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Chuquicamata, Radomiro Tomic, Mina Ministro Hales - MM, Mina Sur (Exotica) |
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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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 (#Location: 22° 17' 10"S, 68° 54' 02"W).
District Setting
The mines of the Chuquicamata district are developed in an elongate ~30 x 10 km zone of alteration and mineralisation that extends along the north-south trending West Fissure fault zone, part of the regional Domeyko fault system, and are members of a string of deposits associated with Upper Eocene to Lower Oligocene porphyry intrusion in northern Chile. Together, they are estimated to have contained (production + reserve/resource) a total of 125 Mt of copper metal (Faunes et al., 2005) including in situ geological resources of fine Cu at the end of 2015 of 63.7 Mt at Chuquicamata, 30.5 Mt at Radomiro Tomic and 14.6 Mt at Ministro Hales totalling 108.8 Mt (Codelco Memoria Anual, 2015).
To 2004, ~38.5 Mt of copper had been mined from these deposits (Faunes et al., 2005). Radomiro Tomic is located ~2 km NNE of the northen margin of the main Chuquicamata pit margin, while Mina Sur is immediately to the south of the latter's southern margin. Mina Ministro Hales is ~7 km to the south of Chuquicamata.
Chuquicamata occurs as an ~3 km long, north-south trending, south-tapering, wedge-shaped body with an average width of ~1 km and depth extent of >1 km. It is truncated on its western margin by the West Fissure fault, with the remaining deposit narrowing southwards. Its northern and southern limits are gradual, passing outwards into weakly mineralised zones of propylitic alteration. To the east it passes into a zone of ductile deformation where it is juxtaposed with an essentially, marginally older intrusion (Camus, 2003).
The Radomiro Tomic deposit is covered by 30 to 150 m of alluvial gravel, and extends for 5 km, north-south and about 1 km, east-west. It comprises an upper 150 to 200 m thick oxide zone, overlying 20 to 150 m of irregular and immature supergene chalcocite enrichment that is, strongly influenced by structure (Cuadra and Rojas, 2001). Hypogene sulphides extend for >400 m below the supergene sulphides (Camus, 2003). See the separate Radomiro Tomic record for details of that deposit.
Mina Ministro Hales occurs as a tabular, subvertical, north-south elongated body, that extends for 7 km, with a variable width of between 200 and 320 m, and vertical extent of >1200 m (Boric et al., 2009). It is decribed separately below following the main Chuquicamata summary.
The Mina Sur (Exotica) deposit is an exotic mineralised sheet within a palaeodrainage channel, the northern end of which is the head of the channel that is located immediately south of of the Chuquicamata mine, at an altitude of ~2650 m. It extends to the south for ~6.5 km, with a slope between 3 to 7°. At its southern end, it is concealed beneath >200 m of alluvial gravel (Munchmeyer, 1996). It has a lenticular shape, and at its widest reaches 1.2 km, with a maximum thickness of 110 m. See the separate Exotica, Mina Sur record for more detail.
The oldest rocks in this part of Chile are a Palaeozoic to Lower Triassic Igneous-Metamorphic Complex that are exposed both within the Mina Sur pit,
and ~1 km east of the Chuquicamata Mine area, the Radomiro Tomic mine and northern Mina Ministro Hales orebody. The oldest of the rocks in the complex is the Mesa granite, a pink, microcline granite with a partially gneissic texture that has also been recognised in the Sierra Limón Verde, south of Calama, where it has been dated at 305±4 Ma in the late Carboniferous (K-Ar; Marinovic and Lahsen, 1984). It is intruded by dioritic rocks that have undergone widespread pervasive chlorite-epidote-calcite alteration that it was interpreted to be retrograde regional metamorphism (Ambrus, 1979).
These rocks are, in turn, intruded along the western edge of the meta-plutonic complex by the East Granodiorite which extends for at least 9 km NNE from the SE edge of the Chuquicamata pit following the crest of the Chuquicamata Hills. It has a medium to coarse equigranular texture, containing plagioclase, micro-perthitic K feldspar, quartz, biotite and hornblende, with dating suggesting a Middle Triassic age (U-Pb zircon; Tomlinson, unpub.,1999). It is locally altered to albite-chlorite-magnetite and sericite-clay, ascribed to the influence of the Chuqui Porphyry Complex (Ambrus, 1979).
These intrusions are unconformably overlain by Mesozoic volcanic and sedimentary rocks, commencing with middle to late Triassic continental facies conglomerate, sandstone and 231.6±0.5 Ma andesitic to dacitic breccia and tuffs of the Agua Dulce formation (Lira, 1989; Mpodozis et al., 1993; Camus, 2003). These are gradationally overlain by a transgressive Jurassic marine sequence of limestone, shale, calcareous sandstones and calcarenites of the Quehuita Formation, discordantly overlain by volcano-sedimentary rocks of the of the Cretaceous Cerro Empexa (sandstones, andesites and dacites) and Eocene Icanche (andesitic and dacitic volcanic breccia) formations.
The geology and structure of the Chuquicamata district is dominated by the major north-south trending West Fissure fault 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 divides the district into a Western and Eastern domain, and 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. It 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.
For a map of the regional setting within Central Chile, see the Central Andes and Bolivian Orocline record.
Chuquicamata
High grade copper oxides that were exposed at surface at Chuquicamata were first worked on a small scale by both pre-hispanic miners, and then by Spanish explorers (Miller and Singewald, 1919). Between 1879 and 1912, narrow, rich, veins of brochantite overlying enargite and chalcocite with pyrite and quartz were exploited in the underground workings of Chilean and English companies that were in what is now the upper to middle benches in the northeastern portion of the open pit. In 1915, open-pit mining on disseminated oxide ore averaging 1.89% Cu was initiated by the Chile Exploration Co., the operating arm of the Guggenheim's Chile Copper Co. Production commenced on an estimated resource of 87 Mt @ 2.41% Cu, which by the end of 1916 had been increased to proven + probable ore reserves of ~700 Mt @ 2.12% Cu (Chile Copper Co., 1917). The Anaconda Copper Mining Co. purchased the deposit from the Guggenheims in 1923 and subsequently managed 48 yr of continually increasing operations. Production was expanded and migrated from dominantly oxide to 1952, to mainly sulphide ores in the mid 1950s as the pit deepened. The Exótica (Mina Sur) deposit was accidentally discovered beneath the oxide tailings dump in 1957, and its subsequent systematic exploration and development commenced in the 1960s. Chuquicamata became the world’s largest copper producer in the 1960s. In 1971, the mine was nationalised and management and operation were taken over by the Corporación Nacional del Cobre Chile, CODELCO, that still in 2017 efficiently operate the mine as one of the world's largest copper producers (summarised from Ossandón et al., 2001).
Western Domain
The oldest rocks to the west of the West Fissure fault include volcano-sedimentary marine lithologies of the Jurassic Quehuita Formation, discordantly overlain by further volcano-sedimentary sequences of the Cretaceous Cerro Empexa and Eocene Icanche formations, as described above. The oldest intrusive belongs to the Late Cretaceous to Paleocene 69 to 63 Ma (mostly 64 to 63 Ma; Tomlinson et al., 2001) Montecristo Intrusive Complex, which is exposed over an area of 120 km2, and is composed of quartz-diorite and quart-monzonite (Camus, 2003).
These are, in turn, intruded by rocks of Eocene to Oligocene age, principally belonging to the Los Picos-Fortuna Intrusive Complex, a polyphase pluton formed, in decreasing order of age, by the Los Picos diorite (K/Ar biotite; 45.1±1.8 Ma; Ambrus, 1979; 43.4±1.2 Ma and 45.1±1.2 Ma; Tomlinson et al., 2001), the Antenna granodiorite (39.6±1.2 and 39.0±1.2 Ma K-Ar and U-Pb in zircon; Ambrus, 1979; Tomlinson et al., 2001), the Fiesta granodiorite (37.9±1.2 and 37.3±1.2 Ma K-Ar and U-Pb in zircon; Tomlinson et al., 2001), the Tetera aplite porphyry and the San Lorenzo granodiorite porphyry (Marinovic and Lahsen, 1984). The volumetrically dominant Fiesta granodiorite is cut by smaller, irregular bodies of San Lorenzo granodioritic porphyry and minor Tetera aplite porphyry that are generally <100 m in length (Camus, 2003).
