INTRODUCTION to ORE-FORMING PROCESSES ITOA01 09/03/2009 14:30 Page Ii ITOA01 09/03/2009 14:30 Page Iii
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ITOA01 09/03/2009 14:30 Page ii ITOA01 09/03/2009 14:30 Page i INTRODUCTION TO ORE-FORMING PROCESSES ITOA01 09/03/2009 14:30 Page ii ITOA01 09/03/2009 14:30 Page iii Introduction to Ore-Forming Processes LAURENCE ROBB ITOA01 09/03/2009 14:30 Page iv © 2005 by Blackwell Science Ltd a Blackwell Publishing company 350 Main Street, Malden, MA 02148-50120 USA 108 Cowley Road, Oxford OX4 1JF, UK 550 Swanston Street, Carlton, Victoria 3053, Australia The right of Laurence Robb to be identified as the Author of this Work has been asserted in accordance with the UK 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. First published 2005 by Blackwell Publishing Library of Congress Cataloging-in-Publication Data Robb, L.J. Introduction to ore-forming processes / Laurence Robb. p. cm. Includes bibliographical references and index. ISBN 0-632-06378-5 (pbk. : alk. paper) 1. Ores. I. Title. QE390.R32 2004 553′.1—dc22 2003014049 A catalogue record for this title is available from the British Library. 1 Set in 9/11–2 pt Trump Mediaeval by Graphicraft Limited, Hong Kong Printed and bound in the United Kingdom by TJ International, Padstow, Cornwall For further information on Blackwell Publishing, visit our website: http://www.blackwellpublishing.com ITOA01 09/03/2009 14:30 Page v Contents Preface vii 2.3 Formation of a magmatic aqueous phase 79 INTRODUCTION: MINERAL RESOURCES 1 2.4 The composition and Introduction and aims 1 characteristics of magmatic- A classification scheme for ore deposits 2 hydrothermal solutions 85 What makes a viable mineral deposit? 4 2.5 A note on pegmatites and their Some useful definitions and compilations 6 significance to granite-related Natural resources, sustainability, and ore-forming processes 93 environmental responsibility 11 2.6 Fluid–melt trace element Summary and further reading 15 partitioning 96 2.7 Water content and depth of PART 1 IGNEOUS PROCESSES emplacement of granites – relationships to ore-forming 1IGNEOUS ORE-FORMING PROCESSES 19 processes 101 1.1 Introduction 19 2.8 Models for the formation of 1.2 Magmas and metallogeny 20 porphyry-type Cu, Mo, and 1.3 Why are some magmas more fertile W deposits 106 than others? The “inheritance 2.9 Fluid flow in and around granite factor” 28 plutons 108 1.4 Partial melting and crystal 2.10 Skarn deposits 113 fractionation as ore-forming 2.11 Near-surface magmatic- processes 37 hydrothermal processes – the 1.5 Liquid immiscibility as an “epithermal” family of ore-forming process 54 Au–Ag–(Cu) deposits 117 1.6 A more detailed consideration 2.12 The role of hydrothermal fluids of mineralization processes in in mineralized mafic rocks 122 mafic magmas 57 Summary and further reading 125 1.7 A model for mineralization in layered mafic intrusions 71 PART 2 HYDROTHERMAL Summary and further reading 74 PROCESSES 2MAGMATIC-HYDROTHERMAL ORE-FORMING PROCESSES 75 3HYDROTHERMAL ORE-FORMING PROCESSES 129 2.1 Introduction 75 3.1 Introduction 129 2.2 Some physical and chemical 3.2 Other fluids in the Earth’s crust properties of water 76 and their origins 130 ITOA01 09/03/2009 14:30 Page vi vi CONTENTS 3.3 The movement of hydrothermal 4.6 Supergene enrichment of Cu fluids in the Earth’s crust 138 and other metals in near surface 3.4 Further factors affecting metal deposits 238 solubility 147 Summary and further reading 245 3.5 Precipitation mechanisms for metals in solution 153 5SEDIMENTARY ORE-FORMING PROCESSES 246 3.6 More on fluid/rock interaction – 5.1 Introduction 246 an introduction to hydrothermal 5.2 Clastic sedimentation and heavy alteration 166 mineral concentration – placer 3.7 Metal zoning and paragenetic deposits 247 sequence 174 5.3 Chemical sedimentation – banded 3.8 Modern analogues of ore-forming iron-formations, phosphorites, processes – the VMS–SEDEX and evaporites 266 continuum 177 5.4 Fossil fuels – oil/gas formation and 3.9 Mineral deposits associated coalification 287 with aqueo-carbonic metamorphic Summary and further reading 307 fluids 189 3.10 Ore deposits associated with connate fluids 197 PART 4 GLOBAL TECTONICS 3.11 Ore deposits associated with AND METALLOGENY near surface meteoric fluids (groundwater) 209 6ORE DEPOSITS IN A GLOBAL TECTONIC CONTEXT 311 Summary and further reading 214 6.1 Introduction 311 6.2 Patterns in the distribution of PART 3 SEDIMENTARY/ mineral deposits 312 SURFICIAL PROCESSES 6.3 Continental growth rates 312 6.4 Crustal evolution and metallogenesis 315 4SURFICIAL AND SUPERGENE ORE-FORMING 6.5 Metallogeny through time 319 PROCESSES 219 6.6 Plate tectonics and ore deposits – 4.1 Introduction 