The genetic relationship between upper Mississippi Valley District lead-zinc mineralization and minor base metal mineralization in Iowa, Wisconsin, and Illinois.
Minor base metal occurrences present in Paleozoic dolomites, and to a lesser extent, in slates and sandstones in a broad zone surrounding the main upper Mississippi Valley Zinc-Lead District in Iowa, Wisconsin, and Illinois are of interest because they may be remnants of fluid pathways of upper Mississippi Valley zinc-lead mineralization. Outlying base metal occurrences contain mainly pyrite, marcasite, sphalerite, galena, and calcite; they occur in a variety of forms including vertical veins, gash veins, disseminated breccias, vug linings, and paleokarst replacement bodies. Two paragenetic types of mineralization are evident: occurrences in which sulfides are generally early and calcite is late, and those in which carbonate (mainly calcite) is early and sulfides are generally late. Similarities exist among mineralogy, geologic setting, and some geochemical aspect of the main Upper Mississippi Valley District and outlying occurrences. However, paragenetic, fluid inclusion, and stable isotope data indicate major differences in the genetic history of the outlying occurrences. This report discusses measurements on fluid inclusion homogenization temperatures, salinities, and sulfur isotopes. The isotope studies support a connate seawater source for the mineralizing fluids from both areas. Sulfur, carbon, and oxygen isotope data suggest that some outlying mineralization was more strongly influenced by local diagenetic processes than by processes that formed the main upper Mississippi Valley deposits.