Diagenetic source of fluids causing the hydrothermal alteration of teschenites in the Silesian Unit, Outer Western Carpathians, Czech Republic: Petroleum-bearing vein mineralization from the Stříbrník site

Calcite-pyrite veinlets containing a small amount of quartz, chlorite, sphalerite and marcasite occur in teschenite which is situated within the Lower Cretaceous flysch siliciclastics of the Silesian Unit. Fluid inclusion study demonstrated the presence of a heterogeneous mixture of a low-salinity aqueous solution and mature methane-rich petroleum during precipitation of calcite and quartz. The δ13C values of methane and dissolved carbonate, δ34S values of pyrite, and δ18O values of parent hydrothermal solutions are all consistent with the external source of the fluid phase, derived from diagenetically modified flysch sediments. The vein formation occurred during an increase in both temperature (from <80 °C to 220 °C) and pressure (∼100–700 bar). The observed discrepancy between the maximum fluid temperature and the regional burial temperature, together with fluctuations in petroleum density and significant variations in chemical and isotopic compositions of fluids are interpreted in terms of episodic tectonically induced generation of the fluid phase related to deeper burial. The observed REE patterns of hydrothermal calcite and pyrite can be attributed to increased concentrations of organic ligands in the fluid phase and/or involvement of sorption/desorption processes.