NATURE AND EVOLUTION OF METAMORPHIC FLUIDS ASSOCIATED WITH TURBIDITE-HOSTED GOLD DEPOSITS - HILL END GOLDFIELD, NSW, AUSTRALIA

The Hill goldfield, NSW, Australia, is an example of a syntectonic, sl ate-belt gold deposit formed in a multiply deformed, Late Silurian sla te-metagreywacke turbidite sequence. Gold is confined to bedding-paral lel veins and discordant leader veins composed of as many as four gene rations of quartz, accompanied by phyllosilicates, carbonates and mino r sulphides. Vein formation and gold deposition was apparently synchro nous with Early Carboniferous metamorphism and deformation. Homogenisa tion temperatures (T(h)) for fluid inclusions in vein quartz demonstra te five groupings in the temperature intervals 350-280-degrees-C, 280- 250-degrees-C, 250-190-degrees-C 190-150-degrees-C, and 150-110-degree s-C, corresponding to a variety of primary and secondary inclusions de veloped during four periods of vein quartz deposition under a generall y declining temperature regime. Inclusion fluids are characterised by a low salinity of around 0.1 to 3.6 wt.% NaCl equivalent. The dominant gas phase present in the inclusion fluids varies from N2 in the early stages of the paragenesis, through CH4 during the main episode of gol d deposition, to CO2-rich fluids associated with late-stage mineralisa tion. DeltaO-18 values for vein quartz (range 15.1-17.1 parts per thou sand) and vein carbonate (range 11.3-13.4 parts per thousand) are typi cal of metamorphic mineralisation. DELTAD composition of hydrous miner als and inclusion fluids (range -53 to -138 parts per thousand) sugges t an influx of meteoric water in the later mineralising fluids. This c onclusion is supported by deltaC-13-data for vein calcite(range -2.5 t o -9.7 parts per thousand). DeltaS-34 composition of vein pyrrhotite a nd pyrite ranges from 6.9 to 7.8 parts per thousand early in the parag enesis, to lighter values (around 4.2 to 5.8 parts per thousand) accom panying late gold deposition from more oxidising fluids. Sulphur isoto pe data imply a sulphur source from underlying turbidites and an incre ase in fluid oxidation state during mineralisation. Lead isotope measu rements on vein pyrite, arsenopyrite, galena and gold are characterise d by two isotope populations with Pb-207/Pb-206 ratios of 0. 862 and 0 . 860, which define two discrete mineralising events during vein forma tion. Consistency between data from vein minerals and lead isotope sig natures for potential source rocks indicate that lead was derived from the sedimentary pile.