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Aznalcollar |
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Seville, Sevilla, Spain |
| Main commodities:
Cu Au Zn S Pb Ag
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Super Porphyry Cu and Au
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The Aznalcóllar and Los Frailes pyrite and polymetallic deposits are located within Seville Province and belong to the Iberian Pyrite Belt of southern Spain. It is located ~45km west of Seville in southern Spain and includes the historic mines of Santiago, Panaleta, Cuchichon, Masa Nueva, Silillos & Higuereta.
In 1990 the Aznalcóllar - Los Frailes operation had reserves of 43 Mt of massive pyrite @ 0.44% Cu, 1.74% Pb, 3.33% Zn, 67 g/t Ag, 1 g/t Au.
Also outlined was a reserve of cupriferous pyroclastics amounting to 47 Mt @ 0.58% Cu, 0.40% Zn, 10 g/t Ag.
Exploitation of massive sulphides and their oxidation products at Aznalcóllar dates to when it was under the control of the Carthaginians in 237 BCE. Mining continued under the Romans from ~205 BCE, when it was known as 'Iptucci', and then by the Visigoths after they conquered the Romans in 410 CE. The Visigoths defended the fortified site from the Moors who defeated them in 844 CE. The name Aznalcóllar comes from the Arabic 'hanz-al-kollar', meaning 'walled enclosure', as dubbed by the Moors. Modern operations at Aznalcóllar were commenced in 1979. The Los Frailes orebody was discovered in 1988, 1 km east of the Aznalcollar open-pit mine. Production ceased at Aznalcollar in 1996, after depletion of its reserves, and operations transferred to Los Frailes. The concentrator was modified to treat the new mine’s ore, and by December 1997, Los Frailes had attained full rated mine output of 4 Mtpa and was successfully milling the new ore. However, in April 1998 Boliden Apirsa was obliged to halt the operation after the tailings dam failed releasing 4 to 5 million cubic metres of mine tailings into rivers traversing the adjacent Doñana National Park. The company was tasked with the clean-up, which was stored in the old open pit, and recommenced production during 1999. Soon after, the concentrator ran into technical problems with the then-available ore. In 2000, Boliden decided to cease operations when the active pit was exhausted, which occurred in September 2001 (Mining-Technology website entry dated 12 June 2000). The Los Frailes mine is now owned by the Andalucían regional government (Boliden, 2022). Plans to restart the operation have attracted opposition on enviromental grounds.
As of January 1999, Los Frailes had (Mining-Technology website entry dated 12 June 2000):
Proved + Probable Ore Reserves of - 43 Mt @ 0.3% Cu, 2.1% Pb, 3.7% Zn, 58 g/t Ag.
Measured + Indicated Mineral Resources of a further - 30 Mt at similar grades.
Almodóvar et al. (1997) suggest the mines of the Aznalcóllar District contained:
130 Mt of massive sulphides averaging approximately 0.4% Cu, 2% Pb, 3.6% Zn, 65 g/t Ag.
Lithologies exposed in the Aznalcóllar district include Upper Devonian to Lower Carboniferous detrital and volcanic rocks of the three main stratigraphic units identified characteristic of the Iberian Pyrite Belt, namely the: i). Upper Devonian Phyllite-Quartzite Group, comprising slates, quartzites, sandstone, limestone and conglomerate that are unconformably overlain by ii). Devonian to Carboniferous Volcano-Sedimentary Complex; and iii). Carboniferous Culm Formation slates and greywackes.
Two sequences have distinguished within the Volcano-Sedimentary Complex. To the south it is mainly detrital, as indicated above, but also includes basaltic pillow-lavas and shallow-water limestone. In contrast, the Aznalcóllar-Los Frailes sequence contains abundant volcanic rocks, correlated with the two main felsic volcanic episodies seen elsewhere in the Iberian Pyrite Belt. Massive sulphides at Aznacollar occur in association with black shales overlying the first felsic volcanic package, which om microfossil evidence is Upper Devonian in age. Field and textural relationships of volcanics suggest an evolution from a subaerial pyroclastic environment for the first volcanic package to hydroclastic subvolcanic conditions for the second. Despite changes in the character of volcanism and deposition, the same dacitic to rhyolitic composition are found in both pyroclastic and subvolcanic igneous series (Almodóvar et al., 1997).
