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UBC Theses and Dissertations

Structural evolution of the Mitchell Au-Cu-Ag-Mo porphyry deposit, northwestern British Columbia Febbo, Gayle Elizabeth


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. As of 2015 it held a resource of 1777 Mt at 0.61 g/t Au, 0.17% Cu, 3.1 g/t Ag, and 58 ppm Mo (0.5 g/t Au eqiv. cut-off; meas+ind). The calc-alkalic deposit is genetically related to multiple diorite intrusions (Sulphurets suite) that cut volcanosedimentary strata of the Stuhini Group (Upper Triassic) and Jack Formation (basal Hazelton Group, Lower Jurassic). Phase 1 plutons (U/Pb, zircon; 196 ±2.9 Ma and 192.2±2.8 Ma) host Stage 1 potassic and propylitic alteration, veins and copper-gold mineralization. A Phase 2 plug (189.9±2.8 Ma; U/Pb zircon) is central and temporally related to a molybdenum halo (190.3±0.8 Ma; Re-Os, Mo) that is accompanied by phyllic alteration (Stage 2). Phase 3 plutonism is temporally related to diatreme breccia, intrusion breccia dikes and Stage 3 massive pyrite veins and advanced argillic alteration. High-level, gold-rich veins comprise Stage 4. Three phases of progressive deformation related to the mid-Cretaceous Skeena fold and thrust belt structurally modify the Mitchell deposit. Deformation Phase 1 is characterized by a steep, easterly striking pervasive pressure solution cleavage (S₁) and steeply west-plunging buckle folds in veins (F₁); fold geometry and flattening degree are a function of alteration type. In rheologically weak alteration types a pressure solution cleavage is associated with loss of silica, mechanical remobilization of chalcopyrite-molybdenite, and passive enrichment of chalcopyrite-molybdenite-pyrite along the cleavage planes. Strain intensity (i.e., S₁ development) is heterogeneous and this greatly affects the shape of the orebody. In Deformation Phase 2, steeply north-plunging F₂ vein folds overprint S₁ and F₁. The Mitchell thrust fault (Deformation Phase 3) offsets the Snowfield deposit ~ 1600 m to the east-southeast and the Mitchell Basal shear zone displaces the Mitchell deposit from its core zone, located ~1-2 km to the west at a depth of ~ 1 km. It is speculated the Mitchell deposit was emplaced into a structurally influenced, north-trending Jurassic basin and subsidiary east-west structures controlled the intrusion, vein geometry, alteration and metal pattern trends.

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