cerro verde

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IntroductionThe contiguous Cerro Verde and Santa Rosa deposits and

the neighboring Cerro Negro prospect constitute the north-ernmost demonstrably economic hydrothermal systems in thecentral Andean upper Paleocene-middle Eocene porphyryCu-Mo belt, which parallels the South American plate bound-ary for at least 800 km in southern Peru and northern Chile(Fig. 1a). Centered at latitude 16°33' S, longitude 71°34' W,30 km southwest of the city of Arequipa, the Cerro Verde andSanta Rosa open pits have been operated since 1994 by So-ciedad Minera Cerro Verde, initially formed by Cyprus Amaxand now a subsidiary of Phelps Dodge Copper Corporation(82.5%) and Compañía de Minas Buenaventura (9.2%). SX-EW recovery attained 84,000 t of fine Cu at US $0.44/lb in2001, and output was expected to rise to 86,900 t at US$0.40/lb in 2002 (Ednie, 2002). Production, 70 percent fromCerro Verde, is almost entirely from reserves of 331 Mt of su-pergene ore grading 0.52 percent copper, but the develop-ment of 464 Mt of largely hypogene material at 0.61 percentcopper is under consideration (Ednie, 2002).

The geology of the deposits is documented by Estrada(1969, 1978), Kihien (1975), Le Bel (1985), Perea et al.(1983), and Phelps Dodge (2000). Stewart (1968) provides adetailed account of the “Caldera Complex,” the cluster ofgranitoid intrusions that hosts much of the mineralization,representing a segment of the Peruvian Coastal batholith

(e.g., Pitcher et al., 1985). Age relationships in the district are,in part, defined through U-Pb zircon (62–67 ± 1 Ma: Mukasaand Tilton, 1985; Mukasa 1986) and Rb-Sr (68 ± 3 Ma: LeBel, 1985) dates for the precursor Yarabamba granodiorites(Fig. 1b), and U-Pb zircon (61 ± 1 Ma: Mukasa, 1986) and K-Ar biotite (56-59 ± 2 Ma: Estrada, 1978) dates for the por-phyry bodies most closely associated with hypogene activity.However, no age data are recorded for the economically crit-ical supergene oxide and sulfide mineralization.

In the present communication, we document new multi-step, laser-induced 40Ar-39Ar dates for sericites directly associ-ated with hypogene chalcopyrite-pyrite mineralization andfor supergene alunite-group minerals from both Cerro Verdeand Santa Rosa, the latter representing the first such data fora porphyry deposit in southern Peru.

Geologic Framework

Host rocks and structural relationships

The Cerro Verde and Santa Rosa deposits crop out at ele-vations of 2,680 to 2,750 m a.s.l. on a subplanar pediment,herein termed the Santa Rosa surface. This was eroded intothe older La Caldera surface (Jenks, 1948), which lies ca. 200m higher (Fig. 2) and is now locally represented by isolatedsummits, including Cerro Verde (2,904 m a.s.l.) and CerroNegro (2,910 m a.s.l.).

Whereas the Santa Rosa deposit (Fig. 1b) is hosted entirelyby Paleogene granitoid units, the Cerro Verde center straddles

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40Ar-39Ar AGES OF HYPOGENE AND SUPERGENE MINERALIZATION IN THE CERRO VERDE-SANTA ROSA PORPHYRY Cu-Mo CLUSTER, AREQUIPA, PERU

CHAN X. QUANG,† ALAN H. CLARK, JAMES K.W. LEE,Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada

AND JORGE GUILLÉN B.Sociedad Minera Cerro Verde S.A., Asiento Minero Cerro Verde-Uchumayo, Avenida Alfonso Ugarte 304, Casilla 299, Arequipa, Peru

AbstractThe contiguous Cerro Verde and Santa Rosa porphyry copper deposits are hosted by Paleogene granitoid

rocks and Precambrian gneiss, and spatially associated with 61 ± 1 Ma (U-Pb zircon: Mukasa, 1986) dacitic por-phyry stocks. The age of hydrothermal activity is constrained by laser-induced incremental-heating 40Ar-39Arsericite (muscovite-2M1) dates of 61.8 ± 0.7 (2σ) and 62.0 ± 1.1 Ma for Cerro Verde, and 62.2 ± 2.9 Ma forSanta Rosa, representing the terminal event in the Arequipa segment of the Coastal batholith.

The deposits crop out on the Santa Rosa erosional pediment, which itself is incised into the older La Calderasurface. Two populations, of ages 36.1 to 38.8 Ma and 24.4 to 28.0 Ma, are identified by multiple analyses of asample from Cerro Verde comprising alunite partially replaced by natroalunite, demonstrating that supergeneactivity had commenced by the latest Eocene, during the Incaic orogeny, thereafter continuing through theOligocene. In the Santa Rosa deposit, deep (ca. 300–350 m) leaching in the late Oligocene is recorded by ca. 26Ma natroalunite that is inferred to have formed beneath the La Caldera surface. At the top of the Cerro Verdepit (2738 m bench), veins of alunite (ca. 23 Ma) and natroalunite (ca. 21 Ma) in a hematitic leached zone aretruncated by the Santa Rosa surface, which is inferred to have developed after 21 Ma. Decreasing ages of alunite-group minerals with increasing depth in the Cerro Verde pit (e.g., ca. 12 Ma at the 2648 m level, and 4.9–6.7 Maat the 2618 m level) are evidence for deepening of the supergene profile through the Miocene beneath thispediment. Jarosite dates (0.7–1.3 Ma) record the persistence of minor supergene activity into the Pleistocene.

Economic GeologyVol. 98, 2003, pp. 1683–1696

† Corresponding author: e-mail, [email protected]

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N 8169000

N 8170000

E22

1000

E22

5000

E22

4000

E22

3000

E22

2000

N 8172000

••

•

••

•

Cerro Verde pit

Santa Rosa pit

SURF-110 ( , )sr al

•

LEGEND“Dacite MonzonitePorphyry”

YarabambaGranodiorite

TiabayaGranodiorite

Augite Diorite

Yura Group

CharcaniGneiss

“SilicaBreccia”

TourmalineBreccia

Pit Outlinein 2000

Location ofDated Sample

SURF-109

alnajrsr

AluniteNatroaluniteJarositeSericite

•

•

Fault

SURF-113 ( )na

A-A’

SURF-114 ( )na

SURF-109 ( )al,na, jr

SURF-119 ( )sr

SURF-112 ( )na

A - A’

B - B’

0 250 500 m

A

A’

B’

B

•

Spence

BOLIVIA

PERU

CHILE

Arica

Arequipa

Iquique20°S

70°W 66°W

200 km

(a)Chapi

CERRO VERDECuajone Quellaveco

Toquepala

Cerro Colorado

N 8171000

SURF-111 ( )sr, al, jr

(b)

FIG. 1. a. Locations of Cerro Verde-Santa Rosa and other Paleocene to middle Eocene porphyry Cu deposits of southernPeru and northern Chile. B. Local geology of the Cerro Verde-Santa Rosa district, showing the locations of dated alunite-group and sericite samples. Heavy lines show limits of panoramas in Figures 6 (A-A') and 8 (B-B'). Modified after PhelpsDodge (2000).

El Misti5822 m a.s.l. Cerro Verde

2904 m a.s.l. Santa Rosa deposit

Cerro Negro2910 m a.s.l.

Looking NE (045°)

Santa Rosa Surface

Dissected La Caldera Surface

Cordillera Occidental

Cerro Verde deposit

FIG. 2. Panoramic view, looking northeast (045°), of the Cerro Verde-Santa Rosa-Cerro Negro district, showing the SantaRosa pediment (in foreground), remnants of the La Caldera surface (accordant summits in middleground), and active anddormant stratovolcanoes of the Cordillera occidental (skyline) in August, 2001. (NB. The term “La Caldera surface” reflectsthe basinal local topography, with no volcanological implications). Approximate width of the foreground is 2 to 3 km.

the contact between these and the Precambrian Charcanigneiss. The latter comprises a series of amphibolite-faciesmetasedimentary and metaigneous rocks, representing partof the Mesoproterozoic (Wasteneys et al., 1995) ArequipaMassif, which constitutes much of the Andean basement insouthern Peru and northernmost Chile. Each of the depositsis associated with ca. 0.12-km2 steep-walled stock of hy-pabyssal quartz- and feldspar-phyric rock, traditionallytermed “dacite monzonite porphyry” or “quartz-bearingmonzonite porphyry” (Fig. 1b), but with quartz, alkalifeldspar, and plagioclase modal contents indicating a daciticcomposition. These are the youngest major intrusive units inthe district. However, a dike of postmineralization dacitic,quartz-feldspar porphyry is exposed in the southern quad-rant of the 2563 m level of the Cerro Verde pit, and a weaklyaltered quartz porphyry exposed in the Santa Rosa pit mayrepresent the same late intrusive event. Tourmaline-ce-mented breccias are widespread only in the Cerro Verdeand Cerro Negro deposits, but small volumes of tourmaline-free “silica breccia” occur at all three centers (Fig. 1b).Northwest-southeast– striking bodies of tourmaline-richgranitic pegmatite and aplite widely cut all granitoid andbasement rocks, but their relationship to the tourmalinebreccias is uncertain.

The northwest-southeast elongation of the Santa Rosahydrothermal system (Fig. 1b), as well as the overall distrib-ution of the Cerro Verde, Santa Rosa, and Cerro Negro de-posits, represent a segment of a linear array of porphyry,breccia, and vein copper deposits that extends at least to theChapi mine, 7 km to the southeast. This trend parallels a sys-tem of northwest-striking and steeply northeast dipping re-gional faults (Phelps Dodge, 2000), plausibly the northwest-ern equivalent of the major Incapuquio fault system in theCuajone-Quellaveco-Toquepala district, ca. 115 km to thesoutheast (Fig. 1a). As at Toquepala (Zweng and Clark,1995), shallow intrusion and mineralization at Cerro Verdeand Santa Rosa are considered to have been controlled bythe intersections of these northwest-southeast structures andlocal northeast-southwest tensional faults. Widespreadnortheast-southwest and east-west postmineral fractures andtensional faults cut the northwest-southeast structures andwere associated with reverse reactivation (Fig. 1b; PhelpsDodge, 2000).

Hypogene alteration-mineralization relationships

Hydrothermal alteration extends over a northwest-elon-gated, 5- by 1.5-km area in the Cerro Verde-Santa Rosa dis-trict, potassic and phyllic zones lying within a propylitic enve-lope. Potassic alteration, characteristically with a blotchydevelopment, is most extensively preserved at depth, but per-sists to shallower levels at Cerro Verde. Two main subfaciesare represented: orthoclase with lesser biotite (ca. 30%) andmagnetite (<5%) in Yarabamba granodiorite and the daciticporphyries; and biotite-magnetite, best developed in Char-cani gneiss and andesite. In addition, pit exposures and drillintersections at Cerro Verde reveal magnetite-biotite-albitealteration, which may be characteristic of the deeper, low-grade (0.1–0.15% Cu) subfacies of the early alteration. Mag-netite-cemented hydrothermal breccias at Santa Rosa proba-bly developed contemporaneously. Perea et al. (1983)

estimate an average sulfide content of 3 percent in the potas-sic zones, with a chalcopyrite/pyrite ratio of ca. 3.

