The deposits at El Peñón are low to intermediate epithermal gold-silver deposits, hosted in steeply dipping fault-controlled veins following rhyolite dome emplacement. Gold and silver mineralization comprises disseminations of electrum, native gold and silver, acanthite, silver sulphosalts and halides, plus accessory pyrite occurring with quartz, adularia, carbonates, and clay minerals. These minerals were deposited from boiling of low salinity fluids circulating in a hydrothermal system driven by the Eocene to Paleocene magmatism. Late stage oxidation has occurred to a depth of 400 m below surface.

The deposit comprises many individual tabular, steeply dipping zones or shoots that are amenable to mining by both underground and surface methods. Vein widths range from decimetre-scale to over 20 m. Individual mineralized shoots measure from less than one kilometre to four kilometres in strike length, and up to 350 m in the down-dip direction. Gold grades range up to hundreds of grams per tonne but are more typically less than 30 g/t. Silver grades are in the order of hundreds to thousands of grams per tonne.

There are thirteen main vein zones and many subsidiary veins in nine vein systems that have supported, support currently, or are planned to support surface and underground mining operations. The veins strike predominantly north south and dip steeply to the east and west. North-northeast to northeast-striking fault zones are also host to mineralized zones, however, the relative proportion of the overall deposit is small. The principal mineralized veins are Al Este, Bonanza, Cerro Martillo/Dorada, Dominador, El Valle/Discovery Wash, Fortuna, Martillo Flats, Pampa Campamento, Playa, Providencia, Quebrada Colorada, Quebrada Orito, and Vista Norte.

Gold and silver mineralization consists of disseminated electrum, native gold, native silver, silver sulphosalts, and silver halides occurring in a gangue of predominantly quartz, adularia, carbonate, and clay. Electrum is the most common form of precious metals in the deposit and occurs as micron- to millimetre-size subrounded and irregular grains. Two phases of electrum are present: a primary phase, which contains approximately 55% Au to 65% Au, and a secondary phase, which has resulted from supergene processes that have remobilized silver, and which typically consists of over 95% Au.

The primary mining method is an underground bench and fill method and all access to the veins is by ramps and crosscuts. Veins are separated by a distance of 100 m to 500m. The application of this method will vary between veins, but it is usually applied to sublevels spaced between 10 m and 18 m. Vein dips are steep and the bench drifts are built along the strike of the vein. A top access drift is driven for drilling, and a bottom access drift is driven for ore extraction. Depending on the vein width, the access drift dimensions are generally 3.5 m wide by 4.0 m high. Both the drill access drift and the lower ore extraction drift are grade-control sampled every drill, blast, load and haul cycle.

Crushed ore is stored in a 1,500 tonne capacity bin, from which it is fed to a 4.72 m x 7.77 m 2,500 kW semi-autogenous grinding (SAG) mill. A new 4.27 m x 6.10 m diameter ball mill was added in series with the SAG mill in 2009 to increase mill capacity. Pebbles from the SAG mill are crushed in a pebble crusher. Cyanide solution and lime are added in the grinding circuit. The grinding mills are in closed circuit with hydrocyclones.

The grinding circuit product, the cyclone overflow at a nominal P80 of 180 µm, is sent to a thickener where the solution is thickened to 50% solids with the underflow reporting to a cyanide leaching circuit. The thickener overflow is sent to the unclarified solution tank. The leaching circuit product is sent to a counter current decantation (CCD) circuit.

The precious metals are recovered in a zinc precipitate Merrill-Crowe process. Unclarified solution is sent to the clarification circuit where it is filtered ahead of reporting to the pregnant solution tank. The solution is then de-aerated in a vacuum tower and zinc dust is added ahead of pressure filters. Gold and silver are precipitated on the zinc dust which is collected from the pressure filters and calcined in a mercury retort to remove contained mercury. The calcined precipitate is then smelted to make doré bars.