Characteristics of post-depositional fluids in the Witwatersrand Basin

Four metamorphic/hydrothermal fluid systems were identified in detailed mic rothermometric and spectroscopic investigations carried out on fluids trapp ed in late-stage quartz veins sampled throughout the Witwatersrand Basin. T hese include aqueous, H2O-CO2-rich, H2O-CH4-CO2-rich and CH4-N-2-rich fluid s. Early inclusion populations are dominated by H2O- and CO2-rich fluid sys tems. Cross-cutting these early fluid populations are trails of inclusions that contain CH4, C2H6, N-2, H-2 and H2S (vapour-rich inclusions). Fault-re lated quartz veins, in particular, are characterized by these later fluid t ypes. A common feature of some of these later fluids is the presence of hyd rocarbons and daughter crystals. A very late phase of aqueous fluids occurs in most samples as transgranular inclusion trails. P-V-T-X considerations as well as chlorite geothermometry, mineral assemblages and burial conditio ns suggest entrapment temperatures of between 200 degrees C and 400 degrees C and locally up to 500 degrees C. Fluid chemistry suggests that the post- depositional C-O-H fluids affecting the sediments have oxygen fugacities be tween Q-F-M and Ni-NiO in the range 300 degrees C to 400 degrees C. This ac counts for the widespread preservation of detrital uraninite and gold remob ilization along major fluid conduits.