Whole-rock lithogeochemistry, Nd-Hf isotopes, and in situ zircon geochemistry of VMS-related felsic rocks, Finlayson Lake VMS district, Yukon

The Finlayson Lake district in southeastern Yukon is composed of a Late Paleozoic arc-backarc system that consists of metamorphosed volcanic, plutonic, and sedimentary rocks of the Yukon-Tanana and Slide Mountain terranes. These rocks host >40 Mt of polymetallic resources in numerous occurrences and styles of volcanogenic massive sulphide (VMS) mineralization. Geochemical and isotopic data from these rocks support previous interpretations that volcanism and plutonism occurred in arc-marginal arc (e.g., Fire Lake formation) and continental back-arc basin environments (e.g., Kudz Ze Kayah formation, Wind Lake formation, and Wolverine Lake group) where felsic magmatism formed from varying mixtures of crust- and mantle-derived material. The rocks have elevated high field strength element (HFSE) and rare earth element (REE) concentrations, and evolved to chondritic isotopic signatures, in VMS-proximal stratigraphy relative to VMS-barren assemblages. These geochemical features reflect the petrogenetic conditions that generated felsic rocks and likely played a role in the localization of VMS mineralization in the district. Preliminary in situ zircon chemistry supports these arguments with Th/U and Hf isotopic fingerprinting, where it is interpreted that the VMS-bearing lithofacies formed via crustal melting and mixing with increased juvenile, mafic magmatism; rocks that were less prospective have predominantly crustal signatures. These observations are consistent with the formation of VMS-related felsic rocks by basaltic underplating, crustal melting, and basalt-crustal melt mixing within an extensional setting. This work offers a unique perspective on magmatic petrogenesis that underscores the importance of integrating whole-rock with mineral-scale geochemistry in the characterization of VMS-related stratigraphy.

Yukon–Tanana terrane in the southern Campbell Range, Finlayson Lake belt, southeastern Yukon: the geological setting of retrogressed eclogite of the Klatsa metamorphic complex

Yukon–Tanana terrane in the southern Campbell Range is composed of rocks that have different metamorphic, exhumation, and structural histories, and that have formed in disparate parts of the Paleozoic Yukon–Tanana volcanic arc. The geological relationships in the southern Campbell Range reveal the tectonic and structural history of the Klatsa metamorphic complex, which represents the remnants of an Early Mississippian subduction zone beneath the Yukon–Tanana arc. The Klatsa metamorphic complex is composed of foliated to massive serpentinite, leucogabbro, amphibolite, and retrogressed eclogitic quartz–muscovite schist with lenses of metabasite. It was structurally juxtaposed on Upper Mississippian to Lower Permian metasedimentary rocks of the White Lake, King Arctic, and Money Creek formations. Regional and local structural and stratigraphic relationships suggest that the Klatsa metamorphic complex is part of the Cleaver Lake thrust sheet, the structurally highest thrust sheet in a north- to northeast-vergent thrust belt that deformed the Yukon–Tanana terrane during the Early Permian. Restoration of the displacement on the Cleaver Lake and underlying thrust faults places the Klatsa metamorphic complex on the western margin of Yukon–Tanana terrane. Late Devonian to Early Mississippian subduction is thought to have occurred along this margin based on previous paleogeographic reconstructions. Generally north- to northeast-vergent D1 to D3 folds deformed the Klatsa metamorphic complex and adjacent metasedimentary rocks. Jurassic(?) D4 imbricate thrust faulting has, in part, reactivated the Cleaver Lake thrust fault contacts and imbricated the Klatsa metamorphic complex with metasedimentary rocks in fault panels that are repeated at a scale of 10 to hundreds of metres.