Fracture mineralization and fluid flow evolution: an example from Ordovician-Devonian carbonates, southwestern Ontario, Canada

Petrography, geochemistry (stable and radiogenic isotopes), and fluid inclusion microthermometry of matrix dolomite, fracture-filling calcite, and saddle dolomite in Ordovician to Devonian carbonates from southwestern Ontario, Canada, provide useful insights into fluid flow evolution during diagenesis. The calculated dOfluid, RREE, and REESN patterns of matrix and saddle dolomite suggest diverse fluids were involved in dolomitization and ⁄or recrystallization of dolomite. The Sr ⁄ Sr ratios of dolomite of each succession vary from values in the range of coeval seawater to values more radiogenic than corresponding seawater, which indicate diagenetic fluids were influenced by significant water ⁄ rock interaction. High salinities (22.4–26.3 wt. % NaCl + CaCl2) of Silurian and Ordovician dolomite–hosted fluid inclusions indicate involvement of saline waters from dissolution of Silurian evaporites. High fluid inclusion homogenization temperatures (>100 C) in all samples from Devonian to Ordovician show temperatures higher than maximum burial (60–90 C) of their host strata and suggest involvement of hydrothermal fluids in precipitation and ⁄or recrystallization of dolomite. A thermal anomaly over the mid-continent rift during Devonian to Mississippian time likely was the source of excess heat in the basin. Thermal buoyancy resulting from this anomaly was the driving force for migration of hydrothermal fluids through regional aquifers from the center of the Michigan Basin toward its margin. The decreasing trend of homogenization temperatures from the basin center toward its margin further supports the interpreted migration of hydrothermal fluids from the basin center toward its margin. Hydrocarbon-bearing fluid inclusions in late-stage Devonian to Ordovician calcite cements with high homogenization temperatures (>80 C) and their C-depleted values (approaching )32& PDB) indicate the close relationship between hydrothermal fluids and hydrocarbon migration.