Strata-bound dolomite occurs in deep-seated Ordovician and Cambrian sediments within the Huron Domain of southern Ontario, Canada. Core samples were analyzed for petrographic, stable and Sr isotopic composition, and fluid inclusion microthermometry to characterize dolomitization and evaluate diagenetic fluid composition. The samples represented a range of host rocks from dolomitized limestones, dolostones, sandy dolostones and sandstones within Ordovician Black River Group and underlying Cambrian formation. The petrographic and geochemical attributes have provided a basis to gain insight on the source fluids that modified these rocks, as well as, the possible timing of formation. Evidence indicates that the formations were subjected to high temperatures (average 115 degrees C) that cannot be explained by burial history alone. This suggests the occurrence and migration of hydrothermal fluids within the low permeability dolomite horizons, possibly during Paleozoic orogenesis. Dolomite and calcite fracture infill isotopic and fluid inclusion data point to two possibly individual diagenetic fluid systems; i) an earlier Cambrian system that is characterized by a pronounced negative shifts in oxygen and carbon isotopic composition (delta O-18 average -9.0%o for dolomite and -13.9%o for calcite, respectively; and delta C-13 -3.06%o for dolomite and -4.82%o for calcite, respectively), a more radiogenic (Sr-87/Sr-86 ratios range from 0.70977 to 0.71100), warm (Th values range from 84 to 156(degrees)C for dolomite; average 113.6 C-degrees and 87-141 C-degrees for calcite; average 101.5 C-degrees) and saline signature (salinity range from 23.2 to 27.2 wt% NaCl eq for dolomite; average 24.3 for dolomite and 23.6 wt% NaCl eq for calcite); and ii) a later Ordovician system that is characterized by less negative shifts in both oxygen and carbon isotopes (delta O-18 average -8.7 %ofor dolomite and -7.5%o for calcite; and delta C-13 average + 0.37%o for dolomite and -0.36%o for calcite, respectively), hypersaline (salinity range from 22.4 to 30.1 wt% NaCl eq.; average 27.0 for dolomite and 27.5 to 29.7 for calcite; average 29.2), comparable homogenization temperature (Th ranges from 85 to 132 C-degrees for dolomite; average 109.6 degrees C and 66-153 degrees C for calcite; average 107.2 degrees C) and a less radiogenic (Sr-87/Sr-86 ratios range from 0.70818 to 0.71000) fluid system. The observation of highly discrete, strata-bound dolomites combined with only trace quantities of saddle dolomite and its associated geochemical signature suggest that diagenesis, as a result of hydrothermal fluids, was neither pervasive in volume or extent within the north western Huron Domain.
