METAMORPHIC DEHYDRATION REACTIONS DRIVE PYRITE BREAKDOWN AND FORMATION OF A SULFUR GENERATION WINDOW: THE SOURCE OF OROGENIC GOLD

The source of metamorphic sulfur, critical in understanding orogenic Au deposit genesis, has long been hypothesised to be derived from breakdown of pyrite (FeS₂) to pyrrhotite (Fe_1-xS) during regional metamorphism. 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 in common metamorphic rocks. The results suggest that most of the continental crust’s metamorphic S is produced in a relatively narrow P-T window corresponding to the terminal breakdown of chlorite at moderate to low pressures. This is because pyrite stability is controlled partly by P and T, 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 T increases, more S is required to maintain equilibrium proportions of H₂O, H₂S and SO₂ in the fluid, and in addition, more S is required at lower P. At low T, little S is required by fluid released during initial chlorite breakdown, whereas at higher T coinciding with the terminal breakdown of chlorite, not only is more fluid present, but the fluid’s S requirement has also increased dramatically. In this way, metamorphic dehydration drives pyrite breakdown and generation of S-rich hydrothermal fluids at mesothermal conditions. Beyond the chlorite stability field there is minimal metamorphic fluid production, except at low P and high T where muscovite can break down without causing melting; conditions that are a long way from typical crustal geotherms. Since hydrothermal fluids are inherently buoyant and consequently tend to migrate upwards and towards cooler temperatures through the crust, these results imply that orogenic gold deposits are most likely to form at lower amphibolite to greenschist facies conditions, an unlikely to form at higher T. The P constraints on metamorphic S generation imply that Archean geotherms would be more effective at generating orogenic Au deposits.