The Los Picos diorite occurs as a 30 km long, north-south elongated strip, intruding the Quehuita, Cerro Empexa and Icanche formations to the west and north, while being intruded along its eastern margin by the Antena granodiorite. It is characterised by pyroxene, biotite, plagioclase, quartz and interstitial feldspar (Tomlinson et al., 2001; Camus, 2003). The Antenna granodiorite is the next most areally extensive intrusive. It is relatively homogeneous, generally fine to medium grained with a weak igneous foliation, composed of plagioclase, hornblende, biotite and magnetite, with quartz and K feldspar. Camus 2003).
The Fiesta granodiorite forms the central part of the complex, where it intrudes the Antenna granodiorite, the Los Picos diorite and Mesozoic volcanic rocks. It is cut by the Tetera aplite porphyry and the San Lorenzo granodiorite porphyry, and is juxtaposed across the West Fissure fault, with the Chuquicamata Intrusive Complex. It is composed of up to 4 cm long K feldspar megacrysts and up to 1 cm hornblende crystals, and contains tabular plagioclase, quartz, K feldspar, biotite, hornblende, sphene and magnetite as accessories. It is fine-grained with only sparse megacrysts in the apex of the intrusive.
The Tetera aplite porphyry occurs as <10 cm to 30 m thick dykes that cut the Fiesta granodiorite with a gradational contact. All are mineralised and outcrop in the northern part of the Fortuna-Los Picos Complex, particularly in the Fiesta granodiorite.
The San Lorenzo granodiorite porphyry cuts the Fiesta granodiorite, although locally the contact is gradational, and has an almost identical mineralogy (Tomlinson et al., 2001), but a higher content of phenocrysts of plagioclase, biotite, hornblende, quartz, K feldspar, magnetite and sphene in an aplitic matrix. Alteration is weaker than that of the Tetera aplite porphyry, suggesting the former is younger.
With the exception of the Tetera aplite porphyry, only traces of mineralisation are found in these porphyries, mainly at contact zones where disseminations and veinlets contain weak chalcopyrite, bornite and magnetite. They are interpreted to have been structurally juxtaposed against the strongly mineralised Chuqui Porphyry Complex to the east by large-scale, postmineral displacement along the West Fissure fault (e.g., Dilles et al., 1997; Tomlinson and Blanco, 1997).
Eastern Domain
The mineralised Chuqui Porphyry Complex is restricted to the eastern side of the of the West Fissure fault, bounded to the east by the Eocene Elena granodiorite, the Triassic East Granodiorite and the Triassic meta-volcanic and meta-sedimentary rocks of the Agua Dulce Formation.
Virtually the entire ore deposit at Chuquicamata is hosted by and related to the NNE elongated, 14 x 1.5 km, 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 (Ossandon, et al., 2001; Faunes, et al., 2005):
i). The East Porphyry, which is the oldest and dominant phase, has been dated at 34.6 Ma (U-Pb in zircon; Ballard et al., 2001), and varies from granodiorite to biotite quartz-monzonite in composition, with phaneritic to incipient porphyritic textures within a medium grained fabric. It contains sparse to increasingly abundant centimetric megacrysts of K feldspar, accompanied by subhedral to euhedral plagioclase (average >2 mm), biotite, hornblende (altered to biotite) phenocrysts, producing a poikilitic texture with interstitial quartz, K feldspar, subhedral biotite and rare hornblende. Euhedral quartz is absent, although elongated polycrystalline and strained quartz blebs are prevalent and probably represent deformed phenocrysts. Magnetite, titanite and zircon are accessories.
ii). The West Porphyry, which occur as a smaller, ~400 m wide body within the East Porphyry on the northern margin of the Chuquicamata pit. It has similar phenocrysts to the East Porphyry, though a little finer (<2 mm), with common quartz eyes in an aplitic groundmass of much finer equigranular quartz, K feldspar and biotite, and has been dated at 33.5 Ma (U-Pb in zircon; Ballard et al., 2001).
iii). The Banco Porphyry occurs as a number of small dyke-like intrusions, generally less than 200 m across, located towards the eastern margin of the East Porphyry. It is finer grained and more porphyritic than the East Porphyry, which it intrudes with the only sharp contact seen between members of this suite of intrusives. It has been dated at 33.3 Ma (U-Pb in zircon; Ballard et al., 2001), and is characterised by 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 the East Porphyry, although it also has a hypidiomorphic-granular texture. It occurs in the north-central section of the Chuquicamata pit, converging southwards with the West Fissure fault, before being obliterated by alteration in the core of the deposit.
On the SE margin of the deposit, the Chuqui Porphyry Complex has a diffuse, gradational contact with the 37.7 Ma Elena Granodiorite (U-Pb in zircon; Ballard, 2001), which is essentially barren, although its mafic minerals are usually chloritised and it locally contains minor disseminated pyrite. The principal petrographic difference between the two is the absence of K feldspar macrocrysts in the equigranular Elena Granodiorite, which is interpreted to be a precursor of the former, based on its age, petrographic similarity and their gradual and diffuse mutual contacts in some locations. However, detailed work has shown that the contact between the two intrusions is a poly-episodic brittle-ductile deformation zone of variable thickness, continuously developed along the entire eastern flank of the deposit.
Structure
The ductile structure marking the eastern flank of the deposit, trends NNE, has a variable thickness and overall dip of 80°W. It is known as the East Deformation Zone, and is a complex of mylonites, cataclasites and cohesive fault breccias, which encroaches into both intrusions. The breccias incorporate clasts of the various lithologies the structure cuts, as well as quartz, (possibly from the early stages of mineralisation), within a recrystallised matrix of chlorite, feldspar and magnetite derived from an igneous protolith. The ductile, penetrative fabric of the mylonites is superimposed on the breccia texture. Bounding lithologies are cut by later veinlets and small breccias of specularite, with traces of pyrite and chalcopyrite. To the north, this deformation zone becomes the Mesabi Fault zone which curves to the NE, away from the Chuqui Porphyry Complex, to parallel the Estanques Blancos fault (see below) and separates the East granodiorite from the Elena granodiorite and the Agua Dulce Formation. This structure, which diverges northward from the West Fissure fault, is taken to be part of the regional Domeyko fault system. Ductile kinematic indicators suggest dextral displacement along the Mesabi Fault-East Deformation Zone (Faunes et al., 2005). Faunes et al. (2005) offer evidence that the Messabi Fault-East Deformation Zone, probably associated with a transpressive dextral tectonic environment, played a key role in localising the East Porphyry intrusive phase of the Chuqui Porphyry Complex.
In the southern section of the Chuquicamata pit, the subvertical, north-south to NNE Americana Fault is expressed as a breccia zone that fractures and deforms quartz-molybdenite veins. It sub-parallels, but diverges northwards from the West Fissure fault that is ~200 m to the west. Like the East Deformation Zone, it is truncated or offset to the south by the ENE trending Portezuelo Fault (see below). It not only controls the location of late stage pulses of quartz-sericite alteration but also appears to exert control over the emplacement of the earliest quartz-molybdenite veining (Faunes et al., 2005).
The NNE trending structures detailed above are linked by a series of NE to ENE trending faults, the most significant of which are the Portezuelo Fault in the south that defines the southern margin of the Chuqui Porphyry Complex and the Balmaceda and Estanques Blancos faults in the central and northern sections of the Chuquicamata open pit. The latter influence the localisation of both early and late episodes of alteration and mineralisation, as well as supergene processes, reflected in the strike of the alteration zones and the anisotropy of mineralised veins and veinlets. All of these faults indicate dextral displacement of 200 to 300 m, and a south block down normal displacement (Faunes et al., 2005).
