THE NATURE AND ORIGIN OF A GRANITOID-RELATED GOLD DEPOSIT AT DARGUES REEF, MAJORS-CREEK, NEW-SOUTH-WALES

Dargue's Reef is a mesothermal, gold sulfide deposit developed in alte red granitoid rocks of the Braidwood Granodiorite in southeastern New South Wales, Australia. Two main lodes consist of narrow zones (0.6-9. 5 m wide) of intense sericitic alteration and pyritization (15-30% pyr ite), enclosed in areas of propylitic alteration. Deposition of barren euhedral-subhedral pyrite accompanied early-stage alteration and was followed by deposition of irregular pyrite containing numerous small i nclusions of silicates, calcite, chalcopyrite, Bi sulfo-salts, galena, gold, trace tellurides, native bismuth, and pyrrhotite. Separate aggr egates of chalcopyrite, Ri sulfosalts, and tetrahedrite are intergrown with the silicate alteration minerals. Fluid inclusion data indicate that gold-bearing hydrothermal fluids were CO2 bearing, of low to mode rate salinity, and medium to low temperature (<350 degrees C). Two K-A r dates on alteration sericite from the main ore zone indicate an age of about 409 Ma for the alterations and show that mineralization was b roadly contemporaneous with emplacement of the host pluton in the earl iest Devonian. Sulfur isotope ratios in pyrite from the mineralization (delta(34)S, -0.4 to -3.4 parts per thousand) and disseminated pyrite in the host granodiorite (delta(34)S, 1.4-2.5 parts per thousand) are consistent with a magmatic sulfur source. Carbon isotope data for cal cites from the mineralization indicate delta(13)C values for the fluid close to 0 per mil. These are not inconsistent with a magmatic C sour ce, but they are also consistent with derivation of CO2 from minor lim estones in the country rocks. Textural and fluid inclusion evidence su ggest that CO2 activity in the fluids increased with time, which could be explained by late stage introduction of country-rock fluids or pos sibly influx of fluid from a later intrusion at depth. Oxygen isotope data for calcites from the ores (6.5-10.9 parts per thousand) and late -stage veinlets (6.5 parts per thousand) implicate fluids similar to t hose in porphyry-mineralizing systems with some intermixing of delta(1 8)O-depleted, probably meteoric fluid. The geometry internal lithologi cal variation, and magnetic features of the Braidwood Granodiorite sug gest that gold mineralization is developed close to the roof zone of t he intrusion. This zone extends at shallow depth to the west. The larg e amount of alluvial gold produced from the Braidwood area (>40 t) was probably derived from small veins and disseminated deposits, similar to Dargue's Reef, in the eroded parts of the roof zone and overlying c ountry rocks.