Constraints of C-O-Sr isotope and elemental geochemistry on the origin of dolomite of the deeply buried Ediacaran sedimentary succession, central Sichuan Basin (SW China)

It is significant and challenging to study the formation mechanism of multistage dolomite and the characterization of various fluid alteration in multi-tectonic cycles, ancient, and deeply-ultra-deeply buried carbonate rocks. The Sichuan Basin is a superimposed multicycle basin with abundant oil and gas resources and complex geological evolution, in which there are multiple dolomite textures in the Ediacaran sedimentary succession (similar to 542 Ma) and the burial depth is more than 7000 m in geological history. Therefore, this study combines petrology, C-O-Sr isotope, ordering degree, fluid inclusions, and elemental geochemistry methods to systematically restore the mineral stages and identify the fluid information to analyze the origin of multistage dolomite. The results show that the host rock -> dolomite-1 -> bitumen-1 -> dolomite-2 -> bitumen-2 -> quartz, accompanied by some metal minerals dominate the Ediacaran Dengying Formation. After normalization by seawater value, the light rare earth elements (REE) were enriched while the heavy REE were depleted in all dolomites. The results also show that the joint analysis of isotopes and elements is reliable under deep burial conditions, whereas the ordering degree needs to be treated with caution as the potential irregularity increases with burial depth or evolution to a certain extent. Dolomite-2 has more negative C-O isotope and higher Sr isotope than Precambrian seawater and Eu anomalies, indicating the origin of formation water and deep thermal fluids during its burial dolomitization. The formation of the host rock is the joint result of microbial action, evaporation, and backflow infiltration under seawater conditions. It is dominated by mud crystals, retaining some primary structures. Dolomite-1 was formed between the host rock and dolomite-2, which is the product of shallow burial and the fluid is a mixture of seawater and basinal brine.