The Sijiaying banded iron formation (BIF)- type iron ore is the largest iron deposit in eastern Hebei, for which a great deal of chronological, petrological, and elemental geochemical work has been carried out, but no Fe isotope study has yet been done. Methods:This article proposes effective constraints on the ore- forming material sources and formation background of Sijiaying BIF through the combination of Fe isotopes, major elements, trace elements, and rare earth elements. At the same time, it supplements the zircon U- Pb age data of Sijiaying BIF. Results:Zircon U- Pb geochronology shows that the Sijiaying BIF was formed at 2537~2531 Ma. Geochemical data shows that Sijiaying BIF ore is mainly composed of TFe2O3 and SiO2, with lower Al2O3 and TiO2 contents and enriched in Fe heavy isotopes (δ56Fe=0.341‰~0.525‰); the rare earth element distribution pattern shows the characteristics of light rare earth depletion and heavy rare earth enrichment, with obvious positive anomalies of Eu, Y, La, and high Y/Ho ratio (Y/Ho=34.96~45.84).Conclusions:These characteristics indicate that Sijiaying BIF is a chemically sedimentary rock with minimal involvement of detrital materials. Rare earth elements are derived from a mixed solution of high- temperature hydrothermal and seawater, while iron is derived from marine hydrothermal fluids. The lack of true negative Ce anomalies and positive Fe isotope compositions in Sijiaying BIF indicates that it was formed in an hypoxic environment. By comprehensively comparing the Fe isotope characteristics of the Archean BIF in other regions of the world, this article believes that the oxygen content in the Earth's oceans gradually increased during the Neoarchean period. At this time, the oceans generally belonged to an anaerobic environment, but some areas had higher oxygen content.