We study a carbonate body looking like a classical fossil travertine which was discovered in the Chebaki–Balakhta basin within the Minusa trough (Khakassia, Russia) and called quasi-travertine. It is a thin layer sandwiched between a basalt–dolerite sill and calcareous siltstone. Comprehensive studies of the quasi-travertine and its comparison with Devonian fossil travertines located a few kilometers away in terms of structure and composition have made the basis for its formation model. According to this model, the quasi-travertine has had a two-stage history: deposition and subsequent hydrothermal metasomatism. Laminated limestone coexisting with calcareous siltstone of the Early Devonian Shunet Formation formed during the first stage and then experienced hydrothermal metasomatism with precipitation of secondary calcite, prehnite, and pyrobitumen (kerite).