Fluid inclusions in the mesozonal gold deposit at Atud mine, Eastern Desert, Egypt
Gold mineralization at the Atud mine occurs as fracture-filling auriferous quartz veins hosted in Neoproterozoic dioritic rocks and along their contact with metagabbro. Gold mineralization is associated with metasomatic alteration zones around shear zones and quartz–carbonate vein arrays. The mineralized veins consist of quartz, carbonate and albite gangue enclosing minor amounts of pyrrhotite, arsenopyrite, pyrite and sphalerite. Trace amounts of galena, chalcopyrite, magnetite and rutile are also present. Paragenetically, the mineralization is divided, with decreasing temperature, into three stages, namely: (1) Early (Au-poor, pyrrhotite–arsenopyrite–pyrite–quartz vein); (2) Main (Au-rich, sphalerite–arsenopyrite–pyrite–galena ± chalcopyrite–quartz vein); and (3) Late (quartz–carbonate–pyrite ± galena). 
 
Gold (15.6–36.2 at.% Ag) is present mainly as discrete grains of native gold (<5–20 μm in diameter). Free gold appears mainly as inclusions in quartz and as microscopic inclusions (<5 μm) in arsenopyrite and pyrite. Gold also occurs in fractures and grain boundaries of pyrite, arsenopyrite and base metal sulphides. These occurrences of gold indicate that several influxes of gold and/or stages of remobilization took place. 
 
Based on temperatures inferred from arsenopyrite compositions by electron microprobe, the estimated temperatures for Early and Main mineralized stages reach 340–430, and 273–368 °C, respectively. The sulphur activity (atm) of ore formation at the Atud deposit was estimated for each stage as 10−6.5–10−9.6, and 10−8–10−12.2, respectively. 
 
Fluid inclusions in quartz intimately associated with the mineralization are dominated by aqueous H2O–CO2 + NaCl types. In most cases, these fluid inclusions co-exist in individual samples and show various CO2 phase volume proportions at 40 °C. Co-existing H2O-rich liquid and CO2-rich vapour fluid inclusions homogenized into liquid and carbonic vapour phases, respectively, over the same temperature range (270–490 °C). The petrographic observations and microthermometric data suggest that fluid inclusions in quartz intimately associated with the early and main mineralizing events were trapped during phase separation of an originally homogenous H2O–CO2 liquid, with low salinity (2.8–8.2 wt.% NaCl equiv.) and high density of 0.8–0.9 g/cc (corresponding to 15–76 mol% CO2). These data are consistent with transportion of gold as a bisulphide complex. Gold deposition occurred over a temperature range of 270–430 °C and pressures of 160–272 MPa (∼6–11 km depths), likely due to decreases in sulphur activity (10−6.5–10−12.2) accompanying fluid unmixing. Gold deposition in the Atud diorites was related to fluid phase separation, sulphidization and carbonatization of host dioritic rocks during hydrothermal alteration and mineralization. The diorites are considered to have acted as preferential sites for fluid flow and ore precipitation due to their brittle nature during regional deformation.
Gold metallogeny and exploration
Distribution of gold in the Earth's crust.- 1.1 Introduction.- 1.2 Gold in rock-forming minerals.- 1.3 Gold in igneous rocks.- 1.3.1 Introduction.- 1.3.2 General characteristics of gold distribution in the lithosphere.- 1.3.3 Gold abundance in mafic and ultramafic rocks.- 1.3.4 Gold in Precambrian rocks.- 1.3.5 Estimates of mantle gold abundance.- 1.3.6 Mafic volcanic rocks in non-orogenic environments.- 1.3.7 Mafic plutonic rocks in non-orogenic settings.- 1.3.8 Orogenic environments.- 1.4 Gold in sediments and sedimentary rocks.- 1.4.1 Introduction.- 1.4.2 Gold in sediments.- 1.4.3 Gold in clastic rocks.- 1.4.4 Gold in chemical sedimentary rocks.- 1.5 Gold in metamorphic rocks.