Rn-generated206Pb in hydrothermal sulphide minerals and bitumen from the Ventersdorp Contact Reef, South Africa

The auriferous conglomerate horizons (reefs) in the Witwatersrand Basin of South Africa are in many places cut by hydrothermal quartz veins that frequently contain sulphide, bitumen, and, less commonly, free gold. New Pb isotopic results for the Ventersdorp Contact Reef which has experienced particularly intense hydrothermal alteration, reaffirm the radiogenically enriched nature of the Pb in this reef and provide additional insight into its origin. This study focuses on analyses of galena, chalcopyrite, pyrrhotite, and bitumen from quartz veins, which presumably formed during the 2.020Ga Vredefort meteorite impact event. The radiogenic, mainly uranogenic, component of the Pb appears to have been derived almost entirely from uraninite in the surrounding reef rock. Assigning a 2.02 Ga age of mineralization and constructing secondary isochrons for paragenetically early galena and chalcopyrite, ages of the source uraninite are calculated as 2.6-2.4 Ga. No special significance is ascribed to these source ages, which likely reflect extensive radiogenic Pb loss from originally somewhat older detrital uraninite during transport, sedimentation, and post-burial alteration. Analyses of detrital(?), syngenetic, and epigenetic pyrite from a reef conglomerate define a subsidiary linear array with a considerably shallower slope. Interpreted as a secondary isochron, the array gives an implausibly young mineralization and/or source age indicative of a superimposed isotopic disturbance. Five analyses of paragenetically late chalcopyrite and pyrrhotite plot on a 2°:Pb/Z°4pb versus 2°6pb/Z°4pb diagram with nearly constant 2°7pb/Z°4pb of 24.1-24.6 despite a huge range in 2°6pb/Z°4pb from 60-230. This trend is further revealed by Pb with similar 2°:Pb/Z°4pb but still higher 2°6pb/2°4Pb ratios (up to 949) in bitumen globules deposited on quartz crystals lining cavities in the veins. This nearly horizontal array cannot be interpreted as a secondary isochron, and requires the addition of virtually pure 2°6pb to a more normal, radiogenically-enriched Pb. The most plausible explanation for this decoupling of the 238U and 235U decay schemes is that an intermediate daughter isotope, most likely 222Rn, diffused from uraninite and was selectively captured by the bitumen where it subsequently decayed to 2°6pb. Whether the 2°6pb was acquired mainly at the time of hydrothermal activity by fluids at elevated temperatures, or more or less continuously until the present remains unresolved