Abstract
The mineralization potential of arc magmas depends, among other factors, on the timing of sulfide melt saturationrelative to magma differentiation and to exsolution of a magmatic fluid phase. In fossil mineralized or barrensystems, understanding the evolution of metals along the magma differentiation path is often hindered by latemagmatic processes and hydrothermal alteration. To better understand the process of metal evolution “caughtin the act” in crustal reservoirs, we analyzed magmatic sulfides and melt inclusions found within eruptive productsfrom the active arc volcano, La Fossa (Vulcano Island, Italy), for the basalt to rhyolite compositional spectrum.We found that, in case of sulfide-undersaturated and volatile-rich arc basalts, metals are scarcelysubtracted by degassing during ascent to shallowcrustal reservoirs and reach the highest abundances in intermediatemagmas (250 ppm Cu). At sulfide saturation the sulfide melt has 34–66 wt% Cu, leading to a dramatic decreasein chalcophile metals dissolved in the silicate melt. After fractionation of only 0.2–0.3 wt% of sulfide in thesolid assemblage, the exsolved sulfide is a monosulfide solid solution (pyrrhotite) containing <3 wt% Cu. Metalsthat do not partition in sulfides (Pb, Zn) increase their concentrations during magmatic evolution until they aresequestered by a Cl-rich aqueous fluid phase exsolved at the rhyolitic stage. The absolute and Cu-normalized concentrationsof metals in sulfide inclusions are similar to sulfide accessories in magmatic rocks associated withworld-class porphyry Cu systems. Our results demonstrate that the mechanisms governing metal evolution inferredfor the magmatic stage in porphyry Cu environments can be also tracked at an active arc volcano, usingeruptive products as snapshots of the magmatic evolution. Arc volcanoes can thus be viewed as ideal activeanalogues when studying these crucial processes for the formation of porphyry Cu deposits.