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Abstract:

The high-grade (8.1??&U+202F;??million ounces of gold; 15.6 Mt grading 16.1??&U+202F;??g/t Au; Pretium, 2016) Brucejack epithermal Au-Ag deposits are located in the Canadian Cordillera of northwestern British Columbia, and formed in association with extensive early Mesozoic island arc magmatism. Porphyry-type Cu-Au-Mo mineralization occurs nearby at the Kerr-Sulphurets-Mitchell (196-190??&U+202F;??Ma; Febbo et al., 2015), Bridge Zone (191.7??&U+202F;??+ or -??&U+202F;??0.8??&U+202F;??Ma, 191.5??&U+202F;??+ or -??&U+202F;??0.8??&U+202F;??Ma), and West Zone (188.9??&U+202F;??+ or -??&U+202F;??0.9??&U+202F;??Ma) prospects. Gold-silver vein-type mineralization at Brucejack is hosted by variably altered and deformed Early Jurassic porphyritic latite lava flows, volcaniclastic rocks, and volcanic-derived sandstones, siltstones, and conglomerates. This study focuses on the Valley of the Kings Zone at Brucejack, where host rocks have been dated at 188-184??&U+202F;??Ma (U-Pb, zircon). Post-mineralization (but not post-alteration) basaltic and trachybasaltic dikes that cut the veins are geochemically similar to the host volcanic sequence and follow the same structural trends of the mineralized vein corridors. We suggest these dikes are broadly coeval with mineralization, and that ore formation at the Valley of the Kings formed during the overall Jurassic tectonomagmatic event.Gold-silver mineralization is hosted by quartz-carbonate veins that cut sericitized and pyritized volcaniclastic rocks. Six stages of veining are defined, with Au-Ag mineralization (electrum) focused in stages III-V: stage I-III veins consist of quartz with minor carbonate, chlorite, sericite, and sulfide minerals; stage IV quartz veins contain more abundant base metal sulfides (pyrite, sphalerite, and galena with minor sulfosalts); stage V veins are dominantly calcitic; late post-mineralization stage VI veins are quartz-calcite and contain sparse pyrite and chlorite, but no electrum. The veins are interpreted to be syn- to late-tectonic, with deformation decreasing from locally penetrative in the host-rocks prior to veining (resulting in local foliation of sericite and pressure shadows around pyrite), to dismemberment and shearing of early stage I-II quartz veins, brittle disruption of stage III-IV quartz and stage V carbonate veins, and minimal deformation of post-mineralization stage VI quartz-carbonate veins. A much younger set of extensional muscovite veins locally cuts the deposit with Late Cretaceous apparent ages, and appears to be associated with a weak thermal overprint that has reset K-Ar and (super 40) Ar/ (super 39) Ar ages in sericite throughout the district.The deformation experienced by the vein system, particularly in the later stages (III-V), was not uniform, and original undeformed vein textures and fluid inclusion assemblages are locally preserved. We report fluid inclusion microthermometric data from carefully selected primary fluid inclusion assemblages from quartz, calcite, and sphalerite from vein stages III-IV, many of which show evidence for boiling (coexisting liquid- and vapor-rich primary fluid inclusions). Liquid-rich fluid inclusions from stage III and V veins have moderate homogenization temperatures ( approximately 170??&U+202F;?? degrees C and approximately 160??&U+202F;?? degrees C respectively) and salinities of 2-8??&U+202F;??wt% NaCl equiv., whereas inclusions from base-metal-sulfide-bearing stage IV veins show evidence of mixing with a cooler, more saline brine ( approximately 140??&U+202F;?? degrees C, approximately 10-15??&U+202F;??wt% NaCl equiv.). Carbon dioxide was observed as clathrate during cooling in some fluid inclusions, suggesting that minor amounts of CO (sub 2) were present in the fluids.Calculated oxygen isotopic compositions of fluids in equilibrium with quartz and calcite from vein stages III-V range from delta (super 18) O (sub fluid) ??&U+202F;??=??&U+202F;??-10.7 to +1.8 ppm, whereas delta (super 13) C (sub CO2) ranges from -9.5 to -4.5 ppm, and delta (super 34) S (sub pyrite) ranges from -1.7 o +0.6 ppm. Taken together, these data suggest a magmatic source for S and some C, carried by a fluid of evolved or diluted magmatic origin, which variably mixed with meteoric-derived groundwater or seawater containing carbon of sedimentary (organic) origin. Mixing is supported by large salinity ranges of vein stage IV fluid inclusions, where individual assemblages range from approximately 1 to 15??&U+202F;??wt% NaCl equiv.These fluid temperatures, salinities, and isotopic compositions are typical of epithermal deposits where distal magmatic fluids mix with local heated groundwaters. The low fluid temperatures but evidence of boiling suggest formation at shallow crustal depths. This is despite the evidence for penetrative deformation in the sericitized volcanosedimetary host rocks, which we attribute to rapid uplift immediately prior to or during the earliest stages of mineralization. The vein system is therefore interpreted to have formed during the later stages of a deformational event related to arc accretion.

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