The main Chuquicamata and associated Radomiro Tomic and Mina Ministro Hales (previously MM or Mina Mansa) porphyry copper-molybdenum deposits and the Mina Sur (Exotica) exotic copper orebody are located in northern Chile, some 240 km north-east of Antofagasta and 1690 km north of Santiago.
These mines are developed in an elongate zone of alteration and mineralisation that extends for 30 km along the north-south trending West Fissure zone and are members of a string of deposits associated with Upper Eocene to Lower Oligocene porphyry intrusion in northern Chile. Together they contain (production + reserve/resource) a total of 86 Mt of copper metal. Radomiro Tomic is located ~5 km NNE of the northen margin of the main Chuquicamata pit margin, while Mina Sur is immediately to the south of the latters southern margin. Mina Ministro Hales is ~7 km to the south of Chuquicamata.
See the separate Radomiro Tomic record for details of that deposit. Mina Ministro Hales is decribed separately below following the main Chuquicamata summary.
The oldest rocks in this part of Chile are Palaeozoic metasediments and meta-plutonic lithologies which are exposed in the Mina Sur pit and just to the east of the main Chuquicamata mine area where they are represented by gneissic granite, meta-diorite, quartz-diorite and minor tonalite. These are unconformably overlain by Mesozoic volcanics and sediments, commencing with late Triassic continental conglomerate, sandstone and andesitic to dacitic breccia and tuffs. These pass up into a transgressive Jurassic marine sequence of shale, sandstone and limestone and a lower Cretaceous sandstone.
The geology and structure of the Chuquicamata district is dominated by the major West Fissure zone (or West Fault) which is part of the regional Domeyko fault system that passes through the Escondida district 200 km to the south, and the Collahuasi district, 200 km to the north. The main movement on this structural zone, which represents a focused corridor of Cenozoic, arc parallel transcurrent and reverse faults, was post mineralisation at Chuquicamata. It changed its sense of movement at least twice in the period it was active from prior to the emplacement of the Chuqui porphyry until after 16 Ma and was a major control on the localisation of the host intrusives, the formation of mineralised structures and subsequent displacement of the ore. It separates and juxtaposes the mineralised intrusives to the east from the basically barren rocks to the west and has a net sinistral transcurrent movement of about 35 km.
The post-Mesozoic rocks to the west of the West Fault are of Eocene to Oligocene age, principally belonging to the older Los Picos diorite and Fortuna Intrusive Complex, being represented by the volumetrically dominant Fiesta granodiorite, which is cut by smaller bodies of San Lorenzo granodioritic porphyry and minor Tetera aplite porphyry. Traces of mineralisation are found in these porphyries, mainly at contact zones where disseminations and veinlets contain weak chalcopyrite, bornite and magnetite.
To the east of the West Fault, pre Chuqui intrusives include the Triassic East Granodiorite and the Jurassic to early Cretaceous (146 to 122 Ma) Elena granodiorite. Both intrude meta-sediments derived from Mesozoic shales, sandstones and minor limestone protoliths.
Virtually the entire ore deposit at Chuquicamata is hosted by and related to the 36 to 33 Ma Chuqui Porphyry Complex which comprises a number of phases, many of which do not have well defined contacts. Where unaltered, all are composed of plagioclase, quartz, K-feldspar, biotite and hornblende with accessory titanite and magnetite. All are modified by the same stages of alteration and mineralisation. The individual intrusives are:
i). The East Porphyry, which is the oldest and largest phase, has a hypidiomorphic-granular texture with plagioclase, biotite, hornblende and locally K-feldspar in a matrix of quartz, K-feldspar and biotite.
ii). The West Porphyry has similar phenocrysts, though a little finer, with quartz eyes in an aplitic groundmass.
iii). The Banco Porphyry is finer grained and more porphyritic than East Porphyry which it intrudes with the only sharp contact seen between members of this suite of intrusives. It has an abundance of small plagioclase crystals in an aplitic groundmass, but with a variable texture.
iv). The Fine Texture Porphyry is distinctly finer grained than East Porphyry, although it also has a hypidiomorphic-granular texture.
Emplacement of the Chuqui Porphyry Complex, the main causative intrusive, was accompanied and possibly facilitated by ductile deformation associated with the West Fault system. Similarly the development, density and orientation of faults and fractures, and the veins and veinlets that fill them were influenced by the extent and sense of movement on the West Fissure zone. Movement on fractures and faults was multistage with most having been opened and mineralised more than once as well as deforming existing veins after the cessation of mineralisation.
The great majority of the mineralisation at Chuquicamata occurs in veins and veinlets, the earliest of which are quartz and K feldspar veinlets with little or no sulphides. These are cut by more continuous quartz veins ranging up to 5 cm in width with molybdenite and traces of chalcopyrite, which are cut in turn by large banded, molybdenite rich quartz veins which are typically 1 m or more (up to 5 m) thick. Both of these quartz vein types are commonly structurally segmented. The next generation are the pyritic main stage veins which carry pyrite, chalcopyrite, bornite and digenite and exhibited progressively reduced amounts of quartz and increasingly better developed sericite halos with time. This transition accompanied the change from early pyrite veins with little copper, through the appearance of chalcopyrite and bornite to late stage enargite ± pyrite with some sphalerite, and finally to a very late stage coarse grained covellite and digenite without pyrite which originated deep in the quartz-sericite zone and flared upwards and outwards.
