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Baja de la Alumbrera
Catamarca, Argentina
Main commodities: Cu Au

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The Bajo de la Alumbrera porphyry copper-gold deposit (Alumbrera) is located within the northern Sierras Pampeanas in the eastern Andes Mountains of Catamarca province in northwestern Argentina. It lies 145 km to the northwest and 150 km southwest of the Provincial capitals of Catamarca and Tucuman respectively.
(#Location: 27° 19' 48"S, 66° 36' 27"W)

It is on the eastern margin of the Miocene to Pliocene belt of porphyry copper mineralisation, some 250 to 300 km to the east of the main belt of Chilean porphyry deposits. The deposit is centred on a two phase dacite-porphyry stock emplaced within a large andesitic strato-volcano. It has a central cylindrical core of K-silicate alteration with high grade Cu-Au zones rich in quartz and magnetite, surrounded by sericite and supergene kaolinite, with an external propylitic zone. The supergene profile is immature and economic ore is principally hypogene (Brown, 2005).

The Alumbrera deposit formed in a tectonically favourable location within a major arc-oblique wrench fault system, the Tucuman Transfer Zone. Initial andesitic volcanism deposited on crystalline Lower Palaeozoic basement, and subsequently emplaced dacitic subvolcanic stocks are directly related to eastward subduction of the Nazca oceanic plate beneath the western continental margin of South America. Structural preparation and shallowing of the angle of subduction of the Nazca plate - related to the arc-normal Juan Fernandez Ridge on that plate - probably aided the ascent of calc-alkaline oceanic arc-related magma into the Tucuman Transfer Zone (Brown, 2005).

The commencement of volcanism was approximately coincident with the shallowing of the angle of subduction beneath the northern Sierras Pampeanas. Intrusion of the mineralised porphyries was contemporaneous with the development of thick-skinned shortening which produced uplift of the Sierras de Aconquija basement block to the southeast at between 10 and 5 Ma. Structural degradation of the crystalline basement brought about by the development of a broad asthenospheric wedge above the descending Nazca plate, aided the development of block uplift caused by generally east-west shortening at that time (Brown, 2005).

The country rock surrounding the deposit are predominantly extrusive rocks of the Farallán Negro Volcanic Complex. This complex dominantly comprises pyroclastic breccias with minor interlayered massive or autobrecciated lava flows, heterolithic mud-flow breccias, and epiclastic sandstones (Llambias, 1972; Sasso, 1997; Sasso and Clark, 1998). Although amphibole-plagioclase andesites are prevalent, the total compositional range of these volcanics extends from basalts (±clinopyroxene phenocrysts which are generally at the base of the sequence) to rhyolite (biotite ±sanidine phenocrysts) occurring as late domes and ignimbritic diatremes (e.g., Hug, 1999; Brown, 2005)).

Volcanic wall rocks in the immediate vicinity of the Bajo de la Alumbrera porphyry have been subdivided into four separate units; i). a lowermost sequence of coarse pyroxene andesite breccias, tuffs and flows that are overlain by ii). amphibole-plagioclase andesites, then by iii). plagioclase andesites and the iv). uppermost unit of pyroxene andesite breccia and hornblende andesite breccia (Ulrich and Heinrich, 2002 after J.M. Proffett). Microscopic features indicate pyroxene and amphibole (titanaugite/oxyhornblende) are Fe- and Ti-rich (Brown, 2005).

