GEOLOGICAL AND STRUCTURAL SETTING OF THE RED CHRIS PORPHYRY COPPER(-GOLD, SILVER) DEPOSIT IN NORTHERN STIKINIA, BRITISH COLUMBIA

The new Red Chris mine is developing one of the largest porphyry copper deposits in the Canadian Cordillera, with measured + indicated resources of 1.2 billion tonnes grading 0.327% Cu and 0.327 g/t Au (0.2% Cu-equivalent cut-off). It is hosted mostly in the 6 x 1-2 km, Late Triassic (LTr) Red stock, composed of several plagioclase + hornblende ± biotite-phyric porphyry phases (P1-4) of subalkaline to silica-saturated alkaline compositions. The earliest and largest phase is pre-mineral P1 leucodiorite. Syn- to intra-mineral P2 quartz monzonite porphyries intrude P1 at depth in the core of the stock. P3 monzonite and P4 monzodiorite are late- to post-mineral dike phases. Quartz veins with bornite and/or chalcopyrite and associated potassic alteration formed mainly during emplacement of P2, waning into P3. The upper levels of the preserved system are strongly overprinted by quartz-sericite-pyrite and intermediate argillic alteration. Dates of 206-204 Ma have been obtained from the stock and mineralization. The ENE linearity of the stock and related dikes suggests structural control on emplacement, although mineralized quartz veins are related to the cupola of P2 rather than to fault kinematics. The regional ENE structural trend continued to influence faulting at Red Chris through the Mesozoic, and localized a Miocene-Pliocene volcanic feeder which penetrated the stock.

The Red stock intruded gently folded LTr Stuhini Group volcanic-sedimentary rocks. Trachyandesitic volcanic and volcaniclastic rocks of imprecise LTr-Early Jurassic (EJ) age unconformably overlie the Stuhini south and west of the Red stock, and are thought to be volcanic equivalents of ~200-Ma intrusions in the Red Chris area. These LTr-EJ volcanic and intrusive rocks including the Red stock are believed to represent very early Hazelton Group arc magmatism. A gently south-dipping post-mineral sequence including the Early to Middle Jurassic Upper Hazelton Group and the Middle Jurassic-Early Cretaceous Bowser Lake Group overlapped the eroded Red stock. Structural calculations suggest the Red stock was not tilted before this sequence was deposited.

Syn-mineral P2 porphyries at Red Chris are high-K calc-alkalic, in contrast to most ~205-Ma porphyry systems in BC that are dominated by alkaline magmatism.