METAMORPHISM, FLUID-FLOW, AND GOLD MOBILIZATION IN THE WITWATERSRAND BASIN - TOWARDS A UNIFYING MODEL

Recent advances in the understanding of metamorphism in the Witwatersr and Basin, particularly in the previously deeply buried central portio ns now exposed in the Vredefort Dome, here contribute to the developme nt of a basin-wide metamorphic model. Low-pressure, high-temperature p eak metamorphic conditions were attained throughout the basin during t he crustal thermal perturbation associated with the Bushveld Event. Du ring this event, prograde devolatilization reactions in the predominan tly argillitic West Rand Group produced pulses of aqueous fluids. Phas e proportion calculations indicate that in an average West Rand Group shale composition, where peak metamorphic temperatures were high enoug h for the complete reaction of chlorite + muscovite to biotite (simila r to 450 degrees C), 70 litres of hydrous fluid would have been produc ed per cubic metre of rock. In these rocks further breakdown of hydrou s minerals to produce the peak metamorphic assemblage recorded in the collar of the Vredefort Dome (similar to 600 degrees C), would have li berated an additional 69 litres of hydrous metamorphic fluid per cubic metre of rock. These calculations, coupled to the peak metamorphic th ermal profile through the basin and the known sub-outcrop limits of th e West Rand Group suggest that at least 7.58 x 10(14) litres of metamo rphic fluid was evolved from within the presently preserved West Rand Group. Data from alteration assemblages and fluid inclusion studies in gold-bearing reef horizons of the Central Rand Group near the basin m argins indicate that fluid flow occurred close to the peak of metamorp hism, as predicted by the modelling of metamorphism in the West Rand G roup. Calculated gold solubility in the measured fluid inclusion compo sitions is between 1 and 10 parts per billion. Thus, metamorphic fluid s derived from within the presently preserved lower portions of the Wi twatersrand Supergroup had the potential to mobilize at least 7600 ton s of gold. In this paper it is proposed that this metamorphic fluid de rived from within the basin mobilized and redistributed a significant portion of the pre-existing gold concentrations, resulting in many of the features interpreted to be in support of an epigenetic origin for the entire deposit.