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Paleozoic (Silurian and Devonian) cherts from Western Bulgaria occur in three tectonic units: Svoge, Lyubash-Golo Bardo and Morava. The SiO2 content in these rocks varies from 76.06% to 96.02%. The silica source and depositional settings of the cherts have been interpreted based on the received geochemical major element data. The calculated Si/(Si+Fe+Al+Ca) ratio and Al-Fe-Mn diagram plotting at non-hydrothermal field suggest that most of silica in the cherts is biogenic or both biogenic and terrigenous in origin. On the other hand, all studied siliceous samples plot into the field of the continental margin sediment environments on the Fe2O3/TiO2 vs. Al2O3/(Al2O3+Fe2O3) diagram. The calculated MnO/TiO2 ratio also indicates deposition in continental shelf or slope environments. [ABSTRACT FROM AUTHOR]