Handbook of gold exploration and evaluation PDF

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Handbook of gold exploration and evaluation Related titles: Valuing mining companies: A guide to the assessment and evaluation of assets, performance and prospects (ISBN-13: 978-1-85573-435-7; ISBN-10: 1-85573-435-4) This book offers a perspective on the international mining and metals industry, fr...

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Handbook of gold exploration and evaluation

Related titles: Valuing mining companies: A guide to the assessment and evaluation of assets, performance and prospects (ISBN-13: 978-1-85573-435-7; ISBN-10: 1-85573-435-4) This book offers a perspective on the international mining and metals industry, from historical details of mines and mining to the potential problems encountered in attempting to value a mining company. Chapters are devoted to accounting practices and taxation and there is valuable information on the financial aspects of mining. It also covers various techniques used to value mining companies and gives instruction on how to set up a portfolio and begin trading in this complex field. The international gold trade (ISBN-13: 978-1-85573-072-4; ISBN-10: 1-85573-072-3) Activity in the gold markets has focused investors' attention on this unique commodity. To provide the reader with a better understanding of the trade, the book is set out in three sections. The first sketches the structure of the gold market from the point of view of the commodity analyst before reviewing in detail the institutions and practices of bullion and futures trading; the second looks at gold mining from a long-term perspective; the third surveys the use of gold, past and present, and discusses the metal's future prospects. Base metals handbook 3rd edition (ISBN-13: 978-1-84569-154-7; ISBN-10: 1-84569-154-7) Described as the bible of the metals trading community, this updated edition of Base metals handbook represents a completely revised version of one of Woodhead's most successful publications. It is a comprehensive handbook ± published in convenient looseleaf form ± covering the structure and conduct of the markets and detailing each of the base metals in separate sections to give a unique view of the entire international trading picture. Like its predecessor, this edition has become the industry standard for all those involved in base metals trading. It contains a substantial amount of data from a wide variety of sources, placing this data in a wide context and using it to explain the operation of these markets in depth. Details of these and Woodhead Publishing materials books and journals, as well as materials books from Maney Publishing, can be obtained by: · visiting our web site at www.woodheadpublishing.com · contacting Customer Services (e-mail: [email protected]; fax: +44 (0) 1223 893694; tel.: +44 (0) 1223 891358 ext. 130; address: Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CB21 6AH, England) If you would like to receive information on forthcoming titles, please send your address details to: Francis Dodds (address, tel. and fax as above; email: [email protected]). Please confirm which subject areas you are interested in. Maney currently publishes 16 peer-reviewed materials science and engineering journals. For further information visit www.maney.co.uk/journals.

Handbook of gold exploration and evaluation Eoin H. Macdonald

Woodhead Publishing and Maney Publishing on behalf of The Institute of Materials, Minerals & Mining

CRC Press Boca Raton Boston New York Washington, DC

Woodhead Publishing Limited and Maney Publishing Limited on behalf of The Institute of Materials, Minerals & Mining Published by Woodhead Publishing Limited, Abington Hall, Abington, Cambridge CB21 6AH, England www.woodheadpublishing.com Published in North America by CRC Press LLC, 6000 Broken Sound Parkway, NW, Suite 300, Boca Raton, FL 33487, USA First published 2007, Woodhead Publishing Limited and CRC Press LLC ß Woodhead Publishing Limited, 2007 The author has asserted his moral rights. NOTE: Every effort has been made to trace and acknowledge ownership of copyright. The publishers will be glad to hear from any copyright holders whom it has not been possible to contact. This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. Reasonable efforts have been made to publish reliable data and information, but the authors and the publishers cannot assume responsibility for the validity of all materials. Neither the authors nor the publishers, nor anyone else associated with this publication, shall be liable for any loss, damage or liability directly or indirectly caused or alleged to be caused by this book. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming and recording, or by any information storage or retrieval system, without permission in writing from Woodhead Publishing Limited. The consent of Woodhead Publishing Limited does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from Woodhead Publishing Limited for such copying. Trademark notice: product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe. British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library. Library of Congress Cataloging in Publication Data A catalog record for this book is available from the Library of Congress. Woodhead Publishing ISBN-13: 978-1-84569-175-2 (book) Woodhead Publishing ISBN-10: 1-84569-175-X (book) Woodhead Publishing ISBN-13: 978-1-84569-254-4 (e-book) Woodhead Publishing ISBN-10: 1-84569-254-3 (e-book) CRC Press ISBN-13: 978-1-4200-4460-7 CRC Press ISBN-10: 1-4200-4460-5 CRC Press order number: WP4460 The publishers' policy is to use permanent paper from mills that operate a sustainable forestry policy, and which has been manufactured from pulp which is processed using acid-free and elementary chlorine-free practices. Furthermore, the publishers ensure that the text paper and cover board used have met acceptable environmental accreditation standards. Typeset by Godiva Publishing Services Ltd, Coventry, West Midlands, England Printed by TJ International Limited, Padstow, Cornwall, England