The dominant West Fissure fault, in general, strikes north-south to NNE with a dip of 75 to 80°W that progressively shallows with depth. This structure is part of the regional Domeyko fault system, which just to the north of the Chuquicamata pit is 5 km wide, from the Tetera fault on the west to the Mesabi fault in the east. It has been active prior to intrusion of the Chuqui Porphyry Complex until after 16 Ma, changing sense of movement at least twice. Observations within the pit however, indicate that the most recent major displacement on the actual West Fissure fault is post-mineral, with a reverse senses of movement and sinistral displacement of ~35 km, as indicated by regional mapping. It defines the abrupt western limit of the ore deposit, juxtaposing the Chuqui Porphyry and early mineralisation with the largely barren Los Picos-Fortuna Intrusive Complex across the fault to the west. The distribution and symmetry of hypogene alteration and mineralisation indicate that it does not represent the axis of the deposit, but that it is located to the west of the centre of hypogene hydrothermal activity, with <30% of the original mineralised system having been removed. The structure is also oblique to the trend of the hypogene mineralisation. However, continued movement on the West fault produced a wide zone of brecciation from 30 to 150 m wide which appears to control very late stage sericite/pyrite mineralising events which are centred on the structure. In addition, a high-grade supergene chalcocite body extends for up to 800 m in depth in this zone of fault brecciation and pervasive main-stage sericitic alteration immediately to the east of the West Fissure fault (Faunes et al., 2005; Ossandon et al., 2001).
A suite of important NNW to NW trending, subvertical faults cross the mineralised system, and are most abundant in the central and southern sections of the Chuquicamata pit. This set of faults were likely active during the emplacement of the deposit, and are a conjugate suite to the Estanques Blancos-Portezuelo Fault System, and were reactivated during late post mineral and recent periods. They are observed to inflict metre scale sinistral displacement of mineralised veins, sinistral north-south faults and geological contacts, as well as cutting and displacing both the West Fissure fault and the supergene enrichment blanket, producing a modified drag fold in the latter with a 'few' metres of sinistral displacement (Faunes et al., 2005).
Mineralisation and Alteration
Both the Banco and West porphyries are crosscut and overprinted to the same intensity, by the same main alteration and mineralisation events as the East Porphyry, although both are similar in age to the early pulses of alteration. The Chuqui Porphyry Complex is taken to be a pre- to early syn-mineral intrusive (Faunes, et al., 2005).
The Chuquicamata deposit has been formed by multiple events of hypogene mineralisation and alteration, which may be temporally divided into: i). early, low sulphidation, characterised by low pyrite within the sulphide assemblage, and ii). late phyllic event, with much higher sulphidation mineralisation, and abundant pyrite (Faunes, et al., 2005).
• Early Low Sulphidation (Low-Pyrite) Associations - The first phase of the Chuqui Porphyry Complex, the 34.6 Ma East Porphyry, appears to have been barren or only accompanied by weak, late-magmatic alteration and mineralisation. The intrusion of this phase was probably associated with a transpressive dextral field related to the East Deformation Zone-Messabi Fault.
Background potassic alteration - The intrusion of the smaller 33.4 Ma West and Banco porphyries was a more pronounced mineralising event, producing an intense stockwork of barren, pre-ore quartz and K feldspar 'A' veins and veinlets with little or no sulphide, mostly in the eastern and northerns part of the deposit, accompanied by a huge background potassic alteration halo of selective biotitisation of mafic minerals, and secondary potassic feldspar partially replacing plagioclase. Relicts of this altreration stage are also scattered through the remainder of the deposit, suggesting it was more widespread, but was obliterated by subsequent overprinting phases. This background potassic alteration preserved original textures, and, depending on the alteration intensity, added copper values ranging from 0.1 to 0.5% Cu, with low total sulphide contents, generally <1%, occurring mostly as disseminated hypogene chalcopyrite ±bornite or chalcopyrite ±pyrite associations. In general, chalcopyrite is the dominant sulphide, with bornite or pyrite only being important locally. Background potassic alteration has been dated at 33.4 Ma, based on a set of Ar-Ar in biotite and K feldspar measurements (Reynolds et al., 1998), and is very similar to the crystallisation age of the West and Banco porphyries. This halo affects most of the Chuqui Porphyry Complex (Faunes, et al., 2005).
Marginal chloritic alteration - Selective chloritic alteration of primary and secondary mafic minerals, and the development of chlorite veinlets, occurs on the fringes of the deposit, accompanied by low grade copper (<0.3% Cu) mineralisation and low total sulphide (<1 vol.%) disseminations, characteristically comprising a pyrite ±chalcopyrite association with local, rare chalcopyrite. In the upper benches of the mine, this chloritic margin is coincident with the occurrence of preserved primary magnetite and marks the outer limit to the background potassic alteration. However, deeper in the system, similar low grade mineralisation is not accompanied by chlorite, but still has preserved primary magnetite and hornblende. In general this zone forms a fringe to the background potassic phase, and predominantly preserves primary textures. This zone also includes intervals of selective albitisation of plagioclase and development of calcite-ankerite veinlets. Epidote is only recognised locally. X-ray diffraction analyses of samples of this chloritic alteration indicate that the mineral macroscopically and microscopically identified as 'chlorite' corresponds to clinochlore, a member of the chlorite group. The distribution of chloritic alteration as a restricted fringe to the background potassic alteration, differs from the typical more extended halo normally encountered on the margins of orthomagmatic porphyry copper deposits. Late specular hematite, both as disseminations and in veinlets, is relatively common, locally occurring in brecciated rock, and is predominantly on the southeastern margins of the deposit. Specularite may, in isolated instances, be intergrown with chalcopyrite (Faunes, et al., 2005).
Intense potassic alteration - The background potassic alteration and peripheral chloritic phase were followed by an intense potassic event, responsible for the main hypogene mineralisation stage at Chuquicamata. This alteration and associated mineralisation occur as NNE oriented 'vein-like' domains or relatively well defined bands, suggesting an essentially structural control, probably related to repeated reactivation of the East Deformation Zone-Messabi Fault. Two alteration assemblages have been recognised:
i). K feldspar-fine quartz, which comprises moderate to intense replacement of feldspar and biotite by secondary K feldspar, sometimes also accompanied by secondary albite and quartz. The protolith texture is partially to totally destroyed and has a characteristic grey colour. This alteration has a strongly cataclastic fabric on a microscopic scale, frequently forming a micro-breccia with a fine matrix of micro- to crypto-crystalline quartz and feldspar. Quartz and K feldspar are also present as micro-veinlets. Deeper in the hypogene sections of the deposit, disseminated, veinlet and massive anhydrite is frequently found accompanying this alteration. Mineralisation associated with K feldspar-fine quartz preferentially occurs as veinlets and micro-veinlets and generally contains significant copper grades, with >1% total sulphides, usually unevenly distributed. Two forms of K feldspar-fine quartz alteration are recognised. The first is fractures controlled, occurring as selvages to sulphide veinlets with a high total sulphide and copper grade, and is characterised by strong destruction of the original host porphyry texture. The second is more pervasive, is not texture destructive, and forms diffuse, irregular zones with a total sulphide content and copper grade marginally greater than that of the background potassic alteration phase. Where K feldspar-fine quartz alteration is >30%, the hypogene copper grade is generally high, ranging from 0.8 to 1.5% Cu.
ii). Grey-green sericite - which resulted in intense destruction of the Chuqui Porphyry texture, and replacement of its mineral assemblage to produce aggregates of sericite, quartz, abundant disseminated copper sulphides and some K feldspar. It occurs as irregularly shaped pervasive zones, or as halos to early, frequently sub-parallel veinlets of quartz, quartz-bornite or quartz-molybdenite. Early sericite has as characteristic greenish-grey colour and a coarse texture, in contrast to later phase sericite alteration. Ore mineralisation accompanying grey-green sericite alteration occurs as abundant fine disseminations. The zones of abundant grey-green sericite alteration (i.e., >10 vol.%), represent significant sections of the deposit and contain >1 vol.% Cu, with grades of consistently >1% Cu, making this the principal hypogene ore event at Chuquicamata.
There is generally a very close spatial coincidence between the distribution of grey-green sericite and K feldspar-fine quartz alteration styles, but also between the grey-green sericite and quartz-molybdenite veining, although locally the latter may also have an inverse relation. These associations suggest a close temporal association, and together are regarded to represent an Intense Potassic event, that is predominantly localised in the eastern part of the deposit, where it occurs as a series of irregular tabular zones, trending NNE and dipping at 70 to 85°W. Within these zones, there is a gradation from grey-green sericite in the north and at depth, to K feldspar-fine quartz at higher levels and in the central and southern sections of the deposit. Cataclastic textures, and locally, ductile deformation features are recognised in these zone.
