Introduction to Ore-Forming Processes.pdf

ITOA01 09/03/2009 1430 Page ii INTRODUCTION TO ORE-ING PROCESSES ITOA01 09/03/2009 1430 Page i ITOA01 09/03/2009 1430 Page ii Introduction to Ore-ing Processes LAURENCE ROBB ITOA01 09/03/2009 1430 Page iii 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 identifi ed 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 retri system, or transmitted, in any 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-ing processes / Laurence Robb. p.cm. Includes bibliographical references and index. ISBN 0-632-06378-5 pbk. alk. paper 1. Ores.I. Title. QE390.R322004 553′.1dc22 2003014049 A catalogue record for this title is available from the British Library. Set in 9/111– 2pt Trump Media by Graphicraft Limited, Hong Kong Printed and bound in the United Kingdom by TJ International, Padstow, Cornwall For further ination on Blackwell Publishing, visit our website ITOA01 09/03/2009 1430 Page iv Prefacevii INTRODUCTION MINERAL RESOURCES1 Introduction and aims1 A classifi cation scheme for ore deposits2 What makes a viable mineral deposit4 Some useful defi nitions and compilations6 Natural resources, sustainability, and environmental responsibility11 Summary and further reading15 PART 1 IGNEOUS PROCESSES 1 IGNEOUS ORE-ING PROCESSES19 1.1 Introduction19 1.2 Magmas and metallogeny20 1.3 Why are some magmas more fertile than others The “inheritance factor”28 1.4 Partial melting and crystal fractionation as ore-ing processes37 1.5 Liquid immiscibility as an ore-ing process54 1.6 A more detailed consideration of mineralization processes in mafi c magmas57 1.7 A model for mineralization in layered mafi c intrusions71 Summary and further reading74 2 MAGMATIC-HYDROTHERMAL ORE-ING PROCESSES75 2.1Introduction75 2.2Some physical and chemical properties of water76 2.3ation of a magmatic aqueous phase79 2.4The composition and characteristics of magmatic- hydrothermal solutions85 2.5A note on pegmatites and their signifi cance to granite-related ore-ing processes93 2.6Fluid–melt trace element partitioning96 2.7Water content and depth of emplacement of granites – relationships to ore-ing processes101 2.8Models for the ation of porphyry-type Cu, Mo, and W deposits106 2.9 Fluid fl ow in and around granite plutons108 2.10 Skarn deposits113 2.11 Near-surface magmatic- hydrothermal processes – the “epithermal” family of Au–Ag–Cu deposits117 2.12 The role of hydrothermal fl uids in mineralized mafi c rocks122 Summary and further reading125 PART 2 HYDROTHERMAL PROCESSES 3 HYDROTHERMAL ORE-ING PROCESSES129 3.1Introduction129 3.2 Other fl uids in the Earth’s crust and their origins130 Contents ITOA01 09/03/2009 1430 Page v 3.3The movement of hydrothermal fl uids in the Earth’s crust138 3.4Further factors affecting metal solubility147 3.5Precipitation mechanisms for metals in solution153 3.6 More on fl uid/rock interaction – an introduction to hydrothermal alteration166 3.7Metal zoning and paragenetic sequence174 3.8Modern analogues of ore-ing processes – the VMS–SEDEX continuum177 3.9Mineral deposits associated with aqueo-carbonic metamorphic fl uids189 3.10 Ore deposits associated with connate fl uids197 3.11 Ore deposits associated with near surface meteoric fl uids groundwater209 Summary and further reading214 PART 3 SEDIMENTARY/ SURFICIAL PROCESSES 4 SURFICIAL AND SUPERGENE ORE-ING PROCESSES219 4.1 Introduction219 4.2 Principles of chemical weathering220 4.3 Lateritic deposits223 4.4 Clay deposits233 4.5 Calcrete-hosted deposits235 4.6 Supergene enrichment of Cu and other metals in near surface deposits238 Summary and further reading245 5 SEDIMENTARY ORE-ING PROCESSES246 5.1 Introduction246 5.2 Clastic sedimentation and heavy mineral concentration – placer deposits247 5.3 Chemical sedimentation – banded iron-ations, phosphorites, and evaporites266 5.4 Fossil fuels – oil/gas ation and coalifi cation287 Summary and further reading307 PART 4 GLOBAL TECTONICS AND METALLOGENY 6 ORE DEPOSITS IN A GLOBAL TECTONIC CONTEXT311 6.1 Introduction311 6.2 Patterns in the distribution of mineral deposits312 6.3 Continental growth rates312 6.4 Crustal evolution and metallogenesis315 6.5 Metallogeny through time319 6.6 Plate tectonics and ore deposits – a summary339 Summary and further reading343 References345 Index368 viCONTENTS ITOA01 09/03/2009 1430 Page vi There are many excellent texts, available at both introductory and advanced levels, that describe the Earth’s mineral deposits. Several describe the deposits themselves and others do so in com- bination with explanations that provide an under- standing of how such mineral occurrences . Few are dedicated entirely to the multitude of processes that give rise to the ore deposits of the world. The main