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IntroductionPORPHYRY Cu-Mo deposits form during very narrow time in-tervals in the life of a convergent margin magmatic arc. Theyfurthermore are not uniformly distributed along the strikelength of most convergent margin arcs. Instead, they tend toform clusters of systems distributed along an arc segment thatformed over geologically narrow time frames during a muchlonger and protracted arc magmatic history. Porphyry Cu de-posits are genetically related to the emplacement of porphyryintrusions that emanate from a larger batholith emplaced atgreater depth (Dilles, 1987; Shinohara and Hedenquist, 1997;Dilles et al., 2000). Explanations for the dynamic environ-ment conducive to formation of a porphyry Cu deposit in-clude subduction reversals, subduction of aseismic ridges ortears in the down-going plate, changes in tectonic environ-ment, and the waning of magmatism at the end of an episodeof arc magmatism (Solomon, 1990; Tosdal and Richards,2001; Richards, 2003; Cooke et al., 2005; Sillitoe and Perelló,2005). Critical to arriving at a trigger for the formation of a

porphyry deposit is the timing of its formation with respect toother events happening along the convergent plate margin. Inthis manuscript, we focus attention on one of the more poorlyunderstood yet major porphyry Cu belts in the Andes, specif-ically the Paleocene to Eocene belt of southern Peru (Fig. 1).Contained within this belt are the significant mines (Singer etal., 2008) at Cerro Verde-Santa Rosa (3,571 Mt at 0.40% Cu,0.01% Mo), Cuajone (2,626 Mt at 0.47% Cu, 0.020% Mo),and Toquepala (3,530 Mt at 0.47% Cu, 0.03% Mo) as well asthe Quellaveco prospect (1,598 Mt at 0.57% Cu, 0.021% Mo),currently under feasibility study by Anglo American (Fig. 1).

Previous work established that the porphyry deposits ofsouthern Peru formed during the Paleocene and earlyEocene. The Cerro Verde-Santa Rosa porphyry systemshosted by Proterozoic gneiss of the Arequipa Massif and thepremineral Yarabamba Superunit batholith (U-Pb ages of62.1 ± 1 and 67.2 ± 1.0 Ma) are associated genetically withporphyry intrusions with 61 ± 1 Ma U-Pb ages (Le Bel, 1985;Mukasa, 1986). 40Ar/39Ar hydrothermal sericite alterationages are 61.8 ± 0.7 and 62.0 ± 1.1 Ma for Cerro Verde and62.2 ± 2.9 Ma for Santa Rosa (Quang et al., 2003). Hydro-thermal sericite alteration at Cuajone has been dated by40Ar/39Ar methods at ~52.8 ± 1.0 Ma (Clark et al., 1990a). AtToquepala, 40Ar/39Ar ages of igneous biotite and hydrothermalsericite suggest the porphyry Cu system formed between 55.9and 55.0 Ma (Zweng and Clark, 1995), with a late pyrite-bear-ing hydrothermal stage as young as 52 Ma (A.H. Clark, 2003,writ. comm.). At Quellaveco, Estrada (1975) reported a K-Arage on sericite of 56.2 Ma (no uncertainty provided), whereasa slightly younger 40Ar/39Ar age on sericite of 54.3 ± 2.0 Ma

Punctuated Magmatism Associated with Porphyry Cu-Mo Formation in the Paleocene to Eocene of Southern Peru*

ADAM T. SIMMONS,1,** RICHARD M. TOSDAL,1,† JOSEPH L. WOODEN,2 RUBÉN MATTOS,3,*** OSCAR CONCHA,4STUART MCCRACKEN,5,**** AND TIMOTHY BEALE5,*****

1 Mineral Deposit Research Unit, University of British Columbia, 6339 Stores Rd., Vancouver, British Columbia V6T 1Z4, Canada 2 Geological and Environmental Sciences, Stanford, California 94305

3 Southern Peru Copper Corporation, Casilla 303, Toquepala, Peru4 Southern Peru Copper Corporation, Casilla 163, Cuajone, Peru

5 AngloAmerican Exploration, Lima, Peru

AbstractThe Paleocene to Eocene southern Peru porphyry belt contains three significant porphyry Cu-Mo deposits

at Cuajone, Quellaveco, and Toquepala. Ten new zircon U-Pb Sensitive High Resolution Ion Microprobe- Reverse Geometry (SHRIMP-RG) ages for Cuajone and Toquepala, together with published ages forQuellaveco, establish a magmatic history characterized by episodic events. Punctuated magmatism at Cuajoneis distributed over approximately 13 m.y., at Toquepala over 8 m.y., and at Quellaveco over 6 m.y. The ages ofthe porphyry intrusions hosting or associated with the introduction of Cu and Mo at the three deposits showremarkable similarity, with emplacement beginning and ending at approximately 56.5 to 53.0 Ma at Cuajone,57.0 to 54.0 Ma at Toquepala, and at 58.4 to 54.3 Ma at Quellaveco. Field relations coupled with the U-Pb agesfor synmineral intrusions suggest very similar timing of the cupriferous hydrothermal systems, with theyoungest pyritiferous and Cu-poor hydrothermal systems being associated with porphyry intrusions as much as2 m.y. younger than significant Cu introduction. Overall, the porphyry Cu-Mo intrusive complexes representthe youngest magmatic complexes formed during the Late Cretaceous and early Tertiary arc, having formedprior to eastward migration of the magmatic locus.

† Corresponding author: e-mail, rtosdal@gmail.com*A digital supplement to this paper is available at http://economicgeol-

ogy.org/ and at http://econgeol.geoscienceworld.org/.**Present address: PI Financial Corporation, 1900-666 Burrard St., Van-

couver, BC V6C 3N1, Canada.***Present address: Anglo American, Quellaveco S.A. Calle Esquilache

371 Piso 10, San Isidro, Lima 27, Peru.****Present address: Anglo American plc, 20 Carlton House Terrace, Lon-

don SW1Y5AN, United Kingdom.*****Present address: Iron Creek Capital Corporation, 830-355 Burrard

St., Vancouver, BC V6C 2G8, Canada.

©2013 Society of Economic Geologists, Inc.Economic Geology, v. 108, pp. 625–639

Submitted: October 22, 2011Accepted: July 23, 2012

has also been reported (A.H. Clark, 2003, writ. comm.). Silli-toe and Mortensen (2010) reported U-Pb ages on zircon be-tween ~54 and 59 Ma from a suite of porphyry intrusionsfrom Quellaveco that, although fairly close, are slightly olderthan the K-Ar and 40Ar/39Ar ages, suggesting that the latterages reflect the overall cooling of the hydrothermal eventrather than the actual age of the porphyry formation, as is typ-ical for most porphyry Cu deposits and districts (e.g.,Richards and Noble, 1998; Gustafson et al., 2001; Harris etal., 2008). Nonetheless, the K-Ar and 40Ar/39Ar ages are suffi-ciently close to the ages of the porphyry intrusions associatedwith Cu introduction to provide constraints to define a broad-scale chronologic framework for porphyry Cu formation (e.g.,Perelló et al., 2003, 2008). The available chronologic data sug-gests at least an 8- to 10-m.y. period during which porphyryCu deposits were emplaced in the southern Peru belt, a du-ration similar to that found elsewhere (Sillitoe and Perelló,2005; Barra et al., 2005; Glen et al., 2007).

