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 (Pearson and Rennie, 2008). Epithermal deposits
represent shallow parts of larger, mainly subaerial, hydrothermal systems (Figure 8-1) 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 (John et al., 2010).

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 range from decimetre-scale to more than 20 metres. The strike length of individual mineralized zones ranges from less than 1 km to 4 km and the downdip extent reaches up to 350 m.

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.

The El Peñón mine (El Peñón) is an underground and open-pit gold-silver mine located in the Atacama Region of Chile, approximately 165 km southeast of the city of Antofagasta.

The current major assets and facilities associated with the mining operations at El Peñón are
listed as follows:
• Mine and mill infrastructure including office buildings, shops, laboratories, stockpiles, tailing storage facility, and equipment.
• Campsite/housing facilities
• Facilities providing basic infrastructure to the mine, such as access roads, electric power distribution systems connected to national power grid, water treatment and supply and sewage treatment.
• Underground infrastructure including portals, access ramps, ventilation raises, maintenance shops, and mobile fleet equipment.
• The open-pit infrastructure including haulage roads, ramps, and mobile fleet equipment.

Ore from underground mines have recently been—and will continue to be—the main source of feed for the El Peñón mill. Currently, ore is sourced from one small active open pit mine (Chiquilla Chica) and from five of the seven major underground mining zones.

The various underground mining zones are accessed by ramps; this type of access is suitable for this mine in light of its shallow depth. The underground workings of the core mine extend approximately ten kilometres along strike and span a vertical extent of approximately 500 m, measured from the highest portal collar elevation to the bottom-most mine workings.

Mine access is achieved via spiral declines generally located in the footwall of veins. The declines have section dimensions of 4.3 m wide × 4.5 m high at a gradient of up to ± 16% and a minimum turning radius of 15 m. Access to the ore is made approximately every 10 to 20 vertical metres via crosscuts. Infrastructure generally included at every intersection of declines with crosscuts consists of service bays, ventilation drifts to connect crosscuts to return air raises, remucks, and dewatering infrastructure when required.

The main mining method utilized at El Peñón is the bench-and-fill method, which is a narrow longhole-stoping method that uses a combination of rockfill and cemented rockfill. The method involves ore development at regular level intervals, which, at El Peñón, range generally between 10 and 20 m. Due to the narrow vein widths, a “split-blasting” technique is used in many areas of the mine to reduce dilution in secondary development of ore zones. The minimum mining width of a split blast is of 0.6 m, plus 0.5 m of total overbreak, generating a minimum blast void of 1.1 m width. Once the split-blast ore is mucked out, the remaining waste is slashed out and used for rockfill purposes. The split-blasting technique has been refined and improved at El Peñón since 2016, reducing the achievable ore mining width from 2.1 m to 1.1 m, minimizing dilution and ore loss, and improving productivities for faster face cycle times. The result is increased gold and silver mining grades. In some cases, development rounds that would have previously been mined as waste if blasted to the full drift dimensions, are now mined selectively as separate ore and waste rounds, resulting in increased mineral reserves.

Stopes are formed by drilling blast holes between levels. After blasting, the broken ore is extracted from the lower level using conventional and remotely operated load-haul-dumps (LHDs). Bench-and-fill is a bottom-up method, in which mining takes place above and adjacent to previously mined and backfilled stope voids. Once the maximum-allowed stope span is reached, and after completion of ore extraction from the blasted stope, stopes are filled with rockfill and selective use of cemented rockfill.

Ground support for development consists of shotcrete, wire mesh, and rockbolts. Shotcrete (50 mm-thick) is applied to the back and walls in areas associated with weak rock mass. Where excavations are located in more competent rock, wire mesh is installed along the back and shoulders. Rock bolts (2.4 to 2.8 m-long) are installed to support the back and sidewalls. The bolt length and spacing vary depending on the condition of the rock mass and on the dimension of the excavation.

