Summary:
The Quellaveco deposit is porphyry copper-molybdenum orebody with an elongated mineralised zone. Primary sulphide mineralisation dominates the deposit and is successively overlain by a secondary supergene copper mineralisation blanket. This is followed by low grade copper-bearing oxides that are capped by barren ignimbrites. At least five stages of intrusion are recognised; the oldest intrusives correspond to regional granodiorite surrounding the main orebody. Three syn-mineralisation intrusions of monzonitic to dacitic composition host most of the mineralisation. All of these are cut by a suite of late post-mineral intrusives.
The Quellaveco deposit, located near the northern end of the Paleocene to early Eocene porphyry copper belt of the central Andes (Sillitoe, 1988; Clark et al., 1990), is centered on a multiphase quartz monzonite porphyry stock emplaced into an equigranular granodiorite pluton, which itself cuts rhyolitic volcanic rocks assigned to the Late Cretaceous to early Paleocene Toquepala Group (Estrada, 1975; Guerrero and Candiotti, 1979; Candiotti, 1995). The pluton, which crops out over >3 km2 in the vicinity of the Quellaveco stock, as well as extending northward beneath postmineral ignimbrite and gravel cover, is part of the granodioritic to monzogranitic Yarabamba superunit of the Coastal batholith (Pitcher, 1985). Phenocrysts in the porphyries are made up of abundant plagioclase, quartz, and biotite, plus scattered K-feldspar and hornblende, all surrounded by aplitic groundmass.
Potassic (biotite-K-feldspar) and sericitic (quartz-sericite) alteration, the former generally overprinted and partly obliterated by the latter, developed extensively throughout much of the quartz monzonite porphyry stock and immediately surrounding granodiorite, and both contain veinlet and disseminated chalcopyrite. The potassic alteration is transitional outward in the precursor granodiorite to a pyrite-bearing propylitic assemblage, which defines the periphery of the system (Estrada, 1975; Candiotti, 1995; Kihien, 1995). Fluid inclusion studies showed that the main potassic and sericitic alteration and associated copper mineralization took place at temperatures of 590° to 340°C (Kihien, 1995).