Coalified bitumens from the Kłodawa Salt Structure (central Poland) as evidence on migration of hydrothermal fluids in Zechstein (Upper Permian) deposits

Abstract

The paper presents the results of petrographic and geochemical studies of coaly matter occurring in Stinking Shale and Main Dolomite (PZ2, Upper Permian) deposits in the Kłodawa Salt Structure in central Poland. The coaly matter is both dispersed in the PZ2 shales and carbonates and concentrated in veins which crosscut these deposits and, depending on the location, depicts contrasting maceral composition and the rank of coal. The veins are built of colotellinite and micrinite, showing a mean random reflectance of coal in the range of 1.38–1.40%, whereas the dispersed organic matter is composed mostly of semifusinite and fusinite with minor vitrinite, showing a mean random reflectance of 0.82%. It is interpreted that the heterogeneous composition and rank of coal result from migration of hot fluids within a fracture system developed in the PZ2 shale and carbonate beds. The influx of hot fluids into the organic matter-rich beds led to the generation of bitumen which catagenetically altered along the hot fluid conduits. The coalification of bitumens took place at an early stage of tectonic deformation of the Zechstein salt series, initiated by regional extension and normal displacement on a basement fault, most probably in the Late Permian–Early Triassic.