Behaviour of gold during partial melting of supra subduction zone mantle wedge

Gold-rich magmatic-hydrothermal deposits form in atypical convergent plate-margin tectonic settings conducive to melting lithospheric mantle. Deposits tend to be associated with K₂O-enriched arc magmas, but ultrapotassic magmas are almost never gold-fertile. Our global compilations of chemical compositions of peridotite xenoliths and of near-primary mafic melts reveal that, during mantle-wedge melting, Au typically has a melt/restite partition coefficient like Yb and Na, much higher than Cu and PGEs. Redox-sensitive element ratios in primitive mafic melts in gold provinces typically indicate ƒO₂ conditions like ordinary arc magmas. Lithophile-element ratios in primitive mafic melts reveal that Au-fertile melts form from garnet-undersaturated, hornblende(±phlogopite)-bearing sources within spinel lherzolite facies, constraining most melting to pressures ≲1.8 GPa and temperatures ≲1100°C, below the solidus of monosulphide solid solution (MSS) of typical mantle composition. Therefore, silicate melts acquire metallogenic fertility primarily by selectively dissolving Au in crystalline MSS. We highlight Au enrichment in lithospheric mantle due to zone refining during orogenic thickening of lithospheric mantle, wherein incompatible-element-rich partial melts are generated by hornblende breakdown at its depth limit and migrate into overlying cooler lithospheric mantle. Fractional crystallisation during ascent of these low-degree melts as hornblende + MSS ± phlogopite-bearing metasomatic veins generates enriched domains within the lithospheric mantle. These low-temperature veins selectively re-melt later upon hydration, decompression, or heating, producing Au-fertile melts in extensional and contractional tectonic settings. Our case studies of gold provinces in rift and orogenic stress settings indicate similar mantle melt compositions but magmatic differentiation trends that diverge according to tectonic stress regime.