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UBC Theses and Dissertations

Geological characteristics and genesis of the Kemess North porphyry Au-Cu-Mo deposit , Toodoggone district, north-central British Columbia, Canada McKinley, Bradley Scott Mason


The Kemess North porphyry Au-Cu-Mo deposit (300 Mt resource @ 0.30 g/t Au and 0.16% Cu)i s situated in the Toodoggone district, along the eastern margin of the Stikinia terrane in British Columbia. Mineralization is genetically related to the ca. 202 Ma, moderately SE-plunging, Kemess North diorite and is also hosted by proximal Takla Group basalt country rock. The nearby 202.7 ± 1.9 Ma Sovereign diorite has a comparable emplacement age, mineralogy, and chemistry to the Kemess North diorite, but is unmineralized. Toodoggone Formation volcaniclastic rocks (199.1 ± 0.3 Ma) crop out as prominent N-trending ridges or as isolated, fault-bounded blocks within Takla Group basalt. The unmineralized, (197.3 + 1.1/0.9 Ma) Duncan pluton intrudes Takla Group basalt. Seven vein types are separated into four stages of formation with respect to Au-Cu-Mo mineralization. Early-stage veins include magnetite stringer veins and later quartz-magnetite-pyrite + chalcopyrite + molybdenite veins. These veins are restricted mainly to the diorite, are associated with locally preserved potassic (biotite) alteration, and resulted in most of the Au-Cu-Mo mineralization at Kemess North. Main-stage quartz-pyrite + chalcopyrite ± molybdenite veins are the most abundant vein type and are present in the diorite and proximal Takla Group basalt. The veins are associated with phyllic (sericite-quartz) alteration and have a Re-Os molybdenite age of 201.8 ± 1.2 Ma. Late-stage pyrite-chalcopyrite and anhydrite ± pyrite ± chalcopyrite veins and associate phyllic (sericite-chlorite-pyrite) alteration occur in diorite and Takla Group country rocks. Lastly, post-mineralizationan hydrite and carbonate-zeolite veins cut all rocks. Fluid inclusion studies indicate that early-and main-stage ore fluids deposited Au-Cu-Mo at similar temperatures (about 400°C to 375°C) and pressures (0.9 to 3.0 kbar), corresponding to crustal depths of 3 to 10 km. Sulfur and Pb isotope compositions suggest that metals from the early-stage fluid were derived from the Kemess North diorite; metals in the main-stage fluid were derived from the diorite and probably Takla Group country rock and meteoric fluids. An E-striking, steeply S-dipping fault truncates the northern extremity of the ore body. Late NW- to NE-striking normal faults vertically displace the deposit resulting in graben-and-horst block shuffling of the stratigraphy.

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