Estudo de minerais pesados dos pegmatitos da Província Pegmatítica de São João del Rei, Minas Gerai

Abstract

The São João del Rei Pegmatitic Province occupies an area of approximately 2500 km2 in the southern portion of the Minas Gerais state and it is characterized by the presence of a pegmatite swarm mineralized in Sn-Ta-Nb-Li. The pegmatites crosscut the rocks of the Rio das Mortes metavolcano-sedimentary sequence, and several paleoproterozoic plutonic bodies, among them the Ritápolis metagranitoid, whose crystallization age of 2121 ± 7 Ma marks the last magmatic pulse in this region. Twelve pegmatitic bodies from this province were studied, three intrusive in the Brumado metadiorite, three in the Ritápolis metagranitoid, two in the phyllites and two in the amphibolites from the Rio das Mortes metavolcano-sedimentary sequence, and another two intrusive in the Cassiterita orthogneiss. The heavy mineral concentrate of each pegmatitic body was characterized by optical stereomicroscopy, Raman microprobe, scanning electron microscopy (SEM/EDS) and electron microprobe (EM/WDS). The studied area is intensely weathered, causing the pegmatite’s alteration, the formation of kaolinite and the partial or total release of the main accessory minerals, corresponding to columbite-tantalite, garnet, Ilmenite, magnetite and zircon, whereas cassiterite, gahnite, microlite, monazite and xenotime occur in most of the bodies, in varying proportions. Andaluzite was found only in the pegmatites that are intrusive into the phyllites from the Rio das Mortes metavolcanosedimentary sequence, while monazite and xenotime present a higher concentration in the pegmatitic bodies intrusive into amphibolites from the Rio das Mortes metavolcano-sedimentary sequence. On the other hand, the pegmatites intrusive in the Cassiterita orthogneiss are characterised by the absence of cassiterite and columbite-euxenite group minerals. The studied pegmatites were classified as belonging to the LCT family (lithium, caesium, tantalum), due to the Ta > Nb concentration and to the presence of muscovite, tourmaline, spessartine-rich garnet, gahnite, andaluzite and REE phosphates (monazite and xenotime). The U/Pb LAICPMS ages, obtained from zircon grains, coupled to the bodies’ mineralogy, suggest two groups of pegmatites: the first, not mineralized, intrudes the Cassiterita orthogneiss and is older, 2489 ± 10 Ma. The second one, mineralized in Sn-Ta-Nb, intrudes the Brumado metadiorite, the Ritápolis metagranitoid and the phyllites and amphibolites from the Rio das Mortes metavolcano-sedimentary sequence and presents was dated between 2129 ± 33 Ma and 2121 ± 9 Ma. The correlation of the second group of pegmatites to the Ritápolis metagranitoid, and the enrichment of these bodies in economic elements (Sn-Nb-Ta) seems to be associated not only to the source of the pegmatitic bodies, but also to its fractionation, and as a result of the interaction with their host rocks