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Geology and ore deposits of the Stewart Mining Camp, British Columbia Alldrick, Dani James


The Stewart mining camp in northwestern British Columbia is abundantly mineralized with widely distributed, texturally and mineralogically varied, precious and base metal deposits. This report documents the geologic setting of the mining camp and the geologic features of the major mineral deposit types. The Stewart camp is underlain by a 5-kilometre-thick Upper Triassic to Lower Jurassic (Norian? to Toarcian) island arc complex of calc-alkaline basalts, andesites and dacites with interbedded sedimentary rocks. Coeval (211-189 Ma) hornblende granodiorite plutons intruded the arc at two to five kilometres depth. Rocks were deformed during mid-Cretaceous (110 ± 5 Ma) tectonism that produced north-northwest-trending folds, penetrative fabric and lower greenschist facies regional metamorphism (290°±20°C, 4.5 ±1.5 kb). Mid-Eocene (54.8-44.8 Ma) biotite granodiorite of the Coast Plutonic Complex intruded the deformed Mesozoic arc complex. Two mineralizing events formed over 200 mineral occurrences in the district. These two metallogenic epochs were brief (< 5 million years), regional-scale phenomena characterized by different base and precious metal suites. The Early Jurassic ore-forming episode produced Au and Au-Ag-Zn-Pb-Cu deposits. The mid-Eocene episode produced Ag-Pb-Zn ± W ± Mo deposits. Early Jurassic deposits have a characteristic lead isotope signature (²⁰⁶Pb/²⁰⁴Pb = 18.816; ²⁰⁷Pb/²⁰⁴Pb = 15.617) and include gold-pyrrhotite veins, gold-silver-base metal veins, and stratabound pyritic dacites. All Early Jurassic mineral occurrences are late- to post-intrusive deposits that were emplaced in andesitic to dacitic host rocks at the close of volcanic activity, about 190-185 million years ago. Transitional gold-pyrrhotite veins (Scottie Gold mine) formed in en echelon tension gashes developed in country rock around Early Jurassic plutons during late magma movement. Epithermal gold-silver-base metal veins and breccia veins (Big Missouri and Silbak Premier mines) were deposited along shallower sub-volcanic faults and in hydrothermal breccia zones formed along dyke contacts. Stratabound pyritic dacite tuffs (Mount Dilworth and Iron Cap prospects) formed where venting fumarolic fluids and hotspring pools deposited abundant fine pyrite in local areas on a cooling ignimbrite sheet. Eocene deposits also have a characteristic lead isotope signature (²⁰⁶Pb/²⁰⁴Pb = 19.147; ²⁰⁷Pb/²⁰⁴Pd = 15.627) and include silver-rich galena-sphalerite veins, gold-silver skarns and, beyond the study area, porphyry molybdenum deposits. These mineral occurrences are related to Middle Eocene plutons of the Coast Plutonic Complex. All are late- to post-intrusive deposits emplaced about 50-45 million years ago. Mesothermal silver-lead-zinc veins (Prosperity/Porter Idaho and Riverside mines) were deposited in brittle zones along major fault structures. Skarns (Oral M and Red Reef prospects) developed where plutons cut limestone or limy siltstone units within minor turbidite sequences. Major porphyry molybdenum deposits (Kitsault mine and Ajax) developed where mid-Eocene stocks were emplaced in thick turbidite sequences. Diagnostic features such as lead isotope ratios, stratigraphic and plutonic associations, alteration assemblages, sulphide mineralogy and textures, and precious metal ratios allow discrimination amoung these different deposit types. Using these criteria, the most prospective areas for each deposit type have been targetted for exploration.

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