justificação do valor científico
The Borralha deposit is located in the GTMZ of Iberian Massif. Country rock underlying the Borralha district consist of folded metasedimentary sequences, regionally metamorphosed to medium grade (andalusite-biotite) during the Hercynian orogeny. The metasedimentary sequences are mostly from Ordovician-Silurian including important volcano-sedimentary complex and organic rich terms (Noronha; Ribeiro, 1983).
Plutonic magmatism is essentially represented by Hercynian synorogenic granites.
Based upon their petrographic, geochemical and geologic characteristics the granites can be schematically divided into two main groups: the two-mica granites and the biotite granites. The first group of granites could be considered a syn-metamorphic peak, syn-collisional granites, which would be approximately syntectonic (Serra da Cabreira granite). The second group of granites is essentially post-metamorphic peak and post-collisional granites and post-tectonic (Serra do Gerês granite).
The deposit consists of mineralised quartz veins associated with two breccia pipes.
The mineralised structures are in the contact zone of the metasedimentary sequences and syntectonic granites. The veins of lenticular shape are synchronous and present a N70 to N120 trend with two different dips, one higher than 45º and another less than 30º (Noronha, 1974, 1983, 1984, 1988).
The breccia pipes with angular elements are cemented by quartz. They resulted from collapse phenomena slightly later than the vein formation.
Quartz, wolframite, scheelite and sulphides, chalcopyrite, pyrite, sphalerite, molybdenite and bismuthinite characterize the mineralogical assemblage. Two main mineralising episodes can be recognized:
- The first with the tungsten deposition,
- The latter with the sulphides deposition, initially with their deposition and a later hydrothermal alteration of pyrrotite into pyrite and marcassite and the deposition of native bismuth, galena and sulphosalts (Cu, Bi and Pb, Pb, Bi and Ag, Ag and Bi, Cu and Bi).
Quartz is the dominant mineral. Muscovite is the early mineral and followed by tungstates. A lamellar chlorite is synchronous of the initial sulphide deposition with a followed ripidolite. Both chlorites are associated with K-feldspar.
Wolframite, sheelite and chalcopyrite were the exploited species.
During its life the Borralha mine was the second tungsten mine in Portugal, after Panasqueira.
Since 1904, the first year of operation, to 1983 the global production of tungsten concentrates (82% of wolframite and 18% of scheelite) was around 19 500 t.
From 1975-1980, the average of production per year was 302.805 t. In the same period the global production of chalcopyrite concentrates was 1711.65 t. These concentrates had some silver content corresponding to 0.3%.
Noronha, F. (1974): Étude des inclusions fluides dans les quartz des filons du gisement de tungsténe de Borralha (Nord du Portugal). Publicações do Museu e Laboratório Mineralógico e Geológico da Faculdade de Ciências do Porto. 4ªSér.,45pp.
Noronha, F., 1983. Estudo metalogénico da área tungstífera da Borralha. Tese Universidade do Porto, 413 pp.
Noronha, F. (1984): Caractéristiques physico-chimiques des fluides associés a la genèse du gisement de tungstène de Borralha - Nord du Portugal. Bull.Minéral.,107, 273-284.
Noronha, F., 1988. Mineralizações. Geonovas,10: 37-54.
Noronha, F. & Ribeiro, M.L., (1983). Notícia explicativa da folha 6A – Montalegre. Serviços Geológicos de Portugal.
Outros valores e sua justificação
Traditional mining village well preserved with beautiful mountain panorama. Many operating equipments and also an old treatment plant near river Zêzere (Lavaria do Rio).
Minerals shop in the office of the Mining company.