Fluid evolution in a slate-belt gold deposit

Abstract

Auriferous quartz veins in the Hill End goldfield, NSW, Australia, comprise bedding-parallel vein sets and minor extension and fault-controlled veins which are hosted by a multiply deformed, Late Silurian slate-metagreywacke turbidite sequence. Fluid inclusions in quartz, either from bedding-parallel veins or from narrow, steeply N-dipping veins (‘leader’ veins) indicate a similar range in homogenisation temperatures (Th) from 350°C to 110°C. Within this range, Th data demonstrate five groupings in the temperature intervals 350–280°C, 280–250°C, 250–190°C, 190–150°C, and 150–110°C, corresponding to a variety of primary and secondary inclusions developed during five periods of vein growth under a generally declining temperature regime. Inclusion fluids are characterised by a low salinity of around 0.1 to 3.6 wt% NaCl equivalent. Laser Raman microprobe inclusion analysis indicates that gas-phase compositions relate to the paragenetic stage of the host quartz. H2O(g) and N2 dominate in the primary inclusions from barren, Stage I quartz; CH4 and CH4 + H2O(g) are important in inclusions related to the early gold forming events (equivalent to Stages II and III quartz), but inclusions developed during the last episode of gold deposition are characterised by H2O(g), CO2-rich and liquid-CO2 bearing fluids. Precipitation of gold was aided by sulphidation reactions or phase separation in response to periods of vein opening. Late in the paragenesis, gold deposition may have been promoted by oxidation of the ore fluid.