Summary:
The Antamina deposit is considered to be an example of a giant skarn deposit. It is unusual in its persistent mineralization and predictable zonation, and has a southwest-northeast strike length of more than 2,500 metres and a width of up to 1,000 metres. The skarn is well-zoned symmetrically on either side of the central intrusion with the zoning used as the basis for four major subdivisions: a brown garnet skarn, a green garnet skarn, a wollastonite/diopside/green garnet skarn and a marbleized limestone with veins or mantos of wollastonite. Other types of skarn, including the massive sulphides, massive magnetite, and chlorite skarn, represent the remainder of the skarn and are randomly distributed throughout the deposit. The variability of ore types can result in significant changes in the relative proportions of copper and zinc produced in any given year.
The formation of the Antamina deposit is attributed to the emplacement of multiple mineralized porphyries (strictly along a northeast-trending fault zone into reactive wall rocks between 10.95 ± 0.20 Ma (oldest uranium–lead zircon age) and 9.68 ± 0.05 Ma (youngest rhenium–osmium molybdenite age).
The main mineralized area consists of three northeast–southwest-oriented contiguous porphyry centres: the Antamina Main zone, the Usupallares zone, and the Condorcocha zone, which is located about 1 km north of the Antamina Main zone. Technically, the intrusive bodies are trachyandesite porphyries in composition, but are referred to as quartz monzonites at the mine and in many published papers on the deposit.
The Antamina Porphyry Complex hosts five porphyry phases. The Usupallares zone occupies a smaller southwest portion of the Antamina Porphyry Complex, and consists of three porphyry phases. The Condorcocha zone consists of at least three porphyry phases. Identification of the first phase is complicated by the presence of Lake Condorcocha, which obscures the earliest intrusion under water. Two sub-parallel porphyry dykes that intersect the Antamina Main and Condorcocha zones along their northeast margins are referred to as the Oscarina dykes.
The Antamina Porphyry Complex is partially altered to prograde, pink/brown-garnet endoskarn, which occurs as a 20–100 m wide annulus around the entire complex, as well as in many internal zones, which can reach as much as 75 m wide, occurring to depths well below 3,000 m above mean sea level. Internal endoskarn zones are irregular but can be followed along strike for several tens of metres.
Mineralisation and metal zoning shows an outward progression from molybdenum ± copper in the central porphyry, to copper (± silver, bismuth)–zinc–lead from proximal to distal exoskarns.
The ore mineralogy is dominated by molybdenite, chalcopyrite, bornite, and sphalerite, with lesser galena and minor bismuth–silver–sulphur minerals.
The main core of the Antamina deposit is about 2,000 m long and approximately 1,500 m wide. The mineralization extends from the surface at approximately 4,500 m above mean sea level and has been intersected in core drill holes below 2,500 m above mean sea level.
The Usupallares zone mineralization lies to the south of the Antamina Main zone, extends from surface at 4,200 m above mean sea level, and has been intersected in drill holes to approximately 3,000 m above mean sea level. The Usupallares zone is about 800 m wide, and approximately 1,000 m long.
The majority of mineralisation at Antamina formed during Stage 2 retrograde alteration and occurs in endoskarns, exoskarns, and in veins cutting across porphyries. A minor amount of mineralisation is noted in marble and hornfels.
The deposit was unroofed by glaciation and is exposed in a glacial valley; hence there is no significant oxidation or enrichment.
The Antamina Main and Usupallares zones are horizontally and vertically zoned over a depth of at least 2 km with respect to major metal components:
• Copper is relatively evenly distributed from endoskarn to the limestone contact, and chalcopyrite is distributed throughout all skarn zones;
• Zinc and bismuth tend to occur within 70 m of the contact of green garnet skarn with limestone/marble/hornfels. The appearance of sphalerite approximately coincides with the transition from brown to green garnet;
• Molybdenite occurs in the intrusion and adjacent skarn, as well as being abundant in the wollastonite-diopside skarn;
• Silver, lead, and bismuth values are highest in the outer part of the copper-zinc zone and adjacent marble. Silver can be present in any of the skarn lithologies. Lead is generally hosted by green garnet exoskarn, diopside exoskarn, and hornfels.
By rock type, the highest concentrations of zinc are hosted in exoskarn, copper in endoskarn and exoskarn, and molybdenum in endoskarn and P2 (Antamina Main and Usupallares). Bismuth displays two prominent zones of high concentration, one in the north–northeast Antamina Main zone and another in the Bornita zone. The Bornita zone is a name used for a region of the deposit where bornite is associated with wollastonite exoskarn. Silver distribution overlaps with copper and zinc.
Zonation of the main ore minerals typically follows the same trend as their constituent metals, with molybdenite concentrated in the Antamina Main and Bornita porphyry centres, chalcopyrite slightly outward of molybdenite in endoskarn and exoskarn, and sphalerite outward from chalcopyrite in exoskarns. The patterns of chalcopyrite and sphalerite are similar in the Usupallares zone; however, the minerals are less abundant. Bornite has a more localized distribution, occurring mostly southwest of the Antamina Main zone in the Bornita zone. Galena occurs in minor amounts in distal exoskarns where its occurrence overlaps with sphalerite. Scheelite was observed in only one occurrence in the northeast Antamina Main and Oscarina zones, although the distribution of tungsten indicates that it may be more common than noted in distal exoskarns.
Copper, molybdenum, and zinc sulphides comprise the principal ore minerals; these include chalcopyrite, bornite, molybdenite, and sphalerite. Pyrite and magnetite are common, but of lesser economic importance. Minor minerals include galena and pyrrhotite. Rare occurrences of chalcocite and wittichenite (Cu3BiS3) have been observed. Sulphides occur disseminated interstitial to garnet, as irregular massive sulphide zones, and as veinlets.