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Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario, Canada K7L 3N6
Rio Tinto Mining and Exploration Limited, Manco Cápac 551 - Lima 18, Perú
Corresponding author: e-mail, charbon{at}geol.queensu.ca
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Abstract |
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 Top Abstract IntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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The landform chronology for the area surrounding the Cuajone, Quellaveco, and Toquepala deposits (ca. 17° S) is revised and extended northwestward through field mapping to the Cerro Verde-Santa Rosa district (ca. 16° 30' S). The 40Ar-39Ar incremental-heating dates of supergene alunite group minerals from the Angostura (38.1 and 38.8 Ma) and Posco (38.8 Ma) prospects and the Cerro Verde deposit (36.1–38.8 Ma) demonstrate that supergene processes were underway in the late Eocene beneath a subplanar topography resulting from uplift and erosion during the Incaic orogeny, now represented by a regional unconformity in the Cenozoic volcanic-sedimentary rock succession. Broadly contemporaneous supergene processes were probably active in the Cuajone-Quellaveco-Toquepala district. Slow erosion and the accumulation of clastic sediments through the tectonically quiescent early to mid-Oligocene are envisaged to have caused a rise in the water table and the widespread preservation of the Incaic supergene profiles. Aymará uplift subsequently led to the incision of the 23.8 to 24 Ma Altos de Camilaca and the 18.8 to 19.1 Ma Pampa Lagunas pediplains and their regional correlatives. The ensuing water-table lowering was associated with intense leaching and sulfide enrichment from the late Oligocene (24.4–28 Ma natroalunite at Cerro Verde, 26–27 Ma natroalunite at Santa Rosa, and 28.6 Ma jarosite at La Llave) to the early Miocene (23 Ma alunite and 21 Ma natroalunite at Cerro Verde, and 19.2 Ma jarosite at La Llave) and was plausibly responsible for much of the upgrading of the Cuajone and Toquepala deposits and thr Quellaveco prospect, which are intersected by both the Altos de Camilaca pediplain and erosional features representing upslope extensions of the Pampa Lagunas pediplain. The younger supergene profiles were widely superimposed on the remnants of those generated during the Incaic orogeny. Middle Miocene (<14.2 Ma biotite age) Chuntacala Formation flows protected the Cuajone supergene profile from destruction by erosion, but at 13.0 Ma interrupted supergene processes at Quellaveco. Revision of volcano-stratigraphic relationships in the latter area reveals that subsequent erosion of the Chuntacala Formation ignimbrites and part of the supergene profile took place prior to the deposition of a 10.1 Ma ash-flow tuff of the Asana Formation. Elsewhere, supergene activity persisted at the Cachuyito prospect through 11.4 Ma, and minor jarosite development occurred at least until 4.9 Ma both there and at Cerro Verde during and following the Multiple Pediment landform stage (ca. 7.9–15.0 Ma).
The occurrence of relics of late Eocene alunite group minerals within considerably younger late Oligocene to late Miocene supergene alteration profiles suggests that the overall physiographic configuration of the Pacific piedmont of southern Perú remained remarkably consistent from the late Eocene to the middle Miocene. Moreover, the new age data confirm that, as in northern Chile, semiarid climatic conditions prevailed along much of the plate boundary from the mid-Eocene until the late Miocene or early Pliocene onset of hyperaridity.
The local geomorphologic and volcanic conditions in southern Perú, however, conspired to generate more complex supergene profiles with lower aggregate enrichment factors relative to the strongly enriched profiles in the late Eocene to early Oligocene porphyry copper belt of northern Chile, which underwent supergene upgrading over relatively brief periods.
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Introduction |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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From the standpoint of mineral deposits geology, the tectonic and concomitant physiographic evolution of the oceanward flank of the cordillera since ca. 58 Ma assumes importance through its influence on the supergene modification of the major upper Paleocene to middle Eocene and upper Eocene to lower Oligocene porphyry copper deposits of the region. The former constitute a narrow metallogenic subprovince (Fig. 1) extending in southern Perú from the 61 to 62 Ma (Quang et al., 2003) Cerro Verde-Santa Rosa cluster in the north (lat. 16°33’ S) to the Cuajone (52.4 Ma; Clark et al., 1990a; Clark, 2003)-Quellaveco (54.5 Ma; Minera Quellaveco S.A., unpub. data)-Toquepala (55.0–55.5 Ma; Zweng and Clark, 1995; Clark, 2003) district at ca. 17° S. An almost uninterrupted Neogene ignimbrite cover mantles the southern extension of this belt in northernmost Chile, but its probable continuity is emphasized by the 58 Ma (Quirt et al., 1971) Mocha prospect (19°49’ S) and the 51.8 Ma Cerro Colorado deposit (20°03’ S), the latter exhibiting a thick and complex supergene profile (Bouzari and Clark, 2002). In southern Perú, Clark et al. (1990a), Quang (2003), and Quang et al. (in prep.) further record the emplacement in the Cordillera de la Costa of both Middle Jurassic and Upper Cretaceous porphyry copper and associated Cu-Au vein systems, similarly affected by middle to late Cenozoic supergene modification.
