Abstract

The Kutcho Assemblage is a latest Permian to Early Triassic volcano-sedimentary sequence within the fault-bounded King Salmon Allochthon. Volcanic and volcaniclastic rocks consist of massive to pillowed flows and tuffs of basaltic to basaltic andesitic composition, as well as flows, mass flows, and pyroclastic flows of rhyodacitic to rhyolitic composition. The volcanic sequence is intruded by gabbro, diorite, trondhjemite, and quartz–plagioclase porphyry. Volcanic and intrusive rocks have tholeiitic magmatic affinities, with die exception of the gabbro sills and dykes, which are chemically similar to alkaline arc magmas. Cu–Zn volcanogenic massive sulphide mineralization at die Kutcho Creek deposit is hosted by rhyolite mass flows near me top of the Kutcho Assemblage. Rhyolite mass flows from the hanging wall and footwall to mineralization have U–Pb ages of 242 ± 1 and graphic, respectively, whereas a quartz–plagioclase porphyritic intrusion to the south of the mineralization has a U–Pb age of 244 ± 6 Ma. The dates determined in this study constrain a previously unknown age for massive sulphide mineralization in the Canadian Cordillera. Furthermore, the age of the Kutcho Assemblage and the primitive Pb isotopic signature of its contained syngenetic mineralization (207Pb/206Pb = 0.83988–0.84112 and 208Pb/206Pb = 2.0517–2.0556) are inconsistent with formation as part of the adjacent Stikine or Quesnel terranes. Primitive Nd isotopic signatures (εNd(initial) = +7.5 to +9.1) and trace and rare earth element chemistry indicate that volcanic rocks of the Kutcho Assemblage, related intrusions, and volcanogenic massive sulphide mineralization formed in an intraoceanic island arc environment, probably directly on oceanic crustal basement. Gabbro sills and dykes, which are interpreted to be slightly younger than volcanic rocks of the Kutcho Assemblage, appear to have formed in response to a change in the tectonic regime, perhaps as a result of a collisional event.

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