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

José Vítor Lisboa, Daniel P. S. de Oliveira

Resumo



O presente trabalho tem por objetivo o estudo de áreas com potencialidade para produção de pedra
ornamental nos maciços graníticos de Esmolfe e Antas-Matança, na região Centro-Norte de Portugal. Os
granitos estudados, tardi-hercínicos e de característica calco-alcalina, têm cor cinzenta azulada, granulação
média e tendência levemente porfirítica. Apresentam acentuada homogeneidade mineralógica, textural e
estrutural e escassos defeitos do ponto de vista ornamental, sendo a principal condicionante à atividade
extrativa a fracturação. Procede-se à análise da informação colhida e tratamento estatístico dos dados de
fracturação, visando ao conhecimento do estado de compartimentação dos maciços nos níveis superficiais,
embora com as reservas inerentes à alietoriedade da rede de fracturação natural. No maciço de Esmolfe, a
textura do granito é mais homogênea, apresentando uma granulação mais grosseira e cor mais clara do que
o granito de Antas-Matança. Os espaçamentos dos sistemas de fracturas em ambos os maciços são
maioritariamente moderados a amplos (1 a 6,25m). A atividade extrativa em Antas-Matança é ainda reduzida,
enquanto em Esmolfe é intensa e sem planeamento. A proliferação exagerada de pedreiras, que raramente
ultrapassam os níveis superficiais (2 a 3m de profundidade), sem atingir níveis mais promissores, causa um
elevado impacto visual. Foram definidas cinco áreas potenciais para exploração de granito ornamental em
Antas-Matança e quatro em Esmolfe.

ESMOLFE AND ANTAS MATANÇA 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-Matança
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-Matança 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-Matança 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-Matança 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-Matança facies. Superficial alteration of the Antas-Matança 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
< 1m although there is considerable dispersion.
The spacing between main fracture sets varies considerably in most cases with no regularity in spacing
patterns.
The total population in the Antas-Matança 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). This implies that
the set 40-50º penalises a few of the areas due to high frequencies of closely spaced fractures (< 0.3m).
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-Matança massifs where, in the latter, reduced spacing predominates. However, if we
compare the average distance frequencies between fractures in the scan-lines, the spacing between fractures in
Esmolfe appears to be greater than in Antas-Matança (Fig. 9B). This is mainly due to the high frequency of reduced
spacing in the Antas-Matança massif that lowers the average distance between fractures. 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. The exaggerated proliferation of quarries that
hardly ever go beyond the superficial levels 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 extracted
resulting in the premature closure of the quarry.
The Esmolfe granite is commercialised under various designations of which Cinza (grey) is the most
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-Matança 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 (Map 1). For the Antas-Matança massif 5 areas are proposed, namely
the outcrops nearby Pisão, Lameira area (station A24), Antas and Abrocedo areas (Map 2).
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.


Palavras-chave


maciço, granito; pedra ornamental; fracturação; área potencial. Key-words: maciço, granito; pedra ornamental; fracturação; área potencial.

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DOI: http://dx.doi.org/10.5380/geo.v52i0.4198

Boletim Paranaense de Geociências. ISSN: 0067-964X
 
 
Programa de Pós-Graduação em Geologia da UFPR