- 1.5.1 Introduction.- 1.5.2 Regional metamorphism.- 1.5.3 Granitoid intrusions in metamorphic terranes.- 1.6 Concluding summary.- 1.7 Mineral data.- 1.7.1 Explanatory notes.- 1.7.2 The gold content of rock-forming minerals.- 1.7.3 The gold content of igneous rocks.- 1.7.4 The gold content of sediments and sedimentary rocks.- 1.7.5 The gold content of metamorphic rocks.- References.- The hydrothermal geochemistry of gold.- 2.1 Introduction.- 2.2 The inorganic chemistry of gold.- 2.2.1 Oxidation states.- 2.2.2 Relativistic effects.- 2.2.3 Coordination chemistry of Au(I).- 2.3 Gold complexing in hydrothermal solutions.- 2.3.1 Which complexes are important?.- 2.3.2 Halide complexes of gold(I).- 2.3.3 Hydrolysis.- 2.3.4 Hydrosulphido and sulphido complexes.- 2.3.5 Additional sulphur-containing ligands.- 2.3.6 Other ligands.- 2.4 Gold deposition.- 2.4.1 Boiling.- 2.4.2 Precipitation on colloid and mineral surfaces.- 2.5 Summary.- References.- 3 Archaean lode gold deposits.- 3.1 Introduction.- 3.1.1 Global distribution and economic significance.- 3.1.2 Genetic concepts.- 3.2 Nature of deposits.- 3.2.1 Introduction.- 3.2.2 Size and grade.- 3.2.3 Structural styles.- 3.2.4 Host rocks.- 3.2.5 Mineralization and wallrock alteration.- 3.2.6 Metal associations.- 3.3 Regional distribution.- 3.3.1 Heterogeneous distribution.- 3.3.2 Structural setting.- 3.3.3 Metamorphic setting.- 3.3.4 Spatial relationship to intrusive rocks.- 3.3.5 Timing of mineralization.- 3.3.6 Peak mineralization age.- 3.4 Constraints on genetic models.- 3.4.1 Introduction.- 3.4.2 Nature of ore fluids.- 3.4.3 Transport and deposition of gold.- 3.4.4 Fluid focusing.- 3.4.5 Source of fluid and ore components.- 3.5 Genetic models.- 3.6 Tectonic setting of gold mineralization.- 3.7 Potential exploration significance.- 3.8 Brief summary.- References.- 4 Phanerozoic gold deposits in tectonically active continental margins.- 4.1 Introduction.- 4.2 Distribution of Phanerozoic lode gold deposits in space and time.- 4.2.1 North America.- 4.2.2 South America.- 4.2.3 Australia New Zealand.- 4.2.4 Asia.- 4.2.5 Europe.- 4.2.6 Africa.- 4.3 Geological and geochemical characteristics.- 4.3.1 Host rocks.- 4.3.2 Structure.- 4.3.3 Ore morphology and textures.- 4.3.4 Mineralogy and paragenesis.- 4.3.5 Hydrothermal alteration.- 4.3.6 Elemental geochemistry and zoning.- 4.3.7 Fluid inclusions.- 4.3.8 Stable isotopes.- 4.3.9 Applications of Sr, Pb and Nd isotope ratios.- 4.4 Genetic models.- 4.5 Comparisons of Phanerozoic mesothermal deposits to other types of gold Mineralization.- 4.6 Conclusions.- References.- 5 Epithermal gold deposits in volcanic terranes.- 5.1 Introduction.- 5.2 Exploration case studies.- 5.2.1 Hishikari, Japan.- 5.2.2 Kelian, Kalimantan, Indonesia.- 5.2.3 Ladolam deposit, Lihir Island, Papua New Guinea.- 5.3 Environment of alteration and mineralization.- 5.3.1 Volcanic association.- 5.3.2 Structural controls.- 5.3.3 Wallrock alteration.- 5.3.4 Fluid inclusions and light stable isotopes.- 5.4 Active geothermal systems.- 5.5 Metal transport in epithermal systems.- 5.6 Physico-chemical conditions in the depositional regime.- 5.7 Epithermal deposits through geologic time.- 5.8 Exploration.- 5.9 Summary.- References.- 6 Intrusion-related gold deposits.- 6.1 Introduction.- 6.2 Geotectonic settings.- 6.3 Intrusion-hosted stockwork/disseminated deposits.- 6.3.1 Porphyry deposits.- 6.3.2 Other intrusion-hosted stockwork/disseminated deposits.- 6.4 Deposits in carbonate rocks.- 6.4.1 Skarn deposits.- 6.4.2 Carbonate-replacement deposits.- 6.5 Stockwork, disseminated and replacement deposits in non-carbonate rocks.- 6.6 Breccia-hosted deposits.- 6.7 Vein-type deposits.- 6.8 Deposit interrelationships and metal zoning.- 6.9 Genetic considerations.- 6.9.1 Magma type.