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

Abstract

In this study we report on the source and evolution of fluids associated with secondary vein and replacement minerals in low-permeability carbonate units in the Michigan Basin. This petrogenetic information was collected using thermometric data from fluid inclusions combined with C-, O- and Sr-isotope data, and focuses on mm- or cm-wide vein and vug minerals from Ordovician limestones of the Trenton and Black River groups and Cambrian sandstones in SW Ontario, Canada.Primary fluid inclusions in dolomite represent fluid stage I and have the highest trapping temperatures (Ttrap) in the sedimentary succession, between 88 and 128 °C. Primary inclusions in calcite (stage II), and celestine and anhydrite (stage III), represent the final stages of secondary mineral formation and have Ttrap values of 54–78 °C. All three stages of vein minerals formed from brines with salinities of 31–37 wt% [CaCl2+NaCl]eq that were saturated in halite and methane gas at the time of mineral growth. Three subsequent stages of secondary fluid inclusions were observed in the samples (stages IV–VI), however, no secondary vein minerals formed during these stages and they are interpreted as re-mobilization and/or minor fluid ingress along grain boundaries and micro-fractures. Notably, stage IV secondary inclusions are gas-undersaturated with salinities of 32–34 wt% [CaCl2+NaCl]eq and minimum Ttrap values of 57–106 °C, and are interpreted to have formed during a Late Devonian–Mississippian regional heating event.Secondary minerals from the Cambrian units and the Shadow Lake Formation have δ13C values of −6.1 to −2.5‰ (VPDB), δ18O values of +14.6 to +24.2‰ (VSMOW) and 87Sr/86Sr of 0.70975–0.71043. Relative shifts of approximately +2 to +3‰ in δ13C, >+4‰ in δ18O and −0.002 in 87Sr/86Sr are observed upward across the boundary between the Cambrian–Shadow Lake units and the overlying Gull River Formation. Samples from the Gull River and Coboconk formations and the Trenton Group (Kirkfield, Sherman Fall and Cobourg formations) have δ13C values of −1.0 to +1.9‰, δ18O values of +18.9 to +28.1‰ and 87Sr/86Sr values of 0.70790–0.70990.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.