MISSISSIPPI VALLEY-TYPE MINERALIZATION IN THE SILURIAN PALEOAQUIFER, CENTRAL APPALACHIANS

This study used lead and sulfur isotope tracers to determine the natur e and geometry of MVT-related brine flow in the Silurian paleoaquifer of the central Appalachians. Although the stratigraphy and lithology o f this sedimentary package is similar to that of the important Lower C ambrian MVT-hosting paleoaquifer of the southern Appalachians, it lack s important mineralization. Lead isotope compositions from MVT deposit s in the Lockport Dolomite, in the upper part of the Silurian paleoaqu ifer, form a cluster at about (206)pb/Pb-204 = 18.3, Pb-207/Pb-204 = 1 5.6, and Pb-208/Pb-204 = 38.2, which is similar in form but slightly d ifferent in composition from those seen in deposits in underlying elas tic rocks, Sulfur isotope compositions for Lockport MVT sulfides range from -19 to - 6 parts per thousand, and have lower delta(34)S values than MVT sulfides hosted by underlying elastic rocks. These relations, in combination with Na-Cl-Br compositions of fluid inclusion leachate s, suggest that MVT deposits in the lower part of the paleoaquifer for med when rising metal-bearing brines intersected reduced sulfur of pro bable thermogenic origin, whereas deposits in the upper part of the pa leoaquifer (Lockport Dolomite) formed where metal-bearing brines inter sected reduced sulfur of biogenic origin that was dissolved from sulfa te evaporites in the overlying Salina Group and transported downward i nto the Lockport.