Mineral Potential of Portugal (1998)

2. Geological and Mining Background

From a geological viewpoint, Portugal is a considerably diverse and complex country, these characteristics providing it with important potential in various mineral resources.

The territory can be subdivided, in geological terms, into 2 large groups: the Hesperian Massif and the Epi-Hercynian Covering, the latter including the western and southern Meso-Cenozoic borders, and also the basins of the Tagus and Sado rivers.

The Hesperian Massif, in which predominantly metallic mineral resources occur, can in turn be divided into various geotectonic units (Fig. 1), as described below (Ribeiro et al., 1979; Quesada, 1992):

• Galicia - Trás-os-Montes Zone - This is characterised mainly by the existence of two mafic and ultramafic polymetamorphic massifs known as Bragança and Morais. The surrounding formations date chiefly from the Silurian period and are characterised by the existence of acid and basic volcanic rock, which make contact with the massifs via larger thrust systems. Binary granite, "alkali" and porphyritic granite, biotite and "calcalkali" granite also occur. The chrome, platinum and, possibly, copper, nickel and cobalt potential of the Morais and Bragança massifs is worth stressing, as is the potential for tungsten, tin, precious metals, uranium and, probably, polymetallic sulphides in the surrounding formations (Goínhas et al., 1992).

• Central Iberian Zone - This is mainly characterised by the predominance of the formations of the so-called Schist-Greywacke Complex, consisting of a flysch-type series dating from the Cambrian and Late Precambrian period. There are also large areas of "alkali" and "calcalkali" granitoids, in which various types of granite can be distinguished. Worth highlighting is the occurrence, in the Douro-Beiras sector, of continental formations from the Carboniferous period, where various coal mines have been exploited (Douro Carboniferous Belt).

  Also worth mentioning in this geotectonic unit are important mineralisations of tungsten and tin, normally associated with the contact between the granite and the metasediments, and the existence of precious metals, frequently associated with arsenic and antimony, as in the Valongo/Gondomar Gold-Antimony Belt, for example. There are also important mineralisations of uranium, many of which have been exploited, related with late tectonic and metallogenetic phenomena that have affected the post-tectonic "calcalkali" granite.

• Ossa - Morena Zone - This is an extremely complex and diverse unit which begins with a polymetamorphic Precambrian, followed by Cambrian and Silurian formations, and ends with a flysch sequence from the Late Devonian period.

Contact with the Central Iberian Zone is carried out via an important shear zone which stretches from Oporto to Cordoba in Spain (Blastomylonitic Belt).

With regard to magmatism, the NE sector has a predominance of granitic rocks, chiefly "calcalkali", porphyritic, biotitic, similar to those in the north and centre.

To the south, the basic character of the intrusions gradually increases, with "calcalkali" becoming predominant: gabbros, diorites, serpentinites and anorthosites (which form the recently named Beja ophiolite complex), various different porphyries, and later intrusions comprising gabbrodiorites, granodiorites, tonalites and granites (which constitute what is known as the Evora Massif).

The most important mineral occurrences are base metals, which are associated with the Cambrian-Ordovician volcanic sedimentary complex, precious metals, chiefly related with the Precambrian formations, and tungsten and tin in the Sta. Eulália granitic complex, not to mention the potential for chrome, nickel, cobalt and platinum in the basic and ultrabasic rocks on the north and south margins of this zone.

With regard to non-metallic minerals, this geotectonic unit is rich in ornamental rocks, particularly marble.

• South Portuguese Zone - The Ossa - Morena Zone and this unit are joined by the Ferreira - Ficalho thrust (partially over the Beja-Acebuches complex), which runs approximately E-W to the east and NW-SE to the west.

The South - Portuguese Zone is characterised by the existence of a volcanic sedimentary complex (VS) from the Late Devonian - Early Carboniferous period, overlain by a Culm flysch sequence; underlying this complex is the so-called "Phyllite-Quartzite Group". The oldest formations in this zone date from the Early Devonian period and belong to the "Pulo de Lobo" Formation, which includes phyllites, quartzites and rare acid and basic volcanic rocks. The acid volcanic rocks in the volcanic sedimentary complex constitutes the metallotect of the massive polymetallic sulphides that are characteristic of the Iberian Pyrite Belt, the most important metallogenetic province in Portugal in which the Lousal, Aljustrel, Neves Corvo and S. Domingos mines are located.

We shall not go into the geological aspects of the Epi-Hercynian Covering, but shall touch on its non-metallic resources, namely sands, gypsum, clay, kaolin, limestone, diatomite and salt, further on.

COMO CITAR ESTA PUBLICAÇÃO (HOW TO CITE THIS PUBLICATION):
Instituto Geológico e Mineiro (1998). Mineral Potential of Portugal.
Versão Online no site do INETI: http://e-Geo.ineti.pt/geociencias/edicoes_online/diversos/potential/indice.htm