Introduction to Ore-Forming Processes

LAURENCE ROBB is Visiting Professor in Economic Geology at the Department of Earth Sciences, University of Oxford. He continues to work on the metallogeny of the mineral districts of the African continent and also in SE Asia. He served a term as President of the Society of Economic Geologists in 2017.

• Preface to the 2nd Edition xiiiPreface to the 1st Edition xvIntroduction: Mineral Resources xviiPart I Igneous Processes 11 Igneous Ore-Forming Processes 31.1 Introduction 41.2 Magmas and Metallogeny 41.2.1 Crustal Architecture and Mineral Wealth 41.2.2 Magma Types and Metal Contents 71.2.2.1 Basalt 71.2.2.2 Andesite 91.2.2.3 Rhyolite 101.2.2.4 Alkaline Magmas, Carbonatite and Kimberlite 121.3 Why Are Some Magmas More Fertile than Others? The “Inheritance Factor” 131.3.1 The “Late Veneer” Hypothesis of Siderophile Metal Concentration – An Extraterrestrial Origin for Au and Pt? 141.3.2 Diamonds and the Story They Tell 151.3.3 Metal Concentrations in Metasomatized Mantle and Their Transfer into the Crust 201.3.4 Metal Enrichment in Carbonatitic and Peralkaline Magmas 211.3.5 I- and S-Type Granite Magmas and Metal Specificity 271.4 Partial Melting and Crystal Fractionation as Ore-Forming Processes 301.4.1 Partial Melting 311.4.1.1 Trace Element Distribution During Partial Melting 321.4.2 Crystallization of Magma 341.4.2.1 The Form and Internal Zonation of Igneous Bodies 361.4.2.2 Trace Element Distribution During Fractional Crystallization 391.4.3 Fractional Crystallization and the Formation of Monomineralic Chromitite Layers 431.4.3.1 The Irvine Model 431.4.3.2 Other Mechanisms for the Formation of Chromitite Layers or Pods 471.4.4 Filter Pressing as a Process of Crystal Fractionation 481.4.4.1 Anorthosite Hosted Ti–Fe Deposits 481.5 Liquid Immiscibility as an Ore-Forming Process 491.5.1 Silicate–Oxide Immiscibility 491.5.2 Silicate–Sulfide Immiscibility 501.6 A More Detailed Consideration of Mineralization Processes in Mafic Magmas 521.6.1 A Closer Look at Sulfide Solubility 521.6.2 Sulfide–Silicate Partition Coefficients 531.6.3 The R Factor and Concentration of Low Abundance Trace Elements 541.6.4 Factors that Promote Sulfide Saturation 561.6.4.1 Addition of Externally Derived Sulfur 561.6.4.2 Fractional Crystallization 561.6.4.3 Injection of a New Magma and Magma Mixing 581.6.4.4 Magma Contamination 681.6.5 Other Models for Mineralization in Layered Mafic Intrusions 691.6.5.1 PGE Clusters 691.6.5.2 The Role of Chromite in PGE Concentration 711.6.5.3 Hiatus Models 721.6.5.4 Fluid-Related Infiltration of PGE 721.7 A Model for Mineralization in Layered Mafic Intrusions 721.8 Summary 75Further Reading 752 Magmatic-Hydrothermal Ore-Forming Processes 772.1 Introduction 772.2 Some Physical and Chemical Properties of Water 782.3 Formation of a Magmatic Aqueous Phase 812.3.1 Magmatic Water – Where Does It Come from? 812.3.2 H2O Solubility in Silicate Magmas 832.3.3 The Burnham Model 852.3.3.1 A Note on the Mechanical Effects of Boiling 882.4 The Composition and Characteristics of Magmatic-Hydrothermal Solutions 882.4.1 Quartz Veins – What Do They Tell Us About Fluid Compositions? 