LEAD-ISOTOPE GEOCHEMISTRY OF MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE CENTRAL APPALACHIANS

Lead isotope compositions of sulfide minerals from Mississippi Valley- type deposits in Cambrian, Ordovician and Silurian rocks in the centra l Appalachians plot in distinct groups, one for each major district. D ata for deposits in the Nittany Arch and Shawangunk districts form cir cular clusters, whereas data for the Timberville and Friedensville dis tricts form more elongated fields. Comparison of these data to isotopi c compositions of possible source rocks suggests that lead in the Nitt any Arch, Shawangunk, and Timberville clusters came largely from Paleo zoic igneous rocks or sediment derived from these rocks, whereas lead in the Friedensville deposits came from Grenville-age basement rocks. Additional lead from the Timberville district appears to have come fro m Triassic diabase, suggesting that as least some mineralization there is unusually young. These results contrast strongly with those for th e southern Appalachians, where different Mississippi Valley-type distr icts have similar isotopic compositions over large areas. Only lead fr om the Ordovician-hosted Timberville deposits in the central Appalachi ans has an isotopic composition similar to that of any of the southern Appalachian deposits, and its composition is similar to that of depos its in the Lower Cambrian paleoaquifer of the southern Appalachians, r ather than the Ordovician paleoaquifer. These relations indicate that although Mississippi Valley-type brines in the central Appalachians pa rticipated in significant cross-formational flow, they were not as reg ionally extensive as those in the southern Appalachians, a factor that might account for the relative scarcity of large Mississippi Valley-t ype deposits in the central Appalachians area.