Conditions during the Formation of Carbon-Bearing Rocks, Transonega Region

In order to determine the physico-chemical conditions for the formation of Precambrian сarbon-bearing rocks (CBR) of the Trans-Onega region, the compositions of their minerals and fluid inclusions (FI) in quartz were studied in samples collected at different mine workings of the Zazhogino ore field. The obtained FI density values in the rocks fluctuate in a sufficiently wide range. This indicates the existence of several stages of changes in the temperature regime of fluids during epigenesis. The densities and concentrations of FI solutions from the host silicified lydites are close to those of the highest-density varieties from quartz veinlets in the CBR. Some FI are partially decrepitated, which indicates superimposed processes in the post-capture period. Based on the studies of eutectic temperatures, it is suggested that mineral-forming solutions in the inclusions throughout the entire ore field contain magnesium chloride. The study of the chemical composition of the minerals of the CBR and host rocks allowed us to identify various sulfides, oxides, carbonates, phosphates, silicates, and aluminosilicates. Among them, mineral assemblages suitable for calculating the temperatures and pressures of mineral formation were selected and the corresponding calculations were carried out. Comparison of the results of the FI study and mineral thermobarometry allowed us to estimate the PT conditions that existed during the CBR epigenesis. The maximum values of the greenschist-facies temperatures and pressures during CBR formation correspond to T = 410–400°C and P = 2.4–2.1 kbar. Chlorites were formed at T = 240–200°C and P = 0.5–0.4.