ORE DEPOSITS AND MANTLE PLUMES

Ore Deposits and Mantle Plumes

by

Franeo Pirajno Geological Survey ofWestern Australia, Perth, Australia

SPRINGER-SCIENCE+BUSINESS MEDIA, B.V.

Library of Congress Cataloging-in-Publication Data

ISBN 978-90-481-4026-8 ISBN 978-94-017-2502-6 (eBook) DOI 10.1007/978-94-017-2502-6

Cover illustration: The image on the cover is a modified version of that shown in Figure 5.7B in the text and represents a computer simulation of mantie processes; hot regions are red and cold regions are blue and green. The hot mantie material rises and uplifts the surface of the planet. The rising hot mantie simulates a mantie plume. This convection simulation model was created by Walter Kiefer Lunar and Planetary Institute, Houston, USA) and Louise Kellogg (University of California, Davis, USA) for the planet Mars, but is not specific to Mars, and is applicable to mantie plumes that occur on Earth. The element symbols represent those that may be directiy or indirectly linked to mantie plume activities on Earth, resulting in anomalous concentrations of these elements in the crust where they form ore deposits. The image is used by permission of the authors.

Printed on acid-free paper

Ali Rights Reserved © 2000 F. Pirajno Originally published by Kluwer Academic Publishers in 2000 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner.

To Mariateresa, my wife

CONTENTS

PREFACE XI

ACKNOWLEDGEMENTS XIII

INTRODUCTION XVII

PARTONE

CHAPTER 1 The Earth's Interna! Structure and Convection in the Mantle 1

1.1 Introduction 1 1.2 Early planetary evolution 2 1.3 The Earth's interna1 structure 5 1.3.1. The crust 7 1.3.2. The mantle 11 1.3.3. The core-mantle boundary (CMB) and D" 1ayer 20 1.3.4. The core 25 1.4 Convection in the mantle; theories and models 27 1.4.1. Theories and dynamics of convection 29 1.4.2. Physical parameters ofmantle convection 31 1.4.3. Whole mantle and two-layers mantle convection models 32 1.5 Mantle geochemistry 41 1.6 Mantle evolution through time and implications for Earth's

h~~cy % 1. 7 Concluding remarks 53 1.8 References

CHAPTER 2 Mantle Plumes and Superplumes; Contineotal Breakups, Supercontinent Cycles and Ore Deposits 59

2.1 Introduction 59 2.2 Hotspots: distribution and relationship to rifting 61 2.3 Labaratory modelling, structure and dynamics of mantle plumes 65 2.4 Doming ofthe crust (hotspot swells) and associated

topographic and drainage features 71

VIII Contents

2.5 Mantle plume-lithosphere interactions and plume-generated melts 77

2.5.1. Crustal stress regimes in response to mantle plumes 85 2.6 Superplumes and continental breakup 86 2.6.1. Gondwana and Rodinia breakups, mantle plumes or plate

forces? 90 2.6.2. Supercontinent cycles and ore deposits 94 2.7 The "other side" ofthe mantle plume theory 100 2.8 Concluding remarks 104 2.9 References 105

CHAPTER 3 Oceanic Islands, Large lgneous Provinces, Mafic Dyke Swarms, and Intracontinental Alkaline Magmatism 111

3.1 Introduction 111 3.2 Oceanic volcanic islands 112 3.2.1. The Hawaiian-Emperor seamounts chain 116 3.2.2. Marquesas Islands 119 3.2.3. Walvis Ridge and Tristan da Cunha 119 3.2.4. leeland 124 3.2.5. Reunion Island 127 3.2.6. Geochemical and isotopic characteristics of oceanic volcanic

island basalts 128 3.3 Large igneous provinces (LIP): oceanic p1ateaux and continenta1

flood basalts (CFB) 135 3.3.1. lntroduction 135 3.3.2. Isotope systematics 139 3.3.3. Oceanic plateaux 140 3.3.4. Volcanic-rifted continental margins 149 3.3.5. Continental flood basalts (CFB) 151 3.4 Mafic dyke swarms 183 3.4.1. Mafic dyke swarms in the Kaapvaal Craton, South Africa 189 3.4.2. The Mackenzie dyke swarm, Canada 190 3.4.3. Parami-Etendeka dykes 192 3.5 Intracontinental alkaline magmatism 193 3.5.1. Teetonic settings, ages and controls of intracontinental

alkaline magmatism in Africa 194 3.5.2. The Damaraland alkaline province, Namibia 198 3.5.3. Carbonatites 199 3.5.4. Kimberlites and lamproites 200 3.6 Concluding remarks 201 3.7 References 202

