Abstract Polaris is a late Devonian Mississippi Valley-type Zn-Pb deposit hosted by 10°-15° east- dipping late Ordovician Thumb Mountain dolomitized limestones rich in macro-and microfossils. Microfossils such as scolecodonts, chitinozoans, graptolites and hydrozoans exhibit anomalously low reflectance values in oil (%R 0 ) which decrease with depth through the 150 m mineralized stratigraphy, in contrast to the expected increasing reflectance with depth. This phenomenon is tentatively attributed to hydrogenation of microfossils by hydrogen evolved, together with other gaseous hydrocarbons, from organic material during the ore deposition process. Reflectances of 1.3%R 0 in rocks above the zone of anomalous decreasing reflectance exceed values expected from maximum estimated burial temperatures of 60°C, and are consistent with observed oil generation and fluid inclusion temperatures of 105°C measured on ore sphalerite and gangue dolomite. Clay minerals exhibit a zonal pattern around and above the orebody. Conversion of sedimentary illites to coarse kaolinite in and around ore was probably driven by acid generation during sulfide precipitation. Illitization of pre-existing sedimentary diagenetic clays above the ore may be related to the influx of potassium in the highly saline evaporative brines identified in fluid inclusions in Polaris ore sphalerites. It is proposed that methane co-generated with hydrogen during hydrothermally-induced maturation of organic matter reduced brine sulfates to H 2 S, allowing precipitation of metal sulfide ores. Reactions inferred from observed clay mineral distributions can be accomplished at the temperatures prevalent during ore precipitation. Brines were probably driven by the late Devonian Ellesmerian orogeny, but ore precipitation occurred while the host rocks were horizontal. Clay mineral analyses and organic matter alteration studies are both potentially useful exploration tools when integrated with available geological data.

Abstract The Polaris Mine is a carbonate hosted lead-zinc deposit located on a small island in the centre of the Canadian Arctic archipelago. Cominco Ltd. discovered the Polaris orebody in 1971 and put it into production in late 1980. Mining and milling at Polaris produce galena and sphalerite concentrates for sale to European smelters. Daily production of ore from the mine averages about 2850 tonnes per day totaling 1,040,000 tonnes a year. Ore reserves at Polaris are 7.6 million tonnes grading 3.6% Pb and 13.6% Zn.