Geochemical Characteristics of silica fillings in Ediacaran cap dolostones of South China: A comparison of different environments

The Neoproterozoic cap dolostones that overlies the Snowball Earth successions host numerous enigmatic structures, whose formation mechanisms remain highly controversial. These structures are cemented by abundant authigenic minerals precipitated during early diagenesis, which preserve critical geochemical signature of post-depositional fluids. In this study, we investigate the geochemical characteristics of silica-fillings in these structures across a suite of sedimentary environments in the Neoproterozic, spanning shallow-water facies, deep-water slope facies and restricted basins. The results indicate that distinct geochemical variations are observed in these silica-fillings across the different depositional settings. The δ18O differences between the coexisting silica and dolomite (△18OQz-Dol) mostly range from –8.0 to –5.03 ‰, indicating that silica-fillings are not in oxygen isotope equilibrium with their host cap dolostones. It should be noted that, despite substantial variations in both δ18ODol and δ18OQz values across different depositional environments, the △18OQz-Dol values fall within a similar range. The mean δ18OQz values of silica-fillings from shallow-water facies are comparable to those from deep-water slope, but significantly higher than those from the restricted basin. Additionally, the δ30Si values of shallow-water facies are higher than those of both deep-water slope and the restricted basin. Rare earth element patterns also exhibit notable variability across the depositional environments. The geochemical signatures reveal that silica cements precipitated from fluids dominated by a mixture of seawater, hydrothermal fluid and glacial meltwater at temperatures ranging from 105 and 120°C. Based on the δ18OQz and δ30Si values of silica-fillings, we quantify the proportional contribution of three fluid endmember across the different depositional environments. Hydrothermal fluids exert a more significant influence in deep-water settings compared to the shallow-water facies and restricted basin. Notably, glacial meltwater contributes a substantial proportion of the fluid budge in restricted basin, possibly exceeding 80 %. Therefore, the distinct geochemical signatures of silica-filling can provide a new tool for paleoenvironmental reconstruction of Neoproterozoic cap dolostones.