Abstract

To understand better the paleoenvironmental controls of placer formation in certain Witwatersrand rocks, a localized study of two marginally economic units was undertaken to relate gold and uranium distributions to sedimentary textures and structures. Both units, the Leader reef and Elsburg No. 5 reef, consist of interbedded quartz conglomerates and sandstones which are interpreted as deposits of proximal braided streams.On the local scale, good correlations exist between gold and uranium concentrations, supporting the contention that both components are concentrated by hydraulic processes. Gold/uranium ratios, however, are not consistent with grade changes in either reef, possibly due to provenance effects and/or unknown sorting mechanisms. In the Leader, thin carbonaceous seams contain high values and consistent gold/uranium ratios, suggesting that the seams formed from algal mats which entrapped heavy minerals as sediment mixtures were transported over them. Placer minerals are concentrated on scour surfaces and as matrix in well-sorted conglomerates. Sandstones and poorly sorted conglomerates are generally poorly mineralized, though moderate values occur in foreset laminae and scour troughs of some sandstone crossbeds. Poor to moderate values are contained by pebble lag deposits within sandstone beds. Two well-mineralized local deposits are examined in detail: a multistoried gravel bar and a constricted channelway produced by bankward movement of transverse bars. Both features present linear placer trends that can be oriented from underground exposures.Irregular distributions of gold and uranium are believed to be due partly to random erosion and temporary redeposition of bars and channel sediment. Braided alluvium in fluctuating flows comprises a series of local "point sources" which individually may have concentrated or dispersed heavy minerals in a previous event. Formation of local placer deposits thus depends both on the sorting mechanisms and the heavy mineral content of the deposits' local upstream point source.

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