Abstract: The Late Cretaceous volcanic rocks around Zigana Mountain (Gümüşhane) in the eastern Black Seametallogenic province in NE Turkey show intensive hydrothermal alteration but less weatheringalteration. The basement of the study area is formed by Late Cretaceous basalt, andesite and theirpyroclastics. These rocks are overlain by dacitic rocks of the same age, namely Dacite-I and Dacite-II.These volcanic rocks are bimodal in character and have developed in a volcanic arc environment.The volcanic rocks in the study area have been altered to the sericite/illite–chlorite facieses,and contain sericite/illite, chlorite, quartz, carbonate minerals (ankerite and calcite), iron-oxide, andrare kaolinite, smectite and epidote as the products of alteration. Sericitization/illitization is themost common type of hydrothermal alteration associated with these volcanics, and chloritization isthe most common alteration type after illitization; pyritisation is seen in all volcanics, and is the mostcommon in dacites. In some fields limonitisation is occasionally present. Epidotization is rare, andespecially seen in basalt and andesite. Isocon analysis was undertaken to estimate the mass gains andlosses of the Zigana Volcanics as a result of hydrothermal alteration. In general terms, the resultsshows that, basalt and andesite have 2-61% mass gain, Dacite-I 71% mass gain and 42 % mass loss,and Dacite-II 44% mass gain and 32% mass loss. Namely, both mass gain and mass loss occurred inthe volcanics during the hydrothermal alteration of the parent materials. From less altered rock tohighly altered rock there was an increase in illite-chlorite-kaolinite whereas there was a decrease incarbonate minerals. In the volcanics, fluids which cause sericitization and chloritization did notincrease metals like Cu, Pb and Zn and, in fact, it can be said that these fluids are poor in point ofthese metals. This also shows that the metals developed under different hydrothermal conditions.