Fluid volatile composition associated with orogenic gold mineralization, Otago Schist, New Zealand: Implications of H2 and C2H6 for fluid evolution and gold source

Abstract

We provide the first large-scale investigation of gold mineralizing fluids from 19 gold deposits distributed in the Mesozoic Otago Schist belt endowed with approximately 580 tonnes of gold (past production and resources). Fluid inclusions from gold-bearing veins were petrographically studied and analysed by solid-probe mass spectrometry for H2O, CO2, N2, C2H6, He, Ar, CH4, SO2, H2S, and H2. Fluid inclusion assemblages are quite similar for all studied gold deposits, a particular feature for orogenic gold districts. Gold-bearing fluids are dominantly aqueous (H2O > 82.13 mol %) with very low contents of CO2 (<12.93 mol) in comparison with other orogenic gold deposits worldwide, with H2 (up to ~8 mol %) and C2H6 (up to ~0.5 mol %). Fluid composition is exceptionally homogeneous at the belt-scale. It is proposed that the relatively low content of CO2 and high content of H2 is related to chemical reactions in the fluids inducing graphite precipitation in the host rocks and mineralized structures and consumption of CO2 and CH4. The unique homogenous composition of fluids through time and space for the Otago Schist is related to the combination of: 1) the relatively homogeneous sedimentary rock source for fluid generation by metamorphic dehydration; 2) large-scale chemical reactions for consuming carbon-bearing volatiles; and 3) lack of external source (meteoric or magmatic) for fluid mixing. The presence of C2H6 is interpreted to be an indication that gold was sourced from primary gold-bearing pyrite hosted in carbon-rich turbidites, which is also the source for fluid generation and its hydrocarbon components.