Mineral Potential of Portugal (1998)

4. Precious Metals

As mentioned earlier, it has always been known that the Iberian Peninsula is rich in precious metals, and these are to be found the length and breadth of Portugal. We shall begin with a description of the various occurrences and deposits, citing the most recent exploration results obtained either by the IGM itself or private companies. We shall also briefly describe the areas that show potential for these metals so as to provide a short to medium term forecast for this mining sector in Portugal.

A description of the most important deposits follows, then, grouped by location within the geotectonic units referred to above (Fig. 4):

Galicia - Trás-os-Montes Zone

• JARIÇA/EDROSA - Auriferous mineralisation occurs associated with sulphides (arsenopyrite, pyrite, sphalerite, galena and chalcopyrite) and embedded in a Silurian volcanic sedimentary complex, clearly controlled by the Hercynian orogeny (Knopf et al., 1990), which causes an alignment of the mineralised structures (quartz and porphyry veins) in a NNW-SSE direction. The company Prominas has been exploring this area since 1986.

• POÇO DAS FREITAS - The mines in this area date back to Roman times and have left behind a series of small open pits, the largest of which is roughly 100 metres long and 80 metres in width. Mineralisation, which is associated with stockworks of quartz veins, occurs embedded in granitic rocks, running parallel to the large Régua/Verin fault and corresponding to a possible shear zone.

Cogema prospected this zone between 1986 and 1990, and carried out an economic viability study on the north structure of the old Limarinho works. This points to the existence of possible reserves of 2.07 tons of gold with an average grade of 2.8 g/t.

An exploration license for this area was recently awarded to the Australian company Northern Territory Gold Mining N.L..

• TRÊS MINAS - This was the largest Roman mine in Portugal, and consists of three open pits running WNW-ESE, two of which are of an impressive size (Ribeirinha and Covas), the largest approximately 500m long, 100m wide and 80m to 100m deep. There are also numerous shafts and galleries (the largest of which is 250 metres in length with a 5 x 1.5m cross-section). It is estimated that 10 millions tons of material have been mined here.

As in the NW of Spain, the Romans employed the "ruina montium" mining technique, which was usual practice for low grade, high tonnage deposits. Gold occurs associated with arsenopyrite and pyrite, in quartzite lenses interstratified with Silurian shales, and appears to be of syngenetic (paleoplacers) and epigenetic (tectonic control) origin. This area was studied by the Portuguese company SPE between 1984 and 1988, and an exploration license is currently awarded to a consortium formed by the Societé Minière du Bourneix, a French company belonging to the Cogema group and the EDM, a state-owned Portuguese company. The area also includes the Jales zone, which we describe below.

• JALES - The first mining operations carried out in this zone also date back to Roman times, and has become this century the largest gold mine in Portugal. Hydrothermal gold-bearing quartz veins occur here in two main directions (NE-SW and WSW-ENE), coincident with sub-vertical fractures embedded in Hercynian granites and schists, greywackes and quartzites from the Schist-Greywacke Complex, and mainly from the Silurian age. These lodes are sometimes as long as 2.5 kms and although their thickness varies, they never exceed 1 metre width. The two main lodes, which run in a NE-SW direction, are known as Campo and Desvio. Gold and electrum occur associated with quartz and sulphides (arsenopyrite, most common, pyrite, pyrrhotite, chalcopyrite, sphalerite, tetrahedrite, galena). Grade is fairly irregular, although figures of 30-40 g/t Au were frequently found. Since 1933, the mine has produced approximately 25 tons of gold and 100 tons of silver; at the time of its closure in 1992, it had reached a depth of 620 m. A consortium formed by Minas de Jales and BP Minerals, which later transferred its position to RTZ, prospected the area between 1984 and 1991; as mentioned earlier, it is currently awarded to Soc. Min. Bourneix/EDM. The latter recently determined that the Campo lode continues approximately 600 m south of the old mining works, in a structure known as the Horta vein, although mineral grade is highly variable. In the Gralheira structure - a shear zone with several veins, running in a WSW-ENE direction, embedded exclusively within metasedimentary Silurian rocks and located NE of the Campo and Desvio lodes - the consortium has also been able to determine the existence of a mineralised zone that extends for at least 1850 metres and is roughly 15 metres thick. A pre-viability study is currently being carried out on this structure; possible future exploitation might also allow complementary exploitation of the Campo zone.

