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Geochemistry and Sm─Nd─Fe─Si isotope compositions as insights into the deposition of the late Neoarchean Qidashan banded iron formation, North China Craton

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Abstract

Banded iron formations (BIFs) are chemical sediments that reflect the composition of the seawater from which they were deposited. Therefore, they provide a key part of the evidence for the modern scientific understanding of paleoenvironmental conditions in Archean and Paleoproterozoic times. Although BIFs have been extensively studied, many aspects (e.g., specific mechanisms controlling iron (Fe) and silicon (Si) precipitations) of their origin still remain enigmatic because of the lack of modern analogues. In China, abundant BIFs occur throughout within the late Neoarchean volcanic and sedimentary succession and therefore are the principal source of Fe for the Chinese steel industry. Here, we examine the ~ 2.53 Ga Qidashan BIF, one of the most extensive BIFs in China, by conducting a detailed petrographic and multi-proxy investigation to well constrain its formation mechanism. The BIF consists mainly of magnetite and quartz with lesser amounts of calcite and various types of silicate minerals, of which the content of Al-rich minerals (i.e., chlorite) is rare, coupled with a low abundance of detrital geochemical indicators (e.g., Al and Ti), suggesting that the BIF is relatively pure with insignificant terrigenous contamination. A wide range of Nd isotope compositions and shale-normalized patterns and specific anomalies of rare earth elements, especially highly positive Eu anomalies, indicate that the BIF precipitated from seawater imprinted by high-temperature hydrothermal fluids. Furthermore, there is a significantly negative correlation between Nd isotope values and total Fe contents of the BIF. This suggests that such enhanced hydrothermal activity provided vast volumes of dissolved Fe(II) necessary for the formation of the BIF via alteration of ancient continental crust. In addition, the Qidashan BIF was deposited under pervasively anoxic conditions, as revealed by the absence of shale-normalized Ce anomalies and the presence of consistently positive Fe isotope values. Hence, anoxygenic photosynthesis is the most plausible mechanism responsible for Fe(II) oxidation. Given that Fe─Si bonding has a strong impact on Si isotope fractionation, the formation of primary Fe(III) oxyhydroxides should have exerted a first-order control on the negative Si isotope signatures observed in the studied BIF samples. It is also noted that the BIF possesses a variation of negative Si isotope values, further implying that diagenetic dissolution and reprecipitation of silica took place after primary Si precipitation associated with Fe.

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Fig. 1

Age data sources: a - Wang and Zhang (1995); b - Li et al. (2016); c - Wang et al. (2017a) (the youngest group of detrital grains of zircon and the oldest group of metamorphic zircon, 2530─2513 Ma)

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Acknowledgements

We thank editor Bernd Lehmann, Alexandre Cabral, and two anonymous reviewers for their constructive comments.

Funding

We acknowledge the financial support provided by the National Natural Science Foundation of China (grant 42150104 and 42302073), Key Research Program of Frontier Sciences, Chinese Academy of Sciences (grant ZDBS-LY-DQC037), the Key Research Program of the Institute of Geology and Geophysics, Chinese Academy of Sciences (grant IGGCAS-201905), and Youth Innovation Promotion Association, Chinese Academy of Sciences.

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C.L.W. conceived the study. C.L.W. collected samples for this study. C.L.W. processed the samples. C.L.W., Z.D.P., X.X.T., and L.G. conducted geochemical analyses. All authors contributed to writing and editing the manuscript.

Corresponding author

Correspondence to Changle Wang.

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All authors certify that they have no affiliations with or involvement in any organization or entity with any financial interest or non-financial interest in the subject matter or materials discussed in this manuscript.

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Editorial handling: A. R. Cabral.

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126_2024_1245_MOESM1_ESM.xlsx

Supplementary Material 1: Table S1. Electron proble microanalyses (wt %) of hematite, magnetite, amphibole, carboante minerals, and calcite within the Qidashan BIF samples

126_2024_1245_MOESM2_ESM.xlsx

Supplementary Material 2: Table S2. Major (wt %) and trace element (ppm) compositions of the Qidashan BIF samples and relevant standards

126_2024_1245_MOESM3_ESM.xlsx

Supplementary Material 3: Table S3. Sm-Nd, Fe, and Si isotopic data of the Qidashan and Dataigou BIF samples along with geostandard samples

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Wang, C., Peng, Z., Tong, X. et al. Geochemistry and Sm─Nd─Fe─Si isotope compositions as insights into the deposition of the late Neoarchean Qidashan banded iron formation, North China Craton. Miner Deposita (2024). https://doi.org/10.1007/s00126-024-01245-8

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