Origin of barite concretions in the West Carpathian flysch, Poland

The Paleogene concretions occurring in the Sub-Silesian nappe of the West Carpathians were investigated to understand their origin and evolution. They were considered previously to be of hydrothermal origin. Three types of concretions were found in marls: (1) phosphorite impregnated, (2) barite impregnated and (3) composed of the mixed types. The sulphur and oxygen isotope determinations of barite (δ34S between 32 to 36‰ and δ18O between 20 and 23‰) as well as the ratio Δδ34S/Δδ18O of 1.5 point out for bacterial reduction of sulphate as the primary cause of enrichment in heavy isotopes of sulphur and oxygen. Another process which could contribute to high δ18OSO4 values was sulphate isotope exchange with seawater oxygen. The 87Sr/86Sr compositions of barite confirmed that they are fairly identical with strontium isotopic composition of Paleogene seawater (O.7076–0.7078). Therefore, barite nodules of the Sub-Silesian unit in the flysch Carpathians can not longer be considered as of hydrothermal origin.