882.4.2 Major Elements in Magmatic Aqueous Solutions 892.4.3 Other Important Components of Magmatic Aqueous Solutions 892.4.3.1 Magmatic Fluid Compositions from Fluid Inclusion Analysis 922.4.4 Carbon Dioxide in Magmatic Fluids 942.4.5 Other Important Features of Magmatic Fluids 952.5 A Note on Pegmatites and Their Significance to Granite-Related Ore-Forming Processes 972.5.1 Early Models of Pegmatite Genesis 982.5.2 More Recent Ideas on the Origin of Pegmatites 982.6 Fluid–Melt Trace Element Partitioning 1002.6.1 Early Experiments on Metal Solubilities in Aqueous Solution 1002.6.2 A More Detailed Look at Fluid–Melt Partitioning of Metals 1022.6.2.1 Fluid–Melt Partitioning During “First Boiling” 1032.6.2.2 Fluid–Melt Partitioning During “Second Boiling” 1032.6.2.3 Partitioning of Metals into H2O-Vapor 1042.6.3 Partitioning of Cu, Mo, and W Between Melt and H2O-Fluid 1062.7 Water Content and Depth of Emplacement of Granites – Relationships to Ore-Forming Processes 1072.8 Models for the Formation of Porphyry-Type Cu, Mo, and W Deposits 1102.8.1 The Origin of Porphyry Cu–(Mo) and Porphyry Mo–(Cu) Type Deposits 1102.8.2 The Origin of Porphyry W(±Sn) Type Deposits 1142.8.3 The Role of Sulfur in the Formation of Porphyry Copper Deposits 1152.8.3.1 The Role of Sulfur in Concentrating Metals in Porphyry Systems 1152.8.3.2 The Role of Sulfur in Precipitating Ore Minerals in Porphyry Systems 1162.9 Near-Surface Magmatic-Hydrothermal Processes – The “Epithermal” Family of Au–Ag–(Cu) Deposits 1162.9.1 Gold Precipitation Mechanisms in Epithermal Deposits 1192.10 Skarn Deposits 1232.10.1 Prograde – Isochemical Contact Metamorphism 1262.10.2 Prograde – Metasomatism and Replacement 1262.10.3 Retrograde – Meteoric Fluid Influx and Main Metal Precipitation 1272.11 Fluid Flow in and Around Granite Plutons 1282.12 The Role of Hydrothermal Fluids in Mineralized Mafic Rocks 1332.12.1 The Effects of a Magmatic-Hydrothermal Fluid on PGE Mineralization in theBushveld Complex 1342.13 Summary 135Further Reading 136Part II Hydrothermal Processes 1393 Hydrothermal Ore-Forming Processes 1413.1 Introduction 1423.2 Other Fluids in the Earth’s Crust and Their Origins 1423.2.1 Sea Water 1443.2.2 Meteoric Water 1443.2.3 Basinal (or Connate)Water 1453.2.4 Metamorphic Water 1493.2.5 Waters of Mixed Origin 1503.3 The Movement of Hydrothermal Fluids in the Earth’s Crust 1523.3.1 Factors Affecting Fluid Flow at a Crustal Scale 1523.3.2 A Note on Hydrostatic Versus Lithostatic Pressure Gradients 1543.3.3 Deformation and Hydrothermal Fluid Flow 1553.3.4 Other Factors Affecting Fluid Flow and Mineral Precipitation 1583.3.4.1 How DoWe Know that a Fluid Has Passed Through a Rock? 1593.4 Additional Factors Affecting Metal Solubility 1603.4.1 The Important Metal–Ligand Complexes in Hydrothermal Solutions 1623.4.1.1 Hard Metals 1623.4.1.2 Borderline Metals 1633.4.1.3 Soft Metals 1653.4.2 More on Metal Solubilities in the Aqueous Vapor Phase 1673.4.3 A Brief Note on Metal–Organic Complexes 1673.5 Precipitation Mechanisms for Metals in Solution 1693.5.1 Physico-Chemical Factors Affecting Metal Precipitation 1703.5.1.1 Temperature 