Mica and tourmaline geochemistry of pegmatites from Conselheiro Pena Pegmatite District, Minas Gerais, Brazil: Implications for pegmatite genesis and economic potential

抄録

Geochemical studies on mica and tourmaline and boron isotopic analysis on tourmaline from pegmatites of the Conselheiro Pena Pegmatitic District—Eastern Brazilian Pegmatite Province—were conducted in order to: 1) analyze their relationship with the Urucum granite, believed to be their source pluton; 2) investigate their origin and evolution; and 3) assess their economic potential. Considering the fractionation from a granitic pluton as the most accepted model for pegmatite genesis, some factors suggest the study pegmatites do not represent residual magmas from the Urucum suite fractionation: the granites from this suite cannot be classified as fertile according to the chemical composition of its facies; the emplacement of rare element pegmatites around and within the Urucum granite is inconsistent with the model of evolution as they fractionate from a common granitic intrusion; and there are no clear geochemical trends among mineral separates from the pegmatites and the Urucum suite. Boron isotope analysis measured on tourmaline crystals yielded δ¹¹B values in the range of –16.3 to –11.3‰ (n = 86) for the pegmatites, –13.8 to –11.5‰ (n = 8) for the São Tomé Formation host schists and –14.0 to –13.1‰ (n = 6) for the Córrego do Onça facies (tourmaline-bearing) of the Urucum suite. This extremely similar isotopic composition suggests all the analyzed rocks come from a common source, but more studies are needed to determine which source may it be. Most pegmatites are classified as complex and some (Boca Rica, Cigana and Pomarolli) are Li-bearing, with spodumene, triphylite and/or amblygonite. However, mica and tourmaline chemical compositions are inconsistent with the presence of such Li-minerals in that where these are present, mineral separates show low values of Li, Rb, Cs and Sn, while mica and tourmaline from pegmatites without Li-minerals (Urucum, Sapo and Jonas) show higher amounts of such elements. Therefore, chemical analysis of these mineral separates seems, in this case, uncorrelated to the economic potential for Li exploration, and the data should be used with caution. Nevertheless, pegmatites are economically attractive for gems and collection minerals, being two of them (Sapo and Itatiaia) potentially suitable for Ta exploration.