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A faceted, multiphase, Neogene landscape, dominated by pediments, is extensively preserved under the hyperarid climatic conditions now prevailing below ca. 3,000 m a.s.l. in this segment of the Atacama-Peruvian coastal desert. The Neogene landforms are particularly well represented in the areas surrounding the valleys of the Ríos Moquegua and Locumba (Tosdal, 1978; Tosdal et al., 1984), where they constitute the llanuras costaneras, or coastal slopes. In this area, moreover, the ages of formation of many of the pediments, and hence the wider landform chronology, may be established through geochronologic study of numerous superincumbent and broadly coeval ash-flow tuff units (Tosdal et al., 1981). These relationships provide a temporal framework for interpretation of the supergene profiles in the Mesozoic and Paleogene porphyry copper centers, although the strictly geomorphologic record extends back only to the latest Oligocene (ca. 23–24 Ma), and the earlier Incaic events must therefore be deduced from tectonostratigraphic relationships and geochronology. The model relating supergene processes and landforms (Tosdal, 1978; Clark et al., 1990b) followed that originally proposed for the Copiapó mining district of the southern Atacama Desert (Fig. 1) by Clark et al. (1967a- b), Sillitoe et al. (1968), and Mortimer (1973), who inferred that episodes of leaching, oxidation, and enrichment of Jurassic to Paleogene copper deposits occurred prior to the late Miocene, under semiarid climatic conditions beneath regionally extensive pediments, and were stimulated by drastic and abrupt descents in the water table caused by uplift and resultant pediment erosion (Fig. 2).
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Previous Model for Landform Chronology and Supergene Profile Evolution in Southern Perú |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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Tosdal (1978) and, in more detail, Clark et al. (1990b) applied the landform relationships described by Tosdal et al. (1984) in developing models for the evolution of supergene profiles in the neighboring variably enriched Cuajone, Quellaveco, and Toquepala porphyry copper deposits (Fig. 3). Chalcocite development at Cuajone was inferred to have occurred during stage 1 uplift and pediplanation in the latest Oligocene and to have been terminated at 22.8 Ma when blanketed by a thick Huaylillas Formation ignimbrite flow. Supergene enrichment at Quellaveco, which is more extensive than at Cuajone, is considered to have commenced beneath the developing Altos de Camilaca surface in the earliest Miocene but to have been interrupted by the deposition of an 18.1 Ma Huaylillas Formation ignimbrite. Following the incision of an open valley at the onset of the Multiple Pediment stage, the chalcocite horizon was deepened in the middle Miocene (Clark et al., 1990b), before accumulation of Chuntacala Formation (Fig. 4) ignimbrites and lahars at 13.1 Ma. At Toquepala, a vertically extensive, multilevel enrichment profile was developed (Anderson, 1982; Clark et al., 1990b), but Neogene ignimbrites are not preserved in the mine area, precluding assignment of direct age constraints for the different supergene episodes. However, the highest altitude chalcocite zones were inferred to have formed during the late Oligocene or early Miocene below the Altos de Camilaca surface, whereas the deepest and most important chalcocite horizon was tentatively correlated with the earliest part of the Multiple Pediment stage.
Two major episodes of supergene enrichment were therefore proposed by Clark et al. (1990b) for the Toquepala-Cuajone district, one in the late Oligocene to early Miocene, associated with the Altos de Camilaca surface, and the second in the middle Miocene, associated mainly with the Pampa Lagunas pediment and hence with the earlier erosional events of the Multiple Pediment stage (Fig. 2). No geochronologic data are available for supergene minerals in these deposits.
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Geochronology of Ignimbrite Flows |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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. Most of the new age determinations are
concordant, within 2 error, with the previous dates, but some
are significantly older or younger, with implications for the ages of eruption
and, hence, of erosional events.
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Four ash-flow tuffs in the precordillera gave ages slightly older than the reported K-Ar dates (Table 1). A welded tuff (SPT-127f) that lies unconformably on the eroded Upper Cretaceous-Paleogene arc terrane gave a plateau age of 24.0 ± 0.8 Ma (Fig. 5d). This flow is assigned to the Moquegua Formation because it falls in the age range of 23.1 to 25.5 Ma determined for ignimbrite flows intercalated with coarse clastics near the top of the upper member of that formation. Three semiwelded porphyritic tuffs (SP-85, SPT-124, SPT-305, Fig. 5) from a higher stratigraphic position relative to SPT-127f gave plateau ages of 18.8 to 19.1 Ma (Table 1). These three lower Miocene flows are assigned to the Huaylillas Formation (Fig. 4), which comprises regionally extensive ignimbrite sheets that blanket the eroded Upper Cretaceous to Paleogene strata in the precordillera (Tosdal et al., 1981) and overlie a sequence of conglomerates in the llanuras costaneras mapped as the Moquegua Formation by Bellido and Landa (1965).
