Geology and genesis of the wolf precious metal epithermal prospect and the capoose base and precious metal porphyry-style prospect, Capoose Lake area, central British Columbia
Creators: name: Andrew, Kathryn Pauline Elizabeth
Publisher: name: University of British Columbia
Year: 2010

description: The Wolf and Capoose prospects represent two distinctly different types of precious metal deposits in volcanic rocks of the Stikinia terrane, central British Columbia. At the Wolf prospect, auriferous and argeniferous metallic minerals are in bladed quartz-carbonate veins and heterolithic breccias within Lutetian calc-alkaline rhyolite of the Ootsa Lake Group. Electrum, native silver, and silver sulphosalts occur as inclusions in and adjacent to pyrite in five silicic zones which have eight recognisable phases of veining and brecciation and are bordered by argillic and sericitic altered rhyolite. Fluid inclusions define growth zones in precious metal-bearing quartz-carbonate veins and precious metal-poor drusy quartz veins. The inclusions are primary, two-phase, liquid-rich, low salinity, and low C0₂. Homogenization temperatures of quartz-carbonate veins are 270°C and 170°C and in the late drusy quartz veins, 250°C. Oxygen and hydrogen isotope compositions of vein quartz, rhyolite, and alkali feldspar phenocrysts indicate that depositional fluids were ¹⁸0 depleted by 4 to 9 0/00. Ootsa Lake Group rocks at Wolf are formed by explosive eruptions and flows, related to a ring fault. Flat-lying rhyolite tuffs and flows are intruded by cogenetic stocks and dykes in a caldera collapse setting. Precious metal deposition occurred as one event related to quartz-carbonate veins. Later drusy quartz veins precipitated from a different fluid. Primary fluid inclusion homogenization temperatures show that fluids which deposited quartz-carbonate were boiling and existed under both hydrostatic and near lithostatic pressures at depths of about 100 m. Oxygen and hydrogen isotope compositions indicate a high degree of isotopic exchange between rhyolite and large volumes of low ¹⁸0 content meteoric fluids. The fluids evolved to a non-boiling, lower salinity, extremely ¹⁸0 depleted, precious metal-poor variety which precipitated late drusy quartz veins. Geological setting, vein and breccia textures, alteration, metal distribution and depositional fluid evolution at Wolf resemble a low sulphur, epithermal hot spring or silicified stockwork deposit. At the Capoose prospect, auriferous and argeniferous metallic minerals occur as inclusions within disseminated galena and sphalerite in calc-alkaline Maastrichtian rhyolite sills intrusive into Lower and Middle Jurassic Hazelton Group volcanic and sedimentary rocks. Flow-banded, spherulitic rhyolite sills are preserved within a minor horst. Spessartines in the sills are similar in composition to plutonic garnets with less than 5% change in end member composition from rim to core. They occur adjacent to disseminated, aggregate and vein galena, sphalerite, pyrite, arsenopyrite and chalcopyrite. Sulphide and spessartine accummulations are commonly surrounded by muscovite and quartz coronas. Sulphide poor quartz and calcite veining is in hornfelsed Hazelton Group rocks peripheral to the sills. Phyllic alteration is restricted to the sills and overprints mineralized zones. Primary two-phase, liquid-rich, low salinity, low C0₂ fluid inclusions from late silicate veins homogenize from 285°C to 335°C. Rhyolite sills are not depleted in ¹⁸0 whereas sericite, quartz and calcite are. Spessartine in rhyolite sills at Capoose crystallized as late phenocrysts stabilized by high manganese content. They provided a nucleus for sulphide deposition shortly after sill emplacement in groundwater saturated, permeable Hazelton Group rocks. Cooling, crystallization and fracture development in the sills initiated hydrothermal circulation and phyllic alteration with late quartz and calcite veins related to collapse of the hydrothermal system. Lead-zinc mineralization occurred as two events, with only one related to precious metal and copper deposition. Oxygen isotope compositions of quartz-garnet mineral separate pairs indicate crystallization of garnets and sulphides from magmatic fluids at temperatures from 528°C to 725°C. Sericite, quartz and calcite precipitated from meteoric fluids. In summary, hydrothermal fluids at the Capoose prospect evolved from early, high temperature magmatic fluids to late lower temperature, low salinity, meteoric fluids. The geological setting, silicate and sulphide mineralogy, alteration, metal distribution and depositional fluid compositions at Capoose resemble a low-grade, epigenetic, intrusion-related, porphyry-style deposit.