Summary:
The Morro da Mina Mn deposit is located in the Minas Gerais and inserted in the Rio das Velhas Supergroup, specifically within the Lafaiete Formation, identified as a volcanic-sedimentary sequence of the greenstone belt type in the Quadrilátero Ferrífero.
The Mn contents in theis deposit is ~ 32% of MnO2.
Morro da Mina, in the Lafaiete district, Minas Gerais, Brazil, has been one of the most productive manganese deposits in the western hemisphere. The ore is residual and consists of manganese oxides formed by the weathering of silicate and carbonate basement lithologies. This protore has been called gondite (Park, 1956) and manganese silicate-carbonate protore (Dorr, Coelho, and Horen, 1956). Dorr and his coworkers state that the protores are metamorphosed sediments that formed a gradational sequence of cherts, mudstones, and manganiferous carbonates. They are in schists of Precambrian age which may be volcanic in origin. The characteristic manganiferous beds have been followed along strike for more than 60 km, and probably continue farther. The irregular distribution of most constituents in the metasediments is compatible with the present erratic distribution of manganoan carbonates in the protores. The sediments have undergone intense regional metamorphism, and locally are invaded by granitic masses and mafic and pegmatitic dikes (Guimaraes, 1935). Igneous metamorphism has been superimposed upon regional metamorphism, particularly near the northwestern part of the mine, a factor that has led observers in the past to relate the mineralization to igneous metamorphic processes. Igneous metamorphism is now thought to have developed patches of the manganoan olivine tephroite (Mn2Si04) and other silicates, mainly at the expense of manganese carbonates.
The ore at Morro da Mina consists of a mantle of manganese oxides, products of the removal of silica and carbonate from the protores by weathering. Only in very few places does the ore grade downward into the protore; rather, the contact is usually a sharp, clearly defined, undulating irregular surface that locally extends downward into the protore along fractures, dikes, and water courses. The presence of carbonates and sulfides that are more easily decomposed than the silicates is thought to aid greatly in the weathering of the protores. Where these materials are present, their relatively rapid weathering renders the protore permeable, and decomposition proceeds rapidly and to considerable depths. Ore has been mined to a depth of about 200 meters. Where the carbonates and sulfides are absent in the protores—for example, in those parts of the district where igneous metamorphism has been activ —oxidation is limited, and generally the manganese oxides form only a thin film or at most a meter or so of ore.