VOLCANIC-HOSTED MASSIVE SULFIDE DEPOSITS OF THE NORTHERN CORDILLERA: DIVERSE EXPRESSIONS OF EXTENSION AND SUBDUCTION ALONG THE PALEOZOIC NORTH AMERICAN CONTINENTAL MARGIN

Most volcanic-hosted massive sulfide (VHMS) deposits in the northern Cordillera formed during Devonian and Mississippian submarine magmatism associated with extension and east-dipping subduction along the North American continental margin. The end product of crustal extension was development of the Seventymile/Slide Mountain ocean basin. VHMS deposits that formed in a parautochthonous (inboard) continental-margin setting include those of the Bonnifield and Delta districts in east-central Alaska, and the Eagle Bay assemblage (Kootenay terrane) in British Columbia. In the Bonnifield Zn-Pb-Ag district, the Dry Creek (DC) deposits are hosted by ~373-363-Ma peralkaline metarhyolite associated with alkali basalt; both have within-plate geochemical signatures and elevated Nd isotopic signatures that indicate an enriched mantle component and formation in an extensional setting. The 363 Ma Anderson Mountain (AM) deposit occurs within metarhyolite that is derived largely from sialic crust, and is associated with intermediate to mafic rocks with arc geochemical signatures; AM and DC likely formed a continental-margin arc and back-arc pair, respectively. The Delta (~372-363 Ma; Zn-Pb-Cu-Au-Ag) and Eagle Bay (~361 Ma; Zn-Pb-Cu-Au-Ag-Ba) deposits occur in felsic to intermediate-composition metavolcanic rocks with arc geochemical signatures; a crustal setting is indicated by Nd isotopes (Eagle Bay) and zircon inheritance (Delta). Most VHMS deposits in the Finlayson Lake district of southern Yukon developed behind a west-facing arc within the continental-margin fragment that formed by the opening of the Slide Mountain ocean. The Fire Lake Cu-Co-Au deposit (~365 Ma) is associated with boninitic metavolcanic and intrusive rocks. To the north, Kudz Ze Kayah and Wolverine Zn-Ag-Cu-Au-Pb deposits (~365-362 and ~347 Ma, respectively) are associated with alkalic felsic metavolcanic rocks derived from the melting of sialic crust during back-arc extension. The final phase of submarine hydrothermal mineralization accompanied late Paleozoic mafic magmatism in the ocean basin: VHMS deposits within thrust sheets of the Slide Mountain terrane include the Permian (~274 Ma) Cu-(Co-Au) Ice deposit, in the Finlayson Lake district, and the Mississippian-Permian Cu-Zn-Au-Ag Chu Chua deposit, west of Eagle Bay.