Sample SPT-211, a semiwelded, pumiceous, porphyritic tuff deposited in a channel incised into the tilted Moquegua Formation (Fig. 3), gave a disturbed age spectrum (Fig. 5h) that shows partial resetting. The high atmospheric argon to radiogenic argon ratio (Fig. 5i) reflects the chloritization apparent in some of the biotite grains. Elsewhere, an unconsolidated tuff (SPT-330, Fig. 3) intercalated with alluvial gravels yielded a disturbed age spectrum showing recoil effects (Fig. 5j) for biotite with interlaminated chlorite.
Four middle Miocene tuffs intercalated with coarse gravels deposited in channels incised into the Moquegua and Huaylillas Formations (Tosdal et al., 1981) also gave 40Ar-39Ar ages that are slightly older than the previously reported K-Ar dates (Table 1). Sample SP-65, a semiwelded porphyritic tuff exposed ca. 5 km east of Quellaveco, gave a plateau age of 14.8 ± 0.5 Ma (Fig. 5k), whereas a slightly welded, pumiceous, porphyritic tuff (SP-73) in Quebrada Chuntacala gave a plateau age of 14.2 ± 0.2 Ma (Fig. 5l). These areally restricted Chuntacala Formation ignimbrites (Fig. 4) from the type locality define a narrow age range of 14.2 to 14.8 Ma. Two unconsolidated, porphyritic tuffs (SPT-323 and SPT 324) located, respectively, in a valley incised into Moquegua Formation sediments and intercalated with alluvial gravels on Pampa Sitana (Fig. 3), yielded identical ca. 10 Ma plateau ages (Fig. 5m-n). An unconsolidated tuff (SPT 308) yielded an integrated age of 9.6 ± 0.5 Ma from a disturbed age spectrum (Fig. 5o) for a biotite separate with inclusions of apatite and zircon and fragments of hornblende, plausibly responsible for the increased Ca/K ratios in the highest temperature steps (Fig. 5p). In addition, the ratio of atmospheric argon to radiogenic argon is significantly higher than for other samples. Thus, the 8.9 Ma K-Ar date reported by Tosdal et al. (1981) for this sample is considered to be unreliable.
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A semiwelded porphyritic tuff (S-16b) ca. 6 km east of Moquegua gave a four-step plateau age of 23.8 ± 0.5 Ma (Fig. 7a). The other four samples from this area are from a sequence of ignimbrites and conglomerates overlying the leached zone of the Quellaveco prospect (Fig. 8). An unwelded, pumiceous, porphyritic tuff (S-40) from near the base of the thick volcanic sequence that overlies the northern half of the deposit (Fig. 8b) yielded a seven-step plateau age of 13.0 ± 0.4 Ma (Fig. 7b). A five-step plateau age of 10.1 ± 0.3 Ma (Fig. 7c) was obtained from an unconsolidated ash-flow tuff (S-38: the "white tuff" of Clark et al., 1990b), exposed ca. 40 m lower than S-40 (Fig. 8b) and discontinuously overlying the Quellaveco Conglomerate, a local accumulation dominated by hematitic, leached cobbles.
Two unwelded, pumiceous, porphyritic tuffs (S-37 and S-41), probably representing a single flow that is discontinuously exposed (Fig. 8), gave identical plateau ages of 7.9 Ma (Fig. 7d-e). This volcanic flow is assigned to the Sencca Formation of Tosdal et al. (1981; Fig. 4).
Farther to the northwest, an unwelded, unconsolidated tuff (S-30) exposed in an arroyo between the Desierto de Clemesí and Quebrada Guaneros (Fig. 6) yielded a four-step plateau age of 10.5 ± 0.2 Ma (Fig. 7f) and is thus assigned to the Asana Formation (Fig. 4).
Three tuffs (S-2, S-59, S-60) from the wider La Joya area (Fig. 6) gave identical ca. 5 Ma plateau ages (Fig. 7g-i) and are assigned to the La Joya Formation (Fig. 4).
A revised stratigraphic column for the Oligocene to Pliocene volcanic and sedimentary units in littoral southern Perú is summarized in Figure 4 (cf. fig. 2 of Tosdal et al., 1981).
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Revised Landform Model |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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The landform chronology for littoral southern Perú requires revision in the light of these data and the improved understanding of the stratigraphic relationships. The proposed modifications mainly concern the earlier erosional events and are shown in Figure 9. Whereas Tosdal (1978) interpreted the Huaylillas Formation ignimbrites (i.e., SPT-124 and SPT-305) as providing a minimum age of 18.3 to 18.4 Ma (K-Ar dates; Table 1) for development of the Altos de Camilaca surface (I), the age of this major erosional feature is now temporally constrained by coeval dates of 23.8 Ma (S-16b) and 24.0 Ma (SPT-127f) for tuffs assigned to the Moquegua Formation (Table 1). The relationships between the Altos de Camilaca surface (I) and the Pampa Lagunas (II) and younger surfaces are shown in Figure 10a, in which ignimbrites (SP-85, SPT-124, and SPT-305) of the Huaylillas Formation (Fig. 4) dated at 18.8 to 19.1 Ma are correlated with erosion resulting in the development of the Pampa Lagunas surface (II; Fig. 10a). In contrast, Tosdal et al. (1984) inferred an age range of 11.2 to 14.2 Ma for the Pampa Lagunas erosional event on the basis of two K-Ar dates (SPT-211 and SPT-308, Table 1). Our analyses of biotite from these tuffs revealed disturbed age spectra (Fig. 5h, i, o, p) which indicate that neither date is reliable.
