THE VOLTA-GRANDE PEGMATITES, MINAS-GERAIS, BRAZIL - AN EXAMPLE OF RARE-ELEMENT GRANITIC PEGMATITES EXCEPTIONALLY ENRICHED IN LITHIUM AND RUBIDIUM

The Volta Grande granitic pegmatites are associated with Transamazonia n granites (Early Proterozoic) hosted by the Archean greenstone belt o f the Rio das Mortes Valley, which is situated at the southern border of the Sao Francisco Craton, in Minas Gerais, Brazil. The pegmatite bo dies, which are usually large, show a dominant intermediate zone conta ining spodumene, microcline, albite and quartz, with an irregular bord er of an aplitic facies surrounded by an extensive metasomatic aureole with zinnwaldite, phlogopite and holmquistite. The spodumene-rich cor e zone is continuous or segmented, and also contains lenses of lepidol ite. These pegmatites are characterized chemically by their high Rb an d Li content. Rubidium is concentrated in the potassic minerals: micro cline, muscovite, lepidolite and zinnwaldite. These minerals show exce ptionally low K/Rb weight ratios, from 1.5 to 4.0. In micas, a positiv e linear correlation exists between fluorine and rubidium. The evoluti on of the concentrations of Rb and Cs in the minerals, in the core of the pegmatite and in the metasomatic aureole, can be explained using e xperimentally determined partition coefficients.