Rocas

INTRODUCTION TO ORE-FORMING PROCESSES

Introduction to Ore-Forming Processes
LAURENCE ROBB

© 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 inaccordance 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 published2005 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– pt Trump Mediaeval2 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

Contents
Preface INTRODUCTION: MINERAL RESOURCES Introduction and aims A classification scheme for ore deposits What makes a viable mineral deposit? Some useful definitionsand compilations Natural resources, sustainability, and environmental responsibility Summary and further reading vii 1 1 2 4 6 11 15 Formation of a magmatic aqueous phase 2.4 The composition and characteristics of magmatichydrothermal solutions 2.5 A note on pegmatites and their significance to granite-related ore-forming processes 2.6 Fluid–melt trace element partitioning 2.7 Water content and depthof emplacement of granites – relationships to ore-forming processes 2.8 Models for the formation of porphyry-type Cu, Mo, and W deposits 2.9 Fluid flow in and around granite plutons 2.10 Skarn deposits 2.11 Near-surface magmatichydrothermal processes – the “epithermal” family of Au–Ag–(Cu) deposits 2.12 The role of hydrothermal fluids in mineralized mafic rocks Summary and further reading 2.3

7985

93 96

PART 1 IGNEOUS PROCESSES
1 IGNEOUS ORE-FORMING PROCESSES 1.1 Introduction 1.2 Magmas and metallogeny 1.3 Why are some magmas more fertile than others? The “inheritance factor” 1.4 Partial melting and crystal fractionation as ore-forming processes 1.5 Liquid immiscibility as an ore-forming process 1.6 A more detailed consideration of mineralization processes in mafic magmas 1.7 Amodel for mineralization in layered mafic intrusions Summary and further reading 2 MAGMATIC-HYDROTHERMAL ORE-FORMING
PROCESSES

19 19 20

101

106 108 113

28

37 54

117 122 125

57 71 74

PART 2 HYDROTHERMAL PROCESSES
3 HYDROTHERMAL ORE-FORMING PROCESSES 3.1 Introduction 3.2 Other fluids in the Earth’s crust and their origins 129 129 130

2.1 2.2

Introduction Some physicaland chemical properties of water

75 75 76

vi

CONTENTS

The movement of hydrothermal fluids in the Earth’s crust 3.4 Further factors affecting metal solubility 3.5 Precipitation mechanisms for metals in solution 3.6 More on fluid/rock interaction – an introduction to hydrothermal alteration 3.7 Metal zoning and paragenetic sequence 3.8 Modern analogues of ore-forming processes – theVMS–SEDEX continuum 3.9 Mineral deposits associated with aqueo-carbonic metamorphic fluids 3.10 Ore deposits associated with connate fluids 3.11 Ore deposits associated with near surface meteoric fluids (groundwater) Summary and further reading

3.3

138 147 153

4.6 Supergene enrichment of Cu and other metals in near surface deposits Summary and further reading 5 SEDIMENTARY ORE-FORMING PROCESSES...