Fluid Compartmentalization and Dolomitization in the Cambrian and Ordovician Successions of the Huron Domain, Michigan Basin

A preliminary study has been completed to examine the paleogenesis of strata-bound dolomite found occurring in deep-seated Ordovician and Cambrian sediments within the Huron Domain of southern Ontario. As part of this study, seven samples were analyzed for petrographic, stable and Sr isotopic composition, and fluid inclusion microthermometry to characterise dolomitization. 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 subject to higher temperatures than can 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. Calcite fracture infill isotopic and fluid inclusion data point to two possibly isolated diagenetic fluid systems; i) an earlier Cambrian system that is characterized by a more radiogenic, cooler and saline signature; and ii) a later Ordovician system that is characterized by hypersaline, more hydrothermal and a less radiogenic 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.