Summary:
Gold-copper skarns have developed mainly along the contact between intrusives and carbonate units. Two different types of skarn have been recognised 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 mineralisation is associated with high magnetite and bornite content that is localised 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, eastwest, and northwestsoutheast orientations) produced widespread brecciation and favoured the emplacement of porphyritic dykes. A low temperature alteration and mineralisation 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 mineralisation, 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 mineralisation overprints previously mineralised gold-copper skarn, forming jasperoid or semi-jasperoids with native copper and minor chalcocite and cuprite. The associated geochemistry is characterised by an increase in As, Sb, and Hg. This low temperature event is the principal gold mineralising episode at El Valle.
Gold, and in some cases base metal mineralisation, has been found in association with late tectonic breccias related to low angle thrust faults at El Valle-Boinás. The origin of the gold mineralisation in these structures is thought to be due to remobilisation of previous skarn or jasperoid related gold mineralisation. Gold associated with low angle structures is important at El Valle-Boinás where a significant percentage of the open pit minable gold mineralisation extracted from the Boinás East Zone came from this type of structure.
EL VALLE-BOINÁS
Mineralisation 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 mineralisation 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 mineralisation 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 sericitised, 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 characterised by enrichment in gold, arsenic, antimony, and mercury. The presence of cuprite and native copper in the structures and breccias suggests the leaching of chalcocite, which is encountered at a depth of approximately 400 m along the A107 structure. This can be viewed as evidence of two-cycle leaching and enrichment.
CARLÉS
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 approximately 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. 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.
Mineralisation is continuous for over 1,000 m, ranging in thickness from 1.5 m to over 25 m, dipping 50° to 90° 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 mineralisation occur. The skarn mineralisation transitions into coarse grained marbles then non-altered limestones away from the intrusive. The latter may show narrow intercalations of distal garnetpyroxene incipient skarn.
Gold mineralisation 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. Mineralisation at Carlés is divided into four areas: Carlés East, Carlés North, Carlés Northwest, and Carlés West.