Unraveling the Effects of Melt–Mantle Interactions on the Gold Fertility of Magmas

Orogenic gold: is a genetic association with magmatism realistic?

“…The refertilization and oxidation of depleted mantle, preserved below continents for billions of years because of its negative thermal buoyancy coexisting with a positive chemical buoyancy, is a consequence of metasomatism due to slab subduction. Tassara et al (2020) describe how the oxidation of silicate melts rising through such an enriched SCLM can lead to S and Au enrichment in magma prior to ascent and magmatic-hydrothermal processes at shallow crustal levels. Subsequent melting of the newly enriched mantle, commonly due to slab delamination or rollback allowing asthenospheric upwelling, is expressed as shoshonitic or at least high-K calc-alkaline magmatism (Feeley 2003).…”

Platinum-group element and Au geochemistry of Late Archean to Proterozoic calc-alkaline and alkaline magmas in the Yilgarn Craton, Western Australia

“…Journal Pre-proof probably derived from low-degree partial melting of lithospheric mantle sources that experienced ancient metasomatic enrichment, reflected in their highly elevated incompatible trace elements and enriched Sr-Nd-Hf isotope compositions (Choi, 2020;Giuliani et al, 2015;Nowell et al, 2004). Also, the metasomatised SCLM is considered to be relatively oxidised (e.g., Yaxley et al, 2017), which would facilitate the breakdown of sulfides and metal alloys and transfer for their metal budget into silicate melts (Tassara et al, 2020). Taken together, the observed flat PGE patterns with moderately high IPGE values may reflect the contribution of IPGE-rich and relatively oxidised peridotitic material from SCLM.…”

“…Alternatively, Au enrichment might be due to the physical entrainment and/or assimilation of locally Au-rich lithospheric material, as indicated by Au enrichment in some mantle xenoliths. In particular, a redox gradient during ascent of asthenospheric-derived mantle melts reacting with lithospheric mantle wall-rock may facilitate the breakdown of sulfides or dissolution of Au-bearing alloys and release of Au (Tassara et al, 2020). A more comprehensive assessment of Au compositions in alkaline ultramafic magmas and mantle xenoliths worldwide will be required to test these hypotheses.…”

Mineralogy of the HSE in the subcontinental lithospheric mantle —An interpretive review

“…The size of this window is a function of fO 2 and to a lesser extent of the FeO content in the silicate melt (Andrews and Brenan, 2002;Bockrath et al, 2004b). Thus, magmatic laurite can crystallize more easily from oxidized silicate melts with fO 2 higher than QFM = -1.5, conditions that apply to the late infiltrating melt (i.e., the glassy veins) that bear the laurites in the Cerro Redondo and Pali-Aike peridotites (Tassara et al, 2020). However, some experiments have shown that a Ru metal nugget preceded many magmatic laurite grains and then reacted with sulfur once fS 2 increased in the silicate melt (Bockrath et al, 2004b).…”

“…2f in Tassara et al, 2017). Decreasing pressure or increasing fO 2 during the melt-rock reaction associated with silicate melt infiltration would increase the S solubility (Mavrogenes et al, 1999;Jugo et al, 2010), promoting the partial dissolution of the Ni-Cu sulfide blebs into the silicate melt and the release of the gold particles (Tassara et al, 2020). If liberated to the surrounding silicate melt, the high Au solubility in the silicate melt would result in the partial resorption of these particles.…”