The Alteration Mineralogy and Mass Change of the Zigana (Gumushane) Volcanics of NE Turkey

The Late Cretaceous volcanic rocks around Zigana Mountain (Gumushane) in the eastern Black Sea metallogenic province in NE Turkey show intensive hydrothermal alteration but less weathering alteration. The basement of the study area is formed by Late Cretaceous basalt, andesite and their pyroclastics. 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, and rare kaolinite, smectite and epidote as the products of alteration. Sericitization/illitization is the most common type of hydrothermal alteration associated with these volcanics, and chloritization is the most common alteration type after illitization; pyritisation is seen in all volcanics, and is the most common in dacites. In some fields limonitisation is occasionally present. Epidotization is rare, and especially seen in basalt and andesite. Isocon analysis was undertaken to estimate the mass gains and losses of the Zigana Volcanics as a result of hydrothermal alteration. In general terms, the results shows 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 in the volcanics during the hydrothermal alteration of the parent materials. From less altered rock to highly altered rock there was an increase in illite-chlorite-kaolinite whereas there was a decrease in carbonate minerals. In the volcanics, fluids which cause sericitization and chloritization did not increase metals like Cu, Pb and Zn and, in fact, it can be said that these fluids are poor in point of these metals. This also shows that the metals developed under different hydrothermal conditions.