Neoarchean Banded Iron Formations in the North China Craton: Geology, Geochemistry, and Its Implications

Abstract

The distribution of Banded iron formations (BIFs) in the North China Craton (NCC) has some obvious characteristics. Large-scale BIFs and BIF-type iron deposits occur mainly in greenstone belts area such as Anshan-Benxi, eastern Hebei, Wutai, Guyang, western Shandong and Huoqiu; formation ages of BIFs in NCC show a wide range spanning from Mesoarchean to early Paleoproterozoic, with a peak in the late Neoarchean (2.6–2.5 Ga); BIFs can be divided into two types, Algoma-type and Superior Lake-type, most BIFs occurring in Neoarchean greenstone belts in NCC belong to the former, whereas only the Paleoproterozoic Yuanjiacun BIF in the Luliang area has typical characteristics similar to Superior-type BIFs. The Neoarchean BIFs are mainly distributed in paleo-intracratonic basins and/or their margins. The BIFs can be developed in either marine volcanic-sedimentary environment or submarine exhalation sedimentary environment, either shallow marine clastic sedimentary environment or marine clastic-carbonate transition environment. All the BIFs in NCC experienced relatively high-grade metamorphism and strong deformation, forming most sedimentary metamorphic iron deposits with magnetite as a major economic mineral. Large amounts of geochemistry results of the Neoarchean BIFs in NCC show that the BIFs are composed predominantly of SiO2 + Fe2O 3 T , and the contents of Al2O3, TiO2, K2O, Na2O, MnO, and P2O5 are very low, suggesting that BIFs belong to typical chemical sedimentary rocks. The Post-Archean Australian Shale-normalized REY patterns of the BIFs display an enrichment in heavy rare earth elements with positive La, Eu, and Y anomalies, indicating that the primary chemical precipitate is a result of solutions that represent mixtures of seawater and high-temperature hydrothermal fluids. The e Nd(2.55 Ga) values (3.0–4.5) in Anshan-Benxi BIFs are close to those of the coeval depleted mantle, implying that the iron was most likely derived from the hydrothermal leaching of oceanic mafic crusts; the e Nd(2.55 Ga) values (2.89 to −2.58) in the Wutai and E. Hebei BIFs suggest that the iron sources are mixing oceanic crusts and continental crusts. At the same time, the BIFs lack negative Ce anomalies with consistently positive δ56Fe values (0.12–1.87 ‰), which suggest that they formed in a low-oxygen or anoxia environment. A possible tectonic setting where the NCC BIFs and associated supra crustal rocks form is a back-arc basin or island arc related to a subducted ocean slab.