Low K rhyolites of the Archean Blake River Group that host the Horne volcanogenic massive sulfide deposit are altered to quartz-sericite-chlorite-albite (quartz-sericite zone) and chlorite + or - quartz (chlorite zone) assemblages. The quartz-sericite zone accounts for approximately 90 percent of the 5 km 3 of the altered hanging-wall and footwall rocks. Chlorite zone alteration is limited to portions of the footwall and flanks of the Cu-rich H orebodies. Clusters of gold-bearing chlorite-quartz veinlets crosscut large volumes of the quartz-sericite zone rocks.Alteration has added an average of 13 wt percent SiO 2 , 2.3 wt percent K 2 O, 430 ppm Ba, and 46 ppm Rb to the system and leached 3.6 wt percent Na 2 O, 2.2 wt percent CaO, and 41 ppm Sr. Middle and light REE were increasingly mobile from Yb to La; they were leached from the chlorite zone rocks and added to quartz-sericite zone rocks. Chlorite was more Fe rich (Fe/Fe + Mg = 0.65-0.85) in the chlorite zone rocks than in other altered lithologies (Fe/Fe + Mg = 0.2-0.5).Sulfur isotope values for massive ores are in the range delta 34 S = -2.1 to 1.9 per mil. Mineral separates from altered rocks yielded delta 18 O values of 10.2 to 12.1 per mil for quartz, 7.2 to 9.5 per mil for calcite, 2.2 to 3.0 per mil for chlorite, and 0.1 to 1.0 per mil for magnetite. Oxygen and sulfur isotope fractionation between coexisting minerals suggests formation from fluids having delta 18 O = 3 to 4 per mil at temperatures between 275 degrees to 380 degrees C. Oxygen isotope compositions of host rocks are delta 18 O = 8 + or - 1 per mil for freshest rhyolite, delta 18 O = 6.6 to 11.6 per mil for quartz-sericite zone rocks, and delta 18 O = 3.9 to 4.4 per mil for chlorite zone rocks. These measured values compare closely to estimated whole-rock delta 18 O values based on mass balance considerations using normative mineralogies.Alteration of wall rocks encompassing the massive sulfide orebodies is characterized by intense sericitization, a high degree of silicification, and a marked positive delta 18 O anomaly. Calculations based on SiO 2 solubilities and probable K contents of the hydrothermal fluids suggest a water-rock mass ratio of 50 to 300 during the mineralization event.