Abstract

Gold quartz veins at the Bralorne mesothermal vein deposit have extensive quartz-ankeritic carbonate-muscovite hydrothermal alteration envelopes that grade outward to chlorite-calcite-albite. Minor pyrite with traces of sphalerite, galena, and tetrahedrite are found in the veins with native gold; more abundant pyrite and arsenopyrite, with lesser pyrrhotite and chalcopyrite occur in altered wall rock adjacent to the veins. Envelopes are characterized by addition of K 2 O, CO 2 , S, As, and Au, with corresponding depletion of Na 2 O, FeO total, and MgO; SiO 2 and CaO are locally depleted and reconcentrated. Fluid inclusions suggest that deposition of gold was from dilute aqueous solutions (<5 wt % NaCl equiv) that had a significant carbonic component (5-15 mole % CO 2 + CH 4 ), at 350 degrees C and 1.75 kbars. Progressive dilution of this fluid by cooler meteoric water led to fluids at 250 degrees C and 0.5 to 1 kbar with 1 wt percent or less NaCl equivalent and no detectable carbonic component. Sulfur isotope ratios of sulfides associated with gold mineralization range from -7 to +9 per mil, clustering about a magmatic signature of 0 per mil. The ore fluid had a delta 18 O value of 13 + or - 1 and delta 13 C of -11 + or - 2 per mil, based on measurements in coexisting vein quartz, carbonate, and muscovite. Temperature of the mineralizing fluids appears to have increased with depth at a normal geothermal gradient of approximately 30 degrees C/km.Gold, sulfides, and the associated alteration assemblages are modeled, using observed mineralogy and thermodynamic data for chloride complexes, as having been deposited from a slightly acid solution (pH 4.5) with an Na/K ratio of at least 8:1 and a high content of dissolved CO 2 (log fugacity = 2.5). Conditions were strongly reducing, as suggested by the log fugacity of CH 4 (0.5), with f (sub O 2 ) about 10 (super -30) bars and f (sub S 2 ) about 10 (super -7) bars. Precipitation of gold in the immediately adjacent, highly quartz-sericite-ankerite-pyrite-altered wall rock, was due to reaction with (carbonation of) the wall rock that caused the pH of the ore fluid to rise, destabilizing aurous chloride complexes. Such a shift is in the direction of slightly increasing or constant stability of gold thiosulfide ions, and at a relatively constant f (sub O 2 ) . The most significant result of this modeling is that in detail, the actual precipitation of gold is by reduction of aurous ions by electrons donated through concurrent oxidation of S (super -2) in H 2 S to S (super -) in pyrite (FeS 2 ). This explains the empirically observed correlation between pyrite and gold in this type of deposit, and in the model between consumption of Fe (super +2) and production of pyrite on the one hand, with precipitation of gold on the other.

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