Ore Deposit Geology AND ITS INFLUENCE ON MINERAL EXPLORATION

To ourfamilies

Veronica, Rachael, Faith, Anna and Rebecca

Lesley, Caroline and Rosalind

VVho had to put up with preoccupied husbands andfathersfor three years

Ore Deposit Geology AND ITS INFLUENCE ON MINERAL EXPLORATION

Richard Edwards and Keith Atkinson Camhorne School ~rMines, Redruth, Cornwall

LONDON NEW YORK

CHAPMAN AND HALL

First pllhlished ill 1986 h)' Chapmall alld Hall Ltd

II ,\Iell l Fetter Lalle, London EC4P .:lEE Pllhlisiled ill the USA h)' Chapman (Iml Hall 29 West 35th Street, ,\Iou York ,\IY1O()() I

© 1986 R. P. Edwards mid K. Atkinson Softcover reprint of'tile hardcover 1st edition 1986

IS13N-13: 978-94-011-8058-0

This titk is available in both hardbound and paperback editions. The paperback edition is sold subject to the condition that it shall not, by

way of trade or otherwise, be lent, resold, hired out, or otherwise circulated without the publisher's prior consent in any form of binding or cover other than that in which it is published and without a similar condition including this condition being imposed on the subsequent

purchaser. All rights reserved. No part of this book may be reprinted, or

reproduced or utilized in any form or by any electronic, mechanical or other means, now known or hereafter invented, including

photocopying and recording, or in any information storage and retrieval system, without permission in writing from the publisher.

l3ritish Library Cataloguing in Publication Data

Edwards, Richard Ore deposit geology and its influence on mineral exploration. 1. Ore-deposits I. Title II. Atkinson, Keith 553 TN263

TSBN-13: 978-94-011-8058-0 e-TSBN-13: 978-94-011-8056-6 DOT: 10.1007/978-94-011-8056-6

Library of Congress Cataloging in Publication Data

Edwards, Richard, 1<J..j.()-

Ore deposit geology and its influence on mineral exploration. Includes bibliographies and index. 1. Ore-deposits. 2. Geology. 3. Prospectint~.

I. Atkinson Keith, 1942- II. Title. TN265.E33 19S5 553'.1 S5-11713 ISBN-13: 978-94-011-8058-0

Contents

ACKNOWLEDGEMENTS

PREFACE

GLOSSARY

INTRODUCTION

1.1 Objectives and reasons for the approach taken 1.2 Mineral deposit or mine? 1.3 A genetic model as the basis for exploration 1.4 The scientific study of mineral deposits

References

2 MAGMATIC DEPOSITS

2.1 2.2 2.3 2.4 ') -_.J

Introduction Chromite deposits Nickel sulphide deposits Kimberlites Concluding statement References

3 MAGMA TIC HYDROTHERMAL DEPOSITS

3.1 Introduction 3.2 Porphyry copper deposits 3.3 Exploration for porphyry copper deposits

page IX

XI

Xli

4 12 15 17

18

18 18 37 54 63 65

69

69 69 87

Contents

3.4 Porphyry molybdenum deposits 3.S Exploration for porphyry molybdenum deposits 3.6 Porphyry gold deposits 3.7 Porphyry tin deposits 3. H Volcanic-associated massive sulphide deposits 3.9 Exploration for volcanogenic sulphide deposits 3.10 Concluding statement

References

-I- HYDROTHERMAL VEIN DEPOSITS

S

6

4.1 Introduction 4.2 Classification of hydrothermal vein deposits 4.3 Classification of hydrothermal gold deposits 4.4 Hydrothermal gold deposits in Archaean terrain 4.5 Exploration for gold in Archaean terrain 4.6 Concluding statement

References

PLACERS AND PALAEO-PLACERS

S.l - ') J._

S.3 5.4 5.5 5.6 5.7

Introduction Placer deposits Eluvial (residual), colluvial and fluvial (alluvial) deposits Beach sand deposits Marine placers Palaeo-placer deposits Concluding statement References

SEDIMENT-HOSTED COPPER-LEAD-ZINC DEPOSITS

6.1 Introduction 6.2 Sediment-hosted copper deposits 6.3 Syngenetic and diagenetic lead-zinc deposits in shales and

carbonates (sedimentary-exhalative deposits) 6.4 Epigenetic carbonate-hosted lead-zinc deposits (Mississippi

