Lead- and sulfur-isotope ratios have been determined for some galenas and iron sulfides from two regions around Yellowknife, Northwest Territories. For the ore deposits, sulfur-isotope ratios are close to the meteoritic value or slightly positive. Lead-isotope ratios fall into two groups, one derived from a source with Th/U = 4.3 and μ = 9–11, and the other from a source with Th/U = 3.5 and μ = 9.0. The ratios fall on an anomalous lead line, which indicates one event of great antiquity (approximately 4000 m.y.), and another about 2800 m.y. ago.Other sulfide samples from areas around Great Slave Lake, primarily outside the ancient (2800 m.y.) Slave Province, indicate events at 2800 m.y. and 1550 m.y., the latter date probably representing the time of the last major tectonic event in the Slave Province. Sulfur-isotope ratios for these samples vary from +1‰ to −13‰ relative to the meteoritic value, suggesting that these leads may have been remobilized by thermal metamorphism, and recombined with crustal sulfur enriched in 32S.