The Cotabambas porphyry copper-gold cluster includes at least four porphyry centers in an area of 5 × 3 km, namely Ccalla, Azulccacca, Huaclle, and Ccarayoc, of which the first two are the best known. The geology of the area is dominated by two large granodiorite and diorite plutons and smaller stocks and dikes of microdiorite and andesite, all of which form part of the middle Eocene to early Oligocene Andahuaylas-Yauri batholith. At Cotabambas, granodiorite (K-Ar age of 39.8 ± 1.5 Ma) intrudes diorite (K-Ar age of 43.2 ± 1.1 Ma) and both, in turn, are intruded by a series of composite, structurally controlled porphyry stocks and dikes of granodioritic to quartz monzodioritic composition associated with porphyry copper-gold mineralization. The main structural features include several intersecting north-northeast- and west-northwest-trending faults, which are interpreted to have controlled the emplacement and architecture of the porphyry centers.
Most of the copper-gold mineralization at Cotabambas is associated with early-stage potassic alteration, including multiphase, magnetite-rich stockworks with quartz, K-feldspar, biotite, chalcopyrite, and bornite. Apatite and anhydrite are common constituents. An early, copper-bearing potassic-calcic assemblage made up of quartz, K-feldspar, biotite, actinolite, hornblende, diopside-hedenbergite, and magnetite is also preserved locally. Early, biotite-rich alteration from Ccalla yielded a K-Ar age of 35.7 ± 0.9 Ma. These mineral associations are overprinted by greenish-colored, intermediate argillic assemblages dominated by quartz, chlorite, illite, smectite, halloysite, and greenish soapy sericite, which have partially to completely destroyed earlier formed chalcopyrite-bornite associations but have contributed pyrite as disseminated grains and veinlets. These associations are spatially related to a series of inter- to late mineral porphyries.
All the previously described rocks and alteration assemblages are cut by a large, late mineral, domelike body of dacitic composition and an associated dike swarm. Both dome and dikes developed incipient alteration to calcite, illite, and chlorite and host a set of centimeter-wide veins with open spaces filled by quartz, calcite, sphalerite, and galena.
Supergene mineralization is present as patchily distributed copper oxides near surface underlain by an irregular supergene enrichment blanket dominated by sooty chalcocite. Supergene kaolinite and alunite are common. A K-Ar age of 3.3 ± 0.2 Ma on alunite shows a late Pliocene age for supergene enrichment and leaching processes in the region.
The mineralization at Cotabambas is assigned to the porphyry copper-gold class because (1) gold grades are typically >0.3 ppm; (2) copper-gold mineralization is accompanied by abundant (>5 vol %) hydrothermal magnetite in potassic alteration; (3) hydrothermal amphibole and pyroxene are present in potassic-calcic alteration; (4) copper and gold display a sympathetic relationship, and all observed Au occurs as micron-sized inclusions in chalcopyrite; (5) intense pyrite-rich intermediate argillic alteration overprints earlier formed potassic and calcic-potassic alteration and associated copper-gold mineralization; and (6) molybdenum contents are low (<0.01%).
The coincident K-Ar and fission-track (apatite) ages (38.6 ± 3.4 and 33.3 ±1.4 Ma) for plutons from the Cotabambas area, together with the late Eocene age of Cotabambas (see above) and regional data, confirm that porphyry copper emplacement in the Andahuaylas-Yauri belt of southeastern Peru took place simultaneously with intense shortening, surface uplift, and rapid exhumation during the middle to late Eocene Incaic orogeny.
