Dolomitization and fluid evolution in the Middle Devonian Sulphur Point Formation, Rainbow South Field, Alberta: petrographic and geochemical evidence

Petrographic and geochemical studies of the Middle Devonian (Givetian) Sulp hur Point Formation in the vicinity of the Rainbow South Field, northwester n Alberta, reveal that dolomitization was a direct result of precipitation by chemically distinct fluids, and that recrystallization of these dolomite s significantly altered their original chemical signatures. Sulphur Point c arbonates were deposited in a restricted peritidal environment. Lithofacies include grainstones, sparsely fossiliferous packstones, mudstones, algal m udstones, and intraclast breccia mudstones. Multiple episodes of calcite ce mentation and dolomitization have affected these rocks to varying degrees. Five dolomite types were identified: 1) dolomicrite, 2) fine-crystalline ma trix dolomite, 3) medium-crystalline matrix dolomite, 4) saddle dolomite an d 5) fracture-lining dolomite. Dolomicrite (2-20 mu m) replaced both micrite and calcite cement in the mud -supported facies before early compaction. A trend toward more negative del ta(18)O values of -9.22 to -3.10 parts per thousand Vienna Pee Dee Belemnit e (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 suppor t this interpretation. Both fine- and medium-crystalline matrix dolomites (40-200 mu m) are usuall y 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 concen trations, and delta(18)O values of -12.20 to -8.34 parts per thousand VPDB. This evidence, in addition to the presence of high salinity fluid inclusio ns (similar to 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 sugges ts 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 ear ly Tertiary Laramide Orogeny. Strontium isotope modelling confirms that sad dle dolomite was precipitated from a two-component hydrothermal fluid incor porating a significant quantity of Middle Devonian brines and radiogenic ba sement fluids. Fracture-lining dolomite (0.2-1.0 mm) was the last dolomite phase to precip itate, and is intimately associated with blocky calcite, quartz, sulphide m ineralization and pyrobitumen. Isotopic and fluid inclusion evidence imply precipitation from slightly saline brines (similar to 8 wt% NaCl equivalent ) at elevated temperatures. Extremely low Fe and Mn concentrations, negativ e delta(13)C values (similar to -5 parts per thousand VPDB), and significan t volumes of H2S gas suggest that fracture-lining dolomite was precipitated from syn- to post-laramide fluids during thermochemical sulphate reduction .