Evidence for mixing of Re–Os isotopes at < 2.7 Ga and support of a remobilized placer model in Witwatersrand sulfides and native Au

Abstract

This study analyzes Re and Os contents of Au rich sulfides from the Carbon Leader (Tau Lekoa and Mponeng), the VCR (Mponeng and Tau Lekoa), and the Black Reef (Modder East), as well as native Au in the Carbon Leader, to further investigate the detrital (placer), modified detrital (placer), and hydrothermal models for the source of Au in mines from the Witwatersrand. An isochron plot of the results from the four locations yields an imprecise isochron age of 2381±140Ma (MSWD=17) and initial 187Os/188Os=0.24±0.21. Isochron plots for the individual reefs also yielded ages that are younger than the deposition of the basin Witwatersrand sediments. Samples that possess higher concentrations of Os (>100ppb) and low 187Re/188Os ratios (<0.02) yield model ages of the same age or older than the basin formation (2820 to 3270Ma).To unify all of the data providing a large variation of chronologic information, Os mixing diagrams for both gold and sulfide minerals produce realistic mixing trends when solving Os initial ratios for 2050Ma. The two most probable end members for the mixing relationships are a high Os concentration non-radiogenic source and a low Os concentration highly radiogenic source. The high concentration non-radiogenic Os source has been measured in multiple studies and has been clearly derived from the subcontinental lithospheric mantle (SCLM). The low concentration, highly radiogenic source is debatable. The measured concentrations of Os in the Witwatersrand ores are much greater than published Os concentrations in other hydrothermal ores. The data could indicate a redistribution of the detrital Os signatures at 2050Ma; thus a modified detrital model best explains the previously published data and this data.