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.

The 3.05 Ga U-Pb dating of uraninite grains in Dominion and Witwatersrand conglomerates has established that they were older than the onset of Witwatersrand sedimentation at 2.97 Ga. and therefore that they are detrital in origin. A precise Re-Os isochron age of 2.99 Ga obtained for rounded pyrite grains associated with the uraninite indicates that the pyrite is also detrital. Evidence of detrital forms in the gold has confirmed its placer origin prior to modification during metamorphism. Furthermore, rhenium-depletion ages ranging from 3.5 to 2.9 Ga for Witwatersrand gold support numerous other lines of evidence that have been used in the past to interpret the gold, uranium and pyrite concentrates as Archean placers. The sedimentary and stratigraphic history of the Witwatersrand succession indicates that uraninite, pyrite, and gold were part of a sub-aerial sediment load over a period of 180 m.y. Net sedimentation rate during the accumulation of the Central Rand Group is estimated at less than 14 m per million years, reflecting stratigraphic losses due to repeated reworking. Individual paleoplacers would have been exposed over areas of up to 400 km 2 but collectively they covered a region exceeding 2000 km 2 and contained more than 243 million tons of pyrite, 1.5 million tons of uraninite, and 80,000 tons of gold. Because there is no record of either detrital uraninite or pyrite in Proterozoic red-bed sediments on the Kaapvaal Craton, it is concluded that a change in the composition of the atmosphere took place there after 2.64 Ga.