Contents

1 1.1 1.2

2 2.1 2.2 2.3 2.4 2.5 2.6

3

Foreword

ix

Preface

xi

Acknowledgements

xv

Nature and history of gold Gold mineralogy Gold through the ages

1 1 32

Geology of gold ore deposits

62

Crustal evolution Tectonic elements of plate movements Hydrothermal gold systems Gold deposition in volcanic terrain Provenance Time rate of unroofing ore bodies

63 80 101 111 126 128

Gold deposition in the weathering environment

134

3.1 3.2 3.3 3.4 3.5 3.6

The plate tectonic rock cycle Earth's atmosphere and climate Agents of weathering Weathering processes Landscape denudation Low-temperature aqueous geochemistry

134 143 162 169 173 179

4

Sedimentation and detrital gold

195

4.1 4.2 4.3

Sediment characteristics Fluvial hydrology Drainage systems

195 205 215

vi

Contents

4.4 4.5

Entrainment, transport and sorting Fluvial gold deposition

5 5.1 5.2 5.3 5.4 5.5

6 6.1 6.2 6.3 6.4 6.5

Gold exploration Geological investigations Exploration geochemistry Remote sensing Shallow land-based geophysics Shallow offshore geophysics

Lateritic and alluvial gold sampling Sampling criteria Prospecting methods onshore Prospecting methods offshore Sample dressing Ore resource estimation

225 236

267 268 285 305 314 330

339 340 353 371 383 395

7

Mine planning and practice

410

7.1 7.2 7.3 7.4 7.5 7.6 7.7

Planning Operational concepts and schedules Sluicing practice Bucketline dredging Hydraulic dredgers Dry mining Miscellaneous dredger types

410 421 439 447 474 477 484

8 8.1 8.2 8.3 8.4 8.5

9 9.1 9.2 9.3 9.4

Metallurgical process and design Theory of gravity concentration Flow sheet and materials balance Feed preparation Gravity processing Lateritic gold metallurgy

Evaluation, risk and feasibility Project management Economic appraisal Risk analysis and uncertainty The feasibility study concept

