Fluid interaction in the Witwatersrand goldfields: constraints from oxygen isotope data

Both placer and hydrothermal origens have been suggested for the Wits gold mineralization, with most recent authors prefering the placer model. δ18O values were determined in both Witwatersrand quartzites and some mafic intrusions which cut the metasedimentary strata. Mafic igneous rocks are far more susceptible to oxygen isotope exchange than quartzite and might be expected to show more evidence for fluid exchange in their δ18O values. Despite the apparent mantle δ18O values, the oxygen isotope geochemistry of the Ventersdorp intrusions considered in this study is equally consistent with a large fluid flux. The present oxygen isotope data do not contradict the model for gold mineralization which requires there have been large scale fluid activity. Low water/rock ratios are equally consistent with the oxygen isotope data, in which case the δ18O values of the dolerites are the original magmatic values.2 figs., 6 refs