Post-depositional alteration of the Ventersdorp Contact Reef at Vaal Reef no. 10 shaft, Klerksdorp Goldfield
Master Thesis
1996
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University of Cape Town
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Abstract
The Ventersdorp Contact Reef (VCR) is an auriferous conglomerate which unconformably overlies the rocks of the Witwatersrand Supergroup. It acted as a palaeo-aquifer and is capped by relatively impermeable metabasalts of the Ventersdorp Supergroup. Three post-depositional alteration events, which form an alteration halo in the footwall and hangjngwall rocks around the VCR, can be recognised. The first of these alteration events is attributed to regional metamorphism and is identified by the formation of pyrophyllite in the footwall quartzites and a lower green schist facies mineral assemblage in the hangingwall metabasalts. The second and third alteration events are interpreted as metasomatic fluid in filtration events which were focused along the VCR horizon. The second alteration event involved K⁺metasomatism which affected the footwall and hanging wall rocks up to a distance of several metres away from the VCR The third alteration event, during which the muscovite was partially replaced, was associated with the formation of chlorite in and immediately around the VCR-Chlorite thermometry suggests a temperature of 307 ± 14°C for this event. The close mineralogical association of gold with chlorite, secondary pyrite and secondary quartz in the VCR is interpreted to indicate that gold remobilisation was associated with the chlorite forming alteration event. The inhomogeneity found in gold particles within a hydrothermal quartz vein indicates a sharp drop in temperature after gold precipitation, suggesting a very short period of hydrothermal fluid infiltration.
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Bibliography: pages 157-167.
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Gartz, V. 1996. Post-depositional alteration of the Ventersdorp Contact Reef at Vaal Reef no. 10 shaft, Klerksdorp Goldfield. University of Cape Town.