We focus attention on the southern end of the southernPeru porphyry belt, specifically the three spatially associatedporphyry Cu-Mo deposits at Cuajone, Quellaveco, and

Toquepala (Fig. 2). We report zircon U-Pb ages at Cuajoneand Toquepala (Table 1) for 10 samples, including severalfrom premineral host batholiths and representative rocksfrom each major intrusive phase present in two of the threeporphyry Cu-Mo systems. Augmenting these new ages are U-Pb ages for four rocks from the Quellaveco deposit (Sillitoeand Mortensen, 2010), as well as published K-Ar and40Ar/39Ar ages. The collective ages define a chronology ofmagmatic events just prior to and including formation ofthese giant porphyry Cu-Mo deposits.

Geologic Framework of Southern PeruMid-Mesozoic rifting along the western margin of Gond-

wana (now western South America) marks the beginning ofthe Andean orogen (Coira et al., 1982; Davidson andMpodozis, 1991; Benavides-Cáceres, 1999). Steep subduc-tion under the western margin of Gondwana led to trench re-treat and formation of intraarc and back-arc rifts filled bymafic-dominated, mantle-derived magmatic rocks (Athertonet al., 1983, 1985) as well as sedimentary detritus derivedfrom the rift margins and volcanic edifices (Benavides, 1956;Wilson, 1983). The rift margins are marked by large-scalefaults to the east, which in southern Peru correspond to theIncapucio fault (Fig. 2); this fault is equivalent to theDomeyko fault system in northern Chile, which localizedmost of the younger Eocene porphyry Cu-Mo deposit there(Camus, 2003). Magmatism spanned much of the Jurassicand Early Cretaceous (Mukasa, 1986; Clark et al., 1990a;Pitcher et al., 1985; Boekhout et al., 2010; Mamani et al.,2010).

The Late Cretaceous marks a time of a major tectonic andmagmatic shift throughout the Andes, coincident with theopening of the southern Atlantic Ocean. In southern Peru,Late Cretaceous shortening led to northeastward thrusting ofJurassic and Cretaceous marine volcanic and sedimentary se-quences over continentally derived clastic rocks (Vicente,1989; Benavides-Cáceres, 1999). Magmatism continuing intothe Paleogene (Boiley et al., 1990; Clark et al., 1990a;Martínez and Cervantes, 2003) obscured the earlier rift se-quence and Late Cretaceous fold-and-thrust belt. This arc ispreserved as dacitic to andesitic pyroclastic rocks and inter-mediate flows included within the Toquepala Group (Bellido,1979; Mamani et al., 2010). Age constraints throughout theregion on these volcanic complexes are generally poor(Martínez and Cervantes, 2003), but within the area of inter-est herein, U-Pb ages suggest volcanism occurred in the LateCretaceous, in the range of 71 to 68 Ma, although, regionally,ages as old as 90 Ma are known (Clark et al., 1990a; A. Sim-mons, unpub. data, 2013).

Large batholiths intrude the Toquepala Group (Bellido,1979; Clark et al., 1990a; Mamani et al., 2010). These LateCretaceous intrusions, grouped into the Yarabamba Supe-runit batholiths (Mukasa, 1986), form a component of theCoastal batholith of Peru that extends from the Peru-Chileborder to central Peru (Beckinsale et al., 1985; Pitcher et al.,1985). Between Cuajone and Toquepala (Fig. 2), thesebatholiths were emplaced prior to the porphyry Cu-Mo de-posits. Emplacement of the porphyry Cu complexes marksthe final stages of the Late Cretaceous and Paleogene mag-matic arc in this part of southern Peru (Clark et al., 1990a).

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Bajo de la Alumbrera

El Teniente

Rosario-Uyina-Quebrada Blanca20°

25°

30°

35°70°75° 65°

0 50

kilometers

N

Bolivia

ArgentinaChile

Peru

Axi

s of

Per

u-C

hile

tre

nch

Mioceneporphyry Cu-Mo

belt

Miocene porphyry Cu-Au belt

Paleocene-Eocene

porphyry Cu-Mo belt

Arequipa

Eocene-Oligoceneporphyry Cu-Mo belt

Santiago

Cretaceous porphyry Cu-Mo belt

La Colorado

Spence

El Salvador

La Escondida

Chuquicamata

Cuajone-Quellaveco- Toquepala

Co. Verde-Santa Rosa-Cerro Negro

FIG. 1. Late Cretaceous and Tertiary porphyry Cu belts of the centralAndes of southernmost Peru and Chile. Modified from Sillitoe and Perelló(2005).

Eocene and Oligocene flat-slab subduction (Sandeman et al.,1995; James and Sacks, 1999) in Peru caused a northeastwardshift in magmatism to the Andahuaylas cordillera (Sandemanet al., 1995; Perelló et al., 2003). Erosion and degradation ofthe older magmatic arc was accompanied by sedimentation ofthe Moquegua Formation in an intraarc or forearc basin. Ex-plosive volcanism of the Huaylillas Formation blanketed theprecordillera from 24 to 18 Ma. The older ignimbrites are in-terbedded within the upper Moquegua Formation whereasthe younger ignimbrites of the formation buried the regionalTertiary erosional surface, preserving and interrupting super-gene enrichment (Tosdal et al., 1981, 1984; Quang et al.,2003, 2005). Miocene to Holocene uplift and oroclinal bend-ing in the Andes (Isacks, 1988; Jacay et al., 2002; Roperch etal., 2006) was accompanied by small-volume pyroclastic vol-canism of the Miocene Chuntacala Formation, intermediatevolcanism of the Pliocene to Holocene Barroso Group, andexhumation of the porphyry Cu deposits (Tosdal et al., 1981,1984; Clark et al., 1990b; Quang et al., 2005).

Deposit Geology

Cuajone

The Cuajone mine, located approximately 30 km northeastof the town of Moquegua (Fig. 2), was first described by Lacy(1958). There have been only a few subsequent studies of thehypogene geology of the deposit (Fig. 3), and this giant de-posit remains poorly documented and largely unknown, ex-cept for summaries by Manrique and Plazolles (1975), Satch-well (1983), and Concha and Valle (1999). Unit names usedherein are those utilized within the mine.

