Windows of metamorphic sulfur liberation in the crust: Implications for gold deposit genesis

Understanding the source of metamorphic sulfur is critical to clarifying the complete cycle of ore genesis, from source to sink, for several mineral deposit types. In this study, a mass balance approach and the thermodynamic computer programs Thermocalc and PerpleX were used to constrain the P-T range of pyrite breakdown to pyrrhotite (which liberates sulfur) in common metamorphic lithologies. The results suggest that most of the continental crusta??s metamorphic sulfur is liberated in a relatively narrow temperaturea??pressure window corresponding to the terminal breakdown of chlorite at moderate to low pressures. This is because pyrite stability is controlled partly by temperature and pressure, and partly by the amount of H2O present. During prograde metamorphism from the greenschist to the amphibolite facies, metamorphic H2O is produced primarily through chlorite breakdown in mafic to pelitic bulk compositions. As temperature increases, more sulfur is required from pyrite to maintain equilibrium proportions of H2O, H2S and SO2 in the fluid, and in addition, progressively more sulfur is required at lower pressures.