The volcanosedimentary Zaonezhskaya Formation of the Ludicovian Suprahorizon (approximately 2.0 Ga) is well-known for grate hosting accumulation and a high content of carbonaceous matter. The rocks had been intruded by numerous mafic sills and later underwent lower greenschist-facies regional metamorphism. High-carbon rocks (up to 70 wt. % organic carbon) near Shunga village have been studied by optical microscopy, scanning electron microscopy and Raman spectroscopy. As a result some spheromorphic and polyhedral microstructures which can, with high probability, be identified as biogenic were found.

The Raman spectrum of carbonaceous matter from the Zaonezhskaya Formation exhibits features typical of poorly crystalline carbonaceous material in lower greenschist-facies regional metamorphosed rocks. In spite of the similar morphology of the Raman spectrum, carbonaceous matter varieties have different values of some Raman parameters. The kerogen (carbon matrix and microstructures) has a more ordered molecular structure in comparison with the migrated carbonaceous matter. We demonstrate that Raman spectral features permit its definition. We also show that some microstructures that have been previously described as sedimentary organic debris were generated by the transformation of carbonaceous matter due to contact metamorphism and hydrothermal activity.