Summary:
The Los Helados deposit is classified as a porphyry copper-gold system.
The deposit is located along the Los Helados fault, a district-scale structure that is part of the structural corridor that defines the Miocene mineralization within the district (Dietrich, 2023). Minor faults of northwest and west-northwest orientation cut across the deposit area, and intrusive bodies and breccias have a preferential northwest trend. In addition, faults in the area have altered gouge and damage zones, suggesting that structural adjustment coincided with porphyry and breccia emplacement and that the structural intersection of the two major trends (north-northeast and northwest) was important for localizing the deposit.
Alteration
Five main alteration types are recorded within the Los Helados system: potassic, chloritesericite, sericitic, advanced argillic, and argillic. Each has a distinctive mineral assemblage, which can be significantly controlled by the host rock lithology with a large difference between assemblages in felsic and mafic rock types. Together, where mapped on surface, these types define an alteration footprint of approximately three kilometres north-south and two kilometres east-west. The steep topography and erosion in the valley bottom into the uppermost part of the deposit allowed for clear alteration vectoring to the central part of the system.
The deepest assemblage, in the potassic domain, is not exposed at surface, and is intersected in drilling from approximately 200 m below surface to the deepest part drilled to date at approximately 1,500 m. Biotite is the defining mineral of the potassic domain where mafic host rocks are altered, while K-feldspar dominates in the more felsic units. Remnants of overprinted potassic alteration are found in the lowermost outcrops at surface, as inherited quartz-sulphide and quartz-tourmaline veins within intense sericitic alteration.
Chlorite-sericite alteration overprints the potassic assemblage and is the predominant alteration within the deposit area. A chlorite-hematite assemblage defines the alteration within mafic lithologies changing to chlorite-sericite-clay in the felsic host rocks.
The predominant alteration at surface around the deposit area is quartz-sericite. The sericitic alteration is strong to intense in the lowermost exposures on the slope above the deposit with a quartz-sericite-pyrite assemblage that gives way upslope and outwards to a sericite-quartzdominant assemblage with no pyrite. Sericite gives way to illite in the outermost regions with a transition to a weak propylitic halo.
Argillic alteration is recorded locally, particularly intense along some fracture zones within the peripheral parts of the system.
Advanced argillic alteration is present at surface in exposures of magmatic-hydrothermal breccia. Small breccia bodies up slope from the deposit to the south also display advanced argillic alteration.
Mineral Zones
Four mineral zones are recognized within the deposit based on sulphide occurrence. Zone definition does not include late pyrite veinlets or the total volume of sulphides present in the rock. In order of increasing depth, the zones are: pyrite-only (Py); pyrite>chalcopyrite (Py>Cpy); chalcopyrite>pyrite (Cpy>Py); and chalcopyrite-only (Cpy).
This sulphide zoning sequence reflects a progressive downward increase in the amount of chalcopyrite relative to pyrite.
Mineralization
Miocene copper-gold mineralization at Los Helados is hosted within the early and intermineral intrusive phases but is volumetrically most significant within the magmatic-hydrothermal breccia.
The breccia limits have been established by drilling to the west, east and south, although the low drilling density still allows for significant room for further high-grade discoveries within the breccia limits. The system also remains open at depth, and the lateral extent of the breccia at depth is also poorly constrained by the current drilling. The eastern contact appears to be subvertical, whereas the western contact dips outwards at approximately 70°, hence the width of the breccia body increases progressively downwards.
The copper grade increases downwards, either in the lower parts of the sericitic zone or in the underlying chlorite-sericite alteration zone, and elevated grades are maintained into the potassic alteration zone. In the central part of the breccia body, within the Condor Zone, consistent grades in the order of 0.5% Cu and 0.2 g/t Au to 0.3 g/t Au in the core zone are flanked by domains of approximately 0.3% Cu to 0.4% Cu and 0.1 g/t Au to 0.2 g/t Au. High-grade zones within mineral cemented breccia exceeding 1% Cu and 1.5 g/t Au are found locally.
The new discoveries of high-grade zones at Alicanto and Fenix, on the margins of the main body, are in chalcopyrite mineral-cemented breccias that return grades in the order of 0.6% Cu to greater than 1% Cu and 0.2 g/t Au to 1 g/t Au. Both of these zones contain significant molybdenum values, which are not typically seen in the Condor Zone. Best intersections include 343.8 m at 0.81% Cu, 0.12 g/t Au and 204 ppm Mo, including 63.8 m at 1.14% Cu, 0.14 g/t Au, and 741 ppm Mo in hole LHDH081-2 in the Fenix Zone, and 122.1 m at 0.94% Cu, 0.14 g/t Au, and 190 ppm Mo in hole LHDH083 in the Alicanto Zone.
Gold grades generally correlate well with copper; however, within the sericitic alteration zones in the upper part of the deposit, where pyrite content exceeds chalcopyrite, high gold grades can be locally independent from copper values and are typically higher than in the underlying potassic zone. High gold grades are also associated with the apex of at least the Alicanto hydrothermal breccia and sporadic very high grade gold-quartz veins have been intersected peripheral to the main breccia (e.g., LHDH086-1 with 4 m at 11.2 g/t Au and LHDH087 with 4 m at 17.9 g/t Au).
Consistently high copper and gold grades are present in the potassic and chlorite-sericite zones where chalcopyrite is more abundant than pyrite.