Abstract

The Late Triassic Southwest zone alkalic porphyry Cu-Au deposit in the Galore Creek district, northwestern British Columbia, is hosted in hydrothermally cemented breccias that overprinted older matrix-rich breccia and megacrystic orthoclase-phyric syenite and monzonite dikes. Synmineralization cemented breccias and biotite-phyric monzodiorite dikes are localized at the contact of the matrix-rich breccia and host syenite and monzonite dikes, as well as in crosscutting faults. Regional shortening tilted the Southwest zone deposit so that the cemented breccias now dip 50° to 60° southwest.

Four paragenetic stages formed the deposit. A premineralization stage (1) consists of the emplacement of coherent syenite and monzonite host rocks that are cut by a matrix-rich breccia. The synmineralization stage (2) produced potassic-altered rocks and phlogopite-magnetite-K-feldspar–cemented breccia and veins, and most of the Cu-Fe sulfides. A halo of stage 2 diopside-magnetite-grandite-phlogopite calcic-potassic veins and altered rocks formed around the potassic domains and Cu ore zones. Alteration during stage 2 probably formed at temperatures of about 500° to 425°C from oxidized saline magmatic-hydrothermal fluids. Contemporaneous high (aCa2+)/(aH+)2 conditions peripheral to the potassic domains are required to form the marginal calcic-potassic and calcic assemblages. The potassic and calcic-potassic alteration domains were overprinted by propylitic and calcic alteration assemblages due to the retrograde influx of an admixture of cooler magmatic and nonmagmatic fluids. Sphene from a stage 2 calcic-potassic vein yielded a 202.8 ± 2.1 Ma age (U-Pb SHRIMP-RG) that is interpreted to reflect the approximate time of hydrothermal alteration.

Late-mineralization stage (3) alteration overprinted the cemented breccias and comprises sericite-anhydrite (sericitic), grandite (calcic), and chlorite-epidote-carbonate-pyrite (propylitic) assemblages. Localized late-mineralization and Cu-poor stage (4) K-feldspar-Fe carbonate-specularite (carbonate-potassic) and quartz-pyrite veins (quartz veins) cut stage 2 and 3 assemblages.

Chalcopyrite and bornite dominate the core of the deposit, and grade outward to chalcopyrite > pyrite and to pyrite > chalcopyrite, flanked by a pyrite-only halo. The core of the system is Cu-Au rich, grading outward to an Au-enriched, Cu-poor halo coincident with pyrite-bearing propylitic alteration. Gold-copper ratios increase upward within stage 2 potassic zones and outward from the core. Variations in pH, redox, and temperature along and across fluid flow paths best explain the change to higher Au/Cu values upward and away from the Cu-rich potassic domains.

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