Mechanism of hydrothermal illite clay mineralization in the southern Ordos Basin, central China

• A genetic shift from sedimentary kaolinite to hydrothermal illite is revealed. • Potassium was introduced by external fluids driving the alteration, as evidenced by pristine K-feldspar and illite zoning. • Mineralization was structurally controlled by fluid flow along an unconformity, creating a distinct “high-low-high” illite vertical zoning. • A practical exploration model is established, highlighting unconformities and permeable layers as key targets for similar illite deposits. Significant illite clay deposits, which are of considerable economic interest and genetic importance, are found in the Chunhua area along the southern margin of the Ordos Basin. This study takes the Yanjiagou deposit as a representative case and integrates field geology, mineralogy, X-ray diffraction (XRD) , scanning electron microscopy, geochemistry, K-Ar isotopic dating, and illite crystallinity analysis to systematically investigate its geological characteristics, age of mineralization, and genesis. The results indicate that the mineralized clay layers are predominantly hosted in the lower segment of the Upper Carboniferous Taiyuan Formation. The protolith is identified as a kaolinite claystone that formed through chemical precipitation, which was subsequently altered to illite through hydrothermal processes. K-Ar dating of illite yields an age of 146.8 ± 2.5 Ma, and crystallinity-based geothermometry indicates a formation temperature of approximately 210 °C, constraining the mineralization to a medium–low temperature hydrothermal event during the Late Jurassic to Early Cretaceous. A genetic model of “hydrothermal diagenetic mineralization” is proposed, wherein potassium-rich fluids migrated along structural pathways and triggered selective illitization of precursor kaolinite, resulting in zoned illite horizons. These findings not only provide key insights into Late Mesozoic hydrothermal mineralization on the southwestern margin of the North China Craton, but also offer practical guidelines for the exploration of clay mineral deposits in similar geological settings.