The Iberian Pyrite Belt (IPB) is the largest sulphide concentration in the earth's crust and forms one of the most prominent massive sulphide provinces in the world. It is a world-class ore province and one the most important metal reservoirs in Europe. It comprises about 1500-1700 Mt total resources of stratiform massive sulphides and stockworks in giant (> 100 Mt, i.e. Tharsis, Sotiel-Migollas, and Masa Valverde) and supergiant (> 150 Mt, i.e. Aznalcollar-Los Frailes, La Zarza, Aljustrel, Neves-Corvo, and Rio Tinto) volcanogenic massive sulphide deposits (VMS) and dozens of orebodies containing > 1 Mt as well as various prospects. The metal potential is summarized to be 14.6 Mt Cu, 34.9 Mt Zn, 13.0 Mt Pb, 46.1 Kt Ag and 880 t Au.
The IPB is located in the southwest of the Iberian Peninsula and forms an arcuate belt about 250 km long and up to 50 km wide, stretching west-northwest from near Seville (Spain) in the east to south Portugal near Grandola. The area correspondes to the South Portuguese Zone and Devonian-Carboniferous volcanic and sedimentary rocks containing massive polymetallic sulphide deposits. Mining commenced in this important massive sulphide district more than 5,000 years ago. Early mining was concentrated only on Cu, Au and Ag, won from supergene enrichment zones (gossans) of the massive sulphides. During the Industrial Revolution in the 19th and early 20th centuries, mining activities and development culminated. At this time, more than 60 mines were operating for Cu, Au, Ag, Zn, Pb and S.
The massive sulphide deposits of the IPB occur on the passive margin of the South Portuguese Zone (SPZ) of the Variscan Fold Belt, related to transpressional deformation and associated igneous activity. The SPZ depicts the southernmost geotectonic unit of the Variscan orogen. It is an exotic terrane belonging to Avalonia continental margin of southern Laurussia which was accreted to the Iberian Autochthon during the Variscan orogeny by the closure of the Rheic Ocean.
In the Variscan orogeny, the IPB formed in a pull-apart continental basin related to crustal-scale left lateral strike-slip faulting related with oblique northwards subduction and later collosion with the Iberian Autochthon. The formation of a major volcanic belt was linked to transpressional deformation, crustal thinning, widespread mafic intrusion, underplating-induced melting and quick ascent of felsic magmas into the upper crust.
The IPB is limited by thrusts with a southwards vergence. The SPZ is to the north separated from the Iberian Autochthon by a suture zone depicted by the Beja-Acebuches ophiolite and the Pulo do Lobo Terrane, while to the south the IPB is thrusted above the Baixo Alentejo Flysch.
Major structural units and tectonostratigraphic domains and location of the major VMS deposits in the IPB.
The SPZ forms a fold and thrust belt and it consists of overlapping allochthonous overthrust units that can be divided into three well-differentiated lithostratigraphic formations. The sequence includes 1,000 to 5,000 m of Late Paleozoic rocks, being the following from the bottom to upwards:
Volcano-Sedimentary Complex (CVS) – Late Famennain to Early Late Visean (Late Devonian to Early Carboniferous). The PQ-Group is overlain by a 100 to 1,300 m thick volcano-sedimentary succession. The CVS includes a complex mafic-felsic (bimodal) volcanic sequence interbedded with mudstone and chemical sediments. The felsic volcanic rocks consist mostly of dacite and lesser amounts of rhyolite. The mafic volcanic rocks are basaltic sills or small stocks and locally andesite. The depth of formation is unknown, but it is suggested that the deposition took place in a shallow marine environment. In general, the entire sequence shows intense pervasive hydrothermal alteration.
The VMS deposits of the IPB are hosted within the CVS. In general, they show the typical structure and features of VMS deposits formed in ancient and modern massiv sulphide provinces, i.e. a lens-shaped accumulation of massive sulphides on top overlays a network of sulphide-rich veins (stockwork) acting as conduit for the hydrothermal fluids on their way up to exhalation at the sea bottom or a favourable level in the host rocks. This whole area is affected by a wide zone of intense hydrothermal alteration.
Corta Atalaya open pit (Riotinto), looking east with massive sulphide mineralization (grey) with drift.
San Miguel open pit, looking north-east with pyritic stockwork, sulphide veins are grey-green and the red colour of the host rock (acidic volcanic) is an oxidation patina from meteoric water.
