Abstract
In Goias and Tocantins States, Central Brazil, several granitic pegmatites were characterized and grouped for the first time. These pegmatites had been intensely explored by hand in the past, producing mainly gemstone varieties of tourmaline and beryl. Barren, beryl- and tourmaline-bearing pegmatites occur across an area of 2,000 km 2 where they intrude regional metasedimentary rocks and peraluminous granites. K‑feldspar (mostly altered to kaolin), quartz and mica (mainly muscovite) are the major minerals. The main accessory minerals are beryl, tourmaline, garnet, albite, Fe-Mn phosphate aggregates, and trilithionite. The paragneiss surrounding the barren pegmatites was affected by thermal metamorphism and later hydrothermal alteration, producing Ca-silicates, Ti-Nb-Y oxides and sulfides. Leucogranites of the Mata Azul Suite are peraluminous and syn- to post-orogenic with geochemical characteristics of the LCT granite-pegmatite group. LA-ICP-MS U-Pb geochronology in monazite yields an age of 519 ± 2.8 Ma. Additionally, U-Th-Pb chemical dating of uraninite reveals a maximum age between 500 and 560 Ma. These ages, the field relationships, the mineralogy and the geochemical data suggest that the granites of the Mata Azul Suite are the probable sources of the studied pegmatites. The mineral associations and the mineral chemistry are used to define the degree of fractionation of the pegmatites. We propose that the group of studied pegmatites represents a pegmatitic field, called the Mata Azul Suite Pegmatitic Field.
Highlights
Pegmatite is a singular group of igneous rocks that usually exhibit exotic mineralogy and geochemistry
Mineral compositions were determined in an electron probe microanalyzer (EPMA) JEOL JXA8230 superprobe with five spectrometers using a voltage of 15 kV and a beam current of 10 mA
The frequent presence of Li-minerals and the scarcity of Y and rare earth elements (REE) minerals in the pegmatites described in the previous sections indicate that they belong to the LCT family (Černý 1990, 1991, Martin and De Vito 2005)
Summary
Pegmatite is a singular group of igneous rocks that usually exhibit exotic mineralogy and geochemistry. Its genesis can be either from an evolved magma or from the anatexis of a protolith. The pegmatites with granitic composition (Li-Cs-Ta (LCT) and Nb-Y-F (NYF) families) are the most studied, either because of their more common genesis or because of their economic potential. Pegmatite occurrences are frequently found as small bodies, pegmatites are considered economically important because of their industrial minerals, rare metals and especially their gems. In the north of Goiás and in the south of Tocantins, pegmatites were found and mined by hand for decades, but have never been properly studied. There are pegmatites with LCT and NYF composition in this area, the minor occurrence of NYF bodies, their limited exposed area and the very different chemical-mineralogical signature from
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