Geochemistry of muscovite from pegmatites of the Eastern Brazilian pegmatite province : a clue to petrogenesis and mineralization potential.

Abstract

Single crystals of muscovite (N = 143) from different zones of selected granitic pegmatites belonging to two wellknown gem-producing districts of the Eastern Brazilian Pegmatite Province (northern Araçuai and western Governador Valadares in Minas Gerais) were analyzed for major, minor and trace elements. Structural formulae display considerable (Fe+Mg) variation within the octahedral site, coupled with tetrahedral Si-Al substitution (Tschermak substitution – phengite component). Tetrahedral Si varies from 3.06 to 3.21 and AlVI from 1.71 to 1.90 apfu. Trace elements in muscovite are significant indicators for the economic potential of pegmatites as well as for the differentiation degree and origin of the magma. Muscovite with higher Li and B contents is characteristic for gem-tourmaline-bearing pegmatites. Lower concentrations in K/Rb, Ti and Mg and higher F contents are found in muscovites from higher differentiated zones within the pegmatite. In comparison with anatectic pegmatites, relatively low K/Rb ratios of muscovite from the studied pegmatites are indicative of derivation by fractional crystallization of granite magma. Zn, Ga and Y, elements rarely analyzed in micas, tend to increase with decreasing K/Rb ratios, showing them to be good indicators of the fractionation degree of magmas.