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Rio Tinto and the Iberian Pyrite Belt

Last Updated: 31st Dec 2018

By Nathalie Brandes

The Iberian Pyrite Belt (IPB) is a very large region of mineralisation that is argued to be the largest massive sulphide deposit in the world (Rona, 1988; Leistel et al., 1994; Carvalho et al., 1999). Extraction of minerals from the IPB has a colourful history that spans at least 5000 years and includes localities such as the famous Rio Tinto mine.

The IPB extends from the Setúbal and Beja Districts in Portugal to the Huelva and Seville Provinces in Spain (Barriga et al., 1997). The east-west trending ore zone is approximately 250 km long and varies in width with an average of around 30-40 km. Thickness of the pyrite belt is up to 100s of metres (Davis et al., 2000). The climate of the region is Mediterranean with Atlantic and Continental influences. Summers are long and dry while winters are short and mild. Precipitation averages 40-70 cm per year, mostly falling from October to February. The region’s average annual temperature varies between 15 and 20º C (Barriga et al., 1997; Fernández-Caliani et al., 2009). Much of the IPB was originally covered with Querqus forests. Mining activities, however, resulted in significant deforestation. Today, scrublands, eucalyptus, pine, and some orange orchards grow in the region (Fernández-Caliani et al., 2009).

The IPB is part of the South Portuguese Zone (SPZ) of the Iberian Massif (Boulter, 1993). The tectonic setting of the SPZ is debated. Some studies suggest it is an accretionary prism related to subduction that occurred to the southwest (Carvalho, 1972; Thiéblemont et al., 1994). Subduction followed by continental collision has also been suggested (Quesada, 1991; Dias and Ribeiro, 1995). Other researchers interpret the setting as a back-arc basin (Munhá, 1983; Ribeiro et al., 1983).

Three major rock units comprise the IPB. The oldest unit is the Phyllite-Quartzite Group (PQ Group), which consists of over 2000 m of slate and quartzite with subordinate amounts of conglomerate and limestone (Barriga et al., 1997; Tornos et al., 2009; Cáceras et al., 2013). Fossils found in the limestones were dated to the Upper Devonian (Boogard and Schermerhorn, 1975). The PQ Group is overlain by the Late Devonian to Early Carboniferous Volcano-Sedimentary Complex (VSC). The VSC consists of up to 1300 m of volcanics and mixed sediments. The volcanic rocks range from mafic to felsic in composition with felsic rocks more abundant. Mafic rocks include diabase and pillow lavas. Felsic rocks include rhyolite domes, rhyolite lavas, and some pyroclastics. The VSC sedimentary rocks include shale, greywacke, impure quartzite, jasper, chert, and limestone (Barriga et al., 1997; Soriano and Martí, 1999; Tornos et al., 2009). This unit is the host rock of the IPB massive sulphides, which are commonly associated with felsic tuff and black shale. Minor manganese deposits also occur in the VSC, usually in association with jasper (Barriga et al., 1997). Overlying the VSC is the Culm Group, also known as the Flysch Group. This is an Early Carboniferous turbidite deposit that includes shale, greywacke, and conglomerate containing volcanic clasts (Oliveira, 1983; Oliveira, 1990; Tornos et al., 2009).

During the Variscan Orogeny, these rocks underwent folding, thrusting, and regional metamorphism (Ribeiro and Silva, 1983; Silva et al., 1990; Quesada, 1996). The maximum metamorphism occurred during to just after the Variscan deformation and reached prehnite-pumpellyite to greenschist facies (Barriga et al., 1997).

The mineral deposits in the IPB are complex. The sulphides were emplaced 300-350 Ma (Moreno, 1993). Most are lenses of massive sulphides overlying hydrothermally altered rock and a stockwork of sulphide-rich veins that were the conduits of ore fluids. In the northern IPB, the sulphides occur as replacement deposits in pumice-rich volcaniclastics and dacite. In the south, ores are slate-hosted with characteristics of exhalative deposits. Rio Tinto is located between the two zones and exhibits characteristics of both (Tornos, 2006; Tornos et al., 2009). Because of these complex variations, Sáez et al. (1999) proposed that they be considered Iberian-type massive sulphides, defined as volcanic hosted massive sulphides transitioning to sediment hosted massive sulphides.

The mineralising fluids that created these deposits were mostly seawater modified by the addition of some magmatic water (Munhá et al., 1986; Sáez et al., 1999). Fluid inclusion studies indicate that the salinity of these fluids was 2 to 13 weight % NaCl and temperatures varied from 130 to 400ºC (Sawkins, 1990; Nehlig et al., 1998; Sáez et al., 1999). The sulphides were likely produced in several stages, the earlier stages having waters <300ºC, the later stage waters being 300-400ºC. The source of the metals in the deposits was the underlying Devonian rocks (Sáez et al., 1999). Sulphur was derived from hydrothermal fluids as well as bacterial reduction of sulphate on the seafloor (Velasco et al., 1998). Rocks underlying the ore deposits have been hydrothermally altered by the mineralising fluids. This alteration has produced chlorite, carbonates, epidote, albite, and actinolite in mafic volcanics (Munhá, 1990). Felsic volcanics have been affected by various processes including, albitisation, sericitisation, chloritisation, silicification, and adularisation (Munhá and Kerrich, 1980; Barriga and Kerrich, 1981, 1984; Barriga, 1983, 1990).

