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The Rio Tinto mines, Huelva, Spain.

Rio Tinto, perhaps the oldest major mine in the world, exploits huge massive pyrite orebodies and their associated gossans. Fine native copper, large barite crystals, gratonite crystals, spectacular formations of post-mining sulfates, and the world's finest voltaite are among the features of this historic deposit.

INTRODUCTION

The famous Rio Tinto mines are located in Huelva province in extreme southwestern Spain. They represent probably the oldest major mining area in the world, and were for many years the leading producer of copper in Europe. A variety of interesting archeological and mineralogical specimens have also come from the mines since the 19th century, and can be seen in museums throughout Europe. The name Rio Tinto ("Red River") comes from the iron-stained local river of the same name.

HISTORY

Mining in the Rio Tinto area seems to have begun during the Phoenician period (before the 10th century B.C.). Archeological evidence is extensive regarding ancient work for the recovery of gold, silver and copper. Many Roman-period mining and metallurgical artifacts are regularly brought up with the ore and come tumbling out into the dumpers. As might be expected, the quantity and quality of these historical treasures have placed archeologists at odds with the mining company; if allowed, archeologists would quite cheerfully bring a complete halt to gossan production and rope off the entire space of the mine for the purpose of detailed archeological excavations. Consequently, in order to remain in operation, the mining company has felt compelled to place limits on archeological access, and has set aside a small area as an archeological reserve. But, in my experience, the areas of potentially greatest archeological interest are much larger.

As an indication of the scope of ancient mining, sixteen million tons of Roman slag have been identified, with a high silver and copper content. It was not until 1873, however, that large-scale modern mining began in the district. British companies set up a colonial-style system of operation and commenced open-cast mining. This was a major departure from the traditional operations involving numerous small-scale pits and shallow underground workings; metal production of the district began to increase.

An industrial complex grew up around the district, making it increasingly self-sufficient. A foundry, electric power station, workshops of all kinds and railway connections to the company-owned port facilities at Huelva were constructed. Eventually over 14,000 workers were employed, utilizing 300 km of railway track, 2,000 ore wagons and 150 locomotives, all within the 23-square-km concession.

In 1954 the mines came under Spanish ownership, and Rio Tinto Company Ltd. later merged with the Zinc Corporation in 1962 to form the London-based Rio Tinto Zinc Corporation. Meanwhile the Spanish-owned Compania Espanola de Minas de Riotinto, S.A., continued to operate the mines, modernizing the facilities and improving working conditions. Over the years the company has changed its name several times (e.g. Rio Tinto Patino, then Union Explosivos Rio Tinto). Technical improvements have continued, particularly the construction of a new gossan concentrator for gold and silver extraction in 1970.

In 1977 the Rio Tinto Minera, S.A, was founded. Under this company the Cerro Colorado workings were expanded and the Alfredo shaft was modernized, including the installation of conveyor belts for ore transport.

In 1987 falling copper prices precipitated a crisis. Work at the Alfredo shaft was halted and the Cerro Colorado operation was restricted to the mining of gossan only; this, however was accompanied by increased production (30 million tons annually), yielding 5,000 to 6,000 kg of gold per year.

The final product of the Rio Tinto operation today is bullion (98% silver, 1% gold, plus minor Cu, Pd, Hg, etc.). The Atalaya open pit, which produced crude pyrite only, was shut down in 1992. Gold reserves are expected to run out in January of 1996, at which time mining may cease at Rio Tinto after over 3,000 years of activity. However, a project turning again to copper extraction is currently being developed under the direction of Professor Pla Ortiz de Urbina, and may extend the life of the mine.

GEOLOGY

Spain has long been a major world producer of pyrite, due almost entirely to production from mines along the Hispano-Portuguese pyrite Belt in Huelva Province. This belt extends for some 230 km, from Sevilla Province to the Atlantic Coast; over this distance it is estimated to have contained over 1 billion tons of ore containing 39-43% Fe, 2-6% Cu+Pb+Zn, 5-40 ppm Ag and 0.2-1.5 ppm Au. Of this total, roughly 300 million tons have been mined to date.

The pyritic deposits are associated with Lower Carboniferous marine volcanic rocks and intercalated slate, quartzite, sandstone and limestone. These rocks are overlain by the Carboniferous Culm Formation (conformable slates and graywackes), and rest upon Upper Devonian slates, quartzites, sandstones, limestones and conglomerates. The entire stratigraphic sequence was folded and metamorphosed during the Hercynian Orogeny, the major Late Paleozoic orogeny in Europe (Vazquez Guzman, 1989).

