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Volcanic framework, geochronology and geochemical evolution of the El Dorado gold district, El Salvador, Central America Richer, Mathieu


The Pliocene El Dorado epithermal Au-Ag vein system, located in northern El Salvador, is hosted in Tertiary volcanic rocks that accumulated along the Caribbean plate margin in response to the subduction of the Farallon/Cocos plate. The volcanic basement rocks in the district form a >400-meters-thick sequence of basaltic to andesitic lava flows and sedimentary rocks that were intruded by porphyritic basaltic to andesitic domes and dikes during the late Miocene. These rocks petrologically correlate with the Eocene to Miocene Morazan Formation of El Salvador. Mineralization in the district is constrained between ~4.7 and 4.0 Ma, and likely occurred closer to the lower limit according to field relationships and ⁴⁰Ar-³⁹Ar ages of bracketing volcanic rocks. The waning stage of hydrothermal activity coincides with the onset of a period of extensive Pliocene felsic volcanism (~4.0 to ~3.3 Ma) based on field relationships and ⁴⁰Ar-³⁹Ar geochronology of hornblende-biotite-phyric dacitic to rhyolitic lava flows, domes and related pyroclastic and sedimentary rocks. Pliocene and possibly younger post-mineral basaltic to andesitic lava flows cap the volcanic sequence. The sub-arc mantle source that generated primary magmas during the Miocene and Pliocene is compositionally constrained between a depleted- and an enriched-MORB. Selected trace element ratios (e.g., Ba/Th) suggest that subducted sediments and/or slabderived fluids account for some of the geochemical variation in volcanic rocks at El Dorado. The Sr, Nd and Pb initial (and measured) isotopic ratios of all volcanic rocks are remarkably uniform, independent of age or rock type, likely indicating that the isotopic composition of Tertiary magmas was buffered by the mantle, and possibly, by a constant input of hemipelagic sediments during the Miocene and Pliocene. Short mixing trends in Pb isotopes further suggest that Pliocene felsic magmas were generated from partial melting of previously emplaced sub-arc igneous rocks, isotopically equivalent to the high ²⁰⁶Pb /²⁰⁴Pb volcanic basement rocks in the district, by introducing low ²⁰⁶Pb/²⁰⁴Pb mantle-derived basaltic magmas in deep crustal hot zones (Annen et al., 2006). The current exposures from north to south at El Dorado appear to represent an oblique cross-section through the volcanic and hydrothermal system associated with the low-sulfidation epithermal vein system. North and Central El Dorado, being dominated by the mafic basement rocks, represent the deeper parts of the system. South El Dorado is clearly the shallow level of the epithermal vein system, preserving the surface and shallow sub-surface environments, including the sinters and contemporaneous to postmineral felsic volcanic rocks. The distribution of volcanic facies along Titihuappa River, a regional fault zone on the extreme south of the district, appears to define the northwestern margin of the Pliocene Rio Titihuappa basin, potentially representing a volcano-tectonic depression that formed during and/or shortly following vein formation. The Miocene to Pliocene transition in Central America is marked by a trenchward shift in subduction and associated magmatic activity (Weyl, 1980). The volcanic record preserved at El Dorado suggests that the Pliocene magmatic event led to the formation of compositionally evolved hydrous magmas that likely ponded in mid- to upper-crustal magma chamber(s). These magma chambers potentially represented the critical heat engine to the once active hydrothermal system and a possible source of precious metals. Extensive felsic volcanism associated with the Pliocene magmatic system, possibly in conjunction with caldera formation, may have suppressed the heat engine to hydrothermal activity. References Annen, C, Blundy, J.D., and Sparks, S.J., 2006, The genesis of intermediate and silicic magmas in deep crustal hot zones, Journal of Petrology, vol. 47, p. 505-539. Weyl, R., 1980, Geology of Central America: Berlin-Stuttgart, Gebruder Borntraeger, 371 pp.

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