| This study details a previously unknown hydrothermal system that operated in the eastern Wisconsin region during Late Devonian to Early Mississippian time. Several constraints have been placed on the system's temperature, fluid composition, timing, burial depth, fluid sources, duration of heating, and regional extent. Petrographic and geochemical constraints suggest that a single water-rock interaction process was responsible for replacement and precipitation of dolomite, precipitation of authigenic K-feldspar, and MVT mineralization. This interaction proceeded at substantially higher temperatures than were expected for the region, based upon its known stratigraphic history. Fluid-inclusion data suggest that much of the dolomitization, silicate mineralization, and MVT mineralization occurred at temperatures ranging from 60°C to 115°C. Early mineralization, including dolomitization and quartz precipitation, likely occurred between ∼60°C and ∼85°C. Later dolomitization and most of the MVT sulfide mineralization occurred at temperatures between about ∼80°C and 110°C. Some of the late-stage barite and calcite contain all-liquid fluid inclusions, suggesting that cooling of the rocks to temperatures below about 50°C took place during the final stages of mineralization. Fluid-inclusion freezing data from sphalerite, dolomite, and quartz suggest that mineralization took place in the presence of a Na-Ca-Mg-Cl brine, which ranged in salinity between about 14 to 28 weight %, NaCl equivalent. Stratigraphic and organic maturity data suggest that the rocks in the study area were not affected by long term burial beneath several kilometers of sediments. In the absence of deep burial, regional advection of warm brines out of an adjacent sedimentary basin is called upon to explain the observations. Time-temperature estimates for organic maturity suggest that the rocks were only exposed to temperatures near 100°C for less than 10 million years. Authigenic K-silicate mineralization present throughout the Wisconsin arch - Michigan basin region and has been dated as Late Devonian to Early Mississippian in age. Field and laboratory data support the idea that warm brines flowed out of the Michigan basin into eastern Wisconsin during the Middle Paleozoic. These data include the regional extent of dolomitization, a peculiar sulfide cement horizon on the western side of the basin, correlation of cathodoluminescent growth bands in dolomites from the study area, and petroleum in the study area that is geochemically similar to Devonian oil from the Michigan basin. |
