El Laco


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The El Laco magnetite-apatite deposits (listed below) are exposed at altitudes between 4700 to 5300 m on the flanks of El Laco, a calc-alkaline volcanic complex in the Central Andes of northern Chile, 300 km east of Antofagasta, near the Argentine border (#Location: 23° 49' 7"S, 67° 29' 58"W).

The El Laco andesitic stratovolcano is located in the Pliocene to Pleistocene volcanic arc of the Andes and has has K-Ar ages of 5.3±1.9 Ma to 1.6±0.5 Ma (Naranjo et al., 2010).

With the exception of the orebodies, El Laco is similar to other volcanic complexes in the area in terms of silicate lava compositions, phenocryst assemblages, general eruptive history, and degree and distribution of alteration. The ore deposits have been dated at 2.1 Ma by apatite fission track analysis (Maksaev et al., 1988). Andesitic to dacitic lavas predate and postdate the ore deposition. Fumarolic activity has continued at El Laco to the present day.

The mineralisation occurs mainly as stratabound lenses of iron oxide interbedded with andesite flows. Individual lenses are as much as 1 km2 in area, composed of >98% iron-oxide and up to 100 m in thickness. The ore deposits are composed mostly of massive, friable and brecciated primary magnetite or martite pseudomorphs after magnetite, accompanied by minor amounts of diopside, anhydrite and apatite. These have been interpreted to represent pyroclastic material, lava flows and related dykes (Laco Norte, Laco Sur and San Vicente Alto), subvolcanic bodies (San Vicente Bajo, Rodados Negros and Laquito) and an intrusive dyke-vein network (Cristales Grandes).

Widespread occurrence of features interpreted as vesicles and subvertical tubes suggests extensive degassing during crystallisation (Henriquez and Martin, 1978; Naslund et al., 2002). These tubes are lined by euhedral zoned magnetite and diopside and a complex assemblage that includes apatite, fluorite and Fe-bearing sulphates.

All of the orebodies are rooted in subvertical metre thick veins of massive magnetite. Where unaltered, the host andesite is vesicle poor and contains phenocrysts of plagioclase, augite and enstatite-pigeonite and small grains of disseminated magnetite in a glassy groundmass of rhyolitic composition. The andesite flows locally preserve their original shape indicating that the mineralisation formed subaerially or at very shallow (<100 m) depths. Locally, plagioclase phenocrysts contain melt inclusions as much as 200 µm in diameter where two immiscible melts coexist, one silica-rich and another that is rich in Fe with minor amounts of Ca, Mg, Si, Ti and P (see Naslund et al., 2009).

The andesite adjacent to the mineralisation is affected by an alkali-calcic hydrothermal alteration. This alteration is similar to what is observed in iron oxide-copper-gold deposits, characterised at El Laco by early albite followed by a pervasive replacement of host rocks by K feldspar, diopside, magnetite, scapolite, fluorapatite and rare quartz. Magnetite within this assemblage accounts for <5% of the total volume of magnetite. The alkali-calcic alteration zone includes large breccia bodies with fragments of altered andesite in a matrix of magnetite, diopside and scapolite. Locally, there are zoned veins as much as 40 cm thick with intergrown euhedral diopside and magnetite with interstitial anhydrite (Tornos et al., 2016).

Tornos et al. (2016) propose that an iron-rich melt likely separated from the parental andesitic magma at depth, and ..... "sub-volcanic [intrusive] and volcanic [extrusive] crystallisation of that iron-rich melt as massive magnetite, promoted the exsolution of a small volume of hydrosaline melt and of large amounts of vapour that led to the formation of an alkali-calcic hydrothermal assemblage replacing the host andesite. This assemblage is capped and overprinted by a large zone of acid-sulphate steam-heated alteration, forming as a whole, a protracted shallow-level magmatic-hydrothermal system. Oxygen isotopic data for the massive magnetite and the alkali-calcic altered rock suggest that these rocks are genetically related to the host andesite".

Within an area of 30 km
2, centred on the El Laco volcano, there are at least seven deposits with total resources of >1.5 Gt of massive iron ore (Tornos et al., 2016).

This deposit and its origin are the subject of a paper in the monograph: "Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective" volume 2, published by PGC Publishing, Adelaide, Australia.

The full abstract and the reference list from the paper can be displayed by selecting options offered below.

For detail consult this paper or other reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 2016.     Record last updated: 29/5/2016
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.

El Laco district

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Naslund H R, Henríquez F, Nyström J O, Vivallo W and Dobbs F M,  2002 - Magmatic Iron Ores and Associated Mineralisation: Examples from the Chilean High Andes and Coastal Cordillera: in Porter T M (Ed.), 2002 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective, PGC Publishing, Adelaide   v.2 pp. 207-226
Sillitoe R H, Burrows D R  2002 - New field evidence bearing on the origin of the El Laco Magnetite deposit, northern Chile: in    Econ. Geol.   v97 pp 1101-1109
Tornos, F., Velasco, F. and Hanchar, J.M.,  2016 - Iron-rich melts, magmatic magnetite, and superheated hydrothermal systems: The El Laco deposit, Chile: in    Geology   v.44 pp. 427-430.

 References to this deposit in PGC Publications: Want any of our books ? Pricelist
Naslund H R, Henriquez F, Nystrom J O, Dobbs F M, 2002 - Magmatic Iron Ores and Associated Mineralisation: Examples from the Chilean High Andes and Coastal Cordillera,   in  Porter T M, (Ed.),  Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective,  v2  pp 207-226
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