Abstract

Minor base metal occurrences are 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. These occurrences 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 occurrences in which carbonate (either calcite or dolomite but predominantly calcite) is early and sulfides are generally late. Similarities exist among mineralogy, geologic setting, and some geochemical aspects 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.Fluid inclusion homogenization temperatures for outlying sphalerite and calcite range from 37.8 degrees to 115.8 degrees C. These temperatures are generally lower than those reported previously for these minerals from the main Upper Mississippi Valley deposits (46 degrees -220 degrees C). Although measured salinities for inclusions in sphalerite in outlying occurrences (15.6-23.8 equiv wt % NaCl) are similar to those for sphalerite from the main Upper Mississippi Valley district (19.6->23 equiv wt % NaCl), salinities for inclusions in calcite in outlying occurrences (5.1-18.1 equiv wt % NaCl) are considerably higher than those determined for calcite (4.8-5.1 equiv wt %) from the main Upper Mississippi Valley district. The delta 34 S values for outlying sulfates range from 21 to 31 per mil (CDT) and are similar to those of the main Upper Mississippi Valley district (22-36ppm). Overall values of delta 34 S for sulfides from outlying occurrences range from -22 to +36 per mil and contrast to that obtained from "pitch and flat" deposits (6-30ppm) and gash veins (approximately 1ppm) from the main Upper Mississippi Valley district. Although sulfides from this district formed from reduced connate water sulfate, the wide range in sulfur isotope data was caused by the ore-forming fluid reacting, to varying degrees, with isotopically light diagenetic pyrite. Values of delta 13 C and delta 18 O for outlying calcites (-9 to +5ppm (PDB), and 21-29ppm (SMOW), respectively) tend to be heavier than calcites from pitch and flat deposits in the main Upper Mississippi Valley district (delta 13 O = -13 to -2ppm; delta 18 O = 17-23ppm). The delta 18 O values for fluid inclusions in outlying mineralization (-8 to +6ppm) are similar to those obtained from the main Upper Mississippi Valley district mineralization (-5 to 6ppm); however, delta D values (-95 to -20ppm) are lighter than values (-47 to +2ppm) for the main district. These isotopic values 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.

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