Metais raros associados ao granito desemborque : estudo mineralogical and chemical study using MEV-EDS

• Autor(a) principal: Castro, Cassiano Costa e
• Data de Publicação: 2007
• Tipo de documento: Dissertação
• Idioma: por
• Título da fonte: Biblioteca Digital de Teses e Dissertações da UERJ
• Texto Completo: http://www.bdtd.uerj.br/handle/1/7073
• topic: Granite with rare metals; Columbite; Zircon rich in Hafnium; Cassiterite; São Paulo; Granito com metasi raros; Columbita; Zircão rico em Háfnio; Cassiterita; São Paulo; CNPQ::CIENCIAS EXATAS E DA TERRA::GEOCIENCIAS::GEOLOGIA

The Desemborque Granite, a body almost of circular shape, placed in the southernmost part of São Paulo State, has an area of ca. 50 km2. and it is represented by a sienogranite biotite. Its peraluminous features with significant contents of K2O and Na2O, together with low CaO contents, high Fe/Mg ratios, elevated values of SiO2 and trace elements as Zr, Nb and Y, assure the granite alcaline affinity that can be classified as type A. The Y numbers reflect the presence of acessory phases as parasite de Ce, yttrofluorite and monazite. Columbite, zircon rich in hafnium and cassiterite presented in the association of accessory minerals of the Granite Desemborque were semiquantitatively analysed(values in weight) by MEV-EDS. Because of this it was possible to detail the two evolutional phases for the generation of some minerals of rare metals associated to the body; one fairly magmatic (with granitic sub-phases and pegmatitic) and an hidrotermal phase. Columbite (Nb2O5 mean value 66,2% and 6,5% Ta2O5) is clearly generated in magmatic phase and in the granitric sub-phase it has ubiquitous presence and disseminated character. Zircon, in its turn, occurs in the two magmatic sub-phases, and in the granitic sub-phase it would have a later character. The mean value of the analysis was of 9,3% for HfO2, classifying it as a zircon rich in hafnium. In the hidrotermal phase, the enrichment proccesses are developed in Hf of this mineral. Cassiterite, the REE minerals and a large number of radiactive mineral have their formation restricted to the hidrotermal phase. Amongst the proposed classification for the mineralized granites, the Granite Desemborque has an peralkalkine mineralized approach to rare metals with a wide tendency to create deposits or Nb and REE occurences. The opposition could be the fact that this body is not peralkaline (it is peraluminous) and the Y and REE mineral potentials only have been recorded in the hosted mineral inclusions in the cassiterite grains (yttrocolumbite) and of zircon (xenotímio). The lack of profitable Nb-Hf-Sn mineral concentrations associated to the Granite Desemborque can be magma anhydrous character (type-A) and the lack or limitation of the hydraulic fracturing. The absence of profitable mineral concentrations because of the erosive removal of the apical zone is not in agreement with the prospective results found, once the presence of chromite in the samples of the active stream bed collected at the draining which is established exactly in the basis of the granite body indicates the preservation of remaints roof pendant the wall rocks (ortognaisses with boudins of ultrabasic rocks).