Geology and Development History of the Antamina Copper-Zinc Skarn Deposit, Peru

Abstract

Abstract Antamina, the world’s largest copper-zinc skarn deposit, entered production in 2001. This paper describes the development of the geologic model for the feasibility study (1996–1998). Antamina is located in the eastern part of the Western Cordillera of northern Peru at latitude 9° 32′ S and longitude 77° 03′ W and 4,200 to 4,800 m in elevation. Antamina has a long history of exploration and is a case study of successful creation of an orebody from a mineral resource. While small-scale mining is recorded intermittently since 1860, the first serious exploration was not begun until a century later by Cerro de Pasco Corporation (1952–1970), followed by a Minero Peru-Geomin (Romania) partnership, which conducted a feasibility study (1970–1976) with a reserve of 128.6 million metric tons (Mt) at 1.6 percent Cu and 1.3 percent Zn. Privatization of the project was won by Compañía Minera Antamina in 1996. This consortium undertook a major exploration program and completed a full feasibility study in 1998 that defined a minable, open-pittable resource of 500 Mt at 1.2 percent Cu, 1.0 percent Zn, 0.03 percent Mo, and 12 g/t Ag within a global resource of 1,500 Mt. Production is by open pit and flotation at 70,000 t/d, producing 270,000 t of copper and 162,000 t of zinc in concentrates per year. This makes Antamina the seventh largest copper and the third largest zinc mine in the world. Antamina is located in the polymetallic belt of central Peru, which comprises copper, zinc, silver, lead and gold deposits related to mid to late Miocene calc-alkaline stocks. The regional geologic setting comprises Late Jurassic to Late Cretaceous siliciclastic to carbonate sequences in a northwest-trending foreland fold-thrust belt of mid-Eocene age, the Incaic II deformation phase. Antamina is hosted by calcareous siltstone and mudstone of the Late Cretaceous Upper Celendin Formation. Skarn mineralization forms a shell over and around a quartz monzonite porphyry stock of late Miocene age, which itself hosts subeconomic porphyry copper-molybdenum mineralization. The skarn body is approximately 2,500 m long in a northeasterly direction and up to 1,000 m wide, with a known vertical extent of 1,000 m. The skarn consists mainly of andraditic garnet. It is symmetrically zoned around the intrusion from proximal brown garnet endoskarn and exoskarn outward to green garnet exoskarn, with peripheral wollastonite-diopside exoskarn. Significant copper mineralization is hosted by endoskarn. Retrograde chlorite skarn and hydrothermal breccia are minor. Metals are zoned laterally from a central copper-only zone to a peripheral copper-zinc zone. Chalcopyrite is distributed throughout all skarn zones. Appearance of sphalerite approximately coincides with the transition from brown to green garnet. The copper-zinc zone thins at depth and originally appears to have closed over the top of the intrusion, although most of it has been eroded. The main copper mineral in the wollastonite-diop-side skarn is bornite, and this zone also has elevated gold values. Silver, lead, and bismuth values are highest in the outer part of the copper-zinc zone and adjacent marble. Molybdenite occurs in the intrusion and adjacent skarn, as well as being abundant in the wollastonite-diopside skarn. Sulfides were deposited during the late prograde and retrograde phases and occur disseminated interstitial to garnet; as irregular massive sulfide zones; and as veinlets. The deposit was unroofed by glaciation and is exposed in a glacial valley; hence there is no significant oxidation or enrichment. Antamina is an oxidized calcic copper skarn related to a calc-alkaline quartz monzonite porphyry stock containing subeconomic porphyry copper-molybdenum mineralization. The outer zinc zone is unusually well developed. Features that appear to have contributed to Antamina’s world-class status include a possible mantle origin of the intrusions, the basin-margin setting of the host sedimentary rocks, favorable structural preparation, limited retrograde alteration, and partial preservation of the intrusion roof zone.