Metasomatites Developed after Shungite-Bearing Rocks of the Maksovo Deposit (Onega Basin, Karelia)

The Maksovo shungite-bearing metasapropelite (maksovite) deposit is located in the eastern part of the Onega Basin that was initiated and formed in the Paleoproterozoic in the southeastern Fennoscandian Shield. The maksovite deposit represents a diapiric fold formed about 2050 ± 10 Ma ago. It is underlain by carbonate sequences and, in turn, overlain by tuffaceous siltstones and intruded by gabbro-dolerites (1956 ± 5 Ma). Unaltered maksovites represent pelitomorphic rocks with a massive or obscure-layered structure containing about 30% Corg. Alterations superimposed on the maksovites and underlying tuffaceous siltstone sequences are developed along brecciation zones within the ridge-shaped fold. The alterations are represented by alkaline Fe–Mg metasomatites with brecciated structures, as well as heterogeneous mineral and chemical compositions. They differ in terms of intense biotitization, chloritization, and development of the apatite-bearing albite–carbonate and sulfide-bearing carbonate–quartz stringers. Alteration zones are enriched in Na, K, P, Ti, Mg, Fe, and other ore components. Based on the Re–Os dating of sulfides, the age of metasomatites is estimated at 1558 ± 61 Ma.