The earliest alteration was the potassic phase, which influenced all of the porphyries. It resulted in K-feldspar development and was accompanied by the early quartz-K feldspar veinlets described above, with associated biotite veining and replacement of hornblende, and by K feldspar alteration of plagioclase. This phase did not destroy existing textures. A second phase of K feldspar development at the expense of biotite and following albitisation of plagioclase was texture destructive and accompanied a period of cataclastic deformation. A 200 x 1500 m band of quartz-K feldspar alteration follows a Banco Porphyry dyke swarm and is the locus of the main bornite-digenite core of the sulphide zoning pattern. Eastward from this band the mineralogy changes first to bornite-chalcopyrite to chalcopyrite-pyrite to barren country rock in the propylitic altered margin of the system, but without a pyritic fringe. Sulphides are only abundant in the potassic zone where there is intense crackle brecciation.
Westward from the main bornite-digenite core this zoning is interrupted and over printed by the pyritic main stage veins which are accompanied by pervasive quartz-sericite alteration. The main stage veining occupies a structural zone adjacent to the main West Fault and is distinctly younger than the earlier potassic phase and the quartz-molybdenite stages, although they occur in the same structural zones. The pyritic main stage veining was developed during dextral shear on the West Fault system, although some of the latest enargite veins were emplaced after the fault reverted to a sinistral sense of movement.
The final phase of mineralisation is represented by a partly preserved leached cap and extensive oxide ore that replaces an upper chalcocite blanket which overlies a high grade supergene blanket that persists to nearly 800 m below surface in the zone of fault brecciation and pervasive pyritic main stage quartz-sericite alteration. Some leached copper was moved laterally to form the Mina Sur (Exotica) secondary deposit comprising chrysocolla, atacamite, Cu-wad and other oxides in younger channel gravels and the underlying decomposed bedrock representing transported Cu in the base and bedrock of a palaeo-alluvial channel. This deposit is around 6.5 km long, has a maximum width of 1.2 km and is up to 110 m thick.
The Mina Ministro Hales deposit is a large, tabular shaped porphyry copper(-molybdenum) deposit located 7 km to the south of, and on the west side of the regional West Fault that truncates and defines the western limit of the Chuquicamata deposit.
The deposit is masked by a blanket of gravels and has a 7 km north-south extent, is 200 to 320 m wide east-west, and extends for >1200 m vertically. It forms a tabular body, adjoining, and structurally sliced by the West Fault.
The host rock at Mina Ministro Hales is predominantly the 237 to 222 Ma (U/Pb) Triassic MM Granodiorite, intruded by the 38.9 Ma (U/Pb age in zircon) Eocene MM Porphyry and 35.5 Ma (U/Pb age in zircon) MM Quartz Porphyry (Boric et al., 2009). The MM Granodiorite is petrographically similar to the Triassic Elena Granodiorite at Chuquicamata. The dyke-like MM Porphyry occurs deep in the deposit where it is adjacent to the West Fault, while the MM Quartz Porphyry is only of limited extent and is characterised by abundant quartz eyes (Boric et al., 2009). Other rocks units described from drill-logs are a Hornblende Porphyry, a Feldspar Porphyry and several generations of Dacitic dykes (Boric et al. 2009)
Eocene porphyry type Cu-(Mo) mineralisation is found at depth, overprinted in its upper levels by younger, high-sulphidation Cu-(Ag-As) hydrothermal breccias (Oligocene). The intruded country rocks are andesitic flows and breccias of Palaeozoic to Triassic age correlated with the Collahuasi Group.
The oldest and deepest mineralisation comprises veinlets of molybdenite and chalcopyrite, bornite-chalcopyrite(-digenite) and potassic alteration with K feldspar, green-grey sericite and anhydrite. Younger, high-sulphidation mineralisation is characterised by hydrothermal breccia bodies and stockworks overprinting the mid- and upper levels of Mina Ministro Hales. These breccias contain chalcocite, enargite, pyrite, bornite, covellite, chalcopyrite, tennantite and sphalerite, accompanied by advanced argillic alteration (quartz, alunite, pyrophyllite, sericite and dickite). High-grade zones (>2% Cu) carry high As, Ag and Zn values (Sillitoe et al., 1996; Boric et al., 2009). Re-Os dates on molybdenite (oldest age 37.3 Ma) suggest that some of the early ores may be associated with the Eocene porphyries, although Wilson et al. (2011) note that no intrusion of Oligocene age had been identified as the parents of the high sulphidation bodies with hypogene alunite that yields 40Ar/39Ar ages of 31.4 to 32.2 Ma (Boric et al., 2009).
The total production from the main Chuquicamata pit to 1997 had totalled 2.035 Gt averaging 1.54% Cu, while Mina Sur (Exotica) had yielded 120 Mt @ 1.25% Cu.
The resource at the main Chuquicamata mine in 2000 was 6.45 Gt @ 0.55% Cu, while Mina Sur had 190 Mt @ 1.12% Cu.
The resources at Mina Ministro Hales published by Codelco (2012) are >1.3 Gt @ 0.96% Cu, with a 289 Mt open pit reserve.
The combined production + reserve/resource for the district, including the main Chuquicamata mine, Radomiro Tomic, Mina Ministro Hales and Mina Sur is approximately 11.4 Gt @ 0.76% Cu.
Chuquicamata, Mina Sur and Radomiro Tomic are operated by the Codelco Division Norte. The main Chuquicamata mine has been in operation on a large scale since 1910 with only oxide ore having been exploited up to 1952. Currently a mix of oxide, supergene blanket and hypogene mineralisation is treated, extracted from an open pit that covers an area of some 3.8 x 2 km, and is near 500 m deep.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2013)
