| Summary: | 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. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === Graduate
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