Cation ordering and stoichiometry of the Upper Ediacaran Dengying dolomites, South China: an extensive diagenesis-controlled pattern

Dolomite cation ordering and stoichiometry are key proxies for formation conditions, yet high-resolution data from Precambrian dolomites remain scarce, hindering understanding of ancient dolomite formation and transformation. The Upper Ediacaran Dengying Formation in the Sichuan Basin, South China, provides an exceptional archive for addressing this gap due to its continuous deposition, significant thickness, and diverse lithologies. We analysed 442 dolomite samples from three stratigraphic sections (Hujiaba, Yangba, and Wuxi), representing microbial dolomite, sand-bearing dolomite, crystalline dolomite and silicified dolomite. Powder X-ray diffraction quantified cation ordering (S) and stoichiometry, revealing a ‘single peak with two shoulders’ pattern linked to microbial (S = 0.65), silicified (S = 0.75), and sand-bearing dolomite (S = 0.53). Stoichiometry ranged from 46 to 54% MgCO3, with lower values (43–45%) in sand-bearing dolomites. Strong correlations between cation ordering, oxygen isotopes, Sr concentrations, and Mn/Sr ratios indicate early to late diagenesis as a dominant control. Elevated ⁸⁷Sr/⁸⁶Sr values exceeding contemporaneous seawater and lighter oxygen isotopes suggest continental fluid involvement. These findings, the first high-resolution dataset on upper Ediacaran dolomites, highlight cation ordering as a more reliable proxy for microbial activity than stoichiometry and emphasize the critical role of diagenesis in shaping dolomite attributes over geological time.