Antofagasta plc owns the Centinela mine through its 70%-owned subsidiary, Minera Centinela S.A., with the remaining 30% held by its minority partner, Marubeni Corporation.
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Summary:
The Centinela district is a 40-km-long segment of the late Eocene to early Oligocene porphyry copper belt of northern Chile. The main mineralization styles in the district include porphyry copper and associated skarns with gold and molybdenum credits, oxide copper in the form of either exotic deposits or in situ oxidation zones, and supergene copper sulfide enrichment blankets.
Structurally controlled, district-wide porphyry copper mineralization formed between 44 and 41 Ma, contemporaneously with transpressional tectonism along the regional-scale Centinela-Limón Verde fault zone, a splay of the major Domeyko fault system. The Domeyko fault system accommodated tectonic uplift of the Cordillera de Domeyko during early stages of the middle Eocene Incaic orogeny. Several productive porphyry copper systems in the district were emplaced syntectonically along reverse- and oblique-slip faults that controlled the shape and attitude of both the dominantly dikelike intrusions and associated alteration mineralization.
All the porphyry copper deposits in the district evolved from early potassic alteration with copper mineralization present as chalcopyrite and bornite, through sericite-chlorite alteration with chalcopyrite and pyrite, to late, overprinting pyritic sericitic alteration. Some systems developed advanced argillic lithocaps in their upper parts. Much of the copper was introduced with the potassic alteration, and all the gold-rich deposits display abundant hydrothermal magnetite and a positive correlation between copper and gold.
Supergene processes gave rise to oxide copper zones above pyrite-poor, chalcopyrite- and bornite-bearing protore and to chalcocite blankets over less reactive, sericitic zones containing pyrite and chalcopyrite. Coeval regional uplift, erosion, and exhumation resulted in widespread piedmont gravel deposition broadly contemporaneous with the supergene activity, with formation of exotic copper deposits alongside the shallow, actively oxidizing, pyritic parts of porphyry copper deposits.
The Mirador deposit is a multiphase porphyry Cu Au-Mo system, within which four intrusive pulses dated by the U-Pb zircon method at 41.3 + 0.4 to 38.4 + 0.7 Ma are currently recognized. These are: feldspar- and biotite-bearing granodioritic Mirador porphyry; dacite porphyry with biotite and quartz eyes; porphyritic granodiorite stock; and late dacite porphyry. The deposit occurs within the Domeyko fault system but is localized by a N65-80°W-trending fault, suggesting the possibility of a new structural system with potential to host porphyry copper deposits in the Centinela district. The porphyry system is emplaced into Jurassic calcareous shale and gypsum, giving rise to peripheral, locally mineralized skarn.
Four mineralization types are recognized: early porphyry Cu-Au with potassic alteration, EB, A and B veinlets, and hypogene chalcopyrite bornite-Au mineralization; Granodiorite stock with Cu-Mo mineralization, sericite-chlorite alteration, fine-grained disseminated sulfides and abundant molybdenite; Cu-Au-bearing garnet skarn with chalcopyrite-pyrite-magnetite; and hornfels and skarn displaying in situ oxidation, which is affected by gravitational collapse due to basal gypsum dissolution. Piedmont gravel and tectonic and solution-collapse breccias then channeled supergene copper-bearing solutions, leading to precipitation of chrysocolla, antofagastite and copper wad. The resultant high grade oxide copper ore is currently being exploited by Minera Centinela.