Porphyry copper-gold-molybdenum deposits in the Island Copper Cluster, northern Vancouver Island, British Columbia

Special Volume, Vol. SV 46, No. 1995, 1995

The Island Copper Cluster (ICC), situated at the northern end of Vancouver Island, consists of five porphyry Cu-Au-Mo systems (Island Copper, Bay Lake, G zone, Red Island and Rupert Inlet) and a porphyry Cu-Mo system (Northwest zone) genetically associated with Jurassic stock and dike-like rhyodacitic porphyries (c. a. 175 Ma) that intruded comagmatic island arc, calc-alkaline basalts, andesites, pyroclastic and sedimentary marine rocks of the Bonanza Group. These share similarities in geometries of alteration and mineralization but exhibit a large range of size and grade. Copper-bearing garnet-pyroxene skarn (e.g., Northwest zone) and vein-type mineralization (A zone) also constitute integral parts of the porphyry systems. The only orebody in the cluster supports the Island Copper mine, controlled and operated by BHP Minerals Canada Ltd. Between start of production in 1971 and the end of 1994 the mine produced 345 million tonnes of ore having average head grades of 0.41% Cu, 0.017% Mo, 0.19 glt Au and 1.4 glt Ag. The Island Copper hydrothermal system evolved from an early, probably juvenile magmatic fluid-dominated stage, to one strongly influenced by meteoric waters, as the main heat source cooled and further intrusion and brecciation took place. Three main stages of alteration and mineralization have been differentiated. Most copper, gold and some molybdenum were deposited under K-si/icate stable conditions during an Early stage related to the intrusion of a Main rhyodacite porphyry. This was followed by a coppermolybdenum-( gold?) Intermediate stage associated with quartz-sericite and quartz-sericite-clay (SCC) assemblages and by a copper-barren, pyrophyllite-rich Late stage under advanced argillic alteration conditions. These stages were assisted by Intra-mineral and Latemineral rhyodacite intrusions. Certain features of Island Copper such as the positive correlation between copper and gold, the association of gold with a potassic, biotite-rich alteration assemblage, and the high magnetite content (> 8% by volume) in the system are characteristic of gold-rich porphyry copper deposits. The spatial a"angement of biotite chalcopyrite ore zones around a copper-ba"en, quartz-magnetite amphibole core is, however, considered to be a unique feature of the Island Copper orebody and other members of the cluster (Bay Lake, G zone, Red Island). Comparisons are also valid between the Pe-rich core of the systems of the ICC and iron ore mineralization of the Kiruna-type.

Mots Clés: Island Copper Cluster, ICC, Porphyry, Jurassic stock, Rhyodacitic porphyries, Orebody, Gold, Copper