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Título

INTERPLAY AMONG LITHOLOGY, DEFORMATION, IRON MINERALIZATION AND HIGH-GRADE METAMORPHISM: THE SERRA DOS ALEMÃES MINE, SOUTHERN SÃO FRANCISCO CRATON

Texto do resumo

Due to the worldwide demand for iron ore and the exhaustion of large mines, small deposits have become viable. The Serra dos Alemães iron ore deposit, discovered and operated by Vórtice Mineral, is located in Oliveira municipality, in the south-central region of Minas Gerais state, Brazil. It is situated in the Southern São Francisco Craton, close to the Jeceaba-Bom Sucesso lineament. The deposit comprises metamorphic banded iron formations (25-48% Fe) and high-grade magnetite ores (>58% Fe), associated with metasedimentary rocks (quartzite, ferruginoues quartzite, muscovite schist, impure marble), suggesting a Superior-type classification. Field observations, thin section studies and X-ray diffraction analysis show high-grade metamorphic assemblages, mainly including corundum, sillimanite, amphibole (ferroactinolite), pyroxene (diopside), and phlogopite, indicating an upper amphibolite-facies metamorphism. Retrograde metamorphism is represented by muscovite rims around corundum, tremolite, chlorite, and talc. All rocks are intruded by undeformed alkaline granitoids, associated with extensional faults. NE-SW-trending mafic dikes cut the previous rocks. Based on detailed drill hole logging, geological mapping, and 3D modelling, the structural architecture of the deposit is formed by superposed folds, similar to type 3 of Ramsay and Ruber (1987): F1 folds are recumbent, north-vergent with sub-horizontal axial planes, refolded by F2 folds, which have south-vergence and inclined axial planes. The structural evolution is interpreted as a progressive event initiated by a north-vergent nappe (F1 folds) progressively blocked, causing vergence inversion and mass transport to the south (F2 folds). The metamorphic banded iron formations are represented by amphibolite/pyroxenite itabirites with minor quartz itabirites (25-35% Fe), both having granoblastic fabric, medium-grain size, and magnetite±hematite as ore minerals. Goethitic itabirites (40-48% Fe) are friable and interpreted to be the weathered product of amphibolite/pyroxenite itabirites. High-grade magnetite ore is massive, granoblastic and has magnetite±martite as ore minerals. The main occurrences are controled either by folds or along the contact between itabirite and muscovite schist. Subordinate ironbodies occur paralel and/or discordant (along F2 axial planes) within quartzites and disseminated in muscovite schists (40% Fe), close to the contact with itabirite. The lithological and structural controls of high-grade iron ore are thought to be formed by open spaces created during shearing and folding, where iron were precipitated, favoured by the relatively impermeable layer of muscovite schist. Fe-mineralized muscovite schist with magnetite inclusions in corundum suggests the iron mineralization is pre-high grade metamorphism.

Palavras Chave

banded iron formations; iron mineralization; high-grade metamorphism; Southern São Francisco Craton.