Witwatersrand Metallogenesis

Abstract The Witwatersrand (WWR) ores contain more gold than could have been derived in particulate form by erosion from any conceivable type of source area as proposed by the modified placer hypothesis. In contrast to this, syngenesis goes further to explain a host of observations from those Late Archean Au-U ores. Although recycling, placer processes, and processes of hydrothermal (diagenetic/authigenic) mobilization all contributed, syngenesis was a major factor contributing to ore genesis in this huge metallogenic province. Over 80% of the gold occurs in the Main Reef and Bird Reef of the Johannesburg Subgroup in the Central Rand Group, and about half of this gold is closely associated with carbon derived from microbial remains. In the principal deposits within the WWR basin, the ore is disposed in thin carbonaceous horizons of extensive lateral continuity upon chronostratigraphic unconformities in otherwise unmineralized siliciclastic metasediments. The ore-bearing horizons are not themselves part of the erosion cycle that gave rise to those paleosurfaces but were generated during the initial phase of renewed cycles of deposition after long intervals of nondeposition. They bear little resemblance to placers, their alluvial character seemingly inherited from reworking in fluvial environments. Most of the gold and probably also part of the uranium were made available for transport in solution under relatively low-temperature, chemically aggressive environmental conditions, a situation favored on the emerging Kaapvaal Craton. Intense chemical weathering was made possible by the influence of the same ionizable gases as occur in geothermal systems, and this was a crucial factor leading to metallization. These elements, together with a host of other heavy metals, were then transported to the edge of the depository. A key confluence of conditions was completed with the blooming of microbial communities during hiatuses in sedimentation. Over large areas, microbial mats developed directly on paleosurfaces upon which the goldfields occupy slight depressions, bounded on either side by clean quartz arenites. The resulting metallization was a complex chemical and biochemical precipitation of gold, uranium, pyrite, and associated Co, Ni, Cu, Pb, and As in thin, areally extensive deposits. Metallization was focused at several carbonaceous horizons along the north and northwestern margins of the WWR basin, depending on the availability of metal-rich aqueous fluids coincident with the stillstand of land surface degradation and the consequent proliferation of microbial mats. Biochemical processes supplemented low-temperature geochemistry of the fluids in helping to concentrate a substantial portion of WWR gold in larger particles, which were transported further downslope and then subjected locally to fluvial processes. Gold precipitated outside of the preserved basin by these processes likewise will have undergone alluvial reworking prior to deposition in the conglomerates without the originally associated carbon; recognition of this feature diminishes the source rock problem. Minor remobilization of metals occurred during diagenesis and metamorphism.

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