CHAPTER 4 Rifting Proeesses, Volcano-Sedimentary Basins and the Role ofMantle Plumes 215

4.1 Introduetion 215 4.2 Rifting dynamies: passive and aetive 220 4.2.1. Passive rifting 221 4.2.2. Aetive rifting 222 4.3 Rifting and basie formation related to eompression in

thiekened erust 223 4.4 Geophysieal signatures of rifts 227 4.5 Stratigraphie sueeessions as reeords ofbasin evolution 230 4.5.1. The Stratigraphie reeord ofintraeontinental basins and

aulaeogens 230 4.6 The East Afriean Rift System and the Afar Triangle: examples

of modern eontinental rifting where mantle plume aetivity is reeognised 239

4.6.1. Introduetion 239 4.6.2. The East Afriean Rift System (EARS) 240 4. 7 Examples of aneient eontinental rifts where mantle plume

aetivity is assumed: Thuli-Sabi-Lebombo hotspotjunetion; Damara and Irumide hotspot junetions 244

4.7.1. Tuli-Sabi-Lebombo hotspotjunetion 244 4. 7 .2. Damara and Irumide hotspot junetions, southwestern Afriea 248 4.8 Sequenee stratigraphy, eustasy and mantle plumes 252 4.9 Concluding remarks 254 4.10 Referenees 256

CHAPTER 5 The P1anetary and Meteorite Impact Context of Mantle P1umes 261

5.1 Introduction 261 5.2 Moon 263 5.3 Mercury 265 5.4 Venus 265 5.5 Mars 269 5.6 Large meteorite impaets and possible eorrelations with mantle

plumes 274 5.6.1. Ore deposits and impaet structures 277 5.6.2. Can meteorite mega-impaets trigger eontinental breakup and

the ascent of mantle plumes? 279 5.7 Concluding remarks 285 5.8 Referenees 286

X Contents

PARTTWO

CHAPTER 6 Intracontinental Magmatism, Anorogenic Metamorphism, Ore Systems and Mantle Plumes 291

6.1 Introduction 291 6.2 Intracontinenta1layered igneous intrusions 291 6.3 Anorogenic prograde metamorphism and hydrothermal

convention in hotspot-related rift systems 299 6.3.1. Anorogenic metamorphism in the Central Zone ofthe

Damara Orogen, Namibia 301 6.3.2. Anorogenic metamorphism in the eastern Pyrenees 302 6.3.3. Anorogenic metamorphism and intraplate magmatism

around the V redefort Dome, South Africa 303 6.3.4. Metamorphism and fluid generation; metamorphogenic

hydrothermal systems 306 6.4 Concluding remarks 317 6. 5 References 317

CHAPTER 7 Direct Links; Magmatic Ore Deposits-Fundamental Features and Concepts 323

7.1 Introduction 323 7 .1.1. Definitions and terminology 323 7 .1.2. Geometry of layered intrusions and magmatic processes 331 7.2 Magmatic oxide ores 342 7 .2.1. Crystallisation of spine1s from mafic-ultramafic magmas 344 7.3 Magmatic sulphides and platinum group elements (PGE) ores 347 7.3.1. The formation ofNi sulphide ores 347 7.3.2. Platinum group elements (PGE) 355 7.4 Concluding remarks 378 7.5 References 380

CHAPTER 8 Magmatic Ore Deposits 387 8.1 lntroduction 387 8.2 Large layered igneous complexes 388 8.2.1. The Great Dyke, Zimbabwe 389 8.2.2. The Bushveld lgneous Complex, South Africa 401 8.2.3. Molopo Farms Complex, South Africa and Botswana 425 8.3 Magmatic ore deposits in igneous complexes associated with

continental flood basalts 426 8.3.1. Duluth Complex, Mid-continent Rift System, USA 426 8.3.2. Noril'sk-Talnakh, Siberian Traps, Russia 428

