Summary:
Chuquicamata porphyry copper-molybdenum deposit 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 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 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 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 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).