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The revised landform chronology for the wider Moquegua region shown in Figures 9 and 10a emphasizes the elimination of an extended early Miocene hiatus between the Altos de Camilaca surface (I) and the Multiple Pediment stage (cf. Tosdal et al., 1984).
Ilo, Clemesí, Tambo, and La Joya areas
The erosional landforms of the Moquegua area have been traced continuously
through field mapping southwest to the Ilo area, and northwestward across the
Desierto de Clemesí and the Río Tambo valley to the La Joya area (Fig. 6).
Geomorphologic relationships in this region are based on the relative positions
of local landforms with respect to the mapped continuous surfaces and are
illustrated in a series of cross sections (Fig. 10), the locations of which are
shown in Figure 6. In each area, planar landforms are given local geographic
names, which can be correlated with corresponding features in the Moquegua area
(Table 3; Figs. 10a, 11a). The ca. 4500-km2 region covered by Figure
12 experienced a consistent history of erosional events. For ease of comparison,
the erosional features are assigned to stages I through V, corresponding to the
Altos de Camilaca, Pampa Lagunas, Cerro de las Chulpas, Pampa Sitana, and Cerro
Sagollo surfaces in the Moquegua area.
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The Cerro Sagollo surface extends west from the Moquegua valley to the base of Cerros Homo, the accordant summits of which are remnants of the Altos de Camilaca surface (Figs. 10c, 11b; Table 3). A prominent bench below the summit of Cerros Homo, immediately above the Cerro Sagollo surface, corresponds to the Pampa Sitana surface (Figs. 10c, 11b). The Pampa Guaneros and Pampa de Congas surfaces (Fig. 10c) are correlated, respectively, with the Cerro Sagollo and Pampa Sitana surfaces (Fig. 10a). A 10.5 ± 0.2 Ma ash flow (S-30) intercalated with clastic sediments in the aggradational facies of the Pampa de Congas surface demonstrates that erosion was active at that time (Fig. 10c).
A southwest-northeast cross section (Fig. 10d) of the Clemesí area is dominated by the well-preserved Desierto de Clemesí surface which, in agreement with Laharie (1976), we correlate with the Pampa Lagunas surface (Fig. 10a; cf. Tosdal et al., 1984). Remnants of the older Altos de Camilaca surface occur in the northeastern sector of the region and in the Cordillera de la Costa (Fig. 10d). The younger Quebrada Chayanto surface (Fig. 10d) is correlated with the Cerro de las Chulpas surface (Fig. 10a).
An east-west cross section (Fig. 10e) illustrates the landform relationships farther to the northwest, between the Clemesí area and the Río Tambo valley. The Pampa Colorada surface (Fig. 10e) is correlated with the Pampa Sitana surface (Fig. 10a). The Cerro de las Chulpas surface is here represented by eroded remnants (Fig. 10e). The Pampa Pacheco and Pampa Yamayo surfaces (Fig. 10e) are both correlated with the Cerro Sagollo surface (Fig. 10a). The summit of Cerro Buena Vista (Fig. 10e-f) is interpreted as a remnant of the Pampa Lagunas surface and is incised by benches that are correlated with the Pampa Sitana surface (Fig. 10a).
A west-northwest–east-southeast cross section (Fig. 10f) shows the landform relationships in the area between the western bank of the Río Tambo valley and the La Joya area. The rounded summit of Cerro Bronce (Fig. 10f) is correlated with the Altos de Camilaca surface and the areally extensive Pampa de La Joya surface (Fig. 10f) with the Pampa Lagunas surface (Fig. 10a). A minimum age for the Pampa de La Joya surface is delimited by three 4.9 to 5.0 Ma ash-flow tuffs (S-2, S-59, and S-60, Table 2). The La Caldera surface (Figs. 10g, 11c), represented by accordant summits in the precordillera (e.g., Cerro Verde, Cerro Negro, Cerro San Martín), as well as eroded remnants (e.g., Cerro Nieves) standing above the Pampa de La Joya surface, is correlated with the Altos de Camilaca surface (Fig. 10a).
Geomorphologic mapping therefore reveals that, as in the Moquegua-Locumba transect, upper Oligocene and lower Miocene surfaces are the dominant landform elements in the Ilo, Clemesí, Tambo, and La Joya areas (Fig. 12). The two older, regionally extensive, planar landforms, assigned to stages I and II, may thus be classified as pediplains, and it is evident that the most intense erosional dissection of the Pacific slope of the Cordillera Occidental was initiated at 25 to 26 Ma and completed by 18 Ma. Subsequently, erosion was focused around the exorheic river systems (Fig. 12). We therefore recommend the restriction of the Multiple Pediment stage (Tosdal et al., 1984) to the Cerro de las Chulpas (III), Pampa Sitana (IV), and Cerro Sagollo (V) pediments and their correlatives to the northwest (Table 3). The revised landform chronology (Fig. 9) and new geomorphologic map (Fig. 12) provide a context in which to examine the age and evolution of supergene processes in littoral southern Perú.
