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El Abra |
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Chile |
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Cu
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Super Porphyry Cu and Au
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The El Abra porphyry copper deposit is located some 40 km to the north of Chuquicamata, and ~280 km north-east of Antofagasta in northern Chile at an elevation of 3900 to 4100 m above sea level (#Location: 21° 55' 10"S, 68° 49' 56"W).
Copper mineralisation has been known at El Abra since pre-hispanic times, when turquoise and chrysocolla had been mined. During the early twentieth century, the British owned Compañia Minera de Calama exploited several vein deposits within the El Abra area. Subsequently, the US company Anaconda explored the area intermittently from 1945 to 1969, and indicated the presence of a large, low-grade deposit. The Chilean Government took control of the deposit as part of its nationalisation of the copper industry in 1971. The Chuquicamata Geological Division of the newly formed state owned company Codelco assumed responsibility for exploration in 1972, and subsequently outlined a significant copper-oxide resource by 1975. In 1994, Codelco entered into a joint venture to develop the deposit, resulting in the formation of the Sociedad Contractual Minera El Abra, 51% owned by the Cyprus Amax Mining Company, with mining commencing in 1995, and the first cathode production in August 1996. The Cyprus Amax share subsequently passed to Phelps Dodge in 1999 and then to Freeport McMoRan in 2007 as a result of successive corporate mergers in the US.
For details of the regional setting, see the Central Andes and Bolivian Orocline record.
Geological Setting - El Abra is part of the middle Eocene to early Oligocene belt of porphyry copper deposit that includes Collahuasi, 100 km to the north and Chuquicamata and Escondida 40 and 255 km to the south respectively.
Mineralisation at El Abra is hosted within the El Abra granodiorite complex, located 2 to 6 km to the east of the north-south striking West Fault, part of the regional Domeyko Fault System, which divides the area into two geological domains. The country rock in both domains if largely composed of volcano-sedimentary units of the Lower Palaeozoic to Upper Triassic Collahuasi formation, composed of basalts, andesites and rhyolites with local volcanic epiclastic units, and plutonic rocks of similar age. These are unconformably overlain by the Upper Cretaceous Tolar Formation, a sequence of red-brown terrestrial clastic sedimentary rocks, and the succeeding 53 to 43 Ma Lower Eocene Icanche Formation, composed of a package of subaerial volcanic and terrestrial volcanogenic sedimentary rocks (Tomlinson et al., 2001).
In the western domain, the Palaeozoic and Triassic volcanic and granitic rocks are partly in fault contact with the younger Tolar and Icanche formations, exposed in NNW trending synclines and anticlines. The Tolar Formation includes two members, a lower which comprises andesitic lavas and breccias, pyroclastic flow breccias and tuffs, and an upper unit, where breccias and conglomerates dominate. The Icanche formation rests conformably on the Tolar sequence, both east and west of the West Fault, consisting of hornblende andesite and pyroxene, hornblende and biotite dacites and, to a lesser extent, sandstones and conglomerates. In the vicinity of El Abra it is dominantly andesites (Tomlinson et al., 2001; Camus, 2003).
Three groups of intrusive rocks are recognised in the district surrounding El Abra, namely i). Palaeozoic granitoids, ii). pre-mineral Eocene intrusives, and iii). the El Abra granodiorite complex (Ambrus, 1977, 1979) that is associated with the economic mineralisation.
The deposit occurs in the core of the late Eocene El Abra granodiorite complex, which is part of a regional unit that comprises a set of intermediate to felsic calc-alkaline intrusions (Ambrus, 1977), divided into the Cerro Panizo de Ojuno quartz-diorite (37.9±1.0 Ma; K-Ar), the El Abra granodiorite and Llareta granodiorite-granite (Tomlinson et al., 2001). At El Abra, this complex includes intrusive units with quartz > K feldspar, hornblende and biotite, with lesser pyroxene and magnetite, and accessory sphene, and ranges in composition from diorite to granodiorite and syenite. It is sub-divided into the Central diorite, the Dark diorite and the Equis quartz-monzodiorite (Barrett, 2004), and is accompanied by the Rojo Grande granite, the local equivalent of the regional Llareta granite, which in turn has a transitional contact with the Llareta granodiorite to the north of the El Abra deposit. All of the units above are intruded by the Clara (or Cotari) granodiorite, the largest intrusion of the complex. This intrusion occupies the centre of the complex, and crops out in the southern section of the deposit, with an upper contact that dips gently to moderately outwards, and is the main host rock of mineralisation at depth. The Clara granodiorite is a coarse to medium crystalline rock with 10 to 15% mafic minerals, 20 to 25% quartz, 15 to 20% of K feldspar, 48 to 54% plagioclase, 2.5 to 6% hornblende and 2 to 7.5% biotite, with minor magnetite, titanite, apatite and zircon.
