We report on the first authigenic siderite (FeCO₃) concretions recovered from near-bottom sediments at gas hydrate-bearing mud volcanoes in fresh water (Lake Baikal, Eastern Siberia). The carbonates appear as firm 'plate-type' formations at the Malenky mud volcano (Southern Baikal Basin) and as soft nodules at the K-2 mud volcano (Central Baikal Basin). Calcium is the main divalent component which substitutes iron in the carbonate lattice (7 to 20 mol%). The δ ¹³C values of the carbonates (+3.3 to +6.8‰ at Malenky, and +16.5 to +21.9‰ PDB at K-2) indicate that their formation is due to methanogenesis. The latter was most likely caused by the microbial methyl-type (acetate) fermentation that is suggested from the isotopic composition of the accompanying methane hydrates and dissolved methane. General depletion of the siderites in ¹⁸O (-11.6 to -9.9‰ at Malenky, and -13.9 to -12.3‰ PDB at K-2) is mainly inherited from the isotope composition of pore water (-15.2 to -15.4‰ SMOW) at ambient temperature (3.5°C).