Basalt, andesite, and quartz-phyric volcanic rocks of theToquepala Group are the oldest rocks at Cuajone, croppingout largely in the Rio Torata, but also the northwestern partof the deposit. Premineral Late Cretaceous (K-Ar age 66.7 ±1.7 Ma; Park, in Concha and Valle, 1999) or early Paleocene(average of concordant 40Ar/39Ar plateau ages of 63.4 ± 0.2Ma on biotite and hornblende; A.H. Clark, 2003, writt.comm.) granodiorite and diorite form the northern margin of

MAGMATISM ASSOCIATED WITH PORPHYRY Cu-Mo FORMATION IN THE PALEOCENE TO EOCENE, PERU 627

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Toquepala Group (Cretaceous)

Granodiorite (Cretaceous)

Porphyry Intrusive Complexes (Paleocene)

Chuntacala/Huaylillas Fm. (Oligocene-Miocene)

Alluvium

Moquegua Fm. (Eocene-Oligocene)

Stratified rocks

Intrusive rocks

N

Barroso Group (Pliocene -Holocene)

Contact Fault

8105000 mN

8090000 mN

8095000 mN

8100000 mN

8115000 mN

8110000 mN

QuellavecoProposed Pit

Quellaveco Fault

Incapuquio FaultToquepala

Mine

315000mE

320000mE

325000mE

330000mE

335000mE

340000mE

345000mE

Micalaco Fault

Cuajone Mine

0 5kilometers

FIG. 2. Simplified regional geology surrounding the Quellaveco, Cuajone, and Toquepala orebodies. Coordinates in Pe-ruvian coordinate system (PSAD56); zone 19S. Internal contacts between formation and members of the larger units alsoshown. Map is simplified from Bellido (1979).

the deposit. Copper is spatially and temporally related to asuite of porphyritic latite (mine term) stocks (Fig. 3) (Man-rique and Plazolles, 1975). The older latite porphyry suite—Latite Porphyry 1—hosts the bulk of the Cu-Mo. Petrologi-cally similar but intramineral Latite Porphyry 2 suiteintrusions host lesser Cu-Mo. Andesite intrusions into thehost Toquepala Group (too small to be shown on Fig. 3) wereconsidered to be a subvolcanic hypabyssal intrusion associ-ated with those rocks (Concha and Valle, 1999). However, U-Pb data presented herein demonstrates that this unit is tem-porally related to the porphyry suites. The Latite Porphyry 1,Latite Porphyry 2, and intrusive andesite suites form the coreof the porphyry Cu deposit (Fig. 3). In contrast, the youngestsuite of latite porphyry, Latite Porphyry 3, which carries low

to no Cu grade but is altered to quartz-sericite-pyrite assem-blages, is largely located to the northwest of the main Cu ore-body (Fig. 3).

Toquepala

The Toquepala mine is located approximately 23 km south-southeast of the Cuajone mine (Fig. 2). Toquepala was recog-nized in the 1930s as a porphyry Cu prospect by CarlSchmedeman (Lacy, 1991), based upon the surface outcropsof leached and limonite-filled veinlets that locally containedsecondary copper oxides and carbonate minerals. Richardand Courtright (1958) first described the geology of the de-posit (Fig. 4). The porphyry Cu-Mo deposit is associated witha complex intrusive center dominated by several dacite

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TABLE 1. Brief Description, Location of Samples, and U-Pb Ages from Toquepala and Cuajone Porphyry Cu-Mo Mines, Southern Peru

Rock type Sample no. Easting Northing (mine units) Notes

Cuajone

Qu-701 316290 8116626 Granodiorite Equigranular, medium-grained (3 mm), biotite- and hornblende-bearing granodiorite; mafic minerals altered to chlorite and epidote; U-Pb age = 65.1 ± 0.8 Ma

Qu-700 315535 8114974 Diorite Equigranular, medium-grained hornblende and biotite; coarse plagioclase; contains fine mafic xenoliths; U-Pb age = 64.2 ± 0.9 Ma

Qu-697 318586 8115509 Intrusive andesite Microporphyritic diorite to monzodiorite; fine anhedral feldspar (20–25%), coarse anhedral, rounded quartz (1%) and small anhedral hornblende (5%) phenocrysts; plagioclase and secondary K-feldspar altered to sericite; hornblende and secondary biotite altered to chlorite; pyrite is common in the mafic sites; level 3175E; U-Pb age = 56.2 ± 0.5 Ma

Qu-696 318694 8115023 LP1 porphyry Quartz monzonite, granodiorite to monzogranite porphyry; phenocryst/groundmass = 40:60, with large euhedral plagioclase being dominant (25–35%), lesser quartz (5–10% of rock), and minor biotite (2–3%); intensely altered to early K-feldspar overprinted by pervasive sericite; biotite and hornblende phenocrysts and hydrothermal secondary biotite altered to chlorite-sericite; averages ~0.4 to 0.5% Cu; early mineral intrusion; level 3175SE; U-Pb age = 55.6 ± 0.6 Ma

Qu-695 317984 8115385 BLP porphyry Quartz monzonite to granodiorite porphyry; phenocryst/groundmass = 30:70, with plagioclase (20–30%) dominating over quartz (up to 10%); trace biotite in groundmass; contains more quartz and is coarser grained than LP1; hydrothermally altered with feldspar partially altered to sericite or illite and biotite to chlorite; intramineral intrusion; level 3190W; U-Pb age = 56.2 ± 0.7 Ma

Qu-699 316778 8116376 LP3 porphyry Monzogranite to granodiorite porphyry; phenocryst/groundmass = 20:80, with rounded, anhedral quartz (10–15%) and finer-grained euhedral plagioclase (12–18%); mafic minerals are not visible; contains more quartz and has a lower phenocryst to matrix ratio than older porphyry intrusions; plagioclase altered to white mica; pyrite common; late-mineral intrusion; sample outside open pit; U-Pb age = 53.5 ± 0.58 Ma

Toquepala

Qu-705 327942 8092833 Diorite Equigranular, fine-grained (3 mm), biotite-hornblende diorite with rare interstitial quartz; mafic minerals altered to chlorite and epidote; U-Pb age = 61.4 ± 0.8 Ma

Qu-704 328529 8093013 Dacite Porphyry Dacite porphyry; phenocryst/groundmass = 40:60, with medium-grained, euhedral plagioclase (30–35%) and lesser rounded, anhedral quartz (5–15%); rare very fine chlorite-pyrite–altered biotite phenocrysts; K-silicate alteration assemblage of anhydrite-biotite-K-feldspar dominates the rock; level 2815W; U-Pb age = 56.8 ± 0.6 Ma

Qu-708 328813 8093861 Dacite Agglomerate Dacite porphyry; medium-grained, phenocryst/groundmass = 30:70, medium-grained, euhedral plagioclase (30–35%), rounded, anhedral to subhedral, large quartz (10–15%); fine, euhedral, chloritized biotite (2–5%); late intramineral intrusion; level 3265N; U-Pb age 56.1 ± 0.4 Ma

Qu-706 328291 8093469 Latite Porphyry Dacite porphyry; fine-grained, phenocryst/groundmass = 35:65, subhedral feldspar (25–30%), rounded anhedral to subhedral quartz (2–8%); chlorite-pyrite–altered biotite; groundmass altered to white mica and quartz; late-mineral intrusion; level 3160N; U-Pb age = 54.3 ± 0.6 Ma

