The Ecocene volcanic rocks of the Hasandag (Firesun area in the eastern Pontides (NE Turkey) have undergone several stages of hydrothermal alteration. These rocks are andesites, andesitic tuffs, brecciated andesites, andesitic dykes and cross-cutting granitic instrusions. Development of breccias was related to penetration of silica-rich hydrothermal fluids along fractures. Silicification and argillisation are widespread as the main types of hydrothermal alteration. The operation of the hydrothermal processes is also evidenced by mineralogical and geochemical characteristics. Alteration of plagioclase and Fe oxidation in amphibole are widespread. Development of silicification, advanced argillisation (alunite +/- kaolinite), and argillic (smectite +/- illite) and propylitic zones (chlorite +/- sericite +/- calcite) - associated with decreasing alunite +/- kaolinite and some increase in chlorite +/- sericite +/- calcite from the central to the outer parts - indicate acidic hydrothermal activity. In the Hasandag area, pyrite, visible gold and other native elements were all determined via ore microscopy. Hypogene minerals include goethite and lepidocrocite. Based on whole-rock chemical analyses, the N-type MORB-normalised trace-element and chondrite-normalised rare-earth-element (REE) patterns of the intrusive and volcanic rocks (except the breccia zone) are similar to each other, and all resemble subduction-related, collisional rocks. Field, mineralogical, whole-rock petrochemical and clay-fraction stable isotope data indicate a high-sulphidation type, gold-bearing epithermal system.