Abstract

The Ventersdorp contact reef is an auriferous conglomerate horizon and economic orebody on top of the Late Archean Witwatersrand Supergroup in South Africa. It differs from the other conglomerate beds (reefs) in the Witwatersrand basin in having a massive metabasalt sequence as its hanging wall and by its intense post-depositional alteration. A geochemical, petrologic, and fluid inclusion study of the Ventersdorp contact reef reveals a polyphase hydrothermal alteration history. Regional metamorphism under acidic conditions was followed by potassic alteration along the reef. The potassic alteration, which also extended into the hanging-wall and footwall rocks, was subsequently followed by a chloritization episode that was restricted to the reef and its immediate contacts. Mass-balance calculations suggest that the latter alteration required only minor fluid influx from an external source, whereas the potassic alteration reflects the progressive dehydration of the siliciclastic footwall rocks and associated hydration of the basaltic hanging wall. Based on microtextural observations on quartz using SEM-cathodoluminescence imaging, the chloritization event is ascribed to the 2020-Ma Vredefort impact event that created the necessary secondary permeability in the form of microfractures particularly within the conglomerate bed. Considering that the Ventersdorp contact reef is generally regarded as the most intensely altered reef in the Witwatersrand basin, the relatively minor fluid infiltration established in this study suggests that, contrary to prevailing opinion, the other auriferous conglomerate beds might have experienced even less influx of externally derived hydrothermal fluids.

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