Geology and Geochemistry of Epithermal Au-Ag Mineralization in the El Peñón District, Northern Chile
Authors: Warren Ian , Zuluaga José I. , Robbins Charles H. , Wulftange William H. , Simmons Stuart F. 
Published: February 2020

Abstract
The El Peñón gold-silver deposit comprises six epithermal veins, which contain a geologic resource (measured + indicated + inferred) of 3.8 million oz (Moz) Au and 63 Moz Ag. Three of the veins are currently being mined by underground and open-pit methods. El Peñón is located in the Central Depression of northern Chile, where the geology is dominated by Paleocene and Eocene mafic to felsic volcanic rocks and minor intermediate to felsic subvolcanic rocks and intrusions. The deposit is located in the central portion of the Paleocene metallogenic belt, 165 km southeast of Antofagasta. The property covers an area of ~440 km2, and the ore deposit occurs within an area of approximately 15 km2.
Late Cretaceous, Paleocene, and Eocene andesitic to rhyolitic flows, domes, tuffs, and minor intrusive rocks characterize the geology of the district. The deposit occurs within flat-lying to gently dipping, andesitic to rhyolitic pyroclastic and flow units, and volcaniclastic breccias of Paleocene and early Eocene age and is partly assignable to the Augusta Victoria Formation. El Peñón veins are partly hosted by, and spatially associated with, a 54 to 55 Ma (40Ar-39Ar, U-Pb) rhyolite dome complex that occurs over an area at least 18 km2, and similar rhyolite lavas occur over an area of tens of kilometers2.
Rocks in the district display two distinct types of hydrothermal alteration: widespread alteration associated with near-neutral pH, reduced fluid and localized alteration associated with acidic pH, oxidized fluid. Near-neutral pH, reduced fluid produced widespread replacement of phenocrysts and groundmass by quartz, adularia, albite, illite, chlorite, smectite, calcite, and pyrite; quartz-adularia flooding and cement to hydrothermal breccia intensify in the vicinity of veins. Where upflow of these fluids was focused along dominantly north- and northeast-trending structures, Au-Ag ± base metal-bearing crustiform quartz ± adularia ± carbonate veins formed, including the six veins that comprise the El Peñón deposit and several outlying prospects. Adularia from the two largest veins has been dated at 52 to 53 Ma (40Ar-39Ar), indicating formation 1 to 3 m.y. later than the host rhyolite domes.
Acidic pH, oxidized fluid produced lithocaps of massive quartz-alunite alteration, quartz-alunite cemented breccia, and, locally, weak Cu mineralization above inferred Late Cretaceous and Eocene intrusions. Isolated occurrences of quartz-alunite alteration covering hundreds of meters2 are located at the periphery of the property, in addition to several larger areas beyond it. Locally, quartz-barite veins occur peripheral to quartz-alunite alteration and contain variable amounts of base metals and Ag with little or no Au.
The veins that comprise the El Peñón deposit range from &lt;50 cm to 22 m wide. Pervasive supergene oxidation extends to 400 m below surface. Limited drill intercepts at deeper levels consist of banded and brecciated quartz, adularia, and massive, bladed, and acicular, Ca-, Fe-, Mn-, and Mg-bearing carbonate minerals, with minor amounts of pyrite, chalcopyrite, sphalerite, and galena. Veins exhibit a wide range of crustiform textures, including comb, colloform, and lattice quartz, rhombic adularia, and abundant massive and bladed Fe and Mn oxide minerals. Recrystallization textures suggest amorphous silica and chalcedony precursors for some quartz. Coexisting liquid- and vapor-rich inclusions, lattice textures, and vein adularia are evidence for boiling conditions that were likely responsible for Au-Ag deposition. Ore minerals observed in oxidized veins consist of electrum (mostly 40–60 wt % Au), acanthite, gold, silver, silver sulfosalts, silver halides, and rarely pyrite, chalcopyrite, and galena. High ore grades are generally associated with massive bands of fine-grained quartz and adularia, breccias composed of vein material in a matrix of fine-grained quartz and adularia, and, less commonly, colloform quartz bands. Supergene processes resulted in local remobilization of Au and Ag, leaving nearly pure gold (up to 98 wt % Au) along fractures and associated with oxide masses.
Fluid inclusion data from the El Peñón deposit indicate vein formation from low-salinity (&lt;2 wt % NaCl equiv), boiling hydrothermal fluid at temperatures mostly from 230° to 260°C. Fluid inclusion data from other mineralized quartz veins in the district indicate formation from commonly boiling, dilute fluid (&lt;3 wt % NaCl equiv) at temperatures between 180° and 330°C. Quartz-barite veins peripheral to quartz-alunite alteration formed from boiling fluid between 175° and 225°C with apparent salinities of 1 to 6 wt percent NaCl equiv.
Geochemical gradients in altered rocks surrounding veins in the El Peñón district indicate that Au, Ag, As, and Sb concentrations increase toward quartz veins, and Au, Ag, As, Sb, and base metal concentrations increase toward quartz-barite veins. Geochemical analyses of altered rocks from drill holes surrounding the Quebrada Colorada vein, the highest grade vein of the El Peñón deposit, show enrichment of Au and Ag and depletion of Ca, Na, and Sr toward the vein. Comparing the geochemical data to elevation, the highest mean values for Au (205 ppb) and Ag (5.9 ppm) occur at mid levels, the highest mean values for Pb (35 ppm) and Zn (183 ppm) occur at lower levels, and the mean values for As (139 ppm) and Sb (34 ppm) increase with elevation. Arsenic (100s of ppm) and Sb (10s of ppm) anomalies occur in rocks above the Quebrada Colorada vein that are barren or contain only low levels of Au and Ag; such anomalies may be useful indicators of blind mineralization.
Epithermal deposits in the Paleocene belt of northern Chile are preserved in rocks located at a long-lived convergent plate boundary with a complex history of compression and extension that has formed linear morphotectonic and metallogenic belts parallel to the plate boundary. In northern Chile, Paleocene and early Eocene epithermal deposits occur west of the uplifted Cordillera Domeyko, which contains younger porphyry Cu deposits and intrusions characteristic of deeper environments. This paradoxical situation is partly explained by a protracted history of structurally controlled basins in the Paleocene belt of northern Chile. From Late Cretaceous to Eocene time, the Paleocene belt was characterized by fault-bounded basins that experienced both subsidence and inversion; however, cumulative postmineral uplift was minimal because fluid inclusion data from El Peñón indicate erosion of only several hundreds of meters. Since the Miocene, erosion has largely ceased due to the onset of hyperaridity.