488 489 502 505 518 546

553 554 559 570 593

Contents

Appendix I Appendix II

Field laboratories and techniques Variogram structural analysis

vii

600 606

Appendix III

Sitework testing

607

Appendix IV

Gold economics

611

References and further reading

614

Index

630

Foreword

I am honoured and extremely pleased to write the foreword for Eoin Macdonald's book Handbook of Gold Exploration and Evaluation. Eoin's first book on this subject, Alluvial Mining ± The Geology, Technology and Economics of Placers, published in 1983, was aimed at a basic understanding of the fundamentals of alluvial mining theory and practice, primarily for beach sand minerals. In the subsequent years Eoin realised that a book based on alluvial gold deposits was warranted and indeed needed. Handbook of Gold Exploration and Evaluation is the result and represents the knowledge and foresight of a man with over 65 years of expertise and practical experience in the fields of exploration and mining of mineral deposits across the globe. Rich alluvial gold deposits have been mined for centuries and continue to capture the interest and imagination of specialists and lay people in the search for the yellow metal. But with increases in demand and depletion of easily identified and won detrital gold deposits, exploration has entered into a new and exciting phase. It is thus pleasing to see a new book that recognises the common relationship of exploration geology, geochemistry and remote sensing techniques in the search for both alluvial and hard rock gold ores. This book contains nine chapters covering topics as diverse as the nature and history of gold, geology of gold ore deposits, gold deposition in the weathering environment, sedimentation and detrital gold, gold exploration, lateritic and placer gold sampling, mine planning and practice, metallurgical processes and design, and evaluation, risk and feasibility. The breadth of subject matter contained in this book is outstanding and I know of no better and more up-todate book on alluvial gold deposits, exploration and mining. Discussions involving the methods, hazards and costs of conducting mining operations of primary gold ores at depth are beyond the scope of the treatment but an indisputable and most important conclusion is that both primary and secondary gold ventures profit equally from the same detailed field investigations. Detrital gold accumulations represent the weathered detritus of their host rocks hence neither primary nor alluvial gold surface features can be studied in isolation without neglecting possibly vital evidence from the other. Ground

x

Foreword

surface studies involve similar geochemical stream and soil sediment surveys. Similar geological and geophysical techniques also apply to sub-surface investigations in both primary and secondary gold settings including geomorphic reconstructions of palaeo-erosional surfaces. Remote sensing techniques employ similar sensors to map relevant geological and geomorphic features and dating of the same key rock types and minerals helps clarify the geological history of any area under investigation in both ancient and modern tectonic settings. Future demands on gold will be very great and additions to resource quantities of both primary and secondary gold ores must increase in order to cope with increasing demand. Definition of that part of a resource that might be mined economically within the guidelines laid down for exploration is an essential requirement of economic evaluation. This book will be of special interest and use to exploration and mining geologists, mining engineers, metallurgists, academics and students alike. However, the book is written in such a manner that non-technical people will find it informative and it will help them gain a thorough understanding of gold exploration. Finally, congratulations Eoin on the tremendous achievement of publishing Handbook of Gold Exploration and Valuation in your 90th year, and may the ultimate compliment to you be the discovery and successful exploitation of a new alluvial gold deposit. Assoc. Prof. J. Bruce Gemmell ARC Centre of Excellence in Ore Deposits (CODES) School of Earth Sciences University of Tasmania

Preface

The intention of an earlier text Alluvial Mining ± The Geology, Technology and Economics of Placers ± was to provide a basic understanding of the fundamentals of alluvial mining theory and practice for placer engineers and geologists to build upon. The beach sand minerals rutile, zircon, ilmenite and middle density minerals cassiterite and tantalite took prominence over gold and other noble metals of higher density and greater unit value. The overall response was encouraging but the amount of fresh evidence emerging from ongoing studies showed that a fresh book was needed based solely upon gold as a unique metal in its own right. This book, the Handbook of Gold Exploration and Evaluation is designed primarily for professional geologists and engineers engaged in gold exploration-evaluation exercises, and as a text for undergraduate and graduate students in higher schools of learning. Since much of the treatment is empirical, it should also provide useful reference material for prospectors and small-scale miners. Presentation of the text commences with a brief description of the nature of gold and of those properties that make it unique amongst all other minerals. The history of gold is reviewed from the earliest times recorded by man until the beginning of the 20th century when geologists were beginning to understand how the Earth's surface was formed and what causes it to change over time. New theories involving the metallogenic roles of orogenic-related volcanics and sedimentation within intracratonic basins were being examined. Geophysicists were investigating the structure of the Earth and its magnetic, electrical and magnetic properties. Classical geology, which could once tell only what happened in a past sequence of events but seldom how or why, was now finding answers in the new and revolutionary approach to mineral exploration, the global theory of plate tectonics. Resurgence of efforts seeking an understanding of universal processes involved with the origin and general structure of the Earth and its neighbours in space was stimulated by discoveries in planetary science. Explanations based upon plate tectonic theory are offered for the evolution and inter-relationship of oceanic and continental crust and the supercontinental cycle in which continental masses continually join together, drift apart and

