Gaby Sur, Gabriela Mistral


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The Gaby Sur porphyry deposit, and the Gabriela Mistral mine that exploits the ore, is located at 2660 m asl., in Antofagasta Region, district of Sierra Gorda, located 120 km SW of Calama.

The geology of the area surrounding the Gaby Sur deposit is dominated by a Permo-Carboniferous volcano-sedimentary sequence, comprising sedimentary rocks that are correlated with the Quebrada Escondida Formation, while towards the W and NW, mafic to intermediate volcanic rocks of the Del Bordo Formation predominate. A complex of tuffs, ignimbrites and rhyolitic acid domes of the Cas Formation are exposed to the north. These sequences they are intruded diorites, monzodiorites, granodiorite and granites, belonging to the Pampa Elvira Plutonic Complex which has yielded numerous K-Ar ages of from 280 to 270 Ma (Mpodozis et al., 1993). Continental red-bed sequences of probable Cretaceous age of the Purilactis Group unconformably overlie the Palaeozoic basement to the North of the deposit.

Small diorite and microdiorite stocks outcrop as 'island-like' hills to the west of the deposit, which on the basis of regional correlations, have been attributed to the Eocene by Mpodozis et al. (1993). The ore deposit is associated with Eocene tonalitic porphyry and intrusive breccias of the Gaby Sur Complex, which have been dated (K-Ar, 40Ar-39Ar and U-Pb) at between 43 and 41 Ma (Martin, 1999). More than 70% of the district in general, and the Gaby Sur deposit in particular, are covered by gravels with intercalations of ash dated at 9.6 Ma (K-Ar; Mpodozis et al., 1993; Aguilar, 2000; Camus, 2001). Faults and lineaments within the district are dominantly oriented NNE to NE and NW.

Three lithological suites are recognised at Gaby Sur (Aguilar, 2000; Camus, 2001):
Volcano-sedimentary sequence, a set of stratified rocks composed of sandstones, fine, tuffs and andesites, all subject to varying degrees of low grade metamorphism. The sequence generally strikes north-south and dips at ~45° to the W, in the West sector of the mine area. Its sedimentary facies have been correlated with the Quebrada Escondida Formation, while the volcanic components are equated with the Del Bordo Formation.
Elvira Pampa Intrusive Complex, composed of granodiorite and tonalite and is cut by mafic dykes. It is the most areally extensive unit, within the Gaby Sur deposit area, and intrudes the volcano-sedimentary sequence. U-Pb dating of zircons from the complex yielded ages of 278±1.8 and 280±1.2 Ma (Martin, 1999; Aguilar, 2000), which confirmed the K-Ar ages of from 280 to 270 Ma (Mpodozis et al., 1993), while sericite was dated at 267±3.64 Ma (
40Ar-39Ar; Dilles et al., 2000).
Gaby Intrusive Complex, composed of the following lithotypes:
Intrusive Breccia, present as a number of separate bodies composed of polymictic fragments (but predominantly granodiorite) set in a fine-grained matrix of biotite and quartz, occurring as irregular, discrete masses that are distributed through the west-central part of the stock. On the basis of the presence in its matrix of abundant secondary biotite, K feldspar, quartz and minor copper sulphides, it is interpreted to be associated with the late-magmatic stage of the system. The sediment-breccia contact often has associated subparallel arrays of thin fragments of intrusive rock.
Tonalite Porphyry, which corresponds to irregular bodies, in part dykes, which are found in the central section the deposit, where they intrude the granodiorite of the Elvira Pampa Intrusive Complex and Volcano-sedimentary sequence. It has a porphyritic texture with 50% of phenocrysts set in a phaneritic matrix. The tonalite porphyry has been dated at 41.9±0.2 and 41.7±0.4 Ma (U-Pb zircon; Martin, 1999)
Gaby Porphyry, that consists of a set of porphyritic diorite dykes, with phenocrysts of feldspar and mafic minerals set in a matrix of quartz and biotite, and is mostly located in the central part of the complex, where it intrudes to all other units. It has been dated at 41.8±0.3 Ma (U-Pb zircon; Martin, 1999)

The deposit lies in a graben, delimited by high-angle boundary faults, and is covered by up to 40 m of gravels (including a ~9.6 Ma tuff; Camus 2001) that completely obscure, all of the units described above. These alluvial gravels comprise subangular, polimictic clasts within a sandy clay matrix, and are cemented by calcrete in the upper 20 m of the profile. The basal section of these gravels is composed of fragments of red sedimentary rocks, which mostly correspond to detritus derived from the Purilactis formation, enclosed in a reddish, cemented matrix that includes sulphates and carbonates.