The immediate host sequence includes rhyolites, felsite, pyroclastics, altered clayey pyroclastics, graphitic black shale and the massive pyrite. The tabular massive pyrite body is up to 100 m in thickness and extends over a NE trending strike length of 1750 m.
The Hercynian orogeny resulted in a complex structural evolution with a major, ductile deformation phase (F1), and development of folds that progressed to thrusts by short flank lamination. These thrusts led to tectonic repetition of massive and stockwork mineralisation. Some of the stockwork mineralisation has been overthrust onto massive sulphides. These structures are, in turn, cut by large brittle overthrusts and by late wrench faults. The original geometry of the massive sulphides is interpreted to have been present as large blankets with very variable thicknesses of from 10 to 100 m, systematically associated with stockwork zones. Footwall alteration is zoned, with an inner chloritic and a peripheral sericitic assemblage. Silica, sulphides and carbonates are also within these assemblages. Hydrothermal alteration is considered too be multi-stage, geochemically characterised by Fe, Mg and Co enrichment and intense leaching of alkalies and Ca. REE, Zr, Y and Hf are also mobilised in the inner chloritic zones (Almodóvar et al., 1997).
Three ore types are recognised, both in stockworks and massive sulphides, namely pyritic, polymetallic and Cu-pyritic. Of these, Cu-pyritic is the more common in stockworks, whereas polymetallic is prevalent in massive sulphides. Many of the alternations of polymetallic and barren pyritic zones are probably due to structural repetition. Although the paragenesis is complex, several successive mineral associations can be distinguished, namely: framboidal pyritic, high-temperature pyritic (300°C), colloform pyritic, polymetallic and a late, Cu-rich high-temperature association (350°C). Statistic population analysis of fluid inclusion data points to three stages of hydrothermal activity, at low (<200°C), intermediate (200 to 300°C) and high temperatures (300 to 400°C) (Almodóvar et al., 1997).
There is an intimate spatial relationship between massive sulphides and black shales, suggesting the massive sulphides were precipitated and replaced black shales which acted both as physical and chemical barriers during sulphide deposition (Almodóvar et al., 1997).The same authors suggest a three-stage genetic model comprising: i). low temperature, diffuse fluid flow, producing pyrite-bearing lenses and disseminations interbedded with black shales with locally, channelled high-T fluid flow; ii). cyclic hydrothermal activity at a low to intermediate temperature, producing most of the pyritic and polymetallic ores, and iii). a late high-temperature phase, yielding Cu-rich and Bi-bearing mineralisation, mainly in the stockwork zone.
The most recent source geological information used to prepare this decription was dated: 1989.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Almodovar G R, Saez R, Pons J M, Maestre A, Toscano M, Pascual E, 1997 - Geology and genesis of the Aznalcollar massive sulphide deposits, Iberian Pyrite Belt, Spain: in Mineralium Deposita v33 pp 111-136
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Leistel J M, Marcoux E, Thieblemont D, Quesada C, Sanchez A, Almodovar G R, Pascual E, Saez R, 1997 - The volcanic-hosted massive sulphide deposits of the Iberian Pyrite Belt. Review and preface to the Thematic Issue: in Mineralium Deposita v33 pp 2-30
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Sanchez-Espana J, Velasco F, Boyce A J, Fallick A E 2003 - Source and evolution of ore-forming hydrothermal fluids in the northern Iberian Pyrite Belt massive sulphide deposits (SW Spain): evidence from fluid inclusions and stable isotopes: in Mineralium Deposita v38 pp 519-537
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Soriano C, Marti J 1999 - Facies analysis of volcano-sedimentary successions hosting massive sulfide deposits in the Iberian Pyrite belt, Spain: in Econ. Geol. v94 pp 867-882
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Tornos F, 2006 - Environment of formation and styles of volcanogenic massive sulfides: The Iberian Pyrite Belt: in Ore Geology Reviews v28 pp 259-307
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Vazquez Guzman F 1989 - Spain (Extracts): in Dunning F W, Garrard P, Haslam H W, Ixer R A (Eds.), Mineral Deposits of Europe IMM, London v 4/5: Southwest and Eastern Europe, with Iceland pp 105-127, 194-196
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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