GEOLOGY AND STRUCTURAL EVOLUTION OF THE MITCHELL AU-CU-AG-MO PORPHYRY DEPOSIT, NORTHWESTERN BRITISH COLUMBIA

The Early Jurassic calc-alkalic Mitchell Au-Cu-Ag-Mo porphyry deposit is located in the Sulphurets mining camp, Stikine terrane, northwestern British Columbia. It is exposed through an erosional window beneath the Mitchell thrust fault that separates it from its offset equivalent in the hanging wall to the southeast, the Snowfield deposit. The Mitchell porphyry hosts 42.8 million ounces of total contained gold resource (0.5 g/t Au equiv. cut-off; meas+ ind+ inf); Snowfield hosts 34.9 million ounces of total contained gold resource (0.3 g/t Au equiv. cut-off; meas +ind+inf); they comprise the two largest undeveloped in-situ gold deposits in Canada. Here we provide the first detailed description of the Mitchell deposit geology with an emphasis on Early Cretaceous deformation (Skeena fold and thrust belt).

The Mitchell zone is characterized by cyclic intrusion of porphyritic diorite stocks (premineral and intramineral 196 ±2.9 Ma and 192.2±2.8 Ma; late intramineral 189.9±2.8 Ma); stockworking and cannibalization of previously emplaced diorite and stockwork. A molybdenum halo peripheral and contiguous with the core copper-gold system overprints both andesite breccia and diorite intrusions (190.3 ± 0.8 Ma; Re-Os). Three phases of progressive deformation related to the Skeena fold and thrust belt structurally modify the Mitchell deposit. The first phase of deformation (D₁) is characterized by: pervasive S₁ foliation that strikes west and dips 80° north, and by non-cylindrical F₁ folds developed in quartz veins that plunge steeply to the west. A pressure solution cleavage (S₁) is developed in phyllosilicate dominated rocks and results in the passive redistribution of tennanante, galena, sphalerite, chacopyrite and gold. F₁ folds are locally overprinted by steep north-northwest plunging, open-gentle F₂ folds (D₂). The deposit is offset by the Mitchell thrust fault (D₃: 110.2 ± 2.3 Ma, Ar-Ar), and imbricated by smaller thrust faults and displaced along the Basal thrust fault located at ~900 m depth below surface. Easterly displacement of the Mitchell deposit along the Basal thrust predicts a copper-rich core zone to the Mitchell-Snowfield porphyry system to lie in the bottom of the western Mitchell valley, an estimated 1-4 km west of the Mitchell deposit.