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Los Chancas

Peru

Main commodities: Cu Mo Au
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The Los Chancas porphyry copper-molybdenum-gold deposit is located on the SW margin of the Andahuaylas-Yauri belt, on the eastern margin of the Western Cordillera, at an altitude of 3400 to 4000 masl in southern Perú, 485 km SE of Lima, 135 km SW of Cusco, 60 km SSW of Abancay.
(#Location: 14° 10' 3"S, 73° 7' 19"W).

  The Middle Eocene to Early Oligocene Andahuaylas-Yauri belt porphyry copper province is situated in southeastern Peru, extending for ~300 km between Andahuaylas in the NW and Yauri to the SE. The belt includes both porphyry related iron-copper skarn mineralisation, such as Tintaya, Atalaya and Las Bambas, and typical porphyry style alteration and copper±Mo±Au mineralisation, such as Antapaccay, Los Chancas and Cotabambas. Other mineralisation includes replacement, and sediment-hosted oxide zinc deposits, and minor epithermal vein-style ores.
  For more detail of the geology of the Andahuaylas-Yauri belt see the separate Las Bambas and Cotabambas records.

The Los Chancas deposit is associated with a cylindrical intrusion of granodiorite and quartz monzonite porphyries, which occur as an inlier of the regional Middle Eocene to Early Oligocene Andahuaylas-Yauri Bathlolith. This intrusion was injected in two phases into country rock of the Late Jurassic to Mid Cretaceous Yura Group, specifically the sandstone, quartzite with interbedded shales and limestone of the latest Jurassic Chuquibambilla Formation, but mostly the white quartzite and lesser shales of the Lower Cretaceous Soraya Formation. Both the main intrusion and country rocks are intruded by dykes that are younger, and compositionally and texturally distinct from the main intrusion.

The main mineralised zone covers an area of ~1000 x 800 m and is located within a deep, conical depression drained by a stream to the west into the Rio Antabamba. The deposit is exposed in the eroded core of a NW-SE trending anticlinal structure. Three fault directions are recognised, with the most important being a NW-SE striking regional regional set which controls the mineralisation.

The porphyry stock is the main host to mineralisation, which is accompanied by a zonal distribution of alteration, comprising an early central potassic assemblage of secondary biotite and K feldspar, overprinted and surrounded by peripheral moderate amounts of phyllic quartz-sericitic alteration. It is primarily a copper deposit, characterised by chalcopyrite with lesser bornite, accompanied by significant amounts of both gold and molybdenum. The hypogene mineralisation is mainly contained within disseminations and B-type veinlets, characterised by semicontinous centre-lines filled by millimeter- to centimeter-sized grains of chalcopyrite and molybdenite. Mineralisation also occurs within the surrounding quartzites, mainly as veinlets.

In addition to the hypogene mineralisation, the deposit is overlain by a leached cap, characterised by goethite, limonite and neotocite, accompanied by antlerite-brochantite chrysocolla and malachite. This is underlain by an irregular, stratigraphically and structurally controlled immature supergene enrichment blanket developed in quartz-sericitic alteration, containing chalcocite, covellite, digenite and a lesser cuprite.

The primary sulfides present are chalcopyrite, bornite, molybdenite and pyrite; is associated with chalcopyrite and molybdenite bornite occurs in veinlets and related chalcopyrite.

Published reserve figures (Southern Copper Corporation presentation, 2013; after MINTEC, 2012) were:
    Mill ore at a 0.3% Cu cut-off
      Proved reserve - 222.963 Mt @ 0.633% Cu, 0.046% Mo;
      Probable reserve - 321.885 Mt @ 0.567% Cu, 0.036% Mo;
      TOTAL reserve - 544.848 Mt @ 0.594% Cu, 0.040% Mo, 0.04 g/t Au;
    Leach ore at a 0.1% Cu cut-off
      Proved reserve - 82.109 Mt @ 0.39% Cu;
      Probable reserve - 99.087 Mt @ 0.33% Cu;
      TOTAL reserve - 181.196 Mt @ 0.357% Cu.

A 2011 report by INGEMMET Peru listed a probable reserve of 200 Mt @ 1% Cu, 0.08% Mo and 0.12 g/t Au. Development of a planned 80 000 to 100 000 tpa copper operation by Southern Copper Corporation was under revision from 2014.

This summary is gleaned from a number of sources, including "Acosta, J., Rodriguez, I., Villarreal, E. and Huanacuni, D., 2011 - Memoria sobre la geología económica de la región Apurimac; Proyecto GE 33, Instituto Geologico Minero y Metalurgico, Lima, Peru, 44p." and "Perello, et al., 2003". A copy of the key source "Corrales, E., 2001, Proyecto Los Chancas: II International Congress of Prospectors and Explorers, ProExplo 2001, Lima, Perú, April 24-27, 2001, Instituto de Ingenieros de Minas del Perú, 9 p. (CD-ROM)" is not readily available and has not been consulted.

The most recent source geological information used to prepare this summary was dated: 2003.    
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.


Los Chancas

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Perello, J., Carlotto, V., Zarate, A., Ramos, P., Posso, H., Neyra, C., Caballero, A., Fuster, N. and Muhr, R.,  2003 - Porphyry-Style Alteration and Mineralization of the Middle Eocene to Early Oligocene Andahuaylas-Yauri Belt, Cuzco Region, Peru: in    Econ. Geol.   v.98., pp. 1575-1605.


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