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UBC Theses and Dissertations

Evolution of hydrothermal alteration and mineralization at the deformed Kerr and Deep Kerr Cu-Au porphyry deposits, northwestern British Columbia, Canada Rosset, Sandra


The calc-alkalic Kerr and Deep Kerr Cu-Au porphyry deposits are located in northwestern British Columbia, and are part of the Kerr-Sulphurets-Mitchell (KSM) property. The deposits comprise a steeply dipping tabular intrusive complex extending N-S ~2.4 km, with a vertical extent >2 km. Ore is primarily hosted within syn-mineral intrusions of monzonite (~197 Ma: Bridge, 1993) which intruded volcanosedimentary rocks of the Hazelton group. The Kerr deposit contains probable reserves of 276 million tonnes at 0.43% Cu, and 0.22g/t Au, for a contained total of 2.6 billion lb’s Cu, and 2.0 million oz’s Au (Seabridge Gold, 2016). The Deep Kerr deposit has an inferred resource estimate of 1.92 billion tonnes grading 0.41% Cu and 0.31 g/t Au for a contained total of 17.3 billion lb’s Cu, and 19.1 million oz’s Au (Seabridge Gold, 2017). Hydrothermal alteration assemblages are zoned, from early high-temperature potassic alteration located centrally at depth, outwards into subsequent dark chlorite-sericite and sericite-chlorite alteration assemblages. The upper ~1.5 km of the deposit is largely overprinted by late phyllic alteration. Potassic alteration was initially more extensive, and has been largely overprinted. The vein paragenesis comprises ten main vein types subdivided into syn-mineral and late- to post-mineral veins. Copper and gold mineralization are primarily associated with A and B type quartz-sulphide veins contemporaneous with potassic alteration. High-grade copper and gold mineralization is correlated with quartz vein abundance above 500 m elevation within the deposit, and is of a more disseminated style below this elevation. Phyllic alteration introduces molybdenum. A hydrothermal zonation in both white micas and chlorites was indicated with short-wave infrared spectroscopy. White mica composition becomes more Fe+Mg rich (phengitic) distally from the hydrothermal centre, and is also more phengitic at depth within potassic and dark chlorite-sericite altered rocks. The Mg content of chlorites relative to Fe increases with increasing proximity to the centre of the hydrothermal plume. A zonation in sulphides and suphates was indicated by sulphur isotope analysis. Sulphides are isotopically lighter (lower δ³⁴S values) proximal to the high-grade core of the deposit, and sulphates become isotopically heavier (increasing δ³⁴S values) with decreasing depth in the hydrothermal system.

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