There are around 90 massive sulphide deposits in the IPB. They occur as lenses or sheets 4-5 km long, 1-5 km wide, and 80-100 m thick, Individual deposits are up to 500 million tonnes, Rio Tinto being the largest (Barriga et al., 1997). In the Cenozoic, supergene alteration affected the ore deposits, resulting in thick gossans at the surface with an enriched zone between the gossan and massive sulphide (Salkield, 1987; Hunt Ortiz, 2003; Tornos et al., 2009). At Rio Tinto, the gossan of Cerro Colorado was up to 70 m deep. It is estimated that there were originally over 1.7 million tonnes of sulphides in the IPB. Approximately 20% have been mined and 10-15% lost to erosion (Barriga et al., 1997).

The sulphides in the IPB are predominantly pyrite with lesser amounts of sphalerite, galena, chalcopyrite, arsenopyrite, and pyrrhotite (Sáez et al., 1999). Less common and minor minerals include Bi-Sb-Pb-As sulphosalts, tetrahedrite-tennantite, stannite-kesterite, cassiterite, magnetite, electrum, cobaltite, bournonite, mawsonite, Co-sulphoarsenides, and stannoidite (Aye and Picot, 1976; Sierra, 1984; García de Miguel, 1990; Kase et al., 1990; Ruiz de Almodóvar et al., 1994; Marcoux et al., 1996; Barriga et al., 1997; Leistel et al., 1998; Almodóvar et al., 1998). Typical gangue minerals include quartz, carbonates, muscovite, and chlorite (Velasco et al., 1998). Average ore grades of sulphide deposits were reported as 45% S, 40% Fe, 0.9% Cu, 2.1% Zn, 0.8% Pb, 0.5 g/t Au, and 26 g/t Ag (Garcia Palomero 1992). Gossan rocks are rich in Au, Ag, Pb, Sb, and Bi with up to 1.8 to 2.5 g/t Au and 35-45 g/t Ag (Tornos et al., 2009).

Mining in the IPB dates as far back as the Chalcolithic Age in the 3rd Millenium BC when minerals were extracted from surface trenches (Barriga et al., 1997; Yeoman, 2010). The Phoenicians arrived around 1100 BC. They introduced improved smelting techniques, resulting in greater silver production from the mines. The Carthaginians controlled the region from around 535 BC until their defeat by the Romans in 206 BC (Yeoman, 2010). It was under Roman rule that operations were significantly expanded. Shafts reached depths of around 450 ft., requiring special ventilation shafts and dewatering techniques that included large underground waterwheels. The most intense period of Roman mining occurred between 0 AD and 180 AD when Rio Tinto was the largest mining operation in the empire (Edmondson, 1989; Ruiz et al., 2008; Yeoman, 2010). It is estimated that during the Roman period 25 million tonnes of ore were extracted (Strauss et al., 1977).

During the 2nd Century AD, the Mauri of North Africa invaded Spain and mining decreased significantly. Under Visigoth rule (~350 AD to ~650 AD) mining was abandoned completely. Minor mining activity resumed in the 10th to 13th Centuries during Moorish rule. In the 1550s, King Phillip II of Spain commissioned a priest, Diego Delgado, to find new mines. While Delgado reported the mineral rich rocks of Rio Tinto to the king, no mining was pursued in the area until the British began large open-cast operations in 1873 (Yeoman, 2010). Peak production from the mines occurred from 1900 to 1930 (Salkield, 1987). The mines closed in 2001 as the prices of commodities dropped (Yeoman, 2010).

The Rio Tinto Mine was named for the river flowing in the region. Water in the Rio Tinto flows red due to the acidic and metal rich water. This pollution began in the Miocene with the natural oxidation of the sulphide minerals exposed in the IPB. It increased with the escalation of mining in the region (Cáceras et al., 2013). The pH of water in the river varies between 0.8 to 4.0 with significant amounts of dissolved heavy metals (Nelson and Lamothe, 1993; Fernández Remolar et al., 2002). Several minerals including melanterite, rozenite, rhomboclase, szomolnokite, copiapite, coquimbite, hexahydrite, halotrichite, and gypsum form on the banks of the Rio Tinto as the acidic waters evaporate (Buckby et al., 2003). The chemistry of the water also supports extremophile microorganisms and algae in a unique ecosystem unknown elsewhere on earth (Garcia-Moyano et al., 2007; Tornos et al., 2009).

Mining at Rio Tinto is dormant at present, but its long history is visible in the many mine pits, crumbling buildings, and rusting equipment. The Rio Tinto Foundation maintains a mining park and museum dedicated to preserving the unique history of this region.


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