The source and evolution of paleofluids responsible for secondary minerals in low-permeability Ordovician limestones of the Michigan Basin

The combined microthermometric and isotopic data for secondary minerals in the Cambrian-Shadow Lake units suggest they formed from hydrothermal brine with a geochemical signature obtained by interaction with the underlying Precambrian shield, or shield-derived minerals in the Cambrian sandstones. Previous U-Pb dating of vein calcite and Rb-Sr dating of secondary K-feldspar from the region suggests that brine ingress occurred during the Late Ordovician-Silurian. The O- and Sr-isotope variability in vein samples from the Gull River and Coboconk formations is interpreted as localized mixing of 18O-enriched, connate fluids with hydrothermal brine. In comparison, isotopic data for the Trenton Group indicate secondary mineral formation from connate fluids, sourced from 18O-enriched, evolved seawater and/or modified hydrothermal brine that experienced fluid-rock interaction during transit through the underlying stratigraphy.