Abstract Tourmaline in the North orebody and altered rocks of the Kidd Creek massive sulfide deposit grew mainly during sulfide deposition; only trace amounts grew during subsequent low-grade regional meta-morphism. Tourmaline retains its original oxygen, hydrogen, and boron isotope compositions, providing a unique record of the origin and evolution of hydrothermal fluids responsible for alteration and mineralization in this deposit. The fluid history at Kidd Creek involved boiling of seawater at ca. 100° to 150°C in shallow recharge zones, followed by heating at depth to ca. 250°C to form a modified seawater fluid; additional heating (>350°C) of the modified seawater, probably accompanied by supercritical phase separation, and water-rock reaction produced further enrichment in 18 O, forming a high-temperature fluid. Tourmaline- and sphalerite-dominated sulfide assemblages precipitated from mixtures of modified seawater and slightly modified seawater over a range of temperatures from ca. 150° to 250°C, whereas tourmaline- and chal-copyrite-dominated sulfide assemblages formed from mixtures of high-temperature fluids and modified seawater at higher temperatures (ca. 275°-425°C). The temperature of the ore-forming fluids varied from 183° to 429°C, based on quartz-tourmaline oxygen isotope fractionations. Cooling and dilution of the hydrothermal fluids were principal factors responsible for sulfide precipitation. Three fluids are identified: (1) slightly modified seawater, with δ 18 O ≈ 0 per mil, δD ≈ 0 per mil, and δ 11 B ≈ 3.0 per mil, and an assumed normal salinity of ca. 3.2 wt percent NaCl; (2) extensively modified seawater with a boiling model-based, estimated maximum salinity of 6.4 to 16 wt percent NaCl and δ 18 O≈4.0 per mil, S D≈20 per mil, and S 11 B≈-3.0 per mil; and (3) a high-temperature, probably supercritical, fluid with a similarly estimated maximum salinity of at least 6.4 to 16 wt percent NaCl and δ 18 O ≈7.0 per mil, S D≈0 per mil, and S 11 B≈-3.0 per mil. The boron isotope composition of slightly modified seawater and the oxygen isotope composition of high-temperature fluid require reaction with rocks previously altered at low temperatures. Mineralizing brines that formed near the surface circulated to greater depths where they were heated to higher temperatures, probably by heat supplied by the ultramafic and felsic dikes and sills in a rifting environment. The high salinities inferred for the hydrothermal fluids were most likely a contributing factor in the development of such a large massive sulfide orebody.