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

摘要

Hydrothermal alteration is widespread in Miocene volcanic rocks in the Esme (Usak 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 Kisladag 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 Kisladag 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 33-170 degrees C and 123-132 degrees C, respectively, consistent with epithermal alteration processes and localised chemical weathering at low-temperature <= 63 degrees C. The negative delta S-34 and positive delta O-18 values for jarosite and gypsum suggest oxidation of pyrite in a supergene environment.