Detrital gold, uranium, and pyrite concentrations related to sedimentology in the Precambrian Vaal Reef placer, Witwatersrand, South Africa

Abstract

The Vaal Reef placer was deposited during the Vaal Stage of the Upper Witwatersrand System, approximately 2,500 million years ago. The stratiform deposit is up to 5 feet thick and covers an area of over 100 square miles. It was discovered in 1934 and today is exploited at eight adjacent gold mines which produce approximately 16 percent of South Africa's total gold output.This study integrates stratigraphical, structural, petrological, and mineralogical data with a comprehensive set of sedimentological data to synthezise a model that describes the genesis of the Vaal Reef placer. Results show that the Vaal Reef placer was a shoreline feature of the Upper Witwatersrand Basin and that its deposition was controlled, firstly, by the major structure of the basin and, secondly, by a minor transverse synclinal structure that induced an entry point. The sediments in the footwall of the Vaal Reef placer regressed into the depository down a southeastern paleoslope in a shallow water fluviodeltaic environment. Lateral depositional limits are evident. A reduced supply of sediment to the technically subsiding shoreline resulted in a transgression, which truncated the consolidated deltaic plain to a very regular erosion surface with the remnants of a shallow braided drainage pattern incised upon it. The braided channels, up to 5 feet but generally less than 2 feet deep and 500 feet wide, meandered down a southeastern paleoslope. Protracted deposition of the Vaal Reef placer, marked by a mature pebbly quartz-arenite and basal algal growth, filled the irregularities of the erosion surface and spread between channels to form a dendroidal belt of sediment containing allogenic gold, uranium, and pyrite. Continuing transgression deepened the aqueous environment and a longshore, northeasterly current buried the Vaal Reef placer under a wedge of sediments that prograded from the southwest.New geological aspects that have emerged are, firstly, that the tectonic framework during Vaal Reef Stage deposition was very much simpler than previously envisaged. There is no evidence of subsidiary incipient basining. Secondly, modifications of previous conceptual models are made which emphasize the importance that detailed dispersal patterns and subtle paleosurface forms had on heavy mineral distribution in the distal fluviodeltaic environment of carbon-seam placers. Thirdly, sedimentary responses involving unconformities, disconformities, and accumulation are evidently related to particular entry points and may not be specifically correlated beyond their local tectonic framework. Fourthly, tectonic instability and basining is dated between Lower and Upper Ventersdorp System time and certainly before deposition of the Transvaal System.