8.3.3. The Insizwa Complex, Karoo lgneous Province 433 8.3.4. Skaergaard and Kap Edvard Holm, East Greenland 438 8.4 magmatic ores in Proterozoic troctolite-anorthosite complexes 441 8.4.1. Voisey's Bay Ni-Cu-Co 442 8.5 Komatiite-related magmatic ore deposits 445 8.5.1. Komatiite volcanology 446 8.5.2. Komatiite mineralogy and whole rock geochemistry 447 8.5.3. Komatiite-hosted Fe-Ni-Cu sulphide ores 449 8.6 Hydrothermal Ni-cu and PGE mineralisation in ultra-

ultramafic rocks 453 8. 7 Concluding remarks 458 8.8 References 459

CHAPTER 9 Indirect Links: Hydrothermal Mineral Deposits 469 9.1 Introduction 469 9.2.1. Ring complexes and carbonatites 471 9.2.2. Proterozoic Cu-Au-U-REE-Fe deposits 473 9.2.3. Mesothermal ore deposits 480 9.2.4. Carlin-type epithermal ore deposits 483 9.2 Ore deposits associated with intracontinental anorogenic

magmatism 471 9.3 Metallogeny ofthe Damara and Irumide orogens, Soutb-

western Africa, and the Mid Continent Rift System, USA 491 9.3.1. Metallogeny ofthe Damara and Irumide orogens 491 9.3.2. Metallogeny ofthe Mid-continent Rift System,

N orth America 497 9.4 Archaean 1ode Au deposits 498 9.5 Concluding remarks 502 9.6 References 504

CHAPTER 10 Indirect Links: Sedimentary Rock-Hosted Ore Deposits. Epilogue 509

10.1 Introduction 509 10.2 Metallogeny in modern rift settings 511 10.2.1. The East African Rift System 511 10.2.2. The Red Sea brine pools 516 10.3 Sedimentary-hydrothermal ore deposits 520 10.3.1. Mississippi Valley-type sulphide deposits 520 10.3.2. Sedimentary exhalative (SEDEX) massive sulphide deposits 523 10.3.3. Stratabound Cu-Ag and Cu-Co ore deposits 528 10.4 Metalliferous black shales 531

XII Contents

10.4.1. Mo-Ni-V-PGE-Au in black shales, southern China 533 10.5 Iron-formations and manganese deposits 534 10.6 Concluding remarks and epilogue 539 10.7 References 540

APPENDIX 547

INDEX 549

PREFACE

PERTH Western Australia

March 2000

Increasingly explorationists are seeking to find new ore deposits in poorly prospected areas, be they geographically remote, such as in the Arctic, or geologically remote, such as those under sedimentary cover. Modern prospecting techniques, including low-detection-level geochemistry and the use of advanced geophysical instrumentation have greatly assisted explorers but fundamental to any soundly based exploration program remains an understanding of the geological framework of ore deposits. This allows the development of deposit models on macroscopic and mesoscopic scales. This book by Dr. Franeo Pirajno draws on his extensive and wide global experience. To set the scene for a discussion of ore deposit generation Franeo details the Earths internal structures and mantle dynamics. He then explores the impact of mantle plumes on the crust and in particular their role in the production of magmatic environments, and in continental scale rifting. This includes a descriptive section on magmatic provinces around the globe, which highlights the importance of plumes.

Any study of Earth processes needs to take into account the effects of extraterrestrial bombardment, and in particular the results from the impacts of large bolides. The effects of these impacts on the atmosphere and on life have now been recognised as profound. It is likely that the effect ofthese impacts on the Earth's crust is as equally profound. It is inter­esting to speculate as Franeo has, on just what these effects are likely to be, but with the energy transfers involved the stability of the crust is likely to be compromised exposing the mantle to modification. The close correlation between the accumulations of ore-forming minerals and impact sites is not always obvious but by examining the wider causal effects of large bolide impacts, as Franeo has, this linkage becomes readily apparent.

There are strong and obvious direct links between mantle plumes and mafic magmatism and these direct links are detailed followed by descrip­tions of associated orthomagmatic ore deposits.

XIV Contents

The less obvious associations between mantle plumes and hydrothermal deposits are described utilising the linkage of rifting and intracontinental magmatism.

Finally Franeo establishes indirect links with sediment-hosted ore deposits, such as SEDEX type, MVT, black shale-hosted and BIF's, and mantle plumes in a wide ranging and thought provoking analysis.

This book integrates the processes and features of mantle plumes, magmatic processes, rifting and ore deposition. It provides the modern explorationist with an opportunity to place their own field Observations into a context involving dynamic earth processes, and will assist this explorationist in ore model development and the subsequent generation of new targets.