Notes: UTM Coordinates in Peruvian coordinate system (PSAD56); zone 19SAbbreviations: BLP = Latite Porphyry 2, LP1 = Latite Porphyry 1, LP3 = Latite Porphyry 3

stocks, a dacitic breccia and dike complex, and extensivehydrothermal breccias (Fig. 4) (Zweng and Clark, 1995; Mat-tos and Valle, 1999). The timing of the intrusive and main Cu-bearing hydrothermal events has been interpreted by40Ar/39Ar methods on igneous biotite and hydrothermalsericite to be between 55.0 ± 0.21 and 55.91 ± 0.4 Ma (A.H.Clark, 2003, writt. comm.). The dacite porphyry stocks anddikes intruded older Paleocene quartz monzodiorite plutonswith a 40Ar/39Ar age on biotite of 58.44 ± 0.26 Ma, located tothe immediate west of the deposit, and others with ages of62.07 Ma (no uncertainty reported) located to the east andsoutheast of the mine (Clark et al., 1990a; Zweng and Clark,1995; A.H. Clark, 2003, writt. comm.). Conventional K-Arages on magmatic biotite from the region are consistent withthese more precise ages, but are characterized by larger un-certainties (see discussion in Clark et al., 1990a). Still olderrhyolitic and intermediate volcanic rocks are known in themine and surrounding region, but they are undated. The de-posit lies about 1 to 2 km northeast of the Micalaco fault,which is a subsidiary and parallel strand of the Incapucio faultsystem that lies still farther to the southwest (Figs. 2, 4).

The deposit is spatially and genetically related to multi-phase dacitic porphyry intrusions (Fig. 4). The Dacite Por-phyry unit is the oldest suite (Richard and Courtright, 1958;Mattos and Valle, 1999), and is divided into three textural andmineralogical porphyry types, each of which is cut by early-stage chalcopyrite-bearing veins (Zweng and Clark, 1995).

Barren tourmaline breccias cut the porphyry stocks, andearly-stage Cu-rich veins. Some quartz-sericite-pyrite alter-ation accompanied breccia formation, but little copper wasdeposited at this time. The main stage of Cu-Mo sulfide de-position followed the barren tourmaline breccia, and was inturn followed by a late stage of Cu-poor veins. Late to post-mineral igneous activity includes a dacite diatreme complexforming the Dacite Agglomerate and dikes of the Latite Por-phyry unit (Fig. 4). The Latite Porphyry dikes, the youngestigneous unit, intruded all older igneous and hydrothermalunits (Zweng and Clark, 1995).

Quellaveco

Quellaveco was recognized as a porphyry Cu deposit in the1930s (Lacy, 1991). It was explored between 1947 and 1952by Northern Peru Mining and Smelting Company and in1970 by Southern Peru Copper Corporation, before being na-tionalized and explored further by MineroPeru in 1972. Cur-rently (2012), Anglo American is exploring the prospect. De-posit details are available in Estrada (1975), Kihien (1975,1995), Guerrero and Candiotti (1979), Torpoco (1979), Can-diotti de los Rios (1995), and Sillitoe and Mortensen (2010).

The Quellaveco deposit is hosted by a supracrustal sequenceof basal andesite overlain by more felsic volcanic rocks of theToquepala Group (Fig. 2), including quartz-feldspar-phyricrhyolites, then by andesite and basaltic-andesite flows andbreccias, and, finally, by rhyodacitic ignimbrites at the upper

MAGMATISM ASSOCIATED WITH PORPHYRY Cu-Mo FORMATION IN THE PALEOCENE TO EOCENE, PERU 629

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LegendStratified Rocks

Intrusive Rocks/Breccias

Alluvium/Talus

Chuntacala Fm. (Miocene)

Huaylillas Fm. (Oligo. - Mio.)

Toquepala Gp. (Late Cretaceous)

LP3 Porphyry (Late Mineral)

Hydrothermal Breccia

BLP Porphyry (Intermineral)

LP1 Porphyry (Main Stage)

Diorite /granodiorite (Pre-mineral)

Qu70165.1±0.8Ma

Qu699

Qu70064.2±0.9Ma

Qu69556.2±0.7Ma

Qu69655.6±0.6Ma

Qu69755.9±0.5Ma

U-Pb location

8115000mN

8117000mN

8116500mN

8115500mN

8114500mN

8114000mN

316500mE

317000mE E

m005

713

Em0

0081

3

Em0

0581

3

Em0

0091

3

Em0

0591

3

Em0

0002

3

Em0

0502

3

open pit limit53.5±0.5Ma

8116000mN

Rio

Torata

FIG. 3. Simplified geology of Cuajone mine, courtesy of Southern Peru Copper Corporation, based upon Manrique andPlazolles (1975) and Concha and Valle (1999). Coordinates in Peruvian coordinate system (PSAD56); zone 19S. Location ofgeochronology samples plotted for all intrusive rocks; uncertainties given are at 2σ levels. BLP = Latite Porphyry 2, LP1 =Latite Porphyry 1, LP3 = Latite Porphyry 3.

stratigraphic levels (Bellido, 1979). Granodiorite and quartzmonzonite intrude the volcanic rocks; Sillitoe and Mortensen(2010) report a 59.46 ± 0.24 Ma age for the large granodior-ite on the property. Numerous porphyritic diorite-granodior-ite stocks intrude the volcanoplutonic complex. The oldestporphyry has a U-Pb zircon age of 58.41 ± 0.53 Ma, an in-tramineral porphyry an age of 55.90 ± 0.31 Ma, and a late-mineral porphyry an age of 54.63 ± 0.63 Ma (Sillitoe andMortensen, 2010). Late quartz latite dikes cut the complex.

U-Pb GeochronologyNew U-Pb ages reported herein utilized the Sensitive High

Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG) at the United States Geological Survey (USGS)-StanfordUniversity facility. Zircons were separated at the Pacific Cen-tre for Isotopic and Geochemical Research (PCIGR) in theDepartment of Earth and Ocean Sciences, University ofBritish Columbia (UBC). Data reduction for U-Pb SHRIMP-RG geochronology uses the Microsoft Excel add-in Squid(Ludwig, 2001) and Isoplot programs (Ludwig, 2008), follow-ing the methods described by Ireland and Williams (2003).For young zircons, U-Pb SHRIMP-RG ages are calculated

from the weighted mean 207Pb-corrected 206Pb/238U ages ofspot analyses on individual crystals, due to the poor ability ofthe SHRIMP-RG to precisely measure small amounts of204Pb, the isotope used to ensure proper correction for anycommon Pb. Individual analyses characterized by excessivecommon Pb are included in the age calculation, even thoughtheir inclusion may degrade the statistical validity of the age.Ages for individual analytical spots that did not overlap theweighted mean age within their analytical uncertainty are ex-cluded from final age calculation. Zircon spot analyses con-taining distinct chemical characteristics, such as elevated Th/Uor high U concentrations from the normal population, were alsoexcluded. Tera-Wasserburg (1972) inverse concordia diagramswere utilized to confirm exclusion of data points, as this diagrameasily evaluates the possibility for Pb loss from zircons and thepresence of older zircons, as well as the influence of commonPb on the interpreted ages; these plots are shown adjacent tothe weighted mean histogram plots for each sample. The con-cordia intercept age calculated for each sample is generallywithin analytical uncertainty of the weighted mean 207Pb-cor-rected 206Pb/238U age. Only analyses that intersect the chordwithin the limits of the analytical uncertainty are included in the

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3290

00m

E

8092000mN

8091500mN

LegendStratified Rocks

Intrusive Rocks/Breccias

Alluvium/Talus/DumpsToquepala Gp.