The most important mines on the Pyrite Belt are the Tharsis, the La Zarza and the Rio Tinto. The several pyrite orebodies exploited at Rio Tinto are related to an anticline within the so-called Rio Tinto Syncline; they are located on the eastern extremity of the anticline. Individual workings include the San Dionisio, Filon Sur [South Lode], Planes-San Antonio, Filon Notre [North Lode] and the Cerro Colorado. These deposits are thought to have originally formed a single, continuous stratum 5 km long by 750 meters wide and about 40 meters thick, containing half a billion tons of ore. But much of it has weathered away leaving an extensive gossan still rich enough to be mined, at least in part.

The largest of the Rio Tinto orebodies, the San Dionisio, was exploited by the Atalaya open pit and also the Alfredo shaft which served the underground workings. The Cerro Colorado workings are in the gossan where copper may be mined in the future. Except for the small Masa Valle deposit (in carbonaceous slates and rhyolitic ash), all of these orebodies occur associated with pyroclastic felsic rocks penetrated by chimneys, pipes and a multitude of small sulfide veins forming stockworks.

The pyritic orebodies occur as massive, stratabound deposits near volcanic centers, either overlying volcanic rocks or intercalated with shales and tuffs (Strauss and Madel, 1974). The mineralization is considered to be of volcanic-sedimentary origin intimately associated with felsic volcanic rocks, primarily sodic to potassic rhyolites and spilitized dacites (Schermerhorn, 1970). Some authorities believe the origin is related to a zone of subduction and associated island arc formation (Williams et al., 1975), whereas others see the volcanicity as representing magmatic activity in an intra-continental basin (Munha, 1979). In any case, all agree that the massive pyritic beds formed as the result of submarine metalliferous volcanic exhalations.

WORKINGS

Corta Atalaya

The Corta Atalaya (= Atalaya open pit) was opened in 1907 on the San Dionisio orebody. It currently has an elliptical or ovoid shape measuring 1,200 meters on the long axis, 906 meters across and 325 meters deep. Atalaya, with 26 bench levels at 12.5-meter intervals, is one of the most spectacular open pits in the world. In recent times it yielded 1 million tons of pyrite a year, but is now closed.

Pozo Alfredo

The Pozo Alfredo (= Alfredo shaft) is reached via a connecting tunnel and access ramp at the bottom of the Atalaya open pit. The workings are extensive, reaching a depth of 562 meters in the porphyritic portion of the San Dionisio orebody. Initially operated around the turn of the century, it employed the room-and-pillar method but later switched to cut-and-fill. The Alfredo workings consist of 45 levels at 12.5-meter intervals, with access down to level 32 via a headframe shaft, as well as the ramp from the 23rd level in the Atalaya pit.

Mine water has become quite acidic (pH = 2 or less) due to reaction with the sulfide ore. In 1987 and 1989 temporary flooding below level 33 literally dissolved a power substation, a mandibular crusher and several hundred meters of transport belts. The Pozo Alfredo is currently closed.

The Grupo Mineralogista de Madrid opposed the closing of the Pozo Alfredo, proposing instead that the shaft be reconditioned as a tourist attraction. As at other important mines in Spain, however, support from the general public and the mining companies was lacking. Nevertheless, many people still hope that the Rio Tinto mine will someday be transformed into a mining museum and tourist site.

Cerro Colorado

The Cerro Colorado (= Red Hill, named for its huge ferruginous gossan) carried many ancient workings but was reopened as an open pit operation for copper in 1966. As copper prices fell, however, the higher-grade portions of the oxidized copper zone were extracted, leaving much of the remainder uneconomical to mine. Gold and silver values in the gossan now support the operation, the only one still active at Rio Tinto.

Others

Numerous other less important workings exist in the area, including the Planes-San Antonio orebody (worked intermittently from Roman times up until 1950), and the Filon Sur and Filon Norte. For the mineral collector and historian there are many fascinating remains including Roman furnaces, abandoned processing facilities, shunting yards for steam engines, and an incredible Railway Museum.

MINERALS

The list of minerals which have been found at Rio Tinto over the years is extensive (see Table 1). Most of them are associated with the gossan and supergene zones of the orebodies which, being the richest in copper, have been heavily exploited. In old collections and museums it is possible to see specimens of azurite, malachite, cuprite, tenorite, native copper and other typical species from Rio Tinto. With the exception of native copper, however, specimens of these minerals are now almost impossible to find on the market.

The zone of secondary enrichment commonly carried chalcocite, bornite and covellite, but only in massive specimens of little interest to the collector. A large suite of complex sulfates occurs in a contact zone between the pyrite orebody and chlorite. The deeper, unaltered portions of the orebodies are occasionally crossed by thin veins containing sulfides and sulfosalts including sphalerite, galena, tetrahedrite and gratonite.

