The origin and evolution of hydrothermal reservoirs implicated in polymetallic massive sulfide deposits are evaluated in terms of the variability in delta 18 O and delta D and the temperature of their ore-forming systems.The Iberian pyrite belt is characterized by numerous extremely large volcanogenic massive sulfide deposits cumulatively in excess of 10 9 metric tons. Throughout the pyrite belt volcanic rocks are characterized by spilitic mineral assemblages and are typically 18 O enriched. At Aljustrel specifically, the regional quartz eye keratophyres are enriched in 18 O up to 18.1 per mil, with quartz eye cores (11.2-13.5ppm) to whole-rock fractionation values of -3 to -5 per mil. The stockwork and its immediate periphery have a uniform whole-rock delta 18 O value of 12.2 + or - 0.6 (1Sigma ) per mil, or 5 per mil less than that of the mean regional felsic pyroclastics (delta 18 O = 16.7 + or - 0.7 (1Sigma )ppm). In the stockwork zone of the Feitais-Estacao orebody, quartz (delta 18 O = 13.3-13.9ppm) and chlorite (delta 18 O = 3.2-7.3ppm) have fractionations corresponding to calculated temperatures of 200 degrees to 250 degrees C with a delta 18 O (sub H 2 O) = 0 to 6.1 per mil. delta D values of chlorites from both stockwork zones are uniform at -32 to -40 per mil, implying delta D (sub H 2 O) = 0 to + 15. Temperatures diminish from 200 degrees to 250 degrees C in the stockwork, to 175 degrees to 160 degrees C in massive ore (delta O quartz = 17.1-18.3ppm, delta O chlorite = 6.8-7.3ppm), and finally, to 120 degrees C in the overlying cherts where delta O quartz = 18.7 to 20.1 per mil. This cooling trend is accompanied by progressive fractionation of Fe from Mn in chlorites and Ba enrichment, collectively due to entrainment of marine bottom water into the hydrothermal discharge.At Rio Tinto, quartz and chlorite in stockwork ore of the Cerro Colorado deposit are isotopically uniform (delta 18 O quartz = 11.1-11.6ppm, delta 18 O chlorite = 2.0-2.9ppm, delta D chlorite = -40 - -45ppm). Hydrothermal fluids streaming through this vent domain had ambient temperatures of 210 degrees to 230 degrees C and delta 18 O = 0 to + 1.3 per mil and delta D = -5 to +8 per mil. Isotopic ratios of stockwork ore at Chanca are isotopically similar to those of Cerro Colorado. For the Salgadihno deposit, where base metal mineralization is present in sericitic felsic tuffs, quartz (delta 18 O = 13.8ppm) and muscovite (delta 18 O = 9.5ppm, delta D = -30ppm) define temperatures of 230 degrees C, with fluids where delta 18 O = 4.0 per mil and delta D = -10 to 0. Manganiferous and Ba-rich cherts overlying stockwork ore at Lagoas de Paco, Casto Verde, and Mertola are characterized by delta 18 O quartz = 20.6 to 22.3 per mil, delta 18 O barite = 12.8 to 13.8 per mil, and delta 18 O rhodonite = 14.8 to 16.1 per mil. Quartz-barite fractionations correspond to temperatures of 130 degrees + or - 20 degrees C, close to those estimated for Aljustrel cherts, corroborating the idea that these units are hydrothermal sediments. At Crandon, chloritic andesitic flows and tuffs of the footwall volcanic section 3 km from the deposit possess whole-rock delta 18 O values of 5.6 to 7.9 per mil. Elevated delta 18 O quartz values (7.4-9.5ppm) along with delta 18 O (sub quartz-feldspar) values of 2.7 to 3.1 per mil indicate that these rocks have undergone pervasive low-temperature hydrothermal alteration. Footwall volcanics have erratic whole-rock delta 18 O values of 5.2 to 7.3 per mil, reflecting the variable proportions of vein quartz to chlorite. Quartz occupies a restricted range of 8.1 to 9.9 per mil, with delta 18 O (sub quartz-chlorite) corresponding to temperatures of 210 degrees to 240 degrees C and delta 18 O (sub H 2 O) = -2.1 to -1.3 per mil. Fluid inclusion filling temperatures in stringer quartz range from 220 degrees to 310 degrees C. Quartz and chlorite isolated from massive ore, lateral tuffaceous cherts, and hanging-wall volcanics are isotopically comparable to separates from the footwall. The total range of hydrothermal temperatures is 210 degrees to 290 degrees C, with fluid isotopic ratios given by delta 18 O = -2.1 to +0.1 per mil and delta D = -2 to -12 per mil.In the stockwork of the Blue Hill mine, quartz (delta 18 O = 13.1-13.7ppm), muscovite (delta 18 O = 9.3-10.0ppm), and chlorite (delta 18 O = 5.9-6.7ppm) locally preserve triple concordancy, corresponding to temperatures of 250 degrees to 310 degrees C for fluids streaming through the footwall. The delta 18 O tourmaline = 11.1 to 11.7 per mil. The delta D values of muscovite and chlorite are uniform at -54 and -60 to -68, respectively. Ore-forming fluids were characterized by delta 18 O = 5 to 6.6 per mil and delta D = -20 to -35 per mil, plausibly representing a composite of approximately 60 percent marine water evolved to delta 18 O = 5.5 per mil, with 40 percent magmatic water. The contiguous Ellsworth volcaniclastic rocks are 18 O enriched at 10 + or - 0.9 per mil. For all of these deposits the ore-forming fluids are regarded as dominated by marine water that had undergone variable positive 18 O shifts up to 6.1 per mil, according to the extent of isotopic exchange with volcanic rocks under high temperature and low water/rock conditions. At Blue Hill a significant proportion of magmatic fluid is present. No covariance is observed between the size of deposits and the fluid delta 18 O values.