Latite Porphyry (Late Mineral)

Diorite Breccia

Dacite Agglomerate (Intermineral)

Anhydrite Breccia (“Ore Breccia”)

Dacite Porphyry (Main Stage)

Diorite Intrusion (Pre-mineral)

Qu70561.4±0.8Ma

Qu70456.8±0.6Ma

Qu70856.2±0.6Ma

Qu70654.3±0.6Ma

8094000mN

8093500mN

8093000mN

8092500mN

3275

00m

E

3280

00m

E

3285

00m

E

3295

00m

E

3300

00m

E

open pit limit

FIG. 4. Simplified geology of Toquepala mine, courtesy of Southern Peru Copper Corporation, Richard and Courtright(1958), and Zweng and Clark (1995). Coordinates in Peruvian coordinate system (PSAD56); zone 19S. Location ofgeochronology samples plotted for all intrusive rocks; uncertainties given are at 2σ levels.

regression. Complete descriptions of the analytical proceduresand isotopic data are included in the electronic supplement.

Ten new U-Pb SHRIMP-RG weighted mean 207Pb-cor-rected 206Pb/238U ages establish a magmatic chronology forpremineral batholithic rocks as well as intrusions related tothe Cuajone and Toquepala porphyry Cu-Mo deposits (Fig.5). The porphyry Cu-Mo–related intrusions, as established bythe mine geologists at Cuajone and Toquepala (Manrique andPlazolles, 1975; Zweng and Clark, 1995; Concha and Valle,

1999; Mattos and Valle, 1999), are the principal focus ofstudy. Unit designations from the mines are used throughout;those unit designations do not always represent the igneouscomposition (see Table 1). The U-Pb ages presented herein(Fig. 5) are augmented by the published U-Pb ages fromQuellaveco (Sillitoe and Mortensen, 2010) as well as available40Ar/39Ar ages (e.g., Clark et al., 1990a).

Zircon is a refractory mineral that can have a complicatedgrowth history, forming xenocrysts, inherited cores withinnewly crystallized zircons, and antecrysts. Xenocrysts are zir-con crystals that are incorporated into magma from the coun-try rock and lack any new zircon overgrowths. In contrast, in-herited zircons form cores to new zircons crystallized fromthe magma. Antecrysts are zircon crystals that formed earlywithin the magma and may be slightly older than the age offinal crystallization (Miller et al., 2007; von Quadt et al.,2011). Studies of recent volcanic rocks suggest antecrystic zir-cons can be as much as 300,000 years older than the age ofthe eruption (Bacon and Lowenstern, 2005; Bachman et al.,2007). Antecrystic zircons are, in essence, part of the crystal-lizing magma that may have been plucked off the walls of aconvecting and deeper crustal-level magma chamber, and in-corporated in the rising porphyry stock (Miller et al., 2007).Because of their slightly older ages, their presence can causeanalytical scatter in zircons of Tertiary age, particularly forspot instruments such as the SHRIMP-RG, thereby increas-ing the uncertainty in the calculated ages. Their presence isan added potential complication to interpreting the crystal-lization ages of the porphyry intrusions at Cuajone andToquepala.

Cuajone

Premineral rocks: At Cuajone, an equigranular granodiorite(Qu701) from approximately 2 km northwest of the center ofthe deposit (Fig. 3) yielded a zircon U-Pb age of 65.1 ± 0.8Ma (Fig. 6a, Table 1) based on eight of nine zircon crystals.The one excluded crystal is slightly younger, plots off the best-fit regression line, and thus is inferred to have undergoneminor Pb loss. Inclusion of that zircon in the age calculationdecreases the calculated age and degrades the statistics (64.0± 1.1 Ma; MSWD = 2.2).

A coarse-grained (~5–10 mm) equigranular diorite intru-sion (Qu700) located 3 km to the west of the center of the de-posit (Fig. 3) yielded a zircon U-Pb age of 64.2 ± 0.9 Ma (Fig.6b, Table 1) based on nine of 10 zircons. One zircon crystal isslightly older and displaced to older ages on the Tera-Wasser-burg diagram. This crystal may be a xenocryst or perhaps anantecryst. Inclusion of that grain in the age calculation doesnot change the age of the rock. The age of diorite sample iswithin the analytical uncertainties of the granodiorite, but it isspatially separated from that rock by younger rocks in out-crop. These rocks are part of a premineral batholith that un-derlies the Cuajone area.

Porphyry Cu-related intrusions: The oldest porphyry suiteat the Cuajone mine, the Latite Porphyry 1 suite, consists ofmultiple intrusions ranging in size from a large 1,000- × 800-m stock to small dikes. The main stock is elongated in an NW-SE direction (Fig. 3). The Latite Porphyry 1 intrusions hostthe majority of the Cu-Mo, with Toquepala Group rocksbeing well mineralized adjacent to their contacts with Latite

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Cuajone

Toquepala

52Ma

53Ma

54Ma

55Ma

56Ma

57Ma

58Ma

59Ma

60Ma

61Ma

62Ma

63Ma

64Ma

65Ma

66Ma

67Ma

A B

A

C

B

D

C

DE

F

A. Qu-701; Regional Granodiorite; 65.1 ± 0.8 MaB. Qu-700; Premineral Diorite; 64.2 ± 0.9 Ma

A. Qu-705; Regional Diorite; 61.4 ± 0.8 Ma

Cuajone ToquepalaQuelleveco

A

B

D

C

Quelleveco (from Sillitoe and Mortensen, 2010)A. Pre-mineral granodiorite; 59.46 ± 0.24 Ma

C. Intermineral porphyry; 55.90 ± 0.31 MaD. Late-mineral porphyry; 54.36 ± 0.63 Ma

B. Early porphyry; 58.41 ± 0.53 Ma

Published Ar ages for pre-mineral rocks (see text)

C. Qu-697; Intrusive Andesite; 56.2 ± 0.5 MaD. Qu-696; LP1 Porphyry; 55.6 ± 0.6 MaE. Qu-696; BLP Porphyry; 56.2 ± 0.7 MaF. Qu-699; LP3 Porphyry; 53.5 ± 0.5 Ma

B. Qu-704; Dacite Porphyry; 56.8 ± 0.6 MaC. Qu-706; Dacite Agglomerate; 56.1 ± 0.4 MaD. Qu-708; Latite Porphyry; 54.3 ± 0.6 Ma

FIG. 5. Summary of ages from Quellaveco, Cuajone, and Toquepala,based on weighted mean averages of 207Pb-corrected 206Pb/238U spot agesusing SHRIMP-RG. Errors shown at 2σ levels. Ages for Quellaveco from Sil-litoe and Mortensen (2010). Available 40Ar/39Ar ages (see text) are includedfor reference. Available K-Ar ages summarized by Clark et al. (1990a) gener-ally have large uncertainties and have been excluded, although these ages areconsistent with the more recently determined 40Ar/39Ar ages. BLP = LatitePorphyry 2, LP1 = Latite Porphyry 1, LP3 = Latite Porphyry 3.

