Witwatersrand Metallogenesis: The Case for (Modified) Syngenesis

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 lowtemperature 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 f...

Cycling of biogenic elements drives biogeochemical gold cycling

“…Geochemical modelling has shown that gold solubility in ancient waterbodies was much higher than today and that gold likely occurred as Au(I)-sulfide complexes (Heinrich, 2015). Different authors have suggested different precipitation mechanisms for these Au-complexes, including: i) adsorption (Mossman et al, 2008); ii) oxidative precipitation triggered by the release of oxygen on the surface of the microbial mats resulting from oxygenic photosynthesis (Frimmel, 2014;Frimmel and Hennigh, 2015); iii) precipitation via reduction by organic hydrocarbons (Heinrich, 2015); and iv) reductive precipitation as a result of bacterial activities (Horscroft et al, 2012). A laboratory study by Lengke et al (2006) has shown that the cyanobacterium Plectonema boryanum can withstand high concentrations of mobile Au-complexes and has the ability to biomineralize them to metallic nano-and micro-phase gold.…”

Witwatersrand gold deposits formed by volcanic rain, anoxic rivers and Archaean life

“…Evidence for gold enrichment during sedimentation, from a slightly older source exposed in the eroding hinterland of the basin, seems to be winning in the debate, and modified placer proponents are claiming victory 12,13 . While the debate has centred on the timing of gold introduction (see Supplementary Methods), the possibility and extent of syngenetic chemical transport of gold in surface water has received little attention 5,14 , despite clear observations that many pyrite concretions precipitated chemically 10,11,15 and delicate films of gold are coating vertical carbon fibres of wellpreserved former bacterial mats 5,9,14 . These textures suggest lowtemperature chemical precipitation in a mature regolith or in a quiescent lake, rather than mechanical accumulation in a fast-flowing river environment that is typical for modern placer deposits 5 .…”

“…Gold may have been adsorbed into living cells 5,9 or was precipitated through abiotic reduction by organic carbon or hydrocarbons (represented by CH 4 ), as the third driver labelled 'life' in Fig. 2.…”

“…However, local rainout of only 3% of the sulphur from such an eruption can raise sulphate and sulphide contents of the rainwater to levels supporting 0.3 ppb Au solubility across our 100 × 100 km catchment area. Considering the recurrence of super-eruptions in the Deccan Trap 22 , Vaal Reef could have formed within a million years, by 100 decade-long periods of active solution transport of gold from the catchment to the deposition site, separated by longer periods of mechanical reworking and growth of microbial mats in shallow pools 5 . Depending on topography and weather patterns, the source of the volcanic degassing may have been hundreds of kilometres away and its lavas are not necessarily preserved on the Kaapvaal Craton, although the eroded Hlagothi Complex 21 is one candidate.…”

Dating detrital zircon from the gold-bearing Ventersdorp Contact Reef in the Ventersdorp Supergroup of South Africa

“…However, the likelihood of acidic conditions in rain and groundwater, enhanced by sulphur-rich volcanism, could also account for the high weathering and alteration indices which they reported. The action of acidic water on freshly ground glacial material could have enabled the dissolution and mobility of gold, which enhanced the grade of gold reefs, as proposed by Horscroft et al (2012) and Heinrich, (2015).…”