All of these intrusions are mostly fine to medium grained equigranular rocks, although the Clara granodiorite may locally be porphyritic. In contrast, younger units are dominantly porphyritic, which at El Abra include an aplitic granite porphyry, the main Lagarto granodiorite porphyry, and the late Lagarto mafic granodiorite porphyry (Dilles et al., 1997). The oldest of these porphyries occur as dykes and sheets that crosscut the Clara granodiorite and include aplitic granite porphyry, the Apolo leucogranite and an aplite-pegmatite unit, which are all interpreted to be differentiates expelled from the Clara granodiorite (Tomlinson et al., 2001). A quartz-monzonitic porphyry, known locally at the mine site as the El Abra Porphyry, is equivalent to the Lagarto porphyries. The main Lagarto granodiorite porphyry is associated with potassic alteration while the late Lagarto mafic granodiorite porphyry post-dates the potassic alteration (Dilles et al., 1997).
The complex is exposed over an area of 125 km2, with north-south and east-west dimensions of 13 and 12 km respectively. The intrusive complex at El Abra is truncated to the west by the West Fault, and is largely bounded along its southern and eastern margins by lower Mesozoic metavolcanic and metasedimentary rocks.
The northern limit of the complex is defined by a sharp contact with the older Pajonal diorite (Dilles et al., 1997). The composite Pajonal-El Abra igneous suite is exposed to the east of the West fault, and evolved over an ~7.5 m.y. period, from the oldest 43.2 Ma Pajonal diorite, through the 40.60±0.30 Ma Central diorite (U-Pb zircon LA-ICP-MS), 37.6±0.7 Ma (U-Pb zircon) Clara granodiorite to the youngest 35.6 Ma Lagarto porphyry (Valente et al., 2006; Correa et al., 2016). Within the El Abra deposit area, there are up to seven texturally distinct intrusive units, as described above (Barrett, 2004), whilst crosscutting relationships, and age determinations suggest multiple magmatic-hydrothermal events (Maksaev et al., 1990; Dilles et al., 1997; Tomlinson et al., 2001; Ballard et al., 2002; Campbell et al., 2006). Age determinations indicate magmatic activity within the immediate deposit area spans an ~3.5 m.y. period, between the older equigranular 40.60±0.30 Ma Central diorite and a dacitic phase of the younger El Abra porphyry (37.16±0.38 Ma; U-Pb zircon LA-ICP-MS; Ballard et al., 2002; Campbell et al., 2006), although additional phases have been encountered at depth which may be outside of this age range. K-Ar data from hydrothermal biotite and sericite associated with potassic and quartz-sericite alteration respectively, suggest a duration of ~2 m.y. (35.0 to 36.8 Ma) for the hydrothermal system (Ambrus, 1977; recalculated by Maksaev, 1990; Dilles et al., 1997; Tomlinson et al., 2001), although additional data generated by Correa et al., 2016 extends the duration of magmatic activity to ~8.6 m.y.
Structure - The dominant structural element of the district is the West fault, which corresponds to the eastern margin of the Sierra de Moreno basement block of Neoproterozoic, Palaeozoic and Mesozoic rocks that is, in turn, bounded to the west by a west vergent reverse fault. The West Fault is part of the regional regional Domeyko Fault System, which also passes through, or close to, the Collahuasi, Chuquicamata, Escondida and El Salvador porphyry copper deposits of Northern Chile.