Pyrite is by far the most common sulphide mineral in the paragensis (roughly more than 90 - 95 % of sulphides) with minor amounts of sphalerite, galena and chalcopyrite and a wide variety of other metals. Base metal grades are generally low, i.e. between 0.5 % and 1.5 % Cu, 2.3 % and 2.6 % Zn, and 0.9 % and 1.0 % Pb. In general, most of the VMS deposits in the IPB belong to the Zn-Pb-Cu subtype of massive sulphide deposits following Large (1992).
The VMS deposits and the styles of mineralization in the IPB can be divided into a southern zone and a northern zone. In the southern zone, the mineralization is hosted in slate and forms large pyrite-rich exhalative deposits. The deposits are considered to be formed by brine accumulation and biogenic activity processes. In contrast, the deposits in the northern zone are hosted in pumice-rich volcanoclastic rocks and are richer in base metals formed by replacement of the volcanic rocks. Ore-forming processes occurred in a short time interval, in the range of a few million years.
As a whole, the mineralization of the VMS deposits in the IPB deviates from ancient and modern VMS provinces distinctly as they were formed in an intracontinental pull-apart basin. In the latter case, formation of VMS deposits is linked to spreading oceanic centers, oceanic back arcs or continental back arcs.
In the IPB, most of the VMS deposits and related stockworks show more or less well developed iron caps (gossans). The gossans are the result of supergene alteration, that began with the exhumation and weathering of sulphides in the Miocene. Erosion and oxidation by surfical water leaches soluble elements and re-precipitates them above the water table forming secondary iron-bearing phases (goethite, hematite and minerals of the jarosite subgroup). The gossans are supergene enriched in Au (1.8-2.5 g/t) and Ag (35.0-45.0 g/t) in relation to massive sulphides (Au 0.44 g/t and Ag 37.0 g/t) and thus being mined since Roman times.
San Miguel open pit, looking north-east with prominent goethite-rich gossan above the stockwork.
The IPB is still an unquestionable target for exploration activity due to its historical importance, both quantitatively and qualitatively, and because of recent mine developments. Currently, there are seven active mining operations in the IPB: Neves Corvo (Somicor), Aljustrel (Almina), Sotiel-Migollas (Matsa), Riotinto (Atalaya Mining), Aguas Tenidas and Mina Magdalena (both Matsa), and Las Cruces (Cobre Las Cruces) and several ongoing exploration and development projects, for example Lagoa Salgada (Portex), Sesmarias (Colt Resources), La Zarza (Ormonde Mining), Lomero-Poyatos (Winmar Resources), Masa Valverde (Cambridge Mineral Resources), Aznalcollar-Los Frailes (Emerita Resources), and La Concepcion (Matsa).
La Zarza open pit (Corta San Pablo) and shaft 4. This project will be developed by Ormonde Mining and it has a preliminary non-JORC compliant global mineral resource of some 61 Mt @ 0.8 % Cu, 2.0 % Zn, 0.6 % Pb, 0.9 g/t Au, and 5.7 g/t Ag (01/2017).
View of the Lomero-Poyatos project. It will be developed by Winmar Resources and has a total Indicated + Inferred Resource (open pit + underground) of 8.1 Mt @ 0.56 Cu %, 1.36 % Zn, 0.68 % Pb, 2.31 g/t Au, and 31.27 g/t Ag (01/2017).
Los Frailes open pit will be developed by Emerita Resources and has a preliminary non-JORC compliant global mineral resource of some 71 Mt @ 0.34 %
Cu, 3.86 % Zn, 2.18 % Pb, and 60.0 g/t Ag (11/2016).
Geological solid model of the Los Frailes deposit (from Emerita Resources). Emerita’s development plan is focused on the high grade portion of the resource > 5 % Zn, i.e. 20 Mt @ 0.26 % Cu, 6.65 % Zn, 3.87 % Pb, and 84.0 g/t Ag (11/2016).
Matsa's La Concepcion project with total resources of 55.9 Mt @ 0.57 % Cu, 0.48 % Zn, 0.19 % Pb, 0.2 g/t Au, and 7.0 g/t Ag.
Above - Remains of old facilities at Aljustrel mine. Aljustrel has been a pre-mining resource in six ore bodies of 189 Mt @ 1.20 % Cu, 3.20 % Zn, 1.20 % Pb, 1.0 g/t Au, and 36 g/t Ag.
Left - Headframe at Aljustrel and Au-bearing gossan with Roman galleries in the foreground.
Processing facilities and tailings pond at Aljustrel operated by Almina.
Mining operations at Cerro Colorado open pit (Riotinto) operated by Atalaya Mining. The Riotinto project has proven and probable reserves of 153 Mt @ 0.45 % Cu, cut-off grade of 0.20 % Cu (11/2016) and is producing currently 5.0 Mtpa and in the expansion phase up to 9.5 Mtpa.