Mineral Potential of Portugal (1998)

5. Base Metals

Portugal's main base metal resources are located in the south of the country in the OssaMorena and South Portuguese zones, the Pyrite Belt in the latter being the most important. Given the different metallogenetic characteristics of these two geotectonic units, we shall sub-divide our presentation in the same manner.

Ossa Morena Zone

Below is a summary of the primordial geological and mineral characteristics of the different "Belts" that occur in the Ossa-Morena Zone, together with a description of the most important mineral occurrences which may in some way reflect the metallogenetic models that are representative of each Belt (Fig. 6). The IGM has carried out exploration work in all of them, having presently an important know-how, obtained during that activity.

I - NORTH ALENTEJO AREA

Given the nature, typology and regional geological setting of the mineral occurrences, this area is split into three belts:

• Arronches - Campo Maior Belt

• Alter do Chão - Elvas Belt

• Sousel - Barrancos Belt

IA - ARRONCHES - CAMPO MAIOR BELT

This belt basically consists of Late Precambrian formations belonging to the Tomar -Badajoz Blastornylonitic Belt.

Occurrences of copper and, more rarely, lead are represented by the old Tinoca, Azeiteiros and Balôco mines. Stratiform mineral occurrences, with disseminated sulphides, are located near the contact between different types of gneiss and associated with a felsic volcaniclastic formation.

The existence of high metamorphic gradients, related with important tectonic accidents and consequent hydrothermal activity, resulting in silicification, chloritization and muscovitization phenomena, appears to have significantly contributed to the concentration of mineralisations of pyrite, chalcopyrite, pyrrhotite, magnetite and, more rarely, argentiferous galena.

IB - ALTER DO CHÃO - ELVAS BELT

This belt essentially corresponds to Cambrian formations. Occurrences of Cu, Pb and Zn are mainly related with intermediate-acid Cambrian metavolcanics, sometimes with associated agglomerated tuffs. Small ocurrences of Pb-Zn are also known to exist in the Early Cambrian carbonated formation, particularly where this is more fractured and silicified.

IC - SOUSEL - BARRANCOS BELT

This basically corresponds to Cambrian, Ordovician, Silurian and Devonian formations.

There are essentially three types of mineralised structure here: vein, stratiform and disseminated types.

In the first, which basically consists of copper mineralisations, there exist subconcordant veins and veins that are completely discordant from the formations in which they are embedded. The most important of the sub-concordant veins are perhaps Miguel Vacas and Mociços ore-bodies, situated at the base of the Silurian, and Urmos, located in basic metavolcanics (spilites) from the Mid Cambrian.

The most important of the discordant veins are Minancos, from the base of the Silurian, Bugalho Mine, in Silurian formations not far from its base and near the Messejana fault, and Zarnbujeira, in Ordovician formations. The Mostardeira, Aparis and Botefa veins are also discordant, and all are situated on the SW flank of the Terena syncline, which is essentially made up of flysch formations from the Devonian period.

With regard to stratiform mineralised structures, these are closely related to a CambrianOrdovician discordance. Mineralisations basically include pyrite, sometimes massive and rare chalcopyrite, sphalerite and galena. Mineralisations of pyrite, sphalerite and galena also occur in dolomitic limestones from the Early Cambrian, in microfractures and particularly in brecciated facies.

The disseminated mineralisations formed essentially of pyrite and chalcopyrite are associated with post-Silurian, intermediate-acid, sub-volcanic structures and constitute the cement of eruptive breccias. Mineralisation is more intense when the elements of these breccias are predominantly of a carbonated nature.

II - ÉVORA - BEJA MASSIFS

In view of the nature, typology and regional geological setting of the mineral occurrences, 3 belts have been defined in these massifs:

• Arraiolos - St. Aleixo Belt

• Magnetitic - Zinciferous Belt

• Porphyry Belt

IIA - ARRAIOLOS – STº. ALEIX0 BELT

Various ocurrences of copper, mainly of vein type, are known to exist here, some of which have been exploited on a small scale (Azaruja, Monte do Trigo, Reguengos and SC. Aleixo). These mineralisations are related with both granodiorite intrusions and basic volcanics commonly spilitized.

IIB - MAGNETITIC - ZINCIFEROUS BELT

This basically comprises Cambrian, Ordovician and Silurian formations, intruded by several Hercynian granitic massifs such as Pias and Pedrogão.

The most important deposits in this belt are Algares (Cu, Pb, Zn) and Balsa (Pb, Zn) in the Portel region, Enfermarias (Zn, Cu, Pb) and Preguiça (Zn, Pb), in Moura and Sobral da Adiça regions respectively, and other occurrences of Pb, Sb, Ag, Zn in the Ficalho-Moura-Vale de Vargo triangle.