purpose of this book is to provide a better understanding of the processes, as well as the nature and origin, of mineral occurrences and how they fi t into the Earth system. It is intended for use at a senior undergraduate level third and fourth year levels, or graduate level North America, and assumes a basic knowledge in a wide range of core earth science disciplines, as well as in chemistry and physics. Although meant to be introductory, it is reasonably comprehen- sive in its treatment of topics, and it is hoped that practicing geologists in the minerals and related industries will also fi nd the book useful as a sum- mary and update of ore-ing processes. To this end the text is punctuated by a number of boxed case studies in which actual ore deposits, selected as classic examples from around the world, are briefl y described to give context and relevance to processes being discussed in the main text. Metallogeny, or the study of the genesis of ore deposits in relation to the global tectonic paradigm, is a topic that traditionally has been, and should remain, a core component of the university earth science curriculum. It is also the discipline that underpins the training of professional earth scien- tists working in the minerals and related industries of the world. A tendency in the past has been to treat economic geology as a vocational topic and to provide instruction only to those individuals who wished to specialize in the discipline or to follow a career in the minerals industries. In more recent years, changes in earth science curricula have resulted in a trend, at least in a good many parts of the world, in which economic geology has been sidelined. A more holistic, process-orientated approach earth systems science has led to a wider appreciation of the Earth as a complex interrelated system. Another aim of this book, therefore, is to emphasize the range of processes responsible for the ation of the enormously diverse ore deposit types found on Earth and to integrate these into a description of Earth evolution and global tectonics. In so doing it is hoped that metallogenic studies will increasingly be reinte- grated into the university earth science curricula. Teaching the processes involved in the ation of the world’s diminishing resource inventory is necessary, not only because of its practical relevance to the real world, but also because such processes an integral and inative part of the Earth system. This book was written mainly while on a protracted sabbatical in the Department of Earth Sciences at the University of Oxford. I am very grateful to John Woodhouse and the departmental staff who accommodated me and helped to provide the combination of academic rigor and quietitude that made writing this book such a pleasure. In particular Jenny Colls, Earth Science Librarian, was a tower of support in locating reference material. The “tea club” at the Banbury Road ann pro- vided both stimulation and the requisite libations Preface ITOA01 09/03/2009 1430 Page vii to break the monotony. The staff at Blackwell managed to combine being really nice people with a truly professional attitude, and Ian Francis, Delia Sandford, Rosie Hayden, and Cee Pike were all a pleasure to work with. Dave Coles drafted all the diagrams and I am extremely grateful for his forebearance in dealing amiably with a list of fi gures that seemingly did not end. Several people took time to read through the manuscript for me and in so doing greatly improved both the style and content. They include John Taylor copy- editing, Judith Kinnaird and Dave Waters Intro- duction, Grant Cawthorn Chapter 1, Philip Candela Chapter 2, Franco Pirajno Chapter 3, Michael Meyer Chapter 4, John Parnell and Harold Reading Chapter 5, and Mark Barley, Kevin Burke, and John Dewey Chapter 6. The defi ciencies that remain, though, are entirely my own. A particularly debt of gratitude is owed to David Rickard, who undertook the onerous task of reviewing the entire manuscript; his lucid comments helped to eliminate a number of fl aws and omissions. Financial support for this project came from BHP Billiton in London and the Geo- logical Society of South Africa Trust. My col- leagues at Wits were extremely supportive during my long absences, and I am very grateful to Spike McCarthy, Paul Dirks, Carl Anhauesser, Johan Kruger, and Judith Kinnaird for their in so many ways. Finally, my family, Vicki, Nicole, and Brendan, were subjected to a life-style that involved making personal sacrifi ces for the fruition of this project – there is no way of saying thank you and it is to them that I dedicate this book. Laurence Robb Johannesburg viiiPREFACE ITOA01 09/03/2009 1430 Page viii INTRODUCTION AND AIMS Given