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Supergene Profiles of Porphyry Copper Deposits and Prospects |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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Prospects from the La Joya to the Ilo area
Six porphyry-style centers that exhibit leached and oxidized profiles have
recently been assessed in the La Joya, Río Tambo, and Ilo areas (Fig. 15).
Leached assemblages rich in hematite and with minor copper oxide and jarosite at the 68 Ma Angostura prospect are truncated by the Pampa de La Joya surface (Figs. 13, 15). Supergene jarosite (S-96) and natroalunite (S-98) from Angostura gave disturbed age spectra interpreted as evidence for leaching at 38.1 to 38.9 Ma (Fig. 14a-b).
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Supergene mineralization in the pyrite-poor, 166 Ma, Cachuyo porphyry copper prospect (Fig. 13) comprises copper and iron oxides that extend vertically for over 100 m and overlie a weak chalcocite horizon (Fig. 15; Andrews et al., 2000). The oxide zone underlies a goethite-hematite leached zone preserved below a remnant of the Altos de Camilaca surface (Cerro Bronce, Fig. 10f). The Pampa de La Joya surface truncates both the mineralized oxide and leached zones (Fig. 15).
Five kilometers east-northeast of Cachuyo, copper oxides associated with a weakly mineralized stockwork in biotitized andesite (Liassic Chocolate Formation) crop out on the Pampa Colorada surface at the 160 Ma Cachuyito prospect (Fig. 13). Supergene natrojarosite (S-12) and jarosite (S-13) from a leached outcrop ca. 500 m to the southeast gave 11.4 and 4.9 Ma dates (Fig. 14f-g). The 11.4 Ma natrojarosite veins are interpreted to have formed beneath a correlative of the Cerro de las Chulpas surface in Figure 10e (e.g., Lomillas del Toro surface, Fig. 15). The 4.9 Ma jarosite veinlets are interpreted as recording local oxidation during the Valley and Terrace stage (VI).
The hematite-goethite leached zone of the undated Posco prospect (Fig. 13), exposed below the Pampa Colorada surface, exhibits abundant hematite-coated fractures and quartz stockwork with an alteration assemblage of quartz-sericite-clay and relict disseminated pyrite. Supergene alunite veins from the leached capping at Posco gave a plateau age of 38.8 Ma (Fig. 14c).
Supergene copper oxide mineralization in the 106 Ma Yaral stockwork of the Ilo area is incised by the Pampa Sitana surface, whereas a higher elevation, hematite-dominated, leached zone associated with minor copper oxides occurs below the Pampa Lagunas surface (Figs. 13, 15).
Cuajone-Quellaveco-Toquepala district
The models proposed by Clark et al. (1990b) relating supergene profile
evolution in the Cuajone-Quellaveco-Toquepala district to local landform
development and the extent of Neogene ignimbrite cover are reproduced in Figure
15, but with age revisions resulting from the new 40Ar-39Ar
ignimbrite dates.
Sulfide enrichment at Cuajone was apparently confined to a single gently southwest-dipping blanket with an average thickness of ca. 20 m and an average grade of 1.5 percent Cu, underlain by a transitional zone characterized by coatings of sooty chalcocite on hypogene sulfides (Manrique and Plazolles, 1975; Clark et al., 1990b; A.H. Clark, unpub. report for Southern Perú Copper Corporation, 2001). The chalcocite zone passes upward into an oxide zone, which is itself overlain by a hematitic leached zone. Widespread remnants of supergene sulfides in the oxide and leached zones are evidence for the oxidation and leaching of preexisting chalcocite. The 23.8 to 24.0 Ma Altos de Camilaca surface truncates the leached zone on the southern benches of the Cuajone open pit (Fig. 15). Subsequently, renewed erosion incised the Altos de Camilaca surface, removing ignimbrites, the leached zone, and the upper part of the oxide zone. This episode of uplift and erosion was associated with the 18.8 to 19.1 Ma Pampa Lagunas pediment (Fig. 15). Development of a broad valley, correlated with the Cerro de las Chulpas surface, eroded the overlying ignimbrites and truncated the leached, oxide, and chalcocite enrichment zones in the northern part of the deposit. Deposition in this valley of the middle Miocene Chuntacala Formation ignimbrite prevented further erosion of the supergene profile (Fig. 15).
Supergene sulfide enrichment at Quellaveco reaches depths of 250 to 300 m below the present surface (Clark et al., 1990b). An irregular chalcocite blanket with a troughlike cross section and a thickness of 50 to 60 m comprises a thicker, upper zone of moderate to strong enrichment overlying a thinner, lower grade zone representing a downward transition to unaltered hypogene assemblages (cf. fig. 12 in Clark et al., 1990b). The enrichment blanket is overlain by a hematite-bearing leached-oxidized zone, extending from the surface to depths of up to 80 m. Middle Miocene Chuntacala Formation ignimbrites were deposited on a correlative of the Cerro de las Chulpas surface that had incised the leached zone (Figs. 8, 15), effectively terminating supergene activity. This sequence is beveled by a valley bench (Fig. 8), correlated with the Pampa Sitana surface, and overlain by an upper Miocene unwelded white tuff (S-40; Fig. 15). Incision of the supergene profile during the development of the presently active Asana valley, correlated with the Valley and Terrace stage, is inferred to have taken place after the deposition of a 7.9 Ma ignimbrite (Fig. 15).