• LATADAS/FREIXEDA - Mineralisation occurs in quartz veins embedded in the Trás-os-Montes volcanic-siliceous and quartz-phyllite Silurian complexes, which are occasionally intruded by alkali granites. The veins that run predominantly in a NE-SW and E-W direction, in a shear zone, were also worked by the Romans. The gold is mostly associated with W, As, Cu, Pb, Zn and Sb polymetallic sulphides. The firms Cogema and Rena prospected this zone between 1986 and 1992; the IGM is currently carrying out a mining evaluation project here.

• VILA VERDE/PONTE DA BARCA - Prospecting carried out by the IGM at the end of the eighties uncovered a NE-SW alignment that had gold-bearing potential, the following areas being the most important:

Marrancos - mineralisation is related with a shear zone which affects the Silurian metasediments, these transformed into hornfels by contact metamorphism. In the zone affected by this shearing, breccias and silicifications are to be found accompanied by sulphide mineralisations (arsenopyrite and pyrite) with gold.

Godinhaços - mineralised structures (gold + arsenopyrite) are located exclusively in the granite of Vila Verde. There appears to be lithological control of the mineralisation, the veins running predominantly in an NE-SW direction.

Grovelas - characterised by the occurrence of a dense network of joints filled with arsenopyrite and running predominantly N 25º - 40º E and N 50º - 70º E. There appear to be two mineralised axes, the intersection of which could prove to be an enrichment zone. Also worthy of mention is the existence of old Roman mining works.

A concession for exploration in this area was recently applied for by the company RTZ.

Central - Iberian Zone

• VALONG0/GONDOMAR - This gold-antimony belt is situated in Baixo Douro region, northeast of Oporto and stretches from Esposende to Castro Daire in a distance of 90km. Besides Sb-Au occurences exist also Au-As, Pb-Zn (Ag) and Sn-W mineralisations, which are located in the large Valongo anticline and in the Carboniferous syncline, embedded in metasedimentary formations dated from Precambrian and/or Cambrian to Carboniferous and more rarelly in hercynian granites.

In remote times, initially the Pheonicians, later and chiefly the Romans, exploited gold in this mining district. At the end of the XIX century, this belt was extensively mined for antimony; and, as a sub-product, gold contained in quartz veins. We can remark the production from the ancient mines of Ribeiro da lgreja (Sb-Au), Montalto (Sb-Au) and Banjas (Au-As) not forgetting the silver Terramonte mine that during the sixties was one of the most important in Europe. Lead and zinc were also mined here.

Studies undertaken by EDM and BRGM, from 1988 to 1993, indicated the occurrence of gold-antimony mineralisations of stockwork type in Alto do Sobrido and of saddle reefs type in Banjas. From recent works carried out by IGM resulted the important discovery that gold is not restricted to the quartz veins but pervades, within shear zones, the pores and fractures of their host rock; in consequence of this discovery quartz stockworks and silicified mineralised bodies with gold and antimony were found in Alto do Sobrido and these orebodies illustrate the excellent mineral potencial of the area.

A consortium between EDM and Connary Minerals, applied recently for an exploration licence, including Alto do Sobrido and Banjas areas.