Both the K feldspar-fine quartz and grey-green sericite alteration styles were accompanied by abundant veining and disseminated assemblages of bornite-digenite-chalcopyrite-covellite, with cumulative grades in the range of 0.6 to 1.2% Cu. Mineralisation occurs in two associations, i). a bornite dominant bornite ±digenite ±chalcocite ±covellite, and ii). a chalcopyrite rich, chalcopyrite ±bornite ±covellite ±digenite assemblage. Although these two associations may be found together, on the basis of detailed studies, they appear to represent two separate, clearly differentiated, ore pulses. In the second episode, when 'D' veins are superimposed on grey-green sericite alteration, early sulphides are replaced by pyrite-digenite ±bornite or pyrite-covellite. It has been suggested these two pulses correspond to the two intense potassic alteration assemblages, where potassic feldspar and grey-green sericite are dominant respectively (Faunes, et al., 2005).
Quartz-molybdenite veining - A series of massive 'blue veins' and banded 'B' quartz-molybdenite veins and veinlets are interpreted to have been emplaced during the transition between the K feldspar-fine quartz and grey-green sericite alteration phases. Where this veining comprises >50 vol.%, it defines a tabular, structurally controlled, north-south to NNE oriented, ~80°W dipping core within the south-central part of the deposit. This core has grades ranging from 0.1 to 0.2%, averaging 0.13% Mo, dimensions of 1000 m in length, 50 m average width and has been recognised over a vertical interval of >600 m. However, while this zone has high molybdenite grades, copper is reduced to generally <0.6% Cu, due to the mass dilution of the early copper mineralisation by the abundant quartz-molybdenite veining, and the lack of permeability and sulphide content of the veins which restricted the emplacement of subsequent fracture controlled and disseminated copper mineralisation. There is a spatial association between this quartz-molybdenite veining and both the grey-green sericite and the late quartz-sericite alteration (described below). However, contact relationships between the quartz-molybdenite veining and high-pyrite 'D' veins directly associated to the late quartz-sericite (phyllic) event, demonstrates that the quartz-molybdenite event is earlier. Whilst molybdenite is also recognised in other alteration/mineralisation pulses, particularly the early background potassic phase, and in very late stage remobilised veins and fractures (Ambrus and Soto, 1974), the quartz-molybdenite vein and veinlet event is by far the most important and introduced the bulk of the molybdenite into the system. The principal geological control on the Mo grade is the volumetric frequency of the quartz-molybdenite veining (Ossandón et al., 2001), reflecting the intensity of the event (Faunes, et al., 2005).
Chalcopyrite veining with phyllic selvages - Toward the end of the intense potassic event, a late pulse of chalcopyrite was zonally deposited on its fringes, generating an average grade of 0.8% Cu, and marking the onset of the more destructive episodes of phyllic alteration. This pulse produced a greyish colouration and massive chalcopyrite veinlets with quartz-sericite halos containing disseminated chalcopyrite. These veinlets contain subordinate to rare to no pyrite, and although they crosscut all of the previously described alteration/mineralisation phases, belong to the early low pyrite association. Copper grades within these zones are ~0.7 to 0.8% Cu, corresponding to >1 vol.% sulphides. Chalcopyrite is locally accompanied by molybdenite. These chalcopyrite veinlets with sericitic selvages differ from the early grey-green sericite alteration as follow: a). chalcopyrite is predominantly within the veinlets and only partially in the selvages; b). the sulphide association comprises chalcopyrite ±pyrite, with no bornite; and c). individual chalcopyrite veins are isolated, the sericite halo is weak and the precursor porphyry texture may at least be partially preserved (Faunes, et al., 2005).
• Late, High Sulphidation (High-Pyrite) Associations - which commenced with:
Intense quartz-sericitic alteration, occurring as at least two consecutive phases, namely the Main and Late sub-events, forming a north-south elongated and vertically dipping zone in the western part of the deposit, adjacent to the West Fissure fault. This alteration is either pervasive quartz-sericite, or occurs as halos to 'D' veins. It has transitional margins, the 'transitional quartz-sericite zone', where quartz-sericite can be seen to be superimposed upon, but not entirely obliterating, earlier textures and alteration assemblages. The Main and Late sub-events are spatially related and are very difficult to separate in mapping. They took place at 31.1 Ma (Ar-Ar determination from sericite; Reynolds et al., 1998), 2 m.y. younger than the background potassic alteration event, and 3 m.y. younger than the crystallisation age of the East Porphyry (Faunes, et al., 2005).
The quartz-sericite alteration comprises sericite aggregates, with quartz and pyrite, which obliterate the original texture of the porphyry through the intensive replacement of feldspar and biotite. Minor amounts of kaolinitic clay replace plagioclase, while rare veinlets and breccia-veins of quartz-alunite are found, especially in the Late sub-event. X-ray diffraction analyses of sericite from the quartz-sericite alteration reveal it comprises aggregates of illite and muscovite (Faunes, et al., 2005).
This alteration obliterated the former mineralogy and generating a telescoped, high sulphidation primary mineral assemblage, accompanied by the addition of abundant pyrite (>1 vol.% pyrite, accounting for >50% of total sulphides). This event involved a series of progressive temporally and spatially overlapping pulses of sulphide mineralisation, reflecting the evolution of the high pyrite association and the degree of sulphidation, as follows: pyrite-chalcopyrite, pyrite-digenite ±bornite, pyrite-covellite ±enargite, pyrite-enargite ±sphalerite, representing a significant increase in S, Fe, As and Cu. The ore and gangue minerals have open space filling textures within the 'D' veins, and occur as disseminations in their halos, and in local breccia-veins. The Late pyrite-enargite pulse was the dominant source of arsenic within the system, and was structurally controlled. It was emplaced in a brittle environment in the form of massive veins or as breccia matrix in zones of abundant tectonised quartz-molybdenite veins, fringed by a pyrite-enargite stockwork of veins superimposed on earlier associations. This late alteration stage apparently terminated with a final post-ore pulse adjacent to the West Fissure, comprising barren to very low total copper bearing pyrite veinlets, with >90% pyrite, and copper sulphides constituting <10% of total sulphides (Faunes, et al., 2005).
There is evidence to suggest that the copper grades following the last phase of intense quartz-sericite alteration largely reflect the distribution inherited from the intense potassic phase with rare remobilisation and the addition of no more than 0.3 to 0.5% Cu (Faunes, et al., 2005).
Late veins, veinlets and micro-breccias - The terminal hypogene mineralising event at Chuquicamata comprises very locally distributed veinlets, micro-veinlets and micro-breccias carrying chalcopyrite ±covellite ±digenite ±red hematite ±anhydrite ±gypsum, but lacking pyrite. It cuts all the events described above, including the high pyrite associations of the Late quartz-sericite (phyllic) events and, in general, is not accompanied by alteration halos. In the areas where this event is observed, it does not apparently influence the copper grade, implying alteration and remobilisation of pre-existing mineral assemblage, without the introduction of additional copper (Faunes, et al., 2005).
• Post-hypogene structure - Dextral-normal, north-east oriented, distributive faulting, e.g., the Estanques Blancos and Portezuelo Systems, produced en echelon, progressive 'south block down' displacement, finally truncating the ore body to the south, while exposing it close to the roots of the mineralised system to the north. This was followed by uplift and sinistral displacement along the West Fissure fault, juxtaposing the barren block on its western side with the orebody to the east. Finally a sinistral-normal north-west fault system was reactivated to produce weak segmentation of both the hypogene mineralisation and the West Fissure fault, while increasing the permeability that influenced the subsequent supergene processes (Faunes, et al., 2005).
• Supergene Mineralisation - Supergene processes modified the hypogene mineralisation throughout the Chuquicamata deposit, mainly in its uppermost portions, with the generation of extensive zones of leaching, oxidation and enrichment of sulphides, and total/partial leaching of sulphates.
The Chuquicamata deposit was subjected to at least two leaching, oxidising and enrichment events between 19 and 15 Ma. The first generated a thick, strongly enriched blanket with grades averaging 2 to 3% Cu, focussed by the late and waning stage quartz sericitic alteration that was responsible for creating non-reactive rocks with abundant pyrite. A relatively thinly developed extension of this blanket persisted beyond the quartz-sericitic zone, into more reactive sectors to the east and north where there was lesser pyrite and predominantly early potassic alteration. Subsequent tectonic uplift and lowering of the meteoric water table oxidised the supergene enriched blanket to produce hematitic leached remnants in the quartz-sericite rich lithologies, and high grade copper sulphates in the potassic alteration of the near surface in the eastern and northern sectors (Faunes, et al., 2005).