Phyllic (i.e., quartz-sericite-pyrite) alteration surrounds thepotassic zones in the upper portions of the deposits andrepresents the major host of economic mineralization, withaverage sulfide contents of 5 to 7 percent and a chalcopyrite/pyrite ratio of 0.3 to 0.7 (Perea et al., 1983). The characteris-tic pervasive assemblage is best developed in Yarabamba gra-nodiorite and the dacitic porphyries, but is also representedin Charcani gneiss. The age relationships between the phyllicalteration and the tourmaline breccia bodies in the CerroVerde deposit are uncertain. Clasts in these breccias exhibitintense quartz > sericite, pyrite-free alteration, appearing sili-cified, as at Toquepala (cf. Zweng and Clark, 1995). The Bo-nanza breccia, the main body of silica breccia at Santa Rosa(Fig. 1b), comprises angular fragments with intense sericite >quartz alteration and disseminated chalcopyrite in a matrixdominated by massive chalcopyrite and minor pyrite, mag-netite, and ferberite.

Supergene mineralization

The earliest large-scale mining in the district was initiatedin 1968 by Mineroperú, who exploited brochantite-domi-nated oxide ores at Cerro Verde, averaging 1 percent Cu, butwith restricted zones exceeding 2 percent. The supergeneactivity attained depths of over 300 m (from ca. 2,750 to2,438 m a.s.l.) within the main body of tourmaline breccia,probably a reflection of its high permeability, but the super-gene profile is significantly thinner at Santa Rosa. In mostareas of the deposits, the main zone of chalcocite mineral-ization exhibits a very irregular and discontinuous distribu-tion. Moreover, the southwestern sector of the Cerro Verdepit reveals the presence of at least two sulfide enrichmentblankets: an older horizon, ca. 15 m thick, discontinuouslypreserved on and above the 2648 m bench; and a younger,more localized, but thicker zone located within a large bodyof tourmaline breccia and juxtaposed with hypogene miner-alization between the 2573 and 2633 m benches. This lowerblanket averages 60 to 80 m in thickness but increases to 100m, and locally 150 m, in the main tourmaline breccia body.In general, the supergene sulfide zone thins to the north andnortheast. At Santa Rosa, the single preserved blanket rangesfrom 20 to 45 m thick, with an underlying transitional sectionin which chalcopyrite is partially replaced by chalcocite, cov-ellite, and bornite.

The chalcocite blankets at both Cerro Verde and SantaRosa are overlain by the brochantite subzone of the oxidezone, in which minor chrysocolla occurs as veins cutting abrochantite stockwork. Chalcedony, antlerite, and malachiteare minor constituents. The highest oxide-Cu grades occur inthe matrix of hydrothermal breccias, and the thickest devel-opment of such ores is in the eastern half of the Cerro Verdedeposit. The brochantite ores are, in turn, overlain by thecopper pitch subzone, in which Cu-Fe-Mn oxides predomi-nate. The distribution of the oxide ores is erratic and com-monly spatially associated with chalcocite-kaolinite orhematitic zones. The uppermost part of the profile is theleached zone, which averages 70 m in thickness, but locallyattains depths of 250 m in the Cerro Verde deposit. Hematite,goethite, and minor jarosite are most abundantly developed

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in breccia zones. The widespread hematite is evidence for theprevious existence of chalcocite (Anderson, 1982), and relictzones of chalcocite-kaolinite occur throughout the supergeneprofiles. Jarosite is locally abundant over the originally morepyritic margins of the two deposits.

Sampling and GeochronologySamples rich in sericite were selected from the Cerro

Verde and Santa Rosa pits to determine the age of the pre-ponderant hypogene Cu mineralization. Fine-grained (<50µm) sericite is distributed throughout a matrix of quartz (±tourmaline) and iron oxides with residual disseminated pyrite.Pure separates, yielding muscovite(-2M1) X-ray powder dif-fraction patterns, were prepared for 40Ar-39Ar incremental-heating analysis. The locations, hypogene and supergene con-texts, and inferred ages of the sericites are summarized inTable 1, and the Ar-Ar analytical data are summarized as agespectra and inverse isochron plots in Figure 3.

The occurrence of supergene alunite-group minerals atCerro Verde and Santa Rosa has been described by Kihien(1975), Cedillo et al. (1979), and Cedillo and Wolf (1982).Representative samples of alunite-group minerals were col-lected from different levels of the supergene profiles thatwere superimposed on the phyllic alteration zones of theCerro Verde and Santa Rosa deposits (Fig. 1b). The locations,petrographic relationships, and inferred ages of the dated su-pergene minerals are recorded in Table 2 and in Figure 1.One critical sample, SURF-110, from the 2648 m bench ofthe Cerro Verde pit, is discussed separately. Full 40Ar-39Ar an-alytical data are recorded in Appendix 1. All dates are quotedwith an uncertainty of ±2σ. For the purposes of this paper, anage plateau is defined as three or more separate outgassingsteps with ages that are concordant at 2σ errors and that ac-count for at least 50 percent of the 39Ar released; ideally,

there should be no consistent increase or decrease in appar-ent age across the plateau.

Age of Hypogene Mineralization The age of hypogene mineralization in the district is estab-

lished by 40Ar-39Ar hydrothermal sericite dates (Fig. 3).SURF-110 and SURF-111, both from Cerro Verde, yieldedidentical correlation ages of 61.8 ± 0.7 Ma and 62.0 ± 1.1 Ma(Fig. 3b, d; Table 1). These dates are preferred to the plateau


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TABLE 1. Locations and Ages of Hydrothermal Sericite Samples from Cerro Verde-Santa Rosa

Mineralogy by Elevation Latitude (°S) powder X-ray Age ± 2σ

Sample (m a.s.l.) Location Longitude (°W) Hypogene setting Supergene setting diffraction Habit (Ma)

SURF-110 2,648 Cerro Verde pit, 16.5326 Q-s-p (quartz- Hematite-goethite Muscovite-(2M1) Fine- 61.8 ± 0.7 upper benches on 71.5980 sericite-pyrite) capping with slightly grained, correlationsouthern wall altered quartz weathered pyrite pervasive age

monzodiorite and minor "sooty" chalcocite and covellite

SURF-111 2,738 Cerro Verde pit, 16.5311 Q-s-p altered Hematite Muscovite-(2M1) Fine- 62.0 ± 1.1 top of western 70.6074 tourmaline leached cap grained, correlation wall breccia pervasive age

SURF-119 2,558 Santa Rosa pit, 16.5420 Bonanza breccia Transitional zone Muscovite-(2M1) Fine- to 61.0 ± 1.2 southwestern 71.5841 comprising intense between secondary medium- plateau wall sericite-quartz– sulfide enrichment grained, age

altered fragments and hypogene pervasivewith disseminated zoneschalcopyrite ±pyrite cemented by massive chalcopyrite associated with sericite, molybdenite ± pyrite

0 . 0 0

0 . 0 2

0 . 0 4

0 . 0 6

0 . 0 8

0 . 1 0

39Ar/4

0Ar

0 . 0 0 0 0 . 0 0 1 0 . 0 0 2 0 . 0 0 3 0 . 0 0 43 6 A r / 4 0 A r

120SURF 111 , Sericite

PA = 61.3±2.7

Integrated Age: 56.2±5.5 Ma

0.0 1.00

20

40

100

120

60

80

SURF 110 , Sericite

PA = 62.3±2.0


0.0 1.0

0

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2 0

3 0

4 0

5 0

6 0

7 0

8 0

9 0

1 0 0

Age(Ma

)

0 . 0 1 . 0F r a c t i o n 3 9 A r

0

20

40

100

60

80

SURF 119 , Sericite

PA = 61.2±1.4

Integrated Age: 60.7±1.5 Ma0.0 1.0

0

20

40

100

60

80

Age

(Ma)

Fraction Ar39

(a)

(f)

(d)

(e)

(c)

(b)

3940

Ar/

Ar

36 40Ar/ Ar

SURF 110 , Sericite

SURF 111 , Sericite

SURF 119 , Sericite

0.02

0

0.04

0.06

0 0.002 0.004

Correlation Age: 61.8±0.7 Ma



0.02

0

0.04

0.06

0 0.002 0.004

0.02

0

0.04

0.06

0.08

0.10

0 0.002 0.004

FIG. 3. 40Ar-39Ar step-heating isochron plots and age spectra for hydro-thermal sericites from Cerro Verde (a-d) and Santa Rosa (e, f). PA = plateauage.

ages (Fig. 3a, c) because the isochron plots display lineararrays that clearly define the age of the samples, whereas thespectra exhibit atmospheric argon and slight recoil effects inthe lower temperature increments, resulting in greateruncertainty in the plateaus. Sericite (SURF-119), from thematrix of the Bonanza breccia at Santa Rosa, gave a correla-tion age of 62.2 ± 2.9 Ma (Fig. 3f), which corresponds closelyto those from Cerro Verde. Although composed of only four

steps, the age spectrum defines a plateau (Fig. 3e), eventhough much of the 39Ar was released in a single step and thelowest- and highest-temperature steps are associated withlarge errors due to the small amounts of gas released.

These dates overlap within error with the 61 ± 1 Ma U-Pb zircon date of Mukasa (1986) for “dacitic-monzoniticporphyries” at Cerro Verde and a 62 ± 2 Ma Rb-Sr whole-rock isochron age reported by Beckinsale et al. (1985) for


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TABLE 2. Locations and Ages of Supergene Alunite-Group Minerals

a. Cerro Verde

Elevation Latitude (°S) Hypogene Supergene Age ± 2σSample (m a.s.l.) Location Longitude (°W) setting setting Mineral Habit (Ma)

SURF-111 2,738 Cerro Verde pit, 16.5311 Q-s-p–altered Hematite (a) Natroalunite 4-cm-wide, white (a) 20.7 ± 0.3 western margin 70.6074 tourmaline leached cap (b) Alunite to pale yellow (a) 21.2 ± 0.3

breccia (c) Jarosite porcelaneous (b) 23.3 ± 0.3 vein comprising (c) 0.74 ± 0.03 aggregates of up to plateau ages20-µm-size alunite (b) crystals cut by finer-grained, massive natroalunite (a); both are cut by mm-scale jarosite (c) veins

SURF-114 2,648 Cerro Verde pit, 16.5285 Silicified and Hematite- Natroalunite 1-cm-wide, pale 12.0 ± 0.5eastern slopes 71.5963 sericite-altered goethite green-light brown, 12.6 ± 0.9

intrusive leached Cap porcelaneous vein plateau ages

SURF-109 2,618 Cerro Verde pit, 16.5308 Q-s-p–altered Jarositic (a) Natroalunite Millimeter-wide, (a) 4.9 ± 0.3southeastern wall 71.5955 dacite leached cap (b) Alunite pale yellow, (b) 6.7 ± 0.2

porphyry (c) Jarosite porcelaneous alunite (c) 1.3 ± 0.2(b) veinlets cut by plateau agescm-scale, tan-white porcelaneous natroalunite (a) veins; both are cut by thin, fine-grained jarosite (c) veinlets

SURF-110 2,648 Cerro Verde pit, 16.5326 Q-s-p–altered Hematite- (a) Alunite Pink, porcelaneous, (a) (36.1 ± 0.3)upper benches 71.5980 quartz goethite (b) Natroalunite fine-grained to (38.8 ± 0.7) on southern wall monzodiorite capping with (1–3-µm size), minimum ages

slightly zoned alunite (b) (24.4 ± 0.3)weathered (a) veins cut by to (28.0 ± 0.4) pyrite and cryptocrystalline, maximum ages minor 'sooty' natroalunite chalcocite (b) veinletsand covellite

b. Santa Rosa

SURF-112 2,588 Santa Rosa pit, 16.5426 Strong quartz- Lower portion Natroalunite Tan, massive clots 26.8 ± 1.7southwestern wall 71.5862 sericite-clay– of the in leached quartz 26.2 ± 0.8

altered secondary veinlets plateau agesintrusive sulfide

enrichment zone

SURF-113 2,573 Santa Rosa pit, 16.5410 Q-s-p–altered Transitional Natroalunite Gray-tan, massive (26.9 ± 0.3)southwestern wall 71.5854 quartz zone between patches in quartz- (27.4 ± 0.3)

monzodiorite secondary tourmaline-sulfide maximum agessulfide enrich- vein with ment and chalcocite-covellite hypogene zones enrichment

13 samples of intrusive rocks in the vicinity of Cerro Verde. Incontrast, the conventional K-Ar dates of Estrada (1969, 1978),56 to 59 Ma, are markedly younger and may reflect the loss ofradiogenic 40Ar*.

