Gold-copper skarns have developed mainly along the contact between intrusives and carbonate units. Two different types of skarn have been recognized at El Valle-Boinás. The first is a calcic skarn related to limestone units and the second is a magnesian skarn, called “black skarn”, that is related to dolomite units.

The gold-copper-bearing skarns at Carlés are generally calcic skarns. Better grade goldcopper mineralization is associated with high magnetite and bornite content that is localized in generally continuous, relatively thin (four metres thick) layers of retrograde skarn.

At the El Valle-Boinás deposit, reactivation of fracture zones (along northeast-southwest, east-west and northwest-southeast orientations) produced widespread brecciation and favoured the emplacement of porphyritic dykes. A low-temperature alteration and mineralization event is spatially and genetically associated with the subvolcanic porphyry dykes, which overprint all previous lithologies. Depending on the host rock, there are different styles of hydrothermal alteration and mineralization, such as: sericite-adularia-carbonates (+py-aspy) in granites and skarns; leaching, enrichment, and silicification in skarns (+ native copper and chalcocite), and silicification (+py) in dolomites

Highest gold grades occur where the low-temperature mineralization overprints previously mineralized gold-copper skarn, forming jasperoid or semi-jasperoids with native copper and minor chalcocite and cuprite. The associated geochemistry is characterized by an increase in As, Sb, and Hg. This low-temperature event is the principal gold-mineralizing episode at El Valle.

Gold, and in some cases, base-metal mineralization, has been found in association with late tectonic breccias related to low-angle thrust faults at El Valle-Boinás. The origin of the gold mineralization in these structures is thought to be due to remobilization of previous skarn or jasperoid related gold mineralization. Gold associated with low-angle structures is important at El Valle-Boinás where a significant percentage of the open pit minable gold mineralization extracted from the Boinás East Zone came from this type of structure.

Mineralization at the El Valle-Boinás copper-gold deposit can be grouped into several significant deposits related to the Boinás granitic intrusive and carbonate rocks of the Láncara Formation (Cambrian age).

The gold mineralization system has a strike length of two kilometres and a width of at least 0.5 km. The intrusive is elongated trending N35°E with a length of 500 m and an average thickness of 300 m. A copper-gold mesothermal skarn was developed mainly along the contact between the igneous rock and the carbonate unit.

Late reactivation of the main northeast-trending fracture system was accompanied by two or more phases of epithermal mineralization as well as the intrusion of porphyry dykes. These events produced hypogene oxidation with further enrichment of gold, arsenic, antimony and mercury (Martin-Izard, et al., 1999). Rhyodacite dykes, which are always sericitized, were emplaced along fractures and breccia zones trending north-northeast. The intense silicification along fractures and breccia zones resulted in the formation of hematitic jasperoid that is characterized by enrichment in gold, arsenic, antimony and mercury.

The Carlés deposit is a gold and copper bearing skarn developed predominantly in the Devonian limestones of the lower portion of the Rañeces Formation along the north margin of the Carlés granodiorite. The Carlés intrusion is approximately circular in plan with a diameter of about 750 m. The intrusion is located at the intersection of major faults (east-west, northeast-southwest and southeast-northwest) and it is bisected from west to east by the Río Narcea River. The northern part of the granodiorite is in contact with the lower part of Rañeces Formation and the southern part of the intrusion is in contact with the siliciclastic Furada Formation. Several barren Permian porphyritic and diabasic dykes crosscut the existing lithologies.

Mineralization is continuous for over 1,000 m. It ranges in thickness from 1.5 m to over 25 m, dipping 50º to 90º away from the granitic intrusion. The skarn is known over a vertical continuity of 400 m and remains open at depth.

The Carlés skarn is of calcic composition and is an exoskarn although irregular endoskarn has developed locally. It consists of layers of garnet (grossularite-andradite composition) intercalated with layers of pyroxene skarn, mostly of hedenbergite composition. Retrograde phases of the skarn results in the formation of irregular magnetite layers associated with amphibole. Inside these bands is where most of the copper sulphides and gold mineralization occur. The skarn mineralization transitions into coarse-grained marbles then non-altered limestones away from the intrusive. The latter may show narrow intercalations of distal garnet-pyroxene incipient skarn.

Gold mineralization at Carlés is closely associated with copper sulphides, which consist of disseminated and patchy chalcopyrite and bornite that precipitated mainly in the magnetite zone. Other minerals common in the skarn are arsenopyrite, löellingite, pyrrhotite, and latestage pyrite.

The current mining methods used at Boinás Mine are overhand drift and fill, and transverse longhole stoping. Due to decreasing thickness of the remaining Boinás skarns, RPA changed the design of the longhole mining from transverse to longitudinal where appropriate. Drift and fill mining will continue to be used in the oxide areas of the mine. Ore is hauled to surface via ramp and/or skipped via shaft, depending on location and ore type. Backfill is placed by truck and scoop, consisting of cemented rock fill or waste fill, as appropriate to the mining sequence and geotechnical demands.

Carlés Mine uses longitudinal longhole stoping methods. Ore is hauled to a surface stockpile via underground truck, and transferred to highway-rated surface trucks for transport to Boinás. Backfill is via waste fill, as stopes are separated by rib pillars.

Ore extracted from EVBC is stockpiled and classified according to mineralogy, gold, copper, and/or deleterious element grades. When possible, a suitable ore blend is prepared to feed the mill with material containing 2 g/t Au to 5 g/t Au and < 1% Cu. A front end loader collects the ore from the various stockpiles and delivers it to the crusher feed bin equipped with a 600 mm aperture slotted grizzly screen. The crusher feed bin has an apron feeder that feeds the primary single toggle jaw crusher and any apron feeder spillage is collected on a conveyor belt below. The jaw crusher set to 153 mm. The spillage and the crushed ore which is less than 153 mm in size are transferred to the next conveyor belt equipped with load cell type belt weightometer and this belt discharges into the 100 ton mill feed bin. A level indicator and controller in the mill feed bin controls the apron feeder feed rate to the jaw crusher to maintain the set level in the bin.

Dry crushed ore at P80 < 153 ........