The Red Chris Porphyry Copper-Gold Deposit, Northern British Columbia, Canada: Igneous Phases, Alteration, and Controls of Mineralization

Abstract

Red Chris is a Late Triassic porphyry Cu-Au-(Mo) deposit in the Stikinia island-arc terrane in northern B.C. Late Triassic sedimentary and volcanic rocks of the Stuhini Group host a series of Late Triassic to Early Jurassic (204–198 Ma) diorite to quartz monzonite stocks and dikes. The Red Chris deposit is hosted in the 204 Ma Red stock, the largest (6.5 × 1.5 km) and most altered intrusion in the area. Volcanic rocks approximately time equivalent with the stock are exposed 2 km to the southwest and west. Postmineral Early and Middle Jurassic sedimentary rocks unconformably overlie the Stuhini Group and Red stock to the south and southeast. The composite Red stock consists of multiple phases of crowded porphyries (30–65% plagioclase + hornblende ± biotite phenocrysts) including abundant premineral “P1” leucodiorite; multiple subphases of synmineral “P2” quartz monzonite porphyries; and volumetrically minor late mineral monzonite (“P3”) and monzodiorite (“P4”) porphyries. The stock evolved from subalkaline to silica-saturated alkalic composition, with the mineral-related P2 porphyries being high K calcalkalic and high Sr/Y. Common septa of Stuhini biotitic hornfels are entrapped within the stock. Minor postmineral, weakly altered mafic dikes cut the stock. The zone of >0.25% Cu equiv (Cu + Au) is >2 km long (WSW-ENE) and up to 650 m wide. Early A-type quartz veins with disseminated bornite accompanied biotitic and K-feldspar-magnetite alteration, which was coeval with P2 porphyries. Higher Cu and Au grades correlate in a general way with zones of more abundant A veins. Chlorite-carbonate-epidote-actinolite replaced hornblende peripheral to biotitic alteration, and chlorite overprinted much of the secondary biotite. Late alteration affected all porphyry phases and is subdivided into earlier sericite-clay-pyrite, and later low sulfide carbonate-clay-hematite. Late sericite-clay-pyrite alteration sulfidized much of the primary bornite to chalcopyrite with or without pyrite, and carbonate-clay-hematite alteration converted most magnetite to hematite. Highest Cu-Au grades (>2% Cu equiv) occur in and around the early and intermediate stage P2 porphyries. Zoning of Cu, Au, Au/Cu ratios, sulfides, and quartz veins is centered and mostly symmetrical around a core of high-grade Cu-Au within and surrounding apices of P2 porphyry. Contact relationships confirm the close association of P2 porphyry phases with Cu-Au. Zones of >50 ppm Mo surround the highest grade Cu-Au zone at depth. West-northwest and east-northeast strike-slip to oblique-slip faults offset sulfide, Cu-Au, and Mo patterns. Several lines of evidence indicate 15° to 20° of southward tilt since the system formed. Due to glaciation, oxidation is very thin and supergene enrichment negligible. Red Chris was previously considered by most workers as an alkalic or “hybrid” alkalic deposit. New chemical data on synmineral porphyry phases instead suggest high K calc-alkalic composition, similar to the monzonitic Cu-(Mo-Au) class of porphyry deposits, which includes Bingham and Bajo de la Alumbrera. Red Chris is also classified as an A vein type deposit due to strong control of higher grade Cu-Au by A vein stockworks with abundant disseminated Cu sulfides.