This final phase resulted in a partly preserved leached cap (after erosion) and extensive oxide ore that replaces the earlier upper chalcocite blanket, overlying 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.
Exotic Mineralisation - Copper dissolved through the leaching of hypogene sulphides also migrated laterally, mainly to the south, via palaeochannels, to be deposited at the gravel-basement interface at the base of the channel, hosted by both channel gravels and the underlying decomposed bedrock to form the Mina Sur (Exotica) secondary deposit comprising chrysocolla, atacamite, Cu-wad and other oxides.
Sulphate leaching - Anhydrite and gypsum are significant components in most of the hypogene alteration zones at Chuquicamata, with anhydrite present as both disseminations and as veinlets, while gypsum is chiefly found in veinlets, the product of late anhydrite hydration. Both are abundant within the intense potassic and late quartz-sericite (phyllic) alteration zones, but are rare in the background potassic zone and are rare to virtually absent in the chloritic fringe. Below the zone of oxidation, these sulphates seal the rocks with little open fracturing and good geomechanical characteristics. However, supergene leaching within the upper levels of the Chuquicamata deposit has resulted in the partial to total dissolution and removal of these sulphates from sealed fractures, making the rock more porous, with lesser competence and reduced geomechanical properties. Based on the sulphate content and degree of sulphate leaching, three main zones are distinguished: i). Primary zone - characterised by a competent, sealed, rock mass, containing anhydrite and gypsum, without cavities; ii). Transitional or partially leached zone - comprising a less competent rock mass with cavities and open fractures, containing gypsum, but only very sporadic and rare anhydrite; iii). Secondary zone - with abundant cavities and open fractures, and an incompetent to less competent rock mass, where gypsum and anhydrite are absent due to complete leaching. Very late selenitic supergene gypsum may occur locally in the latter. The Transitional and Secondary zones are more deeply developed in the late quartz-sericite alteration zone on the western fringe of the deposit, locally persisting to depths of as much as 1200 below the pre-mining surface. This is a directly related to the less chemically reactive nature of the phyllic alteration assemblage and its higher pyrite content, which combined to produce more acid (lower pH) supergene solutions (Faunes, et al., 2005).
Supergene enrichment - The degree of supergene enrichment is most intense within the zones of phyllic alteration with strong permeability, low reactivity and high pyrite, conditions favouring strong acid generation. The primary copper grade was roughly doubled by secondary enrichment in the core of the late quartz-sericite alteration zone, whilst on the fringes and at depth, the enrichment factor progressively declined. The zone of supergene enrichment extends over a length of ~3 km north-south, maximum width of ~500 m and to a maximum depth of of 800 m, structurally controlled by the north-south, NNE and NE faults that predominate (Camus, 2003).
Three zones are recognised, downward from the bases of the leached and oxidised cap (as described below): i). Strong enrichment zone - where secondary sulphides predominate, replacing >75% of the hypogene copper sulphides. Bornite is totally replaced and pyrite is coated by secondary copper sulphides. This zone carries ~2.5% Cu (Camus, 2003), and corresponds to the gypsum/anhydrite 'secondary zone' described above, although the lower limit of the strong enrichment zone is still ~20 m above the 'gypsum roof', the base of sulphate leaching; ii). Weak enrichment zone - with subordinate secondary sulphides, mainly chalcocite and covellite, that occur in fractures, fault zones and partially or wholly replace hypogene copper minerals, while pyrite remains clean, bright and unaltered. This zone assayed ~1.8% Cu (Camus, 2003), and coincides with the gypsum/anhydrite 'transitional sulphate zone' described above; iii). Primary zone - without secondary sulphide, oxidation, leaching of sulphates or any enrichment of hypogene grades, corresponding to the gypsum/anhydrite 'primary zone' described above (Faunes, et al., 2005). These three zones principally represent the second supergene enrichment event, containing Cu sourced from the overlying leached cap and the now oxidised first event supergene sulphide blanket, and overprints hypogene Cu mineralisation (Camus, 2003).
Chalcocite is the dominant secondary copper mineral at Chuquicamata, although supergene covellite is also found in significative quantities. Chalcocite generally predominates in the 'strong enrichment zone' while covellite tends to be more abundant at greater depth within the 'weak enrichment zone', as described above, although there are reduced zones where covellite is important in the 'strong enrichment zone' and there are 'weak enrichment zones' with only chalcocite (Faunes, et al., 2005; Camus, 2003).
In situ oxidation and leaching - the oxidised and leached zone, which includes the leached cap and oxidised supergene blanket from the earlier supergene event, have been largely exploited or eroded, and are now only exposed as remnants at the northern and southern extremities of the current pit (Faunes, et al., 2005). Pre-mining, the un-eroded sections of this zone were up to 200 m thick and comprised a zone of strong leaching, with a remaining grade of 0 to 0.1% Cu. It was characterised by an assemblage of jarosite-goethite with lesser hematite, mostly representing two overprinting, telescoped stages of supergene leaching. The dominant goethite-jarosite mineralogy is indicative of an original low Cu:S ratio sulphide assemblage (Camus, 2003). This zone of leaching enclosed large islands of oxide copper mineralisation, forming a zone with an average thickness of ~150 m. Towards the southern end of the deposit, this oxide zone terminates at the West Fissure fault, whereas toward the north, it narrows, ending in a series of irregular bodies controlled by NE-trending structures. The north-south extent of the oxide body reached ~3.5 km, although it may, in part, have been of exotic origin to the north.
The oxide copper mineralisation, which contains up to 1.5% Cu (Camus, 2003), has a mineralogy, as described by Jarrell (1944), characterised by antlerite [Cu3(SO4)(OH)4], with lesser bronchantite [Cu4SO4(OH)6], atacamite [Cu2Cl(OH)3], chrysocolla [(C2-xAlx)H2-xSi2O5(OH)3•nH2O], copper pitch and kroenkite [Na2Cu(SO4)2•2H2O]. The dominance of hydrated sulphates indicate that the Cu-oxide assemblage was generated as a product of the in situ oxidation of a pre-existing sulphide enriched zone (Camus, 2003). Within this environment, the predominant ironstone is hematite, indicating leaching and oxidation of a very high Cu:S ratio sulphide (e.g., chalcocite) zone. However, the presence in some areas of abundant As-rich minerals such as chenevixite [Cu(Fe3+,Al)(AsO4)(OH)2] and locally olivinite [Cu2AsO4OH] suggest possible in situ oxidation of hypogene enargite at deeper levels within the system (Jarrell, 1944).
The principal current remnant zones of in situ oxide copper are found at the northern end of the deposit, where they occur as subvertical, elongate bands, interspersed with leached zones in which limonites coexist with variable proportions of copper oxides. The general trend of the copper oxides in this area is consistent with structural control related to the northeast striking Estanques Blancos, and north-south to NNE trending C-2 fault systems. The higher grade and volume oxide bodies are developed vertically above, and are the upward continuation of higher hypogene grades located below the 'top of dominant sulphides'. Hypogene alteration in these areas is predominantly a late quartz-sericite (phyllic) assemblage, with secondary copper also replacing veinlets locally superimposed on the widespread background potassic alteration to the north and east of the phyllic zone. The dominant mineralogy in these northern oxide ores comprise the sulphate bearing minerals antlerite-brochantite (±atacamite), with accompanying hematite, lesser sericite and rare clays. Sulphide relicts are abundant within the oxide ores, mainly as leached chalcocite partially replaced by hematite, implying that the current oxide ore zones were developed from an earlier supergene enrichment blanket which was oxidised and leached, and its copper redistributed to a newer and deeper zone of supergene enrichment (Faunes, et al., 2005).
However, on the southern margins of the Chuquicamata pit, copper oxides occur over the southern sections of the Chuqui Porphyry at a deeper level than the exotic oxides at Mina Sur. These oxides, which include brochantite, atacamite and antlerite, are considered to be in situ oxidation of hypogene sulphides, based on their occurrence in oxidised 'D' veinlets with quartz-sericitic halos (Faunes, et al., 2005). This would be consistent with the greater uplift and deeper erosion in the south, exposing structurally controlled supergene enrichment that had penetrated deep into the hypogene ore.
