Walshe, John L.1, Bath, Adam, B.1
1CSIRO Mineral Resources, PO Box 1130, Bentley, Western Australia 6102, Australia
It is possible to think of the mineral systems that created the Earth’s major mineral deposits and provinces as volato-thermally driven chemical engines with roots deep in the mantle. It may be argued that the chemical potential of systems was linked to this degassing history and reflects the interplay of deep-Earth anhydrous fluids with the Earth’s hydrous outer layers. Metal transport and deposition capacity was closely linked to propagation of redox and related physico-chemical gradients through systems. Such arguments imply quite specific links between the formation of the Earth’s resources across time and space, secular changes in architecture and geochemistry of the planet over some 4.5 billion years of evolution and Earth phenomena such as mass extinction events, global anoxia and atmospheric evolution. Conceivably a better understanding of large-scale linkages may lead to better techniques for differentiating productive terrains and epochs of Earth history as well as the links between metallogenesis and other Earth processes.
The Late Archean Au deposits, at ~ 2.7 to 2.63 Ga, can be interpreted as one manifestation of planetary degassing of highly reduced and oxidized volatiles. The Late Archaean gold deposits are known from five continents, are hosted within supra-crustal volcano-sedimentary sequences and are spatially associated with trans-crustal structures, 100s of kilometres in length, as exemplified by the deposits of the eastern Yilgarn Craton, Western Australia. All the productive gold camps of the eastern Yilgarn gold province show evidence of deposit to district scale mineral zoning and systematic patterns in 13Ccarbonate and δ34Ssulfide that can be related to chemical gradients sustained by fluxes of reduced (H2, CH4) and oxidized volatiles (SO2) of mantle origin, triggered by tectonic events and related magmatism. Gold transport and deposition was favoured by chemically zoned volato-thermal plumes with an inner core of reduced, alkaline & quartz under-saturated fluids and outer zones of oxidized fluids. Gradients in activity of H2O were sustained by pulsed injection of anhydrous mantle-volatiles into pre-existing crustal hydrothermal systems. Local electrical discharges occurred over distances of ~10 to 100 m, coupled to rock fracturing in the anhydrous parts of systems. Gold deposition was controlled by water activity gradients, coupled with pH and redox gradients.
Sustaining these gradients was key to sustaining gold transport and formation of high-grade resources. Collapse led to dispersion of the gold ± low grade mineralization.
It is hypothesized that the ultimate driver of the Late Archean Au epoch was an electron flux from the core-mantle boundary that released H from the mantle (e + OH_ = O2- + H), drove redox reactions and sustained the volatile flux of the late Archean.
Dr John Walshe is Chief Research Scientist, CSIRO Mineral Resources. Since joining CSIRO in 1995, Dr Walshe has contributed to developing mineral systems concepts and applications to mineral exploration. Prior to joining CSIRO, Dr Walshe lectured at the Australian National University. He is a graduate of the University of Tasmania.