COMPARATIVE STUDY OF TWO PEGMATITIC FIELDS FROM MINAS GERAIS, BRAZIL, USING THE Rb AND Cs CONTENTS OF MICAS AND FELDSPARS

Abstract

The distribution of alkali elements and F was investigated in minerals of pegmatites and their contact aureole of metasomatism, in two groups of pegmatites from the State of Minas Gerais, southeast Brazil: the Transamazonian-cycle age (1,930 Ma) Volta Grande pegmatites near São João del Rei and the Brasiliano-cycle age (525 Ma) pegmatites of Itinga. The Volta Grande pegmatites are albite and spodumene bearing and their contact aureole of metasomatism, in the amphibolite host rock, are characterized by the presence of holmquistite and zinnwaldite. The Itinga pegmatites correspond to complex lepidolite and pet alite-bearing pegmatites with tourmaline, zinnwaldite and muscovite in their contact aureole developed in quartz-biotite-muscovite-schists. The K-bearing minerals of the Volta Grande pegmatite are exceptionally rich in Rb. Using the experimentally determined partition coefficients between minerals and fluids, the atomic ratio K/Rb of the pegmatitic fluid may be evaluated. This ratio shows homogenous values, around 4. Similar values are found in the metasomatic aureole, at the contact, which increase as a function of the distance from the contact, due to the crystallization of trioctahedral micas that deplete the fluid in Rb. The K/Rb ratios of the fluid could represent the ratio of the initial pegmatitic magma before differentiation. Rb content in the Itinga pegmatites is much lower and strongly differs from border to core. The presence of K-bearing minerals in the wall rocks contributes for the increasing the K/Rb ratios of the neoformed minerals at the contact zone. It is then more difficult to evaluate the K/Rb ratio of the initial magma. Considering the composition of the minerals at the border of each pegmatite, it varies from 25 at Urubu to 50 at José de Linto and Xuxa localities. The Itinga pegmatites are clearly poorer in Rb than the Volta Grande pegmatites. Geochemically, a relationship between the Rb and F contents in the micas is clearly demonstrated in each group of pegmatites. The situation is more complex for Cs because crystallization of all minerals concentrates Cs in the fluid. Urubu pegmatite is the only one where pollucite was found in the core. Cs content in K-bearing minerals is higher in the lepidolite of the core and in zinnwaldite of the aureole. Cs2O in the Volta Grande pegmatite can be as high as 2%, at 1 meter from the contact, as metasomatic fluids become enriched in Cs while the metasomatism proceeds. This element could be used as guide for exploration of the alkali pegmatites.