A detrital model for the origin of gold and sulfides in the Witwatersrand basin based on Re-Os isotopes

The Re-Os systematics of gold and sulfides from the Witwatersrand basin were utilized to determine whether the gold is detrital or was introduced by hydrothermal solutions from outside the basin. Gold from a gravity concentrate from the Western Areas Gold Plant and gold from the Vaal Reef have very high Os concentrations of approximately 73 to 10000 ppb and 3 to 32 ppb Re, resulting in ¹⁸⁷ Re/¹⁸⁸Os ratios of 0.010 to 0.185. The gold has subchondritic ¹⁸⁷Os/¹⁸⁸Os ratios between 0.1056 to 0.1099 and an average value of 0.1067. Rhenium depletion ages (T_RD) range from 3.5 Ga to 2.9 Ga, with a median age of 3.3 Ga. Pyrite from the Vaal Reef have Os concentrations ranging from 0.26 to 0.68 ppb, Re concentrations of 1.7 to 2.8 ppb and ¹⁸⁷Re/¹⁸⁸Os ratios of approximately 14 to 87. The pyrite samples have measured ¹⁸⁷Os/¹⁸⁸Os ratios of 0.84 to 4.7 and define an isochron with an age of 2.99 ± 0.11 Ga (MSWD = 0.77). The Os isotopic data from the direct measurement of gold preclude introduction of gold to the Witwatersrand basin from crustally derived metamorphic or hydrothermal fluids between 2.7 to 2.0 Ga. The unradiogenic ¹⁸⁷Os/¹⁸⁸Os ratios, old T_RD ages of the Western Areas and Vaal Reef gold samples, as well as the contemporaneously old age of the Vaal Reef pyrite are consistent with detrital deposition of gold during the formation of the Witswatersrand basin. The Os data will allow for minor hydrothermal remobilization and/or overprinting of hydrothermal gold on preexisting detrital gold grains but does not support the introduction of gold solely by hydrothermal fluids.