The Balsa and Preguiça deposits are associated with dolomitic limestone; those at Algares and Enfermarias are related with an intermediate-acid volcanic episode which occurred at the base of the Early Cambrian, contemporaneous with a carbonated sedimentation, with which an important hydrothermal system is associated.

The Enfermarias "hidden deposit", located essentially by gravimetry and confirmed by drilling, illustrates nicely base metal prospecting in the Magnetitic - Zinciferous Belt. Deposits of Zn and Pb are also represented in paleokarsts, which contain very high levels of these elements, an example of which is the old Vila Ruiva mine.

The european firm Exminco recently applied for exploration rights for an area that includes these deposits.

IIC - PORPHYRY BELT

This covers a wide area in which sub-volcanic and volcanic rocks - namely rhyodacites, felsites, spilites and basalts - predominate and which are surrounded to the NE and SW by diorites, gabbros and ultrabasic rocks. The most significant mineral occurrences are Corte Pereiro, Caeirinha, Alcáçovas, Peroguarda and Asseiceiras.

South-Portuguese Zone

This zone is characterised by the Iberian Pyrite Belt (IPB), which is known around the world for its large deposits of massive polymetallic sulphides and plays an important role in supplying Europe with base metals.

Up until the eighties, the prospecting of massive polymetallic sulphide deposits was mainly carried out in areas where formations of the Volcanic Sedimentary Complex (VS), potential bearers of sulphide masses, outcropped or were expected to lay at depths not significantly exceeding 300 metres(Oliveira et al., 1998).

These areas were thoroughly prospected during the sixties, during which there was a marked turnaround in the methodology that had hitherto been used as a result of the evolution in the geological models related with the formation of these deposits and the introduction of new prospecting techniques and systematic gravimetric coverages, which in turn brought about great improvements in discovering mineral deposits. The changes in both corporate sector and in official departments led to important discoveries being made, most of them situated near mining centres or known occurrences, as referred above.

The Pyrite Belt has been of great interest to private companies in the mining sector for many years, this clearly reflected in the number of prospecting contracts that have been signed with the Portuguese State.

The discovery of Neves-Corvo in 1977 allowed a new deposit type to be defined for the IPB, in which the levels of base metals, Cu and Sn in particular, are extraordinarily high. As this type of mineral deposit can be economically exploited at depths of more than 500 metres, there was a change of strategy on the part of several mining companies in respect of the geological- structural characteristics of the areas they wished to explore. From that moment on, prospecting at great depths thus became viable, as the discovery of a deposit with similar characteristics would support the necessary investment.

These facts, coupled with the scientific and technological breakthroughs that took place during the eighties (in respect of both the geology and metallogeny of the IPB, and the geophysical techniques applied - advances in high sensitivity airborne magnetometry and time domain electromagnetics, as well as data processing), meant that prospecting in this metallogenetic province moved into areas where the VS ocurred at great depths bellow the flysch sediments and/or the Tertiary. ln recent years this trend continued, the number of companies prospecting in such areas has risen as technological advances and computer data processing have boosted their ability to detect mineral masses at hitherto unreachable depths and/or in particular adverse geological contexts such as conductive overburden.

Targets that might prove of interest in those areas are mostly located at great depth, where potential mineral-bearing horizons occur under a thick covering of flysch or beneath nappes, the thickness of which is often uncertain. The depth of drilling here reach levels that were unthinkable several years ago, these usually around the 600 m and 1000 m mark, occasionally going even further.

Despite those developments, the basic tools used since the sixties - gravimetry combined with specialized geological mapping, and magnetometry (with ground vertical component surveys replaced by high-resolution total field airborne surveys and ground follow-up work) - still play a major role.

Other complementary geophysical techniques are also routinely used, mostly in previously detected gravimetric anomalies. Those include vertical electrical soundings, time-domain electromagnetic soundings and borehole logging, and other electrical techniques such as induced polarization and magnetotelurics. In a limited fashion, reflection seismics has also been used. Another geochemical technique that has been used is the enzyme leaching of soil samples.

Although a few localized helicopter surveys, namely aeromagnetic and EM (INPUT and frequency domain) had been done in the eighties, a recent regional airborne high resolution magnetic and 256 channel spectrometric geophysical survey, has been contracted by one of the main operators in Portugal, RTZ. Those coverages have greatly contributed towards better geological mapping, defining regional geological structures, tectonic acidents, buried volcanic axes, and conductors, providing information that might indicate the presence of mineral masses deep bellow the surface.

