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Timing and tectonic setting of volcanogenic massive sulphide deposits in British Columbia: constraints from U-Pb geochronology, radiogenic isotopes, and geochemistry Childe, Fiona Christina


Volcanogenic massive sulphide (VMS) deposits occur within several of the allochthonous terranes of the North American Cordillera in British Columbia. In this study, the age of mineralization was determined for a number of these deposits. The radiogenic isotopic and geochemical signatures of host rocks and the mineralization were used to constrain the tectonic settings in which these deposits formed. The radiogenic isotopic signatures and geochemical affinities of three of these deposits in Stikinia, the Middle Jurassic Au-Ag-rich Eskay Creek deposit, the Late Triassic Cu-rich Granduc deposit, and the Late Mississippian polymetallic Tulsequah Chief deposit, assist in defining the evolution of this complex terrane. Rhyolite, which hosts and underlies mineralization at Eskay Creek has a primitive radiogenic isotopic and chemical character which is distinct from that of rhyolites of the same age within the region. Similarly, basalt which structurally underlies mineralization at Granduc is isotopically and chemically primitive relative to basalt which occurs within the same group regionally, and indicates formation within a back arc basin or immature island arc. The Tulsequah Chief deposit in northern Stikinia exhibits isotopic and geochemical features indicative of a more evolved island arc setting, which may reflect a variation in the basement to this terrane. The Cu-Zn-rich Kutcho Creek VMS deposit is hosted within felsic-dominated strata of the Kutcho Assemblage. Permo-Triassic to earliest Triassic ages are established for volcanic rocks which host mineralization. These ages, along with primitive radiogenic isotopic and geochemical signatures, indicate that these rocks and their contained mineralization formed in a primitive arc or fore-arc setting built on oceanic basement. Uranium-Pb dating and isotopic and geochemical studies of volcanic and intrusive rocks in fault-bounded slices further south in the Cordillera indicate that this Permo-Triassic magmatic event was widespread, and possibly define a new terrane. Age, isotopic and geochemical information was also obtained for igneous rocks within pendants and belts enclosed within the Coast Plutonic Complex which host VMS mineralization. New data for intrusive rocks from the Scotia-Quaal Belt and Anyox pendant are permissive of formation of these units within Stikinia.

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