Three sets of fractures define the structural framework of Gaby South: i). north-south system dipping 45 to 70°W, that is dominant on the western margin of the deposit, along the contact between the Pampa Elvira intrusive complex and the sediments and the Quebrada Escondida Formation; ii). a NE to NNE oriented system, that control the location of the deposit and defines the distribution of supergene mineralisation, and iii). a NW-trending system of low-angle structures.

Alteration and mineralisation, including an initial late magmatic phase, have affected both the Elvira Pampa Intrusive Complex and the Volcano-sedimentary country rocks. Pervasive biotite (1 to 2 vol.%) replaces phenocrysts of hornblende, and occurs as biotite stringers, probably representing 'EB' (early-biotite) type veins, accompanied by type 'A' and 'B' quartz stringers. Potassic alteration has been dated at between 43.2±1.1 and 42.1±1.1 Ma by K-Ar determinations from biotite replacing mafic minerals in granodiorite of the Pampa Elvira intrusive complex (Aguilar, 2000). The presence of secondary biotite in the matrix of the intrusive breccias suggests these breccias are a late magmatic stage.

Epidote and trace amounts of chlorite, towards the margins of the deposit, affecting the intrusive rocks in particular, suggest the presence of a propylitic halo. The hypogene copper mineralisation associated with the late-magmatic phase is dominated by chalcopyrite, with minor bornite and pyrite, distributed in a zonal pattern comprising a bornite-chalcopyrite-rich core, surrounded by a shell in which chalcopyrite predominates, grading out into a halo where pyrite dominates with progressively decreasing chalcopyrite. This outer pyrite-dominant shell approximately coincides with the halo where epidote predominates (Aguilar, 2000; Camus, 2001).

The main hydrothermal stage is represented by a zone of phyllic alteration, predominantly in the southern part of the deposit. 'C' type stringers with chalcopyrite-pyrite and halos of chlorite-albite-sericite cut all of the intrusive porphyry rocks. '0'-type stringers with pyrite, minor chalcopyrite, and sericite halos also dominate in that part of the deposit. The phyllic alteration was dated at 40.67±0.19 Ma (
40Ar-39Ar) from the sericite selvedge to a veinlet with atacamite and goethite. Another sample of sericite from the halo of a veinlet of molybdenite+chalcopyrite, indicates an age of 40.18±0.22 Ma. Dilles et al. (2000), estimates that the sericite ages indicate cooling of the hydrothermal system associated with the emplacement and crystallisation of the Gaby porphyry, from 700 to 275°C would have happened over a period of ~1 m.y.

The upper part of the Gaby Sur deposit below the gravel cover, comprises an up to 180 m thick copper oxide zone, underlain by hypogene sulphides. The oxide zone contains a supergene mineralised suite, mainly oxides of copper and, to a lesser extent, by limonite, which is restricted to an interval immediately below the gravel overburden, coinciding with the area of underlying primary mineralisation. At the interface between the oxidation and hypogene zones, there is a weak and immature chalcocite blanket and a mixed zone of supergene sulphides and poorly developed copper oxides. The average thickness of oxidation is ~150 m, fluctuating between 60 and as locally much as 250 m. The oxide zone mineralised assemblage dominantly composed of chrysocolla with lesser neotocite, atacamite and malachite, present in fractures and replacing feldspars. Towards the margins of the oxide zone, there is an area of leaching with goethite and lesser jarosite (Aguilar, 2000; Camus, 2001). A moderate to weak zone of structurally controlled argillic alteration, represented by kaolinite, smectite and alunite, accompanies the zone of oxide mineralisation.

The pre-mining Gaby South development project considered the exploitation of an initial reserve of oxide minerals of 566 Mt @ 0.43% Cu at a cutoff of 0.20% Cu, of which 70%, corresponded to proven and probable reserves. Metallurgical recovery forecast for these reserves, exceeded 80%.

In 2102, the Gabriela Mistral mine had an estimated 620 Mt of oxide ore averaging 0.41% Cu. The mine commenced production in 2008 and is operated by Minera Gaby S.A., a 100% owned subsidiary of Codelco (the Corporación Nacional del Cobre de Chile).

This summary is paraphrased from a translation from the original Spanish of an excerpt from Camus (2003) "Geología de los Sistemas Porfíricos en los Andes de Chile" pp. 193-196.

The most recent source geological information used to prepare this summary was dated: 2003.    
This description is a summary from published sources, the chief of which are listed below.
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