Origin of dolomite of the lower Cambrian Longwangmiao Formation, eastern Sichuan Basin, China

Combined elemental and isotopic geochemistry of dolomites in the lower Cambrian Longwangmiao Formation, eastern Sichuan Basin with petrographic observations has been investigated to determine the origin of these dolomites. Our data show that these dolomites are mostly located in the middle and upper parts of the Longwangmiao Formation, and most of them commonly occur as crystalline dolostones, whereas they less commonly occur as a dolograinstone. The dolomite contents of the dolostones range from 64 to 97%. The Mg/Ca ratios of these dolomites vary from 0.6 to 1.2, with an average of 0.8. These criteria divide the Longwangmiao dolomites into two types, according to either a positive correlation (type-A) or negative correlation (type-B), which exist between the MgO and CaO contents. Type-A dolomite consists of micritic-to-fine crystals (< 50 μm) of dull-to-no cathodoluminescence. It occurs in association with evaporites and relicts, and has no direct or indirect evidence of formation from carbonate precursors. These features suggest that the type-A dolomite has a parasyngenetic, evaporitic, and near-surface origin. Type-B dolomite consists mainly of medium-to-coarse (50–250 μm) crystals of zoned to uniform, red, or brown luminescence associated with relict textures. High Na concentrations, low Sr concentrations, and Sr/Ca ratios lower than those of type-A seem to indicate that the salinity of the dolomitizing fluid is similar to that of hypersaline seawater. The strongly positive correlation between δ13C and δ18O values, with the occurrence of karst caves and vugs, shows that the Type-B dolomite has been modified by meteoric water. Estimates from δ18O values suggest precipitation at depths of approximately 308 m. These proxies demonstrate that type-B dolomite formed during burial from seepage–reflux of hypersaline seawater, with a minor influence of meteoric water. Type-A dolomite is characterized by micropores and has a low permeability ranging from 0.00195 to 45.00262 × 10−3 μm2, with an average of 0.03730 × 10−3 μm2. It is volumetrically abundant throughout the region, and thus, it represents a potential gas reservoir. High-quality hydrocarbon reservoirs in the region are closely related to type-B dolomite, which is likely to occur in banks at the margins between open and restricted platforms.