Geology of unmineralized and gold-bearing iron formation, Contwoyto Lake – Point Lake region, Northwest Territories, Canada

Abstract

Numerous gold occurrences, including the Lupin mine, exist in Archean iron formation within the Contwoyto Lake – Point Lake region of the Slave Province. Early studies suggested that gold was a syngenetic component of the iron formation; however, the present study suggests that the gold and sulphides are epigenetic.At both the Lupin mine and small-scale gold occurrences gold is associated with quartz veins and concentrations of pyrrhotite or pyrite and of arsenides (arsenopyrite ± loellingite) in iron formation. The quartz veins contain 0.03–1.00 ppm Au and comprise sulphide-poor quartz. A zoned sequence of hydrothermal alteration is present in iron formation adjacent to quartz veins. Immediately adjacent to the veins a calc-silicate lithology (0.03–1.00 ppm Au) is sometimes developed that comprises hedenbergite + quartz ± epidote ± scheelite ± grossular. Next is an arsenide-rich zone (5–30 ppm Au) comprising hornblende + quartz ± hedenbergite ± epidote ± actinolite. The next zone is an iron-sulphide zone (5–30 ppm Au), lacking abundant arsenides but containing pyrrhotite or pyrite and hornblende + quartz ± hedenbergite ± epidote ± actinolite. Farther from the veins, iron formation is unmineralized (≤0.03 ppm Au), lacks sulphides, and comprises grunerite + quartz ± magnetite. In the transition zone, hornblende replaces grunerite, and iron sulphides replace amphiboles and magnetite. The scale of the sequence of zones varies from millimetres to metres about individual veins. In well-mineralized portions of the Lupin mine, where quartz veins are closely spaced, unmineralized iron formation is absent between the veins.The symmetrical zonal pattern in the mineralogy and gold values about the veins at Lupin and at the small-scale occurrences indicates that mineralization at both scales of gold occurrence formed by an epigenetic process. Mineralization occurred by selective sulphidation of iron formation after most of the Archean deformation and was coincident, or nearly coincident, with peak thermal conditions. Gold was probably transported as an aqueous gold–sulphide complex and deposited as a result of sulphidation reactions.