Magmatic sulfide immiscibility at an active magmatic-hydrothermal system: The case of La Fossa (Vulcano, Italy)

Magmatic sulfide minerals preserved in fresh volcanic rocks can be used to trace sulfur and chalcophile element evolution in magmatic systems and to constrain the potential magmatic contribution to ore-forming fluids. In this work, we present a petrographic and microanalytical study of magmatic sulfides in the products of La Fossa (Vulcano, southern Italy), an active arc-related volcano currently with a shallow acidic hydrothermal system, and discuss the results with petrological data obtained from melt inclusions. While the sulfur-rich basalts feeding the plumbing system were oxidized and sulfide-undersaturated, as common in arc-basalts, a major event of sulfide melt separation from the silicate melt occurred at the trachytic stage, in the shallow reservoir (minimum depth based on H₂O content of melt inclusions is 1200–2200 m), promoted by the lowering of fO₂. Sulfide immiscibility was unrelated to magnetite fractionation and was encountered after a notable sulfur loss at the transition from mafic to intermediate magma composition. The late sulfide saturation implies that the silicate melt became enriched in Cu with differentiation. This is indicated by the relatively high Cu concentration in the analyzed sulfide blebs, comparable to the composition of sulfides found in other active arc volcanoes as well as in magmatic systems associated with porphyry copper mineralization. The newly reported occurrence of sulfide immiscibility in the La Fossa shallow reservoir has implications for the evolution of sulfur in the plumbing system and for the contribution of magmatic sulfur and metals to surface fumaroles, as well as to the deep hydrothermal fluids during non-eruptive periods. The results indicate that La Fossa volcano is a possible active analogue of mineralizing magmatic-hydrothermal systems.