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Absolute age and fluid temperature determinations of secondary calcite at the Bruce nuclear site


Initial uranium-lead (U-Pb) isotopic analyses and fluid inclusion homogenisation temperature determinations were carried out on calcite veins and vugs collected at surface and from drill core at the Bruce nuclear site, including samples from the Devonian Lucas Formation and the Ordovician Trenton and Black River groups. The U-Pb isotopic analyses were carried out by LA-ICPMS and by ID-TIMS. The U-Pb data constrain the age of secondary calcite precipitation and provide information on the minimum timing of fracture formation. The framework for interpreting the fluid inclusion dataset is provided by petrographic anaylsis used to distinguish distinct paleofluid generations and mineral parageneses. The U-Pb and fluid inclusion results are complimentary and, in conjunction with the known burial history, provide constraints on the paleohydrogeological evolution of the Huron Domain. The Lucas Formation surface calcite veins are associated with numerous sub-vertical NNW-, NE- and SE-trending, and lesser sub-horizontal, fractures. The veins exhibit mm-scale widths and in some cases are open. Outcrop cross-cutting relationships suggest that all fractures (joints, veins, faults) were emplaced contemporaneously. Vein morphology is consistent with emplacement under high pore fluid pressure conditions. The Lucas Formation core samples are from an interval ∼ 34 m below ground surface that hosts numerous irregularly oriented, mm-scale, veins. Calcite-filled veins in the deeper Ordovician samples are subvertically-oriented, tight and generally thinner and much fewer in number than surface veins. LA-ICPMS analyses of the Lucas Formation samples defined two age clusters, one with a relatively small number of analyses at around 56 Ma and a broader peak around 100 Ma. Independent confirmation by ID-TIMS, with single-grain analyses at 51±2 Ma and over the range 85±2 Ma to 109±4 Ma, provide confidence in the results. LA-ICPMS analyses of the deeper Ordovician drill core samples suggest a much older age of 445±42 Ma for emplacement that approaches the depositional age of the host rock. Fluid inclusion analyses reveal four episodes of microfracturing and paleofluid infiltration. The first involved post-diagenetic infiltration of a 42-60 °C aqueous solution containing free methane gas and resulted in the precipitation of calcite into veins and vugs. In the second event a 60-89 °C gas-free aqueous solution infiltrated the veins and vugs. The third event involved infiltration of an aqueous solution at some temperature < 70 °C. The final event involved infiltration of a 50-60 °C mixture of light petroleum oil and free methane gas. All four paleofluids were emplaced under conditions of thermal equilibrium with the host rock.