David Blight1

Director Geological Survey of Western Australia

1. Present address: Executive Director Minerals and Energy Resources Dept. of Primary Irrdustries and Resources, Adelaide, South Australia

Acknowledgments

I am indebted to several people, friends and colleagues, particularly at the Geological Survey of Western Australia. I owe a debt of gratitude to Drs. Pietro Guj and David Blight for their encouragement. Dr. Tim J. Griffin has been an understanding manager, whose "behind the scenes" support is greatly appreciated. Peter Carroll manager of the CAD team, Dellys Sutton, Arthur Hoffman, Tony Veale and Sirnon Urbini draughted the figures of this book "in-between" the pressing demands of everyday's work. To them all, I owe a special debt of gratitude for their efforts and patience. The contribution ofthe Geological Survey ofWestern Australia for covering the cost ofthe colour figures is gratefully acknowledged.

Roger Hocking undertook the unenviable task of reading and editing most of the manuscript, during holidays, suburban train tripsandlunch breaks. Roger, who learned something about mantle plumes that he never intended to, also contributed constructive criticism, as did Bruce Groenawald and Andrew Glikson.

My colleagues at the Geological Survey have helped with countless discus­sions in the field and in the office; their enthusiasm and dedication are inspiring. Thanks are due to Nell Stoyanoff and Elizabeth Slater, who braved my handwriting and done much work on the text, especially tables. Brian Knyn, Eunice Cheung and Robert Cross in the library are thanked for their friendly assistance and for managing to get even the most difficult references in record time.

My wife, to whom this book is dedicated, has been a source of strength, always unwavering in her support, not only during the writing of this book, but throughout the many years of my professional career, punctuated by long absences in the field. Words cannot express my gratitude.

Australia, my adaptive country, provided an ideal setting.

INTRODUCTION

This book is about the control and consequences that processes in the Earth's mantle have on mineralising events in the crust. In developing this general topic, I focus on the theory of mantle plumes and the role that they have in generating, directly or indirectly, ore systems. Furthermore, I take a uniformitarian approach by assuming that plate tectonics and mantle plumes have been operative at least since Late Archaean times (e.g. Wyman and Hollings, 1998). This assumption is certainly not shared by all geoscientists (e.g. Hamilton, 1998), and even those who advocate plate tectonics in the Archaean, do not necessarily concur with the idea that mantle plumes existed then (e.g. de Witt, 1998). In fact, some geoscientists completely reject the notion (e.g. Sheth, 1999). Nevertheless, geological evidence embodied in the occurrence of giant dyke swarms, layered mafic­ultramafic intrusive complexes and flood basalts, at least since 2700 Ma, perhaps even 3.0 Ga (Tomlison et al. 1998), and the presence of ancient and modern oceanic plateaux, shield volcanoes, continental uplifts and rifting associated with flood basalts, intracontinental alkaline magmatism and positive gravity anomalies, imply the existence of convective flow in the mantle, upper or lower or both, as we shall discuss in Part One of this book. In addition, tomographic imaging confirms that flow of hot material in the mantle does take place, and more importantly, right where we expect it to be, namely, below regions of uplift and hotspot volcanism. Each year, for the past 20 years, a great number of papers are published in which reference is made to mantle plumes. In the last 10 years, most papers dealing with mantle plumes focussed on isotope systematics to track or monitor tectonic and magmatic activities, interpreted to be caused by mantle plumes.

Important contributions to our knowledge of the Earths' mantle, its composition, dynamics and evolution through geological time include Jacobs (1992), Jackson (1998), Davies (1999) and Orovetskii (1999). These authors are the recognised authorities in their field and the interested reader should consult these books for details on the Earth's mantle, its convective phenomena and the theory of mantle plumes.

My credentials in the writing of this book are a very long experience in

XVIII Introduction

ore deposits in many parts of the world, complete innocence on mantle plumes and a life-long fascination with volcanoes, due to my growing up in the shade of Mount Vesuvius. However, in spite of this innocence and for the reasons given above, I may be forgiven if the plume theory is central to the context of this book. I contend that the theory offers a neat explanation for the vast amounts of melts that are generated in the mantle, in settings other than convergent plate margins.

The generation of these melts has important implications for both magmatic and hydrothermal ore deposits. A direct link between mantle plumes and mineralisation can be found in mafic-ultramafic magmas (e.g. Ni-Cu sulphides associated with flood basalts, such as Noril'sk) and indir­ectly in the high geothermal gradients that are set up in the crust above the plume (e.g. mesothermallodes, such as those of Pilgrim's Rest in South Africa).

