Abstract Middle Ordovician Trenton Group carbonates are fractured and extensively dolomitized along the axis of the Algonquin Arch in southwestern Ontario. Hydrocarbon reservoirs formed where these dolomitized fracture zones penetrate otherwise impermeable host limestones. Three different types of dolomite (D1, D2 and D3) are distinguished. Petrographic characteristics and δ18O values indicate that D1 formed during early diagenesis from Middle Ordovician seawater and recrystallized during progressive burial, whereas fracture-related, replacive matrix dolomite (D2) formed by hydrothermal fluids (68–99 °C). Late-stage saddle dolomite (D3) and calcite (C3) cements occlude fractures. Based on petrographic, fluid inclusion, and stable isotope data, D3 dolomite and C3 calcite formed from warm (68–144 °C), saline (22–24 wt. % NaCl + CaCl2) hydrothermal fluids. The least radiogenic 87Sr/86Sr values (0.70830–0.70842) of D2 are consistent with estimated values of Devonian and Silurian seawater, whereas the slightly enriched 87Sr/86Sr ratios of D2 and D3 (0.70902–0.70918) suggest their precipitation from fluids similar in composition to oil field brines. Rare earth element (REE) results of D2, D3, and C3 indicate enrichment in REEs content of these mineral phases relative to undolomitized host rock. The similarity in the average REEs pattern of D2, D3, and C3 and the overlying Blue Mountain shale and basement rocks suggest progressive water/rock interaction. Magnesium required for dolomite precipitation was supplied by Mg-rich seawater-derived (Silurian and/or Devonian) saline waters from dissolution of Silurian evaporites which descended along faults and fractures, to reservoir depths at the center of the basin while being heated. Hot basinal brines migrated laterally through basal sandstones and ascended into the network of faults and fractures and precipitated fracture-related dolomite. The abundance of fracture-related dolomite in the periphery of Michigan Basin in southwestern Ontario suggests that dolomitizing fluids originated from the Michigan Basin rather than Appalachian Basin.