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El Algarrobo, Los Colorados

Chile

Main commodities: Fe
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El Algarrobo in the Huasco Valley in Chile's 3rd Region is one of the seven large (>100 Mt of high-grade ore) iron oxide deposits magnetite-apatite deposits within the 1000 km long Cretaceous Chilean Iron Belt which lies within the Coastal Cordillera of northern Chile, closely associated with the Atacama Fault Zone.   The mine has recently closed and production in the Huasco Valley is now derived from the Los Colorados Mine which is located ~35 km north of Vallenar, in the southern Atacama Desert.

  At El Algarrobo magnetite-tremolite/actinolite-apatite mineralisation is related to clino- and ortho-pyroxene diorite intrusions and hosted by a suite of andesites between the intrusive masses of the belt.
  El Algarrobo currently only has a remnant reserve stated at 3.6 Mt with an average iron content of 47.5% and a cut-off grade of 26% magnetic iron, although other occurences in the surrounding district are said to contain resources of 130 Mt .

  The Los Colorados Mine, which has a similar geological setting, was recently-opened to replace the old El Algarrobo operation which was closed due to mineral exhaustion.   Los Colorados has reserves of 245 million tonnes with an average yield of 48% iron, while a further reserve amounting to 73 million tonnes is reported in the district, including the Chanar Quemado, Sositas and Coquimbana prospects.
  The magnetite ores at Los Colorados have been dated at ~110 Ma, (K-Ar; Pichon, 1980; Oyazún and Frutos, 1984) and are hosted within volcanic and volcaniclastic rocks of the Punta del Cobre Formation. The ores contain 60 to 65% Fe and predominantly occurs as a high-grade massive tabular body of magnetite on the western side of the open pit, where they are referred to as the 'Western Dyke'. This tabular body, which is predominantly composed of massive magnetite (>90 vol.%) with minor accompanying actinolite, apatite and pyrite, strikes at from 10 to 15° to to north-south, and is 1500 m long and from 90 to 180 m wide. Emplacement of the magnetite orebody was controlled by a district-scale structure, the Los Colorados fault system, part of the southern segment of the Atacama fault system. Within the mine these structures at Los Colorados are locally called Magnetite Fault West and Magnetite Fault East.
  A lower grade, 45 to 52% Fe mineralised body, which is locally brecciated and ~900 x ~280 m in area, envelopes the Western Dyke, forming a disseminated halo composed of magnetite intergrown with centimetre scale crystals of actinolite and apatite.
  A second massive tabular magnetite orebody, the Eastern Dyke, which is similar to the Western Dyke, occurs to the east of the lower grade mineralised breccia halo. This orebody is smaller, at 780 x 50 m, and has grades of 57 to 62% Fe. The Eastern Dykes strikes at 10 to 15° in its central-north section, and 345 to 350° in the central-south section. An elongate 2000 x 600 m diorite-microdiorite intrusion which strikes at 45 to 60° is exposed in the eastern part of the pit. The diorite is fine grained, with up to ~5% disseminated magnetite, variable, but up to ~5% pyrite and traces of chalcopyrite, with local zones of moderate argillic alteration, including jarosite-hematite. Minor (supergene) chalcocite and covellite are also found locally.
  Pyrite is the dominant sulphide at Los Colorados, although minor amounts of chalcopyrite are also present. The pyrite occurs as disseminated <1 mm grains and ~0.2 to 3 mm wide veinlets associated with actinolite and magnetite. The pyrite veinlets generally cut the magnetite ores occurring in both the magnetite dykes and in the diorite intrusive body and post-date an early main magnetite stage. This veinlet pyrite is commonly spatially associated with magnetite and can exhibit corrosion and/or replacement textures, suggesting a late hydrothermal magnetite precipitation event. The modal abundance of pyrite increases from the tabular magnetite bodies (<1 vol.%) to the breccia bodies and the diorite intrusion (up to ~5 vol.%).
  This description of the Los Colorados deposit is paraphrased from Reich et al. (2016).

See also the description on El Romeral the main deposit of this type in the Chilean Iron Belt.

The most recent source geological information used to prepare this summary was dated: 2016.     Record last updated: 19/4/2016
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:
Menard J J  1995 - Relationship between altered pyroxene diorite and the magnetite mineralisation in the Chilean Iron Belt, with emphasis on the El Algarrobo iron deposits (Atacama region, Chile): in    Mineralium Deposita   v30 pp 268-274.
Reich, M., Simon, A.C., Deditius, A., Barra, F., Chryssoulis, S., Lagas, G., Tardani, D., Knipping, J., Bilenker, L., Sanchez-Alfaro, P., Roberts, M.P. and Munizaga, R.,  2016 - Trace Element Signature of Pyrite from the Los Colorados Iron Oxide-Apatite (IOA) Deposit, Chile: A Missing Link Between Andean IOA and Iron Oxide Copper-Gold Systems?: in    Econ. Geol.   v.111, pp. 743-761.


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