Valley-type) 6.5 Exploration for Mississippi Valley-type deposits 6.6 Concluding statement

References

7 ORE DEPOSITS FORMED BY WEATHERING

7.1 Introduction 7.2 Bauxite deposits

VI

92 104 106 106 107 128 137 138

143

143 144 146 146 164 169 170

175

175 175 181 188 197 201 211 212

21S

215 216

230

259 266 267 269

274

274 276

7.3 Lateritic nickel deposits 7.4 Kaolin deposits

Contents

7.3 Supergene manganese deposits 7.6 Supergene sulphide enrichment 7.7 Concluding statement

References

IRON ORES OF SEDIMENTARY AffILIATION

291 298 306 308 311 311

314

8.1 Introduction 314 8.2 Classification of iron ores 313 8.3 General characteristics of iron-formation 317 8.4 Genesis of iron-formation 324 8.5 Enriched haematite ore deposits 325 8.6 The Hamersley Basin - an example of banded iron-formation and

associated enrichment ores 326 8.7 Exploration 331 8.8 Evaluation 332 8.9 Concluding statement 333

References 334

9 URANIUM ORES OF SEDIMENTARY AffILIATION 337

9.1 Introduction 337 9.2 Geochemistry of uranium in the secondary environment 339 9.3 Unconformity-type uranium deposits of the Northern Territory,

Australia and Northern Saskatchewan, Canada 341 9.4 Sandstone-hosted uranium deposits of the western USA 353 9.5 Concluding statement 365

References 366

10 ORES FORMED BY MET AMORPHISM

10.1 10.2 10.3 10.4 10.3 10.6 10.7

Introduction Skarns Skarn deposits Classification of skarn deposits Genesis of skarn deposits Exploration for skarns Concluding statement References

11 THE DESIGN AND IMPLEMENTATION OF EXPLORATION PROGRAMMES

11.1 Introduction

Vll

370

370 371 372 373 392 394 396 398

400

400

Contents

11.2 Who undertakes exploration? 11.3 Factors affecting exploration programmes 11.4 The exploration programme 11.5 Concluding statement

References

APPENDIX

Mineral list

INDEX

Vlll

402 403 407 422 424

427

429

445

Acknow ledgements

The wntmg of this textbook would not have been possible without the help of individuals and organizations in both industry and education.

In particular we wish to thank the Governors of the Camborne School of Mines and the following organizations for financial assistance towards the cost of travel: the Institution of Mining and Metallurgy, the British Council, BP Minerals International, Consolidated Gold Fields PLC, Gencor (UK) Ltd and RTZ Services Ltd. We also wish to acknowledge a contribution by the Minerals Industry Research Organisation towards the cost of typing.

We are indebted to the following individuals for their valuable criticisms of individual chapters:

Dr]. D. Ridge DrC A. Lee Mr A. M. Killick Dr P. H. Nixon Dr P. W. Gregory Dr D. I. Groves Dr R. P. Foster Professor D. A. Pretorius Professor K. F. G. Hosking Professor M. J. Russell Mr C A. J. Towsey Dr S. Patterson Mr R. H. Parker Dr R. C Morris Mrs A. M. Giblin Dr Graham Closs

Acknowle~\!,ement5

We also wish to thank the many geologists who have given their time during mine visits and responded to our queries. We have benefited from the stimulus and advice of our colleagues.

Weare grateful to the students who read the manuscript and suggested those words which appear in the glossary. We would like to acknowledge the extensive help we have received from the Library Staff at the Cambornc School of Mines.

Finally we express our appreciation to Mrs Joan Edwards and Mrs Anne Taylor for their excellent typing.

Figures and tables from the journal Economic Geology have been reproduced with permission of the Economic Geology Publishing Company, USA.

x

Preface

Why another book about Ore Deposits? There are a number of factors which motivated us to write this text and which may provide an answer to this question. Firstly our colleagues are predominantly mining engineers and minerals processing technologists, which provides us with a different perspective of ore deposits from many academic geologists. Secondly we have found that most existing texts are either highly theoretical or merely descriptive: we have attempted to examine the practical implications of the geological setting and genetic models of particular ore deposit types. We have written the text primarily for undergraduates who are taking options in Economic Geology towards the end of a Degree Course in Geology. However, we hope that the text will also prove valuable to geologists working in the mining industry.