• CASTROMIL - This area, which was first mined in Roman times, is located within a Hercynian granite near its contact with Silurian metasediments, part of the Eastern flank of the Valongo anticline. The latter is characterised by the occurrence of various gold/antimony deposits, as mentioned earlier. Two important orthogonal fault systems occur in this zone: NW-SE and NE-SW. The latter, which is more recent, is of great metallogenetic importance and appears to represent the hydrothermal conduits. Gold is essentially disseminated along veins in the silicified granite, running NW-SE, related with a shear zone and frequently associated with sulphides (arsenopyrite and principally pyrite). Besides the silicification, other alteration phenomena, such as sericitisation and kaolinisation, also play an important role. In paragenetic terms, three stages of mineralisation are considered: ferro-arseniferous (quartz + arsenopyrite I + pyrite I + pyrrhotite + bismuth ?), zinciferous (sphalerite + chalcopyrite), and remobilisation (arsenopyrite II + galena + gold).

Mining operations will begin in 1998 by the Irish firm Connary Minerals, PLC in two fields: Covas de Castromil and Serra da Quinta, separated by the Sousa river and the Oporto-Pocinho railway. For the first of these fields, proved reserves of 2,147,000 tons have been determined with an average grade of 1.9 g/t Au (cut-off of 0.5 g/t) and probable reserves of 270,000 tons with 1.8 g/t Au. For Serra da Quinta, probable reserves are put at 743,000 tons with 2.8 g/t Au.

This area was previously prospected by Billiton between 1988 and 1992. In 1994, the IGM carried out a re-evaluation study of the deposit, which proved decisive for the future exploitation.

• PENEDONO - In this area, also worked by the Romans, various gold-bearing quartz veins occur. These are sub-vertical and run predominantly in a NE-SW direction. Gold is chiefly to be found in the arsenopyrite and does not appear to be restricted to lode-beaning structures: it also occurs in the wall rock, normally heavily sericitised Hercynian granite near the contact with the Cambrian metasediments of the Schist-Greywacke Complex. Around 50 years ago, this area produced gold (Stº. António mines) with an average grade of 14 g/t. Prospecting studies are currently being carried out on it by the firm Sociedade Mineira de Moimenta, which has concentrated mainly on the Docotim (Paredes da Beira) and Stº. Ant6nio (Penedono) structures.

• CARAMULO - Another area worked by the Romans, this was recently studied by the IGM. Gold occurs associated with pyrite and arsenopyrite in quartz lenses. There is b tectonic control, suggesting the existence of a NNW-SSE shear zone, parallel and related to the Oporto-Tomar thrust system with a maximum width of 500 metres and a length of several kilometres. Taking both this and the geochemical mapping of gold obtained from soil samples into account, it is likely that mineralisation also occurs in the shist -greywacke wall rocks. Exploration rights for this area were recently applied for by RTZ.

• ESCÁDIA GRANDE - At this old mine, that was also worked by the Romans, there was gold production during the forties. Mineralisation occurs associated with quartz vein structures running NW-SE, dipping SW, probably related with the Oporto-Tomar thrust system, and consists of polymetallic sulphides (pyrite, arsenopyrite, sphalerite, galena and chalcopyrite) in which the gold is finely disseminated. The wall rock belongs to the Schist-Greywacke Complex and is frequently altered by silicification and carbonisation phenomena, with which the mineralisation is associaled.

• PORTALEGRE - This area is located within the Tomar-Badajoz Blastornylonitic Belt. Gold occurs predominantly associated with silicified, chloritised and carbonated metavolcanic sedimentary levels of the Precambrian Série Negra in the Mosteiros and S. Martinho (Alter do Chão) sectors. In the latter, the existence of Plio-Pleistocene covering deposits (gravel) bearing secondary gold mineralisation is also worthy of mention. These facts were uncovered by exploration studies undertaken by the IGM between 1992 and 1994. Exploration rights for the area were awarded to the firm Portuglobal in 1995, which has been studying it since this time. Drilling recently executed by the company in the S. Martinho sector have provided extremely positive results (2m with 49 g/t Au, 4m with 12.7 g/t Au and 33.43m with 1.2 g/t Au).

The gold is usually accompanied by pyrite, pyrrhotite and arsenopyrite.

