Summary:
El Peñón is classified as a low- to intermediate-sulphidation epithermal gold-silver deposit associated with steeply dipping fault-controlled veins emplaced following rhyolite dome emplacement. Gold and silver mineralization consists of disseminations of electrum, native gold and silver, acanthite, silver sulphosalts, halides, and accessory pyrite occurring with quartz, adularia, carbonates, and clay minerals. Epithermal deposits represent shallow parts of larger, mainly subaerial, hydrothermal systems formed at temperatures as high as about 300°C and at depths from about 50 to as much as 1,500 m below the water table. Analogous epithermal gold-silver deposits set in an extensional-transtensional, continental-margin arc are the Comstock Lode in Nevada, Martha Hill in New Zealand, Peñasquito in Mexico, and Hishikari in Japan.
El Peñón is located in the Central Depression (also known as the Central or Longitudinal Valley), that extends for 650 kilometers from the Chile-Peru border in the north to south-central Chile in the south. In the Atacama Desert, this valley corresponds in part to a Late Cretaceous to Paleogene volcanic belt that separates the Mesozoic magmatic arc, exposed in the Coast Mountains located to the west, from the Paleozoic and Triassic volcanic and sedimentary assemblages of the Domeyko Cordillera to the east. The Late Cretaceous to Eocene volcanic and intrusive rocks within the Central Depression consist of an alkali-enriched calk-alkaline bimodal suite. Rocks consist of basaltic andesite to rhyolitic lavas and tuffs, subvolcanic porphyritic intrusions, and granitoid stocks. This belt is host to many epithermal deposits and subvolcanic porphyry systems.
The local area is underlain by a fault-bounded north-south trending panel of Paleocene to Eocene volcanic rocks. This panel is bounded to the east and west by rocks of Permian to Cretaceous age. Formation names and ages as reported below have been updated by extensive recent work by the Servicio Nacional de Geología y Minería, which resulted in significant changes from stratigraphic divisions reported in earlier reports. The Cretaceous sequence (95-90 Ma) dominates and consists of volcanic and minor sedimentary rocks of the Paradero del Desierto Strata Formation and continental sedimentary and volcanic rocks Quebrada Mala Formation. The Paradero del Desierto Strata outcrops northwest of the deposit area. The Upper Cretaceous Quebrada Mala Formation is present to the west, north, and northeast of El Peñón; it consists of volcanic rocks varying in composition from basaltic andesite to high-silica rhyolite; textures vary from flows to ignimbrites (Astudillo et al, 2017; Ferrando et al., 2013). Ignimbrites and other rock types formerly assigned to the Augusta Vitoria Formation are now included in the Quebrada Mala Formation. Away from the deposit, these rocks are intruded by large granitic to dioritic stocks dated at between 40 and 50 Ma.
The gold-silver mineralization at El Peñón is hosted in near-horizontal to gently dipping Paleocene to Eocene basaltic to rhyolitic volcanic rocks. The El Peñón deposit comprises many individual tabular and steeply dipping zones that are amenable to mining by both underground and surface methods. Vein thickness ranges from decimetre-scale to more than 20 metres. The strike length of individual mineralized zones ranges from less than 1 kilometre to 4 kilometres and the down-dip extent reaches up to 350 metres. The veins strike predominantly northsouth and dip steeply to the east and west. Vein textures often display crustiform textures, although the highestgrade gold and silver mineralization are associated with massive banded quartz-adularia. Gangue minerals occur as open space filling as well as replacements of primary host rock mineral phases.
The known deposit consists of 19 main vein zones and many subsidiary veins. They are grouped in vein systems that have previously supported, currently support, or are planned to support surface and underground mining operations. The 19 principal mineralized veins are listed as follows:
• Abundancia/La Paloma;
• Angosta;
• Aleste/Bonanza;
• Borde Oeste;
• Cerro Martillo/Dorada;
• Dominador;
• El Valle/Discovery Wash;
• Esmeralda/Esperanza;
• Fortuna;
• Laguna;
• Martillo Flat;
• Pampa Augusta Vitoria (PAV);
• Pampa Campamento/Sorpresa;
• Playa;
• Providencia;
• Quebrada Colorada;
• Quebrada Orito;
• Ventura;
• Veta NW.
The veins strike predominantly north-south and dip steeply to the east and west. A small proportion of the deposit is also hosted in fault zones striking north–northeast to northeast.
Vein textures often display crustiform textures, although the highest-grade gold and silver mineralization are associated with massive banded quartz-adularia. Gangue minerals occur as open space filling as well as replacements of primary host rock mineral phases.
Gold and silver mineralization occurs as disseminated electrum, acanthite, native gold, native silver, silver sulphosalts, and silver halides; these minerals are hosted in a gangue dominated by quartz, adularia, carbonate, and clay. Precious metals occur mainly as micron- to millimetre-size subrounded and irregular grains of electrum. Two phases of electrum are present: a primary phase, which contains approximately 55 to 65% gold, and a secondary phase where the gold content is usually greater than 95%, due to the supergene remobilization of silver.
Sulphide minerals are relatively rare, except at the northeastern portion of the El Peñón mine area. This paucity of sulphides may be due to oxidation, or to an initial overall low abundance of sulphides as would be expected in a low-sulphidation environment. Iron- and manganese-oxyhydroxides are common, with only trace occurrences of relict sulphides. In order of abundance, trace amounts of pyrite, galena, sphalerite, chalcocite and covellite occur locally.
Age-dating of adularia from the veins at El Peñón suggests that mineralization took place at around 52 Ma to 53 Ma (Early Eocene). Two mineralization and alteration events have been defined from fluid inclusion studies. The principal mineralization event resulted from circulation of neutral reduced fluids that replaced host-rock phenocrysts and groundmass by quartz, adularia, albite, carbonate, clays, calcite, and chlorite. It also produced quartz-adularia flooding and breccia-filling in the vicinity of the veins. Another, more widespread, alteration process was derived from acidic oxidized hydrothermal solutions. This event resulted in the formation of lithocaps of quartz-alunite alteration, quartz-alunite breccia-filling, with minor copper and silver and little or no gold.