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Geology and genesis of the wolverine polymetallic volcanic rock-hosted massive sulphide (VHMS) deposit, Finlayson Lake District, Yukon, Canada Bradshaw, Geoffrey David

Abstract

The Wolverine polymetallic massive sulphide deposit (6,237,000 tonnes grading 12.7% zinc, 1.3% copper, 1.6% lead, 370.9 g/t silver and 1.8 g/t gold) is located in the Finlayson Lake district (FLD) of southeastern Yukon Territory, Canada. Its discovery .in 1995 contributed to the largest staking rush in the history of Yukon and provided the impetus for scientific research in this poorly studied part of the Yukon Tanana terrane. The deposit occurs in a highly deformed, but coherent, stratigraphic succession of early Mississippian to early Permian metavolcanic and metasedimentary rocks. Regional geologic mapping and lithogeochemical studies are consistent with its formation on the margin of an evolving ensialic back-arc ocean basin, between the Yukon-Tanana terrane and the ancestral North American craton. Local stratigraphy consists of four major units including (from oldest to youngest): (1) quartz- and feldspar-phyric volcaniclastic rhyolite, carbonaceous argillite and rhyolite porphyry; (2) interbedded argillite, aphyric rhyolite and magnetite-carbonate-pyrite exhalite; (3) fragmental rhyolite; and (4) interbedded carbonaceous argillite, greywacke, basalt and rhyolite. Sulphide mineralization occurs at the "Wolverine horizon", which is located at the contact between Unit 1 and Unit 2, and marks a significant change in the character of the volcanism between the footwall and the hanging wall of the deposit. The footwall consists of a sequence of carbonaceous shale, quartzand feldspar-phyric volcaniclastic rhyolite, and minor feldspar-phyric rhyolite sills, whereas the hanging wall consists of intercalated aphyric rhyolite, carbonaceous shale, and carbonate-pyrite exhalite. Two separate Zn-Pb-Ag massive sulphide lenses, situated at the same stratigraphic horizon, comprise most of the Wolverine deposit. They are laterally connected by stratabound, semi-massive replacement-style, Zn-Pb-Ag mineralization. Copper-rich mineralization commonly replaces the Zn-Pb-Ag mineralization at the base of the lenses and in the footwall replacement zones indicating that primary metal and mineral zonation are preserved. Multiple zones of sulphide stringer veins and regions of conformable chloritesericite-carbonate alteration are developed within permeable volcaniclastic rocks of the footwall. Bariumrich phengitic mica, biotite, Mg-rich chlorite, and siderite are associated preferentially with massive sulphide mineralization. The Zn-Pb-Ag massive sulphide lenses formed at 264 ± 33°C, based on the arsenopyrite geothermometer. The sulphide stringer zones formed at slightly higher temperatures (mean = 282 ± 7°C) based, on estimates derived from the composition of associated hydrothermal chlorite. Late-stage quartzchalcopyrite- pyrite veins formed at temperatures between 265°and 353°C (mean = 302 ± 22°C), based on fluid inclusion microthermometry. Mineralizing fluids are low salinity (2.1-8.5 wt.% NaCl equiv.; mean = 6.0), two-phase, aqueous solutions with high liquid-to-vapor ratios. Evidence of fluid boiling or phase separation is absent. Mineralization is estimated to have formed at a minimum water depth of ~ 1048 meters based on the average salinity and temperatures of homogenization. In-situ δ³⁴S values of sulphide minerals from massive sulphide lenses and sulphide stringer veins display a pronounced bimodal distribution with modes of 0.8 ‰ and 12.0 ‰. The lighter δ³⁴S values are near the top of the lenses, whereas the heavier values come from the underlying stringer veins. The δ³⁴S values and existence of two distinct populations, suggest that sulfur was derived by a combination of biogenic and inorganic reduction of seawater sulphate within a partly closed, anoxic basin. Mineralization formed through a combination of seafloor hydrothermal venting from multiple sulphide mounds and sub-seafloor replacement processes. The reduced nature of the ambient bottom water in the basin likely contributed to the preservation of the sulphide mounds. The geologic setting, styles of mineralization, and physico-chemical conditions of the mineralizing fluids all suggest that the Wolverine deposit formed in a geological environment transitional between that which hosts classic bimodal volcanic rock-hosted massive sulphide deposits and that which hosts sedimentary exhalative massive sulphide deposits. The genetic model developed for the Wolverine deposit in this study will benefit exploration in the Finlayson Lake district by providing specific details on the geological setting and local depositional environment necessary for the formation and preservation of these unusual polymetallic deposits.

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