1De Bever, Johannes Nicolaas 1Van Eeden, Johan

1Geology, Economic -- South Africa 1Geology, Stratigraphic 1Prospecting -- Research -- South Africa 1Sedimentary basins -- South Africa

11996 11997

1Moore, John 1Recent, successful, local and international, opencast gold mining operations, has led to renewed interest in the Black Reef as an exploration target in parts of the Transvaal Province. This has resulted in renewed research activity on the Black Reef over the last few years. No recent attempt been made, however, to collate available information on the Black Reef, past and present. The purpose of this dissertation, therefore, is to collate relevant data with special reference to the historical and present areas of Black Reef palaeoplacer mining in the Transval Basin. The Black Reef is a sporadically developed, sometimes auriferous basal conglomerate of the predominantly siliclastic Black Reef Quartzite Formation, that occurs at the base of the early Proterozoic, 2460 - 2050 M.a. Transvaal Supergroup, over much of its distribution in the Transvaal Province of the Republic of South Africa. It overlies Archaean basement rocks, the Witwatersrand and Ventersdorp Supergroups and the 'proto-basinal' Wolkberg and Buffelsfontein Groups of the Transvaal Supergroup, which may alternately, be correlates of the Ventersdorp Supergroup. The Black Reef Quartzite Formation is transitionally and conformably overlain by the carbonates of the Transvaal Supergroup. The Black Reef in the Transvaal basin, has an extensive, but intermittent history, of surface and underground mining activity, covering the late 1800's to the present day. At present, successful, opencast operations (Lindum Reefs) are ongoing on the West Rand near Randfontein and underground mining is taking place at Consolidated Modderfontein Mines on the East Rand. Past activities have taken place in the Klerksdorp - Venterdorp area of the southwestern Transvaal, in the Dedepoortrant area of the west - central Transvaal and in the Kaapsehoop area of the eastern Transvaal. It is found that economic Black Reef, is predominantly associated with proximal, footwall, auriferous Central Rand Group sediments of the Witwatersrand Supergroup and occurs in localised palaeochannels, which may be many kilometres in length, up to 600 m wide and up to 13 m deep. Morphology of these channels, is controlled by the footwall palaeotopography at the time of deposition of the Black Reef. The palaeotopography was, in turn, shaped by the strike, dip and differential erosion features of the footwall successions. Channels tend to overlie the topographically negative, softer formations, such as the Booysens Shale Formation and mostly trend parallel to the strike of these formations. In the southwestern Transvaal, the Black Reef developed as braided streams in post-Ventersdorp graben structures, whilst on the East Rand, northwest trending faulting controlled the general direction of the Black Reef channels. Deposition of the Black Reef took place on an erosional surface, in channels incised into pre - Black Reef basement. The overlying, siliclastic, sheetlike facies was deposited in a prograding or braided plain environment. This was terminated by transgressive conditions and the onset of minor shale deposition, followed by deposition of the carbonates of the Chunniespoort Group of the lower Transvaal Supergroup. It is concluded that the economic potential of the Black Reef is confined to areas overlying or bordering the Witwatersrand Basin. Some soil covered areas, overlying the Rand Anticline, may not have been properly assessses in the past. Underground mining of unoxidised Black Reef is not a viable economic prospect at present. The best potential for exploitation of the Black Reef, is as small-scale,opencast operations,similar to the Lindum Reefs operation.

11996 11997