- School of Earth Sciences and Engineering, Nanjing University, Nanjing, China (xcljun@nju.edu.cn)
Original phases of the Proterozoic Iron Formations (IFs) have been considered as essential archives of paleo-seawater geochemistry. It is widely acknowledged that hydrated ferric oxides/hydroxides were the primary precursor phase. However, the significance of high silica concentrations in Precambrian seawater has been receiving increasing attention for understanding the original iron mineralogy. Recent studies have provided compelling evidence that Fe(II)-silicates were the predominant precursor phases of IFs.
In this study, we identified the Fe-illite cortices within ooids from the Paleoproterozoic Chuanlinggou Iron Formation, located on the northern margin of the North China Craton. The Fe-illite, characterized by an Fe(II)/Fetotal ratio of approximately 20%, exhibits tangentially arranged crystals probably formed by wave action in reducing environments. There is considerable hematite within the Fe-illite cortices, which can be categorized into striped and granular types. Striped hematite is tangentially arranged alongside the Fe-illite, and its rare earth element (REE) patterns exhibit heavy rare earth element (HREE) enrichment, similar to Fe-oxide cortices. In contrast, Fe-illite associated with granular hematite exhibits light rare earth element (LREE) enrichments. We propose the Fe-illite cortices reflect the original Fe-smectite precipitation from paleo-seawater under alkaline conditions, and striped hematite represents the original mineral phase during the formation of the Fe-smectite cortices. Furthermore, the Fe-illite cortices exhibit orders-of-magnitude enrichment in biological elements compared to Fe-oxide cortices. It is noticeable that similar clay-hematite association has been reported in the Paleoproterozoic Yunmengshan oolitic ironstone from the southern margin of the North China Craton (Qiu et al., 2020). These findings indicate specific solution chemistry and potential biological influences at the craton margins during the Proterozoic era.
References
Qiu, Y., Zhao, T. and Li, Y. (2020) The Yunmengshan iron formation at the end of the Paleoproterozoic era. Applied Clay Science 199, 105888.
How to cite: Li, Y., Lu, X., Cui, X., Liu, H., Liu, J., and Cai, Y.: Clay-hematite Association in Late Paleoproterozoic Oolitic Ironstones of the North China Craton , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10015, https://doi.org/10.5194/egusphere-egu25-10015, 2025.