Three pumping systems are currently operating at the mine. The auxiliary pumping system collects water at the faces and pumps it to the secondary pumping stations using Grindex portable pumps. The main pumping stations decant water and pump it to HDPE-lined ponds on surface. The main pumping stations consist of a pond with multistage centrifugal 220 hp pumps of 30 L/s capacity. These pumps can handle water columns of up to 320 m, which is approximately equivalent to a maximum operating pressure of 30 bars.

Ventilation of the underground mines at El Peñón is provided through the use of primary and secondary ventilation systems. The primary ventilation system is an exhaust/pull system. Fresh air is supplied through portals, intake ventilation raises, and declines. Return air is exhausted through return air raises (RAR) to surface by main exhaust-air axial fans usually positioned on surface.

Run-of-mine or stockpiled ore is dumped from a 7 m3 capacity (CAT 988H) front-end loader and screened through a 600 mm square-grid grizzly into a 100 t-capacity hopper. Fine material is collected and transported directly to the conveyor belt that carries primary crushed material. A 1,500 mm-wide apron feeder is used to transfer ore from the dump hopper to the jaw crusher. Coarse material is fed into a 950 mm × 1,250 mm jaw crusher and crushed to a P80 size of 63.5 mm. The crushed ore is transported by a conveyor belt to a 1,500 t-capacity silo. Additionally, an auxiliary stockpile for crushing product is located to the northwest of the silo.
The stockpile has a capacity of 10,800 t and covers an area measuring approximately 40 m × 60 m..

The ore stored in the silo is transported by a variable-speed mill-feed conveyor belt, which has a nominal capacity of 250 tonnes per hour (tph), to a transfer chute that discharges onto the conveyor belt that feeds the SAG mill.

The ore from the auxiliary stockpile is fed via a front-end loader to an encapsulated hopper with suppressor system to mitigate dust emissions. The hopper discharges onto a belt which transports the ore to the mill-feeder conveyor belt.

Crushed ore and sodium cyanide process solution are fed into the SAG mill; the sodium cyanide solution is used as leaching agent.

The SAG mill operates in series with a ball mill, which feeds a battery of hydrocyclones. The underflow of the hydrocyclones returns to the SAG mill. Pebbles formed in the SAG mill are discharged by a trommel onto a conveyor belt, which transports the pebbles to a chute returning them to the mill-feeder conveyor belt. Alternatively, the pebbles can be mixed with crushed material to be recirculated to the grinding circuit. The density of the pulp fed to the hydrocyclone circuit is controlled online via density measurements using a nuclear densitometer.

The classification circuit consists of six hydrocyclones. Generally, four hydrocyclones operate while two remain on standby. The cyclone overflow pulp contains between 42% and 46% solids with a P80 of 170 µm. Particle size is measured online through a PSI 300 particle-size analyzer and is controlled by changing hydrocyclones operating pressures. The hydrocyclones overflow is fed to the grinding thickener and the underflow is recirculated to the SAG mill feed. Spills are
pumped to the mill’s discharge sump using a floor pump located in the area.

Flocculant is added to the grinding thickener to promote solid-liquid separation by decantation. The underflow of the thickener, containing 50% solids, is pumped to the first leach tank.

The El Peñón processing plant and associated facilities process run-of-mine as well as stockpiled ore, using the main processes listed below:
• Crushing;
• Grinding and pre-leaching thickening;
• Leaching;
• Counter-current decantation (CCD) concentrate solution recovery;
• Clarification, zinc precipitation, and precipitate filtering;
• Refining;
• Tailings filtering and disposal.

Gold and silver leaching starts at the SAG mill, where sodium cyanide is added as a leaching agent. An extraction of around 75% is reached in this step. Six reactors, with a combined capacity of 7,279 m3 using mechanical agitators, leach the underflow of the grinding thickener. Oxygen is also added to maximize dissolution kinetics. Leach tanks are arranged in series with cascading heights to facilitate the transport of pulp by gravity. The reactors are fed from the bottom to reduce the potential for the leach slurry to shortcircuit between tanks. Under normal oper ........