Abstract

Petrographic and geochemical studies of the Middle Devonian (Givetian) Sulphur Point Formation in the vicinity of the Rainbow South Field, northwestern Alberta, reveal that dolomitization was a direct result of precipitation by chemically distinct fluids, and that recrystallization of these dolomites significantly altered their original chemical signatures. Sulphur Point carbonates were deposited in a restricted peritidal environment. Lithofacies include grainstones, sparsely fossiliferous packstones, mudstones, algal mudstones, and intraclast breccia mudstones. Multiple episodes of calcite cementation and dolomitization have affected these rocks to varying degrees. Five dolomite types were identified: 1) dolomicrite, 2) fine-crystalline matrix dolomite, 3) medium-crystalline matrix dolomite, 4) saddle dolomite and 5) fracture-lining dolomite.

Dolomicrite (2–20 μm) replaced both micrite and calcite cement in the mud-supported facies before early compaction. A trend toward more negative δ18O values of −9.22 to −3.10‰ Vienna Pee Dee Belemnite (VPDB) with respect to postulated Middle Devonian marine carbonate values suggests that dolomicrite was recrystallized by later fluids. Geochemical modelling of the isotope and trace element trends in the dolomicrite support this interpretation.

Both fine- and medium-crystalline matrix dolomites (40–200 μm) are usually fabric destructive. However, some intervals have retained lamination and algal structures. Matrix dolomite was formed during intermediate burial, as suggested by its association with dissolution seams, high Fe and Mn concentrations, and δ18O values of −12.20 to −8.34‰ VPDB. This evidence, in addition to the presence of high salinity fluid inclusions (~ 18 wt% NaCl equivalent), indicates that matrix dolomite was precipitated by basinal fluids between the Mississippian and Late Jurassic.

The precipitation of saddle dolomite (0.5−2.0 mm) is genetically related to fractures and breccia zones where it partially to completely occludes the fractures, breccias and vugs that were developed through the dissolution of the earlier matrix dolomites. Geochemical and petrographic evidence suggests that saddle dolomite was precipitated from a hot, slightly saline (10.5 to 13.3 wt% NaCl equivalent), calcium-rich fluid that was funnelled upward along faults and fractures that developed during the Late Cretaceous to early Tertiary Laramide Orogeny. Strontium isotope modelling confirms that saddle dolomite was precipitated from a two-component hydrothermal fluid incorporating a significant quantity of Middle Devonian brines and radiogenic basement fluids.

Fracture-lining dolomite (0.2−1.0 mm) was the last dolomite phase to precipitate, and is intimately associated with blocky calcite, quartz, sulphide mineralization and pyrobitumen. Isotopic and fluid inclusion evidence imply precipitation from slightly saline brines (~ 8 wt% NaCl equivalent) at elevated temperatures. Extremely low Fe and Mn concentrations, negative δ13C values (~ −5‰ VPDB), and significant volumes of H2S gas suggest that fracture-lining dolomite was precipitated from syn- to post-Laramide fluids during thermochemical sulphate reduction.

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