The Toquepala deposit exhibits the most extensive record of supergene sulfide enrichment in the region, with well-defined hematitic and minor jarositic leached zones separating chalcocite horizons (Fig. 15; Clark et al., 1990b). The absence of ignimbrites in the vicinity of Toquepala probably contributed to the development of a thicker supergene profile but precluded direct age constraints for the landforms that controlled subjacent supergene activity. The highest occurrence of relict supergene chalcocite, however, is documented at the 3,535-m level, 40 to 70 m below the accordant summits of the precordillera, correlated with the Altos de Camilaca surface (I).
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Integrated Model of Landscape Evolution and Supergene Development |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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The above geochronologic database establishes a late Eocene to early Oligocene episode of leaching and enrichment that affected Mesozoic to middle Eocene deposits throughout the central-central Andes. The Incaic landscape and, presumably, much of its associated enrichment have been destroyed by subsequent uplift and erosion, as in the Cerro Verde-Santa Rosa and the Cuajone-Toquepala districts (Figs. 15–16). As argued by Bouzari and Clark (2002), semiarid conditions and deep water tables must have been imposed by 40 to 45 Ma, at least between 16° and 27° S. Such a climate would also have favored pediment erosion in an uplifting terrane. An even earlier episode of supergene activity was proposed by Clark et al. (1967a) and Sillitoe et al. (1968), who argued that enrichment of Mesozoic and Paleocene copper deposits in the Copiapó area (Fig. 1) took place beneath a postulated "Cumbre" (summit) surface. Sillitoe (1969) noted that thick enrichment zones are developed only in deposits now located 1 km or more from erosional remnants of the 53 Ma ignimbrites (Cerro La Peineta Formation) and inferred that their eruption terminated the enrichment. However, remapping by Arévalo et al. (1994) demonstrated that the Eocene ash flows were deposited on downdropped caldera floors (Hornitos hemigraben) and that the landforms below the ignimbrite do not therefore represent remnants of a regional erosion surface.
Late Oligocene
The tectonically quiescent post-Incaic interval (ca. 28–38 Ma; Sandeman et
al., 1995) was presumably dominated by slow relief reduction as the
Cretaceous-Paleogene arcs were further eroded to generate the forearc molassic
sediments of the lower member of the Moquegua Formation in southern Perú
(Marocco and Noblet, 1990). The mid-late Oligocene Aymará (ca. 25–28 Ma)
tectonic event, widely recognized throughout the central Andes (Tosdal et al.,
1984; Sébrier et al., 1988; Sébrier and Soler, 1991; Sandeman et al., 1995),
was responsible for crustal thickening, uplift, and erosion across broad areas
of southern Perú. In the study area, this is correlated (Fig. 4) with the
unconformity between fine shales and sandstones of the lower member of the
Moquegua Formation and the upper member comprising polymictic conglomerates
interbedded with coarse sandstone (Bellido, 1979; Marocco and Noblet, 1990; this
study) and coincided precisely with resumption of arc activity at 25.5 Ma after
a hiatus of more than 20 m.y. These events overlap in age with, and may have
been related to, oblique subduction of the north-northeast–trending segment of
the aseismic Juan Fernández Ridge beneath southernmost Perú during the latest
Oligocene or earliest Miocene. Yáñez et al. (2001) infer a rate of southward
migration of the ridge-continental margin intersection of ca. 200 km/m.y., a
model supported by the only slightly younger, early Miocene ages (<25.5 Ma K-Ar
age of García et al, 1999; <21 Ma Ar-Ar age of Mortimer and Sari
, 1975; and
<23 Ma Ar-Ar ages of C.X. Quang, A.H. Clark, and J.K.W. Lee, unpub. data)
determined for ignimbrites that are intercalated with coarse gravels overlying
beveled Cretaceous-Paleogene rocks in northernmost Chile (18°–19°35' S).
However, the resumption of volcanism at the arc front in southern Perú at 25.5
Ma also coincided precisely with a major expansion of the arc to embrace both
the entire Cordillera Occidental and the Cordillera Oriental (Sandeman et al.,
1995), an event unlikely to have been caused by the initial subduction of the
ridge. Sandeman et al. (1995) ascribe this abrupt areal expansion of magmatism
to the opening of a window in a subhorizontally subducting slab, but it should
be emphasized that James and Sacks (1999) appeal to a gradual steepening of the
slab through the early Miocene.
