Summary:
Lomas Bayas is a low grade Cu-Mo deposit resulting from the intrusion of several porphyry and breccia systems that were later exposed to leaching and subsequent supergene enrichment and in situ oxidation. Green copper oxides, copper sulphates in various forms and less partially mixed ores are the main source of ore for the existing SX/EW operation. The copper oxidessulphate mineralisation is the basis of the Lomas Bayas Ore Reserves (Lomas Bayas I).
The deposit is located in a structural framework formed by three sets of faults along an ENE bend in the Tertiary strike-slip San Cristóbal Fault system. The deposit area is mainly underlain by granodiorite that has been intruded by a cluster of small granodiorite and/or granite to dacite porphyry bodies of the Lomas Porphyry. Later events, focused by the structural framework, include the emplacement of breccia pipes and zones as well as hydrothermal alteration and copper mineralisation.
The deposit area is characterised by a structural regime, the main structural elements of which are NNE, NW, north-south and WNW-trending high-angle faults and fractures, as well as west to SW-dipping, low-angle faults and fractures that occur to the north of the ENE-trending South Fault Zone.
Both primary and supergene mineralisation occurs at Lomas Bayas. The upper parts of the orebody are generally oxidised, with a few zones of mixed oxide-sulphide, and it is principally exploited as a heap-leach operation. Copper mineralisation occurs as an irregular concentric zone around a low grade, hydrothermally-altered core. The economic oxide mineralisation at Lomas Bayas occurs as quick-leaching, low acid consumption copper sulphate minerals. The ore assemblage includes Cu-oxide minerals, principally antlerite and brochantite, with minor chalcanthite, atacamite, chrysocolla and malachite, with limonite and relict sulphides, mainly chalcopyrite and bornite, and minor local pyrite, within early dark mica veins and alteration halos, and in breccias. Magnetite has been partly to completely destroyed in these veins and halos. The oxide minerals are recognised in drillholes to depths of at least 300 m below the surface. A minor amount of preserved secondary enrichment has been observed, mainly as chalcocite and covellite that replaces or coats chalcopyrite, bornite and pyrite.
Hypogene sulphides, which include chalcopyrite and pyrite, with small amounts of bornite and molybdenite, are thought to occur in three different phases, namely: i). a quartz-poor sulphide phase, ii). disseminations within the phyllic quartz-sericite alteration and iii). sulphides in quartz and quartz-tourmaline veins. Molybdenite may occur in the sulphide phase and in the quartz veins. Secondary enrichment processes affected the hypogene mineralisation, which was then oxidised with partial remobilisation of the copper.
The primary hydrothermal alteration is dominantly potassic (including an early dark mica assemblage), defined by secondary biotite and K feldspar, comprising an inner feldspar zone grading outward to biotite. Quartz-sericitic-pyrite phyllic alteration occurs mainly in the Lomas Porphyry and some breccias, while propylitic assemblages of chlorite and epidote are found in a NNE trend located in the northern and southern extremities of the mine area. The higher grade mineralisation occurs as sets of thin, sub-parallel early veinlets with few mm- to few cm-thick "early dark mica type" alteration as sericite-K feldspar and secondary biotite halos, that contain oxides after Cu sulphides. These veinlet swarms do not have a close spatial association with individual porphyry intrusions. Mineralisation is also evident in the breccias.
The Lomas III is the hypogene open-pit sulphide resource below the Lomas Bayas oxide orebody, including a mixed sulphide-oxide transition zone. This resources was the subject of a pre-feasibility study in 2011-12 which considered its potential for milling and flotation.
The Lomas Bayas (III) Mineral Resources estimates sulphides in a pit shell calculated using Measured, Indicated and Inferred Mineral Resources; Oxides-Mixed within this pit are also considered Mineral Resources that will eventually feed the SX/EW operation.
The Lomas Bayas (I) and Lomas Bayas (III) ore bodies, whereby the Lomas Bayas (III) sulphide layer lies beneath the Lomas Bayas (I) oxide layer. Additional oxide-mixed material at depth has been added to the Lomas Bayas (I) resource base during the year.
The Lomas II (Fortuna de Cobre) deposit is a porphyry copper deposit that is lithologically and structurally similar to Lomas Bayas. In addition to features typical of a larger porphyry copper system, Lomas II differs from Lomas Bayas by the presence of abundant amounts of chalcanthite, a water-soluble copper mineral. It also contains an upper enrichment zone rich in water-soluble minerals and a lower zone containing secondary sulphides.
Lomas Bayas (II): This low grade Cu deposit is located 2km south of Lomas I pit, in the same district and geological environment as Lomas Bayas deposit. The main difference is a larger presence of water soluble copper oxides and lower geotechnical rock quality.