PORPHYRY DEPOSITS OF THE CANADIAN CORDILLERA

Porphyry deposits are intrusion-related, large tonnage low grade miner al deposits with metal assemblages that may include all or some of cop per, molybdenum, gold and silver. The genesis of these deposits is rel ated to the emplacement of intermediate to felsic, hypabyssal, general ly porphyritic intrusions that are commonly formed at convergent plate margins. Porphyry deposits of the Canadian Cordillera occur in associ ation with two distinctive intrusive suites: calc-alkalic and alkalic. In the Canadian Cordillera, these deposits formed during two separate time periods: Late Triassic to Middle Jurassic (early Mesozoic), and Late Cretaceous to Eocene (Mesozoic-Cenozoic). Deposits of the early M esozoic period occur in at least three different are terranes (Wrangel lia, Stikinia and Quesnellia) with a single deposit occurring in the o ceanic assemblage of the Cache Creek terrane. These terranes were loca ted outboard from continental North America during formation of most o f their contained early Mesozoic porphyry deposits. Some of the deposi ts of this early period may have been emplaced during terrane collisio ns. Metal assemblages in deposits of the calc-alkalic suite include Mo -Cu (Brenda), Cu-Mo (Highland Valley, Gibraltar), Cu-Mo-Au-Ag (Island Copper, Schaft Creek) and Cu-Au (Kemess, Kerr). The alkalic suite depo sits are characterized by a Cu-Au assemblage (Copper Mountain, Afton-A jax, Mt. Milligan, Mount Polley, Galore Creek). Although silver is rec overed from calc-alkalic and alkalic porphyry copper mining operations , silver data are seldom included in the published reserve figures. Th ose available are in the range of 1-2 grams per tonne (g.t(-1)). Alkal ic suite deposits are restricted to the early Mesozoic and display dis tinctive petrology, alteration and mineralization that suggest a simil ar tectonic setting for both Quesnellia and Stikinia in Early Jurassic time. The younger deposits, late Mesozoic to Cenozoic in age, formed in an intracontinental setting, after the outboard host are and relate d terranes accreted to the western margin of North America. These depo sits are interpreted to occur in continental are settings, and individ ual deposits are hosted by a variety of older country rocks. These you nger deposits also show a spectrum of metal associations: Cu-Mo (Huckl eberry, Berg), Cu-Au (-Mo) (Bell, Granisle, Fish Lake, Casino), Mo (En dako, Boss Mountain, Kit-sault, Quartz Hill), Mo-W (Logtung), Au-W (Du blin Gulch) and Au (Ft. Knox). There may be a continuum between Mo, Mo -W, Au-Mo-W and Au deposits. The distribution and timing of these post -accretion deposits likely reflect major crustal structures and subduc tion geometry. Cordilleran porphyry metallic deposits show the full ra nge of morphological and depth relationships found in porphyry deposit s worldwide. In addition, the Cordillera contains numerous alkalic sui te deposits, which are rare worldwide: the unusual, possibly syntecton ic Gibraltar deposit; and end-member gold-rich granite-hosted deposits , such as Ft. Knox (Alaska).