Parallel to the activity carried out by private companies, the IGM also plays an important role, either through its own projects or through the services provided to these companies. Its work as led to the direct or indirect discovery of several masses of massive polymetallic sulfides and the promotion of certain areas later awarded to private companies.

During the nineties, prospecting on part of mining companies has showed no signs of letting up.

The IGM has continued to foster exploration work in this important province, by moving into less attractive areas, like the Tertiary Basin of the Lower Sado river, to the north of the Grândola fault. This led to the discovery of a new deposit of massive polymetallic sulphides, known as Lagoa Salgada, located far from any mining centre or known occurrence, which increased the mining potential of the IPB. Situated in an area where the VS is covered by sediments, and the interpretation of the gravity anomalies is hampered by the paleorellef and horst and grabens structures, controlled by the Paleozoic basement, this event captured the imagination of a great many firms, and was largely responsible for a wave of new prospecting requests.

All the different actions being carried out on the Pyrite Belt by both private companies and the IGM itself has led to a wealth of new geological information, which is being collected and stored in a general database.

We now move on to the most important mineral deposits and mines in this metallogenetic province (Fig. 7), from NW to SE although other deposits do exist (Caveira, Montinho, Chança, Salgadinho and Cercal, the last currently being mined for Fe and Mn):

LAGOA SALGADA - In this area, located in the Tertiary Basin of the Sado River but included in the NW segment of the IPB, the IGM has carried out varied exploration work which, thanks mainly to gravimetry, has uncovered several targets. In one of them, situated roughly 12 km NE of Grândola, a mass of polymetallic sulphides was discovered in August 1992 at a depth of 128 metres under tertiary covering (Fig. 8).

Boreholes executed in the central and NW nuclei oft he gravimetric anomaly of Lagoa Salgada have revealed a pyrite-bearing acid volcanic sequence accompanied by polymetallic sulphides, the NW nucleus the richer of the two (Oliveira et al., 1998). Here, the mineral mass is embedded in a volcanic sequence highly altered by an important hydrothermal system. The main volcanic facies are composed of intermediate - acid lavas and autoclastic breccias with abundant feldspar megacrystals, and fine, sometimes porphyrous, chloritic-sericitic volcanics; intercalated layers of possible lapilli are frequent. Note that the enormous tectonic complexity that affects the mineralisation and wall rock makes investigation of the zone extremely difficult.

As mentioned, this area was studied by the IGM until 1993, at which time it was awarded to a consortium of two companies, EDM and RTZ.

To date, three types of ore have been distinguished:

a) Complex, massive, banded sulphides, very finely grained, containing interesting grades of lead and zinc and not rare Cu/Au enrichment zones.

b) Stockwork mineralisation with concentrations of base metals.

c) Gold mineralisation associated with silicified shear zones, with arsenopyrite.

The following are some of the figures obtained from boreholes executed in the NW nucleus: during the drilling of LS 22 78.55 m with 0.35% Cu, 4.79% Pb, 7.52% Zn, 94.65 g/t Ag and 1.34 g/t Au were intersected and in LS 04 63.13 m with 0.48% Cu: 2.66% Pb, 2.96% Zn, 0.26% Sn, 69.91 g/t Ag and 0.98 g/t Au were obtained. In the central nucleus, LS 18 intersected 12.05 m with 0.08% Cu, 0.08% Pb, 0.20% Zn, 12.83 g/t Ag and 4.37 g/t Au.

The mineralised mass begins with an iron hat of variable thickness - never greater than 20m -where there is marked enrichment of precious metals, Sn and some times Pb. This enrichment in Au and Ag is also visible in the first section of the mass, immediately underlying, due to supergenic phenomena, or next to important thrusts and consequent remobilizations. The gossan has the following paragenesis: goethite, hematite, amalgams of Ag - Hg, chalcocite, neodigenite, covellite, bornite and mimetite. The primary mineralisation is composed mainly of pyrite with the following minerals: sphalerite, arsenopyrite, tetrahedrite, tennantite, galena, lollingite, chalcopyrite, cassiterite, stannite, meneghinite and pyrrhotite.

LOUSAL - Old mine situated on the SW flank (inverse) of the Lousal anticline. Mineralisation is found in a deep, narrow syncline, characteristic of the VS occurring here, which is deeply folded within isoclinal structures. Several masses have been recognised with tonnages of 100,000 tons to over 3MT, essentially pyrite, generally lenticular, aligned along the axis of the folds, dipping roughly 80º to the SW and extending several hundred metres with variable thickness.

The ore is relatively poor in Cu (<0.7%) and in Pb + Zn (1.1 - 3.5%), with annual production at the start of the seventies totalling between 230,000 and 250,000 tons. The mine was closed down in 1987.

