Estudos Geológicos v. 19 (2), 2009 105 STRUCTURE AND MINERALOGY OF THE MURO ALTO GRANITIC PEGMATITE (VIEIRA DO MINHO - PORTU- GAL) – PECULIAR ASSEMBLAGES OF HIGH-F HYDRO- THERMAL EVOLUTION Dias, P. A. 1 Pereira. B. 2 Leal Gomes. C. 1 Guimarães, F. 3 1 Universidade do Minho – CIG-R - Escola de Ciências- Gualtar-4710-057 BRAGA, Portugal 2 Sinergeo LDA. Edifício IEMINHO Lugar de Casal 4730-575 Soutelo - VILA VERDE 3 LNEG- Rua da Amieira,4465, S. MAMEDE DE INFESTA Keywords: coupled LCT pegmatites; hydrothermal evolution; hydraulic breccia. INTRODUCTION In the place of Muro Alto, in Vieira do Minho, until the 1970s, two contiguous and spatially related pegmatites (hereinafter referred to as W and E bodies) were mined for Ta, Nb, Be, and Sn. Both pegmatites show LCT specialization (Li, Cs, Ta) and are hosted in a syntectonic granite of the 3 rd phase of Variscan deformation (D3) (Fig. 1). However they exhibit sensitive differences in composition: the W body has abundant beryl and Al-Li phosphates; the E body is paragenetically more evolved with abundant lepidolite. The study of the structure and paragenesis of the Muro Alto pegmatites suggests the existence of typical correlations between the original F/OH ratio in successive fluid- magmatic differentiates and protracted late hydrothermal evolution in this genetically interrelated pegmatitic pair. As a hypothesis, the same volatile ratio may have been responsible for increasing paragenetic diversification of phosphates, Nb-tantalates, and micas mineralogical groups. Methodologically the discrimination of minerals (Table 1) used X-ray diffraction analysis and microtextural analysis in reflected light and electron microprobe (EPMA). Compositions of minerals and its variability were determined in electron microprobe. Table 1. Mineralogical associations assigned to the internal structure of the pegmatites, highlighting the most differentiating characters.