219 a summary 339 4.2 Principles of chemical weathering 220 Summary and further reading 343 4.3 Lateritic deposits 223 4.4 Clay deposits 233 References 345 4.5 Calcrete-hosted deposits 235 Index 368 ITOA01 09/03/2009 14:30 Page vii Preface There are many excellent texts, available at both treat economic geology as a vocational topic and introductory and advanced levels, that describe to provide instruction only to those individuals the Earth’s mineral deposits. Several describe the who wished to specialize in the discipline or to deposits themselves and others do so in com- follow a career in the minerals industries. In more bination with explanations that provide an under- recent years, changes in earth science curricula standing of how such mineral occurrences form. have resulted in a trend, at least in a good many Few are dedicated entirely to the multitude of parts of the world, in which economic geology has processes that give rise to the ore deposits of the been sidelined. A more holistic, process-orientated world. The main purpose of this book is to provide approach (earth systems science) has led to a wider a better understanding of the processes, as well as appreciation of the Earth as a complex interrelated the nature and origin, of mineral occurrences and system. Another aim of this book, therefore, is how they fit into the Earth system. It is intended to emphasize the range of processes responsible for use at a senior undergraduate level (third for the formation of the enormously diverse ore and fourth year levels), or graduate level (North deposit types found on Earth and to integrate America), and assumes a basic knowledge in a these into a description of Earth evolution and wide range of core earth science disciplines, as global tectonics. In so doing it is hoped that well as in chemistry and physics. Although meant metallogenic studies will increasingly be reinte- to be introductory, it is reasonably comprehen- grated into the university earth science curricula. sive in its treatment of topics, and it is hoped that Teaching the processes involved in the formation practicing geologists in the minerals and related of the world’s diminishing resource inventory industries will also find the book useful as a sum- is necessary, not only because of its practical mary and update of ore-forming processes. To this relevance to the real world, but also because such end the text is punctuated by a number of boxed processes form an integral and informative part case studies in which actual ore deposits, selected of the Earth system. as classic examples from around the world, are This book was written mainly while on a briefly described to give context and relevance to protracted sabbatical in the Department of Earth processes being discussed in the main text. Sciences at the University of Oxford. I am very Metallogeny, or the study of the genesis of ore grateful to John Woodhouse and the departmental deposits in relation to the global tectonic paradigm, staff who accommodated me and helped to provide is a topic that traditionally has been, and should the combination of academic rigor and quietitude remain, a core component of the university earth that made writing this book such a pleasure. In science curriculum. It is also the discipline that particular Jenny Colls, Earth Science Librarian, was underpins the training of professional earth scien- a tower of support in locating reference material. tists working in the minerals and related industries The “tea club” at the Banbury Road annexe pro- of the world. A tendency in the past has been to vided both stimulation and the requisite libations ITOA01 09/03/2009 14:30 Page viii viii PREFACE to break the monotony. The staff at Blackwell David Rickard, who undertook the onerous task managed to combine being really nice people with of reviewing the entire manuscript; his lucid a truly professional attitude, and Ian Francis, comments helped to eliminate a number of flaws Delia Sandford, Rosie Hayden, and Cee Pike were and omissions. Financial support for this project all a pleasure to work with. Dave Coles drafted came from BHP Billiton in London and the Geo- all the diagrams and I am extremely grateful for logical Society of South Africa Trust. My col- his forebearance in dealing amiably with a list of leagues at Wits were extremely supportive during figures that seemingly did not end. Several people my long absences, and I am very grateful to Spike took time to read through the manuscript for me McCarthy, Paul Dirks, Carl Anhauesser, Johan and in so doing greatly improved both the style Kruger, and Judith Kinnaird for their input in and content. They include John Taylor (copy- so many ways.