The supergiant Witwatersrand Goldfield: A result of anhydrous mantle degassing on Earth’s earliest supercontinent

Abstract

The origin of the supergiant high grade Witwatersrand gold deposits has attracted debate for several decades with modified gold placer models contrasting with orogenic gold crustal metamorphic models. Neither explains the anomalous quantities of gold, pyrobitumen, and ‘carbon leaders’ in the Basin during the otherwise gold- and carbon-poor Mesoarchean. The Witwatersrand, part of a trilogy with the Bushveld Complex PGE-Cr-Fe-Ti-V and Kaapvaal diamond fields as the largest deposits globally, is unique, requiring consideration of a tectonic-scale non-conventional model to explain it.

Host to the Witwatersrand Basin, the Kaapvaal Craton represents a continental block within the first supercontinent Ur. Iron-Ni alloy inclusions and terrestrial-like C isotopes in CLIPPIR diamonds within the Craton provide evidence for the addition of metals, including those that later resided in cumulate slurries in the giant Bushveld Complex, to the mantle during Archean core-mantle-crust overturns. Mesoarchean mantle degassing is considered the most likely key process for transport of gold by anhydrous fluids as indicated by CH₄ and H₂ inclusions in CLIPPIR diamonds. The lithosphere-scale Colesberg suture on the western margin of the Witwatersrand Basin, the entry for sedimentary fans during deposition of the Central Rand gold reefs and carbon leaders, represents reactivation of a subduction zone during breakup of Ur at ca. 3.0–2.9 Ga. It is proposed that this suture, recognized as a break in lithosphere profiles, acted as the conduit for CH₄ and H₂ fluids carrying Au in metal hydrides to the mid-upper mantle where CH₄ and H₂ dissociated to form hydrocarbons which extracted additional Au from mantle sulfides. The gold and hydrocarbons were then exhaled on to the western hinterland of the Witwatersrand Basin to provide the anomalous quantity of Au and C required to explain both the Witwatersrand endowment and why such a unique conjunction of factors has never occurred again in Earth history, although most subsequent primary gold-rich deposits are also now considered to have had a mantle connection.