Auriferous Precambrian conglomerates of Witwatersrand

Abstract

Geological and mineralogical specificity of the world’s largest Witwatersrand gold deposit was defined by geodynamic processes in the Late Archean. The primary crust composed of felsic rocks (“gray gneisses”) was intruded 2.9–3.1 Ga ago by mafic melts that gave rise to greenstone belts. This was followed by the appearance of long-lived granulite mobile zones that promoted the formation of hydrosphere and atmosphere accompanied by an intense acidic leaching (weathering) of rocks in the greenstone belts. Numerous conglomerate interbeds were formed in the Witwatersrand Basin section due to vigorous eolian processes and floodwater washouts that produced a fan channel system mainly filled with quartz pebbles. At present, most researchers support a modified paleoplacer model of the origin of gold mineralization in Witwatersrand, suggesting a hydrothermal-metamorphic redistribution of the primary placer gold. According to various hydrothermal models, gold was introduced into sedimentary rocks in a water medium from outer deep sources or during the filtration of postsedimentary hydrothermal fluids. The present communication suggests that a significant portion of gold contained in reefs could be delivered to the sedimentation basin by the auriferous hydrothermal quartz of pebble or sand dimension that was metamorphosed at approximately 350–400°C. Metamorphism of gold was accompanied by its purification, transfer to matrix, and hydrothermal intrastratal redistribution.