Petrology and alteration of the Bathurst Norsemines central area deposits, Northwest Territories

The deposit is located 303 miles northeast of Yellowknife in rocks of the Archean Yellowknife Group, which consist of metavolcanics and metasediments. The metavolcanics are of the basaltic to rhyolitic association, including their related chemical and clastic sediments, and the metasediments are of the greywacke to argillite type. The rocks have been folded into a moderately southeast plunging synclinorium. They can be grouped, from oldest to youngest, into three conformable sequences; a southern sequence of calcareous basalt-andesite flows and pyroclastics which occur on the south limb of the fold, and a northern sequence of rhyolite pyroclastics and a central sequence of greywacke-argillite which occur on the north limb of the fold. Granites and granodiorites surround the metavolcanic and metasedimentary rocks and dykes of granite, pegmatite and quartz-eye porphyry rhyolite are common.Three massive sulphide deposits occur at the top of the thickest development of rhyolite pyroclastics of the northern sequence. These deposits are located in the Central Area, Boot Lake Zone and East Cleaver Lake Zone and are hosted by the same stratigraphic package which is locally referred to as the "mineral horizon".The area described is the Central Area. It is situated along the north limb of the synclinorium. This area is underlain by andesite, dacite and rhyolite and their related chemical and clastic sediments, and greywacke and argillite. The Central Area contains three mineral zones designated the A, Jo and Jo South Zones. The A and Jo Zones consist of conformable massive sulphides (pyrite, sphalerite, galena, chalcopyrite) underlain by discordant, mineralized (chalcopyrite, pyrrhotite, pyrite, magnetite) alteration pipes. The intimate, stratabound, spatial relationship of the massive pyrite lenses with the volcanic rocks, and more particularly with the "quiescent" marker rock types, indicates that massive sulphide formation was an integral part of, and related to, the volcanism. The Jo South Zone, which is stratigraphically lower than the A and Jo Zones, contains only veined and disseminated sulphides (pyrite, sphalerite, chalcopyrite, galena).Three distinct types of alteration occur intimately associated with the mineral zones and have been designated Subgroups A, B, and C. The rocks within these alteration zones have been extensively leached, silicified, metasomatized and replaced by sulphides, and locally, oxides. The Subgroup A zones surround and enclose the Subgroup B and C zones. Subgroups B and C are equivalent to the quartz-sericite and anthophyllite-cordierite zones commonly observed in other Archean deposits; Subgroup A may represent a weakly developed equivalent to the zones of chloritization noted in many Archean deposits.The mineralogy and textures observed in the various altered and unaltered rock is the result of transformation of the original constituents in the rocks by later isochemical cordierite-amphibolite facies metamorphism.