Gold mineralization within the Witwatersrand Basin, South Africa: Evidence for a modified placer origin, and the role of the Vredefort impact event

The chemical composition of gold within the Archaean metasedimentary rocks of the Witwatersrand Supergroup displays significant heterogeneity at the micro-, meso- and regional scales. A detailed electron microbeam analytical and petrological study of the main auriferous horizons in the Central Rand Group throughout the Witwatersrand Basin indicates that gold has been remobilized late in the paragenetic sequence over distances of less than centimetres. Contemporaneous chlorite formation was strongly rock-buffered. Gold mobilization occurred under fluid-poor conditions at temperatures that did not exceed 350 °C. Widespread circulation of mineralizing fluids within the Central Rand Group is not supported by the gold and chlorite chemical data. Brittle deformation that affects most of the paragenetic sequence of the Central Rand Group late in its post-depositional history is followed by sequences of mineral growth and dissolution that appear throughout the Central Rand Group and have consistent textural relationships with gold. The consistent location within the paragenetic sequence, the wide regional and stratigraphic extent of the brittle deformation, together with mineral chemical and petrological data suggest that the Vredefort Impact Event (2.02 Ga) was the cause of this late deformation, and that post-impact fluid-poor metamorphism resulted in crystallization of a significant proportion of the gold on and within mineral grains that were deformed during this event.