MACIÇOS GRANÍTICOS DE ESMOLFE E ANTAS - MATANÇA (PORTUGAL): RECONHECIMENTO, CARACTERIZAÇÃO E DEFINIÇÃO DE ÁREAS COM POTENCIALIDADE PARA EXPLORAÇÃO DE GRANITO ORNAMENTAL

Abstract

O presente trabalho tem por objetivo o estudo de areas com potencialidade para producao de pedra ornamental nos macicos graniticos de Esmolfe e Antas-Matanca, na regiao Centro-Norte de Portugal. Os granitos estudados, tardi-hercinicos e de caracteristica calco-alcalina, tem cor cinzenta azulada, granulacao media e tendencia levemente porfiritica. Apresentam acentuada homogeneidade mineralogica, textural e estrutural e escassos defeitos do ponto de vista ornamental, sendo a principal condicionante a atividade extrativa a fracturacao. Procede-se a analise da informacao colhida e tratamento estatistico dos dados de fracturacao, visando ao conhecimento do estado de compartimentacao dos macicos nos niveis superficiais, embora com as reservas inerentes a alietoriedade da rede de fracturacao natural. No macico de Esmolfe, a textura do granito e mais homogenea, apresentando uma granulacao mais grosseira e cor mais clara do que o granito de Antas-Matanca. Os espacamentos dos sistemas de fracturas em ambos os macicos sao maioritariamente moderados a amplos (1 a 6,25m). A atividade extrativa em Antas-Matanca e ainda reduzida, enquanto em Esmolfe e intensa e sem planeamento. A proliferacao exagerada de pedreiras, que raramente ultrapassam os niveis superficiais (2 a 3m de profundidade), sem atingir niveis mais promissores, causa um elevado impacto visual. Foram definidas cinco areas potenciais para exploracao de granito ornamental em Antas-Matanca e quatro em Esmolfe. ESMOLFE AND ANTAS – MATANCA GRANITIC MASSIFS (PORTUGAL): GENERAL GEOLOGICAL SURFACE STUDY, CHARACTERISATION AND DEFINITION OF POTENTIAL AREAS FOR ORNAMENTAL GRANITE QUARRYING Extended Abstract The present investigation seeks to study the areas with dimension stone potential in the Antas-Matanca and Esmolfe massifs, in central northern Portugal. There are two massifs: the Esmolfe massif, with an outcrop area of 12 km2 , is circular with a narrow offshoot to the NE and prolongs itself to the SE until the Antas-Matanca massif. This latter massif is elliptic in shape, elongated in an E-W direction, with an outcrop area of approximately 19 km2 . Both granites studied are calc-alkaline, undeformed, blue-grey in colour, medium-grained with a slightly porphyritic tendency. They are regarded as part of the Celorico-Matanca granite that has been dated at 252± 9 M.a. (Pinto et al. 1987). The textural characteristics of the granite massifs are relatively homogeneous although variations do occur. The Antas-Matanca facies shows some colour variations and heterogeneity in grain size. The Esmolfe facies is more homogeneous, coarser-grained, lighter in colour, due to aggregates of feldspar crystals, than the Antas-Matanca facies. Superficial alteration of the Antas-Matanca facies is greater than in the Esmolfe facies. Due to the petrographic and textural characteristics of the granite under study, and small defects in terms of dimension stone production, the main limiting factor in defining favourable areas for dimension stone extraction is the fracture density pattern. Hence, this aspect is highlighted. The paper proceeds to analyse field information and statistically treats fracture density data to understand and define the compartmentalisation of the massifs in the upper levels although limited to the inherent randomness of natural fracture patterns. In the Esmolfe massif the main fracture orientation is N15-30o W (total population) in 16 of the 22 lines considered both as main or secondary fracture sets (Table 1). In this system 50% of the fracture spacing are