In-situ iron isotope analysis of pyrite from coals: Insights into iron isotope fractionation from dissimilatory iron reduction (DIR) and redox reactions

In-situ δ56Fe and δ34S compositions of pyrite from Late Paleozoic coals of the Qinshui Basin, China, were investigated using LA-MC-ICP-MS. The results demonstrate substantial micrometer-scale heterogeneity in Fe and S isotopes, reflecting spatially variable geochemical controls. The subduction of the Inner Mongolia Ocean and the uplift of the northern North China continent led to the formation of the Yinshan Oldland and surrounding highlands, intensifying the weathering of pre-Late Carboniferous rocks and providing the primary sediment source for the Qinshui Basin. While Rayleigh distillation partially explains observed Fe isotopic trends, deviations from theoretical predictions suggest additional fractionation during multi-stage Fe–S cycling. Dissimilatory iron reduction (DIR) mediated Fe(III) reduction leads to Fe(II) enrichment with negative δ56Fe values, which subsequently reacts with H2S to form pyrite. In coal-rich settings, the extent of DIR is governed by organic matter availability and redox conditions. The organic matter in coal largely consists of structurally complex macromolecules such as humic substances and aromatic compounds, which are poorly bioavailable and restrict DIR. Moreover, sulfate reduction preferentially generates hydrogen sulfide (H2S), which rapidly reacts with Fe(II) to form FeS and ultimately pyrite, further suppressing DIR. The δ56Fe values reflect the repeated Fe cycling process under the fluctuation of redox states, driven by processes such as incomplete reduction of Fe(III), partial reoxidation, and late-stage release of heavy Fe2+ via processes such as anaerobic oxidation of methane coupled to Fe reduction (AOM–FeR). This isotopic signature mirrors the persistent “hidden” Fe cycle observed in modern iron-rich basins, highlighting the combined influence of microbial activity, and diagenetic reworking, and SMTZ (sulfate-methane transition zone) dynamics in shaping Fe isotopic signatures in organic-rich sediments.