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Los Pelambres, Frontera

Chile

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The Los Pelambres and satellite Frontera porphyry style copper-molybdenum ore deposits are located some 200 km north of Santiago in Chile, and immediately west of the border with Argentina (#Location: 31° 43' 01"S, 70° 29' 28"W).

The deposit contains more than 20 Mt of copper, and in 2010 produced around 0.325 Mt of copper per annum. It is the northern-most of the trio of giant Miocene to Pliocene deposits (with El Teniente and Los Bronces-Rio Blanco) formed near the boundary between two major Andean segments, the flat slab segment to the north and the steeper subduction angle Southern Volcanic zone to the south.

Some 5 km to the south-east of Los Pelambres the El Pachon porphyry copper deposit (880 Mt @ 0.62% Cu, 0.01% Mo, 0.02 g/t Au) in Argentina is embraced by a continuation of the same alteration zone/mineralised system responsible for Los Pelambres.

The regional basement to the Los Pelambres-El Pachon mineralised system is composed of a Palaeozoic sequence commencing with marine shelf sediments, grading up into continental clastics and volcanics, and cut by common Palaeozoic granitoid batholiths. These were overlain by Jurassic to Cretaceous marine strata and andesitic lavas and flow breccias (with associated batholiths), discordantly followed by Paleocene to Oligocene continental volcanics and lesser continental sediments, which following deformation, uplift and erosion were overlain by Oligocene to Miocene andesite flows and pyroclastics, and interbedded continental sediments.

The dominant rocks within the Los Pelambres district comprise andesitic lavas, flow breccias and volcaniclastic sediments which have been intruded by dioritic and granodioritic masses, including the mineralised stock at Los Pelambres. This local andesitic sequence is subdivided into three units, commencing with the early Cretaceous Los Pelambres Formation (hosting the main mineralised intrusive at Los Pelambres) which is composed mainly of thinly bedded andesitic lava flows, which are generally each only a few metres thick, with some flow breccias and volcaniclastics. These are overlain by the Viñita Formation lava flows, flow breccias and volcaniclastics of pre 24.9 Ma age, followed by the third unit, equivalents of the Farellones Formation, which comprise reddish andesite and andesitic breccia, tuffs and volcaniclastics.

These andesites are intruded by small irregular dioritic to granodioritic plutons and by the 5 x 1 to 2 km composite mineralised stock at Los Pelambres. In the mine area the following phases have been recognised in the latter:
i). A quartz-feldspar porphyry (a granite porphyry) is the oldest intrusive, which occurs as a steeply dipping sill like mass and is believed to predate the main mineralised stock.
ii). Quartz diorite (or tonalite) which comprises 90% of the stock and is the principal host, comprises a light to medium grey, sub-equigranular rock with prominent subhedral plagioclase and has disseminated biotite and biotitised hornblende and disseminated bornite, chalcopyrite and pyrite with accessory magnetite.
iii). Porphyritic quartz diorite (granite porphyry), occurring as a 500 m wide irregular plug to the north and two dykes to the south-east, similar in composition to the quartz-diorite, but with plagioclase and quartz phenocrysts.
iv). Porphyry A (a quartz-diorite porphyry) present as sparse, scattered small dykes and irregular bodies, commonly of less than 5 m in width, concentrated in a north-south band just to the centre of the main stock. It is generally dark grey with plagioclase phenocrysts and a biotite altered mineralogy.
v). Porphyry B (quartz-monzodiorite porphyry) which is more widespread than Porphyry A, occurring as dykes in the central part of the deposit and is of light to medium grey to brown colour with a crowded mass of predominantly plagioclase phenocrysts.
vi). The late porphyry, which closely resembles Porphyry A and is found as a small number of short, thick dykes and plugs in the east and centre of the deposit. It cuts the main mineralised veins but in turn hosts some quartz-chalcopyrite-molybdenite veins and late veins with sericitic halos.

Dating of these porphyries has yielded an average Miocene age of 9.9 1 Ma.

