The mineralogy, texture and composition of rocks associated with the kaolin deposits in the Ahırözü-Hamidiye-Üçbaşlı area, SE of Mihalıççık, Eskişehir, Turkey, were investigated. In the study area, Triassic, blue-green schists and serpentinized ultramafic rocks are exposed. Kaolinization occurs at the contact between a Triassic granitic intrusion and metamorphic rocks. Textural and mineralogical characteristics were identified by X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX) analyses. The effects of hydrothermal alteration on the whole-rock chemical composition were determined by inductively coupled plasma mass spectrometry (ICP-MS) analysis. Homogenization temperature (Th, °C), salinity, (wt.% NaCl) and melting temperature (Tm, °C) were detected by fluid-inclusion analyses.

The argillic alteration zone (Zone A) includes kaolinite, smectite, natroalunite and accessory pyrite, hematite and goethite. This paragenesis corresponds to a low–intermediate argillic alteration caused by fluids having neutral to acidic pH, at a temperature of <200°C. The propylitic alteration zone (Zone B) is characterized by epidote, chlorite, illite and pyrite and was caused by fluids with a neutral pH and a temperature of >250°C. The rocks in Zone C are mainly silicified and represent sinter formation in the study area. The rocks in the kaolin deposit are depleted in Rb, Pb and Y and enriched in Cs, U and Sr, and represent the products of epithermal hydrothermal alteration of granitic intrusions emplaced in a volcanic arc.

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