The following descriptions will deal only with those species which, because of their rarity or quality, are of particular interest to collectors. Nevertheless, this discussion is surely incomplete. Mineral collectors in southwestern Spain have never been particularly active or interested in documenting the minerals from important occurrences, so the available source material is sparse regarding earlier finds. Calderon (1910) mentioned a number of species from Rio Tinto but gave practically no descriptive information.

Barite BaS[O.sub.4]

A plumbian variety of barite has been found at Rio Tinto, in tabular to prismatic-looking crystals up to 5 cm long. The smaller crystals tend to have complex terminations. Many crystals are stained brown by iron oxides, but some excellent colorless, transparent specimens have also been recovered. The lead-rich variety was also quite abundant at the nearby Cerro Salomon area.

Copper Cu

Native copper has been found in large dendritic masses and crystalline arborescent growths; one is illustrated in Calderon's (1910) Los Minerales de Espana. The individual crystals are generally cuboctahedral in habit, sometimes coated with black tenorite. At Cerro Colorado it is still possible to find specimens, although in many cases they are hidden by surface oxides.

Goethite FeO(OH)

Beautiful iridescent goethite is very common at Cerro Colorado, as in most of the other mines in the Pyrite Belt. (Fantastic specimens have come from the Tharsis mine in particular.) It occurs in botryoidal to stactitic habits with beautiful colors and luster. Some exceptionally large cavities lined with goethite have been found in the gossan.

Gratonite [Pb.sub.9][As.sub.4][S.sub.15]

Gratonite, which collectors usually associate with the mines at Cerro de Pasco, Peru, was found at Rio Tinto in the 1980's. Only a single short-lived occurrence on level 23 of the Atalaya open pit has been encountered, extending to the same level of the Alfredo shaft. Unfortunately, although hundreds of specimens were found, they were nearly all badly damaged in transport.

The gratonite forms carpets and druses on massive pyrite, as steel-gray prismatic crystals with three-faced rhombohedral terminations. Most of the crystals are small, not exceeding 3 mm.

Specimens of gratonite are usually overvalued in the Rio Tinto area, and can be purchased more economically in Madrid.

[TABULAR DATA FOR TABLE 1 OMITTED]

Melanterite FeS[O.sub.4][center dot]7[H.sub.2]O and Chalcanthite CuS[O.sub.4][center dot]5[H.sub.2]O

The typical sulfates of copper and iron occur abundantly in the Alfredo shaft area, especially in rooms 6 and 18 on level 33. Spectacular formations of stalactites and stalagmites there reach several meters in length. Nearby acidic pools contain druses of magnificent, 1-cm, transparent, blue-green crystals of cuprian melanterite. Sadly, their conservation is almost impossible because of their strong tendency to oxidize and dehydrate if removed. Many visitors have come down to see the fantastic sulfate formations, including a television crew.

Voltaite [Mathematical Expression Omitted]

The finest voltaites in the world are probably those from levels 29 and 33 in the Pozo Alfredo workings. Brilliant black crystals to 1.5 cm show a complex combination of cube, octahedron and dodecahedron. They have grown in a pyrite-chlorite matrix with capillary white halotrichite and fibroferrite (?). On level 33 it is extremely common in octahedral crystals to 3 mm on halotrichite, pink coquimbite, red botryogen, blue chalcanthite, brown jarosite, yellow copiapite and many other complex sulfates . . . a colorful assemblage. These specimens have unfortunately never been made available on the mineral market, despite efforts to convince the miners of their value to collectors.

VISITORS INFORMATION

The Rio Tinto mines offer the visitor a rich mix of historic sites and facilities, local culture, and minerals. The description given here has been necessarily brief, omitting much more information of archeological and historical interest. Visitors will also enjoy seeing the Rio Tinto Mining Museum, with its extensive exhibits devoted to mining archeology, and may even have the opportunity to take an underground tour. For further information contact the Fundacion Rio Tinto, El Valle, Minas de Riotinto, 21660 Huelva, Spain.

ACKNOWLEDGMENTS

I would like to thank all of my friends at Riotinto and Rio Tinto Minera, S.A., in particular my companions in the Pozo Alfredo, and above all Pablo Fernandez Diaz, who took me on a tour, during an unforgettable month, of the best treasures of the Alfredo mine. My friend Francisco Pina took the photographs, and my friends James R. Catmur and Paloma Fradejas translated my original manuscript into English.

BIBLIOGRAPHY

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CALDERON, S. (1910) Los Minerales de Espana. Eduardo Arias, Madrid, 2 vols., 416 + 563 p.

COLLINS, H. F. (1921-1922) The igneous rocks of the Province of Huelva and the genesis of the pyritic orebodies. Institute of Mining and Metallurgy Transactions, 31, 61-169.

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Author:Garcia, Gonzalo Garcia
Publication:The Mineralogical Record
Date:Jul 1, 1996
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