and sedimentary processes. Thorium is commonly, a by-product of mines exploiting rare earth elements and copper. Thorium concentrations, are found in pegmatites with uranothorite, thorite,, orthite and zircon, in carbonatites (e.g. 0.01%, in ore at Palabora, South Africa) and in hydrothermal veins with thorite, bastnaesite, monazite, and apatite in the vicinity of alkali complexes, and carbonatites. In practice, only coastal, placers with monazite as a by-product of, ilmenite and zircon mining are a relevant source, of thorium., , Uranium ore deposit types, Exploitable concentrations of uranium are formed, in a broad spectrum of geological systems, but, foremost in magmatic, magmatic-hydrothermal,, retrograde-metamorphogenic, supergene alteration and infiltration, and sedimentary settings, (compare Cuney & Kyser 2009, Cuney 2009). The, mobility of uranium in oxic surface waters and, seepage solutions explains a large number of different uranium ore deposits that were formed by, fluids convecting in the shallow crust. This group, includes the richest deposits of uranium (the
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276, , PART I METALLIFEROUS ORE DEPOSITS, , “unconformity type”). Due to the generally, reduced state of common diagenetic and prograde, metamorphic fluids, uranium is rarely mobile or, enriched in these process systems. Exceptions, occur where oxidized solutions were involved., A stringent genetic classification of uranium deposits remains elusive (cf. Cuney 2009, Dahlkamp, 1993). Therefore, generally used descriptive terms, are included in the following brief overview of, significant uranium deposit types, supplemented, by genetic information. The order of the list, reflects economic importance, from highest to, lowest:, . unconformity deposits (saline brines, retrograde, metamorphogenic; e.g. Athabasca District,, Canada);, . magmatic-hydrothermal uranium in iron-oxideCu-Au (U-REE) deposits (IOCG) (Olympic Dam,, South Australia);, . infiltration deposits in sandstone (Kazakhstan);, . ancient alluvial placers (Witwatersrand);, . orthomagmatic deposits in felsic intrusions, (R€, ossing, Namibia);, . post-magmatic hydrothermal uranium in HHP, granites, characterized by protracted cooling and, flooding with meteoric water;, . basinal brine-related, granite-hosted disseminations and veins (Limousin, French Massif Central);, . infiltration/evaporation deposits in dry river, channels and palaeovalleys: calcrete, and palaeochannel deposits (Yeelirrie, Western Australia,, Lake Frome region in southern Australia)., , Unconformity uranium deposits, Unconformity uranium deposits occur in ancient, cratons close to the unconformity between an, Archaean to Palaeoproterozoic igneous and metamorphic basement, and a cover of younger clastic, sediments. Orebodies may be basement and sandstone-hosted. This class contains over 40% of, world reserves, mainly in two provinces: i) the, Alligator River District of the Northern Territory, in Australia (Jabiluka, Ranger, Narbarlek);, and ii) the Athabasca District in Saskatchewan,, Canada (Rabbit Lake, Cigar Lake, Key Lake and the, new McArthur River mine). The Athabasca basin, produces 30% of annual world primary uranium,, , with average mining grades from 0.5 to >20%, U3O8:, Cover rocks in the Athabasca District are fluvial to, marine red sandstones with conglomerates at the, base and several intercalations of evaporites and, felsic volcanic rocks. The suite is called Athabasca, Group and reaches a thickness of 1500 m. Its age is, <1750 Ma. The basement includes Palaeoproterozoic, metasediments in amphibolite and granulite facies,, and Archaean gneiss domes. The unconformity is, marked by lateritic palaeosoil and is hardly tilted., The metasediments display bands of graphite-bearing, rocks, which control uranium orebodies where they, coincide with reactivated permeable basement structures. Several periods of more intensive fluid circulation and perturbation of uraninite in the Athabasca, Basin have been dated by U-Pb ages and neoformation, of alteration silicate minerals (Fayek et al. 1998). The, main uraninite ore formation event, however, took, place at 1590 Ma (Alexandre et al. 2009)., Until discovery of McArthur River, the Key Lake, deposit (exploited 1982–1997) was the largest highgrade uranium mine in the world, with 2.15% U3O8, in run-of-mine ore and considerable reserves, (90,000 t U3O8; compare Chapter 5.3.2 “Ore Reserve, Estimation”). The ore zone had a width and height of, 15 by 100 m, and a strike length of 3.6 km (Figure 2.45)., The ore was dominated by uraninite with some coffinite; gangue included a considerable mass of nickel, and arsenic sulphides, and traces of Co, Fe, Cu, Pb,, Zn, Mo and Se. Occasionally, thucholite occurred, in pockets. The alteration halo displayed outer illitic, and inner illite-chlorite zones surrounding the orebody. Above the mineralized core, rocks are silicified, and tourmalinized. This extends from basement, into overlying sandstones, providing a vector to ore,, which is mainly explored by core logging with infrared reflectance field spectrometry. Alteration was, reducing, made conspicuous by the change from, red to green or grey. Microthermometric investigations imply mineralization at 150–250 C and, 0.75 kbar, and salt contents in the fluids of 30%, NaCl-equivalent., Exploited since 1999, the McArthur River deposit, considerably exceeds Key Lake. Grades are 21%, U3O8 and geological resources are estimated to, 188,000 t of uranium oxide (Jamieson & Spross, 2000). The nearby Cigar Lake mine suffered a crown, pillar failure and underground flooding in October, 2006, from which it had not recovered by early 2010.
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , 277, , Figure 2.45 Schematic section of the, unconformity uranium deposit Key Lake in the, Athabasca District, Canada. After Dahlkamp,, F.J. 1978, Society of Economic Geologists, Inc.,, Economic Geology Vol. 73, Figure 6, p. 1435., , The genesis of unconformity deposits is, undoubtedly hydrothermal and epigenetic (Alexandre et al. 2005, 2009). The uranium may be, derived from uranium-rich phases (e.g. apatite,, zircon, monazite) in both cover and basement, rocks. In the Athabasca district, orthogneisses, appear to be the most likely source. The gneisses, are strongly altered and uranium-depleted to a, depth of 200 m below the unconformity (Hecht, & Cuney 2000). Hydrothermal convection systems were controlled by permeable fault structures in the basement. Oxygen-rich basinal, brines derived from Athabascan evaporites descended into the basement leaching uranium., Heated at depth to >250 C, they were reduced by, contact with graphite schist on the upflow-branch, or by electron gain from Fe2þ of basic rocks, resulting in formation of haematite (Komninou &, Sverjensky 1996). Precipitation of uranium is a, , consequence of reduction. Once initiated by tectonic movements, hydrothermal convection must, have continued until mineral precipitation terminated all permeability, because uranium decay, produces heat (cf. “HHP-granites”), making such, systems independent from external energy. In, conclusion, the formation of unconformity deposits combines elements of ore formation by migrating oxidized saline brines and of retrograde, metamorphogenic hydrothermal processes., Uraniferous iron oxide-copper-gold (IOCG), deposits, IOCG deposits (Groves et al. 2010) are represented, by one single, albeit giant deposit: Olympic Dam, near Roxby Downs in South Australia (Figure 2.13,, cf. Section 2.2.1 “Copper”). Although most IOCG, systems have anomalous uranium and LREE
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278, , PART I METALLIFEROUS ORE DEPOSITS, , traces, typical grades <0.1 wt.% U3O8 are at present too low for profitable exploitation, except in, deposits with economic grades of copper and gold, (Hitzman & Valenta 2005):, At Olympic Dam, host rock is the Roxby Downs, granite (1590 Ma), which is comagmatic with volcanic rocks (Skirrow et al. 2007). The magmas are, magnetite series and include both A- and I-Type, compositions. Immediately after solidification the, granite was brecciated and the breccias were the site, of pervasive magmatic-hydrothermal mineralization, and alteration, probably interacting with surfacederived fluids. Main minerals introduced were haematite, sulphides, uraninite, quartz, sericite and fluorite (Hitzman et al. 1992; Groves & Vielreicher 2001)., Breccia bodies have a wide cross-section and are, steeply plunging. A first batch of mineable reserves, of 450 Mt with 2.5% Cu, 0.08% U3O8, 0.6 g/t Au and, 6 g/t Ag was outlined for initial operations. Total, metal endowment is estimated at >1 Mt uranium,, apart from 30 Mt Cu, 1200 t Au and 7000 t Ag. Hosting, 40% of the world resource base of uranium, Olympic Dam is the largest deposit of this metal yet, known., , USA (Tertiary), Chu-Sarysu and Syrdarya basins, in Kazakhstan (Late Cretaceous to Palaeogene:, Petrov 1998) and the Agadez Basin in Niger (Carboniferous). Lod, eve near Montpellier, France is a, peculiar trap of uranium because the means of, precipitation was bitumen in faults, joints and, pores of tuff and silt bands in Autunian black shale, (Carboniferous-Permian boundary; Schlepp et al., 2001)., , Figure 2.46 shows a typical sandstone deposit: After, initial underground ore extraction, an infiltration, deposit in Cretaceous sandstone of southern Saxony, was exploited by in-situ leaching with dilute sulphuric acid from 1967–1990, yielding 18,000 t uranium., The ore horizon sandstone is draped over Palaeozoic, granite and granodiorite. It includes disseminated, pitchblende and much pyrite. Sandstone beds are, aquifers separated by aquitards. During uranium, leaching, the water table was controlled by pumping, in the shafts. Since 1999, remediation of aquifer 4 is, carried out by flushing with fresh water (Biehler &, Falck 1999). Some uranium continues to be recovered, from this operation and a small production will, extend well after 2010., , Sandstone, or infiltration deposits of uranium, Sandstone, or infiltration deposits of uranium are, very common and occur in all continents, mainly, in sediments younger than 400 Ma. Individual, deposits are usually rather small and have low, grades (0.1–0.2% U3O8). Fluvial arkose sandstones, of epicontinental, semi-arid basins are most frequent host rocks. Sources of uranium are felsic, volcanics or granites that were weathered and, eroded in up-lifted basin margins. Uranium dissolved in surface waters infiltrated aquifers in the, basin fill. Conditions of mobilization, migration, and precipitation can be explored by determination of U-Th nuclides and daughter elements, (Reynolds et al. 2003, Bopp et al. 2009). Uranium, is immobilized by reduction (typically at elongate,, sinuous fronts and at roll-front deposits, cf. Chapter 1.2 “Infiltration as an Agent of Ore Formation”;, Figure 1.57), but also by fluid mixing and pH, change in the form of tabular orebodies (Spirakis, 1996). The world’s greatest provinces of uranium, sandstone deposits include the Colorado Plateau,, , Ancient fluviatile placers, Uranium is a by-product of gold exploited from, ancient fluviatile placers (“quartz pebble conglomerates”) of the Neoarchaean Witwatersrand, gold province in South Africa (cf. Section 2.3.1, “Gold”). In certain gold reefs, the matrix between, pebbles contains silt-sized rounded grains of, uraninite, coffinite and brannerite (UTi2O6). The, last two were probably produced by diagenetic or, metamorphic alteration of primary uraninite. Pyrobitumen bands (thucholite ore, Figure/Plate 2.26), with a thickness between 0.2 and 50 mm were, either algal mats or migrated petroleum (Parnell, 1996). Similar ancient uranium placer deposits, occur at Elliot Lake in Canada and Jacobina in, Brazil. Because of the rapid oxidation of uraninite, in oxic conditions, its alluvial transport in the, geological past is one of the arguments put forward, for extremely low oxygen contents of the, Archaean atmosphere (cf. Chapter 1.3 “Banded, Iron Formations”).
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , SSW, , 279, , NNE, Uranium solution mining area, , m, 400, , Shaft, Aqu, , ifer, , 300, , Aqu, , Shaft, 1, , ifer, , Elbe, River, , 2, Mudstones, & calcareous, sandstones, (aquitards), , 200, 135 m level, , 100, , Aqu, , ifer, , 3, , 94 m level, , Cretaceous, sandstone, , Water table (1992), 50 m level, 25 m level, , 0, , -100, , Variscan crystalline basement, , Aqu, , ifer, , 4, , (U-b, earing), , 1 km, , Figure 2.46 Underground in situ leaching of the sandstone-hosted uranium infiltration deposit K€, onigstein in Saxony., Leaching took place in blasted chambers. With kind permission by Wismut GmbH, Chemnitz., , Uranium deposits related to igneous rocks, Uranium deposits related to igneous rocks occur, in association with granites, rhyolites, alkali massifs and carbonatites. Orthomagmatic uranium, deposits are rare because accessory uranium, minerals are hardly ever concentrated to economic, grades. However, elevated traces may be a source, of uranium in hydrothermal ore deposits if postmagmatic fluid convection systems are established as, for example, in HHP-granites. Higher, uranium concentrations occur only in fractionated late liquids and fluids of felsic magmas., Examples are pegmatites (e.g. Madagascar), which, cannot compete with today’s large and high-grade, deposits:, R€, ossing in Namibia is one of few large orthomagmatic, uranium deposits (Berning 1986). At 500 Ma, posttectonic leucogranites (alaskites) intruded intensively, folded metasediments and mafic metavolcanic rocks, of the Panafrican Damara orogen. The intrusives are, mainly alkali-feldspar granites with <5% mafic minerals and variable texture, including aplitic, granitic and, pegmatitic variants. The intrusion caused contact, metamorphism expressed by cordierite in metapelites, , and skarn at contacts with metabasalt. Alaskites form, sheeted dykes, parallel or oblique to hostrock schistosity, and diffuse larger bodies that attain a thickness, of 100 m. U/Th and 87 Sr=86 Sr ratios in some members, of this suite are high, possibly indicating a derivation of, the melts from metasediments. Uranium is almost, exclusively present as uraninite; small (30–50 mm), rounded grains are disseminated in alaskite, together, with biotite, zircon, monazite, apatite and titanite., Minor phases include fluorite, sulphides (Fe, Cu, Mo,, As) and oxides of Fe, Mn and Ti. Not all alaskites are, mineralized; proximity to methane-bearing semipelitic and carbonate-banded metasediments seems to be a, first-order control (Kinnaird & Nex 2007). Resources, are estimated to ~166,000 t contained U3O8 at the very, low grade of 0.035%., , Granite-related uranium deposits in South, China are an important source of uranium. At, Dalongshan in Anhui province, high-grade uranium ore was found at the contact between a, Cretaceous alkali-feldspar granite and Jurassic, sandstone hornfels in the hanging wall. The pluton is a HHP-intrusion; careful age investigations, showed that cooling was very protracted. Hydrothermal mineralization took place while the
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280, , PART I METALLIFEROUS ORE DEPOSITS, , pluton cooled from 300–150 C, 25–30 million, years after emplacement (Zhao et al. 2004)., Uranium ore veins and disseminations in Variscan basement regions of France (Armorican, Massif and Massif Central) defy a simple genetic, classification:, Post-orogenic (ca. 310 Ma) Variscan S-Type leucogranites with 12.5 ppm U host the uranium deposits. The, ore occurs in pipes and veins of “episyenites”, which, are the alteration product of granite after hydrothermal removal of quartz. The leached rock displays a, connective porosity of 20–30% and striking druses of, adularia, quartz, fluorite and dolomite. The uranium, mineralization is independent of this post-magmatic, Permian hydrothermal activity and much younger, (Middle Jurassic). Apparently, the episyenites offered, preferential flow channels when low temperature, (ca. 100 C) oxic diagenetic brines from adjacent sedimentary basins invaded the crystalline basement., Uranium ore was precipitated with illite-smectite, clay in episyenite (Patrier et al. 1997). The brines, probably took up uranium by dissolution of uraninite, disseminated in fertile leucogranite. The largest, uranium mining district in the Massif Central is, Limousin, based on cross-cutting uranium veins and, mineralized episyenites in leucogranite. Single orebodies are rather small but cumulate production, reaches 35,000 t U3O8, at grades of 0.1–0.6% U3O8., , Rhyolite tuffs, tuffites and volcanogenic limnic, sediments in calderas are frequently mineralized, with uranium. Tertiary U-Th-rich topaz rhyolites, in the western USA display numerous epithermal, (cf. Section 2.5.11 “Beryllium”) and infiltration, deposits as in the McDermitt caldera (NevadaOregon). At McDermitt, breccias near the main, ring fault are epithermally impregnated with uranium and zirconium ore (Castor & Henry 2000)., Playa lake sediments in such settings may be, economically exploitable. The largest rhyoliterelated uranium deposit is probably the Streltsovka caldera in Transbaikalia, Russia (Chabiron, et al. 2003)., Palaeochannel and calcrete deposits, Palaeochannel and calcrete deposits of uranium, form in palaeovalleys with little present groundwater flow. Examples of the calcrete type occur, , in Western Australia (yet undeveloped Yeelirrie:, Cameron 1990) and Namibia (Langer Heinrich,, producing since 2007). Langer Heinrich operates, with reserves of 30 Mt at 0.06% U3O8. Several, significant palaeochannel deposits were recently, discovered in the Lake Frome region of southern, Australia., Due to the dry and hot climate in central Western, Australia, former river valleys hardly ever carry surface water. Drainage is mostly confined to buried, channels in the alluvial sediments. Yeelirrie lies in, the lower reaches of one of the valleys before it enters, a saline lake basin. In this area, the groundwater table, is several metres below surface. High evaporation, in the unsaturated zone leads to precipitation of, metasomatic and concretionary Ca-Mg carbonates, (channel calcrete or caliche). Uranium is concentrated in the lowest calcrete and underlying alluvial, clays, mainly just below the groundwater table. The, uraniferous calcrete orebody is some 6 km long,, 0.5 km wide and up to 8 m thick. Pockets and crusts, of carnotite characterize the ore. Total resources of, the deposit are estimated at 52,000 t U3O8 with an, average grade of 0.15% (Cameron 1990). Its discovery, in 1972 was due to a total-count aeroradiometric, survey financed by the Australian state. Uranium, and potassium at Yeelirrie are derived from weathering of granites at basin margins, but mafic rocks in, the basement of the basin are probably the source of, vanadium. Calcrete formation and mineralization, are geologically young, although radioactive equilibrium is usually present. The ore at Yeelirrie was, precipitated in an oxic environment, not by reduction. Infiltrating uranium was immobilized solely by, formation of the insoluble vanadate. The palaeochannel deposits in South Australia seem to have precipitated uraninite by microbial reduction and show, displacement of radium from uranium (Dickson &, Giblin 2007)., , Hydrothermal vein deposits or uranium, Hydrothermal vein deposits of uranium lost, their former prominent role when the potential, of uranium in weapons and energy production was, recognized during World War II. Intensive prospecting for the new strategic metal soon brought, discoveries of new deposit types (e.g. sandstone, ore) that could be exploited with lower-cost
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , mining techniques compared to underground, stoping of narrow veins. When mining of silver, and lead in the Erzgebirge (ore mountains in German, Krušne hory in Czech language) reached a, peak during the 16th century, by-breaking pitchblende was discarded as useless. Even after the, discovery of uranium by Klaproth in 1789, nearly, 100 years passed before an application for the, element had been found. Beginning in 1863, pitchblende was mined at Joachimsthal (Jachymov) for, the production of dyes. Until 1912, this mine was, the most important source of uranium (and, radium) in the world. It acquired scientific fame, when Marie Curie discovered the new elements, radium and polonium in pitchblende from Joachimsthal (1898). The mine was operated well into, the recent past, until 1968. Over the last 50 years,, it developed into a spa based on radon dissolved in, thermal water, which is recommended for alleviating numerous serious ailments such as rheumatic diseases and asthma. A brief introduction, to the wider geological setting is provided in Section 2.2.3 on “Tin”:, The Joachimsthal uranium deposit is not only of, historic importance but also of great scientific interest. It is peculiar because apart from uranium, five, different metals were present in commercial quantities. In classical economic geology this assemblage, was called the Bi-Co-Ni-Ag-As (U) vein formation, (Baumann et al. 2000, Stemprok & Seltmann 1994)., Some 200 veins occur at Joachimsthal within an, area of 35 km2 but only 8 were major silver producers., Two vein sets are developed: The older prominent, “morning veins” striking roughly east-west and the, smaller “midnight veins” north-south. The latter, contained most of the ore, although their thickness, rarely surpassed 10–30 cm. A Proterozoic metasedimentary sequence of phyllites and mica schists hosts, the veins. Carbonate, pyrite and graphite-rich bands, in the metasediments exerted a strong lithological, and chemical control on ore shoots (Ondruš et al., 2003). The metasediments are underlain by the Variscan Karlovy Vary (Karlsbad) batholith, at depths, from 500–800 m below the surface. The batholith is, composed of several phases intruded between 310 and, 290 Ma, with a uranium-rich younger generation and, ending with highly fractionated, very small rare element-fertile granite intrusions (Breiter et al. 1999)., A gross depth zonation is developed in the veins, with, , 281, , silver (As) nearest the surface, followed by Bi-CoNi-As and uranium below just above the roof of the, batholith. Veins within the granite are barren. The, paragenesis associated with pitchblende is composed, of dolomitic carbonates, pyrite and blackish-violet, fluorite (Figure/Plate 2.47). Uraninite ages from Joachimsthal vary between 5 and 285 Ma (Ondruš et al., 2003). Only the oldest age, 285 Ma, can be considered, primary. Younger ages indicate disturbance of the, system or dissolution and reprecipitation of pitchblende. The absolute age of the polymetallic Bi-CoNi-Ag-As ore is not yet constrained but structural, relations suggest that it was formed after the uranium, (Ondruš et al. 2003). In conclusion, uranium mineralization at Joachimsthal is related to Variscan felsic, magmatism. Protracted cooling similar to the Chinese deposits mentioned above (Zhao et al. 2004) may, best explain the post-magmatic formation age., , Supergene enrichment uranium deposits, Ronneburg in Th€, uringen (Germany) is an interesting example of a supergene enrichment uranium, deposit. During the period 1950–1991, the district, provided the former Soviet Union with a total of, 113,000 t uranium, in spite of low grade (Lange, 1995). Host rocks are strongly folded and very lowgrade metamorphic shales occurred in a region, where an antiform is traversed by a broad fault, zone. Most ore was extracted from shales, banded, cherts and argillaceous limestones of Late Ordovician to Early Devonian age. Unaltered shales are, characterized by high contents of organic C, (5–9%), pyrite (S 2–3.5%) and 30–50 ppm uranium., Orebodies are blanket-shaped but independent of, bedding. Mineralization is clearly controlled by, the density of rock fracturing and the resulting, palaeo-permeability. Pitchblende, chlorite, carbonates, sulphides and arsenides are most important, minerals of the ore. Pitchblende ages fall into two, groups of Early Permian (Rotliegend, 300–270, Ma) and Mid-Cretaceous (120–90 Ma). Ore at Ronneburg had an average grade of 850 ppm U. The, enrichment was induced by supergene alteration, and dissolution of uranium by infiltrating meteoric water. The oxidation zone extends to a depth, of 100 to a maximum of 1000 m. Below the redox, boundary, uranium was immobilized by reduction
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282, , PART I METALLIFEROUS ORE DEPOSITS, , Figure 2.47 Uraninite vein at, Hartenstein mine (1530 m level) in the, Erzgebirge, Germany. Courtesy Volker, L€, uders, GFZ Potsdam. Uraninite in, dark bands, the light gangue comprises, pink dolomitic carbonates, pyrite and, Bi-Co-Ni sulphides. This is one of, nearly 2000 uraniferous veins in the, district. Between 1946 and 1991, total, production was 80,000 t uranium, contained in concentrate. From, outcrops around Schneeberg in the, southwest the vein field plunges to, great depths in the northeast, about, parallel to the roof of an underlying, granite batholith. Height of image, 50 cm., , and concentrated in a secondary enrichment zone, with a thickness between 200 and 400 m., Exploration for uranium is singular because of, the application of radiometric methods. 238 U, 235 U, and 232 Th are long-lived alpha and low-energy, gamma emitters. Together with 40 K and the, respective daughter nuclides they generate most, of the natural radioactivity. Daughter nuclide, chains of uranium and thorium comprise one, or more radioactive gas species. In the case of, 238, U this is 222 Rn (radon), itself an alpha emitter, with a half-life of 3.8 days (Andre-Jehan and Feraud, 2001, Appleton 2005). The thorium decay chain, includes 220 Rn (half-life 51.5 seconds) that is also, called thoron. Radon is quite mobile, for example, by diffusion, and is used for locating buried uranium anomalies, which cannot be detected by, measuring surface gamma radiation. In practice,, a prospect is covered with a grid of shallow (<10 m), open drillholes (>80 mm) and after equilibration, with soil gas, the radon concentration in holes is, determined with commercial probes. Radon dissolved in groundwater can also be a useful guide., Because of its short half-life, thoron mobility is, very limited., The decay series of uranium and thorium reach a, secular equilibrium after 300,000 to 400,000, years. For the more abundant 238 U, the daughter, nuclides 234 Th and 214 Bi have a prominent role,, because both emit high-energy gamma radiation., 214, Bi is the base for the determination of, , “equivalent” U contents (eU) using gamma ray, spectrometry. This is of great practical interest, because gamma radiation penetrates further (up, to 1 metre of low-density material) facilitating, detection. Most common techniques of radiometric uranium and thorium exploration are based on, g-radiation. Provided that uranium and its daughters are in equilibrium, the intensity of g-radiation, is positively correlated with uranium concentrations, which can easily be calculated., However, discrepancies between radiometric, and chemical determination of uranium contents, are not rare (Brooks 2008). These “errors” result, from the different mobility of uranium and its, daughter nuclides 234 Th, radium, radon and, 214, Bi. In the case of geologically young uranium, (e.g. in post-glacial peat, sandstone and calcrete), equilibrium is not fully established and the, daughter nuclide spectrum is incomplete. Consequently, eU will not point to the uranium. The, reverse is not rare in “moving” roll-front deposits,, where uranium is leached on the upstream side,, but not its daughters. In this case, eU will be high, but little uranium may be present. The differential, movement of nuclides of the uranium decay series, is extremely useful for timing transport processes., Reserve estimations of uranium deposits must be, based on a good understanding of the local disequilibrium situation (Bowden & Shaw 2007)., Equipment employed in uranium exploration, includes, thallium-activated, sodium-iodide
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ECONOMIC GEOLOGY OF METALS CHAPTER 2, , scintillation detectors (scintillometers measuring, total gamma counts), which are also available as, spectrometers that allow separate determination, of eK, eU and eTh. Instruments are employed in, cars and aeroplanes to prepare radiation maps of, larger areas. Hand-held versions are used for reconnaissance in difficult terrain and for detailed mapping of prospects. Borehole probes are available,, but note that in many cases it may be better to, work with prompt fission neutron (PFN) logging, systems that allow determination of in-situ uranium contents (Givens & Stromswold 1989), both, in operating mines and in exploration. Mapping, uranium distribution in cut and polished ore specimen is most efficiently carried out with the scanning electron microscope (SEM). Of course,, uranium exploration employs other mineralogical, geochemical and geophysical methods, apart, from radiometric surveys., Identified and inferred uranium resources, at mining costs <130 US $/kg are estimated at, >5.5 Mt (U metal content). Largest resources, occur in Australia, Kazakhstan, Russia, South, Africa, Canada, USA, Brazil and Namibia. Since, 1945, 2.5 Mt uranium has been produced worldwide, of which nearly 50% were destined for military purposes. Annual world mine production is, 50,600 t U (2009). Main producing countries are, Kazakhstan, Canada, Australia, Namibia, Russia, and Niger. The balance to annual consumption of, 70,000 t U required by the world’s nuclear reactors is made up from secondary sources, including, former nuclear weapons, re-enrichment of, depleted uranium and stockpiles. Large potential, resources assure future supplies, but new mines, and technologies will be needed, especially for, eventual recovery from seawater., Specific problems of uranium production and, use start at the mine with strict control of radiation exposure of miners and people living nearby,, both during operations and after closure (Merkel, et al. 2002). In this respect, uranium mines are, highly regulated. Safe keeping of closed mines and, mine waste facilities is complex. Remediation of, uranium mines in Canada, France and Germany, provides exemplary case histories (Daroussin et al., 1998). The public has accepted the results of these, efforts, but disposal of nuclear waste is still hotly, , 283, , discussed, although the geoscience community is, largely supportive of deep permanent geological, storage. Results of comprehensive research programmes are available and field experiments confirm the feasibility of underground storage, (Chapman & McCombie 2003, Gibb & Attrill, 2003, Miller et al. 2000, Hollister & Nadis, 1998). Hazard from high-level nuclear reactor, waste is initially due to the short-lived radionuclides 137 Cs (half-life 30.17 years) and 90 Sr (28.64, years), followed by 241 Am, 243 Am, 239 Pu, 240 Pu and, 237, Np from 103 to 105 years. Afterwards, 99 Tc,, 210, Pb, and 226 Ra dominate the remaining radioactivity that is equivalent to ore containing 0.2% U, (Plant et al. 2005). Generally, the hazard of nuclear, energy production is little different from other, large-scale energy sources and the sum of benefits, and risks may be best balanced by using all available technologies (Lovelock 2006)., , 2.6 SUMMARY AND FURTHER, , READING, , A cursory inspection of this chapter reveals that, the short lists of significant deposit types for specific metals are never identical. Even closely, related metals such as iron and manganese, or lead, and zinc exhibit substantial differences. Yet, if one, would be asked to devise an ore-forming system, for a certain metal, its geochemical properties, should provide valuable clues. Many metals, for, example, are redox-sensitive and their solubility, and transport by aqueous fluids is limited either to, a reduced (Fe, Mn) or an oxidized state (U, Mo)., A drastic redox change immobilizes such metals., Other controls on ore formation include the tendency to form sulphides (Cu, Zn, Ag, Cd, In, Hg,, Tl, Pb, Bi, As, Se, Sb, Te), their “volatility” (Hg, As,, Sb, Te, Tl) and tendency for enrichment in, extremely fractionated batches of felsic melts (Sn,, W, Ta, Mo, U, Th, Rb, Cs, Li, Be). Geochemistry, also helps to conjecture potential source materials, for certain metals. If extraction by aqueous hot, fluids is envisaged, basalt would be a favourable, source of zinc and copper, whereas uranium and, lead should be sought in granite. Clearly, geochemical study of ore formation is vital.
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284, , PART I METALLIFEROUS ORE DEPOSITS, , Yet, understanding the variety of physical constraints that are implicated in ore formation is, not less important. Pressure and temperature, gradients from the Earth’s interior to the surface, drive a flow of liquids and fluids, which is, involved in ore formation. Mantle and crustal, dynamics combine in the global process system, of plate tectonics. Specific ore deposit types (e.g., copper porphyry) occur in well-defined settings., Yet, the apparently simple system of metal, source, transport and trap displays an unlimited, variety, modulated by very many factors. This is, not to discourage the pursuit of ever better understanding but is a word of caution not to take my, simplified presentation of the fascinating world, of geochemical cycling and metal concentration, for the full account., More detailed geochemical information is provided in the Encyclopedia of Geochemistry by, Marshall & Fairbridge (1999) and amplified in the, voluminous Treatise on Geochemistry edited by, Holland & Turekian (2003). To readers seeking, more details concerning deposits mentioned in, , this book or additional examples, I recommend, the extensive and systematic description of thousands of ore deposits worldwide by Laznicka (1985,, 1993). Geodynamic controls of metallogeny can be, studied in Blundell et al. (2005), based on the, example of Europe. For a thorough presentation, of geology and ore formation, the book by Guilbert, & Park (1986) is unsurpassed. Berkman (2001), provides a blend of valuable scientific and very, practical information (watch out for the next edition, which should appear in 2011). To my knowledge, there is no recent comprehensive book on, environmental management of metal mines, but, the two SEG volumes The Environmental Geochemistry of Mineral Deposits edited by Plumlee, & Logsdon (1999a) and Filipek & Plumlee (1999), are very useful. The Essentials of Medical Geology, edited by Selinus et al. (2005) offer a treasure-trove, of relevant information. Production and resources, figures, and concise information about use and, market trends of single metals are available in the, United States Geological Survey’s Minerals Information Webpages (USGS 2010).
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PART II, Non-Metallic Minerals, and Rocks, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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CHAPTER 3, Industrial minerals,, earths and rocks, Synopsis, Industrial minerals are valuable economic raw materials, which are not used in the production of, metals or energy. Compared with metals and other non-metallic resources, they are mainly, processed by physical methods. Both definitions, however, are not without exceptions and some, attributions to the group are rather by tradition. Typical examples of industrial minerals are talc,, mica and andalusite. Several ore minerals such as chromite, bauxite and rutile also have industrial, applications, but the bulk of production feeds metallurgy. Industrial rocks and earths typically, comprise multi-mineralic hard and unconsolidated rocks, which are mainly used in the construction industry (granite, sand, gravel, clay). Together with salts (Chapter 4), industrial raw materials, form the class of non-metallic and non-energy rocks and minerals. Because of multiple and ever, changing uses, and a wide genetic variety (cf. Table 3.1), neither supports a particularly illuminating, subdivision of industrial minerals, rocks and earths. Therefore, the order in this chapter is simply, alphabetical. The selection of about 30 materials is based on geological and economic interest., Uses of minerals and rocks encompass all human activities, from the mundane (e.g. road building, aggregate) to industry (refractories for steel-making), food production (apatite and sulphur) and high, technology applications (quartz for electronics). In many cases, high-technology conversion of fairly, common raw materials into refined products (e.g. micronized fillers in plastics) is the key to, economic success. Therefore, market considerations are a major component from the exploration, stage to evaluation and development of a deposit (Border & Butt 2001). Feasibility characteristically, depends on successful industrial-scale production trials in cooperation with potential customers., Economic data and facts were assembled from Mining Journal, Mining Magazine, Mining Annual, Review, Berkman (2001), Neuendorf et al. (2005), United States Geological Survey (USGS minerals, information webpages) and Walker & Cohen (2007). Discrepancies concerning details are not rare, between different sources. Therefore, where highest accuracy is sought, I advise the reader to verify, critical data., , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , concentrates should have 56–60 wt. %Al2O3,, Fe2O3 <1% and very little alkalis. Coarse-grained, products command higher prices., The thermal expansion of andalusite and sillimanite (6 and 4 vol. %, respectively) is so small that, the uncalcined minerals can be used directly for, blocks and mortars applied as an inner cladding of, furnaces. Kyanite, however, expands by 16–18 vol., % and has to be calcined before application. In, some instances, raw kyanite can be useful, for, example when shrinkage of clay upon burning is, to be compensated. Calcining at temperatures, between 1100 and 1650 C converts the natural, Al2O3SiO2 to a mixture of 88 wt. % needle-shaped, crystals of mullite (3Al2O32SiO2, density 3.17 g/, cm3) and 12 wt. % SiO2 (cristobalite, or glass). Up, to a temperature of 1810 C, mullite is very, stable under thermal, chemical and mechanical, stresses. Due to these properties, mullite is used, to line furnaces for the production of iron, steel,, other metals, glass, ceramic products and cement., Mullite is also synthesized in large quantities by, mixing and calcining kaolin þ bauxite or quartz, þ alumina. Natural occurrences of mullite are, known (Mull, Scotland) but are too small to warrant economic interest., , The aluminosilicates andalusite, kyanite and, sillimanite are metamorphic minerals, which, formed in aluminium-rich rocks by orogenic or, contact metamorphism (Holdaway 1971). Average, pelites have Al2O3-contents of 14–17%, similar to, most igneous rocks, except ultramafics (e.g. 1.82%, in dunite). Higher contents are found in anorthosite (average 25.86%) and nepheline syenite (21%)., The Al2O3-enrichment required for economic, exploitability in many deposits (>20% to 40%) is, an interesting scientific question that is not, always easy to answer. The metamorphic veil, often hides evidence about the origin of the precursor rock (Ihlen 2000). One known agent of, aluminium concentration is surficial weathering, (cf. Chapters 1.2 “Supergene Ore Deposits” and 2.4, “Aluminium”). Former kaolin-rich soil horizons, or sands may be one genetic explanation. Kaolin, also is formed by hydrothermal alteration in granitic and felsic volcanic environments and these, altered rocks are possible precursors, too. Investigations of Norwegian kyanite quartzites favour, , 289, , the latter hypothesis, substantiated by very low, content of alkalis that are always present in sedimentary quartzite (M€, uller et al. 2007). In deep, shear zones of high metamorphic grade, fluids, may transform common granitoids into strongly, peraluminous (Al2O3 > NaO þ K2O) kyanite, mica schists (Sassier et al. 2006). Metamorphic, fluids concentrate aluminium by preferential, leaching of SiO2 and alkalis. This is favoured by, elevated fluorine contents (Nabelek 1997). Hydrothermal kyanite in quartz veins is rather rare,, because of the low solubility of Al in aqueous, fluids (Bucholz & Ague 2010)., Mutual pseudomorphic replacement of the, three closely related minerals is very common and, is a function of the metamorphic P/T/time path., With rising pressure, andalusite is replaced by, kyanite and both by sillimanite, when the temperature increases. Retrograde metamorphism, results in muscovite replacing the aluminium, silicates. The extent of pseudomorphism or, replacement may be one of the controls of product, quality., All three alumosilicate minerals occur in alluvial, placers, which were formerly important sources., At present, kyanite and sillimanite are locally significant by-products of coastal placer mining., 3.1.1 Andalusite, Andalusite (density 3.16–3.20 g/cm3) occurs in, contact-metamorphic metapelites of the heated, aureole of magmatic intrusions and in low-pressure amphibolite facies metasediments (Bucher, & Frey 2002). Undesirable inclusions in andalusite include staurolite, chloritoid, biotite, sericite, graphite, Fe-oxides and TiO2. The main, producer of andalusite is South Africa, from the, vast contact zone of the Bushveld Complex., Initially, the mineral was exploited from alluvial placers. Today, hard rock mining of primary, deposits prevails (Siegers & Lange 1991). The, second major producer is France with the Glomel mine in the Bretagne. Ordovician slates in, the contact zone of a granite intrusion contain, 15 wt. % andalusite in large porphyroblasts, (1–4 by 10–40 mm). The run-of-mine ore is, crushed and pulverized. From this, andalusite
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290, , PART II NON-METALLIC MINERALS AND ROCKS, , Gopane, , Andalusite, mines, , 26°, , Post-Timeball Hill formations, , N, 27°E, Dinokana, , 25°3’, Zeerust, , u, , u, Groot, Marico, , Pre-Timeball Hill formations, , Swartruggens, , Timeball Hill, Formation, Koster, , 26°S, , 50 km, , Limit of andalusite occurrence, , is concentrated by flotation, high-intensity, magnetic and heavy media separation:, Near Groot Marico in the Northwest Province of, South Africa, the Timeball Hill Formation of the, Palaeoproterozoic Transvaal Series hosts andalusite deposits. However, only pelitic metasediments, within certain metamorphic subzones characterized by index minerals are prospective (Figure 3.1),, including zone A (chiastolite, biotite, cordierite,, epidote, minor andalusite) and zone B (chiastolite, and biotite). Zone C (biotite plus minor andalusite), and zone D (spotted slate) are not favourable., Chiastolite is a variety of andalusite with dark, carbonaceous inclusions forming a cruciform, design., , Andalusite is generally preferred to kyanite,, because it can be used without calcination, thus, saving energy. Most of the world production of, andalusite in 2009 originated from South Africa, (260,000 t) and France (65,000 t). The development of new mines depends mainly on markets,, because andalusite-bearing contact aureoles are, not rare., , 3.1.2 Kyanite, Kyanite (density 3.55–3.66 g/cm3) is common in, aluminous metasedimentary and meta-igneous, rocks that experienced medium-pressure and, medium-temperature orogenic metamorphism, , Figure 3.1 Andalusite in the, northwestern thermal contact zone of the, Bushveld Intrusive Complex (modified, from Hammerbeck 1986). Courtesy, Geological Society of South Africa. Black, circles denote andalusite contents in, weight percent: <5%, 5–10% and >10%., , (Bucher & Frey 2002). Metamorphic host rocks, of kyanite are quartzite, schist and gneiss containing kyanite as porphyroblasts or in crystalline masses. Secretion quartz veins developed, by dehydration reactions in these rocks and, certain pegmatites can also be enriched with, coarse-grained kyanite. Minimum exploitable, grades of hard-rock kyanite deposits are 20 wt., %. Muscovite and garnet are often associated, with kyanite and can be economically important co-products. Kyanite contains abundant, solid inclusions (e.g. muscovite) that influence, its value. In the surficial weathering environment, kyanite is very stable and as a heavy, mineral, it is enriched in alluvial and coastal, placers:, Large primary kyanite deposits occur in the eastern, USA. At Henry Knob in South Carolina, a thick bed, of quartzite interbedded with Precambrian gneiss, had contents of 20% kyanite (and pyrite) over a, strike length of several kilometres. Most of today’s, world production of kyanite is derived from the, Willis Mountain mine in Virginia, USA (2009, 80,000 t after 115,000 t in 2008). Indicated reserves at Willis Mountain amount to 65 Mt at, 25% kyanite. Together with many smaller occurrences, Willis Mountain forms a “kyanite belt”, within the central Virginia volcanic-plutonic province. Country rocks include Palaeozoic mafic to, felsic volcanic rocks, volcaniclastic metasediments, granites and tonalites. Kyanite quartzites
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , are thought to have originated by severe leaching in, high-sulphidation (advanced argillic) alteration systems, resulting in quartz-kaolin protoliths (Owens, & Pasek 2007)., , Small quantities of kyanite are produced in, India, Simbabwe, Brazil, Sweden, Spain, Russia,, Ukraine and China. Indian kyanite was renowned, for its large grain size, but export ceased because of, dwindling reserves. In Russia, deposits are hosted, by Archaean metasediments of the Kola Peninsula. Apart from kyanite, the ore schists contain, quartz, sillimanite, staurolite, muscovite and, graphite. Exploitable high-grade lenses with, >20% kyanite occur over a strike length of 200 km, of the schist zone. Several potential deposits are, known in the East African Mozambique belt,, a complex Neoproterozoic orogen. In Kenya, eluvial placers of kyanite were exploited at Murka, Hill near Voi Township until 1960. On the ridge,, the source kyanite quartzites contain dispersed, high-grade pockets. Today, the former mine lies, within Tsavo National Park., , 3.1.3 Sillimanite, Sillimanite (density 3.23 g/cm3) occurs in many, metasedimentary rocks that experienced low- to, medium-pressure and high-temperature orogenic, or contact metamorphism (Bucher & Frey 2002)., Typical rocks that may have exploitable sillimanite grades are biotite-sillimanite hornfels, cordierite-sillimanite gneiss and quartz-mica-sillimanite, schists. Lateral secretion quartz veins in these, rocks and, rarely, in pegmatites, can be enriched, with coarse-grained sillimanite. However,, exploitable grades of hard rock sillimanite are, infrequent. Consequently, sillimanite has the, least significance of the aluminosilicates. The, mineral is probably exploited in China in order to, supply refractories to its steel industry but statistics are not published. In India it is a by-product of, coastal placer mining (24,000 tonnes in 2009)., Very small quantities of the formerly famous, sillimanite-corundum “balls” from Assam are, still traded. In South Africa, minor sillimanite, production is derived from the contact halo of the, Bushveld Igneous Complex., , 291, , 3.2 ASBESTOS, Density, (g/cm3), Chrysotile asbestos, (“white asbestos”), Crocidolite asbestos, (“blue asbestos”, or “Cape blue”,, a variety of, the amphibole, riebeckite), Amosite asbestos, (“brown asbestos”;, the formula, describes, grunerite), , Mg3Si2O5(OH)4, , 2.5, , Na2(Fe2 þ , Mg)3, Fe23 þ Si8O22(OH)2, , 3.2, , (Fe>Mg)7, Si8O22(OH)2, , 3.1–3.25, , Generally, the commercial and industrial term, asbestos refers to minerals of the serpentine and, amphibole groups that occur as bundles of thin,, flexible and separable fibres. All asbestos minerals, are characterized by a length to width ratio (the, “aspect ratio”) of >100:1. “Amosite” is a trade, name; mineralogically it may consist of anthophyllite, grunerite, gedrite or cummingtonite., Tremolite and actinolite are other common, amphibole minerals that may occur in asbestiform, varieties. Exploitable asbestos ore typically contains 5–6 wt. % fibres with a length of 20 mm., For over 4000 years, asbestos was considered as, a magic safeguard against fire. Only a few decades, ago, its hazards for human health were discovered., When handled, for example in asbestos board, manufacturing, asbestos fibres disintegrate readily and release microscopic needle-like particles., Inhalation of these particles causes a number of, serious respiratory diseases and other health, problems, especially benign and malignant lung, diseases (Kane 1993, Gibbons 2000, Selinus et al., 2005). Therefore, mining and the use of asbestos in, industrial countries have been virtually terminated. Naturally occurring asbestos exposed by, weathering and erosion always caused a background of asbestos fibres in the air. Human, intervention (e.g. road building, quarrying and, agriculture) may locally aggravate the risk., Authorities should identify rock bodies that
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292, , PART II NON-METALLIC MINERALS AND ROCKS, , contain asbestos and if needed, establish control, measures such as suppression of dust during excavation and farming (Van Gosen 2007). Exposures of, naturally asbestos-bearing rocks can be detected, by airborne spectral mapping (Swayze et al. 2009)., There is no evidence that ingestion of asbestos, with drinking water is hazardous to health (WHO, 2006). Note, that blue asbestos poses a much, greater health hazard than chrysotile., Useful properties of asbestos include resistance, against heat (permanent temperatures of 400 C, and peaks to 1000 C) and many chemicals, as well, as low conductivity of heat and electricity. Chrysotile asbestos made up >90% of past asbestos, production and represents >99.5% of remaining, world production, due to termination of amphibole asbestos exploitation. For some applications,, fibre length and tensile strength are important., Asbestos’ exceptional insulation and fire-resistance properties have been used in numerous, products and industrial applications (e.g. tiles for, Space Shuttle Columbia). An important sector of, asbestos use is the building industry, because, many countries with a rapidly growing population, continue to use low-cost asbestos cement for roofing and water pipelines., , 3.2.1 Asbestos mineralization types, Asbestos deposits include (1) hydrothermal veinlet stockworks in ultramafic rock bodies, (2), hydrothermal stockworks in mafic intrusive or, extrusive rocks, and (3) hydrothermal-metasomatic alteration of iron formations. Asbestos, occurrences of minor economic role are (4) hydrothermal veinlets in contact-metamorphic dolostone and (5) metasomatized alkalic intrusions, (Libby, Montana) and carbonatites (Mountain, Pass, California). In most cases, either contact or, orogenic metamorphism established the hydrothermal systems, but magmatic fluids are equally, capable of producing asbestos. Essentially, a suitable combination of chemical rock composition,, pervasive rock fracturing, silica-bearing fluids, and favourable (commonly lower greenschist, facies) T/P-conditions cause formation of asbestos, minerals., , Ultramafic-hosted asbestos deposits, These are complex stockwork mineralizations in, partially or totally serpentinized dunite, peridotite, or pyroxenite. With exceptions (e.g. Zimbabwe:, Great Dyke; South Africa: Barberton Greenstone, Belt), these rocks are parts of ophiolite sheets., Extensional fracturing of the rock bodies, possibly, during ophiolite obduction, allowed access of, mineralizing fluids. High fluid pressures and pure, extension favour formation of valuable “cross, fibre” veinlets, where fibre growth is vertical to, the walls of the fracture. Shear-hosted “slip fibre”, or unoriented “mass fibre” have low value. Chrysotile, like most of the asbestos minerals, forms, under P/T-conditions of the lower greenschist, facies. Because low-T hydrothermal chrysotile, excludes iron very efficiently, magnetite is a frequent by-product of the rock-fluid reaction, producing chrysotile. Average olivine with a composition of Fo90Fa10 transforms into serpentine, with 5–10 vol. % magnetite (eq. 3.1)., Chrysotile asbestos formation (serpentinization), from olivine:, 2Mg2 SiO4 þ 3H2 O ! Mg3 Si2 O5 ðOHÞ4 þ MgðOHÞ2, Forsterite, Chrysotile, Brucite, 3Fe2 SiO4 þ O2 ! 2Fe3 O4 þ 3SiO2aq, Fayalite, Magnetite, 3Mg2 SiO4 þ SiO2aq þ 2H2 O ! 2Mg3 Si2 O5 ðOHÞ4, ð3:1Þ, Mines processing ultramafic-hosted ore (e.g. Ni,, Cr, asbestos) sequester enough carbon dioxide, from the atmosphere to more than offset greenhouse gas emissions from operations. This is due, to the formation of carbonates in tailings (Wilson, et al. 2009)., The largest (or after the Ural Mountains in Russia and, Khazachstan, the second largest?) chrysotile asbestos, deposits in the world were exploited near Thetford in, Quebec, Canada. They occur in harzburgitic mantle, of Ordovician fore-arc ophiolites near the western, margin of the Appalachian orogen (Page & Barnes, 2009, Hebert & Bedard 2000; Figure 3.2). Mining was, terminated in 2003.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 293, , The South African crocidolite asbestos deposits occur, along a distance of 400 km in a belt between Prieska, and Botswana. Here also, the deposits are controlled, stratigraphically by iron formations of the Transvaal, Supergroup. The largest mine was Pomfret near the, Kalahari Desert. Thin beds with an extension of 1.5 by, 2 km were exploited. The sodium of the riebeckitelike asbestos was probably derived from mafic tuffs, that are intercalated with the iron formation (Beukes, & Dreyer 1986b). Fibres were short (1–18 mm) and, structurally controlled by competent bands of the, iron formation, similar to saddle veins. The asbestos, ore rock contained 12 wt. % of fibres. Processing, included crushing, airlifting the fibres and classifying, different grades. Nearly the whole production was, used for manufacturing asbestos cement., , Figure 3.2 The asbestos-rich ophiolite (black) near, Thetford south of Quebec, Canada highlights the suture, of Iapetus Ocean closed by the collision between, Caledonian nappes and the Laurentian continent (after, Hebert & B, edard 2000, La Chronique de la recherche, mini, ere). by permission of BRGM, www.brgm.fr., , Asbestos deposits related to Precambrian, banded iron formations (BIF), Asbestos deposits related to BIF were mined extensively in South Africa until closure a few years ago., Large deposits occur in the Palaeoproterozoic, Transvaal Supergroup:, Amosite asbestos (grunerite) was exploited in the, Penge area in northeastern Transvaal province., Certain horizons of the Penge Iron Formation are, mineralized at tectonic highs (domes) that were, formed by the interference of two fold systems., The later phase of folding and a regional thermal, metamorphism are thought to be related to the, Bushveld intrusion (Beukes & Dreyer 1986a). In, this area, rosettes and ubiquitous thick prismatic, masses of grunerite are regional products of metamorphism. Asbestos was only formed where synmetamorphic folding and doming provided lowpressure domains that attracted fluids. Asbestos, orebodies had a thickness of a few metres only,, but a horizontal extension of 3 by 1.5 km. Fibre, length reached 76 cm., , Main producers of asbestos are Russia (50%),, China, Brazil, Khazachstan, Canada and Zimbabwe. Because of regulations, production in most, western industrial countries was terminated., World production in 2009 amounted to 2 Mt, (USGS 2010). Asbestos is substituted by wollastonite (cf. “Wollastonite”), ceramic, glass and, steel fibres, and with fibres made from carbon,, cellulose and synthetic organics. Most of these, materials are too expensive for roofing and similar, applications in buildings, explaining why asbestos, continues to be used in less affluent countries., 3.3 BARITE AND CELESTITE, Density (g/cm3), Barite, Witherite, , BaSO4, BaCO3, , 3.9–4.5 (calculated 4.468), 4.3, , Nearly 300 minerals are known to include barium, but barite and witherite are the only natural, resources used by industry. In its crystal structure,, barite readily incorporates strontium (0.1–5%), lead, (0–0.2%, usually 0.17%) and calcium (CaCO3, 0–5%, usually 2.8%). Often, barite is associated with, Pb-Zn sulphides, fahlore, manganese oxides, stibnite, fluorite, carbonate and quartz. Some of these, may be co-products of barite mining. Elevated trace, contents of mercury are common in barite and have, to be controlled during processing and marketing.
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294, , PART II NON-METALLIC MINERALS AND ROCKS, , Barite is a low-cost mineral and is mainly, extracted in open pits. Underground exploitation, may be economically feasible if the material is, of high quality (e.g. for white filler or medicinal, blanc-fixe production). Veins should have, a minimum thickness of 2 m and the deposit must, include at least 0.4–1 Mt BaSO4. Generally, a high, content of BaSO4, high density and whiteness, facilitate marketing processed barite., Barite is heavy, non-abrasive, inert, non-corrosive, insoluble and non-toxic. This qualifies the, mineral particularly for use in the petroleum and, natural gas industry. More than 85% of world, barite production is employed for regulating the, density of drilling fluids by adding finely ground, (micronized) barite to a maximum of 40% by, weight of the fluid. For similar reasons, heavy, concrete is made with barite. It also serves as, a non-abrasive, dense and chemically inert filler, in plastics, paper, rubber and paints and, due to its, high adsorption capacity for hard radiation, as, a shield in nuclear reactors, X-ray laboratories and, in faceplate glass for television cathode ray tubes,, but not in flat-panel monitors. Barium chemicals, are made by reduction of barite with coal or coke to, produce barium sulphide (“black ash”), which is, reacted to form chloride, hydroxide or carbonate., Chemically precipitated BaSO4 (blanc-fixe) is, employed in medicinal X-ray diagnostics and, increasingly in paints and other specialized fields., Witherite and technical barium carbonate are useful additions in glass and ceramics (as a flux), and, in chemicals, ferrites and photographic papers., Strontium is chemically similar to calcium and barium, and is part of group 2 (alkaline earths) of the, periodic table of the elements. Strontium consists of, four stable isotopes, but only the ratio 87 Sr=86 Sr is, commonly employed in geological investigations., 87, Sr is composed of a primordial and a radiogenic, component from the decay of 87 Rb (Schreiber & Tabakh 2000, cf. Chapter 1.1 “Isotope Geochemistry”)., In the environment, strontium is harmless. The, nuclear fall-out isotope 90 Sr (half-life 28.9 years) is, dangerous, however, but is also used in cancer therapy. Natural celestite (also called celestine, SrSO4,, <43.88% Sr, density 3.9 g/cm3) and rarely, strontianite (SrCO3, density 3.8 g/cm3) are processed into a, range of strontium chemicals, which are essential in, , many products. Previously, strontium was merely, regarded as a provider of red colour in fireworks and, signals. At present, consumption is growing fast, because many applications of strontium carbonate, are similar to those of barium carbonate (see above), but strontium is often preferred. Strontium metal, (density 2.64 g/cm3, melting point 777 C) is alloyed, with aluminium and silicon for improved casting, for, example of car engines. The only source of strontium, occurring in large exploitable deposits is celestite., Celestite deposits are mainly associated with evaporites (Figure/Plate 3.3), more rarely with carbonates, and other rocks., Celestite deposits are typically stratiform, but various, features suggest a replacement origin during diagenesis,, as in Coahuila State, northeastern Mexico. These stratabound celestite deposits occur in Mesozoic sediments, close to horst structures. Homogenization temperatures of fluids vary from 50 to 160 C, salinities from, 0 to 19 wt. % NaCl equivalent pointing to basinal, brines, which leached Sr from basement-derived clastic, rocks and precipitated celestite upon mixing with, meteoric waters. Together with deposits and numerous, occurrences of Zn, Pb, Ba and F, the Coahuila strontium, deposits are considered as one of the nearly two dozen, Mississippi Valley Type provinces of North America, (Gonzales-Sanchez et al. 2009). Low-temperature, hydrothermal epigenetic veins and veinlets occur in, limestone (e.g. Cyprus). The largest producing celestite, district is Granada in southern Spain with the Escúzar, and Montevives mines in Tertiary basins of the Betic, cordillera. In this district, diagenetic celestite occurs in, beds of gypsum and stromatolitic limestone of a flatlying Late Miocene (Messinian) sequence. Strontium, was probably mobilized during early diagenesis by the, conversion of gypsum to anhydrite (cf. “Gypsum and, Anhydrite”). The world production of strontium, amounts to 500,000 t (2008). Main producing countries, are Spain, China and Mexico (USGS 2009)., , 3.3.1 Geochemistry, Barium is lithophile and closely related to calcium, strontium and radium. Like lead, Ba2 þ, , , (ionic radius 1.35 A) replaces K þ (1.38 A) as an, incompatible LIL-element in many rock-forming, minerals to contents of several percent. Basalts, contain 250 ppm, whereas felsic magmatic and, clastic sedimentary rocks attain 600 ppm Ba. Its, crustal average is 430 ppm Ba (Sr 350 ppm) with, a range of 179–1070 (Sr 150–480 ppm; Smith &
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 295, , Figure 3.3 (Plate 3.3) Celestite crystals in roughly bedding-parallel solution cavities of Neogene gypsum at Wadi, Essel, Red Sea Coast, Egypt., , Huyck 1999). Barite forms in the seawater column and its accumulation in bottom sediment, corresponds with biological productivity. Relative to aluminium, barium (or barite) is enriched, in pelagic sediments of highly productive seas., During weathering, diagenesis, metamorphism, and hydrothermal processes such sediments are, a ready source of barium. Reduced and acidic, conditions facilitate barium mobilization from, rocks (Cooke et al. 2000). Therefore, barite deposits are common and barite is a frequent gangue, mineral in hydrothermal mineralization. In, hydrothermal solutions, the element is transported in the form of chloride complexes such as, BaCl2(aq). Upon encountering SO2, 4 ions (e.g. oxidized brines or seawater) barite is precipitated., Witherite forms at lower oxygen fugacity and, alkaline conditions (pH > 8). Witherite and barite, are resistant against weathering and nearly insoluble in water and dilute acids, so that outcropping, deposits are marked by strikingly white exposures and boulders. Some dissolution may, take place, however, caused by sulphate-reducing, , microbes (Bolze et al. 1974). Because of their low, solubility, barite and witherite are not hazardous,, but all soluble compounds of barium are toxic,, especially the chloride. For application as a rat, poison, the chloride used to be prepared from, witherite., SrSO4-concentrations in barite may assist, genetic interpretation. Because celestite and barite are isostructural, a complete solid solution, series exists between both minerals, although, most samples plot near one end or the other of, the series. Higher SrSO4-contents in barite appear, to characterize hydrothermal veins formed at elevated temperatures, whereas synsedimentary and, early diagenetic barite contains little strontium., Because barite is practically free of rubidium,, the isotopic ratio 87 Sr=86 Sr of barite generally represents the composition of the host fluid at, the time of mineral formation. This allows the, distinction of two groups of stratiform barite, (Maynard et al. 1995). The first is associated with, Pb-Zn ore and displays clearly radiogenic Sr, (implying derivation from continental crust),
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , Submarine, hydrothermal-exhalative barite, deposits, These deposits can be very large. Commonly,, barite is a co-product of base metal mining. Barite, is either part of the ore paragenesis or occurs as, a separate nearly monomineralic seam. Kuroko, (Figure 1.46) and sedex deposits (China, Nevada,, Alaska, Figs. 1.71, 1.73) are prominent types. In, chemical sedex stratification, barite occurs with, galena or follows immediately after galena. This, may be due to zone refining by redissolution of, barite in the sulphide zone and precipitation on, encountering sulphate. The volcanic environment, of the Greek island of Milos seems to recall Kuroko features. However, barite at Milos occurs in, the form of beds, tubular and irregular masses, that replace Pliocene volcanic tuff. Nearby kaolin, deposits clearly point to the transitional position, of mineralization from marine to terrestrial and, epithermal. Sulphide-poor barite deposits in, marine black shales and chert, such as those of, South China and Nevada, were probably formed in, a setting comparable to recent submarine barite, sinters, which form at seeps of cold methanerich pore waters on passive continental slopes, (Figure 3.5, Torres et al. 2003):, Barite and witherite deposits of southern China occur, in sequences of Neoproterozoic-Cambrian marine, black shales and chert, those of Nevada in comparable, sediments of Ordovician-Devonian age. In both, , Figure 3.5 Submarine formation of barite, deposits by venting of early-diagenetic, barium and methane-rich fluids on the sea, floor (“cold seeps”, Torres et al. 2003). 1 –, Biogenic barite production in the water, column of coastal upwelling zones; 2 –, Mobilization of sediment-hosted trace, barium by reduction and methanogenesis;, 3 – Migration of fluids with dissolved, barium and methane in permeable, sediments or faults; 4 – Precipitation of, barite by mixing of diagenetic fluids with, seawater at seepage vents on the sea floor,, formation of “milky-white smokers”,, sedimentation, possible mechanical, erosion and redeposition of barite., , 297, , regions, the host sediments formed on submarine, continental slopes. The conspicuous linear arrangement of deposits implies a control by synsedimentary, faults (Clark et al. 2004). One of the largest sedex, barite deposits on Earth is Mangampeta in southern, India, with 37 Mt of reserves. Giant accumulations, of Mississippian-age sedex barite associated with the, largest known massive sulphide deposits (Red Dog, district, Figure 1.73) have been found in the western, Brooks Range, Alaska. They contain as much as, 2000 Mt barite. At present, however, these geological, resources are clearly second to the metal potential of, the region., , Hydrothermal barite veins, An example for hydrothermal barite veins, in this, case formed from basinal fluids, is the Wolkenh€, ugel mine near Bad Lauterberg in the German, Harz Mountains:, The mine exploited a thick steeply dipping vein over, a length of 1100 m and a depth of 350 m. The economic section is but a small part of the total exposed, length of the vein structure (8.5 km). The mine was, worked from 1838 to 2007, with a total production of, 4 Mt of barite. Host rocks include Devonian and, Early Carboniferous greywacke, shale and siliceous, limestone. Near the vein, these rocks are variously, bleached, silicified, carbonatized and chloritized., The vein displays two different parageneses. The, hanging-wall part consists of crumbly quartz and, carbonate, which remained after leaching of
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298, , PART II NON-METALLIC MINERALS AND ROCKS, , anhydrite cement. The footwall part of the vein consists essentially of massive and banded coarsely crystalline barite with some quartz, carbonate and traces, of sulphides (chalcopyrite, pyrite, tetrahedrite). Barite, thickness reaches 30 m. Run-of-mine ore assayed, 89% BaSO4, 3.4% SrSO4, 6% SiO2 and 1% CaCO3., The deposit formed during a Late Cretaceous tectonic, phase that activated faults and induced convection of, meteoric water. On the downflow path, the water, dissolved overlying Permian salt. Barium was leached, at depth and concentrated in the upflow limb, where, the hot brine mixed with cool sulphate solutions., , Terrestrial barite sinter (hot springs) deposits, Genetically interesting as well as economically, significant was the terrestrial barite sinter (hot, springs) deposit of Les Redoutieres near Chaillac, on the northwest margin of the French Massif, Central (Sizaret et al. 2004):, Here, migmatites of the Variscan basement are transgressively overlain by coastal red sands of earliest, Liassic age (203 Ma). In a shallow basin, these sands, contained a stratiform barite deposit, which reached, 20 m thickness and comprised 4 Mt of BaSO4. The, mine closed in 2006. The ore bed displayed both, horizontal and vertical zonation (Figure 3.6)., Thin banding of red to yellow barite and brown, goethite was characteristic, the latter becoming more, , abundant upwards. The banding was affected by synsedimentary brecciation and by deformation due to, dehydration and gravitational consolidation of the, hydrothermal precipitates. On the eastern side of, the barite deposit, a near-vertical fluorite (-barite-), vein (Rossignol) had been mined until recently. In, this vicinity, the barite sinter displayed an elevated, content of fluorite. Obviously, both vein and surficial, sinter deposits were derived from one hydrothermal, convection system. The sinter formed by mixing of, Ba-F-fluids (110 C, 21% NaCl equiv.) of deep origin, with surficial SO4-solutions (Ziserman 1980). A, genetic relation to intensified heat flow, early rifting, and extensional strain associated with the opening of, the Atlantic Ocean is assumed., , Barite deposits in karst, Barite deposits in karst may be epigenetic (ascending hydrothermal) or a product of supergene, weathering and infiltration into existing karst, caves (descending meteoric water):, The former Fleurus mine in southern Belgium was, sited in a karst depression of Visean limestones on, the northern flank of the Namur synclinorium. The, doline was filled with Early Cretaceous terrestrial,, lignitous sediments that enclosed a barite bed reaching a thickness of 25 m. Barite textures resembled, travertine and the bed enclosed sediments, implying, syngenesis. Dejonghe (1989) proposed derivation of, the barium by weathering of surrounding Late Carboniferous rocks and precipitation by sulphate-bearing groundwater in the karst lake. The mine yielded, a total of 1.1 Mt until closure in 1987., , Epigenetic-hydrothermal karst barite and barite, veins occur in Morocco and elsewhere in western, North Africa (the Maghreb region). They are, considered to be products of deep basinal fluid, convection systems that were established in Triassic and Jurassic times, when the Atlantic Ocean, started to open (Valenza et al. 2000; Figure 1.79)., Figure 3.6 Section of Early Jurassic barite sinter deposit, Les Redouti, eres near Chaillac, France, formed from, hydrothermal solutions ascending through Rossignol, fluorite vein (cf. Figure 3.17). After Sizaret, S., Marcoux,, E., J, ebrak, M. & Touray, J.C. 2004, Society of Economic, Geologists, Inc., Economic Geology Vol. 99, Figure 2C,, p. 1109., , Evaporite-hosted barite deposits, The Pessens deposit is located on the southern, margin of the French Massif Central and, like, Chaillac in the North, in transgressive Early Jurassic sediments:
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300, , PART II NON-METALLIC MINERALS AND ROCKS, , earthy sepiolite. The dense, massive and white sepiolite (“meerschaum”) extracted near Eskishehir,, Turkey is a different modification that is used for, carving ornamental objects. The largest deposits of, attapulgite are worked in the Quincy-Attapulgus, region of Georgia and Florida, USA, but the mineral, is also produced in Spain (Kendall 1996). Very large, resources of attapulgite are known in Senegal,, although at 50,000 t/yr present production is moderate. Authochtonous deposits of supergene montmorillonite-palygorskite above basalt rock and, resedimented lacustrine sediments are exploited in, Jiangsu and Anhui provinces, China. The formation, of palygorskite from montmorillonite is explained, by evaporitic pedogenesis, with dissolution of, sodium and calcium and import of magnesium and, silica (Long et al. 1997)., , Industrial classification of bentonite stresses, technical properties including thixotropy, base, exchange, swelling and adsorption capacity, (Christidis & Scott 1996). Two types of bentonite, are distinguished: i) The sodium, high-swelling, type, derived from volcanic ash deposited in shallow marine or playa lake environments; and ii) the, calcium (Mg), low-swelling type that evolved, from volcanic ash settled in freshwater environments (Alther 2004). The second is more common, and less valuable than the first. Note, however,, that Cretaceous calcium-montmorillonite in, southeast England has been used for centuries, under the trade name “Fuller’s earth”. For the, standard swelling test, 1 g of bentonite is added to, 100 mL of demineralized water. One gram of, dry Ca-bentonite swells from 1–2 mL, and 1 g of, Na-bentonite swells up to 16 mL in volume. Nabentonite adsorbs six times its weight in water, (Alther 2004). Swelled water-saturated bentonite, forms a gel with a certain mechanical resistance,, because the platelets are negatively charged at, edges and positively charged on planes. Electrostatic forces cause gelification, but the gel is easily, liquefied by stirring (taking the state of a sol). This, transformation gel-sol-gel (thixotropy) is essential, for use in drilling mud. Poorly swelling betonites, can be improved (“activated”) by homogenizing, moist bentonite (30 wt.% H2O) with 2.5–6%, hygroscopic soda (NaCO3), resulting in replacement of Ca þ 2 and Mg þ 2 by Na þ ., , Natural bentonite (as opposed to processed products) below the ground surface is grey or bluishgreen (reduced). Near the surface, oxidation of iron, causes yellow and brown colours (Figure/Plate, 3.8), and the stiff clay-rock decomposes into small, fragments and soft clay. Bentonite is extracted in, open pits and stockpiled for maturing, the principle of which is to spread and rework the material, periodically in order to advance oxidation and, disaggregation:, In the field, bentonite is not easily recognized because, the rock resembles ordinary marlstone. Identification, may be assisted by unusual colours (red, green, gray, and blue; typically yellow when weathered) and by, a slippery feel or waxy appearance. Polyhedral fragmentation in the dry state and a soapy surface after, wetting and, of course, swelling are characteristic., Samples from bentonite outcrops are deceiving; drilling and trenching is recommended for recovery of, representative material. Wet sieving and XRD analysis of the clay fraction are standard laboratory procedure. Specific tests depend on the intended market., Bentonite seams of 0.5 m thickness can be exploited, if conditions are favourable. However, working, smectite-rich clays is awkward because of their, peculiar hydraulic and mechanical properties (Terzaghi et al. 1996)., , Bentonite is used: (i) raw, after little processing, such as removal of coarser grain fractions and, drying; or (ii) activated by chemical treatment, with acids or alkalis. Heat activation (iii) is a more, recent way to improve desired properties. Sodium, bentonite is used raw as a drilling mud component, a binder for pelletizing ore fines (e.g., 0.5–1.5% in iron ore pellets), for the treatment of, water contaminated with heavy metals or hydrocarbons, for soil improvement, as a filler in paint, and putty, for supporting deep open trenches in, civil engineering and as a binder in foundry, moulds made from sand. Acid-activated bentonite, (bleaching earth) is employed as an adsorbent in, refining and decolourizing oil, fat, wine and beer,, as an inert carrier of insecticides, a fire drencher,, cat litter (the largest market segment) and for, absorption of spilled fuels. Alkali-activated bentonite is mainly used in thixotropic drilling muds, and as a binder in pellets and foundry sand.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 301, , Figure 3.8 (Plate 3.8) Bentonite sample from Moosburg mining district, Germany. Copyright S€, ud-Chemie, AG 2009., , Synthetic smectites are commercially produced, for special purposes (e.g. laponite) as a colourless, gel in medicine, cosmetics and numerous other, small-volume applications., For humans and the environment, bentonite is, harmless. Actually, it is one of the edible earths, used as a medicine and for other reasons, (“geophagy”, Selinus et al. 2005). Bentonite is an, important material in environmental engineering., Its high plasticity and swelling capacity, united, with low permeability make it a most effective, sealing material. Its high adsorption capacity, minimizes the mobility of harmful substances., Typical applications are landfill liners and the, sealing of underground repositories containing, toxic or radioactive waste., 3.4.1 Bentonite deposit types, Bentonite deposits originate by alkaline alteration, of volcanic rocks and especially of glass-rich felsic, , ash. Some deposits are detrital clays with a, high proportion of smectite or of swelling, mixed-layer minerals. Consequently, the following settings of bentonite formation are, distinguished:, . alteration of volcanic ash under alkaline, conditions, by reaction with seawater (Wyoming,, England: “Fuller’s earth”; Milos, Greece) or with, terrestrial evaporative alkaline brines (Bavaria);, . hydrothermal, alkaline alteration by seawater, convection at half-submerged felsic volcanoes, (Milos, Greece);, . autochthonous weathering of basic tuff, basalt, and ultramafic rocks resulting in smectite-rich, soil;, . smectitic clay as a marine or playa lake, sediment., Bentonites of high quality are geologically, young. The name is derived from the Cretaceous, Benton Group volcano-sedimentary rocks in, eastern Wyoming, USA, where swellable sodium
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302, , PART II NON-METALLIC MINERALS AND ROCKS, , bentonite was found more than 100 years ago., The Hardin and Black Hills districts still are the, world’s largest source of high-grade sodium, bentonite., Tertiary and Quaternary tuffs host the large, bentonite and kaolin deposits of the Aegean islands, with the island of Milos the most remarkable of all. Several exposures on Milos show that, pyroclastic rocks were transformed into either, kaolin or smectite, indicating passage of acidic, (hot) and alkalic (cool, seawater?) fluids at different, times. Kaolin formation supersedes the smectite, phase and although pH-boundaries are very sharp,, kaolin pervades bentonite in the form of irregular, tubes, veins and stockwork veinlets. Such complex distribution patterns complicate selective, exploitation. Acidic hydrothermal alteration of, bentonite is in principle similar to acid activation, but the product lacks the desired properties, possibly because of ageing (Kaufhold & Decher 2003)., Rheological and swelling properties are positively, influenced by smectite concentration but decrease, with opal-CT content. Sodium bentonite is mostly, derived from rhyolitic tuff and calcium bentonite, from andesite (Christidis & Scott 1996). Also in, a marine island volcanic setting, low-temperature, hydrothermal alteration of Tertiary andesiticdacitic tuff near Serrata in the Sierra del Cabo de, Gata, Almeria, Spain produced large bentonite, deposits. Pits form a wide circle around the, gold-mineralized caldera at Rodalquilar (cf. Chapter 2 “Gold”)., Economically important bentonite deposits, occur in the East Bavarian Tertiary molasse zone, (the foreland basin north of the Alps). In the Neogene of the region Mainburg-Landshut-Malgersdorf, shallow, hydrographically closed basins, accumulated a suite of volcanic ash-derived, bentonite beds, silt, sand and impure freshwater, carbonates. Hot and dry climate conditions caused, periodic alkalinity. The sediments are undeformed. Laterally, smectite contents of the yellow,, bluish-green to olive-green bentonite beds change, little (Figure 3.7, Figure/Plate 3.8), whereas vertical differences are considerable. The bentonized, rhyolitic tuff beds can be related to the distant, Transylvanian-Pannonian volcanic province (Unger et al. 1990)., , World production of bentonite is 10 Mt, (2009), with USA first (40%) followed by Greece, and Turkey. Reserves and resources are very, large. Fuller’s earth (calcium bentonite) adds, another 3.5 Mt (2009) to world smectite production., , 3.5 BORON, , %, B2O3, Borax (tincal), Kernite, (rasorite), Ulexite, Probertite, Colemanite, Szaibelyite, (ascharite), , Na2B4O5(OH)4, 8H2O, Na2B4O6(OH)2, 3H2O, NaCaB5O6(OH)6, 5H2O, NaCaB5O7(OH)4, 3H2O, CaB3O4(OH)3H2O, MgBO2(OH), , D, (g/cm3), , 36, , 1.7, , 51, , 1.9, , 43, , 1.9, , 50, , 2.1, , 51, 41, , 2.4, 2.7, , The list names the most commonly extracted, boron ore minerals, out of more than 230 that, contain boron (Garret 1998). Minor sources of, boron include some silicate minerals, playa lake, brines and boric acid from volcanic fumaroles., Elemental boron does not occur in nature. In many, cases, water-rich minerals such as borax are aqueous precipitates that age by dehydration to highergrade minerals such as kernite. Near the surface,, kernite readily rehydrates to borax. Colemanite is, often seen to replace ulexite and borax, but can, also be a primary epigenetic phase (Miranda-Gasca, et al. 1998)., Boron is mainly traded as anhydrous boron oxide, (B2O3), as mineral concentrates and boric acid, (H3BO3). Worldwide, most boron is consumed in, the borosilicate glass industry. Certain enamels,, ceramics, agricultural chemicals, pharmaceuticals, detergents, wood preservatives and fluxing, agents also require boron. Grinding media (boron, carbide is harder than corundum) and buildings for, radiation protection (because boron absorbs
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , neutrons) are further sectors of boron use. A future, perspective may be NaBH4 as a hydrogen storage, material for vehicles that run on fuel cells. The, metalloid boron (density 2.34 g/cm3, melting, point 2075 C) is very hard and useful in a number, of specialized applications., Boron is an essential nutrient for humans, animals and plants, but is moderately toxic in higher, concentrations. Human intake is mainly with, food, but increasingly through drinking water, prepared from treated sewage effluent. In 2006,, WHO set a provisional drinking water guideline, value to 0.5 mg/litre. The threshold for normal, plant growth is 15 mg/kg boron in soil., 3.5.1 Geochemistry, Boron is a lithophile element with a crustal abundance of 10 ppm (range 3–50: Smith & Huyck, 1999). Granites (15 ppm) contain more boron than, basalt (5 ppm). Boron has one of the smallest ionic, radii of the common elements and displays incompatible behaviour in magmatic systems, leading to, strong enrichment in hydrous liquids, magmatic, fluids and vapours. Elevated boron concentrations, mark marine pelites (100 ppm), the magmatic gas, phase (BF3, BCl3, etc; e.g. Larderello in Toscana,, Italy) and many terrestrial hot springs (boric acid, H3BO3). Mud volcanoes associated with gas and oil, fields emit much boron. Based on seawater with, 4.6 ppm B, boron is enriched in potassium-magnesium salts of marine evaporites. An example is, boracite (Mg3ClB7O13), which was formerly byproduced from potash seams of the Stassfurt mining district in Germany. In the surficial environment, oxy-anions of boron (II) and (III) are very, mobile., Isotope investigations imply that boron in magmatic rocks, geothermal systems and fumaroles is, mainly derived from pelites. The considerable, mass difference between 10 B and 11 B causes strong, fractionation in many geological processes,, including the formation of boron minerals (Swihart et al. 1996). Boron in terrestrial settings is, characterized by negative d11 B, whereas marine, borates (e.g. in evaporites) exhibit positive values,, similar to modern ocean water with delta, 11, B ~40‰., , 303, , 3.5.2 Boron deposit types, Current commercial boron extraction is mainly, based on boron mobilized (“distilled”) from, crustal rocks by volcanism or deep heat anomalies, (e.g. intrusions). Boron concentration takes, place by evaporation in confined evaporative lake, basins, either: i) syngenetically from lake brines;, ii) during early diagenesis from pore water; or ii) in, some instances as at Sonora, Mexico (MirandaGasca et al. 1998) epigenetically from hydrothermal solutions. Metamorphic borate deposits are, exploited on Liaoning Peninsula, China. A large, boron skarn deposit occurs in the Russian Far East, (Dalnegorsk)., Most boron is extracted in one of four borate, provinces: i) Southwestern USA; ii) western Anatolia; iii) the High Andes (Argentina: Alonso et al., 1991, Bolivia, Chile, Peru); and iv) Kazakhstan and, Tibet., California hosts the major North American, boron deposits, in Miocene playa lake sediments,, as at Boron (Kramer) and the nearby Death Valley, area in the Mojave Desert. A Pleistocene equivalent with brine-filled crystal mush is exploited at, Searles Lake in the Mojave Desert:, Kramer is the largest of these deposits, with resources, >100 Mt of ore. It lies within a borate zone 8 by 1.6 km, that represents a small sub-basin within a large Tertiary playa valley. The orebody is a lenticular mass of, borax and kernite 1600 m long, 800 m wide and, 100 m thick, with several intercalations of smectite, clay. Locally, the borate bed rests directly on basalt., Concentrated (saline) geothermal spring waters are, supposed to have flowed into the lake, where they, sank to the bottom and on cooling precipitated borax, (Siefke 1991). Exploitation takes place in a large open, pit. Elsewhere in California, deep borate orebodies are, extracted by borehole leaching (e.g. the new mine, Fort Cady, with reserves of 138 Mt B2O3 at 410 m, below the surface)., Searles Lake is a Pleistocene equivalent to the Tertiary, borate playa lakes. The deposit consists of muddy, evaporitic sediments, which contain halite, trona,, borax and other salts. These minerals prevail in two, “salt layers” that are exploited by solution mining., The brine contains 1–1.2% B2O3, as well as recoverable, sodium bicarbonate and sodium sulphate. Here, the
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304, , PART II NON-METALLIC MINERALS AND ROCKS, , boron supply is assumed in thermal springs along the, Sierra Nevada frontal faults (Smith 1979)., The Salar de Atacama in the Chilean Altiplano is, a large dry saline flat. Lake sediments contain brines, with lithium, potassium (as well as traces of Rb and, Cs) and boron. Solutes are concentrated in constructed ponds by natural evaporation. Products, include H3BO3 (cf. Chapter 2 “Lithium”)., , Turkey probably hosts the world’s largest boron, resources. Early Miocene lakes occupied a series, of elongate extensional rift basins that formed in, a continent-continent collisional setting. Lake, sediments comprising limestone, tuff, marls and, gypsum are intercalated with felsic calc-alkaline, volcaniclastic beds. Sediments reflect an evaporative setting and changing shallow to deep-water, phases. Borates were deposited in subaqueous settings as seams of laminated borax, as at Kirka, (Figure/Plate 3.9). Towards lake margins, a zone, of ulexite is followed by a broad colemanite rim, (Helvaci & Orti 2004). Five main borate mining, , districts are worked. With resources of nearly, 1000 Mt Bigadiç is the largest deposit of colemanite-ulexite on the Earth. Here, two 30 m thick, borate horizons occur in a sequence dominated, by volcanic tuff that is covered by basalt. Borates, are hydrothermal-sedimentary precipitates in yellow and white banded sediments. Gypsum and, dolomite occur with borates and confirm evaporative conditions. Borate textures show that initial, growth took place in the soft amorphous precipitates (early diagenetic), although later diagenetic, changes are evident (Helvaci 1995). Here, colemanite remains the marginal facies, but the basin, centre is occupied by ulexite (Helvaci & Orti 1998)., Liaoning Peninsula in China is an important, province of magnesite, talc and boron mining. The, marginal facies of a Palaeoproterozoic rift basin is, characterized by clastic sediments and carbonates, including magnesite. The central part of the basin, is built of footwall meta-arkose, alkali rhyolite,, Mg-rich metasediments and hanging-wall metaturbitites. Borates occur in the Mg-carbonates and, , Figure 3.9 (Plate 3.9) Borax mine Kirka in western Turkey is one of the world’s largest boron producers. Courtesy, Walter Prochaska, MU Leoben. Formed in a Miocene evaporitic lake (Helvaci & Orti 2004), the borax ore (lower part) is, enveloped in limestone and overlain by claystone and banded limestone, tuff, marl and chert. Gangue of borates, comprises realgar, orpiment, gypsum, celestite, calcite, dolomite and smectite clay. Ore thickness reaches 145 m,, resources comprise >110 Mt of 45% B2O3.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , silicates that are now magnesite marbles, partly, with phlogopite, forsterite-diopside fels or serpentinite, all with much magnetite. Borate ore consists of, lenses and masses of suanite (Mg2B2O5) and ascharite [also called szaibelyite, MgBO2(OH)]. Orebodies, have the form of breccias, stockworks, masses or, dispersed stratabound nodules in metasediments., Because of amphibolite facies metamorphism and, strong deformation, genetic keys are ambiguous., Oneofthepossibleinterpretationsincludesaformer, playa lake setting similar to the Californian and, Anatolian deposits (Peng & Palmer 1995)., Russia procures 80% of its boron requirements, from the skarn ore deposit at Dalnegorsk near, Vladivostok. Triassic carbonates were intruded by, a Cretaceous granodiorite. The resulting skarn, consists of danburite (CaB2Si2O8), datolite (CaB, (OH)SiO4), wollastonite, hedenbergite and garnet, (Crowe et al. 2001)., Exploration for boron is based on the observation that borate in salt lake sediments occurs in, massive beds or displacive crystals embedded in, mud and gypsum. Along strike, the seams reach, extensions of several hundred metres and appear, to be centred on former hydrothermal springs., After barren intervals, the same horizon may again, host exploitable borate. Margins of arid basins, with volcanic activity are principally prospective., Deposits that can be worked by open pit mining, are preferred. Basinal deeps are explored by drilling. The present limit of underground mining of, borates is probably 400–500 m below the surface., Below this depth, brine and solution mining may, still be an economic proposition., The world production of primary boron in 2009, amounted to 6 Mt B2O3, mainly from Turkey, (35%), USA, Argentina, Chile and Russia (USGS, 2010)., , 3.6 CARBONATE ROCKS: LIMESTONE, CALCITE, MARBLE, MARLSTONE, DOLOMITE, , Density (g/cm3), Calcite, Dolomite, , CaCO3, CaMg(CO3)2, , 2.6–2.8 (calculated 2.7), 2.85–2.95 (2.876), , 305, , The density of carbonate rocks (specimen), varies within a wide range from 1.8–2.85 g/cm3., Large samples of many cubic metres are preferred, for the determination of in-situ rock mass weight, per volume (e.g. for reserve estimation). In the, mineral dolomite, part of the magnesium may be, replaced by ferrous iron and manganese., In this chapter, carbonate and carbonate-clay, rocks are presented that are exploited as industrial, raw materials. Specific requirements of rocks, quarried for building stone, ornamental use and, road ballast are not discussed, nor is magnesite (cf., “Magnesite”). The majority of industrial carbonate production is based on autochthonous, abiotic/, biogenic marine sediments. Viable deposits of, terrestrial (calcrete), lacustrine, hydrothermal, (travertine: Guo et al. 1996) and magmatic origin, (carbonatite: Notholt et al. 1990) are less frequent., With few exceptions such as recent oolite sands, and earthy chalk, sedimentary carbonate rocks are, consolidated by diagenesis and occur as hard, rocks. Metamorphic equivalents including calcite, and dolomite marble are often chemically less, pure but many make excellent white fillers. All, investigations of carbonate rock deposits must, include methods of sedimentology and carbonate, petrology. In practice, carbonate rocks are classified according to calcite and dolomite content, (0–10% dolomite ¼ limestone, 10–50% dolomitic, limestone, 50–90% ¼ calcitic dolomite, and, 90–100% ¼ dolomite), and the percentage of clay, and quartz impurities (limestone with 0–5% clay,, marly limestone 5–35%, marl or calcareous clay, 35–65%, marly clay 65–95%, and clay 95–100%)., If the mudrocks are indurated, add “-stone” as in, marlstone when referring to these rocks., Carbonate rocks are extremely important raw, materials for industry, the building sector, agriculture, forestry and environmental engineering., Potentially exploitable resources should be, mapped, recorded and protected by land use, regulations., Generally, carbonate rocks are processed by, washing, crushing and calcining at temperatures, of 800–1050 C in kilns of varying construction, (Oates 1998). Resulting products are lime CaO, ( MgO) and CO2. Pure calcite loses 43.8 wt. %,, but practice shows that the production of 1 t CaO
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306, , PART II NON-METALLIC MINERALS AND ROCKS, , consumes 1.8–2.2 t limestone. For shipping, lime, is pulverized and often hydrated to calcium, hydroxide Ca(OH)2. In contrast to calcining limestone, natural marl that is destined for cement, production, or a mixture of ground limestone and, clay, is “sintered”. Sintering designates heating to, incipient melting (>1280 C). Only a small part of, carbonate rocks is used in uncalcined form, for, example in forestry as a mild and slowly acting, basic soil conditioner., 3.6.1 Limestone, Limestone consists of calcite (rarely of aragonite), and minor amounts of iron, magnesium, quartz,, clay, pyrite, phosphate and organic matter. Reef, limestones are often quite pure because clastic, silicates are nearly absent, but may have spurious, (diagenetic) dolomite contents. Bedded limestones, tend to have higher clay and quartz fractions. An, efficient field method for determination and visualization of undesirable non-calcitic components, in limestone is short-time immersion in diluted, hydrochloric acid, which dissolves calcite and, exposes a relief of dolomite and silicates. Comparison with samples of known mineralogical and, chemical composition allows on-site provisional, estimation of quality., Desirable properties of limestone include a specified, chemical and mineralogical composition, calcining, performance, grindability, etc. as a function of the, intended use. Low MgO thresholds, for example, are, applied in steel and calcium carbide production and, for de-acidification of water. Generally, low contents, of SiO2, Al2O3 and Fe2O3 are stipulated and their level, determines possible uses. Sulphides and phosphates, interfere with pig iron and steel manufacturing where, carbonate rocks are used as a slag-forming flux. In all, metallurgical and combustion processes, alkali element contents, which are often derived from contact, with saline waters should be <0.05%. Resistance to, size reduction during loading, transportation, unloading and calcination in industrial kilns is advantageous. Note that all limestone deposits comprise, parts with differing qualities, which must be identified. Surficial alteration, and especially karstification, are frequent causes of impaired quality. If at, all possible, higher-grade parts should be separately, extracted in order to maximize economic benefits., , Viable carbonate rock deposits should have a minimum tonnage of 500,000 t (for white filler or lime, production) and ten to several hundred million, tonnes for cement manufacturing., , Limestone is one of the most versatile and, essential commodities. More than 70% of limestone products are used in chemical and metallurgical processes (e.g. iron, steel, glass, paper, sugar),, including large amounts for flue gas desulphurization, water treatment, cement and fertilizer, production. There are numerous smaller and, specialized uses of limestone and lime., High-grade carbonate fillers are micronized, carbonate rocks with a high whiteness. This is, compromised by dark minerals or joint coatings, such as iron and manganese oxides, organic matter, and pyrite. Silicates that give a white powder, upon grinding (e.g. muscovite) are not harmful. In, Europe, numerous suitable deposits are exploited,, based on Cretaceous chalk, other marine limestones and calcite marbles. The last reach the, highest degrees of whiteness., 3.6.2 Metamorphic calcite (and occasionally, dolomite) marbles, Calcite marbles are in high demand for the production of high-whiteness fillers and coatings, (1–10 mm) for the paint, paper and plastics industries. Country rocks are typically amphibolite,, mica schists and paragneiss. A recent example is, the new Kristallina mine near Vitipeno (Sterzing), in northern Italy. The deposit has a strike length of, 4 km and a width of 300 m (Pine et al. 2006). As is, increasingly common in Europe and in North, America, extraction will be by underground mining because of environmental and societal considerations. For more than 2000 years, ornamental, marbles have been exploited from Mesozoic metamorphic carbonates at Carrara in Italy (Meccheri, et al. 2007)., 3.6.3 Marlstone, Marlstone with 35–65% clay and argillaceous, limestone (>50% CaCO3) are the most important, components of cement production (Hewlett 1997).
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , Cements are hydraulic binders reacting with, water to solid “hydrates”. Cement production, begins with sintering ground limestone and clay, in a kiln. The resulting “clinker” is ground to, a fine powder. Cement clinker consists of tricalcium silicate, tricalcium aluminate and tetracalcium aluminoferrite (Saint John et al. 1998)., Because rocks with a suitable chemical composition (“natural cement marls”) are relatively rare,, the feed for cement factories is mixed from limestone, marlstone, claystone, quartz sand and iron, ore. Almost all cements used for manufacturing, concrete are based on Portland cement (the name, originates from the similarity of concrete to Portland stone, a Jurassic oolite used as a traditional, building material in England). The raw mix for, Portland cement consists of 75% CaCO3, 3%, Fe2O3, a maximum of 4% dolomite and silicates, with a ratio of SiO2 to Al2O3 of 3–6. Only small, amounts of sulphur, alkalis (Na, K) and phosphorus are admissible. Elements such as As, Sb and, Hg, which volatilize at the high sintering temperatures in the kiln, must be carefully monitored in, order to avoid emissions into the environment., The described mass ratio requires that cement, factories are located near limestone or natural, cement marl deposits. Other components may be, hauled from greater distances. Supplementary, cementitious materials (SCM), such as pozzolans,, pulverized fly ash from coal-fired power plants or, blast furnace slag, may substitute for part of the, cement formula. Main environmental issues, related to cement production are high energy consumption and carbon dioxide emissions., Among speciality cements, calcium-aluminate, cements are important for applications in underground mining and tunnelling. Also called, “fondu” (French for melted) cements, they are, made from a mixture of limestone and low-silica, bauxite resulting in Al2O3 contents of 40–50%, (exceptionally up to 80%). The raw mix is melted, and after cooling, finely ground. In concrete prepared with aggregate and water, the fondu cement, hydrates to calcium-aluminium hydrate and Alhydrate without secreting free Ca-hydroxide like, Portland cement. Upon curing, water is given off, and strength-developing phases crystallize,, mainly monocalcium aluminate (CaAl2O4). Con-, , 307, , crete made from fondu cement displays a high, chemical resistance against sulphate and acids,, a rapid strength development and no release of, alkalinity to mine and tunnel water., 3.6.4 Dolomite, Dolomite rocks are formed from the mineral dolomite, some calcite and the same minor and trace, minerals as listed for limestone. Dolomite rocks, are rarely primary sediments but are usually, formed by diagenetic Mg-metasomatism (dolomitization) of limestone. This involves the passage of, Mg-bearing solutions, although non-diagenetic, sources of Mg-fluids are possible (cf. “Magnesite”)., Metasomatic dolomites form rocks of a friable, crystalline (saccharoidal) nature near unconformities, faults and fractures. Incomplete conversion or, later recalcitization are quite common. Therefore,, dolomite rock bodies, which are sufficiently large,, pure and homogeneous for utilization, are less, widely available compared to low-Mg limestones., Dolomites are used raw, calcined and sintered., The first includes applications as a filler and, extender, as a fluxing agent in the iron and steel, industry, and as a magnesium-fertilizer in agriculture and forestry. Calcined dolomite is a rapidly, acting fertilizer, neutralizes acid waters and is the, solid reagent in manufacturing seawater magnesite and magnesium metal. Sintered dolomite (at, 1600–1950 C) in the form of mortar or blocks is, a useful refractory in the iron, steel, cement and, lime industry. The sintering aptitude of dolomite, is a function of crystal size and decrepitation, upon heating. Mineralogical methods can give, first indications but full-scale industrial trials are, indispensable. High-purity dolomites are required, for Mg-metal, seawater magnesite and brucite Mg, (OH)2 production. Detailed specifications for, different uses vary widely. Strict requirements, exist for glass-making and refractory dolomite, (high MgO; Al2O3 and Fe2O3 below 1.5%, SiO2, <0.5%). Similar to limestone, an improvement of, the extracted dolomite by mine-site processing is, hardly possible (except the removal of chert, from chalk, or of fine impurities by washing)., Accordingly, the exploration of dolomite deposits, requires closely spaced sampling, preferably by
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308, , PART II NON-METALLIC MINERALS AND ROCKS, , coring. Extraction will be selective so that different qualities can be separately marketed., World production of lime and calcium hydrate, in 2009 was 280 Mt (USGS 2010), 10% less than, in 2008. Of the total, a share of 68% was recorded, from China, followed by USA, Japan, Russia, Brazil and Mexico. Cement production reached, 2800 Mt. Of all mineral raw materials, cement, consumption’s growth is highest., 3.7 CLAY AND CLAY, , ROCKS, , Clay rocks are cohesive unconsolidated or indurated clastic sedimentary rocks with an important, size fraction <0.002 mm (2 mm), which mainly, consists of clay minerals such as kaolinite, illite,, montmorillonite, chlorite and mixed-layer clay, minerals (Figure 3.7 and Figure 3.10). Apart from, clay minerals, clay and claystone contain finegrained clastic silicates (quartz, mica, feldspar),, biogenic matter (carbonatic microfossils, kerogen,, coaly particles) and diagenetic minerals (marcasite, pyrite, carbonate, phosphate). Nearly monomineralic, high-grade clays such as bentonite and, kaolin are separately discussed. Here, emphasis is, on lower-grade plurimineralic clays, which are, products of in-situ weathering, erosion and resedimentation in lake basins or in the sea. These, clays are mainly used for the production of bricks,, stoneware and numerous other essential baked,, fired and sintered products., , Basically, clay rocks consist of three groups of, minerals, which determine possible uses: (1), 35–55% clay minerals that provide plasticity, (2), inert minerals giving strength (e.g. quartz, or flint), between 25 and 45%, and (3) 25 to 55% reactive, fillers (“fluxes”) such as illite and feldspar. Two, main groups are distinguished: refractory and nonrefractory, or ceramic clays. Of course, properties, of naturally occurring clay rocks are modified at, will (within economic boundaries) by an admixture of various components: Kaolin and bauxite,, for example, raise the melting temperature; quartz, and feldspar flour dilutes group (1) minerals, and, barium carbonate prevents bloom defects. Oxides, of Fe, Cu, Ni, Co, Mn, Cr, Ti, Zn and Sn are more, common members of the palette that may be used, to colour fired ceramics. Glazing is applied in order, to seal porous surfaces of stoneware and for decoration. Different glazes are mixed from ingredients, such as illitic or bentonitic clay, talc, ash prepared, from various plants, barium carbonate, halite,, borax, calcite, alkali feldspar and nepheline, syenite., 3.7.1 Clay deposit types, Clay deposits are products of supergene alteration,, erosion and sedimentation. Those still found, above silicate rock from which they formed are, part of autochthonous or moderately relocated, soils (kaolin, loam). Most clay deposits originated, by erosion and short transport in limnic or fluvial, settings. Clay beds in the footwall of peat (or, lignite, coal) typically display reduced iron and, alkali element content, because of the passage of, acidic and humic waters. This favours formation, of kaolin and results in refractory grade. Numerous examples can be cited, including the “fire, clay” in Carboniferous coal basins of England or, in Tertiary basins of Czechia. Marine clays are, rarely of high grade but may be usable after prolonged weathering., Refractory clay, , Figure 3.10 Grain-size distribution and mass percent, of minerals in Cretaceous marine sealing clay from, Dolgen near Hannover, northern Germany (company, data). With permission from www.schweizerbart.de., , Refractory clay (fire clay) displays incipient melting at >1580 C. This is mainly a function of, elevated Al2O3 content provided by kaolinite,
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , diaspore or boehmite. Substances that cause melting at low temperatures (“fluxes”, i.e. Ca, K, Na,, Fe) should amount to <3 wt. %. Some refractory, clays contain more SiO2 (“acidic” clay), whereas, others are Al2O3-rich and particularly suitable for, the production of chamottes (calcined clay). Synthetic calcined mixtures of clay and bauxite, for, example, are manufactured to replace natural, refractories such as andalusite. In most cases, fire, clay deposits are chiefly composed of kaolinite, that does not burn white and therefore, is of lesser, value (cf. “Kaolin”)., Flint clay is a hard compact microcrystalline to, crystalline claystone with a conchoidal fracture, mainly composed of well-ordered kaolinite. Flint, clay is highly refractory, with a quality between, kaolin-illite plastic clays (ball clay) and high alumina nodular clays characterized by diaspore or, boehmite., Geological, mineralogical and technological investigations are required for exploring a fire clay, deposit. Critical properties, which are tested with, standardized methods, include plasticity, the, behaviour during heating and sintering, melting, point and chamotte quality. Chamotte is used as, a refractory lining in kilns and furnaces, whereas, acidic clays are applied as a binder in foundry, moulds, and for bricks, tiles and whiteware of, extra quality., Ceramic, non-refractory clays and loams, Ceramic, non-refractory clays and loams soften, by incipient melting below1520 C. Compared to, refractory clays they display lower contents of, Al2O3 and relatively more iron and alkalis. With, decreasing value, the following subgroups are, distinguished:, . Fine ceramics, or potter’s clay, must have good, plasticity and fluidity for casting ceramic pieces,, low air drying and firing shrinkages, high green, strength before firing (no cracking) and favourable, firing properties. The colour after firing should, be white, or as light as possible (Fe2O3<1%), or, a pleasant red (Fe2O3>7%). Clays with these characteristics consist mainly of kaolinite and illite,, possibly with some mica and montmorillonite., Coarse-grained minerals (of iron, or carbonates), , 309, , and rock fragments are unwanted. In contrast to, kaolin (china clay) that gives a vitreous body,, potter’s clay is porous after firing and must be, glazed. In Devon and Dorset in England, extensive, deposits of poorly to fairly ordered kaolinite-illite, (“ball”) clays are found at deeper levels of Eocene, and Oligocene lacustrine to brackish basins (Bristow & Robson 1994). The source of the clay was, most probably a former kaolin blanket on the, Culm shales and granites of Cornwall. The material is nearly free of iron and gives a white or creamcoloured body. Because of its favourable small, grain size it is very plastic. This makes it an, excellent complement to kaolin of low plasticity., Blending different clays in order to attain desired, properties is an important skill of both artist potters and producers of industrial ceramics and, stoneware., . Coarse ceramic clay contains more silt, sand and, other impurities than potter’s clay (pyrite, iron, hydroxides, carbonate, organic matter). Roofing, tiles and frost-resistant clinker bricks are made, with the best material, whereas lesser qualities are, used for bricks, drain pipes and other construction, stoneware (also called “earthenware”). Geological, aspects of brick clay deposits and brick making are, described in detail by Bell (1993)., . Loam is the common material for brick-making., It is a mixture of sand, silt and clay of brown or, yellow colour, often with humic matter and abundant weathered and fresh rock fragments. Loam is, a product of supergene alteration affecting alumosilicate rocks, loess or clay. In northern latitudes,, unweathered aeolian loess deposited by glacial, dust storms contains no clay minerals and is of, little use. Loam is either found in situ as a soil, horizon, or eroded and resedimented. Most bricks, are fired, but the advantages of using unfired clay, bricks and binder (“earth masonry”), such as a, healthy indoor environment and low environmental impact over the whole life-cycle of the material, lead to increasing use. In some regions people, prefer red-fired bricks requiring elevated iron contents. The red colour will be subdued if the loam, contains calcite; fired bricks turn out yellow. Of, course, the firing colour can be manipulated by, additives: Coal and sawdust cause black, and manganese oxides dark purple colour.
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310, , PART II NON-METALLIC MINERALS AND ROCKS, , Expanding clay, Expanding clay is distinguished by a considerable, volume increase when rapidly heated to, 1100–1200 C. After curing for several months in, exposed dumps, the raw clay is usually shredded, and pelletized. Upon heating, the round pellets, expand and form little balls with a diameter of, up to 5 cm. These balls (“light expanded clay, aggregates”) display a vitreous dense skin and, a cellular inside. Different qualities and processing allow adjustment of bulk density from, 250–950 kg/m3. Low-density expanded clay, makes an ideal aggregate for lightweight concrete., In loose form it is also used for sound and heat, insulation., Expanding clays are normally free of quartz, sand, therefore plastic, and contain little iron., Prevailing components are illite and smectite. The, molten skin should form before the main body, expands. Therefore, melting at relatively low temperature is desired and this requires high contents, of fluxes (up to 30%). The expansion may be, caused by reduction of iron compounds in reaction, with organic matter, liberating CO2. Degassing of, calcite and pyrite may also be involved. Many, deposits exploit expanding claystone and even, Palaeozoic shales are known as adequate raw, materials. Therefore, the suitability of likely rocks, should be tested at a semi-industrial scale, in, addition to laboratory methods. One expanding, clay deposit at Fehring in southern Austria occurs, as a sediment in a Pliocene freshwater maar lake., The material consists of illite, montmorillonite,, kaolinite, vermiculite, muscovite and mixedlayer clay minerals., , Sealing clays, Sealing clays are a traditional material for control, of permeability in hydraulic and civil engineering,, such as building the core of earth dams. Recently,, they are widely employed in the construction of, landfills and the remediation of abandoned hazardous sites, both as hydraulic and geochemical, barriers. The suitability depends on properties,, such as cation exchange and adsorption capacity,, swelling (if smectites are present), high plasticity, , and low permeability. Best practice demands that, sealing clay barriers, which are supposed to protect, ground or surface water, should be composed of, several layers of different mineral composition., Because of economic reasons, sealing clays are, normally not processed apart from homogenization and curing. In northern Germany, a Cretaceous marine montmorillonite-illite clay deposit, at Dolgen near Hannover is the main supplier, (Figure 3.10)., Large (>1 Mt) clay occurrences with a reasonably homogeneous composition and a favourable, location are always potential deposits. Even in an, unexplored state, they should be mapped and protected by land-use planning agencies. Indeed, even, common brick loam resources are limited, at least, in many industrial countries. Detailed investigations of clay deposits must consider the typically, high inhomogeneity and include possibly hazardous trace element contents (cf. Chapter 2, “Molybdenum”). Intensive geological, mineralogical and technological investigations are required, before reserves, possible products and marketing, of different clay qualities can be reliably estimated. Many technological tests are time-consuming and expensive, whereas investors always, expect rapid answers. Note that internationally, leading clay mining districts and companies making clay products typically have a long history of, learning and development., Worldwide clay production amounts to 500, Mt/year., , 3.8 DIAMOND, Diamond (C, Density 3.5 g/cm3) is foremost, a valuable gemstone but large quantities of diamond are used in industry. Diamond is remarkable, because of its hardness (10 on the Mohs scale),, a high refractive index that causes its singular, brilliance, the highest thermal conductivity of any, mineral, mechanical toughness and high electrical, resistivity., Natural diamond occurs in the following, varieties:, . Diamond sensu stricto includes transparent, crystals, of which the highest valued are colourless
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , (“white”) and flawless (without visible inclusions). Different colours are caused by impurities., . Bort and ballas describes globular aggregates of, small diamonds with a confused radial or granular, structure. The term bort is also used to designate, natural diamonds of lowest quality or similar, synthetic stones and waste from gem cutting that, can only be used in industry., . Coated diamonds are characterized by an opaque, outer shell of diamond containing impurities such, as graphite and other inclusions, and a gem-quality, core, . Carbonados are black, sintered, porous diamonds that contain reduced metals or metal, alloys, carbides and nitrides, but no terrestrial, mineral inclusions. They are possibly of extraterrestrial origin., The size (mass) of gem diamonds is measured in carat, (1 ct ¼ 200 mg); the minimum for industrial recovery, is 5 mg. Most rough stones produced from diamond, mines are under 0.5 carat in weight. Diamonds with, 1 ct are relatively common and stones to 300 ct are, rare. Larger diamonds, however, are so extraordinary, that they are usually given a name (e.g. Cullinan, the, largest rough gem-quality diamond ever found with a, weight of 3106 ct or 621 g). The value of gem diamonds, depends on size, transparency, colour, flaws, cut and, other factors., Industrial diamond is foremost an abrasive that is, used for drilling, grinding, sawing and polishing., Useful properties apart from its hardness include, toughness, resilience against aggressive chemicals, and high-temperature stability. Applications are, numerous, ranging from microsurgery to deep drilling, for petroleum and cutting large monolithic dimension stones. Most of this market is served by synthetic diamonds., , Exploitability of diamond deposits depends on, the combination of grade and value of the diamonds produced. Most mines exploit grades, between 0.05 and 2.0 ct/t, corresponding to, 0.01–0.4 ppm C. One of the recent new mines,, Diavik, Northwest Territories, Canada, extracts, kimberlite ore grading 3.9 ct/t in an open pit;, the recovered diamonds have an average value of, 96 $/ct. Letseng in Lesotho, the world’s highest, diamond mine (3200 m above sea level) operates, , 311, , with ore of 1 ct/100 t but an average value of nearly, US$ 1000/ct. Diamond placers of Gbenko, Guinea, have a grade of only 20 ppb; probably, this operation works the lowest- concentrated “ore” in the, world, which is only possible because the recovered diamonds are valuable gems >1 ct. Diamond, ore processing utilizes dense media separation,, cyclones and X-ray fluorescence sorting., 3.8.1 Source and formation of diamonds, Most diamonds formed in the Earth’s lithospheric, mantle, at high pressures and relatively moderate, temperatures. Diamonds are brought to the surface, by volcanic eruptions that originate from these, source regions defined by the “cool” 40 mW/m2, mantle geotherm of Pollack & Chapman (1977)., Such conditions are realized beneath continental, cratons with deep keels (“lithospheric roots”). A, small part of diamonds found in kimberlites and, alluvial deposits, however, has its source below the, lithosphere in the asthenosphere at depths of, 250–410 km, the transition zone (410–670 km), and even in deep mantle (>670 km; Tappert et al., 2009). Sublithospheric diamonds identified in, South Australia, southern Africa and South America are thought to be derived from deeply subducted, Permian crust related to a formerly active margin of, Gondwana (Tappert et al. 2009)., Carbon isotope data of diamonds display a wide, spread of d13 C from þ 4 to 41‰. This points, either to very different sources of the carbon,, including biogenic matter or to strong fractionation during diamond formation (Moore 2009)., Subducted oceanic and sedimentary source material is indicated by sulphur and Pb-isotopes, of sulphide inclusions in eclogitic diamonds, (Eldridge et al. 1991, Tappert et al. 2005). In one, case, even staurolite inclusions have been found, (Daniels et al. 1996), confirming a contribution of, crustal material, most probably by subduction., Comparative analyses of carbon and N-isotopes,, however, led to contrary views (Catigny et al., 1998, Haggerty 1999)., Lithospheric diamond forms in the upper mantle below continents, at pressures between, 43–65 kbar (4–6 GPa, 135–200 km depth) and at, mean temperatures of 1100–1200 C (Gurney et al.
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312, , PART II NON-METALLIC MINERALS AND ROCKS, , 2010, Mitchell 1986, 1991, Kennedy & Kennedy, 1976). The stability of diamond is favoured by, extremely reducing conditions and the presence, of CH4-H2O-H2 fluids or of carbonate melts (Frost, & Wood 1997). In Yakutian kimberlites, the paragenesis of diamond, chromite and Cr-rich garnet is, explained by the passage of CH4 fluids through, harzburgite that originally contained neither diamond nor garnet (Malkovets et al. 2007). Oxidation of methane, as exemplified by eq. 3.2, may, induce diamond formation., Diamond formation by oxidation of methane, reacting with Fe2O3 in chromite:, Fe2O3 þCH4 !Cdiamond þ2H2Oþ2FeOðin chromiteÞ, ð3:2Þ, Microdiamonds with a grain size of <0.4 mm or, a mass <0.001 ct have been found in many rocks,, including ultramafics, basalts, chromitite of ophiolites, ultrametamorphic gneiss (Davies et al. 1993),, meteorites and impact rocks. Some originated in, the lower mantle (>660 km: McCammon 2001), and in ultrahigh-pressure parts of subduction zones, (Griffin et al. 2002, Barron et al. 2005). Of course,, microdiamonds are economically insignificant., , 3.8.2 Diamond deposit types, Most valuable diamonds formed in lithospheric, mantle roots (Gurney et al. 2010). Once formed,, diamonds may be entrained as xenocrysts by mantle magmas that rise rapidly to the surface, where, they form volcanic edifices. If the ascent of the, magma is too slow, diamond will be graphitized or, resorbed. Primary diamond deposits occur commonly in subvolcanic pipes, sills and dykes of, alkalic ultramafic rocks (kimberlites and lamproites). Weathering of diamondiferous rocks liberates diamond, because the mineral is highly, resistant to supergene alteration. Enrichment and, formation of secondary diamond deposits takes, place either in autochthonous weathered material, (eluvial placers) or after erosion and transport, (alluvial and marine placers). Transported placer, diamonds are often more valuable gemstones than, those of primary deposits., , The first primary diamond deposits were found, in South Africa in 1870 (Wilson et al. 2007). Until, then, only secondary sources (placers) were, known. The newly discovered deposits were soon, recognized as “pipes”, vents and diatremes of, explosive volcanism. The diamondiferous rocks, filling pipes in the Kimberley district of South, Africa were called kimberlite., Kimberlites, Kimberlites are petrographically very complex and, always strongly altered K-rich ultramafic volcanic, rocks. Like many volcanic rocks, kimberlites display phenocrysts dispersed in a groundmass. The, groundmass is dominated by carbonate, serpentine/olivine, phlogopite and diopside; phenocrysts, include olivine, phlogopite, picroilmenite, chromium-rich pyrope, perovskite (CaTiO3) and accessory diamond (Mitchell 1986, 1991). Several, unaltered Devonian-Carboniferous kimberlites in, Siberia contain magmatic Na-K chlorides and NaK-Ca sulphates, which may be assimilated from, Cambrian evaporite host rocks, or derived from, the mantle (Maas et al. 2005). The rapid rise of, kimberlites is attributed to high CO2-contents in, the source region of the mantle and high pressures, during ascent (Wilson & Head 2007). Spera (1984), calculated that kimberlites rise with 10–30 m/s, through lithospheric mantle, accelerating to, trans-sonic speed in the crust. When the magma, reaches the surface, volcanic explosions lead to a, sharp pressure decrease in the vent and an implosion of the walls. Sudden freezing of the melt, initiates the fluidization stage, which produces, the final form of the pipe and its brecciated diatreme facies infill. Commonly, kimberlites occur, in volcanic fields that comprise hundreds of pipes, and dykes, with very few, typically the earliest of, successive generations, having economic diamond contents., Before erosion, the surface expression of kimberlite volcanoes must have included craters,, often maar-like, with a low rim of volcaniclastic, ejecta (Figure 3.11). For the first time, this facies of, volcaniclastic kimberlite was observed in the Fort, , a la Corne kimberlite field, Saskatchewan, Canada, (Leckie et al. 1997). At this locality, Early
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 313, , Figure 3.11 At Ekati Mine, Northwest, Territories, Canada, diamonds are recovered, from volcaniclastic lithic kimberlite tuff (here, seen in drill core laid out in the core box)., Courtesy W. Prochaska, MU Leoben., , Cretaceous sediments contain terrestrial diamondiferous lapilli tuff, which is overlain by olivine crystal tuff. Subsequent transgression caused, enrichment of diamonds in the surf zone. Since, 2002, the Ellendale mine in northern Australia, exploits diamonds from Miocene lamproitic pyroclastites. Remains of the crater facies are known, at Mwadui, Tanzania and in the Jwaneng and Orapa, pipes, Botswana. However, most kimberlite pipes, have been eroded to the level of the deeper diatreme, zone, which is characterized by steep and smooth, walls that converge inwards with depth and by a, circular cross-section. The deepest zone attained, by mining is the root zone, which contains a high, fraction of country rock breccias and has a very, irregular form. The root zone gives way to narrow, non-brecciated hypabyssal kimberlite dykes,, which are commonly too low-grade for mining., Some mines in South Africa, however, do exploit, dykes. The whole profile may have had an original, vertical extent of 2000 m. The deepest kimberlite, mines in South Africa reach a depth of 1000 m:, A paragon of diamondiferous kimberlites was the De, Beers Pipe near Bultfontein in the Kimberley district, (South Africa, Figure 3.12). Discovered in 1871, the, mine was closed in 2005 when reserves were exhausted. Total production was 5 tonnes (25 Mct), diamonds. Duringlateryears, mining wasunderground, with average ore grades of 20 ct/100 t. Individual kimberlite generations had different diamond concentrations: DB1 and DB2 contained only 6.6 and 3.5 ct/100 t,, respectively, whereas DB3 had 60.8 ct/100 t., Diamondiferous kimberlite (and lamproite) and the, diamonds always contain inclusions of mantle rocks., Peridotitic mantle inclusions include olivine, pyrox-, , ene and Cr-pyrope, e.g. in the Kimberley district with, additional magnesite, graphite and anatase at the, Finch pipe. Eclogitic inclusions are characterized by, pyrope-almandine and omphacitic pyroxene, and also, by kyanite, coesite and sulphides (e.g. Argyle, Australia). Peridotitic inclusions may have harzburgitic, or lherzolitic character. Extraordinary is the coincidence of all three types of inclusions in diamonds of, , Figure 3.12 De Beers Kimberley diamond pipe, South, Africa (modified from Clement et al. 1986). By kind, permission from Geological Society of South Africa., Hatched: Root zone breccias; DB1-4 are different, kimberlite generations.
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314, , PART II NON-METALLIC MINERALS AND ROCKS, , the Premier Mine, S.A., where the oldest (Archaean), diamonds contain harzburgitic, those of Palaeoproterozoic age lherzolitic and the youngest diamonds, that crystallized shortly before eruption of the kimberlites (1180 30 Ma) display eclogitic inclusions., It is hypothesized that the harzburgites are residues of, komatiite melting; the lherzolites may be related to, formation of basaltic Bushveld melts (Richardson, et al. 1993) and eclogitic inclusions are probably, derived from Kibaran subducted oceanic crust. Part, of the eclogitic diamonds stand out by light d13 C, very, low nitrogen contents, irregular form and absence of, colour (Type II diamonds). Moore (2009) presented, evidence that these diamonds crystallized in pegmatitic melts injected into the thermal aureole of kimberlite melt bodies. Type II diamonds associated with, pegmatitic megacrysts cause the outstanding value of, gemstones from Letseng (Botswana), Jagersfontein, and Premier mines (South Africa)., , Kimberlites are closely related to carbonatites, and other alkali magmas. Kimberlites are thought, to originate by a very small degree of melting of, metasomatized subcontinental lithospheric mantle (Mitchell 1986, 1991). Some kimberlites that, originate in the lower mantle are petrologically, related to mantle plume magmas (Tachibana et al., 2006) or to deeply subducted oceanic slabs (Tappert et al. 2009). The Mainpur kimberlite field in, India is synchronous with and genetically related, to the Deccan flood basalt event (Lehmann et al., 2010), one of the largest igneous outpourings on, Earth, which was emplaced 65 Ma ago. Others, may be sourced in an unusually enriched asthenospheric mantle (Paton et al. 2007). Typically, however, diamondiferous kimberlites occur within old, cratonic nuclei that experienced tensional tectonic deformation (“Clifford’s rule”, Clifford, 1966), for example in the early stages of the, break-up of supercontinents. This hypothesis originated in Africa where Cretaceous kimberlites, seem to be controlled by a system of faults and, lineaments that are related to the opening of the, Atlantic and Indian oceans., Lamproites, The role of K-Mg-rich lamproites as sources of, diamonds was only discovered in 1980, when the, important deposit at Argyle, Australia was found., , Phenocrysts and groundmass of these extrusive or, hypabyssal rocks consist of olivine, K-richterite (a, tremolite) and diopside, often with minor titanium-phlogopite and leucite or sanidine (Peccerillo 1992). Diamondiferous lamproites appear to be, saturated in SiO2, in contrast to kimberlites. Lamproite melting may be controlled by higher partial, pressure of H2O and fluorine, whereas kimberlite, melting is favoured by elevated pCO2., In both kimberlite and lamproite, diamonds are, xenocrysts or exotic fragments, which were entrained from the mantle source and lifted to the, surface. Ages of diamonds and their igneous host, rocks are typically very different; diamond formation in the lithospheric keels is episodic and can be, related to the geodynamic evolution of the host, craton (Gurney et al. 2010). In the Cretaceous (ca., 100 Ma) Orapa and Finch pipes, S.A., diamonds, display a formation age of 990 and 1580 Ma. Yet,, there is ample theoretical and experimental evidence that diamonds do crystallize from fluid-rich,, alkaline carbonate melts similar to kimberlites., Argyle in Northwest Australia is the largest primary, diamond deposit in the world. Mining started in 1985., The AK1 pipe is a volcanic crater filled by olivine, lamproite lapilli tuff covering an area of 1.6 by 0.6 km., At 1200 Ma, lamproite intruded Mesoproterozoic sedimentary siliciclastic rocks. Lamproite dykes cut, across the pyroclastic rocks, which contain the diamonds. The crater fill is the product of multiple, phreatomagmatic eruptions caused by contact of the, lamproite magma with groundwater (Boxer et al., 1989). The diamonds formed at 1.6 Ga, contain, eclogitic inclusions and have a d13 C of 9 to 12‰, PDB (Boxer & Jaques 1990). In 2001, reserves at Argyle, were 54 Mt at a grade of 3.0 ct/t and resources 166 Mt, at 2.5–3.0 ct/t. The open pit operation was terminated, in 2007 and extraction moved underground. Gem, quality diamonds >2 ct are only 1% of production,, but famous for their rare colours (champagne, cognac, and pink stones: Figure/Plate 3.13). Apart from, Argyle, more than 100 other lamproites and kimberlites have been found in Western Australia, including, Ellendale mentioned above. With an age of only, 20 Ma (Early Miocene), this swarm is the youngest, of this rare igneous family in the world., Recently, in the Bunder district of Madhya Pradesh,, India, eight lamproite pipes were discovered. Two
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 315, , Figure 3.13 (Plate 3.13), Champagne-coloured octahedral, diamond crystals from Argyle,, Australia. Note the macroscopic, (eclogitic) inclusions. Courtesy, Rio Tinto Argyle Diamonds., coalesced pipes at Atri display a surface extension of, 17 ha and host inferred resources of 37 Mt at a grade of, 0.7 ct/t, which are amenable to open-pit mining and, conventional diamond recovery., , Diamond placers, Diamond placers owe their origin to the high, density and remarkable mechanical and chemical, resilience of the mineral. This allows for multiple, erosion/sedimentation cycles explaining placer, deposits without known primary sources and, especially, the discovery of spectacular single, stones. Geologically very early fossil placers, include the Palaeoproterozoic conglomerates near, Vila Nova in Brazil. Geologically young and recent, placers occur in higher river terraces and present, river beds. An excellent example for the connection between primary and secondary deposits are, the diamond placers along Bow River in Western, Australia, which occur downstream of the Argyle, AK1 pipe. Based on large reserves with an average, grade of 0.3 ct/t, the annual production was (in, 1991) 960,000 ct. Highest diamond grades, occurred in the oldest, highest level terraces. Gem, quality stones contributed 20% of production,, confirming the experience that transport results in, , higher-quality diamonds. Most alluvial diamonds, have values of the order of US$ 450–500/ct, in, contrast to diamonds from kimberlites pipes and, dykes that commonly fall into the range of US$, 30–150/ct. A famous example of rich alluvial, placers occurs along Orange River in Southwest, Africa. The diamonds were eroded in post-Cretaceous time from kimberlites in central South, Africa. From the Tertiary to the Holocene, the, diamondiferous gravels were reworked due to, a complex evolution of the river system (De Wit, 1999). Note that in 1866, South Africa’s first, diamond (Eureka with 21.5 ct) was found on the, banks of Orange River., Marine (coastal) and submarine placers of diamond,, noted as a source of gems averaging over US$ 350/ct,, are exploited in southwestern Africa (mainly Namibia). Near Gibeon in the upper reaches of the area, draining to the coast, many kimberlite pipes are, known, but none is diamondiferous. Due to the wide, Quaternary sea level variations, both submarine (lowstand, formed during glacial times) and onshore, deposits (high-stand, formed during interglacial, periods) are present, sourced from the Orange and, Olifants rivers. From the Orange River delta, diamonds were transported northwards along the coast, to a distance >150 km. Diamond contents and stone
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316, , PART II NON-METALLIC MINERALS AND ROCKS, , size in the beach complex are closely correlated to, specific sedimentary settings (Spaggiari et al. 2006)., Buried bedrock gullies of the Sperrgebiet host, famously rich gem diamond concentrations (Jacob, et al. 2006)., , Diamond exploration, Diamond exploration uses a combination of geological, geochemical and geophysical methods, (Michel 1996). Clifford’s rule, that most likely old, cratons have diamondiferous lithospheric roots,, still applies with few exceptions (e.g. Argyle), and, crustal tectonics control kimberlite fields (Gurney, et al. 2010). Regional-scale exploration always, includes the systematic collection of heavy mineral samples from regolith or drainage systems., “Indicator minerals” for kimberlites and lamproites include high-Mg ilmenite (picroilmenite, with >8 wt. % MgO and >0.5 wt. % Cr2O3), chromian diopside, chromian spinel or chromite,, Cr-rich pyrope, low-U zircon, phlogopite and, of, course, microdiamonds (Figure 3.14; Muggeridge, 1995). Infrared reflectance spectrometers are routinely used to scan samples, such as drill core, in, order to detect favourable indicator minerals., , Argyle was found by the recovery of diamonds, 20 km downstream from the primary deposit during routine drainage sampling. Major and trace, element compositions of indicator minerals can, be valuable guides to diamondiferous kimberlites, and lamproites (Griffin & Ryan 1995). In humid, climates, soft weathered kimberlites of subcropping pipes form maar-like depressions that can be, detected in satellite images. Other methods, include geochemical soil sampling in order to, spot anomalies of Ni, Cr and Co, supplemented, by airborne and ground geophysical surveys, searching for magnetic, electromagnetic and, gravimetric anomalies. At Argyle, however,, a range of airborne and ground geophysical methods failed to produce a definitive response over, the pipe (Dentith et al. 1994). Once kimberlites or, lamproites have been found, their diamond potential must be evaluated (Rombouts 2003). Routinely, very large samples (several hundred, tonnes) are processed in order to recover sufficient, diamonds for a preliminary evaluation. One, tested sampling method is deep, large-diameter, (584 mm) drilling. Similarly, the evaluation of, diamond placers requires processing of large, , Figure 3.14 (Plate 3.14), Heavy minerals, concentrate from glacial till, in the region around Ekati,, N. T., Canada, where more, than 150 kimberlite pipes, have been found. Courtesy, W. Prochaska, MU Leoben., Indicator minerals on, display include dark green, Cr-diopside, pale or, transparent yellow eclogitic, garnet (?), light grey, picroilmenite, dark grey, chromite, dark red Crpyrope, bright green olivine, and one octahedral, diamond (centre, diameter, 1.8 mm).
