Late-Stage Low-Temperature Hydrothermal Alteration Overprint at the East Zone in the Red Chris Porphyry Cu-Au Deposit, Northwestern British Columbia, Canada

High- and intermediate-temperature alteration assemblages at the East zone in the Red Chris porphyry Cu-Au deposit, northwestern British Columbia, Canada, are varyingly overprinted by a lower-temperature intermediate argillic alteration assemblage composed of illite-kaolinite-hematite-carbonate. The intermediate argillic assemblage extensively overprinted the upper 600 m of the porphyry deposit and is present discontinuously to depths of 1,500 m below the premining surface. Kaolinite is dominant in shallow levels and gradually diminishes with depth, replaced by illite as the dominant clay mineral. Hematite replaced hydrothermal and igneous magnetite, but the intensity diminishes with depth. Mixtures of ankerite, dolomite, siderite, and calcite replaced mafic silicates and formed veins. Oxygen and hydrogen isotopes confirm a magmatic fluid source for the potassic assemblages preserved at depth as well as for the overprinted phyllic assemblage in the upper part and flanks of the East zone. In contrast, the superposed intermediate argillic assemblages formed by a mixture of magmatic and meteoric fluids. Sulfide minerals and sulfur isotopes retain zonal patterns for porphyry Cu deposits and appear largely unaffected by the overprinted intermediate argillic assemblage. Carbon and oxygen isotopes in carbonate vary with depth that may reflect a thermal gradient as a rising fluid cooled. The intermediate argillic assemblage is spatially associated with and overprinted on as yet undated late monzodioritic dikes—the youngest phase in the host Late Triassic Red stock. The relative age relationships and stable isotopic geochemistry indicate the intermediate argillic alteration assemblage represents the flux of magmatic-derived hydrothermal fluid that mixed with external fluid and thus represents either the last fluid pulse in the porphyry Cu deposit or a younger, temporally distinct hydrothermal fluid.