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The Antucoya porphyry copper deposit is located in the Coastal Ranges of western Chile, ~135 km NNE of Antofagasta (#Location: 22° 38' 12"S, 69° 52' 52"W).
It is one of the largest of the deposits of the Early Cretaceous porphyry belt in the Coastal Cordillera of northern Chile. The geology of the Antucoya and neighbouring Buey Muerto porphyry systems is dominated by a sequence of volcanic rocks of andesitic composition assigned to the Middle-Late Jurassic La Negra Formation, intruded by a series of plutons of granodioritic, tonalitic and dioritic composition porphyritic stocks and dykes, part of the broadly coeval 'Coastal Batholith'. Copper-bearing ore occurs as stockworks and disseminations in altered rocks, and as breccia matrix, and is hosted by granodioritic and tonalitic porphyries, and by magmatic to hydrothermal breccias, all within an area of 1600 x 1000 m (Maksaev et al., 2006; Perello et al., 2003).
The Antucoya porphyry copper system is indicated to have been formed following the change in the stress field along the magmatic arc from extensional in the Late Jurassic to transpressive in the Early Cretaceous. The deposit is located immediately west of the major regional, intraarc, north-south trending, sinistral strike-slip Atacama Fault Zone, which was active during the Early Cretaceous (e.g., Dallmeyer et al., 1996; Scheuber and Andriessen 1990). The structural fabric in the deposit area is dominated by several, NNE-trending splays of the Atacama Fault System, which exert a strong control on the geomorphologic features of the area. Field evidence indicates that these splays and several intersecting, NW-trending faults, form part of a larger, sinistral, Riedel-type shear system interpreted to have been active during the time of porphyry copper emplacement and are consistent with a NW-SE compression at the time
of faulting and dyke emplacement (Maksaev et al., 2006; Perello et al., 2003).
It occurs within a multiphase 142.7 ±1.6 to 140.6 ±1.5 Ma granodioritic and tonalitic porphyry intrusive complex cut by late, unmineralised, but potassic altered, NW-SE trending, 5 to 20 m thick dacite dykes, emplaced within Jurassic andesitic volcanic rocks of the La Negra Formation. Dyke emplacement and brecciation at Antucoya are controlled by NW-striking faults which splay from the N-S Atacama Fault Zone and are consistent with a NW-SE compression at the time of faulting and dyke emplacement (Maksaev et al., 2006).
The exposed La Negra Formation strikes at 330 to 335° and dips at from 20 to 30°NE. It is composed of massive aphanitic to porphyritic andesites, locally interbedded with fine-grained, tuffaceous calcareous sandstone and siltstone horizons. All of these rocks are masked by a layer of post-mineralisation gravel, partly cemented by nitrates, regolith, and gypcrete (Maksaev et al., 2006).
Two altered and mineralised porphyry intrusions occur at Antucoya (Maksaev et al., 2006), namely:
i). the dominant granodioritic porphyry (the "Antucoya porphyry") characterised by 25% plagioclase phenocrysts (altered to kaolinite, illite, calcite, and traces of anhydrite) up to 3 mm long, and 5% perthitic K-feldspar phenocrysts up to 1 mm long, both set in a groundmass (70%) composed of a microcrystalline aggregate of quartz and argillised feldspar, with chlorite and disseminated opaque minerals and minor relicts of chloritised fine-grained biotite;
ii). lesser tonalite porphyry (the "fine-grained porphyry") which has minor (~10%) up to 2 mm long phantom-relict plagioclase phenocrysts that have undergone strong argillic alteration and partial calcite replacement. The groundmass (90%) is a microcrystalline aggregate of quartz and argillised plagioclase with abundant chlorite and disseminated opaque minerals completely replacing mafic minerals, and relicts of partly chloritised microcrystalline aggregates of biotite and minor sericite.
The granodiorite and tonalite porphyries have mutual intrusion relationships, suggesting they are either composite intrusions or that they were intruded almost simultaneously (Maksaev et al., 2006).
The contacts between these two porphyries are occupied by magmatic-hydrothermal breccias in the centre of the deposit. These breccias, which are also spatially related to NW-striking faults, are polymictic, matrix-supported, pipe-like, irregular bodies, which range from near vertical to SW-dipping and have vertical extents of 250 to 350 m. The uppermost sections of the breccia bodies have a matrix of hydrothermal minerals, while at lower levels an altered granodioritic igneous matrix dominates. Breccia fragments are angular to subrounded and are composed of altered and mineralised granodioritic and tonalitic porphyries (Maksaev et al., 2006).
