Evidences of hydrothermal fluids recorded in microfacies of the Ediacaran cap dolostone: Geochemical implications in South China

Abstract

The Ediacaran cap dolostones overlying the Marinoan Nantuo diamictite of the Doushantuo Formation in South China contain abundant enigmatic structures. Two generations of carbonate cement (named C1 and C2, respectively) can be observed in these structures at Wuhe of the Guizhou Province. C1 is characterized by dolomite cements that line the wall of the structures, and C2 by equant calcite spars that distribute in the center. The REE + Y patterns and the range of δ13C and δ18O values of C1 are similar to those of the micritic matrix, indicating that C1 precipitated from the same fluid as the matrix. The REE + Y patterns of C1 are characterized by minor MREE-enrichment, positive Ce and Eu anomalies, large variations in Y/Ho ratios. These patterns reflect normal marine origin with the release of Fe-Mn oxides in pore water during early diagenesis. Positive Ce anomalies and Th/U ratios of matrix suggest the anoxic depositional conditions. C2 is characterized by MREE-enrichment, positive Eu anomalies, high Y/Ho ratios and unobvious Ce anomalies, which is different from C1 and matrix. The Eu anomalies indicate that the precipitation fluids of C1 and C2 may have involved in different contribution of high-temperature hydrothermal fluids. The precipitating temperatures of C2 would be from 105 °C to 120 °C based on the δ18O variations of C1 and C2. The positive correlation between δ18O and δ13C values of matrix, C1 and C2 may attribute to the alteration by hydrothermal fluids. However, the negative correlation between δ13C and δ18O values of C2 may result from the oxidation of organic matter and methane seeps. The geochemical features of different microfacies of the cap carbonates reflect enigmatic fluid chemistry after the global Marinoan glaciation.