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , samples. Meticulous investigation of all aspects, of the deposit is required for rational exploration, and extraction. Understanding distribution and, value of the diamonds is crucial. For valuation, of diamonds found, a minimum sample should, comprise at least 2000 ct of macrodiamonds., Microdiamonds cannot be evaluated., After the unexpected discovery of the giant diamond deposit Argyle in Northwestern Australia, (1983) in a geological unit that was thought to be, unprospective, a worldwide exploration rush revealed many new diamond fields. Examples are, Northwestern Canada (Ekati and Diavik mines,, Northwest Territories), Finland and USA (Kelsey, Lake, Colorado). Yet, among more than 10,000, kimberlites known in the world, less than 100, mines have been established and only 15 are really, significant., Mine production of natural diamonds in 2008, was 72 Mct (14.5 tonnes) and little less in 2009, (USGS 2010). Ranked according to weight, the, largest producers were Congo (DRC), Australia,, Russia, South Africa and Botswana. About twothirds of the total is from primary deposits, onethird from placers. Only 20% of the production is, of genuine gem quality. Because of this divergence, ranking according to value puts Botswana, first, followed by Russia, Angola and Canada. The, rapidly growing demand for industrial diamonds, cannot be satisfied by mining natural diamonds., China alone produces 4000 Mct/yr (or 800, tonnes) of synthetic diamonds that can be, designed to specifications of the customer (e.g., grain size from crystals and grit to powder, homogeneity, structure). Most synthetic diamonds are, made from graphite by the High Pressure/High, Temperature (HPHT) technology at T >1400 C, and P >59 kbar in a metallic melt. Heat-resistant, doped semiconductors are produced by the Chemical Vapour Deposition (CVD) technology. Even, the production of synthetic gem diamonds is possible, at a fraction of the price of natural diamonds., The consequences for diamond mining are not, clear. One strategy against the competition by, man-made diamond gems is certification, (“branding”) of natural stones. Unregulated artisanal mining of diamonds is a bane for certain, regions, as it leads to extreme exploitation of, , 317, , people and brutal civil wars, especially in tropical, Africa. Solutions are being sought by industry,, governments and NGOs (e.g. the Kimberley, Process Certification Scheme 2003)., , 3.9 DIATOMITE AND TRIPOLI, Diatomite, (alternative, names, include, diatomaceous earth and kieselguhr) consists of, microscopic skeletons (frustules, 50–100 mm) of, unicellular algae of the phylum Bacillariophyta., Structurally in two halves (valves) – hence their, name – their cell walls are made up of amorphous, opaline silica (SiO2nH2O, density 2.0–2.25 g/, cm3), which diatoms polymerize from dissolved, silica (Figure 3.15). In ocean water, the common, form of silica is orthosilicic acid H4SiO4. The, estimated 100,000 extant species of diatoms, take many forms, as do ancient ones, and this may, influence specific uses. In lakes and oceans,, diatoms are part of the plankton that feeds fish, and other animals. Because of their huge total, mass they are one of the main sources of atmospheric oxygen. It is estimated that 40% of all, organic carbon fixation on our planet (photosynthetic transformation of carbon dioxide and water, into sugars, using light energy: eq. 6.1) is carried, out by diatoms, about equal to all of the world’s, tropical rainforests. A steady rain of diatoms sinks, , Figure 3.15 Electron microscope image of the skeleton, of diatom Nitzschia lancettula f. minor which is, characteristic for Pleistocene diatomite at Adami Tulu, Sida (Ethiopia), illustrating the delicate sieve-like, structure that is the base for many applications of, diatomite. Length of image 15 mm. Courtesy Juliane, Fenner, BGR Hannover.
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318, , PART II NON-METALLIC MINERALS AND ROCKS, , to the floor of water bodies. Under favourable, conditions, diatoms “bloom” and this results in, useful concentrations if other sedimentary input, remains low. Diatoms only became widespread in, the Cretaceous and exploitable deposits are, mainly of Tertiary and Quaternary age., As a sedimentary rock, exploitable diatomite, often includes a minor component of organic substances, quartz, pyrite/marcasite, clay, calcite and, volcanic ash. SiO2 contents are most often >80%,, the best grades have >90% (Lompoc, California)., Of course, gradation into diatomaceous clay or silt, is common. In outcrops, the material resembles, friable chalk with a white to greenish colour. High, organic matter and sulphide contents give it a dark, grey or brown colour. Raw diatomite can hold, 60% water. Dry, loose diatomite has a very low, density of 0.12–0.25 g/cm3., Diatomite is usually extracted in open pits and, crushed in spiked rolls and hammer mills. The, material is then dried and ground at 100 C., Because diatoms are very delicate, enrichment is, preferably carried out in cyclones and air separators, not in aqueous slurry. For some applications, (e.g. polishing), the concentrate is calcined at, 800–1000 C or flux-calcined at 1000–1200 C,, resulting in stronger silica particles. In addition,, this procedure burns organic matter and gives the, product a white or pink colour. Commercial products are divided into grain size classes., Useful properties of diatomite include the high, silica content, low density and high porosity (Kogel et al. 2006). The advantage of silica is its, chemical inertness. Typical uses include heat and, sound insulation, filtering of liquids (e.g. clarification of swimming pool water and beer), filtering of, microbial contaminants such as bacteria, protozoa, and viruses in public drinking water systems, as a, filler in rubber, plastics, paper, paints and bitumen, as an ingredient in cement and as a carrier, powder in nitrogen fertilizer and insecticides., Small amounts find applications in cosmetics and, polishing wares. Similar to sepiolite, much diatomite is used as cat litter and industrial spillage, absorbent., Amorphous opaline diatomite does not cause, silicosis. However, the World Health Organization cautions that diatomaceous earth with a crys-, , talline silica content over 3% should not be, ingested by humans or animals. Crystalline silica, content may be elevated in heat-treated products., 3.9.1 Diatomite deposit types, Diatomite deposits result from localized profuse, growth of diatoms in a geologically short time., This is favoured by a high and continuous availability of dissolved silica, in addition to essential, nutrients (e.g. P, K, N). Geothermal springs in, volcanic settings (Iceland) and settling of glass, tuff into lakes and coastal lagoons provide, soluble silica (Denmark; California). As a consequence, most diatomite deposits occur in volcanic, districts., Freshwater diatomite, Present diatomite formation can be studied in, Iceland. Diatomite is recovered from the shallow, bottom of Myvatn Lake near subaqueous discharge sites of geothermal springs. The French, Massif Central hosts Europe’s largest diatomite, deposits, related to a chain of basaltic-trachytic, volcanoes whose activity lasted from the Late, Miocene to Early Holocene; the Murat deposit, actually is a former maar lake filled with diatomite. In Africa, economically important diatomites occur near volcanic centres along the East, African Rift. In the Neogene and Quaternary, diatomite formed during times of wet climate in huge, lakes in and near the Rift Valley and the Afar, depression of Ethiopia. High-grade diatomite, occurs in Pleistocene Lake Galla sediments at the, foot of Aluto volcano., Glacial/interglacial diatomite, In the L€, uneburg area of northern Germany, diatomite formed during Quaternary interglacial warm, periods, in elongate, narrow lake basins excavated, by the inland ice during glacial phases. Deposits, reach a thickness of tens of metres and several, kilometres length. The material is finely, banded and in deeper parts olive-green to blackcoloured by organic substance and pyrite. Near the, surface, white and grey colours prevail, because of
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , oxidation and leaching above the groundwater, table. Remaining resources comprise >12 Mt, exploitable diatomite, but because of environmental considerations, extraction was stopped in 1994., Considering the absence of proximal volcanism,, elevated silica availability is explained by rapid, interglacial weathering of rock flour exposed after, retreat of the glacial ice shield., Marine diatomite, The Eocene moler earth of northern Jutland, Denmark, is formed by interbedded marine strata of, light diatomite and argillized bentonitic ash tuff., The deposit reaches a thickness of 60 m. Shear, deformation underneath the southward-moving, Quaternary ice sheets left strata folded and disrupted. High clay and iron contents impede use as, a filtering material, but dried, calcined and sintered products find a ready market, which includes, insulation bricks, absorbents (cat litter) and coating material (e.g. for fertilizer). California has large, diatomite deposits of marine and freshwater origin. South of Lompoc, diatomite deposits occur in, Neogene marine sediments. The higher part of the, Middle and Upper Miocene Monterey Formation, comprises hard opaline cherty shale and chalcedonic chert, with intercalated pure diatomite., Diatomite is also hosted by the overlying Sisquoc, Formation in 25 beds between clay layers., World production of diatomite in 2009 was 2.2, Mt. Major producers are USA, China, Denmark, and Japan (USGS 2010)., 3.9.2 Tripoli, Tripoli is similar to diatomite but of different, origin. Like diatomite, it is used as a fine, mild, abrasive and as a filler and extender in elastomeres, paints and plastics. Originally, the term, was coined for diatomaceous material traded at, the port of Tripolis (Libya). Today, “tripoli” designates residual earthy, very fine-grained, nondiatomaceous silica accumulations, which result, from weathering of chert, flint and siliceous limestone (Neuendorf et al. 2005). Constituent silica, phases depend on the source material and may, vary from amorphous opal to low-temperature, , 319, , quartz (often chalcedonic). Typical grain sizes, range from 1–10 mm, but include particles as small, as 0.1 mm. Commercial tripoli grades 98–99% silica, with traces of alumina and iron oxide. White, colour is characteristic but yellow, brown and red, material is also extracted. Tripoli is classified as, carcinogenic and may cause silicosis. This must, be taken into account when the material is, handled. The volume of world mine production, is not recorded; its magnitude is probably several, 100,000 tonnes per year., , 3.10 FELDSPAR, , Orthoclase,, microcline, Albite, Anorthite, , KAlSi3O8, NaAlSi3O8, CaAl2Si2O8, , D ¼ 2.5–2.6 g/cm3, 2.6, 2.8, , Feldspar is a main component of most igneous, rocks and of many metamorphic and sedimentary, rocks. It is one of the most common minerals., However, only the two first listed alkali feldspars, or members of their solid solution series (K-Na, feldspar, perthite) are of major industrial significance. These desired feldspars occur mainly in, felsic and alkaline magmatic rocks. Feldspar of, most intermediate and mafic rocks is rich in calcium (plagioclase, a solid solution series of albite, and anorthite). Plagioclase lacks important properties and is, therefore, rarely of economic interest., Potassium, or K-feldspar has a theoretical composition of 64.8% SiO2, 18.3% Al2O3 and 16.9% K2O, but, always contains some sodium, traces of calcium and, tiny flitters of haematite that give the mineral its pink, to red colour. Pure albite is composed of 68.8% SiO2,, 19.4% Al2O3 and 11.8% Na2O. Minor kaolinization, of feldspar is acceptable., , Use of feldspar is mainly (ca. 85%) in the, production of porcelain and technical or glazed, ceramics (whiteware), and glass. In ceramics and, porcelain, the role of feldspar is that of a fluxing, agent for lowering the melting temperature of, a ceramic body during firing and formation
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320, , PART II NON-METALLIC MINERALS AND ROCKS, , of a glassy phase. As a component of glass melts,, feldspar provides alumina for improved hardness,, thermal endurance, durability and resistance to, chemical corrosion. Increasingly, feldspar is, employed as a filler in plastics, paints and rubber., The intended use controls the precise specification for processed sales products: Main variables, are the contents of alkalis, alumina, free silica and, of iron. Iron is generally unfavourable because it, induces undesired colours. Alumina content of, glass is also lifted by adding kaolin, bauxite or, kyanite, but either higher costs or iron contents, may be a handicap. As a standard flux, the glass, industry uses soda ash so that alkali contents of, feldspar are less critical., Earlier, feldspar was extracted from coarsely, crystalline pegmatites and manual sorting delivered a pure product. Today, processing of run-ofmine ore to concentrate includes methods such as, comminution, flotation, and electrostatic and, high-intensity magnetic separation. Pegmatites, continue to be significant primary sources, but, other magmatic rocks gain significance. This is,, of course, due to the economy of scale resulting, from exploitation of large homogeneous rock bodies. Examples include nepheline syenite (Brazil,, Canada, Norway, Russia and Turkey), aplite (Japan,, Elba), alaskite (USA), albitite (Sardinia, Vysoki, Kamen, CZ), phonolite (Eifel, Germany), rhyolite, (Saar-Nahe, Germany) and Cornish stone (partly, altered sandy granite in Cornwall). In Norway,, white anorthosite is quarried for the production of, mineral wool, fillers and extenders; its use in glass, and ceramics production is limited. The world’s, largest feldspar pegmatite is reportedly Pippingarra, near Port Hedland, Western Australia, with a, length of 1.5 km and a width reaching 200 m., Prospective deposits must have a suitable size,, and reasonably constant processing characteristics and product composition (compare Greiling, et al. 2005). Low iron contents are always stipulated. Remember in this context that many, pegmatites are paragons of inhomogeneity, so that, compliance with these conditions may be very, difficult. Product specifications are best reconciled with potential users. Co-production of quartz, and mica should always be considered. World, mine production of feldspar in 2009 was 19 Mt, , (after 22 Mt in 2008). Largest producers are Turkey, Italy and China. Reserves and resources are, extensive., , 3.11 FLUORITE, , Fluorite, (Fluorspar), , CaF2, , Max. wt. % F, , Density, (g/cm3), , 48.9, , 3.18, , Numerous minerals contain fluorine but only, fluorite and fluorapatite are industrial sources of, the element. Fluorite regularly contains traces of, rare earth elements substituting for Ca (particularly Y and Ce), which activate the namesake, fluorescence (and thermoluminescence) of the, mineral. They also cause many colour variations, of fluorite, but other elements and hydrocarbon, inclusions are also involved. Different colour, bands always differ in trace element geochemistry. Frequent cations in fluorite include Sr and Y,, whereas Al, Ba, Cd, Mg, Mn, Na, K and U þ Th are, less common. Minor and trace element variations, mark certain fluorite generations in any one, deposit, and even different deposit types in a fluorine province. This may assists exploration. The, deeply purple colour of some fluorites is attributed, to uranium content, or more precisely, to radiation, damage affecting the crystal lattice. Fluorite ages, can be determined by the Sm-Nd isotope isochron, method (Munoz et al. 2005). Frequent U þ Th, content of several tens of micrograms per gram, allows application of the (U-Th)/4 He dating, method (Pi et al. 2005). Common gangue minerals, of fluorite ore include quartz, carbonate, barite,, galena and sphalerite. Beware that sellaite MgF2 is, easily mistaken for fluorite but less valued. Fluorite resists supergene alteration and tends to form, visible outcrops. It may even be enriched by dissolution of carbonate gangue and host rocks., Industrial fluorine requirements are mainly satisfied by fluorite. Until closure in 1987, natural, cryolite Na3AlF6 was extracted from a singular, pegmatite deposit near Ivigtut in western
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , Greenland. Meanwhile, synthetic cryolite is made, from fluorite for a flux in electrolytic melting of, aluminium. An increasing source of fluorine is, fluorapatite Ca5[F(PO4)3] processing in the phosphorus fertilizer industry (cf. “Phosphates”),, which results in considerable quantities of hexafluoride silicic acid H2SiF6., Large fluorite deposits with resources of several, million tonnes are exploited at grades of 15 to 20%, CaF2, smaller ones require 30 to >40%., Fluorite’s main use is as a flux in numerous, metallurgical processes and this is the reason for, its name (fluere in Latin means “to flow”). Large, quantities are consumed in the iron, steel and, cement industries. Fluorite concentrate is, accepted as “metallurgical grade” if it contains, >60% CaF2 at good granularity and low contents, of SiO2, S and Pb. The chemical industry requires, >97% CaF2 and very low sulphide and phosphorus, concentrations. This “acid grade” fluorspar is used, for the production of fluoric acid by dissolution in, H2SO4 yielding anhydrite as a by-product. Fluoric, acid HF is the base for many industrial processes,, for example cryolite synthesis, the production of, propellant gas and of perfluorocarboxylates that, are used in large quantities for consumer products, such as Teflon. Gaseous UF6 is the preferred, means for enrichment of 235 U for energy and weapons. Metallurgy and chemical industry consume, most fluorite in about equal shares. Extremely, pure and transparent fluorite (and sellaite) is used, for lenses and prisms, but most optical material is, made synthetically., For humans, fluoride is an essential element, but, within narrow limits. Both too low and too high, intake cause visible damage to teeth and other health, problems (Edmunds & Smedley 2005). Drinking, water should contain from 0.7 to 1.2 mg/l F (Dissanayake 2005), but not more than 2 mg/l. In order to, protect the population, the dissemination of fluoride, into the environment must be strictly controlled. In, industry, recycling or safe disposal is the rule. A, contraction of demand for acid-grade fluorite was, caused by recycling combined with the ban on halone, and CFCs (chlorofluorocarbons) by the Montreal Protocol 1987, because these substances deplete the, stratospheric ozone layer. Production of HCFCs (hydrochlorofluorocarbons) in developing nations will, , 321, , stop in 2013. Newly developed HFCs (hydrofluorocarbons) that are benign to stratospheric ozone, support fluorite mining. Although HFCs are strong, greenhouse gases they may be acceptable, because, at low concentrations they cause very little, radiative forcing in the atmosphere (Shine & Sturges, 2007)., , 3.11.1 Geochemistry, Lithophile fluorine (Goldschmidt 1958) is remarkably enriched in mantle volatiles, as evidenced by, fluorine contents in phlogopite of kimberlites, reaching 8500 ppm. Mantle-derived carbonatites, and igneous alkali rocks also contain much fluorine, commonly in the form of fluorapatite segregations, which may be large enough to support, important mines (e.g. Khibiny, cf. “Phosphate”)., With an estimated crustal abundance of 500 ppm, (range 270–800: Smith & Huyck 1999) fluorine is, the most common trace element. Fluorine is a, component of all magmas. Its concentration rises, from mafic (400 ppm) to granitic rocks (735 ppm),, although actual contents depend on the individual, degassing history of a magmatic body. Metapelites, contain an average of 500 ppm F, carbonates, 330 ppm and seawater only 1.3 ppm. In common, rock-forming minerals, fluorine (F) substitutes, for OH. Therefore, mica, amphibole, apatite,, clay and other hydrous minerals are carriers of, fluorine traces. Accordingly in most rocks,, fluorine is freely available. This explains why, fluorite is a common gangue mineral in hydrothermal ore deposits and why fluorite deposits are so, widespread., Volcanic vapours contain fluorine as an HFo, phase, causing strong alteration of rocks. Cooling, magma bodies may liberate giant amounts of fluorine. Fumaroles in the Valley of the Ten Thousand, Smokes, Alaska are estimated to eject some, 200,000 t/y fluorine into the atmosphere. Carriers, of fluorine in hydrothermal solutions include F,, þ, 2, HFo, HF, and BF, 2 , SiF6 , CaF, 4 . Solutions are, mainly acidic and precipitation of fluorite is, induced by contact with limestone or dissolved, calcium, raising pH. In the surficial environment,, fluorine is equally soluble and mobile, and is, precipitated by earth alkalis.
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322, , PART II NON-METALLIC MINERALS AND ROCKS, , Many fluorite occurrences and deposits occur in, certain stratigraphical and lithofacies units that, are marked by regional geochemical fluorine, anomalies. Examples include Carboniferous sandstone in the central Russian Platform, Zechstein, (Late Permian) saline dolomite in Germany (Ca2, und Ca3: cf. Chapter 4 “Salt Formation in the, Geological Past”), Triassic sediments of the, Morvan near the French Massif Central and in the, Eastern Alps, and Cretaceous limestone in, Mexico. This observation can be explained by, either synsedimentary exhalation and dispersion, of fluorine or by pervasive epigenetic hydrothermal processes., , a Tertiary rhyolite breccia. In South Africa, magmatic, degassing of the Bushveld granites (cf. Chapter 1.1, “Orthomagmatic Ore Formation”) produced the, fluorspar deposits of Vergenoeg and Buffalo mines., Host rocks are contact metamorphic sediments of the, Transvaal Group and rhyolitic Rooiberg felsites, (“leptites”) in the roof of the granitic intrusions., Orebodies consist of vein systems, which are injected, either into bedding planes of quartzites (Buffalo), or, into cross-cutting structures within a pipe (Vergenoeg). Some veins carry only fluorite, others display, a gangue of apatite, siderite, quartz, chlorite and, sphalerite. High trace contents of uranium and rare, earth elements are characteristic. Resources of the, Buffalo deposit are estimated at >50 Mt, those of, Vergenoeg at 100 Mt., , 3.11.2 Fluorite deposit types, , With 12 Mt of ore at 30% CaF2, Amba Dongar, Gujarat, India is one of the largest carbonatite-associated, fluorite deposits. The intrusion is located within, the giant late Cretaceous Indian flood basalt province, (Deccan traps), with which it is genetically related., Numerous veins of fluorite with barite, chalcopyrite,, dickite, galena and pyrite occur at the contact of carbonatite with fenitized sandstone. The mineralization, originated by mixing of magmatic fluorine-rich fluids, with calcium-rich formation water in the sedimentary, country rocks (Palmer & Williams-Jones 1996)., , The formation of fluorite deposits is mainly, a consequence of magmatic and hydrothermaldiagenetic systems. Magmatic-hydrothermal, deposits are connected with carbonatites, alkaline magmatic rocks, granites and rhyolites., Economic fluorite concentrations are commonly located in the roof of intrusions, where, magmatic volatiles reacted with host rocks., However, the majority of fluorite deposits have, no connection with magmatic process systems., They include fluorite-rich members of the, Mississippi Valley Type class, epigenetic hydrothermal veins, stockworks, pipes and metasomatic masses, and stratiform fluorite hosted in, sediments. Diagenetic processes and basinal, fluids are often implicated, but there is no single, genetic explanation for non-magmatic epigenetic fluorite deposits. In some cases, degassing, of the mantle may have transferred fluorine into, crustal hydrothermal systems (e.g. in Europe, during the Triassic break-up of Pangaea)., Magmatic-hydrothermal fluorspar deposits, These deposits are exploited in Mexico, South, Africa, Canada and China:, The volcanogenic deposit Las Cuevas near Zaragoza, in Mexico is the world’s largest fluorite mine, with, reserves of 28–30 Mt at 84.5% CaF2. Fluorite with, some calcite and silica fills hydrothermal karst cavities in Early Cretaceous limestone in contact with, , Diagenetic-hydrothermal fluorite deposits, These deposits are very common. In the English, Peak District, the famous ornamental fluorite, (“Blue John”, Figure/Plate 3.16) was introduced, by diagenetic fluids during a late stage of the main, Variscan deformation. The Illinois-Kentucky district within the type area of Mississippi Valley, type (MVT) deposits was the source of most fluorite produced in the USA. The spatial coincidence, does not, however, support a priori an origin comparable to MVT lead-zinc deposits. Geological, position and geochemical modelling (Plumlee, et al. 1995) make it very likely that these deposits, originated in connection with HF-CO2 degassing, of Permian alkali complexes, although these are, poorly exposed. The authors imply that rising, magmatic gas dissolved in basinal brines, which, migrated along faults and formed the large fluorite, veins, mantos (in limestone) and breccia orebodies, of the cryptovolcanic Hicks Dome.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 323, , Figure 3.16 (Plate 3.16), Banded dark blue, purple, and yellow hydrothermal, fluorite (“Blue John”), filling karst pipes and, replacement bodies in, Carboniferous limestone,, Derbyshire, England., Light crystals in the, central cavity are calcite., Courtesy Volker L€, uders,, GFZ Potsdam., , A remarkable fluorite province occurs around Witkop, mine near the town of Marico in western Transvaal,, South Africa. Orebodies are located below an unconformity of the Palaeoproterozoic Malmani Dolomite, (of the Transvaal Supergroup), where it is covered by, shales, chert with banded iron formations, breccias, and conglomerates of the Pretoria Group. Below the, unconformity plane, the dolomites are severely karstified, which proves emersion to subaerial levels., Stratiform fluorite impregnates stromatolitic dolomite of the karst zone (“algal ore”), and cements, collapse breccias of black dolomite (“block spar”)., Gangue includes quartz, calcite, dolomite, pyrite,, talc, tremolite and sphalerite. The ore contains only, 16% fluorite but open pit mining allows economic, extraction. Potential resources of the district are very, large; at a cut-off grade of 10% CaF2 they amount to, 1000 Mt. Ore formation is bracketed between deposition of the Pretoria Group (because unmineralized, Malmani dolomite forms transgressive breccias) and, intrusion of the Bushveld Complex (because the ore, paragenesis is clearly contact-metamorphosed as, demonstrated by the presence of talc, tremolite and, graphite formed from petroleum droplets). Many of, these observations suggest a genetic affinity to MVT, deposits. Basinal brines are thought to have mobilized, trace fluorine from the sediments. Precipitation, occurred below the impermeable barrier at the unconformity. A connection with a phase of large-scale, fluid mobilization at 2.0 Ga is assumed, triggered, , by the Kheis orogeny along the western margin of the, Kapvaal craton (Duane et al. 1991)., , Hydrothermal veins and hydrothermalmetasomatic deposits, These deposits may be produced by several geological process systems. Magmatic and diagenetic, origins were mentioned above. Tensional tectonics, including continental rifting (e.g. Kenya) and, distension of cratons by a network of smaller, faults and shears (e.g. Late Variscan Europe), are, further activators of fluorine metallogeny. It is, possible that in some of these cases rising mantle, volatiles, degassing of deep magma bodies and, leaching of crustal rocks play a similar role, as, recently proposed for the Rio Grande Rift (Partey, et al. 2009)., The fluorite orebodies of the Kerio Valley in Kenya, occur on the western border fault of the East African, Rift. Proterozoic calcite marbles, gneiss and quartzite, are exposed at the foot of the escarpment, which rises, 1300 m to the highlands above. Near the fault, the, marbles host metasomatic fluorite masses that are, controlled by cross-cutting structures. The gangue, includes chalcedony, adularia, pyrite and chlorite,, indicating epithermal conditions. Kaolinization of
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324, , PART II NON-METALLIC MINERALS AND ROCKS, , feldspar in country rocks is a prominent hydrothermal alteration. This sector of the rift displays Miocene and Recent basaltic to Na-rhyolitic volcanoes,, fumaroles and hot springs that discharge much, fluorine. The precise provenance of the fluorine, remains open, however, because degassing magmas, and older volcano-sedimentary rocks in the rift are, equally possible sources. Note that the alkaline, playa lakes Magadi, Nakuru und Elmenteita in the, Rift Valley of southwest Kenya all have fluorine, concentrations between 1100 and 1700 lg/L. Nevertheless, these lakes provide a favourable habitat for, millions of pink flamingos (Phoenicopterus minor)., Europe north of the Alps experienced several phases, of “reactivation” following its consolidation by the, Variscan orogeny, caused by large-scale tectonic, processes such as the rifting and opening of the, Tethys, Atlantic and Penninic oceans in the Jurassic, and a Tertiary phase of renewed rifting. During, these periods, hydrothermal fluids were repeatedly, generated and invaded both the metamorphic basement and the sedimentary cover. A number of, formerly important epigenetic fluorite deposits resulted from these events, including the early Jurassic (Hettangian) fluorite (-barite) veins in the French, Massif Central (Morvan) near Chaillac (Sizaret, et al. 2004; cf. “Barite”, Figure 3.6 and Figure 3.17)., The Rossignol vein was filled in two stages: The, , Exploration for fluorite is mainly based on geological and geochemical methods. Rock, soil and, water analyses with the ion-selective fluorine, probe are useful guides to prospective anomalies., In contrast to the brightly purple or green fluorite, samples commonly seen, much ore-grade fluorite, is not easily recognized. Because of its mobility,, fluorine is also a useful pathfinder element whenever it forms part of the paragenesis, for example in, many Pb-Zn-Ba deposits in carbonate rocks, in, REE-carbonatites and in certain tin granites., In 2009, 5 Mt fluorspar were produced worldwide after 6 Mt in 2008. With 60% of the total,, China is the largest producer, followed by Mexico,, Mongolia, South Africa and Russia. Identified resources amount to 500 Mt of CaF2 contained, and, global resources of fluorine in phosphate rock are, estimated at 1300 Mt fluorite equivalent (USGS, 2010)., , E, , W, Sulphate-rich, continental water, , first is remarkable by its open space sedimentation, at low velocities of the ascending fluid. The second, stage produced a fluorite-barite breccia and the, barite sinter deposit on the surface. Fluids had low, temperature (110 C) and high salinity (19–22%, NaCl equiv.). Total production by underground, mining amounted to 800,000 t at 50–60 wt. % CaF2., , Yellow fluorite-barite, , Sinter, deposit, , Vein, Barite, Metamorphic, basement, , Yellow fluorite, , Green and, purple fluorite, Basement, fragments, , Figure 3.17 Schematic profile, showing the evolution of, Rossignol fluorite (barite) vein and, its surficial barite sinter deposit, Les Redoutieres near Chaillac,, France (not to scale). After Sizaret,, S., Marcoux, E., Jebrak, M. &, Touray, J.C. 2004, Society of, Economic Geologists, Inc.,, Economic Geology Vol. 99,, Figure 13B, p. 1120.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 3.12 GRAPHITE, Natural graphite (C, Density 2.1 to 2.3 g/cm3,, calculated 2.27) often contains traces of hydrocarbons and of nitrogen. Its melting point is nominally at 3652–3697 C, but graphite sublimes, instead of liquefying. The degree of graphitization, is commonly characterized by X-ray diffraction, and Raman spectroscopy (Lespade et al. 1982)., Related to the grade of structural ordering, several, varieties of graphite are distinguished. Optically,, graphite is defined by reflectance Rmax >9%, semigraphite by Rmax from 6.5–9% (Kwiencinska &, Petersen 2004; cf. Chapter 6 “Coal”). Graphite is, considered to be a “one-mineral” geothermometer, (Luque et al. 1998), because its differential thermal, analysis (DTA) maximum is thought to correlate, with formation temperature. Natural graphite is, intergrown with minerals such as sulphides and, silicates. In practice, determination of carbon content is routinely done by combustion and the mass, of residuum is termed “ash”., Graphite is one of the softest solids known (and, diamond, the other natural crystalline modification of carbon, one of the hardest). The most, important properties of graphite are softness and, low friction, because of weak bonds between lattice layers; it displays high conductiviy of heat and, electricity, resistance to acids and to temperatures, reaching 3000 C in the absence of oxygen (in air,, graphite oxidizes at 400–500 C). Grain size and, carbon contents of graphite concentrate are, main controls of its possible use. Highly valued, flake graphite, with platelet diameters from >1, to 0.018 mm, is distinguished from the finegrained and less valuable microcrystalline, or, “amorphous” graphite. Massive crystalline lumpy, graphite (“Ceylon lumps”) is preferred to graphite, dust (<100 mm). Although for some uses low, carbon contents (say >40%) are acceptable, most, traded concentrates grade >85 and even >90%, carbon. Sulphides and abrasive or reactive minerals in concentrate are generally undesirable., Flake graphite can be extracted at a minimum of, 3–5% graphite in rock, whereas amorphous graphite ore requires grades >45%. Concentrates are, prepared by flotation, wet-mechanical methods,, cyclones and air separators. Very pure grades that, , 325, , reach C >99.9% are made by chemical refining of, concentrates. Graphite resists supergene alteration, so that perfectly preserved flakes are found, in decomposed rock and even in autochthonous, soil. Extraction of weathered deposits is economically very attractive, compared with the high cost, of separating the delicate flakes from fresh and, hard rock. Accordingly, unweathered deposits, stipulate higher ore grade (e.g. 16–18% C in Lac, Knife, Quebec)., Large quantities of graphite are used for, manufacturing foundry crucibles and moulds, and, for recarburizing steel. Modern crucibles are, highly complex composite materials, with, 20–50 wt. % of flake graphite, depending on the, properties required. Other possible ingredients, include tar and pitch, silicon carbide, silicon metal, or alloys, aluminoborosilicate glass and aluminosilicates (mullite). Quartz-rich graphite is used as, an acidic and reducing slagging agent in iron blast, furnaces. Graphite is also used as a lubricant, as a, filler in paints, for manufacturing electrodes and, as a substitute for asbestos in brake lining. Durable, refractory bricks and mortars result from mixing, flake graphite with magnesite or alumina. Small, amounts of high-purity graphite play an important, role in electrotechnology (e.g. lithium ion batteries), as a neutron moderator in fission reactors and, in uranium-carbide nuclear fuel elements. Synthetic graphite and substances resembling graphite (e.g. carbon black) are made from petroleum, coke in electric furnaces at 2600–3000 C. Several, synthetic products have a different market from, natural graphite (e.g. electrodes in electric arc, furnaces, carbon fibres). Others compete with, amorphous graphite (paints and anticorrosive protection). In the future, widespread use of fuel cells, may require much graphite., The source of graphite carbon is investigated by, stable isotope analysis (Hoefs 2009, Faure & Mensing 2004). d13 C of graphite ranges from 2 to, 23‰, indicating a predominantly biogenic, source of carbon with admixture of carbon derived, from marine carbonate rocks, for example by calcsilicate formation (decarbonation) during metamorphism. With an average of 0.1–0.2 wt. %,, graphite is a common accessory or trace mineral, of igneous rocks. Isotope data indicate that even
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326, , PART II NON-METALLIC MINERALS AND ROCKS, , this magmatic carbon is mostly of organic derivation, possibly by assimilation of sedimentary, country rocks (Luque et al. 1992)., 3.12.1 Graphite deposit types, Graphite deposits commonly occur in rocks that, experienced orogenic or contact metamorphism., Most large deposits are (1) metamorphic mineral, deposits sensu stricto, with graphite derived by, nearly isochemical transformation of sedimentary, or diagenetic organic substances, including coal,, kerogen and hydrocarbons. Whereas synthesis of, graphite at ambient pressure requires very high, temperatures, well-ordered graphite of metamorphic rocks typically originates at only 300–500 C,, but at 2–6 kbar (greenschist facies metamorphism). Very large flakes seem to be restricted to, amphibolite facies rocks. Coeval shear strain, boosts graphite formation in experiments (Ross, & Bustin 1997, Bustin et al. 1995) and natural, graphite formation is enhanced by penetrative, shearing during orogenic metamorphism. Apart, from pressure, shear strain and temperature, controls on graphite formation include the type of, organic precursors, the composition of the fluid, phase, the available reaction time and the presence, of minerals that may catalyse the reaction. Graphite-mineralized pegmatites, hydrothermal veins, and shear zone-hosted deposits are clearly (2) epigenetic and mostly metamorphogenic deposits, formed by migrating supercritical carbon-bearing, fluids or fluid-rich magma., The principle of epigenetic graphite formation is, illustrated by the reactions of eq. 3.3 (Frost 1979)., , Isochemical precipitation is a consequence of falling, temperature or rising pressure. Fluid-precipitated, graphite is always well-ordered, even fine-grained, “amorphous” varieties. The formation of high T/P, carbon-bearing fluids is most often a consequence of, metamorphism, but magmatic degassing can also, produce graphite (Luque et al. 1998)., Orogenic metamorphic graphite, Orogenic metamorphic graphite is a common, constituent of metasediments such as schists,, quartzite, marble and paragneiss (Figure 3.18):, In the Eastern Alps, deposits (e.g. Kaisersberg) are, located in Late Carboniferous epimetamorphic sediments of the Upper East Alpine tectonic unit (Figure, 1.89). Host rocks of graphite seams (former coal), include phyllites, slates, quartzite and conglomerates., Intensive folding, faults and shearing complicate, underground extraction. Graphitization of the coal, occurred during a Late Cretaceous-Palaeogene phase, of orogenic deformation. Graphitization of the original coal is variable and some of the material is metaanthracite. Structural ordering and crystallization are, incomplete, because of weak synmetamorphic strain., The result is typical “amorphous” graphite with its, main market in foundry applications., In the Variscan Bohemian Massif, graphite is associated with amphibolite-facies metamorphic rocks., Country rocks of graphite include amphibolite, calcite, marbles, paragneiss and schists that are probably, , Formation of graphite from hydrothermal C-O-H, fluids at high T/P:, 2Csolid þ 2H2 O ! CH4 þ CO2, Mobilization, Transport, CH4 þ CO2 ! 2Cgraphite þ 2H2 O, Precipitation, , ð3:3Þ, , Extremely reducing conditions are not required, for stabilizing and precipitating graphite. Precipitation may be induced by fluid/rock interaction, (e.g. chlorite formation by hydration, thus reducing, xH2O), by fluid mixing and by redox-change., , Figure 3.18 Graphite (dark bands) in folded amphibolite, facies Palaeoproterozoic paragneiss at the abandoned, Kanziku mine near Tsavo National Park, Kenya.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , marine sediments of Early Palaeozoic age. Graphite, occurs throughout the extension of this large crystalline basement block with mining districts in Austria,, Czechia and southeastern Germany. Most of the deposits produced fine-grained flakes, but included, “amorphous” portions. The Moldanubian graphites, are “metamorphic oil shales” (former kerogen and/or, bitumen-rich mudrocks). Accessory minerals in, this ore include alkali feldspar, plagioclase, quartz,, phengitic muscovite, Mg-rich dravitic tourmaline,, kyanite, Mg-calcite and traces of dolomite, rutile,, corundum, apatite, pyrite, pyrrhotite and chalcopyrite., Anomalous geochemical concentrations of Ni, Co, Cr,, Mo, V and Se confirm deposition of the precursor, sediments in a sea characterized by massive proliferation of life and euxinic conditions (cf. Chapter 1.3, “Black Shales”)., , Comparable graphite deposits in metamorphic, sediments are exploited in the Canadian Grenville, Orogen (e.g. Graphite Lake, Ontario with, famously large and pure flakes), in Norway and, in Madagascar that is also a source of highly valued, flakes, although with a low output., Contact metamorphic graphite deposits, These deposits can have very large reserves, but, most exploit material of low crystallinity and flake, diameter. In addition, irregular distribution of, grades, relic coal and anthracite complicate extraction. La Colorada in Sonora, Mexico is reportedly, the world’s largest contact metamorphic graphite, district. Dykes, sills and stringers of white granite, intrude coal seams in Triassic limestone producing, soft, amorphous graphite, natural coke and anthracite. Graphite seams reach a thickness of 8 m., Compared to world output, the district’s production of 10,000 t (2009) is insignificant., Epigenetic graphite deposits, Epigenetic graphite deposits may take the form of, cross-cutting veins or impregnations of shear zone, material. Although not unique, the most famous, deposits of the first occur in the late Archaean, charnockitic terrain of Sri Lanka (Ceylon). Since, 1834, they are a source of highly valued crystalline, lump graphite (apart from chips and microcrystalline fractions):, , 327, , Sri Lanka’s deposits consist of vein fields with tens to, over 100 single veins that reach a length of 500 m and a, width of 3 m. Vein fill is variably either pure graphite,, or graphite associated with a gangue of quartz, biotite,, feldspar, pyroxene, calcite, apatite and pyrite. Some of, the veins seem to be syngenetic with pegmatite., Coarsely crystalline flakes and needles of graphite are, oriented perpendicular to the vein walls and banding, reflects progressive vein growth. Phyllic graphite parallel to vein walls is a product of shearing. Sri Lanka, graphite reaches a reflectance Rmax of nearly 15%., Formation temperatures are estimated at 700–800 C., Genetic interpretation of these deposits is not sufficiently supported by data. With d13 C between 2 and, 9‰, graphite carbon of Sri Lanka is possibly derived, from the mantle, but a mixture between organic, and carbonate carbon cannot be excluded. Granulite, metamorphism at depth or the intrusion of charnockitic magmas are likely means of mobilizing a carbonrich volatile phase (Farquhar & Chacko 1991)., , Exploration for graphite combines geological, and geoelectric methods. Often, chance encounters during geological mapping (and notes in, mapping reports) give initial clues. In weathered, deposits, pitting and trenching is more revealing, than drilling. When embarking on detailed investigations of a prospect, quality criteria (flake size, and purity) and processing properties must be, a parallel part of the work towards establishing, reserves. The achievable array of products decides, the commercial feasibility., World production in 2009 amounted to 1.1 Mt, natural graphite, little different from 2008. China, leads with 70% of the total (both flakes and, amorphous graphite), followed by India, Brazil,, North Korea and Canada. Sri Lanka is still a source, of valuable lumps, although with a low output, (7000 tonnes). Reserves and resources of graphite, are large. Any development of new mines depends, on China’s market control., , 3.13 GYPSUM AND, , ANHYDRITE, , Density (g/cm3), Gypsum, Anhydrite, , CaSO42H2O, CaSO4, , 2.3, 2.96
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328, , PART II NON-METALLIC MINERALS AND ROCKS, , Nearly monomineralic rocks of the two common calcium sulphates are exploited but in most, applications, gypsum is preferred. Impurities in, gypsum rock may be anhydrite, clay and carbonate, with accessory kerogen, bitumen, authigenic, quartz and soluble salt. The same gangue occurs in, anhydrite rock with soluble salts more common., Anhydrite rock is usually strong, massive and, finely crystalline. Alabaster is a very fine-grained,, massive and pure (>99%) gypsum rock that is used, for ornamental purposes, preferably when it is, snow-white and translucent. Alabaster originates, by near-surface supergene solution and reprecipitation of gypsum. In arid and semiarid climate, zones, ascending soil solutions may form gypcrete, similar to calcrete. Gypsite is an earthy, variety of gypsum mixed with sand and soil. Selenite designates crystals of translucent and colourless gypsum., Gypsum and anhydrite cannot be enriched by, mine-site processing; therefore, run-of-mine, material must meet buyers’ stipulations. These, include specification of purity (usually a minimum of 70–80% gypsum or anhydrite), carbonate contents (commonly <5%), MgO <2% and, soluble salt contents (NaCl þ MgCl2 þ MgSO4), <0.02%. For many applications, white colour, (low Fe-Mn concentration) is desired., Gypsum is composed of 32.6% CaO, 46.5%, SO3 and 20.9% H2O. When heated to 65 C,, 11/2 moles of water are lost and the remaining, solid is very reactive “plaster of Paris”. The relic, 1, /2 mole of water is retained strongly until 95 C,, when dehydration is quickly completed and the, structure of the solid transforms into a polymorph of anhydrite (Klein & Hurlbut 1999)., This “dead-burned” gypsum sets (hardens) very, slowly. Most gypsum is consumed in the building industry, in the form of the fast-setting, semihydrate CaSO41/2H2O, for example for, wallboards and mortar. An average house in the, United States is said to contain 7 tonnes of, gypsum. Uncalcined gypsum is used as, a retarder in Portland cement and as a calcium, soil conditioner and sulphur fertilizer in agriculture. Small amounts of high-purity gypsum are, used for industrial processes, such as smelting, and glass-making., , Anhydrite may be added to the cement powder, instead of raw gypsum, in order to retard setting of, concrete. In deep coal mines, anhydrite is used for, fire prevention as a backfill and plugging material, in galleries. Note that anhydrite hydrates strongly, and fast under ambient conditions (eq. 3.4), resulting in a volume increase of 60%. This “heave” is, quite a nuisance in mining, tunnelling and road, building (Bell 1993)., Dehydration of gypsum and hydration of anhydrite:, CaSO4 2H2 O $ CaSO4 þ 2H2 O, , ð3:4Þ, , 3.13.1 Deposits of gypsum and anhydrite, Deposits of gypsum and anhydrite are formed as, chemical sediments of evaporating marine or terrestrial water bodies. Therefore, common country, rocks of the calcium sulphates include dolomite,, saline claystone and salt rocks such as halitite, (cf. Chapter 4 “Salt Deposits – Evaporites”). With, increasing concentration of seawater, calcium sulphates are precipitated after carbonate rocks and, before rock salt. Gypsum is slightly soluble in, fresh water (13.78 103 mole/L at 25 C and, atmospheric pressure). Its solubility displays, a positive correlation with temperature and salinity, but the maximum solubility at 58 C does not, change significantly with fluid salinity. Contrasting the behaviour of gypsum, the solubility of, anhydrite is retrograde; it increases with falling, temperature. The primary precipitate of calcium, sulphate is gypsum; only at temperatures higher, than 56–58 C, anhydrite is the thermodynamically stable phase. In sabkhas, conditions of gypsum and anhydrite stability switch easily so that, multiple transformations are observed (eq. 3.4)., With increasing overburden, however, already in, early diagenesis anhydrite is the stable phase, at, 50–70 C (Jowett et al. 1993). Dehydration of gypsum mobilizes a considerable mass of water, and most of the strontium is abstracted with the, dehydration water. Somewhere along the flow, paths, celestite (SrSO4, the principle ore of strontium) may precipitate and fill joints and rock, cavities, replace beds or form irregular masses that
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , can be economically attractive (cf. “Barite”). Overall, textures, structures and geochemistry of gypsum and anhydrite include the heritage from, sedimentary formation and diagenetic alterations,, and therefore reflect conditions from basin palaeogeography to the history of burial and uplift (Kasprzyk & Orti 1998)., Isotope data (D, O and S) provide a deeper understanding of gypsum and anhydrite formation. We, have seen earlier that sulphur isotopes in seawater, (d34 S of sulphate) display strong variations with, geological time (Figure 3.4). Also, stable isotopes, allow deductions about conditions of sedimentation and diagenesis of calcium sulphate rocks., Gypsum and alabaster of the Paris Basin, for example, were not extracted from Tertiary seawater but, leached from older Permian and Triassic evaporites. Messinian gypsum deposits in the Mediterranean region, by contrast, are direct precipitates, from Neogene seawater. Sulphate ions in the, Canadian McKenzie River system are derived, from pyrite oxidation in the eroding mountains, and not from sedimentary sulphate (Calmels et al., 2007). D and O isotope data usually allow a unique, genetic attribution of associated water. Note that, hydration water in gypsum is enriched in 18 O by, 4‰ compared to the water from which it is, derived. The precise age of geologically young, gypsum (and many other evaporite rocks) can be, determined with the U-series disequilibrium, method (Reich et al. 2009)., Many gypsum deposits originated by hydration, of anhydrite rock, which was uplifted to the near, surface by geological processes. Access of meteoric water initiates supergene gypsification that, may reach a depth of several tens of metres. The, anhydrite underneath often displays large karst, cave systems. In the overlying gypsum, sinkholes, and caves are ubiquitous and the surface resembles karst morphology of carbonate rocks, (“gypsum karst”, Figure 3.19). The processes of, gypsification and karst formation are a function of, the groundwater system, morphology and the, nature and spatial arrangement of rock bodies., Sinkholes and caves may be filled with collapse, breccias comprising gypsum and dolomite, and, often with earthy impurities. Breccias can be, cemented by calcite. In that case, leaching of gyp-, , 329, , Figure 3.19 Gypsum outcrop at Wadi Essel, Red Sea, Coast, Egypt, provides a model of full-scale gypsum karst, morphology. Flutes and joint-controlled furrows also, characterize the karst surface at the scale of tens to, hundreds of metres., , sum fragments is the path to formation of cellular, rocks termed “rauhwacke” (Schaad 1995)., Australia hosts a variety of young gypsum deposits. Its arid coasts are lined by numerous salinas, (coastal salt lagoons), which originated due to the, worldwide post-glacial rise of seawater levels., With rising oceans, valleys of Pleistocene dune, belts were filled with seawater that evaporated, (Figure 3.20). Sediments formed in these salinas, consist mainly of gypsum, including solid masses, of selenite near the base and banded gypsum sands., Near the margins, aragonite may occur. Holocene, dry climate periods caused formation of gypsite, dunes. All these varieties of gypsum are extracted
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330, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 3.20 Schematic section of a, Holocene coastal gypsum lagoon in South, Australia (after Barnes 1990). Water level in, the lagoon is lowered by evaporation., , and stockpiled for several years in order to reduce, salt content by exposure to rain. Coastal Lake, Macleod in Western Australia is essentially a dry, lagoon of 100 by 30 km in size. Here, gypsum is, extracted with a floating dredge, while salt brine is, pumped from a deeper aquifer and harvested from, evaporation ponds. In South Australia, gypsum, was deposited in Pleistocene tidal Lake Alexandrina that developed at a highstand mouth of the, Murray River, today some 50 km from the coast., This gypsum is well drained and free of salt., Gypsum is mainly exploited in quarries and, exploration is limited to near-surface deposits., Geological maps provide first guides. In the field,, karst morphology and sulphate in spring water, (determined by reaction with BaCl2 solution) may, provide clues. Georadar and drilling are employed, to define shape and nature of the karst surface, underneath soil cover, of the base of gypsification, and of waste material filling karst caves. Often,, this can only be done with a very close spacing of, drillholes. Gypsum karst landscapes are ecologically peculiar and mining intentions may therefore be resisted. It is advisable to obtain a valid, mining licence as early as possible in the course of, investigations., Gypsum and anhydrite are low-cost raw materials and deposits are common. Most countries are, self-sufficient. Nevertheless, considerable quantities are internationally traded. World mine production in 2009 was 152 Mt. Largest producers, were China, Iran, Spain, Thailand and USA (USGS, 2010). Synthetic gypsum is a by-product of coalfired electric power plants and other smoke-stack, industries, of phosphate processing and of de-acidification of industrial waste water (e.g. TiO2-production). In the USA, synthetic gypsum accounts, for 22% of the total domestic gypsum supply. It, is used like natural gypsum, except for special, cases where elevated trace contents of heavy, , metals, acids, chlorine, soot and its small grain, size may be problematic. Still unsolved is the, problem of phosphogypsum. The production of 1, tonne of phosphoric acid from phosphorite results, in 4.5 tonnes of calcium sulphate. Unfortunately, the processing of raw phosphate ore specifically concentrates radionuclide (226) radium in, the sulphate phase. 226 Ra emits (222) radon by, radioactive decay and this inhibits nearly all uses, of phosphogypsum, except for those plants that, process phosphate ore with very low contents of, radionuclides., , 3.14 KAOLIN, , Density (g/cm3), Kaolinite, , Al2Si2O5(OH)4, , Dickite, Nacrite, Halloysite, Allophane, , same as K., same as K., Al2Si2O5(OH)42H2O, xAl2O3ySiO2zH2O, , 2.1 (aggregates), 2.6 (crystals), 2.6, 2.6, 2.09, 1.9, , The four first listed kaolin minerals (“kandites”), are dioctahedral phyllosilicates made of 39.5%, Al2O3, 46.5% SiO2 and 14% H2O, corresponding, to a formula Al2O32SiO22H2O. On heating, the, structurally bound water escapes at different temperatures: 390–450 C for kaolinite and 510–575 C, for dickite. Kaolinite forms generally by weathering and low-temperature weakly acidic hydrothermal alteration (50–150 C). Compared to kaolinite,, dickite and nacrite are rare and originate at higher, hydrothermal temperatures (150–300 C). Macroscopically crystalline nacrite is a characteristic, gangue of high-temperature gold and tin ore. Kaolinite may have a well-developed crystal structure
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , or, not surprising for a low-temperature mineral, forming in soil, a disordered structure. Because this, affects some uses, the degree of disorder is routinely measured by X-ray diffraction and expressed, in figures between 0 (disordered) and 2 for nearly, perfectly ordered kaolinite (Hinckley 1963). Allophane is poorly ordered or X-ray amorphous gel and, is mainly found in soil. Allophane and halloysite, are generally precursors of kaolinite and are not, acceptable in high-grade kaolin products. However, halloysite and allophane-rich clay can be, marketable., Whereas mineralogical terms concerning kaolinite are well defined, the application of the geological and commercial term “kaolin” is much, less precise. Kaolin clay in nature displays wide, compositional variations and some commercial, kaolin hardly contains kaolinite. It is important, to remember the origin of the word kaolin from the, Chinese kau-ling, which simply designates a location where clay for porcelain manufacturing was, excavated and which was the source of the earliest, samples sent to Europe. Meanwhile “kaolin” has, captured many other industrial applications so, that its suitability for porcelain-making is of lesser, relevance., Kaolin, or China clay rocks consist mainly of, kaolinite and residual minerals of the precursor, rock, for example quartz, potassium feldspar, very, often muscovite and sericite, or biotite and tourmaline. Frequent accessories include anatase, goethite, haematite and crandallite-goyazite (Ca-Sr, hydrophosphates). Kaolin rocks are commonly, soft and clayey materials in the sand to clay-size, fraction. Highest quality kaolin is brilliantly, white. Commercial kaolin deposits contain, between 10 and 60% kaolinite. By- or co-production of other minerals (mostly quartz sand, but, also feldspar and mica) may materially improve, the earnings of a mine. Kaolinite is concentrated, from run-of-mine material by wet or dry processing, and upgraded by a number of chemical, physical and magnetic methods. Iron oxy-hydroxides,, which are often carriers of toxic heavy metals (As,, Pb, Cd, Hg), are removed by leaching and magnetic, separation. However, few applications require, preparation of a mineralogically pure kaolinite, concentrate. Some admixture of minerals such as, , 331, , illite and sericite may be acceptable. Higher contents of illite, other clay minerals and iron diminish kaolin quality and cause transition to, kaolinitic clays that include fire clay, ball clay, (with 30–80% kaolinite) and flint clay (cf. Section 3.7 “Clay and Clay Rocks”)., Useful properties of kaolinite include its chemical inertness, white colour, opaqueness, softness,, small grain size (0.5–50 mm in nature), plasticity,, high melting point (1850 C), white colour after, burning, low electrical and thermal conductivity,, low absorption and cation exchange capacity, and, good dispersivity. Kaolin’s chemical composition, is suitable for manufacturing synthetic zeolites., By curing with the appropriate fraction of NaOH,, zeolite A, for example, with an atomic ratio of Na:, Si:Al ¼ 1:1:1, can easily be prepared (Dyer 1988)., More than 60% of all traded kaolin is used as, filler in paper pulp and for coating paper surfaces., Coating grade kaolin is the purest and most valuable kaolinite product. In the paper sector, kaolin, is competing with similar white fillers such as, barite, calcite, gypsum and talc, but several properties of these substitutes are not identical with, kaolinite. High-grade kaolin is also applied as, functional filler in paint, rubber, plastics, pharmaceuticals and after calcining (700–1200 C), as the, main ingredient of porcelain (china clay). Lesser, grades, commonly dry-processed, are added to, ceramics of minor quality (for whiteness), to, refractory mortar, cement and fertilizer. Because, of the wide field of applications, the suitability of, kaolin for specific uses can only be investigated by, both mineralogical and appropriate technological, methods., 3.14.1 Kaolin deposit types, Kaolin deposits are typically formed by surficial, alteration of leucocratic feldspar-rich rocks (leucogranite, granulite, rhyolite, andesite, arkose) or, of illite-rich claystone. Iron-rich rocks are unsuitable precursors. Feldspar and other Al-silicates are, kaolinized by acidic meteoric water percolating, near-surface rocks. This induces hydrolysis that, liberates alkalis (Na, K, Ca) and SiO2 into solution,, which leaves the system (eq. 3.5). The reduction of, the original rock’s silica content connects
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332, , PART II NON-METALLIC MINERALS AND ROCKS, , kaolinization with the formation of bauxite. There, are two main process systems where acidic waters, may react with suitable rocks, that is (1) supergene, autochthonous weathering and (2) hydrothermal, alteration. Autochthonous kaolin soils may be, eroded and resedimented, resulting in (3) sedimentary kaolin deposits. Rarely, kaolin forms in, sand as a pore cement from infiltrating supergene, solutions (“kaolin sand”). Discrimination, between the various lines of kaolin genesis is, based on geological arguments, and data such as, the variation of O- and H-isotopes of water in the, kaolinite crystal structure (Harris et al. 1999, Gilg, & Sheppard 1996)., Formation of kaolinite from potassium feldspar:, 2KAlSi3 O8 þ 9H2 O þ 2H þ, Feldspar, þ, ! Al2 Si2 O5 ðOHÞ4 þ 2Kaq, þ 4H4 SiO4, , Kaolinite, , Silicic acid, , ð3:5Þ, , Autochthonous, supergene kaolin deposits, These deposits form extensive, often remarkably, white blankets above unaltered rocks (Figure/, Plate 1.50). Similar to other mature soils, kaolin, is prevalent in pluvial, and wet and dry (savannah), tropical climate. Related soils include laterite,, bauxite and peat. The last case is prone to form, kaolin with very little iron, because this is efficiently leached by organic acids. Unleached kaolin, may contain goethite, siderite or pyrite. The quality of supergene kaolin is often impaired by elevated contents of halloysite. Generally, the lower, part of a laterite profile is clayey and several bauxite mines co-produce high-grade kaolin or smectite clay from the same pits (e.g. Weipa, Australia, with a production of 100,000 t/y of paper-grade, kaolinite). Examples of large provinces of supergene kaolin include giant deposits in Russia, (South Urals: Gorbachev et al. 2004, 2007), Brazil, and USA (Austin 1998):, High grade kaolin of the Amazon Basin was developed, in the last decades. In this province, kaolin is part of, , an ancient lateritic regolith that formed a peneplain, above Cretaceous sediments, magmatic rocks and, a Precambrian crystalline basement. The regolith’s, former kaolinitic B-horizon was exposed by erosion, and iron was removed where tropical forests and peat, covered the land (da Costa & Moraes 1998). Resources, amount to >500 Mt., , Sedimentary kaolin deposits, Sedimentary kaolin deposits are a valuable part of, the world’s largest kaolin provinces in the southeastern USA (Georgia and South Carolina) and in, Brazil (Amazonia). The American deposits are, hosted by transgressive sediments of Late Cretaceous to Early Tertiary age, marginal to the Piedmont province. Cretaceous kaolin (and bauxite) is, a product of in-situ supergene alteration, whereas, the younger, Tertiary deposits have been formed, by resedimentation of eroded older kaolin and by, kaolinization of bauxite., Hydrothermal alteration deposits of kaolin, These deposits occur in both volcanic and plutonic, igneous rocks, provided feldspar content was high., Epithermal kaolinization is a characteristic byproduct of high-sulphidation gold deposits but is,, of course, independent of exploitable gold contents. Associated minerals are commonly halloysite, alunite and silica, but also dickite and nacrite,, as in Mexico. Hydrothermal kaolinization of plutonic rocks characterizes geochemically specialized granites. This may be caused by strong acids, (H3BO3, HCl and HF) in the fluids segregating from, such melts. The result of fluid-rock interaction, within the granite cupolas and their roof is greisen, formation and kaolinization:, The largest hydrothermal kaolin province in the world, is Cornwall, with estimated resources of 5000 Mt, kaolinized rock containing 10 wt. % kaolin. Cumulative production amounts to 100 Mt of wet-processed, kaolin. Deposits are zones of kaolinized granite that, are several 100 metres long, 150 m wide and >250 m, deep. Host rocks are Variscan granites including, specialized lithium granites as the youngest phase. In, the large pits around St. Austell zones of kaolinization, are controlled by sheeted tin-tungsten quartz veins.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , Very thin greisen bands line the vein contacts with, the granite whereas kaolinization extends for tens of, metres. Plagioclase is preferentially kaolinized, in, contrast to K-feldspar that is sericitized. Quartz, grains display lobate solution surfaces. Assuming, that Al2O3 was immobile during alteration, a, removal of SiO2 (27%), Fe2O3 (59%), MgO, (27%), CaO (88%), Na2O (98%), K2O (33%), and F (56%) can be calculated. The total rock, volume has been reduced by 28%. In the Goonbarrow pit the kaolinization affected not only granite, but also felsic dykes (“elvans”) that cut across the, quartz veins. This sets 280 2 Ma as the maximum, age of kaolinization and confirms a hydrothermal, formation process. Other observations, however,, point to supergene processes (e.g. the oxygen and, hydrogen isotope composition of kaolinite that suggests a meteoric origin of the water). The apparent, contradiction is resolved by assuming two major, steps in the formation of these deposits: An initial, hydrothermal kaolinization was later (in the, Tertiary?) overprinted by weathering and leaching, of iron (Dominy & Camm 1998)., , World mine production of 30 Mt kaolin (2009), was provided by USA, Uzbekistan, Czechia, Germany, Ukraine, Brazil, United Kingdom and many, other countries (USGS 2010). Consumption, decreased by 15% compared to 2008, because, of lower demand in world paper markets., World reserves and resources are enormous, (20,000 Mt)., , 3.15 MAGNESITE, Measured densities of the mineral magnesite, MgCO3 vary between 2.9 and 3.2 g/cm3 (theoretically 3.0). Magnesite rock, however, has an average, mass of only 2.7 t/m3, due to porosity, dolomite, contents and tectonic fracturing. Industrial use of, brucite Mg(OH)2 (density 2.37 g/cm3) is similar to, magnesite, especially for environmental applications, because its acid neutralizing capacity is, very high. Exploitable deposits of natural brucite, are rare (China, USA); usually it is made by reacting seawater with calcined dolomite. “Seawater, magnesite” is produced in the same way. The, synthetic products are in some aspects superior, to natural magnesite because they can be, , 333, , “designed” to stringent specifications. In several, plants, seawater as a source of magnesium is replaced by natural brines (Dead Sea, Great Salt, Lake, Utah) and by residual brines of salt solution, mining or of carnallite processing., For most applications, the suitability of magnesite depends on low contents of Fe, Ca, Al and, SiO2. Because of the similarity of the ionic radii, , of Fe2 þ and Mg2 þ (0.83/0.75A), solid solution of, several percent FeCO3 is not rare. Magnesites with, higher Fe-contents (called breunnerite at 5–30%, and mesitite at 30–50% FeCO3) and Mg-rich siderites are not common. Some magnesites display, traces of haematite as a reddish pigment, which, indicates early exposure to an oxidizing (and, saline) environment. Ca-contents may be caused, by substitution in the crystal lattice but are more, often due to the presence of dolomite. Dolomite, occurs in host rock inclusions or in the form of the, conspicuously white, coarse dolomite crystals, (“horse teeth”), which appear in druses and veinlets traversing sparry magnesite rock. SiO2 in, magnesite occurs as quartz or chalcedony and in, a number of silicate minerals such as talc, phlogopite, sepiolite, enstatite or clay. Several of these, minerals bequeath Al-contents to products., Untreated magnesite can be employed as a filler, and slowly acting alkaline soil conditioner. Most, applications, however, rest on its calcination, (decarbonation) to the oxide (magnesia, MgO,, melting point 2827 30 C, density 3.58 g/cm3)., High-temperature calcination at 1500–2000 C results in unreactive, “dead-burned” refractory sinter magnesia, which is used as a basic lining in, containers of molten steel. This consumes 75%, of total magnesia produced. Magnesia sinter of, good quality is expected to contain >95% MgO,, <1.5% CaO, <1% Fe2O3 þ Al2O3 and <1.5% SiO2, with a lime-silica ratio of 2:1. Bone magnesite, reaches these requirements more easily. Sparry, magnesite often needs to be upgraded by removing, contaminants before sintering. Lately, magnesia, sinter for special applications in the steel industry, is melted in electric arc furnaces, resulting in very, dense, granular “fused magnesia”. “Electric-grade, fused magnesia” is valued as an isolating powder, in domestic heating appliances. “Hard-burned”, magnesite is calcined at temperatures between
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334, , PART II NON-METALLIC MINERALS AND ROCKS, , 1000 and 1500 C. Its moderate reactivity is useful, in animal feed and fertilizer. Calcining magnesite, between 700 and 1000 C results in chemically, reactive “caustic” or “light-burned” magnesia,, which is used for floor cements and isolating, boards, and in various applications including, paints, paper, plastics, sealants, surface coatings,, rigid PVC and polyolefin films. In industrialized, countries, environmental applications consume, much of the caustic magnesia produced. These, include de-acidification of forest soil, desulphurization of combustion gas and chemical treatment, of water., “Seawater magnesite” is competing with natural magnesite. In 2009, seawater and natural brines, accounted for 55% of US magnesium compounds production. Synthetic magnesite, magnesia and magnesium metal are produced by reacting, calcined dolomite with seawater (0.13% Mg), or, with brine resulting from carnallite dissolution, and KCl-recovery, or with natural evaporitic, brines (e.g. Great Salt Lake, Dead Sea: cf., “Magnesium”). The intermediate product magnesium hydroxide can be further processed into, magnesia (by dehydration) and calcined to the, desired degree of reactivity. Compared to natural, magnesite, the synthetic product may have higher, MgO-contents, but its production (eq. 3.6) requires, more energy., Production of magnesium hydroxide from seawater or brine:, ðCaCl2 þ MgCl2 þ H2 OÞþðCaO MgOÞsolid þ 2H2 O, Brine, , Calcined dolomite, ! 2MgðOHÞ2 solid þ 2CaCl2aq þ H2 O, Mg-hydroxide, , ð3:6Þ, , 3.15.1 Magnesite deposit types, Magnesite deposits (Pohl & Siegl 1986, Pohl 1990), of economic significance are formed by very different process systems. Virtually all are epigenetic:, . The first is desilication and carbonatization of, Mg-rich ultramafic igneous rocks by CO2-rich, fluids that may have a magmatic, diagenetic,, , metamorphic or mantle degassing origin; the product is cryptocrystalline, or bone magnesite., . The second is essentially the percolation of, meteoric or low-temperature hydrothermal water, through ultramafic rock massifs, dissolving Mg;, the solutions discharge in freshwater or saline, lakes, or infiltrate terrestrial basinal sediments;, this setting is supergene mobilization followed by, infiltration; magnesite precipitates occur in continental limnic sediments as cryptocrystalline,, banded, nodular, or sinter-like magnesite., . The third is characterized by metasomatic, replacement of marine dolomite and limestone by, cation exchange of Mg for calcium; this rests on, the provision of Mg-chloride bearing solutions, whose prevalent origin is probably evaporation, in emerged parts of marine carbonate platforms;, periodically, magnesium-enriched, seawater, (Lowenstein et al. 2003; Lowenstein 2001) may, have played a role; the genetic model is a combination of evaporation, infiltration and synsedimentary (early) diagenesis; this magnesite is, commonly coarsely crystalline (sparry) and, stratabound., Ultramafic-hosted, or Kraubath type deposits, The name Kraubath is derived from a former, mine in Styria, Austria. This magnesite occurs, in strikingly white veins, stockworks, massive, bodies within, or as caps above ultramafic rocks, (Figure 3.21). Common host rocks are dunites,, peridotites and serpentinites. Typically, these are, parts of ophiolites, but mafic intrusions may also, host magnesite occurrences (e.g. Bushveld). Most, deposits of this type are related to a former land, surface, with caps and stockworks near the surface, whereas some veins extend downwards for, hundreds of metres. Magnesite veins can be very, thick (up to 45 m at Mantudi, Euboea, Greece) and, reach a length of 4 km (Susehiri, Turkey). Vein fill, is nearly monomineralic magnesite but occasional, quartz, chalcedony and calcite are observed. Near, veins and stockworks, the host rocks are hydrothermally altered to “brown serpentinite”, which, consists of fine-grained serpentine, chlorite, deweylite, montmorillonite, nontronite, inherited, chromite and goethite:
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336, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 3.22 Principles of the assumed, supergene leaching, infiltration and, evaporation origin of the limnic magnesite, deposit at Kunwarara in Queensland,, Australia (modified from Wilcock 2000)., , water and form hydromagnesite mud (Braithwaite, & Zedef 1996)., At Kunwarara in eastern Queensland (Australia), a, very large deposit of nodular cryptocrystalline magnesite was discovered in Tertiary-Quaternary alluvial, and lacustrine sediments adjacent to hills formed by, ultramafics (Wilcock 2000). The basin is over 60 km, long. Magnesite nodules and lumps displace red finegrained sand and grey magnesitic mudstone which, form the upper section of an infill of 40 m thickness., Black clay covers the exploitable horizon which may, be controlled by fluviatile meanders. The magnesiteenriched blanket is proven in an area of 30 km2, with, an average thickness of 8 m. Total resources approach, 800 Mt. These magnesites probably orginate from, infiltrating meteoric groundwater derived from the, hills (Figure 3.22). Precipitation of the magnesite may, have been induced by evaporation in dry, either seasonal or longer, climate-controlled periods. Organic, matter and bacterial activity in the deeper basin, sediments could also have had a role., , All these observations confirm that Bela Stena, type deposits are formed from low-temperature, or ambient aqueous solutions that discharge in, lakes or infiltrate terrestrial basinal sediments., Magnesium is leached from nearby ultramafic, rocks and transported along variably deep flow, paths. Hydraulic head is probably provided by, topographical elevations surrounding the basins., Deeper convectice systems may be driven by, geothermal heat, as indicated by elevated, trace contents of boron and arsenic in Neogene, lacustrine magnesite-huntite of Southwest, Turkey. The precipitation of magnesite and, related minerals is controlled by physical and, chemical conditions at the sites where spring, waters enter the lakes or react with pore water in, sediments., , Marine carbonate-hosted, or Veitsch type, Veitsch type magnesite (the name is derived from a, formerly important mine in Styria, Austria) forms, large replacement masses and stratiform lenses in, marine platform sediments. Deposits are mainly, found in Proterozoic and Palaeozoic rocks and are, rare in both Archaean and post-Palaeozoic times., Country rocks comprise dolomite, limestone,, black and grey pelites, sandstone, conglomerate, and mafic volcanic rocks. Metamorphism may, vary from very low grade (Eugui-Asturetta in, Spain) through greenschist facies (the majority of, deposits) to upper amphibolite facies (Namdechon, Korea, Liaoning, China) and possibly to, granulite facies (Snarum, Norway, although this, is a somewhat enigmatic occurrence):, Typically, the magnesite rock of these deposits is, massive and sugary to coarse-grained. Its colour may, be white, grey, black, yellowish, reddish or brown., Many deposits have parts with a “pinolitic” texture, (Figure 3.23). The term designates the presence of, white, elongated magnesite crystals (“pinolites”, resemble the edible seeds of Pinus pinea) in a finegrained dark matrix, that consists of clay or chlorite,, organic material, pyrite, talc and dolomite. Clearly,, pinolite growth displaced the dark matrix. The long, axis of the pinolites may form an irregular fabric or, three-dimensional rosettes, or they may grow perpendicular from bedding planes, joints and stylolites, resulting in mono- or bipolar fabrics. In the magnesite, rock, traces of bedding may be preserved; less frequent, are various sedimentary fabrics such as cross and, oblique bedding, ripple marks, erosion channels,, syn-sedimentary breccias and desiccation fissures., Beds of limestone, dolomite and chert may occur, between magnesite rock bodies. In the magnesite,, traces of pyrite (and other sulphides) and bitumen are, common, whereas haematite and plant remains are
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , are much smaller. Brines and seawater, however,, are practically inexhaustible. Leading magnesite, producers are China (56% of world mine production), Turkey, Russia, Slovakia and Austria. World, production of magnesite is 18 Mt per year., , 3.16 MICA (MUSCOVITE, PHLOGOPITE,, , VERMICULITE), , Density, (g/cm3), Muscovite, Phlogopite, Vermiculite, , KAl2(OH)2(AlSi3O10), KMg3(F,OH)2(AlSi3O10), Mg0.3(Mg,Fe,Al)3.0(OH)4, (Si,Al)4O108H2O, , 2.8–2.9, 2.8–2.9, 2.4, , These three minerals of the mica group are, economically significant (Madhukar & Srivastava, 1995). Vermiculite is structurally similar to talc, and is essentially derived by alteration of biotite, and phlogopite. Due to elevated iron-contents,, unaltered biotite is of little practical use. Phlogopite (Mg-mica) resembles biotite in composition, but is nearly free of iron. By substitution of iron (II), for magnesium, a continuous solid solution series, is established, which makes a clear distinction, between biotite and phlogopite impossible. Muscovite is chemically rather simple, with only, minor substitution of K (by Na, Rb, Cs), Al (by, Mg, Fe2 þ , Fe3 þ , Li, Mn, Ti, Cr) and (OH) by F. Its, sodium equivalent paragonite, although isostructural with muscovite, is not miscible at low to, moderate temperatures., , 3.16.1 Muscovite and phlogopite, Muscovite (white mica) and phlogopite (amber, mica) have similar uses. However, there are differences because muscovite is less heat-resistant, compared with phlogopite. Upon heating, water, loss of muscovite commences at 400–500 C,, whereas phlogopite remains stable to 850–1000 C., Obviously, phlogopite is preferable for high-temperature applications. Both minerals display a perfect basal cleavage and can be split into thin, , 339, , sheets. The sheets are flexible, elastic, resistant, against heat shock and exhibit excellent electrical, insulating properties. Mica sheets are mainly used, in the electronic and electrical industry. Smaller, flakes and scrap are produced for many applications, including drilling additives, paint, plastics,, roofing and rubber., Sheet mica is extracted from muscovite, pegmatites (cf. Chapter 1.1 “Ore Deposits in, Pegmatites”). Associated minerals include quartz, and feldspar, minor tourmaline, beryl, tantalite, and spodumene. Large crystals, called “books”, or, crystalline aggregates of mica are usually concentrated in the wall zone of pegmatite bodies, but, also in pockets and lenses within the main mass., Zoned pegmatites are more favourable for mining, than unzoned chaotic bodies, because this facilitates exploration and extraction. Mining and, dressing the sales products to blocks, plates and, sheets requires highly skilled manual labour., This explains why sheet mica production is virtually non-existent in high-wage countries. Sheet, muscovite deposits are known to occur in China,, India, Russia, Madagascar, Zimbabwe and Brazil., Sheet phlogopite is very rare compared with, muscovite; it occurs in pegmatitic pyroxenite, dykes that intrude gneiss and granite (Madagascar, Canada)., Flake and scrap (“ground”) mica: Muscovite and, phlogopite are mass-produced by mechanized, extraction of pegmatites and other micaceous, rocks (e.g. weathered and kaolinized granites and, alaskites, mostly of S-type, sericite phyllites, micaschists, phlogopitized peridotites). They are also, a by-product of kaolin, feldspar and Li-Ta mining., Hard rocks are comminuted (therefore the term, “ground mica”) and mica is enriched by wet or dry, processing methods. Dry grinding results in a, greasy product, which is useful for roofing, paper, and drilling fluids. Wet grinding produces thinner,, glossy platelets, which embellish paint (e.g., metallis, e paints of vehicles), facing plaster, plastics and gypsum wallboard., The Neoarchaean (2.61 Ga) Siilinj€arvi carbonatite, complex in Eastern Finland hosts an important, deposit of glimmerite consisting of phlogopite, (Figure/Plate 3.24), apatite and carbonate.
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340, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 3.24 (Plate 3.24) Concentrate of, phlogopite flakes produced at Siilinj€arvi,, Finland, is processed for use in decorative, paint, special plastics and fire protection, coatings. Courtesy LKAB 2010., , Sericite phyllites are essentially composed of finegrained (<200 mm) white mica. They form at low, metamorphic grades below temperatures of ca., 400–450 C. Above this temperature, grain size increases so that the mineral is “muscovite” and at, 580–600 C white mica finally breaks down to Kfeldspar and aluminium silicate. White micas in, metamorphic rocks include muscovite, paragonite, (sodium mica), celadonite (iron) and phengite (magnesium). Some uses may exclude specific white mica, minerals. Many sericite-rich deposits are products of, hydrothermal (“phyllic”) alteration of feldpathic, rocks (cf. Chapter 1.1 “Hydrothermal Alteration”,, eq. 1.6), including orthogneiss, aplite and felsic volcanics. The fluids responsible for alteration may be, magmatic (La Crocetta, Elba, Italy: Maineri et al., 2003) or metamorphic. An example for the latter is, “leucophyllite” in the Eastern Alps (Austria). Leucophyllites are rocks that consist of sericite, Mg-chlorite and quartz. Precursor rocks were Palaeozoic, granite gneiss and Permian quartz porphyry. During, the Cretaceous orogeny, metamorphic fluids were, channeled along shears and nappe boundaries, and in, some locations caused pervasive sericitization (Prochaska et al. 1997)., , 3.16.2 Vermiculite, Vermiculite of economic quality is visibly micaceous (>3 mm). Microscopic aggregates occur in, soil and sediments but have no commercial value., Vermiculite develops from biotite or phlogopite, through replacement of interlayer K by hydrated, , Mg (Na, Ca) cations resulting in expansion of the, structure. Concurrent oxidation of Fe(II) to Fe(III), is characteristic. This alteration of the micas may, be caused by low-temperature hydrothermal systems or by intensive tropical weathering. The, name of the mineral derives from the observation, of worm-like growth and buckling when it is, heated. Heating vaporizes interlayer water and the, vapour drives expansion. The extent of expansion, is the main measure of quality; a minimum of, 6-fold exfoliation is expected, but 50-fold volume, increase is possible. An extremely important second criterion for marketable vermiculite is the, total absence of asbestos. Raw vermiculite is, processed at the mine site by crushing, screening, and winnowing into a concentrate of large, flakes (16–0.25 mm) and a bulk density of, 640–1000 kg/m3. Near consumer markets, the, concentrate is expanded by flash heating to, 870 C in a furnace, yielding a sales product, density of 52–192 kg/m3. A new microwave, exfoliation process promises to reduce energy, consumption to about one fifth., Expanded vermiculite is extremely light and, porous, and has a high internal surface. This results in properties such as efficient thermal and, acoustic insulation and high adsorption capacity., The heated mineral has a good thermal stability, and no known health risks. Therefore, it replaces, asbestos in many applications. Most vermiculite, is used in the building industry (for fireproofing,
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 341, , heat and sound insulation) and in agriculture (for, soil improvement, replacing peat). This is similar, to applications of perlite and pumice (cf. Section 3.27 “Volcaniclastic Rocks”) or of expanded, clay aggregates. Smaller amounts of vermiculite, are consumed as industrial fillers or in environmental engineering (e.g. absorbents for oil spills)., Vermiculite deposit formation requires the presence of rocks that are chemically predisposed to, formation of massice biotite and phlogopite., Therefore, common host rocks are ultramafic, magmatic rocks. Pervasive flooding by hydrothermal fluids causes transformation of the primary, olivine and pyroxene of these rocks into nearly, monomineralic mica bodies. Vermiculite may be, generated during waning hydrothermal conditions, or by later surficial alteration. One giant deposit in, South Africa (Palabora) and deposits in the USA, and in China produce the bulk of world supply:, Large vermiculite deposits occur in Virginia, South, Carolina and Montana. One of the largest deposits, in the world was the mine near Libby in Montana, (Figure 3.25). An alkalic Cretaceous ring complex with, pyroxenites and nepheline syenites intruded Precambrian metasediments (Boettcher 1967). In the centre, a, pyroxenite plug is altered to biotite and vermiculite., Vermiculite has been drilled to over 300 m depth below, the surface indicating that hydrothermal systems, probably related to later syenite phases, were the cause of, mica formation. A high incidence of asbestos-related, deaths and respiratory diseases affecting miners and, townspeople led to closure of the mine in 1990 when, the vermiculite ore was found to contain tremolite, asbestos (Van Gosen 2007)., At Palabora (South Africa) >200,000 tonnes of vermiculite are produced annually from a pyroxeniticsyenitic and carbonatitic alkali complex., The carbonatites are a source of copper, zircon,, magnetite and apatite (cf. Chapter 2 “Copper”)., Vermiculite is extracted from pegmatitic phlogopite-diopside-apatite-olivine rocks with contents of, >15% vermiculite and a grain size of >0.45 mm., Generally, alteration of phlogopite to vermiculite, reaches from the surface to a depth of 50 m, which is, thought to indicate a supergene origin. In some, drillholes, however, vermiculite was found at, depths exceeding 400 m. Accordingly, a hypogene, hydrothermal formation cannot be excluded., , Figure 3.25 Geological setting of vermiculite in the, Mid-Cretaceous alkaline-ultramafic Rainy Creek, Complex, near Libby, Montana (adapted from Boettcher, 1967). By permission of The University of Chicago Press., Vermiculite ore formed from biotite-pyroxenite (black), surrounds a central pipe of biotite that was intruded by, alkalic syenite dykes. The ore had about 30–50 wt. %, vermiculite content., , Occurrences of vermiculite often line the contacts between ultramafic rocks and intruding pegmatites. Reaction of felsic hydrous melt with, ultramafics produces corundum and kyanite-bearing pegmatites (“plumasite”; in Kenya with ruby, and sapphire: Pohl et al. 1980). Marginal alteration, displays a characteristic zoning of distal anthophyllite and proximal phlogopite, chlorite and, vermiculite., The world’s annual sheet mica production, amounts to 5200 tonnes per year, almost only, from India and Russia. Largest producers of ground, mica are Russia, USA and Finland; world production in 2009 was 380,000 tonnes. Vermiculite is, provided to world markets by only three countries:, South Africa, China and USA. In 2009, world, production amounted to 550,000 tonnes (USGS, 2010).
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342, , PART II NON-METALLIC MINERALS AND ROCKS, , 3.17 OLIVINE, , Olivine, , (Mg,Fe)2SiO4, , Density 3.2 (forsterite), – 4.4 g/cm3 (fayalite), , Forsterite (Mg) and fayalite (FeII) are end members of a continuous solid solution series. Manganese and calcium may substitute part of Mg and, Fe. Valuable olivine is generally forsteritic., Already in antiquity, it was mined as a semiprecious, clear and translucent mineral (peridote), on tiny Zebirget Island (St Johns) in the Red Sea,, Egypt. About 100 years ago, nearly monomineralic, olivine rocks (dunite) were first used as a refractory, material. With a melting point of 1890 C, Mg-rich, forsterite is understandably preferred to Fe-rich, fayalite (1205 C)., Olivine is in demand for manufacturing special, refractories, but mainly (75% of total consumption) as a slag conditioner similar to dolomite in, pig iron production. In this sector, iron content of, olivine is accepted. The use of olivine is advantageous because it replaces dolomite and reduces, coke consumption, thereby diminishing CO2, emissions. Olivine is also used for sintered heatstorage elements in electrical heating appliances, where it competes with magnesia made from, magnesite. Olivine makes good foundry sand, because compared with quartz, the mineral displays a lower expansion when heated. It requires, less binder (e.g. bentonite) so that steam development during casting is reduced. However, other, alternatives such as zircon and chromite sand are, less easily damaged by sudden heat shock. Olivine, sand is also a useful abrasive (hardness 61/2–7) and, poses no hazard concerning silicosis., Olivine is very prone to hydration and weathering (cf. Chapter 1.2 “Lateritic Nickel Ore Deposits”). Hydration is easily measured by the loss on, ignition (weight %). It should be minimal where, refractory application is the target. Unaltered, dunites and olivine-rich peridotites are quite rare., However, the propensity to alteration is very useful if the aim is sequestering CO2 into stable, minerals. Ground dunite rock may be one means, of immobilizing carbon dioxide. The reaction, , (eq. 3.6, Section 3.16.1 “Magnesite”) is exothermic, and the heat might be recovered., Common accessory minerals in dunite including chromite and pyroxene can be removed by, processing. The same applies to joints coated with, serpentine and magnesite, but net-textured serpentinization of olivine grains at the microscopic, scale impedes some applications. When heated,, hydrated minerals give off steam that fractures the, grains., , 3.17.1 Olivine deposits, Pure and unaltered (hydrated) forsterite fels, (dunite) is the preferred olivine raw material., Deposits occur in ophiolites and in complex, intrusions of the Alaska-Urals type., , The world’s largest olivine mine is Aheim in, western Norway (Figure/Plate 3.26). Polymetamorphic Palaeoproterozoic gneisses enclose a, large (5 1 km) ultramafic body with dunite and, several eclogite inclusions (Sturt et al. 2002). The, dunite is enveloped by a narrow (100 to a maximum of 300 m) serpentinization mantle, which is, due to interaction with metamorphic fluids., Exploitable dunite comprises 50 vol. % of the, ultramafic body. The extracted rock displays a loss, on ignition of <2 % and consists of 91% forsterite, and 7% enstatite, as well as some serpentine,, chlorite, chromite and spinel (Figure/Plate 3.27)., , With a capacity of 2.4 Mt per year, Aheim is reported to provide >50% of olivine sand produced, worldwide., International statistics on olivine (dunite) production are incomplete and most is probably comprised in the giant class of “crushed stone and, aggregates”. A large seaside quarry at Atammik, between the capital Nuuk and Maniitsoq in, Greenland, with an annual production of 1 Mt,, was put on hold in 2009., , 3.18 PHOSPHATES, , Apatite, , Ca5(F,OH,Cl), (PO4)3, , 40 wt., % P2O5, , D ¼ 3.2 g/cm3
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343, , INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , , , , , Figure 3.26 (Plate 3.26) Part of the Aheim (Almklovdalen) olivine quarry in southern Norway. Courtesy of Havard, Gautneb, Geological Survey of Norway, Trondheim., , , , Figure 3.27 (Plate 3.27) Aheim dunite in thin section (crossed nicols). Olivine grain diameter is 1–2 mm. Note the, , fractures with weak net-textured serpentinization. Courtesy of Havard Gautneb, Geological Survey of Norway,, Trondheim.
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344, , PART II NON-METALLIC MINERALS AND ROCKS, , Apatite is chemically highly variable. Calcium, may be substituted by Mn, Sr, Na, Mg, U or REE., Fluorine and (OH) can be replaced by Cl and (CO3)., Silica (SiO4) or (SO4) may take the place of (PO4)., Therefore, many different names have been proposed for varieties of apatite (Kohn et al. 2002)., Apatite is very useful for radiometric age determination by the fission-track method (Wagner & van, den Haute 1992). Because of its low closure temperature of 100 C, the mineral supports thermochronological studies of the cooling history of, geological rock bodies. Phosphorites, or phosphate, rocks are apatite-rich weathering crusts and sedimentary rocks that consist of fine-grained apatite, with a gangue of carbonate, clay, quartz, pyrite,, glauconite and organic matter. The crypto-crystalline apatite is called collophane or francolite., Francolites are a very heterogeneous group of carbonate-fluor apatites., The phosphate content of a rock is usually expressed, in weight percent P2O5, or % tricalciumphosphate, (¼ 2.185 P2O5) and % elemental phosphorus, (¼ 0.436 P2O5). Cut-off grades of phosphorite vary, from 15 to 30% P2O5. Carbonatite-hosted apatite ore, can be profitably mined at 8% P2O5., , Wherever feasible, apatite and phosphorite ores, are selectively extracted (“direct shipping ore”)., Elevated contents of carbonate, iron oxides,, kerogen and clay are undesirable in fertilizer, production, as they cause higher sulphuric acid, consumption. Comminution, washing, sieving, and flotation are employed to remove impurities, in order to reach a marketable concentrate grade of, >30% P2O5. Apatite concentrate always contains, minor and trace elements, some of which may be, recovered as by-products (U, Th, F, V, REE: Dill &, Kantor 1997). If left in the product stream, some of, these elements may cause an undesired geochemical heritage in phosphate fertilizer, or contaminate the environment of phosphate mines (Abed, et al. 2008). Problem elements include Cd, Cu, Ni,, Cr, Pb, Zn, Hg, As and Se. For example, waste, rocks left after mining the phosphatic shales of the, Permian Phosphoria Formation in SE-Idaho, release selenium, causing adverse effects to, aquatic resources in the region (Hamilton & Buhl, , 2004, Hein 2004). Sludge from processing phosphate in Togo endangers the coastal environment, (Gnandi et al. 2005). Several of the trace elements, are not associated with apatite but with organic, substance and pyrite in the ore. Elevated cadmium, concentrations of some phosphorite deposits in, western Africa are a serious problem, but many, sources in the same region have acceptable low, concentrations. Of course, the industry strives to, mitigate environmental consequences, but operations with very low contents of hazardous elements find this to their advantage., Phosphorus is one of the six major elements —, H, C, N, O, S and P — that are required to build all, biological macromolecules (Falkowski et al., 2008). P is an essential constituent of desoxyribonucleic acid (DNA), ribonucleic acid (RNA) and, many other parts of organisms (bones, teeth,, shells). It is one of the three main nutrional, elements (P, N, K), which are indispensable for, plant growth. Consequently, over 90% of world, phosphorus production is consumed as fertilizers, in agriculture. In fertilizer production, the concentrate is first reacted with sulphuric acid in a wet, process, in order to make phophoric acid. Calcium, sulphate is a by-product (“phosphogypsum”). The, most common fertilizers ammonium phosphate, and superphosphate are made by reacting phosphoric acid with ammonium or with apatite, concentrate. Untreated phosphate is of little use, to agriculture, because apatite dissolves too, slowly. A minor part of phosphate is used for, manufacturing elemental phosphorus, animal, feed and chemicals., The possibility of marketing the by-product phosphogypsum as a building material is determined by its, contents of radionuclides of the uranium decay series., Radium (226) and its daughter radon (222) exhaling, from plaster are the main hazard. Elevated contents, impede use of the gypsum in the building industry, and much of it is stored as waste. In Florida, phosphogypsum tailings reach a mass of 1000 Mt. Efforts to, exploit this potential resource of nuclear fuel have, recently intensified. Phospho-gypsum from Siilinj€arvi in Finland, in contrast, is sold as a filler for paper., , Phosphorus is foremost an essential element, for all life forms and in natural occurrence is not
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , hazardous. Yet, high concentrations of phosphorus disturb the ecology of rivers and lakes. In the, industrial countries, eutrophication by phosphorus in detergents was common in the years, 1960–1970. Mitigation by building water treatment plants and replacing P-detergents with, zeolites was one of the great environmental, achievements of the period., 3.18.1 Geochemistry, Phosphorus is a minor component of all magmas,, with highest concentrations in mafic (1400 ppm), and intermediate melts (1660 ppm). Its crustal, abundance is between 480 and 1300 ppm, shales, contain 700 ppm. In magmas, the element displays, incompatible behaviour but is precipitated in early, apatite if calcium is available. Enrichment in late, melts (pegmatites) and fluids is only possible at, low calcium activity. Phosphorus is siderophile, and associates with many magmatic and sedimentary iron ores. Hydrosaline melts and saline fluids, may be responsible for the enrichment of phosphorus in spectacular magnetite-apatite ore, as at, Kiruna (Sweden) and in some apatite orebodies of, carbonatites and nephelinites. Apatite crystallizing from hydrous residual melt batches is marked, by elevated fluorine and chlorine concentrations., The element occurs in eight different oxidation, states, but valency (V) is most common., Weathering releases phosphate into rivers and, oceans. At surface conditions, pentavalent phosphorus in oxyanions is extremely mobile, except, in the presence of adsorbing iron oxide particles., Seawater is largely oversaturated (50–100 ppb P), but in wide parts of the oceans, phosphorus occurs in biologically limiting concentrations., Nevertheless, a steady rain of biomass sequesters, phosphorus into oceanic sediments. Main agents, appear to be diatoms and common marine cyanobacteria that accumulate polyphosphates in, their cells. In the bottom sediment, polyphosphate particles nucleate and authigenic growth, of (carbonate-fluor) hydroxi-apatite takes place, (Diaz et al. 2008). Overall, phosphorus is strongly, enriched in the deep ocean as a result of continental runoff, marine phytoplankton production, and the remineralization of biomass raining, , 345, , down to the ocean floor. Marine phosphorites are, formed where cold, more acid and P-enriched, deep water flows up into shallow regions off, continental margins promoting high biological, productivity (“oceanic upwelling”; Robb 2005)., Subrecent phosphate rich sediments have, been found on many continental shelves (e.g., southwestern Africa and Peru). As a result of, biological processing, modern marine phosphates have d18 OP of 19–26‰ (SMOW),, compared to 6–8‰ that are characteristic of, apatite in igneous rocks., Phosphorite of wide shallow epicontinental seas, (e.g. northern Africa, Arabia) cannot be explained, by upwelling, but is due to fluvial import of phosphorus (Glenn et al. 1994). In both cases, phosphorus availability causes a synergetic increase in the, density of life, supporting the food chain from, phytoplankton to fish and birds. Under anoxic, conditions in Corg-rich bottom sediments, large, sulphur bacteria such as Thiomargarita namibiensis gain energy by oxidizing sulphide with, nitrate as an electron acceptor. At the same time,, they release phosphate into the pore waters, which, results in the high degree of supersaturation that is, required to overcome the kinetic nucleation, barrier to apatite precipitation (Schulz & Schulz, 2005)., 3.18.2 Phosphate deposit types, Phosphate deposits include marine (rarely lacustrine) sediments, guano (Nauru and Christmas, Island) and fluid-enriched parts of magmatic, bodies (mainly carbonatite and alkali complexes:, Notholt et al. 1990)., Sedimentary phosphorites, Sedimentary phosphorites represent the largest, part of world P-resources (>90%) and yield most, of annual phosphorus production (northern Africa,, China, Middle East and USA). They are marine, sediments of shallow seas (<200 m depth) and, form lenses or beds of considerable extension., Lateral passage to non-phosphatic rocks is gradual., Host rocks may be pelitic (often oil shale), calcareous or psammitic; associated chert bands are
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346, , PART II NON-METALLIC MINERALS AND ROCKS, , characteristic. As a rule, deposits occur in little, deformed platform sequences (e.g. northern Africa) but also in strongly folded (Kara Tau, Kazakhstan, midwestern USA) and metamorphic, phosphate strata (Siberia: Guliy 1995). Many, deposits owe their exploitable grade to enrichment, by supergene alteration, both by dissolution of, carbonate matrix and by (limited) transfer of, phosphate in solution (“phoscrete”). Mechanical, erosion and concentration of phosphate nodules, similar to coarse detrital iron ore at Salzgitter in, Germany is caused by high-energy submarine, currents. A rapid oscillation between high and low, energy states characterizes many submarine areas, of phosphorite enrichment (the “Baturin cycle”):, Phosphorites are highly variable ranging from soft, and earthy to hard rocks, with a grain size from silt to, coarse pebbles and nodules, or fine-grained pelletoid, and dense. Colours vary from white to green and, brown. Apart from cryptocrystalline francolite, bone, fragments, fish teeth, dolomite, calcite, chalcedony,, kerogen, pyrite and detrital components (quartz, etc.), are common. Some phosphorites consist mainly of, phosphatized bone breccias and coprolites. The macroscopic and microscopic variability of phosphorite, makes it an eldorado of sedimentologic research, (Glenn et al. 1994)., , The earliest marine phosphate deposits occur in, the Palaeoproterozoic of Siberia. Geological periods of enhanced phophate deposition include the, Cambrian (Asia, Australia), Permian (Phosphoria, Formation of North America: Piper & Link 2002,, Hein 2004), Late Cretaceous to Eocene (Saudi, Arabia, northern Africa, northern South America), and Miocene (Florida, North Carolina). The, increase of phosphorite with decreasing age is an, effect of the evolution of life:, One of the oldest of the large deposits occurs in Early, Palaeozoic platform sediments of the Mt Isa region in, Queensland, Australia, with resources of >2000 Mt at, 17% P2O5 that include 40 Mt at 31% of direct shipping ore. Al Jalamid in Saudi Arabia is a new mine, exploiting a Cretaceous deposit with reserves of 213, Mt. The phosphate horizon contains 21% P2O5 and, extends over 18 km2 with an average thickness of, 6.5 m. Planned production is 5.3 Mt/y of phosphate, concentrate containing 32.5% of dry P2O5., , Magmatic phosphate deposits, Magmatic phosphate deposits occur in alkaline and, carbonatite complexes (Figure 1.14, Figure 2.40 and, Figure 3.28) and in layered mafic intrusions. Brazil,, Canada, Russia and South Africa host notable resources. Most carbonatite-hosted deposits are only, exploitable because of supergene enrichment (e.g., Araxa, cf. Chapter 2 “Niobium and Tantalum”)., For many years, the ultramafic-carbonatitic, Palabora Complex in South Africa was an important source of apatite, apart from metals such as, Cu, Zr, Hf and and Fe in magnetite (cf. Chapter 2, “Copper”), and other minerals (vermiculite)., Before the change to underground mining of copper, annual production used to be 3.5 Mt fluorapatite concentrate. Ore rocks included apatiterich pyroxenite and the so-called phoscorite,, which surrounds the copper ore of the central, carbonatite pipe. Typical phoscorite consists of, 25% apatite, 35% magnetite, 18% calcite and, 22% phlogopite, some serpentine and chalcopyrite. Average in-situ grades were 10% P2O5. High, apatite concentrations were correlated with more, phlogopite, implying that fluid-enriched melts, concentrated phosphorus:, Geologically less complex is the alkali ring intrusion, of Khibiny near the town Apatity on Kola Peninsula, (Russia). It was formed after the Variscan orogenic, peak (300–280 Ma), intruding Archaean and Proterozoic basement. Today’s erosion level exposes a cross, section at subvolcanic depth with a diameter of 35, km (Figure 3.28). Rings formed successively from the, margin to the centre comprise nepheline syenite,, ijolite, apatite nephelinite and foyaite. P-enriched, sections of apatite nephelinites are exploitable apatite ore. The ore horizon rocks occur as concordant, intrusive bodies that reach a length of 20 km and, a thickness of 250 m, and were probably injected into, late ring fissures. The ore consists of apatite, nepheline, aegirine and hornblende, and traces of sphene, and titanomagnetite. Ore grades exploited in two, open pits are 12.3 and 16% P2O5. Today, apatite ore, is the only commercial product (900,000 t/a), but, earlier, nepheline was exploited as an aluminium ore, and baddeleyite as a source of zirconium and hafnium. The comparable Kovdor Complex in Russia,, near the border to Finland, produces annually 2.5, Mt of apatite besides iron ore and zirconium oxide.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 347, , Exploration for phosphorus is based on geological models. Both sedimentary and magmatic apatite is marked by traces of uranium and, radiometric methods can help in recognizing Panomalous outcrops. Apatite was so named after, the Greek word apatein that is translated as “to, deceive”, because it is not easily identified. Phosphorite may attract attention because of bone, fragments and fish teeth, but radiometric methods, help at all scales of investigation. Carbonatites,, alkali complexes and layered mafic intrusions are, located by a combination of remote sensing, photogeology and aerial magnetic surveys., Figure 3.28 Generalized geological map of the alkali, ring intrusion at Khibiny, Kola Peninsula, Russia., Exploited horizons of apatite-nephelinite in black., Modified from Notholt et al. (1990). By permission of, IMM London & Maney Publishing (www.maney.co.uk/, journals/aes)., , The ultramafic alkaline Fanshan Complex in northern China is an important source of phosphorus, fertilizer, based on a reserve of 76 Mt of apatite ore, at 11% P2O5, with by-product magnetite (Neng Jiang, et al. 2004)., With a diameter of 80 km, the Cambrian Sept-Îles, Complex in Quebec, Canada is one of the largest, layered mafic intrusions of the world. However, the, greater part of the outcrop is underneath the sea. Of, three magmatic cycles that produced the intrusion,, the oldest terminates with the 250 m thick “critical, zone” of which nearly 100 m consist of magnetiteilmenite-apatite rock (nelsonite). Its formation is, thought to be due to orthomagmatic liquid immiscibility (Cimon & McCann 2000). Mining reserves, amount to 110 Mt with 15 wt. % apatite and 16%, ilmenite., , Phosphate reserves are very large (16,000 Mt), and shared by many regions of the world, although, 40% occur in northwestern Africa (USGS 2010)., Additional very large potential phosphate resources are known in offshore regions of Namibia, and Mexico. World phosphate production in 2009, was 158 Mt. Largest producers were China, USA,, Morocco, Western Sahara and Russia., , 3.19 QUARTZ AND SILICON, , Quartz, , SiO2, , Si ¼ 46.7 wt. %, , Density 2.65 g/cm3, , Quartz is economically the most important, among all silica minerals. Other silica polymorphs, include tridymite, cristobalite, coesite, stishovite, and amorphous silica (opal sensu lato). With the, exception of opal (diatomite), quartz is the only, one of these minerals that is widely used. It is also, the most common of the group, especially in, magmatic, metamorphic and sedimentary rocks., Hydrothermal quartz usually originates at, T>120 C. Many quartz deposits are possibly precipitates from silicothermal fluids (Wilkinson, et al. 1996), which are liquids with 90% SiO2, that coexist with an aqueous supercritical fluid in, a wide temperature field (300 to >750 C). Quartz, (silica) solubility at surface conditions is, extremely low; it rises with increasing T, P and, pH. Quartz occurs in low- and high-temperature, forms. At 573 C and atmospheric pressure, the, first transforms instantaneously to high quartz., This causes a small volume increase (DHQ ¼ 2.53), that may be a problem in certain high-T applications. Most natural quartz is twinned. Critical, properties are chemical purity and crystallinity., Quartz consists of 46.7% silicon and 53.3% oxygen. Although natural quartz is commonly quite, pure, it does contain structurally bound trace elements (B, Li, Al, Ge, Ti, Fe, Mn, Ca, K, Na, P) in its
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348, , PART II NON-METALLIC MINERALS AND ROCKS, , lattice. Common point inclusions are H2O, AlOH, and LiOH. Fluid inclusions (with dissolved salts), are frequent. Variable quantities of inclusions, define sectors of quartz crystals (Ihinger & Zink, 2000). The melting point of natural quartz varies, about 1650 ( 75) C. Note that whenever quartz is, drilled (in the mine) or pulverized during processing, the hazard of inhaling fine dust must be, excluded, because prolonged exposure leads to, lung complaints such as silicosis (Ross 1999)., Silicon is a lithophile element, with an approximate crustal average of 27% (Smith & Huyck, 1999). It is the second most abundant element, after oxygen. Silicon occurring in sand and glass, is made up of stable isotopes 28 Si (92.23%), 29 Si, (4.67%) and 30 Si (3.1%). Native silicon is, extremely rare in nature:, The metalloid silicon (melting point 1410 C, density, 2.329 g/cm3) is currently one of the most important, electronic materials, apart from its mass market in, the metallurgical and chemical industries. For electronic applications, silicon is produced from highpurity quartz (HPQ) raw materials. Melting, followed, by reduction with carbon and reaction with chlorine, is employed in order to produce liquid silicon tetrachloride, that is purified by repeated distillation. It is, then reacted with magnesium in order to precipitate, native silicon, which is further purified by metallurgical zone, or chemical refining to a final impurity, content of <109. Ultrapure silicon is not conductive., For electronic use as a semiconductor, silicon is, doped with other elements (P, Sb, B, In, etc.) constructing negative and positive transistors in minimal space. Nearly all microelectronic equipment is, based on silicon. Yearly world consumption of electronic and photovoltaic grade silicon is estimated at, 100,000 t/year., , Quartz of metallurgical quality is used for the, production of quartz glass, silicon alloys (with Cu,, Al), silicon metal, silicon carbides and ferrosilicon, which is important in the cast iron and steel, industry. Most silicon metal is used in the form of, aluminium-silicon alloys for manufacturing cast, parts in the automotive industry. Fumed silica is, produced at high temperatures, mainly as a filler, for silicon rubber and for paints. Worldwide in, 2009, production of metallurgical silicon (>98%, Si) was 5.4 Mt. Ferrosilicon accounts for about, , four-fifths of this amount. Largest producers of, ferrosilicon were China, Russia, USA, South Africa and Norway, and of silicon metal, China, Brazil,, Norway and France (USGS 2009)., Very pure (>99% SiO2), lumpy (>20 mm) quartz, concentrate or processed quartz sands constitute, the base for chemical grade silicon. One path of, processing leads to glass fibre cables, semiconductor silicon chips for micro-electronic applications,, and photovoltaic (solar) cell panels (solar grade, silica). Compared with metallurgical use, the electronic applications account for only a small percentage of total silicon demand. The alternative, path provides silicon for the chemical industry, (e.g. silica gel, silanes, silicones), which is the, second largest sector of chemical grade silicon use., Silica gel, for example, is an important industrial, adsorbent for drying process air and extracting, heavier hydrocarbons from natural gas., Quartz crystals are required in the electronics, and optical industry. The largest part is consumed, in electronics as frequency-control oscillators,, timers and frequency filters in a wide range of, products, for example communications equipment, watches, computers and television receivers. Properties that make quartz singularly useful, for these applications include its piezoelectricity,, dielectric capacity and a high acoustic hardness., Electronic quartz must be very pure, untwinned, and faultless. As natural quartz of this quality is, rare, crystals are hydrothermally cultured in autoclaves. Natural quartz crystal material is only, required as seeds (lascas) for synthetic crystal, growth. Optical quartz must be transparent and, untwinned. It is used for manufacturing UVtransparent prisms and cells, and for certain, lenses, for example in high-temperature equipment (<1100 C)., 3.19.1 Quartz deposit types, Economic quartz deposits include quartz in granite pegmatites, hydrothermal vein quartz, sedimentary and metamorphic quartzites, and, mature clastic sediments such as quartz sand and, gravel., Deposits of quartz include material of very different quality and value. Sandy gravels for road
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , construction are at the lower end and quartz of, sufficient purity for electronics is the most valued, of all industrial silica materials. High-purity, quartz is defined by a content of trace elements, <50 mg/g. High-purity quartz deposits include, quite varied genetic types, such as silica caps, (“quartzolite”) of specialized granites (Figure 3.29),, granitic pegmatites, hydrothermal quartz veins,, quartzites (cf. Section 3.21 “Quartzite”) and, mature clastic sediments such as quartz sand and, quartz pebble deposits (Desindes et al. 2006)., M€, uller et al. (2007) identified kyanite quartzites, as a new source of high purity quartz (cf., “Andalusite, Kyanite and Sillimanite”). Detrital, deposits are more profitable than hard rocks, because of cheaper extraction, less energy-intensive processing and higher quality due to natural, preselection by weathering and transport. However, the preselection hypothesis is contradicted, by Cox et al. (2002), who propose that non-, , Figure 3.29 The massive silica cap of the Panafrican, Nuweibi rare metal granite, Eastern Desert, Egypt, is, made up of large upright unidirectional quartz crystals., , 349, , siliceous, less durable pebbles are removed by, diagenetic factors (e.g. in-situ dissolution). Major, sources of lascas are Brazil, Canada, Germany and, Madagascar. Cultured quartz crystals are exported, by China, Japan and Russia. Considering that, high-quality quartz raw materials are quite, rare, prospects of favourable size and processing, characteristics should always be explored and, developed. Close cooperation with industrial, customers is essential., , 3.20 QUARTZITE, Quartzites are hard and brittle sediments or metamorphic rocks that consist chiefly (>80%) of, quartz. Rock density is 2.6–2.7 t/m3 and bulk, density of crushed quartzite 1250–1350 kg/m3., Fragments of crushed quartzite are angular with, a rough surface, low porosity, high strength, and, resist wear and weathering. This makes quartzite a, valuable road-building aggregate., However, high-quality quartzite is also an, industrial raw material. Based on its high melting, point (>1700 C) refractory-grade quartzite with, >96% SiO2 is used for the production of “acidic”, refractory bricks and mortars. Note that the transformation of low silica polymorphs (e.g. low, quartz) into tridymite (900 C) and cristobalite, (>1470 C) by heating in the furnace may result in, irregular expansion and consequent damages to, the refractory lining. Therefore, transformation, behaviour must be investigated if the refractory, market is sought., Metallurgical-grade quartzite is used for the, production of silicon metal and ferrosilicon (cf., Section 3.20 “Quartz”). Quartz consists of 46.7%, silicon and 53.3% oxygen. Quartzite (or quartz), are reduced by graphite electrodes at >1780 C in, electric arc furnaces. For this process, a high chemical and pyrometallurgical reactivity is desirable., These properties can be predicted by microscopic, confirmation of a high percentage of strained,, strongly undulatory and highly cataclastic quartz, grains, which provide a large reactive surface when, thermally shocked. Also, favourable reactivity is, correlated with higher solubility of quartzite in, NaOH. Deleterious impurities include Al, Fe, P,
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350, , PART II NON-METALLIC MINERALS AND ROCKS, , Ti and Ca. Of these, only alumina and calcium can, be removed with slag. Some quartzite deposits, may yield highest quality material that can be, used for chemical and electronic products (cf., Section 3.20 “Quartz”)., 3.20.1 Metamorphic quartzite deposits, Metamorphic quartzite deposits are usually, derived from sedimentary quartz sandstone precursors, by orogenic or contact metamorphism., M€, uller et al. (2007) suggest that kyanite quartzites, are a product of hydrothermal alteration and, propose these rocks as a new source of high purity, quartz with co- or by-product kyanite (cf. Section 3.1 “Andalusite, Kyanite and Sillimanite”)., In contact metamorphic zones, silica-rich, cryptocrystalline hornfels can occur near more granular, quartzite., Metamorphic quartzite is commonly massive or, slightly foliated. Quartz grains are typically equidimensional or somewhat elongated and have, clear boundaries, resulting in a simple mosaic, texture. A higher percentage of grains with, a diameter above 2–3 mm impedes a number of, upper-value uses. Cataclastic structures and lattice defects are often observed, without necessarily lowering the value. Quartzites with accessory, mineral content of >4% are not suitable for higher-value uses; common impurities include mica,, feldspar, garnet, pyrite and goethite. Because of, brittleness, many quartzites are densely jointed, and brecciated. Although this facilitates extraction, joints and faults may be pathways for seepage, and groundwater flow and with it, introduction of, impurities. Careful mapping of grade classes and, selective mining is the key to quality control., Remember that whenever quartz is drilled (in the, mine) or pulverized during processing, the hazard, of inhaling fine dust must be excluded, because, repeated exposure provokes silicosis (Ross 1999)., 3.20.2 Sedimentary quartzite, Sedimentary quartzite is an unmetamorphosed, sandstone that consists mainly of quartz grains,, which are cemented by silica. The cement may be, opal (some quartzites have >30% X-ray amor-, , phous silica), chalcedony or microcrystalline, quartz. The rocks are smooth, extremely hard and, tough, and break into sharp-edged fragments with, a conchoidal fracture surface. The silica-cement is, evidence of solution and reprecipitation of SiO2., Two geological process systems provide models, for explaining these observations:, . migrating alkalic formation or shallow groundwater, which is common in tropical wet and dry, (savannah) climate (related to silcrete);, . dissolution, for example by kaolinization, and, transport of silica by complexing with dissolved, organic matter., In Tertiary lignite basins of central and northern, continental Europe, sedimentary quartzite boulder horizons in sand are very common and illustrate the second model. Organic acids combined, with reduction cause very low iron concentrations, (cf. Section 3.22 “Quartz Sand and Gravel”). Bennet et al. (1991) provided analytical and mineralogical proof of the high SiO2-solubility in lower, parts of peat profiles., Exploration for quartzite is best guided by published geological maps and reports, and narrowed, down by geographical factors such as the availability of land for quarrying. High-grade quartzite is, located by wide-spaced regional sampling. Most, of the quartzite produced worldwide makes up, part of the giant mass of “crushed stone and, aggregates” (cf. Section 3.22 “Quartz Sand and, Gravel”). Its share in world silicon production, (5.4 Mt in 2009) is not recorded., , 3.21 QUARTZ SAND AND GRAVEL, Sand is one of the most common natural materials, in our life, but has many little-known fascinating, aspects (Welland 2009). Sand (0.06 to 2 mm grain, diameter, dry loose density 1.3 t/m3) and gravel, (2–63 mm, dry density 2 t/m3) are defined by, grain size. They are sedimentary, non-cohesive, (loose) and unconsolidated clastic rocks. Sand consists mainly of minerals such as quartz, calcite and, silicates, with traces of heavy minerals (cf. Chapter 1.3 “Placer Deposits”). Gravel is more often, a mixture of quartz and rock fragments. Both sand, and gravel grains display shapes that are described
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , as angular to rounded. Of course, gravel and sand, in the ground have higher densities because of, partly or wholly water-saturated pore space, (saturated densities of gravel 2.3 t/m3 and sand, 2.15 t/m3). Concerning possible uses, two subgroups are differentiated:, . industrial sand (glass and foundry sand; quartz, flour, well-packing and cementing sand, abrasive, sand);, . building sand and gravel (concrete aggregates,, construction fill, mortar sand, various concrete, products)., Properties of natural sands and gravels control, eventual use. Favourable are minimal contents, of iron, calcite, clay, salt, humic and bituminous matter, heavy minerals, and suitable grain, form and grain size distribution. Some negative, properties can be alleviated by processing,, which includes washing, classifying, attrition, and flotation., 3.21.1 Industrial sand and gravel, Industrial sand and gravel is defined by higher, quartz contents and lower impurities compared, with building sand. Industrial specifications are, very narrow. Main consumers are glass and metal, casting factories. SiO2 should be at least 96% and, for flat glass >99%. Glass sand implies very low, (<0.008%) to moderate Fe2O3, TiO2<0.03%,, Al2O3<0.2% and CaO þ MgO<0.05%. Accessory heavy minerals (zircon, chromite, kyanite), may be deleterious. Grain size should be, 0.1–0.6 mm (medium sand) and angular grain, shape is preferred. Note that there are glasses, with very different requirements. Sand for, green bottle production may contain as much, as 4% Fe2O3., Quartz sand of the high chemical purity described is, the basic raw material for glass production, with, 70 wt. % in the formula. Because most commercial, glasses belong to the soda-lime-silica variety, main, components of the glass mixture include quartz, soda, (Na2CO3 to 15% Na2O) and calcined limestone or, dolomite (10%). Minor additions are made to improve, properties of melting, forming and use: Alumina, (Al2O3) for chemical durability and reduced devitrification, sodium sulphate for purifying and salpetre for, , 351, , decolourizing. Heat-resistance is improved by adding, boron (B2O3). K2O and BaO enhance the hardness of, optical glass. PbO is added to produce decorative glass, as it increases the refractive index. Traces of metal, oxides such as CoO (blue) and Cr2O3 (orange) colour, glass. Pure silica glass (vitreous silica) is produced for, its excellent high-temperature stability, optical properties and thermal-shock resistance. It is more expensive because of the high temperature required for, melting quartz (1700 C) and the great viscosity of, the melt., , Industrial sand deposits, Pure quartz sand deposits occur predominantly in, repeatedly eroded and recycled mature sediments,, because only quartz and the most durable heavy, minerals (e.g. zircon) survive multiple chemical, and mechanical attacks. Sand deposits form in, coastal marine (Figure 2.5, Figure/Plate 3.30), fluvial and aeolian process systems. High-quality, sands seem to be marked by proximity to peat and, emersion surfaces and this is probably a genetic, factor: Water infiltrating the soil beneath bogs and, tropical forests is loaded with dissolved organic, carbon (DOC) and organic acids that leach alkalis, and cause reduction and microbial methanogenesis in the sand. Iron is reduced to Fe2 þ and dissolved. DOC production and export rise during, geological periods of elevated CO2 content in the, atmosphere (Freeman et al. 2004). Favourable, hydraulic conditions in the sand aquifer, especially, persistant percolation, allow removal of dissolved, impurities. Many glass sand operations extract, fossil sand of raised beaches (deposited and purified, in interglacial warm climate). Recent coastal sand, must be desalinated by washing with freshwater., Quartz flour is prepared from high-purity quartz, sand by metal-free grinding. Micronized to <3 mm,, the flour is used as a functional filler, for example, in ceramics, paint and plastics., Abrasive quartz sand (hardness 7) competes, with garnet, olivine and slag-derived abrasive products. Garnet and olivine are preferred, because, quartz and slags may endanger the health of workers (by silicosis, or hazardous metals such as As, and Pb)., Foundry sand is slightly cohesive sand that, allows manufacturing of moulds and cores for
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352, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 3.30 (Plate 3.30) High-grade industrial sands at Uhry in northern Germany were deposited in a shallow, bay of the Late Cretaceous sea north of the Harz Mountains and chemically upgraded when the sea retreated westwards, and tropical forests covered the area., , metal casting. For this use, sand should contain, cohesive grain fractions (clay and silt) and must, tolerate heat. Specified grain size distribution,, rounded grains and high gas permeability are, expected. Although lower silica content is acceptable (85–95%), carbonate and mica contents, are rejected. Because many of these properties, display considerable variations in natural deposits, foundry sands are increasingly “synthesized”, by mixing pure components such as glass sand and, bentonite or fire clay as a binder. Foundry sands are, also prepared from chromite, zircon, staurolite and, olivine., World production of industrial sand and gravel, in 2009 amounted to 112 Mt. The largest producers (>5 Mt) were USA, Italy, Germany, United, Kingdom, Australia, Poland, France and Spain., 3.21.2 Building sand and gravel, The most important use of building sand and, gravels is the provision of aggregates for concrete., Aggregates are materials composed of natural or, crushed, hard, sound and durable particles of non-, , reactive minerals. This covers sand, gravel and, crushed rock. In comparison with industrial sand,, quality may be lower and the grain spectrum, wider. Alluvial and glacio-fluvial pebble sands are, common near moraines and in terraces and valleys, of rivers. The high competition for land in river, plains, for example by housing and industry,, agriculture, nature preserves, roads and railways,, groundwater extraction, etc., continuously reduces the accessability (Cook & Harris 1998) of, areas for sand and gravel extraction (Figure 3.31)., In spite of substitution by construction and demolition debris, such as used asphalt, bricks and, concrete, by metallurgical slag and combustion, residues of coal power stations, high-quality natural sand and gravel are still needed for critical, concrete structures, such as bridges and high-rise, buildings. Shrinking resources of this material, must be protected, preferably by a strict land-use, planning programme. An alternative is to develop, aggregate sources far from densely populated, areas, such as the megaquarries on the coasts of, Northern Europe and Greenland. This is feasible, for delivery from coast to coast, because maritime
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , Figure 3.31 Various demands of society on fluvial, landscapes diminish accessible gravel and sand resources, to a point were protection by planning authorities and, maritime import from far-away coasts are inevitable., , bulk transport is very economic, but doubts, remain concerning the costs of supplying inland, destinations., Specifications for concrete aggregates are closely regulated and subject to industrial standards (Primel &, Tourenq 2000). Some important points concern:, . the petrographical composition (quartz and, quartz-rich pebbles or fragments are favourable,, whereas weathered, schistose, friable fragments, are deleterious as are pebbles of coal, clay, marl and, alkali-reactive chert, opal and flint: Scott 1993,, Michel et al. 2003)., . Grading, investigated by sieving and illustrated, by cumulative particle size curves. Unfavourable, compositions can be adjusted by adding or substracting a size class, although within limits. A, high fraction of cobbles (>63 mm) or of silt and fine, sand is not favourable., . The grain shape; rounded fragments are preferred to angular or platy ones. Pebbles should, display a high uniaxial compressive strength and, frost resistance., The investigation of potential sand and gravel, deposits is mainly carried out by trenching and, drilling. The aim of investigations is a full understanding of the three-dimensional architecture of, the deposit (Kostic et al. 2007, Miall 1996), of the, quality distribution and of the quantities involved., In a phased approach, widely spaced drilling (500, m) and geophysical line surveys (electrical resistivity sounding) can establish whether sand and, gravel are at all present in the required quantity, (Annels 1991). For detailed exploration, grid sam-, , 353, , pling distance should be less than 200 m. Spacing, can be adjusted (e.g. by calculating variograms) as, soon as sufficient data are available. Although, cheap and quick auger drilling is sometimes, used for sampling gravel and sand prospects, only, wide-diameter, airlift reverse circulation holes, provide reliable samples, both above and below, the groundwater table. Use of water-based drilling, fluids is discouraged because fines will be lost., Vertical sample profiles should be prepared from, a minimum of 5–10 kg material per metre, and this, must be weighed and documented. If larger grains, are present, the minimum size of a truly representative sample expands exponentially – a maximal, grain diameter of 50 mm requires a sample mass of, 250 kg (Berkman 2001). It is important to remember that small samples are always a compromise, that widens error margins (Pitard 1993). Washed, particle size analysis is preferred to dry sieving., The presence and percentage of boulders and, cobbles (>63 mm) is best determined by excavating test pits using a hydraulic excavator. Soil, mechanical properties determine pit slope angles, (Terzaghi et al. 1996) and as a consequence, the, exploitable volume of the deposit., Sufficient volume and homogeneity, acceptable, overburden thickness, depth and a water table, position allowing extraction either in pits or by, dredging, and a favourable distance to potential, buyers are further criteria. Environmental considerations concern foremost the position of the, groundwater table in relation to the deposit and, consequent problems of groundwater protection., It is most important to obtain a valid planning, consent to extract the minerals as early as possible, in the course of investigations. Later restoration, of the land (including pit lakes) must guide all, actions from early stages of development. In the, past, sand pits and rock quarries were too often, abandoned without environmental clean-up., Financial assurance requirements during licensing are one possible enforcing measure., In 2009, 800 Mt of building gravel and sand were, produced in the United States (USGS 2010). This, was one-fifth less than a year before, illuminating, the severity of the financial and economic crisis., Crushed stone, the alternative major construction, aggregate, is increasingly replacing natural sand
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354, , PART II NON-METALLIC MINERALS AND ROCKS, , and gravel, especially in more densely populated, areas of the world. In the USA, a total of 1100 Mt of, crushed stone was produced in 2009, comprising, 70% limestone and dolomite, 14% granite, 7%, basalt and lesser quantities of sandstone, quartzite, marble, volcanic cinder and scoria, calcareous, marl and slate. Most of the total was used as, aggregate for highway and road construction, and, only a small part for chemical and metallurgical, uses, including cement and lime manufacturing., Although reserves and resources are very large,, zoning regulations and alternative claims on land, are expected to cause future hard rock, sand and, gravel operations to be located at increasingly, greater distance from high-population centres., This will, of course, affect prices., , 3.22 SODIUM CARBONATE,, , SULPHATE AND ALUM, , Density g/cm3, Trona, Natron, , Na3H(CO3)22H2O, Na2CO310H2O, , 2.14, 1.45, , In addition to natural sources, the demand for, sodium carbonate (“soda ash” Na2CO3) is satisfied, by synthetic soda made via the Solvay or ammonia-soda process using halite and limestone, (eq. 3.11). “Soda” designates any of the common, sodium compounds such as the oxide, hydroxide,, carbonate and bicarbonate. End use of soda ash is, mainly in glass manufacturing, followed by chemicals, soap and detergents, pulp and paper production, water treatment, bentonite activation,, animal feed and flue gas desulphurization, dechlorination and defluorination., Synthetic soda production by the Solvay, process:, 2NaCl þ CaCO3 ! Na2 CO3 þ CaCl2, , ð3:11Þ, , Sodium carbonate deposits are either beds of, soda carbonate salts (commonly trona) or brines, that are harvested from shallow aquifers and from, playa lake water (cf. Chapter 4.2 “Salt Lakes”)., Trona-forming brine is strongly alkaline with a, , pH >9, alike to existent soda lakes (Eugster &, Surdam 1973; e.g. East African Rift)., The largest deposits of trona occur in the Green River, Basin, Wyoming, USA, with >47,000 million metric, tonnes of exploitable resources. Sediments in the, centre of Eocene Lake Gosiute reflect repetitive, lacustrine expansion-desiccation cycles. They comprise 42 beds of trona and horizons of displacive, intrasediment nodules, formed in shallow playa, lakes. Within an area of >2000 km2, 25 exploitable, trona beds are known (Dyni 1996) with a thickness, attaining 14 m at depths from 120 to over 1000 m, below today’s surface. Interbedded thin dolomitic oil, shales represent deeper water phases. The frequency, of oil shale beds (cycles/metre) and by implication,, the trona beds, is a function of orbital signals including precession, obliquity and eccentricity (Meyers, 2008). Other members of the sedimentary suite are, dolomitic limestone, mudstone and numerous beds, of halite (Fischer & Roberts 1991). The Green River, Basin is the world’s largest producer of soda ash, with, an annual output of 15 Mt of trona ore., , Brines are exploited at Searles and Owens lakes, in California (with co-products salt, sodium sulphate and borax), and from the subsurface in the, Roma area, Queensland, Australia, containing an, average of 14 g of Na2CO3 per litre. In Lake Magadi, in the East African Rift Valley, Kenya, on the foot, of carbonatite volcano Oldoinyo Lengai, Holocene, trona beds are 7 to 40 m thick, sourced from geothermal springs and precipitated by evaporation, (Eugster 1970). Fluorine contents in this trona ore, are significant, occurring in the form of 22 wt. %, villiaumite (NaF) and 6% fluorite. Extraction is, by dredging. The rate of replenishement of the, deposit is reported to exceed production, which, makes Magadi one of the rare examples of literally, sustainable mining. In neighbouring Tanzania, a, project to exploit soda from Lake Natron is disputed, because of possible negative consequences, for the lake’s famous flamingo population., The annual production of soda ash (natural and, synthetic) is 40 Mt/a, but only 12 Mt are primary, mining products, almost wholly from the Green, River Basin. In Turkey, two newly discovered and, very large trona deposits are presently prepared, for production: Kazan with resources of 600 Mt at, 31% trona and nearby Beypazari with 230 Mt at
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 70% trona. China leads in the production of synthetic soda., 3.22.1 Sodium sulphate, In several aspects, sodium sulphate is comparable, to sodium carbonate; both the environment of, formation (as at Searles Lake, USA) and their uses,, such as powder detergents overlap. Two minerals, provide most of industrial supply (apart from, industrial sources, such as lead-acid battery reclamation): Thenardite Na2SO4 with a density, of 2.67 g/cm3, and mirabilite or Glauber’s salt, Na2SO410H2O, D ¼ 1.49 g/cm3. The solubility of, sodium sulphate is a function of temperature., Below 32 C, solubility decreases until mirabilite, precipitates; above the transition point, thenardite, is the stable phase and precipitation is favoured by, rising temperature. This curious behaviour caused, an accumulation of 80 Mt Na2SO4 in Holocene, saline lakes of southern Saskatchewan and southeastern Alberta in Canada. More than 5.5 million, saline lakes are counted in the post-glacial, moraine plains of Canada. Only 100 of them have, permanent salt beds at depth (thenardite, dolomite, and gypsum) or a recoverable harvest of autumnal, coldwater precipitates (mirabilite; Warren 2006)., Other sources of solid sodium sulphate ore are, exploited in Spain and Turkey, and giant deposits, similar to the cold playas of Canada exist in saline, lakes of Siberia and China. In Kara Bogaz Gol, (Turkmenistan), a bed of 3–8 m thick cryogenic, sodium sulphate was exploited that may date from, the ice ages (cf. Chapter 4.2 “Salt Formation, Today”). Primary annual production of sodium, sulphate is estimated at 4 Mt (USGS 2010). Largest, producing countries are USA, Spain, Canada and, Mexico., 3.22.2 Alum salts, Alum salts are a large group of hydrated double, salts, i.e. sulphates of triple-valence ( þ 3) and, single-valence ( þ 1) ions. The alum of greatest, commercial importance is potash alum KAl, (SO4)212H2O. Alum occurrences are not infrequent and for over 2000 years, native alum was, extracted from sulphuric acid-altered felsic volca-, , 355, , nic rocks or weathered pyritic schists. The Swedish alum shales (kerogen and V, U, Mo, Ni-rich, black shales of Middle and Late Cambrian age), were so called for constituting a source of this, important chemical. Since medieval times,, alunite rock (alumstone) KAl3(SO4)2(OH)6 D ¼, 2.6–2.8 g/cm3 was employed for manufacturing, potash alum. Alunite deposits are formed by the, interaction of sulphuric acid with alumosilicates, such as feldspar (eq. 1.5); they are usually volcanogenic (cf. Chapter 1.1 “Volcanogenic Ore Deposits”) or supergene, as a product of acid-generating, sulphide oxidation (cf. Chapter 1.2 “Supergene, Enrichment by Descending Solutions”). With, today’s easy availability of low-cost sulphuric, acid, alum is commonly made from shales, leucite, bauxite or clay. Alum has important uses in, the pharmaceutical, textile, sugar, paper, paints, and other industries. It is also used as a flocculant, in water purification., 3.23 SULPHUR, , Sulphur, , S, , Density 2.05–2.09 g/cm3, , Native sulphur is a pale yellow, brittle solid. It, occurs in two polymorphs: orthorhombic sulphur, is stable at low temperatures, which is the reason, why it is sometimes called “sedimentary” sulphur. The monoclinic polymorph of sulphur is, quite rare in nature. It is stable above 95.5 C and, typically crystallizes from melt (“volcanic, sulphur”). Sulphur melts at 115 C and boils at, 445 C. Sulphur liquid density varies from 1.8 g/, mL just above the melting point to 1.6 g/mL at the, boiling point. All native sulphur ages in time to, the orthorhombic modification. Often, native sulphur contains trace amounts of arsenic, selenium, and tellurium., Native sulphur is not the only feasible source of, the element. Ferrous and base metal sulphides have, contents between 53.3% S (pyrite) and 13.4% S, (galena), and metallurgical fabrication of the metals results necessarily in co-production of sulphur, or sulphuric acid. The metallogeny of these deposits is presented in Part I of this book. Coking plants
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356, , PART II NON-METALLIC MINERALS AND ROCKS, , and desulphurization of coal-burning power plant, fumes also recover by-product sulphur. Sulphur, might be extracted from anhydrite (23.5% S) and, gypsum (18.6% S) (cf. Section 3.13 “Gypsum and, Anhydrite”), but this is economically not feasible., The main source of industrial sulphur, however,, is petroleum refining and sour natural gas processing, which make up >70% of annual production., In sour gas, hydrogen sulphide (H2S) is a natural, component, whereas petroleum yields H2S during, refining. Hydrogen sulphide is converted to elemental sulphur by the Claus process (eq. 3.12)., Almost all of the remaining 30% is produced as, SO2-gas from non-ferrous metal smelters and the, roasting of pyrite, which is the most economic, path of sulphuric acid production. Mining of elemental sulphur is now rare., Conversion of H2S to elemental sulphur by the, Claus process:, 2H2 S þ O2 ! 2S þ 2H2 O, , ð3:12Þ, , Demand for sulphur depends primarily on the, activity of the phosphate fertilizer industry (cf., Section 3.19 “Phosphate”). Sulphuric acid is, needed to produce superphosphate from phosphorite, apatite and ammonium sulphate. Over 90%, of all recovered sulphur is converted to sulphuric, acid (“the work horse of the chemical industry”),, which is one of the most widely used of all chemicals. For example, sulphur and sulphuric acid, are essential for making insecticides, dyes, pharmaceuticals, explosives, rubber and for preserving, food. In the future, climate engineering (Wigley, 2006) may be a new use of sulphur, because injecting aerosols or aerosol precursors (i.e. SO2) into, the stratosphere can provide a negative forcing of, the climate system and offset part of the positive, forcing that is attributed to increasing greenhouse, gas concentrations in the atmosphere., 3.23.1 Geochemistry, Sulphur is a nonmetallic element of lithophile, character. Its crustal abundance is estimated at, 500 ppm (range 260–1200 ppm: Smith & Huyck, 1999), which is about equal to chlorine and, , fluorine. Large crustal reservoirs of sulphur, include evaporites and seawater (oxidized,, S6 þ ), as opposed to sulphides and organic substances, including hydrocarbon fluids (reduced,, S2). Both oxyanions and hydrogen sulphide are, rather mobile, but sulphates and metal sulphides, tend to immobilize sulphur. Intermediatevalence sulphur species prevail in the shallow, marine diagenetic setting. Sulphur is essential to, life. It is one of the six major elements – H, C, N,, O, S and P – that are required to build all, biological macromolecules (Falkowski et al., 2008). The biological fluxes of the first five of, these elements, including sulphur, are driven, largely by microbially catalysed, thermodynamically constrained redox reactions. Geological, cycling of sulphur can be illuminated by studying isotopic fractionation (cf. Chapter 1.1, “Isotope Geochemistry”)., 3.23.2 Deposit types of elementary sulphur, Deposits of elementary sulphur occur (1) by, magmatic degassing in volcanic settings and (2), as a product of microbial sulphate reduction of, gypsum/anhydrite in the presence of organic matter, petroleum and natural gas (cf. Chapter 1.3, eq., 1.15). Occasional encounters of liquid sulphur in, deep drillholes traversing anhydrite-carbonate sediments are of scientific interest (Hunt 1996). This, sulphur is the result of abiotic thermochemical, sulphate reduction (eq. 1.21; for details refer to, Chapter 1.4 “Diagenetic Ore Formation, Systems”)., Volcanogenic sulphur deposits form by sublimation and precipitation of elemental sulphur on, crater walls, near solfataras, at H2S-fumaroles and, hydrothermal sulphurous springs (e.g. Vulcano,, Italy). This is an oxidation/reduction process, (eq. 3.13). Some sulphur may collect in gas flow, channels and in brecciated and porous wall rocks, (Figure 1.47). Later heating to temperatures above, the melting point causes formation of liquid sulphur flows, which are known to occur from time to, time on several volcanoes in Chile and Japan., Mining of volcanic sulphur is, however, mainly, based on disseminated sulphur in volcaniclastic, rocks.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , in Sicily declined and the last mine working to a, depth of over 300 m closed in 1988:, , Abiotic formation of elemental sulphur:, 2H2 S þ 3O2 ! 2SO2 þ 2H2 O, 2SO2 þ 4H2 S ! 6S0 þ 4H2 O, , ð3:13Þ, , Sedimentary or more precisely, diagenetic and, biogenic sulphur deposits are formed by reduction, of SO4-sulphur in sulphate rocks (gypsum, anhydrite). This is catalysed by anaerobe microbes in, oxygen-depleted settings. The process is mediated, by consortia of sulphate-reducing bacteria and, archaea. Common sulphate-reducing bacteria, include Desulphovibrio desulphuricans and Clostridium nigrificans. In this context, organic matter is the source of energy, frequently provided by, diagenetic migration of oil or gas. Bacteria assimilate dissolved (SO4)2 and hydrocarbons into their, system and secrete H2S and CO2. Equation 3.14, describes this process for the case of anaerobic, methane oxidation. The role of archaea is not yet, fully understood., Anaerobic methane oxidation and microbial formation of elemental sulphur:, SO42 þ CH4 ! HS þ HCO3 þ H2 O, 2HS þ 0:5O2 ! 2S0 þ H2 O, , 357, , ð3:14Þ, , HS is oxidized to elemental sulphur, whereas, HCO3 reacts with calcium released from the sulphate rock by precipitating secondary aragonite or, calcite. Oxidation of HS- is possible by reaction with, SO4, by mixing with oxygen-bearing meteoric, water, or by autotrophic bacteria that use part of, the resulting CO2. The participation of biota in, these processes is illuminated by the enrichment, of the light isotopes 32 S and 12 C in the products, (sulphur, limestone) compared with the source, material (gypsum, hydrocarbons). One mole of, gypsum/anhydrite yields 1/4 sulphur and 3/4 white,, massive or cellular limestone. The calcite cap rocks, above salt diapirs are often major hydrocarbon traps., This coincidence led to the discovery of sulphur., Sulphur in shallow marine sediments, such as, the deposits of Sicily formed by similar diageneticbiogenic processes. For many centuries in the past,, Sicily was the leading source of sulphur. Since, 1900, pyrite roasting and sulphur extraction, from salt domes in the Gulf region of the southern, USA began to dominate markets. Sulphur mining, , Sicilian sulphur was exploited from Miocene (Messinian) evaporitic sediments of the Caltanisetta and, other basins. The sedimentary sequence comprises, basal clay, bituminous diatomite (called “tripoli”),, overlain by limestone, gypsum/anhydrite, halitite, (rock salt) and potassium salt, which are covered by, more sulphates, gypsarenite, calcarenite, mudstone, and calcareous marl (Zuffardi 1989). The whole, package is strongly folded and faulted. Sulphur, orebodies occurred in sulphate rocks as stratiform, lenses of 1–2 m thickness (max. 30 m). Ore formed, thin veinlets of sulphur parallel to bedding planes, and joint-controlled pockets. Run-of-mine ore comprised native sulphur, sulphate, carbonate, bitumen,, rare sulphides and celestite. Sulphur contents used to, be 20–25 wt.% S at a cut-off grade of 15% S., In the Gulf regions of southern North America,, native sulphur occurs in the caprock of salt diapirs, that are mantled by Miocene and Pliocene mudrocks, sandstone and limestone. Because some of, the earliest bonanza oil fields in America had been, found in caprock and in updomed host sediments of, diapirs, the occurrence of sulphur was known., Extraction, however, became only feasible when, Herman Frasch introduced a novel hot-water melting process at Sulphur Mine, Louisiana, in 1894., Extraction by the Frasch process involves pumping, overheated water (with 150–165 C far above the, melting temperature of sulphur) at high pressure, downhole into the orebody; molten sulphur is, brought to the surface via an inner pipe. Meanwhile,, more than 100 similar onshore and offshore deposits, were found. Typically, the sulphur-bearing caprock, displays a thickness of 40–120 m and is traversed by, numerous lenses, beds, pockets and veins of elementary sulphur (Figure 2.32). Economic grades were, 20–40%. Remaining resources are gigantic, in, spite of 100 years of intense exploitation. Fraschmining of sulphur ceased, however, because of its, high energy costs compared to by-product sulphur, from the hydrocarbon industry., Large, at present uncompetitive occurrences of native, sulphur resembling those of the Gulf occur in the, Carpathian foredeep of southern Poland, Romania, and Ukraine. The province forms a narrow corridor, that extends over 1500 km length and contains, a sizeable part of world resources of native
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358, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 3.32 Schematic section showing, formation of native sulphur deposits in, caprock of salt diapirs in the Gulf of Mexico, region. Black arrows indicate flow of, hydrocarbon fluids. Not to scale. After, Ruckmick, J.C., Wimberly, B.H. & Edwards,, A.F. 1979, Society of Economic Geologists,, Inc., Economic Geology Vol. 74, Figure 1,, p. 471., sulphur. Concentrations are associated with Miocene, evaporites that are interbedded with clay, marl and, sand of the Neogene molasse basin. Here also, formation of sulphur is related to migrating hydrocarbons, and appropriate flow paths (Parafiniuk et al. 1994)., This connects diagenetic-biogenic sulphur mineralization with petroleum traps, although closure by beds, of low permeability is rarely observed. On the contrary, an open system appears more favourable, because it provides access of meteoric water, oxygen, and seed microbes., , In 2009, world production of sulphur amounted, to 70 Mt. Of this total, about two-thirds were, “recovered sulphur”, which designates by-product, sulphur of sour natural gas processing, sour crude, refining, tar sands processing and stack gas cleanup. “Mined sulphur” includes elemental sulphur, recovered by conventional mining and the Frasch, method. Mined sulphur production continues to, decline. Pyrite mining yields annually 6 Mt and, non-ferrous metal smelters 18 Mt. In the future,, the share of sulphur derived from oil and gas, processing is expected to grow, because of an, overall increase of the quantity treated and of, higher sulphur concentration in crude petroleum, and gas. Sulphur resources are very large., , 3.24 TALC, , Talc, , AND PYROPHYLLITE, , Mg3(OH)2, Si4O10, , D ¼ 2.7–, 2.8 g/cm3, , Mohs hardness 1, , In talc, small amounts of Al and Ti may substitute for Si, and Fe (II) may replace some of the Mg., Traces of iron and nickel in talc lend it a greenish, colour; grey-green talc rock is rich in chlorite. It is, rare to find monomineralic talc in exploitable, volumes. More common are talc rocks with, a gangue of colourless chlorite (leuchtenbergite,, a magnesium-rich clinochlore containing little or, no iron), magnesite, dolomite, amphibole, quartz, and pyrite. Steatite is a dense unfoliated talc rock,, which can be worked by carving. Soapstone is, another term for steatite but is also applied, to metamorphic talc-mica-chlorite-amphibolepyroxene rocks, or in Norway, to talc-carbonate, rocks, which are used for cutting dimension stone,, fire places and ovens (Sturt et al. 2002). Talc, schists are foliated schistose talc rocks., Talc has a number of unique properties including softness, chemical stability, refractory nature, and low thermal and electrical conductivity., Applications of talc are numerous. Much is consumed in production of fine and electrical ceramics, paints (as extender), paper (for stabilizing, resin, as a filler and coater), refractories, plastics, (filler), insecticides, cosmestics, pharmaceuticals,, rubber and roofing fabric. Specifications vary with, use. Talc destined for paper or as filler must have, a high whiteness. Ceramics demand high SiO2 and, MgO contents as well as a favourable firing behaviour. Cosmetics require high talc grade and absolute absence of acicular (asbestiform) minerals. In, most applications of talc, asbestiform fibres are, acceptable but enforce costly precautions during, extraction and processing. The content of talc in
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , some commercial products is of little importance,, whereas colour, whiteness, adherence to narrow, grain size specifications and an extremely high, specific surface may be crucial., Like most sheet silicates, talc and talc rocks, have a very low shearing resistance. Consequently, open cut mining slopes are prone to collapse, as are underground stopes. The economic, fate of a mine may depend on delivering a constant, quality of run-of-mine ore to the processing plant., Even large deposits still use handpicking for highest quality material, apart from grinding, flotation, and optical laser-based sorting for the majority of, run-of-mine material. Talc is an environmentally, benign mineral and talc mining has a very minor, impact on the environment., 3.24.1 Talc deposit types, Most talc deposits originate by hydrothermal addition of silica and water (OH) to Mg-rich ultramafic rocks or carbonates (dolomite, magnesite), resulting in replacement talc ore. The majority of, deposits are generated during orogenic metamorphism (they are metamorphogenic). A smaller, number is due to magmatic-hydrothermal contact-metasomatic processes., Talc is part of the metamorphic paragenesis in, siliceous metadolomite, where it forms in a relatively closed system at high lithostatic pressures., Talc deposits with high concentrations of the, mineral and large volumes are chacteristically, controlled by shear zones and faults (similar to, the San Andreas Fault in California: Moore &, Rymer 2007). Seismic pumping (Sibson 1990) is, probably involved and phases of free hydrothermal, flow and low, hydrostatic pressures may be essential, because import (SiO2, Mg) and export (Ca,, CO2) of matter must be achieved. Hydrothermal, synthesis of talc was demonstrated at temperatures above 300 C. Kinetic limitations occur, below that temperature. In nature, talc is definitely formed at lower temperature, including the, diagenesis of evaporites and the saprolite zone of, soil formed on ultramafic rocks (cf. Chapter 1.2, “Nickel Laterites”)., For the hydrothermal formation of talc from, ultramafics, magnesium-rich rocks such as mag-, , 359, , nesian dunite are more suitable than iron and, aluminium-rich varieties. The talc-forming reaction is summarized in equation 3.15., Hydrothermal formation of talc from dunite:, 3Mg2 SiO4 þ 5SiO2ðaqÞ þ 2H2 O! 2Mg3 ðOHÞ2 Si4 O10, Forsterite, Talc, ð3:15Þ, Clearly, the addition of dissolved silica is needed, for the formation of talc from dunite/olivine., Because most ultramafic rocks contain pyroxene, besides olivine, talc formed from these rocks will be, associated with retrograde amphibole that may, be acicular. Chlorite and serpentine are common, gangue minerals. If the hydrothermal fluids contained CO2, carbonate minerals are part of the, paragenesis (eq. 3.16). Talc derived from ultramafic, rocks often displays iron contents in the percent, range, and traces of nickel and chromium. Some, ophiolites comprise huge masses of metasomatic, talc-carbonate rocks (e.g. those of the Pan-African, Orogen in Saudi Arabia and Northeast Africa). The, bulk of these rocks cannot be utilized, having low, talc and high iron concentrations., Talc-carbonate rock formation by carbonation of, serpentine:, 2Mg3 Si2 O5 ðOHÞ4 þ 3CO2 ! Mg3 ðOHÞ2 Si4 O10 þ, Serpentine, Talc, 3MgCO3 þ 3H2 O þ 1:5 O2, Magnesite, , ð3:16Þ, , Exploitable talc concentrations in ultramafics, typically follow structures that controlled hydrothermal flow, such as stockwork bodies, but many, are relatively small and irregular veins. Worldwide,, 30% of talc is derived from ultramafic rocks:, In eastern Finland along an Early Proterozoic rift and, ophiolite belt marked by copper and nickel deposits, (Outukumpu and Talvivaara) large ultramafic-hosted, talc deposits are exploited. Locally, metamorphic, fluids converted Svecokarelian ophiolites into talcbreunnerite soapstone and talc schist with >50%, talc. Near Lahnaslampi, a vertically elongated lens, of this material with a cross-section of 200 500 m
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360, , PART II NON-METALLIC MINERALS AND ROCKS, , contains >30 Mt of talc. Flotation yields 200,000 t/, year of talc products and 1000 t/y nickel concentrate, as a by-product from accessory sulphides., , Carbonate-hosted talc deposits, Carbonate-hosted talc deposits provide 70% of, world production. Talc is formed from dolomite or, magnesite by reaction with hydrothermal SiO2, solutions. With magnesite, this is easy to understand. However, even replacement of dolomite, by talc is possible without the need to add, magnesium (eq. 3.17)., Hydrothermal formation of talc from dolomite:, 3CaMgðCO3 Þ2 þ 4SiO2ðaqÞ þ 4H2 O þ 6H þ !, þ, Mg3 ðOHÞ2 Si4 O10 þ 3Ca2ðaqÞ, þ 6H2 CO3, , ð3:17Þ, Large metasomatic talc bodies can only form in, an open system that allows evacuation of Ca and, CO2, because otherwise the reaction would soon, cease. Theoretically, talc formation according to, this reaction should cause a volume decrease of, 14% and drusy textures should develop. This, was never observed, however. As an explanation,, some scientists propose hydrothermal supply of, magnesium in addition to SiO2 derived from outside of the system. Observations of talc replacing, non-magnesian rocks such as limestone, and even, non-carbonate rocks (e.g. quartzite and radiolarite, in deposits of northern Spain: Tornos & Spiro, 2000) are also cited as evidence of extraneous, Mg-supply called “magnesium metasomatism”, (cf. Section 3.16.1 “Magnesite”). Most probably,, both alternatives occur in nature – derivation from, , the rock in situ, and hydrothermal supply from, outside. Hydrothermal fluids of carbonate-hosted, talc deposits often are saline brines. One plausible, explanation is that dolomite and magnesite rocks, hosting talc deposits may be residual members of, evaporites. It is also possible that migrating fluids, dissolved salt elsewhere (Powell et al. 2006)., Talc in carbonate rocks is generally of better, quality compared to ultramafic-hosted talc. Common gangue minerals are chlorite (if metapelites, are involved) and carbonate. Trace element concentrations are very low. Dolomites host larger, talc deposits than magnesite, because of the commonly denser fracturing, which made the original, rockmass more permeable for hydrothermal solutions. Of course, dolomite is also more common, compared to magnesite. Talc in magnesite tends to, occur in shells around cores of magnesite rock., Trimouns, Luzenac in the French Pyrenees is the, largest dolomite-hosted talc deposit in Europe (Moine, et al. 1989). At 1700 m above sea level, the talcbearing zone extends for more than 5 km along the, eastern slope of the Massif of St Barthelemy. Its, footwall is formed by mica schists and migmatite, (Figure 3.33, Figure/Plate 3.34). Crystalline basement, rocks are overlain by an epizonal metamorphic suite, of dolomite (Ordovician), sericite schist (Silurian) and, calcite marbles with bands of quartzite (Devonian)., The sediments were overthrusted as a nappe during, the Variscan Orogeny (Late Palaeozoic). In the Cretaceous, an Alpidic (Pyrenean) transtensional deformation affected the region and the thrust plane, experienced renewed intense shearing. The Ordovician dolomites are partially replaced by talc and in the, shear zone, blocks of dolomite, quartzite, mica schist,, gneiss and pegmatite float in a mass of talc. The talc, zone reaches a thickness of 80 m and dips with 40–80o, , Figure 3.33 Simplified geological profile of the talc deposit at Trimouns, Luzenac in the French Pyrenees (adapted, from Sch€, arer et al. 1999). Mg-chlorite fels derived from micaschist and pegmatites; Talc (white) derived from, dolomite; Dol – Ordovician dolomite; Sil – Silurian black schists; Dev – Devonian calcite marbles; Q – Quaternary., For location refer to Figure/Plate 1.89.
