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.