Abstract

A depositional model is presented for the gold-rich Precambrian deposits within the Witwatersrand basin. It is a closed basin inset in a now stable craton of the southern African Shield with an areal extent of approximately 39,000 km 2 The clastic fill has a maximum thickness of 7,500 m and comprises laterally persistent conglomerates, sandstones and shales occurring within thick (up to 800 m), widespread, and continually alternating coarser and finer-grained sequences. The succession as a whole coarsens upward and is divided into a sandstone and shale dominated lower division and a sandstone and conglomerate dominated upper division. Mineralogical maturity also increases upward, the sandstones of the lower division being mainly sub-greywackes, those of the upper division being feldspathic quartzites. Analysis of the abundant sedimentologic data gathered during exploration and development of the gold deposits demonstrates that sedimentation within the basin fits a braided alluvial plain and lacustrine model. The origin and lateral relationships of the different rock types are explained by this depositional setting. The thick alternating coarser and finer grained sequences are explained by a shifting depositional system (sourceward and basinward) controlled by tectonism in the basin and source area. The upward increase in mineralogical maturity and grain size is explained by migration inward of the basin margins, contemporaneous with sedimentation, causing marginal reworking of the lower division sequence as well as regression of the sedimentary succession. The lower division becomes a distal environmental equivalent of the upper division.

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