632 SIMMONS ET AL.

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50

60

70

80

1-107

6-107

9-107

8-107

7-107

5-107

3-107

4-107

2-107

Qu701 Granodiorite Batholith65.1 ± 0.8 MaMSWD 1.45Probability 0.18

40

50

60

70

80

Qu700 Diorite Intrusion64.2 ± 0.9 MaMSWD 1.37Probability 0.20

7-007

01-007

9- 007

2- 007

4- 007

6- 007

5- 007

3- 007

1- 007

8- 007

525456586062

0.04

0.05

0.06

0.07

0.08

0.09

100 104 108 112 116 120 124

602702

bP/ bP

238 206U/ Pb

55.7 ± 0.6 MaMSWD 0.99Prob. 0.42

Inheritedzircons

Pb-Loss

Common Pb

Qu696 LP1 Porphyry55.6 ± 0.6 MaMSWD 0.98Probability 0.43

8-696

7-696

11-696

6-696

1-696

01-696

21-696

4-696

3-696

2-696

5-696

9-696

40

45

50

55

60

72 70 68 66 64 62 60

0.040

0.044

0.048

0.052

0.056

0.060

0.064

88 92 96 100 104 108

Error ellipses are 2σ

207

206

bP/bP

238 206U/ Pb

65.1 ± 0.7 MaMSWD 1.5

To commonPb

Pb loss

72 70 68 66 64 62 60 58

0.04

0.05

0.06

0.07

0.08

0.09

88 92 96 100 104 108 112

602702

bP/bP

238 206U/ Pb

63.9 ± 0.8 MaMSWD 0.80

Inheritedzircon

Qu695 BLP Porphyry56.2 ± 0.7 MaMSWD 1.49Probability 0.18

6-596

8-596

5-596

9-596

1-596

7-596

3-596

11-596

21-596

2-596

01-596

4-596

40

45

50

55

60

65

64 62 60 58 56 54 520.04

0.06

0.08

0.10

0.12

0.14

98 102 106 110 114 118 122 126

602702

bP/bP

238 206U/ Pb

56.3 ± 0.6 MaMSWD 1.50Probability 0.18

InheritedZircons

Common Pb

a.

b.

c.

d.

Age

(Ma)

Age

(Ma)

Age

(Ma)

Age

(Ma)

Pb-Loss

FIG. 6. U-Pb ages for samples from the Cuajone mine, southern Peru. Ages are calculated based on the weighted mean207Pb-corrected 206Pb/238U histograms. Tera-Wasserburg (1972) concordia diagrams shown for each sample demonstrate thepresence of common Pb, Pb loss, and inheritance in some of the analyzed zircons. a) Qu701: Granodiorite batholith. b)Qu700: Diorite intrusion. c) Qu696: Early Latite Porphyry 1. d) Qu695: Intramineral Latite Porphyry 2 (BLP). e) Qu697:Intrusive andesite. f) Qu699: Late Latite Porphyry 3 (LP3). Analyses excluded from the age calculation shown in gray.

Porphyry 1 intrusions. Zircon spot U-Pb analyses from Qu696,collected from the southeast portion of the main Latite Por-phyry 1 stock, show some scatter. Six of 12 analyzed zirconsyield an age of 55.6 ± 0.9 Ma (Fig. 6c, Table 1). Two crystalsexcluded from the age calculation are older than theweighted mean age for the six zircons and displaced to theleft of the chord through those zircons, and therefore are in-terpreted to represent xenocrysts. The remaining four zirconsare younger due to Pb loss because their isotopic systematicsare displaced from the concordia regression.

A second and younger latite porphyry suite (Latite Por-phyry 2) consists of slightly NW elongated stocks (Fig. 3).Based on field relations, the Latite Porphyry 2 suite is in-tramineral, having intruded after the Latite Porphyry 1 suite,but before postmineral intrusions. Several generations of Cu-Mo sulfide-bearing veins are present in these rocks. A por-phyry (Qu695) from the southeast portion of the largest of theLatite Porphyry 2 intrusions in the mine yielded a U-Pb ageof 56.2 ± 0.7 Ma (Fig. 6d; Table 1), based on seven of 12 an-alyzed zircons. Three crystals are clearly younger, presumablydue to Pb loss. The other two crystals are slightly older andmay represent an antecryst or perhaps a xenocryst.

An intrusive andesite unit was considered to be a subvol-canic intrusion roughly equivalent in age to the overlyingToquepala Group volcanic rocks (Manrique and Plazolles,1975; Concha and Valle, 1999). The unit is elongated towardthe northwest along the eastern contact between ToquepalaGroup rocks and the Latite Porphyry 1 suite (Fig. 3). TheLatite Porphyry 1 and Latite Porphyry 2 suites intruded the

andesite. An intensely altered intrusive andesite (Qu697)from immediately west of the main Latite Porphyry 1 stockyielded an age of 55.8 ± 0.4 Ma based on twelve crystals. Twocrystals, grains 697-3 and 697-5, are slightly younger than theother grains, presumably due to Pb loss, because their dataplots to the right of a chord defined by the remaining 10 crys-tals (Fig. 6e; Table 1). Excluding those slightly younger zir-cons results in a weighted mean age of the 56.2 ± 0.5 Ma forthe remaining 10 zircons. There is no evidence for ToquepalaGroup-age zircons, nor is there significant Pb loss, suggestingthat the calculated age is the emplacement age. The U-Pb agefor the intrusive andesite requires that it is part of the por-phyry magmatic suite and an early intrusion, based upon fieldrelation.

The youngest porphyry suite is composed of late-mineralmonzogranite to granodiorite dikes and stocks, known asLatite Porphyry 3 intrusions. They lack significant copper andmolybdenum. The Latite Porphyry 3 intrusions form elon-gated stocks and NW-oriented dikes, largely in the northwestportion of the pit in 2009 (Fig. 3). A moderately supergene al-tered porphyry (Qu699) from the largest stock of Latite Por-phyry 3 yielded an age of 53.5 ± 0.4 Ma (Fig. 6f; Table 1)based on 11 of 12 zircons. One crystal is too young, and musthave lost Pb.

Toquepala

Premineral rocks: At Toquepala, an equigranular diorite(Qu705) in the eastern part of the deposit (Fig. 4) yielded azircon U-Pb SHRIMP-RG age of 61.4 ± 0.8 Ma (Fig. 7a;

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45

50

55

60

Qu697 Intrusive Andesite56.2 ± 0.5 MaMSWD 0.70Probability 0.52

697-

5 3-796

4-796

11-796

1-796

01-796

7-796

6-796

8-796

9-796

21-796

2-796

5254565860

0.04

0.05

0.06

0.07

106 110 114 118 122

602702

bP/ bP

238 206U/ Pb

56.4 ± 0.5 MaMSWD 0.36Prob. 0.94

Pb-loss

40

45

50

55

60

Qu699 LP3 Porphyry53.5 ± 0.4 MaMSWD 0.59Probability 0.81

21-996

9-996

6-996

01-996

8-996

11-996

4-996

1-996

3-996

5-996

2-996

7-996

48525660640.04

0.06

0.08

0.10

0.12

95 105 115 125 13520

720

6bP

/bP238 206U/ Pb

53.5±0.4 MaMSWD 0.76Prob. 0.66

To spot699-7

Common Pb

Pb loss

Age

(Ma)

e.

f.

