Felsic igneous complexes and associated volcano-sedimentary rocks in continental back-arc environments host large-tonnage and/or high-grade volcanogenic massive sulfide (VMS) deposits. The emplacement mechanisms, style, and preservation of these deposits is thought to be partially dependent on depositional rates of the host lithofacies (i.e., discrete volcanic eruptions) relative to the setting of massive sulfide genesis on the seafloor as mounds and/or via subseafloor replacement of existing strata. The localization and occurrence of subseafloor replacement-style VMS deposits is therefore strongly influenced by the characteristics of the volcano-sedimentary facies in the hosting basin and the rates of their emplacement; the latter are poorly constrained in the literature due to the difficulty of obtaining high-precision dates that make this possible in Phanerozoic and older rocks. New high-resolution U-Pb geochronology and detailed regional stratigraphic investigation indicate that Devonian-Mississippian volcanic rocks and associated VMS mineralization in the Yukon-Tanana terrane in the Finlayson Lake district, Yukon, Canada, were erupted or emplaced during distinct time periods (ca. 363.3, 362.8, and 355.2 Ma) in two discrete submarine basins: the Kudz Ze Kayah formation and the Wolverine Lake group. The VMS deposits in both settings are contained within intrabasinal rocks that accumulated at rapid rates of ~350 to 2,000 m/m.y. over 0.6 to 1.4 m.y. Locally, these rates reach peak rates up to 7,500 m/m.y. in the Wolverine Lake group, which are interpreted to reflect facies deposition by mass transport complexes or turbidity currents. These new dates indicate that rapid accumulation of volcanic rocks in the back-arc basins was critical for localizing subseafloor replacement-style mineralization and the development of the Zn-enriched GP4F, Kudz Ze Kayah, and Wolverine VMS deposits. Rapid depositional processes observed in these deposits and their host basins are interpreted to have an important role in developing highly porous and permeable, water-saturated lithofacies that provide optimal conditions for enhancing zone refining processes and subsequent preservation of massive sulfide mineralization, which are key in the development of high-grade and large-tonnage VMS deposits. It is herein suggested that quantitative basin-scale accumulation rates, as a result of new U-Pb geochronological methods and increased precision combined with detailed stratigraphic and facies analysis, may provide important perspectives on the formation of continental back-arc basins and the localization of VMS deposits in other continental margin environments globally.

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The Yukon-Tanana terrane in the Finlayson Lake district, Yukon, represents one of the first arc–back-arc systems that formed adjacent to the Laurentian continental margin in the mid-Paleozoic. Back-arc rocks contain many large and high-grade volcanogenic massive sulfide (VMS) deposits. This study integrates U-Pb zircon geochronology, lithogeochemistry, and Hf-Nd isotopes to establish precise controls on tectonomagmatic activity adjacent to the western Laurentian margin in the Late Devonian to Early Mississippian. High-precision chemical abrasion- (CA-) ID-TIMS U-Pb zircon geochronology defines coeval arc (ca. 363.1 to 348 Ma) and back-arc (ca. 363.3 to 355.0 Ma) magmatism in the Finlayson Lake district that intruded continental crust of Laurentian affinity (e.g., Snowcap assemblage). Mafic and felsic rocks display geochemical and isotopic characteristics that are consistent with being formed from mixtures of depleted asthenosphere and enriched lithospheric mantle sources. These melts variably entrained Laurentian continental crust via high-temperature crustal melting due to basaltic underplating. The high-temperature back-arc felsic magmatism occurs at specific time periods coinciding with VMS deposits and supports previous genetic models for VMS mineralization that suggest elevated heat flow and hydrothermal circulation were due to regional-scale rift-related magmatism rather than from local subvolcanic intrusions. The short timescales and transient nature of tectonomagmatic events in the Finlayson Lake district suggest that rapid and complex subduction initiation of oceanic and continental crust fragments facilitated coeval compression, extension, and magmatism in the arc and back-arc regions. We thus reevaluate the presently accepted tectonostratigraphic framework of the Finlayson Lake district and suggest revised interpretations that shed light on VMS depositional environments and a possible broader association with the ca. 358 Ma Antler Orogeny. Results of this study have implications for incipient tectonics, magmatism, and mineralization along the western Laurentian continental margin and other orogenic belts globally.

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