The early diorites and granodiorites of the El Abra granodiorite complex, and the Clara granodiorite, are interpreted to have been emplaced during a ductile phase of deformation represented by high angle NNE to NE lineations and a high angle NNE foliation. Subsequent brittle deformation is indicated to have accompanied the emplacement of the later porphyry dykes, controlled by NW and less common NNE structures in a WNW directed maximum stress field (Tomlinson et al., 2001).
Mineralisation and Alteration - The El Abra granodiorites and the Clara granodiorite host the copper mineralisation at El Abra, with the majority within the latter. However, from a genetic viewpoint, there is a close temporarily and spatial association with the Lagarto granodioritic porphyries, which includes two varieties, one light and one mafic. The light variety has undergone K feldspar alteration and is rich in tourmaline, whereas the mafic phase is later, has a lower quartz content and post-dates potassic and sericitic alteration. Both contain ~50% phenocrysts of plagioclase, quartz, hornblende and biotite in a groundmass of aplite comprising quartz, alkali feldspar and rare biotite, with accessory apatite, zircon and sphene.
Igneous breccias are developed on the margins of the dykes of Lagarto granodiorite porphyry, commonly with a diameter of ~20 m, containing angular clasts of up to 50 cm in length in the major axis, set in a matrix of coarse biotite, quartz, orthoclase and some chalcopyrite (Ambrus, 1977). Other breccias have been infiltrated by tourmaline and albite. The most important of these is the Inca breccia, located 0.5 km west of El Abra, which passes from an igneous to hydrothermal breccia towards the top. It has a diameter of ~200 m, and contains angular fragments altered to an assemblage of sericite-quartz-pyrite in a matrix of sericite, quartz and tourmaline, with 5 to 10 vol.% sulphides, mainly pyrite.
Hydrothermal alteration occurred in two main pulses, i). an early late-magmatic potassic core, with a partially developed albitic periphery, and a propylitic halo, and ii). the main stage phyllic event. This alteration pattern is temporally and spatially associated with intrusion of the main Lagarto and mafic Lagarto dykes and igneous- hydrothermal-breccias (Tomlinson et al., 2001; Barrett, 2004; Ardila, 2009).
Potassic alteration occurs as pervasive replacement of plagioclase by K feldspar, recrystallisation of primary biotite phenocrysts, and the development of 'shreddy' biotite intergrown with magnetite and relict titanite. This alteration affects >2 km2 at the surface exposure of the deposit, and extends to depths of at least 1000 m below the surface. Much of the secondary biotite at El Abra is younger than the K feldspar alteration, and is probably related to igneous breccias matrix biotite, while the K feldspar alteration is contemporary with the intrusion of the Lagarto granodiorite porphyry. An up to 2 km wide, WSW-ESE trending corridor occupies the southern section of the main El Abra deposit, but continues over a narrower width to the ESE for a further 6 km, through the Veta Maria vein system on the southeastern margin of the El Abra mineralisation, to the Conchi Veijo porphyry mineralised zone (Camus, 2003).
The potassic alteration is accompanied by 'A' and 'B' quartz veinlets. The 'A' veinlets are discontinuous and undulose, typically filled with quartz, anhydrite, magnetite, calcite, biotite and sulphides (chalcopyrite±bornite), and have no halo. The 'B' veinlets have planar walls, have a central suture, and contain little quartz±chalcopyrite and molybdenite. These two groups of veinlets carry ~90% of the copper and molybdenum of the deposit (Ambrus, 1977).
Mineralisation within the potassic alteration is zoned, with a bornite-chalcopyrite±magnetite assemblage that forms a core with ~1 vol.% sulphides in the central parts of the deposit, although the degree of potassic alteration decreases with depth and at ~1000 m below the surface is less intense with a chalcopyrite to bornite ratio ranging from 1:1 to 1:2, and 0.8 to 0.2 vol.% S. Laterally, the central core passes outward into chalcopyrite > bornite±molybdenite, then to chalcopyrite > pyrite±molybdenite in the outer portions of the deposit (Ardila, 2009). The degree of potasic alteration decreases towards the outer margins of the deposit, where it is weak, with <1 vol.% S, and the chalcopyrite to pyrite ratio varies between 1:1 and 1:4. On the periphery of the deposit, the potassic alteration passes out into a broad propylitic halo, represented by an assemblage of epidote, chlorite and actinolite, with local magnetite.