The text is to a large extent based on a review of the existing literature up to the end of 1984. However, we have visited most of the mining districts cited in the text and have also corresponded extensively with geologists to extend our knowledge beyond the published literature. Nonetheless writing a text-book on Ore Deposits is a demanding task and it is inevitable that sins of both omission and commission have been committed. We would therefore welcome comments from readers which can be incorporated in future editions.

RICHARD EDW ARDS KEITH ATKINSON

Cmnhome School (~n\1illcs April 1985

Adit

Area of influence

Astrobletne

Autolith

Bayer process

Blasthole (sampling)

Block caving

Boxwork (structure)

Glossary

A horizontal, or near horizontal, passage from the surface into a mme.

The zone around a borehole to which values of grade, thickness, etc., recorded for that borehole, may be applied.

An eroded remnant of a meteoritic or cometary impact crater.

An inclusion in an igneous rock to which it is genetically related.

Chemical conversion process involving the digestion of bauxite for the production of purified anhydrous alumina en route to aluminium.

Holes drilled during mining for blasting. Holes are usually shallow, closely spaced and uncased. Samples of the dust from these holes may be taken for evaluation and comparison with diamond drill hole exploration samples.

Large sections (blocks) of the orebody are successively undercut and allowed to cave above the undercut portion. Drawing off the caved ore causes further caving, the ore ca'::..es and crushes by its own weight, and weight of overburden, into pieces of suitable size for handling. Drawing continues until hanging wall material appears in the drawpoints.

A network of intersecting blades or plates oflimonite or other iron oxides, deposited in cavities and along fracture planes from which sulphides have been dissolved by processes associated with the oxidation and leaching of sulphide ores such as in porphyry copper deposits.

By-product metal

Caliche

Carbonatization

Chalcophile

China clay

Colloform (banding)

Decline

Doline

Duplex structure

Environmental Impact Statement (EIS)

Epeiric sea

Flotation (froth flotation)

Footwall

Framboidal

Glossary

A subsidiary metal worked from ore deposits that are dominantly characterized by other metals. In some cases, such as porphyry copper deposits, the income from by-products (for example, gold) can equal or exceed that from the major metal type (copper).

Strongly indurated crust of soluble calcium salts plus gravels etc. commonly found in layers on or near the surface of stony soils of arid and semiarid regions. Term broadly applied in the SW USA and South America.

A type of hydrothermal alteration which results in the formation of secondary carbonates in the host rock, commonly developed in intermediate to basic rocks. The main process is the addition of CO2•

Elements having a strong affinity for sulphur, characterized by the sul;>hide ore minerals.

A commercial product predominantly sold to the paper industry. China clay is a mixture of minerals dominated by kaolinite with minor quantities of mica and feldspar. The particle size is mainly less than 10 microns.

Rounded, finely banded kidney-like mineral texture formed by ultra-fine-grained rhythmic precipitation.

A downward inclined tunnel into a mine or potential ore body.

A funnel-shaped depression found in association with karst surfaces in limestone terrain.

A structural complex consisting of a roof thrust at the top and a floor thrust at the base, within which a suite of more steeply dipping imbricate thrust faults thicken and shorten the intervening panel of rock.

Statement prepared by mining companies which assesses the likely effects on the environment of a proposed mining venture. Such statements are legal requirements in certain countries such as the USA.

A shallow sea located within a continental landmass.

A selective process used to separate minerals from complex ores. It uses the differences in physico-chemical surface properties of minerals. After treatment with reagents fine particles of the desired mineral species attach to air bubbles and are separated as the froth, or float, fraction. (Froth flotation.)

The surface of rock beneath an orebody (compare hanging wall).

A textural term applied to sub-rounded pyrite, literally raspberry shaped. The texture is normally developed in carbonaceous pelitic sediments.

X1l1

Fugacity

Garnetite

Gondite

Gossan

Green strength

Greisen

Hanging wall

H ydrofracturing

Iapetus

Inductive-coupling effect

Klippen

Kniest facies

Labile

Ligand

Glossary

A thermodynamic function used when dealing with gas pressure and defined by the equation

FJ - F; = RT InfJ/~

where FJ and F{ are partial molar energies andfJ andf{ are the fugacities of the one substance at two different concentrations. Fugacity may be considered in qualitative terms as an idealized vapour pressure and is equal to the vapour pressure when the vapour behaves as a perfect gas.

A metamorphic rock consisting chiefly of an aggregate of interlocking garnet grains.

