The source processes affect the initial chalcophile element fertility of arc magmas

The chalcophile element fertility of arc magmas that modulates the enrichment of gold in porphyry copper deposits is negatively correlated with magma differentiation depth, which was attributed to sulfide saturation during magmatic evolution. However, the role of magma source processes remains to be evaluated. Here, we investigate the platinum-group element and copper systematics of a suite of oceanic slab-derived adakitic dikes from West Junggar, NW China. Geochemical and petrographic evidence suggests the adakitic magmas underwent sulfide segregation, but invokes a source control on the chalcophile element variations. Modeling indicates that adakitic melts were initially depleted in copper and palladium, and have gained palladium via interaction with peridotite. Our results demonstrate that source processes can greatly affect the initial chalcophile element fertility of arc magmas. High-degree mantle melting, combined with late sulfide saturation, produces ore-forming magmas with high chalcophile element fertility, favoring the gold-rich porphyry copper mineralization under thin arcs. In West Junggar, China, the unique genesis of oceanic slab-derived adakites explains their distinct platinum-group element and copper patterns relative to typical arc magmas, according to isotopic and trace elemental analyses.