K-Ar dating of biotite from potassic alteration yields ages ranging from 35.0 to 35.3 Ma (Ambrus, 1977; Maksaev, 1990; Tomlinson et al., 2001).
Albite alteration is locally developed in the western and southwestern section of the deposit, over an area of 0.2 to 0.5 km2 where it is overlapping and peripheral to the potassic zone, and occurs in two main styles: i). small breccias and dykes associated with tourmaline in thin veinlets with an albitic halo, or ii). in a breccia matrix in the deepest parts of El Abra deposit. Sulphides are rare in the albite altered zone, but where present, include pyrite and chalcopyrite. Albite replaces plagioclase, K feldspar and mafic minerals. This alteration occurs along seams with tourmaline in fractures with little vein fill, and as whitish halos of albite. Most have been subsequently replaced by 'D' veins with phyllic halos of sericite alteration, superimposed upon the albitisation (Camus, 2003).
Phyllic quartz-sericite-clay alteration overprints both the potassic and albitic zones and occurs in hydrothermal breccias and zones associated with NW to WNW-trending structures, and to the SSE along the main structural trend that passes from the El Abra deposit, through the Veta Maria vein to the Conchi Veijo porphyry mineralised zone. Alteration is generally associated with 'D' veins with thicknesses that vary from 1 mm to 1 m, and comprises an assemblage of sericite and chlorite with quartz halos. Quartz-chalcopyrite ±bornite ±pyrite ±molybdenite, with local tourmaline and anhydrite, are the typical vein-filling minerals. However, sulphides are dominated by hydrothermal pyrite, with a low chalcopyrite content, with the ratio of chalcopyrite:pyrite being <1:10. K-Ar determinations on sericite and sericitised whole-rock give ages of 36.0±1.3, 35.7±3.3, and 34.5±1.4 Ma, comparable to the age of potassic biotite alteration (Tomlinson et al., 2001).
Phyllic alteration is best developed at the Conchi Veijo porphyry mineralised zone, ~6 km ESE of the eastern margin of the El Abra deposit, where it persists to depths of as much as 700 m, in contrast to El Abra, where it does not appear to be found deeper than 300 m below the current surface (Camus, 2003).
Evidence of advanced argillic alteration is only seen at the Veta Maria [vein], located immediately to the ESE of the main WNW-ESE mineralised corridor along the southern margin of El Abra, and is characterised by an assemblage of pyrite-chalcopyrite-bornite and enargite-pyrite. This style of alteration and sulphide development has not encountered elsewhere at El Abra or at Conchi Viejo (Camus, 2003).
NW-SE to WNW-ESE structures control the emplacement of the mineralisation at El Abra, including porphyry dykes and both 'A' and 'B' veinlets. 'D' veinlets, which are related to quartz-sericite alteration, have an extensional character. The post-mineral structures, which also trend NW-SE to WNW-ESE, but also NNW-SSE and east-west in the immediate deposit area, and reactivate 'D' veins, match the axes of the principal stress controlling the sinistral displacement on the West fault (Santana, 2010).
The outline of the mineralised zone at El Abra, as defined by the 0.25% Cu contour, comprises a generally NNE-SSW to NE-SW elongated, 3 x 2 km oval shaped area, with an overlapping WNW-ESE trending, 4.5 x <1 km linear zone along its southern margin, to produce a composite 'snail-like' shape.