ALJUSTREL - Mining at this important mineral deposit dates back to Roman times, the target being the outcroping masses of S. João and Algares. An important archaeological find was made here: two bronze tables inscribed with the mining law of those times.

In this area, the VS occupies the axis of an anticlinorium running in a NW-SE direction and is characterised by the existence of an important sequence of acid pyroclastic rocks, subdivided into lower and upper series(Silva et al., 1997). These rocks are in turn subdivided into two laterally equivalent series. In the central part of the structure there is a sequence which includes the lower unit with tuffs with megacrystals, and the upper with green tuffs; laterally, the lower series is characterised by the existence of volcanics with felsic facies; and the upper, known as the tuffs of the mine formation, is the bearer of masses of massive polymetallic sulphides.

The Messeiana fault has a sinistral displacement of roughly 2.5 km, causing locally a change in the direction of the structures from NW-SE to E-W Later it caused the NW block to lower and be covered by tertiary sediments. Thus, the sector of the Gavião deposit, which constitutes the extension of the Aljustrel structure, presents a tertiary cover of 60 - 90m.

The following masses have been recognised: Algares, S. João, Moinho, Feitais, Estação and Gavião, all occurring in well defined structural positions in the Aljustrel Anticlinorium, running parallel and symmetrical to the alignments of the felsic facies, mine tuffs which are: the S. João Syncline, containing the deposits of S. João, Moinho, Algares and the NE Gavião mass; a second alignment symmetrical to the latter includes the SW Gavião mass; finally, the Feitais and Estação deposits are located on the NE flank of the referred anticlinorium.

S. João and Algares were exploited up until the mid-sixties, the latter now exhausted, Moinho and Feitais were mined in more recent decades, while mining operations have never taken place at Estação and Gavião.

Various types of ore can be distinguished in the Moinho and Feitais deposits: coarse "banded", rich in chalcopyrite; massive, essentially pyritic and fine "banded", rich in sphalerite and galena.

To date, the following resources are known, excluding stockwork mineralisations:

(in SEG Guidebook Series, vol. 27)

The Algares deposit, which is currently exhausted, should have contained mineral resources greater than 50 Mt. An evaluation of the remaining deposits is still to be carried out, particularly for S. João and Estação. An exploration licence contract for Estação deposit was recently started by a consortium formed by the firms EDM and Portuglobal; boreholes are currently being drilled and intersecting the mineralised mass.

NEVES CORVO - This important mine is geologically situated at the end of the Neves Corvo - Rosário Anticline, a structure that runs NW-SE, the axis of which dips to SE, with deposits on both flanks (T.Oliveira et al., 1997). Various groups of sub-vertical faults occur which affect the mineral deposits and also low-angled overthrusts, related with repetitions of volcanic sedimentary and flysch units. These deposits are situated in the upper part of a volcanic sedimentary sequence (VS), mainly acid, and composed of three sequences of acid tuffs, separated by shales formations, with a discontinuous level of black shales immediately beneath the mineralised masses. Due to the abovementioned overthrusts, the flysch and volcanic sedimentary units repeat above the top of the mineral mass.

Geological, geophysical and geochemical prospecting work begun by the IGM and carried on by a Portuguese-French consortium (Sociedade Mineira de Santiago, S.M.M. Peflarroya and SEREM - BRGM) led to the drilling and intersecting in 1977 of the first deposit (Neves). Next, three more important deposits were discovered, Corvo, Graça and Zambujal, with a fifth deposit discovered in 1988 (Lombador). In 1980, the consortium gave way to Somincor (Sociedade Mineira de Neves Corvo, SA).

In 1985, RTZ Metals, Ltd (the Rio Tinto - Zinc Group) acquired the French interest in Somincor (49%), the remaining 51% held by the state-owned Portuguese firm EDM. Production of copper was begun at the end of 1988, with tin production in 1990. From 1988 to the end of 1995 the mine produced 8.7 Mt of copper ore, containing 790,000 tons of copper metal, and 2.2 Mt of tin ore, containing 41,140 tons of tin metal. Annual production currently stands at 1.9 Mt of copper and tin ore; plans to extend production to include zinc are being studied.

These deposits are composed of lenticular masses of massive polymetallic sulphides, also containing stockwork mineralisation in the footwall host rock, and are distinguished from other Pyrite Belt deposits by their high copper and tin contents and a highly metal zonation. This latter fact, related with the segregation of base metals, favours the division into copper, tin and zinc ores, as well as massive "barren" pyrite.

