Chapter 13: Cerro Verde-Santa Rosa Copper-Molybdenum Deposits, Peru: Magmatic, Hydrothermal, and Supergene Characteristics of Two Adjacent Porphyry Systems
Ralph J. Stegen, Mark D. Barton, Jerome A. Waegli, 2018. "Cerro Verde-Santa Rosa Copper-Molybdenum Deposits, Peru: Magmatic, Hydrothermal, and Supergene Characteristics of Two Adjacent Porphyry Systems", Metals, Minerals, and Society, Antonio M. Arribas R., Jeffrey L. Mauk
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At the Cerro Verde district in southern Peru, granodiorite porphyry stocks formed two adjacent porphyry copper-molybdenum deposits that collectively form one of the largest copper districts in the world, with current resources of ~17 million metric tonnes (Mt) of copper. The district is located within the Coastal batholith of western Peru. The Coastal batholith in the Cerro Verde area consists of the Cretaceous Tiabaya and the Paleocene Yarabamba granodioritic plutons. Granodiorite porphyry stocks associated with the porphyry coppermolybdenum deposits were emplaced into the Yarabamba plutons.
The granodiorite porphyry stocks are composite, steep-walled cylinders. Breccia bodies of diverse textures are localized in the apical parts of both stocks. The breccia fragments are predominantly of stock composition proximal to the intrusion and grade outward to heterolithic fragments of stock and Yarabamba wall rocks. Hydrothermal matrix breccia bodies are widespread, situated in the outer areas of the breccia column, and contain tourmaline, chalcopyrite, and molybdenite within the matrix. New zircon U-Pb dating confirms and refines earlier work, indicating that magmatism and mineralization at Cerro Verde occurred about 61 Ma. The hypogene mineralization is bracketed by the Yarabamba batholith host (~62 Ma), well-mineralized stocks at Cerro Verde and Santa Rosa (~61 Ma), and postmineral plugs (~60–59 Ma). The U-Pb ages are consistent with all crosscutting relationships.
Each granodiorite stock is associated with similar sequences of alteration and mineralization. Biotite veinlets and halos containing chalcopyrite formed in the deeper areas of both deposits and are cut locally by later quartz veins and quartz-K-silicate veinlets containing chalcopyrite and molybdenite. Tourmaline-bearing sulfide veins with K-feldspar and chlorite envelopes form an inverted cup-shaped shell that overlaps the medial and upper parts of quartz-K-silicate veinlets. In distal positions, tourmaline veins contain sericite and are bordered by sericitic alteration. Quartz-sericite-pyrite veins and envelopes form an expansive stockwork in the upper part of the deposits and are transitional to and overprint K-silicate alteration.
The Cerro Verde and Santa Rosa stocks formed individual copper and molybdenum ore shells within K-silicate alteration that forms thick-walled cylinders in the medial and upper parts of the deposit. The shells merge at depth to form one NW-SE–oriented mass that is 4.6 × 1.6 km in size using a >0.2% Cu value. Copper grades of 0.7 to 0.4% result from chalcopyrite-dominant veinlets occurring with chlorite in quartz-K-silicate alteration and are localized proximal to the stocks. The copper grades into a zone of 0.4 to 0.2% Cu within the biotitealtered zone at depth. Two discrete molybdenum ore shells are contained within the copper ore shell and are located proximal to the granodiorite stocks. The highest abundance of molybdenite is inward toward the stock and is zoned outward to lower grades. Breccia pipes contain abundant chalcopyrite and molybdenite within the matrix and are the source for the highest-grade ores in the district. The pipes truncate the majority of the veins containing copper, molybdenum, and tourmaline veins and bottom within quartz-rich K-silicate veinlets.
Supergene mineralization consists of zones of leached capping, oxide copper mineralization, and an enriched chalcocite blanket developed above the copper ore shells within sericitic alteration. The oxide copper deposits contain isolated brochantite with chrysocolla in tourmaline breccia bodies situated above a laterally continuous and deposit-wide chalcocite enrichment blanket. Whereas molybdenum contents are little affected by supergene processes, copper and silver are generally leached from sericitically altered rocks and concentrated downward in the sulfide enrichment blanket by a factor of 1.5 to 2 compared to the subjacent protore. The oxide ores reflect in situ oxidation of a mature enrichment blanket hosted within rocks lacking abundant pyrite.
The proximal location and approximately synchronous formation of the Cerro Verde and Santa Rosa porphyry copper-molybdenum deposits formed an expansive K-silicate alteration system and related ore shells that encompass both intrusive centers. In detail, multiple episodes of hydrothermal alteration and metal introduction can be inferred spanning no more than about 1 m.y. Although hydrothermal activity began with the emplacement of the Yarabamba granodiorite, most metals were introduced with the composite Cerro Verde-Santa Rosa stocks, and activity waned with formation of late breccias and ceased in late barren porphyries that truncate ore. These patterns are quite similar to those of other giant Andean porphyry systems, notably El Salvador, Los Pelambres, and Toquepala.