Epizonal Ag-Pb-Zn vein mineralization at San Bartolome, in the eastern Andes of Ecuador, is one of a series of mineral occurrences which makes up the Canar-Azuay Ag belt.The mineral assemblage in the veins crystallized in five stages: comb quartz selvages on vein margins; coarse, crystalline aggregates of sphalerite associated with subsidiary amounts of freibergite (avg 23.9% Ag) and galena, with minor stannite; spectacular interpenetrating networks of bladed pyrrhotite with interstitial galena, freibergite, arsenopyrite, sphalerite, and minor stannite; coarse galena and Ag-bearing sulfosalts dominated by plumose masses of owyheeite (Pb 5 Ag 2 Sb 6 S 9 ), boulangerite, and jamesonite; and finally, a gangue assemblage of quartz, rhodochrosite, and dolomitic carbonates. Pyrrhotite is now almost completely replaced by hypogene marcasite and pyrite pseudomorphs. Sulfur isotope geothermometry indicates ore-forming temperatures of 219 degrees to 354 degrees C and delta 34 S CDT (Canyon Diablo troilite) values in the range -3 to +2 implies a magmatic origin for a dominantly reducing, sulfidic fluid. Relative delta 34 S values in the principal vein sulfides suggest that crystallization proceeded under conditions close to equilibrium. Gradual variation in delta 34 S values along individual bladed structures further suggests that steady crystal growth occurred within a system which was closed to sulfur replenishment. The delta 13 C and delta 18 O values in rhodochrosite are consistent with carbonate-depositing fluids also having a primarily magmatic source with a minor meteoric component. Microthermometry of fluid inclusions in quartz, carbonate, and sphalerite shows that three fluids have contributed to mineralization: a hypersaline fluid (30 wt % NaCl equiv), and an intermediate-salinity fluid (6-15 wt % NaCl equiv), both associated with inclusion homogenization temperatures (T h ) in the range of 200 degrees to 400 degrees C; and a low-salinity fluid (<6 wt % NaCl equiv) associated with lower T h in the range of 100 degrees to 300 degrees C. On the basis of these observations, it is most likely that the San Bartolome veins formed from magmatohydrothermal fluids possibly linked to the intrusion of an S-type pluton, close to the palaeocontinental margin. Mineralizing fluids invaded a dilatant fracture system as a series of discrete pulses, where they were subsequently diluted by the later introduction of cooler, meteoric ground waters.