Hypogene mineralization in the Cerro Verde-Santa Rosadistrict was, at ca. 62 Ma, the final event in the evolution ofthe Arequipa segment of the Coastal batholith (e.g., Pitcheret al., 1985).

Age and Geomorphologic Setting of Supergene Mineralization

Whereas the first recorded conventional K-Ar dates forsupergene alunite group minerals (Gustafson and Hunt,1975) were interpreted as problematic, subsequent K-Ar(Alpers and Brimhall, 1988; Sillitoe and McKee, 1996) and,particularly, 40Ar-39Ar incremental-heating (Vasconcelos et al.,1994; Bouzari and Clark, 2000, 2002; Mote et al., 2001) stud-ies have convincingly demonstrated the efficacy of this ap-proach in the elucidation of the history of weathering profiles.The Ar systematics of alunite are documented by Love et al.(1998) and Vasconcelos (1999).

The age spectra determined herein for alunite-group min-erals reveal complex relationships between the Cerro Verdeand Santa Rosa supergene profiles, and the data are thereforediscussed separately below. The dates provide, in turn, ageconstraints for the local landforms that controlled subjacentsupergene processes.

Cerro Verde

Plateau ages for seven samples broadly decrease withdepth (Fig. 4; Table 2a). White to pale yellow alunite (SURF-111b) from the 2738 m level gave an age of 23.3 ± 0.3 Ma,derived from a four-step plateau comprising 74.6 percent ofthe 39Ar released (Fig. 4a). A supergene origin for this sam-ple is supported by a δ34S value of 7.5 per mil, which is sim-ilar to the 8.3 per mil determined for hypogene pyrite fromSURF-110, a value slightly higher than those reported by LeBel (1985) for hypogene pyrite (5.1–6.9‰) from the deposit.Duplicate analyses of a natroalunite vein (SURF-111a) thatcuts, and contains fragments of, the SURF-111b aluniteveins (Fig. 5) yielded ages of 20.7 ± 0.3 and 21.2 ± 0.3 Mafrom five-step plateaus (Fig. 4b, c) comprising 83.0 and 86.7percent of the 39Ar released, respectively. This implies a ca.2- to 3-m.y. history of leaching and alunite-group mineralprecipitation at this horizon of the supergene profile. More-over, a four-step plateau comprising 72.4 percent of the 39Arreleased (Fig. 4d) gave an age of 0.74 ± 0.03 Ma for fine-grained jarosite (SURF-111c), coating fracture surfaces andforming millimeter-scale veinlets that cut both the alunite(SURF-111b) and natroalunite (SURF-111a) veins (Fig. 5),providing evidence for a persistence of supergene activityinto the Pleistocene.

Ninety meters deeper, on the 2648 m mine bench, na-troalunite (SURF-114) gave duplicate ages of 12.6 ± 0.9 and12.0 ± 0.5 Ma from three-step plateaus comprising 87.2 and82.8 percent of the 39Ar released, respectively (Fig. 4e, f).Pale yellow alunite (SURF-109b) from close to the bottom ofthe Cerro Verde pit, at 2618 m, yielded a plateau age of 6.7 ±0.2 Ma, from five steps representing 94.7 percent of the 39Arreleased (Fig. 4g). These veins are cut by cm-wide veins of


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0.0 1.0

SURF 111a, Natroalunite

PA = 20.7±0.3


30

0

10

20

40

0.0 1.0

SURF 109b, Alunite


0

1 0

2 0

3 0

4 0

Age(Ma

)

0 . 0 1 . 0F r a c t i o n 3 9 A r

PA = 12.6±0.9

PA = 23.3±0.3

0

10

20

40

0.0 1.0

30

SURF 111b, Alunite

SURF 111c, JarositeSURF 111a, Natroalunite (2)

SURF 114, Natroalunite SURF 114, Natroalunite (2)

SURF 109c, Jarosite



PA = 4.9±0.3PA = 6.7±0.2

0

10

20

30

0

10

20

0

10

20

30

0

5

10

15

0.0 1 .0 0.0 1.0


PA = 12.0±0.5

Age

(Ma)

Fraction Ar39

0

1

2

3

4

5

Age(Ma

)

0 . 0 1 . 0F r a c t i o n 3 9 A r0.0 1.0

0.0 1.0

SURF 109a, Natroalunite


0.0 1.0

PA = 21.2±0.3


0

1

2

3

4

5

6

7

8

9

1 0

Age(Ma

)

0 . 0 1 . 0F r a c t i o n 3 9 A r


PA = 1.3±0.2

6

0

2

4

8

10

0.0 1.0

(c)

(f)

(d)

(a) (b)

(h)(g)

(e)

(i)


PA = 0.74±0.03

0

1

2

4

3

5

0

10

20

40

30

FIG. 4. 40Ar-39Ar step-heating age spectra for supergene alunite groupminerals from Cerro Verde. PA = plateau age.

Alunite

Natroalunite

Jarosite

43210 5 cm

L

L L

L

M

T

LL

FIG. 5. Slabbed surface of SURF-111 (2738 m level, Cerro Verde pit),showing crosscutting relationships among supergene alunite, natroalunite,and jarosite veins. Fragments of paler alunite and wall rock are included inthe natroalunite vein. L = leached wall rock, M = hypogene molybdenite-quartz vein, T = hypogene tourmaline veinlet.

white to tan natroalunite (SURF-109a), which yielded aplateau age of 4.9 ± 0.3 Ma from three steps representing96.7 percent of the 39Ar released (Fig. 4h). The natroaluniteveins are, in turn, cut by thin jarosite (SURF-109c) veinlets,dated at 1.3 ± 0.2 Ma from two contiguous steps that repre-sent 63.6 percent of the 39Ar released (Fig. 4i).

These data indicate that the major leached, oxidized, andsupergene sulfide zones exposed at Cerro Verde developedover an interval of at least 18 m.y., from the latest Oligoceneto the Late Miocene. The data do not discriminate betweenquasicontinuous or episodic downward encroachment of su-pergene processes, but we favor the latter model in view ofthe clearly episodic landform history of the region, in whichabrupt uplift and erosional events were separated by morequiescent intervals (Tosdal et al., 1984). At two sites, na-troalunite precipitation followed that of alunite.

40Ar-39Ar dates for alunite (23 Ma) and natroalunite (21 Ma)veins associated with pulverulent, supergene hematite at thetop of the Cerro Verde pit, record the time of oxidation andleaching of a preexisting chalcocite blanket (Fig. 6). Thisolder, enriched blanket and its associated higher-altitudeleached zone were eroded by the Santa Rosa pediment that,therefore, is inferred to have formed after 21 Ma. The ca. 23Ma alunite (SURF-111b) and 21 Ma natroalunite (SURF-111a) veins are inferred to have formed beneath the older LaCaldera surface, represented by remanent accordant summitsoverlooking the Santa Rosa pediment. Natroalunite and alu-nite dated at 12.6 to 4.9 Ma were subsequently generated inthe course of continued or, more probably, renewed leaching

beneath the Santa Rosa pediment. The 1.3 Ma (SURF-109c)and 0.74 Ma (SURF-111c) jarosite veins reflect episodes ofminor supergene activity under hyperarid conditions beneaththe fossilized landscape, and were probably not associatedwith significant Cu mobility.

Santa Rosa

Two natroalunites from the Santa Rosa deposit gave ca. 26Ma dates (Figs. 7 and 8; Table 2b). Duplicate analyses ofSURF-112 from the chalcocite zone (2588 m level) yieldedconcordant plateau ages of 26.2 ± 0.8 and 26.8 ± 1.7 Ma, rep-resenting 95.7 and 100 percent, respectively, of the total 39Arreleased (Fig. 7a, b). In contrast, the two age spectra forSURF-113, from the slightly deeper transitional zone (2573m level), display progressively decreasing apparent ages to-ward higher-temperature steps. Such complex spectra may ei-ther record 39Ar recoil or the occurrence of two or more alu-nite-group minerals with different Ar-retention temperatures(Vasconcelos et al., 1994; Bouzari and Clark, 2002). In theformer case, the highest-temperature steps would yield max-imum crystallization ages (25.5 ± 0.6 and 26.4 ± 0.8 Ma; Fig.7c, d) and, in the latter, would reveal the presence of separateminerals with ages of ca. 28 to 29 and 26 Ma.

The natroalunites are interpreted as recording deep (ca.300–350 m) penetration of supergene solutions beneath theLa Caldera surface, remnants of which are preserved on ca.2,900 m a.s.l. summits. A minimum age for this surface is con-strained by the ca. 26 Ma natroalunites associated with chal-cocite mineralization in the Santa Rosa pit on the 2588 m


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2603 m

SURF-111, 2723 mSURF-111, 2723 m2738 m

SURF-109, 2618 mSURF-109, 2618 m

2603 m

SURF-114, 2648 mSURF-114, 2648 m2588 m

2588 m

2573 m

2738 m2723 m

2708 m2693 m

2678 m2663 m

2648 m

2633 m2618 m

2573 m

2588 m2603 m

2723 m 2708 m2693 m2678 m2663 m

2648 m2633 m2618 m

2603 m

2738 m2708 m

2693 m 2678 m2663 m 2648 m2633 m

2618 m2603 m

2753 m(a)

(b)

A A’

Hypogene Zone

Hematite-GoethiteLeached Cap

Transition Zone Chalcocite Zone

HematiteLeached Cap

Oxide Zone

UnconsolidatedMaterial

JarositicLeached Cap

La Caldera Surface

alnaja

AluniteNatroaluniteJarosite

SURF-114, 12.0±0.5 Ma, 12.6±0.9 Mana

SURF-111a Sample number2738 m Bench level (m a.s.l.)20.6±0.3 Ma Plateau age(37.3±3.3 Ma) Minimum age

Santa Rosa Surface SURF-111a, 20.6±0.3 Ma, 21.2±0.3 MaSURF-111b, 23.3±0.3MaSURF-111c, 0.74±0.05 Ma

naalja

SURF-110, 36.1-38.8 Ma,24.4-28.0 Ma

alna

2588 m

2588 m2603 m

SURF-109a, 4.9±0.3 MaSURF-109b, 6.7±0.2 Ma, 6.5±0.2 MaSURF-109c, 1.32±0.15 Ma

naalja

FIG. 6. a. Panorama of Cerro Verde pit, looking southwest (245°), in August, 2001, showing the bench elevations (m a.s.l.).b. Sketch showing the distribution of supergene facies and locations of 40Ar-39Ar dates discussed in the text. The leached capis dominated by hematite with local jarosite. Local faulting coupled with the presence of highly permeable breccia bodiesmay have caused the complexities in the spatial distribution of supergene facies (e.g., chalcocite enrichment occurs on the2618 m bench adjacent to hypogene mineralization). Dashed lines show remnants of the La Caldera surface and the sub-planar Santa Rosa pediment.

mine bench, inferred to represent the lower limit of thesupergene chalcocite development beneath the La Calderasurface in the late Oligocene. The lack of fault offset of eitherthe Santa Rosa or La Caldera surface in the area of the minesprecludes significant relative vertical displacement of the twoporphyry centers in the Oligo-Miocene. The occurrence ofthese upper Oligocene natroalunites more than 150 m belowthe ca. 23 Ma alunite and ca. 21 Ma natroalunites of SURF-111 at Cerro Verde is therefore interpreted as reflecting theimportance of local controls on permeability, such as the dis-tribution of breccia bodies and fracture systems, in focusingmeteoric fluid flow.

