The hydrothermally altered rhyolites (delta 18 O = 10-16ppm) at the Kidd Creek Cu-Zn-Ag-Pb volcanogenic massive sulfide deposit (Archean, Abitibi greenstone belt) are all markedly enriched in 18 O compared to almost all other massive sulfide deposits. A chalcopyrite stockwork zone, forming a concordant "keel" stratigraphically below the massive sulfide, typically has whole-rock delta 18 O = 10 to 12 per mil. Beneath the stockwork keel, rhyolite delta 18 O values systematically increase stratigraphically downward for about 50 m. Below this level, all of the footwall rhyolites in the Kidd Creek area display exceptionally high delta 18 O values of 13 to 16 per mil. The groundmass quartz in the rhyolites ranges from 13.6 to 15.5 per mil, and crosscutting quartz veins associated with the chalcopyrite stockwork mineralization have delta 18 O = 11.9 per mil. By contrast, the postore basalts of the hanging wall are weakly altered, show only modest 18 O enrichments (delta 18 O = 8-10ppm), and are isotopically similar to the volcanic rocks in other parts of the Abitibi greenstone belt.Petrographically, all of the Kidd Creek footwall rhyolites are intensely altered (typically containing 76-90 wt % SiO 2 ), indicating that the high 18 O characteristics are not primary magmatic features but are the result of hydrothermal alteration. Several disequilibrium oxygen isotope relationships are present. For example, volcaniclastic rhyolites consist of extremely 18 O-rich, chlorite-rich, and sericite-rich clasts enclosed within a lower 18 O, sericitically altered rhyolite tuff matrix. This implies at least a two-stage hydrothermal history, involving either different temperatures of alteration or different fluid sources, or both. Using a temperature of 300 degrees to 350 degrees C, the main ore-forming fluid must have had delta 18 O = 6 to 9 per mil. Inasmuch as the associated felsic volcanic rocks were apparently erupted in a shallow subaqueous (locally subaerial?) environment, the most plausible origin of the ore fluid is evaporation and enrichment in 18 O of normal seawater in a closed basin at the western edge of the Abitibi belt. This would produce a strongly 18 O-shifted, high-salinity fluid that would have been capable of transporting large amounts of metals. The high 18 O signature of the Kidd Creek-Timmins volcanic rocks may constitute an exploration target that warrants further study and evaluation in other areas.