The El Pen gold-silver deposit, located 165 km southeast of
Antofagasta in northern Chile, is a low-sulfidation (adularia-sericite
type) epithermal vein system spatially related to a rhyolite dome
complex. Mineralization, K-Ar dated at 59.4 +1.4 Ma using adularia,
is primarily controlled by faults. Mine production began in
September 1999 from three small open pits. Full-scale underground
production will begin by mid year 2000. Proven and probable
reserves are 1,233,000 oz Au and 19,302,000 oz Ag at an average
grade of 8.0 g/t Au and 126 g/t Ag. The total resource contains 2.9
million oz of Au and 48 million oz of Ag.
North-striking faults are the primary control for ore-bearing
quartz vein development. Secondary lithologic control is important
for the position and overall shape of the ore shoots. Northeast-striking
faults are also mineralized, but gold-silver grades are lower and
more erratic. Northwest-striking faults do not contain significant
ore. Displacement on the north- and northeast-striking faults is
dominated by dip-slip offsets, producing horst and graben blocks.
Extensional faults played an important role in the development of
Postmineral fault movement resulted in strongly fractured and
crushed veins in some of the north-striking faults. Horizontal lineations
and slickensides are found on high-angle, postmineral faults.
Low-angle, postmineral faults have minor dip-slip and reverse-slip
offsets. Strike-slip postmineral movement is postulated on the northstriking
Quartz ±adularia replacement is the dominant alteration developed
on the margins of the vein zones. Silicified rock grades outward
into quartz-sericite/illite ±adularia. Pyroclastic, volcaniclastic,
and dacite flow units contain more extensive and better-developed
argillic and propylitic alteration than rhyolite.
Gold-silver ore is associated with a variety of quartz vein textures
and grain sizes resulting from open-space filling.
Microcrystalline to coarse-grained quartz is found in banded, saccharoidal,
comb, and bladed carbonate-replacement forms.
Hydrothermal breccia is commonly found within the vein zones and
is an important preparation feature for vein formation in the tight but
brittle rhyolitic host rocks. The quartz veins formed from episodically
boiling, low-salinity fluids (²2 equiv. wt % NaCl) at temperatures
of <200¡ to 255¡C. Mineralogy of the oxidized zone, which extends to 280 m below the present surface, consists of native gold and silver, cerargyrite, embolite, and electrum of variable fineness. Base metals are not found in significant quantities in either the oxidized or unoxidized zones; however, trace amounts of sphalerite, galena, and chalcopyrite are present in unoxidized quartz, where they occur in early banded vein stages.