Placer formation during progradational fluvial degradation; the late Archean Middelvlei gold placer, Witwatersrand, South Africa

Previous workers have established experimentally that geomorphologic conditions of degradation are favorable for the transport and concentration of heavy minerals and should therefore be conducive to the formation of placers. This principle may apply generally to the late Archean gold-uranium placers of South Africa, most of which rest on erosion surfaces.The Middelvlei gold placer, a quartz-pebble conglomerate which occurs near the base of the Central Rand Group of the Witwatersrand Supergroup, is mined selectively in a gold field known as the West Wits Line situated west of Johannesburg. The placer is considered part of a depositional system which overlies a regional mudstone marker, the Green Bar Member, and comprises three members, the Footwall Member, the Middelvlei placer, and the Hanging-wall Member.The Footwall Member consists predominantly of trough crossbedded quartz wackes, with lag conglomerates and massive mudstone as subordinate lithofacies. Thickness of this member varies from about 35 to more than 100 m and is generally smaller in the eastern parts of the gold field, where it is overlain by a large-pebble-type Middelvlei placer. Paleocurrent distributions of the Footwall Member are unimodal in all parts of the gold field with mean directions toward the southeast in the eastern regions and toward the south-southeast in the western parts. Directional variances are typically smaller than 1,600.The Middelvlei placer overlies the Footwall Member unconformably in most parts of the gold field, but in some southern parts of the western region no sedimentary break between these two members is evident. The placer comprises interbeds of quartz-pebble conglomerate, mostly massive, cross- and horizontally bedded quartz wacke, and thin massive mudstone.Maximum pebble size is about -6.0 phi and sizes are generally greater in the eastern parts of the gold field. Paleocurrent distributions are also unimodal, and in any particular area the means are not significantly different from those of the underlying Footwall Member. Directional variances are typically smaller than 2,000.The Hanging-wall Member consists mainly of trough crossbedded quartz wackes and subordinate lag conglomerates. For the western regions of the gold field information on paleo-currents is sparse, but in the eastern region the distribution is unimodal, with a mean not significantly different from that of the underlying Middelvlei placer.The Middelvlei depositional system is thought to be fluvial in origin and was probably deposited in braided streams. The Footwall Member and the Middelvlei placer constitute a coarsening-upward megacycle and are thought to represent a prograding braid plain. In most areas the transport of coarse-grained gravel during progradation was accompanied by erosion of the underlying Footwall Member.Gold concentrations are generally higher in areas of severe degradation of the Footwall Member than in regions where little or no basal erosion by the placer has taken place and where a conformable relationship between the two members exists. Statistical analyses revealed that the gold-rich facies of the placer is a single, relatively thin bed of massive, clast-supported conglomerate with large pebbles.