Linking Archaean climate change with gold metallogeny

Abstract

First large-scale concentration of gold to ore grade in Earth’s crust took place at around 2.9 Ga in quartz-pebble conglomerates, well before endogenous deposits, such as porphyry and epithermal systems or orogenic-type deposits, started to play a significant role from ca. 2.75 Ga onwards. The conglomerate-hosted gold placers, typified by those in the Mesoarchaeaen Witwatersrand Basin in South Africa, are thought to have been sourced from the leaching of background levels of gold in the Archaean continental crust, promoted by deep chemical weathering under a reducing acidic atmosphere. Gold dissolved in meteoric waters was trapped by possibly acidophile microbes, fossil remnants of which are preserved as kerogen layers in the 2.9 Ga lower Central Rand Group of the Witwatersrand Supergroup. Mechanical reworking of the delicate microbial mat-bound gold led to rich gold placers from 2.9 Ga onwards. Well-endowed gold placers older than 2.9 Ga are conspicuously missing, although suitable siliciclastic host rocks, even containing relics of former microbes, are known from continental sediment sequences as old as 3.22 Ga. A review of climate-sensitive rock types and geochemical data on the extent of chemical weathering reveals that the period from 2.96–2.91 Ga was cold, with repeated glaciations as evidenced by glaciogenic diamicite, whereas warmer and (or) wetter climates reigned from 2.90 to 2.78 Ga. In concert with the temporal distribution of placer gold accumulation, the conclusion is reached that chemical weathering rate, dictated by climate, was a key determining factor in the exogenous binding of Au into sedimentary deposits.