Two well developed hedenbergitic, ilvaite-bearing skarns formed along the northern contact of the Maladeta batholith (Central Pyrenees, Spain) have been studied in order to determine their physico-chemical conditions of formation. The skarns formed at ~3 kbar total pressure following a classical two-stage (prograde/retrograde) evolution. Oxygen fugacity during skarn formation was buffered close to the NNO oxygen buffer while it is likely that sulfur fugacity remained nearly constant (10−8–10−10) throughout the second, retrograde stage. Fluid inclusions indicate that the hydrothermal fluid was a hypersaline brine (25–30 % NaCl eq.), sometimes halite-saturated at room temperature. Based on fluid inclusions and volatile speciation calculations, H2O was the dominating volatile while CO2 was present in low levels (X CO2 < 0.02). Thermodynamic data for ilvaite have been retrieved from published works and available experiments. These data have made possible the construction of several phase diagrams for the Ca-Fe-Si-O-H ± C system with which the stability of ilvaite with respect to a number of parameters is illustrated. The stability relationships render unlikely that ilvaite can form under pressures below 2 kbar and show that this mineral can only develop from fluids with low or very low CO2 content. With the exception of Mn, the presence of significant amounts of Al, Mg and other elements drastically reduces the stability field of ilvaite. Hence, this mineral should not be expected to occur in environments departing significantly from the 6-component system studied here.

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