Escondida is a giant supergene enriched open pit porphyry copper deposit located in northern Chile, some 160 km to the east-south-east of Antofagasta. It is currently the largest copper producing mine in the world with an annual output of more than 800 000 tonnes of fine copper, and capacity of 1.2 Mt pa. The deposit contains in excess of 30 Mt of copper metal.
The Escondida deposit falls within the 50 km wide Cordillera de Domeyko morpho-tectonic province of the northern Chilean Andes, characterised by north-south elongated ranges and intervening shallow gravel filled basins at elevations varying from 2000 to 4500 m asl. It is one of a string of deposits associated with Upper Eocene to Lower Oligocene porphyry intrusion in northern Chile.
The basement sequence below the Cordillera de Domeyko represents a complex continental margin of accreted continental fragments and Palaeozoic to Mesozoic volcanic arcs, but overall coincides with the transition from continental Precambrian and Palaeozoic crust to the east and thick Palaeozoic turbidites and volcanic-sedimentary sequences most likely resting on oceanic crust, to the west. During the Mesozoic a magmatic arc developed to the west, while the Cordillera de Domeyko was within an extensional back arc basin. This sequence was in turn followed by three continental margin Andean-type magmatic arcs from the late Cretaceous to late Cainozoic, with associated large porphyry deposits such as Escondida formed mainly between 42 and 31 Ma during the Eocene-Oligocene phase. From 31 Ma onwards uplift, climatic change and low rates of denudation have favoured the development and preservation of enriched blankets over these porphyries.
The Cordillera de Domeyko province is dominated by the Domeyko fault system, which in the Escondida district is characterised by cymoid shapes producing elongated, north-south basins in a 200 x 20 to 50 km zone, probably representing long lived zones of structural activity, varying in character over time. Both Chuquicamata (200 km to the north) and Escondida lie on strike-slip faults of the Domeyko fault system.
Escondida is the largest of six known porphyry deposits in the district. The others include Escondida Norte, Zaldivar, Pinta Verde, Pampa Escondida, Carmen and Ricardo. All fall within a localised zone of north-south striking, sinistral strike-slip faulting that forms a cymoidal shape.
The hydrothermal activity responsible for the hypogene mineralisation is closely associated with a late Eocene to Oligocene dioritic to quartz-monzonite to granodiorite stock complex composed of at least three intrusive stages. This complex intrudes a sequence of Palaeozoic andesite, cut by late Palaeozoic (265 to 270 Ma) rhyolites, marine-continental Mesozoic sediments (calcareous sandstone, red sandstone and conglomerate) unconformably overlain by late Cretaceous to Paleocene andesites. The mineralised intrusive complex is overlain and cut by Oligocene to Miocene rhyolitic sub-volcanic intrusives and extrusives.
At Escondida the mineralised NW-SE trending, 4.5 x 2.5 km quartz-monzonitic to granodioritic porphyry stock complex cuts Paleocene andesites. The complex is composed of at least three phases, the first two of which are distinguished on their phenocryst size, composition and quantity while the third is a porphyry breccia which contains mineralised clasts but is cut by copper rich veins.
Alteration commenced with an early potassic (biotite rich, particularly within the andesites) stage grading out into a propylitic periphery and a silicified shell around the intrusive, with associated magnetite, chalcopyrite and bornite and a hypogene grade of generally less than 0.2% Cu. The second phase corresponded to sericite-chlorite and then to quartz-sericite alteration with chalcopyrite, pyrite and molybdenite carrying 0.4 to 0.6% Cu and chalcopyrite:pyrite ratios of 3:1. The third and final phase comprised an acid sulphate episode which over printed the previous assemblages with a younger advanced argillic alteration and a range of sulphides including bornite, chalcopyrite, pyrite, chalcocite, covellite, enargite, sphalerite tennantite and galena, which produced grades of 0.6 to 1.0% Cu where overprinting the earlier pulses of mineralisation.
Supergene enriched ore represents 65% of the total resource at Escondida. The upper surface of the supergene blanket is sub-horizontal and is displaced by faulting, while the base is irregular, with thicknesses varying from a few metres to 400 m. The leached cap extends from the present surface to a depth of around 200 m over the chalcocite blanket to only a few metres over the local copper oxide zone. It carries hematite over the deeper higher grade chalcocite blanket and jarosite over shallower, lower grade sections of the blanket. Over the copper oxides goethite dominates. The copper oxide zone represents around 5% of the resource and is composed of oxides, silicates, sulphates and carbonates of copper, mainly brochantite, antlerite, atacamite, chrysocolla, wad and tenorite.
Sulphides within the zone of supergene enrichment occur as disseminations and in veins and include chalcocite, covellite with minor digenite and idaite replacing pyrite, chalcopyrite and bornite. Chalcocite occurs throughout the blanket, while covellite and digenite appear in the middle, and increase downwards. Copper grades vary from 0.3 to more than 2%, but are highest in the thickest part of the blanket over the best developed hypogene mineralisation, in the zone where strong quartz-sericite and subsequent advanced argillic alteration overprints early biotite.
The total reserve + resource at Escondida and its contiguous mineralised systems amounted to approximately 2.62 Gt @ 1.15% Cu in 2000.
Escondida has reserves (proved + probable) & resources (measured + indicated + inferred) at Escondida in 2002 were as follows (Rio Tinto 2002 Annual Report):
Sulphide ore - reserve of 1548 Mt @ 1.21% Cu, + resource of 579 Mt @ 0.9% Cu,
Low grade float ore - reserve of 575 Mt @ 0.60% Cu, + resource of 815 Mt @ 0.6% Cu,
Low grade leach ore - reserve none, + resource of 666 Mt @ 0.4% Cu,
Oxide ore - reserve of 198 Mt @ 0.70% Cu, + resource of 55 Mt @ 0.5% Cu,
Mixed ore - reserve of 49.7 Mt @ 1.03% Cu, + resource of 62 Mt @ 0.6% Cu,
Escondida Norte has additional resources (measured + indicated + inferred) as follows (Rio Tinto 2002):
Sulphide ore - resource of 649 Mt @ 1.3% Cu,
Low grade float ore - resource of 642 Mt @ 0.6% Cu,
Oxide ore - resource of 142 Mt @ 0.8% Cu,
Mixed ore - resource of 43 Mt @ 0.8% Cu,
The mine is operated by Minera Escondida, the principal owners of which in 2003 were BHP Billiton 57.5% and Rio Tinto 30%.
JORC compliant ore reserves and mineral resources in the Escondida group of deposits at December 31 2011 (Rio Tinto, 2012) were:
Escondida sulphide ore - Proven + probable ore reserves - 1.993 Gt @ 0.97% Cu,
Measured + indicated + inferred mineral resources - 5.762 Gt @ 0.53% Cu,
Escondida sulphide leach ore - Proven + probable ore reserves - 3.503 Gt @ 0.50% Cu,
Measured + indicated + inferred mineral resources - 75 Mt @ 0.53% Cu,
Escondida sulphide oxide ore - Proven + probable ore reserves - 0.111 Gt @ 0.86% Cu,
Measured + indicated + inferred mineral resources - 33 Mt @ 0.65% Cu,
Pampa Escondida sulphide ore - Measured + indicated + inferred mineral resources - 7.378 Gt @ 0.47% Cu,
Pinta Verde sulphide ore - Measured + indicated + inferred mineral resources - 45 Mt @ 0.51% Cu,
Pinta Verde oxide ore - Measured + indicated + inferred mineral resources - 130 Mt @ 0.70% Cu.
(Source: Porter Geoconsultancy, www.portergeo.com.au, 2012)