FIGURE 1 Re-Os isochron diagram for gold and pyrite from the 2.8-2.9 Ga Witwatersrand Supergroup, South Africa, redrawn from the data and information in the study by Kirk et al. (2002). Vaal Reef gold and rounded pyrite interpreted to be detrital produce an excellent 3.03 ± 0.02 Ga isochron with an initial ¹⁸⁷Os/¹⁸⁸Os ratio identical to the mantle value at this time (inset). Because both age constraints are older than the <2.9 Ga age of the conglomerates, they confirm the detrital nature of the gold and suggest that the provenance of the conglomerate included magmatic rocks in a nearby 3 Ga green-stone belt. Epigenetic, cubic-morphology pyrite in the hydrothermally altered Ventersdorp Contact Reef (VCR) shows open-system behavior of the Re-Os system.