Geology and petrology of the Catface porphyry Cu-Mo deposit, Vancouver Island, and linkages to the Paleogene Cascade Arc

Abstract

The geology, petrology and geochemistry of Catface porphyry Cu (Mo-Au) deposit, located on the west coast of Vancouver Island are examined in detail. Detailed core logging and sampling was carried out to characterize the geometry and identity of different intrusive phases and alteration styles prevalent during the emplacement and formation of the deposit, as well as their geochemical affinity. Early- and late-stage potassic alteration is identified, as well as main-stage sodic-calcic and calcic-sodic alteration. Four distinct Paleogene intrusive phases vary from quartz diorite to granodiorite in composition. The rocks are broadly calc-alkaline, weakly peraluminous to moderately metaluminous, and have typical arc geochemical affinity. The timing of emplacement and mineralization is constrained by U-Pb and Re-Os geochronology at 40.4-41.4 Ma and 40.9 ±0.2 Ma, respectively. All four Paleogene Catface intrusive phases were emplaced close in time with a direct temporal correlation to mineralization. The chalcopyrite- and pyrite-bearing miarolitic cavities in the Halo Porphyry intrusive, combined with U-Pb and Re-Os dates suggest this intrusive phase is the most likely source of mineralizing fluids. The intrusions were emplaced at depths of less four kilometers in the crust, as evidenced by the presence of miarolitic cavities and confirmed through amphibole-plagioclase thermobarometry, which record conditions of 615–700 °C and <200 MPa. The lack of primary anhydrite and hematite, and the presence of pyrrhotite in the ore system indicate a reduced magmatic-hydrothermal event. The SO3 contents in apatites are <450 ppm, indicative of a degassed and/or sulphate-free (reduced) magma. The assemblage K-feldspar-quartz-biotite-ilmenite yields oxygen fugacities (fO2) which are 0.5 to 3.0 log units below the quartz-fayalite-magnetite (QFM) buffer at an assumed pressure of 300 MPa; orders of magnitude more reduced than typical porphyry deposits. Parental magmas to the Catface deposit were either derived from intrinsically-reduced mantle, or more typical oxidized arc magma that was subsequently reduced during ascent and emplacement. Further isotopic work is required to determine which process contributed to the reduction of these magmas in an arc setting. Nevertheless, recognition of reduced porphyry-related magmatism on west-central Vancouver Island is of similar age to that of North Fork (~36.8-38.9 Ma) deposit in Washington suggesting a consanguinity of reduced magmatism with the Paleogene Cascade arc.