Geology of the El Dorado epithermal vein system, El Salvador, Central America

Rocks hosting the El Dorado epithermal veins system are part of the Oligocene
and Miocene Morazan and Chalatenango Formations that define an east-west trending
volcanic belt in northern El Salvador. Veins strike N50°W to N50°E with the
majority striking about N20°W or N25°E. They have steep dips and pinch, roll and
swell, forming sigmoidal flexures Vein splays and en echelon structures are common,
forming segments from a few hundred to, in rare cases, over 3000 meters long.
Over 400 m of propylitically altered basaltic to andesitic lava flows, volcanic conglomerates
with volcaniclastic wedges make up the Oligocene to early Miocene
Morazan Formation that host the >1 km-long Minita vein, the principal gold resource
in the Central district. Texturally distinct basalt to andesite porphyry dikes and
domes intrude the lavas and volcanic sediments
In the South El Dorado district, the 1.5 km-long NW-striking Nance Dulce vein is
hosted at depth by a thick sequence of volcaniclastic sedimentary rocks unconformably
overlying a basaltic andesite that is more typical of the Central District.
The Miocene Chalatenango Formation includes a post-mineral andesitic to
dacitic dome and flow complex that partially separates the South and Central districts.
Other post-mineral units include a small hornblende dacite dome, basaltic and
andesitic lava flows of Miocene or Pliocene (?) age as well as Pliocene rhyolite tuffs.
Complex polyaxial veins contain crustiform bands of quartz, calcite, adularia and
clay minerals as well as numerous breccias. Shallow portions of veins are dominated
by recrystallized chalcedony locally accompanied by adularia. Massive calcite forms
distinct bands and may be locally intergrown with quartz. In some veins, calcite has
been strongly leached. Deeper portions of veins may be dominantly calcite including
bladed forms. Spherulitic calcite intergrown with quartz locally forms distinct bands.
Clay-rich bands zone from montmorillonite near surface to corrensite in the main ore
zone to nontronite at depth. Vein breccias are common in some areas and, locally,
breccia constitutes the entire vein width.
Metallic minerals are very rare in veins. Mineral species include chalcopyrite,
acanthite, covellite, pyrite, sphalerite, galena, and electrum. Metallic minerals are associated
with adularia-rich bands, rinds on breccia fragments, intergrown in corrensiteor
nontronite-rich bands, spherulitic calcite and as inclusions in massive calcite.