Mineralogy and geochemistry of hydrothermal alterations in Zigana mountain (Torul-Gümüşhane) volcanics

Mineralogy and Geochemistry of Hydrothermal Alterations in Zigana Mountain (Torul - Gümüşhane) Volcanics In this study, hydrothermal alterations observed in the Late Cretaceous volcanics which are occurred from Zigana Mountain (Torul - Gümüşhane) have been investigated. At the bottom of the vocanic sequence, basalts, andesites and their pyroclastics are seen. Dacite - L Dacite - II and andesite sit on the these rocks. All these volcanics are cut by andesite dykes and porphyryc dacites. All volcanics studied show tholeitic - transition - calcalkaline character, and have developed in volcanic arc environment. Trace element contents indicate two different dacites occurences, they are named as Dacite - L Dacite - n. According to fractional crystallization calculations, fractionation of clinopyroxene, hornblend and plagioclase for basalts and andesites, K - feldspar, plagioclase, hornblend and biotite for Dacite - L K - feldspar for Dasit - II, plagioclase, clinopyroxene and magnetite for andesite dykes and plagioclase for porphyryc dacites are important. Volcanics rocks form similar source which has rich with metasomitism in subduction component, differentiated and contaminated later. Ulite + chlorite + kaolinite ± smectite ± (illite - smectite) ± (chlorite - vermiculite) are determined in the studied volcanics. According to b0, illites have occured in low - medium pressure metamorphism grade, namely pressure is lower than 3 kbar, Fe2+ and Mg - poor and as a result of hydrothermal decomposing of feldspars. According to chlorite geothermometer calculations, temperatures range from 100 to 250 °C. Illites are to exist of pH 7 to 8, chlorites higher than pH 8.33 environments. Mass changes calcultions show that the mass of basalt, andesite and Dacite - 1 increase, mass of Dacite - II and andesite dykes decrease. Porphyryc dacite show both mass increasment and decreasment. Generally, illitization - chloritization - kaolinitization increase from less altered rock to highly altered rock, but carbonatization decreases. The source of H and O in illites from dacites have been formed from hydrothermal fluids with mainly magmatic origin. According to K - Ar method, the age of illites in Dacite - 1 have been determined average 78.7 ± 2.3 My and in Dacite - II average 75.3 ± 2.4 My, that is to say they are Katnpaniyen - Daniyen aged. Key Words: Zigana Mountain, Dacite - L Dacite - n, Ulit, Chlorit, Hydrothermal Alteration, Izocon Method, Chlorit Geothermometer, H and O Isotopes, K - Ar Age Analyses