Aymará uplift, pedimentation (stage I), and attendant lowering of the water table promoted further leaching and downward migration of enrichment, confirmed by the ages of supergene minerals in the study area. Supergene dates from Cerro Verde-Santa Rosa and La Llave provide minimum age constraints of 24.4 to 28.6 Ma for the development of the La Caldera surface (Fig. 15), which is correlated with the 23.8 to 24 Ma Altos de Camilaca surface in the Moquegua area (Fig. 10a). Based on the above 40Ar-39Ar data and landform correlations, we recognize a late Oligocene episode of strong leaching and enrichment in a semiarid climate beneath a regional subplanar landform, the Altos de Camilaca-La Caldera surface (I), in response to Aymará uplift in southern Perú.
Similar late Oligocene supergene dates are recorded for alunite group
minerals from the Cerro Colorado (24.2–26.5 Ma 40Ar-39Ar;
Bouzari and Clark, 2002) and Spence (27.7 Ma 40Ar-39Ar;
Rowland and Clark, 2001) deposits (Fig. 1), as well as from El Salvador (23.1 Ma
K-Ar) and several of the other porphyry deposits of the upper Eocene to lower
Oligocene belt (Sillitoe and McKee, 1996) in northern Chile, where late
Oligocene-early Miocene tectonism (Pehuenchean), corresponding to the Aymará
event in southern Perú, stimulated downward propagation of enrichment profiles.
Geomorphologic relationships and semiarid climatic conditions similar to those
occurring in southern Perú may have existed in northern Chile at this time. A
regionally extensive planar erosion surface, the Choja pediplain, inferred to be
late Oligocene in age (Galli-Oliver, 1967; Mortimer and Sari, 1975), eroded
the early leached cap and sulfide enrichment blanket of the Cerro Colorado
deposit (Bouzari and Clark, 2002) but was also responsible for controlling the
development of the extant supergene orebody. This pediplain is tentatively
correlated with the Altos de Camilaca surface (I) exposed in southern Perú.
Farther south, no direct age constraints are available for the Checo del Cobre
pediplain of the wider Copiapó-El Salvador area (Mortimer, 1973). As
demonstrated by Clark et al. (1967a) and Sillitoe et al. (1968), this surface
lies at an elevation slightly higher than the lower, more important chalcocite
blankets developed in numerous small- to medium-sized copper deposits. A late
Oligocene to earliest Miocene age, and hence correlation with stage I erosion in
southern Perú, are plausible, but a younger, mid-early Miocene age cannot be
ruled out.
Miocene
The Miocene in southern Perú was punctuated by Quechuan tectonic events
(McKee and Noble, 1982; Ellison, 1990; Sébrier and Soler, 1991), resulting in
major crustal thickening and uplift. Ignimbrite eruption continued through the
Miocene, providing temporal constraints on the progressively more restricted
landforms generated in response to episodic uplift. The Pampa Lagunas surface
(II) and its correlatives, representing a regionally extensive pediplain, eroded
preexisting supergene profiles but also controlled significant subjacent
enrichment (Figs. 15–16). An early Miocene age is proposed herein (cf. Tosdal
et al., 1984) for the Pampa Lagunas surface on the basis of 18.8 to 19.1 Ma 40Ar-39Ar
dates for ignimbrites from the Moquegua area (Figs. 9, 16). The 21 Ma
natrojarosite veins associated with hematite after chalcocite in the Cerro Verde
deposit were intersected by the Santa Rosa surface (II), providing a maximum age
for the surface, which broadly overlaps with ignimbrite age constraints (Figs.
15–16).
Pedimentation and supergene processes persisted into the middle to late Miocene Multiple Pediment stage (Figs. 9, 16). Supergene activity in the Cerro Verde deposit and at the La Llave prospect during the early to middle Miocene was controlled by correlatives of the Pampa Lagunas surface (II; i.e., Santa Rosa and Pampa de La Joya surfaces, Fig. 15). Supergene oxidation in the sulfide-poor Cachuyo porphyry copper prospect similarly took place beneath the La Caldera surface (I) and, subsequently, the Pampa de La Joya surface (II; Fig. 15), presumably in the late Oligocene and early Miocene, but no supergene minerals suitable for 40Ar-39Ar analysis were identified. Weak copper oxide mineralization at the Cachuyito prospect, probably generated during the early to middle Miocene Multiple Pediment stage, is truncated by the Pampa Colorada surface (IV; Fig. 15).
Farther south in the Cuajone-Quellaveco-Toquepala district, the strongest chalcocite zone developed beneath the late Oligocene Altos de Camilaca surface (I) and beneath the early Miocene Pampa Lagunas surface (II; Figs. 15–16).
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Conclusions |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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40Ar-39Ar dating of alunite group minerals in the Jurassic through Paleocene porphyry copper centers of southern Perú demonstrates that a protracted supergene history began during the Incaic orogeny in the late Eocene beneath an uplifting and eroding topography (Fig. 16). The Incaic landforms are preserved only as unconformities and the extent of early leaching and enrichment is difficult to assess, but coherent, subhorizontal, chalcocite blankets may have developed and supergene solutions may have locally penetrated deeply along active faults and unusually permeable units, such as the tourmaline breccias at Cerro Verde. We propose that the Incaic supergene profiles were widely preserved through water-table rise attending slow clastic sediment accumulation across the Pacific slope during the early through middle Oligocene interval of relative tectonic quiescence. The fine sandstones of the lower member of the Moquegua Formation are inferred to represent remnants of such a cover, which was almost entirely eroded in areas northwest of Quebrada Guaneros.
