Chemical compositions of the Neogene bedded manganese deposits in the Fukaura area, Northeast Japan

Inorganic chemical compositions are determined for a series of ores from three bedded manganese deposits, that is, Kitaichi, Teranosawa, and Maruyama, in the Fukaura area, northeast Japan. The deposits occur as layers or lenses conformably in sedimentary or pyroclastic rocks of the Odoji formation of the Onnagawa stage in the Neogene period. The ores are composed of lower goethite ore and upper todorokite ore. The ores in the bedded manganese deposits are anomalously high in certain elements: t‐Fe2O3 (max. 51.2%), P2O5 (0.34%), As (9200 ppm), and Pb (600 ppm) in the goethite ore, and MnO (48.5%), Ba (28,000 ppm), Co (560 ppm), Mo (660 ppm), Ni (200 ppm), Tl (32 ppm), V (530 ppm), and W (520 ppm) in the todorokite ore. In the Kitaichi profile, there is distinct compositional zoning, that is, Fe‐As‐Y, P‐Pb, Cu, Co‐W‐Tl, and Mn‐Ba‐Mo‐Sr‐V, in ascending order. Based on the occurrences and chemical compositions of the Fukaura manganese deposits and the geological and paleoceanographic backgrounds, hydrothermal input or upwelling of anaerobic stratified water would be a possible source of elements of initial ferromanganese deposits. The zoning would be made by early diagenetic redistribution process of manganese from initial Fe‐Mn deposits, left residual products of goethite ore at the original horizon. Distinct compositional zoning would be made by the different adsorption behavior of goethite and todorokite for minor elements during early diagenesis.