Regional geological and tectonic setting of porphyry deposits in British Columbia and Yukon Territory

Special Volume, Vol. SV 46, No. 1995, 1995
Porphyry deposits in the Canadian Cordillera formed during two separate time periods: Late Triassic to Middle Jurassic and Late Cretaceous to Eocene. Deposits of the Early Mesozoic period occur in at least three different arc terranes: Wrangellia, Stikinia and Quesnellia, with a single deposit in the oceanic assemblage of the Cache Creek terrane. These terranes were outboard from continental North America during the formation of most of their contained porphyry deposits. Some of the deposits of this early period may have been emplaced during terrane collisions. Metal assemblages in deposits of the calc-alkalic suite include Mo-Cu (Brenda), Cu-Mo (Highland Valley, Gibraltar), and Cu-Mo-Au (Island Copper, Schaft Creek) and Cu-Au (Kemess, Kerr). The alkalic suite deposits are characterized by a Cu-Au assemblage (Copper Mountain, AftonAjax, Mt. Milligan, Mount Polley, Galore Creek). The alkalic deposits are restricted to the Early Mesozoic and display distinctive petrology, alteration and mineralization that suggest a similar tectonic setting for both Quesnellia and Stikinia in the Early Jurassic.
The younger, Late Mesozoic to Cenozoic, deposits formed in an intracontinental setting, after the outboard host arc and related terranes accreted to the western margin of North America. These deposits are interpreted to occur in continental arc settings, and individual deposits are hosted by a variety of older country rocks. They also show a spectrum of metal associations: Cu-Mo (Huckleberry, Berg), Cu-Au(-Mo) (Bell, Granisle, Fish Lake, Casino), Mo (Endako, Boss Mountain, Kitsault, Quartz Hill), Mo-W (Logtung), Au-W (Dublin Gulch) and Au (Fort 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 subduction geometry. Cordilleran porphyry deposits show the full range of morphological and depth relationships found in deposits worldwide. In addition, the Cordillera contains numerous alkalic suite deposits, which are rare worldwide; the unusual, possibly syntectonic, Gibraltar deposit; and end-member gold-rich granite-hosted deposits, such as Fort Knox.
Keywords: Porphyry deposits, Geology, formation
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