Mina Ministro Hales
The Mina Ministro Hales (#Location: 22° 22' 45"S, 68° 54' 50"W) is a large, tabular shaped porphyry copper(-molybdenum) deposit located 7 km to the south of the Chuquicamata mine. It is truncated and structurally sliced along its eastern margin on the west side of the regional West Fissure that also truncates and defines the western limit of Chuquicamata on the eastern side of the same structure. Mina Ministro Hales does not appear to represent the faulted western margin of the Chuquicamata deposit (Boric et al., 2009).
The concealed MMH copper deposit was discovered by CODELCO in 1989. It is masked by a blanket of gravels and has a 7 km north-south extent, is 200 to 320 m wide east-west, extends for >1200 m vertically, and has been subdivided into three, the North, Central and South bodies.
Geology
As at Chuquicamata, the West Fissure separates an eastern and western domain. In the north, the eastern domain, adjacent to the North body, comprises the Permo-Carboniferous Cerros de Chuquicamata igneous and metamorphic complex, composed of metadiorite and amphibolite, which is also exposed in the Mina Sur pit to the north, and constitutes the source of the gravels hosting the exotic mineralisation in that mine. Adjacent to the Central body, the eastern domain is composed of ~1000 m of continental sedimentary breccias of the Eocene Calama Formation overlying the Cerros de Chuquicamata complex (Tomlinson et al., 2001).
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 has a Triassic zircon U/Pb LAM-ICPMS age (Bertens, 2005) and is petrographically similar to the 227±2.0 Ma (U-Pb) Triassic Elena Granodiorite at Chuquicamata, although Boric et al. (2009) suggest it is equivalent to the Este Granodiorite of Chuquicamata. It is a multicoloured, medium grained, equigranular rock, composed, mainly of plagioclase (40 to 60%), potassium feldspar (10 to 20%) and interstitial quartz (20 to 35%), with sericite and clay replacing feldspar to varying degrees. The dyke-like MM Porphyry occurs deep in the deposit where it is adjacent to the West Fissure, while the MM Quartz Porphyry is only of limited extent and is characterised by abundant quartz eyes (Boric et al., 2009). The MM Quartz Porphyry cuts mineralised and altered veins of the MM Porphyry, and has been dated at 35.5±0.6 Ma (U/Pb LAM-ICPMS age in zircon; Bertens 2005). The MM Porphyry, which has been dated at 38.9±0.4 Ma (U/Pb LAM-ICPMS age in zircon; Bertens, 2005), and is probably associated with the mineralization and alteration at Mina Ministro Hales, is a generally equigranular rock, which in some areas has a weak porphyric texture. It has a higher K feldspar (30 to 40%) and quartz (30 to 50%) than the MM Granodiorite, and has only been recognised in the deep sections of the deposit, ~600 m below the surface (Camus, 2003). Hydrothermal breccia bodies, rooted in the MM Porphyry, host the highest Cu grades, as well as elevated values of As and Ag. Post-ore pebble dykes of breccia occur within the north-south fault and vein system, some of which are up to 30 m wide. Clasts are angular to rounded in a clastic matrix, barren of mineralisation (Boric et al., 2009).
The MM Granodiorite is cut on its western margin by a series of subvertical, north-south oriented, tabular dacitic dykes. These dykes are light coloured rocks with a porphyritic texture containing ~70% phenocrysts of feldspar and, to a lesser extent, quartz in the form of eyes, and biotite, set in a fine matrix composed of ~70% feldspar, replaced to a greater or lesser degree by sericite, alunite and/or kaolin. These dykes can have a spherulitic texture with quartz-K feldspar (Camus, 2003). They belong to more than one generation and intrude all of the above units along faults, but are cut by hydrothermal breccias. Some are of Triassic age (Tobey, 2005), and are aphanitic with crystals of hornblende, plagioclase and scarce quartz. Other rocks described from drill-logs are a hornblende porphyry, and a feldspar porphyry (Boric et al., 2009). The MM Granodiorite and MM porphyries intrude a volcano-sedimentary suite in the western domain, composed of undifferentiated green andesitic flows and breccias with chlorite, epidote and pyrite, assigned to the Triassic Collahuasi Formation, although zircon dates indicate some rocks may be Palaeozoic in age (Boric et al., 2009).
Structure
The western and eastern domains are separated by the West Fissure fault, which trends north-south and dips at ~75°W. Subsurface the fault zone is ~2 m wide, unconsolidated and shows significant tectonic brecciation (Camus, 2003). This structure does not control the location of any mineralisation, but rather truncates it, and is a post-mineralisation structure that has undergone early dextral and later sinistral transcurrent displacement (Camus, 2003) inferred to be >35 km (Boric et al., 2009). In contrast to Chuquicamata, which occurs within the eastern domain, Mina Ministro Hales is in the western domain. To the west of the West Fissure fault, a series of parallel and subparallel faults are recognised. These structures vary from north-south, parallel to the West Fissure fault to significant cross-cutting NE and NW structures. Towards the West Fissure fault, these structures tend to increase in density to form a complex fault corridor of up to 1 km in width. Some of these structures control bodies of hydrothermal breccia that formed during the late hydrothermal phase, as discussed below (Camus, 2003).
Mineralisation and alteration
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 and the Triassic MM Granodiorite.
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 a parent to the high sulphidation bodies from which hypogene alunite yields 40Ar/39Ar ages of 31.4 to 32.2 Ma (Boric et al., 2009).
The Mina Ministro Hales mineralisation is older relative to that at Chuquicamata and Radomiro Tomic and contemporaneous with the Toki Cluster orebodies a further 6 km to the SW (Boric et al., 2009).
This hydrothermal alteration and mineralisation took place in the following stages:
• Late-magmatic porphyry-style mineralisation, which produced a deep central core of potassic alteration located >300 m below the surface, characterised by the addition of K feldspar, biotite, anhydrite and quartz. K feldspar predominates over biotite, and includes sections where primary plagioclase is completely replaced by perthitic K feldspar and anhydrite. In addition, the presence of 'A' veins of quartz with bornite-chalcopyrite, with or without a visible K-feldspar halo and 'B' veins of biotite are evident. Anhydrite is only preserved >400 m below the surface, in the deep and central parts of the deposit, where, along with K feldspar, it occurs in both early quartz 'A' veinlets and disseminated in the host rock.
This alteration was accompanied by a nucleus of bornite, with varying proportions of chalcopyrite, pyrite, idaite [Cu3FeS4], digenite, primary chalcocite and covellite in the deep parts of the deposit, >300 m below the surface. The associated primary oxides comprise hematite and rutile, the latter occurring as a remnant species in zones of strong potassic alteration. Remnants of the potassic core are found at depths of as little as 300 m, where they are surrounded and overprinted by the subsequent phyllic and high sulphidation alteration and mineralisation and indicate the original extent of this form of mineralisation (Camus, 2003).
The chalcopyrite content increases outward from the central bornite-rich nucleus towards the periphery of the deposit, forming an asymmetric halo of chalcopyrite-pyrite that is best developed to the east, whereas towards the west, there is an incipient and discontinuous development of chalcopyrite, except in the deepest parts of the deposit, at a depth of >500 m (Camus, 2003).
'B' quartz veins with chalcopyrite-molybdenite are observed at >300 m depth, within both the central bornite-rich nucleus and the chalcopyrite-pyrite halo and are spatially and temporally associated with the MM Porphyry (Camus, 2003).
• Main hydrothermal phyllic alteration stage, which is characterised by the introduction of abundant green-grey sericite and pyrite, mainly in 'D' veins. This stage of alteration has its greatest development at depths of <400 m, and truncates the potassic alteration, overlapping the bornite-chalcopyrite and chalcopyrite-pyrite zones and, in turn, is overprinted by the late hydrothermal high sulphidation stage at shallower depths. In those areas of the MM Granodiorite that are affected by strong phyllic alteration, sericite mostly replaces plagioclase with ghost euhedral crystals after K feldspar and plagioclase. Biotite is completely replaced (Camus, 2003).
