Platinum-group element geochemistry of the shoshonitic igneous suite of Vulcano (Aeolian Arc, Italy): implications for chalcophile element fertility of arc magmas

Platinum-group element (PGE) geochemistry of arc-related magmas can be used to constrain the timing of sulfide saturation, which plays a critical role in the chalcophile element fertility of evolving magmatic systems. In this study, we provide new major and trace elements, PGE, Re and Au data for the shoshonitic suite (from basalt to rhyolite) of the active volcanic system of Vulcano (Aeolian Islands, Italy), an excellent study case for investigating mineralizing processes in arc volcanoes. The most primitive magma erupted at Vulcano is characterized by fractionation of Ir-group PGE (Ir, Os, Ru) relative to the Pd-group PGE (Pd, Pt, Rh), which is attributed to the magma being saturated with a Pt-rich alloy and possibly spinel and Os–Ir alloy. A negative Au anomaly, shown by Vulcano magmas, suggests that the Au was lost to an early exsolved S– (and moderately Cl–) rich fluid phase that, upon migration to higher levels of the feeding system, could have enhanced the mineralizing potential of the magmatic system. The Pd content of Vulcano primitive magmas is high (~ 8 ppb) and comparable to values found in other arc-related mineralized systems. The early Pd depletion, relative to Au and Cu, during evolution towards more evolved magmas, suggests that the magmatic system reached sulfide saturation at about 4 wt.% MgO. Chalcophile element fractionation modeling shows that the immiscible sulfide mass proportion was initially very low (~ 0.0015%), and gradually increased to about 0.1% at about 2 wt.% MgO, causing a late depletion of the less compatible chalcophile elements, including Cu. A significant chalcophile metal depletion in the silicate melt occurred once sulfide saturation was achieved, well before hydrosaline fluid exsolution at < 0.5 wt.% MgO. As a consequence, when the evolving melt reached volatile saturation at the rhyolite stage, it did not contain enough Cu and Au to form a mineralized system.