• MONTEMOR-O-NOVO - Gold-mineralisations are concentrated in tabular zones, normally characterised by heavy silicification and embedded in a Precambrian volcanic sedimentary sequence along a belt over 30 kilometres in length. This area, initially studied by the IGM, has been investigated by several private companies (RTZ, BP, Portuglobal) since 1984. Research undertaken has determined the existence of various deposits with marked lithological and tectonic control, the most important of which are located in the SE sector of the area: Banhos, Casas Novas, Chaminé, Ligeiro, Caras, Covas and Braços. Some free gold occurs, but normaly it is associated with grains of arsenopyrite and loellingite.

In 1991, RTZ presented - with a cut-off of 1 g/t Au - measured and indicated reserves in respect of Chaminé, Braços 2 and Casas Novas and inferred in respect of Ligeiro, Caras, Covas, Braços 1 and Banhos, a total of 2.17 million tons with 2.7 g/t Au.

In 1996, based on new works and economic re-evaluation, undertaken by the London subsidiary of the firm A.C.A. Howe International, Portuglobal recorded an increase in gold reserves using a cut-off of 0.5 g/t Au. The company has then presented a global reserves value (measured + indicated + inferred) of 4.9 million tons with an average grade of 1.8 g/t An, and continues to explore the area.

South - Portuguese Zone

• CAVEIRA, ALJUSTREL and S. DOMINGOS - Gold has been mined here since Roman times in gossan zones of polymetallic sulphide deposits, embedded in a volcanic sedimentary complex dating from the Late Devonian - Early Carboniferous period.

Prospecting is currently underway in Aljustrel (EDM/Portuglobal) and S. Domingos (RTZ), mainly concentrating on base metals (and as such shall be dealt with in greater detail in the respective chapter) but without overlooking the potential that may still exist in these areas for precious metals. With similar aims in mind, the Canadian firm Crockite Mining recently applied for exploration rights for the zone of Caveira/Lousal.

Secondary Deposits

Despite the fact that they have been intensely exploited in Roman times, they are not properly studied.

The most important are the deposits of Alva (Arganil), Arouce (Ceira River), Monfortinho (Erges River), Penamacor (Basdgueda River.), Milreu and Martinchel (Zézere River), Rosmaninhal and Constãncia-Abrantes (Tagus River). The volume of sand and gravel extracted from these sites has been calculated at several million cubic metres. Some of them also contain channels that have been hewn in the bedrock to help concentrate the gold.

Based on the above considerations and the metallogenetic characteristics of the country, we can define the following areas as being potentially rich in precious metals (Fig. 5): Vila Verde/Germil(2), Valongo/Gondomar(6), the Blastornylonitic Belt (1), the Alentejo GoldArsenic -Antimony Belt(13) (Au,Ag), Caminha/Braga(l), Gerês/Mogadouro(3), ErvedosalArgozelo (4), the Douro Scheelitic Belt (5), the Paiva Tungstiniferous Belt(7) , Arouca/S. Pedro do Sul(8), Trancoso/Figueira de Castelo Rodrigo(9), Goís-Segura(10) (W, Sn, An), Sousel/Barrancos(12), Magnetitic-Zinciferous Belt(14) and Porphyries of Beja(15) (Zn, Cu, Pb, Ag, An).

We feel that in the future special attention should be given to three metallogenetic scenarios, none of which have been thoroughly investigated in Portugal(Veigas et al., 1992):

a) The mineral deposits occurring in the Blastornylonitic Belt(11) or its dependent areas, characterised by b tectonic control, frequently in shear zones as equally happens in the Galicia-Trás-os-Montes and Central-Iberian Zones, related with the Régua Verin and Vilariça faults.

b) Gold-bearing mineral deposits associated with granitic intrusions and occurring within it or in the contact with the wall rock (especially skarns) and as a result potentially existing in belts also favourable to W and Sn mineralisations, probably in geochemical spatial relationship with these elements (1, 3, 4, 5, 7,8,9, 10).

Potential Areas for Precious Metals

c) Mineralisations occurring in the surrounding formations (Silurian volcanic sedimentary complex) of the Morais and Bragança massifs, characterised by their association with polymetallic sulfides and/or shear zones.

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