Hypogene mineralisation commenced prior to the cessation of magmatism, taking the form of quartz stockwork veining, potassic alteration and breccia pipes. Late mineralisation occurs as pyrite veining with sericite halos. Mineralisation in both types is present in veins, with the only disseminated ore being in the alteration halos of these veins. Vein types include:
i). Granular quartz - with sparse chalcopyrite, minor bornite and traces of molybdenite, but common magnetite. These are the first to appear and are 2 to 10 mm thick with little evidence of an alteration halo.
ii). Green mica - with accompanying biotite and chlorite, while quartz is largely absent. These veins are an insignificant copper source, although they commonly contain chalcopyrite and bornite with local magnetite and are 1 to 5 mm thick with K feldspar envelopes.
iii). Type 4 copper rich - which are complex mineralogically and are responsible for the bulk of the copper ore. They contain quartz and variable amounts of green mica, biotite, K feldspar, magnetite, pyrite, molybdenite, tourmaline and anhydrite and are planar to irregular in shape with widths of millimetres to several centimetres. The bulk of the associated mineralisation is contained within the 1 mm to 1 m (averaging 5 cm) envelopes of the veins which comprises a dark grey, fine grained mixture of green mica, biotite, quartz, K feldspar and copper sulphides, with local magnetite. The copper sulphides are present as disseminated bornite-chalcopyrite, with bornite deceasing in abundance outwards from the centre of the deposit.
iv). K feldspar halo veins and fractures - are abundant in areas of K feldspar alteration, enveloping granular quartz veins a few mm wide and fractures with sparse associated chalcopyrite and bornite with local molybdenite, both within the veins and the potassic halo. They contribute 5 to 10% of the orebodies copper content.
v). Comb quartz veins - which are smooth and planar without a pronounced alteration halo. They only have a small copper content but represent the main molybdenum source being most common in the core of the deposit.
vi). Sericite halo veins - are abundant around the margin of the orebody but contain mainly pyrite with common quartz and overprint the previous vein types.

Two types of breccia are recognised at Los Pelambres, igneous and a hydrothermal. A large, 600 m across triangular shaped breccia pipe occurs near the centre of the quartz-diorite stock and at the centre of the mineralisation, while several 50 to 200 m diameter breccia pipes are found on the eastern side of the stock. Mineralised breccias also flank some of the dykes. The igneous breccias are not enhanced in copper or molybdenum. The hydrothermal breccias however make an important contribution to the copper grade. They are composed of angular to subrounded clasts of quartz-diorite, porphyritic quartz-diorite, fine grained quartz-diorite, andesite and porphyry A and B set in a matrix of quartz, with abundant tourmaline and common biotite, K feldspar, magnetite and sulphides (pyrite, chalcopyrite, bornite and molybdenite).

Supergene enrichment is important at Los Pelambres, with 5 blanket like zones formed vertically as follows, from the surface down:
i). Complete oxidation and leaching from the surface to between 10 and 200 m and carrying <0.05% Cu accompanied by variable to totally leached limonites (jarosite, goethite and exotic varieties).
ii). Partial leaching and oxidation.
iii). Sulphide enrichment - where chalcopyrite and bornite are partially replaced by grey chalcocite and covellite, while pyrite has thin chalcocite film coatings, decreasing downwards,
iv). Hypogene mineralisation with anhydrite leached, and
v). Hypogene mineralisation with anhydrite sealing fractures. Hypogene mineralisation shows a general concentration of bornite in the centre of the deposit, extending to the north-east. A second bornite rich, but low grade zone occurs to the south-east. Pyrite increase to the west from the limit of bornite development and appears to have been superimposed on the earlier bornite-chalcopyrite (and local bornite-chalcopyrite-magnetite) zoning and is associated with the younger sericite halo veining.

Prior to mining, geological reserves/resources at various cut-off grades had been estimated as follows:
  3.311 Gt @ 0.63% Cu, 0.016% Mo (0.4% Cu cut-off),
  1.699 Gt @ 0.76% Cu, 0.018% Mo (0.6% Cu cut-off),
     471 Mt @ 0.95% Cu, 0.022% Mo (0.8% Cu cut-off),
     236 Mt @ 1.06% Cu, 0.023% Mo (0.9% Cu cut-off).

Published ore reserve and mineral resource estimates at 31 December, 2006 were as follows (Antofagasta PLC website):
    Measured + indicated + inferred resource - 2.939 Gt @ 0.61% Cu, 0.016% Mo, 0.033 g/t Au, 0.80 g/t Ag; including
    Proved + probable reserve - 2.1246 Gt @ 0.64% Cu, 0.018% Mo, 0.033 g/t Au, 0.92 g/t Ag.

Published ore reserve and mineral resource estimates at 31 December, 2010 were as follows (Antofagasta PLC website):
    Measured + indicated + inferred resource - 5.8184 Gt @ 0.53% Cu, 0.010% Mo, 0.04 g/t Au; including
    Proved + probable reserve - 1.4330 Gt @ 0.64% Cu, 0.021% Mo, 0.030 g/t Au.