Landform correlations

A simple landform chronology for the Cerro Verde-SantaRosa district is implied by the supergene 40Ar-39Ar data pre-sented above. The La Caldera surface, represented by rem-nant accordant summits, is inferred to have reached its finalconfiguration before ca. 26 Ma. The Santa Rosa pediment,comprising broad, subplanar valley floors on which the CerroVerde and Santa Rosa deposits crop out, is no older than 21Ma. This landform extends west-southwest along the Que-brada Cerro Verde drainage, to merge with the vast Pampa deLa Joya surface to the southwest, the dominant landform inthis area of the Pacific piedmont.

Evidence for Eocene supergene activity

Six analyses of alunite from sample SURF-110, from the2648 m level of the Cerro Verde pit, yielded age spectra withconsistent configurations that record unexpectedly old dates(Table 2a; Appendix 1). The spectra (Fig. 9) are defined by astaircase pattern in the lower-temperature steps and a flatterprofile in the mid- to high-temperature steps, exhibiting pro-gressively rising ages with increasing temperature. Two ap-parent populations of ages, 36.1 ± 0.3 to 38.8 ± 0.7 Ma and24.4 ± 0.3 to 28.0 ± 0.4 Ma, are recognized in, respectively,the highest-temperature steps and in the lowest-temperaturesteps that record less than 10 percent atmospheric argon(Table 2a). The spectra would be in permissive agreementwith the occurrence of a subordinate younger species, with anage of ≤ 28 Ma, and a dominant species at least 38 to 39 Main age (cf. Vasconcelos et al., 1994; Bouzari and Clark, 2002).Whereas cathodoluminescence imaging did not distinguishtwo phases because of pervasive quenching by Fe, backscat-tered electron images reveal a network of dark (i.e., low ag-gregate at. wt) veinlets and patches that cut an extremely finegrained (1–3 µm) intergrowth of zoned crystals with pale


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30

0

10

20

40SURF 112, Natroalunite (2)

PA = 26.8±1.7


0.0 1.0

30

0

10

20

40

0.0 1.0

SURF 113, Natroalunite (2)

Integrated Age: 27.4±0.3 Ma26.4±0.8 Ma

1.00.0

SURF 112, Natroalunite

PA = 26.2±0.8


30

0

10

20

0.0 1.0


Age

(Ma)

Fraction Ar39

25.5±0.6 Ma

(a)

(d)(c)

(b)

60

0

20

40

80

SURF 113, Natroalunite

FIG. 7. 40Ar-39Ar step-heating age spectra for supergene alunite groupminerals from Santa Rosa. PA = plateau age.

Bonanza Breccia

Oxide Zone

Transition Zone


Chalcocite Zone

HematiteLeached Cap

SURF-113, (na 25.5 ± 0.6 Ma), (26.4 ± 0.8 Ma)

SURF-112 Sample number2588 m Bench level (m a.s.l.)26.8±1.7 Ma Plateau age27.4±0.3 Ma) Maximum age(

La Caldera surface

SURF-112, 26.8±1.7 Ma, 26.2±0.8 Mana

UnconsolidatedMaterial

na Natroalunite

2603 m

2633 m2648 m2663 m

2678 m

Santa Rosa surface

B’B(a)(a)

(b)

2573 m

2588

2618 m

FIG. 8. a. Panorama of Santa Rosa pit, looking southwest (205°), in Au-gust 2001. b. Sketch showing the distribution of supergene facies and 40Ar-39Ar dates. The leached cap is dominated by hematite, with minor goethite.Relatively thin zones of oxide and secondary chalcocite enrichment occurbelow the leached cap and pass downward to a zone of transitional sulfidemineralization.

0

1 0

2 0

3 0

4 0

5 0

6 0

Age(

Ma)

0 . 0 1 . 0F r a c t i o n 3 9 A r

0

1 0

2 0

3 0

4 0

5 0

6 0

Age(

Ma)

0 . 0 1 . 0F r a c t i o n 3 9 A r

0

1 0

2 0

3 0

4 0

5 0

6 0

Age(

Ma)

0 . 0 1 . 0F r a c t i o n 3 9 A r

01020

40

0.0 1.0

37.3±3.3 (al)


38.8±0.7 (al )

Integrated Age: 36.2±0.3 Ma0.0 1 .0

30

01020

40

30

5060

6050

30

01020

405060

0.0 1.0

30

01020

405060

0.0 1.0

30

01020

405060

0.0 1 .0

37.7±0.4 (al)


36.1±0.3 (al )Integrated Age: 33.8±0.3 Ma

37.0±0.4 (al )


27.4±1.6 (na) 24.4±0.3 (na)

27.5±2.0 (na) 26.3±0.4 (na)

25.8±0.7 (na)

Age

(Ma)

Fraction Ar39

(a)

(f)

(d)

SURF 110, Alunite (al ) & Natroalunite (na)

0

1 0

2 0

3 0

4 0

5 0

6 0

Age(

Ma)

0 . 0 1 . 0F r a c t i o n 3 9 A r

30

01020

405060

0.0 1.0

37.2±0.4 (al )

Integrated Age: 33.8±0.3 Ma28.0±0.4 (na)

(e)

(c)

(b)

FIG. 9. 40Ar-39Ar step-heating age spectra displaying two populations ofages for porcelaneous alunite veins in sample SURF-110, from the 2648 mlevel, Cerro Verde.

cores and darker rims, recording remanent compositionalzoning (Fig. 10). Qualitative microprobe analyses (EDS) con-firm the presence of phosphorus and strontium in the centersof the zoned alunite crystals. In a similar setting, Stoffregenand Alpers (1987) assigned a supergene origin to cryptocrys-talline aluminum-phosphate-sulfate (APS) minerals in latefractures from the leached cap of the La Escondida porphyrydeposit on the basis of their grain size and geologic setting.Comparative backscattered electron imaging and microprobeanalysis of ca. 6.5 Ma alunite (SURF-109b) also revealedzoned crystals and the presence of phosphorus and calcium.We tentatively accept this as evidence that these elements canoccur in supergene alunite, whereas Watanabe and Heden-quist (2001) considered the occurrence of svanbergite andwoodhouseite cores in supergene alunite from El Salvador torepresent relict hypogene alunite. Elemental X-ray mappingof the Cerro Verde assemblages is precluded by their ex-tremely fine grain size, but the EDS spectra and the modestintensity contrasts in the backscattered electron images (Fig.10) suggest that the P and Sr in SURF-110 are present insolid solution in the alunite structure rather than as discretealuminum-phosphate-sulfate minerals. A supergene originfor alunite from this specimen is supported by bulk δ34S val-ues of 6.4 and 7.7 per mil from duplicate analyses, which aresimilar to the 8.3 per mil determined for hypogene pyrite inthe same sample and overlap with the 5.1 to 6.9 per mil val-ues reported by Le Bel (1985) for hypogene pyrite at CerroVerde. Although the analytical data are probably insufficientto rule out the presence of relict hypogene alunite (see Vas-conselos et al., 1994), there is no record of late Eocene-earlyOligocene hydrothermal activity in this Andean transect(Clark et al., 1990), and we tentatively conclude that the alu-nite group minerals in SURF-110 are wholly supergene.

The 36.1 to 38.8 Ma dates for alunite from SURF-110therefore imply that supergene activity was underway by thelatest Eocene, probably in response to uplift and erosion dur-ing Incaic tectonism (Sandeman et al., 1995). This earlyepisode of deep leaching and sulfate precipitation, controlledby highly permeable structures, occurred long before thelate-Oligocene development of the La Caldera surface andthe concomitant initial development of the main supergeneprofile at Cerro Verde and Santa Rosa. Similar late-Eocene toearly-Oligocene ages for supergene activity are documented

in northern Chile. Thus, at the ca. 51.8 Ma Cerro Coloradodeposit, Bouzari and Clark (2000, 2002) recorded a 35.3 ± 0.7Ma 40Ar-39Ar age for alunite from a hematitic leached cap,and Sillitoe and McKee (1996) determined a K-Ar date of34.3 ± 1.1 Ma for alunite from jarositic leached cap. Farthersouth, in the ca. 57.0 Ma Spence deposit, supergeneprocesses were active as early as 44.4 ± 0.5 Ma (Rowland andClark, 2001). Comparably early initial supergene activity(43.9 ± 2.6 Ma) has been inferred for the El Salvador deposit,near the southern limit of the Atacama Desert, but the signif-icance of the older age data for supergene minerals in thisupper Eocene center (Gustafson and Hunt, 1975; Gustafsonet al., 2001; Mote et al., 2001) has been questioned byBouzari and Clark (2002).

The 24.4 to 28.0 Ma dates in SURF-110 for the late na-troalunite veinlets correspond to the ca. 26 Ma natroalunitedates from Santa Rosa, providing further evidence of leachingand sulfate precipitation during the late Oligocene.

ConclusionsPorphyry intrusion and hypogene mineralization in the

Cerro Verde-Santa Rosa district occurred at ca. 62 Ma, mark-ing the local termination of magmatic activity. Subsequently,supergene activity controlled by local subplanar landforms, aswell as by highly permeable breccia zones, generated profilesin the Cerro Verde and Santa Rosa deposits with variablethicknesses and complex age relationships (Fig. 11). Dates of36.1 to 38.8 Ma for alunite from Cerro Verde are evidencethat deeply penetrating supergene processes were in progresslocally by the late Eocene, to be subsequently overprinted by24.4 to 28.0 Ma activity that generated minor natroaluniteveinlets. Equivalent ages of 25.5 to 26.8 Ma are also docu-mented for natroalunite from the bottom of the Santa Rosasupergene profile, evidence for ongoing leaching and pene-tration of supergene solutions at both Cerro Verde and SantaRosa. The late Oligocene leaching inferred to have occurredbeneath the La Caldera surface provides a minimum age ofca. 26 Ma for the final configuration of this landform, whichis now preserved as accordant summits including Cerro Verdeand Cerro Negro (Fig. 11). The Santa Rosa pediment, com-prising broad, subplanar valley floors, subsequently degradedthe La Caldera surface, truncating a previously hematitizedchalcocite horizon with 21 Ma natroalunite veins now ex-posed at the top of the Cerro Verde pit. These relationshipsprovide a maximum age of ca. 21 Ma for this erosional sur-face, beneath which the Cerro Verde profile continued todeepen through the Miocene (Fig. 11). Minor supergene ac-tivity persisted into the Pleistocene.

