SULFUR ISOTOPE AND FLUID INCLUSION CONSTRAINTS ON THE GENESIS OF MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE CENTRAL APPALACHIANS

Mississippi Valley-type mineralization from two major host formations in the central Appalachians was studied in order to gain a better unde rstanding of depositional conditions, sources of ore constituents, and the distribution of mineralizing fluids in the region. Sulfur isotope data obtained in this study and fi om the literature (Ault and Kulp, 1960; Howe, 1981; Crawford and Beales, 1983; LeHuray, 1984; Wilbur et al., 1990) show that mineralization in each formation acquired sulfur from sources of similar isotopic composition. Barite deposits in the B eekmantown Group have delta(34)S values ranging from 25 to 36 per mil; the values probably closely reflect the isotopic composition of die s ulfur-bearing fluid involved in the barite formation. Sulfide deposits in this formation have more variable compositions and more complex is otopic systematics. In both the Timberville district and Nittany Arch area, main-stage sulfide minerals have delta(34)S values ranging from about 15 to 30 per mil. Each district also has a smaller, isotopically lighter substage of sphalerite mineralization that has delta(34)S val ues between 1 and 19 per mil. Sulfate minerals in the Nittany Arch are a are isotopically heavier than the sulfides, with delta(34)S values b etween 30 and 40 per mil. The lightest sulfur in the central Appalachi an Mississippi Valley-type districts is found in the Friedensville dis trict, where delta(34)S values of sulfide minerals range between -9 an d +1 per mil. Sulfide minerals from the two Silurian Tuscarora-Shawang unk-hosted deposits have similar delta(34)S values of 21 to 35 per mil , Minor amounts of sulfate minerals in this formation are isotopically heavy with delta(34)S values between 34 and 43 per mil. New fluid inc lusion data were also obtained from the Timberville district and revea l the presence of at least two distinct fluids involved in mineralizat ion-a more saline or calcium-rich fluid with final melting temperature s between -42 degrees and -29 degrees C, and a less saline or more cal cium-poor fluid with final melting temperatures of -24 degrees to -12 degrees C. Homogenization temperatures of both inclusion types ranged for the most part between 100 degrees and 170 degrees C. Geologic evid ence suggests that the Beekmantown Group and Tuscarora-Shawangunk Form ations were part of distinct paleoaquifers, and together with the sulf ur isotope data, also suggests that the source of sulfur for Mississip pi Valley-type deposits in each paleoaquifer was connate (formational) fluids derived from seawater, or alternatively in the case of the Bee kmantown Group, intraformational evaporites. Furthermore, evidence of mixing and a temporal increase in the oxidation state of mineralizing conditions in at least two of the districts suggests that much of the Mississippi Valley-type mineralization in the central Appalachian regi on may be the result of mixing of oxidizing tectonically driven Zn-Pb or Ba-rich fluids with sulfur-rich fluids of varying oxidation state p resent in the Beekmantown Group and Tuscarora-Shawangunk Formations.