Abstract: Detailed mineralogical and chemical studies of hydrothermal alteration of the Madenköy, Kutlular, Kotarak Dere, Tunca, Sırtköy and Sarısu volcanogenic sulphide deposits in the Eastern Black Sea ore province of Turkeywere carried out with the aims of obtaining a better understanding of the ore forming environments and establishingfavourable criteria for discovering economic ore bodies. X-ray diffraction and thin-section studies of the footwall andhanging-wall rocks indicate that the orebodies are enveloped by extensive and regular argillic zones which also includesome carbonate minerals such as calcite, dolomite, and siderite. Zeolite minerals were found in volcanics surroundingthe clay zones. However, significance of the Zeolitic zones still remains to be investigated. Continuation of the alteration zone in the hanging-wall rocks demonst-rate that the hydrothermal activity continued for a while after the formation of the orebodies. The most intensive and extensive alteration is, however, foundin the dacitic footwall rocks.The main similarity in all the mineralized areas studied is that the common form of alteration associated with oredeposition is sericitization (including the presence of illite) of the dacitic footwall rocks, which are also commonlyassociated with silicification.The alteration mineral paragenesis suggest that the oresolutions were acidic during the formation of the orebodiesand kaolinite and sericite zones, but became alkaline with declining temperatune towards the end of ore deposition.During this second phaseshemalitization and carbonitization processes took place in the host rocks.Semi-quantitative evaluation of the ``Clay (kaolinite + montmorillonite + illite + sericite)/Feldspar" ratio by aX-ray diffraction method is useful in detecting the jpresenee and intensity of hydrothermal activity related to sulphidemineralization. The Clay/Feldspar ratio as well as regular zonation properties of the alteration minerals can also beused as direction vectors during detailed exploration studies.Geochemical studies of the host rocks support the hydrothermal alteration characteristics determined by themineralogical technigues.