AcknowledgmentsThis study, a component of the senior author’s M.Sc. thesis,

was funded by Rio Tinto Mining and Exploration Ltd., Lima,Peru, who also provided a graduate bursary to C.X.Q., and bygrants to A.H.C. and J.K.W.L. from the Natural Sciences andEngineering Research Council of Canada (NSERC). The datedsamples were collected by C.X.Q. in 2001 during a mappingproject sponsored by the Society of Economic Geologists: itsleaders, William X. Chávez, Jr., and Erich Petersen, pro-vided stimulating discussions in the field. Doug Archibald,Kerry Klassen, Dave Kempson, and Alan Grant assisted with


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2 mµ

Pits onsurfacewith edgeeffects

Natroalunite(b)(a)

Zoned alunitewith APS-rich

core

FIG. 10. a. Backscattered electron image of pink porcelaneous alunite vein(SURF-110). b. Sketch of the same field, showing zoned crystals of alunitewith pale centers, reflecting concentrations of phosphorus and strontium,and irregular veinlets of a “darker” material, inferred to be natroalunite.

Ar-Ar, sulfur isotope, microprobe, and X-ray analyses, respec-tively. Dave Andrews, Bob Harrington, and Tim Moody atRio Tinto are thanked for their unstinting support of this re-search. Paulo Vasconcelos provided an insightful and con-structive review of the original manuscript.

Permission to publish this study, a contribution to theQueen’s University Central Andean Metallogenetic Project(QCAMP), has been given by Sociedad Minera Cerro VerdeS.A. We thank Randy Davenport and Jim Jones for their co-operation.October 22 2002; June 4, 2003

REFERENCESAlpers, C.N., and Brimhall, G.H., 1988, Middle Miocene climatic change in

the Atacama Desert, northern Chile: Evidence from supergene mineral-ization at La Escondida: Geological Society of America Bulletin, v. 100, p.1640–1656.

Anderson, J.A., 1982 Characteristics of leached capping and techniques ofappraisal, in Titley, S.R., ed., Advances in geology of the porphyry copperdeposits, Southwestern North America: Tucson, University of ArizonaPress, p. 275–295.

Beckinsale, R.D., Sánchez, A.W., Brook, M., Cobbing, E.J., Taylor, W.P., andMoore, N.D., 1985, Rb-Sr whole-rock isochron and K-Ar age determina-tions for the Coastal batholith of Peru, in Pitcher, W.S., Atherton, M.P.,Cobbing, E.J., and Beckinsale, R.D., eds., Magmatism at a plate edge: ThePeruvian Andes: Glasgow, Blackie, p. 250–260.

Bouzari, F., and Clark, A.H., 2000, Definition of a protracted history of su-pergene alteration in the Cerro Colorado porphyry copper deposit, Chile,through Ar-Ar dating of alunite-group minerals [abs.]: Geological Society ofAmerica, Abstracts with Programs, v. 32 p. A110.

––––2002 Anatomy, evolution, and metallogenic significance of thesupergene orebody of the Cerro Colorado porphyry copper deposit, IRegión, northern Chile: ECONOMIC GEOLOGY, v. 97, p. 1701–1740.

Cedillo, E., and Wolf, D., 1982 Estudio mineralogico por DRX de las rocasalteradas en la zona de oxidos de Cerro Verde: Boletín de la SociedadGeológica del Perú, v. 69, p. 19–29.

Cedillo, E., Muñoz, C., and Yana, G., 1979, Procesos de alunitizatión en elyacimiento de Cerro Verde: Boletín de la Sociedad Geológica del Perú, v.62 p. 1–18.

Clark, A.H., Farrar, E., Kontak, D.J., Langridge, R.J., Arenas F., M.J.,France, L.J., McBride, S.L., Woodman, P.L., Wasteneys, H.A., Sandeman,H.A., and Archibald, D.A., 1990, Geologic and geochronologic constraintson the metallogenic evolution of the Andes of southeastern Peru: ECO-NOMIC GEOLOGY, v. 85, pp. 1520–1583.

Ednie, H., 2002 Cerro Verde––continuous improvements mean profit:Canadian Institute of Mining, Metallurgy, and Petroleum Bulletin, v. 95, p.14–15.

Estrada, F., 1969, Edades radiométricas en las cercanías de Cerro Verde:Tésis Ingeniería, Universidad Nacional San Agustín, Arequipa, Perú.

––––1978, Edades K-Ar de los principales eventos geológicos de CerroVerde: Lima, Perú, Boletín del Instituto Científico y Tecnológico Minero,p. 1–15.

Gustafson, L.B., and Hunt, J.P., 1975, The porphyry copper deposit at ElSalvador, Chile: ECONOMIC GEOLOGY, v. 70, p. 857–912.

Gustafson, L.B., Orquera, W., McWilliams, M., Castro, M., Olivares, O.,Rojas, G., Maleunda, J., and Mendez, M., 2001, Multiple centers ofmineralization in the Indio Muerto district, El Salvador, Chile: ECONOMICGEOLOGY, v. 96, p. 325–350.

Jenks, W.F., 1948, Geología de la Hoja de Arequipa: Instituto Geológico delPerú, Boletín no. 9, 204 p.

Kihien, C.A., 1975, Alteración y su relación con la mineralización en elpórfido de cobre de Cerro Verde: Boletín de la Sociedad Geológica delPerú, v. 46, p. 103–126.

Le Bel, L.M., 1985, Mineralization in the Arequipa segment: The porphyryCu deposit of Cerro Verde/Santa Rosa, in Pitcher, W.S., Atherton, M.P.,Cobbing, E.J., and Beckinsale, R.D., eds., Magmatism at a plate edge: ThePeruvian Andes: Glasgow, Blackie, p. 250–260.

Love, D.A., Clark, A.H., Hodgson, C.J., Mortensen, J.K., and Archibald,D.A., 1998, The timing of adularia-sericite-type and alunite-kaolinite-typealteration, Mount Skukum epithermal gold deposit, Yukon Territory,Canada: ECONOMIC GEOLOGY, v. 93, p. 437–462.

Mote, T.I., Becker, T.A., Renne, P., and Brimhall, G.H., 2001, Chronology ofexotic mineralization at El Salvador, Chile by 40Ar/39Ar dating of copperwad and supergene alunite: ECONOMIC GEOLOGY, v. 96, p. 351–366.

Mukasa, S.B., 1986, Zircon U-Pb ages of super-units in the Coastal batholith,Peru: Implications for magmatic and tectonic processes: Geological Societyof America Bulletin, v. 97, p. 241–254.

Mukasa, S.B., and Tilton, G.R., 1985, Zircon U-Pb ages of super-units in theCoastal batholith, Peru: Implications for magmatic and tectonic processes,in Pitcher, W.S., Atherton, M.P., Cobbing, E.J., and Beckinsale, R.D., eds.,Magmatism at a plate edge: The Peruvian Andes: Glasgow, Blackie, p.203–207.

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Hypogene Zone


Transition Zone Chalcocite Zone

HematiteLeached Cap

Oxide Zone

JarositicLeached Cap

PreservedLandforms

?

?

?

?

?

?

?

Santa Rosa surface (< 21 Ma)

La Caldera surface (> 26 Ma)

0

100 m

100 m

20.6 - 23.3 Ma(SURF 111a, b)

10.9 - 12.6 Ma(SURF 114)

4.9 - 6.7 Ma(SURF 109a, b)

36.1 - 38.8 Ma24.4 - 28.0 Ma

(SURF 110)

26.9 - 27.4 Ma(SURF 113)

26.2 - 26.8 Ma(SURF 112)

Inferred Fault?

Santa Rosa deposit Cerro Verde depositNW

Cerro VerdeSE

FIG. 11. Idealized cross section, looking southwest, of the Cerro Verde-Santa Rosa supergene profile, summarizing therelationships between supergene mineralization and post-hypogene landform development.

Rowland, M., and Clark, A.H., 2001, Temporal overlap of supergene alter-ation and high-sulfidation mineralization in the Spence porphyry copperdeposit, II Región, Chile [abs.]: Geological Society of America, Abstractswith Programs, v. 33, p. A-358.

Sandeman, H.A., Clark, A.H., and Farrar, E., 1995, An integrated tectono-magmatic model for the evolution of the southern Peruvian Andes(13°–20°S) since 55 Ma: International Geology Review, v. 37, p. 1039–1037.

Sandeman, H.A., Archibald, D.A., Grant, J.W., Villeneuve, M.E., and Ford,F.D., 1999, Characterization of the chemical composition and 40Ar-39Ar sys-tematics of intralaboratory standard MAC-83 biotite, in Radiogenic Ageand Isotopic Studies: Report 12 Geological Survey of Canada––CurrentResearch 1999-F, p. 13–26.

Sillitoe, R.H., and McKee, E.H., 1996, Age of supergene oxidation and en-richment in the Chilean porphyry copper province: ECONOMIC GEOLOGY,v. 91, p. 164–179.

Stewart, J.W., 1968, Rocas intrusivas del Cuadrángulo de La Joya: ServiciosGeología y Minería, Perú Boletín 19, p. 43–95.

Stoffregen, R.E., and Alpers, C.N., 1987, Woodhouseite and svanbergite inhydrothermal ore deposits: Products of apatite destruction during ad-vanced argillic alteration: Canadian Mineralogist, v. 25, p. 201–211.

Tosdal, R.M., Clark, A.H., and Farrar, E., 1984, Cenozoic polyphase land-scape and tectonic evolution of the Cordillera occidental, southernmostPeru: Geological Society of America Bulletin, v. 95, p. 1318–1332.

Vasconcelos, P.M., 1999, K-Ar and 40Ar-39Ar geochronology of weatheringprocesses: Annual Review of Earth Planetary Sciences, v. 27, p. 133–229.

Vasconcelos, P.M., Brimhall, G.H., Becker, T.A., and Renne, P.R., 1994, 40Ar-39Ar analysis of supergene jarosite and alunite: Implications to the pale-oweathering history of the western USA and West Africa: Geochimica etCosmochimica Acta, v. 58, p. 401–420.

Wasteneys, H.A., Clark, A.H., Farrar, E., and Langridge, R.J., 1995, Grenvil-lian granulite-facies metamorphism in the Arequipa Massif, Peru: A Lau-rentia-Gondwana link: Earth and Planetary Science Letters, v. 132 p.63–73.

Watanabe, Y., and Hedenquist, J.W., 2001, Mineralogic and stable isotopezonation at the surface over the El Salvador porphyry copper deposit,Chile: ECONOMIC GEOLOGY, v. 96, p. 1775–1797.

Zweng, P.L., and Clark, A.H., 1995, Hypogene evolution of the Toquepalaporphyry copper-molybdenum deposit, Moquegua, southeastern Peru: Ari-zona Geological Society Digest 20, p. 566–612.


