NA-CL-BR SYSTEMATICS OF FLUID INCLUSIONS FROM MISSISSIPPI VALLEY-TYPEDEPOSITS, APPALACHIAN BASIN - CONSTRAINTS ON SOLUTE ORIGIN AND MIGRATION PATHS

This study evaluated Na-Cl-Br systematics of fluid inclusion-hosted br ines in Mississippi Valley-type (MVT) deposits from the Appalachian Ba sin. Unlike other geochemical tracers such as lead and strontium isoto pes which constrain metal sources, Na-Cl-Br systematics identify sourc es of brine salinity. Saline formation waters can vary systematically within and between basins with regard to their Na-Cl-Br compositions d epending on the importance of halite dissolution relative to retention of subaerially evaporated seawater for the halogen budget. Oil field brine compositions from the Illinois and Appalachian basins are quite distinct in their Na-Cl-Br systematics. Compositions of saline fluid i nclusions in MVT deposits generally are consistent with these regional differences. These results shed new light on the extent of regional f low systems and on the geochemical evolution of saline fluids responsi ble for mineralization. Nearly all fluid inclusions analyzed from the Appalachian MVT deposits have Na/Br and Cl/Br ratios less than modern seawater, consistent with ratios observed in marine brines involved in halite precipitation. The Na-Cl-Br systematics of the brines responsi ble for Appalachian MVT deposits may be inherited from original marine brines refluxed into the porous carbonate shelf sediments that host t hese deposits. The Cl/Br and Na/Br ratios of most fluid inclusion-host ed brines from Appalachian MVT sphalerites and fluorites fall into two compositional groups, one from the Lower Cambrian paleoaquifer and an other from the Lower Ordovician paleoaqaifer. Leachates from most MVT barite deposits form a third compositional group having lower Na/Br an d Cl/Br ratios than the other two. Appalachian MVT leachate compositio ns differ significantly from those in MVT deposits in the Cincinnati a rch-midcontinent region suggesting that these two MVT provinces formed from brines of different origin or flow path.