A sedimentological investigation of the B-reef at Masimong 5 Shaft

[摘要] English: The ultimate objective of all exploration within the Witwatersrand Basin is to locate concentrations of gold which can be exploited economically. Gold in the B placer has high variance. The gold present in the B placer consists of small heavy detrital particles which are contained in the sedimentary host rock. In order to interpret the variable distribution of the gold within the B placer, cognisance must be taken of the sedimentological framework. The aim of this study was to employ a different approach to acquire an insight into the nature of the B placer and to shed more light on the depositional environments that played a major role during the formation of the B placer. Pebbles of the B placer conglomerates were investigated macroscopically in order to determine localities of possible gravel bar formation within the B placer. It was established that the B placer represented a braided river system with three different depositional environments, namely a fluvial environment, braid plain and a braid delta environment. The Upper Shale Marker at Masimong 5 Shaft played a major role in the development of these different depositional environments. The B placer remained river dominant and neither tide nor wave-related processes had an overwhelming influence on the system. The extreme fluvial dominance of the B placer improved the sorting of the braided delta system. The degree of reworking of the gravel bars in the braid delta by waves and current action resulted in the formation of thicker and better sorted conglomerates which, in turn, led to the formation of the B1 facies. Reworking and re-sedimentation of the B1 conglomerate occurred in the subaqueous setting. The B3 facies present at Masimong 5 Shaft were deposited in a purely fluvial braided environment. The improved perceptive of the B placer made it possible to identify four potential scenarios for the development of gravel bars within the braided river system, namely channel junctions, point bars, side bars or lateral bars; midchannel bars and barrier bars.

[发布机构] University of the Free State