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Appendix 1

40Ar-39Ar Analytical Data

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Power Isotope ratios(watts) 40Ar/39Ar 38Ar/39Ar 37Ar/39Ar 36Ar/39Ar Ca/K Cl/K 40Ar (%) 39Ar (%) 40Ar*/39ArK Age ± 2σ

a. Hydrothermal sericites from Cerro Verde and Santa Rosa

SURF-110, sericite, J = 0.002426 ± 0.000072; volume 39ArK = 20.07 × 10–10cm3, integrated age = 61.80 ± 3.27 Ma (2σ)0.75 1831.466 ± 0.329 1.326 ± 0.367 0.168 ± 1.688 6.472 ± 0.330 1.052 0.024 101.67 0.83 -32.215 ± 64.791 -146.81 ± 307.610.90> 1250.463 0.041 0.912 0.090 0.057 0.941 4.283 0.044 0.357 0.023 98.42 0.79 20.931 24.246 89.36 100.991.10> 512.848 0.033 0.370 0.082 0.021 1.134 1.714 0.036 0.116 0.007 95.98 1.61 21.230 8.666 90.61 36.071.30> 331.933 0.018 0.240 0.084 0.019 0.928 1.086 0.025 0.105 0.004 93.83 1.77 21.065 6.155 89.92 25.63

<1.60> 142.716 0.010 0.106 0.103 0.007 0.604 0.444 0.022 0.028 0.001 88.98 3.39 15.998 2.590 68.69 10.91<2.00> 67.648 0.007 0.057 0.072 0.004 0.714 0.185 0.024 0.022 0.002 78.27 6.90 14.840 1.307 63.81 5.52<3.00> 29.978 0.005 0.026 0.068 0.002 0.353 0.054 0.022 0.012 0.000 51.87 20.06 14.513 0.356 62.43 1.51<4.00> 20.291 0.004 0.020 0.063 0.002 0.531 0.021 0.044 0.010 0.000 29.52 19.70 14.379 0.287 61.86 1.22<5.00> 18.508 0.004 0.019 0.089 0.001 0.799 0.015 0.053 0.006 0.000 21.80 17.07 14.564 0.241 62.64 1.02<7.00> 18.893 0.004 0.019 0.069 0.002 0.240 0.017 0.053 0.009 0.000 25.27 27.88 14.190 0.272 61.06 1.15

SURF-111, sericite, J = 0.002426 ± 0.000064; volume 39ArK = 26.70 × 10–10cm3, integrated age = 56.22 ± 5.54 Ma (2σ)0.50> 669.039 ± 0.050 0.525 ± 0.179 0.021 ± 1.787 2.304 ± 0.062 0.119 0.019 99.09 3.22 6.161 ± 25.857 26.77 ± 111.50

0.75> 502.677 0.034 0.394 0.141 0.038 1.245 1.719 0.043 0.227 0.014 98.47 3.92 7.770 14.006 33.69 60.171.00> 345.413 0.037 0.279 0.187 0.027 0.828 1.197 0.046 0.158 0.010 99.71 5.21 0.994 9.872 4.34 43.10

<1.25> 387.986 0.022 0.294 0.087 0.014 0.614 1.298 0.031 0.069 0.009 96.24 3.11 14.797 8.631 63.63 36.47<1.50> 330.103 0.018 0.252 0.084 0.012 1.394 1.099 0.024 0.055 0.007 95.64 2.43 14.609 5.416 62.83 22.89<1.75> 210.627 0.012 0.171 0.100 0.008 1.450 0.693 0.024 0.034 0.006 94.64 3.99 11.414 4.371 49.28 18.61<2.00> 162.036 0.010 0.127 0.057 0.008 0.800 0.519 0.018 0.035 0.004 92.00 4.08 13.091 2.343 56.40 9.94<2.25> 134.685 0.014 0.109 0.075 0.006 0.477 0.422 0.030 0.024 0.003 90.12 3.48 13.477 3.348 58.04 14.19<2.50> 92.503 0.010 0.077 0.081 0.004 1.254 0.274 0.030 0.006 0.003 85.10 4.28 13.938 2.368 59.99 10.02<3.00> 57.729 0.007 0.050 0.080 0.005 0.271 0.151 0.019 0.026 0.002 74.79 8.62 14.657 0.843 63.04 3.56<4.00> 33.782 0.005 0.033 0.062 0.003 0.390 0.067 0.030 0.013 0.002 56.38 14.18 14.819 0.594 63.72 2.51<5.00> 33.111 0.006 0.033 0.066 0.002 0.638 0.066 0.033 0.008 0.001 56.48 13.10 14.498 0.640 62.36 2.71<6.00> 31.609 0.005 0.031 0.061 0.002 0.613 0.061 0.022 0.008 0.001 54.79 13.48 14.376 0.400 61.85 1.69<8.00> 25.511 0.005 0.028 0.077 0.002 0.471 0.040 0.024 0.011 0.001 44.14 16.90 14.319 0.293 61.61 1.24

SURF-119, sericite, J = 0.002195 ± 0.000014; volume 39ArK = 0.72 × 10–10cm3, integrated age = 60.71 ± 1.51 Ma (2σ)<1.00> 16.534 ± 0.047 0.104 ± 0.473 0.069 ± 0.326 0.067 ± 0.293 0.109 0.005 9.21 2.69 17.197 ± 7.216 66.85 ± 27.54<3.00> 16.330 0.008 0.029 0.122 0.008 0.239 0.006 0.208 0.022 0.002 5.13 46.55 15.644 0.383 60.91 1.47<5.00> 15.925 0.007 0.032 0.159 0.007 0.275 0.006 0.227 0.013 0.003 2.68 42.26 15.681 0.439 61.05 1.68

7.00> 15.783 0.018 0.060 0.251 0.029 0.269 0.026 0.287 0.047 0.006 12.46 8.51 14.364 2.397 56.00 9.20


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b. Supergene alunite-group minerals from Cerro Verde

SURF-111a, natroalunite, J = 0.002413 ± 0.000010; volume 39ArK = 64.05 × 10–10cm3, integrated age = 19.50 ± 0.45 Ma (2σ)0.30 181.047 ± 0.023 0.154 ± 0.133 0.012 ± 1.417 0.685 ± 0.035 0.004 0.002 107.10 0.27 -13.397 ± 5.779 -59.28 ± 26.000.75 18.961 0.004 0.059 0.023 0.001 0.991 0.055 0.030 0.003 0.008 82.61 15.57 3.287 0.493 14.25 2.13<1.00> 9.052 0.004 0.063 0.017 0.001 0.648 0.015 0.037 0.002 0.010 48.67 23.16 4.641 0.170 20.09 0.73<1.25> 7.621 0.004 0.060 0.013 0.001 0.375 0.010 0.057 0.003 0.010 36.84 17.62 4.806 0.170 20.80 0.73<1.50> 6.734 0.004 0.065 0.013 0.001 0.570 0.007 0.071 0.002 0.011 28.74 13.19 4.787 0.149 20.72 0.64<2.00> 8.759 0.005 0.062 0.023 0.003 0.396 0.014 0.083 0.005 0.010 43.20 4.03 4.948 0.351 21.41 1.51

<4.00> 6.052 0.004 0.068 0.011 0.001 0.415 0.004 0.091 0.002 0.012 20.12 24.96 4.827 0.120 20.89 0.526.00 14.791 0.007 0.069 0.065 0.005 0.726 0.034 0.123 0.008 0.011 59.19 1.19 5.999 1.252 25.93 5.37

SURF-111a, natroalunite, J = 0.002413 ± 0.000010; volume 39ArK = 90.39 × 10–10cm3, integrated age = 20.08 ± 0.34 Ma (2σ)0.50 188.629 ± 0.041 0.186 ± 0.187 0.020 ± 1.635 0.671 ± 0.072 0.000 0.007 99.42 0.06 1.155 ± 12.748 5.02 ± 55.340.75 194.660 0.016 0.168 0.081 0.008 1.487 0.715 0.029 0.000 0.003 104.45 0.17 -8.994 5.459 -39.59 24.291.00 66.329 0.006 0.064 0.059 0.001 2.458 0.226 0.017 0.001 0.002 97.67 2.35 1.530 1.084 6.65 4.701.20 40.133 0.005 0.050 0.039 0.001 1.321 0.135 0.020 0.002 0.002 96.47 2.66 1.397 0.799 6.07 3.471.50 38.400 0.005 0.054 0.040 0.002 1.160 0.125 0.018 0.005 0.004 93.19 3.42 2.605 0.671 11.31 2.90<1.75> 13.665 0.003 0.055 0.026 0.001 1.022 0.031 0.023 0.002 0.008 65.60 11.57 4.694 0.220 20.32 0.95<2.00> 8.141 0.003 0.059 0.018 0.000 1.226 0.011 0.036 0.001 0.010 39.84 27.43 4.892 0.124 21.17 0.53<2.20> 7.242 0.003 0.061 0.020 0.000 0.851 0.008 0.044 0.002 0.010 31.20 26.13 4.975 0.106 21.53 0.46<2.40> 8.640 0.003 0.064 0.018 0.001 0.521 0.013 0.038 0.003 0.011 43.04 12.09 4.910 0.148 21.25 0.64<3.00> 8.879 0.003 0.063 0.021 0.001 0.370 0.014 0.033 0.005 0.011 44.13 9.52 4.947 0.139 21.41 0.604.20 9.785 0.003 0.064 0.020 0.002 0.391 0.016 0.039 0.006 0.011 46.16 4.59 5.242 0.190 22.67 0.82

SURF-111b, alunite, J = 0.002417 ± 0.000012; volume 39ArK = 138.16 × 10–10cm3, integrated age = 24.06 ± 0.39 Ma (2σ)0.50 39.941 ± 0.162 0.253 ± 0.558 0.166 ± 0.726 0.227 ± 0.497 0.006 0.013 79.05 0.00 34.947 ± 181.043 146.29 ± 727.950.75 152.370 0.099 0.233 0.355 0.098 0.792 0.594 0.150 0.011 0.003 101.58 0.01 -4.712 40.037 -20.66 176.591.00 273.501 0.045 0.259 0.130 0.043 0.768 0.948 0.057 0.021 0.008 96.74 0.04 11.247 12.397 48.39 52.632.00 345.202 0.045 0.261 0.082 0.008 1.402 1.152 0.048 0.041 0.007 95.91 0.75 14.272 5.231 61.18 22.052.50 33.942 0.006 0.056 0.041 0.001 1.571 0.096 0.017 0.005 0.005 81.51 8.17 6.286 0.480 27.21 2.062.75 11.786 0.003 0.040 0.028 0.001 0.327 0.022 0.020 0.003 0.005 52.69 16.41 5.581 0.132 24.17 0.57<3.00> 8.670 0.003 0.040 0.028 0.001 0.343 0.011 0.037 0.004 0.005 38.11 30.36 5.368 0.127 23.26 0.55<3.20> 9.091 0.003 0.042 0.025 0.001 0.477 0.013 0.024 0.005 0.006 40.94 28.63 5.372 0.095 23.27 0.41<3.40> 10.258 0.003 0.043 0.020 0.001 0.652 0.017 0.031 0.005 0.006 47.47 11.51 5.392 0.160 23.36 0.69<3.60> 12.890 0.004 0.047 0.032 0.002 0.256 0.026 0.029 0.008 0.006 57.59 4.12 5.473 0.224 23.71 0.96

SURF-111c, jarosite, J = 0.002188 ± 0.000014; volume 39ArK = 51.09 × 10–10cm3, integrated age = 0.72 ± 0.03 Ma (2σ)0.50 2.004 ± 0.003 0.015 ± 0.027 0.001 ± 0.188 0.006 ± 0.018 0.002 -0.000 91.54 15.19 0.140 ± 0.035 0.55 ± 0.14<1.00> 0.567 0.007 0.014 0.030 0.001 0.105 0.001 0.030 0.002 -0.000 63.67 29.62 0.175 0.013 0.69 0.05<1.50> 0.361 0.004 0.013 0.019 0.000 0.255 0.001 0.059 0.000 -0.000 36.57 30.76 0.198 0.009 0.78 0.04<2.00> 0.428 0.004 0.013 0.027 0.000 0.359 0.001 0.073 0.000 -0.000 48.66 12.06 0.186 0.019 0.73 0.082.50 0.437 0.006 0.013 0.036 0.000 0.280 0.001 0.118 0.001 -0.000 36.30 9.36 0.241 0.029 0.95 0.113.00 0.605 0.021 0.016 0.060 0.002 0.185 0.002 0.123 0.006 0.000 66.45 3.02 0.160 0.077 0.63 0.31

