Chalcophile element systematics of Mariana Trough basalts and implications for sulphide saturation evolution of intra-oceanic back-arc magmatic systems

Magmatic sulphides largely control the behaviour of chalcophile elements in evolving magmas and also play critical roles in ore-forming processes at convergent plate margins. Magmas at the back-arc spreading centre of Mariana Trough are less affected by slab-derived fluids than Mariana arc basalts. However, whether the evolution of chalcophile elements in Mariana Trough magmas resembles or differs from those in arc magmas is poorly known. Based on the detailed petrological and geochemical studies, we present high-precision analyses of whole-rock chalcophile element (S, Cu, Ag, Re, Pd, Pt, Ru, Ir) contents for a suit of basaltic lavas (n = 15) from the central Mariana Trough. The occurrence of sulphides in high-Mg olivine crystals (Fo80) indicates that at least one episode of sulphide saturation occurred during early-stage magmatic evolution. The sulphur contents of most basalts were not significantly affected by degassing processes due to low oxidation state and high hydrostatic pressures. Instead, progressive fractionation of liquid sulphide with magmatic evolution is the dominant process, as reflected by sulphide petrography, the decrease in Cu, Ag, Pd, and Pt contents, and concomitant increase in Cu/Pd ratios (22,500–708,500), as well as nearly constant Cu/Ag (mean of 3137 ± 848, 1s) with decreasing MgO. We attribute early sulphide saturation of Mariana Trough basalts to their generation and evolution under relatively reducing conditions, with the limited effect of slab components. Combined with data from other island arc and back-arc lavas, we further show that the extent of adding slab-derived components, regardless of specific arc and back-arc tectonic settings, are determinants of the sulphide evolution and chalcophile element behaviour in magmatic systems above intra-oceanic subduction zones. Early magmatic sulphide saturation at central Mariana Trough would result in barren magmatic systems that are not suitable for the large-scale Cu-Au mineralization at the seafloor.