Original paper

Epithermal alteration of Miocene volcanic units in the Gedikler region and the area surrounding the Kışladağ gold deposit, Uşak (western Anatolia), Turkey

Erkoyun, Hülya; Kadir, Selahattin


Hydrothermal alteration is widespread in Miocene volcanic rocks in the Eşme (Uşak Province) region. The alteration developed in rhyolitic to andesitic tuffs, rhyolites, andesites and latite-andesites. Feldspar is argillized and sericitized, biotite and hornblende are opacitized, and the groundmass is argillized and Fe-oxide/hydroxidized. Kaolinite decreases, smectite ± illite, gypsum, barite, alunite, jarosite, pyrite, native gold, psilomelane, goethite and lepidocrocite associated with argillisation, sericitization and silicification increase from the Gedikler region towards the Kışladağ porphyry gold deposit. SEM images reveal that tubular halloysite, platy kaolinite, and flaky smectite have marginally replaced volcanogenic material. A depletion of Ca, Na, K and increase of Mg+Fe/Si+Al ratios from Gedikler to Kışladağ resulted in the development of kaolinite and smectite ± illite under acidic and basic conditions, respectively. Enrichment of LREE relative to HREE, depletion of Ba, Rb and Sr, and negative Eu, Nb and Pb anomalies are responses to the alteration of feldspar and hornblende within the volcanic units. The O and H isotopic composition and formation temperatures of kaolinite and illite, based on meteoric water (–4.5 ‰), are 3–170 °C and 123–132 °C, respectively, consistent with epithermal alteration processes and localised chemical weathering at low-temperature ≤ 63 °C. The negative δ 34 S and positive δ18 O values for jarosite and gypsum suggest oxidation of pyrite in a supergene environment.


mineralogystable-isotope geochemistrygeochemistrywestern anatoliahydrothermal alterationmiocene volcanic rocks