The Hallaçlar kaolinite deposit of the Uşak Province (western Turkey) is hosted by dacite, andesite and siliceous materials of the Miocene Karaboldere volcanites. Mineralogical zonation, such as the prevalence of kaolinite at the centre of the deposit, coexisting with silica polymorphs such as quartz and cristobalite along with feldspar and hornblende, and a relative increase of smectite, illite, chlorite, and Fe-oxide/-hydroxide phases outward and upward, demonstrate that hydrothermal-alteration processes resulted in kaolinization. Micromorphologically, kaolinite occurs as pseudohexagonal blocky kaolinite with coarse-grained quartz in microfractures, and the presence of relatively fine-grained vermiform kaolinite that edges volcanic materials indicates that kaolinite developed in several phases coexisting with goethite, lepidocrocite, hematite, pyrite, jarosite, alunite, and gypsum/anhydrite with smectite in fractures. The highly crystallized Hallaçlar kaolinite is identified by: (1) sharp reflections at 7.23 and 3.57 Å, with triplet and doublet non-basal reflections; (2) sharp infrared spectral bands at 3687, 3651 and 3620 cm−1; (3) well defined, pseudohexagonal and hexagonal kaolinite crystal growth; (4) a chemical index of weathering of 98.6–100; and (5) SiO2/Al2O3 ratios between 1.04 and 1.45. Enrichment of Sr and depletion of Rb+Ba, Ti, and HREE relative to LREE, with a distinct negative Eu anomaly, are responses to the fractionation of feldspar and hornblende by the hydrothermal fluid; these results are also supported by the O- and H-isotopic character and formation temperatures of the Hallaçlar kaolinite and smectite; namely, 134.1–183.4°C and 65.6°C, respectively. The negative δ34S (−20.7‰) value of gypsum/anhydrite reflects its formation from geothermal-water-derived sulphur. Under the influence of a tectonically controlled hydrothermal process, feldspar, hornblende and volcanic glass were altered, resulting in the conservation of Al and depletion of Si, Na, Ca, K, Mg and Fe in an open hydrological system such that formation of kaolinite in the central part of the deposit under acidic conditions and development of smectite and illite upward and outward under basic conditions were favoured.