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Response of Supergene Processes to Episodic Cenozoic Uplift, Pediment Erosion, and Ignimbrite Eruption in the Porphyry Copper Province of Southern Perú

Chan X. Quang, Alan H. Clark and James K. W. Lee

Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6

Nicholas Hawkes

Rio Tinto Mining and Exploration Limited, Manco Cápac 551 - Lima 18, Perú

View larger version (21K): [in a new window]   FIG. 1. Locations of the Cerro Verde-Santa Rosa, Cuajone, and Toquepala mines and other porphyry copper deposits and prospects of southern Perú and northern Chile cited in the text. Geochronologic data for hypogene mineralization: 1 = Quang et al. (2003); 2 = Clark et al. (1990b), 3 = Clark (2003); 4 = Minera Quellaveco, S.A. (unpub. data), 5 = Zweng and Clark (1995) and Clark (2003); 6 = Quirt et al. (1971), 7 = Bouzari and Clark (2002), 8 = Reynolds et al. (1998); 9 = Ballard et al. (2001), 10 = Rowland and Clark (2001), 11. A.H. Clark, unpublished report to Minera Escondida Ltda. (2001), and 12 = Gustafson et al. (2001).

View larger version (35K): [in a new window]   FIG. 2. Summary of previously proposed landform and supergene chronologies in northern Chile and southern Perú. References for northern Chile: A.H. Clark, unpub. short course notes for Rio Tinto Mining and Exploration Ltd.: Mendoza (1999), Nishiizumi et al. (1998), Mortimer (1973), Sillitoe et al. (1968), and Clark et al. (1967a-b), and for southern Peru: Tosdal (1978), Tosdal et al. (1981, 1984), and Clark et al. (1990b)

View larger version (58K): [in a new window]   FIG. 3. Major physiographic units of the Cordillera Occidental, southern Perú, showing the locations of dated tuffs documented in earlier studies and the distribution of Tertiary landform elements within the limits of geomorphologic mapping by Tosdal (1978; simplified after Tosdal et al., 1984). Dated minerals: bi = biotite, hb = hornblende, pl = plagioclase.

View larger version (59K): [in a new window]   FIG. 4. Late Tertiary stratigraphy of southern Perú (Tosdal et al., 1981). Solid circles indicate the approximate locations of dated volcanic rocks, the ages of which are shown on the right (in Ma). Minerals dated: bi = biotite and pl = plagioclase. The K-Ar dates are from Tosdal et al. (1981). Relative positions of Moquegua landforms are indicated in the left-hand column. AdC = Altos de Camilaca, CdlC = Cerro de las Chulpas, CS = Cerro Sagollo, PL = Pampa Lagunas, PS = Pampa Sitana.

View larger version (38K): [in a new window]   FIG. 5. 40Ar-39Ar step-heating spectra for ignimbrite samples from the Moquegua-Ataspaca transect dated in earlier studies (Table 1). Min = minimum age, PA = plateau age.

View larger version (58K): [in a new window]   FIG. 6. Major physiographic units of the Cordillera Occidental, southern Perú, showing the locations of the newly studied ash-flow tuffs and the landform profiles in Figure 10.

View larger version (28K): [in a new window]   FIG. 7. 40Ar-39Ar step-heating spectra for newly studied ash-flow tuffs (Table 2). Min = minimum age, PA = plateau age.

View larger version (34K): [in a new window]   FIG. 8. (a). Simplified topographic map of the Quellaveco prospect area, showing locations of dated ignimbrite flows on the northern flank of the Río Asana valley. The northwest-southeast (A-A') line indicates the approximate location of the cross section. (b). Schematic northwest-southeast cross section along the axis of the Quellaveco orebody, illustrating the interrelationships of present topography, dated Miocene ash-flow tuffs, inferred Pampa Sitana terrace pediment, and the Quellaveco sulfide enrichment zone (modified after Clark et al., 1990b). Vertical scale is exaggerated.

View larger version (23K): [in a new window]   FIG. 9. Comparison of the revised landform chronology with that proposed by Tosdal et al. (1984).

View larger version (36K): [in a new window]   FIG. 10. Schematic cross sections, showing the landform profiles for the selected areas of the Pacific slope between Moquegua and La Joya (see Fig. 6). (a). Moquegua valley. (b). Yaral prospect area. (c). Pampa Guaneros. (d). Desierto de Clemesí. (e). Pampa Colorada. (f). Cocachacra. (g). Pampa de La Joya. Pediments are assigned to stages I through V on the basis of the chronology established for the Moquegua area (Fig. 9). Actual or projected locations of mines and prospects are shown.

