Volcanic rocks of the New Vauze-Norbec area, central Noranda volcanic complex, consist of andesites and rhyolites that have been chemically and isotopically altered by synvolcanic semiconformable alteration processes including spilitization, chloritization, silicification, and epidote-quartz alteration. Andesites have whole-rock delta 18 O values of 1.9 to 9.7 per mil; those with megascopic mineralogical evidence of hydrothermal alteration have ones of 5.1 to 9.7 per mil. Rhyolites also show a broad range in delta 18 O values: 4.4 to 13.7 per mil. All of the volcanic rocks are either enriched or depleted in 18 O with respect to fresh rhyolite (8ppm) or fresh andesite (6ppm). A marked upward stratigraphic increase in delta 18 O values and the strata-bound occurrence and regional zoning of alteration minerals provide evidence of district-scale hydrothermal alteration. These alteration patterns formed under the conditions of an increasing thermal gradient and a downward decrease in water/rock ratio controlled by the cooling of the Flavrian Lake synvolcanic intrusion.