The formation of the porphyry copper deposit at El Salvador culminated volcanic activity in the Indio Muerto district. Host rocks for the ore are Cretaceous andesitic flows and sedimentary rocks overlain unconformably by lower Tertiary volcanics. A steep-walled granodioritic porphyry complex and the closely associated main center of mineralization and alteration were emplaced 41 m.y. ago.The oldest of these porphyries, "X" Porphyry, is fine grained, equigranular to weakly porphyritic. Porphyritic textures are seen in deep exposures, whereas strong K-silicate alteration at higher elevations has developed the equigranular texture. Next, a complex series of feldspar porphyries was intruded. These include an early group, "K" Porphyry, and a late group, "L" Porphyry, defined by mapped age relations at intrusive contacts. Strong alteration and mineralization of most "K" Porphyry bodies have partially obliterated the porphyry texture.

The larger "L," Porphyry complex is relatively unaltered and unmineralized. A wide range of textural variation in "L" Porphyry is spatially related to its contacts and evidences reaction with intruded andesite. Relatively minor porphyry dikes and igneous breccia cut the composite porphyry stock and are followed by postmineral latite dikes and clastic pebble dikes. Below the present surface, pebble dikes exhibit a striking decrease in abundance and a change from a radial-concentric to a nearly orthogonal pattern. Compared to average granodiorite, the El Salvador porphyries are low in total iron and have a smaller K 2 O/Na 2 O ratio. Compositional trends in "L" Porphyry correlate with textural variations. The initial 87 Sr/ 86 Sr ratio of early siliceous extrusive rocks and domes, as well as of the main porphyry series and all alteration products, is a consistent 0.704.

Early alteration-mineralization was mostly accomplished before the intrusion of the last major feldspar porphyry ("L" Porphyry) and contributed probably three-quarters of the 5 million tons of copper in the orebody. Early mineralization is characterized by distinctive quartz veins and largely disseminated K-silicate assemblages of alkali feldspar-biotite-anhydrite-chalcopyrite-bornite or chalcopyrite-pyrite. Early quartz veins are typically granular quartz-K-feldspar-anhydrite-sulfide, generally lack internal symmetry, and are irregular and discontinuous. K-silicate alteration of some porphyries appears to have occurred during final consolidation of the melts as well as later. Biotization of andesitic volcanics and an apparently contemporaneous outer fringe of propylitic alteration were produced during this Early period. Except at deepest exposed elevations in the younger porphyries, incipient K-silicate alteration converted hornblende phenocrysts to biotite-anhydrite-rutile, ilmenite to hematite-rutile, and sphene to rutile-anhydrite. Anhydrite deposition occurred through the entire history of primary mineralization, and probably more sulfur was fixed as sulfate in anhydrite than in sulfides.Outward within a central zone of K-silicate alteration with chalcopyrite-bornite, the proportion of bornite decreases until pyrite appears and increases as chalcopyrite diminishes. Pyrite abundance increases, then decreases in an outer propylitic zone with epidote-chlorite-calcite. In the outermost propylitic zone, minor chalcopyrite-magnetite veins give way outward to specular hematite. Pyrite is very closely associated with sericite or sericite-chlorite, and pyrite-sericite-chlorite veining is clearly younger than both K-silicate and propylitic assemblages. The major fringe zone of pyrite-sericite appears to be a relatively late feature superimposed across the transitional boundary of the Early-formed zones.

A Transitional type of quartz vein was formed after consolidation of all major intrusions and prior to the development of Late pyritic and K-feldspar-destructive alteration assemblages. Transitional quartz veins occupy continuous planar fractures, which tend to be flat. They are characterized by a lack of K-feldspar and associated alteration halos and by the presence of molybdenite. The assemblage K-feldspar-andalusite on deep levels is probably a Transitional alteration assemblage. Tourmaline in veinlets and breccias is closely associated in time with Transitional quartz veins. The abundance of tourmaline increases upward toward the present surface.Late mineralization, characterized by abundant pyrite and K-feldspar- destructive alteration, tends to be more fracture controlled than Early and more disseminated mineralization. Late sulfide veins and veinlets cut all rock types, except latite, and all Early and Transitional age veins. They contain pyrite and lesser but upward-increasing amounts of bornite, chalcopyrite, enargite, tennantite, sphalerite, or galena. Quartz and anhydrite are the most common gangue minerals. Alteration halos surrounding these pyrite veinlets are principally sericite or sericite-chlorite. These veins occupy a radial-concentric fracture set at all levels of exposure.Vertical zoning of Late alteration and sulfide assemblages is well developed. Peripheral sericite-chlorite gives way upward to sericite, which encroaches inward on central zones. Upper level assemblages are dominated by sericite and andalusite and are superimposed on Early K-silicate assemblages. Sericite-andalusite assemblages are gradational with underlying andalusite-K-feldspar zones. Deep-level Early sulfide zones, with antithetic pyrite and bornite, are abruptly truncated by later disseminated sulfide zones containing contact assemblages of pyrite and bornite and variable amounts of chalcopyrite and "chalcocite." Evidence for sulfide zoning higher within the leached capping is based on study of relict sulfide grains. Pyrite-bornite sulfide zones are generally found with sericite or advanced argillic alteration assemblages, but the "roots" of these zones extend downward into K-feldspar-bearing lower level alteration zones.

Salvador Division includes the Salvador mine and concentrator and a smelter/refinery complex at Potrerillos, which has a capacity to treat 671,000 tpy of concentrate.

Production began in 1959. Products include electro-refined and electro-won cathodes and concentrates.
• The Salvador Division is currently projected to close mining operations between 2016 and 2021, depending on copper market conditions.
• The Potrerillos smelter and refinery will continue to operate upon cessation of the mining operations.

In view of depleting reserves at Salvador Division’s mine operations, Codelco identified the potential to extend the mine life of the Rajo Inca project, by extracting through an open pit the remaining resources at Indio Muerto mine.

In 2017 the business case of this project was put on hold. The project involves mining the deposit and feeding sulphides to the existing concentrator plant at a rate of 37,000 tonnes per day. The pre-feasibility st ........