the unprecedented growth of human population over the past century, as well as the related increase in demand for and production of natural resources, it is evident that under- standing the nature, origin and distribution of the world’s mineral deposits remains a strategic topic. The discipline of “economic geology,” which covers all aspects pertaining to the description and understanding of mineral resources, is, there- fore, one which traditionally has been, and should remain, a core component of the university earth science curriculum. It is also the discipline that underpins the training of professional earth scient- ists working in the minerals and related indus- tries of the world. A tendency in the past has been to treat economic geology as a vocational topic and to provide instruction only to those individ- uals who wished to specialize in the discipline or to follow a career in the minerals industry. In more recent years, changes in earth science curricula have resulted in a trend, at least in a good many parts of the world, in which economic geology has been sidelined. The conceptual development of earth systems science, also a feature of the latter years of the twentieth century, has led to dramatic shifts in the way in which the earth sciences are taught. A more holistic, process-orientated approach has led to a much wider appreciation of the Earth as a complex interrelated system. The under- standing of feedback mechanisms has brought an appreciation that the solid Earth, its oceans and atmosphere, and the organic life s that occupy niches above, at and below its surface, are intimately connected and can only be under- stood properly in terms of an interplay of pro- cesses. Examples include the links between global tectonics and climate patterns, and also between the evolution of unicellular organisms and the ation of certain types of ore deposits. In this context the teaching of many of the traditional geological disciplines assumes new relevance and the challenge to successfully teaching earth sys- tem science is how best to integrate the wide range of topics into a curriculum that provides under- standing of the entity. Teaching the processes involved in the ation of the enormously Introduction mineral resources GENERAL INTRODUCTION AND AIMS OF THE BOOK A SIMPLE CLASSIFICATION SCHEME FOR MINERAL DEPOSITS SOME IMPORTANT DEFINITIONS metallogeny, syngenetic, epigenetic, mesothermal, epithermal, supergene, hypogene, etc. SOME RELEVANT COMPILATIONS periodic table of the elements tables of the main ore and gangue minerals geological time scale FACTORS THAT MAKE A VIABLE MINERAL DEPOSIT enrichment factors required to make ore deposits how are mineral resources and ore reserves defi ned NATURAL RESOURCES AND THEIR FUTURE EXPLOITATION sustainability environmental responsibility ITOA02 09/03/2009 1431 Page 1 diverse ore deposit types found on Earth is neces- sary, not only because of its practical relevance to the real world, but also because such processes an integral and inative part of the Earth’s evolution. The purpose of this process-orientated book is to provide a better understanding of the nature and origin of mineral occurrences and how they fi t into the Earth system. It is intended for use at a senior undergraduate level third and fourth year levels, or a graduate level, and assumes a basic knowledge in a wide range of core earth science disciplines, as well as in chemistry and physics. It is also hoped that practicing geologists in the minerals and related industries will fi nd the book useful as a summary and update of ore-ing processes. To this end the text is punctuated by a number of boxed case studies in which actual ore deposits, selected as classic examples from around the world, are briefl y described to give context and relevance to processes being discussed in the main text. A CLASSIFICATION SCHEME FOR ORE DEPOSITS There are many different ways of categorizing ore deposits. Most people who have written about and described ore deposits have either unwit- tingly or deliberately been involved in their classifi cation. This is especially true of textbooks where the task of providing order and structure to a set of descriptions invariably involves some of classifi cation. The best classifi cation schemes are probably those that remain as independent of genetic linkages as possible, thereby minimizing the scope for mistakes and controversy. Never- theless, genetic classifi cation schemes are ulti- mately desirable, as there is considerable advantage to having processes of ore ation refl ected in a set of descriptive categories. Guilbert and Park 1986 discuss the problem of ore deposit classi- fi cation at some length in Chapters 1