FLUID EVOLUTION IN A SLATE-BELT GOLD DEPOSIT - A FLUID INCLUSION STUDY OF THE HILL END GOLDFIELD, NSW, AUSTRALIA

Auriferous quartz veins in the Hill End goldfield, NSW, Australia, com prise bedding-parallel vein sets and minor extension and fault-control led veins which are hosted by a multiply deformed, Late Silurian slate -meta-greywacke 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 temperature s (T-h) from 350 degrees C to 110 degrees C. Within this range, T-h da ta demonstrate five groupings in the temperature intervals 350-280 deg rees C, 280-250 degrees C, 250-190 degrees C, 190-150 degrees C, and 1 50-110 degrees C, corresponding to a variety of primary and secondary inclusions developed during five periods of vein growth under a genera lly declining temperature regime. Inclusion fluids are characterised b y a low salinity of around 0.1 to 3.6 wt % NaCl equivalent. Laser Rama n microprobe inclusion analysis indicates that gas-phase compositions relate to the paragenetic stage of the host quartz. H2O(g) and N-2 dom inate in the primary inclusions from barren, Stage I quartz; CH4 and C H4 + H2O(g) are important in inclusions related to the early gold form ing events (equivalent to Stages II and III quartz), but inclusions de veloped during the last episode of gold deposition are characterised b y H2O(g), CO2-rich and liquid-CO2 bearing fluids. Precipitation of gol d was aided by sulphidation reactions or phase separation in response to periods of vein opening. Late in the paragenesis, gold deposition m ay have been promoted by oxidation of the ore fluid.