The Bruce volcanogenic massive sulfide deposit is a conformable lens which lies at an andesite-rhyolite contact within the 1.7- to 1.0-b.y.-old Yavapai Series exposed south of Bagdad, Arizona. The upper part of a funnel-shaped alteration pipe replaces andesite within the stratigraphic footwall of the deposit. This pipe can be divided into three zones based on characteristic mineralogy: a chlorite zone consisting of chlorite (40-90 wt %) and quartz completely replaces the andesite in the central part of the pipe. A sericite zone containing subequal amounts of chlorite, sericite, and quartz is peripheral to the chlorite zone in the upper part of the pipe. The modal abundances of alteration minerals decrease outward through a gradational zone to unaltered andesite. Amphibole and biotite occur as metamorphic minerals; they are most abundant in the gradational zone. Some biotite is texturally and chemically related to the chlorite and sericite and is also probably hydrothermal in origin. All of the rocks within the pipe have been depleted in 18 O with whole-rock delta 18 O values ranging from 2.4 to 7.6 per mil. A linear relationship exists between the oxygen isotope composition and the quantity of hydrothermal chlorite in the alteration pipe. Both the amount of chlorite and the 18 O depletion are functions of the extent to which the rock has reacted with the hydrothermal fluid. The bulk chemistry of the altered rocks, when measured as a function of increasing alteration progress (decreasing delta 18 O), shows strong enrichments in MgO and FeO and depletions in SiO 2 , K 2 O, NA 2 O, and CaO. The most altered and most 18 O-depleted samples are nearly completely altered to chlorite; their bulk chemistry approaches that of the chlorite in the sample. All of the ferromagnesian minerals become more Mg-rich toward the center of the pipe. The Fe/(Fe + Mg) ratio of chlorite ranges from 0.8 in the least altered sample to 0.3 in the center of the chlorite zone. Extrapolating the linear relationship between the amount of chlorite in a sample and its delta 18 O value to 100 percent chlorite gives a value of 2.1 for the pure chlorite. At 250 degrees and 300 degrees C water in equilibrium with this chlorite would be 1.1 and 2.05 per mil, respectively. These data are comparable to delta 18 O values of water issuing from hot springs on the East Pacific Rise and are consistent with a slightly 18 O-shifted seawater as a source for the ore-forming solutions. Prior to entering the alteration pipe, the seawater reacted with volcanic rocks at depth, lowering the pH of the fluid such that chlorite was stable when the fluid entered the pipe. Subsequent formation of chlorite continued to lower the pH and decreased the activity of Mg in the solution, such that the fluid moved into the sericite stability field.