The El Arco Project is wholly-owned by Southern Copper Corporation, Sucursal del Perú, which is a majority-owned, indirect subsidiary of Grupo Mexico S.A.B de CV. (Grupo Mexico). Mexicana del Arco, S.A. de C.V. (Mexarco), a Grupo Mexico/Southern Copper subsidiary company is the in-country holding company.
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Summary:
The El Arco deposit is considered to be an example of a porphyry copper deposit.
Deposit Dimensions
The deposit has a west–northwest to west–southwest strike and dips steeply to the north. Mineralization extends over an approximate 1,500 x 1,500 m area, with a minimum thickness of >300 m. Mineralization has been drill tested to a depth of approximately 600 m, and remains open at depth.
The copper mineralization is concentrated in a core of potassic alteration in a granodioritic porphyritic stock surrounded by propylitic alteration in andesitic lavas.
The main granodiorite porphyry unit is a wedge-shaped body bound by converging, arcuate, west and west–southwest-striking, north-dipping planar features. These features may be formerly vertical contacts of an intrusive body that were likely overturned and broadly folded during obduction of the arc terrane to the continental margin.
The east end of the granodiorite body is an irregular contact with andesite and the continuity and intensity of mineralization is poorer at this end of the deposit. Small fingers of granodiorite extend from the main body of the granodiorite into the andesite beyond north and south bounding structures.
Quebrada El Arco, which has incised into the western portion of the deposit may be following a cross-fault; however, no clear offset in lithological or grade contacts have been noted to date.
Mineralization
Copper–gold mineralization at El Arco occurs in three sub-horizontal zones.
The topmost zone is an oxide cap. The oxide zone is approximately parallel with the surface topography, has a blanket shape, and an average thickness of 40 m (Echavarri, 1975). The main copper oxide minerals are chrysocolla, malachite, dioptase, argillaceous goethite, copper wad, cuprite and neotocite. Chrysocolla is by far the most abundant copper mineral in the oxidized zone.
Underlying the oxide zone, and parallel to it, is a transition zone varying in thickness from 0– 18 m with an average thickness of 9 m. This zone contains a mixture of both secondary and primary iron and copper oxides and sulfides. Supergene minerals that have been identified are chalcocite, digenite, covellite, native copper and chalcotrichite; however, those minerals contribute little to the value of the deposit.
The primary sulfide zone directly underlies the transition zone and comprises the major portion of the deposit. This zone is lenticular in shape, fingering out at its extremities, with a greater horizontal than vertical dimension. The upper limit of this zone coincides with the present water table. The lower limit is not well defined because exploratory drilling to 600 m bottomed in mineralization grading >0.4% Cu.
Pyrite and chalcopyrite are the most abundant sulfides in this zone with bornite in lesser amounts. Molybdenite, galena and sphalerite are also present. Gold is of economic importance. Copper mineralization occurs in two forms: fracture filling (± 60%) and dissemination as discrete grains (± 40%). About 60% of the copper grades are in the porphyry with the remaining 40% in the andesite wallrock.
At the center of the deposit the total pyrite content is <1% by weight, increasing toward the periphery where it forms a halo with 3–9% by weight (Echavarri, 1972). This halo roughly corresponds to the zone with the most intense propylitic alteration.
A smaller copper–molybdenum–gold–silver deposit to the north of the main granodiorite body is mainly hosted in andesite and occurs above a deeper apophysis of granodiorite, but this apophysis is not well defined by drilling.