Mississippi valley-type mineralization in the Silurian paleoaquifer, central appalachians

This study used lead and sulfur isotope tracers to determine the nature and geometry of MVT-related brine flow in the Silurian paleoaquifer of the central Appalachians. Although the stratigraphy and lithology of this sedimentary package is similar to that of the important Lower Cambrian MVT-hosting paleoaquifer of the southern Appalachians, it lacks important mineralization. Lead isotope compositions from MVT deposits in the Lockport Dolomite, in the upper part of the Silurian paleoaquifer, form a cluster at about 206Pb204Pb = 18.3, 207Pb204Pb = 15.6, and 208Pb204Pb = 38.2, which is similar in form but slightly different in composition from those seen in deposits in underlying clastic rocks. Sulfur isotope compositions for Lockport MVT sulfides range from −19 to −6‰, and have lower δ34S values than MVT sulfides hosted by underlying clastic rocks. These relations, in combination with NaClBr compositions of fluid inclusion leachates, suggest that MVT deposits in the lower part of the paleoaquifer formed when rising metal-bearing brines intersected reduced sulfur of probable thermogenic origin, whereas deposits in the upper part of the paleoaquifer (Lockport Dolomite) formed where metal-bearing brines intersected reduced sulfur of biogenic origin that was dissolved from sulfate evaporites in the overlying Salina Group and transported downward into the Lockport.