Secondary minerals in basalts of the Evevpenta gold occurrence(North Kamchatka,Russia)as indicators of ore forming processes

The mineralogy seen in propylitic alteration of basalt host rocks in the Evevpenta gold occurrence in North Kamchatka is based on a vein system consisting of quartz,adularia-quartz,quartz-carbonate veins,and,hydrothermal breccias hosted in dacite.Vein minerals are native gold,tellurides,sulfides,selenides,and Au-Ag chlorides.The propylitic alteration occurs at a distance from the epithermal veins;however,it contains characteristic mineral assemblages useful for the reconstruction of an ore-formation environment.Four distinct hydrothermal alteration assemblages are recognized within the propylitic zone:1.fluorapatite-fluorine-aluminous titanite,2.quartz-calcite-clinochlore,3.quartz-calcite-stilbite-montmorillonite,and 4.calcite-mordenite.The earliest mineral assemblage was formed from an acid F-enriched hydrothermal fluid.The concentration of fluoride in fluid was 0.2-1.2 g/L,and temperature of about 550 ℃.The long-term interaction between the acid hydrothermal fluid and olivine basalt host rocks resulted in increasing pH to near neutral that caused a chloritization process.The chloritization stage hydrothermal fluid was enriched in Fe and Mg with temperatures of about 175-210 ℃,chlorite I,and 120-140 ℃,chlorite Ⅱ.The final stage of alteration consists of zeolite group minerals,which were precipitated from the hydrothermal fluid of pH～9 and temperature＞135 ℃.The high pH of the hydrothermal fluid might be explained by the presence of bicarbonates and carbonates and removal of the iron salt from the solution.Based on the alteration mineral assemblages of the propylitic zone,the physical and chemical conditions of the Evevpenta vein system are discussed.The transport of ore elements took place due to the formation of precious metal Cl-and OH-complexes at the stage of hydrothermal system formation with circulation of alkaline sodium chloride solutions.Chalcogens were also transferred by high pH solutions.