Abstract 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.

Abstract The Greenstone Resources Santa Rosa mine is located in central Panama about 200 km southwest of Panama City. The mine was the largest producer in Central America, but production was temporarily suspended in February 1999 with less than 5 million tonnes (Mt) of remaining reserves at a grade of 1.9 g/t Au ($300/oz). An integrated structural analysis was performed to determine the primary ore controls within the Santa Rosa deposit. The primary control is fault-related fracturing, particularly those fractures associated with the Santa Rosa fault. Although far less important, secondary controls relate to the permeability of the various pyroclastic, epiclastic, and intrusive host rocks. The methodology and results of the integrated structural analysis are described in the text to familiarize the reader with this technique. The application of the integrated structural analysis process was used to identify exploration characteristics that could be used for the field identification of exploration targets. The best set of exploration pathfinder elements were identified, visual field characteristics associated with ore mineralization were described, the optimum drill hole orientation was defined, and individual targets were identified and prioritized. Production issues were also addressed such as slope stability within the Alto de la Mina pit and grade control. Four separate deformation events were identified from a database collected from within the two pit areas and surrounding outcrop exposures. The earliest deformation event produced the Santa Rosa fault, and provided conduits for the intrusion of a porphyritic basalt stock and later latite dikes. Injection of the latite dikes adjacent to porous, wet volcaniclastic debris piles produced a series of funnel-shaped di-atreme pipes resulting from phreatomagmatic eruptions. Fragments of the latite are abundantly incorporated within the diatreme pipes Subsequent movement along the Santa Rosa fault produced additional steeply raking, strike-slip-controlled conduits for later mineralizing solutions during gangue and ore-metal deposition. The main quartz and sulfide deposition occurred during the second deformation event and the precious metals and associated pathfinder elements came into the system during the third deformation event. The fourth event was barren and offset the upper part of the orebody in the Alto de la Mina pit with reverse movement. Ore is typically found within the diatreme pipes or directly adjacent to the Santa Rosa fault plane in the basalt porphyry. With few exceptions ore is found within several tens of meters from the fault, but in one instance in the A zone of the Santa Rosa pit, ore extends to as much as 300 m from the main fault. This distal ore developed during the third deformation event where the fluids were conducted along an earlier reactivated set of northeast fractures created during the first deformation event. The ore produced here is lower grade than the ore adjacent to the main fault, presumably due to the enhanced fluid flow and resultant better availability of ore metals nearer the primary conduit. Strike-slip movement along the Santa Rosa fault dominated the kinematics of the first three deformation events. Inflections in the Santa Rosa fault conduit from its shear direction toward the extensional direction typify the ore zones in both pits. At these strike inflections, ore-related movement along the fault generates a zone of open conduits where associated extension fracture splits and cymoid loops enhance the permeability in the zones. The increased permeability along the fluid conduits controlled the amount of silica flooding. Gold in the form of electrum is often contained within very late crosscutting clear quartz stringers or adjacent to grains of pyrite. Where fluids deposited gold in nonsilicified sediments such as in the A zone orebody of the Santa Rosa pit, grades are significantly lower than the silicified, diatreme-hosted ore of the Alto de la Mina pit. The overall grade of the diatreme-hosted Alto de la Mina ore is higher than the volcaniclastic-hosted Santa Rosa ore at 2.0 g/t Au and 1.4 g/t Au, respectively.