The mineralised porphyries are cut by NW-striking, dark-grey, post-mineralisation porphyritic dacititic dykes. These dykes are composed of 25% plagioclase phenocrysts up to 4 mm long, that are fresh or partly sericitised, as well as partly chloritised microcrystalline biotite, opaque minerals forming pseudomorphs after amphibole phenocrysts (5%), and scarce quartz eyes up to 2 mm in diameter. The groundmass (70%) consists of microcrystalline aggregates of quartz and argillised plagioclase microliths, with fine-grained disseminations of biotite that is largely replaced by chlorite. Although these dykes were intruded after the main hypogene mineralisation, they are affected by potassic alteration (biotite), with partial chloritisation and an argillic overprint (Maksaev et al., 2006).
Four hypogene hydrothermal alteration assemblages have been recognised at Antucoya, the first three of which affect the porphyries and breccias. These are: i). potassic, mainly within the breccia bodies and the tonalitic porphyry, characterised by a biotite, K-feldspar and quartz assemblage which was formed at temperatures of ~450 to 500°C, and fluid salinities from 7 to 68 wt.% NaClequivalent
related to liquid-vapor phase separation; ii). chlorite-sericite; iii). quartz-sericite; and iv). propylitic, restricted to the volcanic country rocks of the La Negra Formation, characterised by chlorite, quartz, albite, epidote, calcite and pyrite (Maksaev et al., 2006).
A stockwork of quartz veins with chalcopyrite, pyrite and molybdenite with quartz-sericitic haloes occurs in the uppermost part of the deposit in both porphyries, while an assemblage of chlorite, sericite, smectite, quartz, pyrite and chalcopyrite occurs within the Antucoya granodiorite porphyry and magmatic-hydrothermal breccias (Maksaev et al., 2006).
Most of the hypogene orebody has been overprinted by a pervasive supergene argillic alteration assemblage of illite, dickite and kaolinite with oxidation of sulphide to produce atacamite, brochantite, chrysocolla, copper wad, jarosite and limonite. This supergene zone extends downward to depths of 300 to 350 m below the surface and is believed to have formed during the development of the Oligocene to Miocene coastal Tarapacá pediplain (Mortimer et al., 1974).
This oxide zone is underlain by a mixed zone, which carries variable pyrite and chalcopyrite, in addition to the oxide minerals listed above. Where structural conditions, quartz-sericite alteration, and pyrite contents are conducive to supergene enrichment, an irregular chalcocite-covellite blanket is developed below the oxide zone. Regional evidence suggests the supergene oxidation and chalcocite development took place during the middle Miocene (Perello et al., 2003).
Maksaev et al. (2006) quoted a resources of: 300 Mt at 0.45% total Cu.
Construction of the mine and associated facilities was completed in 2015, and full production was commenced in the first half of 2016.
The mine was operated in 2016 by Minera Antucoya S.A., which is owned by Antofagasta Minerals PLC (70%) and Marubeni Corp. (30%).
Published resource estimates at 31 December, 2010 were as follows (Antofagasta PLC website, 2012):
measured + indicated resource - 1.1534 Gt @ 0.28% Cu; plus
inferred resource - 0.3557 Gt @ 0.24% Cu.
Published oxide ore reserve and mineral resource estimates at December 31, 2015 were as follows (Antofagasta PLC Annual Report for 2015):
proved + probable reserves - 686.6 Mt @ 0.34% Cu;
measured + indicated resource - 900.3 Mt @ 0.32% Cu; plus
inferred resource - 354.7 Mt @ 0.27% Cu;
TOTAL resource - 1255.1 Mt @ 0.31% Cu (inclusive of reserves).
Note: The ore reserve and mineral resource cut-offs in 2015 were 0.16% and 0.15% Cu respectively.
The most recent source geological information used to prepare this summary was dated: 2006.
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.
Maksaev V, Munizaga F, Fanning M, Palacios C and Tapia J, 2006 - SHRIMP U–Pb dating of the Antucoya porphyry copper deposit: new evidence for an Early Cretaceous porphyry-related metallogenic epoch in the Coastal Cordillera of northern Chile : in Mineralium Deposita v41 pp 637-644|
Perello, J., Martini, R., Arcos, R. and Muhr, R., 2003 - Buey Muerto: Porphyry copper mineralization in the early Cretaceous arc of northern Chile: in 10th Congreso Geologico Chileno, 2003 Universidad de Concepcion, Departamento de Ciencias de la Terra 1p.|
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