Three main types of ore occur:

a) "Rubané" - occurs at the top of massive sulfide masses, particularly at the Corvo orebody, and is composed of chloritic shales, siliceous shales and chert-carbonate breccia, with sulphide veinlets sub-parallel to perpendicular to the stratification and sometimes with tight lenses and bands of massive sulphides. The sulphides contained here are mostly cupriferous.

b) Massive sulphides - composed of approximately 95% of fine grained sulphides, in which pyrite predominates, followed by chalcopyrite, tennantite and bornite. In the polymetallic lenses the main zinc mineral is sphalerite ; galena occurs disseminated or in millimetric bands in the massive sphalerite. The main tin minerals are cassiterite and minor stannite, normally associated with high grade copper ore, at the base of the deposits in particular but also at its top. This ore can be split into 5 sub-types: cupriferous (MC), characterised by disserninations and bands of variable thickness of chalcopyrite in the massive pyrite and with a Cu content greater than 2%; cupriferous with broad bands of tetrahedrite/tennantite or sphalerite (MH), which introduce high levels of penalty elements, such as As, Sb, Hg and Zn; rich cupriferous ore (MS), with more than 1 % tin and composed of massive or banded chalcopyrite containing elements of massive cassiterite; stanniferous (MT), in which there is abundant cassiterite and a tin content of more than 8%; zinciferous (MZ), rich in sphalerite, which occurs as centimetric bands in the massive pyrite, bands which may contain disseminations of galena.

c) Fissural - stockwork - type mineralisation, including the "breecia" of the base of the Corvo mass, which occurs in the footwall shales and in acid volcanic rocks, usually in discordant veins and veinlets of sulphide minerals. Pyrite and chalcopyrite predominate, although cassiterite and sphalerite also occur, always accompanied by intense hydrothermal alteration of the host rocks.

This mineralisation can be subdivided into three sub-types: cupriferous (FC), with more than 2% Cu, present in veins and veinlets rich in chalcopyrite; stanniferous (FT), with more than 1 % Sn, present in veins rich in cassiterite, together with pyrite and, to a lesser extent, chalcopyrite; zinciferous (FZ), with more than 3.3% zinc, present in veins and vemlets rich in sphalerite.

In June 1996, mineral resources were calculated as follows:

S. DOMINGOS - The sequence and characteristics of the mineral-bearing volcanic sedimentary complex (VS) in this area are poorly exposed and understood, their structural aspects being particularly complicated. The upper levels of the VS are covered by a fairly thin turbiditic sequence overlain by Phyllite-Quartzite Group formations.

Nevertheless, a volcanic sequence composed of rhyolites, jaspers and diabases can still be observed in old works. This mine was extremely important in the past, and in fact dates back to Roman times. Between, 1858 and 1966, the "Mason and Barry Company" mined around 25 million tons of ore, by open-cast operations going down to a depth of 120m with additional underground work to a depth of 400m (Carvalho et al., 197 1).

The ore was composed chiefly of massive pyrite with a maximum grade of 10% copper and 14% zinc-lead, and average grades of 1.25% Cu, 2-3% Zn and 45-48% S.

Given the occurrence of massive polymetallic sulphides in the formations (Silurian volcanic sedimentary complex) surrounding the Morais and Bragança massifs, as well as several geological and metallogenetic similarities with the Pyrite Belt, it is our opinion that the Trás-os-Montes zone is worth investigating.

Nevertheless, the main target will still be the Pyrite Belt, in which exploration will continue to be carried out in zones characterised by (Oliveira et al., 1998):

• Areas of high tectonic complexity, where barren allochthonous terranes thrust over more recent geological formations with high mineral potential.

• Structural alignments, in which the VS can be found at great depth beneath more recent Paleozoic Flysch formations in Baixo Alentejo.

• Areas within the Sado River Tertiary Basin, where the VS can occur at depths ranging from a few dozen to several hundred metres beneath Ceno-Anthrepozoic formations.

In the latter, the IGM is currently carrying out a research project in an area between Sta Catarina de Sítimos and Figueira de Cavaleiros, where gravimetry has been used as the basic prospecting technique, although complemented by others. Various gravimetric targets have been recognised (Fresta, Rio de Moinhos, Lameira, Chaparral, Mte. Branco da Loira, Vale Sabroso) and drilled (except the latter two). Two boreholes in Rio de Moinhos (Fig. 9) have intersected formations belonging to the VS, with characteristics that point to the potential existence of sulphide mineral occurrences (chloritization of acid tuffs, small stockworks, brecciated textures, occurrence of a layer of jasper roughly 30m thick) and have revealed the existence of a new volcanic axis in the Pyrite Belt situated NE of the structural alignment which includes the Lagoa Salgada deposit.

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