Shear-Hosted Copper Mineralization Within the Atacama Fault Zone, El Salado District, Northern Chile

The El Salado district is located 35 km east of Cha–aral
in the Coastal Range of northern Chile (Fig. 1). The district is
included in a group of vein and manto-type copper deposits
spatially related to the Atacama Fault Zone (AFZ) that comprise
the Coastal Range copper province (Espinoza et al.,
1996). Two major strike-slip faults, the central and east
strands of the AFZ, bound the district (Fig. 2). Subsidiary
faults and shears host extensive copper vein mineralization in
a >10 km2 area that comprises the district. Minor production
is derived from several vein deposits and reworking of old
mine dumps. Oxide copper ore is processed at the stateowned
ENAMI heap leach facility in the town of El Salado.
Athick sequence of submarine (?) meta-andesite and siliciclastic
rocks (>1500 m) assigned to the Early Cretaceous
Bandurrias Formation (Mercado, 1977) are the principal host
rocks for copper-mineralized veins. Dark-green to black,
fine-grained to aphanitic andesite and minor porphyritic
andesite comprise the volcanic unit. The massive andesite is
strongly fractured throughout the district and in outcrop
appears structureless. Flow layering, pillow structures, and
vesicular or amygdaloidal horizons are absent making structural
interpretation difficult. Several lenses of thin-bedded
platey limestone, up to 30 meters thick, are interstratified
with the andesite on the west side of the district, adjacent to
the central Atacama fault. The limestone is highly recrystallized
and locally displays strong ductile deformation. The
upper sedimentary unit consists of poorly- bedded indurated
quartz sandstone with minor chert, argillite, and volcaniclastic
interbeds. Within the district the Bandurrias Formation
strikes north-northeast and dips moderately (40-65¡) eastward,
except near the central Atacama fault where deformed
limestone beds are vertically dipping or overturned (cross
section on Fig. 2). About 1.0 km north of the Tulipan mine,
folded quartz sandstone and chert beds interfinger with massive
andesite, suggesting a gradational change from volcanic
to detrital deposition.
Pre- and post-mineral dikes and small stocks of intermediate
composition have intruded the volcanic-sedimentary
sequence. A prominent andesite porphyry dike and numerous
monzonite porphyry dikes were emplaced along the Santo
Domingo fault in the center of the district, and some dikes
contain trace amounts of chalcopyrite. Small fine-grained
andesite dikes are common on the east side of the district and
a large altered diorite dike cuts massive andesite near the central
Atacama fault. Though some intrusive rocks are locally
altered and weakly mineralized, a clear genetic relationship
between copper mineralization and specific intrusions has not
been established. Hornblende phenocrysts from a small plug
of weakly altered diorite porphyry cutting Bandurrias
andesite in the southern part of the district yielded a 40Ar/39Ar
plateau age of 107.4±1.1 Ma. Diorite and granodiorite of the
Cretaceous (127 Ma) Las Tazas pluton crop out extensively
around El Salado.