The Findlay Arch district is a cluster of noneconomic, carbonate-hosted mineral occurrences centered in northwestern Ohio. The principal host rocks are vuggy dolostones that range in age from Middle Silurian to Middle Devonian. Reef complexes, beds of nodular gypsum, collapse breccias, and fracture zones provided important stratigraphic and structural ore controls. The main-stage phases are brown, yellow, and purple fluorite, sphalerite, and galena. The sphalerite/galena ratios are 100 or higher. Early dolomite, pyrite-marcasite, and celestite I are premain-stage phases, whereas saddle dolomite, vug-filling calcite, celestite II, barite, and strontianite are postmain-stage phases.T h and T m measurements of fluid inclusions in fluorite and sphalerite indicate that these minerals were precipitated from a warm Na-Ca-Cl brine, likely derived from the northern Appalachian basin. The flow path is inferred from westward-diminishing salinity values that are documented in successive generations of inclusions. The dominant thermal trend was aregional temperature plateau that peaked at about 110 degrees C. Thermal and salinity data imply that the flow system developed during the Alleghenian orogeny when the burial depth of host rocks was at a maximum and when the brine was diluted isothermally as it migrated westward across the arch. Coexisting inclusions of oil and brine indicate that these fluids moved through the district together.Similar sulfur isotope compositions (-0.9 to +4.8 and -3.4 to +0.6ppm) in sphalerite and galena show that these sulfides are genetically related. These data generally agree with sulfur compositions of late Paleozoic seawater sulfate, which is consistent with an Alleghenian age for the Findlay arch sulfur source. Lead isotope compositions of galena (Haden, 1977) fall in the field of the Appalachian ores and diagenetic galena, indicating further that nonradiogenic Paleozoic sedimentary rocks to the east were the likely source of lead.Minerals in vugs lack a radiogenic component of strontium. Nevertheless, a basinal brine is required for the fluorite-sphalerite mineralization, because a rock capable of providing the salinity of the fluid inclusions was absent in the district. The strontium isotope composition of this brine apparently was reset, acquiring the signature of Salina gypsum of Upper Silurian age. A local source of strontium and sulfur is indicated for celestite I, because its S and Sr isotope compositions are nearly identical to those of the Salina anhydrite.Brown calcite in veins associated with the Bowling Green fault zone contains strontium that is slightly radiogenic relative to vug-filling calcite. This, along with the prehydrocarbon age of the brown calcite, indicates that these early veins and the fiuorite-sphalerite mineralization are not coeval.