- 6.9.2 Ore formation.- 6.10 Possible relationships with other gold deposit types.- 6.10.1 Epithermal deposits.- 6.10.2 Sediment-hosted deposits.- 6.10.3 Mother Lode-type deposits.- 6.11 Concluding remarks.- References.- 7 The geology and origin of Carlin-type gold deposits.- 7.1 Introduction.- 7.2 Classification of Carlin-type deposits.- 7.2.1 A historical perspective.- 7.2.2 Current perspectives.- 7.2.3 Relationship of Carlin-type deposits to polymetallic replacements.- 7.3 Regional geological and tectonic setting.- 7.3.1 Regional geological and tectonic setting in North America.- 7.3.2 Magmatism in western North America.- 7.3.3 Regional geological, tectonic, and magmatic settings in south-eastern China.- 7.4 Characteristics of the deposits.- 7.4.1 Nature of the host rocks.- 7.4.2 Structural setting of the deposits.- 7.4.3 Associated igneous rocks.- 7.4.4 Geochronology of the deposits.- 7.4.5 Alteration and metallization.- 7.4.6 Geochemistry of the deposits.- 7.4.7 Geophysical studies.- 7.4.8 Sizes, shapes, and grades of deposits.- 7.5 Ore deposit models.- 7.5.1 Published models.- 7.5.2 A speculative model.- 7.6 Exploration guidelines.- 7.7 Summary.- References.- 8 Auriferous hydrothermal precipitates on the modern seafloor.- 8.1 Introduction.- 8.2 Gold in seafloor polymetallic sulphide deposits.- 8.2.1 Mid-ocean ridges.- 8.2.2 Seamounts.- 8.2.3 Island-arc settings.- 8.2.4 Sedimented-rift environments.- 8.3 Mineralogy and geochemistry of gold in seafloor hydrothermal systems.- 8.4 Gold in sub-seafloor stockwork mineralization.- 8.5 Gold in hydrothermal plumes and associated metalliferous sediments.- 8.6 Transport and deposition of gold in seafloor hydrothermal systems.- 8.6.1 The chemistry of seafloor hydrothermal fluids in volcanic environments.- 8.6.2 The chemistry of seafloor hydrothermal fluids in sedimentary environments.- 8.6.3 The solubility of gold in seafloor hydrothermal fluids.- 8.6.4 The flux of gold in seafloor hydrothermal fluids.- 8.7 Secondary enrichment of gold in supergene sulphides and gossans.- 8.8 Gold in ancient seafloor hydrothermal systems.- 8.8.1 Volcanogenic massive sulphides.- 8.8.2 Metalliferous sediments.- 8.8.3 Gold deposits in auriferous chemical and clastic sediments.- References.- 9 Ancient placer gold deposits.- 9.1 Introduction.- 9.2 Geological setting.- 9.2.1 Pongola.- 9.2.2 Dominion.- 9.2.3 Witwatersrand.- 9.2.4 Ventersdorp.- 9.2.5 Transvaal.- 9.2.6 Jacobina.- 9.2.7 Moeda.- 9.2.8 Tarkwa.- 9.3 The palaeosurfaces.- 9.3.1 Areal dimensions.- 9.3.2 Topographic relief.- 9.3.3 Stratigraphic position.- 9.3.4 Correlation.- 9.4 The placer sediments.- 9.4.1 Gravel and sand fraction.- 9.4.2 Geometry.- 9.4.3 Deposition.- 9.4.4 Heavy mineral fraction.- 9.4.5 Mineralogy.- 9.4.6 Kerogen.- 9.5 Metamorphism.- 9.6 Structural control.- 9.7 Summary.- References.- 10 Geochemical exploration for gold in temperate, arid, semi-arid, and rain forest terrains.- 10.1 Introduction.- 10.2 Geochemical signatures of gold mineralization.- 10.2.1 Attributes of bedrock mineralization.- 10.2.2 Supergene cycle of gold.- 10.2.3 Transposition of the primary characteristics to the supergene environment.- 10.3 Examples and case histories.- 10.3.1 Introduction.- 10.3.2 Temperate terrains.- 10.3.3 Arid and semi-arid terrains.- 10.3.4 Rain forest environments.- 10.4 Operating procedures.- 10.4.1 Sampling and sample preparation.- 10.4.2 Sample analysis.- 10.5 Alternative sampling techniques.- 10.5.1 Lithogeochemistry.- 10.5.2 Heavy-mineral concentrate geochemistry.- 10.5.3 Hydrogeochemistry.- 10.5.4 Atmogeochemistry.- 10.5.5 Biogeochemistry.- 10.6 Conclusions.- References.- 11 Geochemical exploration for gold in glaciated terrain.- 11.1 Introduction.- 11.2 Glacial dispersal.- 11.3 Glacial stratigraphy and ice-movement directions.