Genesis of the Wolverine Volcanic Sediment-Hosted Massive Sulfide Deposit, Finlayson Lake District, Yukon, Canada: Mineralogical, Mineral Chemical, Fluid Inclusion, and Sulfur Isotope Evidence

Abstract

The Wolverine deposit is a high-grade (~6.2 Mt at 12.7% Zn, 1.3% Cu, 1.6% Pb, 370.9 g/t Ag, and 1.8 g/t Au) volcanic and sedimentary rock-hosted massive sulfide deposit in the Finlayson Lake area, Yukon Territory. The area has the potential to become an important mining camp and currently contains more than 30 million tons (Mt) of massive sulfide ore in five deposits. A succession of metamorphosed early Mississippian felsic volcanic and sedimentary rocks that are interpreted to have formed on the margin of an ensialic back-arc ocean basin between the Yukon-Tanana terrane and the ancestral North American craton hosts the Wolverine deposit. The massive sulfide ore occurs at a single horizon that marks a significant change in the character of the volcanism from quartz- and feldspar-phyric rhyolite of the footwall to aphyric rhyolite and chemical sedimentary exhalative rock of the hanging wall. The deposit consists of two discrete Zn-Pb-Ag massive sulfide lenses situated at the same stratigraphic level connected by strata-bound semimassive, replacement-style Zn-Pb-Ag mineralization. Copper-rich massive sulfide commonly replaces the Zn-Pb-Ag massive sulfide at the base of the lenses and in the footwall replacement zones beneath the massive mineralization. Several zones of sulfide stringer veins and chlorite-sericite-carbonate alteration occur within permeable footwall volcaniclastic rocks. Alteration minerals preferentially associated with massive sulfide mineralization include Ba-rich phengitic mica, biotite, Mg-rich chlorite and siderite. Microthermometric measurements of fluid inclusions in the massive sulfide bodies and accompanying stringer zones indicate formation at temperatures of 235° to 353°C. Primary, aqueous, liquid-rich fluid inclusions in hydrothermal quartz from quartz-sulfide stringer veins contain low-salinity fluids (2.1–8.5 wt % NaCl equiv; mean = 6.0). There is no evidence of fluid boiling in the inclusions, and based on the average fluid salinity and temperature of homogenization, mineralization took place under a minimum water depth of ~1,000 m. In situ analyses of δ34SV-CDT in sulfide minerals from massive sulfide lenses (means 0.8‰) and sulfide stringer veins (mean 12.0‰) display a bimodal distribution. Lower δ34S values characterize sulfides near the top of the lenses versus higher (more positive) values in underlying veins may reflect different processes of reduction of seawater sulfate. Sulfur in sulfides from the tops of the lenses was likely derived mainly from bacterial reduction of seawater sulfate, whereas sulfur in the stringer veins and at the base of the lenses was mainly from inorganic reduction of seawater sulfate. Sulfide minerals precipitated both on the sea floor and in the subsea floor from hydrothermal fluids that vented from multiple mounds. The presence of black shales and the strongly positive sulfur isotope compositions of the sulfides provide evidence that the ambient bottom water in the basin in which the Wolverine deposit formed was likely anoxic. This environment may have contributed to the preservation of the sulfide mounds. Based on its tectonic setting, host-rock types, local geologic setting, metal grades, age, and source(s) of sulfur, the Wolverine deposit best exemplifies the volcanic sediment-hosted massive sulfide (VSHMS) class of syngenetic massive sulfide deposits in the district.