Supergene alteration and mineralisation - The upper sections of the El Abra deposit comprise an oxidised sheet that covers an area of ~1 km2, and deepens from ~45 m at the centre of the deposit, to ~300 m on its northern margin, with a relatively planar base. The central part of the deposit, where potassic alteration is more intense, has the highest oxidised copper grades, predominantly occurring as chrysocolla with lesser atacamite, pseudomalachite, antlerite, brochantite and olivenite [Cu2 AsO4 (OH)]. These minerals are contained in both fractures and veins. This central core is surrounded by lower grade copper oxides, coinciding with the shell of less intense potassic alteration, and is characterised by copper impregnated plagioclase, chrysocolla, copper wad and copper pitch, accompanied by montmorillonite group clays and minor kaolinite. The 'D' veins that originally contained significant quantities of pyrite, are enriched in chalcocite to depths of 50 to 150 m beneath the oxidised zone, accompanied by kaolinite. The main oxide zone is underlain by a mixed sulphide and oxide layer of variable thickness, ranging from centimetres to 60 m in the best mineralised area. This zone is composed of an upper chrysocolla, cuprite, tenorite and native copper assemblage, underlain by weak developments of chalcocite and primary sulphide disseminations (Camus, 2003).
The hypogene ore at El Abra is characterised by has a low pyrite content, which, with the mafic mineral content of the hosts, has resulted in little supergene sulphide enrichment, but strong in situ oxidation of the ore.
Exotic copper mineralisation is developed over an interval of >2 km along Ichuno creek, which overlaps the western margin of the main deposit and flows west and then south. This mineralisation comprises chrysocolla, copper wad and some atacamite, and is hosted in fractures and breccia bodies in the El Abra granodiorite, and to a lesser extent, cements alluvial gravels.
The El Abra operation was originally based on a 400 Mt oxide resource with an average grade of 0.75% Cu, treated by SX/EW technologies. It was said to overlie ~800 Mt @ near 1% Cu as hypogene sulphides (mine visit, 1998).
Total production to December 31, 2002 was 300 Mt @ 0.65% Cu Total .
The remaining reserves at December 31, 2002 were 536 Mt at 0.41% Cu Total , when the production capacity was 225 000 tonnes of copper cathode per annum.
Remaining proven + probable ore reserves at December 31, 2015 were (Freeport McMoRan, 2016) - 339 Mt @ 0.44% Cu, with annual production of ~150 000 tonnes of copper cathode per annum between 2013 and 2015.
The most recent source geological information used to prepare this decription was dated: 2016.
Record last updated: 12/3/2017
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.
El Abra
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Selected References:
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Ambrus J 1977 - Geology of the El Abra porphyry copper deposit, Chile: in Econ. Geol. v72 pp 1062-1085
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Ballard J R, Palin J M, Campbell I H 2002 - Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in Zircon: application to Porphyry Copper deposits of northern Chile : in Contrib. to Mineralogy & Petrology v144 pp 347-364
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Campbell I H, Ballard J R, Palin J M, Allen C and Faunes A, 2006 - U-Pb Zircon Geochronology of Granitic Rocks from the Chuquicamata-El Abra Porphyry Copper Belt of Northern Chile: Excimer Laser Ablation ICP-MS Analysis : in Econ. Geol. v101 pp 1327-1344
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Camus, F., 2003 - El Yacimiento El Abra,: in Geologia de los Sistemas Porfiricos en los Andes de Chile Corporacion Nacional del Cobre de Chile; Servicio Nacional e Geologia y Mineria; Sociedad Geologica de Chile, [in Spanish], pp. 161-167.
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Correa, K.J., Rabbia, O.M., Hernandez, L.B., Selby, D. and Astengo, M., 2016 - The Timing of Magmatism and Ore Formation in the El Abra Porphyry Copper Deposit, Northern Chile: Implications for Long-Lived Multiple-Event Magmatic-Hydrothermal Porphyry Systems: in Econ. Geol. v.111, pp. 1-28
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Dean D A, Graichen R E, Barrett L F, Burton W D 1995 - Geological overview of the El Abra porphyry copper deposit, Chile: in Green S M, Struhsacker E (Eds), Geology and Ore deposits of the American Cordillera Geol. Soc. of Nevada Field Trip Guidebook Compendium, 1995, Reno/Sparks, NV pp 457-464
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Park, J.-W., Campbell, I.H., Malaviarachchi, S.P.K. Cocker, H., Hao, H. and Kay, S.M., 2019 - Chalcophile element fertility and the formation of porphyry Cu - Au deposits: in Mineralium Deposita v.54, pp. 657-670.
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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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