SURF-114, natroalunite, J = 0.002147 ± 0.000018; volume 39ArK = 28.28 × 10–10cm3, integrated age = 11.86 ± 0.56 Ma (2σ)0.50> 499.571 ± 0.024 0.440 ± 0.061 0.054 ± 0.178 1.805 ± 0.028 0.132 0.016 103.61 0.26 -19.556 ± 8.597 -77.38 ± 34.76

1.00 99.825 0.007 0.187 0.044 0.011 0.178 0.340 0.016 0.037 0.025 98.29 5.30 1.689 1.466 6.53 5.66<1.50> 15.820 0.004 0.109 0.022 0.003 0.128 0.044 0.015 0.012 0.020 81.07 30.85 2.979 0.199 11.50 0.77<2.00> 13.759 0.004 0.107 0.022 0.004 0.066 0.037 0.016 0.014 0.020 77.48 27.87 3.083 0.179 11.90 0.69<2.50> 16.035 0.004 0.126 0.016 0.005 0.067 0.044 0.016 0.018 0.024 79.13 24.05 3.334 0.204 12.87 0.783.00 22.508 0.005 0.154 0.018 0.007 0.076 0.065 0.017 0.024 0.029 83.37 8.34 3.736 0.323 14.41 1.245.00 45.269 0.005 0.169 0.032 0.010 0.097 0.143 0.016 0.032 0.029 90.67 3.33 4.228 0.658 16.30 2.53

SURF-114, natroalunite, J = 0.002425 ± 0.000028; volume 39ArK = 17.76 × 10–10cm3, integrated age = 12.51 ± 1.09 Ma (2σ)0.50 377.362 ± 0.092 0.411 ± 0.384 0.110 ± 1.233 1.373 ± 0.138 0.456 0.044 95.90 0.06 21.987 ± 59.974 93.71 ± 249.101.00 1230.844 0.049 0.903 0.102 0.011 6.713 4.352 0.052 0.024 0.020 101.52 0.60 -19.568 24.905 -87.71 114.39<1.50> 30.918 0.005 0.126 0.026 0.002 0.346 0.098 0.016 0.014 0.021 91.23 23.65 2.701 0.447 11.78 1.94<1.75> 16.613 0.004 0.120 0.022 0.003 0.231 0.048 0.027 0.014 0.022 81.97 29.40 2.984 0.382 13.01 1.66<2.00> 11.127 0.004 0.108 0.017 0.003 0.151 0.029 0.021 0.018 0.020 73.38 34.12 2.949 0.184 12.86 0.802.25 22.698 0.006 0.142 0.025 0.005 0.273 0.067 0.028 0.027 0.026 83.71 12.17 3.696 0.555 16.10 2.40

SURF-109a, natroalunite, J = 0.002403 ± 0.000014; volume 39ArK = 50.96 × 10–10cm3, integrated age = 4.68 ± 0.38 Ma (2σ)0.50 307.469 ± 0.022 0.304 ± 0.111 0.032 ± 0.458 1.103 ± 0.031 0.011 0.018 101.38 0.18 -4.780 ± 7.746 -20.84 ± 33.971.00 170.257 0.015 0.169 0.060 0.012 0.427 0.602 0.022 0.054 0.009 101.41 1.67 -2.482 3.057 -10.79 13.33<1.50> 8.252 0.003 0.076 0.017 0.003 0.074 0.024 0.017 0.017 0.013 85.02 42.52 1.213 0.122 5.25 0.53<1.70> 4.240 0.003 0.081 0.014 0.002 0.099 0.011 0.021 0.011 0.015 74.61 36.47 1.052 0.072 4.56 0.31

Appendix (Cont.)

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<1.90> 6.212 0.003 0.107 0.015 0.004 0.116 0.018 0.023 0.022 0.020 81.29 17.72 1.139 0.123 4.93 0.532.50> 13.140 0.007 0.106 0.043 0.016 0.164 0.041 0.061 0.070 0.019 84.70 1.44 2.010 0.758 8.69 3.27

SURF-109b, alunite, J = 0.002397 ± 0.000018; volume 39ArK = 50.61 × 10–10cm3, integrated age = 5.35 ± 2.44 Ma (2σ)0.75> 420.870 ± 0.060 0.349 ± 0.128 0.071 ± 0.663 1.497 ± 0.072 0.198 0.013 102.40 3.12 -10.205 ± 18.067 -44.68 ± 80.09

<1.00> 7.100 0.004 0.039 0.016 0.024 0.028 0.019 0.034 0.067 0.005 76.05 17.61 1.677 0.195 7.24 0.84<1.25> 2.349 0.003 0.032 0.010 0.016 0.016 0.003 0.064 0.045 0.004 34.01 32.21 1.524 0.056 6.58 0.24<1.50> 2.036 0.003 0.030 0.013 0.017 0.021 0.002 0.149 0.048 0.004 23.90 20.82 1.518 0.085 6.55 0.37<2.00> 1.904 0.004 0.029 0.021 0.019 0.036 0.002 0.295 0.053 0.003 20.91 9.99 1.462 0.160 6.31 0.694.00 5.005 0.006 0.033 0.086 0.026 0.103 0.006 0.331 0.071 0.004 25.73 2.23 3.624 0.622 15.60 2.67<7.00> 2.083 0.004 0.031 0.017 0.016 0.030 0.002 0.203 0.045 0.004 20.18 14.03 1.627 0.110 7.02 0.47

SURF-109c, jarosite, J = 0.002210 ± 0.000012; volume 39ArK = 36.28 × 10–10cm3, integrated age = 1.56 ± 0.42 Ma (2σ)0.50> 27.797 ± 0.004 0.057 ± 0.031 0.000 ± 0.515 0.094 ± 0.015 0.001 0.006 98.14 25.14 0.489 ± 0.403 1.95 ± 1.61

<1.00> 3.435 0.003 0.060 0.014 0.000 0.226 0.011 0.016 0.001 0.010 89.93 42.18 0.317 0.051 1.26 0.20<1.50> 2.934 0.003 0.071 0.012 0.000 0.231 0.009 0.017 0.001 0.013 86.67 21.43 0.362 0.046 1.44 0.182.50 5.078 0.004 0.077 0.026 0.001 0.227 0.016 0.021 0.003 0.014 89.68 6.05 0.494 0.103 1.97 0.414.00 4.070 0.005 0.078 0.025 0.001 0.281 0.013 0.029 0.004 0.014 86.94 5.20 0.501 0.109 2.00 0.44

c. Supergene alunite-group minerals from Santa Rosa

SURF-112, natroalunite, J = 0.002420 ± 0.000014; volume 39ArK = 6.58 × 10–10cm3, integrated age = 26.82 ± 1.69 Ma (2σ)0.50> 14.300 ± 0.016 0.683 ± 0.025 0.555 ± 0.025 0.036 ± 0.278 1.614 0.159 61.20 4.93 5.459 ± 3.135 23.68 ± 13.51

<1.00> 10.353 0.005 1.243 0.006 0.581 0.008 0.016 0.073 1.635 0.282 42.74 42.91 5.911 0.363 25.62 1.56<1.25> 10.797 0.009 1.115 0.012 0.591 0.013 0.018 0.270 1.689 0.257 36.59 10.56 6.745 1.510 29.21 6.49<2.50> 12.217 0.010 1.078 0.013 0.568 0.016 0.023 0.274 1.632 0.249 46.05 8.81 6.509 1.905 28.19 8.19<5.00> 11.704 0.005 1.090 0.007 0.600 0.009 0.019 0.093 1.698 0.248 43.89 27.59 6.549 0.541 28.37 2.33

7.00> 10.828 0.015 1.118 0.019 0.581 0.022 0.024 0.437 1.693 0.264 46.52 5.20 5.611 3.302 24.33 14.22

SURF-112, natroalunite, J = 0.002420 ± 0.000014; volume 39ArK = 6.29 × 10–10cm3, integrated age = 27.25 ± 0.92 Ma (2σ)0.50 27.377 ± 0.114 0.362 ± 0.624 0.223 ± 0.451 0.193 ± 0.536 1.724 0.108 73.98 0.11 11.232 ± 60.543 48.38 ± 257.331.00 18.736 0.022 0.576 0.060 0.232 0.087 0.066 0.223 1.361 0.131 64.79 1.46 6.782 4.678 29.37 20.09<1.50> 7.917 0.007 1.002 0.011 0.243 0.019 0.010 0.173 1.426 0.227 28.10 12.59 5.676 0.537 24.61 2.31<2.00> 9.651 0.004 1.325 0.007 0.236 0.011 0.014 0.040 1.390 0.300 38.37 54.12 5.955 0.169 25.81 0.73<2.25> 10.726 0.004 1.030 0.008 0.226 0.011 0.017 0.090 1.331 0.233 40.58 28.98 6.387 0.446 27.67 1.923.00 16.739 0.016 0.278 0.079 0.098 0.096 0.022 0.388 0.554 0.061 16.71 2.73 14.254 2.640 61.18 11.14

SURF-113, natroalunite, J = 0.002423 ± 0.000020; volume 39ArK = 34.07 × 10–10cm3, integrated age = 26.88 ± 0.32 Ma (2σ)0.50 7.998 ± 0.011 0.259 ± 0.052 0.067 ± 0.072 0.016 ± 0.402 0.362 0.055 29.95 0.90 5.565 ± 1.964 24.16 ± 8.471.00 7.572 0.005 0.271 0.012 0.053 0.023 0.005 0.164 0.300 0.059 13.64 6.98 6.541 0.245 28.37 1.061.25 7.229 0.004 0.309 0.013 0.048 0.022 0.003 0.162 0.273 0.067 9.50 12.57 6.548 0.152 28.40 0.661.50 7.052 0.003 0.313 0.011 0.048 0.020 0.003 0.116 0.273 0.068 10.10 19.58 6.345 0.106 27.53 0.461.75 6.905 0.003 0.301 0.009 0.048 0.017 0.003 0.102 0.276 0.066 10.38 31.35 6.195 0.090 26.88 0.392.00 6.657 0.003 0.287 0.011 0.051 0.017 0.003 0.074 0.294 0.062 11.90 27.34 5.869 0.072 25.47 0.312.50 6.644 0.009 0.221 0.046 0.074 0.050 0.009 0.474 0.408 0.046 9.04 1.27 6.013 1.233 26.09 5.31

