Chuquicamata, Core of a Planetary Scale Cu-Mo Anomaly

The copper and molybdenum mineralisation of the Chuquicamata deposit has been known since the 19' century. The deposit is located within the Codelco Norte District in the Andes Ranges of northern Chile, 200 km northeast of the city of Antofagasta. Small miners initially worked the exposed oxidised outcrops and high grade oxide veins that were the surface expression of the deposit, although industrial scale mining did not commence until 1915 with open pit exploitation of the main disseminated oxides. Mining has continued to the present day, currently removing approximately 170 000 tonnes of ore and 400 000 tonnes of waste per day.

Exploration since the 1950s has delineated a resource of 67 million tonnes (Mt) of/'« situ fine copper at Chuquicamata. It has also outlined additional deposits within the district, including the Mina Sur exotic accumulation with 5 Mt of fine copper derived from Chuquicamata, and Radomiro Tomic (RT) with a potential for 25 Mt of fine copper, representing the northern extension of the main Chuquicamata orebody. Further drilling has located additional mineralisation to the southwest at the MM deposit and the Toki Cluster. Recent deep drilling in the vicinity of these Eocene-Oligocene age porphyry copper deposits of the Codelco Norte District, and new concepts derived from pre-existing information, indicate that Chuquicamata is the core and main deposit of an exceptional regional scale anomalous concentration of copper, molybdenum and other elements of economic interest, that may be called a "planetary scale anomaly". Prior to mining, this "planetary scale anomaly" is estimated to have contained 125 Mt of fine copper and comprises a 39 to 31 Ma mineralised belt, largely covered by recent gravels, but partially exposed over a 30 km long, NNE trending interval. It is the final product of a very complex sequence of processes related to tectonic permeability and structural architecture, with intrusions and multi-episodic pulses of hypogene alteration and mineralisation, and subsequent supergene leaching, oxidation and enrichment.

The Chuquicamata deposit is hosted by a NNE elongated, tabular, 14x1.5 km intrusive complex, which is subvertical to steeply west dipping. This complex, which extends from the Chuquicamata open pit to the RT mine, is known as the Chuqui Porphyry (Chuqui Porphyry) and comprises three phases, the East, West and Banco porphyries. To the east it intrudes a Palaeozoic igneous-metamorphic basement, Triassic granodiorite, sediments and Mesozoic volcanics, while to the west it cuts the 39 to 38 Ma Tertiary Fortuna Intrusive Complex, and both to the east^and west the 37.3 Ma Elena Granodiorite. The West Fissure, an important district scale branch of the regional Domeyko Fault System, is a regional, north-south trending, subvertical fault located to the west of the deposit. It exhibits post mineral displacement which splits the Chuquicamata deposit, dividing the highly mineralised Chuqui Porphyry to the east, from the barren Fortuna Complex to the west. The West Fissure is believed to be Cenozoic in age. It has a complex kinematic history including transcurrent and probably reverse movements, and has had a strong structural control over the setting of the ore hosting porphyries, the mineralisation itself and the post mineral evolution of the Chuquicamata deposit. The Messabi Fault-East Deformation Zone, which is recognised on the east and north-east margins of the Chuquicamata and RT pits, affects the wall rocks of the Chuqui Porphyry, and produced ductile and semi-ductile fabrics, including mylonites and cataclastic flows, and was active before, during and after the intrusion of the Chuqui Porphyry, and probably during the early stages of mineralisation.

Recent work has supported a new synthesis of the geological evolution of this giant deposit, commencing with the syntectonic intrusion of the 34.6 Ma East Porphyry as a roughly NNE trending dyke. There is evidence that the Messabi Fault-East Deformation Zone, probably associated with a transpressive dextral tectonic environment, played a key role in localising this intrusive phase. The East Porphyry appears to have been barren or only accompanied by weak, late-magmatic alteration and mineralisation. A more pronounced mineralising event was synchronous with the intrusion of the smaller West and Banco porphyries, dated at 33.4 Ma. This event produced an intense stockwork of barren "A-type" quartz veins, mostly in the northern part of the deposit, with a huge background potassic alteration halo of selective biotitisation of mafics, and secondary potassic feldspar partially replacing plagioclase. This halo affects most of the Chuqui Porphyry, carrying weak and mostly disseminated chalcopyrite, pyrite and bornite, while in its outer fringes chloritic alteration with pyrite predominates, with little or no copper. The background potassic alteration preserved the original texture of the porphyry, adding, depending on the alteration intensity, copper values ranging from 0.1 to 0.5% Cu. Following the background potassic alteration, an intense potassic event ensued which was responsible for the main hypogene mineralisation stage at Chuquicamata. This intense potassic alteration occurred as NNE oriented bands, probably related to repeated reactivation of the Messabi Fault-East Deformation Zone. The two resultant alteration assemblages (potassic feldspar-fine quartz and grey-green sericite) shared the same locus as earlier pulses, destroying the porphyry textures and adding abundant veining and disseminated assemblages of bornite-digenite-chalcopyrite-covellite with cumulative grades in the range of 0.6 to 1.2% Cu. 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. During the transition from the potassic feldspar-fine quartz to the immediately following grey-green sericite phase, a series of massive quartz-molybdenite veins were emplaced. This veining produced a tabular, subvertical, north-south to NNE oriented core zone in the central-south portion of the deposit, with average molybdenum grades ranging from of 0.1 to 0.2% Mo.

Clear evidence of brittle-ductile deformation is registered from the period between the early alteration/ barren to weak mineralisation stages and the later main poly-episodic quartz-sericitic (phyllic) event. During this period, which extended from 33.4 to 31.1 Ma, the deposit was subjected to important tectonic stresses. Intense main and late stage quartz sericitic alteration was imposed on a north-south elongated zone in the western part of the deposit at 31.1 Ma. This phase obliterated the former mineralogy and generated a telescoped, high sulphidation, primary mineral assemblage, with the addition of abundant pyrite and variable amounts of digenite, covellite, enargite, chalcopyrite and bornite, representing a significant increase in S, Fe, As and Cu. There is evidence to suggest that the copper grades following this last phase of intense quartz-sericite alteration largely reflect the distribution inherited from the intense potassic phase with rare remobtlisation and the addition of no more than 0.3 to 0.5% Cu. 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, in the direction of the RT mine. This was followed by uplift and sinistral displacement along the West Fissure 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, while increasing the permeability that influenced the subsequent supergene processes.

Between 19 and 15 Ma the deposit was subjected to at least two leaching, oxidising and enrichment events. The first generated a thick, strongly enriched blanket with grades averaging 2 to 3% Cu, which was focussed by the late and waning stage quartz sericitic alteration that produced non-reactive rocks with abundant pyrite. This blanket extended, although relatively thinly developed, beyond the quartz-sericitic zone, into more reactive sectors to the east and north with lesser pyrite and predominantly early potassic alteration. Subsequent tectonic uplift lowered the meteoric water table, oxidising the 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, which were the target of early mining activity at Chuquicamata. Low grade remnants of this oxide cap may still be observed in the northern part of the present pit. The leaching, which originated in the phenomenon of sudden downward fluctuation of the meteoric water table, also produced exotic mineralisation that was principally transported south from the main deposit through a palaeo-channel to form the Mina Sur orebody.