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , 361, , early tectonic phase of deep burial (as shown in Figure, 1.89), whereas talc was formed later at 350–400 C and, 3 kbar (Pohl & Belocky 1999). A connection with deep, fluid systems and metamorphism provoked by the, Cretaceous nappe formation in the Eastern Alps appears likely., , Figure 3.34 (Plate 3.34) Eastward bird’s eye view of, the talc quarry at Luzenac in the French Pyrenees. White, talc marks the working face. Philippe Psaila/SPL/, PictureDesk.com., to the East. The footwall part of the deposit consists of, 70–90% grey chlorite and 10–30% talc. Pure talc, (>80%) occurs only in the hanging wall part. Chlorite, is derived from silicate rocks by magnesium metasomatism (Moine et al. 1989), whereas talc clearly, replaces dolomite which is best illustrated by, banded talc that passes into equally banded dolomite. Fluid inclusions reveal formation by saline, brines (30 wt. % NaCl) at 300–350 C and 2–3, kbar. The hydrothermal system was unusually longlasting, active for at least 16 million years (112–97 Ma)., This was supported by simultaneous Pyrenean tectonics, fluid migration and metamorphism (Sch€arer et al., 1999). Talc resources of the Trimouns deposit are, estimated at 20 Mt., The Rabenwald deposit in Eastern Austria also occurs, in a shear zone, in this case, however, within Lower, East Alpine basement gneisses (Figure 1.89). The shear, zone dips with 6o southeast, almost parallel to the, slope on which talc is exploited in an open pit. The, shear zone reaches a thickness of tens of metres and, consists of large blocks of magnesite that float in, schistose talc-chlorite rock. The magnesites are, thought to be remnants of Palaeozoic sediments (cf., “Magnesite”) which were overthrust and thinned out, by nappe movement. Magnesite was consumed by talc, formation and only single blocks remain (Prochaska, 1989). Host gneiss was hydrothermally altered and, transformed to fine-grained quartz-phengite-chlorite, (-kyanite) rock. Paragenesis and fluid inclusions data, support formation of the deposit at 500–550 C and, 8–9 kbar (Moine et al. 1989). However, the extreme, conditions probably have prevailed only during an, , In several talc districts, granite intrusions, caused formation of talc deposits although, of, course, granites do not release Mg-rich fluids., Consulting the reactions above (eq. 3.15 and 3.17),, the explanation is straightforward: The intrusions, establish hydrothermal systems with dissolved, silica. On interaction with ultramafic or dolomite, rock, talc can be the result (e.g. G€, opfersgr€, un in, Southern Germany:Hecht et al. 1999):, Skarn-related hydrothermal deposits of talc in the, metamorphic aureole of granite occur in Korea (Shin, & Lee 2002). Near Hwanggangri a Cretaceous granite, intruded Cambro-Ordovician metasediments and, produced a wide skarn zone. During the anhydrous, phase of skarn formation, dolomite was transformed, into magnesian calcsilicates. Subsequent flow of, magmatic fluids at T<400 C through permeable, structures in Mg-skarn provoked talc formation. The, authors suggest that the magnesium of the talc is, inherited from dolomites., , 3.24.2 Pyrophyllite, Pyrophyllite Al2(OH)2Si4O10 (density 2.65–2.85 g/, cm3, hardness 1–1.5) resembles talc closely. In a, number of applications it is exchangable with talc., When calcined, its refractory stability extends to, higher temperatures, including those common in, the iron and steel industry. It occurs in foliated, (phyllitic) form and as compact wax-like masses., When heated, pyrophyllite exfoliates strongly but, positive identification requires X-ray diffraction, techniques. With few exceptions, pyrophyllite is, the result of advanced argillic hydrothermal alteration affecting intermediate to felsic volcanic,, mainly volcaniclastic rocks (Herrmann et al., 2009). At Pueblo Viejo, for example, pyrophyllite, was induced by magmatic volatiles condensing, into meteoric water. Similar to kaolinization, the, process is characterized by depletion of silica,, alkali elements, calcium, magnesium and iron.
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362, , PART II NON-METALLIC MINERALS AND ROCKS, , Pyrophyllite is also a common metamorphic, mineral formed at low to moderate pressure, and T <400 C. Pyrophyllite replaces kaolinite, at 200–300 C by reactions such as 1 kaolinite, þ 2 quartz ! 1 pyrophyllite þ H2O. At higher, temperature (400 C) pyrophyllite decomposes to, alumosilicate (kyanite or andalusite), quartz and, H2O (Bucher & Frey 2002). Numerous deposits of, pyrophyllite occur in Japan and Korea. In USA and, Canada, Precambrian volcanics host important, pyrophyllite deposits., World production of talc and pyrophyllite in, 2009 was 7.2 Mt, mainly from China, the Republic, of Korea, India, USA, Finland and Brazil (USGS, 2010). Available statistics rarely differentiate, between talc and pyrophyllite., 3.25 VOLCANICLASTIC, , ROCKS, , Volcanic processes create a wide diversity of, mineral deposits, including numerous metallic, and non-metallic resources. Bentonite, borates, and zeolites are valuable industrial minerals with, close genetic relations to volcanism. The heat of, shallow magmatic bodies is a prolific source of, geothermal energy. Under suitable climate conditions, volcanic soils are among the worlds’ richest, for farming. Here, we shall briefly describe the role, of quite common volcanic rocks that meet essential needs of human societies., Volcaniclastic rocks comprise all fragmented, volcanic material, whatever its origin, dispersion, and depositional sites. One example is naturally, cemented ash (tuffite) which is often used as a, building stone, apart from many other applications, (Heiken 2006). Note that tuffites are the host rock, for the proposed high-level radioactive waste repository at Yucca Mountain in the southwestern USA., Scientific and more practical data resulting from 20, years of investigations at this site are singularly, comprehensive (Stuckless & Levich 2007)., Volcaniclastic rocks provide much of the, “crushed stone and aggregate class” of raw materials for the building industry (cf. Section 3.22, “Quartz Sand and Gravel”). Of course, the chemical and mineralogical composition of volcanic, rocks varies widely. Generally, loose and cemented volcanic ash contains crystals, glass shards,, , pumice and lithic fragments in variable proportions. As concrete aggregates, uncemented ash, (<2 mm grain diameter) and lapilli (2–64 mm), are preferred substitutes for alluvial sand and, gravel. In this application, favourable grain size, and mechanical properties are positive criteria,, whereas the presence of concrete-damaging substances (salt, sulphate, alkali-reactive silica) is, negative. Preliminary petrological examinations, of aggregate resources are indispensible. Any, material that contains >0.25 wt. % opal, >5%, chalcedony, >3% glass and cryptocrystalline, acidic to intermediate material produces a severe, alkali-silica reaction in concrete (expansion and, cracking by formation of a hydrous gel), unless, low-alkali cement is used. The alkali-aggregate, reaction can be avoided by including pozzolanic, supplements in the aggregate mix, which immobilize Na2O and K2O., 3.25.1 Pumice, Pumice consists of loose and unaltered volcanic, rocks that originate by frothing of intermediate to, felsic melts. Pumice particles consist essentially, of highly vesicular volcanic glass. Fine ash and, blocks (>64 mm) are of little use and the main, economic interest lies in lapilli (2–64 mm). In, felsic systems porosity of pumice reaches 90%, and the material floats on water. In the building, industry, grain diameters of 2–16 mm are preferred. Of course, grain classes are separated and, mixed to consumers’ specifications. Weathered or, hydrothermally altered material is rejected (except, for pozzolanic rocks, see below). Bulk weight, should be below 1000 kg/m3, because the most, common application of pumice is the production, of lightweight concrete by mixing it with lime or, cement. Building blocks made from this mixture, have a high heat and acoustic insulation capacity., Minor uses include horticulture, filtering and, artificial aging (stonewashing) of cloths such as, jeans. Industrial pumice mining is centred in Turkey, Italy, Greece and Chile. Resources are very, large. World production in 2009 was estimated at, 20 Mt (USGS 2010)., After fallout, volcaniclastic rocks may experience various alterations, such as the influence of
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , acid (hydrothermal) or alkalic waters (seawater,, playa lake brines), elevated temperature and, humidity (aging, supergene alteration). This leads, to formation of mineral deposits such as bentonite, and zeolite, which are described in special sections, of this chapter. Other useful products of alteration, include perlite and trass., 3.25.2 Perlite, Perlilte is a hydrated glass-rich volcanic rock of, rhyolitic composition, with a perlitic texture. It is, generally derived from lava flows or resurgent domes, rarely from pyroclastics (Shackley & Allen, 1992). The perlitic texture is often a visible onion, peel-like concentric cracking, caused by expansion, of the glass upon hydration. Water molecules diffuse into the glass in exchange for Na þ followed by, K þ , which move out of the system. Perlitization, commences at 3 wt.% H2O. Obsidian (with less, water) and pitchstone (with much more water to, 10%) are different from perlite, but transitions are, reported, for example in Iceland: Perlites formed by, hydrothermal alteration of rhyolite extruded, beneath glaciers in Iceland display 2–8 wt.% water, content, with perlitization increasing at higher, water concentrations. The water is of meteoric, derivation (Denton et al. 2009)., Rapid heating to 900–1150 C at atmospheric, pressure expels the water, while the glass softens, and expands. Expanded perlite is a white, lightweight and durable material that resists decay., Similar to pumice, it is used as an aggregate for, making lightweight concrete for heat and sound, insulation. There are many other applications,, such as producing paint, ceramics, foundry sand,, drilling muds, filters, abrasives, matrices for, hydroponic plant culturing and loose filling, material in insulation and packaging. Usability, is mainly controlled by the swelling capacity,, which is measured by comparing the bulk density, of raw and swelled material. In the field, the, swelling aptitude of prospective rocks can be, estimated by heating grains of 1–2 mm, diameter with a blowtorch. Volume increase, should be a minimum of 6–10 times; even 20, times is quite common. Typical traded perlite, weighs 80 kg/m3., , 363, , Perlite deposits and resources exist in many, regions with widespread felsic volcanism. Similar, to pumice, transport costs tend to limit international trade. Available statistics reveal Greece,, USA, Turkey and Japan as leading producers, (world production 1.7 Mt in 2009). No data are, available for China and other likely producing, countries. Greece hosts very large (the worlds’, largest) and high-quality resources in the Aegaean, Sea, especially on Kos and Milos islands., 3.25.3 Trass, Trass is not a petrological term, but designates, non-welded massive ash and pumice deposits, (rarely lavas), which have acquired pozzolanic, properties by hydrous alteration. Trass is one of, the pozzolanic raw materials. This term is derived, from a deposit of natural cement (leucite tuff) near, the village of Pozzuoli, Naples (Italy). The namesake deposits seem to have been exploited in, ancient Rome as early as 2300 years ago. The, words cement and concrete are derived from Latin, caementum and concretus. Pozzolanic materials, are either natural or man-made siliceous, often, amorphous substances that react readily with calcium hydroxide, which is a by-product of the, hydration of Portland cement. Calcium hydroxide, or calcite produced by ageing are relatively soluble, if concrete is submerged in water. In contrast,, calcium silicates resulting from the reaction with, pozzolans are insoluble in water and the concrete, is durable. The essential property of pozzolans is, an elevated content of reactive SiO2 (and Al2O3), phases. Apart from trass, natural pozzolans, include chert, calcined diatomite and even oil, shale ash. Man-made are fly ash, dehydrated kaolin (“metakaolin”) and silica fume (a by-product of, silicon metal or ferrosilicon alloy manufacturing)., For application, trass is finely ground and mixed, with Portland cement or lime making the concrete, harden, even under water. The pozzolanic properties of hydrated tuffs are best determined by serial, tests in the laboratory. Sampling must make certain that the natural heterogeneity of altered volcanic rocks is sufficiently constrained. Trass, is always extracted in open pits. Upgrading by, processing is not possible.
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364, , PART II NON-METALLIC MINERALS AND ROCKS, , The genetic variability of trass deposits can be shown, by reference to Western Europe: The deposit Andernach at Laacher See, Eifel, Germany originated in the, late Pleistocene during the giant eruption of a maar, volcano (Schmincke et al. 1999). Hot phonolitic pumiceous ash was deposed in a shallow lake. The material consists of analcite, phillipsite and SiO2 glass, phases (Liebig & Althaus 1998). The bed has a thickness of 9 m and covers an area of 8 by 2.5 km. Trass is, also exploited in the Miocene impact structure, N€, ordlinger Ries in southern Germany. This material, is derived from suevite, a polymict impact breccia, containing sedimentary and crystalline lithic fragments, and glassy impact-melt shards in montmorillonite matrix (Liebig & Althaus 1998). Suevite was, pervasively altered by alkaline hydrothermal fluids, that produced montmorillonite, saponite, illite, analcite, erionite and clinoptilolite (Osinski 2005). In, Austria, trass is extracted from the flank of a Miocene, trachyandesite volcano near Gossendorf. Pyroclastic, rocks were strongly altered by acidic hydrothermal, fluids (Klammer 1997). This material consists essentially of alunite, kaolinite and opal-C/CT phases., , As reported above, only pumice and perlite are, considered in international statistics. Volcanic rocks, used in the building industry or in manufacturing, cement are part of the giant mass of crushed rock, that is briefly quantified in the description of, “Quartz Sand and Gravel” in Section 3.22., , 3.26 WOLLASTONITE, , Wollastonite, , CaSiO3, , Density 2.8–3.0 g/cm3, , Calcium in wollastonite may be substituted by, several percent of Fe and Mn, and lesser Mg or Al., This can reduce the value of the product, either by, lowering whiteness, or by causing less desirable, electric, physical and chemical properties., Frequent gangue minerals of wollastonite are diopside-hedenbergite, tremolite (beware of asbestiform varieties), vesuvianite, grossular-andradite,, graphite, quartz, plagioclase and calcite., Exploitable wollastonite ore has a minimum content of 30% CaSiO3. Saleable by-products (e.g., garnet sand, calcite) allow mining lower grades., , Usually, wollastonite occurs in coarse, bladed, masses, but after crushing and grinding the mineral is acicular to fibrous. The needles have a high, strength that is the key to many uses of wollastonite. It is applied as a component or filler in the, production of ceramics (especially tiles), plastics, and rubber, paints, adhesives, isolating material,, in ceramic abrasives and building elements., Fibrous wollastonite substitutes for asbestos, as, in paint, refractory materials and in welding rods., The most important properties of processed wollastonite are: i) percentage and shape of fibres, (length/thickness); and ii) reflectivity and whiteness. Depending on the intended use, very low, chemical reactivity and weight/volume loss due to, heating, and the mineral’s alkaline reaction in, emulsified state (pH 9.9 in 10% water slurry) may, play a role., 3.26.1 Wollastonite deposit formation, Wollastonite is a product of contact-metasomatism (skarn-formation) of limestone and calcite, marble. Isochemical regional metamorphism of, siliceous limestone does not form sufficiently, high grades of wollastonite. Although it is theoretically possible that the silica in skarn is derived, from impurities in the original calcite rock, investigations show that it is commonly introduced, by magmatic-hydrothermal fluids (Grammatikopoulos & Clark 2006). Exotic elements in the, skarn; such as W, Cu and Au; serve as monitors, for the passage of magmatic fluids. Wollastonite, forms according to reaction 3.18, at T 500 to, >700 C and P at 100–500 MPa (1–5 kbar); corresponding to highest-temperature hornblende, hornfels and pyroxene hornfels facies., Contact-metasomatic formation of wollastonite:, Calcite þ SiO2 ðaqÞ ! Wollastonite þ CO2 ðaqÞ, ð3:18Þ, CO2 must leave the system. as otherwise the, reaction would stall. In a closed system, quartz and, calcite can coexist stably side by side, even under, conditions of the granulite facies. Therefore,, mass transformation requires an open system. For, wollastonite formation, the fluid phase must be
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , mainly aqueous with a low CO2-content (XCO2, <0.3). High fluid/rock ratios are needed in order to, dilute and export the CO2 generated by the reaction. This prerequisite for the formation of a wollastonite orebody is achieved by a considerable, mass flow of magmatic water outwards through, the nascent wollastonite zone. Because during, the prograde stage of contact metamorphism,, decarbonation reactions and high XCO2 prevail,, conditions of wollastonite formation are mainly, realized during the retrograde path (Nabelek 2007)., Exploitable wollastonite concentrations are not, common. The first mines were established in, 1943, in the Willsboro district of the Adirondack, Mountains in New York State. Until today,, this district is a globally significant producer. The, deposits occur in the Mesoproterozoic Grenville, Orogen (Tollo et al. 2004):, Many of the wollastonite deposits in the Adirondacks, occur near the contact of anorthosite with calcite marble. The Valentine deposit and many smaller occurrences are the product of a quartz-syenitic intrusion, into granulite-facies metasediments (Gerdes & Valley, 1994). At the contact, calcite marbles were transformed, into a skarn shell 60 m wide, that contains coarsely, crystalline wollastonite and minor amounts of calcite,, diopside and secondary prehnite. Reserves are >7 Mt, ore. Megascopic boundaries between the wollastonite, skarn and calcite marble are irregular in shape but, sharp. Oxygen isotope ratios in calcite marble remain, unchanged right up to the skarn boundary. As there is, hardly any silica in the marbles the SiO2 must have, been delivered by hydrothermal fluids., An important wollastonite deposit is exploited near, Lapeenranta in southern Finland, not far from calcite marble deposits at Ihalainen. At Lapeenranta, a, large body of calcite marble with bands of finegrained quartzofeldspathic metavolcanic rock, (“leptite”) floats in a giant intrusion of Mesoproterozoic Rapakivi granite. The marbles are traversed by, swarms of granitic, pegmatitic and mafic dykes. The, whole assemblage experienced strong thermal, metamorphism that provoked the formation of wollastonite orebodies (with a gangue of quartz, grossular, diopside and serpentinite). The average, wollastonite grade is 25–30%; grinding and flotation, are employed to produce a concentrate of >90%, wollastonite with very little iron. Resources are, estimated to >30 Mt., , 365, , Wollastonite production continues to increase., In 2008, processed wollastonite world output was, 600,000 t. China was the leading producer (70%),, followed by India, USA, Mexico, Spain and Finland. Spain has several potential deposits (Galan &, Caliani 1997; Figure 3.35). Prospecting is based, on geological models. It is advisable to carry out, processing tests early in investigations, because, many occurrences do not yield a product of sufficient quality (Gracia et al. 1999). Considerable, tonnages of wollastonite are produced synthetically from quicklime and quartz sand (“cyclo-,, pseudo-, or b-wollastonite”, a high-temperature, phase)., , 3.27 ZEOLITES, , Density, g/cm3, Analcime, (analcite), Chabazite, Clinoptilolite, Erionite, Ferrierite, Mordenite, Phillipsite, , NaAlSi2O6H2O, , 2.3, , Ca (K,Na,Sr)Al2Si4O12, 6H2O, (Na, K,Ca)2-3(Al,Si)18O36, 11H2O, (Ca,K2Na2)2(Al4Si14O36), 15H2O, (K,Na,Mg)4.4(Si,Al)36O72, 20H2O, (Na2Ca,K2)Al2Si10O247H2O, (K,Ca,Na)2(Si,Al)O166H2O, , 2.1, 2.1, 2.0, 2.2, 2.1, 2.2, , This list includes the few economically prominent zeolites from more than 70 zeolite minerals, in nature (Bish & Ming 2001). Note that cation, ratios may vary so that formulas and specific, densities provided are only indicative. Zeolites are, crystalline hydrated alumosilicates of alkali and, alkaline earth elements with a zeolite structure, and with the capability of cation exchange and, reversible dehydration. The zeolite structure is a, three-dimensional framework of anionic tetrahedra (AlO4 and SiO4) similar to feldspar, with interconnected micropores, which contain water and, exchangable cations. When heated, many zeolites, fuse readily and display marked swelling, giving
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366, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 3.35 Schematic geological map of, wollastonite prospects (black) near, Merida, southwest Spain, at the contact, between Early Palaeozoic carbonate rocks, and Late Palaeozoic granite (modified after, Galan & Caliani 1997)., , rise to the name zeolite, from the Greek word for, boiling stone. Contrary to dehydration of minerals, with (OH) groups, zeolite water is lost continuously with increasing temperature. At ambient, temperature, water is readily re-absorbed. This, behaviour singles out thermogravimetric analysis, as a suitable method for identification of zeolites., Zeolite use is based on the mobility of water and, cations in the pores and channels of the framework. The ease of cation exchange is the precondition for numerous applications, such as water, softening (removal of dissolved earth alkalis)., Because the overall size of zeolite channels accepts, only molecules below a certain diameter, the separation of large and small molecules (“molecular, sieving”) is another application of zeolites., Designed zeolites are synthesized for industrial, purposes, exemplified by reacting a saturated kaolinite solution with NaOH or sodium silicate at, elevated temperatures. Upon cooling, gel agglomerates form first, within which nucleation and, crystallization of zeolite take place (Mintova, et al. 1999). Other applications of zeolites include, nitrogen fixation in drinking water treatment,, desiccation, deodorization (cat litter), adsorption, of spilled fuel and of radioactive contamination, (e.g. for purifying water and milk, as at Chernobyl, after the 1986 accident, and as a filler between, waste containers in storage facilities). Zeolites are, also used for filtering oil, desulphurization of stack, gas and as filler in paper, fertilizer, cement and, toothpaste. K-rich clinoptilolite is useful as, a slowly acting fertilizer. For environmental reasons, synthetic zeolites are preferred as cleaning, agents instead of sodium phosphate in powder, detergents. Synthetic zeolites are essential as cat-, , alysers in cracking heavy hydrocarbons such as, petroleum and methanol in order to produce, light fuels. In many countries, zeolitized tuff is, extracted as a dimension stone, because such, rocks are easily worked in the natural moist state, but harden by drying. Furthermore, zeolite-rich, rocks are much employed for the production of, lightweight building materials and as pozzolans in, manufacturing seawater-resistant concrete., Zeolite-rich rocks are extracted in open pits., Zeolites forming part of natural rocks cannot be, enriched. No processing is done apart from drying,, milling, sieving and in some cases, acid treatment, in order to remove exchangable cations., Zeolite minerals are ubiquitous in nature,, although commonly only in very small concentrations, for example in joints and vesicles of volcanic, rocks and disseminated in sediments. Subgreenschist metamorphic rocks may contain appreciable, amounts of zeolites, in the case of mafic rocks, typically represented by low-T stilbite (CaAl2Si7O187H2O) and heulandite {CaAl2Si7O186H2O}, and high-T laumontite {CaAl2Si4O124H2O} plus, wairakite (CaAl2Si4O122H2O). Associated minerals include analcime, mixed-layer clays, albite and, quartz (Bucher & Frey 2002). Overall, metamorphic zeolite formation is the result of hydrous, alteration at low temperatures. As temperatures, increase beyond 250–300 C, zeolites disappear, and prehnite-pumpellyite take over., 3.27.1 Zeolite deposit types, Generally, only rocks with over 50 wt. % zeolite, content (zeolitites) are considered as economically, exploitable. High grades like this are favoured by
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INDUSTRIAL MINERALS, EARTHS AND ROCKS CHAPTER 3, , high reactability of the hydrating rock, provided by, small grain size (e.g. volcanic ash) and an amorphous state of particles (e.g. silicic volcanic glass)., An alkaline aqueous environment (pH 8–10) with, desirable cations is best provided by terrestrial salt, lakes (Hall 1996, 1998). In addition, moderately, elevated temperatures (50–300 C) favour zeolitization (Hall 1998). Geothermal waters and groundwater in terrestrial volcanic settings are, commonly acidic to extremely acidic. This inhibits zeolite formation and geological factors must, act in favour of alkaline conditions. Common, causes of alkalinity are evaporation in closed, inland lake basins and ocean water with a pH of, 8.4. Further possibilities include mobilization of, alkalis from volcanic glass, or advection of alkalis, from elsewhere (e.g. groundwater that dissolved, limestone)., Most zeolitites are geologically young (TertiaryQuaternary), because zeolites age with time to, form denser minerals. However, exploitable heulandite and mordenite deposits were reportedly, found in Carboniferous rocks of New South Wales,, Australia (Flood 1991)., Erionite deposits are exploited from alkaline lake, deposits in the western USA. Layers of rhyolitic, tuffs in the lacustrine deposits were zeolitized, by lake brines or pore fluids. In some cases, lowtemperature hydrothermal alteration is evoked. The, famous landscapes of Cappadocia in Turkey are, formed of erionitic rhyolite ignimbrites. However,, the residents of the towns of Karain and Tuzk€, oy in, the area suffer from an elevated incidence of pleural, mesothelioma, lung cancer and asbestosis. This is, attributed to the asbestiform shape of erionite dust,, with very thin (<0.5 mm) and long (>10 mm) needles, (Ross 1999). Other fibrous zeolites include natrolite, and scolecite., Mordenite is present in playa lake deposits (e.g. Green, River Basin, Wyoming) but is particularly characteristic for alteration related to near-surface hydrothermal, systems with temperatures of 100 C. In Bulgaria,, mordenite deposits are exploited that originated by, deposition of volcanic ash into an Oligocene sea., Clinoptilolite is extracted from alkaline lake sediments. Its precursor rocks are typically rhyolitic glass, tuffs. Several deposits are worked in the western, , 367, , USA, which display a zoned alteration: A central zone, with highest pH and authigenic alkali feldspar is, surrounded by an analcime zone. This is followed by, the broad zeolite zone with erionite, clinoptilolite, and mordenite. The outer margin is characterized by, chabazite and phillipsite. Outside of the lake, where, groundwater chemistry is nearly normal, volcanic, glass is fresh or partly altered to montmorillonite., The same succession is observed vertically because, pore water in playa lake sediments acquires higher, alkalinity with increasing depth. Most probably,, the zonation is also a product of changing environmental conditions in geological time. Several of, the large Neogene borate basins in Turkey contain, important clinoptilolite resources in the footwall, of borate seams (e.g. Bigadiç, cf. “Boron”; Sirkecioglu, & Erdem-Senatalar 1996). Clinoptilolite deposits are, quite common so that numerous applications, have been developed for this material and research, is continuing., , In 2007, world production of natural zeolites, was an estimated 2.5–3 Mt. Main producers, were China, the Republic of Korea, Japan, USA,, Indonesia, Turkey and Hungary (USGS 2008)., Most of the production was probably used in, low-value applications, including lightweight, aggregate, pozzolanic cement and soil conditioners. Areas of geologically young volcanism, and adjacent playa lakes with tuff beds are most, prospective. The potential to locate additional, large and high-quality zeolite resources is high., However, zeolite minerals can only be ascertained and precisely characterized by advanced, mineralogical methods., , 3.28 SUMMARY AND FURTHER READING, In my view, industrial minerals and rocks are, deeply interesting. Like metal deposit, they occur, in all genetic process systems (Table 3.1). Let us, summarize the genetic setting of significant, resources presented in this chapter:, 1 Orthomagmatic: Alkali feldspar (in granite),, apatite, limestone (carbonatite) and olivine (in, dunite);, 2 Pegmatites: Alkali feldspar, Li-minerals and, quartz;
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368, , PART II NON-METALLIC MINERALS AND ROCKS, , 3 Magmatic-hydrothermal: Alunite, kaolin and, phlogopite;, 4 Skarn and contact-metasomatic: Boron, fluorite, and wollastonite;, 5 Volcanogenic: Exhalative barite (VMS), diamond, in kimberlites and lamproites;, 6 Supergene: Kaolin, phosphate and vermiculite;, infiltration/evaporation brines of playa lake basins, with boron, iodine, lithium and soda;, 7 Sedimentary: Gravel, sand and clay; volcaniclastic rocks, diatomite, phosphorite; most limestones; diamond placers, borate minerals, trona,, zeolites and sedex barite;, 8 Diagenetic: Dolomite and magnesite; sulphur, in evaporites;, 9 Metamorphic: Andalusite, some asbestos,, graphite, kyanite and sillimanite;, 10 Metamorphogenic: Part of asbestos, graphite,, magnesite and talc., The evironmental record of exploiting and, utilizing this group of resources is inconsistent., Consider the carbonates dolomite, limestone, and magnesite. These bases are essential for, food production, environmental engineering and, , numerous other utilizations. Magnesite and, dolomite are the means for producing the light, metal magnesium. which is central to minimizing the CO2 footprint of all liquid fuel-based, traffic. Yet, the three minerals must be calcined, for further processing and are, therefore, an, important source of carbon dioxide. The only, seriously hazardous substances are probably, amphibole asbestos, fluorine and certain zeolites., Unwanted traces of problematic elements occur, in phosphorite and even in quite ordinary clay., Investigations and precautionary measures are, compulsory., More information on this group of raw materials, is found in the exhaustive volume by Kogel et al., (2006), in Ciullo (1996) and Harben & Bates (1990)., Genetic processes are at length explained in Robb, (2005). Metamorphism, which is important for a, number of the described minerals and rocks, can, be explored in Bucher & Frey (2002). Continental, saline lakes, which are the main habitat of several, of the discussed materials, are revisited in Chapter, 4 of this book and thoroughly described by Warren, (2006).
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CHAPTER 4, Salt deposits (evaporites), Synopsis, In economic geology, the term “salt” generally refers to rocks consisting of halite (sodium chloride), and other water-soluble minerals. Salt is a common member of evaporitic sequences. Evaporites are, precipitations from a saturated brine in sedimentary systems driven by solar evaporation (Warren, 2006) and are chemical sediments. They form along coasts or on continents, resulting in the terms, marine as opposed to terrestrial, or continental salt deposits. Most hard rock halite and potassium, salt deposits are derived from seawater. About 50% of the total mass of evaporites in the Earth’s, crust are salt rocks sensu stricto (mainly consisting of halite), which are the subject of this chapter., The other half is composed of gypsum and anhydrite (cf. Chapter 3 “Industrial Minerals”)., In several respects, salts are very peculiar. They are water soluble, for example, but have the, lowest permeability of all common rocks. Consequently, they confine crustal water flow and form, effective traps for petroleum and natural gas deposits. Cycling of saline waters in the crust is an, important agent of ore formation (cf. Chapter 1 “Diagenetic Ore Formation Systems”). Also, salt is, one of the few sensu stricto sustainable natural resources, because its mass contained in ocean water, is very large indeed., Currently, the role of salt rocks for building inexpensive underground spaces is rapidly growing., Large leached caverns dedicated to the storage of petroleum products and natural gas are an, important example. Wind energy may be temporarily stored in salt caverns in the form of, compressed air from which it is easily reconverted to electricity. Toxic industrial waste is safely, disposed in deep salt bodies (cf. Chapter 5.5 “Deep Geological Disposal”). Critical public reactions, to underground storage of radioactive waste were particularly fruitful by spurring research on salt,, which generated a wealth of new knowledge., The aim of this chapter is to introduce important salt minerals and salt rocks, their formation as, observed today and in the geological past, their diagenesis, metamorphism and deformation, characteristics, and specific aspects of applied salt geology., , Rock salt (NaCl, common salt, halite, sodium, chloride) is not only an essential part of the diet, of humans and most animals (20% of salt, consumption), but primarily an important raw, material for the chemical industry (60% of, , consumption). Most halite is used for the production of chlorine, soda ash (Na2CO3) and sodium, hydroxide (NaOH, caustic soda), which are basic, chemicals for glass, paper, PVC and aluminium, metal manufacturing. Sodium chloride is an, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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370, , PART II NON-METALLIC MINERALS AND ROCKS, , essential nutrient for humans and grazing animals, because a plant-derived diet must be supplemented by salt. Plant growth, however, is impeded, by elevated salt concentrations in soil. In humans,, high salt intake (1 g per kg of body weight) in a, short time is fatal, whereas 4 g per day is the, recommended intake (Paracelsus, 1493–1541:, “The dose makes the poison”). Some of salt’s, applications pose potential risks for the environment, such as the production of organo-chlorides, (Thornton 2000). Salt is needed for de-icing roads, (in the United States 40% of total consumption:, USGS 2010). De-icing may locally increase, Cl-concentration in surface and groundwater,, which might impact on sensitive biota and, resources of drinking water. Note, however, that, WHO 2006 does not set guide values for chloride, and sodium in drinking water because the elements are “Not of health concern at levels found, in drinking-water”. Salt deposits are very common and few countries rely on imports. Harvesting salt from seawater in coastal salt pans by solar, evaporation is feasible and inexpensive along all, low-latitude coasts. This technology must have, been discovered when Neolithic humans changed, from nomadic to agricultural life and first felt the, need for salt. Rock salt is mainly extracted by, solution methods. Therefore, salt is cheap and, rarely traded over long distances. Yearly world, production of sodium chloride is 260 Mt (2009), from 110 different countries. Largest producers, are China, USA, Germany, India, Canada and, Australia. Resources are practically unlimited,, because rock salt beds are huge and widely distributed, and this is backed up by salt dissolved in, ocean water, salt lakes and subsurface brines., Present production is based on all four sources., In contrast to sodium chloride, potassium salt, rocks are relatively rare and primary production is, restricted to only 12 countries, although about 40, different potassium-bearing salt basins are known, worldwide (Goncharenko 2006). Major producers, are Canada (Saskatchewan), Belarus, Russia,, China, Germany, Israel and Jordan. The latter, extract potassium by solar evaporation from Dead, Sea brines, precipitating carnallite (K-Mg chloride), with by-product bromine (cf. Chapter 2.4, “Magnesium”). Potassium is the fifth most impor-, , tant element in terrestrial biomass, after C, O, H, and N. It is one of the three essential plant nutrients (N-P-K) and consequently, agriculture consumes over 95% of production. K þ is the most, abundant cation in the human body and an essential nutritional element for humans and animals., Potassium is also needed for the production of, various industrial products (soap, glass, building, materials, drilling fluids). The minimum grade for, hard rock and deep solution mining of potassium, salts is 10% K2O. Most potassium deposits are, formed by chloride minerals (90%, mainly sylvite) and KCl with 60% K2O is the most common, form of K-fertilizers. Potassium sulphates contribute only 4% of world production; they are recommended for Cl-sensitive crops such as tomatoes,, tobacco and potatoes. In 2008, primary world production of potassium salts amounted to 36 Mt, (K2O), but dropped to 25 Mt in the crisis year, 2009 (USGS 2010). Resources are very large., Agriculture and forestry have increasing, demand for magnesium. This is partly satisfied, by co-production of kieserite (MgSO4H2O) that is, common in certain potassium salt beds (e.g. Sigmundshall, near Hanover, Germany). Other possible by-products of processing potassium salts, are occasionally rubidium and caesium (with, 50–1700 ppm in K-salts), boron (e.g. boracite) and, bromine (partly replacing Cl in chloride minerals,, concentrations reach 5000 ppm). Processing of, carnallite for potassium fertilizer production, results in waste brines with elevated sodium, chloride and Mg-chloride content. These brines, can be used for the production of magnesium, metal or of synthetic magnesite. Separation and, concentration of specific salts is carried out by, flotation, electrostatic separation, thermal dissolution-crystallization and heavy media separation. Waste brines of salt processing are, regarded as an environmental hazard. In central, Europe, geological disposal in deep saline aquifers, is the preferred option., Terrestrial salt deposits and brines also contain, exploitable halite and potassium salts, but their, striking feature is the chemical variability of precipitated compounds. Of course, this reflects the, geochemically and environmentally diversified, setting of salt lakes on land as opposed to the
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372, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 4.1 Potassium seam Stassfurt in the Asse mine,, Northern Germany, made up of intensively folded beds, of carnallite (dark reddish-brown, highly ductile), grey, halitite and white kieserite (thin bands). The long and, narrow Asse salt wall is the product of compressional, intraplate deformation (inversion) during the Late, Cretaceous. Near-parallel scratches and ridges are, typical traces of excavation by a continuous mining, machine., , investigate deformation within deposits. Some, rocks such as carnallitite and minerals such as, polyhalite are usually brilliantly red, but salt rock, colours are not generally diagnostic. Colouring is, due to tiny inclusions or to lattice defects (Sonnenfeld 1995). As in other rocks, a reduced state is, , Figure 4.2 Anhydrite-banded Stassfurt halitite below, carnallite seam in Asse mine, Northern Germany., Each couplet consists of 1 cm thick light-grey, anhydrite and 10 cm halitite; bedding dips moderately, to the left. Near-vertical structures are grooves left by, a continuous miner., , Figure 4.3 (Plate 4.3) Haselgebirge sample from Bad, Aussee mine near Salzburg, Austria. This variety displays a, red halite matrix with dispersed angular fragments of black, claystone., , expressed in grey and greenish colours, which are, caused by clay, bitumen and iron sulphide phases., The much more common reddish colours of salt, rocks are due to an oxidized state expressed by the, presence of tiny haematite needles and platelets., Conspicuous blue halite is produced in the laboratory by irradiation that causes lattice defects and, liberates minute particles of native sodium. In, nature, the passage of diagenetic solutions may, have leached bromine from halite, inducing the, same effect., Most salt rocks contain a small proportion, (<0.5%) of organic matter derived from algae,, microbes and airborne remains of terrestrial, higher plants (e.g. pollen, black carbon). Higher, contents of kerogens and bitumen are restricted to, clayey and anhydritic rocks associated with salt., Salt rocks are investigated by petrographical (in, thin and polished sections) and by chemical methods. Dubious potassium minerals in drill core and, mine exposures can be made visible by spraying, with a solution of Mg-dipicrylamine inducing a, red colour. A portable scintillometer helps to, identify the higher g-ray activity of potassites., Fluid inclusions, Salt rocks in little deformed platform sediments, contain >1 wt.% H2O, but deformed salt (e.g. in, diapirs) has normally <0.1% water. In both cases,, the water occurs in microscopic and sub-
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , microscopic fluid inclusions. Inclusions in diapiric salt are mainly located at grain boundaries., Little deformed salt is more likely to show intracrystalline inclusions along growth zones (Horita, et al. 1996). Vreeland et al. (2000) found viable, “spores” of bacteria in inclusions of this type, within Permian salt crystals from Carlsbad,, New Mexico. However, recrystallization of salt, rock is so common that original brine inclusions, are rare. Prevalent inclusions have been sealed in, at various times during diagenesis and deformation of the salt rocks. Investigations of brine, inclusions by measuring the temperature of, homogenization (TH, heating) and salinity (freezing) allow only limited interpretation. Informative, data are rather expected from diagenetic minerals, that are more stable than salt, for example sulphate and authigenic quartz (Attia et al. 1995)., Formation temperature can be estimated from TH, after strong cooling (Roberts & Spencer 1995). The, chemical composition of inclusions is measured, by EDS Electron Microscopy (Energy Dispersive, X-ray Spectroscopy) and ICPMS (Inductively, Coupled Plasma Mass Spectrometry) at temperatures below 190 C (Garcia-Veigas et al. 1995,, Zimmermann 2001)., , 373, , Geochemical investigations enhance insights, into many processes related to salt formation and, assist practical applications., , Bromine, This halogen element is a very useful measure of, the degree of evaporation (the concentration) of a, seawater-derived brine and an important tracer of, the origin of diagenetic fluids (Botrell et al. 1988;, cf. Chapter 1.4). During halite precipitation, bromine is enriched in the liquid phase relative to, chlorine, with increasing concentration of a brine, (Herrmann 1980). The distribution coefficient for, bromine between precipitated halite and the brine, varies from 0.12 to 0.3. Early halite contains, little bromine (50 ppm in contrast to 500 in, co-existing brine; seawater 67, continental crust, 3–7 ppm Br), whereas halite of potassites contains, over 10 times as much (up to 600 ppm; Figure 4.4)., Epsomite-precipitating brine concentrates Br to, 2500 ppm. Consequently, bromine analysis of, 300, , Rock salt, alternating, with, , In some salt mines, explosive gas-driven salt outbursts are not rare. This happens when mining, exposes salt that contains gas under high pressure,, comparable to coal outbursts (Guan et al. 2009). In, the Permian salt strata of the Werra district in, central Germany, “crackly” salt (giving off a, crackling noise when submerged in water) indicates hazardous areas. In this region, CO2 and N2, were introduced by sills, veins and pipes of Tertiary basalt volcanism. Salt in Wielicka, Poland,, contains nearly pure CH4. Nitrogen is common in, salt of the Caspian basin and in Carlsbad, New, Mexico. Deep German Zechstein salt occasionally, includes thermochemical H2S (Chapter 1.4, eq., 1.21). Trace amounts of gas occur in all salt rocks, (Potter et al. 2004). Similar to salt rock colour,, there is a clear disparity of salt with a prevalence of, oxidized (O2, CO2) in contrast to reduced gas, (alkanes, i.e. CH4, H2, H2S)., , marlstone, , Thickness (m), , Gas in salt, , 200, , 100, , Upper K-seam, Lower K-seam, 0.007, , Footwall anhydrite, , 0, 0, , 0.01, , 0.02, , 0.03, , wt. % Br in NaCl, , Figure 4.4 Bromine concentrations in halite of the, Oligocene salt formation in the Upper Rhine Valley, (Reproduced from Herrmann 1980 with permission from, Elsevier)
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374, , PART II NON-METALLIC MINERALS AND ROCKS, , halites is the best tool for potassium salt exploration. In thick packages of halite sediments, bromine profiles assist stratigraphical correlation and, support derivation of mass balance models for salt, basins concerning inflow, outflow and evaporation. However, disturbance of original bromine, characteristics is possible by post-sedimentary, processes, such as recrystallization, diagenetic, brine migration and the influence of meteoric, water (Siemann & Schramm 2002). Yet, a comparative study of little deformed bedded Zechstein 2, halitite and the same stratigraphical interval in, diapiric salt of Northern Germany displayed the, same trend of bromine concentrations rising, towards potassium seams; in diapirs, only the, variance of data is reduced (K€, uster et al. 2009)., Most non-marine salts display low bromine, contents., Bromine (Br2, liquid density 3.1028 g/cm3, toxic) is an, important by-product of brine exploitation and to a, minor extent of seawater and rock salt processing., Annual world production is estimated at 440,000, tonnes, mainly in USA, Israel, China and Jordan., Reserves and resources are very large, and considering, the total mass of seawater, practically unlimited., Bromine and bromine compounds are foremost used, as flame retardants in plastics of electronic and electrical equipment. Numerous other applications, include drilling fluids, brominated pesticides, mercury capture at coal-fired power stations and water, treatment., , Isotopic geochemistry of evaporites, Several isotope systems contribute to a deeper, understanding of formation, diagenesis and metamorphism of evaporites. Similar to bromine,, boron is enriched by evaporation and salt precipitation. Fractionation of 10 B/11 B allows the discrimination of marine and terrestrial evaporites, even, after salts have been dissolved and only tourmaline remains (Perez Xavier et al. 2008). Sulphur, isotope analyses of recent and fossil marine sulphates (mostly anhydrite) allow a reconstruction, of seawater sulphur isotope characteristics in the, geological past (Figure 3.4). The variations can be, explained by two major factors driving the global, sulphur cycle:, , 1 34 S of the oceans reaches a minimum at times of, profuse precipitation of marine sulphate rocks,, because heavy sulphur is preferentially incorporated in sulphate (e.g. in the Late Permian); and, 2 heavy sulphur is enriched in ocean water when, light sulphur is extracted by formation of sulphides in marine bottom sediments (as in the Late, Neoproterozoic and Early Palaeozoic)., Earlier than 2100 Ma, the sulphur cycle’s control was atmospheric and fundamentally different., Higher precision of past seawater d34 S can be, reached by analysing deep-water barite (Paytan, et al. 2004). Chlorine isotopes are useful for analysis of geologically young salt, because of the, occurrence of cosmogenic radioactive 36 Cl with, a half-life of 301,000 years. Different stable Clisotope ratios 37 Cl/35 Cl characterize earth reservoirs (crust and seawater d37 Cl 0‰; mantle, slightly negative; standard is mean ocean chlorine, SMOC), mark certain genetic processes and preserve this evidence through strong deformation, (Eastoe & Peryt 1999), subduction and magmatism, (Nahnybida et al. 2009, Banks et al. 2000). The, method was employed to prove that deep brines in, country rocks of the Palo Duro halite in Texas did, not originate by meteoric dissolution but are original Permian residual brines (Eastoe et al. 1999)., Strontium isotopes are used for investigating, complex sedimentary and diagenetic processes, (Schreiber & Tabakh 2000). Carbon isotopes of, methane in potassium salt provide arguments for, a perfect preservation of sedimentary and early, diagenetic features (Potter et al. 2004)., Stable oxygen and hydrogen isotopes of water in, fluid inclusions of salt rock provide clues to the, sedimentary environment (e.g. terrestrial or, marine) and eventual post-sedimentary fluid passage through salt rocks. This is a point that is, especially important in connection with radioactive waste disposal in salt rocks. Compared to, seawater, evaporation shifts water isotopes to, higher concentrations of 18 O and 2 H (Figure 4.5)., Red Sea water, for example, displays d18 O þ 2 and, dD þ 11‰. Strong evaporation of brine in the, halite precipitation phase causes a decline in, heavy isotope content, because heavy water volatilizes more easily from concentrated salt solutions. This “salt effect” is caused by stronger
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375, , SALT DEPOSITS (EVAPORITES) CHAPTER 4, , +40, , A, , +20, , δ D‰, , Figure 4.5 Stable hydrogen and, oxygen isotope ratios in evaporating, seawater and brine (modified from, Kyser & Hiatt 2003 with permission, from Elsevier). A demonstrates initial, changes by evaporation in humid, B in, arid climate. C and D illustrate, experiments and natural systems, evaporating until halite precipitation., Trajectory E is derived from salt pans, on the Caribbean Bonaire Island;, sample x1 shows situation just before, gypsum is precipitated, x2 at the onset, of halite formation and x3 after the, main halite stage with first K-Mg salt, deposition. SMOW ¼ Standard Mean, Ocean Water, MWL ¼ Meteoric Water, Line., , D, , C, , B, SMOW, , 0, , x1, x2, x3, , E, , -20, , MWL, -40, , -10, , attachment of light water molecules in hydration, spheres of ions and in changes of the structure of, water that are caused by dissolved salts. The, strength of the salt effect depends on concentration and the prevailing salt (CaCl2 > MgCl2 >, MgSO4 > KCl ¼ NaCl > Na2SO4; Horita et al., 1993). The result is the characteristic evaporation, loop in the water isotope system (Figure 4.5). Note, that primary isotope characteristics can be disturbed by the ingress of meteoric and formation, waters, or by passage of dehydration water of, gypsum interbedded with salt., Isotope dating, The precise age of geologically young gypsum and, many salts can be determined with the U-series, disequilibrium method (Ku et al. 1998). The, method is based on the inclusion of 238 U in authigenic minerals under exclusion of its decay chain, (uranium series) nuclides. The earliest products of, radioactive decay include 234 U and 230 Th. Their, increase towards secular equilibrium with the, parent nuclide is a function of time and is used, for dating. However, the common recrystalliza-, , -5, δ, , 0, O‰, , 5, , 10, , 18, , tion of evaporite minerals severely limits general, application of the method., The age determination of geologically ancient, salt is normally based on palaeontological examination and radiometric age-dating of host rocks., Considering the mobility of salt, this may be, questionable and wind-blown pollen or spores of, surrounding vegetation enclosed in salt are used as, controls. Radiometric age dating is done with, systems such as K/Ar, K/Ca, Rb/Sr and U/He, but, the resulting model ages may be sedimentary or, diagenetic. Salt rocks rarely contain minerals that, are reliable vessels for the intact preservation of, mother and daughter nuclides., Impermeability, The impermeability of salt rocks in respect to, water, petroleum and natural gas is extremely, important, because salt-related traps confine, many giant hydrocarbon deposits (Warren 2006,, cf. Chapter 7.3 “Petroleum and Natural Gas, Deposits”). Radioactive waste disposal projects, prompted much research on salt rock permeability. Data on halitite in the Asse mine
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376, , PART II NON-METALLIC MINERALS AND ROCKS, , (Northern Germany) confirm extremely low, intrinsic permeability [k]. In situ and distant from, mine openings, k is below 1.1021 m2, whereas, destressed and dilated salt on side walls of tunnels, displays a mean k of 1.1016 m2 that equals 1.104, Darcy (Peach 1991). Note that during rapid upflow, of a diapir, or under the influence of a volcanic field, (e.g. Hesse, Germany), salt may be temporarily, quite permeable, comparable to rocks undergoing, penetrative shearing during metamorphism. In, this environment, fluid migration is part of a, dynamic chemical, energy and mechanical flow., Permeability is provided by processes such as, dissolution, precipitation, grain boundary sliding, and creep cavitation., 4.2 THE, , FORMATION OF SALT DEPOSITS, , 4.2.1 Salt formation today, Examples of present salt formation are salt lakes in, California, Chile, Tibet and Utah, the Dead Sea,, sabkhas on the coast of the Arabian-Persian Gulf, and numerous salt lagoons on arid coasts throughout the world. However, there exist no large, marine basins of salt formation as in the geological, past. This is one reason why early research on salt, formation started with coastal salt works (Figure/, Plate 4.6; Usiglio 1849). Salt production, however,, aims at monomineralic solids, and processes in, salt works are not full replicas of natural salt, formation, because back reaction between precipitates and brine is prevented. Yet, the back reactions are prime controls on the mineralogical, composition of natural salt rocks., The solubility of salt in water is expressed either, as mass fraction (g/kg) or concentration (g/L). Note, that resulting figures are different and should not, be confused (Table 4.4)., The term “salinity” is used to describe the salt, content of water. Here also, the units of Table 4.4, are used, and others such as per cent (%), per mil, (‰, equal to grams per thousand) and ppm (parts, per million, or grams per tonne). The density of, seawater measures between 1020 and 1050 kg/m3., Water of the high seas has a mean salt content of, 3.5 wt.% (35 g of salt in 1000 g of ocean water) with, a range of 3.1–3.8%. Lower salinity is observed in, , Figure 4.6 (Plate 4.6) Solar seawater evaporation pans for, industrial salt production on the West Australian coast., Courtesy Dampier Salt Ltd and Rio Tinto Minerals. Seawater, is first concentrated to specific gravity 1.21 in order to, precipitate carbonate and gypsum. Different grades of salt are, crystallized between brine gravity 1.21 and 1.275., Harvesting is visible in the foreground. The remaining, K-Mg brine (“bittern”) may be processed or pumped back, into the sea., , polar seas (2.9%) and values of more than 3.5% are, reached in confined seas at low latitudes (e.g. Red, Sea). Independent of concentration, composition, and proportion of dissolved salts display little, variation: Important cations are Na þ , K þ , Mg2 þ, , Table 4.4 Solubility of NaCl in water at different, temperature (cf. eq. 1.20), T C, g/L, g/kg, , 0, 356.85, 263, , 10, 358.70, 264, , 20, 360.54, 265, , 100, 388.89, 280
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 377, , Figure 4.7 Simple demonstration of the, major stages of evaporite formation by, evaporation of seawater in a beaker, (inspired by Rouchy & Blanc-Valleron, 2006). S is the brine’s salinity, D its, density in g/cm3. The brine in beaker no. 4, has a very bitter, unpleasant taste which is, at the origin of the term “bittern salts”., , and Ca2 þ , anions include Cl, SO42 and HCO3., CaSO4 and MgSO4 may be present in non-ionized, compounds. One litre of seawater with the density, of 1035 kg/m3 contains the following salts: 27.2 g, NaCl, 3.35 g MgCl2, 2.25 g MgSO4, 0.74 g KCl,, 0.12 g CaCO3 and 1.27 g CaSO4. Ten-fold concentration is needed for halite precipitation to start., Saturation of NaCl in water at 25 C is reached at a, concentration of 26.6% (Table 4.4). The total, mass of salt dissolved in the oceans is gigantic., The volume of ocean water amounts to, 1370.106 km3 (Walker & Cohen 2007). Palaeozoic, salt basins contain 13% of NaCl in present, seawater., Undisturbed evaporation precipitates salts as a, function of solubility. First to reach saturation are, calcite or aragonite (often later dolomitized), followed by gypsum or anhydrite, halite and ending, with highly soluble potassium-magnesium chlorides and sulphates (Figure 4.7). The high solubility, of the latter, called “bittern salts”, is the reason, why they are rarely formed and once precipitated,, are easily redissolved., More than 150 years ago, the precipitation of salt, minerals caused by increasing concentration of, brines was experimentally investigated by the, pioneers Usiglio, Van’t Hoff and D’Ans. Braitsch, (1971) summarized their results. Thermodynamic, modelling by Eugster et al. (1980) and Harvie et al., (1984) confirmed most of the earlier deductions., Details on bittern salt precipitation changed, from earlier simple models. The precipitation, sequence had been thought to be bloedite, epsomite, epsomite and kainite, and finally carnallite, and kieserite. However, several variables were not, , considered in the laboratory, which cause variations in the precipitation path from seawater. One, example is the formation of polyhalite (e.g. by, replacing earlier anhydrite) that changes brine, composition so that epsomite is followed, by carnallite and kieserite (Figure 4.8; Eugster, et al. 1980). Generally, back reactions between, brine and solids are decisive (Figure 4.9), but, may remain incomplete. The terminal paragenesis, is principally bischofite, carnallite, kieserite,, halite and anhydrite. Observations show that bloedite and kainite are rare as primary minerals in salt, deposits. However, kainite can be formed when, polyhalite cannot crystallize, because SO4 is, depleted, for example due to an excess of calcium, as supposed for the Caltanisetta district, Sicily, (Garcia-Veigas et al. 1995). SO4 depletion may be, forced by bacterial reduction., Deviations from the normal precipitation, sequence are also possible by depletion of magnesium caused by earlier dolomitization or by formation of Mg-rich clays. Carnallite precipitation, is inhibited by algal blooms that enrich nitrogen, hydrides (e.g. urea), which complex MgCl2. This, might explain primary sedimentation of sylvinite, and of langbeinitic hartsalz that are the most, frequently exploited “potassium ore”, for example, in Western Canada. Secondary, early to late diagenetic sylvite is typically the product of carnallite, exposure to brines undersaturated with MgCl2., Tachhydrite cannot be precipitated from modern, seawater but only from brines rich in CaCl2. It is, striking that worldwide, Cretaceous potassium, salts are associated with tachhydrite. The most, probable solution for this conundrum is to admit
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Table 4.5 Mineral associations forming six stages or, zones of a progressive salt precipitation sequence, Top, , 6. Bischofite (bischofite, boracite, carnallite,, halite, kieserite, anhydrite) or tachhydrite zone, (sylvite, tachhydrite, halite), 5. Carnallite zone (carnallite, halite, kieserite,, anhydrite), 4. Mg-sulphate zone (halite, polyhalite, kieserite,, anhydrite, magnesite), 3. Halite zone (halite, anhydrite, dolomite,, magnesite), 2. Gypsum zone (primary gypsum or anhydrite), Footwall 1. Carbonates (limestone, dolomite), , 5-6 m, , 12 m, , 379, , 18 m, , Halitite + K, and Mg salts, , that past seawater composition was not constant, (see below)., Whatever the details, increasing seawater, evaporation produces a suite of salt minerals and, rocks that is termed the progressive salt precipitation sequence. In nature, however, the last, members of the sequence will rarely be formed, and very often, a more advanced paragenesis is, overlain by salts produced by less concentrated, brine. The regression is called a recessive, sequence. Both combine to form an evaporative, cycle. Most salt basins comprise a number of, cycles. A complete progressive sequence can be, subdivided into distinct stages or zones, which, follow the facies rule by either forming a vertical, (in time), or a lateral (two-dimensional) succession (Table 4.5)., Complete evaporation of a seawater column of, 1000 m produces a salt layer of only 18 m thickness (Figure 4.10), with thickness ratios of, Table 4.6. Clearly, it is impossible to explain the, formation of hundreds of metres of monotonous, halitite by simple static evaporation in closed, basins. Likewise, the thickness relations between, various salt rocks (e.g. halitite vs. potassites) in, salt deposits deviate considerably from the theoretical ratios. Natural evaporites obviously formed, in dynamic and partially open systems, with both, inflow of fresh seawater and outflow of concentrated brines., Terrestrial evaporites are even more variable, than marine salt formations, because local conditions in the catchment region control available, ions. Exploitable deposits of salt minerals such as, , Halitite, , SALT DEPOSITS (EVAPORITES) CHAPTER 4, , Gypsum, Carbonates, , 0.5 m, 0.05 m, , Figure 4.10 Evaporation of a seawater column of, 1000 m in a closed system precipitates a salt layer of 18 m, thickness, including 6 m of potassium-magnesium salts., Because halitite sequences often reach a thickness of, hundreds of metres with very little K-Mg salts it is, obvious that salt formations formed in open systems., , soda, trona, mirabilite and thenardite are only, known in terrestrial salt formations., Climatic parameters, Salt formation is only possible when evaporation, exceeds precipitation and inflow. Today, this is, , Table 4.6 Thickness of salts deposited by complete, evaporation of seawater in a closed system (normalized, to 100 m halitite; Braitsch 1971), Thickness (m), , Evaporite facies, , 38, 3.6, 13.7, 10.8, 100, 4.8, 0.37, , Bischofite (halite, kieserite, carnallite), Carnallitite (carnallite, halite, kieserite), Mg-sulphates (kieserite, halite), Polyhalite (halite, polyhalite), Halitite (halite, anhydrite), Gypsum (3 m if dehydrated to anhydrite), Calcite
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380, , PART II NON-METALLIC MINERALS AND ROCKS, , most common in the Earth’s arid latitude belt, between 15–35 with a mean at 23 4 (Evans, 2006). Air involved in tropospheric Hadley-Ferell, circulation descends on these regions of both, hemispheres and induces desert climate conditions. Near the equator, evaporites are rarely, formed because high precipitation surpasses, evaporation. Distance from the oceans (“continentality”) and rain shadow effects may cause, deserts at higher latitudes and independent of tropical atmospheric dynamics. Examples are deserts, and recent salt deposits in China and Tibet. Cool,, upwelling seawater contributes to desertification, in coastal areas, for example in Namibia and Chile., Halite can be formed at relative air humidity >65%, but K-Mg salt precipitation is restricted to humidity <35% (Schreiber & Tabakh 2000)., Exceptional loci of salt formation are lakes and, mud flats in polar deserts of Antarctica, for example hypersaline lakes such as Don Juan Pond of the, McMurdo Dry Valleys, which is probably the most, saline lake on Earth, with 413 g of CaCl2 and 29 g of, NaCl per kg of water. Neoproterozoic equivalents, were discovered in Svalbard, where glacial diamictites of the 635 Ma Marinoan Snowball Earth glaciation display carbonate and sulphate contents, (Bao et al. 2009). Thousands of Holocene salt lakes, are known in the moraine plains of Canada, and, about 100 contain exploitable sodium sulphate, beds (Warren 2006). In this climate, salt precipitation is “cryogenic”, not solar evaporative, because, sodium sulphate solubility decreases with falling, temperature. Calcium sulphate solubility reaches, a maximum at 40 C and drops with both rising, and falling temperature. Purely as a function of, temperature (disregarding salinity), the stable, mineral below the maximum is gypsum, above it, anhydrite., However, profuse marine salt formation is, restricted to subtropical high-pressure zones., Cooler parts of the globe are rather sites of much, smaller terrestrial salt pans or salt lakes. In this, context, ancient salt deposits can generally be, considered as indicators of specific palaeoclimatic, and palaeogeographical settings (Evans 2006)., Only in the Late Permian, evaporites formed, throughout Pangaea, spanning equatorial regions, and high latitudes (Torsvik & Cocks 2004)., , The physical environment, Salt can be formed from terrestrial or sea water and, from mixtures of both. Terrestrial salt formation is, prominent in endorheic (that is hydrographically, closed or nearly closed) continental basins and, depressions with high evaporation. In deserts,, efflorescence from soil is ubiquitous due to evaporative pore water rise to the surface. Crusts of, salt, gypsum and calcrete form in this way or, deeper in the soil profile. Dry or marshy salt, mudflats develop in shallow depressions and on, the shore of salt lakes. Examples of the first, include the salt marshes of Kazakhstan between, Wolga River and the Urals, the shotts (also spelled, “chotts”) of Northern Africa and the Great Kavir, in Iran. The Great Kavir is the lowest swampy part, of the giant salt desert of Central Iran, Dasht-e, Kavir, which is 800 km long and over 300 km, wide., , Salt lakes (playa lakes, salinas), These are typically ephemeral lakes in lowest, parts of undrained desert basins. In the geological, past, many present-day salt lakes were large, freshwater lakes (e.g. Lake Bonneville, the pluvial, predecessor of Great Salt Lake, Utah). This fact, highlights the surprising conclusion that present, global climate is of exceptional aridity. Worldwide, tectonic rifts and transform faults preferentially host salt lakes (e.g. Dead Sea). These, lake basins are surrounded by mountains where, most precipitation takes place. On entering, the basin, downflowing streams build alluvial, cones and fans of sediment fining away from the, escarpment (Figure 4.11). Wide sandy and clayey, plains (these are the namesake “playas” sensu, stricto) slope down to the lake. Springs occur, where groundwater is forced to the surface by, encountering less permeable sediment. Along, faults, geothermal springs are aligned with typical precipitates of travertine, SiO2 and Si-Al, oozes. Mudcracks, efflorescent crusts and early, diagenetic (authigenic) salt crystals characterize, dry supralittoral mudflats. Salt content increases, towards the lake with the concentration of the, pore-fluid brine until the sedimentary banding of
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Fluvial channels, , Efflorescent crusts, Mudcracks, , 381, , N, , SALT DEPOSITS (EVAPORITES) CHAPTER 4, , Evaporation, , Saline mudflat, , Figure 4.11 Schematic block diagram, showing environmental elements of, sedimentation in Late Miocene playa lakes of, the Beypazari Basin, Central Anatolia, (Yagmurlu & Helvaci 1994). This basin, contains important deposits of trona, gypsum,, oil shale and lignite. GW – groundwater inflow, from alluvial fans., , Ephemeral playa lake, GW, , the clays is totally disturbed by displacive crystalline salt., Apart from siliciclastic and volcanogenic sediments, subaqueous sediments of salt lakes include, banded carbonates, black clay, gypsum, halitite,, trona and other salts, depending on climate and, the chemical composition of inflow. In the Dead, Sea, for example, 15–20 varves are present in carbonate mud, which was deposited on the lake floor, between 1900 and 1975. Humid years and extreme, floods are thought to have caused the few prominent bands. In 1979, the meromictic (permanently, stratified) Dead Sea overturned and entered a mode, of alternating holomictic/meromictic states, (Enzel et al. 2006). Since 1982, coarsely crystalline, halite accumulates on the floor of the lake, for the, first time since more than 300 years. Most playa, lakes are very shallow with just a few metres of, water, but the Dead Sea reaches a depth of 400 m., In the fresh sediments, bacterial sulphate reduction produces H2S or FeS2-phases. Methanogenic, microbes turn out CH4 and the gas forms bubbles, in the sediment that can be fossilized. Although, the salinity of perennial saline lake waters varies, considerably (in the Dead Sea up to 300,000 mg, TDS/L ¼ total dissolved solids per litre), a lower, boundary of 5000 mg/L is often used to define, saline lakes. At this salinity most ordinary freshwater organisms cannot survive. However, many, specialized (halophile and/or alkaliphile) organisms thrive at high salinity and alkalinity, including species of algae, bacteria and archaea., Halobacterium, for example, is an archaeon, known to be extremely resistant to UV and gamma, radiation. Microbes and halophilic algae floating, , Capillary migration, Green, gypsiferous claystone, , Green claystone with, desert rose gypsum and, gypsum filled cracks, , Thenardite (Na2SO4), and glauberite, (Na2 SO4. CaSO4) zone, , Crystalline, and bedded, gypsum, , in the sun-lit, aerobic, nutrient-rich brines are, characterized by carotenoid and rodopsin pigments, which cause the deep red colour of salt, lakes and salt pans (Figure/Plate 4.12a; Boetius &, Joye 2009). These organisms contribute to the, frequent occurrence of oil shale and petroleum, source rocks in fossil playa lakes (Figure 4.12b),, e.g. in the Eocene Green River Basin, USA. Alkalinity can reach extreme pH 12 as in the soda, lakes of East Africa and South America., Although most terrestrial salt lakes are alkaline,, acidic saline lake waters do exist (e.g. Lake Tyrell, in Victoria, Australia, where water is a dilute, solution of H2SO4 with a pH of 2–4). This can be, caused by sulphide oxidation and production of, acidity in the watershed and a lack of acid-consuming rocks. Acidic lake waters are common in, volcanic regions. An ancient example are fluid, inclusion brines in Permian halite of the midwestern USA with a pH < 0 (Counter Benison, et al. 1998)., The provenance of ions concentrated in salt, lakes is varied, but the larger part is provided by, weathering of rocks making up the watershed., Surface and groundwater carry the dilute inflow, into the basin (Figure 4.11). Provenance from surrounding rocks was verified for Neogene evaporites in the Atacama desert of Northern Chile, (Pueyo et al. 2001). Another source may be dissolution of older evaporite beds, which can be recognized by inherited isotope characteristics of the, younger sulphates. Import of matter by volcanic, and geothermal processes is quite common (e.g., tungsten, fluorine, boron, lithium, or volcanic ash, transformed into bentonite or zeolite).
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382, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 4.12 a (Plate 4.12a) Salt rafts floating on brine at, the shore of Lake Katwe, western Uganda. Katwe is a, large maar lake due to Pleistocene volcanism. It draws, seepage water from close-by fresh Lake Edward. The dark, red colour of the brine is caused by teeming microorganisms. Upper left corner is organic-rich mud., , Figure 4.12b Salt harvested from the shallows of Lake, Katwe. Note alternating layers of black organic mud and, salt, and the bipolar growth of elongated clear halite, crystals. Voids are, of course, brine-filled when in situ., , The evolution of brine chemistry and precipitates with increasing evaporative concentration is, primarily a function of the HCO3/Ca þ Mg ratio, (Eugster & Hardie 1978), modified by other parameters and processes. Three major geochemical, paths have been recognized:, 1 Inflow with a high HCO3/Ca þ Mg ratio produces alkaline Na-CO3-Cl-(SO4) brine (the soda, lake type);, 2 a low ratio causes gypsum precipitation and, residual brines characterized by Ca-Na-Cl-(SO4),, , final precipitates are halite, glauberite and, mirabilite;, 3 an intermediate ratio leads first to precipitation, of alkaline earth carbonates (calcite, aragonite,, Mg-calcite), followed by a rich diversity of brine, evolution and evaporite deposition., Precipitation of K-Mg chlorides and sulphates, takes place in salt pans at the stage of near-dryness., On the shores of Dabusun salt lake in the Qaidam, basin, China, potassium salt is forming today, (Casas et al. 1992). Poikilitic carnallite crystals, occur in halitite layers of a rhythmic halitite-clay, succession. In clay bands, carnallite forms displacive crystals. Obviously, this carnallite precipitated from pore solutions within the sediments., Saturation is explained by evaporation on the, surface causing high Mg-K concentration. The, heavy brine seeps downwards, where lighter and, less concentrated pore water is displaced. As the, brine cools, carnallite is precipitated 0–13 m below, the surface. Potassite seams in ancient salt, sequences may have formed in a similar way., Marine salt formation can be observed on the, world’s coasts, in two differing morphological, settings – salt lagoons and sabkhas. The first are, bays disconnected from the open sea, which, receive fresh seawater only through a narrow, channel or by seepage through a barrier (e.g. a sand, spit; Figure 3.20). Sabkhas are saline flats hostile to, most life forms, which occur along desert coasts, throughout the world (e.g. the Arabian coast of the, Persian Gulf)., Sabkha, A sabkha (Arabic for salt flat; also written sebkha) is a coastal saline surface dropping down to, the highwater level beach. An essential part of, the definition is the specific hydrological regime, of salt formation within the sediment by capillary evaporation (Warren 2006). The sabkhas of, Abu Dhabi were first intensely investigated. It, was found that high evaporation causes an, upward flow of increasingly concentrated water, from a saline water table to the surface. Within, the capillary fringe, gypsum or anhydrite, dolomite, magnesite and halite are precipitated., The derivation of water and dissolved ions was
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , sought in the nearby sea, either by occasional, flooding (“sea-water flooding model”) or by, lateral suction of seawater and groundwater, (“evaporative pumping”). More recent quantitative hydrogeological investigations indicate that, most dissolved matter is supplied by inland, groundwater (“ascending brine model”, Wood, et al. 2002). Seawater seems to have only a minor, role. Anyway, because of several reasons, foremost the minor salt accumulation, sabkhas cannot explain the formation of salt deposits., Marine salt lagoons, Marine salt lagoons (saltern, salinas) are omnipresent features on arid and semi-arid coasts. They, illustrate many phenomena that have been, observed in the ancient salt giants, but are orders, of magnitude smaller. Already Ochsenius (1877), used this approach in an attempt to explain the, origin of the Late Permian salt formations in, northern Europe, when he drew parallels with the, Garabogazk€, ol (Kara Bogaz Gol) on the eastern, shore of the Caspian Sea in Turkmenistan:, The Garabogazk€, ol bay covers a surface of 18,000, km2 and has a maximum depth of 15 m. Originally, a, 100–500 m wide channel connected the bay with the, Caspian Sea and evaporation lowered the water level, in the bay, probably by 0.5 m. Annual water flow, through the channel was 18–26 km3. In 1878, total, salt content of the Caspian Sea was 13‰ (about onethird of ocean water), but with a notably different, composition. Salinity in Kara Bogaz Gol was 285‰, and precipitates included gypsum, mirabilite and, some halite. At times, a 3–8 m thick bed of cryogenic, sodium sulphate (cf. Chapter 3 “Sodium Carbonate”), was exploited that may date from the ice ages. Ochsenius (1877) especially studied the role of the barrier, between sea and bay. In later years, the Caspian Sea, suffered from a drawdown caused by drought and, human intervention and the bay fell dry, but recently, the former lake level was restored by a climatic return, to wet years. Considering the chemical difference, between the parent water body and ocean water, and, the preponderance of mirabilite in the precipitates, it, is obvious that Garabogazk€, ol is a poor example for, marine salt deposition., Lake Assal in Djibouti may much better represent, a salt basin fed by seawater and comparable ancient, , 383, , equivalents are known along passive continental, margins (Jackson et al. 2000). Assal is a crater lake, in the Afar Depression of northeastern Africa, close, to the Gulf of Aden. In this region, the giant African, rift system passes into an oceanic spreading ridge., The setting is similar to Salton Sea in Southern, California. Lake Assal region is very arid and hot,, with resulting high evaporation. Lake Assal is 155 m, below sea level. Through a barrier of Pleistocene, volcanic rocks, sea water seeps into the lake and, convects through hot rock and rises in hot springs., Lake Assal water is a saturated brine with a salinity of, 34.8%. A broad salt flat supporting several commercial salt diggings occurs along its northwestern, shore. Starting up in 2010, a large-scale solar salt, project is planned to produce 4 Mt/year of salt from, sea water., , Geologically young marine salt lagoons have a, history of oscillating water levels that deposit a, succession of shallow-water and subaerial sediments. Evaporites with these characteristics represent a marine shallow water facies. This is in, contrast to evaporitic sediments laid down in, deeper water and in central parts of large salt, basins that have no modern equivalent., , Shallow water evaporites, Characteristic sediments of shallow water evaporites are laminated sulphates interbedded with, algal mats, clastic gypsum sands with graded bedding, broken crusts (tempestites) and gypsum, dune sands (Figure 3.20). In-situ grown poikilitic, and replacive crystals, and nodular sulphates form, in the soft sediment from pore brine. In places,, profuse growth of white sulphate nodules leaves, only stringers of dark matrix. Resulting textures, are called “mosaic”, or “chicken wire texture”., Similar soft-sediment processes lead to enterolithic textures of thin sulphate or more rarely, magnesite bands, presenting the appearance of, intestinal convolutions. Selenitic, that is clear, colourless gypsum crystals with typical swallow-tail twins grow to a length of several metres., They form palisades and rosettes near the surface,, pushing up small hillocks. Repetitive cycling, between subaqueous and subaerial conditions is
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384, , PART II NON-METALLIC MINERALS AND ROCKS, , caused by changes of water level (e.g. storm-driven, flooding) and climatically controlled rhythmic, salinity variations (Peryt 1996). In the characteristic shallow water evaporite successions, structure and texture of gypsum and anhydrite deposits, are the key to understanding the sedimentary, environment., In a salt lagoon, halite can crystallize at the brine, surface or on the bottom. At the surface, halite, first nucleates in the form of tiny hopper crystals., These are very distinct inverted pyramidal skeletal crystals that grow mainly along the water, surface (in two dimensions). Hopper crystals, assemble to plates (Figure/Plate 4.12a) and when, their weight is too large for floating, founder and, end up as detrital halitite on the lagoon bottom., Halite precipitating in shallow water (<2 m) on the, bottom grows in the shape of elongate vertical, crystals that appear like palisades in a cross-section (“chevron texture”). Under the microscope,, both halite types display growth zones by linear, and planar arrangement of brine inclusions. Displacive halite crystals in clay grow both in subaqueous and subaerial conditions. Clastic halite, sand may be formed by brine flow. Halite pisoids, originate on beaches by wave action. Halite ooids, indicate involvement of bacteria (Castanier et al., 1999). The porous mushy halite sediment is rapidly lithified by clear halite cement. Only centimetres to decimetres below the surface, halitite is, commonly a hard rock., In 1970, salt reefs started to grow in large solar, evaporation ponds that had been built in the, southern Dead Sea. The aim of these basins was, to precipitate halite and to use the residual brine, for production of potassium, magnesium and trace, elements such as bromine. These salt reefs are, mushroom shaped, round or elongate build-ups, with a height measured in metres. They coalesce, and tend to partition the ponds into thousands of, small compartments that enclose stagnant brine., Instead of being extracted according to planning,, carnallite precipitates uselessly in the small, basins enclosed by reefs. The epidemic afflicted, several hundred km2 of salt works both in Israel, and Jordan (Talbot et al. 1996). Salt reefs have not, been reported from ancient salt deposits. Most, probably, cross-sections of such structures have, , been observed in salt mines but their significance, may have been missed., 4.2.2 Salt formation in the geological past, Similar to the deposition of large masses of coal,, large salt deposits cluster in distinct geological, periods and regions. Traces of halite are very common in the geological record. Large salt formations, however, with a thickness of hundreds of, metres and an areal extent of thousands of km2,, and with valuable potassium salt seams are quite, restricted. Major controls for their formation are, climate and a favourable tectonic configuration of, sedimentary basins. Before discussing this in more, detail, let us briefly look at the chemistry of seawater through geological time., Seawater in the geological past, Ample evidence demonstrates that salinity, chemical characteristics and the composition of dissolved matter in seawater varied considerably, during geological history (Coggon et al. 2010, Anbar, 2008, Horita et al. 2002, Knauth 1998). Therefore,, many observations deduced from modern seawater, may not be applicable to the geological past. The, average water temperature probably declined from, 70 C in the Archaean to 20 C 800 million years, ago (Robert & Chaussidon 2006). Sulphate was rare, in Precambrian seawater until the Mesoproterozoic, when it started to rise and reached 75% of, today’s concentration with the onset of the Cambrian (Kah et al. 2004). Since the Cretaceous, seawater is highly undersaturated with respect to, silica, because of the emergence of silica-consuming diatoms (Grenne and Slack 2005). The isotopic, composition of the oceans is a function of the size of, polar ice sheets. Changing CO2 content in the, atmosphere shifts seawater pH. The pre-industrial, state is estimated at pH 8.17 and since, rising CO2, lowered pH by 0.1 units (“acidification”). Considering that CO2 contents in the geological past were, nearly always much higher than today (Normile, 2009), significantly less alkaline seawater was certainly the norm. d11B in marine carbonates is a, proxy of ancient ocean pH which varied between, 7 and 8.5 (Kasemann et al. 2010). Ocean water, probably was virtually free of oxygen and iron-rich
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 385, , Figure 4.13 Secular variation of Ca2 þ and, SO42 concentration in seawater during, the Phanerozoic (after Lowenstein et al., 2001, 2003). These changes controlled, deposition of marine sulphate, and, chloride or sulphate type potassium salts., , in the first half of Earth history. The oxidation state, of atmosphere and ocean in the Precambrian controlled the seawater SO42 concentration (Fike, et al. 2006) and the bioavailability of essential trace, elements (Anbar 2008). Between 1.8 and 0.8 Ga, H2S, sulphur probably prevailed in ocean water and only, after complete ocean oxygenation at 700 Ma, dissolved SO42 sulphur became the norm (Anbar, 2008). In the Permian, oxygen concentration in the, atmosphere reached 35% compared with today’s, 21%. This induced unusually high oxygen concentration in the oceans. At other times, anoxia was, prevalent (e.g. Mid-Devonian, Early Jurassic) and, biota abstracted sulphur, which was fixed in, reduced sediments. Low sulphate concentration, in seawater can also be caused by mass precipitation of gypsum (Wortmann & Chernyavsky 2007)., Apart from abstraction in evaporites, changes of, major cation concentrations in seawater can be, induced by:, . varying input of river water and sediment (continental weathering);, . fluctuating rates of spreading and hydrothermal, convection at mid-ocean ridges (stripping Mg from, seawater, Coggon et al. 2010, Conrad & LithgowBertelloni 2007); and, . changing rates of seawater alteration by reactions on ridge flanks and across the ocean floor in, general (Demicco et al. 2005)., Conspicuous are the secular variations of the, Mg/Ca ratio and of the Na þ and SO42 concentrations, which have caused systematic variation of, potassium salt rock composition (e.g. the chloride/sulphate facies: Hardie 1996, Horita et al., , 2002). During the Late Precambrian, in the, Permo-Triassic and from the Tertiary until today,, considerable MgSO4 (kieserite) contents in potassium salts are evidence for high Mg/Ca ratios, (>2.5) and for elevated Na concentrations. The, reverse is observed from the Cambrian to the, Devonian and in the Cretaceous, with deposition, of chloridic, sylvite-rich K-Mg-Ca salt rocks, (Figure 4.13)., Periods of evaporite formation, In the Archaean, only traces of evaporites are, known; some of the Isua rocks of Greenland display evaporitic features. Cherts with an age of, 3.3–3.5 Ga in the Pilbara Nucleus of Western Australia contain pseudomorphs after halite crystals., In the Proterozoic, the former presence of salt, rocks is suggested by relatively frequent proxies,, such as anhydrite, albite and scapolite (Evans, 2006). The oldest known halitites occur in Neoproterozoic sediments of Australia, Pakistan (Salt, Ranges), Iran and Oman (Hormuz Salt). Together, with earliest Phanerozoic evaporites, these giant, salt accumulations mark the termination of the, 1.5 to 2 times higher salinity of Precambrian compared to later ocean water (Knauth 1998)., Beginning with the Cambrian, salt formation, and its preservation was much more common, than before (Evans 2006, Warren 1999, 2006)., During the Cambrian, the first and one of the, largest of the Earth’s salt giants (exceptionally, large evaporite basins) was formed in Siberia., Other important Cambrian salt formations occur
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386, , PART II NON-METALLIC MINERALS AND ROCKS, , in Pakistan and in the Persian Arabian Gulf region., Silurian salt is known in the Baltic States, USA, and Australia. Devonian salt formations in western Canada (the Prairie Evaporite) are the base for, the country’s role as the world’s largest potassium, producer. Thick Devonian salt is present in the, Dniepr-Donets Basin of Belarus and Ukraine. On, the supercontinent Gondwana, Permian salt was, formed in several areas, including the mid-western USA (Carlsbad, New Mexico), the forelands of, the Ural Mountains, in the north European Zechstein basin (England, Northern Germany, Poland), and in the Eastern Alps (Ziegler et al. 1997). Triassic salt is exploited in southwestern Germany, in, the North African Maghreb province, Britain and, Spain. Jurassic salt of the Smackover and Louann, salt formations seals many of the extremely, rich oilfields of the Gulf of Mexico province. With, 2.4 million km3, this is the largest of all saline, giants on Earth (Evans 2006). Cretaceous salt formations in the Khorat Plateau, Thailand, host, Halitite, , potassium salt seams of significant economic, potential (El Tabakh et al. 1999). Cretaceous salt, also occurs in the Amazon province and in, Egypt. Of Palaeogene age are potassium deposits, in the Spanish Ebro basin and in the Upper, Rhine Graben. Neogene salt is exploited in the, northern foreland of the Carpathian Mountains, and has an important role by sealing hydrocarbon reservoirs in the Red Sea, the northern, Arabian Gulf (the Fars evaporites above the, Asmari limestone reservoir) and southern Iran, (Gachsaran evaporites). With an age of 5.9–5.3, Ma, the Messinian evaporites in the Mediterranean region are the youngest of the marine, saline giants (Ryan 2009)., Environments of evaporite formation, in the geological past, In geodynamic terms, large masses of evaporites, are likely to occur in four main settings:, , Karlsruhe, , Potassite, , Colmar, , orest, Black, F, , Rhin, e Riv, , er, , Vosg, es, , Strassbourg, , Freiburg, , Mulhouse, Potash Basin, Mulhouse, , Jura Mountains, , 50 km, , Figure 4.14 Early Tertiary (Middle Eocene-Early, Oligocene) syn-rift evaporites in the southern Upper, Rhine Graben contain potassium salts of the chloride, type. Dotted areas include a gypsum-zone surrounding, the salt, and marginal non-evaporitic rift sediments of, alluvial fans and plains, and lacustrine limestone, (modified from Warren 1999).
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 1 in continental rifts (e.g. Upper Rhine Graben:, Blanc-Valleron 1997; Figure 4.14);, 2 in rifts that precede the opening of a new ocean, and in transitional early stages of the newly born, sea (Red Sea, passive continental margins);, 3 in late-orogenic foreland basins of mountain, belts (e.g. the Carpathian foredeep);, 4 in wide epicontinental basins (e.g. Zechstein, within Pangaea)., A somewhat unique case is the intracollisional, setting of Mediterranean oceanic basins hosting, the Neogene Messinian evaporites:, When the Straits of Gibraltar closed in the Late, Miocene (5.96–5.33 Ma: Krijgsman et al. 1999) by, collisional movement of Africa and Eurasia, the, Mediterranean Sea was isolated from world oceans, and quickly dried out (the “Messinian salinity, crisis”: Hs€, u 1972). Marginal shallow salt basins, such as in Sicily and Spain formed first, followed, by halitite on the sea floor at >3 km below the, Atlantic Ocean surface (Clauzon et al. 1996). Salt, precipitation started under deep basin/deep water, conditions (see below) and ended with a deep basin/, shallow water stage and the desiccation of the Mediterranean to residual salt pans on the former sea, bottom (Ryan 2009). Messinian basinal evaporites, reach a thickness of 2000 m and extend over wide, areas beneath the Mediterranean Sea. With a volume of 1 million km3 (Evans 2006), the Messinian is, the fourth-largest of the presently known “saline, giants” (Figure 4.15b)., , Deep water evaporites, Many fossil evaporites display a sedimentological, character that points to deposition in deep water,, (a), , in contrast to the shallow setting of virtually all, present evaporite formation. Striking evidence, for a deep-water facies includes turbidites, (Rimoldi et al. 1996), mass flows (East Alpine, “Haselgebirge”, see below), and the bedding and, lamination of salt rocks that is constant for hundreds of kilometres (German Zechstein, see, below). Laminites of the shallow-water facies are, laterally confined; wedging out of single bands, occurs after a few metres and usually can be ascertained within one exposure., Laminites of deep-water halitite are typically, banded by thin dark bands in a lighter mass of, salt. Usually, the dark laminae consist of sulphates with accessory clay, carbonate and organic, substance. Certain stratigraphical levels may be, characterized by cm-scale, often folded or boudinaged anhydrite bands (Figure 4.2). In certain periods during formation of the Zechstein salt,, the system oscillated 100 to 1000 times between, halitite and anhydrite deposition. Certain layers of, this anhydrite can be nodular or acicular, indicating replacement of primary gypsum. With, 1–10 mm, dark bands in halitite are commonly, thinner than light pure salt bands with a typical, thickness of 10 cm. These rhythmic sulphate, bands are laterally very persistent and obviously, represent planar bedding planes., Halite of the deep-water facies crystallized, either on the brine surface, where evaporation, is strongest, or in the case of stratified salt, concentrations at depth where supersaturated, brine flowing down from basin margins precipitated halite during cooling. Mass-flow of, salt mush from basin margins is a further, , Platform evaporites, , Mudflat, ≈ 100 m, , Figure 4.15 Coastal salt lagoons, or, saltern (a) are common in hot and dry, regions of the world, but no present-day, example exists for the immensely large, salt-filled basins (b, saline giants) of the, geological past (after Warren 1999)., , 387, , Saltern, , ≈ 5 - 10 km, , Exposed barrier, , Ocean, , Sea level, , Reflux, , (b), Ocean, , 10 to several 100 km, , Saline Giant, (basinwide evaporites), , Se, ep, age, , > 1 km, , Mudflat/saltern evaporite, Brine level, Slope evaporite, Basinal evaporite
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388, , PART II NON-METALLIC MINERALS AND ROCKS, , possibility. Regularly laminated halite, however, is, rather due to a steady rate of salt formation, not to, singular events. Either chevron crusts forming on, the floor or a rain of hopper crystals and halite, plates are the most probable salt depositing processes. The lamination of deep-water salt rocks, resembles glacial lake varves. In melt water lakes,, one varve (a couplet) consists of a light and coarse, summer layer and a thin, dark winter layer. Consequently, one glacial couplet represents one year., The “halovarves” of salt rocks were equally, assumed to reflect an annual rhythm, hence the, German salt miners’ term “Jahresringe” (annual, growth rings) that is also used in English papers., Confirming this hypothesis, Messinian gypsumdominated couplets were most likely deposited, annually and even reveal a strong influence of El, Niño Southern Oscillation (Galeotti et al. 2010)., The causal relation would be dry and wet seasons,, with dark bands forming during more humid periods. Generally, the 11-year solar cycle (sunspot, cycle) and even longer climate cycles caused by, orbital oscillations are equally possible causes., Palaeogeography, The spatial distribution of evaporite zones, or, facies (Table 4.5) is a function of morphology and, hydrology of a salt basin. Often, a vector of increasing concentration can be observed, for example, from a leaky barrier to more distal parts of the, basin. In other cases, the bottom reflux of hot brine, from margins causes salt precipitation of high, evaporative facies in deeper, central parts of the, basin. In the case of desiccating basins without an, influx from the sea, evaporite zones are arranged in, rings with increasing concentration from margin, to centre. Apart from the scientific quest for, understanding, mapping and analysing the spatial, and temporal distribution of evaporite facies is of, great practical value. Exploration and extraction of, evaporite resources, such as potassium salt, are, substantially facilitated., Basin models and isolation models, Two hypotheses can explain the great thickness of, fossil salt formations: i) synsedimentary tectonic, , subsidence similar to orogenic siliciclastic flysch, and foreland basins; and ii) previously formed,, deep depressions, which are gradually filled with, sediments. It has been argued against the first that, the high sedimentation rate of halite reaching, 14 cm/year in present salt lagoons surpasses measured tectonic subsidence rates by several orders of, magnitude. Furthermore, the predominant deepwater facies of ancient evaporites can only be, explained by formation in deep basins. But, of, course, even deep basins may be sites of tectonic, subsidence. We now know that the specific combination of tectonic and non-tectonic contributions to evaporite sedimentation varies between, individual basins. Shallow basins display salt formations of lesser thickness. Three principle basin, models cover most natural variations of evaporite, deposition (Sonnenfeld 1991; Figure 4.16), but, complex and mixed systems are the common case:, . deep basins with deep-water facies evaporites, (e.g. part of Zechstein, see below);, . deep basins with shallow-water facies evaporites, (basinal Messinian salt in the Mediterranean);, . shallow basins with shallow-water facies evaporites (Danakil Rift: Hardie 1990; marginal Messinian salt in the Mediterranean)., At first sight, the predominance of certain salt, rocks such as halitite or sulphates over hundreds, of metres of evaporite sedimentation is a paradox., Table 4.6 and Figure 4.10 provide thickness relations that theoretically result from closed-system, evaporation of seawater. This is, however, hardly, ever observed in nature. The rule is the formation, of one facies (e.g. halitite in the Zechstein) as a, background, which is punctuated by intercalations of higher and lower concentration. This, implies preservation of a narrow concentration, range for a long period, which may require backflow (reflux) of brines with salts of higher solubility to balance inflow as already realized by, Ochsenius (1877). Reflux may occur below the, inflowing seawater (near-surface reflux) or, through permeable rocks of the barrier (seepage, reflux, Figure 4.15a). If reflux is not possible, for, example in desiccating deep basins (Figure 4.15b),, a constant concentration grade can only be preserved by a delicate equilibrium between seawater, inflow, evaporation and the mass of water and
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , Brine level, , Saltern and, mudflat, , (a), , 389, Sea, , Sill, , Deep water, Deep basin, Cumulates, laminites and, gravity-displaced deposits, , Prograding mudflat, , (b), , Saltern, , Shallow water, Shallow basin, Periodic overflow, Sea, , (c), Figure 4.16 Schematic basin models of, deep and shallow evaporite basins,, including possible combinations with, deep-water and shallow-water, sedimentation (modified from Warren, 1999)., , Sill, Continental, groundwater inflow, Brine level, , Shallow water, Deep basin, , salts in the basin. Mathematical modelling of the, mass balance in natural settings is most instructive (Eugster 1984)., The Zechstein (Late Permian) salt formation, in Northern Europe, The major salt formation of Europe is the, Zechstein (originally a German miners’ term,, translating as pit rock). In Germany, England, The, Netherlands and in Poland, former and presently, active mines provide access to ancient deepwater, evaporites, although potassium salt seams represent desiccation phases., In wide parts of Europe, the Variscan orogeny, ended with deposition and folding of giant coal, measures in the Late Carboniferous. Extension, and gravitational collapse of the orogen in the, Permian and thermal contraction of the lithosphere induced formation of the wide north-central European Permian rift basin. This was an arid, land below sea level, which accumulated continental red sandstones, shales and conglomerates,, felsic volcanic rocks, minor coal and playa-lake, evaporites of the Rotliegend sequence. The arid, setting is a stark contrast to the organic-rich, , e, pag, See, , Saltern and mudflat with, occasional deeper depressions, , sediments of the tropical Late Carboniferous coal, measures. Burial of peat depressed atmospheric, CO2 to very low concentrations, amplifying continental glaciation within Gondwana, which, lasted from 326 (late Early Carboniferous) to, 267 Ma (Mid-Permian, Horton & Poulsen 2009)., Waxing and waning of the extensive ice sheets, controlled climate and sea levels. The end of, Gondwanan glaciation caused a rapid climate, change from humid-equatorial to desert conditions in wide parts of Pangaea. The global, Late Permian climate was hot and dry. Boreal, forests and coal swamps only formed at highest, latitudes and reptiles lived within 15 of the, South Pole (Torsvik & Cocks 2004, Ziegler, et al. 1997). Final ice melting led to sustained, sea level rise and the Zechstein basin was flooded., The first sediment deposited was a black shale, bed (the European Copper Shale). Because the, connection to world oceans was restricted and, arid conditions unbroken, the basin developed, into a large evaporite pan receiving the Zechstein, sequence., Exploitable Permian salt is restricted to the, Zechstein. At this time, ocean water flowed, through a restricted channel between Norway and
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390, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 4.17 Palaeogeographic sketch of the, Zechstein Basin in Pangaea before the Atlantic Ocean, began to open (adapted from Ziegler 1982 and Warren, 1999). The basin was at about 20 northern latitude, (Torsvik & Cocks 2004). Inside this giant landmass,, the climate was extremely dry and displayed a, seasonal change of monsoonal wind direction., , Greenland (a graben?) from today’s Arctic into the, centre of northern Pangaea (Figure 4.17). Nearly, everywhere in the basin, four cycles of evaporite, sedimentation (Z1–4) can easily be distinguished., Two rather stunted ones (Z5–6) have been added in, recent years by deep drilling for oil and gas. Each of, the cycles starts with clastic sediment (commonly, clay indicating high water levels), which is followed by carbonate, sulphate, halitite and Mg-K, salts (reflecting progressive desiccation). Above, the potassium seams, a thin recessive package of, halitite and anhydrite forms the hanging wall,, before a new cycle starts with a clay bed. Most of, the formation names date from pioneer times of, potassium salt mining in Germany during the, second half of the 19th century, after Liebig, (1842) had discovered the principle of inorganic, plant fertilizers, and K-salts became a lucrative, commodity. In terms of sequence stratigraphy, (Miall 1997), a new stratigraphical classification, of the Zechstein suites comprises ZS 1–8 (Strohmenger et al. 1996), somewhat different from the, traditional cycles as represented in Table 4.7. With, a total volume of only 200,000 km3 (Evans 2006), , the Zechstein evaporites are not among the saline, giants., Near the margins of the basin and around islands, emerging from the saline inland sea (e.g. Harz, Mountains), shallow-water carbonate and sulphate “reefs” or “walls” were building up. Bryozoa, formed the Werra Carbonate (Ca1), whereas Ca2, and Ca3 were built by algae. In deeper parts of the, basin, the carbonates pass gradually into dark,, banded and thinner limestones or dolomites, (Figure 4.18). In certain deeps, thin bituminous, “smelly shales” represent the time horizon of, marginal carbonates. These rocks are a source of, exploitable hydrocarbons. Marginal sulphates display shallow-water facies characteristics. From, the coastal sulphate walls, mass flows and turbidites descended into the deeper basin. Water, depth, however, was not constantly deep but oscillated between deep and shallow. A moderate drawdown can be assumed for the halite-sulphate laminates. Nearly dry conditions are indicated for the, formation of potassium salts, mainly by sedimentary features of the seams. Potassium seams consist of carnallite or sylvite layers, with bands and
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392, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 4.18 Characteristic cross-section of, a sulphate “wall” on the southern margin of, the Zechstein Basin (after Braitsch 1971)., With permission from Springer, Science þ Business Media. In this setting, little salt was formed, and non-saline rocks, of Zechstein Cycles 1 (Werra) and 2, (Stassfurt) are in direct contact. Not to scale., , Permian salt in the Eastern Alps (Austria), Late Permian (to earliest Triassic?) salt deposits, occur in the Northern Calcareous Alps, a tectonic, subunit of the Upper East Alpine thrust sheets, (Figure 1.89). Anhydrite and gypsum deposits are, exploited over the whole outcrop length of this, unit (some 600 km) but salt is restricted to a central part from Salzburg to Tyrol. The sediments, imply a rift setting within Pangaea that later, developed into the Meliata Ocean, which is one, of the ephemeral Tethys-related oceans that, opened and closed during formation of the Alpine, orogen in Central Europe (Neubauer et al. 2000)., Terrestrial sediments and bimodal volcanics of, Permian age resemble the Rotliegend and display, remnants of arid alluvial fans, flood plains, siliciclastic sabkhas and playa lakes. First orogenic, deformation of the Calcareous Alps took place, in the Middle Jurassic, when nappes formed, and during advance, shed and buried thick turbiditic wildflysch (a tectonic-sedimentary melange), along their fronts. Kilometre-sized blocks of Permian evaporites and marine shelf sediments were, embedded in the wildflysch (Frisch & Gawlick, 2003). In the following prolonged period of Alpine, orogenic deformation, the salt was very mobile, and is now found along nappe thrust-soles, in, anticlinal traps, or in stocks that faintly resemble, salt diapirs., Salt mining in the region started in Neolithic, times. An ancient term for the peculiar salt, formation of the region is Haselgebirge (in, German), which translates as “salt rock” or “salt, mountains”. The characteristic Haselgebirge, , rocks are debris-sheets and mega-breccias within, bedded and massive halitite. Rock debris includes, fragments of anhydrite, dolomite, claystone,, basalt and halitite. Opinions on the origin of the, breccias are still not settled. Interpretations vary, from synsedimentary mass flows entering salt, lagoons (with a tectonic overprint) to a purely, tectonic origin during overthrusting and nappe, propagation. The intensive deformation, or more, probably the chaotic incidence of mass flows, severely limits stratigraphical subdivision. The, following lithological units are discerned (Schauberger 1986):, . Gruntongebirge, €, (white halitite, anhydrite,, greenish clay and sandstone), and, . Rotsalzgebirge (red banded halitite with anhydrite, polyhalite, glauberite and Mg-Na sulphates;, black and red clay; Figure/Plate 4.3) make up most, the total thickness (possibly 1000 m) of the salt, formation. The second displays the highest K-Mg, contents of all units. It is assumed that these two, units are synchronous but deposited in different, facies, the first in coastal lagoons with considerable terrigenous siliciclastic import, the second in, areas of repeated desiccation;, . Buntes Salztongebirge (red and white halitite, with black, green and red clay, and numerous, horizons of rounded to subangular fragments of, vesicular basalt and mafic tuffite) may be an intermediate facies between the first and the second, (Schauberger 1986);, . Grausalzgebirge (grey halitite with red and grey, clay fragments, black dolomite) indicates reduced, conditions, probably in deeper parts of the basin.
