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Abstract: Illitic clay deposits of Esteghlal Mine that represent the largest refractory clay deposit in Iran were subjected to petrographical, mineralogical and geochemical investigations to examine their origin. Geological investigation showed that the illitic clay deposits of Esteghlal Mine intercalated with limestone beds rich in marine conodonts range in age from early to middle Famennian indicating the deposition of these illite deposits in marine environment during the early to middle Famennian. The deposits are composed mainly of illite with traces of kaolinite. Non-clay minerals are mainly very fine quartz and altered feldspars phenocrysts disseminated the very fine illite matrix. Scanning electron microscope revealed the occurrence of illite as crumbled plates with undefined outlines of sizes around 2 μm. Kaolinite occurs as flakes of deformed morphology and unclear outlines or as curved and bent flakes. These morphologies along with the abundance of detrital quartz and altered feldspars and low crystallinity of illite suggest a detrital origin of these clays. Nb/Y–Zr/TiO2 discrimination diagram indicated that the studied illitic clay deposits were derived from trachy–andesite source. The relative enrichment of light REE over heavy REE with negative Eu anomalies supports this interpretation. Absence of smectite and illite/smectite mixed layers and detrital nature of illite and kaolinite suggest that illite was formed as a result of hydrothermal alteration of acidic to intermediate igneous rocks then transported to the depositional basin rather than its formation as a result of diagenesis of precursor clays. [Copyright &y& Elsevier]