Fluid diversity in the gold-endowed Archean orogenic systems of the Abitibi greenstone belt (Canada) I: Constraining the PTX of prolonged hydrothermal systems

Abstract

Although many aspects of orogenic gold systems are well understood, considerable debate surrounds the chemistry of the ore-forming fluids and their relevance to mineralization. This may in part relate to the lack of both regional-scale studies and a uniform approach in studying fluid inclusions in these complex and protracted hydrothermal ore systems. In order to address this problem, over 30 orogenic-type deposits covering a variety of geological settings were systematically studied using the fluid inclusion assemblage (FIA) protocol integrated with microthermometry in the gold-endowed Abitibi greenstone belt, Canada. A diversity of fluid inclusion types are documented (i.e., carbonic CO2-CH4, aqueous-carbonic, aqueous), in addition to complex textural features, in particular decrepitation clusters which are rarely reported in the literature or the deposits studied; together these data provide new insight to these ore systems. Furthermore, we note that the fluid composition is significantly affected by the proximity of carbon-rich sedimentary units which are capable of producing a carbonic-only fluid. In later stages of hydrothermal evolution, variable XCO2 in aqueous-carbonic inclusions reflects variable degrees of fluid-rock interaction. Pressure cycling is an integral part of these systems, but petrographic evidence of in situ fluid unmixing is lacking, and cannot be accounted for gold mineralization. Vein-style gold is not related to fluid chemistry and, where best constrained with petrography (i.e., visible Au near fluid inclusions), P-T estimates based on fluid inclusion microthermometry link the mineralization to the later, low-pressure (<1.5 kbar) stages of deposit evolution. Accordingly, gold is coeval with quartz vein formation in epizonal deposits whereas it postdates quartz in the ore dominant mesozonal ore systems.