Polaris as a guide to northern exploration: Ore textures, paragenesis and the origin of the carbonate-hosted Polaris Zn–Pb Mine, Nunavut, Canada

The Polaris Zn–Pb Mine in Nunavut, Canada was one of the largest single Mississippi Valley-type ore deposits in the world. Over 20 Mt of sphalerite (ZnS) and galena (PbS) was hosted in brecciated carbonate rocks of the Upper Ordovician Thumb Mountain Formation. Three paragenetic stages are recognized: 1) early dolomite and marcasite; 2) main stage sulphide and dolomite; and 3) late calcite, marcasite and barite. Ore mineral textures range from discrete crystals to massive crystal aggregates and formed as replacements of the dolomite host rock or as fracture- and open space-filling mineralization. Zinc concentration is highest in the core of the deposit where botryoidal aggregates predominate, whereas iron is concentrated in the upper part. Observations of temperature and in situ sulphur isotope fractionation support a genetic model for the Polaris deposit in which thermochemical sulphate reduction occurred within the deposit, with locally generated hydrocarbons acting as a reducing agent. Information from the Polaris Mine indicates that hydrothermal alteration including dolomite, marcasite and barite; complex paragenesis with numerous ore textures; Th values > 100 °C associated with organic-rich strata; and a geochemical signature that includes in situ sulphur fractionation are effective predictors for determining which showings are prospective in the vast central Arctic Pb–Zn district.