At the Tynagh mine, County Galway, four major stages of mineralization can be recognized that span the diagenetic and postlithification history of the host Waulsortian limestones: (1) growth of colloform and granular pyrite clots during the early diagenesis of the micritic host; (2) rapid geopetal precipitation of microcrystalline sphalerite, galena, barite, and dolomite within a dilatant fracture system that developed as a consequence of the forceful injection of metalliferous fluids into the coherent Waulsortian carbonate mudbanks adjacent to the Tynagh fault; (3) veining and replacement of the host and previously precipitated phases by tennantite, chalcopyrite, bornite, arsenopyrite, medium to coarsely crystalline galena, and coarse sparry barite; and (4) finally, precipitation of calcite within veinlets and cavities and dolomitization of large bodies of previously unmineralized Waulsortian limestones.Sulfur isotope ratios indicate that contemporaneous lower Carboniferous seawater acted as the source of sulfate for precipitation of barite (delta 34 S: 21.1 to 17.4ppm). Stage 1 (delta 34 S: -3.1 to -15.5ppm) and stage 2 (delta 34 S: -4.1 to -26.0ppm) sulfides were precipitated by reduced sulfur generated during open-system bacterial reduction of this sea-water sulfate source. Isotopically heavier (delta 34 S > or = 0ppm) reduced sulfur of deep-seated origin was introduced from the fault zone during stage 3. This mixed with bacteriogenic sulfur migrating into the Waulsortian bank limestones to precipitate stage 3 sulfides and sulfosalts (delta 34 S: +11.1 to -23.0ppm).Mineralized limestones (delta 13 C: 2.1 to 4.0ppm; delta 18 O: 17.6 to 27.1ppm), ore-stage carbonates (delta 13 C: -2.8 to +3.4ppm; delta 18 O: 17.1 to 26.9ppm), and postore carbonates (delta 13 C: -4.0 to +2.6ppm; delta 18 O: 15.9 to 25.8ppm) are depleted in the heavier isotopes of carbon and oxygen with respect to unmineralized limestones (delta 13 C: 3.1 to 4.1ppm; delta 18 O: 22.2 to 28.1ppm). The depletion in 13 C is attributed to the mixing of host-derived carbon with bacterially oxidized carbon and, during stage 4, deep-seated carbon. The 18 O depletion of the mineralized host is a consequence of isotopic exchange with the ore fluids. An oxygen isotope fractionation temperature of 200 degrees C and a delta 18 O (SMOW) value of 10.5 per mil for the coexisting water is obtained from a single stage 3 quartz-albite pair collected from the fault zone. If these values are accepted, the delta 18 O values of stage 3 carbonates can only be interpreted in terms of mixing of this isotopically heavy water introduced from the fault with isotopically lighter water already present or migrating into the bank limestones.