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 393, , ceous). Tectonic deformation was intense when, the frontal wildflysch was overridden by the, nappes. However, already Schauberger (1986), presented arguments for a sedimentary origin of, the peculiar claystone debris-sheets in salt. Most, workers now agree that the haselgebirge is a modified mass flow sediment. Supporting arguments, include the continuity of certain lithofacies over a, considerable distance. In the Hallstatt mine, for, example, basalt and basalt tuffite occur in an, indentifiable lithostratigraphical horizon of the, salt body. Some of the debris flows display, rounded, others only angular fragments; this can, be explained by different water-solid ratios of distinct mass flows. Round pebbles of clear halite are, part of the debris. Halite, clay and sulphates in the, debris are derived from basin marginal shallowwater settings, or from a siliciclastic sabkha., Affirmative observations include cross-bedded, sulphates, mosaic-textured nodular anhydrite,, sulphate pillows that may have been selenite hillocks, polyhalite replacing anhydrite, rare calcrete, and red carnelian fragments, and the common, occurrence of displacive clear halite crystals in, claystone. Most of the debris fragments are, densely jointed but not deformed; joint fissures, are filled by fibrous or scaly halite. The combined, evidence points to haselgebirge formation in a, halite-stage restricted sea within a rift basin that, was bordered by arid alluvial flood plains, desert, mountains and distant volcanoes. Accumulating, shore sediments had the character of a siliciclastic, sabkha. Earthquakes related to rifting released, debris flows from clastic basin slopes and sulphate, walls (Figure 4.19)., , Relative age relations of the lithological units, remain unresolved., The preponderance of red colours indicates prevailing shallow water and oxic conditions. Thick, anhydrite and dolomitic anhydrite bodies are marginal equivalents of the salt formation. S and Sr, isotope data confirm that the Alpine evaporites, were formed from Permian seawater (Sp€, otl & Pak, 1996)., Bromine concentration in halite varies between, 50 and 350 ppm (Sp€, otl 1989). The higher figures, imply concentrations typical for the Mg-sulphate, zone and approaching the potassium facies. Potassite rocks, however, are unknown. In blocks of, anhydrite and clay, which slumped into salt, basins, polyhalite and accessory kieserite are not, rare. This makes it probable that high salinity was, locally reached in marginal positions, but not, sufficient for precipitation of carnallite or sylvite., An alternative possibility is loss of former potassium salt minerals by later dissolution, considering that Alpine salt formations must have, experienced several events of pervasive fluid passage. Yet, viable halobacteria and haloarchaea, have been isolated from Haselgebirge halite and, were successfully cultured (Stan-Lotter et al., 2003)., The Alpine salt bodies also display features of a, tectonic melange that originated when, during the, Late Jurassic, the evaporites acted as the ductile, sole of the (higher) Hallstatt nappes moving over, deeper tectonic units of the Northern Calcareous, Alps (Sp€, otl 1989). Many observations support this, statement, such as inclusions of giant fragments, of much younger country rocks (Triassic to Creta-, , Variscan desert mountains, , Shallow-water facies, Alluvial fans, , Figure 4.19 Schematic sketch of, Permian Haselgebirge formation, by salt-anhydrite-clay debris, mass flows in rifts of the nascent, Eastern Alps., , (Clay, silt, gypsum/anhydrite,, polyhalite, halite), , Deep-water halitite, deposition, Mass flow, , Anhydrite, gypsum, Extensional deformation,, Debris sheet, earthquakes, basaltic volcanism, Banded rock salt
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394, , PART II NON-METALLIC MINERALS AND ROCKS, , Sandling 1717 m, , W, Malm, er, Dogg, , ne, limesto, rite, la, io, rad, stone, rl, a, e-/m, , ic, , Liass, , lim, , E, Roter Kogel 1244 m, L, H, , Central shaft, Steinberg 945m, , H, , Franzberg 677 m, , Sinkhole and water, inflow, , L, , H, , Erbstollen (basal drainage tunnel), , Altaussee Lake 712 m, , Zlambach marl (Norian-Rhaetian), , Scree and soil, Hallstatt limestone, (Carnian-Norian), , L, , H, , Lustrous schists and leached, salt rock (Haselgebirge), , Haselgebirge (Permian), , 500 m, , Figure 4.20 Geological section of Altaussee salt mine, Austria. With kind permission from Salinen Austria AG., , Facies and thickness variations of post-salt sediments near Alpine salt deposits suggest that, already in the Triassic, salt pillows and possibly, diapirs were formed. The position of the salt at the, base of nappes is supposed to have facilitated, movement of thrust sheets, which imprinted, strong deformation on earlier salt structures. Illite, crystallinity, vitrinite reflectance and fluid/rock, reactions record a complex deformation and thermal history differing between, and even within, individual outcrops (Sp€, otl et al. 1998). The resulting “salt mountains” (Figure 4.20) faintly resemble common salt diapirs., In the Eastern Alps, the extraction of salt from, the salt-clay rocks was traditionally carried out by, underground construction of solution chambers, (“Laugwerk”). Low grade salt-clay rock (Haselgebirge) contains 30–40% NaCl, medium-grade, 40–60% and rich portions have 60–70% salt., Masses of pure salt were exploited by hard rock, mining methods. Today, the Alpine salt mines, have been converted to borehole leaching operations, as in the Altaussee salt mine (Figure 4.20)., The mine lies above the Altaussee Lake level and, is drained by a basal tunnel. The Haselgebirge salt, body is enveloped by a thin mantle of impermeable, claystone (“lustrous schists”). Several decades, ago, ancient near-surface leaching caverns in the, west had caused local roof collapse (a sinkhole) and, inflow of water, that was successfully plugged., , 4.3 POST-DEPOSITIONAL, , FATE OF SALT ROCKS, , 4.3.1 Diagenesis and metamorphism, of evaporites, Like other sediments, salt rocks are altered by, recrystallization, neogenesis of minerals and the, passage of various fluids, beginning at precipitation. In the past, scientists studying the highly, soluble potassium and magnesium salts considered even their early diagenesis as a kind of metamorphism. However, this term should be restricted, to truly metamorphic evaporites (see below):, Calcium sulphate is commonly precipitated as gypsum, except at elevated temperature in highly saline, pore waters when anhydrite is stable. Diagenesis, always alters gypsum to anhydrite, so that anhydrite, may be either sedimentary or diagenetic (Jowett et al., 1993). In some cases, replacement of gypsum by, anhydrite can be seen under the microscope. Chemical clues for a replacement origin of anhydrite include, lower strontium contents, because some strontium is, always abstracted when dehydration takes place., Mechanically and chemically, anhydrite is very stable, but CaSO4 undersaturated brines may leach, anhydrite and deposit halite. The conversion of gypsum to anhydrite induces a 40% volume reduction., However, the liberated water plus solid anhydrite, occupies 11% more volume than the original gypsum. If the system is closed, overpressures may result, that can cause intraformational brecciation.
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 395, , Table 4.8 Dehydration reactions of common salt minerals, Mirabilite (Na2SO410H2O), Epsomite (MgSO47H2O), Hexahydrite (MgSO46H2O), Carnallite (KClMgCl26H2O), , ), ), ), ), , Thenardite (Na2SO4) þ 10H2O (at 32.4 C), Hexahydrite (MgSO46H2O) þ H2O (at 26 C), Kieserite (MgSO4H2O) þ 5H2O (at 31 C), Sylvite (KCl) þ 6H2O þ MgCl2aq (max. 167.5 C at 32 MPa, function of pressure), , Ordinarily, the CaSO4 brines resulting from dehydration leave the system., , directly as geological thermometers, except for those, that are also a function of pressure (Table 4.8)., , Halitite: Newly formed salt sediment is a loose mush, of salt crystals with a high pore fraction, which is, filled with brine. More solid salt crusts also contain, large brine inclusions (Figure 4.12b). Investigations in, present salt-forming lagoons revealed that the pore, volume is rapidly diminished by continuing salt precipitation. In geologically very short time and at a, depth to a few metres, lithified salt rocks are formed., Increase of temperature and pressure during main, stage diagenesis provokes recrystallization, often to, equilibrium grain boundaries (foam structures)., , Water originating by dehydration (e.g. of gypsum) within a salt formation, or entering a salt, body from outside is quickly saturated with NaCl., In principle, this inhibits further flow within halitite, except under strong temperature or pressure, gradients. Experiments in the Asse mine demonstrated that saturated brine in tiny fluid inclusions, moves towards a heating element, obviously, because salt solubility rises with temperature., Another possibility is flow within induced porosity, when NaCl-brines pass through soluble K-Mg, salts. Flow takes place until the brine is in equilibrium with the surrounding salt rocks. The accumulated evidence shows that the passage of, diagenetic brines through salt bodies is common., Numerous exposures in potassium mines display, low-temperature fluid-induced sylvite formation, from carnallite (Figure 4.21). This is an obvious, contradiction to the earlier statement of the, extremely low intrinsic permeability of salt,, proved by high-technology measurements as well, as geological observations (e.g. salt trapping highpressured methane). The solution to the conundrum can only be found in geologically short, events that mobilize or deform salt strongly,, including faulting, volcanism and diapiric upflow, (Peach 1991) within a field of steep pressure and/or, temperature potentials., The alteration of K-Mg salts and sulphates starts, at the moment of precipitation (cf. back reactions, of Figure 4.9) and continues through early and, main stage diagenesis. The accumulated petrologic history of potassites is complex. For example,, the distribution of carnallitite, sylvinite, hartsalz, and residual halitite in the undulating K-seam, Th€, uringen in the Werra District (Figure 4.21) is, controlled by relative elevation (the primary, , For over one hundred years, the precise origin, and diagenesis of potassium-magnesium salt rocks, challenged numerous scientists and our present, understanding is far from complete. D’Ans (1969), and Braitsch (1971) summarized details about, results acquired during the pioneering times of, salt research. In the earlier papers, diagenetic reactions with passing fluids at stable temperature, (“solution metamorphism”) are distinguished from, alterations caused by rising temperature (“thermal, metamorphism”):, . Fluid-related diagenesis of potassites is best, explained by an example: If a sodium chloridesaturated but Mg-poor solution passes through, carnallite, incongruent dissolution of the carnallite extracts MgCl2aq and leaves a solid residue, of sylvite. Carnallitite will be transformed into, sylvinite or hartsalz. If the brine-flow continues,, KCl is leached also and the potassium seam is impoverished. Thin bands of red halitite may be the only, witness to the former presence of a K-seam., . Thermal diagenesis describes neogenesis of minerals by heating. The most common case is dehydration, which is a strong proof for temporary, permeability and an outflow from the observed system. Many reactions of this type are known (D’Ans, 1969, and earlier papers). The reactions can be used
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396, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 4.21 Facies map of near-horizontal, potassium seam Th€, uringen (Zechstein 1) in the, Herfa mine, Hesse, Germany (Braitsch 1971). In, elevated positions of the seam, residual halitite, (1Na), langbeinitic hartsalz (2HL) and sylvinitic, hartsalz (3HS) replace the “basinal” carnallitite, 4C). The alteration is attributed to early, diagenetic supergene leaching or to migrating, fluids mobilized by Tertiary volcanism., , carnallite only occurs in depressions) and, to some, degree, by proximity to faults. Tectonics and fluid, migration associated with volcanism are the, accepted interpretation of observations, but early, diagenetic processes such as meteoric leaching, cannot be excluded., Less changeable minerals in salt, mainly carbonates and silicates, provide further restrictions, concerning conditions of diagenetic change. Chlorite is formed from kaolinite and montmorillonite,, phengite from illite and albite from NaCl and clay., Mg-rich tourmaline, talc and sepiolite attest to, higher Mg-concentration in pore fluids, as does, magnesite replacing dolomite. Authigenic quartz, crystals are useful for microthermometry. Finely, fibrous SiO2 aggregates (e.g. in chert bands) consisting of lutecite with >20% moganite indicate, the former presence of salt (Heaney 1995)., , rifting and volcanism are often closely related., Numerous basalt/salt contacts have been exposed, by mines in the Werra District, Germany. At the, surface, the region is dotted with Tertiary volcanic, cones and lava flows. In the salt mines below, the, dykes widen in salt and cause a halo of alteration., At 80 m from the dyke, distal carnallite is first, altered to kainite. With decreasing distance, K-Mg, chlorides are impoverished until only residual, halitite is observed. At the actual contact, many, rare minerals are formed. Note that this alteration, is hydrous, melting of salt is not observed (the, melting temperature of halite is 801 C). The basalt, in the dyke is profoundly altered and soaked with, salt. A similar situation was described from eastern Siberia by Grishina et al. (1992)., , Orogenic metamorphism, Contact metamorphism, Contact metamorphism of salt rocks in the vicinity of subvolcanic dykes is not rare because salt,, , Orogenic metamorphism of evaporites usually, results in the removal of soluble salts with the, metamorphic fluid flow. Anhydrite, however, is
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , more stable and is reported from amphibolite, facies metamorphic rocks. When salts are leached, during metamorphism, relatively subtle clues, remain that point to the former evaporitic nature, of rocks. Indicators are higher concentrations of, minerals with unusual contents of Na, Mg, Cl, B, and SO4, including scapolite (Frietsch et al. 1997),, lapis lazuli (lazurite), Mg-tourmaline, phlogopite, and albite (Cook & Ashley 1992). Talc-kyanite, rocks called “white schists” are believed to represent Mg-rich pelites metamorphosed at conditions, of high pressure and low temperature during lithospheric subduction (>8 or 9 kbar and 700 50 C:, Bucher & Frey 2002). Meta-pelites of evaporitic, lineage may be chemically distinct by elevated, contents of Mg, Li and F, and higher Li/Mg and, B/Al ratios compared to averages. While evaporites are changed by metamorphism, so are the, metamorphic fluids that have passed through, them. They acquire the character of brines and, consequently cause alteration of large rock, volumes in the periphery, such as albitization and, scapolitization. Also, they may induce mineralization (cf. Chapter 1.4 “Ore Formation by Migrating Saline Brines”)., 4.3.2 Deformation of salt rocks, Salt is highly deformable. Nearly all salt rock, bodies display folding and flow structures, even, those that are interbedded with little deformed, sediments. Potassium seams in flat, tabular evaporites display internal recumbent folds. In salt, diapirs, their high ductility is emphasized by flow, fold patterns that resemble banded calcite marbles, in high-grade metamorphic settings., Underground mine openings in salt slowly close, by ductile convergence of walls. After tens or, hundreds of years, only traces of former human, presence are seen in solid salt, such as timber or, iron rails. This illustrates that salt rocks can be, considered as highly viscous Newtonian fluids., Enduring, weak differential stresses cause ductile, flow, whereas sudden loading at high stress ratios, provokes brittle fracture (Hunsche & Hampel, 1999). The flow of salt is termed “creep”, because, it cannot be seen. Depending on the relative values, of shear and normal principal stress, creep may, lead to volume increase (dilatation) or decrease, , 397, , (contraction). Dilatation (expansion) is regularly, observed near mine openings. It causes elevated, permeability and mechanical damage, and often, ends in brittle failure (e.g. spalling). Dilatant strain, may explain some cases of the above-mentioned, episodically higher permeability of salt for passing, diagenetic fluids., Creep of salt is macroscopically free of fractures, or fissures. Microscopic examination of etched, sections reveals, however, that single salt grains, are disjointed by microstructures into numerous, subgrains. Integrated movement of subgrains produces the overall creep. Microfractures “jump”, across grain boundaries. This results in the characteristic polygonal textures of mechanically, strained salt. A second mode of creep is achieved, by solution transfer, grain boundary migration and, diffusion. Migration of dislocations (one-dimensional crystal lattice defects) constitutes a third, component of creep (Miguel et al. 2001). High rock, and fluid pressure, but also higher humidity and, temperature of mine air promote the rate of viscoplastic deformation of salt. Seasonal variations, of these parameters in a mine at 650 m below the, surface still influence the rate of creep (Kwon &, Kim 2005). There is no lower boundary of loading, for inducement of creep. In geological time spans,, salt deforms at very low stress ratios. This is, illustrated by the Hormuz salt glaciers of Iran, (Kent 1979), which flow under their own weight., In this desert region, glacier movement accelerates, after a little rainfall., Any change of humidity, temperature and the, stress field such as the opening of a new mine, tunnel cause a variation of the deformation, rate (“transient creep”), which returns to steady, state creep only after adjustment to the changed, parameters. Mathematical modelling of salt, and associated rocks is possible with non-linear, physically-based material laws exemplified by, eq. 4.1 (Hunsche & Hampel 1999)., Creep of salt rock:, e* ¼ A: exp½Q=ðR:TÞ:sn, , ð4:1Þ, , e ¼ rate of creep; A ¼ structural parameter (type of, salt); Q ¼ activation energy; R ¼ universal gas constant; T ¼ temperature in K; s ¼ stress deviation (MPa);, n ¼ stress exponent (usually 5–7). A, Q and n are
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398, , PART II NON-METALLIC MINERALS AND ROCKS, , determined by experiments. All modes of salt deformation, from elastic-brittle behaviour to creep and, failure, are described by composite material laws, (Swift et al. 2001)., , Experiments as well as practical experience in, salt mines show that at comparable conditions,, different halitite rocks display characteristically, unequal creep rates. In the Zechstein mines, Z2, Stassfurt Halitite (Table 4.7) displays a distinctly, higher creep rate compared with Z3 Leine Halitite., This is not always a function of visible petrographical parameters, although in this case, grain size, and admixture of clay and anhydrite are different., Strength and deformation characteristics of haselgebirge are clearly a function of salt content. Carnallitite creeps like halitite but with a ten-fold rate, and fails after much less strain. This explains the, sudden collapse of whole extraction panels in, potassium mines, which may cause earthquakes., An earthquake of this type originating in the, former potassium salt mine Teutschenthal near, Halle (Germany) had a local magnitude of 4.8, (1996)., During deformation, salt rocks may acquire, directional grain textures but recrystallize (“heal”), readily. Traces of deformation, such as foliation in, Leine Halitite of the Asse mine (Germany) and in, the Khorat Plateau (El Tabakh et al. 1999), macroscopic fissures, or microscopic translation planes, and pressure twinning are rarely preserved. Normally, separation planes in salt including faults,, fissures, joints and schistosity are only recognizable if marked by some fill material. Common fills, include secondary fibrous halite, bischofite, carnallite, sylvite and anhydrite, as well as gas, petroleum and brine., Although the deformation of salt rocks is generally ductile, some are strikingly brecciated., Reports describe breccias of halite and even of, highly ductile carnallitite. It is speculated that, very low fluid pressures during deformation might, explain these observations., Different deformation characteristics distinguish the various salt rocks and the intercalated, clays, sulphates and carbonates. Salt rocks, are less competent and tend to flow, i.e. deform, ductilely. Competence (resistance to flow) increases from sylvite to carnallite to halite. Whereas, , salt rocks flow even at near-surface conditions,, associated sulphates, claystone and carbonates, tend to react by brittle deformation and display, jointing, fissures and boudinage. Typical disharmonic folding results from the widely different, mechanical properties of evaporites and their, country rocks., 4.3.3 Forms and structures of salt deposits, In epicontinental platform settings of littledeformed sedimentary suites, and with less than, 1000 m overburden, basin-wide salt-bearing evaporites occur in the form of essentially flat tabular, strata. As mentioned above, even in little, deformed salt beds, internal folding is observed., Associated more competent and brittle country, rocks display fractures and limited tilting, but, no ductile deformation. Examples include the, Devonian Prairie Evaporite in Saskatchewan, (Canada), Devonian salt in the Moscow basin, (Russia, Ukraine) and Permian (Zechstein) salt, in the Werra district of Hesse (Figure 4.21 and, Figure 4.22), which occupies a marginal platform, position relative to the main Zechstein basin., Note that in this region, only the first Zechstein, cycle is fully developed. Mining is based on potassium seam Th€, uringen with a thickness of 2 m. A, mechanized room and pillar mining method is, used in order to minimize the hazard of roof failure, and water inrush. Waste brines of potassium salt, processing are injected into the Flaggy Dolomite, aquifer (Figure 4.22)., Extensional tectonic deformation often causes, considerable movement of salt masses. Agents, include modification of the load and stress field, and the opening of flow paths along faults. In the, Mesozoic Basin of Northern Germany, pronounced mobility of Permian salt first occurred, during tectonic east-west extension in the Late, Triassic (Mazur & Scheck-Wenderoth 2005),, which was an initial phase of the break-up of the, supercontinent Pangaea. During this time, salt, glaciers formed on rift flanks (Mohr et al. 2007), and allochthonous salt intruded into higher stratigraphical levels., Compressional tectonic deformation shifts salt, into locations of lower pressure, generally
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 399, , Neurode Shaft, Holocene, Holozän, , Lower Triassic, Buntsandstein, , Groundwater, in sandstone, , 401 m, Bröckelschiefer, Upper Zechsteinletten, , Flaggy Dolomite, Lower Zechsteinletten, Rock salt, , Zechstein, aquitard, shale, horizons, , Reddish brown, shale and halitite, , Upper Werra halitite, , Figure 4.22 Geological profile of, the potassium salt mine HerfaNeurode in the Werra region, Hesse,, Germany, indicating the position of, the storage area for toxic and, chemically aggressive industrial, waste. Note the characteristic, pattern of room-and-pillar mining., With kind permission from Kali &, Salz AG, Kassel., , Hessen potassium, seam, , 704 m, , Waste storage, area, , anticlinal fold cores and salt ridges (Ings & Beaumont 2010). Often, the salt-bearing evaporites, constitute a detachment horizon between folded, cover rocks and brittle, faulted basement underneath. An excellent example of the resulting thinskinned fold and thrust belts provide the Jura, Mountains (Switzerland-France: Philippe 1994)., Similar effects show seismic images produced for, gas exploration in northeastern Germany; this, region experienced north-south compressive tectonic deformation (“inversion”) in the Late Cretaceous-Early Palaeogene (Kley & Voigt 2008, Mazur, & Scheck-Wenderoth 2005, Kossow et al. 2000)., Flow of salt into anticlines forming elongate salt, , 3 km, , Middle Werra halitite, , Zechstein, salt rocks, , Thüringen potassium, seam, , 731 m, , Lower Werra halitite, , 825 m, , Werra shale, carbonate and, anhydrite, , Rotliegend sandstone and shale, , pillows is balanced by abstraction of salt below, adjoining synforms., Salt in Alpine-type fold and thrust belts induces, detachment and facilitates movement of nappes, (c.f. haselgebirge in the Eastern Alps). The form of, salt deposits in this setting is the combined result, of early diapirism, salt flow, tectonic shortening, and possibly, late diapirism. The melange of, Alpine thrusting, nappe movement, folding and, repeated salt flow is not easily resolved, (Figure 4.20)., The characteristic form of salt deposits in thick, epicontinental sedimentary basins and in passive, continental margin settings are salt domes, or
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400, , PART II NON-METALLIC MINERALS AND ROCKS, , SW, , NW, 0, , Recent, , m, , -1000, -2000, -3000, , Gorleben diapir, Tertiary, , Jurassic, Late Triassic, Middle, Early Triassic, , Late Crt., Early Crt., Zechstein, Late Permian salt, , -4000, , 0, , Base Tertiary, , m, , -1000, -2000, -3000, , 0, , Base Cretaceous, , m, , -1000, -2000, -3000, , 0, , Base Keuper (Late Triassic), , m, , -1000, -2000, -3000, , diapirs. In diapirs, the salt pierced its former roof, and intruded younger sediments, quite similar to, small granite stocks (Figure 4.23). Host rocks in, contact with the up-flowing salt are dragged, upwards and ductilely bent, and slabs of the roof, are pushed to the side and occasionally overturned. Plug-like parts of the roof may be uplifted, for several kilometres (Perthuisot & Rouvier, 1992). Phenomena like this are thought to originate during a stage of early, active diapirism,, which is the process of forceful intrusion of salt, into roof rocks. During this stage, the permeability, of salt is relatively high, similar to rocks undergoing metamorphic shearing. The crest of an active, diapir may break through the sediments to the, surface, on land or under water cover., Active diapirism is usually followed by a downbuilding phase, when the diapir rises passively, (passive diapirism), while sediments accumulate, in the rim synclines. At shallow depths where, horizontal stresses are small, or at the surface,, diapirs expand horizontally. This results in carrot, or mushroom-like cross-sections of diapirs, the, formation of salt tongues and coalescing salt canopies (e.g. Great Kavir Basin of northern Iran:, Jackson et al. 1991). Salt glaciers that flow laterally, off outcropping diapirs, either below the sea or, subaerially, may result in allochthonous salt, , Figure 4.23 Characteristic development, stages of a salt dome, here illustrated by, the Gorleben diapir, Northern Germany, (modified from Zirngast 1991). With, permission from www.schweizerbart.de., This salt body is under investigation as a, future repository for high-level, radioactive waste. The salt-pillow stage, was reached in the Late Jurassic. Active, diapirism took place in the Early, Cretaceous and was followed by passive, diapirism, which continued into the, Tertiary. The salt diapir in the West was, formed much earlier., , sheets. On the top, the sheets may carry a sedimentary package (Figure 4.24). The feeder stems, beneath a diapir may close so that supply of salt, from below is cut off (“pinched-off salt domes”:, Jackson et al. 1998). Manifold are the variations, of diapiric salt structures; even in one district,, tower-like diapirs occur near elongated walls, (Figure 4.25)., The internal structure of salt diapirs is characterized by ductile flow deformation. In spite of the, drastic change of the outer form from tabular, shape to a plug or wall, the lithostratigraphical, order is commonly well preserved. Bromine profiles are little different from undeformed stratigraphical equivalents (K€, uster et al 2009). The, oldest, originally deepest salts occur in the centre,, mantled by younger and higher beds (Figure 4.26)., Many salt diapirs display internal folding with, nearly vertically plunging fold axes (“curtain, folds”). Certain diapirs, such as some in the American Gulf Coast region, are composed of several, individual intrusions separated by “anomalous, zones”, which contain inclusions of host rocks,, brines, gas and petroleum., The thermal conductivity of salt rocks (6 W m1 C1), is about three times higher compared with common, country rock sediments (1.5–2.5 W m1 C1). As
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 401, , Figure 4.24 Schematic profile of a composite salt-sediment glacier, showing one possibility (a gravity slide or flow), of the origin of allochthonous salt sheets (Fletcher et al. 1995). AAPG [2010] reprinted by permission of the, AAPG whose permission is required for further use. Salt glaciers that formed on the sea floor in the geological, past are not easily recognized. Here, tensional and compressional features are pointed out that may assist in, discrimination from laterally intruded salt sheets., a result, heat flow in salt is higher and temperatures exhibit characteristic anomalies. Relative, to adjacent host rocks, salt is warmer in the upper, part of the diapir and cooler at its base (Figure 4.27,, Manhenke & Beer 2004). Because temperature is a, primary control on kerogen maturity and hydrocarbon, formation, the thermal properties of salt exert important constraints on petroleum and gas deposits (Thomson & Lerche 1991). Also, the fluid flow regime around, diapirs is determined by temperature gradients. The, difference of temperature and salinity that is induced, by the contact of salt with permeable sediments, causes convection in adjoining aquifers. This may, induce clogging of pores by precipitation of minerals, such as anhydrite (K€, uhn & G€, unther 2007)., , Salt intrusions are initiated in either compressive or distensive tectonic settings. The first are, mostly pierced salt-cored anticlines, the second, are rooted in normal faults and rifts. Salt intrusions connected to anticlines are very prominent, , in Romania, in Tunis and Algeria, and in the, Zagros fold belt of Iran. Often, the linear tectonic, control is expressed by a long strike length of salt, structures (“salt walls”) that appear on maps as, swarms of “salt lines”. In Northern Germany, the, pattern can be simplified to (1) Late Triassic N-S, directed transtensional structures that parented, more isolated diapirs, and (2) Late Cretaceous, cross-cutting transpressional salt walls striking, NE-SW., Another large group (3) is constituted by salt, diapirs that have no recognizable tectonic trigger, and whose upflow and intrusion appears to be, totally autonomous. These stocks are round or, elliptic in map view, appear to be randomly distributed through a region and cut through little, disturbed cover rocks. Part of the Gulf Coast and, German diapirs conform to this description. For a, long time, the origin of non-tectonic salt diapirs, was hotly disputed., , Gorleben, m, 30, 00, , Figure 4.25 Salt structures, in the Gorleben region,, Northern Germany, (modified from Zirngast, 1991). With permission from, www.schweizerbart.de., Overhangs of diapirs are not, shown. Prior to salt flow, the, total thickness of the, Zechstein salt was about, 1400 m., , Dömitz, Arendsee, , 2200, , 20, 10, , 00, , 1800, 1400, , 00, , 1000, 600, 200, , 0, , 10, , km, , 40, , 200, , 0, , 600 1000, , 1800, 0, 140, 1000, 600, , 200, , 200, , 200, , 400, , 600
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402, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 4.26 Geological profile of the Gorleben salt diapir, Northern Germany, which is explored as an underground, repository for highly radioactive waste (modified from K€, othe et al. 2007). With permission from www.schweizerbart., de. The envisaged storage depth (“exploration level”) is 800 m below ground surface, within a massive body of, Z2/Z3 halitite. PA ¼ Pegmatite Anhydrite, RT ¼ Red Salt Clay, HA ¼ Main Anhydrite, GT ¼ Grey Salt Clay,, SF ¼ Stassfurt Seam potassite (carnallitite), HS ¼ Main Salt, caprock black. Note the easterly overflowing body, of Z2 salt; is this a former submarine salt glacier?, , The physical base for an explanation of the, upflow of salt was provided by Arrhenius & Lachmann (1912), who recognized that salt rocks are, lighter (less dense) than clastic sediments and, , carbonates forming their hanging wall. This is a, system of inverted density like the classical Rayleigh-Taylor (RT) instability. After early diagenetic pore filling, the density of halitite deviates, , Figure 4.27 Crosssection of geology and, temperature field, imposed by the Kotzen, salt diapir, Brandenburg,, Germany (Manhenke &, Beer 2004).
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , little from the average of 2.2 g/cm3. Unconsolidated sediments display a density of 1.6 to 1.9 and, in fully compacted state 2.6 to 2.8 g/cm3. The, difference illuminates the potential energy of such, a system. Very subtle factors may trigger lateral, and upward flow, including small elevation differences of the footwall, either of tectonic (faults) or, morphological nature (buried hills: Figure 4.26). In, a similar manner, weight differentials in the hanging wall of the salt (e.g. lateral facies change), induce flow. At the stage of thin cover before RT, instability is established, a system of viscous pressure ridges and minibasins may form, e.g. in the, Gulf of Mexico (Ings & Beaumont 2010)., When Trusheim (1957) analysed characteristics, of salt structures in Northern Germany, he divided, the process of diapirism into several stages, (Figure 4.23):, 1 Once the “critical overburden thickness” (density inversion) is exceeded, salt starts to flow, towards lower pressure domains, first forming, “salt pillows”. Updoming above the pillow crest, and lateral abstraction of salt from the source salt, layer result in formation of “primary rim basins”, (sag, or minibasins), which allow precise timing of, salt movement. Using this method, updoming, of Zechstein (Late Permian) salt pillows (i.e., Gorleben during the Jurassic) was measured at an, average of a few millimetres per year., 2 “Active diapirism” designates the breakthrough of salt into cover rocks. At the same time,, “secondary rim basins” are initiated and filled., The piercement may be associated with high, upflow velocities of the salt, possibly reaching, metres per year., 3 Once the high potential energy of active diapirism is spent, the diapir continues to grow in the, mode of “passive diapirism”. Sediment accumulating in “tertiary rim basins” displays unconformities, off-diapir debris flows, salt extrusions, (Jackson et al. 1991) and salt glaciers. In Northern, Africa, salt tongues extend laterally to >1000 m, from the Triassic diapirs along unconformity, planes, probably formed as submarine extrusions, (Perthuisot & Rouvier 1992)., In this connection, Trusheim (1957) introduced the term halokinesis and defined it as, salt flow that is only due to gravity potential., , 403, , He contrasted halokinesis with salt tectonics,, which is salt flow related to regional tectonic, processes. Of course, the terms designate opposites that in nature may not be clearly separated., More intensive salt flow occurs nearly always in, times of tectonic strain (Kossow et al. 2000). In, conclusion, diapirism is basically the product of, compensation processes following density, inversion, but is very often triggered by tectonic, events (Daudre & Cloetingh 1994, Nalpas &, Brun 1993)., 4.3.4 Supergene alteration of salt deposits, Salt solubility in water is very high. Accordingly,, salt on the surface is only preserved in an arid, climate. In humid climate zones, salt is leached, by infiltrating precipitation and moving groundwater. Because of its extremely low permeability, massive salt rock is an aquiclude. Therefore, salt, will be removed by dissolution until a deep saturated brine pool (in the pores of an aquifer), becomes stable and very little flow or additional, solution take place. In such settings the salt, forms the lower boundary of a brine aquifer. The, boundary plane is commonly a level, mirror-like, surface below the brine aquifer, which often is a, cap-rock dissolution breccia (Figure 4.28), but, may include clastic rocks as at Gorleben. In some, cases, the salt surface displays channels or is, inclined. Dissolution of salt reaches considerable, depths. At Gorleben, for example, the salt surface, occurs 250–300 m below sea level, with the local, land surface and River Elbe at 20–30 m a.s.l., (Figure 4.26)., Subsurface solution of salt has been termed, “subrosion”. The removal of mass by subrosion, enforces adjustment of overburden, often leading, to collapse and formation of sinkholes. Depressions fill with groundwater (forming lakes: Figure 4.28) or with younger sediments., The rate of subrosion is of high interest, because, it is one of the critical parameters of a security case, for salt domes destined to take up nuclear waste., Its determination is not simple; methods used, include groundwater flow models and mass balance calculations, geomorphological analyses,, geological data (e.g. the subsidence of precisely
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404, , PART II NON-METALLIC MINERALS AND ROCKS, , Figure 4.28 Formation of a large collapse lake by, Pleistocene and Holocene subrosion of salt at depth., Arendsee Zechstein salt diapir, Northern Germany. With, permission from www.schweizerbart.de. A catastrophic, collapse event last happened in 1685. Black ¼ caprock., , dated rocks) and the rate of caprock formation., In most investigated locations of Northern, Germany, the rate is clearly <1mm per year., At Gorleben, the Holocene rate of subrosion is, minimal (0.01–0.05 mm/a), after relatively high, rates during glacial periods of the Pleistocene, (0.1–0.4 mm/a: K€, othe et al. 2007), when rivers, surged beneath the ice shield. At present, stable, and upward-moving diapirs are known in the area., If the rate of diapir rise is higher than the rate of, subrosion, morphological elevation is the result., Dissolution of salt at depth produces nearly, stationary saline groundwater and brines (Table, 1.4, Chapter 1.4 “Diagenetic Ore Formation Systems”), which display a density stratification., Hydraulic anomalies, however, may cause upflow, of saline tongues and the formation of salt springs, and licks. In earlier times, salt springs were an, important source of salt for humans and wildlife., Today, they are often used for therapeutic baths., Note that little disturbed saline springs and wetlands are biological treasures, which host rare, forms of life., As a by-product of subrosion, the less soluble, components anhydrite and clay of the salt rocks, form residual enrichment bodies of “cap rock”. In, this supergene setting, most anhydrite is rehydrated to gypsum. Exceptions occur in highly, saline and warm brine pools in lowermost cap, , rock, which stabilize anhydrite. At higher levels, above the salt surface and at lower salt concentration in the groundwater, a breccia of grey,, crystalline gypsum is the prevailing rock (gypsum cap rock). Salt rocks with higher contents of, argillite, such as the Alpine haselgebirge, develop, a residual clay envelope, which is advantageous, in mining practice because of low permeability,, whereas gypsum cap rock is commonly highly, permeable., Cap rock, Cap rock bodies above salt domes should be carefully studied when a deposit is to be developed,, whatever the aim of the operation (Figure 4.20 and, Figure 4.26 and 4.28). The thickness of cap rock, fluctuates from a few to hundreds of metres. Cap, rocks are commonly bedded and preserve a record, of the subrosion history. Remember that the youngest beds in the cap rock are the lowest, directly, above the salt, and the oldest lie uppermost. Stable, isotope investigations of crystal water in neogenetic gypsum provide information about: i) the, origin of the water (ocean, or meteoric); and ii) the, climatic background of past subrosion. In some, cases, brine channels occur between cap rock and, the salt, which transport gypsum or anhydrite, sand and pebbles. This is one origin of the typical
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , banding of gypsum cap rock. The supergene alteration may affect flanks of diapirs too, forming, an anhydrite, clay or gypsum mantle. When hydrocarbons gain access to gypsum cap rock, biogenic, sulphate reduction takes place with consequent, formation of secondary calcite/dolomite, sulphides and native sulphur (Figure 3.32)., The thickness of the cap rock is a measure for, the volume of leached salt rock, if the percentage, of anhydrite and clay in the dissolved volume can, be estimated. Modifying factors have to be considered, such as glacial erosion or dissolution of, gypsum. In the higher parts of gypsum cap rock, bodies, percolating seepage and groundwater may, produce fissures, pipes and caves, which are filled, with water, rubble, sediment or soil, originating by, earth falls. Deeper down, cavities are filled with, brines. The surface, if exposed, displays all the, characteristics of a karst landscape. Because of, these properties, gypsum cap rock causes considerable mechanical and physical difficulties for, surface installations, drilling and shaft building,, which are aggravated by the saline brines that, lower the required temperature for the freeze, shaft-sinking technology., Supergene alteration, Supergene alteration of salt rocks beneath the salt, surface affects the more easily soluble potassiummagnesium salts. Generally, potassium seams, that crop out on the salt surface may display, alteration to more than 100 m below the halitite, surface. Below the glacial channel filled with, sand and gravel at Gorleben, Germany (Figure 4.26), carnallitite was locally found to be, altered at 170 m below the top of the salt. This, alteration is zoned: K and Mg salts are totally, dissolved immediately below the salt surface and, only coarsely crystalline residual halite is, observed. Red colouring by haematite marks this, halite as an equivalent of the K-seam. Kieserite is, rehydrated to hexahydrite. This surficial zone is, underlain by an interval with sylvite derived from, carnallite. If the original carnallite contained, kieserite, sylvite is associated with nodules, and crusts of sch€, onite (picromerite K2Mg, [SO4]26H2O). This is underlain by a kainite zone,, or cap above the unaltered carnallite. Kainite, , 405, , forms either from sylvite þ kieserite according, to eq. 4.2, or from carnallite þ sulphate by, abstraction of easily soluble MgCl2., Supergene formation of kainite from hartsalz:, MgSO4 :H2 Oþ KCl þ 2H2 O ! MgSO4 : KCl:3H2 O, kieserite, , sylvite, , kainite, , ð4:2Þ, Because kainite is easily processed to KCl fertilizer, it was preferentially exploited in the early, days of potassite mining. However, its proximity, to water-bearing anhydrite caprock caused a number of crown pillar failures and consequent water, inrushes that usually enforced the closure of, afflicted mines., , 4.4 FROM EXPLORATION, , TO SALT MINING, , 4.4.1 Exploration and development of salt, deposits, Indications for the presence of evaporitic rocks in, the search area are outcrops of gypsum, gypsum, karst surface features (e.g. sinkholes), wetland, depressions or lakes above rock salt beds, salt, licks, ponds and saline springs, and saline biotopes. Salt in groundwater can be a guide to shallow salt rock bodies:, Because of its higher density, saline water resulting, from leaching of buried salt principally collects in, deepest parts of basins or of aquifers. Nevertheless,, even in lowland plains like Northern Germany, with, saltwater usually >200 m below the surface, saline, springs are not rare. This is due to forced hydraulic, convection, for example by an elevated hydraulic, head induced by infiltration from basin-marginal, hills. Possibly even more common are barriers that, constrict the cross-sectional area of flow, as at Gorleben. Some brines may rise because they were heated, at depths resulting in lower fluid density., , Geophysical surveys serve to outline prospective areas of near-surface salt. Gravimetric methods are most efficient because of the density, difference between salt and most country rocks, (Figure 4.29). Seismic reflection and refraction, surveys define depth and borders of the salt rock
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0, -1, -2, -3, -4, -5, -6, -7, -8, , PART II NON-METALLIC MINERALS AND ROCKS, , mGal, , 406, , Gravity, , S, , N, , 0, Tertiary, Late Cretaceous, Early Cret., , -1, , Liassic, Keuper, Muschelkalk, , -2, -3, , -5, , Zechstein salt, , km, , -4, , body. Geophysical data collected for hydrocarbon, exploration are valuable tools for assessing salt, features. However, the methods described reveal, little of the internal structures of a salt body, so, that drilling is indispensable. Note that the, world’s largest potassium salt province in the, Devonian Prairie Evaporite Formation of Saskatchewan, Canada, had been found accidentally, in the 1940s by petroleum exploration drilling., The same happened later in northern Thailand,, but development of the large (10,000 Mt), highgrade, “world-class” carnallite resource is yet to be, realized. Resistance by affected communities and, the less favourable processing costs of carnallite, compared to sylvite are cited as causes., Salt’s high solubility is a specific problem for drilling., In order to avoid caving, the flush fluid must be, saturated with NaCl and if potassites are encountered, in addition with KCl, MgCl2 and MgSO4., Because many salt minerals are transparent, geological drill core logging is facilitated by illumination, across the core. For precise lithostratigraphical attribution, down-hole geophysical surveys are standard, procedure. Natural gamma radiation, acoustic velocity, density and neutron porosity logging are especially useful. Sampling for bromine analyses should, always be included., , Even near-horizontal and little deformed tabular, potassite seams may exhibit laterally changing, facies of significant economic impact (Figure 4.21),, as well as slight undulations or small faults affect-, , Buntsandstein, Rotliegend, , 5 km, , Figure 4.29 Geological profile of the, Arendsee Zechstein salt diapir in, Northern Germany, and the Bouguer, anomaly caused by the low density of the, salt body (modified after Gabriel &, Rappsilber 1999). Note that gravity is, generally expressed in gravity units (g.u.),, with 1 mGal equal to 10 g.u., , ing mining. Therefore, preliminary exploration, drilling needs a minimum of 2–3 holes per km2., This is later improved by variography (cf. Chapter, 5.3). In order to reach a similar understanding of, the internal structure of a salt diapir, many more, holes are necessary. Early drillholes are planned to, penetrate only 10–20 m into the salt, as the, induced hydraulic paths may later cause grave, problems in spite of careful plugging. Oriented, coring and geophysical logging provide sufficient, data for drawing preliminary geological maps and, sections. Based on these and as a function of the, project’s aims (e.g. salt mining, hydrocarbon or, waste storage), a minimal number of deep-penetration boreholes are drilled., Exploration and reserve calculation are supported by basin models. These models integrate, data and observations (e.g. lithostratigraphy,, structures, geochemistry, isopach maps and diagenetic facies) into a holistic image (Sonnenfeld, 1991)., 4.4.2 Geological practice of salt mining, For any underground salt mine, detailed geological, maps are indispensable. This aims at establishing, the local lithostratigraphical standard profile (and, deviations from it) and is supported by geochemical (e.g. bromine) and physical parameters. Rational mine planning depends on geological maps,, sections and block diagrams. Nature and spatial, structure of cover rocks, and their hydrogeology
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , 407, , Figure 4.30 (Plate 4.30), Historic salt exploitation by, borehole solution at, Haraucourt (Meurthe-etMoselle, France) caused these, flooded collapse craters. The, deposits are subhorizontal Late, Triassic (Keuper) salt beds at, shallow depth. Courtesy, Christian Wolkersdorfer, CBU,, Sydney, Canada., , have to be explored in sufficient detail to understand potential hazards. Inclusions of non-salt, rock bodies (e.g. dolomite, anhydrite, clay, basalt), and tectonic disturbance zones deserve special, attention, because they may allow inflow of, or, hold water, brine and gas. Quality and quantity of, potassite seams is illustrated by isopach maps,, maps of K2O grade and of potassite rock facies., Mining blocks, reserves and grades are calculated, both by conventional and by geostatistical modelling methods., Important raw materials exploited from recent, or ancient salt lakes include trona, halite, mirabilite/thenardite, borates, gaylussite, celestite,, nitrate, zeolites, clay, sepiolite, oil shale and, brines. Extraction often relies on pumping brines, from lakes or from subsurface aquifers of dry salt, lakes (I, Li, Mg, KCl, NaCl). Underground mining, of solids is rare (exceptions include trona in the, USA and rock salt in the Atacama Desert), and, open pits are the standard. Exploration and exploitation of salt lake resources are always based on, extensive sedimentological and chemical investigations. Salt lake systems are rather fragile and, human interference can cause dramatic changes., In most cases, however, diverting freshwater, inflow for other uses such as agriculture is the, major impact and mining activities are of little, consequence., Solution mining, Today, rock salt is usually extracted by solution, mining. The most common variant is drilling a, , vertical hole into the salt body, either from underground or from the surface. Fresh water is injected, through an outer tube into the drillhole section, where salt is exposed, whereas brine is pumped, from near the bottom of the growing solution, cavern. Critical geological parameters include, nearby non-salt rock bodies (affecting safe enclosure and the mechanical stability of the cavern),, the presence of potassites (causing irregular and, unstable parasitic cavities) and the risk of surface, subsidence and collapse (Figure/Plate 4.30). Solution caverns are developed and widened to design, size and shape by skilful management of freshwater inflow and brine level. Progress is mainly, controlled by sonar echometric surveys. Specific, problems occur in the presence of carnallite,, because the system easily converts to incongruent, solution resulting in solid-phase sylvite. Using, heated fluid may alleviate this problem. In contrast to carnallite, sylvite-rich potassites are more, easily dissolved. In Canada and USA, borehole, solution mining of sylvite salt reaches depths of, more than 1400 m., The establishment of storage caverns for oil, gas,, compressed air and industrial waste follows the, same principles., Hard rock mining, Hard rock mining of rock salt is characterized by, the excavation of large underground chambers, that are separated by pillars. Cavern dimensions, of 70 by 50 m (horizontal) and 25 m (vertical) are, not rare. As mentioned earlier, potassium salt, rocks display a lower mechanical stability and the
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408, , PART II NON-METALLIC MINERALS AND ROCKS, , supporting pillar surface may have to be larger, than the excavated rooms (Figure 4.22). Engineering geological and rock mechanical methods are, essential tools to control the stability (Jeremics, 1994). In most cases, long-term stability is aimed, for, including the time after mine closure. Collapse is a hazard because it may lead to ingress of, unsaturated formation or meteoric waters from, outside and to damaging earthquakes. Therefore,, the calculation of pillar dimensions of salt must, include stress state and properties of country, rocks, not only salt rock mechanics. In order to, minimize hazards for the local population and for, groundwater protection, near-surface underground openings of a salt mine may have to be, backfilled with salt tailings and fines., Among the greatest hazards in underground, mining of salt are water and brine inflows. It is, important to distinguish between waters sourced:, (i) from host rocks of the salt deposit; (ii) brine, pockets within the salt rocks; and (iii) mininginduced moisture (e.g. condensation)., Only inflows from the country rocks, such as, carbonates, sandstone, shale and cap rock are dangerous. As a function of their hydraulic parameters, and structural position, these rocks may be conduits for formation water, shallow groundwater or, surface water. In all three cases, salt along flow, paths will be dissolved, so that plugging is, extremely difficult or simply impossible. Typically, the flow rate increases steadily from an, initial trickle and if the country rocks are profuse, aquifers (as in the case of most gypsum caps), the, mine will have to be abandoned. The only efficient, precaution is to design the mine in a way that a, thick salt pillar is left intact adjacent to the country rocks. A rare example of successful plugging, was reported from a salt mine in Austria, (Figure 4.20). The most efficient counter measures, are screens of sealing injections., Brines that are occluded by salt rocks may suddenly drain into mine openings or drillholes. This, can be damaging but is mostly a nuisance. If larger, brine-filled cavities are expected, geophysical, detection and forward drilling should be employed, as a precaution. It is important to regularly measure brine flow rate, temperature, closure pressure, and its chemical composition. Main questions, , concern: i) the brine’s origin; and ii) its ability to, dissolve salts occurring in the deposit. Halite-saturated brine, for example, is perfectly able to dissolve potassium salts, potentially causing grave, damage. Isotopic tracers of inflowing water of, unknown origin should always be examined, (including 87=86 Sr, 18 O, 2 H and 3 H; Botrell et al., 1996). This will be the only reliable base for judging hazard and risk. Mining-induced moisture (e.g., condensation from ventilation air, drilling fluid,, backfill humidity) is usually harmless but may, accelerate deformation., Gas in salt mining, All salt rocks contain a mixture of gas including, N2, O2, hydrocarbon gas, CO2 and H2S. Usually,, this gas occurs in trace concentrations and presents no risks. Higher contents that are easily, diluted by the ventilation air flow may locally, occur in all mines. Some mines, however, experience sudden inflow of hazardous gas volumes. H2S, is toxic at concentrations above 0.007 vol.% in air,, highly concentrated CO2 and N2 are asphyxiating, and CH4 is flammable and explosive. Both slow, degassing and sudden gas-driven blow-outs or, “outbursts” can be dangerous. High-pressured gas, trapped in salt rock “explodes” from the working, face into mine openings, filling them with salt grit, and gas. This is very similar to coal outbursts, (Guan et al. 2009). One of the largest salt outbursts, on record took place in 1953 in the potassium mine, at Menzengraben in Germany, where a volume of, 700,000 m3 CO2 and 65,000 t of salt were suddenly fluidized. Advance recognition of gas-rich, salt bodies is not simple but is possible with a, combination of geophysical methods and forward, drilling. Proximity to clay, anhydrite and basalt, rocks is a general indicator. Destressing gascharged salt rock by drillholes and safety management measures for personnel are recommended, precautionary procedures., Disturbance of the groundwater system, Disturbance of the groundwater system around, salt mines is hazardous because any change of the, hydraulic potential (e.g. by pumping) may draw, unsaturated water to the salt and intensify
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SALT DEPOSITS (EVAPORITES) CHAPTER 4, , dissolution (subrosion). Resulting earth falls and, general subsidence may induce costly damages., Drillhole disposal of waste brines from salt processing plants (e.g. MgCl2) must target deep saline,, not freshwater aquifers. Salt mining waste retention facilities include surface dumps of anhydrite/, gypsum, halite, clay and slimes that result from, potash ore beneficiation. This may cause release of, solutes into surface or groundwater. Permissible, salt concentrations of emissions must not be, exceeded. Note, however, that in regions with, profuse precipitation, local brine seeps are quickly, diluted. Natural and man-made saline springs may, be ecologically singular and valuable sites, because, they support rare and specialized life forms., Closure of mines, Closure of underground salt mines is foremost a, decision between “dry” and “wet” variants. Overall, sealing a dry mine (Fuenkajorn & Daemen, 1996) against water inflow is probably preferable, to controlled flooding. A dry mine that is mechanically stable and absolutely water-tight is safe. In, Northern Germany, hundreds of salt mines that, operated over a century ago were flooded, either, intentionally or by accident. The first modern, monitoring programme of a flooding process was, applied at the Hope mine near Hannover in 1984. It, confirmed that water in the shaft (and presumably, in the mine) very soon reached a stable density, layering with: i) slightly saline fresh water at the, top; ii) a saturated NaCl brine; iii) NaCl-KCl brine;, and iv) a lowermost body of MgCl2-NaCl-KCl, brine (Wolkersdorfer 2008). The safety of flooded, mines may be compromised by slow strength loss, of salt pillars, changing water levels, inflow of, fresh water that causes renewed solution and convective brine overturn because of temperature, differences. Sudden earth falls are the main hazard,, with highest incidence in the immediate perimeter of old shafts., , 4.5 SUMMARY AND FURTHER READING, Solutes derived from continental weathering are, concentrated in terrestrial salt lakes and in the, , 409, , oceans, mixed with matter vented by hydrothermal activity. In hot and dry climate zones, intensive evaporation induces precipitation of salt, minerals and rocks. The available ions and the, degree of evaporation determine the precise paragenesis of salts to form. Economically significant, salts derived from ocean water include chlorides of, Na, K and Mg, sulphates of K and Mg, and minor, components such as bromine. Continental salt, lake resources overlap with the first but are particular by producing boron, I, Li, Mg, K and Na, chloride, Na and Li carbonate, and Na, Mg and Sr, sulphate. Salt lakes and brines are an important, source of environmentally useful metals such as, lithium and magnesium., Only in the last 600 million years of earth, history, salt rocks are common, due to profound, changes in ocean water state and composition., Chemical changes of ocean water continue into, recent geological time. By definition, salt rocks are, easily soluble in water. Yet, salt rocks in the upper, crust tend to be well preserved for geological time, periods. One explanation for unexpected survival, in basinal sediments is the prevalence of saline, pore fluids derived from seawater evaporation, instead of undersaturated water. Salt rocks function as seals for moving subsurface water, petroleum fluids and natural gas. Salt rocks conserve, geologically ancient life; viable bacteria and, archaea have been isolated from Permian salt,, more than 250 million years old., Formation and preservation of large salt formations require a favourable physical setting. Arid, climate, a wide sea cut off from the main ocean and, a narrow channel providing continuous inflow is, the basic model. Deep and shallow basins are, distinguished. Evaporation lowers the water level, in the saline sea, resulting in steady-state ocean, water inflow. In time, concentration levels for, halitite and finally potassite precipitation requiring near-dryness are reached. Intermittent or final, isolation may lead to total desiccation., Coastal solar salt operations are a tangible, model of marine salt formation, although with, one important difference. Salt production aims at, pure products. Therefore, K-Mg bitterns are, pumped from the halite pans before bittern salts, precipitate. In nature, however, back-reaction
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410, , PART II NON-METALLIC MINERALS AND ROCKS, , with previously formed solids is the rule and is, part of earliest salt diagenesis. Like sediments, salt, rocks change with increasingtemperature and overburden pressure, and survive into metamorphic, sequences, although rather in chemically anomalous rock compositions than as actual salt beds., Salt rocks are very ductile compared with carbonates, sulphates, shales and sandstones. Small, pressure differences cause salt to flow from higher, to lower potential. This results in many exotic salt, intrusions distant from the source bed. The most, flamboyant products of salt flow are the diapiric, salt domes – salt pillars vertically rising to the, surface from the source bed through pierced hanging wall rocks. Unstable inverted density layering, is the physical driver for diapir formation. Salt, diapirs are a rich source of benefits for humans,, because they provide near-surface exploitable, halite and potassite, sulphur and oil, deep natural, , gas and safe storage for liquid fuels, methane, wind, energy and hazardous waste (cf. Chapter 5.5 “Deep, Geological Disposal of Dangerous Waste”)., To francophone readers I suggest Rouchy &, Blanc-Valleron (2006) for a discerning introduction to evaporites. A profound and detailed, description of evaporites is offered in the two, books by Warren 2006, 1999). Warren is also the, best source to study the relations between hydrocarbon deposits and evaporites. The volume, about salt diapirs in Iran, by Jackson et al., (1991), is full of wonderful images and offers a, framework for the mechanics of diapirism., Applied salt rock mechanics by Jeremics (1994), still is the only complete treatment of the subject,, but should be supplemented by reading papers, such as Kwon & Kim (2005). Salt, a world history, (Kurlansky 2002) is illuminating, comprehensive, and entertaining.
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PART III, The Practice of, Economic Geology, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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CHAPTER 5, Geological concepts and, methods in the mining cycle:, exploration, exploitation and, closure of mines, Mente et malleo (“With intelligence and hammer” – the venerable motto of geology), , Synopsis, Civilization depends on freely available raw materials. To assure supply, ores and minerals, extracted by mining must continuously be replaced by new reserves. Beyond satisfying the present, demand, an additional requirement results from the growth of the world’s population and its, increasing standard of living. This implies that exploitable reserves are identified and prepared for, mining, out of previously assumed but not sufficiently known, or undiscovered mineral resources., Commonly, this activity proceeds in several stages, from reconnaissance exploration for potential, mineral deposits (target generation), through detailed follow-up exploration (investigation of, targets), to evaluation (development of a new mine, or final rejection). Stages may be omitted, for, example in cases of companies buying prospective mineral occurrences, or when new orebodies in, active mining districts (“brown fields”) are discovered and developed., The secure provision of metals and minerals depends not only on geological and technical, availability but also on economic, institutional, societal and legal conditions. In an ideally free and, well regulated market economy, raw materials supply would never be a problem. Indeed, it is, wonderful how well market forces work in our real world, although many regions are far from free., Conflicts and crises surrounding the petroleum and gas markets, however, are a warning that, national and international political efforts to ensure essential supplies should never cease. Of, course, the same is true for scientific and technological work to provide needed metals and minerals., , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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414, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , All activities that serve mineral raw materials supply, from exploration to extraction, including, mine closure and reclamation, must be optimized according to the three criteria:, 1 environmentally sound;, 2 sustainable; and, 3 socially compatible., The same criteria rule the provision of underground waste repositories, mines that are specifically, designed for deep disposal. Waste storage mines are constructed in analogy to natural toxic and, radioactive rock bodies that have been conserved for many million years., This chapter presents essential geological concepts and practices concerning the life-cycle of, a mine, from reconnaissance exploration to closure., , 5.1 ECONOMIC, , CONSIDERATIONS, , Economic extraction of minerals requires that, all costs (for exploration, detailed investigation,, development, mining, processing, credit financing, taxes, environmental mitigation and closure), are covered by returns (gross revenue from selling, mine products) and that reasonable profits remain, for investors (Wellmer et al. 2007). This basic rule, may be disregarded, but always at a cost to somebody. Overriding reasons may concern the national, economy (e.g. the need of foreign currency income),, socio-economic conditions (employment), or the, wish to retain a national supply and knowledge, base for certain strategic metals and minerals., Possible returns are conditional on the price of, metals and minerals. Normally, prices are found, by free interplay of supply and demand, for, example on the London Metal Exchange. This, applies to a number of metals including gold,, lead and tin. Strong price fluctuations are a, problem that severely complicates the financial, planning of mines. The risk is particularly difficult for new mining projects and is aggravated, by the long lead time (often 5–10 years) from the, decision to open a new mine until the first, products can be sold. Other mining sectors typically conclude long-lasting contracts with reliable price agreements (e.g. many industrial, minerals). Because the competition is lively,, mining companies hardly ever have the market, power to dictate prices. Some states, however,, have a near-monopoly on specific raw materials,, which may disquieten purchasers (e.g. European, Commission 2010)., , Apart from prices, the size of reserves, the geological situation, beneficiation costs, geographical, situation, infrastructure, social conditions and, political risk are important parameters of the, feasibility of a mining operation., Resources and reserves are the primary factor, of the viability of a mine. The terms describe, estimated mass and quality of ore, and imply, a lower (resources) or a higher degree of confidence, (reserves). Hardly any obstacle will prevent exploitation of large and rich deposits, whereas lowgrade or small occurrences cannot be mined, even, if all other parameters are optimal. Average or, minimal (so-called “cut-off”) grades for various, ores are provided in Chapter 2 of this book. In, Table 5.1, order-of-magnitude estimates illustrate, the smallest total metal content of a viable deposit, and an appropriate ore grade., The mass of metal contained is essential information, because the investment for establishing, a mine must relate favourably to returns from, selling the metal (as contained in concentrate)., Even small mines commonly require an initial, Table 5.1 Minimal total metal mass and corresponding, metal concentrations in ore (“grade”) for economic ore, deposits, Iron, Manganese, Copper, Tin, Gold, , 1 000,000 t Fe, 30,000 t Mn, 50,000 t Cu, 5000 t Sn, 1 t Au, , 60% Fe, 45% Mn, 3% Cu, 1% Sn, 0.001% Au
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , investment of 50 million US $. Very large operations, such as giant copper or iron mines, cost well, over 1000 million US $. Therefore, already at the, stage of exploration, figures like those of Table 5.1, are important guidelines., In addition, reserves are an essential control on, the optimal output capacity of a mine expressed, in tonnes/day or per year. Most mines operate for, a period of 3 to 30 years. Based on the same reserve, figure, high annual production shortens the life of a, mine compared to low extraction rates. The search, for an optimal rate is carried out by calculating costs, and returns for different production rates. The rate, offering the highest overall profit is the best choice,, but may be moderated by other considerations., Exceptionally large deposits (“giants”, Laznicka, 2006), such as several porphyry copper deposits in, Chile, are economically most attractive. Overall,, large-scale mining costs less per unit of metal produced, even if ore grades (e.g. kg copper per tonne), are relatively low. Very large reserves allow investment in technologies that reduce costs of all mining, and processing activities. Open pit operations are, more economic than underground mines. The, result is that small underground mines need higher, grade ore (e.g. 3% Cu) compared to giant open cut, mines, which are profitable at low grades (e.g. copper, porphyries with 0.4% Cu but reserves of several, hundred million tonnes). Yet there are many small, mines that are quite profitable. An interesting development is the trend of co-producing several minerals from the same deposit. In this case, unit costs of, extraction are balanced by returns from more than, one product. Examples are co-production of clay,, sand and lignite in large open pits, or tantalum,, kaolin, feldspar and quartz sand from pegmatite., The geological situation controls expenditure, from early field investigations to mining and final, mine closure. Form, spatial arrangement and variability of orebodies, as well as mechanical and, hydrogeological properties of ore and host rocks,, influence profitability. Simple unfaulted bodies of, a mineral resource (e.g. a coal seam), the quality of, which hardly changes with distance, is economically very favourable. This allows for wide-spaced, drilling (saving costs) as a base for reserve estimation and mine planning. After an initial phase of, adjustments, operating conditions during the, , 415, , period of exploitation remain stable for a long, time. The contrary applies to strongly deformed, (e.g. metamorphic sulphides) or to highly discontinuous orebodies, including many metasomatic, ores such as lead-zinc sulphide in carbonate rocks., Ore deposits of this kind always cause higher, costs, from exploration to production. Highly fractured or hydrothermally altered host rocks may, have unfavourable and therefore costly mechanical and hydraulic properties. High inflow of water, leads to rising costs for pumping that have caused, the economic collapse of innumerable mines., However, a good grade of the ore allows mining, even under extremely difficult conditions as at, Lihir, with ore temperatures of 100 C and the, ever-present danger of geothermal outbursts., The feasibility of beneficiation is an important, precondition for the economic success of a mineral, resource project (Table 5.2). Factors that control, processing, concentration and the resulting recovery include grain size, type of intergrowths, and, the physical characteristics of ore and gangue, minerals. Contents of minor and trace elements, in the concentrate may be advantageous (e.g. gold, in copper ore) or very costly and a useless liability, Table 5.2 Mine to metal – five important terms, Run-of-mine ore is commonly a mixture of ore and, gangue minerals, often diluted with by-breaking host, rocks (e.g. copper ore with 1.2 wt. % Cu), Ore dressing (or processing) describes mine-site operations, that reduce waste (gangue) and enrich ore minerals to, “concentrates” of metals (e.g. a copper concentrate with, 62 wt. % Cu), which can be sold to smelters, Metal recovery is the ratio between the mass of metal, contained in a specific in-situ volume of ore and in, concentrate produced from this volume, Tailings describes the useless part of ore removed by processing, (say 95% of the above-mentioned copper ore) consisting, of gangue minerals such as silicates, carbonates and quartz;, safe tailings storage is an economic and environmental, liability; because of ever-improving processing methods,, older tailings may be today’s ore, Metallurgical processing isolates metals from concentrates, (by smelting, electrolysis, leaching, etc.) and refines them, to marketable grade or purity. Mines are rarely large, enough to warrant the establishment of mine-site metallurgical operations, but exceptions do exist (e.g. Mt Isa,, many gold mines).
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416, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , such as arsenic. Gold ore with large gold grains, (free-milling gold) can be processed with cheap, gravity methods. The extraction of fine-grained, gold usually involves cyanide leaching, which imposes considerably higher costs for plant and operating expenses. Also, the use of cyanides enforces, environmental monitoring and risk reduction, measures that imply further costs. Sub-microscopic gold in sulphides induces an additional, processing step, that is roasting (oxidizing) the ore, before leaching (Milham & Craw 2009). The flue, gas from roasting kilns may have contents of SO2,, As, Sb, Hg and other volatile elements, which, cannot be emitted into the environment. Clearly,, when all these costs have to be covered, gold content per mass unit of ore must be much higher in the, last case compared with free-milling gold., The geographical situation comprises parameters such as remoteness, morphology, altitude, and climate, which are complemented by infrastructural factors including accessibility by road, and railway, free availability of, or competing, claims on land, water and energy. Social conditions are increasingly the crucial factor for the, decision to launch an exploration project or a new, mine. Social aspects include the density of population, its acceptance of, or aversion to industry,, the availability of a suitable work force and environmental regulations. All the above combined, dictates the minimum size and grade of an ore, deposit. The long-term political stability will be, the final test for a project – a high risk of doubtful, security, financial, fiscal and legal conditions imposes a very different business model compared, with expectations of long-term stability and peace., , 5.2 THE SEARCH FOR MINERAL, (EXPLORATION), , DEPOSITS, , In my view the greatest value is added by the geologist who starts with nothing other than some ideas, and goes out into the desert and finds an orebody, J.K. Ellis (Chairman of BHP Australia, 1998), , 5.2.1 The pre-exploration stage, Every search for mineral deposits starts with, collection and interpretation of existing data, , concerning the geological, metallogenetic and, mining background. Geological maps are indispensable sources because to the expert, they, reveal potentially occurring deposits. Terrestrial, volcanic centres with advanced hydrothermal, alteration, for example, may host gold mineralization. Small, late-phase granite cupolas can have, extractable contents of tin and tantalum. Volcanic, playa lakes are favourable settings of zeolite,, borate and lithium deposits. Many exploration, successes, even most recent ones, were based on, a knowledgeable re-interpretation of public geological maps and a careful reading of the explanatory notes. For example, an incidental reference to, an alunite occurrence on Lihir Island, Papua New, Guinea, in a mapping report induced a search for, epithermal gold mineralization. The results were, overwhelming, as Lihir is now one of the largest, gold concentrations in the world, with a total gold, content exceeding 1300 tonnes:, Allow me here a plea for geological mapping: It is, regrettable that the relevance of systematic geological mapping for a reliable supply with minerals and, metals, but also for the rational management of land,, water and the environment is not more widely recognized. In industrial nations the mining industry may, be expected to finance such services, although within, limits. In developing parts of the world, however,, geological map coverage is often poor and dated., Although they are very useful, relatively low-cost, satellite images and geophysical coverage cannot, replace the experienced geologist’s groundwork., Good geological maps are a significant factor for the, transition from agricultural to industrial societies., This is demonstrated by the first geological map ever, made, by William Smith in England, and published in, 1815, for entirely practical reasons (in this case canal, building)., , The choice of target regions for exploration is, often based on large-scale geological models that, use information published in the scientific, domain. Regions with favourable conditions are, selected, such as large sedimentary basins with, synsedimentary tensional tectonics and welldeveloped reduced and oxidized compartments, (prospective for base metals), or belts of late-orogenic highly differentiated felsic intrusions (rare, metals such as tin), or metamorphic massifs
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , affected by large shear zones (gold). Large deposits, are typically formed where the geological structure is generously proportioned., Seafloor exploration for metals and minerals is, still rare, except for aggregate and submerged, coastal placers of diamonds and cassiterite. With, rising prices of minerals, deep-sea resources, including phosphorite, sulphides, gold and manganese nodules gain economic attraction. Several, seafloor mining projects are currently developed,, for example gold-rich volcanic massive sulphide, deposits at 2000 m water depth offshore of Papua, New Guinea and New Zealand., Geological concepts may be supplemented by, formal quantitative mineral resource assessments, (Singer & Menzie 2010). The pre-exploration desk, study presents a concept of the proposed future, work, including geoscientific, logistic, legal and, financial components, as well as previews of the, physical and social environment. Remember,, however, that exploration is not only a scientific, and technical enterprise but foremost an investment. As with any investment, potential rewards, and risks must be clearly defined and continuously, updated (Kreuzer et al. 2008)., In 2008, a total of 14,400 million US$ was spent, on exploration for non-ferrous metals and industrial minerals (excluding coal), but expenditure, dropped to 8400 millions in the global financial, crisis year of 2009. Most is dispensed in work, near existing mines and very little in unspecified, “grass roots” exploration. About one-fifth of the, annual budget is destined for gold exploration., Significant funds are spent in the search for base, metals, iron ore, coal, uranium, diamonds and, platinum group metals. A small part of the total, exploration budget targets other metals and, minerals., 5.2.2 Geological exploration, About thirty years ago there was much talk that, geologists ought only to observe and not theorize., How odd it is that anyone should not see that all, observation must be for or against some view if it is to, be of any service!, Charles Darwin (Letter from the Beagle,, September 18, 1861), , 417, , Geological concepts and methods are the indispensable base for any exploration project (Kreuzer, et al. 2008, Kelley et al. 2006, Bevier 2005, Sillitoe, 1995, Glasson & Rattigan 1990). Geology is both, the starting point and the unifying frame for, merging and analysing data resulting from a wide, variety of methods. Today, standard practice in, mineral exploration is to analyse data in two, dimensions using GIS (maps and cross-sections in, Geographic Information Systems) and to identify, targets by empirical means. Increasingly, threedimensional and four-dimensional (three-dimensional plus time) modelling is employed by, explorers for integrating multiple datasets, such, as geology, geophysics, geochemistry and drillhole, data, including concepts such as past fluid flow., This approach emulates the “petroleum systems”, method (cf. Chapter 7) in the oil and gas industry., Three-dimensional numerical models of coupled, fluid flow and deformation, for example, assist in, orogenic-gold exploration (Potma et al. 2008)., Resulting models can be visualized and tested., Based on these tools, predictive capabilities are, considerably advanced., Due to increasing mineral raw materials, demand and prices, exploration frontiers are shifting to deeper and lower grade mineralization in, more remote locations. Worldwide, new discoveries will increasingly be of this nature., Fundamental strategies of mineral exploration, include:, . exploring large areas systematically, either, searching for all possibly occurring metals and, minerals (“grass roots exploration”) or, more, commonly, for only a few attractive resources;, . selecting specific regions for their geological, resemblance to known metallogenetic provinces, or districts;, . investigating districts hosting known mineralization and mines, motivated by improved geological understanding that suggests the presence of, undiscovered ore;, . exploration for specific raw materials (e.g. marble of high whiteness) is guided by the search for, locations with geological properties that resemble, commercially viable deposits., Several spectacular exploration successes of the, last decades illustrate different strategies:
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418, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Olympic Dam, Australia: A large Neoproterozoic, volcano-sedimentary basin in South Australia was, selected for its resemblance to base metal-rich basins in Africa and North America. The presence of, satellite-image lineaments and copper-depleted, basaltic rocks similar to the Keweenawan rift in, Michigan was felt to be especially encouraging. The, reward was the discovery of the giant Cu-Au-U, deposit near Olympic Dam (c.f. Chapter 2 “Copper”,, “Uranium”)., Mt Isa, Australia: This is one of the largest base, metal deposits of the world (c.f. Chapter 2 “Lead and, Zinc”, “Copper”). Geological research motivated, exploration along strike of the major structure that, controls ore at Mt Isa. This resulted in the discovery, of several deposits and the development of new mines, such as Hilton, George Fisher and Valhalla, recalling, the motto “if you wish to hunt elephants, go to, elephant country”., Skaergaard, Greenland: The benefit of applying, new concepts to well-known geological sites was, demonstrated by geologists who investigated the, platinum potential of large layered mafic intrusions, based on genetic models of the Bushveld deposits., Many new discoveries were made, but one especially, remarkable (though still undeveloped) Au-Pt deposit, was found at Skaergaard (c.f. Chapter 2 “Gold”)., Previously, this intrusion had been described in all, petrology textbooks and was the site of much scientific work, including the discovery of sulphide immiscibility (Wager et al. 1957)., Voisey’s Bay, Canada: Even today, some mineral, deposits are found by accident. In 1994, diamond, prospectors in eastern Labrador, Canada happened, upon a small gossan. Trace metal contents and its, location within olivine-gabbros suggested an, exploratory electromagnetic survey. The results, revealed the presence of sulphides, which are now, known to be part of the Ni-sulphide deposit, Voisey’s Bay. Total published resources are 137 Mt, at 1.59% Ni, 0.85% Cu and 0.09% Co (Naldrett &, Li 2000, Naldrett 2004)., Lisheen, Ireland: The location of Irish base metal, deposits is known to be controlled by stratigraphy, (usually the base of Early Carboniferous stromatolitic, Waulsortian limestones) and tensional synsedimentary growth faults (c.f. Chapter 4 “Lead and Zinc”). A, combination of geochemical and geophysical methods was used to scan areas underneath overburden, reaching a thickness of 300 m above the prospective, horizon. This depth had been hidden from earlier, explorers. Suggestive anomalies were soon found and, , examined by diamond drilling. Only the seventh, drillhole revealed ore. In 1994, 22 Mt of ore at, 11.5% Zn, 1.9% Pb and 26 g/t Ag were proven and, a new mine was born., , Earlier, the discovery of new deposits was, pursued by geologists, miners and enterprising, non-professionals who scanned the land systematically for traces of ore or other indications of, mineralization such as gossans (“prospecting”,, Figure 5.1; Locke 1921). Promising indications, were examined by trenches, shafts and adits, usually with the aim of immediate exploitation., Meanwhile, however, the chance to discover hitherto unknown ore outcrops on the land surface is, very small, because early prospectors worked, quite thoroughly. Today, large buried deposits are, targeted and the search is guided by geological, concepts. Modern exploration is characterized by, multi-professional teams and systematic methodology (Kreuzer et al. 2008, Sillitoe 1995, Magoon &, Dow 1994). Of course, experienced individuals, with exceptional metallogenetic knowledge may, still be the key to success., The essence of geological concepts is descriptive, and genetic models of mineral deposits (Cox &, Singer 1986). Appropriate geological, geochemical, and geophysical parameters that characterize, targeted deposit types are combined to build, “exploration models”. In practice, locations are, sought that display anomalous indicator parameters within the ordinary geological background. These locations are called “anomalies”,, or “prospects”. Exploration projects commonly, expose many anomalies that have to be ranked, for prospectivity and investigated in more detail., Experience shows that most anomalies are found, to be of no economic interest. Exploration teams, strive for early rejection of barren prospects, (negatives) in order to avoid useless expenses., Reliable identification of good prospects is essential, whereas both false negatives and false positives must be avoided:, The distribution of formerly exploited copper deposits and mineralizations in the Cloncurry-Mt Isa, district provides an instructive example. Apart from, the world class Mt Isa deposit, 262 sites produced, >10 t, 70 >100 t, 14 >1000 t and only 6 including Mt
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Figure 5.1 Prospecting in Europe during the 16th, century (Agricola 1556). Courtesy Thomas Hofmann,, Geological Survey, Vienna, Austria. Agricola explains, that the divining rod (Virgula A) is useless and that only, physical indications of hidden ore should be regarded,, such as ore fragments in trenches (Fossa B) and streams,, stressed vegetation, indicator plants and tiny, temperature differences of the soil, which are made, visible by frost and dew., , Oxide (Figure/Plate 1.56) yielded >10,000 tonnes of, copper (J.H. Brooks in Glasson & Rattigan 1990)., Some of the tiny mines may have enriched a lucky, artisanal miner, but would never support an industrial mining operation. The essence of exploration is, to identify potential mines among the large number, of anomalous locations., , The strategy of exploration programmes directs work from the initial regional scale to, smaller areas, with costs increasing as reconnaissance methods are succeeded by detailed investigations, and the pursuit of resemblance to, models changes to collecting hard data. Different, metal and mineral deposits require distinct approaches, but a general pattern can be sketched as, follows:, 1 Reconnaissance exploration. Reconnaissance, aims at rapid and low-cost sorting out of prospective and unprospective parts of an area. Typical, , 419, , methods used include interpretation of published, geological maps, satellite images and aerial, photographs, aerogeophysics, heavy (indicator), minerals and geochemical sampling of stream, sediments (e.g. for diamond exploration), and, other regional geochemical surveys. On-theground geological reconnaissance and verification, mapping, and on-site inspection of prospects are, indispensable., 2 Detailed follow-up exploration. In this phase,, prospective locations and anomalies (prospects), are examined to a degree that allows a preliminary, appraisal of their potential. Essential data include, the geological setting, contours and nature of, the suspected orebody. Useful methods include, detailed geological mapping, geochemical and, ground-based geophysical investigations, shallow, trenching and some drilling. This work will rapidly expose the low potential of most locations., Note, however, that in some famous cases perseverance, in spite of disappointing first drillholes,, was well rewarded (e.g. Olympic Dam). Retained, prospective locations are submitted to a prefeasibility study, which presents the case of potentially, profitable exploitation by comparison with mines, working similar deposits., 3 Evaluation. Evaluation aims to provide comprehensive data that allow the final decision to, develop a mine or to defer development. In this, phase, drilling is intensified and first mine exposures are made in order to provide large samples for semi-industrial scale processing trials., Results are indispensable for the assessment of, metal or mineral recovery and of product quality. Access to ore and host rocks facilitates, determination of rock mechanical and geohydrological behaviour. Together with drilling and, assaying results, these data serve to estimate, reserves (and resources) of the deposit. The next, step is realistic planning of the future mine and, its processing plant and infrastructure. At this, stage, investment, operating costs and the probable future income can be calculated. Assessment of environmental and social costs is, possible. Of course, evaluation is done by, a team of professionals. Evaluation of a mining, project concludes with the compilation of a, feasibility study. A feasibility study is the
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420, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Today, Landsat TM and more recent satellites, producing high-resolution images are standard, exploration and mapping tools., Only part of the electromagnetic spectrum, between 0.3 mm (micrometre) and 50 cm wavelength is useful for remote sensing. This span, comprises visible light (0.38–0.78 mm), near and, middle (“thermal”) infrared, and microwaves, (Figure 5.2). Radiation emitted from the surface, of interest is recorded. “Passive” remote sensing, uses reflected sunlight, whereas “active” methods are based on reflected induced radiation (e.g., by radar equipment mounted on aircraft and, satellites)., Different minerals, rocks, soils and plants, reflect radiation in specific wavelengths, which, is obvious considering our subjective colour, perception of visible light (from short-wave violet, to long-wave red). By dividing the spectrum, between 0.3 and 20 mm into more than 200 distinct spectral bands (“hyperspectral mapping”,, e.g. NASA’s Earth-observing satellite Hyperion),, specific reflection characteristics are recorded., Based on comparative spectral data at different, scales, from space to ground truth in the field and, laboratory measurements, the method allows, identification of minerals and rocks, different, , required base for a decision to develop a mine, and for an investor (e.g. a bank) to finance the, project., Note that wherever exploration is likely to lead, to new mining activities, environmental studies, must be taken up as early as possible (Plumlee &, Logsdon 1999b). It would be a costly error to defer, this work to the last stages of developing a new, operation., 5.2.3 Geological remote sensing, The term “remote sensing” refers to techniques, that are used to measure and interpret the interaction between distant matter and electromagnetic, energy. Some of these techniques (e.g. electromagnetic methods) are commonly assigned to geophysics. Geological remote sensing is mainly, based on natural electromagnetic waves radiating, from the Earth’s surface. Main observation platforms are aeroplanes and satellites. Interpretation, focuses on geospatial features (Sabins 1999, Drury, 2001). Photogeology was the first remote sensing, method widely employed and remains a useful, tool. A new dimension of remote sensing, opened up in 1972, however, when the first satellite images (Landsat ERTS-1) became available., , Ultraviolet Visible, light, , No observations possible, due to atmospheric, water vapour, (next window occurs, in the microwave region, above 0.1 cm), , Infrared, (near), , (thermal), , Panchromatic, Infrared thermography, , Infrared, colour film, , Multispectral scanners, e.g. Thematic Mapper, Landsat MSS, Band 4 5 6 7, 1 2 3, 0.2, , 0.4, , 0.7, , 8, , Spot, 1, , 2, , 4, , 6, , 10, , 20, , 40, , 60, , 100, , Wavelength (µm), Figure 5.2 Wavelength regions in the lower electromagnetic spectrum frequently used in geological remote sensing., Note that the infrared region extends to a wave length of 0.1 cm, but this long-wave IR heat radiation cannot be observed, from space because of absorption in the atmosphere (resulting in the greenhouse effect).
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , soils, types of hydrothermal alteration (Mauger, et al. 2007), gossans and the discrimination of, healthy and stressed plants. From 20–1000 mm,, atmospheric interference is too strong for reliable, interpretation. Radar bands used in Earth observation include 3 (X), 5.6 (C) and 25 cm (L). These, waves penetrate clouds and vegetation and provide, morphological images. Synthetic aperture radar, (SAR) sensors excel by high resolution. Radar, is most valuable for mapping topography, that reveals geological and man-made structures, (e.g. the paired system of TerraSAR-X and, TanDEM-X launched in 2010)., The foundation of modern remote sensing was, the development of optomechanical radiometers, in the years after 1960. Before, only analogue, photographic films with high resolution were, used. The new technology allowed building multispectral scanners(MSS), which excel in spectral, resolution but have relatively poor spatial resolution. These systems record several sections, (“bands”) of the electromagnetic spectrum synchronously for the same target area (“pixel”)., Landsat pixels cover a surface area of 30 30 m,, but modern satellites have a much better resolution. Reflection intensities for each band are, digitally registered. For better evaluation, ground, surveys with the new generation of portable, spectrometers are invaluable. The instruments, produce data across the full spectral range, (350–2500 nm, visible light to infrared) at very, high resolution and allow mineralogical characterization of soil, rocks and alteration zones which, occur in the survey area., MSS technology and hyperspectral mapping, are also possible from aircraft, but only satellites, provide nearly total coverage of the Earth that is, available to the general public. Much used are the, workhorses of the LANDSAT series (1–7). The French, SPOT series offers better resolution (10 10 m, pixels) and stereographic capabilities. The USJapanese ASTER ( 1999) satellite carries 5 bands in, the short-wave infrared (SWIR) range, enabling, detection of different clays, carbonates, sulphates and other minerals. Typical products, offered are:, . paper images (e.g. black-and-white of single, bands, or colour-coded composites);, , 421, , . electronic copies of the digital dataset of one, scene; and, . variously processed data., One Landsat scene covers an area of 185 185, km. In dry and arid lands, geological evaluation, with simple visual methods provides excellent, insight into large-scale features, which are best, visible in near infrared images. However, the full, potential of satellite data can only be achieved by, combining all available spectral bands in digital, processing. This allows enhancing contrasts,, linear structures, gossans and hydrothermal alteration, correcting topography and the production, of artificial stereo pairs (similar to aerial photographs). Digital image processing combined with, GIS makes it possible to combine results of remote, sensing with topographic (e.g. digital elevation, models, DEMs), geophysical and geochemical, information., In geologically well explored areas, satellite, images have mainly assisted in the recognition of, large-scale structures (lineaments) that defied, earthbound mapping. After the first elation, it was, soon realized that large structures rarely control, the location of ore deposits. Yet, tectonic control is, frequent and aids rational exploration. Geologically less explored regions of the Earth, possibly, making up 75% of the land surface, can be economically and quickly surveyed with satellite, data. Resulting maps at 1:50,000 to 1:1000,000, support an efficient and effective exploration programme. TM data allow easy recognition of gossans and of hydrothermal clay, alunite and mica, zones (Ruitenbeek et al. 2005). Famous successes, include the discovery of large copper porphyries in, northern Chile (Collahuasi and Ujina; Sabins, 1999). More recently, remote mapping at a scale, of 1:10,000 became feasible, because digital panchromatic and colour images of IKONOS (1999) and, QUICKBIRD (2001) have a resolution as small as, 0.6–4 m. Space photographs in cartographic quality taken from manned platforms are available for, certain regions of the Earth., The geological interpretation of aerial photographs is employed for smaller areas that are to, be mapped in great detail. Most countries offer, a full coverage of black-and-white photographs at, scales between 1:20,000 and 1:50,000. Repeated
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422, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , runs many years apart may be available and are, very useful for retracing the landscape evolution,, which is essential for environmental work. In, addition, different vegetation, moisture and, illumination may reveal subtle features. In most, cases, panchromatic film and high-resolution, cameras provide excellent pictures. Lateral, (30%) and longitudinal overlap (60%) allow stereoscopic viewing and mapping. Very precise, (orthorectified) topographic and geological maps, are prepared from aerial photographs. Amended by, groundwork, these maps are an excellent means, for detailed exploration. In special cases, colour or, infrared colour films are employed for better recognition of certain features (e.g. hydrothermal, alteration zoning)., Optimal results of remote sensing are obtained, in arid and semi-arid regions that have little soil, and vegetation cover. Humid landscapes yield, little geological information, apart from structures. Vegetated hydrothermal alteration cannot, be mapped, although anomalous heavy metal, contents may be discernible by stressed plants, because their reflection deviates from that of, healthy ones., 5.2.4 Geochemical exploration, A first modern overview of distribution and mobility of elements in the Earth was written by, V.M. Goldschmidt and posthumously published, (Goldschmidt 1958). More recently, Holland &, Turekian (2003) edited a voluminous and comprehensive presentation of geochemistry. Although, not directed at practicians, both are valuable, sources for applied geochemistry, such as exploration and environmental investigations., Geochemical methods of exploration are based, on the observation that most ore deposits are, surrounded by zones (halos), which deviate chemically from ordinary host rocks. Chemical deviations may be expressed by enrichment or depletion, of certain minerals, elements, isotopes and by, other systematic differences. Various ore deposit, types display characteristic halos, which can be, found by analysing samples of rocks, soil, plants,, water, soil gas (Figure 5.3) and of sediments in, streams and lakes. Graphical and statistical, , Vegetation, , Soil, horizons, , A, B, C, , Cover rocks, , Basement, , Ore, Figure 5.3 The variety of geochemical samples that can, be collected and analysed to assist in the search for buried, ore deposits., , processing of geochemical data helps to define, locations that may indicate ore (“geochemical, anomalies”). The scale of investigations, for example the density of sampling, varies from geochemical mapping at the continent scale (1 sample/, 5000 km2) through an intermediate mesh, (1 sample/300 km2, Reimann et al. 2007) to very, detailed local sampling of, for example, soil, above a prospective geophysical anomaly., Geochemical exploration results in a large number of analytical data. Statistics are used to discern, anomalous and therefore potentially prospective, results, such as simple frequency plots (Figure 5.4), and more advanced methods (Carranza 2008)., Concentrations of elements in unmineralized, rock bodies always fluctuate around a mean value, (background). Samples with higher concentrations, (above a certain threshold) may indicate subtle, or very clear geochemical anomalies. Of course,, anomalies must be considered within their, petrological context. Ultramafic bodies, for example, within metasediments must cause Ni and Cr, anomalies that have no prospective value. In such, cases, data have to be sorted by source rocks into
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Mean value m, (background m ± s), , s, m, , +, , +, , 3s, , 2s, , Standard, deviation, , m, , Frequency, , A, , Possible, Probable, anomaly, , B, , Threshold, , 423, , changes. About the central Cu-Mo ore zone,, porphyry copper ore deposits display shells and, caps of elevated Pb-Zn, Au and an extensive, outer halo of more mobile elements such as As,, Ag, Sb, Hg, Tl, Te and Mn. These halos are threedimensional, whereas others are essentially, two-dimensional, such as those associated with, sedex deposits, which are restricted to the same, stratigraphical horizon. Primary trace element, halos considerably enlarge the targets of geochemical exploration. Consequently, a higher, sampling distance can be set, reducing costs., Also, as demonstrated by the halos surrounding, copper porphyries, associated and more mobile, elements (“pathfinder elements”) may be more, suitable for finding prospective locations than, the elements concentrated in ore., , Log concentration (e.g. ppm), Figure 5.4 Schematic diagram of the frequency, distribution of two different populations of geochemical, data. The distribution of geochemical data is typically, lognormal. Population A may be considered as an, ordinary geological background (e.g. the serpentinized, mantle section of an ophiolite), population B as, expression of a mineralized zone such as chromite in, serpentinite. Note that the concentrations which mark, background and threshold are not necessarily defined as, shown in the plot., , different populations, which are then evaluated for, anomalies indicating possible mineralization. In, the same project, a lower threshold can be used for, regional exploration (e.g. finding mineralized, zones) and a higher one for locating the best targets, for drilling., Primary geochemical anomalies, Primary geochemical anomalies are formed as, a by-product of the processes that concentrate, ore. Geochemical halos enveloping the actual, ore are caused by the “primary dispersion” of, elements. When, for example, hydrothermal, solutions deposit ore in a vein, some of the fluid, permeates into wall rocks causing different, alterations (cf. Chapter 1.1 “Hydrothermal Host, Rock Alteration”), which include chemical, , Secondary geochemical anomalies, Secondary geochemical anomalies are formed by, processes that acted on the deposit after its formation. Most frequent are chemical consequences of, near-surface mobilization, weathering and erosion, which transfer elements from the orebody, or its primary halo to till, soil, plants, groundwater, and soil gas. Erosion moves particles and dissolved, matter into streams, where traces may be detectable at great distances from the source. This is why, stream sediment sampling is a most effective, method of reconnaissance and regional exploration. The post-formation redistribution of elements from an ore deposit is called “secondary, dispersion”. In the processes that cause secondary, dispersion, the variable mobility of elements is of, great significance. Elements with a higher mobility under surficial conditions enlarge the anomalous zone. A project targeting polymetallic, deposits of Pb, Zn and Cu, for example, would use, mobile Zn for regional sampling with a low density, whereas dense sampling of Zn-anomalies for, Cu and Pb should reveal the drilling targets, (Figure 5.5)., The mobility of elements in secondary dispersion is strongly influenced by factors including the, nature of rocks, climate, vegetation, relief and, groundwater flow. The complex interaction of, these natural factors has been called “landscape
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424, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , geochemistry” (Fortescue 1992). Nearly ubiquitous influence of human activities, such as industry, agriculture and building, overprints the, natural state. Geochemical exploration projects, must consider the possible presence of perturbance by “anthropogenic dispersion”., Effective distinction between prospective and, non-significant geochemical anomalies is desirable. To this purpose, non-geochemical data such, as the geological setting are consulted. Based, solely on the geochemical data, the contrast, between background and the anomalous data is, employed. High contrast is considered to affirm, the significance of an anomaly. Controls of the, contrast include the primary metal contents in, ore compared to host rocks, the mobility of the, elements investigated and dilution with barren, material. Because contrast is so important, most, geochemical work in exploration starts with an, orientation phase, which is expected to identify, the most suitable sample material and other constraints for the main phase work. For unconsolidated soil, stream and lake sediments, contrast is, , a function of the chosen grain size, the soil horizon, (depth) and the extraction method. If sufficient, contrast of target element concentrations cannot, be reached, possible pathfinder elements should, be tested., Geochemical exploration programmes, Geochemical exploration programmes may be, designed for the reconnaissance of large areas or, for detailed investigation of prospective locations., Regional sampling is done along roads and, water courses, whereas sampling grids are typically designed for local investigations. In the first, case, sampling distances are measured in kilometres, in the second rather in metres. The orientation of the sampling grid is best chosen to, support geological mapping, geophysical surveys, and later drilling. Orientation sampling serves to, select suitable field methods and the most, appropriate analytical methods. This allows final, planning of the main phase of the sampling programme, including logistics., , 350, 300, 250, 200, 150, Zinc, , 100, , Copper, Lead, 0, , Sampling locations, , Surface, Soil, Host rocks, , Ore, , ppm, , 50, , Figure 5.5 Differential secondary, dispersion of zinc, copper and lead in, soil above a tabular base metal, orebody in siliciclastic host rocks., The profile is drawn along a, geochemical sampling, or grid line.
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , 425, , treated apart from common siliciclastic sediment., Water pH, T and Eh should be measured at each, sample site, because large variations strongly, influence the mobility of elements. Interpretation, of data should always consider the geochemical, characteristics of rock types occurring in the, watershed as a geogenic background. If a reference, to common average contents of elements is intended, one of the worldwide clay rock standards, is more appropriate than the crustal average, (Reimann & de Caritat 1998, Gromet et al., 1984). Fine-grained sediments should only be, compared with material of similar grain size., Figure 5.6 (Plate 5.6) Environmental stream sediment, and water sampling (including in-situ determination of, pH, T and Eh) in the Gatumba tin-tantalum mining, district, Rwanda., , Stream sediments, Stream sediments (Figure/Plate 5.6) are extremely, efficient means to discover geochemically anomalous zones in large regions and with low sampling, density, but only if a well-developed drainage, system is present. Where suitable water courses, are absent, remarkable results are achievable with, wide-spaced soil samples (e.g. Australia), till and, lake sediments (Canada, Finland). The sediment, sample from an active river bed is considered to, represent an average of its upstream watershed. If, mineralization or its dispersion halo is exposed in, the drainage area, chemical traces must occur in, the sample. Because coarse material dilutes trace, element concentrations (and thus lowers the contrast), fine-grained stream sediments (clayey and, silty mud) are preferred. Samples are sieved in, order to submit a homogeneous fraction for analysis (often 80 mesh corresponding to 180 mm)., Samples for indicator mineral investigations, (McClenaghan 2005) are collected parallel with, the stream sediments. In specific cases, such as, expected loss of fine-grained gold, or environmental work aiming at volatile pollutants, freeze-sampling is employed (Petts et al. 1991). Organic, substances and Fe-Mn ooze in surface waters, adsorb dissolved metals more than clay minerals., Geochemical results from such samples have to be, , Soil sample geochemistry, Soil sample geochemistry calls for detailed orientation work, because success depends on sufficient understanding of soil layering and genesis, (Butt et al. 2000), which control element mobility, (cf. Chapter 1.2 “Supergene Ore Formation Systems”). Often, present distribution patterns are, a legacy of several superposed soil formation, phases. Autochthonous soils must be discerned, from transported ones. Allochthonous soil is of, little use in exploration, although exceptions do, occur: In Western Australia, nickel concentrations are anomalous in the B-horizon of transported soil above buried Ni-mineralization. This, totally unexpected feature is explained by nickel, transfer from deep roots into leaves and from, rotting litter back into the soil. Many elements, are enriched in the B-horizon of a regolith profile,, but there are important exceptions. Ferriferous, nodules or pisoids, which are common in areas of, lateritic cover, can be useful geochemical guides to, ore in bedrock (Smith et al. 2000, Smith & Singh, 2007). In South Australia, even pedogenic calcrete, is sampled for locating subcropping gold-quartz, veins (Mauger et al. 2007)., Geochemical exploration with rock samples, Geochemical exploration with rock samples, or, selected minerals is based on specific geologicalpetrological models.Examples include the regional, sampling of granites in order to locate fertile intrusions, the discrimination of prospective and barren
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426, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , porphyries by analysing copper in biotite, and the, identification of rare metal pegmatites by muscovite analyses. Also, rock geochemistry is useful for, tracing orebodies in complex structural settings., Certain hydrothermal alteration zones (cf. Chapter, 1.1 “Hydrothermal host rock alteration”) are easily, recognized and help to point development adits or, drillholes towards ore. Isotope investigations complement data on elemental distribution, mainly at, a more local scale. Whole rock stable isotope mapping around centres of epigenetic mineralization,, for example, often reveals very clear anomalies that, are useful for finding orebodies (Hoefs 2009, Holk, et al. 2008). Remember that with all solid materials, the mass of a representative sample is a direct, function of the grain diameter. The rules of sufficient sample mass and careful diminution, homogenization and sub-sampling must be strictly, followed (cf. Geological Mapping and Sampling;, Gy 1992, Pitard 1993)., Soil air and atmospheric air sampling, Soil air and atmospheric air sampled near the, surface may contain traces of Hg, H2S, SO2 and, metals, which are possible keys to buried sulphide, and gold deposits, and radon may indicate uranium ore (Klusman 1993, Hale 2000). Although, technological innovation in this field is intense,, persuasive case histories are not widely known, and more common geochemical methods seem to, suffice. It appears possible, however, that future, search for deep orebodies buried below today’s, commonly shallow targets between the surface, and 400 m depth will profit from these methods., Biogeochemical exploration methods, Biogeochemical exploration methods (“phyto-exploration”) have a demonstrated success rate but, are not equally often used as stream sediments and, soil samples. Plant roots “sample” soil and soil, water, and thus transfer geochemical information, to their organs above the ground. Samples are, usually taken from live plants. Orientation surveys assist to find suitable plant species and, because of organ-specific accumulation, favourable parts of individual plants. Possible choices, , include leaves, twigs (that must have the same, age), or bark. Sampling is relatively cheap because, drilling and digging is not needed. Regional and, local anomalies of metals and pathfinder elements, can be ascertained., Water samples, Water samples collected from springs, wells, boreholes and streams are rarely useful for exploration., Dissolved metal contents in water are usually very, low (in the ppb range) and vary strongly with pH, and Eh. This makes interpretation difficult. Yet,, regional datasets on ground and surface water, chemistry may provide important clues to several, deposit types. Of course, water geochemistry is, always an essential part of environmental monitoring of mine sites (Ficklin & Mosier 1999)., Analytical methods of exploration geochemistry, Samples are dried, sieved, sub-sampled and ground, until 100% pass an 80 mesh (180 mm) screen. For, analysis, a small aliquot (0.2–2 g) is usually prepared in aqueous solution. Partial solution by, weak acids, which only dissolve weakly adsorbed, elements, is one common procedure. The other is, complete dissolution by aqua regia, or a multi-acid, mixture combining hydrofluoric, hydrochloric,, nitric and perchloric acids at low temperatures, and pressures. The choice is guided by the speciation of the elements of interest in the sample. If the, main interest concerns metals weakly adsorbed on, clay and organic matter, partial solution is recommended. For determination of elements sited in, the crystal lattice of minerals, such as barium in, muscovite of metamorphosed distal sedex exhalites, total dissolution is indicated. Methods of, sequential elution (sequential extraction) provide, an understanding of metal speciation in soil, lake, and stream sediment samples: Total metal content of a sample is the sum of several species, occurring:, . as exchangeable ions;, . carbonates;, . adsorbed to Fe-Mn oxy-hydroxides;, . in sulphides; and, . silicate minerals (Rao et al. 2008).
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Defining the most suitable dissolution variant for, a specific project is one of the aims of an orientation survey., Geochemical laboratory equipment for exploration and environmental studies is nearly identical, (Crock et al. 1999). The workhorses of instrumental analysis are inductively coupled plasmaatomic emission spectroscopy (ICP-AES) and, inductively coupled plasma-mass spectrometry, (ICP-MS). Other useful methods include X-ray, fluorescence (XRF), instrumental neutron activation analysis (INAA), gamma-activation analysis,, atomic absorption spectroscopy (AAS) and, electrochemical means such as specific ion, electrodes., In the field and in mines, portable X-ray fluorescence (XRF) analysers are increasingly used for, on-site data acquisition. Colorimetric and other, simple field methods remain useful, but deliver, semi-quantitative data for elements (e.g. As, Cu,, Zn, Mo, W, Ni) and ions (e.g. SO2, 4 ). They are, chosen when quick results are more important, than accuracy, for example in remote regions. A, decision to use such methods should only be made, after trials with preliminary samples and consultation with an experienced analyst., Analytical data in exploration geochemistry, need not in all cases equal the absolute element, content in a sample, or in other words, accuracy, may not be essential. Deviations of 30% from, the absolute figure (e.g. an international laboratory, standard) are tolerated, if the relative error remains, within narrow limits. In contrast, excellent reproducibility of results, that is high precision, is, absolutely required. This is the base for any data, evaluation, especially if the contrast between, background and anomalies is small. In all geochemical programmes, error control is a fundamental aspect. Errors may be introduced during, sampling, sample processing and transport, and in, the laboratory. It is good practice to repeat at least, 10% of sampling. Analytical errors are revealed by, inserting the same sample or a standard of known, composition repeatedly into the series (Arbogast, 1990). Control by another laboratory is advisable., Based on a collection of control data, it is possible, to calculate total error margins and the confidence, interval (Taylor 1997)., , 427, , Figure 5.7 Tourmalinization is common around, pegmatite and granite-related ore. Near cassiterite vein, deposits at Rutongo, Rwanda, the growth of black, tourmaline needles in quartzite was controlled by folded, bedding planes and a weak schistosity., , Indicator minerals, Indicator minerals are increasingly used as, a complementary tool to geochemistry. Indicator, minerals are characterized by elevated density, (>2.8 g/cm3) and a good preservation potential in, the weathering environment (McClenaghan, 2005). Of course, their application in diamond, prospecting has a long tradition. Other deposit, types, however, also display specific indicator, minerals. Porphyry copper systems, for example,, shed gold, rutile, tourmaline, garnet, jarosite and, alunite. Many rare metal deposits are enveloped by, tourmalinization halos (Figure 5.7)., Presentation and interpretation, Data resulting from geochemical exploration, should always be presented in maps, because the, search is first of all for spatial variation. Statistical, calculations are a useful complement, if geology, and structure are not too diverse. Point-symbol, maps (Howarth 1983) represent the basic tool, (similar to Figure 3.1), which can be amplified by, wavelength filtering to produce a map of residual, anomalies (Ludington et al. 2006). Principle component analyses of multi-element data and plotting the factor distribution may provide valuable, guides for regional exploration. The correlation of
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428, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , geochemical results with geology, geophysics and, topography is investigated using GIS (Carranza, 2008). Examples for complex presentation and, interpretation can be found in geochemical atlases, which provide data at different scales., Environmental geochemistry, Geochemical work is not finished with the discovery and preliminary quantification of a potential ore deposit, but reaches a second climax during, detailed investigations and preparation of the, feasibility and environmental impact studies. The, pre-mining environmental state of the future mining area must be documented, including natural, and anthropogenic characteristics of bedrock,, regolith, surface and groundwater. Establishing, the geochemical landscape (Fortescue 1992) is a, central task. Considering this requirement, costs, of re-sampling may be saved by imposing, high accuracy standards on all exploration geochemistry. High resolution, multi-media and, multi-element mapping is an important part of, environmental impact studies, which are the, foundation of operational and administrative decisions throughout the life of a mine, including, closure., The methods of environmental geochemistry, differ little from its application in exploration. In, fact, investigations and interpretations of nonmining anthropogenic dispersion might profit, from the study of the highly developed methods, and accumulated experience of geochemical, exploration. One example concerns the unreflected reporting of heavy metal concentrations, without reference to natural (geogenic) boundary, conditions- such as background and variance., 5.2.5 Geophysical exploration, The foundation of geophysical exploration methods are the varied physical properties of ore and, gangue minerals, fluids and rocks (Ellis & Singer, 2007, Keary et al. 2002). Passive geophysical, survey methods use natural potential fields (e.g., magnetism, gravity). Active methods rely on, interaction of induced artificial fields with the, subsurface (e.g. electrical conductivity, seismics)., , “Inversion” designates the computation of, geophysical models purely from measurements., These models are inherently ambiguous., Useful interpretations are only obtained when, independent constraints are available, for example, geological models, drillhole and petrophysical, data. Petrophysical properties of rocks and ore, are the critical link between geophysics and, geology., Geophysical methods with a depth penetration, to several hundred metres below the surface are, commonly employed in the search for solid, mineral deposits. Geophysics extend the validity, of geological and geochemical data to this depth,, which currently limits the economic exploitability of most minerals and ores. However, the reason, for using geophysics is not always depth penetration. Geophysical investigations of near-surface, ores such as coastal placers, for example, contribute valuable continuity to deposit modelling, for, example between drillholes. Also, identification, and mitigation of mining-induced environmental, problems may profit from geophysical surveys, (Ackman 2003, Rucker et al. 2009)., In cases of strong incentives to extend the search, to greater depth, which common surface-based, geophysical methods cannot reach, magnetotelluric methods (MT) are used. Another approach was, developed in Canada, where base metal deposits, such as those of the Sudbury District are explored, to more than 3000 m depth: Deep drillholes are, sited on geological evidence to penetrate a prospective rock body, which is scanned by downhole, geophysics for signs of ore., Geophysical surveys complement other exploration methods at all scales. Regional geological, and geochemical work is supported by geophysical, data measured from aircraft and helicopters. Frequently used aerogeophysical methods include, magnetics, electromagnetics, radiometry and gravimetry. For detailed and more local exploration, on the ground, many more methods are available, that allow a high density of observations at, higher accuracy and improved validity. Borehole, geophysical surveys result in the highest resolution of data (Ellis & Singer 2007), especially in, conjunction with geological, physical and chemical core logging results.
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , As with geochemical surveys, geophysical, methods reveal a background that is characteristic for ordinary rocks of an area and distinctive, anomalies, which illuminate physical contrasts., The magnitude of an anomaly is a function of: i), the contrast between host rocks and the anomalous material; ii) the size, spatial orientation and, shape of the anomalous body; and iii) the depth, from the surface. The last is essentially due to, the general law of inverse-square attenuation of, a geophysical signal as a function of distance., Identification and interpretation of geophysical, anomalies are often quite challenging, not unlike, weak geochemical indices., ‘A considerable number of geophysical exploration methods are available for mineral exploration, and each method exists in several variants. The, specific choice is a function of the geological and, exploration model of the targeted deposits (Shen, et al. 2008), of general conditions such as remoteness, climate and human land use, and of the costs., Methods that require the placement of electrodes, in the soil, for example, cannot be deployed in, permafrost regions. Electrolyte-rich highly conductive soil and groundwater in semi-arid lands, limits the depth penetration of most electrical, methods. Some well established geophysical, methods in exploration include the following, (Milsom 1996)., Magnetometry, Measuring the magnetic field is relatively, straightforward, with portable instruments on the, ground, borehole probes and instruments for, aerial surveys (Airo et al. 2004). Various types of, magnetometers are available, including “scalar”, sensors that measure total magnetic field and, “vector” sensors providing directional data. The, latter include the traditional fluxgate and the new, supersensitive SQUID (Superconducting Quantum Interference Device) magnetometers. The, intensity of the magnetic field is measured in, nanoTesla (previously gamma; 1 nT equals 1, gamma). The Earth’s total field varies from, 30,000 (equator) to 65,000 nT (poles). Regular, diurnal variations of the field are enforced by, currents in the ionosphere and reach 10–30 nT., , 429, , Solar activity, such as spots and flares, cause shortterm irregular disturbances (“magnetic storms”),, with amplitudes that may surpass 1000 nT. Field, work must be suspended during magnetic storms., The magnetic properties of rocks differ by several, orders of magnitude. Exploration tends to look, for strong deviations from the background field,, which are commonly caused by minerals of high, susceptibility, such as magnetite, pyrrhotite and, maghemite. Haematite has a very small susceptibility and many iron ore deposits do not produce, magnetic anomalies. Magnetite and sulphide ore, deposits can be located with magnetic surveys, but, also kimberlites and other rocks that host magnetic minerals. Airborne magnetic surveys benefit, geological mapping, especially in regions with, thick cover of soil, moraine sheets or water (on, land and offshore). One of the most relevant tools, for detailed mapping are measurements of the, anisotropy of magnetic susceptibility (ASM;, Mezeme et al. 2007). Results of magnetometry are, presented in maps and in sections with distance as, horizontal and the magnetic signal as vertical axis., Electric current methods, Conductivity (Siemens per metre, S/m) or its, reciprocal, resistivity (Ohm), are determined by, measuring voltages associated with electric currents flowing in the ground, either induced or, natural. Rocks and minerals have widely varying, resistivity, with lowest values displayed by clay,, saline pore water, acid rock drainage, sulphide ore,, native metals and graphite, whereas common, rocks and minerals have low conductivity. This, contrast is used for exploration. Electric current, methods rely on placing electrodes in the ground,, commonly two metal stakes (e.g. steel rods) for, passing current into the subsurface and two nonpolarizing electrodes for measuring the induced, potential in volts. Several time-tested arrays of, electrode layout are possible. Moving cables, electrodes and equipment from one point (traverse, station) to the next makes these methods laborious and slow. They are typically used for local and, detailed investigations., . Spontaneous potential methods (SP) rely on, electrochemical processes caused by weathering
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430, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , orebodies that straddle the groundwater table. The, conductive material concentrates the flow of, oxidation-reduction return currents, producing a, negative anomaly at the surface. Typical targets for, SP-surveys are sulphides, graphite and magnetite., The method is very cheap and simple; only two, non-polarizing electrodes, cables and a voltmeter, of very high impedance are needed. Today, SP, surveys are little used in exploration, because, methods such as EM (see below) more reliably, detect orebodies. However, SP is useful for locating, flowing groundwater (e.g. in a leaky tailings, dam) by an effect that is termed “streaming, potential”. Large-scale natural potentials in the, earth are investigated with telluric and magnetotelluric methods (MT). Practical application finds, MT mainly in the hydrocarbon industry. Hybridsource magnetotelluric systems are capable of, measuring electrical resistivity and the magnetic, field in great detail, over depth ranges of a few, metres to greater than one kilometre. This method, is successfully employed for exploration of coal,, petroleum, uranium and other metals (Shen et al., 2008)., . Electric resistivity surveys require the four electrodes described above. Two different aims are, pursued:, 1 “Resistivity profiling” is in principle a tool for, mapping the shallow subsurface. In this variant,, the distance between electrodes is not changed, and the whole array is moved across the country,, allowing recognition of gravel, sand and clay,, massive orebodies, faultsandsteep rockcontacts., 2 “Resistivity sounding” utilizes the larger, depth penetration of currents as electrodes are, set farther apart. The method reveals the vertical, sequence of different rocks but only works well, if interfaces between beds are largely horizontal., Resistivity surveys have been “mechanized” for, hydrogeological mapping in Denmark by placing, the electrodes on a long trailer pulled by a tractor, (Thomsen 2004). Improvement is also possible, by placing electrodes in boreholes. Resistivity, methods help to locate massive sulphide bodies,, acid rock drainage (Rucker et al. 2009), graphite,, salt water intrusions on the coast, water-filled, sand and gravel aquifers and indirectly, alluvial, tin and gold placers., , . Induced polarization methods (IP) are much, utilized in exploration because they are able to, detect sulphide ore minerals (e.g. of Cu and Mo in, porphyries) and other conducting minerals (graphite flakes in gneiss), which are disseminated in a, matrix with high resistivity. Induced polarization, is activated by passing a pulse of current via two, electrodes into the ground. This charges electronically conducting particles, not unlike capacitors., When the activating current is terminated,, discharge from the particles produces currents,, voltages and magnetic fields. The transient voltage spike (polarization effect) is measured at the, surface via two non-polarizing electrodes. It is a, measure of the number, or more precisely the total, surface, of conducting particles in the ground. A, higher signal indicates more intense mineralization. IP systems work either in the frequency, domain or the time domain; frequency domain, equipment is generally lighter and more portable., Different electrode configurations are possible., Note that clay minerals display “membrane, polarization”, which is the cause for most IP, effects encountered in the field. IP is successfully, employed to a maximum depth of 600 m., , Electromagnetic methods (EM), EM are very often utilized in geophysical exploration. EM works without a physical contact to the, ground (no electrodes), which is an advantage, for use above ice, water, swamps, frozen or arid, ground. Many different surveying systems are, available, for aerial and surface deployment., Today, even highly conductive surface zones such, as salt lakes can be penetrated with equipment, like MagTEM (magnetic field sensor transient, electromagnetic technology). The principle of, TEM is that an alternating current is passed, through a square loop of cable, which induces, an electromagnetic field in the ground. Decaying, currents in the subsoil are measured with, a receiver coil or a magnetometer. If the primary, field encounters a good conductor, “eddy” currents flow and this produces a secondary electromagnetic field. Its strength and relative phase, compared with the primary field indicate possible, ore. Typical targets are kimberlites, sulphides (e.g.
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Ni-Cu in Voisey’s Bay, Canada), graphite (as, a guide to unconformity deposits of uranium) and, water-bearing faults. Increasingly, natural electromagnetic fields are measured concurrently with, induced fields. The magnetotelluric method (MT), is a passive electromagnetic technique used for, exploring the conductivity structure of the Earth, from tens of metres to a depth of several hundred, kilometres. Main applications are in hydrocarbon, exploration., , Gravity methods, Gravity methods in exploration are based on detecting variations of the Earth’s natural gravity, field. The measure of gravity is acceleration, expressed in gravity units (1 g.u. ¼ 1 mm/s2; earlier,, named after Galileo, 1 Gal ¼ 1 cm/s2 or 1 mGal ¼, 10 g.u.). Modern instruments (gravimeters) reach, an accuracy of 0.1 g.u., which is approximately, a 100 millionth of the Earth’s field. Gravity gradiometers are designed to measure gradients, of geological gravity changes. Surface gravity at, a specific location is a function of the rocks underneath, of the distance from the Earth’s centre of, gravity, of latitude and relief. Because one metre, difference in height of the gravity station causes, a change of 3.086 g.u., elevation relative to the, theoretical sea level reference surface must be, determined to an accuracy of 3 cm. Surveying is, usually the most costly part of gravity operations., Geological factors cause relatively small changes, of gravity. Therefore, the above-mentioned effects, on readings must be removed by calculations,, including the tidal drift of gravity (1 g.u.). The, corrected, “residual” or “extended” Bouguer, anomalies, and in some cases gravity gradients,, are presented in maps and in profiles. Interpretation of shape, density and depth of bodies, which, cause the measured gravity pattern. are derived by, calculations based on geological models., Gravity can be determined by airborne systems, on the, ground, in the sea and in underground mines. Regional, gravity maps serve mainly science and hydrocarbon, exploration, but are increasingly utilized in exploration for minerals and metal ores (Hildenbrand et al., 2000). Orebodies of solid minerals are distinguished, , 431, , from ordinary rocks by greater (chromite, sulphides) or, smaller density (kimberlites, salt diapirs: Figure 4.29)., One of the notable achievements of gravity exploration in the recent past was the discovery of the base, metal deposit Neves Corvo in the Iberian Pyrite Belt, (cf. Chapter 2.2 “Copper”; Leca 1990)., , Radiometric methods, Radioactive decay of uranium, thorium and potassium, and of certain daughter nuclides of the first, two releases gamma radiation, which is measured, with portable instruments on the ground, with, equivalent systems on board of aeroplanes and, helicopters (Airo et al. 2004) and with borehole, probes. Scintillometers or spectrometers are, usually employed. The second use energy sills of, c-radiation to distinguish between the three elements and to estimate their concentrations. Measurements of c-radiation in the field are snapshots, of a random process and readings vary, even with, the same instrument. Geologically-sourced radiation forms peaks superimposed on a background of, scattered cosmic (mainly solar) and terrestrial, radiation. The high geochemical mobility of K and, U in surficial environments, compared to the, nearly immobile Th, is the motive for the common, use of ratios (U/Th, K/Th) in maps. Application is, primarily in the search for uranium, but numerous, other utilizations have been found. Determination of gamma-radiation is a very convenient and, low-cost tool to distinguish rocks, from regional, mapping to borehole logging. It facilitates recognition of potassium salts in halitite, beach placer, horizons in sand and phosphorite in marine sediments. Potassium-rich rocks, such as certain granites or zones of hydrothermal K-alteration, can, be detected. Aerial and ground use, however, are, restricted to areas with little soil cover, because, most radiation on the surface comes from, the uppermost 10–50 cm; deeper sources below, soil remain undetected. Read more about radiation, surveys in Chapter 2.5 “Uranium”., , Tomographic methods, Remarkable new developments in exploration, geophysics include tomographic methods, which
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432, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , produce spatial images of geological bodies, for, example ore between the surface, drillholes and, mine tunnels. Various means of excitation are, used, including acoustic, sonar (ultrasonic), radar,, radio and seismic waves. Tomography allows, larger distances between exploration adits and, drillholes, and considerably lowers costs. In addition, the risk of overlooking orebodies is reduced., A convenient method of near-surface reconnaissance is ground penetrating radar (GPR), which, maps interfaces of materials having different electrical properties, elucidating bedding planes,, moisture and clay content, voids and man-made, objects. Applications include imaging the internal, structure of dunes, beaches, fluvial deposits, karst,, buried channels and lake deposits (Baker & Jol, 2007). Multifunctional geophysical systems are, designed to measure several physical parameters, at the same time or in rapid sequence. Seismic, methods are rarely used in hard-rock mineral, exploration but are the workhorse of hydrocarbon, search. Yet, successful location of uranium orebodies with three-dimensional seismics is reported from the Athabasca Basin, Canada. Coal, and lignite seams are routinely scanned by seimics, in great detail, in order to guide mechanized mining. The future of exploration geophysics will be, ever greater resolution and increasing depth penetration in order to locate the next generation’s, mineral deposits., , mic wave velocity, the direction of joints and, borehole breakouts, which reveal horizontal stress, directions (Bartlett & Edwards 2009). Optical scanners (“borehole videos”) provide information on, orientation, frequency and aperture of fractures,, bedding and lithology. A recent development is, logging while drilling, for example the application, of prompt gamma neutron activation analysis, (PGNAA) that allows determination of main and, minor elements of rocks and fluids in the borehole, walls. Uranium is measured with prompt fission, neutron (PFN) logging systems that allow determination of in-situ uranium concentrations, (Givens & Stromswold 1989), both in operating, mines and in exploration., Dentith et al. (1994) provide a useful collection, of some 50 case histories of geophysical exploration in Western Australia, illuminating both, advantages and limitations of most methods mentioned above. In any one project, the variety of, geophysical methods and their wide availability, rapidly lead to copious data, which can only be, processed by computing with appropriate software. The required expertise, both in the field and, office, is best obtained from specialized companies. The highest efficiency in identifying good, prospects, however, can only be expected if, cooperation between geophysicists and the field, geologists is ensured., 5.2.6 Trenching and drilling, , Geophysical borehole surveys, Most geophysical methods have been adapted for, drillholes in mineral resources exploration with, the typically small diameters (1/10th compared, to petroleum and gas drilling, for which they were, originally developed; Ellis & Singer 2007(Figure,, 5.8); cf. Chapter 7.4 “Exploring for Petroleum and, Natural Gas”). Wireline logging methods (socalled because probes suspended from a wire cable, are lowered into the hole) include total gamma,, gamma spectroscopy, density by gamma/gamma,, laterologs (resistivity and SP), electromagnetic, induction (conductivity), borehole deviation,, hydrochemistry and magnetic susceptibility., Acoustic (sonic) scanners are employed for the, determination of mechanical rock properties, seis-, , The difference between successful and unsuccessful, exploration companies is a dramatic difference in the, amount of diamond drilling they do., S. Muessig 1998, , Potential orebodies that are indicated by geological, geochemical and geophysical exploration, methods must be examined by physical exposures., In most cases, this is done by drilling, but trenching (costeaning) through the overburden may, reveal valuable information. Exploration pits,, deep trenches and adits are not regularly made, in this phase of investigations. Similar to the, preceding reconnaissance exploration work, the, objective of detailed follow-up exploration is, identification of the most promising prospects,
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KN GN, 10, , 150, , 250 mm, , 2, , GR, , NN, , 30 50 70, , GG.D, , WU, 1.5 2.0 2.5 3.0 3.5 4.0 4.5, , 3, , 10 Ohm.m, , 90 110 API 2.4 2.2 2.0 1.8 1.6 1.4 g/cm3, , 433, , Depth (m), , CAL, , Lithology, , GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , M,Li, 40, S, M, 50, M,c, M,s, M,c, , 60, , Li, S,m, , M,Li, , 70, , 80, , Figure 5.8 Geophysical characteristics of lignite and host rocks in a borehole log. With kind permission by, Geophysical Services GGD Leipzig. CAL (caliper) records the borehole diameter; GN (natural gamma) displays, g-radiation (usually of 40K in clay; API units); KN and GN are resistivity logs (Ohm) with electrode distances at 0.5 and, 1.0 m, respectively; NN (neutron/neutron) provides porosity and water content (in water units, WU), GG.D (gamma/, gamma) reports density. Clearly, sand (S), silt (M, Ms sandy, Mc clayey, MLi lignitous) and lignite (Li) can easily be, distinguished., , whereas locations with insufficient potential are, discarded. For prospects that display essential, characteristics, such as resources (tonnage) and, ore grade which resemble profitable working, mines, the detailed follow-up phase ends with the, preparation of a prefeasibility study., Planning for the trenching and drilling programme is based on the presumed geological, , model. The programme includes maps and, sections showing the required drillholes and, exposures, and their description. Technical details, and the sequence of execution are proposed. Intermediate targets (milestones) that can be assessed, are described. Fund-controlling recipients of the, programme proposal often stipulate a comparison, of costs and potential rewards of the planned work.
