Raman spectroscopy of organic matter and rare minerals in the Kłodawa Salt Dome (Central Poland) cap-rock and Triassic cover - Indicators of hydrothermal solution migration

The occurrence of organic matter (OM) in a salt dome cap-rocks is very rare in Zechstein formation in the Europe. Within the Kłodawa Salt Dome (KSD) cap-rock and Triassic cover OM occur relatively often and it is accompanied by some rare minerals. The OM in cap-rocks occurs in two forms i.e. as a finely dispersed matter not recognizable in a microscopic image and as organic particles or small veins. Based on the Raman spectroscopy and measured reflectance both forms show similar thermal maturation but finely dispersed matter display higher variability. The temperature of the maturation reaches up to 220 °C and mostly is about 150 °C. Within the solid bitumen (SB) was also found OM particles which show thermal transformations above 300 °C. Within the Triassic cover SB veins and particles prevail. The finely dispersed matter is occasionally found. In terms of thermal maturation, the SB is similarly developed as in the cap-rocks. It also contains OM particles which show higher thermal transformations, as well as graphite-like particles. Therefore, it indicates the continuation of bitumen migration from cap-rock. Additionally, clasts of brown coal which show low thermal maturation occur in Triassic sediments. The SB is sometimes accompanied by two types of hydrocarbons (HC) i.e. with prevailing aliphatic or aromatic compounds as well as titanium minerals (anatase, rutile). Two source rocks for SB and HC generation seem to be: 1) the Stinking Shale (T1) and Main Dolomite (Ca2) members that occur in KSD, 2) the deep basement of Zechstein salt formation. The thermal maturity of OM confirms that the migration of hydrothermal solutions took place during KSD development. The origin of such a hot solution is connected with tectonic activity in the basement of salt formation during Mesozoic.