Reduced granitic magmas in an arc setting: The Catface porphyry Cu–Mo deposit of the Paleogene Cascade Arc

The Catface porphyry Cu (Mo–Au) deposit, Vancouver Island, British Columbia was studied to characterize the age, geometry and geochemical affinity of its different intrusive phases, and their tectonic setting. Four different intrusive phases of quartz diorite are broadly calc-alkaline, moderately metaluminous, and have typical arc geochemical affinity. U–Pb age dating of zircons showing two intrusive phases was emplaced at 41.26 ± 0.11 and 41.15 ± 0.10 Ma, and a second two 40.93 ± 0.11 and 40.88 ± 0.10 (95% confidence). The latter ages are identical to the Re–Os age of molybdenite mineralization of 40.9 ± 0.2 Ma. The depth of emplacement is less than 4 km, as determined by amphibole–plagioclase thermobarometry (630–815 °C and 50–300 MPa). A reduced magmatic-hydrothermal system is evidenced by: (1) presence of pyrrhotite and absence of anhydrite and hematite, (2) low SO3 (< 450 ppm) in apatite, and (3) oxygen fugacities (fO2) of 0.5–3.0 log units below the quartz–fayalite–magnetite (QFM) buffer recorded by the assemblage K-feldspar–biotite–ilmenite–quartz. Reduced porphyry-related magmas on Vancouver Island of similar age to those at North Fork, Washington (37 Ma) suggests consanguinity of reduced arc magmatism and related ore deposits within the Paleogene Cascade arc of the Pacific Northwest. Reduced evolved magmas at Catface are atypical in an arc setting, but can be attributed to thorough degassing of S from the magmas as evidenced by low S in apatite.

► Reduced granitoid magma (FMQ-2) recorded by K‐feldspar–biotite–ilmenite ► Atypical reduction in a typically oxidized Paleogene Cascade arc setting ► Low S in apatite suggests thorough SO2 degassing to cause magma reduction.