The following intrusions have been recognised, from oldest to youngest (Ulrich and Heinrich, 2002):
NE Porphyry dyke which may be the oldest intrusion in the deposit area, although no clear relationship has been determined with other porphyries. It dips to the west and intrudes andesites in the northeast of the deposit. It is intruded by, and has a similar mineralogy to the Early P3 Porphyry, but with fewer quartz phenocrysts. The dyke is partly mineralised and has undergone complete propylitic alteration.
P2 Porphyry, which is occurs as a stock-like body in the core of the deposit, characterised by plagioclase, biotite and a few quartz phenocrysts that are rarely preserved. Most of the P2 Porphyry has been altered to a quartz-magnetite ±K feldspar rock is strongly veined and is generally well mineralised, and has been cut by the Early P3 and Quartz-Eye porphyries.
Los Amarillos Porphyry, known previously as the 'pre- Main-Stage Porphyry' (Stults, 1985). This porphyry is exposed in the northwestern and western part of the deposit, exposed as light-coloured, strongly altered hills. Its relative timing is unclear, although it is cut by Early P3 and younger porphyries. In many areas it more closely resembles an igneous breccia than a porphyry, containing vein fragments of an older intrusion, possibly P2 Porphyry.
Early P3 Porphyry, the largest porphyry in the deposit area, although it is probably a composite intrusion of several phases. It crosscuts P2 Porphyry that had been mineralised already, but also contains all of the alteration and mineralisation types in very variable intensities. Vein and textural relationships have been derived mainly from this porphyry.
Quartz-Eye Porphyry, which is exposed in the southeastern and southwestern part of the deposit and contains up to 8 mm phenocrysts or xenocrysts of quartz with altered plagioclase, biotite and hornblende. It is strongly veined by characteristic purple vuggy quartz and is pervasively altered to quartz-sericite and moderately mineralised. These purple veins occasionally contain bornite and chalcopyrite but little or no magnetite. The Quartz-Eye Porphyry cuts P2 Porphyry, although its timing relative to the Early P3 Porphyry is unclear.
Late P3 Porphyries that comprise a series of intrusions into the Early P3 and Quartz-Eye Porphyries. There are at least two phases identified, the Campamiento and the North Porphyry, occurring as north-south elongated dykes that vary in width from 40 to 160 m. They are characterised by a greater abundance of phenocrysts of hornblende and biotite, and by large quartz crystals, although there are fewer of the latter than in the Quartz-Eye Porphyry. The mineral assemblages are somewhat variable from south to north, although this may largely be a reflection of the distribution of different alteration styles. Textures are commonly well preserved, although they are slightly altered to quartz-sericite and propylitic assemblages. These porphyries are barren or weakly mineralised.
Younger porphyry dykes, which include the Northwest Porphyry dykes and the Post-Mineral Porphyries that are essentially unmineralised and cut most of the other intrusions with an ~NNW-SSE trend, commonly extending beyond the outer edge of the deposit. Irregular bodies found in the centre, and dykes in the NW of the deposit have been correlated with this stage. One dyke of late porphyry can be traced northwestward through the feldspar-destructive alteration zone and the Los Amarillos Porphyry, where it splits into two.
Northwest Porphyry dykes, composed of biotite, plagioclase, partly fresh hornblende, minor magnetite, and scarce quartz phenocrysts set in a grey matrix of plagioclase and quartz.
Post-Mineral Porphyries, which occur as small discontinuous NNW trending dykes. Near the surface, these dark dykes cut all other rocks, are largely unaffected by alteration, and usually contain no veins or sulphide mineralisation.

Ar-Ar dating of the various mineralised intrusive phases indicates intrusive activity lasted for approximately 270 000 years. The upper age of mineralisation is interpreted to be at the onset of feldspar destructive alteration (phyllic-argillic styles) at 6.75 ± 0.09 Ma (Brown, 2005). The principal syn-mineral intrusions are the P2 and Early P3 porphyries, with weak to moderate mineralisation in the late-mineral Quartz-Eye Porphyry and only weak mineralisation in the Late P3 porphyries (Ulrich and Heinrich, 2002).

Mineralisation was focussed on the intrusive centre, largely defined by the P2 and Early P3 porphyries, and the surrounding andesitic host rocks. Reconstruction of the original geometry of the deposit indicates that the cluster of porphyry stocks and dykes that define the intrusive centre formed a sub-circular body with a diameter of around 500 m, while the overall dimensions of the mineralised system (at greater than 0.15% Cu) was approximately 800 x 800 m. The vertical dimension of the mineralisation is less easily measured, but was probably in the range of 800 to 1000 m. Approximately 3.36 million tonnes (Mt) of copper and 409 tonnes of gold were deposited within this volume.

Fluid inclusion studies indicate that deposition of mineralisation seems to have been strongly controlled by cooling of the mineralising fluids, with sulphide phases being formed as the fluid cooled below a 400 to 360° C temperature threshold.

