The Evolution and Structural Modification of the Supergiant Mitchell Au-Cu Porphyry, Northwestern British Columbia

Abstract

The calc-alkalic Mitchell Au-Cu-Ag-Mo porphyry deposit, hosted in intrusive rocks of the Stikine volcanic arc terrane of northwestern British Columbia, is the largest undeveloped gold resource in Canada, with 40.72 Moz of contained gold. It lies within the KSM trend, a 12-km-long linear porphyry array in the Sulphurets district. It is genetically related to Early Jurassic Sulphurets stocks: phase 1 diorite to monzodiorite hosts Cu-Au mineralization in potassic assemblages (stage 1), a phase 2 granodiorite plug cores a molybdenum halo (190.3 ± 0.8 Ma, 191.3 ± 0.7 Ma; Re-Os, molybdenite) that is accompanied by phyllic alteration (stage 2), and a poorly mineralized phase 3 diorite plug temporally overlaps with quartz-pyrophyllite alteration at shallow levels (stage 3). Two deformation events (D1 and D2), attributed to orogen-scale mid-Cretaceous transpression, structurally modified Mitchell. D1 deformation, expressed as steep E-striking pressure solution cleavage (S1) and related W-plunging folded veins (F1), is heterogeneously developed as a function of alteration type. D2 is divided into two progressive events: D2a, defined by N-plunging, folded veins (F2a), and D2b, defined by E-vergent thrust faults, including the Mitchell thrust fault, which offsets the updip continuation of Mitchell (the Snowfield deposit, 192.0 ± 1.0 Ma, 191.1 ± 0.8 Ma; Re-Os, molybdenite) ~1,600 m to the east-southeast. Host structures for the KSM trend may have been long-lived, N-striking basement lineaments that provided transcrustal magma and fluid pathways. East-trending intrusions, hydrothermal veins, alteration and metal distribution at Mitchell are attributed to subsidiary E-striking cross faults. These original anisotropies in turn influenced the geometry of Cretaceous faults and flattening domains within the deposit.

Latest Triassic–Early Jurassic Stikine–Yukon-Tanana terrane collision and the onset of accretion in the Canadian Cordillera: Insights from Hazelton Group detrital zircon provenance and arc–back-arc configuration

“…Sample 13JN11-6 is from an enclave of Jack Formation grit within the Iron Cap intrusion on the northeastern flank of the anticlinorium (Fig. 5C; Nelson and Kyba, 2014;Febbo et al, 2019a). Although highly altered, it yielded 42 concordant grains, with all except the youngest forming a broad peak at ca.…”

“…Major, long-lived structural corridors of Stikinia played an important role in controlling its internal stratigraphy and paleogeography (Febbo et al, 2019a;. The terrane is transected by orthogonal, orogen-normal structural corridors: the easterly Stikine and Skeena arches, the northwesterly Nilkitkwa trough, and a through-going northerly lineament and fault corridor in the Iskut-Stewart-Kitsault region (Figs.…”

“…A 25-m-thick sequence of quartz-feldspar grit, sand stone, and shale overlies deformed Stuhini Group strata north of the Johnstone fault, which marks the northern limit of the Kerr-Sulphurets-Mitchell (KSM) porphyry trend north of Iron Cap (Fig. 5C; Febbo et al, 2019a). The Johnstone fault is interpreted as a south-side-down Early Jurassic normal fault, with 1-2-km-thick sections of Jack and Betty Creek Formation strata to the south and a thinly veneered Stuhini horst block to the north (Febbo et al, 2019a).…”

The Piuquencillo fault system: a long-lived, Andean-transverse fault system and its relationship with magmatic and hydrothermal activity

“…These difficulties can be overcome with a multidisciplinary approach, combining detailed, field-based structural mapping with geologic observations at different scales and various types of geophysical datasets; such an approach has led to the recognition of this type of lithospheric-scale structures in different geological settings, improving our understanding about their complex reactivation histories and their relationship with magmatic-hydrothermal activity (e.g. Chernicoff et al, 2002;Gow and Walshe, 2005;Cembrano and Lara, 2009;Acocella et al, 2011;Lanza et al, 2013;Richards et al, 2013;Fox et al, 2015;Febbo et al, 2019).…”

Potential for Volcanogenic Massive Sulfide Mineralization at the A6 Anomaly, North-West British Columbia, Canada: Stratigraphy, Lithogeochemistry, and Alteration Mineralogy and Chemistry

“…The uppermost Lower Hazelton Group is "capped" by a turbiditic siltstone to sandstone and heterolithic cobble conglomerate [12]. Numerous hydrothermal deposits are hosted in the volcanic and volcaniclastic sequences of the Lower Hazelton Group, including the Au-rich epithermal Brucejack mine and the KSM porphyry deposits (Figures 2 and 3, Table 1; [1,7,[33][34][35]). The Lower Hazelton Group is interpreted as an arc complex similar to the modern-day Philippine archipelago, characterized by volcanic and volcaniclastic sequences [8,28].…”