Metallogeny of subduction initiation revealed by chalcophile element behaviour in the Samail ophiolite

Abstract

Intra-oceanic subduction initiation marks the transition between the mature divergent and convergent stages of the Wilson cycle, each with its relatively well understood metallogeny. The metallogenic systematics of this transition are less clear, however, with uncertainties regarding precious metal enrichment in volcanogenic massive sulfide (VMS) deposits and the potential for magmatic-hydrothermal mineralization in nascent arcs. The Samail ophiolite is increasingly accepted to represent a ‘proto-arc’ formed during subduction initiation. Due to its volcanic glass record of magmatism and resource estimates for its VMS deposits, this ophiolite is well suited for elucidating this metallogeny. New analyses of these volcanic glasses reveal a systematic enrichment in melt chalcophile elements over the course of ophiolite magmatism, with enrichment of Au > Cu ≈ Ag > Zn correlated with sulfide-melt affinity. This enrichment can be explained by sequential remelting of the proto-arc mantle, with initial melting concentrating Au into residual sulfide and later remelting extracting this Au into sulfide-undersaturated melts. Subsequent melt fractionation further increased Au/Cu ratios relative to sulfide-saturated early melts. Despite clear slab contamination, transfer of subducted S into the mantle was minimal. These observations demonstrate that tectono-magmatic evolution drives the coupled Au-enrichment of proto-arc lavas and the VMS deposits they host, with footwall lava composition controlling the metal endowment of these deposits. Despite H₂O-saturation of late boninitic melts, their low S contents and weak Cl fluid/melt partitioning inhibited magmatic-hydrothermal metal extraction, limiting potential for high-sulfidation mineralization. Subduction maturation is necessary to trigger such mineralization, as seen in Pacific intra-oceanic arc systems.