Published ore reserve and mineral resource estimates at December 31, 2015 were as follows (Antofagasta PLC Annual Report for 2015):
    Proved + probable reserves - 1.3087 Gt @ 0.61% Cu, 0.019% Mo, 0.05 g/t Au;
    Measured + indicated resource - 3.414 Gt @ 0.55% Cu, 0.018% Mo, 0.05 g/t Au; plus
    Inferred resource - 2.6901 Gt @ 0.46% Cu, 0.015% Mo, 0.06 g/t Au;
    TOTAL resource - 6.1041 Gt @ 0.51% Cu, 0.016% Mo, 0.06 g/t Au (inclusive of reserves).
Note: The ore reserve and mineral resource cut-offs in 2015 were 0.42% and 0.35% Cu respectively.

The Los Pelambres operation is 60% owned by Antofagata PLC, and 40% by a Japanese consortium.

Frontera

The Frontera deposit is ~1.5 km south east of the Los Pelambres pit, and is centred on a quartz diorite porphyry outlier of the Los Pelambres intrusive complex, emplaced into early Miocene andesites. It comprises a northwest-trending zone of magnetite-rich potassic alteration that is characterised by hydrothermal biotite and an intense quartz-magnetite stockwork.

The mineralised system comprises an upper, oxidised zone containing erratic green and black copper oxides plus pitch limonite, underlain by a weakly developed supergene enrichment blanket of chalcocite and covellite, draping hypogene mineralisation dominated by bornite and chalcopyrite. A phyllic sericite-pyrite assemblage forms a poorly defined annulus between the central potassic and external propylitic zones. Molydenite is locally present within the ore zone, although the average Mo content of the deposit of ~60 ppm is only half that of at its neighbor at Los Pelambres.

Published resources at Frontera comprise (Perello et al., 2011):
    700 Mt @ 0.52% Cu, 0.1 g/t Au using a 0.4% Cu cutoff.

For more detail consult the reference(s) listed below which were the principal source of the information on which this summary was based.

The most recent source geological information used to prepare this summary was dated: 2011.     Record last updated: 13/12/2012
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.


  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Atkinson W W, Souviron A, Vehre T I, Faunes G. A  1996 - Geology and mineral zoning of the Los Pelambres porphyry copper deposit, Chile: in Camus F, Sillitoe R H, Petersen R, (Eds) 1996 Andean Copper Deposits: New Discoveries, mineralization, Styles and Metallogeny Soc. Econ. Geologists   Spec. Publ. No. 5 pp 131-156
Hollings, P., Cooke, D. and Clark, A.,  2005 - Regional Geochemistry of Tertiary Igneous Rocks in Central Chile: Implications for the Geodynamic Environment of Giant Porphyry Copper and Epithermal Gold Mineralization: in    Econ. Geol.   v.100, pp. 887-904.
Mpodozis, C. and Cornejo, P.,  2012 - Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes: in Hedenquist J W, Harris M and Camus F, 2012 Geology and Genesis of Major Copper Deposits and Districts of the World - A tribute to Richard H Sillitoe, Society of Economic Geologists, Denver,   Special Publication 16, pp. 329-360
Perello J, Brockway H, Posso H, Sillitoe R, East p and Matthews F,  2011 - Discovery of the Frontera porphyry Cu-Au deposit at Los Pelambres, central Chile: in Barra F. et al., (eds,),  Lets Talk Ore Deposits, 11th Biennial Meeting, SGA 2011, September, 2011, Santiago, Chile   Proceedings volume, 3p.
Reich M, Parada M A, Palacios C, et al.  2003 - Adakite-like signature of Late Miocene intrusions at the Los Pelambres giant porphyry copper deposit in the Andes of central Chile: metallogenic implications: in    Mineralium Deposita   v38 pp 876-885
Sillitoe R H,  1973 - Geology of the Los Pelambres porphyry copper deposit, Chile.: in    Econ. Geol.   v68 pp 1-10
Sillitoe, R.H.,  1977 - Permo-Carboniferous upper Cretaceous and Miocene porphyry copper-type mineralization in the Argentine Andes: in    Econ. Geol.   v.72, pp. 99-109
Stern C R and Skewes M A,  2005 - Origin of Giant Miocene and Pliocene Cu-Mo Deposits in Central Chile: Role of Ridge Subduction, Decreased Subduction Angle, Subduction Erosion, Crustal Thickening, and Long-Lived, Batholith-Size, Open-System Magma Chambers: in Porter, T.M. (Ed), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide,   v.1 pp. 65-82

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Stern C R and Skewe M A, 2005 - Origin of Giant Miocene and Pliocene Cu-Mo Deposits in Central Chile: Role of Ridge Subduction, Decreased Subduction Angle, Subduction Erosion, Crustal Thickening, and Long-Lived, Batholith-Size, Open-System Magma Chambers,   in  Porter T M, (Ed),  Super Porphyry Copper and Gold Deposits: A Global Perspective,  v1  pp 65-82
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