Portugal - Exploration and Mining (2000)

Executive Summary

Portugal is today one of the more privileged and relevant European countries in terms of exploration, exploitation and valuation of mineral resources.

Conceded Areas for Prospecting and Exploration

This privileged context can be explained by various factors notwithstanding an obvious favourable Geo-Mining framework and a long history of metal mining. Of these factors, the new mining law since 1990 as well as the privatisation and social market economy ruling of mining deserve to be highlighted.

As such, Portugal has seen as authentic exploration boom in the 90's with both major and junior companies coming from Canada, North America, Europe and Australia to start the search for gold and base metals. In reality, there occur various auriferous showings and deposits in a variety of metalogenetic scenarios of which the epithermal, structurally - controlled, vein - type mineralizations predominate. However, the spot-light falls on the Iberian Pyrite Belt; producer of massive polymetallic sulphides, that is one of the most important mining provinces and within which occur, on the Portuguese sector, various orebodies such as the worldclass Neves Corvo orebody.

Neves Corvo has become a reference for the role the IGM plays in the valuation and promotion of mineral resources by developing various research tasks in the fields of exploration, evaluation, characterisation and beneficiation. The IGM also carries out the organisation and management of the information generated by its activities as well as from the activities developed by private exploration companies. A further task involves the licensing for private companies, which is unusual for geological surveys, that has proved very important in a country with the characteristics of Portugal.

Finally, general other factors make Portugal more attractive in this sector: an excellent road communication network , the fact that it is a member of the Economic Community; one of the most important world markets , as well as a high technical and linguistic level help by the country's workforce.

Investments in Prospecting and Exploration

A Long Tradition in Mining

The mining of mineral resources in Portugal was initially carried out by the Phoenicians, but was intensely and mainly developed by the Romans.

The first mining operations would have taken place in "gossan" type oxidation zones (for copper, zinc, lead, gold and silver) and gold-bearing placers. The Romans would later intensely exploit gold and polymetallic sulfide vein deposits. To better comprehend the size of the work involved, one need look no further than Três Minas, to the north of Vila Real, from which approximately 10 million tons of material was extracted; and in Aljustrel (southwest of Beja), mine shafts reached a depth of 120 metres.

PRODUCTION OF MINERAL SUBSTANCES AND MINERAL MASSES (TONS)

Then followed a lengthy break in the mining of mineral resources in Portugal which was only occasionally interrupted, the exploitation of alluvial gold in Adiça near Lisbon during the XII century is one example of this.

From the middle of the last century, coinciding with the start of the industrial revolution, mining once more became an important industry in Portugal, with the first mining concessions being granted in 1836.

By the end of the XIX century, approximately 300 concessions had been awarded, the main substances exploited being polymetallic sulfides (Aljustrel, S. Domingos), tungsten and tin (Panasqueira), and antimony and gold (Valongo/Gondomar).

At the start of the XX century, there was a marked increase in coal annual production.

Main Mining Concessions (Situation as of 31/12/1999)

The two world wars, particularly the Second World War, led to a great demand for tungsten, which in turn led to increased exploration and exploitation of not only this element but tin as well, as these two elements are generally related spatially and/or structurally. Production of the concentrates of these substances peaked during 1942 at 5700 tons for tungsten and 4400 tons for tin, the main producing mines being Panasqueira, Borralha, Argozelo, Montesinho, Vale das Gatas and Ribeira.

The country also possesses important uranium deposits, with approximately 4200 tons of U₃0₈ produced between 1950 and 1990.

From the start of the fifties but mainly in the eighties, exploration underwent somewhat of a boom in Portugal at first including tungsten and tin in the centre and north of the country and later concentrating to precious metals and base metals, the latter sought principally in the Pyrite Belt in the south, where a remarkable number of mineral masses have been discovered: Moínho (1955), Feitais (1963), Estação (1968), Gavião (1970), Salgadinho (1974), Neves Corvo (1977) and Lagoa Salgada (1992).

The most important of these discoveries, in which the IGM (Geological and Mining Institute) has always played an important role, was the Neves Corvo deposit: since the mine came into operation, Portugal has become the largest producer of copper (1988) and tin (1990) in Europe as well as tungsten from the last operational mine (Panasqueira). Portugal is also an important producer of ornamental rocks, particularly marble, even at the international level.

Neves Corvo Mining Complex

Portugal's Geology

Iberian Geotectonic Units

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, as described below:

• 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, litium uranium and, probably, polymetallic sulfides in the surrounding formations. This unit is also very important, concerning ornamental granites, like in Viana do Castelo, Braga, Porto and Vila Real areas and also some talc deposits occur at the Morais and Bragança massifs.

• 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, which are related with late tectonic and metallogenetic phenomena that have affected the post-tectonic "calcalkali" granite.

Important feldspar occurences are mostly associated to granitic massives, like in Viseu (where ornamental granites are very relevant) and Guarda areas, not only here but also in Galicia - Trás-os-Montes (Chaves, Braga) and in Ossa-Morena (Évora) Zones.

• 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 (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 Évora 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, specially in the Estremoz-Borba-Vila Viçosa anticline , as well as in granites, like in the Portalegre district.

• 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 sulfides 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.

A final word for the Epi-Hercynian Covering, where non-metallic resources, namely limestones in Maciço Calcário Estremenho and in Algarve, kaolin in Viana do Castelo, Braga, Porto, Aveiro, Coimbra and Santarém regions, clays in Aveiro, Leiria, Lisboa, Santarém and Faro areas, sands in Coimbra, Leiria, Lisboa, Santarém and Setúbal districts, gypsum and salt in the centre and in the south and diatomites in Rio Maior, are very important.

Geological Map of Portugal

COMO CITAR ESTA PUBLICAÇÃO (HOW TO CITE THIS PUBLICATION):
Instituto Geológico e Mineiro (2000). Portugal - Exploration and Mining. Lisbon.
Versão Online no site do INETI: http://e-Geo.ineti.pt/geociencias/edicoes_online/diversos/portugal_geology/default.htm