Age

(Ma)

FIG. 6. (Cont.)

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Qu705 Regional Diorite61.4 ± 0.8 MaMSWD 2.96Probability 0.01

705-

4 3-507

1-507

21-507

7-507

2-507

11-507

9-507

8-507

6-507

01-507

5-507

40

50

60

70A

ge(M

a)

545862667074

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

85 95 105 115 125

602702

bP/bP

238 206U/ Pb

61.5 ± 0.8 Ma MSWD 2.9Prob. 0.01

Pb lossCommonPb

30

40

50

60

70

Qu704 Dacite Porphyry56.8 ± 0.6 MaMSWD 1.66Probability 0.08

8-407

3-407

11- 407

7-407

5- 407

9- 407

6- 407

21- 407

1- 407

31-407

4- 407

01- 407

2- 407

5054586266707478

0.0

0.1

0.2

0.3

0.4

80 90 100 110 120 130602

702bP

/ bP238 206U/ Pb

56.8 ±0.8 MaMSWD 1.7Prob. 0.08

45

50

55

60

7-807

4-807

11-807

2-807

6-807

21-807

3-807

1-807

01-807

5-807

8-807

9-807

Qu708 Dacite Agglomerate56.1 ± 0.4 MaMSWD 1.2Probability 0.27

66 62 54 500.04

0.06

0.08

0.10

0.12

95 105 115 125238 206U/ Pb

602702

bP/ bP

55.9 ± 0.6 MaMSWD 0.39Prob. 0.86

57.8 ± 0.8 MaMSWD 0.40Prob. 0.67

40

45

50

55

60

11-607

3-607

6-607

4- 607

21- 6079- 6075- 60701- 607

2- 6077- 6071- 60761- 607

8- 60741- 60751- 60731- 607

Qu706 Latite Porphyry54.3 ± 0.6 MaMSWD 1.21Probability 0.30

54.3 ± 0.6 MaMSWD 1.2Prob. 0.3

58.5 ± 0.7MaMSWD 1.08Prob. 0.36

68 64 60 56 52 48

0.04

0.05

0.06

0.07

0.08

0.09

90 100 110 120 130 140

602702

bP/bP

238 206U/ Pb

a.

b.

c.

d.

Age

(Ma)

Age

(Ma)

Age

(Ma)

Pb loss

CommonPb

CommonPb

58.5± 0.7 Ma

58

FIG. 7. U-Pb ages for samples from the Cuajone mine, southern Peru. Ages are calculated based on the weighted mean207Pb-corrected 206Pb/238U histograms. Tera-Wasserburg (1972) concordia diagrams for each sample demonstrate the pres-ence of common Pb, Pb loss, and inheritance in some of the analyzed zircons. a) Regional diorite (Qu705). b) SynmineralDacite Porphyry (Qu704). c) Late-mineral Dacite Agglomerate (Qu708). d) Late-mineral latite porphyry (Qu706). Analysesexcluded from the age calculation shown in gray.

Table 1) based on seven of 12 zircon crystals. Five of the crys-tals gave ages that are too young, presumably due to Pb loss.

Porphyry Cu-related intrusions: The Dacite Porphyry unitat Toquepala consists of multiple granodiorite to monzogran-ite intrusions forming large stocks and volumetrically smallerdikes cut by numerous breccia bodies and younger porphyryintrusions (Fig. 4). A porphyry sample (Qu704) from thesoutheast part of the main dacite porphyry body yielded a U-Pb age of 56.8 ± 0.6 Ma (Fig. 7b; Table 1), based on 11 of 13zircon crystals. One of the crystals gives an age of ~68.8 Ma,consistent with it being a xenocryst from the country rocks—presumably the volcanic rocks of the Toquepala Group. Theother crystal is only slightly older, and may be an antecryst.Inclusion of that crystal results in a slightly older and less pre-cise age of 57.2 ± 0.9 Ma, but with unacceptably large MSWDof 3.1. We interpret the statistically more robust and slightlyyounger age of 56.8 ± 0.6 Ma to more accurately reflect thecrystallization age.

The Dacite Agglomerate unit forms a second porphyrysuite consisting of a dike swarm mainly restricted to thenortheast part of the mine (Fig. 4). A sample (Qu708) from anarea of weak white mica (± secondary feldspar) and chloritealteration yielded an age of 56.2 ± 0.6 Ma (Fig. 7c; Table 1)based on eight of 12 zircons. Inclusion of the two older zir-cons results in an age of 56.6 ± 0.8 Ma, with a slightly higherMSWD of 2.0. Two crystals are younger and are inferred tohave lost Pb. Two other crystals are slightly older, and may bexenocrystic. A discordia regressed through the two older datapoints suggests an older age of 57.8 ± 0.8 Ma. This age is in-teresting in view of obvious older zircons in the younger por-phyry suite (see below). Nonetheless, as with the Dacite Por-phyry unit, we interpret the statistically more robust andslightly younger age of 56.2 ± 0.6 Ma to more accurately re-flect the crystallization age.

The youngest porphyry suite, the Latite Porphyry, formssmall dikes of fine-grained monzonite to quartz monzoniteporphyry and microporphyry that are elongated north-north-east and northwest (Fig. 4). An altered sample (Qu706) char-acterized by hydrothermal white mica and chlorite yielded arange of 206Pb/238U ages between 50.74 ± 1.47 and 63.52 ±0.79 Ma for 16 zircons (Fig. 7d; Table 1)—a scatter in the an-alytical data not seen in any of the other samples. Regressionof all the analyses yields a statistically invalid although geo-logically reasonable age (56.1 ± 1.8 Ma; MSWD = 23). Thedistribution of individual zircon spot ages (Table 1), their dis-tribution on the weighted mean plot (Fig. 7d), and the con-cordia diagram (Fig. 7d) suggest the potential presence of twopopulations of zircon. An older population represented byfive crystals has an age of 58.5 ± 0.7 Ma whereas the youngerpopulation has an age of 54.3 ± 0.6 Ma based on six crystals(Fig. 7d). Two crystals older than 60 Ma are interpreted to bexenocrysts, whereas two crystals are distinctly younger, pre-sumably due to Pb loss. Based upon field relations, we inter-pret the 54.3 ± 0.6 Ma age to best represent the emplacementage of the rock, although, in view of the scatter in the ages,this could be a minimum age. Regardless, the rock can be noolder than ~56 Ma, based on field relations and the U-Pb ageof the older rocks. The possible significance of the older zir-cons in the Latite Porphyry as well as the Dacite Agglomer-ate is discussed below.