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434, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , This is founded in the principle that for a company, drilling is an investment that must be, justified., Pits, trenches and shallow drillholes, Pits, trenches and shallow drillholes are made in, soil and soft rock in order to expose near-surface, ore, alteration zones and host rocks for detailed, geological mapping and sampling. Trenches, tens, to hundreds of metres long, may be excavated by, manual labour, trench excavators and bulldozers., Pits and shallow shafts provide large samples (e.g., of gold or diamond ore). Shallow drillholes can be, sunk with various tools. In unconsolidated loose, rocks, such as tin placers, scoops with a valve,, light-cable and tool-boring rigs, or augering equipment may be used. Hole depths of 60 m are, possible. Experiments with radiotracer gold particles showed that in non-cohesive material, dry, augering is the best method to recover representative gold contents (Clarkson 1998)., Rotary percusson air blast drill rigs, Rotary percussion air blast drill rigs (RAB), also, called top, or down-the-hole hammers (DTH), are, commonly used in quarries and open cut mines for, drilling blast holes at diameters from 25–400 mm, to depths of 100–200 m. In exploration, low costs, may be an argument for using this method, aiming, at quick data acquisition. Compressed air is used, in order to lift rock cuttings and dust from the bit, to the surface, between the drill pipe and the wall, of the hole. The hole is typically open and casing, (lining), which keeps the wall rocks from falling, in, is not installed. Rock flour and cuttings from, DTH-holes are useful samples, but higher accuracy is obtained with reverse circulation (RC), hammers (Figure 5.9). The drill-string of RC-hammers consists of two pipes: The compressed air, flows down between the inner and the outer pipe,, whereas the rock chips are lifted in the inner pipe., This avoids erosion of wall rocks, which may mix, with the cuttings, resulting in diluted samples., The large sample size and the high speed of penetration are important advantages of DTH compared to diamond drilling. RC-drilling has, , Figure 5.9 Reverse circulation hammer bit. Photograph, by Leon Bird, copyright Sandvik 2010. Note the wide, openings, which guide air flow and cuttings into the, inner tube., , become a standard where high accuracy is essential and coincides with closely spaced (and therefore expensive) drilling, as in gold exploration and, mining. Most RC drill rigs are constructed for, holes to 500 m depth, but the largest machines, are capable to drill to 1500 m., , Sonic drilling, Sonic drilling is a new technology that produces, excellent cores from mixed hard and soft rocks, (e.g. nickel laterite, alluvial overburden, mineral, sands, coal spoils) and friable ore such as the, manganese oxide ore at Moanda, Gabon, down to, 60 metres depth. It is based on using high frequency vibration and some rotation., , Diamond core drilling, Diamond core drilling is the most common, method employed in exploration for mineral, deposits in hard rocks. This technology is two to, three times more expensive than RC, but has, several advantages compared with hammer, drilling, not least the smaller disturbance of the, environment. Small diamond drills can be carried
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Figure 5.10 Sandvik DE 130 portable exploration, diamond drill rig in Turkey. Photograph by Muzaffer, Bayazitoglu, copyright Sandvik 2008., , on foot by geologists (Figure 5.10) and averagesized rigs can be pulled by common four-wheel, drive vehicles. RC rigs, in contrast, are much, heavier. Some machines are produced that can, alternatively be equipped for diamond or RC operation. Diamond coring is relatively easy in solid, rock, but asks for considerable skill in the more, common cases of rapidly changing hard material, (quartz), solid rock and soft material (clay, fault, gauge, etc.). Ideally, the core taken from a hole is, a complete and coherent sample of the ground., Cores provide a wealth of information, such as, lithology, rock boundaries and structural data., Core samples are used to determine (assay) ore, grade as well as geochemical, mineralogical and, petrophysical parameters. The essential determinant of the significance of core-derived data is core, recovery (Annels & Dominy 2003), defined as the, ratio of the length of recovered core to the length, , 435, , drilled. Low core recovery impedes quantitative, interpretation of important properties, such as ore, grade and ore boundaries. Therefore, core recovery, of more than 90% is stipulated with drilling contractors. Other strict rules must be agreed, such as, careful extraction of the core and its packing in, properly labelled core boxes, and marking individual core runs. Drilling should be supervised on site, by experienced geologists. Proper storage of core is, needed for the duration of the project (if the prospect is rejected) and for the whole life of the, resulting mine if the deposit is feasible. Note that, although onerous, storage is much cheaper than, repeat drilling., Exploration drillholes for ore and minerals are, rarely sunk to depths greater than a few hundred, metres. In cases of well-known geology and, a reasonable expectation of high rewards (e.g. Witwatersrand, Sudbury), diamond drillholes are, driven to more than 5000 m depth. Not only for, deep holes, directional core drilling and borehole, deviation surveying are standard procedures., Inclined and deep drillholes (deeper than 200 m), must be surveyed for deviation, which is too, frequent to be ignored. Reliable estimation of resources and reserves, and precise mine planning is, impossible if the location of orebodies, faults and, risky zones, such as water-rich or weak rocks, is not, accurately known. Normally, low-cost electronic, multishot (EMS) downhole instruments based on, magnetometers are used, but in steel-cased holes,, or in the presence of magnetite and pyrrhotite, nonmagnetic survey instruments (e.g. gyroscopebased) must employed. The first are prone to considerable errors, which may cause wrong resource, estimates. Nordin (2009) advises to control all, EMS data by occasional gyroscope runs., Neglect of deviation surveying can have drastic consequences: In 1989, resources of the newly discovered, base metal deposit Louvicourt, Quebec, Canada were, estimated, based on diamond drilling to 830 m depth,, to >30 Mt of ore at 3.1% Cu and 1.34% Zn. After, underground development and additional drilling of, 80,000 metres (similar to Figure 1.80), the reserves, were recalculated in 1994 to 15.7 Mt. The company, had to admit that the grave error was due to drillhole, deviation.
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436, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Geological logging, Considering the high costs of drilling, a maximum, of information must be extracted. Geological logging of core and drill cuttings is common practice., State-of-the-art standard procedures are best learnt, from large companies and reputed geological, state surveys. Digital processing is the norm, but, intricate geological features may be most faithfully recorded by hand-drawings at scales larger, than 1:100 (1 cm ¼ 1 m). Whenever samples are, removed from the core box, markers with a sample, number must be put in the empty places. Sample, numbers are composed of the drillhole name and, an individual number that must be singular in, the company’s database. Colour photographs of, the core complement other documented data, but, cannot replace the above-mentioned sketches., The differentiation of macroscopically similar, minerals, such as carbonates, may be facilitated, by colouring techniques (Hitzman 1999). Semiautomated methods of mineralogical analysis, are increasingly employed in field geology, core, logging and metallurgy. Host rock and hydrothermal alteration minerals of core and drilling chips, can be determined by portable infrared field, spectrometers, such as TerraSpec. Automated, instruments (HyLogger) developed by the Australian Commonwealth Scientific and Industrial, Research Organization (CSIRO) measure simultaneously 190 bands in the 400–2500 nm (visible, light to infrared) or 7000–14,000 nm (thermal, infrared) range of the electromagnetic spectrum,, which allow a very precise assessment of minerals, their crystallinity and chemistry. Portable, XRF (X-ray fluorescence) analysers can be employed to provide an initial overview of metal, contents of the core., Geophysical borehole logs are ideally complementing the geological description. They are, indispensable for percussion drillholes and for, cases of high core loss (Ellis & Singer 2007). The, choice of methods depends primarily on physical, properties of ore and host rock, with other modifying factors., A basic rule in planning drillholes is that whenever possible, orebodies should be penetrated at, right-angles. Accordingly, steeply dipping orebo-, , dies are drilledby nearly horizontalholes, andthose, with low-angle dips by vertical or inclined holes., Local constraints, such as steep terrain that limits, the choice of collar sites, may enforce disregard of, this rule. Often, drillholes are arranged in fans (cf., Figure 1.80) parallel to gridlines, preferably along, profiles that were established for geochemical and, geophysical surveys. The distance between drillholes is essentially a function of the assumed continuity and variability of the orebody. As soon as, sufficientdataare available, geostatistical methods, help to adjust distances (cf. “Ore Reserve Estimation and Determination of Grade”) but geological, experience remains indispensable. The first drillholes are aimed at the centre of the assumed orebody, in order to confirm the presence of significant, mineralization. Large pipe-like deposits (kimberlites, copper porphyries) are explored and sampled, by vertical holes in quadratic or rectangular grids., However, such unidirectional drilling may induce, serious errors, if oredistribution is non-isotropic. In, the Cu-Au porphyry deposit Bougainville (Papua, New Guinea), ore grades calculated from vertical, drilling were later found to be lower than excavated, ore. Vertical pipes of rich ore had not been sufficiently sampled. Note that this was a lucky case,, with more metal present than originally estimated., Too often, the reverse ruined a mine., The decision to abandon a prospect, or to continue with detailed exploration in spite of a series, of negative results, may be very difficult. A wrong, decision may cause financial loss, either because, of costs for an unjustified prolongation of investigations, or because the income from a good, orebody is not realized. Of course, every drillhole, will be judged on meeting expected targets. Yet,, negative results should not lead to precipitous, abandonment of a prospect. One example of the, rewards of persistence is Olympic Dam, where, only the tenth drillhole hit exploitable copper ore., Naturally, every new drillhole is carefully evaluated in a drilling campaign and results suggesting, variations of the programme are incorporated., Hydrogeological and geotechnical data, In mine-development projects, groundwater and, surface water investigations are hitherto carried
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , out very late, i.e. usually during feasibility studies. In many cases, this is a costly mistake. It is, much better to routinely collect water data, already during detailed follow-up exploration, drilling. Key data needed by the hydrogeologist, and the geotechnical engineer can easily be, assembled by the team carrying out the drilling., The geologist should always record observations, concerning the overburden (lithology) and the, depth of weathering. Mines have been lost by, mud or water inrushes caused by insufficient, information on the thickness and nature of the, crown pillar. For the fresh rock, minimum data, include core recovery, rock quality designation, (RQD: Deere 1964) and basic observations on, fractures and joint systems. Drillers should, record data on penetration rates, rest water, levels, water inflow and fluid loss in the drillhole. This will form the basis of a conceptual, groundwater model and allows preparation of, hydrogeological contour maps with little additional costs, compared with the need to drill new, holes for the same purpose later. At the same, time, risks concerning water supply for the mine,, its management and security will be exposed, as, well as hydrogeological and geotechnical hazards, affecting the project., With completion of the detailed follow-up, drilling and trenching phase, and availability of, laboratory reports, a formal report must be, prepared that comprises: 1) a concise technical, description of work done; 2) a detailed presentation of geological, geophysical and geochemical, results; and 3) three-dimensional graphic presentations of all essential parameters of the supposed, deposit. On this base, 4) a first valuation of the, mineralization is possible, mainly by comparing, the deposit to key data of currently working, mines. The whole report (1–4) is called a prefeasibility study. A positive outcome of this study, may lead to the decision to start a full feasibility, study, with the aim to establish sufficient reserves, and all information needed to develop the new, mining operation. The feasibility study will not, be complete without an environmental impact, assessment (EIA), the consideration of native, title and cultural heritage sites, and an extensive, consultation of all stake holders., , 437, , Never forget that every drillhole must be safely, sealed (Fuenkajorn & Daemen 1996) when it is to, be abandoned. If several aquifers are present at, depth, they must be separated by barriers (seals)., Holes that are to be accessed again at a later time, must be plugged in such a way as to preclude, hazards for humans and animals., 5.3 DEVELOPMENT AND VALUATION, , OF, , MINERAL DEPOSITS, , Getting the geology right is fundamental, Roy Woodall 2007 (Director of exploration,, Western Mining Corporation WMC), , The science of economic geology is preoccupied, with rather theoretical problems, foremost the, details of ore forming processes. The practice of, economic geology is dedicated to make mining, operations economically profitable. In open-cast, or underground mines (Figure 5.11), this includes, many diverse obligations (Marjoribanks 1997,, Berkman 2001). Some more important examples, of geological tasks in mines are:, . planning, management and interpretation of, drilling programmes, of geochemical and geophysical surveys, and of exploratory drifts, in order to, locate new resources and to outline reserves;, . geological mapping of all rock exposures, underground and in the open pit, at a detailed scale, (commonly used are 1:100–1:500); establishing, a geological surface map of the mining area, (1:1000–1:10,000); logging all drill core from the, mine and its surroundings at a suitable scale (e.g., 1:100), with due consideration for rock mechanics, and hydrogeology (mining), grade (economics) and, ore mineralogy (processing); drafting geological, serial sections;, . sampling of exploration drill core, adits and, trenches, and of day-to-day ore extraction sites, (grade control);, . data management of returns from the laboratory,, including data validation;, . geological and mineralogical investigations of, ore, in order to improve processing and maximize, recovery (“geometallurgy”; Petruk 2000);, . calculation, valuation and reconciliation of, reserves;
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438, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Overburden, , Ventilation, shaft, , Main shaft, , Working level I, Stope, , Under development, for production, Underground production, , le, hic, ve ss, r, o, e f acce, clin, De, , Crown pillar (horizontal), , Ramp, , Sublevel, , Level II, Waste, pass, , Orebody, , Hanging, wall, , Footwall, , Ore pass, ass, , Underground, crusher, , Dip, Winze, , Exploration, drift, dri, ft, , Skip, , Ore bin, in, , Skip filling station, tation, , Core, drilling, , . design and continuous improvement of the, genetic model of the deposit;, . engineering geological examinations of soft and, hard rocks, ore, waste rock dips, tailings dams,, open cut slopes, etc. (cf. Jaeger et al. 2007, Hoek &, Bray 1981 for opencut work; Hoek & Brown 2003, and Brady & Brown 2004 for underground mining);, , Sump, , Exploration and reserve definition, , Open pit, , Figure 5.11 Simplified section, of a mine that first exploited a, steeply dipping planar orebody, in a pit (open cut) and is now, extracting ore underground., Note the vertical segmentation, between (1) ongoing, production, (2) development, for future extraction, and (3), exploration aiming at the, definition of new mining, reserves as a replacement for, extracted ore., , geological input into subsidence damage cases;, resolution of geological water problems, either, concerning the draw-down cone caused by mining, or unexpected hazardous inflow;, . acquisition of environmental data, including, appraisal, based on the environmental impact, statement (EIS) that had to be submitted with the, ., .
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , application for the mining licence; day-to-day, tasks in mining operations include environmental, monitoring of the mine and its surroundings., Of course, a number of these tasks are executed, in teamwork with other company professionals, and with external consultants. In this chapter,, only those duties are briefly sketched that relate, to economic geology., 5.3.1 Geological mapping and sampling, Geological mapping of natural outcrops and of, man-made exposures is the most important task, of the mine geologist. A complete and up-to-date, documentation of all geological data is the precondition for both day-to-day management and for, serious decisions concerning, for example, a large, drilling programme. Field mapping should be carried out at suitable detail (e.g. at 1:100), whereas, display scales of geological mine maps and sections should be equal to the geodetic mine plans., Never neglect preparing geological overview sections through the mine. One mine I knew suffered, a catastrophic mud inrush because a stope was, driven upwards into water-logged hanging wall, sediments. Nobody had ever asked where exactly, the hard rock/overburden boundary was. This, mine had to be abandoned. Modern mine management software includes components for geological, work. Integrated digital processing of geological, data and of mine planning is standard in most, mines. The practical arts of geological mine mapping have been admirably described by McKinstry, (1948), but consult also Marjoribanks (1997)., , Sampling, The mining industry devotes considerable efforts, to acquire accurate analytical data for resource, estimation, grade control, metallurgical accounting and commercial transactions. However, sampling and sample preparation are not always given, the attention they deserve. If the samples submitted to the laboratory are not representative, there, is little point in using expensive analytical equipment. The most important rule for correct sampling is that all parts of the ore, concentrate, slurry, , 439, , or other material being sampled must have an, equal probability of being collected and becoming, part of the final sample for analysis. The logic of, sampling is to collect a minimal mass (of grams,, kilograms, tonnes) that equals a certain parameter, (e.g. gold content) of a much larger mass (hundreds, or thousands of tonnes). In the laboratory, sample, preparation for analysis again includes carefully, controlled diminution of the mass (subsampling),, until fractions of a gram will be used for actual, analysis. This final aliquot must still replicate, targeted properties of the original large mass., Note that the term sampling may include specialized measurements with tools appropriate to the, minerals under investigation, such as downhole, gamma probes and prompt fission neutron borehole probes for in- situ determination of uranium, in ore., Most of the sampling problems occur in the, field, where conditions cannot be as strictly controlled as in the laboratory. The problems are, often of a very practical type, such as cutting, a slit of a given geometry through a succession of, hard quartz veinlets alternating with soft mineralized host rock. Random chip samples may favour, soft over tough rock. Of fundamental interest is, the need to estimate the smallest sample mass, that will guarantee that the sample is representative of the whole. One controlling variable that has, been recognized very early is the grain size. When, comparing a very coarse-grained granite (e.g. displaying feldspar laths 10 cm long) with aplite from, the same melt, but consisting of small equigranular grains of less than 1 mm diameter, it is obvious, that the aplite can be represented by a much, smaller sample compared to the granite. Another, variable is the frequently irregular distribution, of the ore mineral in the ore rock. In many gold, orebodies, most of the gold resides in rich pockets, whereas the overwhelming mass of the ore, rock is of low grade. This type of occurrence is, commonly compared to gold nuggets in alluvial, mining. When statistical processing of sample, data indicates such an unpredictable behaviour,, a nugget-like distribution is implied. There are, many theoretical papers on how to find solutions to both grain size and nugget problems. In
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440, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , mining practice, the use of nomograms or specific software is common (Gy 1992, Pitard 1993,, Berkman 2001)., , mitting well-chosen and unrecognizable doubles, of samples to the laboratory., Geometallurgy, , Grade control, The practical execution of sampling depends on, the conditions at the site and the precision to be, attained (Gilfillan & Levy 2001). The lowest quality have samples that are picked at random from an, outcrop or from an ore heap. Slits of a defined, geometry (e.g. 10 cm square) are recommended if, they can be cut to plan. Drillhole or reverse circulation blast hole dust and cuttings may suffer from, differential settling of the (usually heavy) ore, minerals and the (light) gangue minerals, or of, problems with moisture. Diamond drillhole cores, may have a diameter too small for representativity, core-splitting may introduce serious errors, (Brooks 2008) and core loss is an ever-present risk., In spite of this, continuous sampling methods are, generally preferable to discontinuous procedures., But no method is without problems and this has to, be considered before starting a sampling campaign. If possible, different sampling techniques, should be tested and compared before settling on a, specific method. And a final advice – never underestimate the importance of accurately measuring, the density of samples (Lipton 2001)., Subsampling, Subsampling designates procedures that reduce, the total mass sampled (e.g. from a slit in an orebody exposure) to the few grams of powder in, a small bottle that is all a modern laboratory, requires for analysis. The key are stepped cycles, of grain size diminution (by crushing and pulverizing), splitting (halving or quartering) and rejection of one half or three quarters of the sample. The, choice of the correct splitting device is crucial,, especially in cases where the ore is much heavier, than the gangue (gold, uranium, etc.). Traditional, riffle splitters work badly in these cases, because, they are prone to cause gravity separation. Rotary, splitters avoid this trap. Whatever the technique,, error control is the central task in a sampling, campaign. Always try to check on errors by sub-, , Geometallurgy describes procedures to model the, distribution of different ore types in a mine (Petruk, 2000). We have seen earlier, that mine-site, processing of run-of-mine ore is an important, determinant of economic success or failure of an, operation. During a feasibility study, not only the, variability of metal contents in ore, but also of, metallurgical recovery over the life-time of the, future mine, must be studied and modelled., Automated methods of mineralogical analysis,, such as QEMSCAN (an electron beam technique, that combines a scanning electron microscope,, four X-ray detectors and a software package developed by CSIRO, Australia), provide the required, data. Geometallurgical (ore) domains comprise, parts of the orebody with similar geochemical,, mineralogical, textural and processing properties., Predicting grindability and metallurgical performance is critical. The development of small-scale, tests based on diamond drill cores allows multiple, sampling of an orebody opposed to the traditional, pilot-plant scale sample of several 100 tonnes, taken from one accessible location of yet undeveloped reserves., 5.3.2 Ore reserve estimation and, determination of grade, From the scale of a single mine or a mining, company to national and supranational stock, markets and resource planning, knowledge about, the physical and economic availability of raw, materials is needed for rational decisions. Therefore, international efforts to standardize measuring and reporting of mineral reserves and resources, have reached a wide acceptance., The total geological quantity of a specific raw, material is termed the “mineral resource base”., This includes both known and unknown quantities. However, only ore that has reasonable, prospects for eventual economic extraction may, be included in any estimate. Based on increasingly, reliable data on quantity, quality and
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , exploitability of minerals, McKelvey (1973) proposed a matrix (“McKelvey Box”), which differentiates between prognostic (undiscovered), resources, identified resources and reserves. Reserves are only that part of resources that is thoroughly investigated and proved to be exploitable, with a high degree of confidence., , Based on this correlation, undiscovered resources can be estimated. A different mathematical approach (density functions) was applied by, Gerst (2008), aiming at an estimate of cumulative, grade-tonnage curves for continental copper, resources., Discovered mineral resources, , Prognostic resources, Strategic planning for the future availability of, metals and minerals relies on estimates of prognostic resources. Often, dire predictions are made, about the world running out of specific metals, (Club of Rome: Meadows et al. 1974), coal or crude, oil. In 1974, the reason for the prediction of depletion in the 1990s was the erroneous use of reserve, instead of prognostic resource figures. Even today,, this mistake is commonplace. Because of obvious, reasons, quantification of undiscovered resources, is notoriously difficult. Most methods proposed, are based on extrapolation of identified resources, (including reserves and past production) in well, investigated tracts to geologically similar, less, well-known parts of the globe (Singer & Menzie, 2010, Gautier et al. 2009, Singer et al. 2005) or to, greater depth beneath the surface (Kesler & Wilkinson 2008). Such exercises demand geological and, metallogenetic maps, and mining databases., In estimating undiscovered resources, statistics, have an elementary role. Studying the negative, correlation between tonnage and grade of metal, ores, Lasky (1950) proposed a log-normal distribution (Lasky’s Law). In a log-log diagram, the same, data pairs form a line with a slope determined by, an exponent D between 0 and 3 reflecting a fractal, nature (equation 5.1)., Fractal distribution of ore tonnages and related, grades (Turcotte 1997):, Core =Cmin ¼ ðMmin =More ÞD=3, , 441, , ð5:1Þ, , Core is the average grade of the tonnage More, Cmin the, minimal grade included of the mass Mmin, D is the fractal, dimension. Mmin may be the mass of ore exploited at the, lowest-grade mine, or even source rock from which ore in a, district is thought to be derived (consider the metamorphogenic model of gold deposits leached from crust with trace, contents)., , Discovered mineral resources result from exploration and detailed follow-up work. According to, increasing geological certainty, they are subdivided into inferred, indicated and measured, resources (Figure 5.12). The last category infers, that mass and grade are known with a high level of, confidence. Methods for measuring confidence are, typically based on drillhole or sample spacing and, geostatistical criteria (Abzalov & Bower 2009)., Resources are normally not acceptable as a base, for commercial mining. There are, however, some, projects, such as in-situ leaching (ISL) of uranium,, which must rely on drilling only, so that mere, resources and no reserves at all can be estimated, until production starts., Mineral, or ore reserves, Mineral, or ore reserves are only that part of an, indicated or measured resource that can be, economically mined. Only reserves justify commercial mining. Investigations supporting this, attribution must include mining, metallurgical,, economic, marketing, legal, environmental,, social and political factors (the “modifying, factors”). The modifying factors are time-bound, variables. Mineral reserves are subdivided into, probable and proved, the first with a lower degree, of confidence., Statistical and geostatistical methods are indispensable in the determination of different levels of, confidence. The principle can be illustrated by, varying density of geological observations, (Figure 5.13). In a mine exploiting a simple planar, orebody (e.g. a fluorspar vein, or a steeply dipping, sedex ore deposit), proved reserves must be physically outlined on three or four sides. Probable, reserves are those parts that are only exposed along, two mine openings (or closely spaced drillholes).
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442, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Exploration Results, Mineral Resources, Inferred, Increasing level, of geological, knowledge and, confidence, , Ore Reserves, , Indicated, , Probable, , Measured, , Proved, , Consideration of mining, metallurgical, economic, marketing,, legal, environmental, social and governmental criteria, (modifying factors), Figure 5.12 General relations between exploration results, mineral resources and mineral reserves as defined in the, JORC Code (AusIMM 2004). It is noteworthy that proved reserves can fall back to the measured resources category., , Measured resources are usually the result of, detailed drilling but of insufficient quality or, incomplete data for classification as reserves., Indicated mineral resources are derived from the, geological orebody model and supporting physical, observations., Investigations for resource estimation include, work in six stages:, 1 provide a sufficient quantity of data of appropriate quality (precise and accurate);, 2 develop a well-founded geological deposit, model (including data that link mineralogy and, metal recovery);, , 3 use statistical methods in order to understand, the distribution of analytical data in the deposit;, 4 choose a suitable interpolation model for grades,, considering both the geological model and the, statistics;, 5 calculate tonnage and grade, either globally or, for parts of the deposit;, 6 prepare the report, clearly outlining resource, categories and respective confidence of figures, presented., Each of these points represents a complex system of scientific and technical approaches, which, influence the results. Not all uncertainties can, , Old diggings, , Probable, reserves, Proved, , Proved, , Fault, , Barre, , n are, , a, , Proved, , Indicated resources, , Deepest ore in nearby vein, 100 m, , Figure 5.13 Proved and, probable reserves of a vein, deposit in relation to geometry, of actual underground, exposures, and indicated, resources supported by the, geological model and some drill, hole intersections (not shown)., Shaded areas are extracted parts, of the orebody.
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , be resolved and some assumptions must be made., This is why the product is termed a “resource, estimate” (not a calculation), even if advanced, mathematical methods are used. It must, always be remembered that users (e.g. banks) of, the resulting figures will take them at face value, and derive potentially serious financial and, organizational decisions. Note also, that resource, estimates are essentially numerical models of orebodies. Similar to other such models in the earth, sciences, they can hardly ever be true replicas of, nature (Oreskes et al. 1994)., Because of the economic impact, for example on, financial markets and industry, all major industrial countries developed norms for estimating and, classifying reserves. In Europe, a common standard is prepared by the Pan-European Reserves and, Resources Committee (PERC 2008). A widely, lauded and internationally adopted example is the, Australian system (Joint Ore Reserves Committee, or JORC Code: Edwards 2001, AusIMM 2004). The, United Nations Economic and Social Council, (1997) presented a proposal that was hoped to meet, the requirements of private and state-controlled, , mining industries, as well as governmental needs, for mineral inventory classification. In this system (UN International Framework, Figure 5.14),, three axes are presented, including: i) the degree of, geological assessment (geological axis); ii) the, degree of economic viability (economic axis); and, iii) the stage of feasibility assessment (feasibility, axis). The latter is a new aspect specifically aimed, at potential investors. Another proposal for a third, axis was put forward by BGS (British Geological, Survey), concerning information on the accessibility of deposits considering environmental, legal,, social and political factors (Cook & Harris 1998)., The intention is to visualize which part of reserves, is really available for mining, in contrast to those, that are blocked by other claims. In the future,, mining will increasingly depend on modifying, factors. Purely geological reserves unavailable for, exploitation are meaningless (Weatherstone, 2005)., Practical procedures of calculating quantity and, grade of a mineral deposit are modified according, to type, form, raw material contained and mode of, data collection (Annels 1991). In simple cases,, , Fe, an asib, d M ility, Pr, efe, ini Stu, as, ng, Ge, ibi, Re dy, olo, lity, po, gic, S, rt, tud, al, Stu, y, dy, 2, , 3, , lity, , Figure 5.14 The tri-axial, UN Framework, Classification of mineral, reserves and resources., Courtesy UN Economic, and Social Council (1997)., Numbered codes apply to, each block. Block 111, represents proved reserves,, 121 and 122 probable, reserves (coordinates E-FG). All other blocks are, various resource classes., , ibi, as, e, F is, ax, , c, mi, no, o, Ec, lly, tia, ten omic, o, P on, ec, lly, ica, ins mic, r, t, In ono, ec, , F, 1, , 443, , 1, , 111, , ion, rat, o, l, on, xp, ati, lor, de, p, e, l, x, tai, le, De, era, ng, n, e, cti, ce, 1, G, pe, an, s, o, iss, 2, a, Pr, nn, 3, co, Re, , 4, , G, , 2, 3, , E (Economic axis), , Ge, olo, gic, ax al, is
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444, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , such as homogeneous planar bodies (e.g. a seam of, chromite or of coal), a simple volume-by-density, formula can be applied (eq. 5.2)., , those shown in Figure 5.11. In such cases, exploitable ore surface in cross-sections and distance, between sections are measured (Figure 5.15). The, ore volume can be estimated by applying eq. 5.4., , Simple calculation of in situ tonnage:, Q ¼ FDM, , ð5:2Þ, , If the recoverable metal mass in a specified, volume of ore is required, the formula is enlarged, by the introduction of factors that take account of, ore left in place (mining loss, for example in safety, pillars) and the loss during metallurgical processing (eq. 5.3)., Simple calculation of recoverable metal content:, P ¼ QGAV, , ð5:3Þ, , Q ¼ ore in situ (t), F ¼ surface to be mined (m2), M ¼, average thickness of ore (m), D ¼ specific weight or bulk, density of ore (t/m3), P ¼ recoverable metal content (t),, G ¼ average metal content in ore (% or kg/t), A ¼ correction factor for mining loss, V ¼ correction factor for, processing loss., , Note that the specific weight of ore (D) must be, determined using large samples, because in this, case the aim is to quantify a physical property, of the rock mass including joints and fissures, (Bieniawski 1989, Lipton 2001), in contrast to ore, rock (a specimen, for example). If appropriate,, dilution of the ore by unavoidable extraction of, host rock must be included. Dilution is, of course,, a factor that increases costs. Polymetallic ore, grade is often reported in terms of a single equivalent grade of one major metal such as gold or, copper. It is usually obtained by taking the in-situ, value (grade multiplied by price) of each of the, individual metals, adding these values and calculating the grade of the same value of the primary, reported metal. The result can be very misleading, if the recovery of individual metals is not considered. Therefore, the preferred measure of equivalent grade is the net smelter return (NSR)., Usually due to practical considerations, parts of a, deposit (single orebodies or blocks of ore) are separately submitted to ore reserve estimation. Some, orebodies with a longitudinal continuity but varying contours are depicted by serial sections (wire, frames) that may be based on drilling fans, such as, , Simple estimation of ore volume from serial, sections:, Volume ðm3 Þ ¼ 0:5 ðF1 þ F2 Þ b12, þ 0:5 ðF2 þ F3 Þ b23 þ . . . etc:, , ð5:4Þ, , The role of weighting, Variability of some factors of special importance,, such as varying thickness of a gold vein, sample, length along core, or density of ore impose the need, for weighting (Wellmer et al. 2007). Average grade, of an ore vein with varying contents of lead and,, therefore, variable density can only be correctly, calculated by weighting (eq. 5.5)., Weighted average content (Gw) as a function of, thickness (M) and density (D):, Gw ¼, , M1 D1 G1 þ M2 D2 G2 þ . . . etc:, M1 D1 þ M2 D2 þ . . . etc:, , ð5:5Þ, , The determination of the lowest grade in an, orebody that can be economically mined (the, “cut-off grade”) is of paramount importance, (Rendu 2008, Lane 1997). By definition, this is the, grade where mining and processing costs are equal, to proceeds from the sale of the product. The cutoff grade is the limit between ore and waste rock., Its determination is not simple, as the parameters, that determine the optimal cut-off grade are not, only the given geological properties of the deposit, but include time-dependent factors such as varying metal prices, shallow or deep location of stopes, and the cash-flow strategy of the operation. Therefore, different cut-off grades will apply during the, life-cycle of a mine and at one point of time, in, different parts of the mine. When setting cut-off, grades, the aim will always be to maximize the, profit of a mine. In recent years, maximization of, the net present value (NPV) of the mining operation is the main measure of optimizing the cut-off, grade (Nieto & Bascetin 2006).
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , 445, , b, b, b, b, , F5, F4, F3, F2, , F1, , Figure 5.15 Serial profiles of an orebody (wire frames) are one of several methods to estimate the volume of ore. This, drawing is inspired by the Deilmann orebody at Key Lake uranium mine, Canada., , Geostatistic modelling, Geostatistical modelling is an important part of, ore reserve estimation. Geostatistics were first, developed by Matheron (1971) and Krige (1981),, originally aiming at a higher accuracy of gold, reserve estimates. The difference between geostatistics and ordinary statistics is that geological, parameters depend on the site of a measurement, (e.g. the laterally changing thickness of a coal, seam), whereas each throw of a dice is independent, from the previous one. Commonly in geology,, smaller distances between sample locations cause, a higher correlation of measured values. This can, be used for various predictive purposes, especially, in reserve estimation. The most important step in, geostatistical modelling is therefore the quantification of the spatial correlation of sample properties (“regionalized variables”):, Semivariograms are computed in order to quantify, the spatial correlation and directional properties of, , various parameters, such as ore grade, mineral, paragenesis, thickness, etc. (Figure 5.16). Variography (or “structural modelling”) is an important, tool that reinforces geological understanding of a, deposit (Guibal 2001). Typical uses of variograms, are: i) optimizing the sampling density (e.g. the, drilling grid distances); and ii) helping to define the, geological model (“domaining”) for resource evaluation. Domains are then subdivided into blocks, for calculation of tonnage and grade of ore contained. Common computational methods employed include kriging (i.e. minimizing the error, of estimation; Matheron 1971, Isaaks & Srivastava, 1990) and conditional simulation, allowing, extremely complex models and providing a measure of precision and probability (Abzalov & Bower, 2009, Khosrowshahi & Shaw 2001)., , It is very important to remember that geostatistical methods cannot replace meticulous geological data acquisition and interpretation. They are, computational tools that rely on good geology and, extend its reach. Erroneous applications include, calculating a variogram with data that comprise
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446, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , γ (h), , Sill value (C), •, , 9, 8, , •, , •, , •, , •, , •, , 7, , •, , 6, 5, 4, 3, , •, Range (a), , 2, , Erratic,, no continuity, , "Continuity”, , 1, 10, , 50, , 80 m(h), , distinct domains. Practical experience with geostatistical resource evaluation has shown that, results (e.g. of kriging) should always be checked, with different methods, including conventional, “manual” ones (Sinclair & Blackwell 2002). In, Australia, comparative runs with commercial, software resulted in severe differences, provoking, the demand that all resource announcements, should be accompanied by a manual calculation, (Swain 1997). Much can be learned from a comparison of reserve estimates and quantity produced. For the uranium mine Key Lake, (1982–1997, cf Chapter 2.5 “Uranium”), the original reserve estimate systematically deviated from, later production figures. The estimated tonnage, had been too low and the predicted grades were, hardly ever reached. The explanation for this disparity was an unplanned dilution of the ore by, 25% of host rock (Mistry et al. 1999)., Reserve management and reconciliation, The lesson of Key Lake and many similar cases is, that reserve management and reconciliation is an, important task. Predicted grade of reserves, in-situ, grade of ore produced, grade delivered to the processing plant and mass of metal in concentrate, must be carefully supervised (Fouet et al. 2009,, Gilfillan & Levy 2001). The results are a measure, of the overall metal recovery of the operation, but, also of problems at different points in the mining, process. For some time now, reconciliation, results (“factors”) are required information by, , Figure 5.16 Example of a simple experimental, semivariogram with a fitted model (black line), that may, be used to determine the optimal sampling distance in an, orebody. g (h) is the variance of values at different sample, distances (lag-distance [h] here in 10 m steps). When, reaching the sill at range (a), g (h) equals the variance, of the total population (C) and the predictive value of g (h) –, the covariance – is lost. Optimal sample distance, would be a/2., , international reserves and resources reporting, codes such as JORC., Many placer deposits of gold and diamond, but, also some lode gold deposits, show a high nugget, effect. A nugget is a large lump of gold lying about, that will certainly please the finder. Of course,, one lucky find would hardly be a rational cause, for opening a mine. If gold in a mine is said to, have a nugget-like distribution, an important part, of the total content in the ore is present in erratic, pockets of high concentration. This causes severe, problems in ore reserve estimation. In such cases,, channel sampling of the four walls of a pit may, yield four different results, which in turn show, little relation to the bulk sample from the pit., Common geostatistical methods cannot solve, this problem, because in this case the area of, influence of single samples is negligible. At the, Bendigo gold mine (New South Wales, Australia),, most of the gold content in each tonne of ore is, present in just 5–15 very coarse gold particles, (Johansen 2005). Investigations of large bulk samples of placers at Bendigo have shown that the, grain size distribution is relatively stable at different total grades and can be presented in a type, curve (Figure 5.17). Estimation of the grade of any, sample from the mine appears to be possible by, determination of gold in the lower part of the, grain size curve. The method is reported to allow, reliable grade estimates based on small samples, like drill cores., Reserves of a mineral deposit are a first-order, control on economic evaluation, whether
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , 447, , decisions concern the opening of a new mine, the, value of company shares, the sale of a deposit, or, securing a credit. Therefore, the execution of, reserve estimation carries considerable responsibility. This translates into the condition that experts involved must have a demonstrated high, professional standard (“competent person”), independence and ethical integrity. Australian rules, are an excellent example (AusIMM 2004, 2005;, Edwards 2001)., , Investing in a mine is basically providing funds, in the expectation of being repaid in the future and,, of course, with interest. This translates in economic terms to “the present value or worth (P) of, a sum (A) to be received or paid at some future date, is such an amount as will, with compound interest, at a prescribed rate (i), equal the sum to be received, or paid in the future (n years)”. Accordingly, the, present value is less than the future income,, because that must be discounted (eq. 5.6)., , 5.3.3 Valuation of mineral deposits, , Present value of future income calculated by, discounting:, , The term “mineral resource wealth” tempts the, non-professional to consider undiscovered or, undeveloped minerals in the ground as an economic value. This is not strictly correct because, an income from buried minerals can only accrue, when they are extracted. Only mining creates, wealth for investors, miners, contractors and the, whole economic space. It is true, however, that, minerals in the ground are an economic potential, and therefore can be the object of trading and, speculation., , P ¼ A=ð1 þ iÞn, , ð5:6Þ, , In periods of high interest rates, the present, value of future income falls rapidly to near zero., With an interest rate of 15% per year, an income, (A) in 15 years from now has a present value of only, 0.123 A and 0.001 A for 50 years. At common, interest rates of 7.5% and a period of 15 years, the, result is a present value of 0.338 A. This illustrates, why the development of ore reserves (a future, income) beyond a period of 15–20 years is, , 5, 4.5, , Gold particle size (mm), , 4, 3.5, 3, 2.5, , Bendigo, , 2, 1.5, 1, "Typical" gold deposit, , 0.5, 0, 0, , 10, , 20, , 30, , 40, , 50, , 60, , 70, , 80, , 90, , 100, , Cumulative Au metal (%), Figure 5.17 Gold particle size distribution at Bendigo, Australia compared to a “typical” gold deposit (Johansen 2005)., At Bendigo, coarser (and fewer) grains control the total grade. Determinations of the characteristic grain size/metal, content relations in a deposit allow improved grade estimations in spite of high nugget effects.
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448, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , economically wrong, except in special cases (e.g. a, coal mine producing for an attached thermal, power plant with an amortization time of 25–30, years). In the recent past in Australia, exploring, and developing a new mine caused average costs of, 50 million US$. It is obvious from the above given, figures that ore reserves drilling for a time 30–50, years beyond the start of operations of this mine, just adds to the costs, but does not contribute to, earnings. Capital markets would not support such, wilful waste, because investments in exploration, and mining compete with other uses of capital., Of course, future income and interest rates are, uncertain and estimates should be given with error, margins. Often, average figures of the recent past, are used, but cannot be projected with 100% confidence into the future. The volatility of markets, for many raw materials (e.g. tungsten, tin, cobalt,, lead, zinc, gold, petroleum) is proverbial, whereas, industrial minerals have relatively smaller price, fluctuations. The consequence is that the present, value of an ore deposit can be large or small, simply, as a function of the assumed future metal price., Because the time of high prices cannot be predicted, the estimate and an investment related to, the deposit (e.g. buying shares of the mine) are, subject to a risk. Apart from the risk due to the, product market, further risk factors are political,, social and fiscal. All risks must be identified for a, specific project and included in a formal risk analysis. High risks always reduce the present value of, a project (e.g. the share price). In economic calculations, risk can be allowed for by using higher, discounting rates., , Valuation of a mining project with discounted, cash flow analysis:, NPV ¼ ðR0 C0 Þ þ, , R1 C1, R2 C2, Rn Cn, þ ..., þ, 2, ð1 þ iÞ, ð1, þ iÞn, ð1 þ iÞ, ð5:7Þ, , A positive NPV indicates that expected income, is higher than projected expenses and the difference, illuminates the presumed profit which determines, the feasibility of an investment. A negative NPV, indicates a non-profit or loss situation so that the, project should be abandoned, although the option, of higher product prices in the future might be, considered. Apart from NPV, other guide values, include the internal rate of return(IRR) and the, payback period. The IRR is determined by calculating the interest rate (i) for a net present value of, zero. The payback period is the number of years, that is needed to recover the sum initially invested, by net returns. This exposes clearly when the first, true profits may be expected. Typical payback, periods for metal mines are from 3–8 years., Feasibility study, The phase of development and valuation of, a mining project concludes with the compilation, of a feasibility study. This is a full documentation, of geology and reserves, mine planning, processing, methods, infrastructure, work and construction, plans, costs, markets, cash flow analysis, social,, legislative and fiscal frame, financing, environmental management during the extraction period, and closure plans including landscaping, recultivation and renaturalization., , Methods of cost-benefit analysis, Founded on these basic principles, various methods, of cost-benefit analysis were developed that, can be used for the valuation of mining projects, (Rudenno 1999). Taking into account that in the, first years most mines cause high costs, whereas, income accumulates in later years, the practical, evaluation method termed cash flow analysis, contrasts yearly costs (C) with expected returns, (R). Resulting figures are discounted and their, sum represents the net present value(NPV) in, the year zero (eq. 5.7)., , 5.4 MINING AND, , THE ENVIRONMENT, , The mine of the future will be a waste management, project, N. Weatherstone 2005 (Rio Tinto), , Large mining operations (Figure/Plate 5.18), affect surrounding communities, flora and, fauna, land and water, similar to other major, industrial operations. Yet there are differences, that are clear to any casual observer. Typically,
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Figure 5.18 (Plate 5.18) Lignite pit Sch€, oningen in, northern Germany with its captive power station. Coal, extraction takes place at the pit bottom. Overburden and, combustion residues are used to refill nearby exhausted, pits. Flue gas cleaning yields by-product elementary, sulphur. Note white sands on bench to the right. Strata, are limnic and marine due to marine transgression, during the Palaeocene-Eocene thermal maximum., , mines change the landscape profoundly and, they bequeath much visible waste. This is one, of the main reasons for public misgivings about, mining. Concern for future societal acceptance, of mining inspires visions, such as the Zero, Waste Mine (Wolkersdorfer 2009)., Large quantities of waste are a consequence of, most mining and quarrying operations. Although, the major part of this is inert and non-hazardous,, disposal, at least in densely populated areas, is, often a space problem. Possible solutions include, storage of waste in underground mining voids, and converting it into useful products such as, aggregate for the building industry. Today, the, relatively small mass of potentially hazardous, waste produced by mines is properly regulated by, the authorities and managed by the operators. In, the past, this was quite the reverse. The wealth, created by mines was of first importance, whereas, their impact on people and the environment was, hardly noticed., The earliest anthropogenic heavy metal spike, known occurs in sediments of the estuary of Rio, Tinto in southern Spain; it is due to Copper Age, mining (2500 BCE; Leblanc et al. 2000). In our, time, the mining industry invests heavily in procedures that promise sustainability and minimal, , 449, , environmental risk. It is not rare to encounter, post-mining landscapes that are truly improved, compared to their virgin state. Examples are the, refuges for rare species of animals and plants in, re-naturalized quarries, clay pits and pit lakes, in, contrast to surrounding agricultural land. Citizens, and media have hardly taken notice of this new, face of the industry, but are shocked by images of, careless mining that is often a by-product of weak, and corrupt governance. To call for an end of all, mining is hardly helpful. Environmental improvement of working mines and reclamation of abandoned exploitations is only possible if sufficient, funds are available. It makes no difference if private or public funds (Figure/Plate 5.19) are used; in, both cases, quantitative and qualitative growth is, the precondition. Economic growth, however,, cannot take place without mineral raw materials., The rational conclusion is that exploitation of, mineral resources is not the problem, but in its, “green” and modern execution, represents the key, to sustainable development., , Figure 5.19 (Plate 5.19) Recultivation of the lignite pit, Geiseltal in northern Germany during the flooding, operation in 2005. Photograph by Christian Bedeschinski, 2005. LMBV (Lausitzer und Mitteldeutsche BergbauVerwaltungsgesellschaft mbH). The sunny slope on, footwall limestone in the foreground was planted with, grapevines. The Eocene lignite seam attained a thickness, of 100 m in a large salt subrosion depression within, Triassic limestone measuring 5 15 km. Because of the, induced alkalinity, the coal was famous for exceptional, preservation of vertebrate fossils and of chlorophyll in, green leaves. Exploited through nearly 300 years, original, lignite resources were 1600 Mt.
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450, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Environmental impact assessments, Environmental Impact Assessments (EIA) are, complex studies that must be carried out and, approved by the authorities when operations are, licenced. The potential environmental problems, specific to mining, which have to be considered are, listed below. The same list will serve for preparing, a mine-closure project or the reclamation of abandoned mines. Mining may disturb local society,, natural flora and fauna (the biosphere), soil and, regolith (pedosphere), the geological environment, below the regolith (the geosphere) and surface or, groundwater (the hydrosphere). Possible impact, on the spheres must be studied by a multidisciplinary group. Their role includes the responsibility of asserting the precautionary principle (Foster, et al. 2000) by predicting hazard and risk, and by, cooperating with the management to prevent, damaging incidents. Note that in technical context, the term “hazard” designates a damaging, event that threatens people, environment or property with a specific probability (e.g. once in ten, years). “Risk” is the financial or other consequence of the damaging incidence multiplied by, the probability., , 5.4.1 Potential environmental problems related, to mining and mine-site processing plants, Visible alterations of the landscape:, T Temporary. Disturbance of the ecosystem, (e.g. by clearing the site), relocation of water, courses, drainage of wetlands, removing soil,, laying new roads;, T Lasting. “Scars” in the landscape; new water, courses, ponds and lakes; lowering, tilting or, raising the land surface; landfills, waste rock, tips; pit lakes, settling ponds, tailings dams., . Effects concerning the aquatic and/or hydrologic, environment, T Temporary. Because of dewatering, all mines, (except dredging operations) act as a well, (Figure 5.20). The consequence is often a wide, draw-down cone, influencing nearby groundwater, drinking water well-fields, wetlands,, or medicinal and recreational springs. Water, pumped from a mine into rivers and lakes or, ., , issuing from ore processing may be loaded, with dissolved matter, suspended particles, (e.g. iron hydroxide ochre, clay, mica) and, acidity from oxidizing sulphides (AMD or, ARD – acid mine or rock drainage: Verburg, et al. 2009, Younger & Robins 2002, van Geen, et al. 1997; concerning the formation conditions of AMD refer to Chapter 1.2 “Supergene, Enrichment of Pre-Existing Mineralization”)., T Lasting. Partly as above; long-wall underground coal mining induces subsidence and, lowering of the land surface, so that, undrained depressions may fill with water;, a concurrent reversal of the hydraulic gradient may enforce expensive surface and, groundwater management. Lakes filling former open pits can have problems with acid, rock drainage from the pit walls or from waste, rock and with unstable underwater slopes., Even pit lakes in rocks that contain no toxic, or acid-producing compounds, such as sand, pits in river valleys, cause chemical changes, in passing groundwater (e.g. loss of CO2, oxidation); the quality of drinking water pumped, from wells downstream of the pit can be, affected., Note that “empirical studies of mineral, deposits in the context of their surrounding, watersheds are thus an important and, needed next step in the development of, improved predictive methods to help anticipate, mitigate, and remediate the potential, environmental effects of mineral-resource, development” (Plumlee et al. 1999a)., . Surface damages affecting private or public, property:, T Legally, mines are responsible for compensating damages and losses that are caused by, their activities. Subsidence above coal and, salt mines, gas and oil fields may damage, roads, canals, railways and buildings (Brady, & Brown 2003, Bell 1998). The large underground iron ore mine at Kiruna, for example,, induces considerable subsidence that is predicted to last 100 years, eventually enforcing, a relocation of Kiruna town. Mine dewatering, can lead to drinking water wells falling dry., The attribution of damages to mining is often
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Groundwat, , er surface, , g), in, in, m, re, (p, , H, , GW after, draw-down, Flo, w, , 451, , lin, e, , h, , Seepage su, , rface, , L, R, Figure 5.20 Cross-section of a mining tunnel drawing groundwater (Wittke 1990). With permission from Springer, Science þ Business Media. Once the hanging-wall rocks are fully drained after establishment of the tunnel, steadystate seepage into the tunnel will take place. The rate of water inflow Q (in m3/s per length-metre of the tunnel, without, vertical unsaturated seepage from above) can be estimated by equation (5.8) Q ¼ 2k.(H-h/R).F (k ¼ rock mass, permeability in m/s, R, H and h in m; cf. sketch; F ¼ potential seepage surface in m2). For total quantity of water, entering, multiply Q with L ¼ length of tunnel drawing seepage in metres., , dubious and such cases have to be settled in, court, involving legal and technical experts., . Collapse of underground mining voids or of pit, walls, affecting the surface:, T Disastrous accidents of this kind have, occurred in working operations, but sudden, cratering is more frequent above old, abandoned mines. A general rule is that only open, mine workings situated between the surface, and 100 m depth are likely to break through, to the surface. In rare cases, large voids at, much deeper levels have triggered chimney, caving, which reached the surface (Brady &, Brown 2004). The resulting earth falls, collapse craters and crown holes endanger, people, buildings, infrastructure and groundwater (Goetz et al. 1994)., , . Hazards associated with rock tips, tailings dams, and settling ponds:, T Many waste facilities contain problematic, material, which may be a source of contamination of soil and water by hazardous, compounds and elements (Figure 5.21). This, subject is enlarged on below., T Physical hazards include dam breaks, landslides and mudflows, in most cases caused by, exceptionally heavy rains. Flotation tailings, consist of gangue and by-breaking host rock of, the ore (Figure/Plate 5.22). They are typically, very fine-grained; the silt-like material, tends to liquefaction and in that state can, form destructive mud-flows (Ritcey 1990)., Accidents of failing tailings dams occurred, in the base metal mine Los Frailes in the
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452, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Precipitation, , Groundwater table, , Dam, , Evaporation, Oxidation, , Tailings, Spring, , Colluvial, sediments, , Reduction, , Bedrock, , Receiving, water course, , Contaminated, acidic plume, Figure 5.21 A tailings dam receiving fine-grained flotation tailings. After Alpers, C.N. & Nordstrom, D.K.,1999,, Society of Economic Geologists, Inc., Reviews in Economic Geology 6, Figure 14.8 b, p. 314. Material containing pyrite, and heavy metals with a small acid-neutralizing capacity (low Ca and Mg) is a potential hazard for the environment., Seepage water (shades of grey) can spread contamination., , South Iberian Pyrite Belt (April 1998: Ollas, et al. 2005) and near Baia Mare in Romania, during reworking of an old settling pond in, order to recover gold (February 2000). Dam, failure by overflow or by piping is the most, frequent cause for these accidents (Richards, & Reddy 2007, Terzaghi et al. 1996). The, impact of tailings on the environment can, be minimized if they are stored underground as hydraulic or paste fill in disused, mine openings., , T Pyrite-rich black shale (“reactive shale”), oil, and coal-bearing shale are prone to spontaneous auto-ignition, usually after a lengthy, period of slow temperature increase. Old coal, washing tips may contain so much unrecovered coal that they self-ignite and burn., . Contamination of the environment by toxic, elements and/or heavy metals:, T Apart from the main metals and minerals, targeted by extraction, many orebodies and, immediate host rocks display minor, , Figure 5.22 (Plate 5.22) The dam retaining the tailings, pond of Baia de Aries gold mine in Valea Sartas, Romania., Courtesy Wolfhart Pohl, Washington. The image, demonstrates the usual technology of “upstream, building” with the main tailings discharge pipe placed, along the crest of the dam, and a series of outlet risers, which dispense the slurry into the reservoir. Coarser, material settles near the dam, preserves stabilizing, permeability, and serves to build it up, whereas slimes are, washed to the centre of the pond. Permeable dams are not, advisable for toxic material (e.g. cyanide).
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , concentrations of hazardous elements (Alloway 1990, Fergusson 1990, Plumlee & Logsdon 1999a). Examples include the semimetals Hg, Se and Te in the hanging wall, above epithermal gold ore (Figure 2.23, and, As, Co, Cr, Cu and Ni of gold mine tailings in, South Africa (Aucamp & Schalkwyk 2003)., The hazard posed by toxic elements can be, better evaluated if sequential extraction, methods are used, because total contents may, be readily soluble, or safely hosted in resilient, minerals (Rao et al. 2008). Remember also, that mining-derived contaminants have to be, evaluated in the context of the local natural, background geochemical landscape (Fortescue 1992), which is nearly always anomalous, , ., , 453, , in mineralized areas (Selinus et al. 2005)., Geochemical fact sheets useful for environmental practice were provided by Reimann &, Caritat (1998)., Gas seepage:, T Abandoned coal mines may be a source of, CO2 that collects in nearby depressions or in, the basement of buildings. The lighter methane is no less dangerous, although it will be, retained in different traps such as unaired, rooms. Enhanced gas flow is typically related, to changes of meteorological air pressure., Methane can also be emitted from old petroleum and natural gas wells that were, not properly sealed. Former uranium mines,, their waste rocks and settling ponds may be, , Final, discharge to, Stanley Burn, , Appropriated natural, juncus aerobic wetland, , Cascade for aeration, , rse), , Aerobic willow pond, , Compost, wetland, , Teflon-PFA dam, , Figure 5.23 Constructed wetlands at Quaking Houses, near Newcastle, UK, which serve for passive treatment of, acidic water seeping from spoil heaps of former hard coal, mining (after Younger 2002). By permission of IMM, London & Maney Publishing (www.maney.co.uk/, journals/aes). The acid rock drainage waters have pH 4.5, and contain dissolved SO4, Fe, Al, Mn and Zn. Main agents, of remediation are aeration and anaerobe microbes, which, thrive in a compost and horse manure substrate., Discharge water is purified and has a pH of 6.7. Juncus, are rushes., , Stanle, , y Bur, , n (rec, eiving, , wate, , r cou, , Cascade, , Islands, Central, weir, , Compost, wetland, , on bed, iph, d sstream, e, t, r, e, Inv neath, be, Acid rock drainage, (ARD) source, , Influent, distribution
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454, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , sources of radon. Radon’s hazard lies in its, a-radiation that acts on the lungs when the, gas is inhaled, increasing the risk of cancer., . Dust emissions:, T Dust is a frequent complaint of people who, live near operating pits, quarries, transport, routes and bulk handling sites such as ports., For mapping dust distribution, hyperspectral, methods using aerospatial data are most, efficient. Spraying roads and tracks with, water (possibly mixed with a coagulant) and, other dust-suppressing technologies bring, relief., . Shaking, vibration, ground motion, earthquakes:, T At close distances from a mine, blasting of, rock and ore can be felt and buildings may, suffer minor damages. The effect is minimized by special blasting techniques. Large, rock falls and the collapse of underground, voids spread uneasiness. Vibrations and, noise of heavy duty traffic can incense the, population., T As a secondary effect of mining, faults may be, reactivated, resulting in earthquakes (up to, magnitude 6 in parts of the Witwatersrand, mining district). Similar magnitudes are, reached when large sections of underground, mine voids collapse (e.g. potassium salt, mines in Germany). In exceptional cases,, damages to buildings and loss of human lives, may occur (Gibowicz & Kijko 1994, Klose, 2007)., . Hazards associated with mine-site processing, plant:, T Some ore processing routes imply the use of, chemicals that have to be very carefully handled. Examples include cyanides (Smith &, Mudder 1999), strong acids, alkalis, organic, compounds and mercury. Several accidental, cyanide releases have been widely reported, (e.g. in 2000 at Baia Mare in Romania, affecting rivers Tisza and Danube). Less known is, the worldwide contamination of rivers with, mercury, caused by artisanal gold miners, employing the amalgam method of extraction. In some rivers, alluvial gold particles, are rimmed by mercury derived from this, source (McCready et al. 2003)., , . Environmental hazards by emission of gases or, of trace metals from metallurgical plants (Coppin, et al. 1996):, T Regulated gaseous emissions include CO2,, CO, SO2, SO3, H2S, NO, NO2, F and dioxin, (halogenated organic compounds). In historic mining and smelting districts, downwind contamination of land with hazardous, trace elements such as As, Cu, Pb and Zn (in, the Iberian Pyrite Belt: Chopin & Alloway, 2007), or Se, Cd and Hg (Harz Mountains,, Germany) is an unwelcome heritage., Today, large efforts are expended to minimize emissions., . Greenhouse gas (GHG) emissions:, T such as CO2 and CH4 arise throughout the, life of a mine. Well-planned management of, energy use can reduce emissions and energy, costs. GHG compensation is possible by, acquiring carbon credits through increasing, carbon storage in waste land, for example,, by temporary biofuels production or reforestation. Due to the formation of carbonates in tailings, mines that process, reactive rock, such as ultramafics, sequester more carbon dioxide from the atmosphere than needed to offset greenhouse, gas emissions from operations (Wilson, et al. 2009)., . Societal problems:, T In an abstract way, new mines pitch the, interest of the consumers of minerals who, wish for low prices and assured availability,, against the affected population who fear the, loss of their familiar quality of life. The, consumer of mineral products, however, is, anonymous and will deny any joint guilt, but, the mining company is an easy target for, opponents of change. It is, of course, true that, for the local population, very large mines, fundamentally change all aspects of life., From the landscape to the infrastructure,, bewildering alterations take place. The daily, way of life and relations between locals are, under pressure. Part of the workforce will be, professional migrants. Management and staff, of the mine have to understand the stress, this causes and act in such a manner that the
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , population is not antagonized. Ethical action, and the quest to mitigate negative outcomes, must always be the rule. It may not be sufficient to act according to laws and regulations,, because minor problems can be understood, as major injustice and finally threaten the, project., T Similar to the installation of a new mine,, mine closures can have grave social consequences and minimizing these must be part of, the planning. Social reconciliation must, always be ranged at top priority., Meanwhile, a rich experience in solving the, listed problems has been acquired. Most mines, blend unobtrusively into their environment and, the local society. Many communities are proud of, “their” mine. In some cases, aspired solutions, could not be found:, Such an example is the tailings management of the Ok, Tedi Cu-Au mine in Papua New Guinea. Mining is, based on a copper-gold porphyry and skarn deposit, related to a monzonite intrusion (Rush & Seegers, 1990). The deposit had been found by a weak (<500, ppm) copper anomaly and chalcopyrite-bearing magnetite pebbles in stream sediments (Sillitoe 1995). Ore, grade is low (0.7% Cu and 0.8 ppm Au) but resources, total several hundred Mt. The operation’s viability, depends on throughput of large masses. Because of the, mine’s location in steep mountains, extremely high, precipitation (8–10 m/year) and frequent earthquakes,, safe tailings and waste rock disposal near the mine is, impossible. In order to prevent future acid rock drainage, waste is mixed with limestone. The only feasible, disposal method is tipping the waste material,, amounting to 30,000 m3/day, into the headwaters, of the Ok Tedi River. Near the mine, this causes buildup of mine-derived sediment in the river valley. Further downstream, a high load of suspended matter and, copper contamination have negative consequences for, the river and the people (Hettler et al. 1997). The mine, expends many efforts to mitigate the damage. In view, of the economic advantage for local people and the, whole country and in spite of the negative effects, the, mine enjoys majority support., , Waste rock and tailings, Waste rock and tailings of many mines, even if, mechanically safe, are a potential source of, , 455, , environmental impact (Lottermoser 2007). This, is because weathering of such materials may liberate: i) toxic or harmful elements; ii) salinity; iii), acid water; and iv) radioactive nuclides (Jambour, et al. 2003). Geological, morphological, climatic, and mining parameters are different for every, individual site and must be considered (Hartwig, et al. 2005). The acid formation potential is an, important characteristic of waste; it is controlled, by gangue and ore minerals. During oxidation,, arsenopyrite, pyrite, chalcopyrite, pyrrhotite, marcasite and sphalerite produce much acidity,, whereas galena, chalcocite and sulphates do not, (Jennings et al. 2000). However, many sulphates, are only stable in dry conditions and dissolve, during wet seasons, liberating heavy metals and, acidity (Alpers et al. 2000). Wastes may contain, acid-neutralizing agents such as calcite. If net, acidity results from experiments (e.g. humidity, cell tests) and geochemical modelling, preventative counter-measures may be considered. These, include, . liming (application of Ca(OH)2 or of pulverized, limestone, dolomite, brucite and magnesite) in, order to raise pH, which also immobilizes many, metals;, . minimizing access of water and oxygen, for, example by covering the waste with clay, gravel, and soil, or by flooding it with a few metres of, water; and, . installing active or passive treatment if the seepage water can be captured., “Active treatment” implies dosing the seepage, with chemicals in a water treatment plant,, whereas “passive treatment” is typically done by, means of constructed wetlands (Younger 2000,, Figure 5.23, Figure/Plate 5.24 and 5.25). In many, of these systems, ferric iron is first precipitated, (e.g. in cascades, Cravotta 2007, and stepped settling ponds) before acidity is neutralized by passing the contaminated water over open beds or, through buried drains of crushed limestone. The, next stage is precipitation of chalcophile metals in, a reducing environment, provided by shallow, ponds packed with compost, peat and animal, manure that support a teeming phytoplankton and, microbial community (Ledin & Pedersen 1996)., Their main function is to reduce the sulphate of
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456, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Figure 5.24 (Plate 5.24) Cascades for aerating, mine water from the abandoned Dominion number, 25 coal mine on Cape Breton Island, Canada. Oxidation, initiates precipitation of colloidal red oxyferrohydrate,, which gradually matures into minerals such as goethite,, ferrihydrite and jarosite. Courtesy Christian, Wolkersdorfer, CBU, Sydney, Canada., , the acid drainage. In compost beds, phosphate is, often the limiting nutrient. Treatment in constructed wetlands needs little human intervention,, except to renew the reactive materials. Contaminated groundwater flowing off a site can be treated, by permeable, reactive barriers (Lapointe et al., 2006). Best practices and mitigation technologies, related to sulphide mineral oxidation, acid mine, drainage and metal leaching are described in the, online GARD Guide (Verburg et al. 2009)., Generally, a well-planned water management is, indispensable throughout the whole life-cycle of, every mine, from the stage of feasibility and environmental impact studies to post-closure care, (Younger & Wolkersdorfer 2004, Younger 2006,, Kumar et al. 2010). A worldwide investigation of, past mine water studies presented as part of environmental impact statements revealed significant, errors and usually underestimated environmental, impacts (Brown 2010). This can be illustrated by, the example of hydrological studies, which are, often based on only one annual cycle. It is impossible, however, to predict extreme precipitation, and flooding from one year’s data. Yet, extreme, events are the common cause of dam breaks, landslides and mudflows. Clearly, error analysis of data, is required (Taylor 1997) and upper and lower, bounds of hydrological conditions affecting a mine, must be determined., , Figure 5.25 (Plate 5.25) Polishing reed bed as the last, element in a passive treatment system consisting of, a combined reducing and alkalinity-producing (RAPS),, and wetland system for acid mine drainage from an, abandoned coal mine (Bowden Close near Durham,, County Durham, UK). Courtesy Christian, Wolkersdorfer, CBU, Sydney, Canada., , Remediation of soil, Remediation of soil contaminated by mining or, metallurgy is possible by cultivating and harvesting metal-accumulating plants (“phyto-remediation”, Whiting et al. 2002). Of course,, suitable species must be identified for every contaminant and location. Based on successful decontamination examples, the method is now applied, to natural metalliferous soil, in order to extract, metals (“phyto-mining”, e.g. Ni in soil surrounding nickel laterite deposits, with certain Alyssum, species). A lower-cost alternative for numerous, orphaned mines may be “phyto-stabilization”,, which only aims at covering the hazardous material with plants so that erosion and surface run-off, are reduced.
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Closure of mines, reclamation of post-mining, landscapes and ecosystems restoration, For more than 100 years, the closure of mines is, regulated by the authorities. In the past, legal, provisions aimed at protecting the public, against physical or property damage (e.g. by, open shafts and tunnels) and the land was to be, restored to benefit the post-mining economy., Enforced measures were mainly technical, such, as filling shafts and erecting fences, allowing, partial recultivation (Aston 2001). In more, recent times, mine-closure has to be carried out, in a way that protects people and the environment durably against damages. Reclamation is, enforced by the law. The target of reclamation is, a landscape that is useful for industry, residential or recreational purposes, agriculture or forestry (recultivation). Wherever possible, parts of, the land should be dedicated as oases of nature, and wildlife habitats with minimal human, intervention (renaturalization). Although every, mine is a specific case, reclamation often includes the following actions:, . Take appropriate measures against physical, danger to people (e.g. by permanently sealing, shafts and tunnels: Fuenkajorn & Daemen, 1997)., . Check and substantiate the stability of all, near-surface and of large deep underground voids, (Brady & Brown 2004)., . Investigate possible alternative uses of mine, openings, such as disposal of moderately hazardous waste, geothermal energy production (in the, case of flooded mines: Watzlaf & Ackman 2006), as, a collector of drinking water, a tourist attraction,, etc., . Carry out landscaping while the mine is still, active and heavy equipment is available (creating a new surface in consideration of destined, use, average and extreme rainfall, erosion control, geotechnical and hydrogeological properties of rocks and soils present, including, revegetation possibilities); if feasible, plan for, topographic diversity, springs, step-pools, wetlands and lakes, that will commonly raise the, land’s value., . Design suitable cover systems and final landforms for mine waste storage facilities, , 457, , . Replace and amend topsoil for seeding and, planting., . Restore ecosystems in such a way as to provide, humans with ecosystems services (e.g. food,, flood and erosion control, areas for recreation, and aesthetics, and clean water: Palmer & Filoso, 2009)., . Model hydraulics and if needed, the water chemistry of the post-mining surface, flooded mine, voids and ground water systems (Wolkersdorfer, 2008)., . Construct a new surface drainage system., . Plan for treatment (active or passive) of mine, water and of seepage from mine waste, if needed., . Envisage and model the geochemical landscape, that will be the result of reclamation; note that the, baseline to be attained (e.g. the concentration of, hazardous substances) can hardly be lower, or, better, than the pre-mining natural state of the, land., . Analyse hazard and risk of earthquakes that may, be caused by flooding after mine closure (Klose, 2007)., . Devise a monitoring programme., , Geoscientific aspects of reclamation, Chemically unstable waste rock and tailings, (e.g. containing sulphides) should be either stabilized with pH-buffering materials, or covered, so that oxygen access is minimized (Tordoff, et al. 2000, Patterson et al. 2006). Cover material may be water, clay or residual organic waste, from industry and agriculture (including energy, crops). Surface run-off should conform to standards of ordinary surface water, suitable for, fishing, bathing and water birds. The time, needed for filling mine pits and country rock, aquifers to a new post-mining groundwater, level (“groundwater rebound”: Adams & Younger 2001) has to be calculated based on hydrogeological models (Gandy & Younger 2007,, Fontaine et al. 2003). Flooding of opencut and, underground mines may degrade groundwater,, mainly by oxidation and dissolution of minerals, in mine openings and adjacent permeable host, rocks. Appropriate planning will often be based, on forward modelling of the geochemical evolution of the water (Marcuello et al. 2006,
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458, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , Figure 5.26 Stamp mill tailings (white) left by the, former Gebeit gold mine, Red Sea Hills, Sudan. Tailings, such as these are not only a possible source of, contamination, but also a profitable resource if, reprocessing is feasible., , Bowell 2003, Davis 2003). Based on this, preventive measures or remediation methods may, be designed. Acidification of pit lakes can be, prevented or remediated either by chemical, treatment (addition of lime, sodium hydroxide), or by supporting microbial alkalinity production (Totsche et al. 2006)., Note that mine waste and abandoned mines, may not only be an environmental problem,, but develop quite often into important assets, (Figure 5.26). A first point is their role as a, habitat for many rare and threatened species,, from metal-tolerant and extremophile archaea, and bacteria to plants and animals, thus helping, to preserve biodiversity (Batty 2005). The second, concerns the frequent case that presently uneconomic mines may enter a new productive phase., Because of this, it is paramount to include the, investigation of a possible mining future in all, “final” mine closure projects. As commodity, prices rise and more efficient technologies are, employed, cut-off grades fall steadily, leading, to reclassification of former in-situ low-grade, mineralization, current and historical waste, to “ore deposits” in their own right. Novel, technologies are available, including low-cost, microbial techniques (e.g. heap leaching of, waste rock in Western Australia with 0.4%, , nickel). Even simple aggregates of low quality, are so expensive that waste rock recycling may, be profitable. Therefore, reprocessing of tailings, and waste rock is increasingly seen both as an, attractive investment and as a means to finance, environmental remediation of abandoned, mines. A striking example is the Kolwezi tailings project in the D.R. Congo, with 113 Mt, measured resources at an average grade of, 1.29% copper and 0.32% cobalt. A third positive, aspect is that certain mine wastes sequester so, much carbon dioxide from the atmosphere as to, more than offset greenhouse gas emissions from, operations (Wilson et al. 2009). This is due, to supergene neoformation of carbonates from, silicates, for example tailings and waste rock, from ultramafic-hosted asbestos, chromium,, diamond and nickel operations., Clearly, principles and tools for socially, and environmentally sustainable mining and, mine site restitution are highly developed., Already, they have been embraced by most, large mining companies. Yet, the transition to, “green” mining has only started. For the near, future, we may expect an avalanche of innovation and penetration of best practice execution throughout the world. Wise national and, international governance should aim to support this change, , 5.5 DEEP GEOLOGICAL, , DISPOSAL OF, , DANGEROUS WASTE, , With the preservation of numerous deposits and, occurrences of toxic or radioactive substances over, geological time-scales (e.g. the natural fission reactors in Gabon, Figure 2.44), nature demonstrates, the feasibility of secure disposal of waste in deep, repositories (Rempe 2008, Miller et al. 2000)., Among professionals, the consensus is that surface disposal of dangerous industrial waste that is, only confined by technical systems is not sufficient to guarantee long-term safety. In contrast,, waste that rapidly loses its hazardous properties, (e.g. processed urban or low-level radioactive, waste) may safely be disposed of in shallow pits, and subsurface vaults. Of course, even this
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Surface, Depth (km), , requires geological guidance for appropriate sites., The advantage of deep disposal is the large envelope of rock that protects the repository against, exogenic geological processes (e.g. erosion, weathering) and against intentional or accidental human, intervention. In addition, deep rocks contain very, little water that hardly moves because of low, permeability and flow potentials. This inhibits, mobilization and dispersion of dangerous substances, and restricts movement out of the repository to geological time-scales., Essentially, deep geological disposal simulates,, with technical means, conditions that allowed, preservation of mineral deposits in the geosphere., Deep repositories may be considered to be, “constructed mineral deposits”., Most important conditions for safe repositories, include:, . a mildly reducing environment (natural analogues for this demand are deposits of sulphide ore,, coal, petroleum and gas, uranium and natural, fission reactors);, . absence of, or extremely small hydraulic flow;, . any water present should be in chemical equilibrium with the host rocks (formation waters);, both ascending hydrothermal and descending oxygen-rich surface waters should be absent;, . tectonically disturbed rocks must be avoided, because hydraulically conductive faults may, cause oxidation to depths more than 10 times of, the average surficial alteration., Various technical solutions of deep disposal, have been devised (Figure 5.27). Frequently, underground voids of producing or exhausted mines are, used (including caverns that result from solution, mining of salt). For disposal of high-heat producing, (“hot”) radioactive waste, new, specially engineered mines are constructed. In mountainous, areas, tunnels or caverns may be prepared (e.g., Yucca Mountain: Stuckless & Levich 2007). The, oil and gas industry disposes of brines and CO2 in, deep drillholes (Klusmann 2003). Similarly, many, factories pump waste water into deep saline aquifers. In the future, deep sequestration of CO2 will, probably increase as this is one solution that allows continued use of coal for energy production, without the hazard of aggravating climate change., At much greater depths, reaching 6 km below the, , 459, , 0, , Tunnels,, caverns, , 1, Dedicated, repository, mines, 2, , Salt solution, caverns, , Deep, drill holes, , 3, , Figure 5.27 Technical variants of geological disposal, of hazardous waste (modified from Herrmann &, Knipping 1993). With permission from Springer, Science þ Business Media., , surface, borehole disposal of plutonium in granite, is considered., As a rule, deep disposal in mines utilizes the, concept of multiple barriers (Figure 5.28). At, depth, mine openings suitable for receiving waste, are newly made, or voids remaining after extraction of minerals (e.g. salt) are refurbished. The first, , Earth's surface - Biosphere, Groundwater / Hydrosphere, , Geological barrier / Geosphere, Geotechnical, Chemical, Mechanical, Toxic or adioactive, wast e, , Figure 5.28 The concept of multiple barriers is, employed to protect biosphere and groundwater from, dangerous contaminants. Deep geological disposal relies, mainly on a geological barrier that is supported by, additional measures (constructed barriers). Chemical, barriers, for example, may be concrete that maintains an, alkaline environment inhibiting solubility of many, elements. Physical barriers include absolutely tight, containers.
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460, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , (innermost) barrier concerns conditioning the, waste in order to minimize its reactivity and, mobility (e.g. by controlled oxidation of organic, matter; vitrification; solidification of all liquids)., The second barrier inhibits dispersion of dangerous substances by mechanical means (e.g. stainless steel containers, or a capillary barrier as, proposed for Yucca Mountain: Carter & Pigford, 2005). The third barrier employs chemical, reactions (e.g. bentonite as an alkaline absorbent)., The forth barrier provides geotechnical stability of, storage voids and reliable sealing of access shafts, and tunnels (Fuenkajorn & Daemen 1996, 1997)., This is to assure long-term functionality of the, fifth and most important, the geological barrier., The geological barrier is required to provide, sustained protection of the biosphere, similar to, natural systems, for example high-grade uranium, deposits. Nuclear waste requires highest standards but constitutes only about 1% of the total, mass of dangerous industrial waste. For nuclear, waste repositories, the geological barrier should, be thick and homogeneous and exclude water., Appropriate rock bodies can be thick halitites, (Gorleben, Germany, Figure/Plate 5.29: Bornemann et al. 2008; Carlsbad, New Mexico),, mudstones (Mol, Belgium; Bure, France; Mont, Terri, Switzerland: Arnould 2006; Konrad,, , Figure 5.29 (Plate 5.29) Gorleben mine in northern, Germany is destined (although not yet licensed) to, function as a repository for heat-producing radioactive, waste in salt rock. The aerial view shows surface, installations, the two shaft buildings, waste disposal and, Elbe River in the far distance to the left. Bundesamt f€, ur, Strahlenschutz, Germany., , Germany), gneiss and granite (Olkiluoto, Finland;, € o, Schweden; Grimsel, Switzerland), or volcaAsp€, nic rocks (Yucca Mountain, Nevada: Stuckless &, Levich 2007). The choice among these possibilities is often limited by the geological availability., Finland, for example, has no thick salt or mud, rocks, but ample granite., The former oolitic ironstone Konrad mine near, Salzgitter, Germany is destined to receive intermediate and low-level radioactive waste. Geology makes it, an ideal deep repository. Late Jurassic sediments are, here buried to >1000 m depth in a broad trough adjacent to a salt diapir. A bed of oolitic ironstone is, interbedded with impure limestone of the Coral, Oolite Formation. In Konrad mine, the iron ore bed, had a thickness of 6–18 m with 33% Fe. From, 1965–1976, 7 Mt of ore were produced. Chambers, in the ore horizon are to be filled with waste. The, geological barrier consists of overlying Cretaceous, mudrocks with a thickness of 750–1000 m (Figure 5.30). Because of overconsolidation, the mudrocks are very dry., , For many years, moderately dangerous waste, has been buried in underground mine voids or in, open cuts created by the extraction of minerals,, accumulating a large body of theoretical and practical experience. In Germany, most of the slag, ash, and filter dust of coal power stations and communal waste incinerators are disposed of in potassium, salt or in coal mines. Toxic industrial waste is, preferentially buried in salt mines, including Herfa-Neurode, Hesse (Figure 4.22). In some mines,, waste serves as a fill, supporting the mechanical, stability of underground voids. Until recently,, intermediate and low-level radioactive waste was, stored in the former salt mines at Morsleben and, Asse in northern Germany., Thick salt formations are well suited for the, disposal of toxic and radioactive waste (Herrmann, & Knipping 1993). Salt rocks have an extremely, low permeability for water and gas, which ensures, enclosure and inhibits contaminants entering the, hydro- and biosphere. In a relatively short time,, mine voids in salt rocks close by plastic flow, (creep), thus strengthening the rock barrier. Furthermore, rock salt is resistant against moderate, heating (200 C) and radiation.
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , km, 0, , Engelnstedt, , Bleckenstedt, , ir, diap, , Early Creta, , ceous, , an, m, Mal meridgi, m, i, K, ger, Dog, Oolitic iron ore bed, sic, Lias, , Salt, Keupe, , 2, , Quaternary, , Late Cretaceous, , SL, , 1, , E, , Konrad repository, , W, , r, , 461, , Keuper, , Keu, , l oolite, , "Cora, , SL, , ", , per, , Figure 5.30 Geological profile of the Bleckenstedt Trough north of the Harz Mountains, Germany, with the Konrad, repository for intermediate and low-level radioactive waste, in a former oolitic iron ore mine (iron ore black). SL ¼ Sea, level. With permission from www.schweizerbart.de., , Repositories for radioactive waste, Repositories in salt rocks for heat-producing highlevel radioactive waste require extremely detailed, and meticulous investigations. An exemplar case, is the enormous scientific and technical exploration work at the Gorleben salt diapir in Northern, Germany (Figure/Plate 5.29, Figure 4.26; Klinge, et al. 2007, K€, othe et al. 2007, Bornemann et al., 2008). Important geoscientific aspects of this kind, of work comprise:, . The selection of adequate salt rock bodies is, primarily controlled by geological parameters. In, many regions of the world, existing geophysical, data acquired during hydrocarbon exploration are, very valuable data sources (Jenyon 2008). Ideal, targets are large halitite masses with a minimum, of tectonic disturbance, which are covered by lowpermeability mudrocks. However, as with establishing a new mine, infrastructure, economic and, social parameters will be crucial for a successful, project., . The tectonic stability of a salt rock body refers, to the inherent buoyancy of light salt compared to, surrounding sediments. Within a short geological, time, resulting upflow might lift the repository to, the surface. Many (“active”) salt diapirs in Northern Germany rise at rates of between 0.01 and, , 0.5 mm/year. Stable, that is tectonically, “inactive”, diapirs have no measurable upflow., This can be controlled by long-term geodetic, surveys, but the geological upflow history of a salt, body should be established in order to strengthen, the case for a durably inactive state., . The hydrologeological stability of a salt body, depends on the rate of salt dissolution (present, and past) where it is in contact with aquifers. Salt, diapirs with a history of strong subrosion such as, Arendsee (Figure 4.28) are unsuitable. In many, cases, however, the surface morphology will be, less striking and may be of no help. A thorough, investigation of the caprock and the hydrogeological situation is absolutely required (Gorleben:, Klinge et al. 2007)., . A favourable internal structure of the salt formation, including a large dry salt body (halitite), with a minimum of hydrous mineral components, at the required depth. Large anhydrite masses are, better avoided because they may contain brine., Also, because of anhydrite’s high density, large, blocks tend to gradually sink downwards (Koyi, 2001)., Results of geoscientific investigations are building blocks for assessing the longevity of the geological barrier between waste and biosphere. For, some nuclides that are contained in radioactive
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462, , PART III THE PRACTICE OF ECONOMIC GEOLOGY, , waste, a “proof” of safety (the technical term is, “safety case”) is required for periods approaching, one million years. Clearly, the nature of possible, changes in that time and the associated risks, cannot be known with certainty (Macfarlane &, Ewing 2006). Yet, earth scientists commonly agree, that deep disposal is the safest option, but disagree, often on the safety of a specific site. Considering, the relatively high level of natural background, radiation, however, even a nuclear repository need, not be absolutely tight. Small masses of nuclides, seeping out may be lost in the ubiquitous natural, background (Selinus et al. 2005)., , 5.6 SUMMARY AND FURTHER READING, Metals and minerals serve human needs such as, housing and in use, are consumed. The mining, industry provides the required raw materials. For, continuity of supply, a steady rate of replacement, is required involving new mines. The search for, new deposits is an important part of the industry’s, activities and of economic geologists’ professional, duties and responsibilities., Let us here acknowledge the drivers in this field., Worldwide, about 2000 stock exchange-listed companies are active in non-ferrous metal exploration., International and state organizations (e.g. China),, unlisted companies and specialists for iron ore and, minerals may add another 1000 to this figure. It is a, great achievement, that this relatively small industry sector supplies the world with an adequate, number of new prospects, mines and raw materials., Strategies and methods of the search for minerals, and metals (“exploration”) build on the large body, of metallogenetic science and utilize all available, technologies, from electron microscopy to interpretation of satellite images. Nearly all deposits, display subtle halos of anomalous geochemical and, mineralogical composition, and many have physical properties such as high density or conductivity,, which allow detection through thick sterile overburden. Different deposit types are characterized, by specific sets of geological environment and, properties which are combined in “exploration, models” that guide the search., , Potential deposits indicated by preliminary, work (“prospects”) are investigated in more, detail, usually by drilling. Drilling is first aimed, at the centre of the assumed orebody, in order to, confirm the presence of significant mineralization. Core samples obtained are thoroughly analysed and as data density and coverage increases,, a detailed image of the mineralized body, evolves. Ore resource quality and quantity are, estimated. In a prefeasibility report, the results, are compared with working mines of a similar, type. Very often, this leads to final rejection of, the prospect., In the case of accumulating evidence that the, prospect resembles orebodies supporting flourishing operations, work turns to assembling all data, that are needed to prove the viability of a new, mine. This includes more drilling and bulk sampling, in order to affirm sufficient reserves which, justify the investment – remember that establishing new mines costs from 50 to 2000 million US$., Mine planning, processing and geometallurgy,, environmental state and management of the, future mine, financing and economic viability,, social reconciliation and many more aspects have, to be investigated to such a degree that the decision to open the new mine is fully supported. The, resulting feasibility document equally serves, licensing authorities and investors., It is very important to understand the precise, meaning of the terms “reserves” and “resources”., Reserves are only that part of resources that is, thoroughly investigated and proved to be, exploitable with a high degree of confidence. Reserves are essentially determined by economic, recoverability and are the foundation of mining, operations. Reserves provide collateral security to, investors and stock markets. This role provoked, the elaboration of reserve reporting codes, e.g. by, JORC in Australia. Outlining reserves in the, ground is a costly undertaking, which must, be financed before any profit is made from future, income by extracting and selling the ore. The, present value of future income is a function of, time, because it results from discounting. Income, due in 40 years from now is practically worthless., Therefore, reserves are hardly ever substantiated, for a period beyond this mark.
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GEOLOGICAL CONCEPTS AND METHODS IN THE MINING CYCLE CHAPTER 5, , Resources are principally less well-known or, even unknown. Measured resources may be, disclosed for a deposit in order to show that the, mineralized body is much larger than reserves, but, are not bankable. Unknown resources, however,, are the subject of scientific enquiry and the hopes, of explorers. This is that part of the total geological, endowment of the Earth with a certain metal or, mineral that may one day satisfy human needs. It, is the stock from which new discoveries are, wrested., Few mines have no visible impact on people, and environment. During the working period of, a mine, unaesthetic changes to the land, artificial, water bodies, increased traffic and dust, noise and, vibration, and perturbation of the local community all may cause rejection of the operation. Yet,, the “social licence” is essential. Why are some, communities proud of their mines and others wish, them away? Mine closure, however, is not without, its own social problems., Technical problems of mine closure first, comprise measures to ensure safety for people, and animals, such as sealing tunnels and shafts., Land and ecosystems are restored to some use,, ranging from brown field industrial plots to, nature preserves. Reprocessing old mine waste,, such as shale tips with 10% coal fines, may pay, for remediation. Tailings should be landscaped, and covered in order to avoid leaching of hazardous elements. Parts of mines may be destined to, allow nature its course, in order to create islands, of biodiversity., Principles and tools for socially and environmentally sustainable mining and mine site restitution are pioneered by the large mining, companies. The transition to “green” mining is, on its way. The face of the future extractive industry will be very different from that in the past., , 463, , Readers searching for a more profound introduction to mineral exploration are directed to Moon, et al. (2006). If you have access to a good library you, might enjoy Evans (1993). In order to learn more, about geological concepts, models and methods, that are the indispensable base for any exploration, project, read Kreuzer et al. (2008) and Kelley et al., (2006). My favourite sources, however, for better, understanding geological exploration, are Glasson, & Rattigan (1990) and Sillitoe (1995). The first, assembled 50 valuable case studies from Australia, tracing geological exploration from the time, when in 1850 the first geologists arrived in, the colony. Sillitoe provides a wealth of useful, detailed information and concludes that the key, to finding ore deposits is “experienced geologists, in the field and a generous drilling budget”. In my, view, these two books should be compulsory reading in all exploration training programmes. Quantitative Mineral Resource Assessments by Singer, & Menzie (2010) shows how to estimate number,, tonnage and grade of undiscovered deposits., Modern insight into geochemical exploration is, provided by Carranza (2008) and geophysical, methods are explained in more detail in the book, by Keary et al. (2002). Various facts and figures that, may be needed in practical work are assembled in, Bell 1993, 1998), Berkman (2001) and Walker &, Cohen (2007). Environmental problems of mining, and possible solutions are broadly covered in, Bell & Donelly (2006) and Warhurst & Noronha, (2000). I recommend the two Environmental, Geochemistry of Mineral Deposits volumes by, Filipek & Plumlee (1999) and Plumlee & Logsdon, (1999), for a scientific yet accessible level. Readers, who wish to explore the economic aspects of, environmental issues should consider reading, Gilpin (2000) and Lomborg’s (2001) provocative, Skeptical Environmentalist.
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PART IV, Fossil Energy Raw, Materials – Coal, Oil, and Gas, Carbon is the element of Life on Earth – six major, elements, carbon, hydrogen, nitrogen, oxygen, sulphur and phosphorous are required to build all, biological macromolecules. Carbon dioxide in the, air, and water are used by plants in photosynthesis, of carbohydrates, such as cellulose. By photosynthesis, terrestrial global gross primary production, (GPP) removes 123,000 million tonnes carbon, (MtC) of CO2 per year from the atmosphere (Beer, et al. 2010), compared to human emissions by, burning fossil fuels of 7,000 MtC/yr. Most bacteria and archaea, and all higher animal life forms, including humans, use the reduced carbon of carbohydrates as food and breathe out carbon dioxide., Life is an important part of the Earth’s carbon, cycle. However, the planet contains other reservoirs of carbon and carbon dioxide, including the, mantle, the crust, ocean floors and permafrost, lands with methane gas hydrates. The Earth constantly degasses carbon dioxide and peaks occur, with large outpourings of volcanic eruptions at the, surface. Yet, during the last 450 million years,, carbon dioxide concentration in the atmosphere, has decreased steadily (Normile 2009), mainly, , because of the proliferation of plants on land,, carbonate-fixing biota in the oceans and carbon, burial in the subsurface. By burning fossil fuels,, humanity reversed this trend from a pre-industrial, 280 to a present 388 ppm CO2 in the atmosphere., Beginning in Pleistocene glacial times, the role of, reduced carbon for humans has increasingly changed from food to fuel., Fossil fuels include coal, petroleum, natural, gas, tar (heavy crude oil) and oil schists. Coal’s, peak role as an energy raw material was reached, early in the 20th century. Since then, oil has, displaced coal to second rank. In 2009, the world’s, primary energy supply (ca. 11,164 Mt oil equivalent) was provided by 34.8% from petroleum,, 23.8% natural gas, 29.4% coal, 6.6% hydroelectricity and 5.5% nuclear power (BP Statistical Review of World Energy 2010). BP sources are, limited concerning renewable energy, but do, report that geothermal, wind and solar electricity, generation combined account for approximately, 1.7% of global electricity generation. The International Energy Agency (IEA) provides more, detailed data and projections about renewables, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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CHAPTER 6, Coal, . . . plants are a significant geological force of nature – a critical missing feedback linking biology,, chemistry, and physics that has shaped our planet’s history . . ., David Beerling 2007, , Synopsis, For industrial nations, abundant energy in the form of coal was the fundamental precondition for, development (Freese 2003). Considering examples such as China and India, this is not different, today. Development requires bounteous and low-cost energy, which involves sacrificing the preindustrial farming society with its perceived peaceful and sustainable life. For liberating the, inherent energy, coal must be combusted, resulting in various hazardous emissions that have to, be controlled. Coal’s most widely discussed emission is carbon dioxide because of its influence on, global climate. This feeds a lively debate about the future of coal., This chapter starts with a concise introduction to coal utilization, production and resources,, including information on possible carbon dioxide mitigation strategies. To lay the base, the ensuing, sections discuss the substance of coal, its formation from peat deposited in ancient wetlands and the, processes that make coal from peat. We then look at weathering of coal, which often involves selfignition and natural coal seam fires. Practical aspects of coal science, such as its application in, exploration, reserve estimation, mining and environmental mitigation, conclude the chapter., , At present, the major utilization of coal is electricity production. China, for example, derives, 80% of its electricity from coal and the United, States 50%. Cokes produced from certain hard, coals, and anthracite, are ideal reductants for processing oxide metal ores and are also used in many, other industrial processes (e.g. soda ash and carbide manufacture, cement-making, hydrogen production by steam reforming). Domestic heating is, a minor sector of coal use. In the near future, the, , production of liquid fuels from coal and lignite,, based on the Fischer-Tropsch synthesis and similar processes (“coal-to-liquids technology”), is, expected to expand considerably. This is due to, increasing economic viability, as prices of petroleum and natural gas rise. South Africa has a long, tradition of providing its liquid fuel needs by this, technology. Today, many more countries, which, have limited natural hydrocarbon but large coal, reserves, plan to take this route (e.g. Australia,, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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468, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , because additional giant resources are available. It, illustrates only the difference between coal and, other raw materials, concerning the rules for defining coal reserves and the long-term nature of planning. At face value, coal reserves are assured for a, much longer time than petroleum (at the end of, 2009 46.7 years, the Canadian oil sands not, included) and natural gas reserves (63 years). As, explained in Chapter 7, oil and gas production, will continue far beyond this period. Additional, giant energy resources are available if today’s, uneconomic coal occurrences can be made useful,, for example by new extraction technology or insitu gasification., More than 60% of world coal reserves occur in, the USA, Russia and China. With nearly 30%, the, USA hosts the largest reserves of all countries., Black coal (anthracite and bituminous coal), reserves occur in USA, China, India, Russia, Australia and South Africa. Significant brown coal, (sub-bituminous coal and lignite) reserves occur, in USA, Russia, China, Australia, Ukraine,, Colombia and Canada., Carbon capture and storage must result in a, higher price of electricity and requires giant, investments. Economists are divided on the best, mitigation path to take. Should the huge sums, needed for a reduction of anthropogenic CO2, , China, India, New Zealand, USA). Gasification is, emerging as an improved technology for electricity production from coal, resulting in lower sulphur dioxide, nitrogen oxide, mercury and, particulate emissions, compared to a conventional, pulverized coal power plant. The IGCC (integrated, gasification combined cycle) process also uses less, water and can be more economically retrofitted for, carbon capture., This scenario implies that in the future, production and use of coal will continue to grow. Is this, supported by sufficient coal resources? Today’s, deposits that can be exploited under existing economic and operating conditions (“proven, reserves”) contain 826,000 Mt. Of this total, one, half comprises black coal and anthracite, the, remainder brown coal and lignite (BP Statistical, Review of World Energy 2010). World production, of coal and lignite in 2009 was 7023 Mt (equal to, 3409 Mt of oil equivalent). The largest producers of, hard coal are China (45.6%), USA, India, Australia,, Russia and South Africa. Germany is the leading, lignite producer, followed by USA, Greece and, Australia., Dividing reserves by annual production (R/P), gives the so-called “static period of availability”, of coal reserves at 120 years (Figure 6.1). Of, course, the ratio does not define the end of coal,, , Reserves/Production ratio, 450, 400, 350, 300, , Coal, , 250, , 70, , Natural gas, 60, , Petroleum, 50, 40, , Lead, , Zinc, , 30, 20, 10, , 46, , 19, , 50, , 19, , 55, , 19, , 60, , 19, , 65, , 19, , 70, , 19, , 75, , 19, , 80, , 19, , 85, , 19, , 90, 19, , 95, , 19, , 00, , 20, , Figure 6.1 Evolution of the, reserves to production ratio R/P, (also called “life-index” or “R/C, ratio”) of energy raw materials, coal, petroleum and natural gas,, compared to lead and zinc, from, 1945 to 2000 (adapted from, Wellmer 2008). With, permission from www., schweizerbart.de. The rapid fall, of the R/P ratio of coal is not due, to depletion, but to gradual, world-wide acceptance of, stricter rules for declaring, reserves (cf. Chapter 5.3.2 “Ore, Reserve Estimation and, Determination of Grade”). In, 2010, R/P ratios were 47 years, for oil, 63 for gas and 120 for coal, (BP 2010).
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COAL CHAPTER 6, , BOX 6.1, , 469, , Coal and carbon dioxide, , Worldwide, there are 2100 coal-fired power plants. Burning coal implies the generation of waste gases, including CO2,, SO2 and NOx. The two latter can be reduced to low levels by better combustion and flue gas cleaning technologies., Reduction of greenhouse gas (GHG) carbon dioxide emissions progresses only gradually, however, mainly by more, efficient combustion and by saving energy. The energy yield was improved from 33% in the past to 55% in modern coalfired power plants. Even so, coal-burning accounts for roughly one-third of the CO2 generated by human activity (a total of, 7 Gt per year, Normile 2009). One tonne of black coal containing 80% C generates nearly 3 t CO2. Today, the public,, governments and industry are prepared to take more radical counter measures against CO2 (and other) emissions into the, atmosphere. This is certainly desirable, even if a leading role of CO2 as a control on climate is not undisputed and the, climate’s sensitivity to a doubling of CO2 may be commonly overestimated (Plimer 2009, Kirkby 2008, Royer et al. 2007)., About 75% of the greenhouse effect is due to clouds and water vapour and 20% to CO2 but without CO2 and minor GHGs,, Earth would rapidly fall into icehouse state (Lacis et al. 2010). Present data are insufficient to decide if climate change, increases or decreases evaporation (Dolman & de Jeu 2010) which is a significant factor of the Earth’s surface energy, balance. Stratospheric water vapour is unexpectedly revealed as an important climate driver (Solomon et al. 2010). For the, last five years, the energy balance of the Earth defies climate modellers (Trenberth & Fasullo 2010)., Apart from reducing fossil fuel use, two mitigation strategies against the increase of atmospheric CO2 concentration are, mainly pursued (Lackner 2003): i) Net carbon storage in forestry and agriculture, including soil improvement by adding, biomass or char; and ii) carbon-capture and storage systems (CCS) that include underground sequestration in mature oil, and gas fields, in deep coal beds and in deep saline aquifers. The last has been practised for many years in the Norwegian, offshore gas field Sleipner (Bickle et al. 2007, Figure/Plate 6.2). Worldwide available pore space is thought to be very large, (Figure 6.3). Less credible, CO2 might be disposed of as a liquid in deep oceans, in zeolites, and in magnesite made from, serpentinite and dunite. CO2 capture technologies, more than sequestration, are costly. Two technologies of capturing, CO2 are currently favoured: The first binds CO2 on monoethanolamine (MEA) or ammonium carbonate in order to, separate it from nitrogen in the flue gas, whereas the second burns the coal with pure oxygen., , Figure 6.2 (Plate 6.2) Sleipner platform in the North Sea off Norway is the world’s first large-scale geological, CO2 sequestration operation. On the platform, carbon dioxide is separated from natural gas and pumped into a, sub-seafloor aquifer. Ó Øyvind Hagen, Statoil.
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6, , 10, , 5, , 10, , 4, , 10, , 3, , Ocean, carbon, , Soil carbon, Ocean (CO3-), , Characteristic storage time (years), , 10, , Biomass carbon, Atmospheric CO2, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Annual, emission, , 470, , Carbonates, , Underground, injection, , EOR, Ocean, turnover, , Soil, carbon, 10, , 2, , Wood, , Infrastructure, lifetime, , Leaf, litter, , 10, , 1, 1, , 10, , 100, , 1000, , 10,000, , Carbon storage capacity (Gt), , 100,000, , Figure 6.3 Comparison of several, significant variants of CO2, sequestration, their estimated, storage capacity and time of, immobilization (modified from, Lackner 2003). Reprinted with, permission from AAAS., Carbonates guarantee the, maximum of isolation, time, whereas biomass is, short-lived. Advisable seem, to be EOR (enhanced, oil recovery) and deep injection, with its large capacity., , At ambient pressure and temperature, carbon dioxide is a gas (density 1.98 g/cm3) that is 1.53 times heavier than air. At, moderately low temperatures (31.1 to 56 C) and a pressure from 5–73 bar, CO2 is a liquid heavier than water. Above its, critical point at 31.1 C and ca. 73 bar, carbon dioxide transforms into a low-viscosity supercritical fluid. This is the state of, concentrated CO2 in geological reservoirs at depths below 500–1000 m. With a density of 0.5–0.7 g/cm3, supercritical, CO2 is less dense than brine or petroleum but more dense than methane. Within the reservoir, injected CO2 is gradually, dissolved in formation water and commonly occurs as ionic HCO3 (eq. 1.14). In time, it may be immobilized by, precipitation of carbonates such as calcite. Leakage from CO2 repositories to the surface must be minimized, not only, because escape defies the purpose of storage, but also in order to prevent hazards to human and animal life. Carbon, dioxide gas is an inert asphyxiant and because of its density, collects in calm depressions and basements. Humans react to, CO2 concentrations of 4–5% in air by accelerated breathing and discomfort, at 7–10% persisting for several minutes by, fainting (Weinstein & Cook 2004). Maximum work exposure is set to 0.5%. Because of the perceived hazard, offshore sites, such as Sleipner are probably more acceptable than onshore storage in densely populated regions (Schrag 2009)., Haszeldine (2009) presented a map of CO2 emission sites in Northwest Europe and a draft of possible pipelines and, storage fields in the North Sea., Once captured, CO2 is liquefied, dried and pumped down into geological reservoirs. This technology is well-known in, the oil and gas industry because for many years, supercritical carbon dioxide has been used to displace hydrocarbons from, reservoir pore space in order to enhance oil recovery. One example is the almost pure magmatic CO2 from Bravo Dome, gas field in New Mexico, which is used for carbon dioxide injection to maximize oil recovery in western Texas. The, comparable but less practised CO2-enhanced methane recovery (ECBM) from deep unexploitable coal seams may offer, another path of CO2 geosequestration (Ozdemir 2009). In the Rangely Oil Field, Colorado, loss of injected carbon dioxide
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COAL CHAPTER 6, , 471, , by micro-seepage to the atmosphere is a fraction of natural methane seepage (Klusmann 2003). The safety case for, geological CO2 storage is gaining strength (Houston et al. 2007) and in a North Sea oil field CO2 pool, seal performance for, >70 million years has been demonstrated (Lu et al. 2009). For prospective storage sites, the main questions that must, be answered concern: i) the pressure management; and ii) the fate of the displaced pore fluid (Schrag 2009). Although, large-scale capture and geological sequestration of CO2 still present technological challenges, CCS may soon be widely, employed, provided that clear regulations assure companies of a long-term business perspective (Haszeldine, 2009). Overall it is expected that in a few decades, CO2 emissions from coal, oil and gas power stations will be reduced, to unproblematic levels similar to the successful mitigation of SO2 and NOx. On that base, fossil fuels can yet, provide long-term sustainable energy for the world’s industry and humans (Haszeldine 2009, Jaccard 2006)., , emissions be provided now because of ethical, considerations (Stern & Taylor 2007), or “ramp, up” slowly as conventional economic analysis, (Nordhaus 2007) indicates? For the second more, realistic case, a strategy might be prepared to arrest, a possible “run-away” warming of the Earth’s, climate, when the upper limit of the IPCC (UN, Intergovernmental Panel on Climate Change), temperature rise is near. This includes various, techniques of climate engineering (Wigley 2006)., Injecting aerosols or aerosol precursors (i.e. SO2), into the stratosphere can provide a negative forcing of the temperature and offset part of the positive forcing due to increased greenhouse gas, concentrations. Volcanic eruptions provide natural examples of this effect. In June 1991, Mt Pinatubo blew 10 Mt sulphur into the stratosphere, and caused detectable cooling without disruption, of the climate system. Another method might be, fertilizing the upper oceans with iron in order to, amplify biomass production, enhancing carbon, storage by burial of organic matter in ocean floor, sediments (Boyd 2007). Such actions could slow, warming and provide time for the huge investments needed to reduce human CO2 emissions., , 6.1 THE, , SUBSTANCE OF COAL, , 6.1.1 Coal types, Coals are solid, combustible, fossil sedimentary, rocks that formed from land plants (Embryophyta), profusely growing in ancient wetlands. After deposition, the peat was covered by sediments, usually, clay and sand. Diagenetic processes acting on the, material caused an overall enrichment of carbon, , (coalification). The difference between coal, peat, and coal shale is important because in many countries the state controls coal mining, whereas peat, and coal shale belong to the land owner., Peat, the soft sediment in mires and fens, is the, precursor of coal, which is a hard rock. Low-grade, lignites (bog coal) are intermediate between, mature peat and mature lignite. The limit between, peat and lignite is rather a matter of convention. In, some countries peat may be an important source of, energy., Coal shale and bituminous shale can be valuable, raw materials, for example in cement production, (cf. Chapter 3 “Carbonate Rocks”). These rocks are, remarkable because they self-ignite and burn, slowly when dumped on waste rock tips without, special precautions. Generally, the limit between, coal and coal shale may be set at 50 wt. % combustible substance (dry basis). However, coal with, more than 30 wt. % of non-combustible matter is, rarely used as a fuel. Coals with higher content of, mineral matter (e.g. clay, silt, sand, carbonates,, sulphides), but not exceeding 50%, are called, “impure coals”. If feasible, mine-site processing, is often employed to upgrade the coal content of, low-grade impure material., The distinction between brown coal and black, coal originated in Europe, because there is such an, obvious difference between Tertiary low-rank, brown and Carboniferous high-rank black coal,, with rare intermediate quality deposits. The first, has a low calorific power and its use is restricted to, energy and heat production, whereas the second, is a prime fuel and a valuable industrial reductant, (e.g. for iron melting). The terms brown coal and, lignite are widely considered as synonymous., The terms “black” or “hard coal” are little used
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472, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , C vitrinite, Reflect. VM, Bed, dmmf, dmmf, Ro, moisture, wt. %, wt.%, wt. %, %, , Rank, General, , Detail, , Cal. val., kcal/kg, (MJ/kg), , 0.2, , Peat, , 68, , Peat, , 64, 0.3, , ca. 60, , ca. 75, , 60, , Lignite, ca. 35, , 4000, (16.75), , ca. 71, , ca. 25, , 5500, (23.03), , ca. 77, , ca. 8-10, , 7000, (29.30), , 56, , Brown, coal, , Sub- C, bitum. B, A, , C, , 48, , bituminous, , High volatile., , 0.7, , coal, , Medium, volatile, bitumin.., , Low, volatile, bitumin., Semianthracite, , 44, 40, , 0.8, 36, 1.0, 32, 1.2, , Black, , 52, , 0.5, 0.6, , B, , A, , 0.4, , 28, , 1.4, , 24, , 1.6, , 20, , 1.8, , 16, , 2.0, , Anthracite, , 3.0, 4.0, , 8650, (36.20), , 12, 8, , Anthracite, , ca. 87, , ca. 91, , 8650, (36.20), , 4, , Meta-anthr.., , in English; the common terms are “bituminous, coal” and “anthracite” (Ward 1984, Thomas 2002)., Low rank coals (lignite and sub-bituminous, coal, Figure 6.4) are subdivided according to water, content. Bituminous coals are mainly classified, by % volatiles liberated at high temperature in, the absence of oxygen. This underlines that at, differing rank, suitable parameters for coal classification vary. Some observations on macroscopic, , characterization, following:, , Figure 6.4 Classification of coal, according to rank with reference to, important properties (modified, from Taylor et al. 1998). With, permission from www., schweizerbart.de. The, international unit for calorific, value in the SI-System is Joules., Conversion: 1 kcal ¼ 4.1868 kJ, [d.m.m.f. ¼ calculated to dry and, mineral matter free, i.e. pure coal, substance]., , of, , coal, , types, , include, , the, , Lignites, Lignites are yellow or brown coals that can be cut, with a knife. Lowest rank soft, earthy or crumbly, lignite is called “bog coal”; it occurs in thick, banded seams that are not jointed. In certain
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COAL CHAPTER 6, , horizons, upright tree stumps or fallen logs are, numerous. Exposed bog coal crumbles quickly, because of drying and shrinkage. Seams occur at, little depth below the surface and country rocks, are unconsolidated sediments. “Lumpy” lignites, are harder and more consolidated and on extraction break into large fragments. This type is made, up of brown tree stumps, logs and branches. The, seams are banded, either by thin layers of clastic, sediment, detrital coal, or by charcoal and charred, woody debris (fire horizons). Lumpy lignite occurs, at a depth to a few hundred metres below moderately consolidated sediments. Today, lignites are, exclusively used for large electricity production, units., Sub-bituminous coal, Sub-bituminous coal (“black lignite”) is markedly, brittle. With increasing rank (from C to A,, Figure 6.4) its colour varies from dull brown to, lustrous bright black. Fossilized wood can still be, recognized. The seams are closely jointed and, banded. On drying, dull coal disintegrates into, small cubic lumps, whereas lustrous sub-bituminous coal with lower water content changes little., Sub-bituminous coal occurs at a depth of several, hundred metres below medium-consolidated, sediments., Bituminous coal, Bituminous coal occurs in well-consolidated sediments (e.g. sandstone, shale) and both coal and, host rocks are jointed. From low to high rank, the, main varieties are:, . high volatile bituminous (gas coal);, . medium volatile bituminous (coking coal); and, . low volatile bituminous coal (steam coal)., There is little macroscopic difference between the, three; all bituminous coals are brittle and composed of thin dull and bright bands. As volatile, substance content decreases, banding and layering, of coal wanes and disappears when the anthracite rank is attained. Natural coking coals and, commercial blends that provide good coke are, most valuable. Non-coking bituminous coals are, burned in power plants and general industry; gas, , 473, , coal is especially suitable for the production of, liquid fuels., Anthracite, Anthracites are black, and display a metallic lustre, and a conchoidal fracture. Due to the very low, content of volatile matter, anthracites burn with, little smoke and with short flames. They are preferred for steam and heat generation. Meta-anthracite is transitional to graphite. In fact, a sizeable, part of traded “graphite” is meta-anthracite., The coal varieties described so far are humic, coals. They originate from land plants and peat,, and represent the majority of all coal extracted., Many deposits of humic coal contain a minor part, of less common coal varieties. These include, sapropelic (boghead and cannel coal, schungite), and liptobiolite coals. Sapropelic coal is formed as, a bottom sediment in stagnant anoxic water from, small particles of plant residues, spores and foundered mats of floating algae. The sediment is, putrefied rather than peatified. Liptobiolites consist of plant material that resists oxidation on the, peat surface (e.g. spores, pollen, wax, resin, amber), and accumulates in situ or after transport by wind, and flowing water., Boghead coal, Boghead coal (torbanite) is composed of characteristic algal colonies (e.g. Pila, Reinschia). Boghead, is of blackish-brown or greenish-black colour and, gives a brown streak. Similar to cannel coal, it is, very fine-grained and homogeneous, and presents, a conchoidal fracture. Boghead occurs with humic, coals or forms independent seams that may grade, into oil shale and petroleum source rock as mineral matter content increases. Volatile matter content of boghead is very high, so that the material is, preferably used for “synthetic oil” production. On, heating in a retort, up to 800 litres of oil per tonne, may be recovered (cf. 7.7 “Oil Shale”)., Cannel coal, Cannel coal burns like a candle when ignited,, explaining its name given by miners. It is black
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474, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , with a greasy dull lustre and has a black streak., Often found in the upper part of humic coal seams,, it forms thick slab-like beds. Constituents are, mainly derived from vascular plants and comprise, uniformly fine-grained particles such as spores,, resins, plant debris, black carbon and algae, (Hutton & Hower 1999). Siderite lenses and bands, are frequent in cannel coal and beds may pass into, massive siderite. Cannel coal occurs in all major, coal fields of the world. Jet (gagatite) is similar to, cannel coal and both can be worked into black, jewellry and ornamental articles. However, jet, originates from tree logs that were embedded in, bituminous shale (e.g. in the Jurassic beds of Yorkshire). Schungites are Palaeoproterozoic equivalents of sapropelic coal, formed from mats of, Cyanobacteria and of exuded bitumen (Medwedev, et al. 2001, Melezhik et al. 2004)., Liptobiolites, Liptobiolites are concentrations of plant constituents that resist aerobic weathering. In Eocene, lignite seams in Germany, bands of yellow, light, brown or pink pyropissite are common. Pyropissite is essentially composed of pollen, wax, resin, and leaf cuticles in variable ratios. Often, the, strata are more prevalent near margins of coal, basins, yet formed as aquatic sediments. Pyropissite can be ignited similar to cannel coal. Amber is, re-sedimented fossil resin, occurring in large, lumps that the plants produced to seal and protect, wounds and to discourage herbivores. Earliest, amber dates from the Carboniferous (Grimaldi, 2009). In the Palaeocene-Eocene, major amber, deposits were formed in deltas and shallow water, along the southeastern shore of the Baltic Sea, (Kharin et al. 2004). Baltic amber was already, highly valued by Neolithic people 13,000 years, ago and traded throughout Europe. Amber is used, for the production of ornaments, special lacquers, and chemicals., Saline coals, Saline coals contain elevated gypsum, NaCl and, KCl, because of contact with brines (Yudovich &, Ketris 2006a). In Northern Germany, chloride-, , sulphate brines are common in the vicinity of, subcropping Permian salt formations (Figure/, Plate 5.18). Where brines infiltrate lignite and, sub-bituminous coal, severe problems arise for, boiler operations, because alkalis, silica and iron, tend to combine in fouling surfaces of steam, generators in power plants. Leaching of salt deposits is also the reason for the salinity of underground water pumped from black coal mines, near Cracow, Poland; coal quality, however, is, hardly affected because the small porosity of bituminous coal inhibits absorption of salt. Saline, mine waters can also be a result of aridity, as in, Australia (Côte et al. 2007)., This overview shows that coals are described, according to the three parameters:, 1 rank (grade of diagenesis);, 2 type of organic precursor; and, 3 content of inorganic substance., Traded coal is often a mixture from different, seams and deposits. Its characterization is based, on technological and commercial parameters (calorific power, water content, volatile content, coking properties, etc.) that are translated into, numeric codes (Gayer & Pesek 1997, Thomas, 2002). These codes are shorthand descriptions of, quality and possible use., 6.1.2 Petrography of coal, Most coals are very heterogeneous, both macroscopically and under the microscope. This is aggravated by the fact that organic constituents exhibit, a continuously changing chemical composition as, rank increases, very different from the stable properties of minerals. Because of these impediments,, it took more than 100 years to develop an internationally recognized system of coal petrography., Its base is the Stopes-Heerlen System, which was, first presented by Marie C. Stopes at Heerlen,, Netherlands in 1935., A full description of the composition of a “coal, rock” can only be carried out with a microscope, (Taylor et al. 1998). Yet, macroscopic features are, useful for a preliminary approach, similar to the, field description of ordinary rocks. In this way,, mine exposures or drill cores of coal are examined, and the occurring coal varieties (“lithotypes”) and
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COAL CHAPTER 6, , interbedded non-coal rocks are recorded in a plyby-ply manner. Under the microscope with a 25- to, 50-fold magnification, the organic constituents of, coal (“macerals”) are determined. The microscopic quantification of different macerals making, up coal layers (“microlithotypes”) provides a precise characterization of coal., , 475, , coal, surplus oxygen remains in the atmosphere, (Garrels et al. 1976, Falkowski & Isozaki 2008)., Documentation of macropetrographical analysis is commonly presented in the form of seam, sections that are valuable tools for investigating, lateral facies variations. Brightness logs (ply-byply % of vitrain) are used in exploration to characterize the coking potential of individual seams., , Lithotypes, Lithotypes are layers in coal that can be macroscopically distinguished. Bituminous coal always, consists of thin bright and dull layers that trace, sedimentary bedding. Bright layers are vitrain, dull, ones durain. Vitrain is more brittle than durain and, narrowly jointed by cleats. Thick vitrain displays, conchoidal fractures. Parting planes of coal are, often covered with numerous soft and friable fragments of coal with a silky lustre, resembling charcoal. This is fusain. Clarain consists of alternating, thin laminae of vitrain and durain., These four coal lithotypes form pure coal; many, seams, however, contain mineral matter as thin, strata of clastic sediment, in mixed coal/mineral, material, and as concretions or joint-fillings. Clay, and silt are the most common diluting materials;, with increasing content, impure coal (<20 vol. %, clay, D < 1.5), carbonaceous shale (20–60 vol. %, clay, D ¼ 1.5–2.0) and coaly shale (>60 vol. % clay,, D > 2.0) are distinguished., Lignite lithotypes cannot be described in the, same terms as bands of bituminous coal. Visible, wood remains (“xylite”) are often present and, these sections of a seam are called xylite-rich coal., Stratified or unstratified detrital humic groundmass is matrix coal. Charcoal horizons consist of, charcoal-rich coal, and coal containing clastic, minerals, carbonates and sulphides is mineralrich coal. Colour and degree of gelification can be, used for a subdivision into lithotype varieties (e.g., black gelified matrix coal). Charcoal (fusain) is, very common in Tertiary coals, possibly because, oxygen content in the atmosphere was higher, (23% in the Eocene) favouring the incidence of, peat fires. Remember that by photosynthesis, (eq. 6.1), vigorous plant growth causes increasing, oxygen concentration in the air parallel to depletion of CO2. When reduced carbon is buried, as in, , Macerals, Microscopic investigations of coal started with, thin sections. The introduction of polished sections and microscopy under incident light (Stach, 1935), later completed by the techniques of oil, immersion, luminescence and fluorescence, microscopy, was the foundation of modern coal, petrology (Taylor et al. 1998, Sykorova et al. 2005)., Macerals are organic coal constituents visible at, magnifications between 25 and 50 times and have, differing optical properties, hardness and shape, (Stopes 1935). The term was deliberately chosen, for its similarity to the word “minerals”. Macerals, may be recognizable parts of plants or products of, their degradation. Organic macerals are not crystalline substances. In bituminous coal, three, groups of macerals are distinguished: i) vitrinite, group; ii) liptinite group; and iii) inertinite group:, Vitrinite group macerals are mainly derived from, wood. Vitrinite is very bright and is either cellular, (Figure 6.5, telinite) or homogeneous (Figure 6.6,, collinite). Vitrinite is the purest and most constant, constituent of coal. This is the reason for its use as a, sensitive and precise means of coal rank determination. Huminites (Figure/Plate 6.7, Figure 6.8) are, those macerals of lignites and sub-bituminous coals, that would form vitrinite at higher rank., Liptinites (earlier called exinite) comprise macerals, that originate from plant matter, which resists humification, including spores, pollen, resins, waxes and, fats. Liptinite is common in dull coal bands. It is rich, in hydrogen and, together with hydrogen-rich, (“perhydrous”) vitrinite, is the main source of volatile, matter (and of natural gas and petroleum: Wilkins &, George 2002). Illuminated with UV or blue light,, liptinite demonstrates fluorescence. Cannel and, boghead coal are predominantly liptinitic. During
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476, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Figure 6.5 Microphotograph of, the coal maceral telinite (a, vitrinite; clearly recognizable cell, walls of more or less intact but, humified plant tissue) in, Pennsylvanian bituminous coal,, USA. Reflected light, oil, immersion. Courtesy of Maria, Mastalerz and Indiana Geological, Survey., , coalification, beyond coking coal, liptinites produce, much methane. The concurrent loss of hydrogen, leads to an appearance that approaches vitrinite. Liptinite group macerals derived from plants include, sporinite (Figure 6.9), cutinite (epidermis of leaves),, resinite and alginite. Fluorescence distinguishes the, , macerals fluorinite (derived from fats and oils), bituminite (decomposed algae, bacteria, proteins) and, exudatinite (diagenetically mobilized bitumen). Liptinites are indicators of wet conditions or even open, water in mires, so that elevated clay content of liptinite-rich coal is not unusual., , Figure 6.6, Microphotograph of the, coal maceral collotelinite, (vitrinite, more or less, homogenized) in, Pennsylvanian bituminous, coal, USA. Reflected light,, oil immersion; long side of, image 0.5 mm. Courtesy of, Maria Mastalerz and, Indiana Geological Survey.