1713.5.1.2 Pressure 1713.5.1.3 Phase Separation (Boiling and Effervescence) 1723.5.1.4 Fluid Mixing/Dilution 1733.5.1.5 Fluid/Rock Reactions (pH and Eh Controls) 1763.5.2 Adsorption 1763.5.3 Biologically Mediated Processes of Metal Precipitation 1793.5.3.1 Biomineralization 1803.6 Fluid–Rock Interaction – Introduction to Hydrothermal Alteration 1833.6.1 Types of Alteration and Their Ore Associations 1873.6.1.1 Potassic Alteration 1873.6.1.2 Phyllic (or Sericitic) Alteration 1903.6.1.3 Propylitic Alteration 1903.6.1.4 Argillic Alteration 1903.6.1.5 Silication 1903.6.1.6 Silicification 1903.6.1.7 Carbonatization 1913.6.1.8 Greisenization 1913.6.1.9 Hematitization 1913.7 Metal Zoning and Paragenetic Sequence 1913.7.1 Replacement Processes 1943.8 Modern Analogues of Ore-Forming Processes – The VMS–SEDEX Continuum 1953.8.1 “Black Smokers” – A Modern Analogue for VMS Deposit Formation 1963.8.2 The Salton Sea and Red Sea Geothermal Systems – Modern Analogues for SEDEX Mineralization Processes 2043.8.2.1 Salton Sea Geothermal System 2043.8.2.2 The Red Sea and the VMS–SEDEX Continuum 2063.9 Mineral Deposits Associated with Aqueo-Carbonic Metamorphic Fluids 2093.9.1 Orogenic Gold Deposits 2103.9.1.1 Archean 2103.9.1.2 Proterozoic 2113.9.1.3 Phanerozoic 2113.9.2 Carlin-Type Gold Deposits 2113.9.3 Quartz Pebble Conglomerate Hosted Gold Deposits 2143.10 Ore Deposits Associated with Basinal Fluids 2173.10.1 Stratiform Sediment-Hosted Copper (SSC) Deposits 2183.10.2 Mississippi Valley Type (MVT) Pb–Zn Deposits 2223.11 Ore Deposits Associated with Near Surface Meteoric Fluids (Groundwater) 2303.11.1 A Brief Note on the Aqueous Transport and Deposition of Uranium 2303.11.2 Sandstone-Hosted Uranium Deposits 2313.11.2.1 Colorado Plateau (Tabular) Uranium–Vanadium Type 2313.11.2.2 Roll-Front Type 2333.12 Summary 235Further Reading 237Part III Sedimentary/Surficial Processes 2394 Surficial and Supergene Ore-Forming Processes 2414.1 Introduction 2414.2 Principles of Chemical Weathering 2424.2.1 Dissolution and Hydration 2434.2.2 Hydrolysis and Acid Hydrolysis 2444.2.3 Oxidation 2444.2.4 Cation Exchange 2454.3 Lateritic Deposits 2454.3.1 Laterite Formation 2454.3.2 Bauxite Ore Formation 2464.3.3 Nickel Laterites 2514.3.4 Gold in Laterites 2534.3.5 A Note on Platinum Group Element (PGE) Enrichment in Laterites 2574.4 Clay Deposits 2584.4.1 The Kaolinite (China Clay) Deposits of Cornwall 2594.4.2 “Ion-Adsorption” Rare Earth Element (REE) Deposits in Clays 2614.5 Calcrete-Hosted Deposits 2654.5.1 Calcrete-Hosted or Surficial Uranium Deposits 2654.6 Supergene Enrichment of Cu and Other Metals in the Near Surface Environment 2674.6.1 Supergene Oxidation of Copper Deposits 2674.6.1.1 A Note on Supergene Enrichment of Other Metals 2724.7 Summary 275Further Reading 2765 Sedimentary Ore-Forming Processes 2775.1 Introduction 2775.2 Clastic Sedimentation and Heavy Mineral Concentration – Placer Deposits 2785.2.1 Basic Principles 2795.2.2 Hydraulic Sorting Mechanisms Relevant to Placer Formation 2815.2.2.1 Settling 2815.2.2.2 Entrainment 2835.2.2.3 Shear Sorting 2855.2.2.4 Transport Sorting 2855.2.3 Application of Sorting Principles to