Summary:
The Toquepala deposit is considered to be an example of a copper–molybdenum porphyry deposit.
The deposit is ovoid, extending over a 3.3 x 3.5 km area. Mineralization has been drill tested to about 950 m from the original land surface. Drilling from within the pit base indicates primary mineralization extends for an additional 930 m below the pit bottom.
Mineralization is closely associated with a complex, 1,500 m diameter intrusive centre dominated by four phases of porphyritic dacite plugs, referred to as "T", "Main", "L/M" and "Late", a dacite diatreme and agglomerates, extensive hydrothermal breccias and latite porphyry stocks and dikes.
In the mine area, the Toquepala Lineament appears to control some aspects of mineralization. The lineament is based on the distribution of three porphyry stocks, two breccia bodies, numerous pebble and thin intrusive dikes within the mine, as well as the Cerro Toquepala, Cerro Azul and Totoral breccia pipes, along a 20º azimuth. The lineament is controlled by a north–northeast-trending pre-mineral fault system.
Mineralization consisted of leached capping, oxide, enriched, transitional and primary mineralization. The first three types have largely been mined out.
Mineralization types included:
• Leached capping was predominantly limonite (jarosite 30%, hematite 60% and goethite 10%) mostly disseminated and in fractures. It is mostly mined out; however, remnants occur around the periphery of the pit.
• Oxide mineralization was very sparse and may be entirely mined out.
• Enriched mineralization consisted of 90% chalcocite, and 10% digenite, covellite, and bornite in breccia matrix, disseminations, and veinlets. This material had the highest copper grades and is mostly mined out.
• Transition mineralization was about 60% chalcopyrite, 25% chalcocite and 15% molybdenite and bornite as breccia matrix, disseminations, and veins associated with quartz. It is mostly mined out, but remnants are localized on the west edge of the pit.
• Primary copper mineralization occurs as hypogene sulfides mainly restricted to the dacite porphyry and breccias. Chalcopyrite is the dominant economic mineral (90% of mineralization) with 10% bornite, molybdenite, and enargite as dissemination fracture fillings, and breccia matrix. It is associated with potassic alteration (biotite and/or K-feldspar) with about 1% of pyrite (pyrite to chalcopyrite ratio of about 3:1) associated with magnetite and anhydrite. Economic molybdenite mineralization is associated with quartz veinlets and locally, with disseminated chalcopyrite.
In the phyllic zone, pyrite is present as 10% of the rock. The pyrite to chalcopyrite ratio is about 12:1, associated with minor molybdenite, bornite, chalcocite, sphalerite, and galena.