Lomas Bayas, Fortuna de Cobre


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The Lomas Bayas porphyry copper deposit is located at an altitude of 1500 m in the Atacama Desert, within the San Cristobal mountains of northern Chile, ~85 km ENE of the port of Antofagasta (#Location: Lomas Bayas - 23° 25' 58"S, 69° 30' 41"W; Lomas II - 23° 27' 3"S, 69° 32' 24"W).

The Lomas II deposit, formerly known as Fortuna de Cobre, is 3 km SW of the Lomas Bayas pit.

The low-grade Lomas Bayas copper porphyry copper deposit is hosted by Late Cretaceous volcanic-arc rocks and associated back-arc sediments, that are intruded by the Upper Cretaceous to Paleocene (~64 to 61 Ma at Lomas Buyas) composite San Cristóbal batholith, which is largely composed of medium-grained granodiorite.

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 Bayas deposit has been divided into geological domains to optimise mining and recovery of copper. These domains are determined by analysis of lithology, structure, alteration and mineralisation. Copper grades increase with the intensity of structure and alteration, which are the strongest controls on mineralisation.

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.

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 deposit was discovered in 1880, and produced intermittently until 1992, exploiting high-grade veins and breccias by underground workings. During 1991, Empresa Minera Mantos Blancos SA (EMMB) optioned the property from Compania Doña Ada SA, although the latter maintained the right to continue mining by open-pit and block-caving methods and hauling the ore to its plant located 60 km to the north. In 1994, Compania Minera Gibraltar Ltda (CMGL) signed an option agreement to acquire the Lomas Bayas property and conducted a program of geologic mapping, drilling and metallurgical testing. The property changed ownership several times before and after it was commissioned as a large-scale heap-leach operation in mid-1998. In 2005, the mine produced 63 147 t of Cu by leaching 13.5 Mt of 0.5% Cu ore, and 22.4 Mt of 0.22% Cu of run-of-mine ore in the heap-leach operation.

JORC compliant ore reserve and mineral resource figures at 31 December, 2011 (Xstrata Copper, 2012) were:
  Lomas Bayas oxide and mixed ore -
      Proved + probable reserves - 41 Mt @ 0.30% Cu (0.19% Soluble Cu);
      Measured + indicated + inferred resources - Nil;
  Lomas II (Fortuna de Cobre) oxide and mixed ore -
      Proved + probable reserves - 473 Mt @ 0.27% Cu (0.19% Soluble Cu);
      Measured + indicated resources - 566 Mt @ 0.26% Cu (0.18% Soluble Cu);
      Inferred resources - 77 Mt @ 0.15% Cu (0.08% Soluble Cu);
  Lomas III sulphide ore -
      Proved + probable reserves - Nil;
      Measured + indicated resources - 245.7 Mt @ 0.41% Cu;
      Inferred resources - 190 Mt @ 0.36% Cu;
  Lomas III oxide and mixed ore -
      Proved + probable reserves - Nil;
      Measured + indicated resources - 379.1 Mt @ 0.27% Cu;
      Inferred resources - 28 Mt @ 0.21% Cu;
Measured and indicated resources include reserves.

The most recent source geological information used to prepare this summary was dated: 2012.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

Lomas Bayas

Lomas II

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