xii

Preface

rejoin to form other and larger supercontinents. Many aspects of Earth's geology and history are discussed, including the opening and closing of ocean basins, origins of mountain ranges, geological structures, hydrothermal activity and gold distribution in volcanic and tectonic settings. All past and present processes relating to the modification and release of gold grains in the weathering environment are shown to be paragenetically related to both primary and secondary rock types of all ages, and to the local and regional geologic setting at the time of their formation. Of particular relevance is recognition of a fundamental rock cycle in which all types of gold ores are seen as integrated parts of a process in which the rocks are related to one another and can be transformed one to the other. Occurrences of detrital gold, saprolitic and lateritic-hosted gold ores are integral parts of a weathering cycle in which the global energy balance is largely dependent upon the influence of the Earth's oceanic-atmospheric heat circulation. The effects of climatic change and extremes of change on stresses that produce weathering and erosion have all played significant roles in the formation, sorting and deposition of secondary gold accumulations. Deep-seated lateritic weathering became a topic of great interest to geochemists as well as to geologists and new fields of gold exploration were opened up towards the close of the 19th century. The long-term weathering of regolith has delineated geochemical provinces and defined chalcophile corridors of strategic importance to explorers. The basic concepts of viscosity and boundary layer flows are examined as they relate to sediment characteristics of size, shape and density and to sedimentation in natural stream channels, and expressions are developed for computing the relative magnitude of forces acting in solids/fluid flow. Examination of the complex relationships of channel geometry leads naturally to consideration of erosion, transport, sorting and deposition in the development of alluvial gold concentrations. As a function of topography, sedimentation is both space and time-related as demonstrated in the formulation of models, which establish the relationships between accumulations of heavy minerals, the dynamic conditions of transport and direction of sediment transport. Ground geological surveys provide essential data for investigating the nature and extent of surface and sub-surface ore horizons; both for open-cast mine planning and treatment and for the exploration and development of any primary orebodies that might be revealed as a result of the surface operations. Changes in exploration methods have resulted from new theories of paragenetic relationships and the development of remote sensing and geophysical techniques for regional and sub-surface examination. From identification of the principal sedimentary controls on the distribution of pay zones in some Alaskan gold placers, the course is charted for further exploration of gold deposits in other areas of Quaternary and pre-Quaternary glaciations. Geophysical surveys in shallow offshore areas are typical of those used to investigate the extent of

Preface

xiii

seabed features such as those that contain concentrations of gold in drowned beach sand deposits at Nome, Alaska. A statement of the laws of sampling is followed by a brief description of the essential features of bench scale testwork required by mineral-processing engineers to write down all of the quantitative data needed for the design of each plant component. These data include all essential measurements of the gravel, sand and clay contents of a particular alluvial-type ore and the processing characteristics and percentages of gold grains in the different ore types involved. The vexed question of sample reliability and representivity is examined in relation to the methods employed in the computation of ore resource quantities. Space is devoted to some of the many geological and geographic constraints that may be encountered in the field. Mine planning deals with various principles and factors involved in the choice between wet and dry systems of surface mining, development access, stripping and production requirements and environmental protection. Elements of the design and operation of selected methods are discussed in sufficient detail to make intelligent judgements of the most suitable types for any given case. In land-based operations the nature of surface features, topography and ground water, and the dimensions and location of pay horizons are of particular relevance. Offshore dredging is concerned additionally with the need to compensate for variable wind, wave and current conditions. Determining the most effective recovery methods for specific ore types during the mine-planning phase enables gold-processing plant...