View larger version (114K): [in a new window]   FIG. 11. (a). Panorama looking south from Cerro Baúl. The Pampa Lagunas pediplain (PL) forms the prominent surface stretching across the llanuras costaneras, whereas the precordillera (on the extreme left and beyond the photo) is dominated by the Altos de Camilaca (AdC) pediplain. A discontinuous, residual bench just below the Pampa Lagunas pediplan represents the Cerro de las Chulpas (DdlC) pediment, which is better preserved in the lower llanuras costaneras. The younger Pampa Sitana (PS) pediment forms dissected ridge spurs below the Pampa Lagunas pediplain, and benches just above the Moquegua valley are remnants of the Cerro Sagollo (CS) pediment. The summits of the Cordillera de la Costa, scarcely visible in the far-right background, are remnants of the Altos de Camilaca (AdC) surface incised by the Pampa Lagunas pediplain. (b). Panorama looking southeast across Quebrada Guaneros (Fig. 6). (Pampa de Congas pediment = PdC; Pampa Guaneros pediment = PG) (c). Panorama looking northwest across the La Llave prospect. (La Caldera surface = LC; Pampa de La Joya = PLJ).

View larger version (109K): [in a new window]   FIG. 12. Geomorphologic map, showing the distribution of Tertiary landforms in the study area. The precordillera and llanuras constaneras are dominated by, respectively, the Altos de Camilaca (dark gray) and Pampa Lagunas (light gray) pediplains and their correlatives (numbers in brackets refer to stages in Fig. 9). In contrast, the areally restricted landforms of the Multiple Pediment stage (Fig. 2), comprising the Cerro de las Chulpas, Pampa Sitana, and Cerro Sagollo pediments and their correlatives, are concentrated along throughgoing rivers. The relationships between the landforms of the Multiple Pediment stage and remnants of the Altos de Camilaca and Pampa Lagunas pediplains in the Moquegua-Pampa Guanero transect are shown in greater detail in an inset at the top right of the figure, whereas those in the area around the Cachuyo and Cachuyito prospects, north-northeast of Cocachacra, are shown in an inset at the top center.

View larger version (55K): [in a new window]   FIG. 13. Map showing the locations of dated supergene alunite group samples from Cu deposits and prospects in southern Perú (Table 4).

View larger version (24K): [in a new window]   FIG. 14. 40Ar-39Ar step-heating spectra for supergene alunite group minerals (Table 4). Min = minimum age, PA = plateau age, TPA = "technical plateau" age (comprising 3 or more steps but less than 50% of total 39Ar released).

View larger version (63K): [in a new window]   FIG. 15. Simplified schematic cross sections, showing the geomorphologic setting and age of supergene mineralization in selected profiles from the porphyry copper belt of southern Perú. Toquepala and Cerro Verde exhibit complex profiles with multiple chalcocite horizons, reflecting polystage histories of leaching, oxidation, and enrichment. Alunite group mineral dates in the Cerro Verde deposit and Angostura and Posco prospects indicate that supergene activity began in the late Eocene beneath a regional, subplanar topography. Late Oligocene supergene activity is recorded in the Cerro Verde-Santa Rosa district and the La Llave prospect beneath the La Caldera surface, a correlative of the 24 Ma Altos de Camilaca surface, which is considered to have controlled late Oligocene leaching and enrichment in the Cuajone-Toquepala district. Deepening of supergene profiles during the early Miocene is considered to have been controlled by the Pampa Lagunas pediplain and its correlatives. The development of the Multiple Pediment stage eroded preexisting profiles over wide areas but was responsible for continued supergene activity at the Cerro Verde and Toquepala deposits, prior to the onset of hyperaridity in the late Miocene.

View larger version (21K): [in a new window]   FIG. 16. Schematic northeast-southwest profile of the Pacific slope in the porphyry copper belt of southern Perú, showing the relative positions of the documented landform surfaces: the Altos de Camilaca (I) and Pampa Lagunas (II) pediplains and equivalents, the Cerro de las Chulpas (III), Pampa Sitana (IV), and Cerro Sagollo (V) pediments and equivalents. The landform chronology is constrained by 40Ar-39Ar dates for Tertiary ignimbrite ash flows, and the age of supergene mineralization is summarized by 40Ar-39Ar dates for alunite group minerals for mines (solid black square) or deposits and prospects (light gray square).