A metamorphic rock consisting of spessartine and quartz, probably derived from manganese-bearing sediment.

A Cornish term which describes the iron-oxide-rich zone which lies above a sulphide-bearing ore deposit.

The mechanical strength of ceramic ware after it has been shaped but before it has been dried and fired.

A light-coloured, coarse-grained rock dominated by quartz and white mica with accessory fluorite and topaz. It is normally formed by the hydrothermal alteration of granite but pelitic sediments may also be altered to greisen.

The surface of rock above an ore body (compare footwall).

Fracturing of rock by water or other fluid under pressure. The effect is to increase permeability. Although this occurs naturally, as in the genesis of porphyry deposits, it is also undertaken artificially to increase permeability in oil, gas and geothermal reservoirs.

An ocean which occupied the site of the present North Atlantic and which was at its widest extent in Lower Cambrian times.

The coupling of two bodies by a common magnetic field.

A nappe or pile of nappes detached by erosion or gravity gliding from the parental mass.

A facies developed in the footwall of the Rammelsberg ore deposit, FOR. The facies results from the localized silicification of shale horizons and is believed to be late syngenetic or early diagenetic in age.

A term applied to minerals (such as feldspars) that are easily decomposed.

Anions or molecules which contain at least one unshared pair of electrons. Ligands may combine with a transition metal to form a complex.

XIV

Lithophile

Photosynthesis

Polygons method

Proppants

Protolith

Pseudo section

Pyritization

Raise

Ripping (rippability)

Saprolite

Sericitization

SIROTEM

Stockwork

Glossary

Elements having a strong affmity for oxygen, concentrated in the silicate minerals.

A process in which the energy of sunlight is used by green plants to build up complex substances from carbon dioxide and water.

A method used in reserve estimation where the area ofinfluence around a borehole extends halfway to the next borehole. The value for the borehole is applied to the resulting polygon.

Materials used in hydraulic fracturing of deep rocks to increase porosity and therefore improve recovery of petroleum and gas. Propp ants made of fused abrasive-grade bauxite have been found to be more effective in this use than such materials as quartz sand, plastic balls, steel shot or ground walnut shells.

The unmetamorphosed, or parent, rock from which a given metamorphic rock was formed.

A display of resistivity or induced polarization data where the values are assigned to the intersection points of 45° lines drawn from the mid-points of the current and potential electrode pairs. The result is a vertical geophysical 'section' below a traverse. Depths in this section bear no simple relationship to the true geological section.

A process of hydrothermal alteration, normally developed in iron-rich host rocks such as basalts, which involves the crystallization of pyrite.

A shaft excavated upwards for connecting levels in a mine or ore body.

The removal of overburden, soft or fractured rock, using mechanical excavators without recourse to drilling and blasting. Rippability is an index which measures if this is possible.

That part of the weathering profile which remains ill situ and retains original structures of the parent rock such as jointing.

A hydrothermal alteration process which results in the development of potash mica, generally sericite, as a result of the hydrolysis of feldspar. Chemically it involves the addition of KeO and HeO and usually some removal of SiO" FeO and CaO.

A large loop, time-domain, fixed-transmitter, electromagnetic (EM) geophysical system. Designed in Australia by CSIRO (hence name).

Mineral deposit composed of a three-dimensional network of veinlets usually oflarge enough scale, and so closely spaced, so that the whole mass can be mined.

xv

Strategic mineral

Stylolite

Talus

UTEM

Wehrlite

Wire-line (drilling)

Glossary

A mineral considered essential to the military and industrial strength of a nation but which is oflimited availability.

An irregular suture-like boundary developed in some limestones which is generally independent of the bedding planes. Stylolites are considered to be the result of pressure solution followed by immediate local deposition.

An accumulation of rock fragments, usually coarse and angular, derived from the mechanical weathering of rocks and lying at the base of a cliff or steep rocky slope. (Synonym of 'scree'.)

A large loop, time-domain, fixed-transmitter electromagnetic (EM) geophysical system. The name UTEM is the registered trademark of Lamontagne Geophysics Ltd.

An ultramafic and ultrabasic igneous rock containing essential augite and olivine with accessory plagioclase and ortho­pyroxene.

A diamond drilling technique in which the inner tube of the core barrel is attached to a wire rope which passes within the drill rods. This enables the sections of core to be retrieved without withdrawing the entire string of drill rods each time.

XVI