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COAL CHAPTER 6, , 477, , Figure 6.7 (Plate 6.7), Microphotograph of the, coal maceral textinite (a, huminite; ungelified woody, tissue with intact botanical, cell structures) in Tertiary, lignite, Poland. Reflected, light, oil immersion; long, side of image 0.5 mm., Courtesy of Maria, Mastalerz and Indiana, Geological Survey., Inertinites are coal constituents whose chemical, composition changes little as rank increases and that, do not melt during coking. All inertinites have high, carbon and low hydrogen content and reflect strongly, , under incident light. Many inertinites originate from, precursors of vitrinite group macerals. Their notable, properties are due to partial oxidation during early, stages of coal formation (Moore et al. 1996). The most, , Figure 6.8 (Plate 6.8) Microphotograph of the coal maceral ulminite (a huminite; more or less gelified woody tissue) in, Tertiary lignite, Poland. Reflected light, oil immersion; long side of image 0.5 mm. Courtesy of Maria Mastalerz and, Indiana Geological Survey.
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478, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Figure 6.9 (Plate 6.9), Microphotograph of the, coal maceral sporinite, (liptinite; wax-coated fossil, spores and pollen) in, Pennsylvanian bituminous, coal, USA. Fluorescent, light; long side of image 1.0, mm. Courtesy of Maria, Mastalerz and Indiana, Geological Survey., frequent inertinitic component of coal is fusinite, (Figure 6.10). Under the microscope fusinite exhibits, the cellular structure of wood and its anatomic, details. Former cell lumina may be filled with humic, gel or mineral substance (clay, carbonate, pyrite, etc.)., Semifusinite is intermediate between fusinite and, vitrinite. Most fusinite is fossil charcoal produced by, forest fires that were ignited by lightning (Guo &, , Bustin 1998). The degree of carbonization correlates, with temperature, as illustrated by tree logs that were, externally carbonized but preserve a core of ordinary, vitrinite. Small rounded particles of inertinite are, called macrinite. Micrinite particles are very tiny and, originated probably as airborne soot or carbon black., In Tertiary brown coals, a very common inertinite,, besides fusinite, is sclerotinite that consists of round, , Figure 6.10, Microphotograph of the, coal maceral fusinite (an, inertinite; highly reflecting,, well-preserved cellular, structure inherited from, ligno-cellulosic cell walls), in Pennsylvanian, bituminous coal, USA., F-fusinite, C-collinite., Reflected light, oil, immersion; long side of, image 0.5 mm. Courtesy of, Maria Mastalerz and, Indiana Geological Survey.
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COAL CHAPTER 6, , or oval, brightly reflecting particles with one or several cavities. Sclerotia are remains of mycelia, spores, and other fungal tissues., , Microlithotypes, Microlithotypes are defined by common associations of macerals. They can be mono-, bi- and trimaceralic, for example vitrite, vitrinertite and, vitrinertoliptite (Taylor et al. 1998). Microlithotypes are determined with the microscope and, recorded in seam sections. They are useful for an, interpretation of conditions in the former swamp., Diessel (1986) proposed that the degree of decomposition of plant tissue (TPI ¼ tissue preservation, index measured by the ratio humotelinite/humodetrinite) and the relative water level vs. peat, oxidation (GI ¼ groundwater index, the huminite/inertinite ratio) can be used for the determination of facies and plant communities. Controls, using peat of known depositional facies showed, that pH and the microbial decomposition of peat, are additional obligatory parameters of facies, interpretations (Dehmer 1995). Coal petrography, alone is probably not sufficient for identification, of swamp type and facies (Moore & Shearer 2003)., , 6.1.3 The chemical composition of coal, Coals are extremely heterogeneous mixtures of, various organic substances and of inorganic material, including water, inorganic colloids and, minerals. The chemistry of the organic matter, depends on the fraction of different macerals (the, coal type) and the degree of diagenetic changes, (rank, or maturity of coal). Full chemical analysis, of coal is little used in practice but its complex, organic compounds are determined for scientific, elucidation (Bustin et al. 1997; Hatcher & Clifford, 1997). In chemical terms, coal consists of aromatic, nuclei, cross-linking bridges and adhering functional groups containing oxygen, sulphur and, nitrogen., In practice, large numbers of coal samples are, analysed by standard procedures that include proximate and ultimate analysis. This is sufficient to, control the quality from underground grade control to the final shipping product. Fewer samples, , 479, , will be analysed for data, such as potential atmospheric pollutants in the coal, the chemical composition of ash, the grain size distribution and, coking properties. Proximate analysis includes the, determination of moisture, volatile matter (fixed, carbon), ash content and calorific value (heat, energy per unit mass). Ultimate analysis serves, to reveal the content of the major elements C, O,, H, N and S in coal. Online X-ray fluorescence, analysis of lignite and coal destined for power, plants targets mainly elements that contribute to, slagging and fouling of boilers (Si, Ca, Fe, K, Al, S,, Na). The data are used for overall coal quality, management, blending and automatic alarms, if, critical parameters are exceeded., Organic precursors and chemical, compounds of coal, Cellulose (C6H10O5)n is the major component of, higher plants. By means of the chlorophyll, plants, synthesize carbohydrates such as cellulose from, carbon dioxide in the air plus water (eq. 6.1). In, wood and bark, cellulose is reinforced by lignin., Lignins are a group of complex organic polymers, containing aromatic nuclei with side chains., Molecular weight of lignin macromolecules surpasses 10,000 units. They are more resistant, against microbial attack compared to cellulose., Different plants and even parts of plants have, diverse lignin types and concentrations (5–45%, in dry substance). Plants use wax, cutine (leaf, surfaces) and suberine (bark) as a protection of the, surface of vegetative organs. Sugars (glucose, C6H12O6), starch (polysaccharide carbohydrates), and proteins are rapidly decomposed by microbes, in the peat. Much of the nitrogen in coal is inherited from protein in plants and microbes., Simplified formula of cellulose production by photosynthesis – The fundamental reaction of life:, 6CO2 þ 5H2 O ! C6 H10 O5 þ 6O2, , ð6:1Þ, , Lignin and cellulose are precursors of huminite and, vitrinite. Cellulose and lignin are detectable in, xylite and humotelinite of lignites but not in subbituminous coal. They are transformed by humification into humic substances (K€, ogel-Knabner 1993)
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480, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , that form the bulk of coal. Humic substances are, heterogeneous mixtures of organic compounds, that are divided into fulvic acid, humic acid, and humins. They are colloidal dark-coloured, complex substances, with a high molecular, weight built from the elements carbon, oxygen,, hydrogen and nitrogen. All contain an aromatic, nucleus and various functional groups. When, fulvic acid transforms into humic acid, both oxygen content and solubility in KOH decrease;, humins are insoluble:, Yellow to red-brown hydrophilic fulvic acids occur in, soil, peat and wetland water. They have a molecular, weight of 400–1500 and dissolve in both acidic and, alkaline water. Humic acids are brown to black,, hydrophobic and have a higher molecular weight., Soluble in alkaline water, they flocculate under acidic, conditions. Both organic acids form salts (fulvates,, humates) with cations including Na, Ca, Mg, Fe and, Al. Black humins are the product of ageing of humic, acids and their salts. Among the humates, dopplerite, (the calcium salt of humic acid) is relatively common., It is a gel-like substance filling cavities in lignite, seams. It shrinks and cracks on drying, and turns into, a brilliant black material with conchoidal fracture., Generally, the early transformation of peat by humification is followed by a stage of more or less pervasive, biochemical gelification., , Spores, pollen, resins, waxes and fats are lipids, formed by higher plants. Lipids change little during humification and biochemical gelification., Severe alteration begins only with the onset of, maturation to bituminous coal. In brown coal, part, of the lipids can be determined under the microscope, but much remains undetected and is only, revealed by organic solvent extraction (e.g. wax, by benzene). Chlorophyll is normally destroyed, before peatification. Macroscopically visible chlorophyll in lignites is very rare; one famous locality, was Geiselthal in Germany (Figure/Plate 5.19)., Humins evolve during coalification by increasing aromatization. As functional groups, bridges, and side-chains of nuclei are broken down, and, islands of well-ordered structural elements, (“crystallites”) grow larger. During coal maturation, aromatic clusters are increasingly arranged, into flat sheets that resemble the graphite lattice., , Volatile matter, fixed carbon and chemical, composition, Run-of-mine coal contains water and minerals, (ash). Data on moisture and ash content allow the, calculation of the fraction of pure coal (dry, ashfree ¼ d.a.f., or more precisely, dry, mineral matter-free ¼ d.m.m.f.) in a sample. Data on coal’s, chemical composition and yield of volatile matter, are of both practical and scientific use., Heating a water-free coal sample under exclusion of air induces processes of pyrolysis and carbonization. This results in release of flammable, gas (“town gas” as opposed to natural gas), liquids, and vapours that condense to tar. Because high, temperatures are attained, liberation of volatiles, from minerals (e.g. water from clay, CO2 from, carbonate) must be accounted for. The term, “volatile matter” (VM) describes the mass loss of, the sample due to abstraction of organic and other, volatiles. Volatile matter content of coal decrease, with increasing rank (Figure 6.4), whereas the, share of non-volatile (or “fixed”) carbon increases., Fixed carbon is therefore as useful as volatile, matter for measuring coal rank., The elements making up coal are predominantly inherited from the peat-forming plants., Main elements are carbon, hydrogen and oxygen,, with nitrogen and sulphur as minor components., Pure coal of high-volatile bituminous A-rank with, 84% C has an average chemical composition that, can be expressed by the “formula” C10H7O., The continuous enrichment of carbon and the, decrease of hydrogen (and oxygen) with increasing, rank have been known for a long time (Figure 6.11)., During lower rank coalification, oxygen, diminishes rapidly, because of the loss of CO2 and, H2O. A strong decrease of hydrogen occurs, between high-volatile bituminous A-rank coal, and anthracite. During this stage of coalification,, a large volume of methane (CH4) is released. The, resulting bend in the coal band is sometimes, termed a “jump of coalification”. In parallel with, chemical changes, several physical alterations are, also observed, such as an increase of reflection and, a decrease of liptinite fluorescence., Sulphur occurs in many forms in coal, including, massive, coarse, or very finely disseminated
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COAL CHAPTER 6, , Figure 6.11 An updated rendering of Seyler’s (1933), illustration of coal chemistry in carbon-hydrogen, space. The diagram displays the typical composition, of humic coals (the coal band), the increase of carbon, with higher rank, and the rapid drop of hydrogen, content at about the rank of coking (medium, volatile) coal (d.m.m.f. ¼ dry mineral matter free)., , % Hydrogen (d.m.m.f.), , 6.0, , pyrite, soluble sulphate, acid-volatile sulphides, (AVS) and organic sulphur. Only a small part of, organically-bound sulphur is inherited from plants,, which host the element in the form of aminoacids, (C-S-form) or as ester-sulphate (C-OSO3; e.g. in, polysaccharides, phenol, etc.). In coals that are Srich, most sulphur is derived from bacterially, reduced sulphate. Pyritized bacterial colonies in, coal have been demonstrated (Southam et al. 2001)., Sulphate is commonly imported: i) by flooding of, the peat mire with seawater; and ii) by sulphatecarrying groundwater that seeps into the peat from, the surroundings, either during or after formation, of the peat. In the first case, sulphur is concentrated, in the upper part of the seam, whereas in the, second, sulphate infiltrates from the footwall., Sulphur content can be exceptionally high in, coal seams with carbonate-rich host rocks. This, setting induces alkaline conditions in the mire (e.g., in the Everglades, Florida, USA, up to pH 8.6)., Seawater flooding has a similar effect. Higher pH, favours sulphate-reducing microbial activity and, the resulting H2S will combine with iron or with, organic substances in the peat. Alkaline swamps, may even conserve calcareous fossils and skeletons, of vertebrates (Geiseltal is famous for its Eocene, fauna; Figure/Plate 5.19). In the Sch€, oningen lignite, pit (Figure/Plate 5.18), numerous boulder-like calcite concretions occurred in the seam where its, footwall was formed by karstified Triassic limestone. Extraction of the lignite was severely, impeded. This problem had not been recognized, when mining reserves were defined by drilling., , Medium volatile, bituminous, , 481, , High volatile bituminous, coal A - C, , Low vol., bit., 5.0, , A, , B, , C, , Anthracite, 4.0, , 94, , 90, , 86, , 82, , 78, , % Carbon (d.m.m.f.), , Seams that were inundated with seawater are, characterized by elevated sulphur, ash and nitrogen content. Coals that are derived from unusually, acidic peat (with a pH as low as 3.3 compared with, average peat of pH 4.8–6.5) contain little sulphur., Acidic swamps may be indicated by kaolinized, footwall rocks of coal seams. Exceptionally low, sulphur content of 0.06% (air-dried) has been, reported for the high-volatile bituminous B-coal, from the Tanjung mine in Borneo. The product is, sold under the trade name of “Envirocoal”;, resources are estimated at >1000 Mt:, Sulphur in coal is undesirable because in boilers it is, oxidized to SO2 and enters the flue gas. Likewise,, sulphur in coking coal diminishes product quality., Mitigation of inorganic sulphur is possible by minesite coal processing (e.g. removal of pyrite) but is not, feasible for organically-bound sulphur. Several technical solutions for withholding sulphur from the, atmosphere are applied in power stations, although, not without repercussions on the price of electricity., , Nitrogen is foremost inherited from proteins in, the peat. During coalification, it enters the coal, macromolecules. Coking or gasification of coal, generates ammonia that can be a valuable byproduct. For burning, however, low-nitrogen coal, is preferred, because elimination technologies, such as reduction of NOx to N0 or NH4 are costly., Nitrogen is a biogenic element of coal. Plants, contain a number of trace elements, including Ca,, N, K, P, S, Mg and Fe, but also Na, Si, Al, Mn, B, Ba,, Sr, Zn and Cu. Several of these elements are
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482, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , selectively enriched in biomass and the resulting, coal. The enrichment may be quantified by the, biological accumulation coefficient (eq. 6.2)., Biological accumulation coefficient Bsas a measure of preferential enrichment of elements in, organic substance:, Bs ¼, , % in ash of organic substance, % Clarke crustal average, , ð6:2Þ, , Bs-values >10 are commonly determined for, P, S, Se, Cl, Br and I, values between 1 and 10, for Ca, Na, K, Mg, Sr, Zn and B, whereas most, other elements are comparatively depleted (Bs <1)., World average chlorine content of coal amounts to, 340 40 (hard coal) and 120 20 ppm (brown coal:, Yudovich & Ketris 2006a). The world average for, ash is 1435 ppm Cl compared to a Clarke of, 126 ppm. The selenium average is 1.6 0.1 ppm, and 1.0 0.15 ppm, respectively for hard and, brown coal, compared with a Clarke value of, 0.05 ppm (Yudovich & Ketris 2006b). Note that, the enrichment is not literally “biological” but, represents accumulation from the live plants, (“inherent elements”) to late diagenesis (Swaine, 1990). Furthermore, comparison with average, shale should be more relevant than with Clarke, values, which include basalts (Reimann &, de Caritat 1998). Of course, the geogenic background of the watershed must be considered in, the interpretations., Accessory metal content in coal has little economic significance, mainly because of low and, fluctuating in-seam content. In power stations,, however, some metals may be enriched in the ash, to extractable grades (both bottom and fly ash)., Metals that were occasionally produced from coal, ash include vanadium, germanium and uranium., Coals with anomalous germanium content occur, in America, Australia, China and Russia (Seredin, et al. 2006). Metalliferous (Seredin & Finkelman, 2008) and especially uraniferous coals are ubiquitous. In coal, uranium is adsorbed on organic, matter. This may be due to: i) syngenetic enrichment from waters draining the former hinterland, of the peat swamps; or ii) later, epigenetic input., The first has an equivalent in today’s uraniferous, mires in Sweden, whereas the second is typically, , due to the later passage of uranium-bearing, groundwater (e.g. Wackersdorf, Figure 2.43). REE, are enriched to 300–1000 ppm in giant coal deposits of the Russian Far East, possibly sourced from, nearby volcanic rocks (Seredin 1996)., Trace elements in coal are determined by sedimentary and diagenetic conditions. Their distribution is usually irregular, so they are rarely useful, for identifying specific seams in multi-seam, deposits. An exception may be the ratio Al/La-Sc, (Chyi & Medlin 1996). Several trace elements are, harmful, either for power station operation, (Na, Cl, etc.) or for the environment (e.g. arsenic,, mercury, radioactive elements). Therefore, a thorough geochemical investigation of all coal deposits is mandatory. This must include the, determination of the preferred carrier (mineral or, maceral) of significant elements and of speciation., In many cases, ecologically sensitive elements are, concentrated in pyrite, kaolinite and illite, in, which case removal by coal processing may be, feasible (Palmer & Lyons 1997). The fate of, hazardous trace elements in power stations and, especially the level of stack emissions must be, monitored (Huggins & Goodarzi 2009). Anticipatory mitigation is required., Clearly, peat and coals are extremely efficient, geochemical barriers, due to processes that, include biochemical accumulation, reduction,, sorption and ion exchange. Import of elements, into swamps may occur by surface water, seawater, and groundwater. Seawater influence is indicated, by higher content of B, Sr, Mg and, of course, Cl,, Na, SO4, Ca, K and Br. Groundwater may contribute U, V, Mo and in case of contact with saline, brines or evaporite rocks, Na, K, Ca, Mg, Cl, B,, Sr, Br and SO4. Hydrothermal epigenetic metal, enrichment in lignites is rare (Seredin et al., 2006), whereas most hard coals exhibit traces of, mineralization by the passage of diagenetic, (basinal) fluids., Isotopic investigations of coal are mainly based, on the elements C, O, H and N. Isotopic composition is a function of coal age, geographical latitude,, mean annual temperature (reflected in the D/H, ratio), dominant plants (carbon isotopes), isotopic, character of precipitation including seasonal, variations (Jahren & Sternberg 2008) and coal
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COAL CHAPTER 6, , maturity and degassing history (Whiticar 1997)., Carbon in coal is clearly enriched in 12 C relative to, the source (atmospheric and mantle) carbon,, because photosynthesis strongly discriminates, against 13 CO2 . The Middle Miocene Yallourn, seam lignites (Latrobe Valley, Australia), for, example, have d13 C (PDB) values from 25.3 to, 27.3‰ (Holdgate et al. 2007). Cyclicity correlates, well with the deep-sea d13 C marine organic record., Contrary to earlier assumptions, the oxygen isotope ratio (d18 O) of cellulose is not a reliable proxy, of ambient temperature and relative humidity, (Helliker & Richter 2008). The source of nitrogen, may be discerned, because little fractionation, takes place when plants or microbes assimilate, nitrogen from air (d 15 NPlants from 3.1 to 0.4‰),, whereas uptake of nitrates enriches heavy nitrogen. The release of methane during coalification, initially mobilizes light carbon and hydrogen., Theoretically, this should translate into an enrichment of heavy isotopes in coal. However, depletion of 12 C during coalification cannot be, measured, because the seams are a giant reservoir, of carbon. Deuterium, however, is slightly, enriched (Schoell 1983). Rhenium-osmium ages, of pyrite may be used to investigate the diagenetic, evolution of coal deposits., Water in coal, Water is an essential component of brown coal., Lignites contain between 30 and 75 wt. % water, (Figure 6.4). This figure reveals why power stations, burning these low-grade “wet” coals are built near, the mines.The water content of bituminous coal and, anthraciteislow(from9–2.5%)andthisexplainswhy, they are shipped across the world’s oceans., Coal contains water in four possible forms:, 1 surface, or adventitious moisture;, 2 hygroscopic moisture in capillaries;, 3 adsorbed water; and, 4 mineral moisture., Drying higher rank coal in air removes surface, water. Hygroscopic and adsorbed water are termed, “inherent moisture” and are determined by heating to 105oC. Mineral moisture is mainly a component of clay in coal. Surface water content, reflects conditions of sampling, for example rain, , 483, , or drought. Yet coal samples are always hermetically sealed on site, because adventitious moisture, influences mass calculations and processing of, coal. Compared with inherent moisture, lignites, have little surface moisture. However, on drying in, air, lignite looses even hygroscopic and adsorbed, water and may crumble into low-value coal dust., Gas in coal, Methane is the most common gas occluded or, adsorbed in coal seams. It may be accompanied, by minor fractions of ethane and propane, and by, traces of hydrogen and helium. Some coal seams, contain large amounts of carbon dioxide. Whereas, the source of CO2 is usually external to the coal, measures, for example volcanogenic, methane is, produced during coalification within the seam., Abiotic methane production from liptinites, begins with the coalification jump from sub-bituminous coal B to A rank. Medium volatile bituminous (coking) coals have the highest content, (10–30 m3 gas per tonne of coal). Methane mixed, with 5–14% air is highly explosive (“fire damp”)., The largest mass of gas is formed from humic, substances during continuing maturation to, anthracite. It is estimated that 1 kg of coal produces 200 litres methane during maturation, from high volatile bituminous coal B to anthracite., Because country rocks of higher-grade bituminous, coals are increasingly permeable as jointing intensifies, 90–95% of methane produced migrates, away from the seams. This is one reason why the, actual gas content of coal in situ cannot be predicted (Bustin & Clarkson 1999). Only the ashcontent correlates (negatively) with gas in coal,, because gas is almost exclusively hosted by, organic material. If the migrating gas encounters, a trap capped by impermeable rocks (in northern, Germany, usually Zechstein salt rock, cf., Chapter 4), important and even giant gas deposits, can form (e.g. Groningen, Netherlands). Most lignites and sub-bituminous (C and B) coals are, methane-free. Yet, underground extraction of, brown coals is not without its own dangerous, gases – poisonous CO and asphyxiant CO2 can, form by slow oxidation in parts of the mine, with stagnant air. CO2 collects in hollows, as it is
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484, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , 1.53 times heavier than air. Increasingly, methane, gas released from coal mines is used for heating,, power generation, as a vehicle fuel, or for chemical, manufacturing. Mine methane that cannot be used, should be flared off, as conversion to CO2 lowers, the greenhouse effect (Gayer & Harris 1996)., Only a fraction of the carbon dioxide adsorbed, in coal seams is derived from coalification. If, high content is encountered, the source is usually, igneous or otherwise endogenous. Faults and, permeable sandstones have been identified as conductors. In the past, several mines suffered catastrophic carbon dioxide blow-outs, filling the, mine openings not only with the asphyxiating gas, but also with coal dust. The precondition for gasdriven coal outbursts are high in-situ gas pressures, in the coal. Coal outbursts are violent and spontaneous ejections of powdered coal and gas from the, working coal face (Guan et al. 2009). Coal dust in, air is by itself explosive and very destructive but, methane is often involved. Seams with a clayey, hanging wall are prone to trap gas and to impede, natural degassing. In that case, a slow advance of, extraction, deliberate fracturing and loosening of, the rock mass, and drainage boreholes may, decrease gas pressures. Ordinary degassing of CH4, and CO2 is not dramatic. Gas blowers occur along, joints, or gas bubbles rise in water channels along, mine tunnels. Coal mines and oil or gas fields tend, to be well separated, but in a few cases, a near, neighbourhood did cause accidents (Nottinghamshire, UK, Stevenson 1999)., Ash and minerals in coal, It is important to understand the difference, between ash and minerals in coal (Ward 2002)., Ash represents incombustible matter in coal measured by controlled oxidation (burning) of a sample, at 700–850 C (“high-T ash”). This causes loss of, CO2 from carbonate, SO2 from sulphides and H2O, from clay. The mass of ash is less than the original, mass of minerals in coal. Because coal ash consists, essentially of oxides, its chemical composition is, investigated with analytical methods used for, silicate rock analysis. Calculation of norm, percentages of minerals in coal from high-T, ash data is very imprecise. It is recommended to, , determine minerals directly by appropriate methods after careful drying and oxidation at low temperature (150 C: “low-T ash”)., Ash of run-of-mine coal has various sources:, . wall rocks extracted with coal;, . minerals that were transported into the swamp,, diluting the organic material (e.g. clastic or volcanic sediments);, . diagenetic minerals introduced after sedimentation;, . salts dissolved in pore water; and, . the inherent biogenic mineral matter content of, plants (e.g. silica of equisetum). Matrix coal (of, lignite) or dull, bituminous coal contains more, ash than bright coal:, Ash content and calorific value of coals are negatively, correlated. Generally, high ash coal is economically, less desirable and is a bigger environmental burden. If, possible, ash content of run-of-mine coal is reduced, by one of three technologies: i) Processing in a dense, liquid medium washing plant; ii) in heavy media, cyclones; or iii) in jigs (separating particles of different, density by pulsations immersed in water). All methods rely on the increase of coal density as a function of, ash content. Because of the increasing need to manage, scarce water resources (e.g. in Australia), novel dry, processing methods for upgrading coal are employed, wherever possible., , Minerals in bituminous coal include silicates,, carbonates, sulphides, sulphates, phosphates and, others. Apart from minerals, immature lignites, may contain amorphous and semi-ordered substances, such as allophane instead of kaolinite., Quartz is very common in coal, mostly of detrital origin, but also as a chemical precipitate such, as silicified tree logs, which occur in many lignites. Kaolinite is the most frequent clay mineral, in coal, both as clastic sediment and as an authigenic and diagenetic mineral. Other clay minerals, that are not rare in coal include montmorillonite,, illite and mixed-layer clays. Illites in anthracite, are remarkable because of high content (45–80, mole %) of NH4 instead of K, due to pronounced, liberation of nitrogen in this coalification stage, (Daniels et al. 1994). Coal seams formed in volcanic surroundings may have high zeolite content, (e.g. Beypasari, Anatolia: Querol et al. 1997)., Pyrite (and other sulphides including sphalerite,
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COAL CHAPTER 6, , marcasite, and galena) can be found filling cell, lumina, or as framboids, idiomorphic crystals,, small bedding-parallel lenses, thin coating of joints, and in cross-cutting veins. All this indicates that, sulphide formation starts with early diagenetic, microbial sulphate reduction (Southam et al., 2001) and continues into late diagenetic stages, with inflow of basinal hydrothermal fluids. Carbonates in coal comprise siderite, ankerite, calcite, and dolomite. Siderite and dolomite form syngenetic layers or concretionary horizons in coal, seams. The preservation of uncompressed cells in, concretions is due to very early lithification. Carbonates in cleats (joints) of coal must have been, introduced during late diagenesis. Sulphates in, coal are products of natural pyrite oxidation by, meteoric waters. Some may form by exposure to, humid mine air. Exceptions are sulphates in, saline coal that are derived from invading brines., Authigenic apatite and Sr, Ba and Ca aluminophosphates are trace minerals in all coals (Ward et al., 1996)., Minerals in coal influence the feasibility of, exploiting the coal (Ward 2002). Some minerals, may cause geotechnical problems, for example, montmorillonite clay in opencut mining in a, high-precipitation landscape. Processing, transport, storage, combustion, coking and coal liquefaction can all be affected. Pyrite is critical,, because its oxidation induces acidity from the, mine through processing to coal and waste rock, tips. Also, pyrite is a common carrier of undesirable trace elements such as As, Hg and Sb. In the, boiler, the fusion temperature of the ash is very, important. Most combustion chambers cannot, deal with low-melting ash due to higher content, of montmorillonite, chlorite, carbonates, sulphates and pyrite. As a radical solution of the, problems caused by unfavourable minerals in, coal, acid-alkaline leaching has been proposed. Its, product is nearly pure organic matter (UCC ¼ ultra, clean coal). Note that fly ash and slag from largescale burning of coal can be useful by-products, that find application in producing cement, concrete and ceramics, as a source of alkalinity in, environmental technology, as a road construction, material and as a self-hardening fill in underground mines., , 485, , Physical properties, The density of coal increases with both rank and, ash content. Lignites have a density of 1.2 (range, 1.1–1.25), bituminous coals 1.35 (1.2–1.5) and, anthracites 1.5 (1.34–1.8) g/cm3. As a rule, coals, are less dense than clastic country rocks at the, same level of diagenesis (lignite-associated sand, 1.8 and silty clay 2.0; sandstone hosting bituminous coal 2.0–2.4, shale 2.0–2.5 g/cm3). The density of coal of a specific rank correlates positively, with ash content. The density difference between, low-ash and impure coal is the base for coal processing methods that strive to reduce the percentage of pyrite, country rock material and impure, coal., The hardness of brown coals increases with, lower water content. Bituminous coals are hard, but brittle and this may cause excessive comminution by handling. Generally, markets prefer, lumpy coal to fines. Grindability of vitrain is, higher than that of durain, so that vitrinitic coals, are more susceptible to fining., Optical translucence of coal decreases with, higher rank, whereas reflectance and anisotropy, increase. This is caused by more pervasive ordering of aromatic rings and clusters. Reflectance is, determined quantitatively under the microscope, using polished mounts of coal (Taylor et al. 1998)., The degree of reflectance [R] is determined as a, percentage of the incident light. Because reflectance correlates closely with coal rank and chemistry, its determination provides a very precise, measure of coalification (Figure 6.4). The same, technology is used to determine the maturation, level of dispersed organic particles in sediments, and due to its predictive capabilities, this method, has a central role in petroleum and natural gas, exploration., Under the microscope, coals are birefringent., With crossed Nicols, extinction positions are normally parallel and at right-angles to bedding planes., The same anisotropy is exhibited by reflectance., The maximal reflectance [Rmax] is found parallel to, bedding, and Rmin at 90 to sedimentary stratification. This is caused by the anisotropic molecular, structure of coal, which is imprinted by vertical, lithostatic stress sV and associated strain. Coal
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486, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , seams that were deformed in an orogenic stress, field with horizontal stress sH larger than sV, display oblique extinction angles (Petrascheck 1954)., The inherent heat energy is one of the first, parameters for assessing coal. It is expressed as, specific energy, or calorific value of coal, which, is the amount of heat available per unit mass, [kJ/kg]. The calorific value increases during coalification to the stage of medium volatile (coking), coal; further maturation causes no little change, (Figure 6.4). Water and ash reduce the available, energy. The first because of the energy needed to, heat and evaporate the moisture, the second, because of several endothermic reactions, but foremost the reduced amount of pure coal mass., Because of these relations, coal for power production is not traded per mass unit but for its inherent, energy yield., Cokeability, Coke is the vesicular and fused solid residue that, forms when coking coal is heated in the absence of, air to a temperature of several hundred to 1000 C., If non-coking coals are treated in the same way,, the residue is not coke but char (less porous or, unfused solid). Coke is a highly valued energy, carrier and reducing agent of the iron and steel, industry. Important properties of coke include, lumpiness, mechanical strength and durability., Apart from coke, saleable by-products of the process of carbonization are coal, or town gas (hydrogen, methane, other hydrocarbon gases, carbon, monoxide), crude benzole, coal tar and ammoniacal liquor (Ward 1984)., Cokeability depends on the coincidence of coal, becoming plastic (“fluid”) when heated coinciding, with volatile release forming gas bubbles. The, result is swelled and porous coke. The coking, process can be adapted to different coals by technological parameters, but rank and the ratio of, macerals control the feasibility. Coke is often, compared to concrete; inert components (the pebbles) are bound by a reactive matrix (cement)., Liptinite is an excellent swelling agent, vitrinite, a moderate one. Inertinite and most minerals, remain unchanged. Vitrinite determines plasticity, and gas release, because liptinite content is rarely, , above 5% of the total coal mass. In the case of high, inertinite content, it is preferred to have it mixed, with vitrinite, because this will stiffen the pore, walls of the coke. Generally, vitrinite content, (brightness) and its maturity are the main controls, on cokeability., Lignites and anthracites are not cokeable. However, many ingenious ways of making coke from, non-coking coal have been developed. One example is the blending of reactive material such as, coking coal or bitumen with inert additives, (anthracite, carbon black). The mass is then, formed into high-pressure briquettes. After coking, the briquettes are a valuable smokeless fuel., Synfuels (coal-to-liquids technology), With rising petroleum prices, worldwide production of liquid fuels from coal is rapidly expanding., One of the largest plants is still the facility at, Secunda, South Africa, that was built to cope with, an apartheid era fuel embargo. Unfortunately,, underground storage of the waste product CO2, was not planned and today the plant is the Earth’s, single largest point source of carbon dioxide emissions (20 Mt/y)., Synfuels production is carried out by several industrial, processes, basically variations of the Fischer-Tropsch, synthesis. The principle is gasification of coal in the, presence of steam and some oxygen at high temperature and pressure. Two separate gas streams are produced, “synthetic gas” with the composition CO þ, H2, and waste CO2. The synthetic gas (or syngas) is, then converted by hydrogenation in the presence of, catalysts into products such as gasoline, diesel fuel, jet, fuel or chemical feedstocks. Combined with underground CO2 sequestration, synfuels have a smaller, carbon footprint than petroleum-based fuels., , New developments in underground and in-situ, gasification of coal support hopes that this technology may finally reach profitability. Underground production of syngas eliminates all costs, of mining and transport. Syngas can be directly, burned to produce electricity, or converted to synfuels and to pure hydrogen by removal of CO2., Since nearly 50 years, the world’s only commercial, plant of underground gasification of coal (UGC)
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COAL CHAPTER 6, , combined with electricity generation operates in, Uzbekistan. In the Surat Basin, Australia, a demonstration plant produces diesel fuel from UGC, syngas. In Britain, in-situ gasification of deep coal, below the Firth of Forth, combined with carbon, dioxide capture and sequestration, was licensed in, 2009. It is claimed that the UGC process can be, applied to deep, high ash, conventionally unexploitable coal, which would immensely widen the, resource base of coal., , 6.2 PEAT, , FORMATION AND COAL DEPOSITS, , Even though coal is chemically and petrographically very different from other sediments, it is, an integral constituent of many sedimentary, sequences. Peat deposition occurs in diverse terrestrial, limnic and coastal settings. Sufficient, understanding of a coal deposit, including very, practical aspects, can only be acquired by a comprehensive study covering all aspects of the evolution of a coal basin., , 6.2.1 Types and dimensions of coal seams, Coal occurs in beds (strata) that are called seams., Many coal seams have a tabular shape characterized by a large areal extension but low thickness., Very thin strata that cannot be economically, extracted are referred to as coal bands, or stringers., , 487, , Two main types of coal seams can be discerned,, which impose quite different approaches, from, exploration to extraction:, . Concordant seams occur over large areas intercalated among strata of host sediments of the same, age; lateral changes in seam thickness and facies, are very gradual., . Discordant seams typically occur above a stratigraphical unconformity and constitute the base, of transgressive sediments; discordant seams display considerable and rapid lateral changes., Discordant seams originate by:, . regional subsidence of previously formed landscapes with depressions, which are semi-closed, and support a high groundwater table such as karst, and broad valleys;, . local subsidence of the surface on the downthrow side of normal (tensional) faults; or, . above salt rock bodies undergoing subrosion., The thickness of coal seams typically amounts, to a few metres (black coal) or a few tens of metres, (lignite). Exceptions include the Tertiary, (Eocene-Miocene) brown coal seams in the Latrobe Valley, Victoria, Australia, which are supposedly the thickest on Earth, with a maximum, of 320 m of coal (Holdgate et al. 2007, Barton, et al. 1993; Figure 6.12). The district’s youngest, (Middle Miocene Yallourn) seam alone is 100 m, thick and covers a stratigraphical record of, 1.3 My. A black coal seam at Fushun in China, reaches 200 m (Figure 6.13). However, extremely, thick coal strata are very rare (Volkov 2003)., , Ombrogenic peat domes, change location with time, , W, Hills, , Clayey, deposits, , Peat, , Lake, , Hills, , Coastal sand barrier, Sea, , Figure 6.12 Schematic reconstruction of the palaeogeographic setting of Miocene peat formation in the Latrobe, Valley, Victoria, Australia, in a wide tectonic graben (Holmes 1993). Ponding of water favourable for peat growth was, caused by a coastal sand barrier and high global sea levels. Recoverable resources of the District are estimated at, 37,000 Mt of earthy lignite with low ash contents and calorific value, and 60–70% moisture.
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488, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , NE, , SW, , Western open pit, , m, 0, , Tuff and, basalt, (Oligocene), , 400, , Precambrian, 800, , Cretaceous, 500 m, Clay and marlstone (Oligocene), , Coal, , Oil shale, , Figure 6.13 Geological section of the Palaeogene bituminous coal and oil shale deposit Fushun in Liaoning,, China, illustrating the extraordinary thickness of the seam (Volkov 2003). With permission from Springer, Science þ Business Media., , Compaction of peat, Like all sedimentary rocks, peat, lignite and coal, are compacted when covered by overburden rocks., In engineering geology, the consolidation theory, explains and describes mathematically the, increasing density and decreasing porosity, permeability, water content and thickness during compaction (Terzaghi et al. 1996). The same, phenomena affect peat but at shorter time-scales., In addition, a part of peat consolidation is a, function of synsedimentary biological activity., Therefore, most geological methods that aim to, determine the original thickness of peat transformed into a coal seam target the (total) peat, thickness at the time of deposition of the first, overburden rocks (Widera et al. 2007). Consolidation during peat formation (“self-consolidation”), is generally disregarded. Nadon (1998) vividly, argued that self-consolidation of peat explains, most of the total compaction and that this takes, place in the few uppermost metres of the peat, profile. Deep, mature peat is assumed to have, compacted to 0.2–0.3 m, compared to near-surface, material 1 m thick. The main means of measuring, consolidation in buried peat or coal are lenses of, clastic sediments of the same age, either on top of, the down-warped coal surface or interbedded with, it (Widera et al. 2007, Nadon 1998). Of course, for a, precise determination of the coefficient of coal, , consolidation, the compaction of the clastic sediments should also be determined. Because it is, much smaller than that of organic substance, it is, often disregarded. If measurements are impossible, the traditional rule of thumb may be helpful:, Six metres of peat consolidate to 3 m of lignite and, to 1 m of black coal., The areal extension of coal seams is typically, quite small for local discordant seams (a few km2),, whereas concordant seams reach very large dimensions. The Pennsylvanian (Late Carboniferous), Pitsburgh seam in the Appalachian foreland basin, (Virginia) occurs discontinuously over an area of, 30,000 km2. It is historically one of the most, important sources of coal in the United States, with, an exploitable area of 5600 km2 (Cross 1993). The, spread of the Katharina seam in northern Germany, (210 50 km) is comparable. In the Tertiary lignite, district of Saxony and Thuringia (Germany), the, main seam is developed over 750 km2., , 6.2.2 Concordant and discordant clastic, sediments in coal seams, Thin strata of clastic or volcanogenic sediments in, coal seams (dirt bands, partings, tonstein) are quite, frequent. They divide the seam into several coal, sub-layers that are called plies. Non-coal bands, display a thickness from 1 to more than 50 cm.
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COAL CHAPTER 6, , Dirt bands are an economic disadvantage, because, the material dilutes run-of-mine coal. Finegrained clay and silt predominate, whereas bands, of sand, carbonates and volcanic tuff are less frequent. In black coal, sideritic ironstones and even, siderite oolites have been reported. In many cases,, certain bands form recognizable key horizons that, assist coal seam correlation. In that respect, tonstein bands (kaolinized pyroclastic tuffs) can be, useful marker beds, because they often possess, specific properties (e.g. crystals, glass shards, pumice), which designate particular beds. In addition,, they contain trace minerals such as zircon and, sanidine that allow precise radiometric dating., Thick kaolinitic partings may be a source of valuable fire clay. Bentonitic bands are rare in coal, seams, because of the generally acidic environment of peat-forming wetlands., The lateral evolution of coal thickness, number, of plies and partings is depicted by seam sections,, which are positioned on mine maps (Figure 16.6)., , 489, , The resulting seam facies maps reveal genetic and, practical information, such as estimates of the, likely dilution of coal in reserve blocks. Partings, may be very useful when thick seams are to be, exploited in benches (slices), if they can be utilized, to delimit roof and floor of the slice. Dirt bands, may laterally thicken to the extent that a seam is, split in two or more unexploitable coal stringers., Clastic strata in coal implicate shallow lakes, and inflow of sediment-laden water into the, swamp. Thick coal seams form when subsidence, is such that profuse plant growth is maintained., For optimum conditions, groundwater levels, should oscillate little about a mean, just covering, the peat surface. In the case of higher subsidence,, the swamp surface and with it the plants drown, in open water. Clastic sediment deposition is, the consequence, especially towards basin centres, where seams tend to split by increasing thickness, of dirt bands (Figure 6.14 and Figure 6.15). Mineral, matter content of the coal (ash) rises in parallel, NE, , SW, , Kohlgrube - Wald, 630, , Gebetsleithen South, 633.96, , Heisslerfeld (Central), 620.55, Margarethenfeld West, 619, , wL, , wL, , MS, 619.9/10.1, , MS, US, , US, , US, , US, , wL, , MS, , 625.15/8.81, , 619, , Hanging wall elevation (m above s.l.), Sand, quicksand, , LS, 607/13.55, , Clay, with sand and silt, Clay plastic, , MS, LS, 599/20, , Clay and silt, varved, Coal (lumpy, wood-rich lignite), 599/20 Footwall elevation/ Total thickness (m), , Figure 6.14 Part of the seam facies map of a lignite basin. The sections indicate a more limnic centre in the southwest, and dryer conditions in the northeast (Hausruck, Austria; Pohl 1968). Thickness of seams (LS ¼ Lower, MS ¼ Middle, and US ¼ Upper Seam) and facies of clastic sediments are clearly a function of the relative altitude. wL is a thin, kaolinitic marker bed in the Upper Seam (volcanic tuff).
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490, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , (m) above sea level, , 800 SW, , NE, , Göblberg, Hofberg, , 700, , Coarse gravels, (Late Miocene), , Margarethenfeld, , Heisslerfeld, , 600, 500, , Marine marl (Early Miocene), , Hausruck Lignite Formation, (Late Miocene), , 1 km, , Figure 6.15 Geological section of the southwestern part of the Hausruck lignite district, Austria (Pohl 1968). The, drawing uses different vertical and horizontal scales in order to single out the thin Hausruck Lignite Formation, (seams in black). Observe that its thickness is greater in basin centres than at higher elevations., , until content surpasses acceptable levels. Dry, swamp areas (deep groundwater levels) and, basin-marginal positions of the swamp display, increasing oxidation and microbial destruction of, plant matter, which thins the coal seam to a band, resembling forest floor litter. Swamp margins may, be associated with clastic fans built up by creeks,, which reach out into the seam in the form of sand, and gravel beds., The different facies zones of a coal swamp are, more profoundly distinguished by the inclusion of, petrographical, chemical and palaeobotanical, methods. Two papers by Teichm€, uller 1958,, 1991) about the Lower Rhine Valley lignites in, Germany stimulated research and discussion of, swamp facies. In this broad graben basin, Oligocene and Early Miocene sediments pass from a, southern freshwater swamp to a northwestern, marine bay of the North Sea (similar to the setting, of the Latrobe Valley, Figure 6.12, Sch€afer et al., 2005). Swamps occupied a broad tectonically subsiding embayment, which was surrounded on, three sides by land. Teichm€, uller postulated three, main wetland facies zones: (1) a marginal broad, zone of forest swamp which graded into (2) wide, reed expanses and (3) shallow water with deposition of humic sapropels. Alternating dark and light, bands of woody lignite and assumed reed coal, make up most of the seam. Like other sedimentary, facies changes, this was considered as the vertical, expression of a horizontal oscillation of facies, zones (1) and (2). Moosbrugger et al. (1994), described in great detail one of the forest bands,, which is mainly composed of conifers of the Taxodiaceae and Cupressaceae families with a few, , interspersed palm trees. Teichm€, uller based her, interpretation on a comparison with the cypress, swamps of the southeastern coastal USA. Meanwhile, supposedly better equivalents were discovered in the swamp forests of Southeast Asia., Tropical swamps in Indonesia consist of woody, dark layers, which are the base for ombrogenic, peat domes made of pale, earthy and oxidized, material (Esterle & Ferm 1994). The same interpretation is used to explain colour-banding in the, giant Morwell lignite deposit (part of the Latrobe, district, Figure 6.12), which is interpreted to, reflect orbitally forced climate oscillations (Large, et al. 2004)., In the Hausruck lignites, nearly all coal is characterized by a preponderance of Sequoia wood,, including giant tree trunks and numerous forest, fire horizons (Pohl 1968). Near the basin margin in, the northeast, the lignite seam is quite thin and, displays high ash content. Towards the south, coal, thickness increases until it reaches a maximum of, 7 m. The Hausruck Lignite Formation attains a, total thickness of 30 m in the southwestern depocentre and the seams contain increasing mineral, matter, both as ash and in dirt bands, until the, impure coal is not marketable. A peculiar varved, limnic clay above the Middle Seam is a useful, marker (Figure 6.14). Taken together, these observations imply a high and relatively dry setting in, the northeast that grades into a lower and mostly, limnic basin in the southwest. Geochemical, methods confirm this model (Bechtel et al. 2003)., Some bodies of clastic sediments are drastically, different from the thin clastic strata in coal seams,, by exhibiting a short lenticular shape (in cross-
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COAL CHAPTER 6, , section) but a great length in map view. The sediments include clay, sand and coal fragments., Cross-sections of these clastic bodies in coal can, be very complex, because their compaction is, small compared to peat. Contacts are often slickensided and fractured. These structures are, infilled channels of meandering creeks and rivers, in the swamp. Actually, vegetation is the clue for, stabilization of river banks and of meandering, single-thread channels (Tal & Paola 2007). In the, Pennsylvanian coal fields of Illinois, USA, a giant, swamp river is known with a width of 3–8 km and, a length of 275 km., It is important to distinguish clastic inclusions, formed during peat formation from clastic infill, deposited by streams, which cut into the seam, from a base in the hanging wall (incised channels,, or washouts). In Illinois, Middle Pennsylvanian, channels in coal measures were incised when a, glacial climate period induced low sea-levels and, seasonally dry climate (Falcon-Lang et al. 2009)., Very deep incisions were made by rivers flowing, beneath inland ice shields, which covered northern Europe in the Pleistocene. The subglacial, erosion was well exposed in open pits exploiting, Miocene lignites in the Lausitz District, (Germany). Fluviatile washouts are identified by, sharp erosive contacts instead of the transitional, character between peat (coal) and synsedimentary, channels. In the Hausruck District, washouts take, the form of round potholes (scours) of 30 m diameter and 10 m depth, which are filled with alluvial, gravel and sand resembling the overlying coarse, sediments of the large Late Miocene alluvial fan of, a large river originating in the Alps (Figure 6.15)., Some seams are cross-cut by clastic dykes,, which usually originate in a bed of sand or mud, that is present in the hanging wall. Arguably this is, an effect of synsedimentary earthquakes. Dykes,, swamp channel fill bodies and washouts are, always serious and costly challenges to mechanical coal extraction., 6.2.3 Peat formation environments, Coals originated in former wetlands of coastal and, inland settings (Greb & DiMichele 2006). The, formation of peat deposits requires that:, , 491, , a profuse growth of higher plants is possible;, dead plant material is covered by water so that, oxidation and microbial decomposition is, restricted; and, . that only a minimum of clastic sediment enters, the peatland., Most important is a suitable water level; if the, water is too deep, higher plants cannot survive. If, the swamp falls dry, peat and plant matter will be, rapidly decomposed. Also, a water level that is, favourable for the growth of higher plants will, indirectly limit introduction of clastic sediment., Both transgression and regression of the sea may, promote the formation of peatlands. In the first, case, the rising sea dams up coastal freshwater, so that swamps form. As the waterline slowly, migrates landwards, the swamps drown and peat, beds are covered by marine sediment (e.g. coastal, sand or shallow water carbonate mud). Many, Palaeozoic coal seams in Northern Europe and in, Eastern Australia (Herbert 1997), but also Tertiary, lignite deposits in the Mediterranean realm attest, to this course of events. Regression causes peat, formation when the retreating sea exposes wide, coastal flats where swamp forests thrive (e.g. the, giant Miocene lignite deposits of Lausitz, Eastern, Germany: Standke et al. 1993). In this case, marine, sediments are found below the seam that is covered by terrestrial deposits., Low-lying swamps (fens) develop in depressions, or vales of the land, such as alluvial interchannel, areas, moraine hills, constricted river bends, volcanic craters and in drowning karst dolines. Even, in such terrestrial surroundings, the water table, rise may be caused by eustatic increase of the sea, level. Shallow lakes first develop a stage of floating, peat that accretes on the lake margin. Next, a, luxuriant and diverse flora expands across the, former lake. Such low-lying swamps promote, minerotrophic and eutrophic conditions with a, high rate of organic production, but result in coal, with high ash and elevated sulphur content. In, contrast to ombrogenic bogs, fens are important, sources of CH4 and contribute to CH4 and CO2, fluctuations in the atmosphere. As greenhouse, gases, both are factors of climate variations., Ombrogenic, oligotrophic raised swamps (bogs), occur in regions of high precipitation. Raised bogs, ., .
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492, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , (a), Raised bog, , (b), Low swamp, , Lake, Coarse clastics, , Palaeozoic phyllite, , are independent of morphology and may even, grow on mountains, although with a small chance, of preservation. The transition from an original, low-lying swamp into a raised bog is often, observed (“domed peat deposit”, Figure 6.16). The, domes reach impressive dimensions. On the, coastal plains of Sarawak Island, PleistoceneHolocene peat domes attain 20 m thickness and, a surface of 1000 km2 (Staub & Esterle 1994)., Raised bog peat differs from low-lying swamp peat, by low ash and sulphur concentrations, acidic pH, and less decomposition. However, low concentration of limiting nutrients (Ca, P, K, N) may, severely curb productivity. Remember that six, major elements – H, C, N, O, S and P – are required, to build all biological macromolecules (Falkowski, et al. 2008). Coastal raised swamps of the Indonesian islands are most probably the modern equivalents of ancient low-sulphur and low-ash coal, (Cobb & Cecil 1993). In the Northern Hemisphere,, lignin-free sphagnum (a moss) is at present the, most important peat-producer of raised bogs., The majority of coals are autochtonous, derived, from plants that grew at, or near their present, location in the seam. The opposite, allochthonous, formation of coal from transported plants or peat is, quite rare. Upright trees, tree stumps and seat, rocks (root horizons in footwall sand or clay), are indicators for autochthonous coal. Stump, horizons in German lignites are ubiquitous, (Moosbrugger et al. 1994), and some stumps display over 1500 annual growth rings (Kurths et al., 1993). This implies coal formation periods with, very stable conditions and little incremental peat, deposition. Allochthonous seams are characterized by a high siliciclastic fraction, high ash,, , Clayey peat, , Figure 6.16 Simplified evolution of peat, formation in the Miocene coal basin at, Leoben, Austria, from an early, rather, unproductive stage of a low-lying swamp, (a) depositing high-ash peat to (b) a highly, productive raised bog (modified after, Gruber & Sachsenhofer 2001). Copyright, (2001) with permission from Elsevier., , rounded coal fragments, non-oriented driftwood, masses, rapidly varying seam thickness, discordant boundaries and cross-bedding within the, coal. Allochthonous origin is relatively more common in Gondwana coal (Begossi & Della Favera, 2002, Glasspool 2003). Of course, much plant, matter in all swamps is transported for short distances and may be called “parautochthonous”., This is especially obvious for the char fragments, of fire horizons, which form continuous laminae, of the lithotype fusain (Lamberson 1996)., The formation of swamps and extensive peat, deposits, and the preservation of peat and its transformation into coal, require a set of favourable, factors including climate, tectonics and, palaeogeography., Climate is an essential control on peat-forming, plant communities. Sufficient precipitation is the, basic precondition. It should not fall below, 40 mm/month, even in dry seasons. Monthly average temperatures should remain above 10 C,, because this is the minimum that supports lush, plant growth. Tropical, warm and humid forest, swamps are optimal for a high production rate of, plant substance, where trees grow within 7 to, 9 years to heights of 30 m. Growth in cooler, climates is much slower, with an estimated peat, increment of 0.5–1 mm/a compared to 1–4 mm/a, in the tropics (Falcon-Lang et al. 2009). Although, many major coal deposits originated in tropical, climate zones, significant coal resources formed, in temperate and even cool climates. The Late, Carboniferous (Pennsylvanian) coals of Europe,, North America and China were deposited in, swamp forests of Sigillaria and Lepidodendron at, tropical latitudes (Ziegler et al. 1997). These rain
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493, , COAL CHAPTER 6, , mation in northeastern Greenland just straddles, the end-Triassic extinction and displays a 12 C, spike that is attributed to sudden release of methane from gas hydrates on the ocean floor (Beerling, 2007). At the same time, foundering and rifting of, Pangaea started with the outpouring of basalts in, the Central Atlantic Magmatic Province (Marzoli, et al. 1999). Exceptionally fusinite-rich (40–60%), Mid-Cretaceous seams in British Columbia,, Canada, testify to unusually frequent swamp fires,, which were probably caused by dry climate periods (Lamberson et al. 1996). The iridium anomaly, and botanical extinction caused by the Chixculub, impact at the Cretaceous/Tertiary boundary were, confirmed in a coal seam of New Zealand (Vajda &, McLoughlin 2004)., Wetlands are significant factors of climate regulation, by preserving large masses of carbon in the, form of peat and coal (Beerling 2007). Today’s, wetlands cover a surface of 250–500 Mha and, contain 110–450 Gt Corg. In comparison, the mass, of C(CO2) in the atmosphere is 780 Gt. Clearly,, an important function of peat and coal sedimentation is abstraction of CO2 from the atmosphere, and concurrent cooling of climate. At the same, time, burial of organic matter is the cause of a, net oxidation of the atmosphere via photosynthesis (Falkowski & Isozaki 2008). When CO2 concentration in the atmosphere decreased to very, low levels in the Late Palaeozoic (Figure 6.17),, the Earth’s climate fell into glacial mode. The, , forests were ecologically quite heterogeneous, (DiMichele et al. 2007). Glacial climate cycles,, however, induced long phases of dry tropical vegetation, which are poorly represented in the fossil, record (Falcon-Lang et al. 2009). The giant Permian coal deposits of the Southern Hemisphere,, with their boreal Gangamopteris-Glossopteris, flora, formed during the same Permo-Carboniferous glaciation in a cool and humid climate,, although possibly with tropical summer temperatures (Rayner 1996). Burial of peat depressed atmospheric CO2 to very low concentrations. Waxing, and waning of the ice sheets controlled climate, and eustatic sea levels., The typical Permian flora of southern continents was, one of the main arguments when Suess (1885) recognized the former land connection between Africa,, Madagascar and India, and called this landmass, “Gondwana”. Today we know that Australia, Antarctica and much of South America were also part of, Gondwana. Gondwana and Laurasia finally collided, at 300 Ma, resulting in the formation of Supercontinent Pangaea (Torsvik & Cocks 2004), which was, the stage for Permo-Carboniferous glaciation and peat, formation., , Coal seams are terrestrial climate archives, just, like recent peat deposits (Large et al. 2003). The, density of stomata on fossil cuticles allows an, estimate of atmospheric CO2 concentrations back, into the Palaeozoic (Retallack 2001). A coal for-, , Figure 6.17 Reconstruction of CO2levels in the Earth’s atmosphere from the, Cambrian until today (cf. Normile 2009), based on geochemical models (Berner &, Kothavala 2001) and various proxy data, (Retallack 2001). Adapted by permission, from Macmillan Publishers Ltd: Nature, copyright (2001). Error margins are not, shown. PAL is the present atmospheric, level at 0.039 volume % CO2 in air. At, about 635 Ma in the Neoproterozoic, Marinoan Snowball Earth glaciation,, pCO2 had been as high as 1.25–8% (Bao, et al. 2009)., , Carbon dioxide (estimated vol.%), , 0.5, , 0.4, , 0.3, , 0.2, , 0.1, , PAL, -600, , C, , Ord, -500, , S, , D, , -400, , Carb, -300, , P, , Tr, -200, , Time (Myr), , J, , Cret, -100, , T, 0
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494, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , resulting Gondwanan continental glaciation, lasted from 326 to 267 Ma (Horton & Poulsen, 2009). At the end of the Permian period, the eruption of the giant Siberian trap basalt province, (Kamo et al. 2003) terminated the period of low, CO2 concentration in the atmosphere, although, the long-term draw-down was only interrupted., The reverse – warming of the climate – is a consequence of swamps falling dry and oxidation of peat, and coal, including burning it in power stations, (releasing CO2). Warming is also caused by methane released into the atmosphere through the, activity of microbial methanogens in wetlands., However, the Earth’s carbon cycling remains, incompletely understood (Normile 2009). Our, planet encloses other reservoirs of carbon and, carbon dioxide, which share in the control of, atmospheric carbon dioxide, and climate is influenced by a plethora of processes apart from the, CO2 cycle (Plimer 2009, Solomon et al. 2010)., During the last ice age and its numerous glacial/, interglacial periods, carbon dioxide varied from, 180–300 ppm, but the variations were not the, cause but the consequence of temperature amplitudes between 9 and þ 3 C (Shackleton 2000)., Tectonic processes provide the slow subsidence, that enhances peat thickness by balancing swamp, growth. This includes all scales of tectonics, from, wide epeirogenic flexure of intracratonic basins, and platforms to the formation of orogenic foredeeps and local rift valleys (Figure 6.12). Postorogenic intramontane basins are formed by tensional or shear strain of thickened orogenic crust., Embryonic folding of a basin during peat sedimentation may cause thinned seams over future anticlines and thickened coal in the depressions (e.g., Sydney District, Australia; Wise et al. 1991). Rim, synclines of salt diapirs and walls (Figure/Plate, 5.18) may collect large volumes of peat., Palaeogeographical reconstructions of important coal basins of the Earth lead to an enhanced, understanding of the conditions that support the, formation of large coal deposits (Rowley et al., 1985). Palaeogeographical reconstructions at the, scale of coal-forming wetland landscapes are useful for exploration and exploiting coal. A problem, of such reconstructions for the geological past is, the absence of recent examples of giant wetlands,, , because in the Holocene, continents are in a state, of emergence and general dryness. Today, there is, no wetland resembling the Westphalian coal belt, in Europe with an East-West extension of 3000, km, and a North-South width of 800 km (Ziegler, 1982). Coal “giants” were typically deposited in, periods of marine ingression into continents, at, relatively higher sea levels than today. Therefore,, instead of relying on the rules of actualism, facies, analysis of the sediments associated with coal, seams is the basis for modelling the sedimentary, environment. Work in the foreland of the Appalachian Mountains resulted in the establishment of, the Allegheny model that gained worldwide, acceptance., The Allegheny model is based on the observation that thick coal formations consist of disconformity-bounded, stratigraphical, repetitive, sequences of sediments that have been termed, “cyclothems” (Figure 6.18). Cyclothems were, originally defined as “designating a series of beds, deposited during a single sedimentary cycle of the, type that prevailed during the Pennsylvanian period” (Wanless & Weller 1932). The concept and its, significance are still discussed today (Horton &, Poulsen 2009, Weibel 2004, Wilkinson et al. 2003)., Sediments making up cyclothems can be related to, several depositional environments, which are the, key to the recognition of favourable peat-forming, areas (Thomas 2002, Ward 1984):, . Marine shelf Fossiliferous shale, sandstone and, carbonates, rarely dark phosphatic shale, all deposited in relatively deep water and far from the, shoreline; no coal;, . Delta front, coastal barriers and brackish, lagoons Mainly quartzites that are interbedded, either with shale and carbonates (seawards) or, with dark shale and thin coal beds (landwards);, the shoreline may be marked by the occurrence of, flint;, . Lower delta plain Lagoonal or deltaic shales, that are overlain by silt and sandstone, incised by, sandy alluvial channels; thin coal beds are ubiquitous, either in abandoned channels or in wide, coastal swamps between active flow channels;, . Upper delta plain and broad alluvial plains, Terrestrial clastic rocks and thick coal seams; peat, formed in wide swamps covering inundation
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COAL CHAPTER 6, , 495, , 1, , 10, , Marine limestone, , 9, 8, 7, 6, 5, 4, 3, 2, , Black shale, Grey shale, Coal, , Underclay, Non-marine limestone, Sandy shale, , Figure 6.18 The “ideal” cyclothem (modified from, Wanless & Weller 1932 and Wilkinson et al. 2003)., Wilkinson et al. argue, however, that the apparently, regular succession of sediments in cyclothems is, accidental. The variable width of the column, emphasizes different lithologies and grain size., , plains, lakes and abandoned channels; coal seams, contain clastic interbeds; generally, fining-upward, sediments are characteristic., The formation of Pennsylvanian cyclothems, may have been controlled by: i) eustatic glacial, forcing of sea levels; or ii) tectonic impulses. The, first can be caused by orbital control of sea levels, and climate oscillations related to the Milankovitch theory (Heckel 1986), or by greenhouse gas, control. There is little doubt that most cyclothems, reflect global change of sea levels due to waxing, and waning of continental ice sheets. Estimates of, the glacial-eustactic amplitudes during the Pennsylvanian vary around 100 m, similar to the, Pleistocene. Glacial-eustatic cyclothems are separated by lowstand terrestrial units or by exposure, surfaces. Orbital insolation changes are controlled, by the Earth’s precession, obliquity and eccentricity, with recurrence sinusoidal periods of 20, 40, and 100 kyr, 400 kyr and 2.5 Myr. The short cycles, provide a possible time-scale to single cyclothems., In the Pleistocene, a combination of orbital control and consequent greenhouse gas decrease to, less than 280 ppm CO2 equivalent, is thought to, have caused growth of ice sheets and fall of sea, , 1, , Sandstone, Cyclotheme base, 10, , levels by 120 m. Simulations of Late Palaeozoic, orbital cycles, glaciation and sea levels at different, pCO2 (Horton & Poulsen 2009) suggest that orbital, cycles alone only account for 10 m sea level, change; greenhouse gas forcing must be induced, to replicate observations., Global extent of glacial eustasy caused by Late, Palaeozoic glaciation of Gondwana translates into, the probability that single cyclothems should be, datable events worldwide (Miall 1997). The origin, of specific cyclothems is still discussed. The Carboniferous coal measures of northern Germany,, for example, were deposited in coastal swamps, of the Variscan foredeep (Figure 6.19) and display many cycles of marine transgression and, regression. Resulting cyclothems are considered, eustatic-tectonic by S€, uss et al. (2000) and glacialeustatic by Hampson et al. (1999)., 6.2.4 Host rocks of coal, A siliciclastic nature of rocks hosting coal is, the norm (Figure/Plate 6.20). Exceptional cases, include Late Permian coal seams in southern, China, which rest on marine platform carbonates.
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496, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Clastic sediments in, the upper delta plain, , Extensive low-lying swamps, , Sea, , River mouth, Inactive delta, , Figure 6.19 The largest expansion of Late Carboniferous paralic swamps in northern Germany occurred at times of, beginning transgression of the sea. In sequence stratigraphic terms, this is an “early transgressive systems tract”, (Hampson et al. 1999). Black – peat areas., , This coal contains marine fossils and is enriched, in organic S, Na, Mg and Ca. A setting similar to, recent coastal mangrove swamps is assumed (Shao, et al. 1998)., An appropriate study of host rocks is a vital part, of scientific and practical investigations of coal, deposits. Sedimentological data are needed for, palaeogeographical models and for resource estimation, physical properties determine mining, methods and costs. Note that rocks associated, , with coal always display grey colouring; red (haematitic) sediments are free of coal. The grey colour, of host rocks is, of course, simply a result of the, reducing sedimentary-diagenetic environment, and is caused by fine-grained organic substances, and pyrite. In rare cases, the connection between, former swamps and nearby hills is exposed and, lateritic (red) soil is observed. An example is the, paralic (coastal) Late Carboniferous seams of the, Sydney basin on Canada’s Atlantic seaboard,, , Figure 6.20 (Plate 6.20) Outcrop of Permian Great Northern coal seam below fluvial conglomerate on the, Pacific shore, Sydney basin, New South Wales, Australia. Note vertical joints and subdivision of seam into plies., Courtesy of Keith Bartlett, Minarco-Mineconsult, Tuggerah, NSW.
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COAL CHAPTER 6, , which are interbedded with transported red soil, and calcrete nodules (Tandon & Gibling 1997)., Autochthonous calcic vertisols are attributed to, glacial dry climate spells in the Mid-Carboniferous of Illinois (Falcon-Lang et al. 2009)., Floor (footwall) of coal seams, Coal seams may be in direct contact with older, basement (Figure 6.16), but are more commonly, underlain by clastic sediments of the same age., Plant roots and their symbiotic fungal associates, secrete organic acids that dissolve mineral particles in the soil to liberate nutrients needed for, growth. This enhances the rates at which continental silicate rocks undergo abiotic chemical, weathering (Berner et al. 2003, Berner 1997)., Organic acids dissolved in seepage water from the, mires act as reductants and break down soil minerals. In the case of good drainage, iron and alkalis are, removed, resulting in bleaching, kaolinization and, the upgrading of quartz sands. In consequence,, valuable kaolin, illitic fireclay and high-grade sand, or quartzite deposits in coal districts are not rare., Also, the downward seepage of water removes, large quantities of CO2 from the atmosphere, for, example by calcite formation in deeper parts of, the aquifers. At the same time, dissolved matter, entering the sea favoured proliferation of marine, carbonate-forming life. The combined effect is the, steady drop of atmospheric CO2 during the Palaeozoic (Figure 6.17), while land plants emerged and, colonized the Earth’s terrestrial surface and as a, side-effect, lowered ambient temperatures., A characteristic footwall rock of seams is seat, earth, or underclay, which is the soil that supported the peat-forming vegetation. The material, is typically fine-grained (clay and silt) but may be, sandy. Coalified remains of roots are abundant and, reach a depth of 10 m below coal seams. Siderite, concretions mark many of these horizons as former gley and semi-gley soil., Roof (hanging wall) of coal seams, Thinly stratified carbonaceous shale with numerous leaf fossils, cannel coal and bituminous shale, are characteristic roof rocks of Late Palaeozoic, , 497, , coal seams in the Northern Hemisphere. These, rocks indicate deglaciation, rising oceanic and, local water levels, and drowning of the swamp. In, contact with air, as for example on waste dumps,, these rocks are prone to self-ignition. Sand as a roof, rock of seams in orogenic foreland basins may, have originated by subduction earthquakes and, consequent tsunamis. Also possible is a genetic, equivalence with the mud deposited by Hurricane, Katrina over wide wetland areas of Louisiana, USA, (Turner et al. 2006). Coals with marine hanging, wall rocks contain relatively more ash, sulphur, and nitrogen, and more volatile matter than is, expected from the rank of nearby freshwater coal., The elevated bitumen concentrations shift coking, behaviour. These negative consequences of seawater inundation require a considerable time; peat, in Panama, which was submerged by an earthquake below sea level in 1991, displayed little, alteration three years later (Phillips et al. 1994)., A relatively thin roof of freshwater clay and silt, provides sufficient sealing to prevent negative, consequences of marine transgression on coal, quality (Eble & Greb 1997)., 6.2.5 Marker beds in coal formations, At scales of less than 10,000 years and 10 km, distance, coal seams must be considered as diachronous beds, except if the opposite can be proved, for, example by a tuff marker bed. At larger time spans, and distances, seams may be used as time markers., They are, however, a rather poor basis for chronostratigraphy but serve very well in lithostratigraphical work. In mining applications such as seam, correlation, the latter has proved its high worth., Lithostratigraphical orientation is most important in coal districts with numerous seams, which, are deformed by folds, faults and overthrusting., Lithological marker horizons, fossil-rich bands,, characteristic host rocks, palaeosoils and recognizable dirt bands in single seams, such as tuff and, fire horizons, all serve to build a lithostratigraphical division, and to match an unknown seam into, the system, for example after driving a tunnel, through a fault with uncertain throw., Marine marker horizons are the product of, transgression of the sea over coastal peat swamps
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498, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , or freshwater mud flats. In the Ruhr District,, Germany, 40 exploitable seams and 18 marine, marker horizons are distinguished. These marine, flooding events are traceable throughout Europe, and already long ago served to establish international stratigraphical boundaries. Typical rocks, are dark banded shales with a rich fauna, in part, open marine pelagic, whereas others display a, fauna of a few species in brackish settings., Volcanic tuff horizons are very useful correlation markers, because ash falls mark actual time, horizons. When a tuff band cuts through a seam, at an oblique angle, the seam is diachronous. In, European and North American, Carboniferous, coal seams, tonstein (German for “claystone”), horizons are relatively frequent. They represent, kaolinized rhyolitic tuff with recognizable characteristic rhyolite quartz, former glass shards,, sanidine, zircon and ilmenite. With a thickness, of <10 cm, many tonstein bands are known over, thousands of square kilometres. Kaolin originated, in the acidic swamp; later diagenesis transformed, much kaolin into illite (e.g. Sabero, Spain: Knight, et al. 2000)., 6.2.6 Coal formation in geological space and time, A large number of coal basins were formed in, coastal swamps and are called paralic (Greek for, “by the sea”). Marine marker horizons clearly, indicate the proximity of the sea and its occasional, invasion. Limnic coal basins formed in intracontinental and often intramontane swamps and display freshwater-dominated rocks. The term, “limnic” is not fully satisfactory because little, coal originated in lakes. The majority was deposited in a “telmatic” (wetland) or terrestrial (raised, bog) environment. Of course, host rocks of, “limnic” coal are mainly of limnic and fluviatile, facies, with occasional volcanic and marine, influence., The geodynamic nature of coal basin settings, suggests a classification along the following lines:, 1 Foreland and molasse basins are caused by, downflexure of the lithosphere, due to subduction, and the advancement of an orogenic front. Sediment fill is very thick (up to 8000 m in the North, European Carboniferous) and of continental deri-, , vation, but mainly deposited in the sea. Paralic, seams are numerous, although thin (1–2 m) and, cover a large surface. The sedimentary package is, folded and faulted, with a decrease of strain from, the orogenic front towards the foreland. Examples, are giant coal deposits in the Appalachian foreland, (Cross 1993), the North-Variscan foredeep of, Europe (Drozdzewski 1993), the Bowen Basin west, of the Tasman orogenic belt in Australia and the, Karoo Basin north of the Cape belt in South Africa, (Cadle et al. 1993)., 2 Intramontane and intermontane basins are, the result of late-orogenic distension and collapse, of mountain belts, and of large transform or shear, structures, which create transtensional, transpressional and simple pull-apart basins. Sediments are, predominantly limnic and alluvial with marine, intercalations. The base of the infill may be a, discordant seam. The number of seams is rarely, more than 10, but the thickness of individual, seams may surpass 100 m. The total thickness of, coal formations is rather limited. Tectonic strain, varies widely. This type is well illustrated by the, Early Tertiary Raton and Powder River basins in, North America, although the two originated during the Laramide orogenesis as isolated fragments, of a former Cretaceous foreland basin., 3 Intracontinental and platform basins Coalbearing sediment sequences display a thickness, of a few hundred metres only. Few seams are, developed but some may reach great thicknesses., The seams are constant and attain large areal, extensions to several 10,000 km2. Bedding is, nearly horizontal to moderately inclined. This, type of coal basins is the result of slow, wideranging epeirogenic subsidence of stable cratonic, regions. Examples include Gondwana coal basins, north of the sensu stricto Karoo Basin in South, Africa and India, the Tunguska Basin in Central, Siberia and the lignites of Eastern Germany., Times of peat (coal) formation, Earliest rocks resembling coal are known from, the Archaean Witwatersrand gold placers (cf., Chapter 2 “Gold”); the so-called carbon leaders, are graphitic pyrobitumen seams, which may be, derived from algae. Plants first invaded the land
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COAL CHAPTER 6, , 470 million years ago (in the Silurian). In the, geologically short time since, they have diversified, and profoundly moulded the Earth’s climate and, the evolutionary trajectory of life (Beerling 2007)., Humic coal became only a possibility with this, conquest. The earliest known vascular plant is, fragile and leafless Cooksonia (425 Ma). In the, early Devonian, the first tiny leaves are found, (Eophytophyllon bellum). Later during the Devonian, terrestrial vascular plants became so widespread that atmospheric CO2 concentration, fell rapidly, caused by intensified weathering, (Figure 6.17, Berner 1997). At the same time, the, oxygen concentration in air started to rise, reaching a peak of 35% in the Permian (compared with, the present atmospheric level of 21%)., The oldest coal deposits date from the Late, Devonian of Arctic regions (Asia, northern, Europe, Canada), with fossils of genus Archaeopteris as the earliest tall trees. Well-developed seat, rocks are known in Quebec (Elick et al. 1998)., Since then, coal was formed in all geological, stages. Two main periods of coal formation, however, stand out because of the sheer mass of coal, contained: The Late Carboniferous-Permian and, the Late Cretaceous-Tertiary, which contain 55, and 26%, respectively, of world resources. Both are, related to late orogenic dynamics of continental, collision (the Variscan and Alpidic cycles, respectively). Late Palaeozoic coals of the northern hemisphere (China, Europe, North America) formed in, a relatively short time span of 30 Ma in the Late, Carboniferous. During this time, the region passed, the tropical rain forest zone while Pangaea drifted, northwards. Most of the coals of the Gondwanan, part of Pangaea (Australia, Africa, Antarctica,, India, South America) were deposited later, in the, Early and Mid-Permian. Coal formation in both, hemispheres is intimately controlled by continental glaciation lasting from 326 to 267 Ma (Horton, & Poulsen 2009)., , 6.3 THE, , COALIFICATION PROCESS, , The natural process system, which transforms, plant material into coal, is called coalification, as, proposed by G€, umbel (1883). G€, umbel coined the, , 499, , term in order to stress the contrast with carbonization that is provoked by anoxic heating of wood, in a charcoal pile. Coalification can be considered, to consist of a first stage that takes place in the, swamp during peat formation, and a second stage, that begins with deposition of cover rocks above, the peat. The first is also called “peatification” and, is primarily a biochemical process. The second,, geochemical coalification is essentially abiotic,, driven mainly by increasing temperature and pressure. The domain of coalification is largely equivalent to that of diagenesis, the term applied to, inorganic sediments (cf. Chapter 1.4 “Diagenetic, Ore Formation Systems”), and to “maturation”,, which is used in the hydrocarbon industry to, describe changes of dispersed organic matter in, sediments related to oil and gas generation., 6.3.1 Biochemical peatification, Most peat-forming wetlands have an oxidized, near-surface layer but at little depth, reducing, conditions prevail. A fully developed vertical, redox-stratification may include oxic, suboxic,, sulphidic and methane zones with increasing, depth. The aerobic oxic zone is defined by the, presence of dissolved oxygen in pore waters and, is generally very shallow. The suboxic zone is, identified by accumulation of dissolved Fe(II) and, Mn(II) in the pore waters and is anaerobic. Due to, sulphate reduction, dissolved H2S builds up in the, sulphidic zone. Organic matter degradation by, anaerobic methanogenesis sets in when sulphate, is depleted to levels that do not support microbial, respiration. Lateral variation of the zonation is, considerable (Koretsky et al. 2007)., Dead plant matter in the swamp is decomposed, by bacteria, archaea, actinomycetes and fungi. Synthesis of humic macromolecules occurs by microorganisms proliferating on organic residues (K€, ogelKnabner 1993). Different products are due to the, respective redox environment and the fate of various building materials of the plant. Sugars, starch,, pectine and protein are rapidly metabolized, as is, cellulose, which is not protected by lignin. Lignin, is preferentially attacked by fungi; the effect is, transformation into solid humic substances under, preservation of shape (Hatcher & Clifford 1997).
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500, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Cork (suberine) and tannine-impregnated tissues, are only in part microbially decomposed. Wax and, resins remain unaffected. Lipids such as pollen,, spores and cuticles preserve both shape and material composition. Some peats display enrichment, of carbon to 60% (d.m.m.f.) already near the surface; the increase of carbon towards depth is slow, and in rare cases attains a maximum of 64% C., Clearly, carbon is not a useful parameter for the, determination of peat maturity., Only abiotic changes such as condensation,, polymerization and reduction take place below, 10 m under the surface. Most conspicuous are, decreasing porosity and water content of the peat, as a function of increasing consolidation. Some, observations suggest that water content is reduced, by 1% for every 10 m; such experimental data, collected from a specific deposit may serve well, for studying the maturity distribution. The transition from peat to bog coal and lignite is a matter, of convention, with no natural limiting mark., Water, free cellulose and carbon content, or the, ease of cutting may be parameters taken into, account (Figure 6.4, Taylor et al. 1998)., The most significant process of peatification as a, precursor of coal formation is humification, which, is a limited oxidation of lignin, cellulose and proteins. Humification may be followed by biogenic, gelification. Microbiota are the key to more, intense decomposition. Preconditions for high, microbial activity include: i) a rather basic pH of, the swamp: ii) the availability of sufficient nutrients (Ca, P, K, N): iii) elevated temperature; and iv), free oxygen in surface water. Therefore, high biogenic gelification is one of the indicators of swamp, facies. In some cases, clearly variable gelification of, different plies in one coal seam has been observed., Burial of the peat by alluvial, limnic or marine, sediments terminates the biochemical peatification process, and geochemical coalification, takes over., 6.3.2 Geochemical coalification, Petrographical, chemical and physical properties, of coal change with increasing rank. Peat and bog, coal are little different, but already in sub-bituminous coal organic constituents are markedly chan-, , ged. This observation provoked remarks that, coalification may be better considered as a kind, of metamorphism and not of diagenesis (Hower &, Gayer 2002). Taking into account, however, that, host rocks of coal experience diagenesis while coal, rank increases, using the term metamorphism, appears highly impractical., Humins evolve during coalification by increasing aromatization, condensation and clustering, (Hatcher & Clifford 1997). As functional groups,, bridges and side-chains of nuclei are broken down,, islands of well-ordered structural elements, (“crystallites”) grow larger. During coal maturation, aromatic clusters are increasingly arranged, into flat sheets that resemble the graphite lattice., During coalification, elemental ratios of H/C, and O/C change, and decomposition reactions, liberate water, CH4 and CO2 or CO (Kopp et al., 2000). Note that in certain districts, C2-4 hydrocarbon gas may occur apart from methane. In the, Ruhr District, Germany, this “wet gas fraction”, attains 70%. In addition, all humic coals exude a, little bitumen similar to crude oil. Coal as a source, rock for oil deposits, however, is uncommon, (Wilkins & George 2002)., 6.3.3 Measuring the degree of coalification, Quantitative determination of coal rank relies on, chemical and physical changes during coalification. Because these changes are not equally distinct over the whole rank scale, certain methods, are preferred in specific rank regions., In lignites and sub-bituminous coals, moisture, decreases rapidly with increasing rank (Figure 6.4),, which makes its determination a useful parameter, of maturity. In Australian (Holdgate 2005;, Figure 6.12) and Lower Rhine bog coal, water, content decreases 3–5% per 100 m depth increment, and in the sub-bituminous coal of Borneo at, 1% per 100 m. This loss of water is caused by a, reduction of the pore volume, but also chemical, loss of hydroxyl (-OH) and other organic groups., Factors that influence gradual loss of water, include burial depth, time, seawater contact,, lithotype and tectonic strain (e.g. folding). Moisture of bituminous coal changes little and is of no, use for rank determination.
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COAL CHAPTER 6, , In mining practice, the rank of black coal is, measured by volatile matter yields. Because macerals display different content of volatiles, bright, lithotype layers (vitrain) are preferably used for, precise rank measurements. The reverse, i.e. fixed, carbon (% C ¼ 100 % V.M.) may also serve to, express rank. Increasing coal rank reduces volatile, matter and increases fixed carbon content., Already in 1873, Hilt had reported that the rank, of black coal in a shaft increases with depth (Hilt’s, law). The gradient is locally different but the, average decrease of V.M. is 1.4% per 100 m. Sivek, et al. (2008) show that in the Upper Silesian coal, basin, deviations from the average are due to elevated pressure in tectonically deformed areas, to, thermal influence from igneous intrusions and, even to palaeoweathering., The standard method of rank measurement,, however, is the determination of vitrinite reflectance (Taylor et al. 1998). This is carried out with a, photometer mounted on a microscope, by illuminating a polished surface of coal with non-polarized light under oil immersion and measuring, the reflected fraction of the incident light. In, routine work, particle mounts are prepared and, reflectance is measured. Statistical evaluation, reveals the mean of random reflectance (Rr or in, oil immersion Rr,o, often written Ro), which correlates closely with coal rank. Reflectance determined with polarized light on oriented coal, samples reveals an anisotropy of high (Rmax), intermediate and low (Rmin) values. In little deformed, coal seams, Rmax is oriented parallel to bedding, and Rmin across it at a right-angle. This is due to, the gravity-induced natural stress field, which, causes growth or adjustment of structural lamellae into near-horizontal bedding planes. Deviations from this ordinary spatial arrangement, may occur in folded seams and are important keys, for resolving the relative timing of coalification, and deformation, and for reconstruction of the, palaeo-stress field. Average reflectance increases, from 0.3% (of incident light) for lignite to 4%, for anthracite. The increase is much steeper, in high-rank compared to lower rank coals, (Figure 6.4)., Changes of macerals of the liptinite group can, also be employed for studies of coal rank (Taylor, , 501, , et al. 1998). Illuminated with blue or long-wave, UV light under the microscope, liptinites exhibit, fluorescence (Figure/Plate 6.9). The intensity of, fluorescence is highest in peat and lignite and, decreases with higher rank. Quantitative measurement of rank is possible up to the so-called, coalification break at 29% V.M. (high volatile, bituminous coal). At higher rank, in medium volatile bituminous coal, liptinites approach vitrinite, in chemical and optical properties and the two, cannot be distinguished. Precursors of liptinites, are mainly paraffinic (aliphatic) waxes and lipids of, plants, which are very stable during peatification., In the sub-bituminous stage, liptinites exude bitumen, just like petroleum source rocks. Subsequently, methane is released until liptinites, loose their petrographical identity., Physico-chemical investigations of structural, change during coalification, such as aromatization,, condensation and clustering, also reveal much, about the coalification process (Hatcher & Clifford, 1997). This is, however, hardly ever applied in, practice, in contrast to the methods presented, above. Visualization of coalification data in relation to geological features is provided by maps and, sections, which display rank by contours of equal, water and V.M. content, or reflectance., 6.3.4 Causes of coalification, Many observations in coal mines provide arguments concerning the agents of coalification,, including:, . time, because older coal is commonly of higher, rank;, . temperature, because coal in proximity of igneous rocks is upgraded and deeper coal is subject, to higher heat flow;, . pressure, which also rises with depth;, . tectonic strain, as in the same basin undeformed, coal tends to display lower rank compared to, folded seams; and, . swamp facies, which partly controls volatile, matter content., Currently, a dominating role of temperature, is generally accepted, but the influence of other, factors is not negated (Sivek et al. 2008, Hower &, Gayer 2002).
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502, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Thermal metamorphism of coal, Thermal metamorphism of coal in contact with, magmas continues to provide a deeper understanding of coalification (Mastalerz et al. 2009). Of, course, quantification of the temperatures that, acted on coal excites much interest. Estimates are, made by comparison with results from laboratory, experiments, or are derived from the assumed liquid and solidus temperature of the magma and the, heat conductivity of host rocks and coal. Actually,, with a high heat capacity and one of the lowest heat, conductivity coefficients of geological materials, (0.016–0.148 W/[m.K], Bychev et al. 2004), coal is, an excellent insulator. To give one example, coal, intruded and covered by Permo-Triassic trap basalt, in the Tungus Coal Basin, Siberia, was heated to a, maximum of 650–700 C (Pavlov et al. 2005)., Generally, coal affected by the heat aureole of, magmas displays reduced moisture and volatile, matter content (higher rank), whereas ash, yield increases. Black coal is visibly altered above, 300 C and, above 500 C, obtains an appearance, that resembles industrial coke. Natural coke and, graphite often occur as a thin fringe with hexagonal jointing along the contact. Natural coke can be, formed from coal that is not cokeable, because of, too high V.M.; this observation indicates that, confining pressure had an influence. The coke, fringe is followed by thermally altered coal, which, is dense, hard and dull, and may be hexagonally, jointed. Vitrinites in the intermediate zone exhibit, incipient melting, loss of volatile matter and exudation of tar. Softening induces viscous plastic, flow of coal that destroys sedimentary bedding., Fluid coal may be injected into host rocks resembling a dyke. Haematite may replace pyrite and, abundant CO2 causes deposition of much carbonate, both in coal and in the igneous rock body,, which is itself heavily altered. This raises the ash, content of coal. Water vapour or supercritical fluid, may form at the igneous contact, impeding vitrinite alteration (Barker et al. 1998)., Lignites are strongly dehydrated near the contact, which induces cracks and joints. Coke formation is less frequent but confirmed. Anthracites, hardly melt and, without exhibiting visible alteration, are upgraded to meta-anthracites, which are, , commercially extracted and marketed as graphite, (cf. Chapter 3, “Graphite”):, The West Siberian Tungus Coal Basin extends over, 1.2 M km2 and hosts a thick Permo-Carboniferous, coal-bearing series with many important coal seams., Total coal resources are estimated to reach 40,000 Mt, (Pavlov et al. 2005). Trap basalts covered and intruded, the coal sequence at the Permo-Triassic boundary., This induced a short pulse of thermal metamorphism, that upgraded previous lignites and sub-bituminous, brown coal to coking coal rank, anthracite, and even, graphite. At the same time, the process must have, released enormous volumes of CO2, methane, water, vapour and hydrogen sulphide that contributed to the, near-extinction of life on Earth. In the northern part of, the Tungus Basin, the coal series is covered by lowpermeability rocks and Pavlov et al. (2005) speculate, that giant resources of coal-bed methane may be, trapped in this area., , Deep igneous intrusions may impose a wide, halo of regional thermal coalification, with a, peak in the anthracite stage (e.g. in the western, Lower Saxony Basin of northern Germany: Kus, et al. 2005). Rapid rise and denudation of metamorphic massifs may also impose a regional coalification gradient (Sachsenhofer 2000). In the, Eastern Alps, Tertiary coal occurs in many intramontane basins (Figure 6.16). Over a distance of, 100 km from east to west, rank increases from, lignitic to high volatile bituminous B. The highest rank occurs proximal to the Tauern Window,, a tectonic unit that was metamorphosed in the, Eocene and unroofed in the Miocene (Neubauer, et al. 2000)., Geothermal coalification by subsidence, In contrast to spatially circumscribed heat, sources, the geothermal field of the Earth provides, ubiquitous heat energy. Heat flow from depth, provokes a temperature rise with increasing depth,, which is called the geothermal gradient, with an, average of 25–30 C/km. However, the geothermal gradient varies with lithology; it may be as, low as 14 C/km for highly conductive salt and, 47 C/km for insulating coaly shale. Other variations are due to the tectonic setting. Subsidence
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COAL CHAPTER 6, , 503, , topographic gradient (Gayer et al. 1998). Another, hypothesis suggests that the commonly moderate, extension of anthracite fields might be explained, by an early synclinal downwarp, which is now, concealed by later strong deformation such as folding and inversion (Lyons 1991)., The coalification temperature can be calculated, from vitrinite reflectance data (e.g. from a drillhole, profile) and an estimate of palaeoheat flow derived, from specific geological constraints. One of several models was proposed by Sweeny & Burnham, (1990). The model is based on chemical kinetics, (eq. 6.3). Calculations based on this model demonstrate that coalification represented by reflectance (% Ro) is mainly controlled by temperature,, whereas the reaction time exerts a much smaller, influence (Figure 6.21)., , of a coal basin shifts coal into higher temperature, conditions, which intensify coalification. The, depth arrangement of rank reflects the wide or, narrow spacing and the shape of isotherms (contours of equal T). Coalification isolines may be, used to reconstruct the geothermal gradient, responsible for coalification., The coalification gradient is a function of geothermal gradient and lithologies at a specific location. Based on quantitative estimates of, overburden and heat flow at the time of coalification, calculation of coalification temperatures is, possible. If time is not a limiting kinetic factor (see, below), typical coalification temperatures are, 50–200 C (sub-bituminous and bituminous, coal). Anthracite is formed at 200–300 C, which, is the reason why this rank is relatively rare. Often,, anthracitization is clearly related to magmatic heat, sources, but there are other genetic possibilities. In, the Appalachians, hot tectonic brines, which are, expulsed from the westward moving nappe pile, are, considered to have been the agents of heating, (Figure 1.74, Daniels et al. 1994). Anthracite occurring on the northern margin of the South Wales, Basin (UK) was formed by the passage of hot basinal, fluids, which were displaced by cool water due to a, , ARRHENIUS-equation, , describing the kinetics of an, endothermic chemical reaction (here coalification):, k ¼ A eE=RT, , ð6:3Þ, , k ¼ reaction rate (1/m.y.), A ¼ specific reaction constant (frequency factor, 1/m.y.), E ¼ activation energy (kJ/mol), R ¼, universal gas constant (J.K1 mol1) and T ¼ temperature in, Kelvin ( C þ 273)., , EASY % R r, o, 250, , Temperature (°C), , Anthracite, 3.0%, Low volatile, bituminous, , 200, , 150, , 0.7%, , Lignite, , 50, , 0, , 1.3%, , High volatile, bituminous coal, , 100, , Figure 6.21 Vitrinite reflectance in a temperaturetime diagram as calculated with the numeric, coalification model EASY % from Sweeny &, Burnham (1990). Comparison with coal rank is only, indicative. AAPG [1990] reprinted by permission of, the AAPG whose permission is required for further, use., , 2.0%, , Medium volatile, bituminous, , 1, , 5, , 10, , 50, , Time (My), , 100
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504, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Duration of coalification, Many investigations of coalification in geologically young sedimentary basins constrain the, main parameters temperature, heat flow and duration of heating to narrow confidence limits., Results show that duration of coalification varies, widely. For magmatic heating, coalification time, is normally very short (<1 Ma). Long duration, (25–35 Ma) was reported for anthracitization in, eastern Pennsylvania (Daniels et al. 1994). The, reaction rate of coalification is still not fully, known, so that the effective heating time needed, to reach equilibrium at a certain temperature cannot be predicted. The reverse is clearly displayed, by observations: Rank once reached is hardly, altered by lower temperatures, even if they act for, a very long geological time. In this case, activation, energy is the limiting factor. An example is the, Ruhr District coals in Germany, where coalification patterns were imprinted during the Latest, Carboniferous/Earliest Permian before the rocks, were folded. In spite of elevated rock temperatures, of >50 C for the last 250 Ma, coal rank was not, changed (Figure 6.22)., , The role of lithostatic stress and fluid pressure, Considering the increase of coalification as a function of depth, lithostatic stress might play a role., This is certainly the case for lignite, which, matures by compaction and dewatering. Bituminous coals attain higher rank mainly by chemical, processes, but also by physical compaction such as, decreasing porosity and this may be favoured by, , higher stress. High confining (fluid) pressure, however, may inhibit maturation (Carr 1999) by, impeding expulsion of coalification by-products,, such as H2O, CO2, CH4, higher hydrocarbons, and N., Tectonic stress and strain, Many observations prove that folding of coal, seams does not cause coalification (Figure 6.22)., In fact, this was already documented by Petrascheck (1954) when he defined pre-, syn- and, post-tectonic coalification based on microscopic, investigations of the optical anisotropy of different, folded coal seams (Figure 6.23). In the southern, Ruhr District, where seams are most intensely, folded, coalification is even lower than in less, deformed regions (N€, oth et al. 2001). Nevertheless,, tectonic strain that favours permeability, such as, pervasive shearing, advances maturation of bituminous coals and of anthracite, and graphitization, (Ross & Bustin 1997). With shearing, graphite, formation seems to be possible at temperatures, little over 300 C (Bustin et al. 1995). M. & R., Teichm€, uller (1966) observed a sharp coalification, increase proximal to an overthrust fault in the, Ruhr District. They suggested that friction heat, may have been the agent. Heating by circulating, hot hydrothermal fluids could be an alternative, explanation (Golytsin et al. 1997)., Swamp facies, Seams that display marine influence on peat are of, higher rank than close-by freshwater seams. This, , NW, , SE, Post-orogenic cover rocks, , 0, , m, , Isovole 22%, , 500, , 1000, , Coal seam Sonnenschein, , Figure 6.22 The parallel pattern of folded seam Sonnenschein in the Bochum synclinorium (Northern Germany), and the isovole 22% V.M. proves that coalification took place before deformation. Modified from M. & R. Teichm€, uller, (1966, 1981) in Taylor et al. (1998). With permission from www.schweizerbart.de.
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COAL CHAPTER 6, , 505, , (a) Pre-orogenic, coalification, , (b) Syn-orogenic, (or combined pre-/post-orogenic), , 1, , 2, , sea, m, , 3, , Isorank line, , (c) Post-orogenic, coalification, , Figure 6.23 Pre-, syn- and post-tectonic relations between folded seams and the position of lines of equal coal, rank (isovoles or isoreflectance limits). Modified from M. & R. Teichm€, uller (1966) and W.E. Petrascheck (1954) in, Taylor et al. (1998). With permission from www.schweizerbart.de., , observation of coal miners was often verified, for, example in the Netherlands, where Carboniferous, coal rank decreases from deltaic to alluvial floodplain coal (Veld et al. 1996)., , 6.3.5 Coal maturity and diagenesis of, country rocks, Studies of thermal evolution and burial history of, sedimentary basins are not only based on vitrinite, reflectance of coal and the maturation stage of, dispersed kerogen in sediments. Detailed mineralogical investigations of pore cements, pore fluids, and of pelitic rocks add another component. The, field experience of the relations between coal rank, and lithification grade of host rocks, such as lignite, occurring in plastic clay and loose sand, black coal, in hard shale and sandstone, and graphite in phyllite and quartzite, is resolved in numerous intricate details of diagenesis. Only one aspect is the, chemical change and increasing crystallinity of, clay minerals, from amorphous “clay” colloids, and kaolin in the peat mire, to illite, chlorite and, sericite near the transition to metamorphism, (Bucher & Frey 2002)., Yet, the determination of coal rank and kerogen, maturity provide a tool for highly accurate and, high-resolution investigations of the thermal evolution of a basin and of the diagenetic grade of its, components. Very low grade metamorphism, begins approximately with the formation of, meta-anthracite at Rmax >4%, and the greenschist, facies at Rmax >5% with semi-graphite and, graphite., Reflectance studies of disseminated coal particles and of kerogen in sediments are an important, , tool of petroleum and natural gas exploration., Already White (1935) had recognized that oil formation occurs mainly within a specific stage of, diagenesis (the “oil window”) that relates to coal, rank. The oil window is confined between approximately 44 and 24% of volatile matter in coal, and, 0.6 to 1.4% reflectance of vitrinite (Figure 6.4)., Coal of higher rank is related to natural gas deposits (cf. Chapter 7.2 “The Origin of Petroleum and, Natural Gas”)., , 6.4 POST-DEPOSITIONAL CHANGES OF COAL SEAMS, After burial underneath siliciclastic sediments,, peat is submitted to endogenetic alterations, foremost those of coalification. Meteoric and diagenetic fluids affect coal by epigenetic deposition, of minerals in pores, joints (called cleats) and, faults, raising ash content. Tectonic deformation, of seams leaves structures that influence product, quality and mining conditions. Exhumation and, erosion expose coal to exogenetic alterations that, reduce coal quality., , 6.4.1 Tectonic deformation, Under tectonic strain, coal exhibits a more ductile, response than its common siliciclastic host rocks., However, even peat and low-rank lignite may, develop extensional fissures, which are intruded, by liquefied mud and sand forming clastic dykes,, probably caused by earthquakes. At higher rank,, the ductility of coal promotes thinning and thickening of seams similar to marble bands in
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506, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , metamorphic rocks, and the injection of coal, dykes into host rocks (Thomas 2002)., Brittle deformation of black coal at low temperature results in brecciation and mylonitization,, yielding a fractured and powdery material. Shearing at elevated temperature causes structural, adjustment, such as changes of the orientation of, optical axes and locally increased coalification, (Taylor et al. 1998)., Dense jointing (joints in coal are called cleats) is, characteristic for black coal. Most lignites display, no joints at all, just like their host rocks. In artifical outcrops (e.g. open pit walls), some intensely, gelified lignites develop polygonal fractures soon, after exposure, which are not tectonic but an effect, of dehydration and shrinkage. Cleat systems of, black coal form in response to tectonic stress. This, is easily verified by the spatial and genetic relations to joints in host shales and sandstones. Spacing of joints depends on bed thickness and, mechanical properties of rocks. The same is, observed in coal, with a higher cleat density (lower, spacing) in bright coal (vitrain) compared to dull, coal (durain). Joints in siliceous wall rocks occur at, even larger intervals. The orientation of cleats is, skilfully exploited by miners for easier, that is less, energy-consuming, extraction of the coal. Cleats, (and many bedding planes in seams) are planes of, mechanical separation. This is the reason why, aperture and density of cleats (e.g. number per, m) and the degree of penetration, or in other terms,, their frequency, persistence and connectivity control properties such as gas flow, porosity, coal rock, strength and grain size distribution of run-of-mine, coal (Laubach et al. 1998). The careful documentation of cleat data is an important aspect of coal, geological practice., 6.4.2 Epigenetic mineralization of coal seams, Coal cleats and other fractures may be coated by, minerals such as carbonate, barite, clay, markasite, pyrite, galena, sphalerite (in the Illinois, District), uraninite (in the western US) and quartz., This raises the ash content of coal. Rarely,, exploitable veins were encountered. In the Ruhr, District, Germany, veins with zinc and lead, sulphides were commercially exploited until, , 1962. The base metal veins were hosted by faults, cutting anticlines at right-angles, clearly indicating a late syntectonic (probably diagenetic) origin., Generally, hydrothermal ore deposits in coal are, hardly ever of economic interest, although many, mineralized coals are reported (Laznicka 1985). To, the scientist, hydrothermal minerals in coal reveal, properties of fluids, which migrated through the, rock body, their relative and absolute timing, and, the conditions of mineral precipitation (Daniels, et al. 1994)., In contrast to black coal, peat and lignites are, porous and reactive rocks that act as physicochemical traps for trace elements, which are transported with surface or groundwater. Potentially, economic concentrations of vanadium, uranium, and germanium were mentioned earlier. Laznicka, (1985) reports coals anomalous in copper, gallium,, molybdenum and nickel., 6.4.3 Exogenetic alteration of coal, Near-surface oxidation may induce self-ignition of, seams and is the most common agent of reducing, coal quality., At the surface, sub-bituminous and bituminous, coals alter to black, soot-like soft soil. Apart from, solid residues of oxidation, reaction products, include CO2 and soluble organic matter (DOC –, dissolved organic carbon: Chang & Berner 1999),, which enters the groundwater. Lignites, however,, rot and turn into humic soil constituents. The, original thickness of the seam cannot be deduced, from observations in the surface zone. In most, settings, destructive weathering (supergene alteration zone I) only affects coal to a depth about, equal to the saprolite zone of the regolith profile., Below the first, a second zone of alteration, leaves coal macroscopically intact. The seam displays its full thickness, and the coal is physically, fresh. Only disseminated, or fracture-coating gypsum and other sulphates may be visible indicators, of oxidation, but these often extend beyond degradation. The severe reduction of coal quality in this, zone is only recognized by determination of, calorific value and cokeability. Easily soluble sulphates are a nuisance for extraction and processing, and seepage water activates swelling clays in
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COAL CHAPTER 6, , the seam, which cause additional problems. The, vertical thickness of supergene alteration zone II is, commonly several tens of metres., Micropetrographical methods are not suited to, map the extension of zone II, because vitrinite, reflectance remains unchanged. Possible keys, such as microfractures with higher reflecting rims, occur only above an oxidation temperature of, 150 C (Taylor et al. 1998)., Meteoric seepage may reach hundreds of metres, below the surface, as jointed seams behave as, aquifers. This can be beneficial because infiltration initiates secondary microbial methanogenesis, enhancing gas production from Coal, Bed Methane operations (c.f. Chapter 7.5, “Exploitation of Petroleum and Natural Gas, Deposits”)., Natural coal seam fires, Natural coal seam fires cause destruction of large, panels of near-surface coal seams. In fact, the, “fires” are rather smouldering sections with an, activity that changes from hardly visible to quite, dramatic. Ash and cinder are remains of burnt, coal. Footwall rocks may be baked, but hanging, wall shales are usually fired and sintered (cf., Chapter 3 “Clay and Clay Rocks”), and display, attractive colours ranging from purple-violet, through brick-red, orange, yellow to white. Very, dense varieties are called porcellanite or porcelain, jasper. Others resemble volcanic scoria. The fired, clay rocks resist weathering and erosion so that, long chains of hills mark the former presence of, coal seams (Alberta, Colorado, North Dakota)., Reduction of iron (by reaction with heated coal or, with CO) causes widespread formation of magnetite (and some native iron), which facilitates aeromagnetic mapping of former seams, even under, young cover., Oxidation and self-ignition of coal is an exothermal process. The oxidation of pyrite produces, 1546 kJ/mol and of coking coal 5176 kJ/mol, (assuming a composition of C10H7O), which, results in heating. Coals rich in vitrinite and fusinite are especially vulnerable. Mylonitization, multiplies the hazard, because a high internal, surface favours reaction with oxygen. The thermal, , 507, , conductivity of coal is the smallest among all, rocks (<0.50 [W/(m.K)] compared to 1.98 (shale), and 3.12 (sandstone) supporting heat accumulation. Usually, temperatures rise to 30–80 C, but, then drop off slowly, which may be due to: i), complete oxidation of all accessible surfaces; or, ii) consumption of all available oxygen. Oxidizing, coals with a high internal surface in contact with, ample oxygen display continuing heating until, self-ignition occurs at 120–250 C., , 6.5 APPLICATIONS, , OF COAL GEOLOGY, , An engineer, who has built a bridge, can strike, you nearly dead with professional facts; the, captain of a Ganges river steamer can, in one, hour, tell legends sufficient to fill half a book,, but a couple of days spent on, above and in a, coal mine yields more mixed information than, two engineers and three captains, Rudyard Kipling 1888, , Practicians in the earth sciences serve the coal, industry in the full cycle from exploration to, extraction, environmental mitigation and mine, closure, similar to other sectors of mining. However, some aspects of working in coal are specific,, which is the motivation for this section., 6.5.1 Exploration, Exploration for buried coal seams is based on, geological hypotheses that rely mainly on palaeogeography and facies distribution in space and, time. Geophysical detection of coal at depth is, only indirectly possible, for example by delineating bedding and structures utilizing methods such, as seismic, gravity and magnetic surveys. At moderate depth, the low conductivity of coal may, permit mapping by electrical methods. Oxidation, of coal in the near-surface raises ground temperatures, allowing helicopter-mounted thermal, (radiometric) mapping of coal seam outcrops., Soil gas analyses are a rarely used geochemical, component of prospect exploration. Combined, with geological outcrop mapping, facies models, and geophysical data justify the design of a
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508, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , reconnaissance drilling grid. From the early planning stages through detailed exploration to coal, extraction, geological, engineering and environmental perspectives must be equally considered, (Bartlett et al. 2009)., Geological and geophysical indications allow no, more than estimates of prognostic resources. Definition of identified resources requires quantitative data such as depth, thickness, structures and, quality of coal. Defining measured resources, implies drilling (Figure 6.24), supplemented by, physical exposure and sampling of coal in deep, trenches or underground exploration drifts. The, distance between drilling sites may be based on, geostatistics of nearby mining fields. If such data, , are unavailable, a relatively wide grid is initially, chosen, which is filled in as the need arises. In the, Latrobe Valley brown coal, initial drillholes were, spaced at 1 to 2 km distance; mining reserves are, defined by a 400 m drilling grid (Waghorne 2001)., Continuous geostatistical control assures that, an optimal solution is found as soon as possible, (Figure 5.16; Saikia & Sarkar 2006)., In black coal regions with simple large-scale, structure, such as the western margin of the Late, Palaeozoic Bowen Basin, Queensland, Australia,, drillhole spacings of 1 to 2 km are sufficient for the, first phase of exploring underground mining, resources. Reserve drilling at the Burton Downs, Mine was initially laid out in the form of profiles, , Figure 6.24 Truck-mounted drill rig, here, employed in coal exploration in the Sydney, basin, New South Wales, Australia. Courtesy of, Keith Bartlett, Minarco-Mineconsult,, Tuggerah, NSW.
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COAL CHAPTER 6, , across the strike of the seam at a 100 m distance,, with intervals along the profiles of 50 m. Geostatistical variography soon demonstrated that the, lateral homogeneity of the deposit allows a separation between profiles of 400 m. Considerable, reduction of costs was the consequence. Note,, however, that actual extraction of the coal at, Burton (and elsewhere) is increasingly assisted by, detailed three-dimensional seismic imaging of, faults, unstable roof conditions and gas accumulations in advance of longwalls (Walton et al. 2000)., Core drilling (commonly using slimline, 63.5 mm diameter) is advisable in all reconnaissance work, because the wealth of data that can, be extracted from the core, such as lithology,, stratigraphy, facies, spatial orientation, tectonic, deformation, hydrogeological and geotechnical, properties, justifies the higher costs (Figure 6.25)., Subsequent infill drilling may employ down-thehole hammers (DTH) or other percussion technologies through overburden, but should change to, coring near the seam. A near 100% recovery of core, in coal must be aspired. Because friable parts of, the seam are easily lost during coring, triple-tube, core barrels may be needed. Aluminium or plastic, liners protect the samples from damages during, removal from the barrel and later handling. After, logging, e.g. brightness and inorganic partings, the, containers are sealed in order to guard the coal, , Figure 6.25 Slimline (HQ), drill core section of Permian, Wallarah-Great Northern, coal seam in the Sydney, basin, New South Wales,, Australia. Courtesy of, Keith Bartlett, MinarcoMineconsult, Tuggerah,, NSW. Light shales in the, footwall (foreground right), grade into bedded coal, which is overlain by matrixsupported roof, conglomerate (back, left)., Figures marked in white on, core denote brightness., , 509, , from oxidation and desiccation, but allow, X-ray radiography of the core samples with, no additional manipulation. X-ray radiography is, preferable to photographs, because it provides, clear images of the distribution of minerals (ash), in the coal., Geophysical borehole logging is commonly, applied in order to provide in-situ control of seam, boundaries and depth information reported by, the drilling team, and data such as water and ash, content. Frequently, radiometric, density and, electrical methods are employed (Figure 5.8). Infill, percussion (DTH) holes are routinely surveyed, by geophysical borehole logs (Fullagar 2000)., Recently, acoustic scanning technology emerged, as an important wireline tool for investigating the, orientation of joints, fractures and borehole breakout, which is a proxy revealing the horizontal, stress direction (Bartlett & Edwards 2009). In, mechanized underground mining and coal seam, gas projects, for example, this is vital information., Samples of coal, especially of lignite and brown, coal are prone to desiccation and must be protected. Cool storage of low rank coals prevents, the growth of fungi and bacterial attack. However,, the only guarantee of long-term preservation of, initial properties is coal sample storage under, argon immersion. For petrographical and technological sampling, cores are split or sawed in half
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510, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , parallel to the core axis. One half is archived for, eventual later requirements. Samples of the, seam’s immediate wall rocks are often needed for, geomechanical investigations and must be as carefully preserved as the coal. In all cases, the needs of, the hydrogeologist and the geotechnical engineer, for data must be accounted for (cf. 5.2 “Trenching, and Drilling”)., Worldwide, exploration for coal bed methane, (CBM), also called coal seam gas (CSG), is a rapidly, growing field in the quest for energy. In principle,, data on geology and volume of seams, and the, properties of coal are required for an estimate of, potential gas in situ. Rank, macerals, lithotype and, moisture content are main controls on the methane adsorption capacity of coal and must be determined. In bituminous coals of the Pennsylvanian, Weibei Coalfield, Ordos Basin, China, for example, the adsorption capacity (on a dry and ash-free, basis) varies from 14 to 30 m3/t but gas in situ is, <15 m3/t (Yanbin Yao et al. 2009). The permeability of these coals is between 0.1 and 10 mD and the, porosity ranges from 2 to 7%. The authors combine regional data on seam thickness, gas content,, coal rank, methane concentration, permeability,, porosity, burial depth and deformation pattern to, define targets for detailed investigations., 6.5.2 Reserve estimation, Geological coal resources and exploitable reserves, are estimated and classified with the same methods that are stipulated for other solid minerals (e.g., JORC-Code: AusIMM 2004, cf. Chapter 5.3.2, “Ore Reserve Estimation and Determination of, Grade”). Geological certainty and economic viability are the most important parameters., The economic viability of a coal deposit is a, function of: i) the geological situation; ii) coal, quality; and iii) technical and economic conditions (the “modifying factors”). Geological situation and coal quality are objective determinants, and independent of time. They include the geometry of the deposit, as well as geomechanical,, hydrogeological, geochemical and geothermal, parameters. The modifying factors are timedependent and prone to change, for example by, new technologies in mining, processing and utili-, , zation, and by new factors in economy, markets,, law, environmental regulations, society and politics (Kininmonth & Baafi 2009, Waghorne 2001)., Reserve estimates only conform to international, standards if the modifying factors are accounted, for. This is the main reason why coal reserves, decrease consistently (Figure 6.1), whereas those, of other minerals hardly change, as ever more coal, producers adopt international bankable reporting, standards., Coal reserve estimation incorporates several, distinct features. Unlike black coal, low rank coal, (lignite and brown coal), for example, is hardly ever, traded. Low energy (6–16 MJ/kg) and high water, content (>30%) of run-of-mine coal preclude, transport to any distance from the mine. Mines, are usually linked to adjacent captive power stations, which are specifically designed for each, deposit (Figure 5.18). In addition, low rank coal is, hardly ever upgraded by processing. The consequence is that the feasibility of mining combined, with electricity production must be demonstrated, for a reserve. Utmost diligence must be used to, determine the in-situ density, moisture and ash, content, because this defines coal delivered to the, power plant. Even then, small deviations are still, to be expected: In the Latrobe Valley (Figure 6.12),, run-of-mine coal is typically 1% higher in ash and, 1% lower in moisture than indicated by drilling, (Waghorne 2001). Certain variations of the coal, quality, due to lateral or vertical facies change of, the seam, may be unacceptable for the power, station. If this cannot be mitigated by blending, from different faces, such coal is not included in, reserves. The overall economy of a lignite mine, cannot be established by reference to a market., The only available economic measure is the price, of electricity and its competitiveness. Resources, and reserves need to be extensive, because amortization of power plants is measured in decades., Higher-rank bituminous coal resource estimates are based on determination of the volume, and in-situ bulk density at in-situ moisture of the, coal (Lipton 2001). Large-diameter (100–200 mm), drilling provides advance samples for determination of additional important properties, such as, washability and coal fragmentation (Figure 6.26)., As soon as possible, semi-industrial scale bulk
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COAL CHAPTER 6, , 511, , Figure 6.26 Large diameter core, (200 mm diameter) is drilled for, quantifying technological parameters of, coal such as fragmentation and, washability. Courtesy of Keith Bartlett,, Minarco-Mineconsult, Tuggerah, NSW., , samples are acquired by trial box cuts and underground work. Oxidized parts must be outlined and, clearly separated from unaltered coal. High-rank, coal seams occur typically at an inclination to the, horizontal. Therefore, the volume is best calculated from true thickness and true area of the, seam. High ash plies or partings can be included, or excluded, but consistency is vital. Dilution, during mining (e.g. from hanging wall, footwall,, partings, longwall edges) increases the tonnage,, but mining losses decrease it. Black coal is typically crushed and washed, or submitted to density, separation in order to lower ash content. Certain, grain fractions of traded coal command higher, prices. An optimum grain size distribution of the, saleable product demands care from extraction, and through processing. These are only some of, the points that have to be considered when estimating in-situ tonnages and marketable products., , Present mining technologies pose a lower limit, to the extractable thickness of black coal seams., Cut-off thickness may be 50 cm in an opencast mine, and 100 cm for underground mining., Also, costs preclude coal mining at great depth., Northern Germany’s indicated coal resources at, <1500 m depth comprise 25,000 Mt; extraction, to this depth is technically feasible, although, not economically competitive. Very deep coal, (>1500 m) in Northern Germany is estimated to, total 350,000 Mt. Similar relations are known, from many regions of the world., Motivated by this situation, the potentially elegant method of in-situ coal gasification was examined by numerous research projects in the past,, although with moderate success. The principle is, to drill holes into a coal seam, pump in air and, steam, ignite the coal so that gasification starts, and extract the gas produced (“syngas”). Today,
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512, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , directional drilling is envisaged in order to divide a, seam into panels (600 30 m) that allow better, control of the process. The recovery of the energy, inherent in coal is estimated as between 50 and, 80%. Trial runs in the Surat Basin, Australia, (2008), support hopes that this technology may, finally reach profitability. Giant tonnages of deep,, currently unmineable coal might be converted, into profitable resources., 6.5.3 Coal mining geology, All available geological information is collected in, a database, which is the foundation for a threedimensional geological deposit model. Basic datasets include spatial information, coal rank, in-situ, heat, moisture, ash and methane content, bulk, density and the spatial orientation of cleats. Maps, displaying contour lines of seam floors and isolines of seam thickness, and sections illustrating, spatial geometry of stratigraphy, bedding planes, and structures, assist in communicating the, model. Hydrogeological and geomechanical information is integrated. Much of this work can be, done in Geographic Information System (GIS), space, but dedicated specialized programmes, allow integrated mine planning and scheduling., Note that the geological model must be validated, by a competent person (cf. Chapter 5.3 “Ore, Reserve Estimation and Determination of, Grade”)., In multi-seam deposits, the correlation of individual seams is crucial for the accuracy of the, geological model. Only a correct correlation truly, supports reserve estimates and mine planning., Modern underground mining technology and, future automated mining methods require reasonably constant conditions. Severe production loss is, hardly avoidable if longwall faces run unprepared, into faults, erosion channels or magmatic dykes., Supporting geology, high-resolution seismic and, tomographic methods are routinely employed to, investigate and image the seam in front of the, advancing face., Methane and CO2 content in coal and host rocks, of a deposit are spatially varied. The gas distribution must be documented in order to assist mine, planning. Gas is, of course, a security risk, , (Hargraves 1997), but also a major cost factor,, because the necessary precautions such as higher, air flow and gas drainage drillholes make extraction more expensive. Methane in mine return air, should always be used, by burning in a power, station or in a fuel cell unit. Apart from methane, gas explosions, instantaneous coal outbursts are a, deadly danger in many deep black coal mines. Coal, outbursts are violent and spontaneous ejections of, powdered coal and gas from the working coal face, (Guan et al. 2009). They may be followed by, methane explosions. The mechanism of coal outbursts is similar to other gas-driven solid eruptions, (e.g. of magma, salt and other rocks) insofar as high, internal gas pressure is released into expansion, and fragmentation when external confining pressure is suddenly removed (Guan et al. 2009)., In most lignite and coal mining districts, hydrogeological modelling is an absolutely essential, component of investigations, from prefeasibility, studies to mine closure. Commonly, host rocks of, coal include aquifers that enforce dewatering in, order to avoid water inrushes and unstable ground, conditions. Appropriate dewatering studies are, highly critical (Brown 2010). Drawdown cones, may be very wide and affect whole basins, with, the consequence that regional water management, is required (Kumar et al. 2010)., Geotechnical investigations of coal and host, rocks (Hoek & Brown 2003, Terzaghi et al. 1996), are needed to design mine openings sufficiently, safe but at lowest cost. In opencasts, this is mainly, a question of pit slope angles. In underground, mining, sections of rock mass quality below average must be identified in order to minimize the, hazard for personnel, as well as costs. Often, seepage water raises fluid pressure (u) and reduces, shear resistance (Figure 1.39; Brady & Brown, 2004). Underground longwall operations should, be oriented in a way as to minimize the hazard of, spalling caused by joints, fractures and horizontal, stress (Bartlett & Edwards 2009)., Underground extraction of coal causes subsidence that may induce lowering of the surface,, open fractures and damage to buildings and infrastructure. Earth falls or slope instability can be, triggered. Often, the pre-mining surface water, regime is fundamentally changed. Abstraction of
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COAL CHAPTER 6, , coal causes various reactions to the newly opened, void (Figure 6.26). In the immediate hanging wall, of the extracted coal, the rocks fracture and a, loosened destressed zone develops to a certain, height above the seam. Broken roof strata loosely, fill the goaf (Zone 1). At a higher level, subsidence, is enacted rather by bending than fracturing, and, the rock mass is little disturbed (Zone 2). On the, surface, extensional fracturing and faulting may, occur. As the extraction front passes underneath, buildings or infrastructure, induced strain is initially tensile followed by compressive deformation (Zone 3). For a stringent geomechanical, analysis, refer to Brady & Brown (2004)., The total subsidence of the surface may reach, 80% of the coal thickness extracted, mainly as a, function of nature and thickness of the overburden. Duration of subsidence and surface extent of, mining influence are controlled by several factors., which include the geomechanical properties of the, overburden rock mass. The limits can be defined, by an experimental angle a drawn from the edge of, the underground void (Figure 6.27). Thick competent sandstones in the hanging wall above the, seam may inhibit loosening and caving in zone, (1). A possible consequence is the accumulation of, , Tension cracks on surface, , 513, , stresses over a wider area, which are suddenly, released when the strength of the rock mass is, exceeded. The resulting shockwaves may cause, damage in the surroundings (e.g. an earthquake, of magnitude 4 in the Saar Province, Germany,, in 2007). In mining practice, preventive counter, measures such as filling the goaf with tailings are, taken in order to minimize subsidence, hazard, and damages., 6.5.4 Environmental aspects of coal mining, Black coal, green future. . ., Materials World 2003, , Already during reconnaissance exploration, the, influence of future mining on landscape, water, and humans should be considered in order to avoid, investing in sensitive areas. When entering the, phase of detailed exploration, a preliminary environmental impact study should be done in order, to design and install all required monitoring systems, such as groundwater wells and surface water, discharge weirs. Studies of rare species are advisable. For the definition of reserves, large datasets, must be acquired, including coal chemistry., , Subsidence area, Compressive < >, , extensional strain, , (3), Surface zone, , (2), Intermediate zone, , Strata bend downward, , Overburden, collapses into goaf, , (1), Roof failure, , α, Goaf, , O, , Direction of mining, , Figure 6.27 Underground longwall extraction of a coal seam panel causes surface subsidence (adapted from Ward, 1984). Three affected depth zones are distinguished: (1) Loosened and destressed rock mass falls into the goaf, (2) zone, of intermediate ductile deformation and (3) zone of surficial fracturing and damage by the “strain wave” passing, from tension to compression. Angle a describes the width of mining influence. The square with the inscribed O, is the longwall opening.
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514, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Standard coal quality is described by proximate, analyses (ash, moisture, volatile matter and fixed, carbon by difference), ultimate analyses (C, H, N,, S, ash and O by difference), calorific values and, sulphur forms. This allows estimating the potential atmospheric emissions of sulphur, carbon and, nitrogen, and the potential contribution of these, elements to environmental problems and global, climate change. For environmental impact studies, properties including major, minor and trace, element concentrations, modes of occurrence of, environmentally sensitive elements, cleanability,, mineralogy, organic chemistry, petrography and, leachability, should be determined (Finkelman &, Gross 1999). Trace element analyses of coal are the, base for mitigation measures in the power station, where the coal is combusted (Finkelman et al., 2002). Critical elements specified as hazardous air, pollutants (HAPs) by the US Clean Air Act (1990), are As, Be, Cd, Cl, Cr, Co, F, Hg, Mn, Ni, Pb, Sb, Se, and U. One example is mercury emissions from, coal-fired power plants in the United States, which, in 2004 amounted to 48 tonnes. Novel scrubber, , Zone in front of the face, Forward wells, , technologies similar to those used for SO2 control, are installed to alleviate this problem., Operating coal mines, Lowering water tables by pumping, both in underground mines and in open pits (Figure 6.28),, enforces discharge of large amounts of water into, nearby streams. This may induce unwanted degradation of water quality (e.g. pH, total dissolved, solids, iron, manganese, sulphate and trace metals, such as Se). Treatment of the lifted groundwater is, commonly required. Undisturbed coal seams normally act as weakly permeable aquitards; the permeability of host rocks depends on their nature,, degree of consolidation and the characteristics, of joint systems. Coal mine water is typically, characterized by low pH and high content of Fe(II), or Fe(III), suspended particles, SO42 (from pyrite, oxidation), Cl (from formation or geothermal, waters) and in the absence of sulphate, elevated, content of Ba, Sr and Ra. When such water is mixed, with common surface water, “radiobarite” is, , Moving open cut, Main well gallery, Water level gauges, , Recultivation, , Original water table of aquifer I and II, Original water table of aquifer III, , Water level, gauge, , Aquifer I, Dewatering well, Clay, , Face advance, , Aquifer II, Footwall gauge, and well, , Overburden dump, , Lignite seam, , AquiferIII, , Figure 6.28 Schematic illustration of water management in a lignite open pit (not to scale). Aquifers I to III consist of, poorly consolidated sand and gravels. In front of the advancing open cut, water tables are lowered by well screens,, in order to minimize inflow into the pit and to stabilize pit slopes. Below the bottom of the pit, water pressure is reduced, to avoid inrush of water or of liquefied sand. Water level gauges are for control, and alert in the case of rapid rise.