SURF-113, natroalunite, J = 0.002423 ± 0.000020; volume 39ArK = 31.12 × 10–10cm3, integrated age = 27.37 ± 0.32 Ma (2σ)0.50 8.327 ± 0.010 0.281 ± 0.031 0.071 ± 0.059 0.017 ± 0.247 0.377 0.061 37.42 1.11 5.150 ± 1.260 22.37 ± 5.441.00 7.758 0.004 0.282 0.014 0.057 0.032 0.006 0.154 0.331 0.061 17.52 5.43 6.384 0.299 27.69 1.291.25 7.486 0.003 0.306 0.008 0.048 0.015 0.004 0.118 0.281 0.067 11.32 14.68 6.640 0.127 28.79 0.551.50 7.280 0.003 0.326 0.011 0.049 0.018 0.003 0.133 0.288 0.071 10.69 20.92 6.504 0.132 28.21 0.571.75 7.127 0.003 0.326 0.008 0.051 0.017 0.003 0.058 0.300 0.071 12.23 29.62 6.259 0.063 27.15 0.273.00 6.988 0.003 0.313 0.009 0.053 0.015 0.004 0.088 0.312 0.068 13.04 28.23 6.080 0.095 26.38 0.41

d. Alunite sample SURF-110 from Cerro Verde

SURF-110, alunite, J = 0.002409 ± 0.000012; volume 39ArK = 115.45 × 10–10cm3, integrated age = 36.17 ± 0.25 Ma (2σ)0.75 35.254 ± 0.038 0.083 ± 0.440 0.056 ± 0.448 0.136 ± 0.158 0.270 -0.004 93.56 0.04 2.583 ± 7.628 11.19 ± 32.951.50 82.325 0.031 0.111 0.178 0.043 0.323 0.300 0.076 0.142 0.005 103.30 0.07 -3.054 6.936 -13.32 30.372.50 9.625 0.004 0.030 0.039 0.014 0.049 0.012 0.053 0.091 0.003 34.00 2.79 6.352 0.188 27.40 0.813.00 8.484 0.004 0.023 0.036 0.011 0.052 0.002 0.138 0.069 0.002 5.98 6.66 7.989 0.088 34.39 0.373.50 8.802 0.003 0.022 0.041 0.012 0.044 0.001 0.326 0.079 0.002 3.09 6.47 8.558 0.125 36.82 0.534.25 7.832 0.003 0.023 0.034 0.011 0.038 0.001 0.352 0.072 0.002 2.56 10.40 7.651 0.095 32.95 0.415.00 8.383 0.003 0.021 0.037 0.009 0.062 0.001 0.293 0.058 0.002 1.43 21.26 8.285 0.051 35.65 0.226.00 8.730 0.003 0.019 0.034 0.007 0.050 0.000 0.869 0.045 0.001 1.29 24.23 8.641 0.125 37.17 0.537.00 8.818 0.003 0.019 0.043 0.005 0.072 0.001 0.557 0.036 0.001 1.17 13.46 8.741 0.090 37.60 0.388.00 9.033 0.003 0.018 0.061 0.006 0.070 0.000 0.759 0.040 0.001 0.53 14.61 9.013 0.083 38.75 0.35

Appendix (Cont.)

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0361-0128/98/000/000-00 $6.00 1696

SURF-110, alunite, J = 0.002173 ± 0.000014; volume 39ArK = 120.24 × 10–10cm3, integrated age = 34.28 ± 0.23 Ma (2σ)1.00 11.735 ± 0.006 0.036 ± 0.069 0.022 ± 0.042 0.027 ± 0.028 0.090 0.003 64.70 0.73 4.142 ± 0.226 16.17 ± 0.881.50 6.892 0.003 0.027 0.022 0.009 0.028 0.002 0.047 0.039 0.003 8.99 4.40 6.273 0.040 24.42 0.152.00 7.459 0.003 0.023 0.030 0.010 0.024 0.001 0.097 0.041 0.002 3.40 11.02 7.211 0.038 28.05 0.152.50 8.887 0.003 0.019 0.032 0.007 0.029 0.000 0.189 0.030 0.001 1.25 17.41 8.789 0.039 34.13 0.153.00 9.153 0.003 0.019 0.041 0.008 0.033 0.000 0.296 0.032 0.001 1.18 14.52 9.059 0.051 35.17 0.203.50 9.274 0.003 0.019 0.040 0.008 0.027 0.000 0.292 0.034 0.001 0.95 16.54 9.200 0.045 35.71 0.174.00 9.575 0.003 0.018 0.040 0.011 0.033 0.001 0.145 0.046 0.001 1.49 10.93 9.449 0.042 36.67 0.165.00 9.424 0.003 0.019 0.040 0.014 0.020 0.000 0.180 0.059 0.001 1.08 13.00 9.338 0.040 36.24 0.156.00 9.801 0.003 0.018 0.042 0.011 0.026 0.000 0.234 0.048 0.001 1.06 11.45 9.714 0.047 37.69 0.18

SURF-110, alunite, J = 0.002409 ± 0.000012; volume 39ArK = 45.41 × 10–10cm3, integrated age = 34.20 ± 0.45 Ma (2σ)0.40 20.642 ± 0.094 0.107 ± 0.773 0.102 ± 0.775 0.102 ± 0.776 0.167 0.003 112.52 0.07 -2.224 ± 29.984 -9.69 ± 131.010.75 37.848 0.073 0.118 0.595 0.106 0.655 0.170 0.371 0.155 0.004 112.09 0.09 -4.780 22.409 -20.90 98.531.00 63.703 0.057 0.122 0.420 0.101 0.459 0.248 0.182 0.212 0.006 103.93 0.13 -2.776 14.869 -12.10 65.062.00 7.572 0.005 0.023 0.041 0.020 0.044 0.005 0.171 0.055 0.002 15.43 8.57 6.366 0.234 27.46 1.003.00 8.449 0.004 0.020 0.028 0.021 0.032 0.001 0.550 0.057 0.001 2.86 45.50 8.222 0.161 35.38 0.693.25 8.109 0.005 0.022 0.036 0.024 0.031 0.002 0.319 0.065 0.002 5.04 14.63 7.695 0.180 33.13 0.773.60 8.034 0.005 0.022 0.033 0.020 0.042 0.001 0.601 0.056 0.002 2.21 9.43 7.837 0.212 33.74 0.904.50 8.414 0.005 0.020 0.035 0.017 0.044 0.001 0.725 0.048 0.001 1.73 17.20 8.270 0.171 35.59 0.736.00 8.736 0.005 0.020 0.084 0.017 0.096 0.001 0.986 0.046 0.001 -0.02 4.39 8.681 0.390 37.34 1.66

SURF-110, alunite, J = 0.002152 ± 0.000016; volume 39ArK = 61.63 × 10–10cm3, integrated age = 33.81 ± 0.26 Ma (2σ)0.50 25.732 ± 0.017 0.070 ± 0.070 0.063 ± 0.053 0.091 ± 0.031 0.220 0.008 94.05 0.27 1.533 ± 0.768 5.94 ± 2.971.00 8.331 0.004 0.024 0.013 0.023 0.015 0.006 0.023 0.085 0.002 17.97 9.81 6.835 0.049 26.34 0.191.50 8.977 0.003 0.020 0.013 0.022 0.013 0.001 0.065 0.081 0.001 3.49 20.39 8.675 0.039 33.37 0.152.75 9.065 0.003 0.020 0.013 0.020 0.014 0.001 0.255 0.076 0.001 1.93 25.66 8.902 0.065 34.24 0.252.00 9.318 0.003 0.021 0.015 0.024 0.015 0.001 0.092 0.091 0.002 2.36 11.23 9.111 0.042 35.03 0.162.50 9.449 0.003 0.021 0.015 0.028 0.013 0.001 0.141 0.104 0.002 1.93 7.93 9.279 0.048 35.67 0.183.50 9.340 0.003 0.020 0.016 0.023 0.014 0.001 0.160 0.087 0.001 1.48 9.56 9.214 0.048 35.42 0.185.00 9.528 0.003 0.021 0.015 0.028 0.013 0.001 0.122 0.106 0.001 1.53 15.16 9.396 0.040 36.11 0.15

SURF-110, alunite, J = 0.002147 ± 0.000018; volume 39ArK = 86.75 × 10–10cm3, integrated age = 33.83 ± 0.29 Ma (2σ)0.50 36.487 ± 0.040 0.095 ± 0.426 0.083 ± 0.270 0.142 ± 0.128 0.259 0.008 88.76 0.06 4.411 ± 5.747 17.01 ± 22.050.75 36.385 0.021 0.085 0.286 0.070 0.201 0.139 0.076 0.204 0.005 91.41 0.06 3.288 3.297 12.69 12.681.25 15.489 0.005 0.040 0.071 0.032 0.046 0.039 0.033 0.110 0.004 67.95 0.70 4.965 0.383 19.13 1.472.00 7.836 0.003 0.026 0.037 0.021 0.019 0.002 0.073 0.078 0.003 7.06 13.67 7.285 0.051 28.00 0.192.25 8.108 0.003 0.023 0.028 0.012 0.025 0.001 0.109 0.044 0.002 2.27 11.40 7.929 0.038 30.45 0.142.50 8.879 0.003 0.020 0.036 0.013 0.021 0.001 0.160 0.048 0.001 1.21 10.01 8.780 0.041 33.69 0.162.75 9.165 0.003 0.019 0.044 0.010 0.029 0.000 0.212 0.036 0.001 0.68 9.81 9.113 0.043 34.96 0.163.00 9.347 0.003 0.020 0.035 0.016 0.022 0.000 0.213 0.057 0.001 0.84 12.62 9.282 0.044 35.60 0.173.25 9.351 0.003 0.019 0.036 0.014 0.022 0.000 0.331 0.051 0.001 0.90 11.08 9.279 0.056 35.59 0.213.50 9.587 0.004 0.020 0.033 0.018 0.022 0.001 0.190 0.068 0.001 1.21 11.62 9.485 0.047 36.37 0.184.00 9.391 0.003 0.022 0.034 0.026 0.016 0.001 0.275 0.097 0.002 0.91 9.57 9.318 0.055 35.73 0.215.00 9.777 0.003 0.020 0.038 0.020 0.018 0.001 0.267 0.075 0.001 0.90 9.40 9.703 0.054 37.20 0.21

SURF-110, alunite, J = 0.002173 ± 0.000014; volume 39ArK = 46.44 × 10–10cm3, integrated age = 34.27 ± 0.23 Ma (2σ)0.50 11.612 ± 0.021 0.066 ± 0.348 0.059 ± 0.283 0.055 ± 0.161 0.115 0.007 49.22 0.09 5.774 ± 2.942 22.49 ± 11.391.00 8.714 0.005 0.031 0.087 0.020 0.064 0.013 0.061 0.069 0.003 36.02 1.38 5.542 0.242 21.60 0.941.50 7.089 0.004 0.027 0.047 0.015 0.030 0.002 0.131 0.052 0.003 6.46 5.62 6.618 0.091 25.76 0.352.00 7.766 0.003 0.023 0.034 0.011 0.026 0.001 0.085 0.041 0.002 3.03 11.51 7.533 0.038 29.29 0.152.25 8.716 0.003 0.021 0.041 0.009 0.031 0.001 0.160 0.034 0.001 1.24 10.55 8.616 0.044 33.46 0.172.50 8.834 0.003 0.020 0.040 0.009 0.031 0.000 0.173 0.032 0.001 0.90 14.67 8.763 0.040 34.03 0.152.75 9.528 0.003 0.017 0.040 0.008 0.031 0.000 0.352 0.030 0.001 0.59 19.08 9.486 0.049 36.81 0.193.00 9.758 0.003 0.018 0.041 0.011 0.029 0.000 0.234 0.039 0.001 0.75 11.17 9.698 0.049 37.63 0.193.50 9.146 0.003 0.019 0.053 0.010 0.029 0.001 0.261 0.037 0.001 1.26 11.44 9.040 0.056 35.10 0.215.00 9.607 0.003 0.018 0.046 0.013 0.024 0.000 0.188 0.049 0.001 0.80 14.48 9.543 0.043 37.03 0.17

1 "<" indicates step used in plateau age calculations, ">" indicates step used in inverse correlation calculationsNeutron flux monitor: 24.36 ± 0.17 Ma MAC-83 biotite (Sandeman et al., 1999)Isotope production ratios: (40Ar/39Ar)K = 0.0302, (37Ar/39Ar)Ca = 1416.4306, (36Ar/39Ar)Ca = 0.3952, Ca/K = 1.83(37ArCa/39ArK)

Appendix (Cont.)

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