Abstract

Dargue's Reef is a mesothermal, gold sulfide deposit developed in altered 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% pyrite), 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 inclusions of silicates, calcite, chalcopyrite, Bi sulfo- salts, galena, gold, trace tellurides, native bismuth, and pyrrhotite. Separate aggregates of chalcopyrite, Bi sulfosalts, and tetrahedrite are intergrown with the silicate alteration minerals. Fluid inclusion data indicate that gold-bearing hydrothermal fluids were CO 2 bearing, of low to moderate salinity, and medium to low temperature (<350 degrees C). Two K-Ar dates on alteration sericite from the main ore zone indicate an age of about 409 Ma for the alterations and show that mineralization was broadly contemporaneous with emplacement of the host pluton in the earliest Devonian. Sulfur isotope ratios in pyrite from the mineralization (delta 34 S, -0.4 to--3.4%) and disseminated pyrite in the host granodiorite (delta 34 S, 1..4-2.5%) are consistent with a magmatic sulfur source. Carbon isotope data for calcites from the mineralization indicate delta 13 C values for the fluid close to 0 per mil. These are not inconsistent with a magmatic C source, but they are also consistent with derivation of CO 2 from minor limestones in the country rocks. Textural and fluid inclusion evidence suggest that CO 2 activity in the fluids increased with time, which could be explained by late-stage introduction of country-rock fluids or possibly influx of fluid from a later intrusion at depth. Oxygen isotope data for calcites from the ores (6.5-10.9%) and late-stage veinlets (6.5%) implicate fluids similar to those in porphyry-mineralizing systems...

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