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COAL CHAPTER 6, , precipitated, which may be an environmental, problem (Schmid & Wiegand 2003). In Silesia,, Poland, radium concentrations reach a maximum, of 390 kBq/m3, but Ra is precipitated underground, by reaction with phosphogypsum before the water, is evacuated. The acid formation potential of, waste rock and overburden destined for surface, disposal must be investigated (Verburg et al. 2009)., Any hazardous material is covered with standing, water or with clay in order to prevent contact with, oxygen., As a greenhouse gas in the atmosphere, methane, is 21 to 70 times more potent than CO2 (depending, on how the comparison is made). Conventionally,, methane mixed with return ventilation air (VAM), was released into the atmosphere. In the recent, past, VAM represented 10% of all anthropogenic, emissions of methane and 60 to 70% of all greenhouse gas emissions from underground mines., After agriculture, black coal mining was the largest human source of CH4, followed by household, refuse dumps. Today, the small concentrations of, methane in ventilation air (0.2–1.2%) are combusted in mine-site power stations, for example, in Australia, together with coal shale or with, impure coal. Gasification of VAM with coal processing waste offers the greatest efficiency of, energy production and the lowest environmental, footprint (Clarke 2010). Mine methane that cannot be used must be burnt, because conversion to, CO2 lowers the greenhouse effect., Nearly all coal mines move large masses, (Figure 6.29) that must be transported, temporarily, stored or ultimately disposed of, including coal,, , (a), , 515, , overburden, waste rock and processing sludges., This offers numerous possibilities to raise environmental compatibility, not least by wellplanned recultivation (Richards et al. 1993). Landscaping and recultivation must be carried out as, far as possible during mining when large earthmoving machinery is available. Of course, renaturation of artificial ponds, streams and barren, land should also be considered, because spontaneous invasion of species often provides much higher, ecological value., The burning of coal produces a considerable, mass of combustion residues, including fly-ash,, boiler bottom ash and slag, and flue-gas desulphurization sludge including gypsum. Disposal of, these materials requires well-planned strategies., Their possible environmental impact has to be, carefully considered, not least because they concentrate potentially hazardous trace elements of, the coal (Huggins & Goodarzi 2009). As far as, feasible, the residues should be utilized, for example as cement raw materials and construction, aggregates in the building industry (in America, currently 37% of the total coal mining waste)., This preserves limited resources of sand and, gravel, and saves energy. Combustion residues, that cannot be marketed are buried in exhausted, opencast mines and surface impoundments (58%),, and only 5% are backfilled in underground coal, mines. The latter procedure improves the stability, of mine openings (Michalski & Gray 2001),, reduces surface subsidence (Brady & Brown, 2003) and is an important option in mine reclamation. Because coal ash is highly alkaline, it is, , Overburden stacking, , Overburden removal, , Coal extraction, Overburden, Coal seam, , Figure 6.29 Lignite open pit, in operation (upper section), and after landscaping,, recultivation and rebound of, groundwater levels (lower, section)., , (b), External overburden dump, , High dump, Low dump, Pit lake
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516, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Figure 6.30 (Plate 6.30) Post-mining lignite open pit lakes in the Lausitz region, Germany, in the last stages, of filling and rehabilitation. Courtesy of P. Radke, Ó LMBV, Lausitzer und Mitteldeutsche, Bergbau-Verwaltungsgesellschaft mbH., , suitable for treatment or prevention of acid mine, water generation (Canty & Everett 2006)., Closure of coal mines and remediation, Considerable investments are needed in order to, avoid post-mining environmental damage. Main, hazards include chemically (pyrite, markasite) or, mechanically unstable overburden tips and processing sludge dams, coal fires in dumps or residual coal in situ, rising groundwater levels, outflow, of methane and carbon dioxide, and of acid mine, water with a high iron concentration (Younger, 2002, Bell 1998). Best practices and technologies, related to sulphide mineral oxidation, acid mine, drainage and metal leaching are described in the, online GARD Guide (Verburg et al. 2009). The, severe reduction of coal mining in Europe within, the last decades left many exemplarily remediated, sites, but unfortunately, also a large number of, brown fields, which still await action., Underground openings of historic shallow mining can be prone to sudden collapse. Where maps, are insufficient, ground geophysics or airborne, , remote sensing methods are used for localization., LIDAR (Light Detection And Ranging) is, employed to create high-resolution digital elevation models (DEM), which illuminate slight, depressions that develop above voids., Rising groundwater levels after cessation of, pumping might be considered a blessing, but in, some coal mining districts surface subsidence, accumulated to an extent that large areas were, flooded, if nature would be allowed to take over., Although the resulting wetlands might be an, ecologist’s dream, densely populated countries, cannot afford the loss of land and settlements., Water management by pumping may be the only, long-term solution. Another potentially damaging, effect of rising groundwater is uplift of land, which, is mainly due to buoyancy forces (Terzaghi et al., 1996, Brady & Brown 2004)., Spoil heaps that have been built in the past from, tailings of less efficient coal washing plants can, today be a source of revenue by “tip washing” (i.e., recovery of remaining coal). Because this type of, land cannot be developed before the coal is cleaned, out (considering the threat of self-ignition) and the
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COAL CHAPTER 6, , surface is levelled, tip washing by modern plants is, an important step towards restoration., Coal mine fires, Outcropping coal seams burned naturally in the, geological past (e.g. in the Pliocene of the Powder, River basin, USA: Heffern & Coates 2004). Today,, thousands of coal mine fires are burning on abandoned mine lands throughout the world (Stracher, 2004, 2007). They threaten the health and safety of, local populations, destroy property values and, consume a non-renewable resource. In abandoned, mines, fires started by self-ignition or by human, action may be unnoticed for many years, until, fumes and collapse holes become a threat to, nearby residents. In the Permian coal fields of, Bihar in India, 2000 km2 of land and one million, people are endangered by underground fires, (Michalski & Gray 2001). Some of the fires have, been burning since 1916. Remediation would be, possible, but the costs are prohibitive. Burning, coal can only be extinguished by airtight sealing,, whether in a mine, on coal dumps or in natural, outcrops. Mitigation technologies include total, excavation, trenching, flooding and quenching,, bulk filling of mine workings, saturation grouting,, surface seals, inert gas injection and chemical, foams. Fubao Zhou et al. (2006) report successful, extinction of a mine fire by borehole injection of a, foam made of water, clay and nitrogen. Quenching, with water alone is not possible., Opencast lakes (Figure 6.29, Figure/Plate 6.30), Closure involves cleaning up and shaping the, interior of the pit, foremost the slopes. The stability safety margin acceptable for a mining operation, is commonly not sufficient for post-mining use. In, fact, the changing hydraulic regime may bring, about large landslides. Many pits fill by natural, groundwater inflow, which in others is supplemented with water from nearby streams. The, latter is chosen in order to avoid a long period of, slope weathering and acid formation, and to top up, water quickly with inflow of an acceptable quality. Additional technical measures may be necessary, for example neutralization of low-pH lake, , 517, , water by coal ash (Loop et al. 2003). Well-planned, pit lakes are often an enrichment of the landscape, and provide refuges for nature as well as benefits to, the people. Very instructive examples can be studied in all German lignite mining districts., Post-closure uses, Recultivated land is assigned to forestry and agriculture. Brown fields may be valuable sites for, other industries. Closed deep coal mines can be, utilized for the storage of natural gas. Moderately, toxic waste, such as fly and bottom ash from, coal-fired power stations, is very suitable as a, stabilizing fill of underground mine openings, (Wendland & Himmelsbach 2002). CO2 sequestration in deep coal mines is considered. Deep waterfilled workings of former coal mines may provide, abundant cheap geothermal energy (see below)., Gas seepage, (CH4, CO2) from closed coal mines is not rare,, especially during the period of flooding after closure. The main hazard is accumulation of gas in, buildings. Falling barometric air pressure causes, expansion of gas in the mine, which can result in, overflow. Mitigation methods include filling of, near-surface mine openings and sealing by injections, if needed. Methane should never be released, into the atmosphere but must be used. A pilot, plant in Ohio, USA, demonstrates burning methane from a closed mine in a fuel cell for electricity, production. Note that methane seepage (containing traces of ethane and propane) from the ground, is a widespread natural process; with 42–64 Tg/, year, geological sources such as petroleum and, coal basins are second only after wetlands (Etiope, & Ciccioli 2009)., Geothermal use of mine water, This is an economically alluring option for heating, and cooling buildings based on heat pump systems, (Watzlaf & Ackman 2006). Flooded coal mines are, particularly suitable because of their broad-based, accessibility. Coal seams are commonly mined, over large areas, so that the warm water filling
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518, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , mine tunnels, collapsed longwall voids and loosened rock can be pumped from wide-spaced boreholes. The large contact surface between water, and rocks increases the sustainability of heat, exchange. Various designs of heat extraction are, possible, for example closed loop systems (cycling, a refrigerant fluid contained in thin-walled pipes, through a water-filled shaft) and open-loop systems (pumping water from the mine and re-injecting it through a second well). Flooded parts of the, Pittsburgh coal seam in the northern Appalachian, coal basin of the United States might potentially, heat and cool hundreds of thousands of homes, (Watzlaf & Ackman 2006)., , 6.6 SUMMARY AND FURTHER READING, Coal supplies a third of the world’s primary energy, consumption, especially base load electricity, and, is an important fuel and reductant in many industries. It is the foundation of the present rapid, development of China, India and other nations., In that role it is a blessing, yet coal is highly, controversial because most mines are very large, and profoundly alter the land and adjacent communities. Burning coal contributes to anthropogenic carbon dioxide emissions (Box 6.1). Coal, resources and reserves are very large and can support humanity for hundreds of years. Acceptance, of continuing coal mining and combustion, however, depends on minimizing emissions and negative environmental effects. Progress is already, made but critics expect faster action. Several elegant solutions are investigated, ranging from carbon dioxide capture and geological sequestration, to in-situ gasification., Coals are solid, combustible, fossil sedimentary, rocks formed from land plants profusely growing, in ancient wetlands. Early (biochemical) and late, diagenetic (geochemical) processes enriched carbon and produced the coalification array, which, reflects a continuous evolution of properties such, as colour, transparency, reflectance, molecular, structure, specific density, calorific value, organic, chemistry, elemental composition, moisture, content and many other parameters. Coal rank, , increases from lignite to bituminous coal and, anthracite; the transition to graphite marks the, boundary to metamorphism., The chemical composition of coal is determined, by its derivation from plants and comprises carbon, hydrogen and oxygen, with nitrogen and sulphur as minor components. Low-rank bituminous, coal can be characterized by the “formula”, C10H7O. During diagenesis, carbon increases from, 60 to 90% C, whereas hydrogen decreases from, 5.5 to <3%. This is due to the release of a large, volume of methane (CH4). Part of the methane, remains absorbed in the coal, where it is a bane for, underground miners as “fire damp”, but a favourable resource for gas producers as coal bed methane (CBM). Most gas migrates towards the surface,, acting as a powerful greenhouse gas in the atmosphere. A small part is trapped in the crust and, forms many of the world’s giant natural gas, deposits., Climate is an essential control on peat-forming, plant communities and consequently, coal seams, are terrestrial climate archives. At the same time,, however, wetlands are prime factors of climate, regulation by the fixation of carbon in peat and, coal. The characteristic alternating dark and light, bands of many lignites, and the repetitive, sequences of host sediments that have been called, “cyclothems” in black coal districts might be, controlled by: i) eustatic forcing of sea levels; or, ii) tectonic impulses. The first is probably the, general background to occasional interference of, the second. Presently, eustatic glacial forcing is, attributed to orbital control of sea levels and climate, modulated by greenhouse gas control. With, a better understanding of palaeoclimate, the influence of other climate-forcing processes will certainly be revealed., At geological time-scales, coal formation is, restricted to the last 10% of earth history. Earliest, exploitable coal deposits date from the Late Devonian (ca. 370 Ma) of the Arctic realm, with fossils, of genus Archaeopteris representing the first tall, trees. In the Late Carboniferous (Pennsylvanian), of the Northern Hemisphere, coal was formed in, extensive tropical coastal swamp forests of Sigillaria and Lepidodendron. The giant Permian coal
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COAL CHAPTER 6, , deposits of the Southern Hemisphere with their, boreal Gangamopteris-Glossopteris flora originated at high latitudes in a cool and humid climate. In both hemispheres, coal formation is, intimately controlled by continental glaciation, lasting from 326 to 267 Ma., Coalification is preceded by humification of, peat, which is a limited oxidation of lignin, cellulose and proteins by microbiota feeding on plant, matter. Humins evolve during diagenetic coalification by increasing aromatization, condensation, and clustering. Rank of lignites and sub-bituminous coals is measured by water content. Volatile, matter yield differentiates between higher grade, black coals. The standard method of precise rank, determination is reflectance. Temperature, heat, flow and duration of heating are controls of coalification. The process can be modelled applying, the Arrhenius equation, which describes the, kinetics of an endothermic chemical reaction., Results indicate that typical coalification temperatures are 50–200 C (sub-bituminous and, bituminous coal). Anthracite is formed at, 200–300 C., Technologies in applied coal geology, coal mining and environmental mitigation are in a phase, of revolutionary change. Because of the size of, , 519, , the industry, the impact of coal extraction and, use affects large regions. Pioneers in the industry, already approach the ideals of green mining but, the bulk trails behind. Let us not forget that, investments in green mining raise costs that, have to be recovered from the electricity market., Clear national and international regulations, are the precondition for industry to broadly, invest in carbon capture and sequestration, (Box 6.1), calling for governments to agree on, actions. Let them act in such a way, however, as, not to block human inventiveness in overcoming, obstacles., For a synopsis of the geological diversity of 13, world-scale coal mining districts, I recommend, the volume edited by Cross (1993). As an insight, into the present state of industrial practice, the, book by Kininmonth & Baafi (2009) is singularly, valuable. Beerling (2007), a climate scientist,, recounts in a lively scientific style How Plants, Changed Earth’s History. Freese’s (2003) Coal, a, Human History is a very readable historical, account of the significance of coal for economic, development by fuelling the Industrial Revolution, which started in 18th-century England, its, social impact and further fate until today’s preoccupation with carbon dioxide.
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CHAPTER 7, Petroleum and natural, gas deposits, The Stone Age did not end because the world ran out of stones, and the oil age will end long before the, world runs out of oil. The problem of oil is not its shortage but its concentration., The Economist 2004, , Synopsis, Petroleum and gas deposits are economically exploitable natural accumulations of hydrocarbons in, the Earth’s crust. Their most important role in our civilization is the supply of energy that is highly, concentrated, easily distributed and versatile. Hydrocarbon products guarantee mobility. Public, perception of oil and gas is marked by price fluctuations, disruption of society in producer regions,, oil spills and occasional threats to stop delivery. Media announce depletion of resources for the, middle of the current century., The aim of this chapter is to provide a concise systematic survey of the economic geology of, hydrocarbons. A short introduction presents published figures of production, reserves and resources. The scientific base is laid in the following section, with a presentation of hydrocarbon, species and related compounds. Next we look at the preconditions of oil and gas formation, which, start with the deposition of sedimentary source rocks with high content of organic matter, followed, by diagenetic processes that induce exudation of oil and gas from the organic particles. The, hydrocarbons are entrained by fluid flow in sedimentary basins and are usually dispersed in the, shallow crust and in ocean water feeding microbes. A small part of generated hydrocarbons is, trapped in a variety of geological structures, some of which develop into exploitable deposits. This, chapter describes the nature and fluid fill of traps and their geological controls, and methods of, exploration and exploitation that connect science with practice. In consideration of the crucial role, of hydrocarbon availability, reserve and resource estimation methods are subsumed. Nature and the, present role of unconventional petroleum and natural gas sources, such as tar sands, oil shale and, tight gas shale, allow an outlook into a possible future. The chapter concludes with exposing typical, environmental problems in the hydrocarbon industry and their prevention or mitigation., , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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522, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Hydrocarbons are the base of liquid fuels supply., Also, they are important chemical raw materials, for the production of numerous organic basic chemicals and final products, from plastics and pharmaceuticals to carbon fibres. With a share of only, 3% of total crude oil sold, however, chemical uses, pale in comparison. The large hydrocarbon provinces of the world occur in regions that are distant, from the centres of consumption and a significant, part of current production and resources is statecontrolled. This is conceived as a possible risk, concerning prices and availability, driving competitors to invest in hydrocarbon fields in lower-risk, regions and in alternative fuel sources. Fuels that, may replace natural gas and petroleum products, include liquid hydrogen, methanol (CH3OH), produced from carbon dioxide and water (Olah, et al. 2006), liquids made by fermenting biomass, (“biofuels”, e.g. ethanol C2H5OH from cellulose, or sugar), and hydrocarbon “synfuels” produced by, Fischer-Tropsch synthesis from biomass and coal., At present, only ethanol and synfuels appear to be, economically feasible. The environmental costs of, biofuels may exceed those of fossil fuels (Scharlemann & Laurance 2008). Because of the generally, high cost of crude oil, many coal-to-liquid synfuel, plants are being constructed worldwide., Much hope rests on a future “hydrogen economy”, which implies that hydrogen replaces oilbased liquid fuels in vehicles and in stationary, distributedapplications.Hydrogenwouldbe burned, in fuel cells generating electric energy. Large challenges remain to be solved, however, concerning, hydrogen production,distributionandonboardstorage. In principle, hydrogen contains more energy on, a weight-for-weight basis than any other liquid fuel,, but has a very low energy-density per unit volume., Today, 10 Mt/year of hydrogen are produced by, steam reforming, from natural gas at 800 C and, catalysed by nickel (eq. 7.1). This hydrogen is commonly contaminated with CO and CO2, which, poison the platinum catalysts of fuel cells., Production of hydrogen by steam reforming:, CH4 þ 2H2 O ! CO2 þ 4H2, , ð7:1Þ, , Hydrogen produced according to eq. 7.1 is not, climate-neutral, unless the waste CO2 is stored in, , deep geological isolation. Alternatives of hydrogen, production include biological or catalytic degradation of biomass and electrochemical or photochemical splitting of water (electrolysis). In all, cases, the process is endothermic and requires, a considerable energy input. Therefore, large-scale, hydrogen production is only feasible if cheap electric energy can be provided. This appears to depend, mainly on increasing the number of coal-fired and, nuclear power stations (Jaccard 2006)., Petroleum and gas contain more hydrogen, than coal and yield more energy per carbon unit, than coal. Therefore, burning petroleum and gas, releases relatively less CO2 than coal. Of course,, the sheer mass of hydrocarbons combusted is much, higher than that of coal, so that total emissions from, petroleum and gas are higher. Also, considering the, typically dispersed use of oil products (e.g. in vehicles, buildings), it is very difficult to develop economically viable ways of carbon capture., The world’s measured (proved) reserves of economically exploitable petroleum at the end of, 2009 were estimated at 1333 thousand million, barrels or 181.7 thousand million tonnes. That, was an increase of 12.3% over the end 1999, figure, despite the large cumulative production, during the intervening 10 years (BP Statistical, Review of World Energy 2010). BP defines the, term “proved reserves of oil” as those quantities, that can be recovered in the future from known, reservoirs under existing economic and operating, conditions. Note that discovered resources (indicated and measured) are not reported. World production of oil in 2009 was 3820 Mt. Dividing the, reserves by this figure gives an R/P-ratio of 46,, which is the highest for many years (before 1989, it, was much lower, cf. Figure 6.1). In 2009, the largest, oil producers were Russia, Saudi Arabia, USA, Iran,, China, Canada and Mexico. Main consumers of oil, were USA, China, Japan and Russia., One barrel [bl] has a volume of 158.99 litres or 42, US gallons and contains 0.1364 tonnes of petroleum, of average density. The barrel serves as a measure, of volume since oil was transported to the market, in wooden whisky containers from the world’s, first oilfield near Titusville, Pennsylvania, in the, year 1870.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , The term “conventional” petroleum (and natural gas) implies primary recovery by free flow from, wells, by pumping, or by water and gas injection., Enhanced recovery methods can produce more oil, and gas from the same deposit, but at higher costs., Of the total conventional reserves, 57% occur, in the Middle East, followed by Venezuela and, Russia. Kazakhstan, Libya, Nigeria, Canada, USA, and China share a notable part of the remaining, reserves. Total historic production until 2009 is, estimated at 160 Gt petroleum., Additional undiscovered resources of conventionally recoverable petroleum are estimated, by some at up to 400 Gt, with a geographical spread, similar to present reserves. There is no agreed, method by which to estimate undiscovered resources. Many scientists, the public and the media, are fascinated by the so-called Hubbert Curve, (Hubbert 1962), which was originally used for, oilfield engineering. Hubbert first transferred its, application to estimation of total oil production, from single wells and fields to the whole of the, United States. The production of oilfields typically rises to reach a peak (“depletion mid-point”), and then falls to the point of exhaustion and, closure. A plot of production over time produces, a bell-shaped curve. The same method is used for, estimating total world oil production and implicitly, total world oil resources. Results vary because, of various factors and events that influenced, past world production, such as war, nationalization and economic crises, which are differently, weighted. A more problematic variable is the, changing nature of oil production, from conventional to advanced technologies. Overall it seems, that the Hubbert Curve method produces estimates that are decidedly too low., , 523, , The depletion mid-point of conventional oil, (“peak oil”), the time when 50% of the total, recoverable geological resources will have been, exploited, is generally assumed to be reached, around the year 2020 (Wellmer 2008). After the, depletion mid-point, some forecasts presume, a rapid decline of production. Already now,, however, unconventional production is rapidly, increasing. Generally, the availability of oil and, gas is not only a function of resources, but also of, political decisions, prices and advances in science, and technology. This is why after the depletion, midpoint, a plateau or a gradual decline of production is much more probable. Maugeri (2004), defended this view vividly, and points out that, the future evolution of production and prices will, be dominated by the market, politics and human, inventiveness., Today’s proved economically exploitable, natural gas reserves are estimated at 187.1012 m3, (normalized to a calorific value of 9500 kcal/m3,, this is 168,000 Mt oil equivalent). Additional conventional resources are assessed at 240.1012 m3., Yearly consumption is rising (although with, a dip in 2009 by 2.1% to 2940 billion m3 or, 2646 Mt oil equivalent) and total accumulated, production is 76,000 billion m3. The largest, proved reserves occur in the CIS (30%, mainly, Russia and Turkmenistan) and the Middle East, (41%, Iran and Qatar). The European Union hosts, no more than 3.2% of world reserves. Main, consumers of gas are USA, Russia, Iran, Japan,, Germany and Saudi Arabia (BP Statistical, Review of World Energy 2010). For comparing, the calorific value of different energy raw materials, conversion into oil equivalent mass is often, used (Table 7.1)., , Table 7.1 Average conversion factors for comparing mass of coal (t) and lignite (t), and volume of natural gas (m3) and, petroleum (m3, barrels) with tonnes of oil equivalent (toe), 1 tonne of oil equivalent, 1.5 metric tonnes of average bituminous coal, 3 tonnes of average lignite, 1 billion cubic metres natural gas, 1 tonne of average oil, , !, !, !, !, !, , 42 Gj (gigajoules) or 10 Gcal, 1 tonne of oil equivalent, 1 tonne of oil equivalent, 0.9 million tonnes of oil equivalent, 1.165 m3 or 7.33 barrels
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524, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , A comparison based on toe units shows that, proved reserves of fossil fuels at the end of 2009, amounted to 412 Gtoe of coal and lignite combined, and 182 plus 170 Gtoe of conventional oil, and natural gas, respectively., Conventional oil and gas can be economically, exploited with today’s technologies and at about, present costs. It is estimated, however, that conventional oil and gas make up less than 5% of the, total mass of hydrocarbons in sediments. Future, technologies may generate giant exploitable, resources from the total geological endowment., Less speculative unconventional resources are, the Canadian oil sands that contain more recoverable oil (50.109 m3) than Saudi Arabia, (36.109 m3). The Orinoko Belt in Venezuela hosts, 186.109 m3 of very heavy oil; serious development has not yet commenced. Gas hydrate deposits below permafrost land and beneath the, oceans contain more than twice the mass of, carbon of all known fossil fuels; industrial-scale, trial production was successfully tested in 2008, in Arctic Canada. Submarine hydrates in sand, reservoirs enclosed by mudrock represent substantial resources, which can be extracted by, conventional technologies (Boswell 2009). In the, USA, natural gas in “tight” (practically impermeable) sandstone, shale and in coal seams emerged, as a giant new economic source. The world’s, potential for comparable deposits is enormous –, global recoverable unconventional gas resources, are estimated at 400.1012 (or T) m3 (IEA 2009), compared to present annual consumption of 3, Tm3. Oil shales may provide 500.109 m3 of oil,, compared to end 2009 proved world oil reserves, of 212.109 m3. Even conventional oil is far, from finished – only a few percent of submarine sedimentary basins at more than 500 m, water depth have been explored. The CircumArctic region contains large potential oil and gas, resources (Gautier et al. 2009). Of course, much of, the unconventional oil and gas will be more, expensive. The reported observations confirm, the introductory statement from The Economist., Our present dependence on hydrocarbons will, not end because of exhaustion of geological resources, but because of new energy sources and, technologies superior to oil and gas., , 7.1 SPECIES OF NATURAL BITUMENS, GAS AND, KEROGEN, AND THEIR PROPERTIES, Hydrocarbons may occur in gaseous, liquid, waxlike and solid phase. Most natural hydrocarbons, are insoluble in water but soluble in alcohol and, ether. Common species that form mineral deposits are:, . petroleum (liquid both at depth in the reservoir, and on the surface);, . natural gas (free gas, not associated with oil or, dissolved in water);, . natural gas hydrate (solid, currently first experimental exploitation);, . associated gas (dissolved in oil, or as a free gas cap, above reservoir oil);, . tar (highly viscous, low-quality petroleum);, . earth wax (ozocerite; solid, soft to brittle, yellow, to brown);, . pyrobitumen (solid, black, hard);, . asphalt (solid, black, brittle);, . kerogen (organic matter in oil schists that yields, “synthetic” oil on retorting; in source rocks,, kerogens are the generators of oil and gas)., , 7.1.1 Crude oil, or petroleum, Crude oil, or petroleum, is a liquid of yellowishbrown to black colour, with a green fluorescence, under UV-light. Its odour is rarely aromatic but, usually unpleasant due to traces of sulphur compounds. Petroleum consists predominantly of, hydrocarbons, with a minor share of sulphur,, oxygen and nitrogen compounds. The chemical, differences between oil and black coal (high, volatile bituminous A) are much higher hydrogen, and lower oxygen contents of petroleum, (Table 7.2)., The residue (ash) from burning oil contains a, number of elements, some of which may be, recoverable (e.g. vanadium). Trace elements, include Si, Fe, Al, Ti, Ca, Mg, V, Mo, Ni, Ba, Sr,, Mn, Pb, Cu, Cr, U (plus daughter nuclides) and, noble metals. Some elements may foul up the, catalysts in refineries (e.g. arsenic). Several of the, trace elements (V, Ni, Mo) originate in organic, substance, whereas others reflect geochemical
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Table 7.2 Average elemental composition of fossil fuels,, asphalt and kerogen (Hunt 1996, in wt. %), , C, H, S, N, O, Sum, , Gas, , Petroleum, , Asphalt, , Black coal, , Kerogen, , 76, 24, 0, 0, 0, 100, , 84.5, 13, 1.5, 0.5, 0.5, 100, , 84, 10, 3, 1, 2, 100, , 83, 5, 1, 1, 10, 100, , 79, 6, 5, 2, 8, 100, , characteristics of source rocks, and diagenetic, reactions., Petroleum is a mixture of hundreds of chemical, compounds, which are investigated by ionization, methods, liquid chromatography and mass, spectrometry (Hunt 1996, Panda et al. 2009). Main, groups are normal and branched-chain alkanes, (paraffins), cyclo-alkanes (naphtenes), mono- and, polyaromatic hydrocarbons and the rare olefins, (Figure 7.1). All oils contain some asphaltics,, or nitrogen-sulphur-oxygen (N-S-O) compounds., The latter give petroleum its colour and odour., Most petroleum components are products of, decomposition, condensation and polymerization, reactions after burial of organic substances. Their, chemical diversity can be described as follows:, , ., , ., , ., , Paraffins (CnH2n þ 2, alkanes, or aliphatic hydrocarbons) are economically most prominent,, because they are easily transformed into the, most valuable refinery products (gasoline, kerosine). Paraffins comprise gases at n ¼ 1 4, (methane, ethane, propane, butane), liquids at, n ¼ 5 15 and solids at n > 15 (e.g. ozocerite)., Paraffins are “saturated” hydrocarbons, because, all available carbon bonds are saturated with, hydrogen. Second only to naphthenes, they are, the most common molecular structures in, petroleum. Paraffins predominate in the oldest,, most deeply buried reservoirs. Typical precursors are paraffinic waxes and lipids of plants., Cycloparaffins (CnH2n, cycloalkanes, or naphtenes) form 50% of an average crude oil, with, the percentage increasing in the heavier fractions. Naphtenes are built from carbon rings of, 5 or 6 atoms. Like paraffins, they are saturated, hydrocarbons., Aromatic hydrocarbons (CnH2n-6, benzenoids, and arenes) amount rarely to more than 15% of, crude oil. In distillation, they are concentrated in, the heavy fraction. In spite of their name, aromatics have very little odour. The strong smell, of crude oil is due to traces of non-hydrocarbons., All aromatic hydrocarbons contain at least, one benzene ring, a flat 6-carbon ring (C6H6) in, , Publisher's Note:, Image not available, in the electronic edition, Figure 7.1 Simplified classification of crude oil, by compositional variation of paraffinic,, naphtenic and aromatic hydrocarbon fractions., Position of average oil in important provinces, after Miles (1994). This diagram was first, proposed by Tissot & Welte (1984). By permission, of Oxford University Press., , 525
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526, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , which the fourth bond of each carbon atom is, shared. Aromatic hydrocarbons are unsaturated,, i.e. they react by adding hydrogen or other elements to the ring., . Asphaltics (non-hydrocarbon nitrogen, sulphur, and oxygen compounds) predominate in the, residual fraction of crude oil distillation., Typically, they contain very small amounts of, N, S or O, in addition to C and H, in compounds, such as thiol, phenol, pyridine and quinoline., The asphaltic residuum comprises resins,, waxes and asphaltenes. The latter are the heaviest molecules of crude oil. A typical asphaltene, molecule may comprise groups of condensed, aromatic and naphtenic rings that are connected, by paraffin chains and sulphur or oxygen bridges., The condensed aromatic structures of asphaltenes have free-radical sites that are capable of, complexing metals (vanadium, nickel, etc.)., Average contents of asphaltenes in ordinary, crudes are 3 wt. % and may reach 15%, but, some Middle East oils are reported to contain, >50%. Asphaltenes are part of the primary composition of crude petroleum, but are also a product of secondary bacterial degradation. They, contain clues to the formation temperature of, oil (Di Primio et al. 2000)., Biomarkers, or geochemical fossils are specific, organic compounds found in petroleum and, sediments, which can be useful biochemical, indicators for the former presence of certain organisms (e.g. bacteria, diatoms, dinoflagellates)., Biomarkers are derivatives from original substances of live organisms, such as haemoglobin, and chlorophyll. These compounds (porphyrins), are extremely useful for reconstructing sedimentary conditions, for correlating oil to oil, or oil to, source rock, and as indicators of oil maturity,, because they change with rising temperature. Biomarkers can even be determined from oil in fluid, inclusions, as at Elliott Lake, Canada, where pebbles of the uraniferous conglomerate with an age of, 2.4 Ga contain oil that is derived from cyanobacteria and eukaryotes (Dutkiewitz et al. 2006)., Sulphur concentration of petroleum rises with, higher nitrogen contents and density. If reservoir, rocks are limestone, dolomite or anhydrite, oil, will be rich in sulphur. Petroleum may contain, , sulphur in elemental form, in organic compounds,, or as dissolved H2S gas (typically 5–14 wt. %). The, source of sulphur can be organic precursors of oil or, the host rocks. “Sweet” oil contains <0.5% S, but, today this applies to <30% of well production. Oil, with higher sulphur is “sour”. In refinery products, sulphur is unwanted. Desulphurizing oil, and natural gas, and the resulting by-product sulphur, provide a large part of world sulphur supply., The technical characterization of crude oil is, based on density and the yield on standardized, fractional distillation. Most oils fall into a density, range between 0.7 and 1.06 g/cm3 (at 20 C). In the, industry, API degrees based on density at 15.6 C, are more commonly used (eq. 7.2)., Computation of API (American Petroleum Institute) gravity from density data:, Degrees of API gravity, 141:5, ¼, 131:5, Density at 15:6 C ð60 FÞ, , ð7:2Þ, , 10 API is the density of pure water. Increasing numbers, indicate lower densities (e.g. 30 API equals D ¼ 0.8762 g/, cm3)., , Typical conventional crude oils have 22–35, API. Markets distinguish classes of extra heavy, (<10 API), heavy (10–22.3 API), medium, (22.3–31.1o API) and light oil (above 31.1 API)., Condensates (“natural gasoline”) have >55 API;, they are gaseous in the reservoir and clear to strawcoloured liquids at surface conditions. Compared, with average oil (1.5–1.9) the H/C-ratio of heavy oil, is less than 1.5, which reduces the recovery of, gasoline considerably., Fractional distillation is the common processing technique for petroleum. In distillation, towers of refineries, crude oil is separated into, various molecular size fractions. The smallest, molecule in petroleum is methane (CH4), with a, molecular weight of 16, and the largest molecules, are asphaltenes that reach molecular sizes in the, tens of thousands. Chemical processes such as, cracking, polymerization and reforming are, employed in order to improve output of the most, valuable marketable products from crude oil. Gasoline, kerosine and diesel fuel are manufactured
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Table 7.3 Example of products and boiling ranges for, crude oil distillation (Hunt 1996), Light gasoline, Gasoline, Kerosine, Light gas oil (diesel fuel), Heavy gas oil and lubricating oil, Asphaltic residuum, , 0–120 C, 120–190, 190–260, 260–360, 360–530, (not boiling), , from smaller or larger molecular fractions. Products, and boiling range standards of refineries vary; the, example in Table 7.3 is from a US Gulf Coast, refinery., Crude oil is classified as predominantly paraffinic, naphtenic or aromatic (Figure 7.1). Generally, the fraction of paraffins increases with depth, as the density of oil decreases by thermal maturation. There is speculation that the autochthonous, microflora (thermophilic bacteria and hyperthermophilic archaea) of deep oil pools may have a role, in this (e.g. in the Paris Basin: Haridon et al. 1995)., Even the conversion of long-chain alkanes into, methane at great depth may be catalysed by anaerobe microbes (Zengler et al. 1999). Oil in older and, more tectonized sediments also tends to be richer, in paraffins, whereas younger and little deformed, rocks are more likely to contain aromatic oil. This, is probably caused by a different time of maturation. Exceptions from the general rule are common, however. From lacustrine source rocks, for, example, paraffinic and high-wax oils are formed,, even at low temperature. Contact with meteoric, water and microbes causes enrichment of aromatics and N-S-O compounds and reduces the, quality of oil (degradation)., Viscosity is a vital parameter for the extraction of, oil from the reservoir, because it controls recovery., Viscosity is a function of temperature and pressure,, and of the composition of the oil. Density of oil and, viscosity are positively correlated. Accordingly,, heavy and extra heavy oil have viscosities so high, that flow is extremely slow and with conventional, technology, commercial extraction is not possible, (cf. Chapter 7.5 “Petroleum Mining”)., Particular geochemical indices characterizing, oil include the carbon preference index (CPI; chain, length of certain paraffins), the pristane/phytane, , 527, , ratio, isotopic data and the V/Ni ratio. They assist,, for example, source rock-oil correlation and determination of maturity, which are essential for, understanding the petroleum system of an oilfield, and of whole basins. Also, they allow differentiation of oil families that are derived from source, rocks of similar sedimentary facies., Isotopic characterization of petroleum is mainly, based on the systems 12C/13C, H/D, 32S/34S and, 14, N/15N. The organic predecessors are marked by, light d13C and dD, with lowest negative values in, lipids. Kerogens that are derived from this biomass, contain slightly changed isotope ratios, due to, exchange with formation water (Schimmelmann, et al. 1999). Petroleum that is derived from kerogen inherits its isotope characteristics, supporting, source rock-oil and oil-oil correlations. 12C in oil is, somewhat enriched compared to kerogen. Accordingly, the wide spread of petroleum isotope data,, with d13C from –20 to –32‰ and dD from 60 to, 180‰, mainly reflects different biomass and, sedimentary environments., 7.1.2 Natural gas, Natural gas is always a mixture of more than one, gas. In addition to gaseous hydrocarbon compounds, CO2, H2S (toxic), N2 and several noble, gases are always present. With the exception of, helium, these admixtures have no market value, and at high percentages impede economic extraction. The hydrocarbons in natural gas are mainly, methane and other low-molecular paraffins (C1C4) and their isomers. Temperature and pressure, determine the composition of the gas phase,, because several higher hydrocarbons are gaseous, in the reservoir but condense when the pressure is, lowered (e.g. in the production well during the rise, towards the surface). These condensates characterize “wet gas” with over 4 litres condensable, liquids per 100 m3. Dry gas consists mainly of, methane (commonly 90 mole %). The “dryness”, of gas can be characterized by the percent methane/percent ethane ratio. “Sour gas” has elevated, fractions of sulphur and CO2, “sweet gas” contains less than 2% CO2 and no H2S. Only sweet gas, can be directly used, sour gas must first be refined., In the recent past, by-product sulphur made the
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528, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Lacq gas deposit in southern France (methane with, 15% H2S and 10% CO2) one of the largest sulphur, producers in the world. Today, annual production, of elemental sulphur from gas operations is 25, Mt (mainly in Russia, Canada, the Middle East). In, the future, much H2S may be re-injected if, a projected sulphur oversupply arrives. Natural, gas can have trace contents of toxic metals such, as mercury (e.g. in Northern Germany up to 1 mg/, m3) that must be removed before use., High nitrogen or carbon dioxide contents of, deep natural gas may be explained by different, speed of flow: As methane (density of gas at, surface pressure 0.72 g/L) migrates towards the, surface, heavier nitrogen (N2 1.25 g/L) and carbon, dioxide (1.98 g/L) lag behind at depth. CO2 may be, derived from several sources, which include maturation of organic substance and degassing of, deep magma bodies. 3He/4He and CO2/3He ratios, are employed to discern between the two variants, (Ballentine et al. 2001). The Bravo Dome gasfield, in New Mexico, USA, produces almost pure, magmatic CO2 from depths of 600–700 m. This, gas is used for carbon dioxide injection to maximize oil recovery from oilfields in western Texas., Metamorphic devolatilization of carbonate rocks, (e.g. calcsilicate formation) may also contribute, CO2 (Huang et al. 2003). Gas with higher nitrogen, contents and almost pure nitrogen gas may be, a product of organic matter maturation. Littke, et al. (1995) demonstrated that after peak, methane production, coal enters a stage of, maximal nitrogen discharge. Alternatively,, much nitrogen can be liberated by K þ substitution from illite containing NH4 and subsequent, oxidation of ammonium (Friberg et al. 2000,, Mingram et al. 2005). Less credibly, descending, meteoric waters with dissolved atmospheric, nitrogen may be a source. Determination of, isotope ratios 14N/15N in gas can assist in identifying the origin of nitrogen (Gerling et al. 1997;, Williams et al. 1995)., Nitrogen-rich natural gas may contain up to 2.5, vol. % helium, as in parts of the Hugoton-Panhandle gasfield (Kansas, Texas: Ballentine & Lollar, 2002). Small, uneconomic concentrations of, helium, neon, argon and xenon occur in all natural, gas deposits. They are important genetic tracers., , For example, relative contents of the stable isotopes 4He and 3He indicate the source of the gas., Radiogenic 4He is a product of a-decay of various, members of the U and Th nuclide series that are, enriched in crustal rocks. The “primordial” 3He,, in contrast, originates predominantly from the, Earth’s mantle where it was captured during planetary accretion. Crustal helium has a ratio of, 3, He/4He of 0.02 RA (RA is the 3He/4He ratio, of air equal to 1.4 106). Higher ratios reaching, 8 RA show that degassing of the mantle contributes to He-contents in gas deposits, most frequently in tensional tectonic settings (Kennedy, & van Soest 2007) and in volcanic provinces with a, component of mantle magmas:, Helium (He) is the lightest (0.1785 kg/m3 at standard, pressure) of the six noble gases (helium, neon, argon,, krypton, xenon, radon). All are chemically inert and, serve many purposes, but helium is especially important. Apart from filling air ships and party balloons, it, is employed in industrial processes such as extremely, low-temperature cryogenic applications, or for flooding semiconductor fabrication tables in order to, prevent contamination of the chips. Although, noble gases can be produced from air, most of the, traded helium is derived as a by-product from, North American natural gasfields (e.g. Hugoton)., Exploitable contents range from 0.3 to 1.5 vol %. In, the near future, Qatar and Algeria are expected to, become important producers., , Gas hosted in sulphate or carbonate rocks is, usually enriched in hydrogen sulphide, sometimes, to >90% H2S. Most of this is a product of thermochemical sulphate (anhydrite) reduction by reaction with hydrocarbons at >100 C (cf. TSR in, Chapter 1.4 “Diagenetic Ore Formation Systems”,, eq. 1.21). The main field of microbial sulphate, reduction lies below 100 C. As native sulphur, melts at 113–120 C, many wells penetrating into, deep carbonate-anhydrite rocks have encountered, (and produced) liquid sulphur plus H2S gas., Natural gas is formed by several different, processes from the Earth’s surface down to great, depths:, 1 Near the surface and at low temperature, bacterial fermentation of organic substances produces very pure bacterial, or biogenic methane;
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529, , PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Rotliegend dune sands, Zechstein (Ca2) carbonates and Buntsandstein. Of course, most of the, methane formed during coalification dissipates, towards the surface. Gas remaining absorbed in, coal (“coal bed methane”) is increasingly exploited, and is of significant economic impact (Gayer &, Harris 1996)., Isotopic geochemistry provides fundamental, information on natural gas formation (Whiticar, 1999, Schoell 1983; Figure 7.2). Bacterial/biogenic, methane is characterized by very light carbon (low, d13C). Exceptionally heavy carbon was found in, methane inclusions of German potassium salt,, due to evaporation in the bittern brine (Potter, et al. 2004). Gas produced by thermal maturation,, including associated and dry gas, contains heavier, carbon with increasing maturity. Because of different precursors, hydrogen isotopes of biogenic, methane exhibit a wide spectrum. Similar to carbon, the D/H ratio increases with maturity. This, conforms to the general observation that light, isotopes are first mobilized. Of course, mixing of, gases of different origin, later bacterial influence, (e.g. bacterial methane oxidation) or migration, may complicate this simple pattern., , 2 With increasing depth and elevated temperature, kerogen is partly converted to petroleum, and associated, or primary petroleum gas;, 3 Deep below the surface and at high temperature,, oil breaks down into secondary wet gas ( condensates) and pyrobitumen., Humic kerogen is transformed into dry gas., Parallel to (2) and (3), a large mass of methane is, expulsed from coal seams., Associated gas is dissolved in oil; only when, saturation is reached, a free gas cap forms on top, of the oil pool. Although one of the principles, of reservoir management is always to conserve, energy, oil cannot be extracted without some loss, of gas pressure. The dissolved gas makes oil frothy, as pressure decreases and at the surface, several, 100 m3 of gas may be separated per m3 oil. Earlier,, flaring-off petroleum gas was common industrial, procedure, but is very rare now., Many large and one giant gasfield (Groningen) in, continental Europe have been formed from methane that originated in deeply buried Late Carboniferous (Westfalian) coals and older black shales, below a Permo-Mesozoic cover blanket. Reservoirs include Carboniferous jointed hard rocks,, , δ D Methane (‰), -65, , -60, , -340, , -300, , -260, , -220, , -180, , -160, , -100, , Microbial gas, , Figure 7.2 Isotopic differentiation of methane, origin as a function of diagenetic grade (maturity), of source rocks. The scale on the right provides a, measure of maturity (Ro ¼ vitrinite reflectance in, oil immersion). Modified from Schoell (1983)., AAPG [1983] reprinted by permission of AAPG, whose permission is required for further use., , -50, , -45, , Wet, thermogenic, gas, , Mixed microbial, & thermogenic, gas, , M, , 0.5, , at, , ur, at, , io, , 0.7, , n, , 1.0, -40, , -35, , Dry, thermogenic, gas, , 1.3, 2.0, 3.0, 4.0, , Source rock Ro (%), , δ 13C Methane (‰), , -55
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530, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , 7.1.3 Natural gas hydrates (clathrates), Natural gas hydrates (clathrates) are ice-like solids, with determinate crystal structures that are, characterized by cages of water molecules. The, cages may contain methane or higher molecular, hydrocarbons as “guest molecules”. Complex gas, hydrates with isobutane, propane and/or cyclopentane, in addition to methane, are more stable, than simple methane hydrate (Hailong Lu et al., 2007), extending the P/T stability field of natural, gas hydrates from a minimum of 200 m water, depth down into oceanic sediments. Generally,, the bottom of hydrate stability depends on the, local geothermal gradient, which causes temperature to rise above dissociation conditions as depth, increases. This occurs commonly at 200–500 m, below the sea floor (Pecher 2002). Pressure release, (e.g. a sharp drop in sea level), or rising temperature, liberate the occluded gas. Sudden melting of the, Earth’s gas hydrates increases greenhouse warming. Events of this kind were identified by strong, spikes of atmospheric 12C at the end-Triassic, extinction (Beerling 2007) and in the early Tertiary, (Dickens et al. 1997a), including the PalaeoceneEocene thermal maximum. Giant beds of gas, hydrates have been found in continental arctic, sediments, in seafloor sediments worldwide and,, less frequently, as exposed white “rocky mounds”, on the sea floor (e.g. Gulf of Mexico: Max 2000). At, some locations, hydrates occur in sea-floor gas, , Depth (m below seafloor), , 0, , SW, , 994, , 995, , vents, which have a diameter of hundreds to thousands of metres (Haacke et al. 2009). The methane, in hydrates is derived from sediments beneath the, hydrate layer. The source of the gas in hydrates can, be narrowed down using (129) iodine age determination (Tomaru et al. 2007). Natural gas hydrates, contain remarkable methane volumes, at normal, (surface) pressure up to 164 times the hydrate, volume. In spite of giant resources, experiments, for economic recovery of hydrate gas were only, recently initiated. Estimates range from 500 to, 5000 Giga-tonnes (Gt) of carbon in methane, compared to 140 Gt of carbon in natural gas and, 700 Gt in coal reserves. The occurrence near Blake, Ridge, off the southeastern coast of America (Figure 7.3), is thought to contain a volume of methane, that could supply the total gas demand of the, United States for over 100 years (Dickens et al., 1997b). In northern Canada, an ongoing large-scale, pilot project aims at commercial production of gas, from hydrates below the land surface (Mallik,, Richards Bay Island, Beaufort Sea)., , 7.1.4 Tar, Tar is a term for heavy and extra heavy oils (6–12, API), that are highly viscous and sulphur-rich. Tar, is a low quality and unconventional hydrocarbon, resource. Only now, the exploitation of tar begins, to reach a scale that is significant. Tar is the, , Seafloor, , 997, , NE, , Gas hydrate, 400, , BSR, , Free gas, , 800, , 5 km, , Figure 7.3 Seismic profile of the giant methane deposit at Blake Ridge, situated East of the North Carolina coast in the, Atlantic Ocean, under 2800 m of water (Fleming et al. 2003). Courtesy Geological Society of America. The remarkable, feature is that trap and seal are formed by a layer of low permeability gas hydrates with a thickness of 250 m. BSR ¼, bottom simulating reflector, 994-997 are drill holes.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , residuum of a degradation of normal petroleum., Degradation is essentially the loss of light hydrocarbons and an increase of N-S-O compounds., Several pathways of degradation are possible. The, formation of the tar in the Canadian tar sand, deposits, arguably the world’s largest hydrocarbon, resource after Russia, is due to biogenic processes, (Head et al. 2003). At T <80 C, anaerobic microbes catabolize alkanes and produce organic, acids. Metals, asphalthenes and resins are enriched in particles surrounded by liquid hydrocarbons, resulting in a colloidal suspension. An, important co-product of oil degradation is biogenic, methane that may form gas deposits if suitable, traps are present., 7.1.5 Earth wax (ozocerite), Earth wax (ozocerite) is essentially composed of, high-molecular paraffins. Earth wax precipitating, from paraffinic petroleum typically clogs pipes, and drillholes. Its consistency ranges from a soft, paste resembling medicinal salve to a brittle solid., Light yellowish to brown colour is common. The, density of earth wax is between 0.84 and 0.93,, melting occurs at 85–100 C. It is soluble in petroleum, gasoline and organic solvents. Earth wax, deposits are veins and stockwork bodies similar to, hydrothermal ore deposits. Wax is concentrated in, upflow channels of paraffinic oil, where pressure, or temperature changed rapidly. Today, earth, wax mining is hardly competitive with products, extracted from petroleum., 7.1.6 Pyrobitumens, Pyrobitumens are black, hard, infusible (unlike, ozocerite), insoluble, non-volatile hydrocarbon, substances that occur in vein deposits. Pyrobitumens are commonly formed at high temperatures, and pressures, which induce conversion of petroleum to dry gas and pyrobitumen. The term is,, however, non-genetic and physically and chemically similar material formed by surface degradation of oil seeps is also called pyrobitumen (Hunt, 1996). Thermal pyrobitumens originates from, petroleum at T >150 C, by sudden pressure drop, and loss of dissolved gas and condensate (crack-, , 531, , ing). Therefore, these substances are remains of, former deep petroleum occurrences (Mossman &, Nagy 1996, Stasiuk 1998)., 7.1.7 Natural asphalt, Natural asphalt is a product of petroleum degradation similar to tar, but more advanced. It is formed, at shallow depth or the surface under the influence, of meteoric water and aerobic microbes. Note the, increase in oxygen compared to oil (Table 7.2)., Asphalt contains only a small fraction of hydrocarbons and is mainly composed of N-S-O compounds including asphaltenes. Asphaltene, colloids with resins and waxes (micelles) form, a solid, three-dimensional framework. The term, asphalt is also used for the residual fraction, of industrial oil distillation. Natural asphalt is a, brown to black brittle solid with conchoidal fracture. Its density is 1–1.3 g/cm3 and melting occurs, at 100–140 C. Natural asphalt capping oil seeps is, commonly mixed with clay and sand., 7.1.8 Kerogen, Kerogen is that part of organic matter in a rock that, is insoluble in organic solvents. This is the difference to bitumens (hydrocarbons) which are soluble, but in most rocks are only found in trace, amounts. Kerogen originates from many different, organisms. The main habitat of natural hydrocarbons – marine sediments as opposed to coal that is, formed on land – implies that marine plankton, should be the most common precursor of kerogen, and petroleum. Marine plankton has a high percentage of lipids with elevated hydrogen contents,, including fats, resins, waxes and oils. However,, four types of kerogens are commonly differentiated according to different contents of C, O and H, (Figure 7.4). The affinity with certain coal macerals (cf. Chapter 6.1 “The Substance of Coal”) is, indicated in Table 7.4., Because kerogen types I and II have the highest, hydrogen contents, they are prime sources of, petroleum, although type I is relatively rare. Most, oils are derived from type II kerogen, which is, characterized by the “formula” C515H596O72 (on, the verge of generating oil; Helgeson et al. 2009). It
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532, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , little hydrogen and is unable to generate oil, but, may contribute some gas at high temperatures., Kerogens are investigated by coal petrographic, methods (especially fluorescence microscopy:, Taylor et al. 1998), infrared spectroscopy and various methods of organic chemistry (pyrolysis, gas, chromatography, mass spectrometry, etc.). Industry standard determinations for rapid throughput, of a large number of samples (e.g. drill cuttings), include vitrinite reflectance yielding a measure of, kerogen maturity, and anhydrous or hydrous, (Rock-Eval) pyrolysis (characterizing kerogen, types and their potential for generating hydrocarbons: Espitali, e et al. 1984; Figure 7.5). In addition,, Rock-Eval pyrolysis provides an indication of the, maximum temperature to which a sample of kerogen was equilibrated., , H/C, , 2.0, , I (limnic), Oil shale, , O, , 1.5, , II (marine), , P, III (humic), , 1.0, , IV (inert), G, 0.5, , (Anthracite,, graphite), , 0, 0, , 0.1, , 0.2, , O/C, , Figure 7.4 Progressive chemical changes of kerogen, types with increasing temperature in the Van Krevelen, diagram (modified from Tissot & Welte 1984). With, permission from Springer Science þ Business Media., Only kerogen types I and II produce important amounts, of oil. Type III and IV are almost exclusively sources of, natural gas. Arrows stress the direction of chemical, evolution with increasing temperature (maturation). O ¼, Early formation of oxygen-rich products (CO2, H2O); P ¼, Generation of petroleum; G ¼ Generation of natural gas., Note the field of typical oil shales., , Publisher's Note:, Image not available, in the electronic edition, , is often labelled as representing “marine organic, matter”, but more precisely its dominating precursor is marine phytoplankton. Type III represents particles of higher plants similar to coal and, is mainly a source of gas. Type IV contains very, , Table 7.4 Main types of kerogens, their atomic H/C, ratio and common precursors, Type 1, Type 2, Type 3, Type 4, , H/C 1.9–1.0, H/C 1.5–0.8, H/C 1.0–0.5, H/C 0.6–0.1, , algae and bacteria, liptinite and marine plankton, vitrinite, inertinite, , Figure 7.5 Petroleum and gas source rocks with, different kerogen types in the hydrogen and oxygen index, diagram as a result of Rock-Eval pyrolysis (after Miles, 1994). By permission of Oxford University Press. Dots, represent hydrocarbons and CO2 generated from kerogen, by cracking upon heating from 350–550 C.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , During early diagenesis of sediments, dispersed, organic matter is transformed into kerogens that, generate CO2, H2O, oil and gas as they mature. At, high pressure and temperature near the transition, to low-grade metamorphism, kerogens approach, the composition of graphite (Figure 7.4) and lose, their hydrocarbon generation potential. Kerogens, that are in the initial stages of maturation release, hydrocarbons when heated. This motivated the, term kerogen (Greek for “generating wax”) and is, the reason why rocks that have a high content of, low maturity kerogen are considered to be significant future resources of hydrocarbons (cf. Chapter, 7.7 “Oil Shales”)., , 533, , several oil deposits in China (Bing-Quan et al. 2001)., Many oceanic serpentinites and gabbros contain, traces of methane and other low-molecular paraffins,, which are formed abiotically by Fischer-Tropsch, reduction of CO2 dissolved in convecting seawater, and oxidation of Fe2 þ in host rocks. This process is, a by-product of the serpentinization of ultramafics, that liberates large amounts of H2. The methaneforming reaction between dissolved HCO3 and H2, is catalytically accelerated by Ni-Fe or Fe-Cr phases, (Foustoukos & Seyfried 2004). d13C values of this, “catalytic” methane resemble biogenic gas (Horita, & Berndt 1999), but at the Lost City hydrothermal, field, methane d13C is much heavier (9 to 14‰:, Proskurowski et al. 2008)., , 7.2.1 Petroleum source rocks, 7.2 THE, , ORIGIN OF PETROLEUM, , AND NATURAL GAS, , Nearly all deposits of crude oil and natural gas are, derived from organic matter, which was buried, with sediments. Sediments hosting the source, organic matter are typically fine-grained and display low permeability. Siliciclastic source rocks, (clay and silt) are most common, but carbonate, pelite and algal mats in evaporites are also economically significant (Katz 1994). Early diagenesis, near the surface in still unconsolidated sediments, produces much microbial methane (Figure 7.2),, but this is rarely conserved. At larger depths,, increasing diagenesis releases petroleum and wet, gas from lipid-rich kerogen. Kerogen “spent” after, oil generation, dispersed humic kerogen and bituminous coal are sources of dry thermogenic gas:, However, there are exceptions and an abiotic origin of, hydrocarbons was repeatedly proven, although hardly, ever concentrated to economic significance., Recently, Fiebig et al. (2009) demonstrated the abiogenic origin of volcanic-hydrothermal methane in the, Aegean island arc. Modelling the composition (H2O,, CO2, CH4, H2, CO, O2 and C2H6) of the COH fluid, system in the Earth’s mantle clearly shows that, generation of hydrocarbons such as methane and, ethane is perfectly possible (Zhang & Duan 2008)., Long ago, degassing of the mantle as a source of, methane was proposed for natural gasfields in Miocene volcanoclastic rocks (“green tuffs”) of Japan., Clear mantle signatures mark the geochemistry of, , Petroleum source rocks include any rock that may, generate crude oil. Commonly, marine and lacustrine source rocks are distinguished, but because, terrestrial organic substance is carried far into the, oceans, petroleum sources of a transitional mixed, character are quite frequent. Although not exclusive, black shales are the most common hydrocarbon source rocks apart from their, metallogenetic role (cf. Chapter 1.3 “Sedimentary, Ore Formation”). Global source rock horizons (e.g., Jurassic Figure/Plate 7.6) are the product of major, perturbations of carbon and climate cycles caused, by orbital forcing, endogenetic (plate tectonic) and, exogenetic processes (Emeis & Weissert 2009)., Marine source rocks, Modern oceans hold an estimated 700,000 Mt of, carbon as dissolved organic matter (DOC), more, than biomass on land and a little less than carbon, in the atmosphere. Phytoplankton converts atmospheric CO2 to 60,000 Mt/y of organic carbon,, equivalent to plant growth on land; 95% of marine, DOC is refractory, i.e. hardly bio-digestible., Annually, dead organisms and DOC, such as complex polysaccharides and humic acids, deposit, 300 Mt carbon in seafloor sediments. This compares to 5500 Mt/y C extracted in the form of, oil and gas. Of course, ocean productivity was, much higher during hothouse states of the, Earth, when source rock formation peaked. As a
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534, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Figure 7.6 (Plate 7.6) Early Mesozoic bituminous rocks in the upper Kali Gandaki valley of the Annapurna-Dhaulagiri, zone, western Nepal. This is part of the unmetamorphosed sediments of the Tethyan zone above the crystalline, Greater Himalayan Sequence. Courtesy Krishna Karki, Travel-to-Nature Asia. This image serves as a paradigm for, hydrocarbon source rocks. Further up in the mountains, natural methane seepage feeds eternal flames in Jwala Mai temple., , consequence, all fine-grained marine sediments, contain a fraction of organic matter. For its preservation, reducing conditions are required. A euxinic, environment (i.e. H2S dissolved in deep water) is, not essential, but does promote high hydrogen, contents. For the production of a good source rock,, bottom sediments should certainly not be affected, by oxidation. Low-energy seafloor areas provide, favourable conditions. Marine clays display an, average of 2.1% total organic carbon (TOC), carbonates 0.29%, but sandstone only 0.05%. Of course,, sapropels (organic sludges) and comparable consolidated rocks include high TOC fractions and constitute important petroleum source rocks. In some, regions (e.g. the Arabian Gulf), algal mats in evaporitic sediments such as dolomites, sulphates and, salt generated much oil. Evaporites display typical, lamination, which reflects periods of higher and, lower salt concentration (cf. Chapter 4.2.2, “Environments of Evaporite Formation in the Geological Past”). At lower salinity, halotolerant organisms (mainly cyanobacteria and algae) flourish, in surface water (Figure/Plate 4.12). The character-, , istic thermal and density stratification of brine, pools favours anoxic conditions at depth and preservation of organic matter (Warren 2006)., A Holocene example of sapropel formation is the, Black Sea. At 7000 years BP, the previously dry basin, was flooded with saline water through the Bosporus., The water inflow caused a burst in surface-water, productivity, inducing anoxia and preservation of, organic matter (Arthur & Dean 1998). This initiated, deposition of finely laminated black mud, which, contains 23–35% organic substance and a maximum, of 10% soluble bitumen. Incidentally, this event is, possibly the background to the Biblical flood narrative. Since 5500 years BP, organic productivity is, reduced but even today, black sapropel is laid down in, two deep (>2000 m) basin centres around which, marine currents revolve. Annual couplets comprise, one clay and one TOC-rich varve. Generally in euxinic basins, higher organisms thrive in the upper,, oxic seawater layer. Dead phyto- and zooplankton, drifts to the seafloor, traversing the deep anoxic, H2S water layer. Remember that the term “euxinic”, is derived from the Roman name of the Black Sea,, Pontus euxinus (the hospitable sea).
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Sedimentation of organic particles and refractory DOC is only one path of organic carbon (Corg), enrichment. During early diagenesis, dissolved, Corg may be adsorbed to clay minerals and incorporated into interlayers of smectites and smectiteillite minerals (Kennedy et al. 2002). H2S and HSin the pore fluids cause an early diagenetic, “vulcanization” of the organic substance, preventing bacterial decomposition but increasing sulphur contents. Sapropels and sapropel rocks, display a characteristic trace element composition, (cf. Black Shales in Chapter 1.3 “Sedimentary Ore, Formation Systems”). Vanadium and nickel have, a special role, because these elements complex, with porphyrins, which are transferred into crude, oil. During oil generation and migration, the V/Ni, ratio remains unchanged and allows correlation of, the whole cycle from source rocks to petroleum,, oil seeps and natural asphalt (Hunt 1996)., Petroleum source rocks are not restricted to, confined marine basins. Upwelling of cold, phosphate-rich water in oceans causes a synergetic, increase of life, which delivers a large mass of, organic matter into bottom sediments (cf. Chapter 3 “Phosphates”). Fluvial import of nutrients, into shallow warm epicontinental seas promotes, organic production and deposition in platform, sediments. Warming of the oceans in greenhouse, periods of the Earth’s past, for example in the, Cretaceous with its much higher sea levels, may, trigger ocean-wide mass production and associated anoxic conditions at the bottom. Resulting, sediments with elevated organic matter fractions, are termed “oceanic sapropels” in contrast to the, confined euxinic facies. Even a rather ordinary, TOC content of marine sediments of 1.5% can, be the source of oil deposits, although after the oilgenerating stage, most good source rocks have, residual TOC values between 2 and 3%. The loss, of organic carbon in source rocks is correlated to, increasing diagenesis, due to formation and expulsion of petroleum and gas. The loss is measurable, by the C/S ratio, which decreases with increasing, diagenesis. Source rock sulphur is immobilized, early, mainly in pyrite, so that it constitutes, a reference for the diagenetic mobilization of, organic carbon. Oil from purely marine organic, substance is typically sulphur-rich, low-wax,, , 535, , dominated by naphthenes, aromatics and shortchain (liquid) normal paraffins., Lacustrine oil source rocks, Both saline and freshwater lakes often display, stages of high organic production leading to formation and preservation of sapropelic sediments,, supported by thermal and density stratification., Modern examples include the great lakes in, Central and Eastern Africa (Johnson et al. 1996)., Well-studied ancient equivalents are the Tertiary, lakes of Colorado, Utah and Wyoming, with, oil shales of the Green River Formation (Meyers, 2008; cf. Chapter 3 “Sodium Carbonate”). Coals, and coal measures can source petroleum, although, restricted by the relatively low hydrogen content, of most coals, and the relative rarity of sapropelic, (hydrogen-rich) coal, which is estimated at <10%, of world coal mass (Law & Rice 1993). In laboratory experiments, the quantity of expelled oil, correlates directly with the hydrogen content of, coal (Hunt 1996). In nature, liquid bitumen generated is largely retained in the coal (Taylor et al., 1998). However, several oilfields in the world are, derived from coal, for example in northwestern, China from Jurassic coal, which is not even especially rich in liptinite (Shao et al. 2003). Petroleum, derivation from terrestrial organic matter in, marine rocks is common, for example in the offshore Gippsland Basin of southeastern Australia., The source of Gippsland oil and gas is the Tertiary, Latrobe Formation, which contains the giant, onshore lignite deposits (Figure 6.12). Compare, this with Amazon River, which exports 70 Mt, carbon per year into the sea. Freshwater and brackish-water organic substance is prone to form lowsulphur and high-wax oil (containing pasty longchain paraffins). Waxy paraffins are inherited from, waxes that land plants use to protect their external, surface. High-wax oil is very common and attests, to the large share of land-derived organic matter in, coastal marine sediments., With sufficiently detailed geochemical data, (Hunt 1996), nearly all oil deposits can be matched, with specific source rocks. The world’s largest, petroleum deposit, Ghawar in Saudi Arabia, and, many other accumulations in the Middle East are
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536, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , 2007). The world’s largest dry gas field, South, Pars (Iran)– North Field (Qatar), is supposedly, sourced from deeply buried Early Silurian marine, shales rich in kerogen type II (Aali et al. 2006)., The trap is a large arch trending northwards, across the Arabian-Persian Gulf and the reservoir is formed of Permo-Triassic carbonates, sealed by evaporites., A remarkable number of gasfields in the world, are derived from coal. Beginning at 37% volatile, matter or 0.8% vitrinite reflectance, profuse, amounts of methane (with some ethane, propane,, butane) are generated from coal as coalification, progresses (Figure 7.7). Much of the gas moves, away through pores, joints and faults of country, rocks towards the surface and dissipates in the, atmosphere. Some may be trapped in appropriate, structures. Gas that remains adsorbed in coal is, part of the important energy resource of coal bed, methane (CBM)., Near-surface kerogen-rich rocks flooded by, meteoric water may host considerable gas deposits, which are generated by microbes (McIntosh, et al. 2004; cf. Antrim Shale). Shallow and deep, “gas shales” are both source and reservoir. Because, of recently developed novel extraction technologies, natural gas hosted in shale of low permeability emerges as a vast new resource., , derived from Late Jurassic evaporitic-sapropelic, strata (Kulke 1994). Oil of the Maracaibo Basin, in Venezuela is sourced from the Late Cretaceous, La Luna Formation (Mongenot et al. 1996) and, the giant heavy oil belt (640 60 km) in east, Venezuela from the Cretaceous Querecual series., In Europe, Early Jurassic Posidonia Shale (Toarcien), Late Jurassic Kimmeridge Shale (especially, in the North Sea) and laminated Early Cretaceous, mudstones are prominent source rocks that, have been correlated with numerous oilfields, (Littke 1993). Oils generated from lacustrine, source rocks (e.g. Singliao and Bohai Basins), represent 95% of all petroleum resources in, terrestrial China., 7.2.2 Dry gas source rocks, Methane source rocks are mainly coal seams,, clastic sediments with dispersed humic particles, and so-called “post-mature” kerogen, which is, beyond the stage of generating oil and wet gas, (Rice & Schoell 1995). Deeply buried shales may, produce gas from oil when heated above the, stability boundary of petroleum (at 160 C), where it is thermally decomposed. This methane, may remain in situ as in Barnett Shale or, migrate away (Tian et al. 2008, Pollastro et al., , Subbituminous, coal, , 0.5, , High volatile, bituminous coal, Med. vol. bitum., , Semi-anthracite, , R0 (%), , 1.5, Low vol. bitum., , 2.5, Anthracite, , Meta-anthracite, , 3.5, 0, , 1, , 2, , 4, , 3, , 5, , 109 m3 gas/km3 clay with 1 wt. % kerogen of type III, , 0, , 100, 9, , 200, 3, , 3, , 10 m gas/km coal, , 300, , Figure 7.7 Cumulative methane production of a, clay rock with 1% vitrinite or of massive coal (lower, scale), as a function of increasing rank (modified, after Glennie 1998). Data like this allow appraisal of, total gas production of a basin provided that kerogen, and coal mass can be estimated and maturity (rank), is known.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , logical properties of kerogen macerals are increasingly lost. However, contents of typical source, rock/black shale trace metals do not change, measurably (Mongenot et al. 1996)., , 7.2.3 Eogenesis and catagenesis of kerogen, Eogenesis is the early and near-surface process, system that includes biological, chemical and, physical alteration of minerals and organic matter, in rocks. It ends when the rock is heated to above, 50 C and temperature takes over as the dominant driving force. Eogenetic processes include, reduction, compaction, dehydration, some geochemical dissolution and reprecipitation, and biogenic gas generation. Organic matter in the rocks, is transformed into kerogen. Note that the term, “diagenesis of kerogens” (Tissot & Welte 1984) is, sometimes used to describe the eogenetic stage., This usage can be misleading because diagenesis, of sediments extends to 220 C. “Eogenesis” is, preferable because it is clear without ambiguity., Catagenesis comprises all processes that act on, rock matrix and organic matter after considerable, burial and that result in petroleum generation., Higher pressure and temperature (50–200 C) are, essential factors of change. The main result of, catagenesis is the generation of oil and wet gas, (Figure 7.8) while kerogen “matures”. At the same, time, source rocks are altered, for example by, darkening, disappearance of fine lamination and, formation of a dense network of micro- and macrocracks, which are filled by fluid-precipitated, minerals and bitumen (Littke 1993). The morpho-, , Eogenesis, Eogenesis, the early diagenesis of organic, substance is essentially a biochemical process., Initially, anaerobe microbial fermentation produces methane (biogenic gas), water, CO2 and, kerogen (Figure 7.8). Two stages can be distinguished: i) an initial bacterial enzymatic conversion of easily digestible organic substance into H2, and CO2 is followed by ii) reduction of CO2 that is, dominated by methanogenic archaea (eq. 7.3)., Microbial methanogenesis by reduction of CO2:, CO2 þ 4H2 ! 2H2 O þ CH4, , ð7:3Þ, , Isotope data suggest that much hydrogen of, biogenic gas is derived from formation water, (Martini et al. 1996). Economic deposits of eogenetic gas are rather rare because it is re-oxidized by, synbiotic consortia of methane-oxidizing archaea, and sulphate-reducing bacteria (Delong 2000)., However, in low-sulphate environments, such as, kerogen-rich rocks flooded by meteoric water, (McIntosh et al. 2004), generation of considerable, Hydrocarbons, Immature zone, , biochemical CH4, , 1, , Depth (km), , Oil zone, Wet gas, zone, , Catagenesis, Metag., , Dry gas, zone, , Eogenesis, , 0, , Figure 7.8 Formation of oil and gas during eogenesis, (<50 C), catagenesis (50–160 C) and metagenesis, (160–250 C) of kerogens in sediments (modified from, Tissot & Welte 1984). With permission from Springer, Science þ Business Media. Black area indicates the, presence of “geochemical fossils”, i.e. biological, markers. Depth limits depend on the local geothermal, gradient and other factors., , 537, , 2, , Oil, 3, , Gas, 4
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538, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , gas deposits is possible. About 20% of the world’s, gas resources may be of microbial origin:, The Antrim Shale (Late Devonian, Michigan Basin,, USA) is an outstanding example of biogenic methane, formation long after sedimentation of the source, rocks. The shale contains up to 20% of low-maturity, organic substance derived from algae. Daily, >5000, wells pump 1.2 107 m3 gas from <600 m depth. The, shale is both source and reservoir rock. Gas resources, are estimated at 1012 m3 methane. Flooding of basinmarginal shale with meteoric water to 200 m below, the surface and consequent dilution of basinal brines, favoured microbial degradation of kerogens and hydrocarbons in the shale (McIntosh et al. 2004). Pleistocene ice cover and increased hydrostatic head are, thought to have acted as a seal retaining methane in, the shale. 14C dating implies a Late Pleistocene age of, methanogenesis (<22,000 years; Martini et al. 1996)., , the source rock at various temperatures and, recording the yield of hydrocarbons (Hunt 1996)., Applying the method, it was soon recognized that, the classical four kerogen types (Figure 7.4 and, Figure 7.5) must be further subdivided for a correct, simulation of burial, maturation and hydrocarbon, yield in nature. Reaction rates of subtypes diverge, widely. Also, other factors modify maturation,, including elevated organic sulphur concentrations, that lower the onset of oil generation (Lewan, 1998), reactions with minerals of the source rocks, and with pore fluids. The participation of ubiquitous pore water in the reactions is facilitated by, disproportionation (eq. 7.4)., Disproportionation of water as a source of hydrogen for HC-generation from kerogen:, H2 OðliquÞ $ H2ðaqÞ þ 0:5O2ðaqÞ, , Catagenesis, At 50 C (and 1500 m overburden, the precise, depth being a function of the local geothermal, gradient), eogenesis changes without a break into, catagenesis. Geochemical processes generate, petroleum and thermogenic gas from kerogen., Catalysis by clay minerals, especially montmorillonite, may have a role. The mature, i.e. oil-generating kerogen loses its capacity to yield petroleum, at 160 C (5500 m depth). Only gas is generated, at higher temperatures (Figure 7.8). This may not, be simply due to continued heating of post-mature, kerogen. Catalysis by transition metals (e.g. Ni, V), is assumed to amplify the action of heat (Mango, et al. 1994). The stage of dry gas production, between 160 and 250 C is called “metagenesis”, (Tissot & Welte 1984). In it, the organic substance, is increasingly transformed into pyrobitumen,, graphite and gas. Organic metagenesis equals the, highest grade of rock diagenesis and constitutes, the bridge to metamorphism., Like coalification, the generation of hydrocarbons from kerogen is described by the Arrhenius, equation as a series of decomposition reactions, (eq. 6.3, c.f. Chapter 6.3 “The Coalification, Process”). Temperature constitutes an exponential factor and the kinetic factors E and A are, measured in laboratory experiments by heating, , ð7:4Þ, , Diagenesis consumes oxygen, mainly by oxidation of Fe2 þ , but also by formation of much CO2, liberating hydrogen. The hydrogen may react with, kerogen and generate hydrocarbons, even if the, kerogen has already lost most of its inherent, hydrogen. This “extraneous” hydrogen is thought, to considerably enlarge the generation potential of, kerogen (Seewald 1994)., Recently, a voluminous study based on thermodynamics presented a novel approach to the theory, of petroleum generation (Helgeson et al. 2009):, Kerogen maturation is modelled as a series of, oxidation/reduction disproportionation reactions., Mature kerogen melts incongruently with, increasing burial to produce crude oil, CO2 gas, and a kerogen with lower H/C. Depleted kerogen, reacts with any water present by hydrolytic disproportionation to produce new kerogen with, higher H/C and CO2. The study is innovative and, rigorous. It confirms part of the prevailing kinetic, models and their application. Some conclusions of, Helgeson et al. (2009) are thought-provoking, for, example the suggestion that 75–80% of oil generated remains trapped in source rocks., The characteristic temperature and depth, ranges of hydrocarbon formation described in, the preceding text (and in Figure 7.8 and Figure 7.9), are only indicative. The maturation of kerogen is
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , 539, , Exploration drillhole, 0, , 1, , Depth (km), , Figure 7.9 Profile of a simple petroleum, system (time not shown): A hydrocarbonproducing depression, or “kitchen”,, displays a steeply dipping source rock, layer that exhibits immature, mature and, post-mature portions (modified from, Hunt 1996). Maturity is depicted by, vitrinite reflectance in oil immersion (Ro)., Oil generation is thought to start at RO ¼, 0.62%, and gas with condensate below Ro, ¼ 1.0%. Hydrocarbons migrate upward, and collect in a structural trap. The, effective drainage area is delimited by, sufficient maturation and geological, constraints of fluid flow., , 0.62, , 2, , %RO, , Oil generation, , 0.80, , 3, , urc, So, , el, ev, kl, c, o, er, , 1.00, 1.20, , Gas, 4, , controlled by factors such as mentioned above,, but also by physical criteria (e.g. overpressure, delaying maturation; Carr 1999). The heating rate, is a strong control on the evolution of vitrinite, reflectance and hydrocarbon generation: Rapid, heating accelerates petroleum formation compared to vitrinite reflectance and the reverse is, observed for slow heating (Mukhopadhyay 1992)., The consequences of the multiple controls are, quite variable depths and temperatures of hydrocarbon formation in different provinces:, Considerably higher heat flow is characteristic for, marine rift basins compared with ordinary epicontinental sedimentary basins. A present example is, the Gulf of California straddling the Pacific-North, America plate boundary, where submarine hydrothermal vents produce abundant gas bubbles and, drops of petroleum, which reach a diameter of 2 cm., Barite is precipitated around the vents, but the hydrocarbons are quickly dispersed Above a synrift, metamorphic-igneous crust (Dorsey 2010), the basin, contains young sediments with a total thickness of, <500 m, which should exclude hydrocarbon generation. The sediments are, however, heated to >315 C, at a low pressure of only 200 bar. It is assumed that, this setting is an exceptionally efficient environment of oil generation. Similar observations are, reported by Svenson et al. (2007) from the nearby, Salton Sea geothermal field and by Mark et al. (2010), for the UK Atlantic margin. The North Sea oil and, gas province displays parallels to the Gulf of Cali-, , Effective drainage area, , fornia: The Viking Graben, a major hydrocarbon, kitchen in the province, was formed by extensional, tectonic movements related to the opening of the, Atlantic Ocean, which lasted throughout the Mesozoic. Higher heat flow between the Mid-Eocene and, the Pliocene caused profuse hydrocarbon generation. The Ekofisk field was filled from an oil window, at 3–5 km depth (Glennie 1998). Further to the, North, in the Atlantic Ocean off Norway at about, the latitude of Iceland, thousands of large fluid, escape pipes occur that were probably formed at the, dawn of the Eocene (55 Ma). In this region, basaltic, sills intruded TOC-rich Cretaceous-Palaeocene sediments at a depth of 1–3 km. The sudden heating of, the sediments (<10,000 years) liberated giant amounts, of methane, which possibly triggered the extreme, globalgreenhouseclimate ofthe Early Eocene (Svensen, et al. 2004)., With 6–12 km, the oil kitchen in the southern, Caspian Basin (Azerbaijan) is unusually deep. The, basin fill consists mainly (10 km) of Pliocene and, Pleistocene under-consolidated sediments and displays a very low geothermal gradient (Feizullayev, et al. 1998)., , 7.2.4 The oil window, We have seen that oil generation may take place, at shallow or at deep levels, and that the effect, of temperature is exponential. The only true measure of kerogen maturity in reference to oil generation is its chemical composition determined
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540, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , by pyrolysis techniques such as Rock-Eval, (Figure 7.5). Vitrinite reflectance and the fluorescence of liptinites provide an approximation. The, oil window is the stage of maturation dominated, by oil generation from kerogen (“catagenesis”,, Figure 7.8). The upper boundary of the oil window, where petroleum generation begins varies between, a vitrinite reflectance of 0.3 and 0.7% (Ro). The, lower boundary of the oil window is usually, between 1.0 and 1.3% Ro, where condensates and, wet gas take over (Figure 7.9). The wide span of, these figures is also due to the very different generation behaviour of kerogen types and subtypes., The precise maturity of source rocks and their, upper and lower oil window boundaries must be, determined as a base for petroleum system analysis, (cf. see below). In order to understand the relations, of oil generation with migration and traps, the fate, of the source rocks in geological time needs to be, reconstructed and the generation time span fixed., The construction of the required time-temperature-generation history of a source rock comprises, the combination of burial history (“geohistory”), data with Arrhenius equation kinetics. The crucial, input is thermal history. Time-temperature index, graphs (Hunt 1996) and three-dimensional computer programs (Schneider & Wolf 2000) are based, on quantitative burial models and Arrhenius parameters A and E determined by laboratory methods, such as hydrous pyrolysis., Some authors discuss the term oil window, regarding the stability boundaries of petroleum., In this interpretation, the upper boundary is given, by the many destructive processes, which affect, oil near the Earth’s surface (see below). The lower, boundary is postulated at 160 C where oil enters, the field of thermal decomposition (cracking). In a, first stage of cracking, C2–5 wet gases are produced., The second stage results in methane and pyrobitumen, and ends with dry gas, which may remain in, situ or migrate away (Tian et al. 2008). Cracking, generates vast amounts of methane that cause, supralithostatic pressures and reservoir fracturing, confirmed by observations in pyrobitumen, deposits. Increasing contents of diamondoids are, a mark of deep thermal cracking of oil. Diamondoids reach a maximum concentration at about, Ro >4.0% and are decomposed at higher temperatures (Zhibin et al. 2006). Determination of dia-, , mondoids helps to predict the deepest oil deposits, in a given district. This is, of course, of highest, economic interest., , 7.3 FORMATION OF PETROLEUM, , AND NATURAL, , GAS DEPOSITS, , During maturation, the natural hydrocarbons, exude from dispersed kerogen particles in source, rocks. Tiny droplets move into nearest pores of the, host sediment. Source rock-hosted oil is hardly, ever exploitable but in situ gas was recently discovered as a giant new resource. Conventional, deposits of hydrocarbons were formed by oil and, gas moving out of the source rock, migrating some, distance and collecting in the reservoir space of a, trap structure. Fluid expulsion and migration are, initiated and sustained by heating, compaction,, maturation and dehydration of source and host, rocks. Similar to coalification, the oil generation, and migration phase is rather short, measuring in, millions to tens of million years. An interesting, aspect is the efficiency of the system, expressed by, the ratio of known oil deposits to the estimated, total generated petroleum. The efficiency factor is, commonly not more than 0.1–1%, higher figures, are rare (e.g. 5–6% in the Arabian Gulf, 10% in the, Maracaibo Basin)., Clearly, the formation of oil and gas deposits is, the product of favourable combinations of subsystems, including source rock formation, burial and, heat flow, maturation, tectonic deformation,, migration and the presence of reservoir rocks and, traps. The subsystems combine to form complex, master systems that have been called “oil play”,, or “petroleum system” (Magoon & Dow 1994,, Figure 7.9). Once formed, petroleum deposits may, be exposed to thermal, microbial or tectonic, events, which often cause detrimental alteration, or even total destruction of hydrocarbon accumulations (Figure 7.10)., 7.3.1 Migration, Comprehension of hydrocarbon migration from, the source rock into traps is enhanced by application of hydrogeological concepts and models, (Verweij 1993). Geological and geochemical data
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541, , PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Time (Ma), 250, , 200, , 150, , 100, , Formation of source rocks, , 50, , 0, , 0, , Uplift curves, , 2, , 50, Burial curves, , 4, , Depth (km), , Figure 7.10 Sedimentation, burial, and, hydrocarbon generation in the foreland, of the Brooks Range, Alaska modelled in, a temperature-depth-time diagram, (modified from Parris et al. 2003). Oil and, gas were generated before the source rocks, reached peak temperature (250 C at, 10 km depth). The chance to find, surviving oil and gas deposits is small,, except if the hydrocarbons migrated away, into shallower parts of the basin., , 100, , Oil generation, (Ro = 0.7 - 1.3), , 6, , Condensate and gas, generation (Ro = 1.3 -2.0), , 150, Uplift trajectory, , 8, , 200, , Zircon fission-track age 64 ±3 Ma, , 10, , 12, , Temperature (°C), , 0, , Based on geothermal gradient of 25°C/km and surface temperature of 0°C, , document that in some cases oil and gas migrated for, horizontal distances of hundreds of kilometres and, that other deposits originated by very short vertical, migration. Compaction, heating, maturation and, dehydration generate the fluid flow that is controlled by pressure (head) differences and rock permeability. Flow vectors generally point upwards and, to basin margins. In the fluids, hydrocarbons are, transported in dispersed and dissolved form. Apart, from physical flow, diffusion acts on hydrocarbons., Because of its limited transport capacity, however,, diffusion’s role is insignificant, except for natural, gas deposit formation., Note that “hydrocarbon fluids” are a variant of, diagenetic fluids and share most of the properties, that are described in Chapter 1.4 “Diagenetic Ore, Formation Systems”. Fluids consist essentially of, hot water with dissolved inorganic and organic, matter. A hydrocarbon fluid may be dominated, by an oil phase if very dense kerogen-rich rocks, containing little water such as pelitic carbonates, subside to oil generation depth. In this case, light, hydrocarbons exuded from kerogen cause a rise of, internal pressure (generation pressure) in the, rocks. Oil and water in the pore fluids expand with, heating. Liberation of CO2 and organic acids from, kerogen provokes dissolution and recrystallization of carbonate (Heydari & Wade 2002). All these, processes create temporary permeability, which, allows drainage of oil and gas., Rock permeability is not only a function of, intrinsic rock properties such as pore space and, , 250, , 300, , connectivity, but also of physical properties of the, fluid (eq. 7.5). This has the consequence that light, (low-viscosity) oil flows readily, whereas heavy, (high-viscosity) oil in the same rock hardly moves., In the oil industry, permeability is usually expressed in Darcy units (1 Darcy equals an intrinsic, permeability k ¼ 1012 m2). Reservoir rocks in oil, deposits typically have permeabilities in the range, of tens to thousands of mD (millidarcy); the minimum permeability for conventional gas deposits is, 0.1 mD. Halitites and very dense clayrocks display, the lowest permeabilities of all rocks, down to, about k ¼ 1023 m2., Permeability kfluid of rocks for fluids of different, density and viscosity:, kfluid ¼ ðk rf Þ=m, , ð7:5Þ, , k ¼ intrinsic permeability of the rock (m2), rf ¼ density of, the fluid (g/cm3), m ¼ dynamic viscosity of the fluid, (water at 10 C: m ¼ 1.31106 m2/s)., , The migration of oil and gas from source rocks, into permeable country rocks is called “primary, migration” (Levorsen 1967). Early and late primary, migration are distinguished:, 1 The first occurs during the principle phase of, compaction and dehydration (eogenesis), which, mainly moves water out of the system, before, the source rock reaches maturity. Oil or gas, deposits hardly ever form at this stage., 2 Late primary migration takes place while oil and, gas form from kerogen (catagenesis). At the
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542, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , onset of catagenesis, water content in finegrained rocks is already very low. Therefore,, volume expansion of expelled hydrocarbons is, probably the main driving force., “Secondary migration” (Levorsen 1967) describes the flow of hydrocarbons in permeable, country rocks. Because permeabilities vary, even, within one bed, preferential flow paths evolve that, may be envisaged as streams or rivulets. Subtle, traces of the passage of hydrocarbon fluids are, recorded, including residues of hydrocarbons in, pores and in fluid inclusions of pore cement, thermal alteration (because deep fluids are hotter) and, reduction of Fe3 þ replacing haematite and goethite with magnetite. The hydrocarbon fluids, themselves are altered by reactions with the host, rocks, for example by loss of CO2, which precipitates pore-cement calcite. Hydrocarbon fluids may, be fractionated by density (because higher density, causes lower flow velocity) or by degassing (evaporative fractionation). Fluid inclusions in diagenetic cements containing oil and gas provide, evidence on timing and physical conditions of, hydrocarbon migration (Mark et al. 2010). Oil, saturation is estimated by counting the frequency, of hydrocarbon inclusions (Lisk et al. 2002)., , In some instances, petroleum migrated across, layering in sedimentary basins. If beds of low permeability are present, this observation is counterintuitive and always requires careful investigations. Often, faults explain the connectivity, but, faults may be either highly permeable fluid conduits (Haney et al. 2005) or impermeable barriers., Migrating hydrocarbons are trapped when the, fluid flow meets an adequate structure. Trap structures consist of a permeable reservoir rock, which is, open to receive fluids but is semi-closed by impermeable seal rock. The best seals are gas hydrates,, evaporites and shale, but dense sandstone or saline, dolomite may suffice. The separation of hydrocarbons from an aqueous fluid flow can be envisaged as, oil and gas bubbles floating up into a cupola-shaped, trap and the water passing on underneath the base, (“spill plane”) of the cupola (remember that conventional crude oil is lighter than water)., Within the cupola of the trap structure, the, components of the hydrocarbon fluids separate as, a function of density, resulting in water at, the bottom, overlain by layers of oil and gas. The, boundaries of the density stratification are generally planar but not necessarily horizontal, as in, Figure 7.11. Capillary forces and wetting, , W, , E, , Depth below sea floor (m), , Troll West, oilfield, , 1400, , Tertiary, claystones, top, , Oil, 1600, , Troll West, gasfield, , owc 1569, , Troll East gasfield, Cretaceous, marls and shales, , Tertiary, , top V, iking Cretaceous, , Gas, , Draupne shales, Silt, , Vik, in, , g, , hwc 1549, , owc 1559, , Water, Coarse, clean sands, 1800, , 2000, Fine, micaceous sands, , 2200, 5 km, , Figure 7.11 The offshore gas field Troll ca. 80 km northwest of Bergen, Norway contains giant gas reserves and, considerable oil. Production is expected to last until about the year 2050 (Bolle 1992). The field lies on the eastern flank, of the Jurassic Viking graben, in sandstones of the Middle to Late Jurassic Viking Group. Source rocks are Kimmeridgian, Draupne shales in deeper parts of the graben in the West. owc ¼ Oil/water contact, hcw ¼ Gas/water contact with 0–4 m, oil. Note that the horizontal scale is strongly shortened.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , properties of the fluids often inhibit complete, separation, resulting in gradual transition of relatively clean gas, oil and water through zones of, mixed pore fluids., 40, Ar/39Ar, Pb-Pb and 187Re-187Os dating assist to, determine the time of migration and emplacement of hydrocarbons (Mark et al. 2010, Selby &, Creaser 2005). Uranium, rhenium and osmium are, enriched in organic matter of source rocks and are, transferred into the hydrocarbon fluids. Geochemical markers provide estimates of the migration, distance (“oedometers”, Isaksen 1996)., The term “tertiary migration” is sometimes, used for the flow of oil from a first into a second, trap (Lisk et al. 2002) or its drainage to the, surface, usually caused by tectonic events. In, upflow channels, pressure release may lead to, precipitation of pyrobitumen, ozocerite or asphalt. Rising light hydrocarbons, mostly methane, mixed with overpressured formation water, erode the walls of flow paths and form mud, volcanoes at the surface. Oil may reach the surface and form conspicuous oil seeps and asphalt, lakes (e.g. Trinidad). Submarine seeps are ubiquitous, for example in the Gulf of Mexico. They, are islands of rich life on the desert-like bare, seafloor. In the Gulf, colonies of Mytilus and, tube worms characterize the seep locations. Bubbles of gas and oil drops rise from the seeps,, asphalt and gas hydrates occur nearby. On the, surface of the sea, natural oilfilms produced by, the seepage can be mapped in satellite images, (MacDonald et al. 2004)., 7.3.2 Reservoir rocks, Reservoir rocks of oil and gas deposits must be:, i) porous (with a large volume for storing hydrocarbons); and ii) permeable in order to allow, economic extraction. To be effective, porosity, requires sufficient connectivity between pores., Porosity and permeability of rocks are measured, in situ (by geophysical probes) and on core samples, in the laboratory. In the laboratory, in-situ stress, conditions are simulated because the samples, expand during drilling and extraction. Generally,, porosity and permeability decrease with increasing geological age and depth, as the grains are, forced into a denser packing pattern. This is, , 543, , described by an exponential porosity reduction, model; note, however, that there are exceptions, to this rule (Davies 2005). The formation of pore, cement is a frequent cause of lower than expected, permeability and pore space. Typical petroleum, reservoir rocks display porosities of 10–40 vol. %, and permeabilities of tens to thousands mD., Exploitable gas deposits have a minimum porosity, of 7 vol. % and a permeability >0.1 mD. Even, lower permeabilities characterize so-called, “unconventional” gas resources in tight sandstones and shales. Note that in this case, the gas, is retained in its low-permeability source rock., Tight gas is increasingly redefined as a profuse, and valuable resource. The key to this development are improved drilling and fracturing, technologies., The most frequent and economically important, reservoir rocks are sand, sandstone, limestone, and dolomite. Many other rock types have been, found to host oil and gas deposits, such as fractured shale and chert (California), mafic intrusions (Texas), weathered granite regolith, (Panhandle-Hugoton field in Texas) and Precambrian gneiss offshore of the Shetland Islands., Permian volcanics enclose gas in Northern, Germany, as do the Miocene Green Tuffs in, Japan. Worldwide, coal seams are economically, significant gas reservoirs (CBM)., Porosity and permeability of sand are mainly, determined by grain diameter and grain size spectrum, but also by grain shape and the packing, of grains. Newly deposited sand has a porosity of, 40–42%, which decreases during burial until at, 2500 m depth a minimum of 26.5% is reached, (Figure 7.12). Coarse sands often have higher, porosities than finer sands. Well-sorted sands,, such as aeolian dune deposits (e.g. Rotliegend, sands hosting gas in Northern Germany, The, Netherlands and the southern North Sea), display, a higher porosity and permeability compared to, sand with a broad grain size distribution. In the, latter, small grains occupy pore space between, the larger grains. The permeability depends on the, diameter of connections between pores and on, surface properties of the pore space. Reservoir, sands of the giant Troll gasfield in the North Sea, (Figure 7.11) for example, have 17–36% porosity, and permeabilities of 0–10 Darcy.
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544, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Porosity = intergranular volume (%), 0, , 10, , 20, , 30, , 40, , 0, , 1, , Quartz sand compaction, (mean ± 2σ), , Depth (km), , 2, , 3, , 4, , 5, , 6, , Figure 7.12 Compaction of quartz sand with increasing, burial, in the absence of pore cement (Paxton et al. 2002)., Curves depict the mean and 2 standard deviations., Below 2500 m depth, very little further compaction, takes place., , Sandstones are characterized by partial or complete cementation of the pore space. Cements are, usually quartz, K-feldspar, calcite or clay minerals. At very small porosities, the formation of oil or, gas deposits is not possible. In certain regions of, Northern Germany, illite cement in aeolian Rotliegend sands inhibited production of locked-in, gas. In the southern North Sea, the same sands are, impaired by anhydrite and barite cement, where, faults allowed mixing of fluids derived from Zechstein evaporites and Carboniferous coal measures., When cement reduces permeability of pore throats, but a sizeable pore space is filled with hydrocarbons, technical measures may enable extraction, (e.g. acid treatment, hydraulic fracturing). Pore, space filled with oil or gas is not accessible for, later cement-forming fluids (e.g. in the Athabascan oil sands), in contrast to the water-filled reservoirs rocks beneath the oil. This explains cases, of widely different permeability and porosity of the, same rock in oil zones compared to aquifers below, the oil-water contact., Important aspects of sand and sandstone reservoirs include the original palaeogeographical and, , sedimentary setting, and depositional variations, that need to be fully understood in order to determine the optimal pattern of drilling and extraction. These subjects are treated extensively in, sedimentology books (Nichols 2009)., Carbonate reservoir rocks equal sand and sandstone in economic significance. Because calcite, and dolomite are much more soluble than quartz,, the diagenetic and petrophysical evolution of, limestone and dolostone is more complex compared with sandstone. Reef rocks make outstanding reservoirs with primary porosities reaching, 80%. Carbonate reefs are remarkable as one of the, few sediments directly built as solid rock, not, unconsolidated granular material. However, the, high primary porosity and permeability of reefs, favour the repeated passage of fluids (meteoric,, marine, diagenetic and formation waters), which, often leads to a complex history of solution and, cementation events. The vuggy and fractured Oligocene-Miocene Asmari limestone of southern, Iran, Permian reefs in Western Texas, Late Devonian reefs in Western Canada and the Ordovician, Tazhong reef in the Tarim Basin, China (Zhou, et al. 2009) are famous reservoirs. Karst cavities, in limestone may be filled with oil, as in the, submarine Rosso Mare field near the eastern coast, of Italy. Pure and massive carbonates are normally, too dense to be good reservoirs but there are exceptions such as the Late Cretaceous chalk in the, central North Sea with the giant Ekofisk oilfield., Ekofisk chalk developed a dense network of microfractures when active diapirism of Permian salt, at depth produced tensional domes (Glennie 1998)., Elsewhere in the North Sea, Cretaceous chalk acts, as a seal. Although primary porosity in carbonates, is often lost by mineral precipitation, several diagenetic processes may produce secondary porosity. Calcite in dolomite, for example, is dissolved, by CO2 and carboxylic acid-rich hydrocarbon, fluids (Seewald 2003), producing important reservoir horizons such as the upper portion of the, Neogene Asmari Formation in the northern, Arabian Gulf and southern Iran, and Late Cretaceous carbonates from Iraq to Qatar (Sadooni &, Alsharhan 2003). Dolomitization of limestone, reduces the volume of solids by 13%, increasing, the porosity of the bulk rock mass. Secondary
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , ciency of the seal is a function of its geometry,, integrity and sealing capacity (controlled by capillary pressure: Vavra et al. 1992), or in different, terms, its permeability (Luo & Vasseur 1997,, Schl€, omer & Krooss 1998). The widely varying, forms of hydrocarbon traps capped by a seal may, be generalized as bell-shaped. In the context of “oil, and gas traps”, or structures, the word “structure”, has no connotation of tectonic origin. Because, closure of hydrocarbon structures must be, upwards, it is common to use contour line maps, (and sections) of the reservoir rock hanging wall, boundary to characterize a structure (Figure 7.13)., The comparison with a bell illustrates the finite, volume of traps. Sealing capacity determines the, height of the hydrocarbon column that a structure, can hold, with excess oil escaping through a leaky, seal. Once a well-sealed trap is filled, oil remains, in the by-pass flow and may fill other traps, , dolomites form metasomatic bodies of a friable, crystalline (saccharoidal) nature near unconformities, faults and fractures. With porosities exceeding 30%, such rocks are world-class reservoirs,, from the Middle East to Alberta, Canada., Not unlike sand and sandstone reservoirs, the, original palaeogeographical and sedimentary, setting, and depositional variations of carbonate, reservoirs must be investigated. At least as important, however, is the study of diagenetic evolution, in time and space., 7.3.3 Petroleum and gas traps, Reservoir rocks allow flow of hydrocarbons by, buoyancy and hydrodynamic forces. In order to, halt flow and concentrate hydrocarbons from the, dilute flow, the reservoir must be half-closed by, rocks of low permeability (the “seal”). The effi-, , 3300 m, Zb -18, , Depth below, sea level, Well location, , 3150, , Zb-23, , Zb-18, , Zub, , aila, , 3150, , N/Rum, , R-29, , N, , air, Zb-10, Zb-14, , 00, 33, , 3300, , Zb-11, , 0, , 330, , RU-29, , 3150, , a, T u b, , R-34, , Zb-15, RU-11, RU-36, , 0, , S/R, , 330, , RU-2, , 0, , aila, , RU-8, , 300, , um, , Figure 7.13 Contour lines of the hanging, wall boundary of the Early Cretaceous, reservoir horizon “Upper Sandstone, Member” in the oil fields Zubair and, Rumaila in southern Iraq as an example, of a structure map (modified from, company data). With permission from, www.schweizerbart.de. S. Rumaila is, Iraq’s largest oil field and contains, reserves of around 18,000 million barrels, (2009). Note the relatively narrow NNWstriking anticlines and the broad, depressions. Production wells (black dots), in the structurally high regions provide an, indication of oil distribution., , 545, , Zb-17, , 0, , 345, , Zb-39, , RU-9, , RU-13, , 10 km, , Zb-19
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546, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Oil, Oil, , Gas, , Figure 7.14 The finite volume of a trap, causes by-passing of hydrocarbon fluids, which may form downstream deposits, (Lisk et al. 2002). Gas is retained in the, first trap where it displaces petroleum., , Water, , Oil, Hydrocarbon fluids, , (Figure 7.14). Incoming gas, however, displaces oil, and lowers the petroleum fraction of the upstream, deposits. The time span of trap filling is probably, between a few to tens of millions of years., Because of its small molecule size (effective, diameter 0.38 nm) and low density, methane is, much more mobile in the subsurface than oil., Methane can flow or diffuse through seal rocks, that are impermeable for oil, but is effectively, retained by gas hydrates, salt and less efficiently,, by shales. Many of the giant Siberian gasfields, for, example, are sealed by permafrost-related nearsurface gas hydrates., In the pioneer times of petroleum geology, salt, domes and anticlines were the only traps known, and constituted the main target of exploration., Today, anticlines are still important (Figure 7.15),, , SW, , Stratigraphical hydrocarbon traps, Stratigraphical hydrocarbon traps are the result of, lateral or vertical changes in permeability, which, are due to changes of lithology. Characteristic, examples include vuggy reefs within marine mudrocks, sand bars or meandering fluvial channels,, sand with laterally increasing clay contents and, unconformities. As oil extraction moves out into, deep water, buried delta sediments and masses,, which flowed down continental slopes, gain, , +1, , Iagifu, , Darai, , SL, , Oligo-Miocene limestone, , Darai, , Ieru, Toro, , -1, -2, , NE, , Hedinia, oilfield, , Iorogabaiu, , km, , but a large diversity of traps is known. Classification of this diversity defies a simple approach and, we shall here consider only the most important, common classes and selected examples of special, interest (Table 7.5)., , Cretaceous, , Ieru, , Darai, , Ieru, Toro, , -3, , Darai, Darai, , Jurassic shale, siltstone, , -4, , Ieru shales, , Toro, , Jurassic, source beds, , -5, , Basement, , -6, 5 km, -7, , Figure 7.15 Hedinia oil field in the thin-skinned Cenozoic fold-and-thrust belt of Papua New Guinea was found, by mapping geology and hydrocarbon seeps (after Matzke et al. 1992). Main reservoir rock is Early Cretaceous, Toro sandstone with 12–15% porosity and a permeability of several hundred mD. Source rocks are Jurassic shales in the, internal zone of the orogen.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Table 7.5 A simple classification of hydrocarbon traps, , (Figure 4.26) induces substantial lateral temperature and cementation differences in the abovementioned Rotliegend reservoir dune sands., Dolomitized porous bodies within tight calcite, mudrocks are another illustration of diagenetic, trap formation., Unconformity traps contain a considerable, share of world petroleum. The structural discordance between beds below and above an unconformity provides a large variety of spatial, arrangements of reservoir and seal rocks. Two, main groups are distinguished: i) reservoir rocks, in the lower unit sealed by fine-grained sediments, deposed above the discordancy plane; and ii) reservoir rock bodies above the unconformity forming traps where they pinch out between overlying, shale and sub-unconformity impermeable rocks., , 1. Stratigraphical and diagenetic traps, 2. Diapir-related traps, 3. Tectonic traps, 4. Impact-related traps, 5. Hydrodynamic traps, 6. Traps formed by self-sealing, 7. Combination traps., , increasing importance (e.g. in the Gulf of Guinea, sourced by rivers Niger and Congo)., Elongated sand bodies within pelite host rocks, may be meandering channels of former deltas and, tidal flats, or coastal barriers (bars; Figure 7.16) and, dune belts. A broad Early Permian dune belt in, Northern Germany and The Netherlands, for, example, formed between a large playa lake in, the north and a wide, graben-dissected land in the, south, is an outstanding reservoir of gas. Today,, with exploration moving to deeper and to offshore, targets, elongate gravity flow channels and lateral, slope fan deposits in deep axial parts of marine, basins promise to provide large hydrocarbon reservoirs (Hubbard et al. 2005). Internal sedimentary structures, shape, spatial situation and, genetic details are vital information required for, exploration and exploitation., Inclined sand horizons (sheets) between marine, mudrocks may trap oil and gas where the bed, passes updip into less permeable lagoonal and, intertidal shales. Increase of pore fill by more, intense cementing and consequent loss of permeability forms geometrically similar traps, but, this characterizes diagenetic hydrocarbon traps., The thermal “chimney” effect of salt diapirs, Figure 7.16 The offshore Draugen, oil field 150 km west of, Trondhejm, Norway lies in a faultbounded horst of the Viking graben, at 240–280 m water depth, (modified from Provan 1992)., Reservoir is a sandbar that is, enveloped by Late Jurassic shales., The shales are simultaneously, source and seal rock. Reservoir, sandstones display a porosity of, 26–30% and a permeability of, 700 mD to 10 Darcy., , 547, , The emersion of Cretaceous carbonates followed, much later by sedimentation of Messinian (Miocene), evaporites and thick Pliocene mudrocks formed the, sub-unconformity palaeokarst trap of the Rospo Mare, field in the Adriatic Sea (Soudet et al. 1994), the, largest offshore oil deposit of Southern Europe, with, resources of 75 Mio t., , Reefs are frequently transgressed by marine, shales, which may act as source rocks. Their high, primary and secondary porosity (dolomitization,, leaching, fracturing) provide excellent reservoir, properties. Important reef oil occurs in Libya, the, Arabian Gulf, western Canada and Mexico. From, a Mid-Cretaceous barrier reef of 10 km length and, 1 km width in southern Mexico, for example, more, than one billion barrels (136 Mt) of oil was extracted. Pay zones occurred in the highest parts of, Draugen oilfield, , WNW, , ESE, km, , Pliocene, , 1, , Miocene-Eocene, , ne, Palaeoce, , Cretaceous, , 2, , Jurassic, Triassic, , Caledonian, Basement, , Late, Jurassic, Early, , 3, 4, 5, , Triassic, , Coarse clastic sediments, , 6, 20 km, , 7
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548, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Munster gasfield, , N, , Oilfield, , S, Quaternary, Tertiary, Cretaceous, , Salt diapir, , Zechstein, 5 km, , Volcanic rocks, , the reef. The same observation is reported from the, Ordovician Tazhong reef in the Tarim Basin,, China (Zhou et al. 2009), which is 220 km long, and 5–10 km wide, but only 100–300 m thick., The relations between the growth of reefs, submarine hydrothermal vents, cold seeps and hydrocarbon deposits in reefs are quite fascinating, (Hovland 1990, Roberts & Carney 1997). Exploration of ocean floors resulted in the discovery of, numerous fluid discharges with CH4, CO2 and, heavier hydrocarbons of diagenetic origin. The, vents are high-biodiversity islands of burgeoning, micro- and macro-fauna on the bare seafloor (cf., Chapter 1.1.2 “Ore deposits at mid-oceanic ridges, and in ophiolites”). This results in the build-up of, organic mounds that may grow into reefs. If the, flow of diagenetic fluids continues after the reef is, covered by pelites, an early diagenetic formation of, a hydrocarbon deposit is possible., , Late, Middle, Early, , Triassic, , Rotliegend, , Figure 7.17 Schematic profile of a, typical gas deposit in Northern, Germany (after Kulke 1994). With, permission from www.schweizerbart., de. Sealed by Zechstein salt, Rotliegend, dune sands are faintly domed. Methane, is sourced by Carboniferous coal at, depth. Gas pay zone in black., , cias; the last mentioned, and carbonates, associated with biogenic sulphur formation (cf., Chapter 3 “Sulphur”) are often the only good, reservoir;, 2 in updomed sediments above the top of diapiric, salt;, 3 in pierced country rocks, which are dragged, upwards and ductilely bent; in this case, reservoirs are efficiently sealed by salt;Underneath, the diapir in footwall rock (Figure 7.17)., Many salt structures in Northern Europe were, deformed by compressional intraplate deformation (“inversion”), which is related to the Late, Cretaceous-Early Palaeogene convergence of the, European and African plates (Kley & Voigt 2008,, Figure 7.18). Because of the high thermal conductivity of salt rocks, the maturation of organic, matter in sediments surrounding salt diapirs at, depth is retarded, extending to a distance of several, kilometres (Figure 4.27)., , Diapir-related hydrocarbon traps, Oil and natural gas deposits occur trapped below, little-deformed flat-lying salt formations, nonpenetrating salt pillows, salt-cored anticlines and, associated with intrusive salt diapirs (cf. Chapter, 4.3.3 “Forms and structures of salt deposits”)., Diapir-related traps are variegated and complex, (Warren1793, 1792). These structures yield an, important part of world oil production. Examples, include the Arabian Gulf, parts of Iran, the Gulf of, Mexico, Texas and the Pricaspian Basin in southern Russia (Volozh et al. 2003)., Oil and gas may be trapped in several positions, relative to a diapir:, 1 in the cap rock, which usually comprises anhydrite, gypsum, carbonates and collapse brec-, , Tectonic traps, Many of the Earth’s giant hydrocarbon deposits are, hosted in anticlinal tectonic structures. Their origin is due to various types of folding, reverse faults,, overthrusts (Figure 7.15) and nappe tectonics., Others are tectonic highs within rifts (Figure 7.11, and Figure 7.16). Very rare are oil and gas deposits, in a synclinal position, which seems to be due to, dryness of reservoir rocks. Note that doming of, reservoir rocks may be caused by several nontectonic processes, such as buried hills of an, ancient erosion surface, deep magmatic intrusions, or halokinesis resulting in salt pillows and salt, diapirs (Chapter 4.3.3 “Forms and structures of, salt deposits”).