• Late hydrothermal high sulphidation stage, characterised by advanced argillic alteration, located predominantly at depths of <400 m below the current surface. This alteration stage is developed over the length of the deposit and widths of from 100 to 200 m. It produced an assemblage of pyrophyllite, alunite, kaolinite and sericite, with enargite, tennantite, bornite, chalcopyrite, pyrite, chalcocite and primary covellite occurring in fractures and a series of multipe, elongate, sub-parallel hydrothermal breccias that form north-south to 345° trending elongated tabular bodies up to 700 m long that persists over ~600 m in vertical extent, parallel to the West Fissure. Individual hydrothermal breccias are up to 25 m thick. They tend to fade with depth before disappearing, while towards the surface they branch out. These breccias are composed of isolated angular to subangular to rounded granodiorite or dacite fragments, strongly altered to quartz and sericite, which are totally or partially cemented by a matrix of sulphides (chalcocite, enargite and pyrite), rock flour, silica and alunite. Where the breccias show evidence of rotation of clasts, and banding, Cu grades are highest. They pass outwards into mineralised random stockworks of veins and fractures that surround and separate individual breccias. Advanced argillic alteration (silica, alunite, pyrophyllite, sericite and dickite) accompanies these breccias bodies. They represent the most important part of the copper mineralisation of the deposit and commonly have grades of ~3% Cu. A geological resource, centred on a block of these breccias, contains in the order of 40 Mt @ 4.5% Cu (Camus, 2003).
Within these breccias there is an overall vertical zonation of alteration and ore mineralogy. In the upper parts, at <400 m depth, the association of quartz, alunite, kaolinite and pyrophyllite with chalcocite enargite and pyrite prevails with subordinate bornite and covellite. Laterally, the enargite gives way to tennantite and/or sphalerite. Below these assemblages, sericite accompanied by chalcopyrite-tennantite-pyrite bornite, pyrite, digenite, chalcopyrite and primary covellite predominates with subordinate enargite (Camus, 2003). Within this latter zone, chalcopyrite-tennantite-pyrite predominate at medium depths, underlain by bornite-chalcocite-pyrite (Boric et al., 2009). Microscopic studies have confirmed the presence of silver associated with the copper minerals, occurring as stromeyerite, argentite, electrum and native silver, principally associated with chalcocite. A drill intersection through the lower sections of one of the best developed hydrothermal breccia bodies of the deposit encountered bornite, covellite, chalcocite and pyrite with 40 m @ 3.07% Cu Total and 1.04 g/t Au.
The age of the late stage of the Mina Ministro Hales system has been estimated as 32.9±1.2 and 33.8±1.0 Ma (K-Ar) on the basis of two samples of primary alunite with enargite, taken in the North and Central bodies, respectively (Sillitoe and McKee, 1990). These ages are similar to those of the late hydrothermal stage at Chuquicamata, by the same authors (Camus, 2003).
Other minerals such as ankerite, siderite and andalusite have been observed locally at Mina Ministro Hales, but their temporal and spatial relationships are uncertain. The association of ankerite and siderite with anhydrite is common. These carbonates, together with sphalerite and specularite, are located, both in the deep and shallower parts of the deposit, as opposed to the anhydrite that only occupies a deep position (Camus, 2003).
• Supergene mineralisation and alteration - Below the alluvial gravels that cover the deposit, there is a preserved 50 to 60 m thick leached capping whose base is significantly irregular and controlled by faulting. The upper high sulphidation mineralisation has been oxidised, so that while the copper has been totally leached, leaving values of <0.1% Cu, the arsenic remains as scorodite [Fe3+(AsO4)•2(H2O)], which locally preserves high arsenic grades. The limonites are predominantly jarosite and, to a lesser degree, hematite. Below the leached cap, a zone of mixed mineralogy has been identified, where sulphides, oxidised copper and limonite minerals coexist. The most important of these are chrysocolla and chenevixite [Cu(Fe3+,Al)(AsO4)(OH)2] with lesser malachite and atacamite. Below this mixed zone, there is an immature sheet of chalcocite, which partially replaces pyrite and enargite. The vertical extent of this zone is controlled by young structures and varies over a vertical range of up to 200 m. The only supergene alteration mineral recognised is kaolinite, although the presence of subordinate alunite is observed locally. Two K-Ar dates of these alunites yielded ages of 20.8±0.6 and 20.4±0.6 Ma (Sillitoe and McKee, 1990) documenting the final stages of the supergene process at Mina Ministro Hales (Camus, 2003).
• Exotic ores - Eocene and younger gravels are present east of the West Fault, and contain a 10 to 50 m thick blanket of exotic copper mineralisation in the lower MM Gravel unit below the contact with an upper thin brown unit. The unit contains chrysocolla, Cu-bearing cryptomelane, malachite, azurite, conichalcite [CaCu(AsO4)(OH)] and smectite.
Reserves and resources
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. Production of copper metal to 2000 (Camus, 2003) amounted to 25.036 Mt from Chuquicamata (from 1915), 0.547 Mt from Radomiro Tomic (from 1997).
Published mineral resource estimates for the main deposits of the Chuquicamata district at the end of 2000 (Camus, 2003 from Codelco records) were as follows:
Chuquicamata at a 0.5% Cu cut-off
Measured resource - 1.877 Gt @ 0.66% Cu,
Indicated resource - 1.458 Gt @ 0.60% Cu,
Inferred resource - 4.186 Gt @ 0.49% Cu,
TOTAL resources - 7.521 Gt @ 0.56% Cu,
TOTAL reserves - 1.634 Gt @ 0.78% Cu,
Radomiro Tomic at a 0.5% Cu cut-off
Measured resource - 0.590 Gt @ 0.48% Cu,
Indicated resource - 2.909 Gt @ 0.41% Cu,
Inferred resource - 1.476 Gt @ 0.31% Cu,
TOTAL resources - 4.976 Gt @ 0.39% Cu,
TOTAL reserves - 0.755 Gt @ 0.53% Cu,
Mina Sur at a 0.2% Cu cut-off
Measured resource - 41 Mt @ 1.37% Cu,
Indicated resource - 11 Mt @ 1.04% Cu,
Inferred resource - 119 Mt @ 1.11% Cu,
TOTAL resources - 171 Mt @ 1.18% Cu,
TOTAL reserves - 32 Mt @ 1.71% Cu,
Ministro Hales - Central at a 0.5% Cu cut-off
Measured resource - 9 Mt @ 1.15% Cu,
Indicated resource - 49 Mt @ 1.06% Cu,
Inferred resource - 343 Mt @ 0.89% Cu,
Ministro Hales - North at a 0.5% Cu cut-off
Inferred resource - 139 Mt @ 1.17% Cu,
Toki, 6 km SW of Ministro Hales, at a 0.5% Cu cut-off
TOTAL resources - 1.5 Gt @ 0.5% Cu,
Genoveva, 1.5 km NW of Toki, at a 0.2% Cu cut-off
TOTAL resources - 150 Mt @ 0.35% Cu,
Opache, 5 km SSW of Toki, at a 0.3% Cu cut-off
TOTAL resources - 319 Mt @ 0.60% Cu.
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 (Ossandón et al., 2001). Camus (2003) estimated the same deposits to have production + resources of 92.7 Mt of fine Cu. Faunes, et al. (2005) estimated that the district originally contained a total of ~125 Mt of fine copper, of which close to 38.5 Mt had been
extracted to 2000, leaving around 86.5 Mt in situ.
Published ore reserve and mineral resource estimates at the end of 2015 (Codelco Memoria Anual, 2015) were as follows:
Chuquicamata
Measured resource - 729 Mt @ 0.85% Cu,
Indicated resource - 653 Mt @ 0.70% Cu,
Inferred resource - 320 Mt @ 0.68% Cu,
TOTAL resources - 1702 Mt @ 0.76% Cu,
Proved reserve - 699 Mt @ 0.86% Cu,
Probable reserve - 702 Mt @ 0.60% Cu,
TOTAL reserve - 1401 Mt @ 0.73% Cu.
Radomiro Tomic
Measured resource - 1310 Mt @ 0.50% Cu,
Indicated resource - 1294 Mt @ 0.45% Cu,
Inferred resource - 1210 Mt @ 0.43% Cu,
TOTAL resources - 3814 Mt @ 0.46% Cu,
Proved reserve - 819 Mt @ 0.52% Cu,
Probable reserve - 1356 Mt @ 0.49% Cu,
TOTAL reserve - 2174 Mt @ 0.50% Cu.
Ministro Hales
Measured resource - 174 Mt @ 1.15% Cu,
Indicated resource - 54 Mt @ 1.06% Cu,
Inferred resource - 688 Mt @ 0.89% Cu,
TOTAL resources - 916 Mt @ 0.95% Cu,
Proved reserve - 184 Mt @ 1.05% Cu,
Probable reserve - 55 Mt @ 1.04% Cu,
TOTAL reserve - 239 Mt @ 1.05% Cu.