Subsequently, the development of the Altos de Camilaca-La Caldera and early Miocene Pampa Lagunas-Desierto de Clemesí-Pampa de La Joya-Santa Rosa pediplains in response to the late Oligocene Aymará tectonic event was associated with intense leaching and enrichment, largely of hypogene assemblages which had survived the Incaic weathering. It is also probable that extensive erosion of Incaic supergene ores took place at this time, with resulting dispersal and depletion of the copper budget. At least at Cuajone and Quellaveco, this stage generated laterally extensive supergene sulfide blankets, probably reflecting the uniformly subplanar configuration of the Altos de Camilaca surface. However, blanket deepening and thickening were persistently interrupted by ignimbrite eruption. In southern Perú, the late Oligocene to early Miocene arc was essentially superimposed on the older mineralized igneous provinces (Clark et al., 1990a), and major ignimbrite eruption occurred throughout the interval of most intense supergene activity. This upper Oligocene to middle Miocene ignimbrite cover is almost unbroken in northernmost Chile but, farther south, ignimbrite blanketing apparently did not occur until late in the supergene evolution of the Cerro Colorado porphyry system, at 19.1 Ma. This largely terminated normal supergene activity but stimulated lateral meteoric water flow and hence the formation of exotic chrysocolla mineralization (Bouzari and Clark, 2002). At Cerro Colorado, moreover, the erosion of Incaic leached assemblages, themselves formed through oxidation of chalcocite horizons, is confirmed by the occurrence of 35.3 Ma alunite in hematitic clasts in gravels overlying the lower Miocene ignimbrite (Bouzari and Clark, 2002).
Clark et al. (1990b) proposed that the enrichment blanket at Quellaveco was extensively eroded in the late Miocene following the cessation of middle Miocene Chuntacala volcanic activity. This is now confirmed by our 40Ar-39Ar dating and geomorphologic mapping, which demonstrates that a 10.1 Ma unwelded ash flow on the north slope of the Río Asana is not part of the sequence of Chuntacala Formation ignimbrites overlying the Quellaveco profile, as had been inferred on the basis of drilling to assess the western extent of the orebody (J. Romero, Minera Quellaveco S.A., pers. commun., 1999). This 10.1 Ma tuff instead lies on a valley bench that locally incised the Chuntacala Formation and the enrichment profile. In contrast, the deepest enrichment zone at Toquepala, thicker and higher grade than those at higher elevations (Clark et al., 1990b), probably developed in the middle Miocene during the Multiple Pediment stage. Moreover, supergene activity at Cerro Verde continued until 4.9 Ma, indicating that semiarid climatic conditions persisted until the latest Miocene (Fig. 16).
The occurrence of remnants of upper Eocene alunite group minerals in the deeper parts of considerably younger late Oligocene to late Miocene supergene alteration profiles (e.g., Cerro Verde) indicates that the overall physiographic configuration of the Pacific piedmont of southern Perú remained remarkably consistent through much of the Oligocene and Miocene and, as noted above, that water-table levels must have risen significantly following late Eocene supergene activity. It is also implicit that semiarid climatic conditions, and hence deep water tables, persisted in this transect of the Pacific slope, as in northernmost (Bouzari and Clark, 2002) and northern (Rowland and Clark, 2001) Chile, at least from the middle Eocene to the middle Miocene. At least, this implies the generation and preservation of a cordilleran upland of sufficient altitude to create rain-shadow conditions throughout the middle Tertiary. The existence of such a climatic barrier is supported by both the tectonic evolution of the Pacific slope and the sedimentary history of the altiplano basin (Horton et al., 2001). There is no evidence at the latitude of southern Perú that supergene activity was underway prior to the late Eocene, but it should be emphasized that Kohn et al. (2004) documented climatic stability across the Eocene-Oligocene boundary (ca. 33.5 Ma) in southern South America.
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APPENDIX |
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TopAbstractIntroductionPrevious Model for Landform...Geochronology of Ignimbrite...Revised Landform ModelSupergene Profiles of Porphyry...Integrated Model of Landscape...ConclusionsAPPENDIXReferences |
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Acknowledgments |
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Permission to publish this contribution to the Queen’s University Central Andean Metallogenetic Project (QCAMP), has been given by Rio Tinto Mining and Exploration Ltd. We thank Dave Andrews, Bob Harrington, and Tim Moody at Rio Tinto for their generous support throughout this research. Economic Geology reviewers Richard Tosdal and William Chávez, Jr., provided unusually detailed and constructive reviews of the original manuscript, while Pepe Perelló promptly coordinated the review process and contributed his own insightful observations. Editor Mark Hannington proposed numerous key amendments.
It will be evident that this research builds upon the seminal 1978 study of Richard Tosdal, but the authors are alone responsible for their observations and interpretations.
July 21, 2003; October 29, 2004
July 21, 2003; October 29, 2004
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