Four petrographic alteration mineral assemblages have been identified by Ulrich and Heinrich (2002).
Intense quartz-magnetite ±K feldspar alteration, which forms an innermost core of intense alteration that replaced the magmatic mineralogy and largely obliterated the igneous texture. This phase is characterised by barren quartz-magnetite veins of irregular shape, length and thickness, indicating vein formation in a ductile rock at least for some of these veins;
Potassic alteration comprising an assemblage of biotite-K feldspar-quartz ±magnetite ±anhydrite, which was near simultaneous with, and closely related to, the intense quartz-magnetite phase, and occurs in the surrounding low-lying dark-weathered porphyries and proximal andesite country rock. Copper and gold mineralisation is closely associated with potassic alteration. The combined quartz-magnetite and potassic zones cover an area of >1 km2.
Propylitic alteration, occurring as an unmineralised halo developed in the andesites beyond the limit of potassic alteration. It is characterised by an assemblage of chlorite-epidote-albite ±calcite ±magnetite that affects the andesitic country rocks for up to ~1 km from the composite intrusive stock, including and locally extending beyond a high ring of dark peaks rising up to 300 m above the 'Bajo' (the central 'basin' of subdued relief reflecting the main alteration zone).
Feldspar-destructive phyllic and argillic alteration, comprising an assemblage of sericite + quartz + pyrite ±kaolinite ±illite overprinting sections of the potassic and propylitic zones, and exposed as a low ring of white hills at the base of the surrounding steep andesite peaks. This phase is interpreted to have formed in response to degradation of the thermal plume and consequent induction of increasingly acidic phreatic water into convection cells adjacent to the intrusives. Feldspar destructive alteration is usually accompanied by decreasing grade, suggesting that at Alumbrera this alteration stage remobilised and removed mineralisation.

The density of quartz veins ±magnetite ±K feldspar ±sulphides) varies systematically between different porphyries. The largest volume of veins are hosted by P2 Porphyry (20 to 50 vol.%) and Early P3 Porphyry (5 to 20 vol.%) as well as the andesites (1 to 5 vol.%) in contact with these two porphyries. Later intrusions have a much lower density of mineralogically different veins (<2%).

Early Cu mineralisation occurs as quartz-magnetite-chalcopyrite ±bornite veins related to the alteration assemblage of K feldspar-quartz-magnetite ±biotite (Proffett 2003). This pyrite-absent, rare bornite mineralisation is only preserved in rocks with a minimal later alteration overprint. The principal Cu mineral in the deposit is chalcopyrite, occurring as centerline filling in quartz veins or patches between the quartz and magnetite grains of the veins, i.e., petrographically late relative to most of the vein quartz. These veins are more commonly associated with biotite-K feldspar-quartz alteration, with very narrow selvages of K feldspar. Both vein types are irregular in shape, fine to coarse grained, and range from 3 to 10 mm in width. No crosscutting relationship between these two vein types has been observed. Chalcopyrite also occurs as dissemination in the potassic alteration and in parts of the quartz-magnetite alteration where it is intergrown with magnetite or lies along grain boundaries between magnetite and quartz grains. Limited petrographic observations of gold in polished section showed small grains within or attached to chalcopyrite (Ulrich and Heinrich, 2002). Dawson (1994) described gold concentrations in magnetite, pyrite and K feldspar.

Massive chalcopyrite-pyrite ±quartz veins that are up to 7 mm thick and planar thin veinlets and joints coated with pyrite and chalcopyrite, generally have associated halos of chlorite ±sericite alteration. They contain pyrite overgrowing chalcopyrite to variable degrees, with occasionally earlier magnetite and later hematite. These veinlets cut older veins and intrusive contacts, and extend into later, largely unmineralised porphyries of the Late P3 and the Northwest Porphyries. Although they clearly postdate the main copper deposition represented by the quartz-chalcopyrite veining, they may, nevertheless, host an economically significant fraction of the copper in the deposit. This indicates late redistribution on a local scale (<10 m), rather than significant introduction of copper during the formation of the late sulphide veins (e.g., Brimhall, 1980). Gypsum veins with or without pyrite are the latest. They are 1 to 10 mm in width, commonly straight with various orientations, and some are associated with late faults (Ulrich and Heinrich, 2002).

Structural rotation of the northern Sierras Pampeanas during the Upper Miocene to Lower Pliocene resulted in the reactivation of earlier structures in a transpressional regime that caused dismemberment of the porphyry mineralisation at Alumbrera. Strong alteration of the earliest of the major faults suggests that structural disruption occurred during the development of feldspar destructive alteration, and may have been responsible for the termination of this alteration event. Subsequent movement resulted in displacement of the ore blocks across both normal and reverse faults (Brown, 2005).

Active uplift of the Sierras Pampeanas and consequent rapid erosion prevented the development of significant secondary oxide or supergene mineralisation at Alumbrera.

The pre-mining measured resource was: 695 Mt @ 0.51% Cu, 0.66 g/t Au, with a high grade core of 118 Mt @ 0.64% Cu, 0.92 g/t Au.
Production to the end of 2004 amounted to: 220 Mt @ 0.66% Cu, 0.86g/t Au.
Remaining reserves + resources at the end of 2004 were: 404 Mt @ 0.47% Cu, 0.51g/t Au (Brown, 2005).

