Regolith Exploration Geochemistry in Tropical and Subtropical Terrains (eBook)

Front Cover 
1 
Regolith Exploration Geochemistry in Tropical and Subtropical Terrains 4
Copyright Page 5
Table of Contents 12
Editor's foreword 6
Preface 8
List of contributors 10
Introduction 24
PART I: CHARACTERISTICS OF TROPICALLY WEATHERED TERRAINS 26
Chapter I.l. Climate, geomorphological environment and geochemical dispersion models 28
Climate 28
Climatic geomorphology 32
Geochemical dispersion and exploration models 42
Chapter 1.2. Chemical weathering 50
Introduction 50
Types of weathering reaction 54
Development of a weathering profile 62
Transfer of elements during weathering 63
Chapter 1.3. The ferruginous latentes 66
Introduction 66
The latente profile 67
Chemical mass balances and differentiation of lateritic horizons 72
Rates of chemical weathering and mechanical erosion 74
Stability fields of hydrated and dehydrated minerals 75
Chapter 1.4. Soil formation in tropically weathered terrains 82
Introduction 82
Dynamic equilibria in soils 83
Factors and processes of soil formation in the tropical zone 83
Transformation systems in the tropical zone 90
Classification of tropical soils 101
Chapter 1.5. The chemical mobility and transport of elements in the weathering environment 104
Introduction 104
Solubility 106
Formation of inorganic and organic complexes 113
Adsorption 116
Co-precipitation 118
Chelation by humic materials 118
Microorganisms 119
Environmental effects 120
Chapter 1.6. Physical weathering and dispersion 122
Introduction 122
Disintegration of minerals and rocks 122
Physical translocation within weathering profiles 124
Physical transport on slopes 131
Environmental influences on physical dispersion 137
PART II: GOSSAN FORMATION AND GOSSAN SURVEYS 140
Chapter II.i. Introduction 142
Chapter II. 1. The mechanisms of sulphide oxidation and gossan formation 146
Introduction 146
Oxidative electrochemical weathering 146
Effects due to the mineralogy of the ore 156
Binding of base metals with gossan (iron bearing) minerals 157
Control by pH 160
Weathering of specific minerals 162
Conclusion 164
Chapter II.2. Gossan and ironstone surveys 166
Introduction 166
Gossan search 166
Gossan identification: basic principles 167
Gossan evaluation: geochemistry 170
Data interpretation 179
Lead isotope techniques 183
Case histories 188
Summary 228
PART III: EXPLORATION IN AREAS OF LOW TO MODERATE RELIEF 230
Chapter III.l. Seasonally humid tropical terrains (savannas) 232
Introduction 232
A-type dispersion models: pre-existing lateritic profile mostly preserved 234
B-type dispersion models: pre-existing profile partly truncated 253
Conclusion 269
Chapter III.2. Humid tropical terrains (rainforests) 270
Distribution and description 270
Dispersion models: low recent alteration 272
Dispersion models: moderate recent alteration 290
Dispersion models: strong recent alteration ("stone-line" weathering profiles) 304
Conclusions 321
Chapter III.3. Semiarid and arid terrains 324
The warm arid zone 324
Characteristics of the weathering profile 328
A-type models: pre-existing profile essentially preserved 333
B-type models: pre-existing profile partly truncated 360
Model C: profile completely truncated 388
Summary and conclusion 419
Acknowledgements 420
PART IV: EXPLORATION IN AREAS OF MODERATE TO HIGH RELIEF 422
Chapter IV. 1. Dissected terrains and tropical mountains 424
Introduction 424
Distribution 425
Rainforests and wet savannas 426
Dry savannas, semiarid and arid terrains 434
Conclusions 448
PART V: SPECIFIC COMMODITIES AND TECHNIQUES 450
Chapter V.l. Diamond exploration in tropical terrains 452
Introduction 452
Petrology and geochemistry of kimberlitic rocks 452
The tropical weathering of kimberlitic rocks 453
Distribution of selected elements in near-surface samples over and near kimberlitic rocks 457
Pathfinder mineral sampling 466
Discussion and conclusions 469
Chapter V.2. Uranium exploration in tropical terrains 472
Introduction 472
Geochemical and genetic considerations 472
Classification and examples of lateritic uranium concentrations 478
Exploration and evaluation of lateritic U concentrations 491
Acknowledgements 493
Chapter V.3. The geochemistry of gold in lateritic terrains 494
Introduction 494
Gold in the weathering environment 495
The formation of supergene gold deposits in lateritic terrain 502
Gold dispersion and the surface expression of mineralization 508
Summary: recommendations for gold exploration 512
Chapter V.4. Heavy mineral surveys in exploration of lateritic terrain 516
Introduction 516
Heavy minerals in lateritic environments 516
Methods of sample preparation 517
Identification of transported weathering material 517
The use of heavy mineral surveys in geological mapping 520
The use of heavy minerals in mineral exploration 528
Concluding remarks 530
PART VI: SYNTHESIS AND CONCLUSIONS 532
Chapter VI. 1. Summary and procedural recommendations 534
Introduction 534
Exploration problems in tropically weathered terrains 534
Exploration objectives 536
Recognition of the weathering and geomorphological environment 537
Application of the appropriate dispersion models 538
Selection of exploration procedures 540
Effectiveness of geochemical exploration in tropically weathered terrains 543
Exploration geochemistry and anthropogenic environmental change 545
APPENDICES 548
Appendix 1. Sample media used in geochemical exploration in tropically weathered environments:definitions and use 550
Gossans 550
Ironstones 551
Saprolite 552
Lateritic cuirasse (duricrust), lateritic gravels and lag 555
Soils 557
Transported overburden 559
Appendix 2. Sample preparation and analysis 560
Sample preparation 560
Analysis 563
Appendix 3. Profile nomenclature and glossary 568
Profile terminology 568
Glossary of terms 570
References 582
Author index 616
Place index 623
Subject index 630