Phenyl derivatives of polycyclic aromatic compounds as indicators of hydrothermal activity in the Silurian black siliceous shales of the Bardzkie Mountains, Poland

•High abundances of phenyl derivatives of PACs were found in Silurian sedimentary rocks.•Ph-PACs proposed to originate from OM alteration by migrating fluids•Peri-condensed aromatics formed by dehydrocyclization of Ph-PAC precursors.

High relative concentrations of phenyl (Ph) derivatives of polycyclic aromatic compounds (PACs) were found in samples from the Silurian Żdanów section (Bardzkie Mountains, Poland). These sedimentary rocks were characterized by the occurrence of all previously described Ph-PAC groups, including: phenylnaphthalenes, phenylphenanthrenes, terphenyls, phenyldibenzofurans, phenyldibenzothiophenes, and quaterphenyls, among which the phenylnaphthalenes and phenylphenanthrenes were most abundant. In all samples the thermally more stable isomers clearly dominated, while those with ortho- or α-substitution were present only as traces or absent. This suggests a high maturity of the investigated section, which was also confirmed by other molecular and petrographic thermal maturity parameters. Considering the highest abundances of Ph-PACs occur in the older part of the section, we suggest that the major factor controlling OM maturation was ascending hydrothermal fluids. Moreover, the significant peri-condensed aromatic compounds detected in the samples may have formed by dehydrocyclization of phenyl- and naphthyl-PAC precursors. If the formation of the PhPACs is strictly connected with alteration by ascending solutions, the interpretation of the inorganic geochemical results should be expanded to consider the impact of hydrothermal fluid migration.