Geology of the Mitchell Au-Cu-Ag-Mo porphyry deposit, northwestern British Columbia, Canada

The Mitchell Au-Cu-Ag-Mo porphyry deposit, hosted by Early Jurassic volcanosedimentary and intrusive rocks in the Stikine terrane of northwestern British Columbia, is considered the largest undeveloped gold resource in Canada. It contains 1740 Mt of measured and indicated resource grading 0.61 g/t Au, 0.17% Cu, 3.1 g/t Ag, and 58 ppm Mo based on a 0.5 gold-equivalent gram per tonne cut-off. The deposit is genetically related to multiple diorite intrusions (Sulphurets suite) that cut sedimentary and volcanic rocks of the Stuhini Group (Upper Triassic) and sandstones, conglomerates, and andesitic rocks of the Jack Formation (basal Hazelton Group; Lower Jurassic). Mineralization and accompanying alteration and stockwork proceeded in four stages. Hosted by Phase 1 plutons (196.0 ±2.9 Ma and 189 ±2.8 Ma), Stage 1 sheeted veins and stockwork contain most of the copper-gold mineralization and potassic and propylitic alteration. A Stage 2 disseminated and stockwork-hosted molybdenum halo (190.3 ±0.8 Ma; Re-Os) is peripheral and contiguous with the core copper-gold system. It is associated with phyllic alteration and is temporally related to a Phase 2 pluton (192.2 ±2.8 Ma) that outcrops central to the halo. Stage 3 consists of poorly mineralized massive pyrite veins associated with advanced argillic alteration and is related to Phase 3 diorite, diatreme breccia emplacement and intrusion breccia dikes. Stage 4 consists of high-level, gold-rich veins that are lateral to, and overprint, the main deposit. The geochemistry of the Sulphurets intrusions, nature and extent of alteration assemblages, high silica content of the ore zone and Mo mineralization, indicate that the Mitchell porphyry is calc-alkalic rather than alkalic. The subalkaline Sulphurets suite is here interpreted as the progenitor to the Mitchell deposit, rather than the alkaline Premier suite. The Premier and Sulphurets suites accompanied siliciclastic sedimentation and andesitic volcanism recorded by the Jack Formation. The deposit was deformed during development of the Skeena fold and thrust belt (mid-Cretaceous) and is now exposed in an erosional window beneath the Mitchell thrust fault, which separates it from its interpreted offset (Snowfi eld deposit) in the hanging wall to the southeast.