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[Display omitted] • The dominant Fe sources at Kukeate were derived from mixed seawater and low-T hydrothermal fluids (<200 °C). • There exists a dynamic redoxcline, which separated shallow oxygenated waters and deep, reducing, Fe2+-rich waters at Kukeate. • The abiotic oxidation of Fe2+ to Fe3+ via free O 2 could have occurred at Kukeate. • The Kukeate IF provides evidence for episodic anoxic ferruginous deeper water conditions during the early Silurian. Phanerozoic iron formations (IFs) provide a unique opportunity to understand both the contemporaneous chemical composition and redox state of seawater where they precipitated. However, the absence of detailed reports of Phanerozoic IFs inhibits a robust understanding of their depositional conditions and dominant Fe sources. Here, we describe a hematite- and siderite-rich IF from Kukeate hosted within lower Silurian submarine siliciclastic and carbonate sedimentary succession in NW China. The chemical composition of our analysed samples is dominated by high TFe 2 O 3 and variable SiO 2 contents, with low Al 2 O 3 , TiO 2 , Sc, Nb, Th, and Zr concentrations, indicating an authigenic origin. Distinct depletion in LREE relative to HREE, super-chondritic Y/Ho ratios, and slightly positive Gd anomalies, are diagnostic for derivation from surface seawater. The lack of strong positive Eu anomaly, along with coupled Fe and O isotopic characteristics for the hematite, point to a significant low-T hydrothermal input. Slightly negative to no Ce anomalies, weakly negative to mildly positive Eu anomalies, homogeneous δ56Fe values (near zero), and large positive δ13C V-PDB excursions, together suggest that there exists a dynamic redoxcline, which separated oxic surface water and ferruginous deeper water conditions, where dissolved Fe2+ in deep, reducing, Fe-rich hydrothermal fluids, was oxidized and precipitated as Fe(OH) 3 in shallow oxygenated seawater, leading to the formation of hematite. Accordingly, the occurrence of Kukeate IF suggests intermittent ferruginous deeper water conditions during the early Silurian. [ABSTRACT FROM AUTHOR]