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , 0, , km, , S, , 549, , N, Quaternary, Tertiary, , 1, Late Cretaceous, , Zechstein, salt diapir, , 2, Early Cretaceous, 3, , 4, , Keuper, Muschelkalk, Buntsandstein, , so+m, , su+sm, 5, , su+sm, , r, 5 km, , Figure 7.18 Inversion structure of the Taaken diapir in Northern Germany, with a gas deposit in Rotliegend aeolian, sands underneath its base (modified from Baldschuhn et al. 1998). With permission from www.schweizerbart.de., The diapir formed in the Early Cretaceous. In the Late Cretaceous, it was deformed by compressive tectonics, (“inversion”) that is most clearly diagnosed by the overlap of Buntsandstein strata (su þ sm). Very characteristic are, also the narrow stem of the diapir and its wide overhang, which may have been amplified by submarine salt glaciers., Rotliegend (r); Zechstein (Z)., The Earth’s largest oilfield, with >70 billion barrels, (9500 Mt) recoverable resources, is Ghawar in Saudi, Arabia. It occupies an anticline with a length of, 250 km and an area of 2300 km2. The Iraqi field of, Figure 7.13 is smaller but still impressive. Both discoveries are due to early geological and geophysical, mapping. Some petroliferous anticlines are not only, tectonic but also morphologic highs, such as the, Island of Bahrain. The Qatar Peninsula is formed by, a very flat arch trending northwards across the, Arabian-Persian Gulf; the structure hosts the world’s, largest gasfield., , Faults are often flow channels that allow, hydrocarbon fluids to vent into the sea or to, discharge on land. This escape is probably the, main reason for the low efficiency of petroleum, systems. In contrast to common permeable, faults, however, clay or shale fault gouge can, transform faults into very effective seals for, hydrocarbon reservoirs (Egholm et al. 2008)., Truncated reservoir rock units sealed by impermeable clay rocks and fault gauge may contain, sizeable hydrocarbon deposits. Exploration for, this type of trap is expensive and the worldwide, percentage of fault-trapped hydrocarbon reserves, is small., , Impact-related traps, The supergiant (Table 7.6) Cantarell oilfield in the, Gulf of Mexico is probably the largest deposit of, this kind, produced by the Cretaceous-Palaeogene, boundary Chicxulub impact (Grajales-Nishimura, et al. 2000). The bolide shattered the submerged, carbonate platform that crops out in the western, Yucatan Peninsula. The resulting breccia is the, reservoir for oil sourced by Late Jurassic rocks. It is, sealed by overlying dolomitized ejecta. Obviously,, extraterrestrial impacts may generate structures,, seals and reservoir rocks that can host very large, oil resources., , Table 7.6 Quantification of the terms giant and supergiant hydrocarbon deposits, , Petroleum, Natural gas, , Giant, , Supergiant, , >0.5 109, barrels (68 Mt), >0.5 109, BOE (68 Mtoe), , >5 109, barrels (682 Mt), >5 109, BOE (682 Mtoe), , BOE ¼ barrels of oil equivalent (calculated in thermal units,, cf. Table 7.1)
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550, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , W, , E, , Red shale, and sandstone, , Pennsylvanian, reservoirs, , Mid, , dle, , Late Permian, , Perm, , Groundwater, , ian, , Sea, , flow direction, , l, , carb, , ona, , anhydritic dolo, , mite (seal) 100 m, , te re, , serv, oir, , 10 km, , Hydrocarbon fluids, , Figure 7.19 The gas field at Hugoton, Kansas is one of the world’s giants (Ballentine & Lollar 2002). Copyright (2002), with permission from Elsevier. The field lies at the western border of the Palaeozoic Anadarko basin. Methane migrated, from the East over a distance of about 250 km into a stratigraphic (or hydrodynamic?) trap. Nitrogen and helium were, added by deep groundwater flowing from the West., , Hydrodynamic traps, Hydrodynamic traps occur where deep groundwater or formation waters move downdip against the, updip flow of hydrocarbons. In this type of trap,, there is no seal except the footwall and hanging, wall of the aquifer bed. The hydrodynamic force of, the water is sufficient “closure” if it surpasses the, buoyancy force of the hydrocarbons. An inclined, oil-water contact is thought to characterize hydrodynamic traps. The gasfield at Hugoton, Kansas, (Figure 7.19) displays the typical setting and earlier, was considered as a type locality of a hydrodynamic trap. This is now doubted because tilted, (inclined) hydrocarbon-water contacts may be due, to several causes, including:, . hydrodynamics as described, with an inclined, potentiometric surface;, . oil and gas extraction;, . tectonic tilting of an oil-water contact with, reduced permeability because of a tar mat or, cemented pores in the water zone; and, . gradually changing capillary properties of a reservoir rock unit., Self-sealing of oil reservoirs by tar or asphalt, Self-sealing of oil reservoirs by tar or asphalt is, possible when permeable strata channel the, hydrocarbon fluids to the near-surface, where, , meteoric water, oxygen and aerobic biodegradation (see below) affect the oil. Tar and asphalt clog, the pores and diminish or arrest continuing discharge of the oil (e.g. Kern River Field, California:, Coburn & Gillespie 2002). The giant tar resources, of Athabasca probably have a similar origin as they, were apparently emplaced before sedimentation of, the mudrock seal (Selby & Creaser 2005)., Self-sealing of gas reservoirs by gas hydrates, Nearly all of the world’s giant gasfields are sealed, either by evaporites (in the Middle East) or by gas, hydrates related to permafrost (in Russia). In the, Messoyakha gasfield in western Siberia, the gas, hydrate interval has been measured from 350 to, 870 m below the surface (Hunt 1996). Gas hydrates, form from methane flowing towards the surface,, where appropriate PT-conditions and water are, encountered. In the Laptev Sea offshore of northeastern Siberia, permafrost apparently attains, a depth of 500 m below the seafloor (Cramer &, Franke 2006), which implies a wide distribution of, gas hydrates at depth. Although economically less, attractive, methane hydrates themselves will, soon be exploited (cf. Chapter 7.1)., “Combination traps” result from the interaction of two or more of the trapping mechanisms, described above.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , 7.3.4 Formation and reservoir waters, In hydrocarbon basins, the overwhelming part of, pore fluids is water (formation water), and potential reservoirs are often found filled with water., Even within hydrocarbon deposits, water is an, important component of pore fluids., Formation waters reside in pores and joints., Usually they only move sluggishly and display, the temperatures of surrounding rocks. If the flow, is pronounced, the term “deep groundwater” is, sometimes used. Flow may induce considerable, temperature anomalies. In sedimentary basins, with a mainly marine history, formation waters, typically originated as enclosed seawater or evaporative brine. In other basins, meteoric water may, have displaced primary seawater, or terrestrial, surface water was the primary pore fluid. Water, enclosed during sedimentation is “connate”., However, water in sedimentary basins often has, a complicated history, both concerning its origin, and its further evolution by mixing and diagenetic, reactions (cf. Chapter 1.4 “Diagenetic Ore Formation Systems”). Like other diagenetic basinal, fluids, formation waters are characterized by Eh,, pH, T, P, salinity (cf. Table 1.4), mole ratios of, dissolved halogens and electrolytes (Botrell et al., 1988) and isotopic characteristics (14C, H, O, 129I,, Sr, Li and other systems; Tomaru et al. 2007)., The pressure of formation water is essential, information for a sufficient understanding of any, basinal system. Usually, freely moving water at, hydrostatic pressure predominates in the near, surface zone. Deeper down, individual closed domains are found that display different pressure, levels. Hydrostatic to moderate overpressure is, most common. “Abnormal pressure” is defined, as a pressure above the mean between hydrostatic, and lithostatic values (cf. Section “The Exploitation of Petroleum and Natural Gas Deposits”). In, extreme cases such as uncemented loose sand,, which is totally enclosed by an impermeable, formation, overpressure may reach lithostatic, magnitude and the overburden weight is wholly, carried by the pore fluid., The precise in-situ chemical composition of, formation and reservoir water is not easily established, because drilling always disturbs the system, , 551, , by inducing drilling fluids, cement or acids. Yet,, water sampled from drillholes is routinely analysed for major cations and anions. Interpretation, relies more on ratios, such as Na/Ca, Cl/Br and, Na/Cl, than on absolute concentrations. Of, course, technologies exist for sampling of nearly, uncontaminated pore fluid through perforated, borehole casing., Elevated NaCl concentrations increasing with, depth are common in formation and reservoir, water. Concentrations vary from <1000 mg/L, (freshwater: Table 1.4) to saturated brines, which, are either evaporative or a product of halite dissolution. Common cations of formation water, include Na þ , K þ , Ca2 þ , Mg2 þ , Fe2 þ and NH42 þ ;, important anions are Cl, I, Br, HCO3 and, SO42 in order of decreasing frequency. Elevated, traces of Ba, Sr, B and V are always present. Compared with parental seawater and evaporitic, brines, NaCl, Ca, N, Sr, Li, Br and I are typically, enriched in reservoir water, whereas Mg and sulphate are depleted. Microbial sulphate reduction, explains sulphate depletion. Magnesium is probably consumed by dolomitization or by formation, of Mg-clay minerals releasing Ca and Sr. The, reverse, higher Mg-concentration in pore fluids is, brought about by precipitation of calcite cement, (e.g. by microbial methanogenesis, McIntosh et al., 2004). Oilfield brines are usually moderately alkaline and reduced., Routine water data from wells exploring or extracting hydrocarbon deposits provide valuable, insight into an essential component of the hydrocarbon system. They assist interpretation of geophysical well logs and stratigraphical correlation., Hydraulic and genetic investigations of waters, expose important features at all scales, from whole, basins to single reservoirs., 7.3.5 Alteration of petroleum in reservoirs, (degradation), More than half of the world’s crude oil resources, are to some degree degraded. Much of the production from the next-generation deepwater fields on, continental shelves will be biodegraded oil (Head, et al. 2003). Biodegradation is principally a loss of, value, but as it commonly includes the production
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552, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , of biogenic methane, the additional gas may be, a limited compensation., The quality of petroleum in reservoirs can be, impaired throughout its existence, starting with, the filling stage. The prime agents of degradation, are water, oxygen and microbes. Aerobic biodegradation by bacteria and archaea is due to flow of, oxygen- and nutrient-bearing meteoric water, into shallow reservoirs. Degradation of shallow, petroleum is ubiquitous. Similar processes, affecting coal and kerogen-rich shales produce, economically important microbial gas deposits, (McIntosh et al. 2004). Associated reservoir, waters are strikingly sulphate-depleted (Van, Voast 2003). Aerobic biodegradation is the traditionally prevalent hypothesis to explain degraded, oil but deeper and hotter oil pools are similarly, degraded, although dissolved oxygen is unlikely, to reach this environment and microbes found, are anaerobes. Generally, biodegradation was, assumed to end at 80 C (Wilhelms et al., 2001), but hyperthermophilic methanogenic, archaea are now known to thrive at 110 C and, to still be alive at 140 C (Schippers et al. 2005)., Obviously, anaerobic hydrocarbon degradation is, more common in oil reservoirs than hitherto, thought (Aitken et al. 2004) and may actually, be the prevailing mode of petroleum degradation, (Jones et al. 2008):, Biodegradation of petroleum leads to the loss of nalkanes (paraffins), isoprenoid alkanes, waxes and, part of cycloalkanes, aromatics and biomarkers. Several anaerobic microbes reduce sulphate in order to, satisfy their oxygen requirements (cf. Chapter, “Sulphur”). The resulting reduced sulphur can be, found in the form of native sulphur (e.g. in caprock, of salt diapirs), H2S, sulphides of iron, zinc, lead, etc.,, or in sulphur-enriched residual degraded petroleum., As a consequence of biodegradation, resins and asphaltenes prevail in the residual oil and contents of, sulphur, nitrogen, vanadium and nickel increase., Also, density and viscosity rise. These changes handicap recovery and make processing more expensive., By-products of anaerobic biodegradation are methane, and CO2. Therefore, the process is also called, “methanogenic biodegradation”. Much of this carbon, dioxide is probably converted to more methane. CO2, is also produced by aerobic biogenic oxidation of, , methane (eq. 7.6), provoking the precipitation of carbonate cement in near-surface reservoir rocks., , Aerobic biogenic oxidation of methane:, CH4 þ 2O2 ! 2H2 O þ CO2, , ð7:6Þ, , Petroleum that is affected by large amounts of, water or natural gas can be degraded by selective, solution of light hydrocarbons (“water washing”,, “gas washing”). Typically, water washing removes, water-soluble low molecular hydrocarbons such as, benzene, toluene and xylene. At temperatures of, 100–140 C and in contact with anhydrite hosted, by salt structures or by carbonate reservoir rocks,, oil and gas are oxidized by abiotic thermochemical, sulphate reduction (eq. 1.21 in Chapter 1.4, “Diagenetic Ore Formation Systems”). Magnetite, and haematite, for example in sandstone, are also, hydrocarbon-oxidizing agents (Seewald 2003)., , 7.3.6 Tectonic environments and age, of hydrocarbon provinces, Hydrocarbon deposits are formed in all sedimentary basins on the Earth, which display: i) the, presence of source rocks; ii) a total thickness of, sediments and heat flow sufficient for kerogen, maturation; and iii) adequate traps. Several geodynamic settings provide the suitable background:, . epicontinental platform and shelf basins (Saudi, Arabia: Alsharhan & Nairn 1997; Sirte Basin,, Libya, Western Siberia);, . continental or marine rift systems and aulacogens (Lambiase 1995; Nigeria, Gulf of Suez,, North Sea: Glennie 1998, Sudan);, . back arc basins (Yellow Sea, Okhotsk Sea);, . passive continental margins (Arabian-Iranian, Gulf: Konyuhov & Maleki 2006; the Campos, Basin and the new deep-water supergiant Tupi, field offshore of Rio de Janeiro; West Africa);, . orogenic thrust and fold belts (Zagros orogen in, Iran, Alpine belts in Albania and Romania;, Rocky Mountains, USA);, . foredeeps of orogenic belts (north of the Caucasus, south of the Zagros Belt in Iran: Alsharhan &, Nairn 1997, eastern foreland of the Rocky Mountains in Canada);
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , intramontane basins within orogens, often, related to large transform or shear structures,, transtensional, transpressional or pull-apartbasins (Southern California, Maracaibo in, Venezuela)., Orogens may be both a location of oil and gas, deposits and the source of migrating hydrocarbon, fluids (Figure 7.15). Far-reaching horizontal fluid, migration of “tectonic brines” including hydrocarbons may be induced by advancing nappe piles,, such as depicted in Figure 1.74., More than 75% of the world’s conventional and, heavy oils were generated from Jurassic and Cretaceous source rocks (Hunt 1996). Oil and gas, deposits occur predominantly in geologically, young positions (Jurassic to Neogene), because, conservation of these delicate features through, long geological periods is not favoured. Destruction is caused by uplift, faulting and erosion as, well as by subsidence, deformation and metamorphism. Several graphite and pyrobitumen deposits, bear evidence of representing former hydrocarbon, deposits (cf. Chapters 3 “Graphite”; Section 7.5, “Pyrobitumen”)., Geologically ancient remains of hydrocarbons, are of high scientific interest. Although for most of, the Earth’s history, life was restricted to simple, forms, potential source rocks such as black shales, and stromatolites are common. Of course, most of, the ancient organic substance is over-mature and, oil and gas have been dissipated long ago., Archaean shales in the West Australian Pilbara, craton, for example, display features pointing to, pervasive petroleum generation (Rasmussen, 2005). The earliest giant deposits of oil shale and, (former) petroleum, called shungites, occur in the, Palaeoproterozoic of the Onega basin (Northwest, Russia; Melezhik et al. 2004). Similar rocks rich in, over-mature organic matter were formed at 2 Ga, in North America, Greenland and West Africa, (defining the worldwide “Shunga Event”). At the, White Pine copper mine in Michigan, oil was, actually found seeping from the 1.0 Ga Nonesuch, Shale (Chapter 2.2 “Copper”). Oil traces of about, the same age occur in the Roper Basin, Australia, (Dutkiewicz et al. 2003)., The earliest economic oil and gas deposits, however, are extracted from Neoproterozoic rocks in, ., , 553, , Oman, China, Sudan, Australia’s Amadeus Basin, and in eastern Sibiria. With the explosion of, oceanic multicellular life in the Ediacaran and, Cambrian, and the conquest of land by higher, plants in the Devonian, sources of organic matter, multiplied and diversified at the same time., , 7.4 EXPLORING FOR, , PETROLEUM AND NATURAL, , GAS DEPOSITS, , An introduction into underlying principles and, practice of exploration for ore and minerals is, presented in Chapter 5.2 “The Search for Mineral, Deposits”. The principles are much the same, but, the practice of exploration in the hydrocarbon, industry is different. Prospective depths, scale of, operations, available funds, financial risks and, expected profits are all considerably larger., The search for new oil and gas deposits increasingly extends into the seas and oceans of the world., In plate tectonic terms, these regions are mainly, submerged rifts, passive continental margins and, back arc basins. Offshore production is from ever, deeper water, currently about 3000 m. Because, present oceans are geologically young, Mesozoic, and Cenozoic source and reservoir rocks have, a dominant role. In the deep basins, mass flow, sediments are important hosts and traps are mixed, stratigraphic and structural. The marine environment poses new challenges that are met by, developing adapted versions of exploration, methods founded in terrestrial technology (Jahn, et al. 1998)., Yet, even on land, new hydrocarbon discoveries, are still being made. One example is TOC-rich, Neoproterozoic-Cambrian stromatolites enclosed, in Huqf (Hormuz) evaporites that host valuable, light petroleum in Oman (Schr€, oder et al. 2005). In, the Sichuan Basin and the eastern Tarim Basin in, China, considerable gas accumulations derived, from cracked oil have been found beneath the oil, window (Tian et al. 2008). New extraction technologies unlock vast gas resources in low-permeability shale, exemplified by the Barnett and, Antrim shales in the USA (McIntosh et al. 2004)., The foundation of all hydrocarbon exploration is, an understanding of the geological evolution of the
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554, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , target basin (Allen & Allen 2005). Without this, base, even hydrocarbon seeps, which are “the best, guide to oil”, may be wrongly interpreted. Today,, it is rare to locate new deposits by geological, methods alone. One exception is the large oilfields, in the tropical mountains of Papua New Guinea,, which were discovered by mapping structures and, oil seeps (Figure 7.15). Usually, geophysical and, foremost seismic surveys are the most efficient, discovery tool of the oil and gas industry, combined with geological, modelling and organic geochemistry methods. However, the geological, synthesis of basin evolution with petroleum and, gas systems in space and time (four-dimensional), is the overarching goal (cf. Figure 7.10, which, shows part of the required data)., General search criteria in oil and gas exploration, include:, . basins with sediments of >2000 m thickness (at, average heat flow);, . presence of potential source and reservoir rocks;, . sufficient maturity of the organic substance in, the source rocks (Ro 0.7–1.3 for oil);, . presence of trap structures;, . favourable timing of oil and gas generation,, migration and formation of traps;, . tight shale gas is sought in source rock matured, to the thermogenic gas window (Ro 1.3–2.0)., Hydrocarbon seeps and other pointers to oil,, such as H2S springs, bituminous rocks and mud, volcanoes (Hovland et al. 1997, Hein et al. 2006),, may be caused by ongoing secondary, or more, likely tertiary migration from leaking oil and gas, deposits. At the surface, oil loses light hydrocarbons (<C15) within two weeks by evaporation and, C15-24 in a few months. Water-soluble compounds, are leached and microbial degradation sets in., Oxidation and polymerization harden the oil into, tar, asphalt or pyrobitumen. However, carbon isotope and V/Ni ratios are well preserved and allow, asphalt-oil correlations. Famous examples of, active seeps with bubbling methane are the Tar, Pits in Los Angeles (California), Lake Bermudez in, Venezuela and La Brea asphalt lake of southwest, Trinidad., Fresh oil samples collected from seeps are analyzed for the hydrocarbon spectrum and biological markers. Already in the field, ultraviolet, , illumination distinguishes natural hydrocarbons, from pollution with refined products. UV light, of 302–366 nm wavelength (Figure 5.2) activates, crude oil luminescence in yellow to brown colours, whereas fuels and lubricants appear white, to violet. Remote determination of UV-spectra, of oil films on the surface of the sea by laser, fluorescence sensors allows correlation with, known oil types and even with specific source, rocks. Cold methane seeps on the sea floor, (Figure 3.5), some of which are products of microbial methanogenesis (eq. 7.3), display vent towers, or pockmarks built from carbonate and barite, precipitates. Methane bubbling from the sea floor, is detected during seismic surveys by sensors, towed near the bottom (“sniffer”, Figure 7.20)., Asphalt-rich oil produces tar pits on land and, asphalt flows on the sea floor (MacDonald et al., 2004). Paraffinic oil residue is earth wax. Submerged containers are used to sample gas bubbles, in water. Isotopic characteristics help to distinguish shallow biogenic methane, coal seam gas, and oil-associated gas (Figure 7.2). Fresh kerogenrich rocks (potential source rocks) are brown, but, ash-like white in weathered outcrops. Leaching, in chloroform or boiling in water may yield a little, oil that can be made visible by UV-light. Diffuse, microseeps of hydrocarbons on land can be expressed by:, 1 bleaching of red rocks (haematite ! pyrite or, siderite);, 2 kaolinization (feldspar ! illite ! kaolinite);, 3 formation of calcrete;, 4 enrichment of uranium; and, 5 sickly vegetation (Schumacher 1996)., All these guides are detected by remote sensing, methods (Meer et al. 2002)., In little explored prospective basins, tectonic, and salt-related traps are first sought. Regional, magnetic and gravimetric surveys are able to, map structural highs and lows, and to identify salt, domes (Figure 4.29). Simple stratigraphic traps, such as sandbars, channels and reefs are detected., In this early stage, recognition of complex stratigraphic, diagenetic, hydrodynamic and combination traps is not expected. With more data,, foremost by a number of wells and seismic surveys, prospects of success are much better.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , 7.4.1 Geophysical methods, Geophysical methods in hydrocarbon exploration, include first of all the relatively inexpensive and, highly efficient reflection seismics (Gadallah &, Fisher 2009). On land and offshore, airborne magnetic, magnetotelluric and gravimetric methods, serve early reconnaissance (Gibson & Millegan, 1998; cf. Chapter 5.2 “Geophysical Exploration, Methods”). Seismic refraction is employed to, investigate targets that do not respond well by, seismic reflection., On land, seismic reflection methods use signals, created by detonating explosives in shallow drillholes or by vibrating a large mass (Figure 7.21,, Figure/Plate 7.22 and Figure 7.23). Reflected signals are recorded by a chain of geophones. At sea,, a boat produces sonar signals (e.g. by underwater, air guns) and reflections are recorded by 8–10, towed streamers of hydrophones (Figure 7.20). The, seismic signal is an elastic longitudinal compression sound wave (a P-wave). Reflections are due to, lithological contrasts and depend on the difference, of the acoustic impedance (density seismic, velocity, Table 7.7) of two adjacent rock units. By, processing, depth and dip of lithological contrasts, , 555, , are imaged. Initially, seismic profiles are laid out, crossing major structures. Tie lines and eventually, polygonal infill lines are added in order to, produce a spatial image of the subsurface geology, (“three-dimensional seismics”). At this stage, tectonic and salt diapir- or salt pillow-related traps, are confirmed. Less conspicuous traps can only be, determined if a seismic stratigraphy frame can, be established, which requires information from, drilling. With special technology, very highresolution three-dimensional images and models, can be produced. Remember that seismic data, interpretation was the foundation of global seismic stratigraphy, which is now called “sequence, stratigraphy” (Miall 1997, 2000; Einsele 2000)., This method allows advanced geological modelling with little well control. In response to the, problems caused by salt bodies concealing sub-salt, structures, marine seismic surveys increasingly, rely on three-dimensional data acquisition such, as “wide-azimuth” (several survey vessels moving, in parallel) and “multi-azimuth” methods (repeat, passes at different angles)., Seismic refraction methods are distinguished, by using a specific path of seismic waves: Upon, encountering a lithological interface between, Seismic streamer, , Seafloor, , Seafloor, gas seeps, , Sniffer, , Calcite cementation produces, anomalously high seismic velocity zones, , Eocene aquifer, , Methane-oxidizing microbes, , Figure 7.20 Submarine gas seeps and, seismic detection of calcite-cemented, portions of Eocene sandstone in the Timor, Sea facilitate localization of deep oil, deposits which leak because of Miocene, inversion tectonics (modified from AGSO, Research Newsletter 21, 8–11, 1994). , Commonwealth of Australia (Geoscience, Australia) 1994. The calcite-cementation, is caused by methane-oxidizing microbes., , Leaking gas cap, , Leaky Mesozoic, petroleum, system, , Overmature, source rock, , Oil, Residual oil
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556, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Recording truck, , First arrival of energy, , Vibrator truck, , Geophones, , Source point, instant, , Seismic wave paths, , Reflection from bed A, Reflection from bed B, Sandstone bed A, Corresponding idealized, reflection seismogram, Oil, , Water, , Shale beds, Gas, , Reservoir, , a bed with low P-wave velocity above one with, high Vp, impulses radiating downwards from the, seismic source are refracted into a path parallel to, the interface. From this travel path in the interface, some seismic energy returns to the surface., Nearly horizontal position of the interface is, advantageous. The method is mainly used for the, investigation of cover rocks and water table in, engineering and hydrogeology. In oil and gas, exploration, elucidation of relatively shallow salt, structures is the main object. Salt has a high Vp of, 5000 m/s, resembling hard rocks, whereas other, sediments display lower velocities (Table 7.7)., Telluric currents in the lithosphere are induced, by particle flow from the Earth’s ionosphere., Telluric currents and the associated magnetic, fields display pulsation of direction and strength., The magnetotelluric method (MT) measures the, natural field variations at different frequencies., Lower frequency data provide information about, deep conductors, such as graphite and sulphide, rich rocks. MT cannot replace seismic and potential methods but provides additional constraints,, especially concerning the presence of hydrocarbon, source rocks in deep parts of sedimentary basins, and in their basement., Early distinction between barren and hydrocarbon-filled traps helps to avoid the expense of futile, drilling. Specialized variations of geophysical, methods have been developed to assist in this task., One approach is employing the higher resistivity, , Sandstone bed B, , Figure 7.21 Principles of, seismic reflection field work, the, most common geophysical, method in hydrocarbon, exploration. The inset displays a, typical time-distance graph, resulting from one survey line., , of hydrocarbon pore fluids compared to saline, waters. For the detection of such low-conductivity, bodies, electromagnetic (EM) and magnetotelluric, methods were adapted for submarine conditions., In seismic surveys, advanced analysis of P- and, S-waves may differentiate hydrocarbon from brine, pore fluid in reservoir rocks. This is the seismic, “direct detection technique” (AVO ¼ amplitude, vs. offset)., 7.4.2 Geochemical methods of hydrocarbon, exploration, Geochemical methods of hydrocarbon exploration, range from organic geochemistry of source rocks,, oil and seeps (Hunt 1996) to regional reconnaissance targeting buried deposits on land (Peters &, Fowler 2002). Past efforts to detect specific, guides for surface prospecting included measuring hydrocarbon gas in soil and near-surface air, (Klusman 1993), trace metal analyses and radiometric measurements. Detected hydrocarbon, anomalies at the surface are, of course, just microseeps. which have the same probability to, indicate the location of a hydrocarbon deposit, as macroseeps. Considering that hundreds of dry, wells have been drilled in the past near strong, seeps (Hunt 1996). there is little reason to accept, microseeps as stronger guides than the first., With due care and sufficient geological data, the, significance of seeps can be evaluated and
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , 557, , Figure 7.22 (Plate 7.22) Laying out, a seismic reflection survey line of, geophones in the desert of Cyrenaica,, Libya. Øyvind Hagen, Statoil., , mapping of light hydrocarbon (C1 to C4) distribution on the surface may help exploration (Zhang, 2003). Only a clear understanding of the source of, hydrocarbons seeps and the flow paths assists in, directing drillholes to the right structure, (Figure 7.20)., 7.4.3 Exploration drilling, Data acquired by methods introduced above are, the foundation for choosing the first drilling tar-, , get. A “structure hole” provides a wealth of stratigraphical and lithological data and at the same, time targets a seismically defined prospective trap, structure. Seismic parameters measured in situ, and petrophysical properties of the encountered, rocks are invaluable information for refining the, interpretation of seismic surveys and, of course,, for an improved understanding of the investigated, hydrocarbon system., During drilling, all firms employ sophisticated, data collection procedures. Lithology, fossils and
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558, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Figure 7.23 (Plate 7.23) A fleet of heavy vibrator trucks waiting for the deployment order. Cyrenaica, Libya. Øyvind, Hagen, Statoil., , hydrocarbon contents of drilling mud samples are, recorded. Kerogen particles are separated and comprehensively analysed. Main parameters sought, are kerogen type and maturity. Technical data, such as the rate of penetration are recorded. Selective coring during drilling, or later by perforating, borehole casing, provides little-disturbed samples, for the determination of petrophysical properties,, grain size spectrum, wetting characteristics,, porosity and permeability of potential production, horizons. In-situ fluid pressures are measured and, oil (reservoir fluid) is sampled in order to study, fluid behaviour under reservoir (and production), conditions. If oil or gas is found, flow rate and, pressure are determined (Chierici 1994)., Table 7.7 Range of sonic P-wave velocity (m/s) of common rocks, water and air (gas), Sand, Coal, , 200–2000, 1000–3000, , Clay, Shale, , 500–2500, 2500–5000, , Sandstone, Salt, Water, , 2500–5000, 4500–5000, 1500, , Limestone, Anhydrite, Air, , 3000–7000, 4800–7000, 330, , 7.4.4 Geophysical borehole measurements, Geophysical borehole measurements are used in, all wells drilled for hydrocarbon exploration. The, technology was introduced by Conrad and Marcel, Schlumberger when they opened an office in Paris, (1920) offering geophysical surveys to the oil, industry. A wide spectrum of methods is available,, aimed at providing:, . data supporting geological description and, analysis;, . petrochemical and petrophysical parameters,, and;, . technical information, which is decisive for drilling and hydrocarbon production., Well logging serves stratigraphical correlation,, the distinction and localization of important single horizons (Figure 5.8), the identification of gasoil and water-oil contacts, the determination of, rock properties and the nature of pore fluids in a, reservoir. Chapter 5.2 “Geophysical Exploration, Methods” introduced borehole logging at a general, level. Here, several methods are addressed in more, detail. For professional information, consultation
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , of works such as Ellis & Singer (2007) and Serra &, Serra (2004) is recommended:, Conventional resistivity logs measure the apparent, electric resistivity of borehole wall rocks. Resistivity, is the inverse of conductivity and is measured in Ohm, metres (ohm/m). It is controlled by pore water content, salinity of the water and the presence of drilling, fluid, oil, gas, sulphides or coal. Focused arrangement, of electrodes (e.g. laterologs) renders bed boundary, definition more precise. The main aim of resistivity, surveys is to locate oil by its very low conductivity, compared with formation water, but the data facilitate general formation correlation and evaluation. An, example is the distinction between effective (macro-), and ineffective micro-porosity (Smith et al. 2003)., Formation resistivity is also measured by inducing, an alternating magnetic field and consequent current, loops in the formation. The resulting signal is proportional to the conductivity and is recorded by, a receiver coil. Application is typically related to, formation of a non-conducting mudcake in the walls, of the hole, or mud invasion in the wall rocks., Spontaneous potential logs display the natural difference in electrical potential (mV) between an electrode in the borehole and a fixed reference electrode, on the surface. The SP potential results mainly from, differing ion concentrations in drilling mud and formation water and the clay content of permeable beds., The logs are used to detect permeable beds, to estimate formation water salinity, formation clay content and, although data are not absolute values, for, correlation with nearby drillholes., Dipmeter logs are indispensible for the geological, interpretation of a borehole. Tools for this task are, micro-resistivity devices with 3 or 4 electrodes set at, angles of 120 or 90 . Another instrument (e.g., a magnetic compass) records the orientation of, the probe as it is drawn up the hole. Data plots, are evaluated in context with other information, (e.g. seismic and sedimentological models)., Borehole deviation logs are needed for technical, control of the drillhole and especially in connection, with directional drilling. Dip and azimuth of the, borehole axis are recorded at predetermined depths., The orientation of the tool is given by a magnetic, compass or by gyroscope-based instruments. Deviation and dipmeter surveys are typically performed in, combination., Natural gamma radiation surveys can be run, through casing (the steel tubes of wells). In open holes,, gamma radiation is measured together with the hole, , 559, , diameter,which is recorded bya caliper logging device., This is needed because gamma readings are affected by, missing mass in the wall, for example by wash-out or, caving. Natural radioactivity is due to the radioactive, decay of uranium, thorium and potassium (40K). In, hydrocarbonexploration, recognition of certain lithologies is the major aim, such as uranium-enriched, phosphate, common shale, oil shale, source rocks and, sapropelic coal, thorium lodged in heavy minerals, concentrated in coastal sand bars, and potassium in, potassium salts, feldspar, illite, mica and glauconite., Of course, using spectrometer detectors instead of, scintillometers provides more meaningful data (cf., Chapter “Uranium”)., Bulk density logging (Gamma-Gamma, or GG, method) is only run in uncased holes. Essentially,, the reduction of the gamma ray flux in wall rocks, due to Compton scattering between an emitting, source, usually 137Cs, and a detector is measured., Compton scattering reflects electron density and, not bulk density, but calibration provides density, logs, which apply to the majority of sedimentary, rocks. Assuming that the instrument measures only, that part of the wall rocks that is invaded by drilling, fluid (of known density), porosity is easily calculated, if the rock type and the densities of its dry and solid, constituents are known (e.g. quartz grains of a sandstone, D ¼ 2.65 g/cm3)., Neutron-neutron porosity logs record the effect of, wall rocks on fast neutrons emitted by a source in, the hole. Slowing down and capturing of neutrons is, most intensive on hitting hydrogen atoms. Because, in carbonates and sandstone hydrogen occurs, mainly in the pore fluid, the measured values of, thermal or epithermal neutron detection correlate, essentially with porosity. Clays and other hydrated, minerals give relatively higher readings. Because of, the low hydrogen density, natural gas as a pore fluid, yields very low apparent porosity. This effect can be, used to determine the hydrocarbon fluid composition in oil deposits (“geochemical reservoir analyser”)., Caliper logs display the measured diameter of a, drillhole along depth. The instruments are commonly, multifinger mechanical devices. Borehole wall, irregularities (rugosity) assist lithological and geotechnical interpretation. The characteristic elliptic crosssection of open holes, for example, reflects the orientation of rock mass stress vectors. These data are of, high scientific (Cloetingh et al. 2005) and practical, interest, for example when high-pressure fracturing is, planned for raising gas flow from a dense reservoir.
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560, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Temperature logs in boreholes are produced by determination of bottom hole temperatures (BHT) during, drilling, or by logs run later. Circulating drilling fluid, always induces cooling compared to the ambient, temperature of surrounding rocks. Measured temperatures are corrected in order to establish the, geothermal gradient in the well, which is essential, information (e.g. for maturation modelling). Later, temperature runs serve foremost the detection of, anomalies, or departures, from the reference gradient., Anomalies are usually connected with outflow (of, drilling fluid) or inflow (of pore fluid) into the hole., Cementing, acid stimulation and hydraulic fracturing induce transient anomalies that assist evaluation, of these operations. Because most rocks and pore, fluids have relatively low thermal conductivities, the, temperature log records the behaviour of a well over, a longer time period than other measurements., Sonic, or acoustic velocity logs display velocity (or, some other property) of acoustic P- and S-waves vs., depth. The instrument carries both an emitter of, sound and several receivers. Data serve foremost to, determine porosity, which is directly correlated with, acoustic properties, because the solid components of, rocks transfer most of the P-wave energy. With the, rock type known (e.g. sandstone, limestone), the, measured sound velocity is compared to the pure, mineral at 0% porosity (e.g. quartz, calcite). Typical, ranges of sonic velocity are listed in Table 7.7 (Milsom 1996). Sonic logs are also used for lithological, correlation, the determination of permeability and, potential gas zones, and for early recognition of overpressured zones. Other sonic tools have been developed, which produce borehole wall images that, allow inspection of fracturing, breakouts and bed, characteristics., , BOX 7.1, , Measurement-while-drilling (MWD) is employed, to determine pressure, temperature, electrical resistivity and mechanical rock parameters for real-time, management of the drilling operation. Borehole, seismic tools, which comprise seismic sources and, receivers, are applied in order to refine spatial models, derived from surface surveys., , Potentially economic hydrocarbon-bearing horizons in the well are investigated in great detail., This includes first flow tests (drill stem test, DST),, although a full evaluation of production characteristics is only possible after well completion, (installing hardware and technologies for extracting hydrocarbons from the reservoir). In both oil, and gasfields, preliminary resource estimates can, be based on only one well. However, reliable, reserve determination is only possible when the, full variation of reservoir properties and its fluids, are known, requiring a number of wells. Even if, the first exploration well located hydrocarbons,, appraisal and development drilling will go on. Of, course, all data acquired are integrated in qualitative, quantitative and dynamic analyses of the, reservoir, the effective drainage area (Figure 7.9), and the whole basin (Miall 2000, Allen & Allen, 2005). A model of the petroleum system generating the new field is assembled. The system’s yield,, efficiency and potential resources can be estimated. In mature basins such as the North Sea, (Box 7.1), past production plus remaining measured resources are compared with estimated total, generation (Balen et al. 2000). Strategies are developed to explore for “missing” amounts., , Discovery and Development of the North Sea Oil, and Gas Province, , The North Sea oil and gas province provides an illustrative tale of exploration and development (Glennie 1998)., For >100 years, small and medium-sized oil and gasfields had been exploited on both shores of the North Sea, in, Germany (starting with Wietze, Hannover in 1859) and in England. The inducement for expanding exploration into the, sea was the realization that the gasfield Groningen in the Netherlands was one of the world’s largest deposits. In 1964, the, first offshore drillhole was sunk by a German consortium (Nordsee B-1). The first large finds, however, were the gasfield, Leman in the British sector (1965) and the oilfield Ekofisk (1969: 200 106 m3 oil) in the Norwegian sector. The oil price, spikes in the years around 1970 hastened developments. Exploration successes peaked in 1973–1979 and more than, 50 giant oil and gasfields have been found. Until 2002, when annual production peaked at 450 Mtoe, over 5.1 Gt of oil, and 3.2 Tm3 gas had been produced. Today, the North Sea province is mature, concerning both oil and gas. Production of
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , 561, , oil is declining, whereas gas output is about equal to 2002 (BP Statistical Review of World Energy 2010). Few significant, finds have been made for many years; recent discoveries average around 10 million barrels (1.4 Mt)., Prominent source rocks for oil and gas in the North Sea include:, . Late Carboniferous coal for gas in the southern sector;, . Early Jurassic shales for oil in the southern sector;, . Late Jurassic and Early Cretaceous shales (Kimmeridge Clay) in the northern sector; with 2 to >12% organic matter and, elevated uranium contents, called “hot shale”, because of a distinct signature in g-logs., Reservoir rocks display ages from the Devonian to the Eocene, but the largest oil deposits are hosted in Early and Middle, Jurassic sandstones and in Late Cretaceous chalk. Formation and deposition of these sediments are controlled by, a complex Mesozoic graben system with a width of >100 km. This is a failed rift related to the opening of the Atlantic, Ocean. Source rock maturity sufficient for hydrocarbon generation is confined to more central parts of the rift. Oil and gas, generation started in the Late Mesozoic. Hydrocarbons migrated mainly upwards and short distances to traps. In the, southern sector, compressive tectonic overprint (inversion) destroyed a number of deposits formed. Most traps are, stratigraphic, although influenced by a complex interplay of graben faults, halokinesis and palaeogeography., Exploration and development of offshore oil and gas in the North Sea continue, although targeted traps are increasingly, more complex and deposits found smaller (Figure 7.11 and Figure 7.16). An important part of future production is foreseen, to result from field growth, for example by applying improved recovery technologies. Until recently, heavy oil and sour, gas were hardly considered. This is expected to change with higher hydrocarbon prices on the market. Better sales, revenues would considerably expand resources. Optimists believe that a paradigm shift in exploration concepts may yet, end the decline and are supported by discovery of the Catcher fields (June 2010) with an estimated 300 Mb oil in place. In, the future, deep aquifers in the North Sea province provide Britain and continental Europe with a huge volume for offshore, geological sequestration of CO2 (Figure/Plate 6.2; Haszeldine 2009)., , 7.5 THE, , EXPLOITATION OF PETROLEUM, , AND NATURAL GAS DEPOSITS, , The rational exploitation of hydrocarbon deposits, is the result of teamwork between petroleum, engineering, geology, geochemistry and geophysics. The overall objective is not different from that, of a mine operation (cf. Chapter 5.3 “Valuation of, Mineral Deposits”) and can be summarized by, “optimization of the operation so that the investment is recovered and profits accrue”. The recovery of hydrocarbons from a porous or fractured, rock body at depth is based on geological input, and managed by reservoir engineers (Chierici, 1994, Dake 1994)., , 7.5.1 Reservoir conditions, Before the influence of drawing fluid from, a reservoir can be evaluated, the initial conditions, must be established. Integration of geometrical,, physical and chemical data allows reservoir characterization and the application of numerical, modelling., , Porosity and permeability are of overriding, importance. Although they are determined by, a number of other methods, investigations of, reservoir rock core under simulated reservoir conditions such as P, T, fluid pressure and rock mass, stresses are paramount. With these data, earlier, wireline measurements can be calibrated and reinterpreted. The physical “intrinsic” permeability, k (m2) of reservoir rock is determined with an inert, gas. “Effective” permeability of a rock depends on, the composition of the hydrocarbon fluids (eq. 7.5)., Wetting characteristics (water-wet or oil-wet, grains) and capillary pressure related to different, widths of pore throats are other factors that influence fluid flow. Permeability is greater parallel to, bedding and smallest for fluid moving across bedding. This is especially important for today’s trend, to bedding-parallel production wells. Double, porosity characterizes many hard rocks; the term, describes the presence of both matrix and fracture/, joint porosity. Fluid extraction reduces porosity, because falling fluid pressure results in rising, effective stresses (Figure 1.39). Grains are forced, into a denser packing, fractures close and the solid, framework takes up a higher share of total stress.
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562, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , This is why hydrocarbon production induces, compaction of the reservoir rock, which may, result in seismic activity and subsidence of the, land surface., Petroleum reservoir fluids constitute a complex, system of water, dissolved salt and hydrocarbon, compounds, which may occur in liquid, gas and, supercritical state (Danesh 1998). Subtle changes, in T and P, which are unavoidable during extraction, may cause drastic changes such as foaming of, fluids. Therefore, the fluids’ behaviour is investigated in the laboratory at simulated reservoir and, projected production conditions. At depth in the, reservoir, fluids are compressed (more dense) and, expand as they rise to the surface. Production, volumes are measured at the surface, where oil, and gas are separated and stored in tanks. For, converting oil in place to tank oil (m3), the “oil, formation volume factor” is determined (oil, plus dissolved gas volume at reservoir conditions, divided by oil volume at surface conditions)., Because of dissolved gas, oil formation volume, factors are commonly >1 and reach >2 for volatile-rich oil and high gas/oil ratios. Similar rules, are followed for natural gas deposits, but because, of the high compressibility of gas, formation volume factors are <1. Gas at the surface is measured, in standard cubic metres (Sm3)., The viscosity of reservoir fluids is the principle, property that controls flow through rocks of a, given permeability to the well. Viscosity of crude, oil is reduced by increasing amounts of dissolved, gas and by rising temperature. Considering that, the reservoir temperature changes little during, production, it is mainly pressure and gas loss that, cause gradually diminishing flow of oil from pore, space to the well., Saturation describes the relative amounts of, water, oil and gas in pore space as a percentage, of volume. Water saturation is the volume of, water contained in a volume of pore space. It is, essentially a measure for the boundary between, the oil or gas zone and the water zone. Oil-water, (OWC) or gas-water (GOC) contacts are always, gradual and require a precise determination., Lithologically inhomogeneous reservoirs may, display water-rich lenses within the oil or gas, zone., , Reservoir pore fluid pressure is generally, near hydrostatic conditions. Freshwater imparts, an average pressure increase of 9.8 kPa/m., An increase in salinity of 0.01 g/cm3 adds, 0.098 kPa/m to the hydrostatic value. Vertical, rock pressure (correctly “rock mass stress”) is, normally the upper bound of possible (abnormal), fluid pressures. In principle, the vertical, lithostatic stress at a depth z (m) from the surface is, calculated by the weight of a rock column with a, cross-section of 1 m2 (eq. 7.7). For sandstone, this, amounts to 22 kPa/m, or in units of stress, 22 kN/m., Calculating lithostatic and hydrostatic stress:, sðvÞ or ðhydÞ ¼ g zðmÞ, , ð7:7Þ, , s(v) is the lithostatic, s(hyd) the hydrostatic stress, g the, rock unit weight calculated from density multiplied by, gravitational acceleration (9.81 m/s2, or rounded for, estimates, 10 m/s2). Inserting pore fluid unit weight, for g, hydrostatic stress (or pressure in kPa) will be, the result., , In rock, the vertical stress induces smaller horizontal stresses, so that a complete stress field is, defined by the three vectors s (v), s (H) and s (h)., The direction of the larger horizontal stress s (H) is, usually imposed by the regional plate tectonic, stress field. However, the complex load path and, geological history of most rock bodies cause frequent deviations from this idealized pattern (Brady, & Brown 2004)., Average reservoir pressure is equal to a water, column with the density of 1.1 (a brine with, 8 wt. % dissolved NaCl). Abnormal fluid overpressures are defined by a value above the mean of, hydrostatic pressure and lithostatic stress. Even, higher pressures are possible, but very rare. Overpressures typically occur in sand lenses enclosed, by mudrock, but may also be a regional feature, as in Late Cretaceous chalk in the North Sea, (Mallon et al. 2005). In sand enclosed by clay,, total vertical stress rests in part or fully on the, pore fluid. In the latter case, the sand grains float, in the fluid and have hardly any contact with each, other. This is due to inhibited dewatering and, compaction of the sand when it was rapidly, buried (under-compaction or disequilibrium
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , compaction). A second mechanism for creating, overpressures is expansion by increasing fluid, volume in a restricted pore space, for example, conversion of kerogen to oil, gas and water (Swarbrick & Osborne 1998). Overpressures dissipate, naturally by fluid flow out of the system. The, required time is a function of permeability. Compressive tectonic stress fields favour longer conservation of high fluid pressures (Sibson 2003)., Supralithostatic high-pressure fluids, such as gas, generated by the decomposition of oil and kerogen in very deep (>3 km) compartments, may, cause hydraulic fracturing and fluid flow in fractures (Grauls & Cassignol 1992). Transmission of, high fluid pressure through fissures, or by uplift, and inversion can cause unexpected overpressure, at shallow depths (Tingay et al. 2007, Luo et al., 2003). Formations and reservoirs with unexpected, overpressure endanger drilling because of the, blowout risk. In spite of precautions, fatal accidents, high costs and environmental damages too, often result from blowouts (Tingay et al. 2007,, Fertit et al. 1994)., Abnormally low pressures (below hydrostatic, conditions) are the result of over-consolidation,, with dense packing and low pore volumes that, were imprinted when the rocks were at greater, depth than at present. Neuzil (2000) suggested that, osmotic action may also induce abnormally low, pressures. The retention of subhydrostatic pressure after erosion and uplift is evidence of very, effective sealing against migrating fluids for, geological time periods. Rock bodies with these, characteristics are suitable hosts and barriers for, toxic and radioactive waste (e.g. the Early Cretaceous shales, in Figure 5.30)., The subsurface temperature/depth profile, reflects the local geothermal gradient but is modified by other factors. The average geothermal gradient in the crust approximates 25–30 C/km., However, as a function of local heat flow and the, thermal conductivity of rocks, it varies between, extremes of 20 and 250 C/km. As rock salt is an, excellent conductor of heat, higher temperatures, are measured on top of large salt bodies (e.g. diapirs). Locations with high temperature near the, Earth’s surface are prime targets for geothermal, energy exploration., , 563, , 7.5.2 Oil and gasfield development, After successful discovery of a new hydrocarbon, field and its confirmation by a first well, resources, and reserves must be established. This requires that, the oil or gasfield is delineated and that spatial, distribution and variation of essential parameters,, such as porosity, permeability and hydrocarbon saturation, are known. These tasks necessitate drilling, more wells. Apart from supporting reserve estimates, the new wells are later to serve production., Development and production of a simple homogeneous oilfield is theoretically possible from only, one well. In the case of gas drive and gravity flow,, production would be from the base of the oil zone,, and from the top with water drive. In good reservoir rocks, oil and gas may flow for considerable, horizontal distances. In the Middle East, the first, production wells were several kilometres apart, (Figure 7.13). In practice, however, much smaller, intervals are the rule, caused by changing reservoir, properties, complex geometry and the need to, support extraction by injection of gas or water., Similar conditions determine the development of, a conventional gasfield., Worldwide, coal bed methane (CBM) production, is rapidly increasing (Figure/Plate 7.24). Because of, porosity and jointing (cleats), most gas-bearing, coals are moderately permeable aquifers. In order, to facilitate gas flow to the wells, pressure is, lowered by pumping water (Figure 7.25). Vertical, production wells in the Powder River Basin are, spaced in a grid, which allows degassing an area of, 160 ha. Production is often higher than original, sampling indicated. This may be explained by flow, from a greater distance than expected, or on-going, microbial gas generation (Ayers 2002). Improved, extraction technology incorporates in-seam, or, horizontal wells with lengths of 1000 m, drilled, perpendicular to the fracture system of coal seams., This design maximizes drainage of the gas, resulting in many times greater flow rates than can be, achieved from vertical, fracture-stimulated wells., Lowering pressure in permeable reservoirs by, pumping is also the most promising technology, for the development of gas hydrate deposits, (Boswell 2009). Reduced pressure causes hydrate, dissociation and releases methane.
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564, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Figure 7.24 (Plate 7.24) Drilling for CBM gas in the Rocky Mountains near Pinedale, Wyoming, USA (2008)., Copyright Shell plc., , in the Fort Worth region in Texas, where production started in 1999. The Mississippian (Early, Carboniferous) Barnett Shale at Fort Worth is, a thick organic-rich shale (TOC 3–5 wt. % of kerogen type II), which hosts giant recoverable gas, resources (850 109 m3: Pollastro et al. 2007), and currently provides 50% of US gas production. Although extraction is more expensive, , Tight shale gas development consists of drilling, a pattern of parallel holes that run thousands of, metres within beds, in order to enhance drainage., Shale in the thermogenic gas maturity stage (Ro, 1.3–2.0) is commonly lithified and fractured. If, permeability should be too low, hydraulic fracturing is used to increase gas flow. The pioneering, discovery and technological innovation took place, W, , Surface mine, , E, , Gas, cap, , m, hF, satc, Fm, Wa, n, o, i, t Un, For, , r, , nd, rou, , te, wa, , flow, , Compactional, anticline, , ow, all, Sh al, co, , G, , l, , oa, , c, ep, , Depressurized/dewatered, zone, , De, , No scale; great vertical exaggeration, , Figure 7.25 Strategies of exploration and extraction of coal seam methane in the Palaeocene-Eocene Powder River, Basin, Wyoming (Ayers 2002). Initially, production started in low-pressure zones near open pits and in shallow traps., Meanwhile, drill holes reach a depth of >1000 m. The coal is of low rank (<0.4% Ro), and the gas is of microbial origin., Because of well-developed cleats, the seams are aquifers with a permeability between 10 mD and several Darcy.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , compared to conventional deposits, the economic, success triggered a worldwide search for tight, shale gas deposits:, The giant gas play in the Barnett Shale occurs in an, area of northwestern Texas where for >100 years, conventional hydrocarbon deposits had been, exploited, many of which were sourced from Barnett, Shale. For some time now, the province was considered to be mature. Innovative thinking led to the, recognition that source and reservoir may be one, within this 300 m thick unit. Seals are provided, by dense footwall and hanging wall limestones. The, newly found gas resources are in shale with a maturity Ro >1.1%. Wet gas occurs in the maturity zone Ro, 1.1–1.4%; near the Ouachita Structural Front at Ro, >1.4%, only dry gas is found (Pollastro et al. 2007)., The strong heat gradient may have been caused by, fluids driven from the Ouachita orogen. In-situ gas, was generated by cracking of oil and bitumen and is, unassociated (no oil)., , N, , 565, , Water, injection, template, , Offshore, loading, buoy, , Platform, , Oil producer, Oil producer, (subsea), Water injector, , 7.5.3 Oil and gas production, In many fields, initial oil production is by free flow, from the well. The pressure in the reservoir is, sufficient to drive the oil to the surface. Three, main mechanisms cause a natural drive, and support primary production:, 1 water drive (recharge of bottom water from an, aquifer);, 2 gas cap drive (expansion of the gas cap);, 3 depletion, or dissolved gas drive (no gas cap,, expansion of the oil and its dissolved gas)., Note that in practice these driving mechanisms, are often combined. Gravity and compression of, the pore space with decreasing fluid pressure have, a secondary role:, Water drive is more favourable than the other mechanisms, because oil and gas recovery is highest,, with a range of 20–80% and an average of 50% of oil in, place (Chierici 1994). Bottom water under a high, hydraulic head forces the hydrocarbons upwards and, to the wells. If the aquifer is continuously recharged,, the original reservoir pressure may hardly change., With less effective recharge, the pressure drops. Heterogeneous reservoir rock may induce irregular rise of, the water-oil or water-gas contact. If the natural water, drive is insufficient or irregular, injection of water is, a frequent corrective (Figure 7.26)., , Gas injector, , 5 km, Figure 7.26 Development plan of offshore oil field, Draugen (cf. Figure 7.16), with support of oil production, by gas and water injection (Provan 1992). In this field,, reservoir pressure is low and the oil is, gas-undersaturated., Gas cap drive occurs at oilfields with an important, gas cap (Figure 7.14). Expansion of the gas displaces the, oil downwards, where it is produced from wells near, the bottom of the oil zone. Oil production and reservoir pressure fall steadily while the gas/oil ratio rises., Oil recovery is between 20 and 58%, with an average of, 33%. Injection of gas (gas recycling) supports pressure, and improves production and oil recovery., Depletion gas drive characterizes oil deposits without a gas cap or a contact with an aquifer. This may be, the case for a sand reservoir enclosed in tight shale., Oil and dissolved gas are lifted together so that pressure falls rapidly. Careful management must avoid, effervescence of a gas phase in the reservoir, which, drastically impairs oil production. Recovery by depletion gas drive is poor, with an average of 20% and a, range of 13–30%. Secondary (e.g. gas injection) and, tertiary production methods (see below) must be used, at an early stage.
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566, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Crude oil, Water, , Oil or water (103 barrels/day), , 3, , 2, , 1, , 0, Year 1, , 5, , At many oilfields, drive mechanisms change, during production. Production vs. time diagrams, characterize types and reveal the change. In, Figure 7.27, initial drive by dissolved gas was, quickly exhausted but replaced by water drive, until depletion. If the reservoir pressure falls to, a level too low for oil to flow to the surface, gravity, may assist recovery by supporting a steady flow, , 10, , Figure 7.27 Production history of an oil, well in North America showing change, from dissolved gas drive to water drive., Note increase of co-produced water with, time. Courtesy of Mobil Oil Germany., , towards production holes. Gas injection and various stimulation methods, such as hydraulic fracturing, are often employed to assist oil flow., Pumping the oil is typical for many mature oilfields, often marked by hundreds of “nodding, donkeys” (well-head pumps; Figure 7.28). The oilfield is depleted when the costs of pumping surpass earnings., , Figure 7.28 The symbol of, mature oil fields: A nodding, donkey in Oman. Copyright, Shell plc.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Secondary, artificial lift methods, Secondary, artificial lift methods are employed to, preserve reservoir energy, which is depleted during, production. Primary oil recovery is usually low and, injection of water and gas (predominantly CO2, N2), are widely utilized. Water “flooding” (injection) is, very common. Wells for water injection are sited at, oil zone margins or in the bottom aquifer. They, support production and are also useful as a means to, dispose of the saline water that is lifted with oil., The movement of injected water is controlled with, tracers (Zemel 1995). Secondary production methods are essential but overall, average oil recovery, still remains at unsatisfactory levels., Seawater for flooding is treated with biocides in order, to prevent microbes from intruding the deposit. With, untreated seawater, hyperthermophilic bacteria and, archaea, some of which live at black smokers on the, sea floor, are induced into the reservoir where they, thrive in spite of high T and P, and reduce SO4 to H2S, (Stetter et al. 1993). The result is simultaneous anaerobic biodegradation of the oil in situ and detrimental, H2S contents in the extracted oil. To avoid this,, sulphate may be separated from seawater before injection, especially if barite precipitation in the reservoir, is possible, which risks reducing porosity and permeability. Reactions of injected seawater in the reservoir, are surprisingly rapid (Houston et al. 2007)., , Flow management in the reservoir determines, production rate and oil recovery. Important controls of production flow include the pressure gradient to the well, the viscosity of the pore fluid and, the permeability of the reservoir rock. The latter is, subject to geological inhomogeneity and anisotropy. Water-wet conditions (water films enclosing, grains and narrowing pore throats) and free gas, bubbles in the pores reduce permeability for oil., In the common water-oil systems, permeability, for oil approaches zero when the share of oil in, the pore space falls below 30–20%. A total recovery of oil in place with well-production methods, is impossible. Compared with traditional fluid, management and primary plus secondary methods, four-dimensional, or time-lapse seismic survey methods raise oil recovery by 10–15%. This is, possible by close monitoring and improved man-, , 567, , agement of the changing boundaries of gas, oil, and water in the reservoir, comparing threedimensional seismic data measured at time intervals (e.g. first during exploration, repeated during, production). Time is the fourth dimension., Advanced visualization and modelling methods, are the key to increased oil recovery., Technical measures influence fluid flow in, the immediate surroundings of a production well., They include “stimulation” by hydromechanical, fracturing of the rocks or by injection of mixtures, of hydrochloric and hydrofluoric acid creating dissolution voids. Frac methods (i.e. fracturing the, rocks around a well) are important for the production from reservoir rocks with low permeability,, for example tight shale and sandstone. For flow, stimulation, a fluid (usually water) is injected, under very high pressure. Induced fractures reach, a length of 1000 m and a surface in the order of, km2. The fractures are kept open with co-injected, sand or corundum pellets. By this method, the, flow of gas and oil from “tight” formations to the, well is dramatically improved., Tertiary production, or improved oil recovery, (IOR) Methods, Tertiary production, or improved oil recovery, (IOR) methods raise the recovery by another, 10–15% of initial oil in place, in addition to the, advantage of 10–15% gained by four-dimensional, seismics. This adds up to today’s average oil recovery of 50%. Enhanced recovery methods include, (De Haan 1995):, . Chemical additives to flood water such as surfactants and polymer gels assist precise management of the oil-water boundary. Surfactants, reduce the difference between the surface tension of oil and water, liberating oil trapped in, narrow pores and improving flow. Polymers, block preferential water flow through high permeability zones in the reservoir., . Hydrocarbon gas-based recovery methods, especially the combination of hydrocarbon gas (typically propane and butane) and water injection,, which are combined because water and gas displace oil from different parts of a reservoir. The, technology is long proved and known as water-
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568, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , alternating-gas method (WAG). Supercritical, CO2 liquid for IOR (Klusmann 2003) is expected, to develop into an attractive alternative to, hydrocarbon gas, once sequestration will be, rewarded by carbon credits., . Thermal flooding with hot water or water, vapour (Coburn & Gillespie 2002) and in-situ, combustion are extremely effective. Heating, reduces the viscosity of oil and combustion, creates a front of liquid hydrocarbons that are, driven to production wells. Vapour flooding in, Kern River oilfield, California, raised recovery of, oil in situ to 75–80% (Coburn & Gillespie 2002)., . Microbially enhanced oil recovery (MEOR), works by injecting microbes or supporting oilfield-indigenous microbes in selective biodegradation of large molecules (reducing viscosity),, production of surfactants (lowering interfacial, tension) and increase of the driving force by, biogenic methane production., The economic significance of improved oil, recovery methods rests in the increase of reserves, (“field growth”, see below). The methods are deployed in deposits that are mature (past peak, production), well-known and fully developed., Additional investment for IOR is limited. Employing IOR methods is profitable and contributes to, responsible management of natural resources., 7.5.4 Petroleum mining, Mining of near-surface petroleum is rapidly expanding (cf. tar sands). Future high prices and, increasing shortage of oil may one day justify, conventional mining of the giant amounts of oil,, which are left behind in conventionally depleted, deposits (George 1998). One version is to build, a network of drainage galleries underneath reservoirs and to use vapour flooding to lower oil viscosity allowing seepage into collector adits. It is, thought that reservoirs to a depth of 1000 m might, be exploited by underground mining. Similar concepts exist for exploitation by means of horizontal, drillholes, avoiding the costs of mining and the, risks for workers. Alternatively, cost and risks, might be alleviated by automated mining, which, is already in development. In-situ steam-assisted, gravity drainage based on boreholes is practised to, , recover oil from the Canadian tar sands situated, below opencast depths. If this technology can be, adapted for deep reservoirs, huge additional oil, resources will be the result. In Germany, for example, estimated resources plus past production add, up to 765 Mt of oil in place, but with present, technologies 430 Mt are left in the ground., 7.5.5 Reserve and resource estimation, Reserve figures of oil and gas fields, provinces,, countries and of the whole world are essential, information for rational planning. Share values of, large international companies are in part a function of their reserves. States controlling giant, reserves and resources are sought as allies and are, able to control markets. Oil and gas impart economic and political power. With so much at stake,, it is hardly expected that published reserve figures, can always be trusted., In contrast to the political level, there is a highly, developed professional standard of reserve estimation in the hydrocarbon industry. Refer to Chapter, 5.3 “Ore Reserve Estimation and Determination of, Grade” for general definitions and methods. There, is little methodical difference between ore and hydrocarbons, including the extensive use of geostatistics. Only scale, complexity and financial risk are, generally higher, compared with the average mine., Here, allow me to point out some principles of, reserve and resource estimation, which are, specific to the oil and gas industry. Details are, provided by Chierici (1994) and Dake (1994). Foremost, only proved and audited reserves may be, published by companies whose shares are traded, on the stock exchange (e.g. the Valmin code,, AusIMM 2005). A large part of the figures reported in the introduction to Chapters 6 and 7,, however, are not audited (BP Statistical Review, of World Energy 2010), because controlling, states allow no independent evaluation. The, term “proved reserves” is defined by a high certainty of viable production under present economic conditions and with existing technology., Methods of reserve estimation in the hydrocarbon industry fall into the three categories:, i) volumetric; ii) material balance; and iii) production performance methods.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , Volumetric methods are the only way to estimate reserves in newly found hydrocarbon deposits, based on the geometry of the reservoir, its pore, volume and oil (or gas) saturation. Because reserves are principally expressed in oil at surface, conditions (tank oil), the volume change of oil, from reservoir to the surface must be determined., Most critically, the overall recovery factor must be, estimated. The principle of volumetric reserve, calculation is illustrated by eq. 7.8., Volumetric calculation of recoverable oil in place,, measured at tank conditions:, N ¼ A h F ð1Sw Þ, , ro, ER, Bo, , ð7:8Þ, , N ¼ recoverable oil (t), A ¼ area of trap (m2),, h ¼ average thickness of oil zone in reservoir rock (m),, F ¼ porosity of reservoir (), Sw ¼ share of water in pore, fluid (), ro ¼ density of oil (), ER ¼ recovery factor for, oil () and Bo ¼ formation volume factor ()., , The material balance method works by carefully, monitoring changes in the reservoir during exploitation. The extraction of hydrocarbon fluid imparts changes such as falling pressure, extension of, the gas cap and a rising oil-water contact. Based on, these data, a material balance is established for, each of the phases in the reservoir. The material, balance equation consists of a set of terms (eq. 7.9),, which represent all physical changes that are balanced in such a way as to yield a very precise, measure of extractable oil remaining in place, (the reserves)., A simplified example of the term for oil in a, material balance equation:, Oil remaining in situ ¼ ðNNp ÞBo, , ð7:9Þ, , N ¼ initial volume of oil in the reservoir, in m3 tank oil,, Np ¼ cumulative tank volume (m3) of oil produced, Bo oil, formation volume factor., , During the productive lifetime of a field,, the material balance calculation is regularly, repeated and refined. The deposit is imaged by, a numerical model, which consists of spatial, elements (blocks). Measured dynamic changes, are used to simulate the past and future behaviour of the system in every block of the model., Parameters that cannot be measured are derived, , 569, , by approximation or Monte Carlo simulations., History matching (adjusting model parameters, in order to improve simulation of past behaviour), is employed to improve models. The results, produce ever more precise figures for oil (or, gas) in place. Yet, the validity of numerical simulation of a reservoir system is still limited to a, near future., Reserve estimation is very similar for an ideal,, closed dry gas deposit within a homogeneous reservoir and without contact to water. A single, borehole is sufficient: Initial production will cause, a pressure drop. The extent of the fall in pressure is, a function of the initial pressure, the volume of gas, produced and the initial volume of gas in situ (the, reserves). The results of preliminary calculations, of this kind are normally insufficient for reserve, declaration but may be critical for the decision to, abandon the project or develop the field. With, more wells and a better database for the reservoir,, volumetric and material balance calculations, are employed. Development of gas in tight (lowpermeability) reservoirs relies on very close, drillhole distances and reserves depend on highly, efficient fracturing technology., Production performance methods are a tool, proved in oilfield engineering for estimates of, remaining reserves. By definition, however, they, are limited to projection of past performance into, the future. This is the same principle as applied in, constructing the Hubbert Curve (picturing production vs. time: Hubbert 1962). Utilization of, the method must be guided by a deep understanding of the production history and its parameters. It, is limited when different drive variants and production technologies were involved which, incidentally, produce rather plateaus than curves., Extrapolation of the Hubbert Curve is at best a, measure of the most easily accessible resource,, given technological, political and economic, constraints (Cavallo 2004). Is it not thoughtprovoking that the Hubbert Curve and the Peak, Production Method of resource estimation are, little used in the non-hydrocarbon minerals community? The terms are not even mentioned in the, remarkable volume on reserve and resource estimation by A.C. Edwards (2001). Also, the frequent, occurrence of reserve growth in oil and gasfields
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570, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , falsifies blind acceptance of production history, projections:, Kern River oilfield in California was discovered in, 1899. In 1942, after a production period of nearly, 50 years, its remaining reserves were calculated at, 54 Mbl. Production until 1986, however, amounted to, 736 Mbl and remaining in situ reserves were declared, at 970 Mbl (Maugeri 2004). Today, resources recoverable by steam flooding are estimated at >1000 million, barrels., , Kern River is possibly exceptional. Yet, oil and, gasfield reserve growth is widely observed. Causes, of field growth include economic, technical and, non-technical parameters (Attanasi & Root 1994)., Steam flooding in Kern River field illustrates how, the introduction of a new technology (human, inventiveness) may change all apparently rational, forecasts., Estimates of undiscovered (prognostic) oil and, gas resources are calculated with similar methods, that have been described in Chapter 5.3 “Ore, Reserve Estimation and Determination of Grade”., An exemplary estimate of undiscovered oil and gas, resources in the Arctic realm illustrates current, best practice (Gautier et al. 2009)., 7.5.6 Post-production uses of oil and gas fields, When an oil or gas field is abandoned because, further production is an economic loss, all boreholes must be carefully sealed (Fuenkajorn &, Daemen 1996) in order to minimize risks for, humans, the biosphere and the geosphere (e.g., groundwater). Before taking the decision to close, operations, companies will undoubtedly analyse, the costs of preserving boreholes for later use, against a possible future profit. In Chapter 5.3, “Valuation of Mineral Deposits”, we have seen, that future income, which accrues after a time of, 15 years, is nearly worthless. This is why the, decision to dedicate drillholes to a different future, utilization will rarely be positive. Some wells, may be converted for CO2 sequestration (Figure, 6.3) and deep storage of fluid waste (Figure 5.27)., In countries that have only limited resources of, gas and oil (e.g. most of Europe), the underground, storage of a strategic reserve of imported natural, , gas and oil (in salt caverns and depleted hydrocarbon reservoirs) is common practice. Transformation of deep oil and gas wells near large heat, consumers such as industry and big cities, into, geothermal energy producers may be economically attractive., , 7.6 TAR, , SAND, ASPHALT, PYROBITUMEN, , AND SHUNGITE, , Two totally different environments of petroleum, degradation cause formation of the substances, treated in this chapter:, 1 oil reservoirs are heated beyond the stability of, liquid hydrocarbons, usually caused by subsidence to great depth; and, 2 deep, shallow or supergene biogenic degradation, oxidation, water washing and evaporation of volatile fractions increase the density, of oil., Compared to conventional crude oils with, 22–35 API, the latter results in heavy (10–22.3, API) and extra heavy (<10 API) petroleum. Tar is a, term for heavy and extra heavy oils with 6–12 API,, whereas asphalt, pyrobitumen and shungite are, the products of such severe alteration that any, use resembling that of liquid hydrocarbons is, precluded., 7.6.1 Tar sand, Tar is a highly viscous mixture of heavy and extra, heavy petroleum. The chemical composition of tar, is characterized by loss of aromatics and saturated, hydrocarbons, whereas resins and asphaltenes are, enriched. Tar is a source of valuable petroleum, products but at a higher cost compared with oil., Therefore until recently, tar deposits have been, shunned in favour of conventional oil. Giant tar, deposits occur in Canada and Venezuela, and large, resources are known in most oil provinces of the, world., The largest tar province known is the region, around Athabasca, Peace River and Cold Lake in, Alberta, Canada. Along the eastern margin of the, Western Canada Sedimentary Basin east of the, Rocky Mountains, an Early Cretaceous sandstone
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , 571, , Figure 7.29 (Plate 7.29) Oil sand mining at Muskeg mine, Athabasca, Canada (2009). Copyright Shell plc., , unit (McMurray Formation) underlies an area of, >140,000 km2. In this area, channels and bars in, the fluvial lower member of the McMurray Formation are impregnated with extra heavy oil, (7 API gravity) and asphalt. The sandstone was, deposited in a large coastal delta above Devonian, limestones and covered by marine shales that later, acted as the seal. Natural outcrops of the tar sands, are known near Lake Mildred but the largest part is, covered by 50 to >200 m thick glacial and fluviatile Quaternary sediments. The average thickness, of tar sands is 50 m, but attains 275 m. The sand, consists of quartz and clay, and up to 19% tar. The, source rocks are buried beneath the Rocky Mountains and appear to include sediments of Devonian, to Jurassic age. Earliest migration took place at, 112 5.3 Ma, synchronous with sedimentation of, the Albian-Aptian host (Selby & Creaser 2005):, The bitumen cements quartz grains of the sand,, although pore space is mainly water-wetted. Leaching the bitumen leaves sand and a maximum of 30%, clay. Various exploitation processes have been, devised. At present, ca. 55% of oil is produced by, excavating near-surface tar sand in large open pits, , (Figure/Plate 7.29). In extraction plants, the bitumen is separated from sand and clay by agitation in, hot water with dissolved caustic soda, and flotation., The bitumen concentrate is cleaned, upgraded to, crude oil and shipped to refineries. Deeper deposits, are exploited by several in-situ technologies. Steamassisted gravity drainage (SAGD) based on boreholes, is the source of 45% of oil from tar sands. The, bitumen is liquefied in situ and pumped to the, surface. This process consumes little water and its, GHG footprint is comparable to conventional petroleum. In spite of high sulphur content of the bitumen, the final product is sweet light syncrude with, 40 API. Recovery is between 70% and 100%. In, 2008, the combined production of three well-established mines reached 1 Mbl/d (160,000 m3). With, today’s technology, the volume of total recoverable, oil in the district is thought to surpass the conventional oil reserves of Saudi Arabia (George 1998). The, in-situ volume is estimated at >1.7.1012 barrels, (>270,000 Mm3). At the end of 2009, proved reserves, amounted to 23,300 Mt crude oil (143,000 Mbl: BP, Statistical Review of World Energy 2010). Heavy, minerals of the sand fraction, clay and trace metals, (including V, Sc, Ni, Mo, Ga) in the ash of the, residual bitumen are planned to be future byproducts.
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572, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , 7.6.2 Asphalt, Asphalt (Abraham 1960, Yen & Chilingarian 1994,, 2000) is a term used for the substance found in, nature and the similar but not identical residual, fraction of crude oil distillation. Natural asphalt, contains less hydrogen but more nitrogen and, oxygen than conventional oil (Table 7.2). High, sulphur contents are characteristic. Asphalt is, enriched in naphtenes, aromatic hydrocarbons, and asphaltenes (N-S-O compounds). It is fusible, and soluble in carbon disulphide. Asphalt is modelled as a suspension of asphaltene particles and, micelles in liquid hydrocarbons. Structurally,, asphalt is a gel. Many asphalts contain mineral, substances, such as sand and clay, which is mainly, due to flowage erosion. The distinction between, tar and asphalt is based on rheological behaviour., At ambient temperature, asphalt is apparently, solid but highly viscous, whereas tar is less viscous, and liquid., Asphalt originates where petroleum is subject, to supergene alteration. It is a common product of, oil seeps and oil-rich mud volcanoes. Oil at the, surface loses light hydrocarbons (<C15) within two, weeks by evaporation and heavier HC up to about, C24 in a few months. Water-soluble compounds, are leached and microbial degradation sets in., Oxidation and polymerization harden the oil into, asphalt and pyrobitumen., Asphalt and the comparable residual fraction of, petroleum distillation (sometimes referred to as, “bitumen”) are used in large amounts as a thermoplastic binder of aggregate materials in the, construction of road surfaces and runways of, airports. For manufacturing asphalt concrete, the, material is heated to 150 C. Roofing, anti-corrosion pipe covering and the underbody protection of, motor cars are a few examples of other uses. Properties that control asphalt use include the chemical and molecular composition, colloidal and, physical characteristics. Road construction, for, example, requires asphalt with a suitable thermal, behaviour. At deep temperatures, asphalt should, not harden to the state of easy fracturing, nor, should it become plastic on hot summer days., Modifications of natural and artificial asphalt are, employed in order to improve resistance against, , ageing, water, bacteria and UV-light, and to, increase thermal and structural stability., The earliest known use of asphalt occurred in, the Middle East and in India. Mud volcanoes on, Apsheron Peninsula, Azerbaijan produced profuse, amounts of an asphalt-clay mixture. Industrial, production was initated in the 19th century from, the 1 km2 asphalt lake near La Brea on the island of, Trinidad and the larger Bermúdez lake in Venezuela. At Bermúdez, a thick layer of asphalt with, 25% H2O and up to 50% of mineral substance, floats on water. Although both sites continue to be, worked, traded asphalt is dominated by oil residuum from refineries., Asphalt limestones are not rare. Like oil in, carbonate reservoir rocks, these limestones host, asphalt in pores, and in fractures and veins., Asphaltic limestones may be regarded as exhumed, oil reservoirs. Former mining areas in Europe, include Dalmatia, Slovakia, the Jura Mountains, of Switzerland, the Monte Abruzzi and Ragusa in, Italy, and Lower Saxony in Germany. In the latter,, Late Jurassic limestone with a thickness of, 3.5–14 m and an average 2.5% asphalt is extracted, by underground mining and manufactured into, special floor tiles., , 7.6.3 Pyrobitumens, Pyrobitumens are defined according to physical, properties and uses, not by origin, maturity or, chemical composition (Hunt 1996). Pyrobitumens, are black, solid, non-volatile, infusible (unlike, ozocerite) and insoluble in organic solvents, including carbon disulphide. Pyrobitumens are, chemical raw materials, sealing compounds, a, component of control rods in nuclear reactors and, of black pigments. Recent mining is reported from, Utah, Argentina and China. Deposits are mostly, veins and stratiform bodies (Parnell et al. 1996,, Bing-Quan et al. 2001). Pyrobitumens form in at, least two fundamentally different environments:, i) Most pyrobitumen originates at a late stage of, thermal oil cracking; and ii) physically similar, material results from surface degradation of oil, (Hunt 1996). Thermal pyrobitumen originates at, T >150 C by sudden pressure drop and loss of
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , dissolved gas and condensate (cf. “cracking equation” 7.10). This pyrobitumen is evidence of the, former presence of petroleum (Mossman & Nagy, 1996, Stasiuk 1998)., Schematic reaction describing the thermal conversion of oil to gas and pyrobitumen:, C5 H9 ! 2CH4 þ, ðoilÞ, , ðgasÞ, , C3 H, , ðpyrobitumenÞ, , ð7:10Þ, , Under the microscope, thermal pyrobitumens, exhibit characteristic features. In reservoir rocks,, they mimic the shape of pores. Vein material is, very homogeneous with a conchoidal fracture,, displays flow textures and shrinkage cracks like, coke and is partly vesicular. Small inclusions of, authigenic pyrite, illite and carbonates have been, reported. Trace metal contents are elevated and, include V, Ni and Fe, but also Cd, Cr, Co, Cu and, Zn. In-situ degassing is implied by many details., Flow-textures recall aspects of magmatic dykes,, because the precursor oil is injected at high pressure and temperature into tectonic structures. As, the rock yields, pressure drops, gas and condensate, escape and the remainder freezes to pyrobitumen, (or “migrabitumen”, Jacob 1989). Under the, name impsonite, pyrobitumens are quite common, as a minor component of many hydrothermal vein, deposits., 7.6.4 Shungite, Shungite is a non-crystalline, black, glassy and, dense mineraloid, with a semi-metallic lustre, reaching >98% C (with traces of N, O, S and H)., With an age of 2000 Ma, shungite deposits are the, earliest giant accumulations of organic matter and, of (former) petroleum. Hosted in a 1000 m thick, volcano-sedimentary sequence, shungite formed, in the brackish environment of an active continental rift. Shungite occurs in stratified beds (a, former oil shale), in organosiliceous diapirs, or in, redeposited clasts of migrated oil (Melezhik et al., 2004). It is remarkable that the organic matter, was not graphitized, in spite of a Svekokarelian, (1.8 Ga) greenschist metamorphism. Shungite, is exploited near Onega Lake in four large opencast, mines. It is not combustible, but is used as, , 573, , a replacement for metallurgical coke, and as an, absorber and filler., , 7.7 OIL, , SHALES, , The increasing depletion of conventional, cheap, and easily accessible oil may intensify exploitation of oil shales. Like tar sand, oil shales contain, giant potential energy resources that can be made, available, although only with considerable technological and financial exertion (Russell 1990)., The earliest industrial-scale oil retorting was, founded by James Young at Bathgate, Midland, Valley, Scotland, in 1847 (cf. torbanite in Chapter, 6.1 “The Substance of Coal”). Raw materials were, lacustrine and lagoonal oil shales interbedded, with Early Carboniferous deltaic sandstones., Small artisanal operations, however, had provided, humans with oil and pitch for medicinal purposes,, water proofing and warfare since antiquity., The designation “oil shale” is only used in order, to communicate that a certain sedimentary rock, may be useful to produce “synthetic” oil. The, mineral matrix is irrelevant, but oil shale must, contain organic substance (kerogen) that yields oil, upon heating. Fine-grained sediments of almost, any origin can contain high percentages of organic, matter (cf. “Petroleum Source Rocks”). The petrographical range comprises siliciclastic pelites,, carbonates and sapropelic coal, or coal shale. The, presence of considerable amounts of authigenic, pyrite is always an encumbrance. The organic, substance is investigated with methods of organic, chemistry and petrology (Taylor et al. 1998). Most, oil shales are distinctly anomalous in uranium, and in boreholes, can be detected by gamma ray, logging., Oil shale rarely contains oil or natural gas (cf., Antrim Shale). Most of the organic substance in oil, shale occurs in the form of immature to lowmaturity kerogen. Oil shale is a petroleum source, rock that has not yet generated oil, or that retained, a considerable potential of oil generation after, going through a phase of partial catagenesis., Potentially economic total organic carbon (TOC), contents of oil shale range from 10 to 50 mass %., Oil yields vary from 40 to 600 litres per tonne.
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574, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Because some of the energy contained must be, spent on extraction and heating, mining and, processing costs control the precise position of, the lower boundary (the cut-off grade)., Kerogen rich rocks are commonly brown or, black (Figure/Plate 7.6). When weathered, they, stand out by whitish ash-like crusts that provoked,, for example, the name “White Band” for an oil, shale horizon in the Permo-Triassic Karroo of, Southern Africa. Oil shales may be flammable, with a simple pocket lighter, or at least release, a strong aromatic odour. Fischer assay is the most, common method of ranking oil shale in terms of, potential oil produced (after Fischer-Schrader, 1920). Conditions of the Fischer assay are similar, to those of conventional pyrolysis under nitrogen., Also used are Rock-Eval pyrolysis (Espitalie et al., 1984; Figure 7.5) and the determination of wholerock calorific values similar to coal. Because, organic substances are considerably less dense, than the minerals in shale, the determination of, density is a shortcut whenever large numbers of, samples are to be processed. Of course, a calibration curve for the project-specific correlation, between TOC and density must be established., Heated to 500 C, oil shale generates liquid, and gaseous hydrocarbons that are raw materials, for petroleum refineries, just like natural oil and, gas. Technologies of oil extraction from shale, include retorting, flushing with hot gas, and, underground methods that were sketched in, Chapter 7.5 “Mining of Petroleum”. The largest, current operations exist in China and Estonia., Estonian oil shale is mined in open pits and, underground as a fuel for electric power stations., An increasing part of mine output, however, is, the basis of synfuel production. Reserves amount, to several thousand million tonnes. Across the, border in Russia, mining the same shale bed, recommenced in 2008. Depending on composition, the ash produced in power stations can also, be used, for example as a hydraulic component of, the cement formula (cf. Chapter 3 “Carbonate, Rocks”). In many plants, oil shale is directly, mixed with cement raw materials before sintering, saving fuel and replacing parts of clay or, carbonate in the cement formula. Elevated metal, , contents may yield valuable by-products of oil, shale processing (e.g. vanadium, uranium)., Petrologically, oil shales are organic matter-rich, pelites or mineral matter-rich sapropelic coals. Oil, shale formation is favoured by humid and hot, climate. The frequency of oil shale beds (cycles/, metre) in the Green River Basin, for example, is, a function of orbital signals including precession,, obliquity and eccentricity (cf. Chapter 3 “Sodium, Carbonate”; Meyers 2008, Fischer & Roberts, 1991). To be economically attractive, oil shale, TOC needs to be higher compared with average, hydrocarbon source rocks (e.g. 13% at Julia Creek:, Lewis et al. 2010). Preservation of low-maturity, kerogen is geologically more likely in settings, that were not affected by deformation, subsidence, and heating. This is typically realized in little, deformed epicontinental platform sediments, (e.g. Estonia) and in post-orogenic lake basins., Characteristic depositional settings of important oil shales include:, . large inland lakes (the Eocene saline lakes of the, Green River Basin in the western USA; Carboniferous lakes in New Brunswick, Canada; Triassic of the Congo Basin near Kisangani; the, Songliao and Bohai basins of China); lacustrine, oil shale typically displays low sulphur contents, and yields a waxy, paraffinic oil (Hunt 1996);, . shallow epicontinental seas that deposit variegated sediments including shale, sandstone,, carbonates and phosphates (the Ordovician kukersite of Estonia; Karroo of Brazil and Southern, Africa; Middle Cretaceous in the Great Artesian, Basin, Australia);, . paralic, coastal swamps where torbanite is associated with humic coal (Fushun, Manchuria,, Figure 6.13; Liaoning, China; western margin of, the Permian coking coal district of New South, Wales, Australia);, . continental and marine rifts in a low heat flow, setting (Carboniferous of the Midland Valley,, Scotland)., Many oil shale occurrences are found in volcanic crater lake sediments (Neogene of Pula, Bakony in Hungary), in impact craters (Miocene, N€, ordlinger Ries, Germany) and in maar lakes, (Eocene at Messel, Germany).
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , The largest potential resources of oil shale occur, in Brazil, China and USA. Many other countries, such as Australia, Syria, Russia and Morocco have, considerable volumes of oil shale. None of these, states is a member of OPEC (Organization of, Petroleum Exporting Countries). The world’s, prognostic oil shale resources are very large., Although estimates vary, extractable (synthetic), oil in place probably amounts to 500,000–, 700,000 Mm3, counting oil shale with a minimum, yield of 42 litres/tonne. This is about three times, as much as conventional world oil reserves, (212,000 Mm3 in 2009) and might satisfy world, consumption for >100 years. Because of relatively, low current prices of conventional oil, the incentive to exploit oil shales as a source of liquid fuels, is still moderate. Under changed conditions,, exploration will expand and many new deposits, will be discovered. Favourable economics are expected to encourage the development of new and, better technologies, including extractive operations and environmental mitigation., 7.8 ENVIRONMENTAL ASPECTS OF OIL AND, , GAS, , PRODUCTION, , The environmental disaster caused in 2010 by the, offshore oil well blow-out in the Gulf of Mexico, tragically demonstrated hazards and risks associated with oil production. Generally, however, the, industry does not cause serious environmental, problems. There are practically no emissions, no, remaining waste and its footprint (e.g. land use) is, minimal. Often, the lay person will not even, notice the presence of producing fields. Sensitive, landscapes, including permafrost tundra, or large, cities, demand special precautions. In its own, interest, the industry executes precautionary, environmental management (Wilson & Richardson 1999, Orszulik 2008). Los Angeles is probably, the city with the longest experience (since 1892) of, oil and gas production within its perimeter. Its oil, production is 6.5 Mbl (2006) from 4000 onshore, and offshore wells. The most critical problem, encountered during the past was the seepage of, methane and hydrogen sulphide into tunnels,, basements and buildings (Bilodeau et al. 2007),, , 575, , caused by natural processes but also because formerly, oil wells were not properly plugged when, abandoned. The city responded by publishing, maps of risk zones and by enacting a Methane, Mitigation Ordinance that prescribes building, codes for proper venting, methane detection systems and automatic alarms when the methane, concentration in air reaches 20% of the lower, explosive limit (methane is only explosive, between 5–15 vol. % concentration in air):, One of the largest oil-contaminated landscapes in the, world is Apsheron Peninsula in Azerbaijan, both by, nature (mud volcanoes) and man. The peninsula is, a ridge of the Caucasus Mountains plunging towards, the southeast underneath the Caspian Sea. It was the, site of the first major oil rush outside of the United, States after oil had been found near Titusville, Pennsylvania, in 1870. When in the same year the Russian, empire relinquished the oil monopoly, an explosion, of entrepreneurship was released (Yergin 1991). Three, years later, Robert Nobel, the oldest of the three, famous brothers, visited Baku on Apsheron with the, intention to buy walnut wood for army rifle stocks., He realized at once the business opportunity, bought, petroleum wells and a distillery instead of wood, and, launched a business that was in a short time closely, competing with Rockefeller’s American enterprise., Ludwig Nobel became the “Oil King of Baku”;, brother Albert (the founder of the Nobel Price) helped, with financing. In the 19th century, oil drilling was in, its infancy and scientific understanding was limited,, so that oil spills and outbreaks of oil (“gushers”, or oil, fountains) were lightly accepted. Baku was proudly, called “the black town”, because of spilled oil and the, smoke of hundreds of distilleries. Apsheron was, famous for producing many oil fountains, and the, heritage from the pioneer days followed by 100 years, of state management of the oil industry is quite, impressive: Several thousand square kilometres of, land and sea bottom are contaminated with oil, tar, and asphalt. Abandoned industrial ruins, rusty derricks, oil pools, asphaltic soil and modern rubbish, dumps serve as a paradigm of industrial pollution. It is, a great achievement of free Azerbaijan that part of the, present oil and gas income is invested in a giant land, remediation project., More recently, in 1991, Saddam Hussein ordered, the destruction and ignition of oil wells in Kuwait., For several months, 950,000 m3/day of oil were lost by, burning and flowing away over the desert. This
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576, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , CO2, 2-, , SO4, , S, , CO2, -, , 2-, , NO3, , O2, , N2, , Unsaturated zone, Groundwater, 2SO4, , -, , NO3, Contaminant, , Methanogenic, CH4, CO2, , O2, Fe(III)-reducing, CO2, , a, Sulf, , ing, uc, d, e, te-r, , Fe(III), , d, e-re, Nitrat, , Aquitard, , aerobic, , Fe(II), CO2, , g, ucin, O2, , H2O, , Direction of groundwater flow in aquifer, , bequeathed petroleum lakes, asphalt and soot covering 6000 km2. Because of the arid climate, natural, biodegradation is virtually nil. Despite considerable, mitigation work, negative impacts and hazards persist (Omar et al. 2009). Oil reaching the Gulf, however, was quickly decomposed by natural processes, due to favourable conditions in a warm sea. Beaches, are white as before, and life in the shallow sea is, teeming., , Oil spills at production facilities, along pipelines and transport routes arouse public attention., Great care must be taken to avoid such accidents., The main problem is a sudden release of a large, mass of petroleum; small diluted amounts are, quickly decomposed by aerobic microbes (similar, to eq. 7.6). This is confirmed by investigations of, natural oil seeps. Every year, natural seeps introduce 1.5 Mbl oil into the oceans and 0.5 Mbl on, land (Hunt 1996), with other sources suggesting, a ten-fold upper bound (15 Mbl, 2.38 Mm3 or 2 Mt)., Biodegradation takes its course without human, intervention. However, large concentrated, quantities of oil defy microbes, because mineral, nutrients are quickly depleted (especially phosphorous and nitrogen) and the contact surface is, limited. Mechanical removal is therefore the first, , Figure 7.30 In-situ bioremediation of oil,, gasoline or diesel fuel contaminated, groundwater by supporting specialized, anaerobe microbes that decompose, hydrocarbons to harmless CO2, N2 and, CH4. Reprinted with permission from, AAAS. Aerobe microbes are also capable, of converting hydrocarbons (lower right),, but it is difficult to inject enough oxygen, into an oil-bearing aquifer (Lovley 2001)., , remedial action followed by application of mineral, fertilizers. This was one of the lessons learned, from the disaster of the Exon Valdez in the Prince, William Sound, Alaska, in the year 1989. For oil in, groundwater, anaerobic microbes are better, qualified for biodegradation. They are assisted by, injection of electron acceptors such as sulphate or, nitrate (Figure 7.30)., Remarkable oil spills in the deep sea include, IXTOC 1 in the southern Gulf of Mexico in 1979, (560,000 m3), Ekofisk in the North Sea in 1977, (32,000 m3) and Santa Barbara off California in, 1969 (16,000 m3). At Macondo in the northern, Gulf of Mexico in 2010, outflow amounted to, 4.4 Mbl (700,00020% m3; Crone & Tolstoy, 2010) of oil within the 85 days until the well, could be plugged. The composition of the oil determines much of the damage; the light paraffinic, oil in the Gulf of Mexico, for example, evaporates, in part and the remainder is more rapidly dispersed, and biodegraded compared to heavy, asphaltene, and NSO-rich oil. Natural seafloor vents of oil and, gas support a specialized chemotrophic flora and, fauna of high biodiversity. At a depth of 3000 m, in the Gulf of Mexico, active oil seeps and tar, flows (“asphalt volcanoes”) have been discovered
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , surrounding salt domes. Their species-rich fauna, is comparable to the amazing ecology of black, smokers at mid-oceanic ridges (MacDonald, et al. 2004)., Oil slicks floating on water are removed with, skimmers and booms. Applying dispersants, microbes and fertilizer may be considered. Several, methods are available for the decontamination of, soil and sediment:, . soil washing (soil is agitated in large containers,, oil and asphalt are removed by hot water and, steam; the oil recovered can pay for part of the, costs; this technology is currently applied at an, industrial scale for syncrude extraction from, Canadian tar sands);, . incineration (destruction by burning);, . thermal energy recovery (using the inherent, energy by combustion in cement works or power, plants)., In the past, little attention was paid to the, contents of natural radioactive substances in oil,, gas and formation waters. Many source rocks, and hydrocarbons are geochemically enriched in, uranium and to a lesser extent, thorium. As both, elements are insoluble at the reduced conditions, of reservoirs, their daughter nuclides are of higher, interest. Problematic are 226Radium, 228Radium, and 210Pb, which are dissolved in reservoir water, and lifted together with oil. In pipes and settling, ponds, they are concentrated in scales and muds., 222, Radon is dissolved in both hydrocarbons and, reservoir water, and is concentrated in liquid gas, during processing (Schmidt 2000)., , 577, , 7.8.2 Induced seismic activity, Oil and gas extraction at depth cause decreasing, pore fluid pressures (Figure 7.31) and correspondingly, increasing effective stress (Brady & Brown, 2004). This induces ductile consolidation or brittle, fracturing, probably mainly along existing discontinuity planes. The subsidence of the centre above, a field causes characteristic steep reverse (thrust), faults whose activity produces the earthquakes, (Segal 1989). Subsidence can be predicted and mitigation measures can be planned, but timing and, magnitude of earthquakes remain unpredictable., The first and one of the largest gas productionrelated earthquakes to date occurred in 1951 at the, Caviaga field (Northern Italy), with a magnitude, of 5.5. Because seismic events at most oil and gas, fields are very weak, good long-term records of, seismic activity are rare. Excellent data exist on, the seismicity of the Lacq field in the northern, foreland of the Pyrenees (Maury 1997). At Lacq, a, long suite of numerous micro-seismic signals was, interspersed with a few earthquakes of magnitude, 4.0 to 4.3. This is in a striking contrast to seismicity induced by injection of fluids (e.g. in geothermal frac-operations) or by large dams, which both, raise fluid pressures in contrast to hydrocarbon, production. Higher fluid pressures cause more, frequent earthquakes that reach much higher, magnitudes (the maximum measured until today, was M ¼ 6.3). In addition to the seismic risk, using, former oil and gas fields for natural gas storage or, CO2-sequestration may lead to enhanced gas flow, to the surface. This possibility must be carefully, investigated as a potential hazard for people and, environment., , 7.8.1 Water resources protection, In the upstream oil and gas industry, formation, water is carefully re-injected into deep aquifers in, order to protect the environment and to support, reservoir pressure. This is not possible when gas is, produced from coal seams (coal bed methane ¼, CBM operations), because much water has to be, lifted before gas can be recovered. Water abstraction may affect the wider groundwater regime and, entails problems of disposing of pumped water, commonly rich in solutes. An average CBM-well, in the USA is said to deliver 20 t/y of salt., , 7.8.3 Tar sand mining, Tar sand mining in the Fort McMurray region,, Alberta, Canada, is rapidly expanding because of, improved economic feasibility and the chance to, diversify future petroleum supply. Oil shale mining shares a number of environmental aspects, with the first: The useful substance in the rock is, a very small fraction of the total volume, so that, extraction and processing move very large masses, indeed. Management of waste rock and tailings
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Monthly gas production (106 Sm3), , 578, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Cumulative gas production (109 Sm3), 1000, , 250, , 800, , 200, , 600, , 150, , 400, , 100, , 200, , 50, , 0, 1956, , 1966, , 1976, , 1986, , 0, 1996, , Pressure at -3700 m (bar), , 700, 600, 500, 400, 300, 200, 100, 0, , 0, , 50, , 100, , 150, , 200, , 250, , Cumulative gas production (109 Sm3), , must consider possible self-ignition as with coal, shale. Large masses moved translate into severe, interference with the landscape. Recultivation, and renaturalization are integral to all such operations. Tar sand processing uses large amounts of, water. Both extraction and safe disposal of waste, water have to be sustainable. Fossil organic substance is commonly associated with elevated, contents of potentially toxic compounds, because, organic matter-rich pelites are generally characterized by elevated contents of redox-sensitive, elements, including Fe, Mn, U, V, Mo, Ni, Co,, Cr, Cu, Pb, Zn, Cd and As (Morford & Emerson, 1999). Dispersion of dangerous elements during, processing, retorting and incineration must be, avoided, preferably by recovering and selling these, substances as a by-product. At present, the tar sand, industry is changing very fast. Both the discovery, of unforeseen environmental problems and of, hitherto unknown solutions may be expected., One major advance is the newly established, , Figure 7.31 Production history (upper diagram) and, concurrent pressure loss at 3700 m depth (lower diagram), in the French gas field Lacq Profond (modified from Maury, 1997). Reproduced by kind permission of Total. Initially,, the gas was highly overpressured (hydrostatic pressure, should have been about 400 bar), promptly causing a gas, outbreak when the discovery drill hole hit the reservoir., With declining pressure during production, earthquakes, with a maximal magnitude of 4.3 first occurred in 1969., The surface above the field subsided 5–6 cm. Sm3 is the, methane volume measured at surface standard, conditions. For location refer to Figure/Plate 1.89., , in-situ recovery of oil from tar sands by steamassisted gravity drainage (SAGD) based on boreholes. SAGD results in lower production costs and, a reduced environmental impact. Advantages, include a lower carbon footprint and the elimination of large pits and tailings. One example of, comprehensive oil shale use is a cement factory, at Dotternhausen in southwestern Germany,, where both the inherent energy and the mineral, substance of Jurassic Posidonia shale are utilized, for cement production., 7.8.4 Hydrocarbons and climate, As mentioned in the introduction to Chapter 6,, climate is only one reason why the quest for, a reduced dependence on oil and gas is rational., Hydrocarbons are considered less harmful for the, climate, because they have a higher hydrogen, content compared with coal. Yet, considerable, efforts are under way to further minimize release
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , of CO2 from oil and gas power stations, for example by injecting it into depleted hydrocarbon, reservoirs or into saline aquifers (cf. introduction, to Chapter 6 “Coal”). The technology is executed, since 1996 at the Sleipner Field in the North Sea, off Norway (Figure/Plate 6.2), where nearly 1 Mt, CO2 per year is separated from natural gas and, sequestered at depth (Bickle et al. 2007, Klusmann 2003). This is a paradigm for future deep, geological storage of CO2 captured from flue gas, of coal, oil and gas-fired power plants or hydrogen, production (eq. 7.1). Methane is the third largest, contributor (after H2O-vapour and CO2) to radiative climate forcing, causing 40% of the total, anthropogenic change since pre-industrial times, (Shine & Sturges 2007). Today, CH4 is a valued, product and is hardly ever wasted; unavoidable, emission is converted by flaring. Liquid hydrocarbon fuels for traffic, however, remain a major, source of CO2 emissions, because no technology, is foreseen that might capture the gas from a, multitude of individual small sources. Immediate solutions include reduction of the fuel consumption of vehicles, ships and aeroplanes, and, increasing the share of alternative fuels. Replacement of gas and oil in power stations and transport by sustainable fuels depends on technologies, that themselves require a large variety and mass, of mineral materials (e.g. metals, fertilizers, chemicals and energy). It appears that even if the role, of individual mineral raw materials is certain to, change in the future, geogenic resources will, always be indispensable for the welfare of human, societies., 7.9 SUMMARY AND FURTHER READING, Crude oil and gas are natural hydrocarbons occurring in the shallow crust. Processing of oil yields, liquid fuels, which are the foundation of economic, activities, most importantly of civilization’s, mobility. Reserves and resources of oil and gas are, very large but not inexhaustible. Many experts, agree that the depletion mid-point of conventional, oil is not far in the future. Yet the latest (2009), reserves/production (R/P) ratio is 45.7, little, changed for the last 20 years; before 1989, it, was even lower. This demonstrates vividly that, , 579, , the R/P ratio is not a measure of “the end of oil”., Giant unconventional sources of oil are available,, but will be more expensive and some may not, gain society’s consent. Global recoverable gas resources are enormous and promise to cover several, hundred years of consumption. Overall, there is no, reason to panic because of a geological oil and gas, shortage. There are, however, good reasons to, reduce our reliance on oil, which are not geological, but strategic., Oil and gas deposits are the result of hydrocarbon-forming systems consisting of many, components that may be narrowed to source,, hydrocarbon generation, fluid migration, reservoir, and trap structure., Source rocks of crude oil and natural gas are, organic matter-rich sediments, which were, formed in parts of oceans with proliferating life,, typically during Earth’s greenhouse states. Early, diagenesis transformed organic particles into, kerogens (“eogenesis” at <50 C). Different organic matter converts into four main types of kerogen: (I) Alginite; (II) liptinite and marine plankton;, (III) vitrinite; and (IV) inertinite. Like coal macerals, kerogens are amorphous and chemically, indeterminate., Hydrocarbon generation from kerogen starts, when source rocks are heated by increasing burial., As they mature through “catagenesis” at temperatures of 50–160 C, kerogens generate CO2, H2O,, oil and gas. The main source of oil is type II, kerogen, which at the onset of oil generation is, characterized by the “formula” C515H596O72., Gas is mainly derived from type III kerogen. At, 160–250 C during “metagenesis”, oil is cracked to, produce methane, pyrobitumen and condensates., This is also the domain of dry gas formation from, vitrinite. Kerogens gradually lose their hydrocarbon generation potential and approach the composition of graphite., Like coalification, the generation of hydrocarbons from kerogen is modelled as a series of, decomposition reactions. Temperature constitutes an exponential factor. Kinetic factors are, measured in laboratory experiments. The state of, diagenetic alteration of kerogens (“maturity”) is, commonly measured by: i) vitrinite reflectance;, and ii) Rock-Eval pyrolysis.
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580, , PART IV FOSSIL ENERGY RAW MATERIALS – COAL, OIL AND GAS, , Fluid migration is initiated by exudation of, droplets of water, oil and gas from kerogen. Compaction, heating, maturation and dehydration, combine to support the resulting fluid flow that, is controlled by pressure (head) differences and, rock permeability. Preferential flow paths may be, envisaged as streams and rivulets. Flow vectors, generally point upwards and to basin margins., Trap structures consist of a reservoir rock that is, open to receive fluids but is semi-closed by impermeable seal rock. The best seals are gas hydrates,, salt rocks and shale. The separation of hydrocarbons from a passing aqueous-HC fluid flow can, be visualized as oil and gas bubbles floating up into, a cupola-shaped trap, whereas the water passes on., Economically significant reservoir rocks, such, as sand, sandstone, limestone and dolomite, are, characterized by high porosity and favourable permeability. Typical petroleum reservoir rocks display porosities of 10–40 vol. % and permeabilities, of ten to several thousand milli-Darcy (mD). Con-, , ventional gas deposits have a minimum porosity, of 7 vol. % and a permeability >0.1 mD., Hydrocarbon trap structures occur in many variations. Major process systems of trap formation, include sedimentation, diagenesis, salt diapirism,, tectonic deformation and self-sealing. A curiosity, are impact-related traps associated with the Cretaceous-Palaeogene boundary Chicxulub impact, in the Gulf of Mexico, hosting the supergiant, Cantarell oilfield. Most remarkable is the significance of self-sealing in the Russian North, where, very large gas resources are trapped by gas hydrates, formed in permafrost regions., A large natural flow of oil and gas reaches the, Earth’s surface. Greenhouse gas methane dissipates in the atmosphere, whereas oil is degraded, and decomposed by water, oxygen and microbes., In shallow reservoirs, much of the oil is aerobically, biodegraded by preferential consumption of the, more valuable hydrocarbon compounds. At oil, seeps, tar and asphalt form as residues., , Figure 7.32 (Plate 7.32) Floating production storage and loading vessel in the Bonga field offshore Nigeria. The field, lies 120 km from the River Niger mouth in water more than 1000 m deep. Copyright Shell plc.
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PETROLEUM AND NATURAL GAS DEPOSITS CHAPTER 7, , The search for new hydrocarbon resources, moves into ever deeper water of the world’s oceans, (Figure/Plate 7.32, Box 7.1). In this demanding, setting, technologies of exploration, development,, exploitation and environmental protection must, be re-invented. Former poor practices bequeathed, several seriously oil-contaminated landscapes, that defy easy clean-up methods. Modern fields, are unobtrusive and even blend into city areas. The, Norwegian offshore gasfield Sleipner is a pioneering demonstration of deep geological storage of, CO2 captured from flue gas of coal, oil and gasfired power plants, or from hydrogen production., More details on petroleum science and its, application are best sought in Selley’s (1997), , 581, , Elements of Petroleum Geology and Hunt’s, (1996) Petroleum Geochemistry and Geology., Relations between hydrocarbons and salt are analysed at depth in Warren (2006). Systematic short, descriptions and illustrations of the world’s oil, and gas deposits are available in Kulke 1994,, 1995). Valuable insights into regional petroleum, geology are offered by Glennie (1998) for the, North Sea and Alsharhan & Nairn (1997) for the, Middle East. The world oil resource situation is, lucidly analysed by Gorelick (2010). My favourite, is Yergin (The Prize, 1991) who tells the story of, oil in all its fascinating aspects, including outstanding players, technology, war, finance and, the battles for supremacy.
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Epilogue, Minerals support some of the most basic needs of, humanity. Fertilizers, machines and fuels are, essential for food production. Cement, rocks and, aggregates are used to make dwellings, and a multitude of natural and processed substances is employed in building the infrastructure and, manufacturing the tools, which help us to survive., Of course, minerals are also indispensable for, advanced technologies to tap alternative energy, sources, to make luxury goods and high-technology electronic devices., The main groups of mineral raw materials, include metal ores, industrial minerals and rocks,, salt, coal and hydrocarbons. Energy is the most, important natural resource of all. The extraction, of minerals is only possible if all involved costs are, recovered and value is added. These are the crucial, conditions that determine the difference between, terms such as “deposit”, “reserve” and “ore”, as, opposed to “mineralization” and “resource”., Economic geology fulfils a central role in discovery, development and extraction of mineral, raw materials, and – in partnership with environmental engineering – in the restoration of the land, after mining. Economic geology is rooted in the, natural sciences and in theory and practice includes many disciplines., The core task of economic geology is the search, for an ever greater understanding of the origin of, mineral deposits. Typically, a certain substance, such as gold must be concentrated in small rock, volumes in order to be economically extractable., This concentration to exploitable ore results from, dynamic interactions between the Earth’s core,, , mantle and crust, of the hydro-, bio- and atmosphere. The key are energy gradients, which drive, all of the Earth’s processes, from the slow and, inexorable movement of lithosphere plates to, magmatic flare-ups and the migration of hot aqueous and hydrocarbon fluids in the crust, as well as, climate and surface dynamics. The heat inherent, in the Earth and the energy received as radiation, from the sun are causative agents. Both result from, nuclear processes, the first by radioactive decay, and the second by nuclear fusion., The formation of mineral deposits is an integral, part of the Earth’s major rock-forming process, systems, such as magmatism, supergene alteration, erosion, sedimentation, diagenesis and, metamorphism. Most mineral deposits, however,, are uncommon rock bodies, which originated, from complex combinations of several processes,, boundary conditions and modifying factors that, are subsumed as metallogenetic, or minerogenetic, systems. Full understanding of these systems, which operated millions of years ago in the, geological past, is probably not achievable. Modern economic geology, however, does provide concepts, data and computational models, which, considerably advance both theoretical comprehension and practical application. Successful, search for hidden mineral deposits at ever greater, depth is the critical test for progress., The provision of metals, minerals and energy raw, materials is the ultimate purpose of economic, geology. The Earth’s growing population and its, constant struggle for higher living standards, cause an ever-increasing consumption of natural, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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584, , EPILOGUE, , resources. As demand rises and the pace of extraction quickens, more deposits must be found, developed and exploited. Is this supported by a, corresponding endowment with undiscovered, resources?, The Earth is finite. Yet the mass of, for example,, copper or reduced carbon in the accessible uppermost 10 km of the crust is immense. In the foreseeable future, however, only relatively highgrade new finds will be able to compete with, deposits such as the Chilean copper porphyries or, the Arabian oil fields. It is justly doubted that, similarly rich provinces await discovery elsewhere. For many decades from now, a combination of mining, recycling and increasing the, efficiency of mineral raw materials’ use will have, to guarantee sufficient availability. In the far, future, supply will need to diversify to new, probably deep sources and new technologies such as, automated mining and in-situ leaching. A key, condition for this path is affordable energy. And, the key to all of this is human inventiveness and, adaptability., Can an expanding extractive industry be reconciled with the aspirations of affected people and a, sound environment? In this respect, the past performance of mining has certainly been poor. However, although impacts of large mining operations, still affect communities, flora and fauna, land, and water, there are best-practice performers, that already implement the concepts of “green, , mining”. Green mining contributes to communal, and individual wealth of all stakeholders, minimizes the impact of operations on humankind and, the environment, and restores sites at closure., Green mines achieve lower emissions of greenhouse gases and offset their unpreventable climate footprint by buying carbon credits. Deep, geological sequestration of carbon dioxide in, saline aquifers and depleted hydrocarbon reservoirs can buy the time needed for the transition to, new energy technologies. Its broad application,, however, is subject to national and international, regulations that avoid drastically falling standards of living in countries rushing ahead. More, of civilization’s unavoidable waste will be, buried in suitable mines. Specially engineered, underground repositories are constructed for sustainably isolating hazardous waste from the, biosphere. Future mining operations bequeath, enriched landscapes, which provide a variety of, ecosystem services (e.g. food and biomass production, flood and erosion control, areas for recreation and aesthetics, and clean water). All this is, achievable if exploitation of the shallow geosphere is intelligently coordinated and diligently, managed., In conclusion, allow me to reiterate that wellmanaged extraction of minerals has every potential to contribute to communal wealth, a sustainable and vital social and natural environment,, and peace.
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Index, Bold numbers point to colour plates, Abu Dabbab Sn-Ta, Egypt 204, 264, 2.18, acidity 75ff, 85, 90, 93, 455, 541, acid rock or mine drainage (ARD, AMD) 187, 450, 453,, 456, 516, 5.24, 5.25, adularia 76, Agricola 419, , Aheim olivine, Norway 342, 3.26, 3.27, albitization 53, alkaline igneous complexes 23ff, 255, 262, Almad, en Hg, Spain 242, Alpine type Pb-Zn 199, Alquife Fe, Spain 62, Altaussee salt, Austria 394, 4.3, alum salts 355, aluminium 233ff, alunite 53, 76, 80, 233, andalusite 288ff, anhydrite 57ff, 71, 73, 112, 114, 327ff, 383, 394, anomaly 422ff, 429, anorthosite-ferrodiorite complexes 16, antimony 243ff, Antrim Shale gas, Michigan 538, apatite 80, 260, 342ff, Araxa Nb, Brazil 261, 262, 346, archaea see microbes, argillic alteration 53, Argyle diamonds, W.A. 25, 314, 315, 3.13, Arrhenius-equation (modelling coalification), 503, 538, arsenic 245ff, asbestos 291ff, asphalt 572, Aswan Fe, Egypt 104, Athabasca District U, Canada 276–277, atmospheric carbon dioxide evolution 493, , atmospheric emissions 30, 272, 469, 494, 514, aulacogen 134, background (geochemical) 422, 429, bacterial sulphate reduction (BSR) see microbes, Bad Aussee salt, Austria 394, 4.3, Bajiazi Zn-Pb-Ag, China 223, 224, 225, Bakal siderite, Russia 154, banded iron formation (BIF) 81–82, 87, 100ff, 123, Algoma type 100–101, Rapitan type 100, 103, Superior type 101ff, 1.67, banded sulphide ore 1.72, 1.81, barite 38, 293ff, Barnett Shale gas, Texas 564–565, bauxite 8, 9, 81, 233ff, 1.1, Bayan Obo REE, China 24, 259, Beauvoir kaolin-Ta, France 29, 264, bentonite 299ff, 3.8, Bergslagen Cu-Au, Sweden 124, beryllium 268ff, Besshi Cu 73, Bingham Cu, Utah 188, biodegradation of oil 552, 567, biodiversity 20, 21, 458, 577, biofuels 522, bismuth 250, bituminous coal 473ff, 6.20, black shales 93–94, polymetallic 169–170, 179, black smoker 19ff, 1.11, 1.12, Bleiberg Pb-Zn, Austria 44, 91, Boddington Au, W. A. 81, boiling 38, 42, 49, 71, 72, 75, 222, 240, 321, first b. 31, second b. 31, , Economic Geology Principles and Practice, Metals, Minerals, Coal and Hydrocarbons – Introduction to Formation and Sustainable Exploitation of Mineral Deposits, Walter L. Pohl, © 2011 Walter L. Pohl. Published 2011 by Blackwell Publishing Ltd.
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656, , INDEX, , borehole deviation 435, borehole geophysics 432–433, 509, 558ff, boron 302ff, 3.9, Bougainville Cu-Au, Papua New Guinea 87, breccia ore 64, 71, 73, 1.33, 1.76, brine 16, 22, 30, 32, 36, 37–38, 46–47, 59, 71, 75,, 110ff, 118, 119–120, 128, 266–267, 274, 280, 334,, 338, 408, 551, brittle 65, 124, 130, Broken Hill Pb-Zn, Australia 110, 121, 122, 124,, 197–198, bromine 111, 373, 374, 551, Brunswick Cu, Canada 124, Bugarama W, Rwanda 67, Bushveld Pt-Cr, South Africa (S.A.) 13, 32, 121, 133,, 166, 185, 230ff, 290, 322, cadmium 195ff, 247ff, caesium 265, calorific value (coal) 486, CanTung W, Canada 56, cap rock 404–405, carbon 43, 310 ff, 325ff, 465ff, carbon capture and storage (CCS) 469–471, 517, 561,, 579, 6.2, carbon dioxide 24, 31, 36, 38, 43, 46, 55, 60, 77, 85, 90,, 93, 112, 113, 128, 135, 138, 203, 241, 258, 273, 305,, 312, 326, 335, 359, 364, 373, 408, 453, 454, 459,, 469–471, 483–484, 517, 528, 533, 537, carbonatites 23–25, 80, 260, 322, 339, 341, 344ff, carbonatization 54, Carlin type Au, Nevada 212, Carlsbad cave, New Mexico, USA 90, Caroline Pb-Ag, Germany 50, catagenesis 538, celestite 293ff, 328, 3.3, cement 307, 328, cementation 86–87, Central African Cu-Co Belt 192–193, Central Andes profile 136, Cerro de Mercado Fe, Mexico 17, Cerro Rico de Potosı Ag-Sn, Bolivia 224, 225,, 226, 2.30, Challenger Au, S. Australia 125, Chicxulub impact, Gulf of Mexico 133, 549, chromium 14, 19, 163ff, 231, 2.9, Chuquicamata Cu-Mo, Chile 57, 58, 1.31, 1.33, clay 308ff, refractory 308, ceramic 309, expanding 310, sealing 310, , climate 78, 380, 389, 492–494, engineering 356, greenhouse 78, 535, Climax Mo, Colorado 178, coal 467ff, 536, coal bed methane (CBM) see methane, coalification 500–505, coal lithotypes 475, coal macerals 475–479, coal rank 472, coal seam 487ff, 6.20, coal seam fires 507, 517, cobalt 173ff, Cobalt District Ag-Ni-Co, Canada, 175, Coeur d’Alene Ag, Idaho 227, collapse structures see earth falls, colloform see gels, colloids 37, 50–51, 71, 273, Colorado Plateau U, USA 89, combustion residues 484–485, 515, 516, Conolly diagram 67, constructed wetlands 455–456, contact metamorphism 121, 327, 396, 502, copper 15, 21ff, 24, 56ff, 70, 76, 85, 114ff, 185ff, Copper Shale 114ff, 232, core logging 436, 509, 511, Cornwall, England 69–70, 332–333, cosmogenic 10Be-dating 269, Coulomb-Mohr stress diagram 65, Cowra Au, NSW, Australia 48, cut-off grade 444, cyanide 454, cyclothem 494, 495, Cyprus Cu 23, 74, 189–190, Dachang Sn-Zn, China 206, Dalnegorsk boron, Russia 305, Dampier salt, W.A. 376, 4.6, Dead Sea Mg-salt 239, De Beers diamonds, S. Africa 313, dehydration see devolatilization, devolatilization 125, 121, 127–128, 136, 138, diagenetic 110–121, diagenetic crystallization rhythmites (DCR) see zebra, textures, diamond 25, 32, 310ff, 3.13, 3.14, diamond core drilling 434, diatomite 317ff, differentiation of magmas 29, dolomite 305ff, dolomitization 54, Draugen oil field, offhore Norway 547, 565
Page 693 :
INDEX, , drilling 406, 432ff, 508–509, 7.24, ductile (plastic) 122, 124, 130, 397–398, 1.81, Dwars River Cr, S.Africa 14, , exploration 416ff, extinction 493, extraterrestrial 10, 15, 133, 228, 549, , earth falls, sinkholes 329, 404, 407, 409, 512, 4.30, earthquakes, induced 408, 513, 577, Earth systems 132ff, economic 414ff, 441, ecosystem services 457, 584, effervescence 38, Ekati diamonds, Canada 313, 3.14, elements, chalcophile 29, 186, compatible, incompatible 29, essential to human health 151, 370, essential trace elements 177, granophile 29, halogens 40, hazardous air pollutants (HAPs) 514, high field strength (HFSE) 29, 30, immobile 125, 255, large ion lithophile (LILE) 29, 30, lithophile, or oxyphile 30, major e., essential for all life 344, 356, 465, native 86, 208, noble 38, 86, 209, redox-sensitive 93, siderophile 150, volatile 213–214, El Romeral Fe, Chile 153, El Teniente Cu, Chile 57, emerald 270, endogenetic 10, endothermic 128, 503, 538, environment 428, 437, 448ff, 513ff, Environmental Impact Assessment (EIA) 450ff, eogenesis 537, epigenetic 10, epithermal (volcanogenic) 74ff, 2.30, epizonal 48–49, Erzberg Fe, Austria 61, Erzgebirge, Germany 29, 141, Ni 170, Sn 205,, U 281, euxinic 93, 107, 114, 534, evaluation 419, 437ff, evaporation 376ff, 4.6, 4.12a, evaporites 369ff, exhalative see submarine exhalative, exhalite 70, 123, exogenetic 10, exothermic 128, 130, exotic oxide ore 86, , fahlband 74, feasibility study 448, Felbertal W, Austria 182, feldspar 319ff, fenitization 25, fluid inclusions 45–48, 372–373, fluorite 320ff, 3.16, foam textures 121, fossil fuels 465ff, fractional crystallization 10, 29, 34–35, fracturing 65, fracture-stimulation of wells 567, framboidal 71, 93, 109, fusain 475, fusinite 478, Fushun bituminous coal, China 488, , 657, , gallium 247ff, Ganges plains mass poisoning 247, garnierite 83, 84, gas hydrates (clathrates) 530, gas phase 30, 32, 38, 186, 356, Gash Emir W, Sudan 52, Gatumba Sn-Ta, Rwanda 97, 264, Gebeit Au, Sudan 458, gels 37, 50–51, 116, geochemical exploration 422ff, geological exploration 417ff, geological carbon sequestration see carbon capture, and storage, geometallurgy 440, geophysical exploration 428ff, geostatistics 445, geothermal gradient 138, 401–402, 502–503, geothermal systems 32, 35–37, 40, 74, 75, 241, 1.21, germanium 247ff, Gifurwe W, Rwanda 66, glaciation 103, 389, 493–495, 499, gold 26, 27, 56–59, 66, 76, 81, 98, 207ff, orogenic 125, 129, 139, 140, 213, Golden Mile Au, W. A. 39, 2.25, gondite 82, Gondwana 134, 389, 493, Gora Magnitnaja Fe, Russia 153, Gorleben waste repository, Germany 401, 402,, 460, 5.29, gossan 85, 418, 1.56, granites 25ff, high heat production 28, 273
Page 694 :
658, , INDEX, , granitoids see granite, graphite 325ff, gravitational settling 11, 50, gravity 406, Great Oxidation Event 77, 102, Greenbushes Sn-Ta-Li, W.A. 263, 266, greenhouse climate 78, 535, greenhouse gas (GHG) 454, 515, 578, greisenization 54, Grimsel Alpine veins, Switzerland 126, Groote Eylandt Mn, 162, Guanajuato Ag-Au, Mexico 223–224, Guaymas Basin vents, Gulf of California 38, gypsum 327ff, 383, 394, 506, haematite, high-grade 81–82, 157, hafnium 251, halite 369ff, halokinesis 403, Hamersley Fe-province, W.A. 101, 156, Harz Mts. Pb-Ag-Zn veins, Germany 68, 201, Haselgebirge salt 392ff, 4.3, Hausruck lignite, Austria 498, 490, hazard 450, hazardous air pollutants (HAP) 514, heavy minerals 96, Hedinia oil field, Papua New Guinea 546, helium 528, Hemlo Au, Canada 66, 123, 124, Herfa-Neurode K-salt & waste repository,, Germany 399, Hohentauern magnesite, Austria 337, hot spots see rifting, Hubbert Curve 523, Hugoton gas field, Kansas 550, humic substances 480, huminite 475, 477, 6.7, 6.8, Huntley bauxite, W.A. 8, 9, 1.1, 1.2, HYC-McArthur River Pb-Zn, Australia 107, hydraulic equivalence 97–98, hydrocarbons 524ff, in hydrothermal fluids 38, hydrocarbon fluids 541, seeps 554, hydrogen fuel 522, hydrothermal alteration 34, 51ff, 57, 123, 421, 436, hydrothermal systems 35–54, hydrothermal water 31, 40, liquid, gaseous (vapour) and fluid state (supercritical, “gas” or “liquid”) 37, hypogene 10, hypozonal 48–49, , Iberian pyrite belt Cu 76, 190–191, immiscibility, hydrothermal fluids 38, melts 11, 16, 34, impact, see extraterrestrial, Inagli Pt, Russia 230, 232, indicator minerals 316, 427, indium 247ff, infiltration 88–91, Ingessana Hills Cr, Sudan 167, 2.9, in-situ leaching 271, 279, inversion 549, Irish type Pb-Zn-Ag 199, iron 60–62, 81, 87–88, 100–103, 120, 149ff, oolitic 103–105, iron oxide-copper-gold (-U-REE) deposits (IOCG) 16,, 138, 140, 188, 277, isotope geochemistry 41ff, boron 374, carbon 43, 311, 325, 357, 483, 529, chlorine 374, dating 41–42, 79–80, 269, 329, 375, helium 271, 528, lead 44, strontium 43–44, 294, sulphur 43, 296, 329, 357, 374, uranium 272, water 42–43, 329, 374–375, Itabira Fe, Brazil 156, Jinchuan Ni-Cu-PGM, China 172, Jinding Zn-Pb, China 200, Joachimsthal Ag-U, CZ 281, Joma Cu, Norway 122, JORC Code 442, 443, Julia Creek oilshale-Mo-V, Australia 184, Kambalda Ni, W.A. 12, 172, kaolin 79, 330ff, 350, 1.50, karst 89ff, 236, 329, Kenticha Ta-Li, Ethiopia 33, 264, Kerio Valley F, Kenya 323, Kern River oilfield, California 570, kerogen 114, 531ff, Key Lake U, Canada 276–277, Khibini apatite, Russia 24, 346, 347, kimberlites 23–25, 312ff, Kipushi Zn-Cu-Ge, DR Congo 200, Kirka boron, Turkey 304, 3.9, Kiruna-Malmberget Fe, Sweden 16–17, 152, 450, Kiya-Shaltyr nepheline, Russia 237, komatiites 12, 172
Page 696 :
660, , INDEX, , Mount Tom Price Fe, W. A. 157, 158, 2.4, Mountain Pass REE, California 24, 259, Mt. Isa Cu-Pb-Zn, Australia 128, 193–195, 197, 418, Mt. Oxide Cu, Australia 87, Mt. Weld REE, W.A. 260, Munster gas field, Germany 548, Murray Basin Zr, Australia 252–253, muscovite 339ff, Muskeg oil sand, Canada 571, 7.29, Muzo emerald, Colombia 270, natural gas see methane, natural nuclear reactors 275, Nchanga Cu-Co, Zambia 193, nelsonite 347, Neves Corvo Cu-Zn-Sn, Portugal 191, 431, Ngara Sn-Ta, Rwanda 96, nickel 12, 15, 81, 168ff, lateritic 82–84, Nikopol Mn, Ukraine 105, niobium 24, 261ff, Noril’sk-Talnakh Cu-Pd, Sibiria 14, 171, North Sea oil and gas 560–561, Nsuta Mn, Ghana 163, nugget 446, Nuweibi quartz, Egypt 349, ocean floor hydrothermal vents, see black smoker, ocean floor metamorphism 19, oceanic spreading 18ff, offshore oil 7.32, oil see petroleum, oil sand 570, 7.29, oil seeps, submarine 543, 555, oil shale 573ff, oil spills 575–576, oil spill remediation 576, 577, oil window 539–540, Oklo U, Gabon 275, Ok Tedi Cu-Au, Papua-New Guinea 455, Oldoinyo Lengai volcano, Tanzania 24, olivine 342–343, 3.26, 3.27, Olympic Dam Cu-U, S. Australia 188–189,, 277–278, 418, oolitic ore 103–1054, 157–159, 161, 1.68, open pit (lignite) 449, 5.18, 5.19, 6.30, ophiolite 18ff, 74, 83, 293, 359, orbital cycles 495, 574, ore, ore deposit 1–2, 1.33, ore grade 414, ore microscopy 49, 13, ore processing 415, , ore reserves see reserves, organic matter 93–94, orogenic gold see gold, orthomagmatic 11ff, Ouenza Fe, Algeria 61, 119–120, Outokumpu Cu-Zn-Co, Finland 190, overpressure 562–563, oxidation 84–86, 121, oxidized fluids 110, 112, 115, Pacific “ring of fire” 137, Palabora Cu, S.A. 24, 187–188, vermiculite 341,, apatite 346, Panasqueira W, Portugal 48, 183, Pangaea 116, 119, 139, 161, 390, 493, paragenesis 49, 66, passive mine water treatment 453, 455, 456, 5.24, 5.25, pathfinder elements 221, 423, peat 487ff, 499–500, pegmatites 32ff, 204, 263ff, 269, perlite 362, permeability 64, 375, 541, petroleum 524ff, 539, 7.32, conventional 523, unconventional 524, degraded 551–552, petroleum system 539, Phanerozoic carbon dioxide evolution 493, phlogopite 339ff, 3.24, phosphate see apatite, phosphogypsum 330, 344, Pine Point Pb-Zn, Canada 38, pit lakes 516, 517, 6.30, photosynthesis 479, placers, aeolian 95, 1.61, alluvial 96–97, 256, 265, 1.64, coastal 98–100, 252, 256, 2.39, minerals 96, residual 81, 1.62, plant expansion 499, plate tectonics 134ff, platinum group metals (PGM) 14, 228ff, 2.32, pneumatogenic 31, porosity and permeability 561ff, porphyry deposits 56ff, 136ff, 188, potassium 369ff, prefeasibility study 437, propylitization 53, prospecting 418, protore 84, Pueblo Viejo Au-Ag, Dominican Rep. 213–214
Page 697 :
INDEX, , pumice 362, pyrobitumen 572, 573, pyrophyllite 361, quartz 347, quartzite 349–350, quartz sand & gravel 350, Rabenwald talc, Austria 361, radioactive decay 41, 44, 271, 272, 282, radioactive waste 275, 283, 458ff, 5.29, radioactivity in exploration 282, 347, 431, radiobarite 514, radium 271, 330, 577, radon 271, 577, Rammelsberg Zn-Pb-Cu, Germany 108, 123, 191–192,, 1.72, 1.81, rare earth elements (REE) 24, 257ff, Rayleigh distillation 129, reclamation (mine) 457, 516, 5.19, 6.30, reconnaissance 419, Red Dog Pb-Zn, Alaska 107, 109, 110, red mud 234, 271, reduced fluids 110, 112, 118, 295, 541, reduction 39, 121, reforestation 1.2, regolith 78, 83, remediation see reclamation, remote sensing 420ff, reserves 440ff, 510–512, 568ff, reserve growth 570, reserves/production ratio 468, reservoir rocks 543ff, resources 440ff, 510–512, 570, 575, residual 80–82, reverse circulation drilling 434, rhenium 176, Riecke’s principle 124, rifting 134–5, risk 450, rock salt see halite, Rodinia 139, 264, R€, ossing U, Namibia, Rossignol F, France 50, 324, Rutongo Sn, Rwanda 26, 63, sabkha 383, Salar de Atacama Li, Chile 267, Salar de Uyuni Li, Bolivia 268, 2.44, saline 42, 110, 405, 469, salinity 37, 46–47, 110, 111, 376, salt diapirs 119–121, 400ff, , 661, , salt giants 387, salt lagoons 383, 387, salt lakes 380ff, Salton Sea, California 36, 229, sampling 220–221, 353, 424ff, 439, 509, 5.6, San Rafael Sn-Cu, Peru 205, saprolite 84, sapropel 534, Sarbai Fe, Kazakhstan 153, scandium 261, scheelite 182, 2.12, seals (oil and gas) 454 ff, Searles Lake boron-sodium, California 303, 354, seawater, convection 19, evaporation 111, 376ff, 4.6, modern 42, 93, 151, 274, 345, 376–377, 533, past 151, 384–385, sedex see submarine exhalative, sedimentary exhalative see sedex, segregation 11, 31, 34, seismic pumping 38, 65, seismic reflection geophysics 555–556, 557, 558, 7.22,, 7.23, selenium 247ff, Sempaya hot springs, NW Uganda 36, Sept-Îles Complex Ti-apatite, Canada 347, sericite 340, sericitization 54, Shizhuyuan W-Sn-Bi-Mo, China 181, 250, Shuiximiao Sn-Ta, SE China 26, shungite 573, Siilinj€arvi carbonatite, Finland 339, 3.24, silicification 53, silicon 247ff, 347–348, silicothermal fluids 51, sillimanite 288ff, silver 39, 50, 65, 68, 76, 115, 221ff, Silvermines Zn-Pb-Ag, Ireland 109, 199, Skaergaard Au-Pt, Greenland 418, skarn 54ff, 71, 153, 181, 364, 366, slab rollback 73, Sleipner gas, offshore Norway 469, 579, 6.2, Snowball Earth 103, sodium 369ff, sonic P-wave velocity in rocks 558, Southern Cross Au, W.A. 55, source rocks (oil and gas) 533ff, Spor Mountain Be, Utah 269, Sri Lanka (Ceylon) graphite 327, Steyn Au, West Rand, S.A. 99, stockwork ore 63
Page 698 :
662, , INDEX, , Stoke’s law 50, 97, stream sediments 425, 5.6, strontium 43–44, 294ff, 328, 3.3, subduction 136ff, submarine exhalative 70, volcanic-exhalative 70, sedimentary-exhalative (sedex) 70, 107ff, 1.72, subrosion 403, 404, subsidence 110, 403ff, 494, 513, Sudbury Igneous Complex (SIC) Ni-Cu, Canada 15,, 103, 133, 170–171, 428, sulphate reduction, microbial 90, 92, 116, 118, thermochemical (TSR) 112, 118, 528, 552, sulphidation, desulphidation 39, 75, 76, 121, 124, sulphide melt 12, 66, 124, 169, 229, sulphur 43, 76, 121, 124, 138, 296, 355ff, 481, Sulphur Bank Hg, California 241, supercontinents 139, supercritical fluid 31, 37–38, 138, 347, 470, supergene 10, 76ff, alteration of salt 405, degradation of coal 506, enrichment 84–88, 1.56, vertical zonation 85, 86, Superior type BIF 100ff, 1.67, sustainability 3, 463, 466, 579, 584, synfuels 486, 522, 571, 573–574, syngenetic 10, Taaken gas field, Germany 549, Tabba Tabba Ta, NW Australia 34, tailings 415, 451ff, 458, talc 358ff, 3.34, Talvivaara Ni-Cu-Co-Zn, Finland 179, tantalum 26, 29, 34, 261ff, 2.18, tar (heavy and extra-heavy oil) 570, 7.29, Taupo Volcanic Zone, New Zealand 36, tectonic control 65ff, 71, 73, 137, telescoping 74, Tellnes Ti, Norway 253, tellurium 247ff, thorium 270ff, Tikhvin bauxite, Russia 263, tin 26, 27, 63, 202ff, tin granite 27, 52, 202–203, 204, titanium 16, 29, 254, 2.39, Tongkeng-Changpo Sn, S-China 63, Torres del Paine fluids, Chile 46, tourmalinization 54, 427, trap 130, 542, 545–550, 454 ff, trap basalt 14, 135, 141, 502, , trass 363, Trepca (Trepc€e) Pb-Zn-Ag, Kosovo 201, tripoli 319, Tri-State Pb-Zn, USA 44, 61, 118, Troll gas field, offshore Norway 542, trona 354, Tsumeb Pb-Ge, Namibia 87, 135, tungsten 26, 27, 48, 52, 56, 66, 67, 179ff, 2.12, Tungus coal, Siberia 502, Tyndrum Au, Scotland 131, Uhry quartz sand, Germany 352, 3.30, underground mining 438, underground waste disposal 399, 402, 458ff, unidirectional solidification textures (USTs) 34, 178, uranium 28, 88–89, 270ff, Uston bauxite, France 237, valuation 437ff, 447ff, vanadium 14, 183ff, vapour see boiling, veins 62–68, hydraulic permeability 64, sheeted 64, vermiculite 339ff, Viburnum Pb-Zn, Missouri 61, 117–118, vitrinite 475, 476,, vitrinite reflectance 501, Voisey’s Bay Ni-Cu-Co, Canada 171, 418, volatiles 30ff, 75, 122, 176, volcanogenic deposits 70–76, Wackersdorf U, Germany 274, Wadi Essel celestite, Egypt 294, 3.3, waste repositories 458ff, 5.29, water connate 111, critical density and temperature 31, diagenetic 111ff, 395, 551, formation and reservoir 42, 111, 551, geothermal 35–37, 40, 75, 240, stable isotopes 42, 80, 111, 374–375, juvenile 40, magmatic 30, 42, 47, 55, 75, metamorphic 127ff, meteoric 42, mine 450ff, 514, saline 110, 405, surface 42, 77, 274, volcanic 42, water management (coal mines) 514, weighting 444, Weipa Al, Australia 235
