The existence of a tungsten and tin metallogenetic province on the Peninsula, which in Portugal overlays the Galicia - Trαs-os-Montes and Central - Iberian Zones, has long been known. In this province, gold and silver occur with a certain zonality at regional scale in relation to tungsten, hence the fact that the potential areas in these two groups of substances sometimes coincide.
We shall not cover the tin contained in the polymetallic sulphide deposits of the Pyrite Belt in this chapter as these occur in a completely different metallogenetic context.
The primary deposits or occurrences of W-Sn (Fig. 10) are for the most part either directly or indirectly linked with granites, with differing typology: aplite-pegmatites (Lagares de Estanho - Queiriga), intra and extra-batholithic stockworks (Bejanca-Vouzela and Fonte Santa - F. Espada ΰ Cinta) and vein type. The latter are more common and account for most production of wolframite in the country( Goνnhas, 1987).
They are generally found in contact metamorphic aureoles, sometimes a great distance from the granite outcrops, in which case it is assumed their relationship with fairly deep, non-outcropping granitic domes (Panasqueira, Argemela, Gσis, Borralha, Vale das Gatas, Ribeira, Argozelo, etc.).
Besides these examples, there is also the special case of the skams with scheelite, formed by contact metasomatism, which - in accordance with the wall rock structure -some geologists divide into two types: stratoid (Cravezes-Mogadouro, St Leocαdia-Armamar, S. Pedro da guias-Tabuaηo, etc.) and vein type (most of the skams in the Castelo Melhor-Escalhγo region, near to the River Αgueda and the Spanish border, however the most important ones in the Escalhγo - Barca d'Alva sector are from the stratoid type).
The most promising areas for the occurrence of large deposits correspond to various types of geostructural settings (Fig. 11):
a) Areas covered by monotonous schist series where the existence of non-outeropping granitic dome is presumed. An example of this is the Schist-Greywacke Complex situated to the south of the Estrela Mountain, and in particular the belt running E-W between Gσis and the Spanish border, which covers the Gσis, Panasqueira and Argemela mines. In this area, the main model is probably the Panasqueira deposit: mineralised sub-horizontal veins related with a non-outcropping, partially greisenized, leucogranitic dome.
b) Structural alignments of regional dimension containing various types of Sn-W deposits and small outcropping or sub-outcropping dome of differentiated granitoids. An example of this is the extensive WNW-ESE alignment that runs from near Ervedosa, through the Tuela, Agrochγo and Murησs mines, and then, after being rejected by the Vilariga fault, continues eastwards towards the Ribeira and Argozelo mines in Trαs-os-Montes area. The standard deposit type has not yet been fully defined but could be similar to that of Tuela, with vast stockwork zones with a high density of mineralised veins and remobilisation along strike-slip faults running subparallel to the regional structural alignment.
c) Granitic areas containing multiple intrusions differing in terms of mineralogy, metallogeny and chronology. In some cases, specific parageneses for a certain type of intrusion can be defined, distributed according to specific schemes of zonation, generally accompanied by hydrothermal alteration phenomena.
d) Circumscribed granitic massifs, with differentiated composition from regional granites and its metamorphic aureoles. Outcrops are small in size, circular in shape or slightly elongated along one of the axes and may resemble outcropping domes, the possible metallogenetic models fitting any of the types described in a) and b). The circumscribed massifs in the Arouca region, running in a NW-SE direction, are - in this context - one of the areas with greatest potential for the occurrence of intra- or extra-batholithic stockwork deposits.
A brief mention to the secondary alluvial-type tin and tungsten deposits, of reduced economic interest given their small size. The most important of these, however, are Nave de Haver (Sabugal), Gaia (Belmonte) and, to a lesser degree, Bejanca (Vouzela), Massueime (Pinhel) and St Eulαlia (Elvas), and Vale do Tβmega (Ribeira da Pena) alluvium, amongst others.
In relation to the skarns with scheelite, special mention should be made of the potential of the so-called Douro Scheelite Belt, which includes the whole of the area where the formations of the Schist-Greywacke Complex and the sintectonic granites make contact. This is situated to the north and south of the Douro river between the meridians of Freixo de Espada ΰ Cinta and Rιgua. Besides numerous occurrences, several interesting deposits have also been recognised: Cravezes (Mogadouro), St Leocαdia (Armamar) and S. Pedro da Αguias (Tabuaηo).
Another zone or belt of interest from an exploration point of view located in a similar geological context is the so-called Trancoso-Figueira de Castelo Rodrigo Belt.
