The effects of pervasive dolomitization on the geochemical records of Cambrian series-3 evaporative carbonates in Bachu Uplift, China

Abstract

The effects of pervasive dolomitization on geochemical records and the mechanism of formation of Cambrian series-3 evaporite-bearing dolomites in the Bachu Uplift are assessed from petrography, geochemistry, and fluid inclusion. Near-surface diagenetic dolomites (dolomicrite (DM), microbial (algal) dolomite (MD)) and shallow-deep diagenetic dolomites (fine- to medium-crystalline dolomite (FM) and medium- to coarse-crystalline dolomite (MC)) were identified. Secondary fracture and pore-filling cements (dolomite (CD), early-stage calcite (ESC), later-stage calcite (LSC), gypsum/anhydrite, and authigenic quartz were identified. Weak relationships exist between δ¹³C, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, and Mg/Ca versus Mn/Sr ratios, which are tools used to evaluate the impact of the diagenetic environment on C-O-Sr isotope composition of the carbonates. The replacement dolomites (MD, FM, MC) and DC with high Mg/Ca ratios (0.61–1.23) have low depleted δ¹⁸O, δ¹³C values, compared to the ESC and LSC with more depleted δ¹⁸O, δ¹³C values, indicating that the C-O-Sr isotope composition suffered diagenetic alteration by higher-temperature and equilibrium isotopic fractionation. The decreasing trend of halogens/(Ca + Mg) ratios in DM and MD compared to the increasing trend in these ratios in FM, MC, and CD confirms that dolomitization had substantial effects on halogens/(Ca + Mg) ratios of dolomites formed in sabkha and evaporative-lagoon environment due to the mixing of dolomitization fluids with halite derived from seawater and halogen-enriched saline fluids during fluid-rock interaction. The DM and MD mechanisms of formation were respectively seepage-reflux penecontemporaneous dolomitization and microbial dolomitization of the underlying carbonate sediments and organic substrates at near-surface diagenetic burial settings in ancient sabkha environment. The ⁸⁷Sr/⁸⁶Sr, δ¹⁸O, δ¹³C values, and δ¹⁸O VSMOW values revealed that the FM and MC were formed during shallow-deep burial dolomitization from radiogenic-strontium, and Mg-enhanced saline dolomitization fluids at relatively high-temperatures. The growth of dolomite cement in fractures was the result of higher-temperature ⁸⁷Sr/⁸⁶Sr, Mg-enhanced saline basinal fluids squeezed upward and laterally along fractures and other permeable horizons by geothermal squeegee-fluid flows dolomitization mechanism from deep to shallow burial settings.