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average beneficiation braunite Brazil Bulgaria Bureau of Mines concentrate contained manganese cost countries crude cryptomelane depth developed district economic estimated exports ferromanganese furnace Gabon gondite grade Groote Eylandt hematite high-grade increased India industry International Geological Congress iron formation Kuruman layers lenses located low-grade lower m.t. of manganese Mamatwan manganese carbonate manganese content manganese deposits manganese minerals manganese nodules manganese occurrences manganese oxides manganese reserves manganese resources manganese-bearing manganiferous manganite material metal metallurgical meters Mexico million m.t. million tons Mineral Resources Moanda Nikopol nodules occur open-pit open-pit mining operations panel phyllites pisolites potential Precambrian processing production protore psilomelane pyrolusite ranges recovery reported reserves and resources rhodochrosite rocks sedimentary sediments Serra Serra do Navio shales siliceous South Africa steel steelmaking supergene Symposium on Manganese Tchiatura thick tonnage U. S. Government Printing U.S. Bureau underground mining USSR Washington zone