Biochemogenic origin of the Ordovician chamosite in the Northern Urals

The results of lithological-mineralogical and geochemical studies of the Middle-Upper Ordovician clay-carbonate rocks with chamosite ooids in facies different sections of the Bol’shaya Kos’yu Formation in the Ilych River basin of the Northern Urals are presented. In the siderite-chamosite ooids we identified nanococci and nanotubular bacteriomorphic and mineralized microbial films. The latter showed great similarity with the glycocalyx a by-product of modern bacteria. The main minerals in ooids are represented by three species of chamosite, carbonates of a calcite-siderite number, phosphates, sulfates, oxides, sulfides, including framboidal pyrite. Isotopic composition of carbon and oxygen in rocks (d13С = 0.6-0.9 ‰, d80 = 21.7-22.6 ‰) and ooids (d13С = -2.86...-0.16 ‰, d180 = 20.57-22.31 ‰) reflects a supply of fresh river water in a sedimentation basin. Isotopic composition of carbon in the organic matter of rocks (-28.72 ‰) reflects a significant influence in the formation of precipitation of biogenic carbon. Formation of the Middle-Upper Ordovician siderite-chamosite ooids originated in subequatorial coastal-sea basin with a gas-fluid seeps and activity of sulfate-reducing bacteria.