Teck Resources Ltd. indirectly owns 22.5% of the Antamina copper/zinc mine in Peru, with the balance held indirectly by BHP Billiton plc (33.75%), Glencore plc (33.75%) and Mitsubishi Corporation (10%). The participants’ interests are represented by shares of Compañía Minera Antamina S.A. (CMA), the Peruvian company that owns and operates the project.
Summary:
The Antamina polymetallic deposit is skarn-hosted. 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 Antamina intrusion (9.8 Ma2) is a multiple phase, quartz monzonite porphyry and has the Antamina Cu-Zn skarn developed around it. There is a nearby intrusion of similar composition with narrow, weakly mineralized skarn 1.5 kilometers northwest at Condorcocha.
Antamina is a polymetallic (copper, zinc and molybdenum predominate) skarn deposit resulting from complex multiple intrusive events. Copper mineralization occurs mainly as chalcopyrite except for some areas of bornite, representing approximately 5% of the deposit. Zinc mineralization generally occurs as sphalerite. Other significant sulphide minerals include molybdenite and pyrite, while trace amounts of numerous silver and bismuth bearing minerals and local areas of galena (lead sulphide) are also found within the deposit.
Antamina deposit is unusual in its persistent mineralization and predictable zonation, and has a SW-NE strike length of more than 2,500 metres and a width of up to 1,000 metres.
The Antamina and Usu Pallares deposits comprise a very large copper skarn with zinc, silver, molybdenum, lead, bismuth and arsenic formed by the intrusion of quartz monzonite body into limestones. Characteristics of the skarn zone depend upon the nature of both the intruded rock and the emanations from the intrusive body. Two types of alteration are recognized; recrystallization, or rearrangement of the constituents already in the rocks, and addition of chemical elements. Most skarn deposits show both features. The most striking skarn deposits are developed when the host rock is a sedimentary rock of carbonate composition. Mineral deposition in skarn deposits typically displays structural and stratigraphic controls. Skarn lithology is usually zoned from the fluid source both inward and outward. A typical sequence is; intrusive altered to garnet rich endoskarn, garnet rich exoskarn, diopside exoskarn, wollastonite exoskarn, hornfels, marble, and unaltered limestone.
The Antamina and Usu Pallares orebodies have proven to be consistently well mineralized and predictable, both in terms of grade and metal zoning over large areas, however short distance variability is quite high as is typical in most skarn deposits. Very little of the skarn lithology comprising the deposits is unmineralized.
As with the skarn silicate mineralogy, Antamina and Usu Pallares are horizontally and vertically zoned with respect to major metal components. This lateral zoning is clearly related to the orientation of the intrusive and limestone contacts and continues throughout the nearly one kilometer of vertical range of the deposit explored to date.
Metal zonation is quite distinctive within the deposit. Copper occurs relatively evenly distributed from endoskarn to the limestone contact. Zinc and bismuth tend to occur within 70 meters of the contact of green garnet exoskarn with limestone/marble/hornfels. Molybdenite is generally located within the intrusive core and surrounding endoskarn. Silver is present in any of the exoskarn lithologies. Lead is generally located in green garnet and diopside exoskarn and hornfels. Cobalt is generally associated with sphalerite mineralization. However veins and blebs of tennantite and other minerals can be found as rare occurrences in any rock type at Antamina. Arsenic occurs in solid solution within the copper minerals and in a variety of arsenic bearing minerals, contained within a valley parallel structural corridor and within a zone at the intersection of this structural trend with the Oscarina Dike structural trend.
Chalcopyrite is the predominant copper sulfide mineral. Approximately five percent of the copper mineralization is in the form of the copper sulfide mineral bornite. Zinc occurs primarily as the sulfide mineral sphalerite. Silver is normally associated in solid solution with chalcopyrite. However, it is also associated with galena, bismuth sulphosalts and tennantite. Molybdenum exists as the sulfide mineral molybdenite.
The most common bismuth minerals are bismuthinite, cosalite, whittichenite, cuprobismutite, aikinite, and kobellite, although various other Bi minerals as well.
There was little exposure of the deposit to oxidation and the evidence is in the way of a very limited oxidation cap and minimum supergene enrichment. This is due to the recent glaciation of the deposit, and the cold climate. However, there were zones of acetate soluble copper mineralization in Phase 2, resulting in material unsuitable for the production of salable quality concentrate. This oxidized portion has been almost completely mined to date.