Geologia e gênese dos depósitos de manganês da provincia de Aracoiaba - Pacajús, Ceará

José Vitorino de Souza, Evaristo Ribeiro Filho


The manganese deposits of Aracoiaba-Pacajus district are located in northeast of the State of Ceará, Brazil. The protore and the ore of some of the manganese deposits studied occur between belts of manganiferous rocks extending 60 by 15 km. The Precambrian regional rocks consist of gneisses, garnet-sillimanite-quartzites, metabasites and some pegmatites. These are the host rocks of the manganiferous rocks. All these rocks belong stratigraphically and structurally to the Caico Complex and are part of the Jaguaribeana folding belt. The silicatic protore is composed of spessartite, rhodonite, quartz, manganocummingtonite with subordinated amounts of pyrite and pyrrhotite. The granulite facies of the regional metamorphism was superimposed by amphibolite facies. The original sediments which altered to form the protore were deposited in a restrict basin of reducing condition and underwent the action of two different phases of metamorphism. Surface weathering altered the protore forming an ore consisting of an assemblage of manganese oxides such as lithiophorite, pyrolusite, mangano-nsutite and aMnO2 . The ore presents typical depositional and pos-depositional textures, which show that the ore was formed by replacement and cavity filling. The manganese content of the ore varies from 18 to 38%. The reserves of the district are 2.500.000 metric tons, making economic explotation viable with a small, local siderurgical industry in Ceará. The manganese minerals were formed according to the following sequence: spessartite, lithiophorite and/ou cryptomelane, manganite, pyrolusite, mangano-nsutite.

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