Geological characteristics of the Serra do Navio Formation, Guiana Shield, Brazil, and its manganiferous ores: queluzites as a source rock of oxide and carbonate manganese ores
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Abstract
The Serra do Navio manganese deposit in the Amapá State, Eastern Guiana Shield, northernmost Brazil, had reserves of high-grade oxide ores exhausted, after the production of 33 million tonnes, assaying more than 45% Mn. However, possibly with greater tonnages, the carbonatic protores, the queluzites, remain practically intact, available for exploration and mining. The queluzites occur as lenses, in the upper part of each cycle of the metamorphosed megacyclothem that constitutes the Serra do Navio Formation. Each cycle initiates with sediments of a marine near-littoral quartzose facies, represented by metamorphosed limestones, cherts, and silts, overlain by an intermediate argillaceous, now biotitic, facies, and closing up with a coastal organic material-rich clay, now graphitic, facies, host of lenses of manganese carbonates, now queluzites. The sequence was metamorphosed on three occasions. The first was regional, with folds verging to the north-northeast. The second was thermal, marked by porphyroblasts as diopside, andalusite, staurolite, cordierite, sillimanite, almandine, spessartite, picrotephroite, rhodonite and others. Some porphyroblasts preserve remnant minerals and structures of the former metamorphism. The third metamorphism was regional, with fold axis to the northwest, being marked by a shear zone causing strong deformation along its trace. Broken and displaced minerals and rock fragments occur in deformed areas. In some areas, weak retrograde metamorphism is marked by veinlets of quartz and calcite, chloritization, tourmalinization, and dispersion of iron sulfides.
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