Composition and evolution of fluids during skarn development in the Monte Capanne thermal aureole, Elba Island, central Italy

We describe the chemistry of the fluids circulating during skarn formation by focusing on fluids trapped in calc-silicate minerals of the inner thermal aureole of the Late Miocene Monte Capanne intrusion of western Elba Island (central Italy). Primary, CH4-dominant C-O-H-S-salt fluid inclusions formed during prograde growth of the main skarn-forming mineral phases: grossular/andradite and vesuvianite. The variable phase ratios attest to heterogeneous entrapment of fluid, with co-entrapment of an immiscible hydrocarbon - brine mixture. Chemical elements driving skarn metasomatism such as Na, K, Ca, S and Cl, Fe and Mn were dominantly partitioned into the circulating fluid phase. The high salinity (apparent salinity between 58 and 70 wt% NaCl eq.) and the C-component of the fluids is interpreted as evidence for a composite origin of the skarn forming fluids that involves fluids derived from the crystallizing intrusion with contributions from metamorphic devolatilisation. Oxidation of a Fe-rich brine in an environment dominated by fluctuation in pressure from lithostatic to hydrostatic conditions (maintained by active crack-sealing) contributed to skarn development. Fluid infiltration conformed to a geothermal gradient of about 100 °C km–1, embracing the transition from high-temperature contact metamorphism and fluid-assisted skarn formation (at around 600 °C and 50-200 MPa) to a barren hydrothermal stage (around 200 °C and 20 MPa).