The source and depositional environment of early Silurian iron formation in NW China: Constraints from element and isotope (Fe, C, O) geochemistry

Abstract

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 TFe2O3 and variable SiO2 contents, with low Al2O3, TiO2, 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 δ13CV-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.