The geology and geochemistry of base metal sulfide mineralization in the Foster River area, northern Saskatchewan, Canada

The Foster River area, northern Saskatchewan, is one of several Pb-Zn and Zn deposits (e.g. George Lake: 7.8 Mt 3.9% Zn and 0.5% Pb) along the SE margin of the highly deformed and metamorphosed Wollaston domain, Hearne Craton. Zinc-lead sulfide mineralization in the Foster River area is hosted by feldspathic quartzites within a unit of psammites and pelites near the middle of the Paleoproterzoic (2.0-1.86 Ga) Wollaston Group. These sediments were metamorphosed to the upper amphibolite facies and subjected to at least four episodes of deformation during the 1860-1750 Ma Trans-Hudson Orogeny. Drilling indicates that the Sito Lake East prospect contains 50,000 t of 4.5% Zn, with one intercept containing 11m of 4.2 % Zn and 0.6% Pb. Boulders from a boulder train near the Fable Lake prospect contain up to 13.2% Zn, 4.0% Pb, and 11 g/t Ag. The sulfides are spatially associated with a package of rocks similar to that found with Broken Hill-type (BHT) deposits (quartzite, gahnite-rich rocks, iron formation, and quartz garnetite). A nodular sillimanite rock that occurs in the vicinity of the Sito East, Sito West, and George prospects is likely to be part of a stratabound hydrothermal alteration zone. Hydrothermal sulfide mineralization consists of pyrite, sphalerite, pyrrhotite, galena, and chalcopyrite, locally spatially associated with graphite. Gangue minerals include garnet, gahnite, magnetite, tourmaline, calcite, and augite.;Chondrite-normalized REE patterns of silicate-facies iron formation (garnet-pyroxene-amphibole-magnetite rock) and quartz garnetite show light REE (LREE) enrichment and heavy REE (HREE) depletion, with moderate to large negative Eu anomalies and no or slightly positive Ce anomalies. Such patterns are consistent with meta-exhalites that have a high detrital component (>30%) in the source rock. The compositions of garnet (spessartine-almandine) and gahnite at Foster River are similar to those spatially associated with BHT deposits. Sulfur isotope compositions of pyrite, pyrrhotite, and sphalerite from the Foster River area range from 26.2-38.1‰ (n=20) and are consistent with sulfur being sourced from Paleoproterozoic seawater that was modified by microbial sulfate reduction in a restricted basin. The low Pb/(Pb+Zn) and Ag/(Ag+Pb+Zn) ratios of sulfide mineralization, common graphite-sulfide intergrowths, the high sulfur isotope values, and the lack of bimodal volcanics in the stratigraphic sequence suggest that the mineralization in the Foster River area has a sedimentary-exhalative origin with BHT affinities.