Epigenetic Dolomitization and MVT Mineralization in Paleozoic Rocks of Eastern Wisconsin: Implications for Regional Fluid Flow
J. A. Luczaj
Department of Geosciences,Western Michigan University, Kalamazoo, MI
Paleozoic sedimentary rocks in eastern Wisconsin preserve a pervasive hydrothermal signature and contain abundant epigenetic dolomite and ubiquitous trace Mississippi Valley Type (MVT) mineralization. A similar sequence of mineralization is present throughout the region in rocks ranging in age from Early Ordovician through Late Devonian. Petrographic and geochemical evidence suggests a genetic link between the massive dolomite, MVT minerals, and K-silicate minerals in these rocks. A similar sequence of mineralization exists in Ordovician rocks of western lower Michigan, suggesting a related origin. Constraints were placed on the conditions of water-rock interaction using fluid-inclusion methods, cathodoluminescence and plane light petrography, isotopic analyses (C, O, S, and Pb), and organic maturity data. Homogenization temperatures of two-phase brine inclusions in dolomite, sphalerite, and quartz range between 65 and 120oC. Freezing data suggest a Na-Ca-Mg-Cl-H2O fluid with salinities between 13 and 28 weight percent, NaCl equivalent. Late calcite and barite precipitated below ~50oC. Existing dates on authigenic K-feldspar and illite indicate the timing of water-rock interaction to be 380 to 322 Ma. Stratigraphic reconstructions and Devonian vitrinite reflectance (Ro=0.5 to 0.62%) indicate low thermal maturity for these sediments and are consistent with short-term heating rather than long-term sustained burial. The data are best satisfied by a model in which fluids moved out of the Michigan basin into easternWisconsin during the Middle Paleozoic and were responsible for massive dolomitization, MVT mineralization, and K-silicate mineralization. Evidence supporting this hypothesis includes: 1) similar paragenesis in eastern Wisconsin and western lower Michigan, 2) regionally correlative CL zoning in dolomite, 3) a regional sulfide cement horizon in the St. Peter Formation, and 4) late migrated oil geochemically similar to Devonian Michigan basin oil. Mechanisms capable of forcing warm brines out of the Michigan basin are unclear, but may involve gas-displacement or long distance topography-driven fluid flow along regional aquifers.
AAPG Search and Discovery Article #90900©2001 AAPG Eastern Section Meeting, Kalamazoo, Michigan