THE WESTERN (ANCESTRAL) CASCADES MAGMATIC ARC ON VANCOUVER ISLAND? A NEWLY RECOGNIZED AND UNDEREXPLORED LATE EOCENE PORPHYRY CU-MO-AU EVENT

Porphyry Cu-Mo-Au deposits of the Western (ancestral) Cascades magmatic arc in northwestern Washington State are considered to be part of a magmatic belt that extends northward into the Coast Mountains of southwest British Columbia. The ca. 37 Ma North Fork porphyry Cu-Au deposit (80.4 million tonnes @ 0.44% Cu and 0.1 gram/tonne Au) is the oldest porphyry deposit in the Western Cascades, but similar late Eocene porphyry deposits are not recognized in the Coast Mountains of British Columbia, which calls into question the existence of a late Eocene porphyry event in the Coast Mountains. West of the Coast Mountains on Vancouver Island, however, is the ca. 41 Ma Catface porphyry Cu-Mo deposit (320 million tonnes @ 0.4% Cu). In addition to a shared late Eocene age, both the Catface and North Fork deposits are related to ilmenite-bearing I-type calc-alkaline granitic magmas that crystallized at oxygen fugacities ranging from 1.0 log unit above to 3.0 log units below the quartz-fayalite-magnetite (QFM) oxygen buffer. The mineralizing magmatic-hydrothermal fluids that formed the deposits are also relatively reduced as demonstrated by veins containing hypogene pyrrhotite, but lacking primary hematite and anhydrite. Reduced granitic magmas and associated porphyry deposits are atypical of an arc setting. The existence of the Catface deposit implies that the locus of the late Eocene magmatism extends northwest towards Vancouver Island and not north into the Coast Mountains. Recent plate tectonic models propose that a large slab window lies beneath much of British Columbia between 40 and 35 Ma. The calc-alkaline magmas forming the Catface and North Fork deposits, however, have classic arc signatures and lack geochemical traits that imply derivation as “slab melts” (adakites). Rather, these porphyry magmas probably formed in the fertile mantle wedge above the subducting Farallon plate in the late Eocene, although heat for the partial melting may have been caused by upwelling asthenosphere related to a slab window or slab tear. This new interpretation has important implications for mineral exploration because it implies that undiscovered late Eocene porphyry deposits could lie along a 400km long northwest-trending magmatic belt linking North Fork in Washington State to Catface on Vancouver Island, British Columbia.