Placer Deposits 2885.2.3.1 Small Scale 2885.2.3.2 Intermediate Scale 2885.2.3.3 Large Scale 2885.2.4 A Note Concerning Sediment Sorting in Beach and Eolian Environments 2905.2.4.1 Beaches 2935.2.4.2 Wind-Borne Sediment Transport 2955.2.5 Numerical Simulation of Placer Processes 2965.3 Chemical Sedimentation – Iron-Formations, Phosphorites, and Evaporites 2985.3.1 Iron-Formations and Ironstones 2985.3.1.1 Bog Iron Ores 2995.3.1.2 Phanerozoic Ooidal Ironstone (POI) Deposits 3005.3.1.3 Banded and Granular Iron-Formation – An Enigmatic Rock Type 3025.3.1.4 Mechanisms by Which BIFs Are Deposited 3045.3.1.5 The Periodicity of Iron-Formation Deposition 3085.3.1.6 Transformation of BIFs into Viable Iron Ore Deposits 3095.3.2 Bedded Manganese Deposits 3135.3.3 A Note on Ocean Floor Manganese Nodules 3145.3.4 Phosphorites 3165.3.4.1 A Model for Phosphogenesis Based on Present Day Deposition 3195.3.5 Black Shales 3225.3.6 Evaporites 3235.4 Fossil Fuels – Oil/Gas Formation and Coalification 3305.4.1 Basic Principles 3305.4.2 Oil and Gas Formation (Conventional) 3315.4.2.1 Source Rock Considerations and Organic Maturation 3355.4.2.2 Petroleum Migration and Reservoir Considerations 3375.4.2.3 Entrapment of Oil and Gas 3405.4.3 Coalification Processes 3455.4.3.1 Coal Characteristics 3495.4.3.2 A Note Concerning Formation of Economically Viable Coals 3525.4.4 Unconventional Hydrocarbons – Shale Gas, Oil Shales, and Tar Sands 3545.4.4.1 Shale Gas and Oil Shales 3545.4.4.2 Tar Sands (or Oil Sands) 3545.4.5 Gas Hydrates 3565.5 Summary 359Further Reading 359Sedimentology and Placer Processes 360Chemical Sedimentation and Ore Formation 360Fossil Fuels 360Part IV Global Tectonics and Metallogeny 3616 Ore Deposits in a Global Tectonic Context 3636.1 Introduction 3636.2 Patterns in the Distribution of Mineral Deposits 3646.3 Continental Growth and the Supercontinent Cycle 3666.3.1 Estimations of Continental Growth Rates 3666.3.2 Supercontinent Cycles 3696.3.2.1 Kenorland 3706.3.2.2 Nuna (also referred to as Columbia) 3706.3.2.3 Rodinia 3706.3.2.4 Pangea 3726.4 Geological Processes and Metallogenesis 3756.4.1 Evolution of the Hydrosphere and Atmosphere 3756.4.2 Secular Decrease in Global Heat Production and Mantle Temperature 3766.4.3 Long-Term Global Tectonic Trends and Mantle Convection 3776.4.4 Eustatic Sea Level Changes and “Continental Freeboard” 3796.5 Metallogeny Through Time 3806.5.1 The Archean Eon 3806.5.1.1 The Hadean (>4000 Ma) and Eoarchean (>3600 Ma) stages 3816.5.1.2 The Paleo-, Meso-, and Neoarchean stages (3600–2500 Ma) 3816.5.1.3 Shield formation (pre-3100 Ma) 3826.5.1.4 Cratonization (c. 3100–2500 Ma) 3826.5.2 The Proterozoic Eon 3846.5.2.1 The Paleoproterozoic Era (2500–1600 Ma) 3856.5.2.2 The Mesoproterozoic Era (1600–1000 Ma) 3866.5.2.3 The Neoproterozoic Era (1000–541 Ma) 3866.5.3 The Phanerozoic Eon 3886.5.3.1 Phanerozoic Tectonic Cycles and Metallogeny 3926.5.3.2 Time-Bound and Regional Aspects of Phanerozoic Metallogeny 3936.6 Plate Tectonic Settings and Ore Deposits – A Summary 3966.6.1 Extensional Settings 3966.6.2 Compressional Settings 3966.7 Summary 399Further Reading 400References 401Index 439