Fate of gold and base metals during metamorphic devolatilization of a pelite

Scavenging of gold during metamorphic devolatilization is a widely accepted model for fluid and metal sourcing in orogenic gold deposits. In order to further constrain this process and quantify the capacity of autogenous fluids to release metals from pelites, we investigated the behaviors of S, Au, As, Cu, Pb, and Zn during pelite metamorphic devolatilization using thermodynamic modeling within the Al–As–Au–Cl–Cu–Fe–H–K–Mg–Na–O–Pb–S–Si–Ti–Zn system over a P–T range of 350–650 °C and 0.8–5 kbar.The model revealed that S, Au and base metals are predominantly released via partitioning into the fluid phase during reactions that liberate H2O: the dehydration of chlorite and muscovite, and to a lesser extent, the replacement of pyrite by pyrrhotite. Negligible sulfur is liberated during the pyrite–pyrrhotite transition, because the excess sulfur reacts with Fe in chlorite and muscovite to form pyrrhotite. The sulfidation of chlorite/muscovite releases water, so that a significant amount of Au can be liberated from S-rich pelites at the pyrite to pyrrhotite transition: up to 0.5 ppb Au (as a proportion of bulk rock) can be stripped from a pelite containing 1 wt.% sulfur, whereas only trace amounts of base metals can be mobilized under these conditions. Chlorite dehydration is the most important process in metal extraction; up to 2 ppb Au, 1.5 ppm Cu, 1 ppm Pb and 2 ppm Zn (as proportions of bulk rock) can be extracted from a pelite by autogenous fluids upon crossing the greenschist–amphibolite facies boundary. In comparison, an average pelite contains ∼3 ppb Au, indicating that most Au within an average pelite can be scavenged as a result of the breakdown of chlorite.Prograde metamorphism is an efficient mechanism for generating Au-bearing ore fluids: most Au can be extracted during chlorite dehydration from a source rock of average pelitic composition. In contrast, only a small portion of base metals can be released in autogenous fluids, and therefore only minor enrichment in base metals is expected within large orogenic gold deposits in metamorphic terrains, which is consistent with observations globally.