Petrologic and geochemical evolution of carbonates of South Bruce region, Southwest Ontario: Dolomite petrogenesis and fluid flow evolution

Abstract

Cored samples from two deep boreholes in the South Bruce Region, Ontario, Canada were analyzed to assess the dolomitization process and other diagenetic features of rocks spanning from the Ordovician to Devonian periods. These samples, including dolomitized limestone, dolostones, and evaporites, were compared with previous results from the Bruce Nuclear site and adjacent areas. The analyses focused on mineralogy, stable isotope composition, geochemistry, fluid inclusions microthermometry, and Sr-isotopic ratios, offering insights into source fluids and the timing of dolomitization. Dolomitization in the region is characterized by two main generations: non-stoichiometric replacive dolomite and saddle dolomite cement. The replacive dolomite includes microcrystalline matrix dolomite (D1) that replaces wackestone, packstone, and occasionally grainstone facies. A later formed medium to coarse crystalline dolomite (D2) replaces grainstone facies and is associated with dissolution seams, stylolites, and recrystallized dolomite. Both dolomites formed from warm, saline basinal fluids. Saddle dolomite (SD) is found in minor vugs and fractures in Ordovician and Silurian strata, postdating D1 and D2. Isotopic and microthermometric data indicate multiple diagenetic fluid events with the formation of D1 at shallow burial depth followed by the formation of D2 and SD at an intermediate burial depth likely occurred during Taconian and Acadian orogenies. The negative δ¹⁸O values, enriched ⁸⁷Sr/⁸⁶Sr ratios, and high homogenization temperatures for D2, and SD dolomites suggest formation from connate fluids influenced by basinal brines and hydrothermal fluids. In contrast, Devonian dolomite and blocky calcite exhibit lower salinity and higher homogenization temperatures, reflecting fluid evolution linked to the Alleghenian Orogeny. Geochemical similarities between the Bruce Nuclear site and South Bruce region highlight their geographic proximity, with variations due to different depositional settings, diagenetic fluid compositions and the presence of hydrothermal fluids.

The integration of the petrographic, isotopic and geochemical data in this study reveals the connection between fluid flux history and diagenetic processes, highlighting their role in the regional tectonic evolution of the Michigan Basin.