- 11.4 Sampling and analytical methods.- 11.5 Occurrence of gold in till and soil and the effects of weathering.- 11.6 Drift prospecting for gold.- 11.7 Source of placer gold in glaciated terrains.- 11.8 Conclusions and future trends.- References.- 12 Geophysical exploration for gold.- 12.1 Introduction.- 12.2 Geological and geophysical models.- 12.3 Exploration strategy and methods.- 12.3.1 Reconnaissance.- 12.3.2 Regional.- 12.3.3 Detailed.- 12.4 Examples.- 12.4.1 Veins, stockworks and lodes.- 12.4.2 Skarns.- 12.4.3 Auriferous volcanogenic sulphides.- 12.4.4 Auriferous granitoids.- 12.4.5 Disseminated deposits in igneous, volcanic and sedimentary units.- 12.4.6 Palaeoplacers.- 12.4.7 Placers.- 12.5 Conclusions.- 12.6 Glossary of geophysical terms.- References.- 13 Economics of gold deposits.- 13.1 Introduction.- 13.2 Market setting.- 13.3 Gold-mine production.- 13.3.1 World-wide trends.- 13.3.2 Brazil.- 13.3.3 United States.- 13.3.4 Australia.- 13.3.5 South Africa.- 13.3.6 Canada.- 13.3.7 The 1980s.- 13.4 Economic evaluation of gold deposits.- 13.4.1. The decision process.- 13.4.2 Economic evaluation techniques.- 13.4.3 Estimation of cash flow.- 13.4.4 Cash-flow criteria.- 13.4.5 The cost of capital.- 13.4.6 DCF criteria.- 13.5 Characteristics of economic gold deposits.- 13.5.1 Canadian case study.- 13.5.2 Presentation format.- 13.5.3 Deposit input variables.- 13.5.4 Cash-flow criteria.- 13.5.5 DCF criteria.- 13.6 Conclusion.- References.
O2 constraints from Paleoproterozoic detrital pyrite and uraninite
Redox-sensitive detrital grains such as pyrite and uraninite in sedimentary successions provide one of the most conspicuous geological clues to a different composition of the Archean and early Paleoproterozoic atmosphere. Today, these minerals are rapidly chemically weathered within short transport distances. Prior to the rise of oxygen, low O2 concentrations allowed their survival in siliciclastic deposits with grain erosion tied only to physical transport processes. After the rise of oxygen, redox-sensitive detrital grains effectively vanish from the sedimentary record. To get a better understanding of the timing of this transition, we examined sandstones recorded in a scientifi c drill core from the South African 2.415 Ga Koegas Subgroup, a mixed siliciclastic and iron formation–bearing unit deposited on the western deltaic margin of the Kaapvaal craton in early Paleoproterozoic time. We observed detrital pyrite and uraninite grains throughout all investigated sandstone beds in the section, indicating the rise of oxygen is younger than 2.415 Ga. To better understand how observations of detrital pyrite and uraninite in sedimentary rocks can quantitatively constrain Earth surface redox conditions, we constructed a model of grain erosion from chemical weathering and physical abrasion to place an upper limit on ancient environmental O2 concentrations. Even conservative model calculations for deltaic depositional systems with suffi cient transport distances (approximately hundreds of kilometers) show that redox-sensitive detrital grains are remarkably sensitive to environmental O2 concentrations, and they constrain the Archean and early Paleoproterozoic atmosphere to have <3.2 × 10 –5 atm of molecular O2. These levels are lower than previously hypothesized for redox-sensitive detrital grains, but they are consistent with estimates made from other redox proxy data, including the anomalous fractionation of sulfur isotopes. The binary loss of detrital pyrite and uraninite from the sedimentary record coincident with the rise of oxygen indicates that atmospheric O2 concentrations rose substantially at this time and were never again suffi ciently low (<0.01 atm) to enable survival and preservation of these grains in short transport systems.