Stable isotope and fluid inclusion evidence of multistage fluidal activity in Baltic paleobasin: Silurian carbonate sequence in Kalana, Estonia

During the last half billion years Estonia and more generally Baltic paleobasin haven’t experienced tectonic or volcanic activity that could heat up rocks significantly. Nevertheless, some geological evidences like dolomitization, sphalerite/galena mineralization, diagenetic gradients and secondary magnetization of rocks indicate influence of hydrothermal fluid movement. The aim of this thesis is to study the fluid-driven mineralization, mineral assemblage and paragenesis, and fluid characteristics in fracture-controlled cave and vein systems in the Silurian carbonate succession in Kalana, central Estonia. Formation of calcite-sphalerite veins and speleothemes formed of barite and isotopically very light calcite were under special focus. Sphalerite (ZnS) occurs in two generations. Microthermometry of fluid inclusions suggest temperature of formation not less than 200°C and 70-100°C respectively. It was a CaCl2-type fluid with high salinity (ca 26%). The mineralization could be connected to formation of Caledonides around 400 Ma, and is similar to Mississippi Valley type mineralization. Barite (BaSO4) appears together with calcite (CaCO3) and pyrite (FeS2) on the walls of fractures and cave like structures. Calcite in those speleothemes has exceptional isotopic composition which suggest that its carbon source is methane which is anaerobically oxidized by microbes. Such process going on in caves hasn’t been described before. Barite is formed due mixing of two fluids: deeper and warmer Ba-bearing and shallower sulfate rich fluid. Sulfate originates from seawater or from oxidation of sulfide formed due mineralization of organic matter. Speleothem calcite and barite are formed due to activity of relatively low temperature (ca 70°C) hydrothermal fluids