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Granitos de Esmolfe e Antas-Matança (Portugal): petrografia e influência da fracturação no estabelecimento de áreas potenciais para exploração de granito ornamental

Year: 2005
Type: article
Abstract: The present investigation seeks to study the areas with ornamental/dimension stone potential in the Esmolfe and Antas-Matanca granitic massif, in central northern Portugal. Both granites studied are calc-alkaline, undeformed, blue-grey in colour, medium-grained with a slightly porphyritic tendency. 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, and 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. In the Esmolfe massif, the main fracture orientation is N15-30o W (total population). In this system, 50% of the fracture spacing is < 1m although there is considerable dispersion. The total fracture population in the Antas-Matanca massif shows there to be two main orientation directions 40-50o and 150-160o that are sub vertical in nature. In both sets spacing is < 1m (65% of total). Fracture spacing in both massifs are considered moderate (1-2.5m) to ample (2.5-6.25m). Comparison of the class frequencies of distance between consecutive fractures shows that the distribution of intervals is similar in both the Esmolfe and Antas-Matanca massifs. However, if we compare the average distance frequencies between fractures in the scanlines, the spacing between fractures in Esmolfe appears to be greater than in Antas-Matanca. In Esmolfe, one third of the lines studied, the average spacing is < 2m. These factors, in conjunction with the textural homogeneity of the rock, mineralogical characteristics and other factors, such as favourable topographic conditions, easy access and large reserves demonstrate the high ornamental potential of these granites. The quarrying activity in Esmolfe is intense and chaotic which causes a high negative visual impact. The lack of technical management in the majority of quarries implies that rarely the more promising levels are ever 1 1 e Daniel P. S. de Oliveira Jose Vitor Lisboa Cadernos Lab. Xeoloxico de Laxe Coruna. 2005. Vol. 30, pp. 11-38 extracted resulting in the premature closure of the quarry. The Esmolfe granite is commercialised under various designations of which Cinza (grey) is the more common. It outcrops generally in large boulders that due to their dimension alone allow the installation of the quarries. Alteration profile is generally < 0.2m. The extraction activity of the Antas-Matanca massif is presently restricted to three quarries, being the granite commercialised under the designation Cinza (grey) or Cinzento Antas (Antas grey). The existence of extensive slabs of this facies with a reduced alteration profile creates very favourable conditions for dimension stone extraction. This study shows that the Esmolfe massif has potential for extraction in four areas located SW of Pedras Altas and S of Ponte do Ferreira. For the Antas-Matanca massif 5 areas are proposed, namely the outcrops nearby Pisao, Lameira area (station A24), Antas and Abrocedo areas. The importance of this study is clearly shown not only for territorial land planning uses, in the selection and hierarchisation of potential areas but also to better plan exploration methodologies (e.g. use of georadar or percussion drilling), if the raw material warrants it.
Language: Portuguese
Open Access status: green
OpenAlex ID: W69875174
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Cited by: 6
Related to: 10