Geology, wallrock alteration, and characteristics of the ore fluid at the Bralorne mesothermal gold vein deposit, southwestern British Columbia

The Bridge River gold camp produced more gold than any other camp in British Columbia over its 70 years of operation (130 tonnes or 4 million oz of Au, from 7 million tonnes of 18 g/t ore), mainly from the Bralorne-Pioneer mesothermal vein deposit. The deposits are hosted in the accreted Bridge River and Cadwallader Terranes, which are of Permian to Triassic age and of oceanic and island-arc character, respectively. Mineralization is temporally and spatially related to a suite of early Late Cretaceous albitite dykes of 36-91 Ma age, and thus occurred long after and is genetically unrelated to the emplacement of the major host intrusives (Bralorne diorite and soda granite), dated as Early Permian (>270 Ma U-Pb; 284 Ma K-Ar). The major gold-bearing veins at Bralorne strike about 110° and dip north 70°, with slickensides that plunge 45° east and indicate that the last movement was reverse. Major ore shoots in the veins occupy somewhat less than 20% of the vein and plunge steeply west, roughly perpendicular to the slickensides. The most common host rocks for productive veins are the competent Bralorne diorite and the Cadwallader greenstone; soda granite may have been too weak to sustain large fractures, or too low in Fe for pyrite precipitation. Hydrothermal alteration envelopes around the veins are up to 10 m wide and grade outwards from intensely foliated quartz - ankeritic carbonate - sericite (+fuchsite) to less sheared calcite - chlorite - albite to unsheared epidote -calcite. Chemical studies of the alteration on a constant volume basis (normalized to Al₂O₃ and TiO₂, which have remained relatively immobile), show addition of K₂O, CO₂, S, As and Au, but depletion of Na₂O, FeO (total.) and MgO as the vein is approached. SiO₂ and CaO are locally depleted and reconcentrated. Disseminated pyrite, pyrrhotite, and minor chalcopyrite occur within envelopes for up to several meters from the veins. Arsenopyrite is confined to vein selvages. Occasional sphalerite, and especially galena, appear to correlate with gold-rich portions of the veins. Traces of tetrahedrite and stibnite have been observed but tellurides have not. Gold is found principally as thin smeared flakes of the native metal in the black sulfidic septae of the strongly ribboned veins. Gold is only rarely found by itself in the quartz, usually in extensional veins rather than shear veins, where it locally forms extremely rich pockets. Based on fluid inclusion and stable isotope studies, primary ore deposition appears to have been from fluids of low salinity (less than 5 wt. % NaCl equivalent) with a significant CO₂ and minor CH₄ content, at temperatures of 350°C and pressures of up to 1.75 kb (7 km depth). Later secondary fluids were even more dilute, with much lower CO₂ contents and no CH₄, at temperatures below 240°C and pressures of 0.5 kb. Sulfur isotopic ratios of sulfides associated with the gold mineralization range from -7 to +9 per mil, clustering about 0 per mil. The ore fluid had a d¹⁸O of +13 ± 1 and d¹³C of -11 ± 2 per mil. Homogenization temperatures increase with depth at approximately 30°C/km, a normal geothermal gradient. Computer modelling of the fluid responsible for the observed alteration assemblages and gold deposition used Helgeson's 1969 and 1978 data, and chloride complexes. The results suggest that the fluid had a pH of about 4.5, a Na:K ratio of at least 8:1, and a high content of dissolved CO₂ (log fugacity +2.5). Conditions were strongly reducing, as suggested by the CH₄ (log fugacity +0.5), with fO₂ about 10⁻³⁰and fS₂ about 10⁻⁷. The model predicts precipitation of gold in the immediately adjacent, highly quartz-sericite-ankerite altered wallrock, in response to reaction with the wall rock. It also predicts a strong correlation between gold and pyrite, but not with pyrrhotite. These predictions are supported by observed alteration assemblages, and by the high CO₂ and minor CH₄ observed in fluid inclusions. The veins at Bralorne may have formed by mineralization of faults that originally developed within a Riedel shear zone (the north-west trending Bralorne fault zone) with an east-west maximum compressive stress and north-south, horizontal minimum compressive stress. Sinistral movement is implied for this fault system in the Late Cretaceous. A "fault-valve" model best explains the ribboned, yet euhedral, coarsely crystalline milky quartz veins. Cyclic build-up of fluid pressure in a geopressured reservoir below the brittle-ductile transition caused overpressuring, invoking failure by reactivation of the previously formed steeply-dipping faults, unfavourably oriented for failure at a high angle to the maximum compressive stress in a transpressive regime. Failure provided openings for discharge of fluids, and the coincident drop in pressure promoted deposition of quartz and sulfides; zoned quartz crystals were deposited in space held open by the high pore pressures. Sealing of the fault by this mineral deposition allowed fluid pressure to build and the cycle to repeat. Each of the ribbons of sulfide, with minor gold, probably represents a sliver of highly replaced wall rock that was included in the vein when the next episode of fracturing and mineral deposition occurred.

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