This study evaluates examples of hydrothermal dolomitization in the Middle Cambrian Cathedral Formation of the Western Canadian Sedimentary Basin. Kilometerscale dolomite bodies within the Cathedral Formation carbonate platform are composed of replacement dolomite (RD), with saddle dolomite-cemented (SDC) breccias occurring along faults. These are overlain by the Stephen Formation (Burgess Shale equivalent) shale. RD is crosscut by low-amplitude stylolites cemented by SDC, indicating that dolomitization occurred at very shallow depths (<1 km) during the Middle Cambrian. Clumped isotope data from RD and SDC indicate that dolomitizing fluid temperatures were >230 °C, which demonstrates that dolomitization occurred from hydrothermal fluids. Assuming a geothermal gradient of 40 °C/ km, due to rift-related basin extension, fluids likely convected along faults that extended to ~6 km depth. The negative cerium anomalies of RD indicate that seawater was involved in the earliest phases of replacement dolomitization. 84Kr/36Ar and 132Xe/36Ar data are consistent with serpentinite-derived fluids, which became more dominant during later phases of replacement dolomitization/SDC precipitation. The elevated 87Sr/86Sr of dolomite phases, and its co-occurrence with authigenic quartz and albite, likely reflects fluid interaction with K-feldspar in the underlying Gog Group before ascending faults to regionally dolomitize the Cathedral Formation. In summary, these results demonstrate the important role of a basal clastic aquifer in regionalscale fluid circulation during hydrothermal dolomitization. Furthermore, the presence of the Stephen Formation shale above the platform facilitated the build-up of fluid pressure during the final phase of dolomitization, leading to the formation of saddle dolomitecemented breccias at much shallower depths than previously realized.
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