In the Serra de Arga zone (Minho district) - besides the existence of tin-niobium-tantalum mineralisation- small deposits of scheelite stratoid type occur in skarns, interstratified in Silurian formations and in relation with the post-tectonic granites of Vila Nova de Cerveira and Covas: Cerdeirinha, Covas, Lapa Grande, Argela and Cabraηγo.
We have already referred to the production of tungsten and tin concentrates, which reached maximum levels during the Second World War and the main producing centres. Of these, only the Panasqueira mine remains in operation (we continue to exclude the special case of Sn at Neves Corvo), which in 1996 produced 1342 tons of wolframite concentrates with a W03 grade of 72.96%, 15 tons of cassiterite concentrates with an Sn grade of 70.77% and 550 tons of chalchopyrite concentrates with a Cu grade of 28.50%.
Future production should stabilise around traditional figures: 2100 tons of wolframite, 45 tons of cassiterite and 600 tons of chalcopyrite concentrates.
The mineralised zone consists of series of sub-horizontal quartz veins, which overlap and fill the joints of fractures occurring in schist rock. These veins vary in thickness from 1 to 150 centimetres, the average thickness of the veins currently being mined around the 30-40 cm mark.
Besides the main minerals being mined - wolframite, cassiterite and chalcopyrite - various other minerals occur, such as sphalerite, topaz, apatite, fluorite, triplite, marcasite, siderite, arsenopyrite and muscovite, not to mention many others. The mine is in fact, famous for the occurrence of valuable collection samples.
The known uranium deposits in Portugal are situated in the Central Iberian Zone (Fig. 12). They are located within Hercynian granitic batholiths or in the metasediments of exocontacts and occupy a structurally high position, which can be deduced from the presence of numerous pegmatites and metasedimentary enclaves (Dias et al., 1970).
The granites, occurring in the form of circumscribed massifs, belong to a calcalkaline series, are generally post-tectonic and are heavily fractured, particularly the NNE-SSW to ENE-WSW and NNW-SSE to NW-SE systems, pointing to a N-S maximum compression direction (Goνnhas, 1987).
In the Beiras region, the largest and most productive area for uranium, the granite is intersected by numerous dykes of basic rock.
In this region, the intragranitic uranium bearing veins can be any of the following types:
In the equally productive Alto Alentejo region, intragranitic uranium deposits are of the quartz veins and granitic breccia types.
Jasper type deposits have a characteristic paragenetic association: quartz, hematite, sphalerite, pitchblende, pyrite, galena, ankerite, chalcopyrite and coffinite. The pitchblende dates from the late Hercynian.
Mineral occurrences of all the other types essentially consist of hexavalent uranium minerals (secondary uranium minerals).
Support for uranium mineralisation in peribatholithic deposits, which also occur in the Beiras and Alto Alentejo regions, is provided by the formations of the Schist-Greywacke Complex.
These deposits are also almost totally composed of secondary uranium minerals, with pitchblende fairly rare.
Although this type of uranium dissemination deposit in metamorphic schists that contact with granite represents a very important percentage of total known uranium reserves, its genesis is controversial. Deposits such as Nisa (Alto Alentejo), Azere (Beira Alta) and Horta da Vilariga (Trαs-os-Montes) are important examples of this type of mineralisation.
Occurrences related with concentrations of soil-leached uranium have also been detected in the Western Meso-Cenozoic Margin.
Geologically favourable zones for prospecting uranium in Portugal are described below. These are organised by their geological settings and in decreasing order of their potential (Fig. 13):
In the portuguese uranium metallogenetic province demonstrated reserves in the known deposits in the Beira and Alto Alentejo regions are around 8200 tons of uranium metal.
No mines are currently operational, although environmental processing of water originating from the Quinta do Bispo and Bica deposits produces around 20 tons Of U308 concentrates per year.
Portugal occupies an important position in terms of lithium production. This is mainly due to the exploitation of aplite-pegmatite veins, rich in lepidolite, embedded in a late Hercynian granite, porphyritic, monzonitic, in the region of Gongalo (Guarda).
In this zone, mining at Alvarrυes produced a total of 3155 tons of ore in 1996. After processing, this provided 892 tons of milled ore with a content of 1.05% of Li20 and 1527 tons of crushed ore with a content of 0.65% of Li20.
Lithium-bearing lodes are characterised by a geochemical association of A1-Na-Ca-PRb-Li-B-Sr and occupy a system of sub-horizontal fractures running predominantly NS, with cassiterite, tantalite and beryl also occurring.
