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

摘要

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 crust's metamorphic sulfur is liberated in a relatively narrow temperature-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 H_2O present. During prograde metamorphism from the greenschist to the amphibolite facies, metamorphic H_2O 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 H_2O, H_2S and SO_2 in the fluid, and in addition, progressively more sulfur is required at lower pressures. At low temperatures, little sulfur is required by metamorphic fluid released during initial chlorite breakdown, whereas at higher temperatures coinciding with the terminal breakdown of chlorite, not only is more fluid present, but the fluid's sulfur requirement has also increased dramatically. In this way, metamorphic dehydration drives pyrite breakdown and generation of sulfur-rich hydrothermal fluids at mesothermal conditions. Beyond the chlorite stability field there is minimal metamorphic fluid production, except at low pressures and high temperatures where muscovite can break down without causing melting; conditions that are a long way from typical crustal geotherms. However, deformation also plays a key role in pyrite breakdown. Without deformation, small amounts of fluid in chemical communication with individual pyrite grains will quickly acquire equilibrium concentrations of the sulfur species and minimal pyrite breakdown is necessary. Whereas during deformation, there may be a continuous fluid flux past pyrite grains, promoting ongoing sulfur liberation. In this way, periods of deformation may be the major sulfur-liberating episodes during a metamorphic cycle. 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 prehnite-pumpellyite facies conditions, and unlikely to form at higher temperatures. The pressure constraint on metamorphic sulfur liberation implies that tectonic settings that allow prograde metamorphism to follow low pressure P-T-t paths in an occasionally compressional or transpressional environment are necessary. Settings that promote extensive injection of felsic magma into a mid-crust that contains a significant proportion of pyritic carbonaceous metasediment are shown to be ideal for orogenic gold deposit genesis. Inverted back-arc basins are interpreted to be the most favourable of these.