Ore Deposits and Mantle Plumes

by

Franco Pirajno Geological Survey of Western Australia, Perth, Australia

KLUWER ACADEMIC PUBLISHERS

DORDRECHT / BOSTON / LONDON CONTENTS PREFACE XI

ACKNOWLEDGEMENTS XIII

INTRODUCTION XVII

PART ONE

CHAPTER 1 The Earth's Internal Structure and Convection in the Mantle 1 . 1 Introduction 1 .2 Early planetary evolution 2 .3 The Earth's internal structure 5 .3.1. The crust 7 .3.2. The mantle 11 .3.3. The core-mantle boundary (CMB) and D" layer 20 .3.4. The core 25 .4 Convection in the mantle; theories and models 27 .4.1. Theories and dynamics of convection 29 .4.2. Physical parameters of mantle convection 31 .4.3. Whole mantle and two-layers mantle convection models 32 .5 Mantle geochemistry 41 .6 Mantle evolution through time and implications for Earth's history 46 .7 Concluding remarks 53 .8 References

CHAPTER 2 Mantle Plumes and Superplumes; Continental Breakups, Supercontinent Cycles and Ore Deposits 59 2.1 Introduction 59 2.2 Hotspots: distribution and relationship to rifting 61 2.3 Laboratory modelling, structure and dynamics of mantle plumes 65 2.4 Doming of the 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" of the mantle plume theory 100 2.8 Concluding remarks 104 2.9 References 105

CHAPTER 3 Oceanic Islands, Large Igneous 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. Iceland 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 plateaux and continental flood basalts (CFB) " 135 3.3.1. Introduction 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. Parana-Etendeka dykes 192 3.5 Intracontinental alkaline magmatism 193 3.5.1. Tectonic 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 Contents IX

CHAPTER 4 Rifting Processes, Volcano-Sedimentary Basins and the Role of Mantle Plumes 215 4.1 Introduction 215 4.2 Rifting dynamics: passive and active 220 4.2.1. Passive rifting 221 4.2.2. Active rifting 222 4.3 Rifting and basic formation related to compression in thickened crust 223 4.4 Geophysical signatures of rifts 227 4.5 Stratigraphic successions as records of basin evolution 230 4.5.1. The stratigraphic record of intracontinental basins and aulacogens 230 4.6 The East African Rift System and the Afar Triangle: examples of modern continental rifting where mantle plume activity is recognised 239 4.6.1. Introduction 239 4.6.2. The East African Rift System (EARS) 240 4.7 Examples of ancient continental rifts where mantle plume activity is assumed: Thuli-Sabi-Lebombo hotspot junction; Damara and Irumide hotspot junctions 244 4.7.1. Tuli-Sabi-Lebombo hotspot junction 244 4.7.2. Damara and Irumide hotspot junctions, southwestern Africa 248 4.8 Sequence stratigraphy, eustasy and mantle plumes 252 4.9 Concluding remarks 254 4.10 References 256

CHAPTER 5 The Planetary and Meteorite Impact Context of Mantle Plumes 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 impacts and possible correlations with mantle plumes 274 5.6.1. Ore deposits and impact structures 277 5.6.2. Can meteorite mega-impacts trigger continental breakup and the ascent of mantle plumes? 279 5.7 Concluding remarks 285 5.8 References . 286 X Contents

PART TWO

CHAPTER 6 Intracontinental Magmatism, Anorogenic Metamorphism, Ore Systems and Mantle Plumes 291 6.1 Introduction 291 6.2 Intracontinental layered 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 of the Damara Orogen, Namibia 301 6.3.2. Anorogenic metamorphism in the eastern Pyrenees 302 6.3.3. Anorogenic metamorphism and intraplate magmatism around the Vredefort 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 spinels from mafic-ultramafic magmas 344 7.3 Magmatic sulphides and platinum group elements (PGE) ores 347 7.3.1. The formation of Ni 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 Introduction 387 8.2 Large layered igneous complexes 388 8.2.1. The Great Dyke, Zimbabwe . .. 389 8.2.2. The Bushveld Igneous 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 Contents XI

8.3.3. The Insizwa Complex, Karoo Igneous 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 of the Damara and Irumide orogens, South- western Africa, and the Mid Continent Rift System, USA 491 9.3.1. Metallogeny of the Damara and Irumide orogens 491 9.3.2. Metallogeny of the Mid-continent Rift System, North America 497 9.4 Archaean lode 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