Deposition of a newly identified Mesoproterozoic iron formation from the Dabie orogen: Influenced by high-T hydrothermal fluid and redox stratification

Abstract

The mid-Proterozoic Eon (1.8–0.8 Ga; so-called “boring billion”) is a key interval in Earth’s history. Iron formations (IFs) have been used extensively as proxies for understanding the chemistry of the ancient seawater. However, there is limited information on IFs in this time window. Therefore, the dominant Fe source and depositional environment of IFs during this period remains enigmatic. Here, we report the first case of Mesoproterozoic IF, from Jiamiao in the Dabie orogen. Zircon U-Pb dating of two metavolcanics samples constrain the depositional age of the Jiamiao IF to be ca. 1359 Ma. The chemical composition of the IF samples, especially the strong positive Eu anomaly (Eu/Eu*PAAS = 3.94–6.96), suggest the role of high-T hydrothermal fluid in the IF deposition. The REY (rare earth element and yttrium) patterns and Ceanom (Ce anomaly) data in the studied IF and associated carbonates suggest a redoxcline separated the oxic surface ocean (at least > 1 % of the present atmospheric level) from ferruginous deeper water. In such a water column, the transport and dissolution of Mn-Fe (hydr)oxide particles across the redoxcline from oxidized surface water into deeper anoxic water, would lower the dissolved Y/Ho ratios, raise the LREE/HREE ratios, and increase the concentration of Ce relative to La and Pr. Integration of geochemical and isotopic data (C and O isotopes of marine origin) demonstrates that the composition of the Jiamiao IF and associated carbonates experienced mixing between seawater and a hydrothermal solution and Mn-Fe (hydr)oxide shuttling across a redoxcline.