Note: Resources are inclusive of reserves.
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.
For more detail consult the reference(s) listed below which were the principal source of the information on which this summary was based.
The most recent source geological information used to prepare this decription was dated: 2009.
Record last updated: 19/3/2017
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.
Chuquicamata Radomiro Tomic Mina Sur Mina Ministro Hales
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Alvarez O, Flores R 1985 - Alteration and hypogene mineralisation in the Chuquicamata porphyry copper deposit (Entirely in Spanish): in 4th Chilean Geological Congress, August 1985 Universidad del Norte, Antofagasta 23p
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Arcuri T, Brimhall G, 2003 - The Chloride Source for Atacamite Mineralization at the Radomiro Tomic Phorphyry Copper Deposit, Northern Chile: in Econ. Geol. v98 pp 1667-1681
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Ballard J R, Palin J M, Campbell I H 2002 - Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in Zircon: application to Porphyry Copper deposits of northern Chile : in Contrib. to Mineralogy & Petrology v144 pp 347-364
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Barra, F., Alcota, H., Rivera, S., Valencia, V., Munizaga, F. and Maksaev, V., 2013 - Timing and formation of porphyry Cu-Mo mineralization in the Chuquicamata district, northern Chile: new constraints from the Toki cluster: in Mineralium Deposita v.48, pp. 629-651.
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Boric, R., Diaz, J., Becerra, H. and Zentilli, M., 2009 - Geology of the Ministro Hales Mine (MMH), Chuquicamata District, Chile: in XII Congreso Geologico Chileno, Santiago, 22-26 Noviembre, 2009, Faculty de Ciencias Fisicas y Matematicas Universidad de Chile, Proceedings, 4p.
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Campbell I H, Ballard J R, Palin J M, Allen C and Faunes A, 2006 - U-Pb Zircon Geochronology of Granitic Rocks from the Chuquicamata-El Abra Porphyry Copper Belt of Northern Chile: Excimer Laser Ablation ICP-MS Analysis : in Econ. Geol. v101 pp 1327-1344
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Camus, F., 2003 - El Yacimiento Chuquicamata: in Geologia de los Sistemas Porfiricos en los Andes de Chile, Corporacion Nacional del Cobre de Chile; Servicio Nacional e Geologia y Mineria; Sociedad Geologica de Chile, [in Spanish], pp. 182-189.
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Camus, F., 2003 - El Yacimiento M&M: in Geologia de los Sistemas Porfiricos en los Andes de Chile, Corporacion Nacional del Cobre de Chile; Servicio Nacional e Geologia y Mineria; Sociedad Geologica de Chile, [in Spanish], pp. 189-193.
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Cuadra, C.P. and Rojas, S.G., 2001 - Oxide mineralization of the Radomiro Tomic porphyry copper deposit, Northern Chile: in Econ. Geol. v.96, pp. 387-400.
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Cuadra, P. and Camus, F., 1998 - The Radomiro Tomic Porphyry Copper Deposit, Northern Chile: in Porter T M (Ed.), 1998 Porphyry and Hydrothermal Copper and Gold Deposits - A Global Perspective PGC Publishing, Adelaide pp. 99-109
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Dold, B., Pinget, M.-C. and Fontbote, L., 2023 - Genesis of the exotic chrysocolla - copper pitch/wad - atacamite/ brochantite ore at the Exotica (Mina Sur) deposit, Chuquicamata, Chile: in Mineralium Deposita v.58, pp. 569-591. doi.org/10.1007/s00126-022-01147-7.
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Faunes A, Hintze F, Sina A, Veliz H, Vivanco M and Geological Staff (of 2003), 2005 - Chuquicamata, Core of a Planetary Scale Cu-Mo Anomaly: in Porter, T.M. (Ed), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide, v.1 pp. 151-174
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Guilbert J M, Park C F 1986 - Chuquicamata, Chile, extract from Chapter 11 - Deposits Related to Intermediate to Felsic Intrusions: in The Geology of Ore Deposits Freeman, New York pp 418-426
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Lambiel, F., Dold, B., Spangenberg, J.E. and Fontbote, L., 2023 - Neoformation of exotic copper minerals from gel-like precursors at the Exotica deposit, Chuquicamata, Chile: in Mineralium Deposita v.58, pp. 661-680.
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Lindsay D D, Zentilli M, Ossandon-C G 1995 - Evolution of permeability in an active ductile to brittle shear system controlling the mineralisation at the Chuquicamata porphyry copper deposit, Chile: in Clark A (Ed), Giant Ore Deposits - II Dept. of Geol. Sciences, Queens Univ., Kingston, Ontario pp 62-89
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McInnes B I A, Evans N J, Fu F Q, Garwin S, Belousova E, Griffin W L, Bertens A, Sukarna D, Permanadewi S, Andrew R L and Deckart K, 2005 - Thermal History Analysis of Selected Chilean, Indonesian and Iranian Porphyry Cu-Mo-Au Deposits: in Porter T M (Ed), 2005 Super Porphyry Copper & Gold Deposits - A Global Perspective, PGC Publishing, Adelaide, v.1 pp. 27-42
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McInnes, B.I.A., Farley, K.A., Sillitoe, R.H. and Kohn, B.P., 1999 - Application of Apatite (U-Th)/He thermochronometry to the determination of the sense and amount of vertical fault displacement at the Chuquicamata Porphyry Copper deposit, Chile: in Econ. Geol. v.94, pp. 937-948.
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Mpodozis, C. and Cornejo, P., 2012 - Cenozoic Tectonics and Porphyry Copper Systems of the Chilean Andes: in Hedenquist J W, Harris M and Camus F, 2012 Geology and Genesis of Major Copper Deposits and Districts of the World - A tribute to Richard H Sillitoe, Society of Economic Geologists, Denver, Special Publication 16, pp. 329-360
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Munchmeyer C 1996 - Exotic deposits - products of lateral migration of supergene solutions from copper deposits: in Camus F, Sillitoe R H, Petersen R (Eds), Andean Copper Deposits: New Discoveries, Mineralisation, Styles and Metallogeny Soc. Econ. Geologists Spec. Publ. No. 5 pp 43-58
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Nelson, M., Kyser, K., Clark, A. and Oates, C., 2007 - Carbon Isotope Evidence for Microbial Involvement in Exotic Copper Silicate Mineralization, Huiquintipa and Mina Sur, Northern Chile: in Econ. Geol. v102, pp. 1311-1320.
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Ossandon, C.G., Freraut, C.R., Gustafson, L.B., Lindsay, D.D. and Zentilli, M., 2001 - Geology of the Chuquicamata Mine: A progress report: in Econ. Geol. v.96, pp. 249-270.
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Park, J.-W., Campbell, I.H., Malaviarachchi, S.P.K. Cocker, H., Hao, H. and Kay, S.M., 2019 - Chalcophile element fertility and the formation of porphyry Cu - Au deposits: in Mineralium Deposita v.54, pp. 657-670.
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Reich M, Snyder G T, Alvarez F, Perez A, Palacios C, Vargas G, Cameron E M, Muramatsu Y and Fehn U, 2013 - Using Iodine Isotopes To Constrain Supergene Fluid Sources In Arid Regions: Insights From The Chuquicamata Oxide Blanket: in Econ. Geol. v.108 pp. 163-171
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Sillitoe R H, McKee E H 1996 - Age of supergene oxidation and enrichment in the Chilean Porphyry Copper Province: in Econ. Geol. v91 pp 164-179
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Wilson J, Zentilli M, Boric R, Diaz J and Maksaev V, 2009 - Geochemistry of the Triassic and Eocene igneous host rocks of the MMH porphyry copper deposit, Chuquicamata District, Chile: in Poster sessions, Porphyry Systems and Related Mineralization Styles, 11th Biennial Meeting, SGA 2011, September, 2011, Santiago, Chile Proceedings Volume, 3p.
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Zentilli M, Graves M, Lindsay D 1995 - Recurrent mineralisation in the Chuquicamata porphyry copper system: Restrictions on genesis from mineralogical, geochronological and isotopic studies: in Clark A (Ed), Giant Ore Deposits - II Dept. of Geol. Sciences, Queens Univ., Kingston, Ontario pp 90-113
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