This book consists of two parts. Part One reviews the Earth's internal structure (Chapter 1) and the theory of mantle plumes, superplumes, laboratory models, plumes' effects on the movement of tectonic plates, continental aggregation and breakups, and ends with an incursion into the ideas of geoscientists who do not agree with proponents ofthe plume theory (Chapter 2). Chapter 3 examines the surface manifestations of plume activ­ities, such as oceanic islands and oceanic plateaux, continental flood basalts, intracontinental igneous complexes and giant dyke swarms. Rifting processes and the inception ofvolcano-sedimentary basins and their Strati­graphie successions are treated in Chapter 4. In a short extraterrestrial trip andin view of our common parentage with our planetary companions in the Solar System, in Chapter 5 I discuss the surface manifestations ofuplift and volcanism in terrestrial planets, in terms ofthe plume model. The "spectre" of meteorite or cometary impacts is raised as a possible mechanism for mantle melting and other events of catastrophic nature. In the absence of concrete evidence, the idea of mantle plumes being triggered by mega­impacts is indeed speculative, but nevertheless thought-provoking. One has only to pause and think that impacts have been, and perhaps still are, a major planetary geological process. The evidence was dramatically shown by the Shoemaker-Levi cometary impacts on Jupiter.

In Part Two of this book, I propose that links, direct and indirect, exist between mantle plume events and ore systems. In Chapter 6, I begin with intracontinental magmatism and the effects of high geothermal gradients in regions of the crust. These result in prograde high temperature metamorphism not related to orogenic events. I have called this "anoro­genic metamorphism". The large layered intrusions of the world, with the Bushveld Complex being the undisputed sovereign, are perhaps some of

Introduction XIX

the best expression of mantle plumes; for this reason I decided to devote a fuil chapter (Chapter 7) to magmatic processes of layered intrusions and their association with oxide and sulphide ores. Chapter 8 looks at details of the geology and mineralisation of selected layered intrusions, such as the Bushveld Complex and the Great Dyke, as weil as other types of mafic-ultramafic complexes, such as komatiites and anorthosite-troctolites.

Indirect links between mantle plumes and mineralisation are considered in Chapters 9 and 10. I begin with magmatic-hydrothermal and hydro­thermal ore deposits that are associated with anorogenic complexes that are possibly distal expressions of mantle plumes. I also thought it useful to look at the metallogeny that accompanies some of the ancient great rift systems, for which there is reasonable evidence to show that they were formedas a result of mantle plumes impacting the lithosphere. For these I have selected the Damara and Irumide orogens of southwestern Africa. First because of my first-hand knowledge of them and second, because of the intriguing hypothesis put forward by Borg (1988), in which he proposed that these rift systems result from the movement of the African plate over a hotspot.

The long journey ends with Chapter 10, where I take a look at sedimentary-hydrothermal deposits (SEDEX, MVT, stratabound) as weil as metalliferous black shales and banded iron-formations, the major iron resource of the world, all of which owe their presence to the effects of tectonism (e.g. rifting) and continental-scale magmatism (e.g. flood basalts) that may ultimately be related to mantle plumes.

References

Borg, G. (1988) The Koras-Sinclair-Ghanzi rift in southern Africa. Volcanism, Sedimentation, age relationships and geophysical signature of a late-middle Proterozoic rift system. Precambrian Research, 38: 75-90.

Davies, G. F. (1999) Plates, plumes and mantle convection. Cambridge University Press. de Witt, M. J. (1998) On Archean granites, greenstones, cratons and tectonics: does the

evidence demand a verdict? Precambrian Research, 91: 181-226. Hamilton, W. B. (1998) Archean tectonics and magmatism. International Geology Review,

40: 1-39. Jackson, 1., (ed) (1998) The Earth's mantle- Composition, structure and evolution.

Cambridge University Press. Jacobs, J. A., (1992) Deep interior ofthe Earth. Chapman Hall, London. Orovetskii, Yu. P. (1999) Mantle plumes. A. A. Balkema. Sheth, H. C. (1999) Flood basalts and !arge igneous provinces from deep mantle plumes: fact,

fiction, and fallacy. Tectonphysics, 311: 1-29. Tomlison, K. Y., Stevenson, R. K., Hughes, D. J., Hall, R. P., Thurston, P. C. and Henry, P.

(1998) The Red Lakegreenstone belt, Superior Province: evidence of plume-related magmatism at 3 Ga and evidence of an older enriched source. Precambrian Research, 89:

XX Introduction

59-76. Wyman, D. and Hollings, P. (1998) Long-1ived mantle-p1ume influence on an Archean

protocontinent: geochemica1 evidence from the 3 Ga Lumby 1ake greenstone be1t, Ontario, Canada. Geo1ogy, 26: 719-722.