The Ventersdorp Contact Reef, one of the major gold-bearing conglomerate horizons in the Witwatersrand Basin, occurs as a distinct horizon between the overlying Klipriviersberg Group lavas (2714 Ma) and the underlying Central Rand Group rocks (<2894 to >2714 Ma). The Ventersdorp Contact Reef has been metamorphosed and hydrothermally altered under greenschist facies conditions (290–350 °C and 0.2–0.3 GPa). A study of S, O, and H isotopes was carried out to constrain the sources of hydrothermal fluids and gold mineralization in the Ventersdorp Contact Reef. A narrow range of δ 34 S values (−1.5‰ to +1.8‰) for authigenic pyrite, pyrrhotite, chalcopyrite, sphalerite, and galena from the Ventersdorp Contact Reef and its hanging-wall and footwall lithologies suggests a reconstitution of detrital sulfides during fluid circulation at peak metamorphic conditions. The δ 18 O values of authigenic quartz and calcite range from +8.9‰ to +11.3‰, and the δ 18 O and δD values of muscovite separates are between +7.2‰ and +8.4‰ and −62‰ and −31‰ respectively. The estimated δ 18 O (+4.8‰ to +6.1‰) and δD (−27‰ to −39‰) values of the fluids in equilibrium with these minerals under the relevant P-T conditions suggest a metamorphic origin for the fluid. It is concluded that the metamorphic fluids involved were probably derived from the Basin itself, and allogenic sulfides in the Ventersdorp Contact Reef were reconstituted during fluid circulation at peak metamorphic conditions. Accordingly, gold appears to have been locally remobilized and possibly derived from pre-existing placer concentrations.

Gold and uranium mineralization within the Archean Witwatersrand Basin exhibits a close association with carbonaceous matter. In order to understand the gold and uranium mineralization associations, it is necessary to understand something about the carbonaceous matter itself as well as the fluids involved in remobilizing both hydrocarbons and mineralization within the basin. Diagenetic maturation of primitive bacterial material released hydrocarbons that were able to migrate through the sediments of the economically important Witwatersrand Basin. Migrating hydrocarbons underwent polymerization and condensation due to the effect of ionizing radiation in the proximity of detrital uraninite during catagenesis. Isotopically heavy hydrocarbons precipitated out as bitumen seams where uraninite concentrations were high or as “fly-speck” bitumen around isolated uraninite grains, giving rise to sediment-hosted bitumen types. Lighter hydrocarbons, liberated during catagenic radiolysis, were incorporated into circulating basinal fluids. During late-catagenic/early-metagenic processes, hydrocarbon-bearing fluids migrating along aquifers (faults, bedding planes, and unconformities) precipitated insoluble carbonaceous matter, or distal/vein-related bitumen, in suitable sites. Light hydrocarbons precipitated as crosscutting veins where the vein intersected uraniferous seam bitumen, as bitumen nodules within late-stage quartz veins and as nodules and/or inclusion linings within fluid inclusions in late-stage quartz veins. Where fault zones permeated the Archean granitic basement, hydrocarbons were precipitated by radiolytic processes around high-U phases as nodular, granitoid hosted, or distal bitumen. Distal- and vein-related bitumen exhibits a higher degree of structural ordering than sediment-hosted bitumen as a result of elevated pressure-temperature conditions experienced during metagenic and fault-related processes. Liberated light hydrocarbons in basinal fluids formed organo-urano-gold-, thio-gold-, and/or organo-urano-gold-sulphide complexes. Circulation of these hydrocarbon-rich fluids and subsequent precipitation of bitumen and mineralization provide an explanation for the apparent hydrothermal component of Witwatersrand mineralization.