Abstract

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. Early rhyolite domes, formed about 50 m.y. ago and roughly contemporaneous with voluminous rhyolitic and andesitic volcanics, were followed by irregularly shaped subvolcanic intrusions of quartz rhyolite and quartz porphyry about 46 m.y. ago. Minor copper-molybdenum mineralization accompanied this event. 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.Petrologic trends are obscured because most intrusive rock types are not exposed away from the area affected by alteration and mineralization and because chemical and mineralogic variation within a single fresh major intrusive unit, "L" Porphyry, is apparently greater than it is across the entire porphyry series. However, rhyolitic volcanism in the district was clearly more felsic than younger granodioritic porphyries and produced higher K 2 O/Na 2 O ratios. 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. Patterns of alteration-mineralization are strongly influenced by the intrusion of "L" Porphyry, which removed part of the previously formed Early pattern and largely controlled subsequent Late events.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. Advanced argillic alteration assemblages containing abundant pyrophyllite, diaspore, alunite, amorphorous material, and local corundum are strongly developed at high elevations. These assemblages, present in postore pebble dikes, were formed very late in the evolution of mineralization. Where preserved, the associated sulfide is pyrite.Two types of fluid inclusions are found in Early and Transitional quartz veins but never in Late pyritic veins. They contain high-salinity fluid coexisting with low-density fluid. Both exhibit homogenization temperatures in the range of 360 degrees to >600 degrees C. A third type of inclusion is found in veins of all ages, contains low-salinity fluid, and homogenizes at less than 350 degrees C.Supergene enrichment formed the commercial orebody, roughly 300 million tons of 1.6% Cu. Secondary Cu-S minerals extensively replaced chalcopyrite and bornite but coated pyrite with little or no replacement. Kaolinite and alunite are the principal supergene alteration products. Kaolinite replaces feldspar, biotite, and chlorite but not sericite. The zones of supergene kaolinite are developed beneath the upper level zones of strong sericitic alteration and within the upper preserved portions of the underlying K-silicate and sericite-chlorite zones. Magnetite is oxidized to hematite by supergene alteration. Anhydrite is hydrated to gypsum and then dissolved by supergene water to depths as great as 900 m beneath the present surface.Sulfides originally present in the leached capping have been oxidized to limonite, composed mostly of jarosite, goethite, and hematite. A dominantly jarositic capping overlies most of the orebody and the inner pyritic fringe. This is surrounded by a goethitic capping. A thin hematite-goethite capping between the jarosite and the enrichment blanket was apparently formed during a second stage of oxidation and leaching. Copper was mostly removed from the sericitic capping, but iron, molybdenum, and gold were relatively immobile during supergene leaching.Interpretation of the space-time patterns and relations of the mineralization, alteration, volcanism, and intrusion allows reconstruction of the depositional environments of the El Salvador porphyry copper deposit. The bulk of the primary mineralization and alteration accompanying emplacement of the porphyries was accomplished in less than one million years, at the end of an extended period of volcanism. The granodioritic stocks intruded their cogenetic volcanic pile, which extended probably less than 2,000 meters above the present surface, Early mineralization-alteration formed simultaneous with, adjacent to, and within recurrent intrusions of porphyry. The pressure-temperature environment was close to that of the final crystallization of the melt. The saline aqueous fluids responsible for the bulk transport of metals and sulfur at this time were boiling, limited in quantity, and of largely magmatic origin. They were generally depleted at present levels of exposure prior to the emplacement of the last porphyry mass. The relatively oxidized state of sulfur during this Early period probably reflects leakage of hydrogen from the mineralizing system.As cooling of the intrusive complex progressed, the structural and chemical character of the mineralizing environment shifted, largely in response to the inflow of meteoric water. This water was part of a deep convective system driven by heat from the cooling intrusive center. With continued cooling, upper and peripheral zones of Late alteration and mineralization progressively collapsed inward and downward over zones of Early mineralization, penetrating deepest along continuous vein structures. There was extensive reworking of previously deposited sulfides and wall-rock alteration, especially at high elevations. In the last stages, an acid hot-spring system was established in the upper portions of the deposit. Final and relatively minor intrusion of latite dikes into this hot-spring system caused pebble brecciation along Late vein structures. Erosion and supergene leaching and enrichment followed within 5 m.y. and may have overlapped the final stages of hot-spring activity.A genetic model is proposed for the emplacement and deposition of porphyry copper deposits in general. Essential elements of this genetic model are (1) shallow emplacement of a usually complex series of porphyritic dikes or stocks in and above the cupola zone of a calc-alkaline batholith; (2) separation of magmatic fluids and simultaneous metasomatic introduction of copper, other metals, sulfur, and alkalies into both the porphyries and wall rocks; and (3) the establishment and inward collapse of a convective ground-water system, which reacts with the cooling mineralized rocks. The well-known similarities of porphyry copper deposits from many parts of the world are variations on a common theme. The differences and unique features exhibited by individual deposits reflect the imprint of local variables upon the basic model. The local variables include depth of emplacement, availability of ground water, volume and timing of successive magma advances, and the concentration of metals, sulfur, and other volatiles in the magmas, as well as depth of exposure.

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