The Duvertepe district located on the western end of the Simav Graben is the largest known fossil hydrothermal kaolin deposit in Turkey. Active hot springs occur to the north of Simav city (Eynal, Citgol, Nap) and geothermal fields occur to the west of the graben (Hisaralan). Kaolin in the Duvertepe district formed at the expense of Miocene rhyolites-rhyodacites and tuffs emplaced in a tectonically active system undergoing N-S extension. Upward fan-shaped deposits of silicification and kaolin are found along fault zones and define areas of enhanced kaolinization. Silica sinters located above the kaolin zones suggest that hydrothermal fluids followed the fracture systems and mark outflow of geothermal waters. Fractured volcanic rocks rich in glass shards bear sufficient porosity and permeability for hydrothermal circulation. Two mineral facies, "kaolinite and alunite" dominate the deposits, which respectively include: (1) kaolinite-alunite-quartz and (2) alunite-opal-CT-quartz-halloysite. Kaolinite crystals are well-formed hexagonal vermiform habits and alunite crystals are idiomorphic rhombohedral forms. Needle- and tubular-shaped halloysites are common in the alunite fades. The delta S-34 values for alunite range from -1.55 to +6.18 parts per thousand. Kaolinites have delta D ranging from -49.0 to -943 parts per thousand and the delta O-18 values range from +5.8 to +14.8 parts per thousand. Calculated formation temperatures of kaolin mineralization (using delta O-18 values) suggest that hydrothermal alteration occurred in the range of 38 degrees to 129 degrees C. Independent K/Ar and Ar-40/Ar-39 ages of alunite are concordant and indicate that alteration occurred from 20.1 to 20.6 Ma in the southern part of the graben and 17.3 to 19.2 Ma in the northern part. These results suggest that metasomatism by steam-heated hydrothermal fluids sourced by rhyolitic magma is responsible for the Duvertepe kaolin and alunite deposit origins. (C) 2013 Elsevier B.V. All rights reserved.