Geologic Characteristics and Exploration Significance of Gold-Rich Porphyry Copper Deposits in the El Salvador Region, Northern Chile
Sergio L. Rivera, Tomás Vila, Jorge Osorio, 2005. "Geologic Characteristics and Exploration Significance of Gold-Rich Porphyry Copper Deposits in the El Salvador Region, Northern Chile", Andean Metallogeny: New Discoveries, Concepts, and Updates, Richard H. Sillitoe, José Perelló, César E. Vidal
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The porphyry copper systems of the El Salvador region, represented mainly by El Salvador, Potrerillos, Exploradora, Sierra Jardín, and Coya, are located in the southern part of the middle Eocene to early Oligocene porphyry copper belt of northern Chile. They have distinctively higher gold grades (between 0.1 and 0.5 g/t) and slightly lower molybdenum grades (<100 ppm) than the systems located in the northern parts of the belt. The El Salvador and Potrerillos deposits each contains resources of >600 million metric tons (Mt) at >0.6 percent Cu and 0.1 to 0.2 g/t Au. The Exploradora porphyry copper-gold prospect contains geologic resources of ~100 Mt at 0.3 percent Cu and 0.2 g/t Au, with an overlying leached capping enriched in gold, averaging ~0.5 g/t. The Sierra Jardín and Coya prospects show the lowest Cu contents (0.1–0.2 %), but the Au tenor is in the 0.1- to 0.5-g/t range.
The deposits are related to discrete magmatic pulses emplaced in different lithotectonic environments, including the borders of Paleocene volcanic structures (El Salvador, Sierra Jardín; 45–40 Ma) and dilational jogs and reverse and transfer faults (Exploradora, Potrerillos, and Coya; 37–31 Ma). Host rocks are mainly volcanic and marine sedimentary rocks of Jurassic to Cretaceous age. The most remarkable feature of the deposits is their relationship to multiphase, syntectonic tonalite to granodiorite intrusions. There is strong superimposition (telescoping) of intrusion and alteration-mineralization phases, including early, magnetite-rich, potassic alteration-mineralization, moderately to weakly developed sericitic alteration, and late-stage overprinted advanced argillic alteration. Postmineral phreatomagmatic activity is characterized by pebble dikes and/or diatreme breccias.
The appreciable number of gold-rich porphyries (as opposed to their gold-poor counterparts) discovered during the past ten years in the Chilean Andes (e.g., La Fortuna, Cerro Casale, Esperanza) implies that geologic conditions that favor their development are more widespread than previously considered. Several key characteristics of gold-rich porphyry copper deposits in the El Salvador region can be used for exploration purposes and open new ground to prospecting.
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A variety of metals and deposit types define the metallogeny of the Andes from Colombia through Ecuador, Peru, and Bolivia to Argentina and Chile, although porphyry copper and epithermal gold deposits undoubtedly predominate and will continue to do so. Discoveries over the last 30 yrs or so, predominantly in the central Andes and especially Chile, have been made using routine, field-based geologic and complementary geochemical methods, a situation that is considered unlikely to change radically in the foreseeable future. The only clearcut evolutionary change is the increased number of deposits being discovered beneath pre- and postmineral cover. The predictive capacity of conceptual geology has had minimal impact on the Andean discovery record but is thought to offer much promise for the future. This introductory article selects mineralization styles and relationships as well as some broader metallogenic parameters as simple examples of geologic concepts that may assist exploration. Emphasis is placed on porphyry copper ± molybdenum ± gold and high-, intermediate-, and lowsulfidation epithermal gold ± silver deposits, although reference is also made to several carbonate rock-hosted precious and base metal deposit types and styles as well as subvolcanic tin, volcanogenic massive sulfide, and slate-belt and intrusion-related gold deposits. Particular emphasis is placed on the potential for exceptionally high grade porphyry copper, porphyry gold, epithermal gold, and subvolcanic tin deposits. Deposits resulting from the oxidation, enrichment, and chemical transport of copper and zinc and mechanical transport of gold and silver during supergene weathering are also briefly highlighted.
Si bien la metalogenia de los Andes de Colombia, Ecuador, Perú, Bolivia y Chile se encuentra definida por una gama de metales y estilos de mineralización, son los depósitos tipo pórfido de cobre y epitermal de oro los que dominan en el presente y continuarán prevaleciendo en el futuro. Los descubrimientos de los últimos 30 años, predominantemente en los Andes centrales y especialmente en Chile, han sido realizados mediante métodos geológicos rutinarios de campo, generalmente complementados satisfactoriamente por métodos geoquímicos. Se estima que esta situación difícilmente experimentará variaciones radicales en un futuro cercano. El único cambio destacable en esta historia evolutiva está dado por el aumento apreciable de descubrimientos de depósitos cubiertos, bajo cobertura pre o postmineral. A nivel andino, la capacidad predictiva de la geología conceptual ha tenido un impacto mínimo en el número total de descubrimientos, aunque se piensa que su uso debiera garantizar buenas perspectivas futuras. El presente artículo