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

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.