Remaining Ore Reserves and Mineral Resources at 31 December, 2019 (Miningdataonline.com) were:
  Proved + Probable Reserves underground - 69 Mt @ 0.4% Cu, 0.39 g/t Au, 0.013% Mo;
  Measured + Indicated Resources open pit - 16 Mt @ 0.2% Cu, 0.66 g/t Au;
  Measured + Indicated Resources underground - 123 Mt @ 0.37% Cu, 0.35 g/t Au, 0.014% Mo;
  Inferred Resources open pit - 6 Mt @ 0.2% Cu, 0.48 g/t Au;
  Inferred Resources underground - 1 Mt @ 0.29% Cu, 0.28 g/t Au, 0.014% Mo;

The mine was operated in 2004 by Minera Alumbrera Limited which is owned by Xstrata plc 50%, Wheaton River Minerals Ltd. 37.5 % and Northern Orion Resources Inc. 12.5 %.
Ownership in 2019 was Glencore plc 50%, Newmont Corp. 37.5%, Yamana Gold, 12.5%.

Open pit mining was completed in 2018. The mine then went to underground sub-level caving with a mine life of 10 years to 2028. The infrastructure of the operation was to be used in the development of the nearby Agua Rica deposit which is 30 km to the east.

The most recent source geological information used to prepare this decription was dated: 2006.     Record last updated: 8/3/2021
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.


    Selected References
Brown S C,  2005 - A Review of the Geology and Mineralisation of the Alumbrera Porphyry Copper-Gold Deposit, Northwestern Argentina: in Porter, T.M. (Ed), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide,   v.1 pp. 115-131
Guilbert J M  1995 - Geology, alteration, mineralisation and genesis of the Bajo de la Alumbrera porphyry copper-gold deposit, Catamarca Province, Argentina: in Pierce F W, Bolm J G (Eds),  Porphyry Copper Deposits of the American Cordillera Arizona Geol. Soc.   Digest 20 pp 646-656
Harris A C, Allen C M, Bryan S E, Campbell I H, Holcombe R J and Palin J M,  2004 - ELA-ICP-MS U-Pb zircon geochronology of regional volcanism hosting the Bajo de la Alumbrera Cu-Au deposit: implications for porphyry-related mineralization: in    Mineralium Deposita   v39 pp 46-67
Harris A C, Bryan S E and Holcombe R J  2006 - Volcanic Setting of the Bajo de la Alumbrera Porphyry Cu-Au Deposit, Farallon Negro Volcanics, Northwest Argentina: in    Econ. Geol.   v101 pp 71-94
Harris A C, Golding S D and White N C  2005 - Bajo de la Alumbrera Copper-Gold Deposit: Stable Isotope Evidence for a Porphyry-Related Hydrothermal System Dominated by Magmatic Aqueous Fluids: in    Econ. Geol.   v100 pp 863-886
Harris, A.C., Dunlap, W.J., Reiners, P.W., Allen, C.M., Cooke, D.R., White, N.C., Campbell, I.H. and Golding, S.D.,  2008 - Multimillion year thermal history of a porphyry copper deposit: application of U-Pb, 40Ar/39Ar and (U-Th)/He chronometers, Bajo de la Alumbrera copper-gold deposit, Argentina: in    Mineralium Deposita   v.43, pp. 295-314.
Proffett J M  2003 - Geology of the Bajo de la Alumbrera Porphyry Copper-Gold Deposit, Argentina: in    Econ. Geol.   v98 pp 1535-1574
Sillitoe R H  1995 - Baja de La Alumbrera, Argentina: in Sillitoe R H  Exploration and Discovery of Base- and Precious-Metal Deposits in the Circum-Pacific Region During the Last 25 Years Metal Mining Agency of Japan    pp 21-23
Ulrich T, Gunther D, Heinrich C A,  2001 - The evolution of a Porphyry Cu-Au deposit, based on LA-ICP-MS analysis of fluid inclusions: Bajo de la Alumbrera, Argentina: in    Econ. Geol.   v96 pp 1743-1774
Ulrich T, Heinrich C A  2001 - Geology and alteration geochemistry of the Porphyry Cu-Au deposit at Bajo de la Alumbrera, Argentina: in    Econ. Geol.   v96 pp 1719-1742

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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