Other potential lithium-bearing areas exist: the region between the Barroso and Alvγo mountain ranges, for example, in which the IGM recently carried out exploration. Mineralisation deposits are associated with aplite-pegmatite veins, rich in spodumene, embedded in Silurian metasediments composed of micaschists and quartziferous schists with intercalations of black schists (Fig. 14). Regional exploration, both geological and geochemical, led to the selection of three structures, Afijσ, Veral and Adagoi, which were recognised by drilling (Fig. 15). The following table shows their main characteristics and the most significant values of Li found within them.
|Structure||Direction||Dipping||Maximum thickness||Outcrop extension|
|Alijσ||N 15Ί W||75Ί W||37m||380m|
|Veral||N 10Ί E||18Ί to 35Ί W||8.5m||280m|
|Adagoi||N 25Ί E||50Ί TO 65Ί NW||35m||300m|
|Mineralised structure||Borehole||Length||Li Grade|
|Alijσ||ALJ – 1||12.50||0.21|
|Alijσ||ALJ – 2||12.00||0.51|
|Alijσ||ALJ – 3||37.00||0.58|
|Veral||VR - 1||3.88||0.40|
|Veral||VR – 2||1.50||0.33|
|Veral||VR – 3||2.95||0.37|
|Adagoi||ADG – 2||11.45
As we have mentioned, the lithium occurs chiefly in the form of spodumene, but also in amblygonite and, more rarely, petalite, eucryptite and montebrasite, the pegmatite facies clearly the. richest in lithium.
The results of this phase of the project have led to the reconnaissance of a large extension of the aplite-pegmatite field with spodumene, a NW-SE alignment of more than 20 km identified parallel to the 3rd stage of Hereynian deformation. This type of lithium mineralisation is independent in space and in time from others of Sn-Nb-Ta and W, occurring in the zone.
Portugal's geostructural characteristics lend it great potential for the occurrence of rare earths. This has been confirmed by mineralometric studies and radiometric surveys.
With the exception of the Alter Pedroso zone, where hyperalkaline rocks have been investigated by the firm ERCROS, this group of elements had never been prospected in Portugal.
Regional exploration is currently carried out in the Beira Baixa and North Alentejo regions by IGM, involving geological mapping, alluvial and stream sediment sampling and radiometric surveys, the aim being the selection of targets to investigate at a later stage.
Prospecting is aimed at detecting Rare Earth bearing minerals, such as monazite -nodular monazite in particular - in sedimentary rock areas (more or less metamorphised) and xenotime, without overlooking other minerals such as apatite and allanite.
Mineralometric data on the North Alentejo (Fig. 16) suggests that, to date, nodular monazite rich in light rare earths has originated chiefly from the disintegration of the ridges of the Ordovician quartzites on the southwest flank of the Portalegre Syncline, even in the case of Reveladas (Marvγo), where the alluvial samples were collected in the midst of the Silurian formations, but close to these ridges.
Normal monazite appears to be chiefly associated with granite, as is the case of the Fronteira granite and tertiary gravel, originated from it.
As far as the lithogeochemistry of the Ordovician is concerned, it is the radioactive quartzites interstratified in the schists immediately above the thick base quartzites that are rich in rare earths, and not the schists itself (Fig. 17). This finding for the radioactive quartzites, previously known in Alegrete (Portalegre), mapped in detail in the Vale de Cavalos area as part of an IGM project and now also detected in Penha Garcia, goes against existing data on the levels of schists with nodular monazite, rich in rare earths, particularly europium, in other places of Europe and America.
The occurrence of these metals in Portugal has not been sufficiently investigated, although various metallogenetic scenarios point to their potential existence. These are briefly described below:
a) Morais and Braganηa mafic and ultramafic massifs. The IGM is carrying out geological, geochemical and geophysical exploration around Alimonde, where chromite-bearing peridotite formations (dunites) occur. The first phase of this research is nearing its conclusion.
b) Ιvora Massif of the Ossa-Morena Zone. Parts of this area have geological and metallogenetic characteristics that are similar to those of the Αgua Blanca zone (Monasterio - Spain), in which an important deposit of Ni was recently discovered with geological reserves of 23 Mt with 0.75% Ni and 0.55% Cu at a cut-off of 0.3% Ni.
c) Basic and ultrabasic complexes occurring on the north and south edges of the Ossa-Morena Zone. In the north sector, the Finnish firm Outokumpu holds an exploration license for an area which has shown potential for Ni, particularly in the Alter do Chγo, Elvas and Campo Maior massifs.
Several firms are interested in prospecting the Beja ophiolite complex to the south, although the area remains free.
ZDROJ: Instituto Geologico e Miniero, Portugalsko