Abstract

The Carbon Leader Reef is a ~1 m thick conglomeratic unit with a thin bituminous base, and is one of the major gold-bearing conglomerate horizons in the Central Rand Group of the Witwatersrand Basin. It consists of alternating conglomerate and quartzite layers and was metamorphosed under greenschist facies conditions. Bulk rock δ18O and δD values of the Carbon Leader Reef range from 7.2‰ to 10.8‰ (mean = 10.6‰) and −27 to −65 (mean = −41‰), respectively. The narrow range in δ18O values, together with the lack of correlation between the δ18O value and the modal % matrix minerals, suggests that the original detrital minerals and the authigenic matrix minerals have similar oxygen isotope composition.

Calculated δD values of the fluid, assuming that it was in isotope equilibrium with the bulk rock at the estimated peak metamorphic temperature of 350°C, range from −1‰ to −40‰. This suggests that the ultimate origin of the fluid was a mixture of meteoric and metamorphic water. These data are not consistent with the ingress of large quantities of externally derived fluid post burial metamorphism. Gold-rich sections of the Carbon Leader Reef do not have significantly different δ18O values than the adjacent gold-poor section, which suggests that gold mineralization is not related to interaction with significant amounts of externally derived hydrothermal fluids.

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