Discussion

Timing of porphyry Cu-Mo formation

U-Pb ages reported herein of intrusive rocks at theToquepala and Cuajone Cu-Mo porphyry deposits in south-ern Peru are remarkably similar (Fig. 5). This similarity is best

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2

4

6

8

10

12

14

40 45 50 55 60 65 70 75 80 85

Cuajone

a

Age

Toquepala

b

0

5

10

15

20

40 45 50 55 60 65 70 75 80 85

All

c

Age

0

2

4

6

8

10

12

40 45 50 55 60 65 70 75 80 85

Age

Freq

uenc

yFr

eque

ncy

Freq

uenc

y

Pre-mineralbatholiths

Early crypticmagmatism

LP1 and BLP porphyriesLP3 Porphyries

Pb-loss

(53.5 Ma) (56.4 Ma)

(~58 Ma)

(64.6 Ma)

InheritedToquepalaGroup

Pre-mineralbatholiths

Early crypticmagmatism

Dacite Porphyry& DaciteAgglomerate

LatitePorphyries

Pb-loss

(53.6 Ma)

(56.6 Ma)

(58.9 Ma)

(61.8 Ma)

InheritedToquepalaGroup

Pre-mineralbatholiths

Earlymagmatism

“Mineralizing”intrusions

Post-mineralporphyries

Pb-loss

(53.7 Ma)

(56.5 Ma)

(58.9 Ma)

(61.9 Ma)

FIG. 8. Probability distribution plots for a) Cuajone and b) Toquepalamine. c) Combined data from Toquepala and Cuajone. BLP = Latite Por-phyry 2, LP1 = Latite Porphyry 1, LP3 = Latite Porphyry 3.

visualized through probability density plots (Fig. 8) of all zircons from the various intrusions, including the premineralrocks. Combined with the published ages (Sillitoe andMortensen, 2010), the ages indicate protracted but episodicmagmatism in the area of each porphyry Cu deposit. The U-Pb ages also suggest the potential for a very narrow time offormation for two of the porphyry Cu deposits (Fig. 5).

The episodic intrusive history at Cuajone covers a timespan of approximately 13 m.y., from the premineral regionalgranodiorite batholiths to the late-mineral porphyritic stocks(Figs. 5, 8). Three temporally distinct episodes are recognizedfrom the data, with the oldest event being the premineralbatholiths at approximately 65 Ma, the second being the min-eralized Latite Porphyry 1 and Latite Porphyry 2 porphyriesat approximately 56 Ma, and the last being the late mineral in-trusions occurring at approximately 53 Ma (Fig. 5). The ageof the volumetrically most important mineralizing event atCuajone is constrained to postdate emplacement of LatitePorphyry 1 (55.6 ± 0.6 Ma) and predate the emplacement ofLatite Porphyry 2 (56.2 ± 0.7 Ma); as noted previously, thecalculated ages agree within their analytical uncertainty. Thedensity distribution plot of the zircon U-Pb data from Cua-jone (Fig. 8a) shows several zircons with ages between thoseof the premineral batholiths and the mineralized intrusions,suggesting that, although based on only limited data, therecould be an unrecognized magmatic phase of intermediateage between the regional batholiths and the synmineral por-phyry intrusions in the Cuajone area. Rocks with these agesare known near Toquepala and Quellaveco.

At Toquepala, intrusions were also emplaced within threetemporally distinct magmatic episodes. Magmatism occurredover a shorter time period from approximately 62 to 54 Ma(Fig. 5). Premineral batholiths in the vicinity of the Toquepalamine are younger than at Cuajone, with ages of 61.4 ± 0.8 Ma(this study) and ~58.4 Ma (Zweng and Clark, 1995) (Fig. 8).The mineralized porphyry intrusions at Toquepala are, how-ever, slightly older than those at Cuajone. The main mineral-izing event at Toquepala postdated emplacement of theDacite Porphyry (56.8 ± 0.6 Ma) and preceded emplacementof the Dacite Agglomerate (56.1 ± 0.4 Ma). Late-mineral in-trusions at Toquepala (54.3 ± 0.6 Ma) are similar in age to theyoungest magmatic event at Cuajone. Additionally, a densitydistribution plot of the zircon U-Pb data (Fig. 8b) suggeststhe presence of a ~58 Ma igneous event after the emplace-ment of the regional batholith and before the emplacement ofthe Dacite Porphyry (Fig. 5). If real, these zircons are pre-sumably xenocrystic, being derived from wall rock to amagma chamber at depth.

The timing of magmatic events at Quellaveco (Sillitoe andMortensen, 2010) is similar to that documented for Cuajoneand Toquepala (Fig. 5). A regional batholith has an age of~59.4 Ma, and the oldest and most mineralized porphyry in-trusion an age of ~58.4 Ma. An intramineral porphyry intru-sion has a U-Pb age of 55.9 ± 0.31 Ma whereas the late por-phyry has an age of 54.63 ± 0.63 Ma. In contrast to Cuajoneand Toquepala, where the majority of the Cu can be inferredto have been deposited at about 56 Ma, Sillitoe andMortensen (2010) propose a much longer period of mineral-ization at Quellaveco, with Cu being deposited in episodicevents over about 3 to 4 m.y. However, at each of the three

porphyry Cu deposits, the youngest porphyry suite was em-placed 1 to 2 m.y. after the bulk of the Cu-bearing sulfide wasdeposited, as these young porphyry intrusions contain littleCu, even though they are significantly altered to quartz-sericite-pyrite assemblages.

The timing of mineralization at the Cuajone and Toquepalamines is constrained by pre- and post-ore intrusive rocks to ap-proximately 56 and 56.5 Ma, respectively (Fig. 5). Additionally,the ages of the Cuajone and Toquepala synmineral porphyryintrusions fall within the range observed at Quellaveco, andare very similar to ages of what is interpreted as intramineralporphyry intrusions at Quellaveco (Sillitoe and Mortensen,2010). The ages reported from the syn-Cu porphyry intru-sions at Toquepala and Cuajone also suggest that mineraliza-tion may have been of short duration, similar to the durationof the hydrothermal systems interpreted for Batu Hijau (Gar-win, 2002), Bajo de la Alumbrera (Harris et al., 2004; vonQuadt et al., 2011), Elatsite (von Quadt et al., 2002), Bingham(von Quadt et al., 2011), and Boyongan-Bayugo (Braxton etal., 2012), although the thermal effects of the hydrothermaland magmatic systems may be of longer duration (Richards etal., 2001; Harris et al., 2008; Campos et al., 2009). In contrastto the possibility that Cu was introduced during a singleevent, Sillitoe and Mortensen (2010) argue for episodic Cumineralization at Quellaveco over the entire magmatic cycle.

The hydrothermal systems at these three major porphyryCu-Mo deposits clearly are part of a protracted but distinctlyepisodic magmatic event, with arguably the bulk of the Cu inat least two of the deposits being introduced at very similartimes. Such a protracted but episodic magmatic history ischaracteristic of many, but not all, porphyry Cu districtsaround the world (e.g., Cornejo et al., 1997; Richards et al.,2001; Harris et al., 2004; Padilla-Garza et al., 2004; Deckartet al., 2005).

Magmatic events associated with porphyry Cu formation in southern Peru

At Quellaveco, Toquepala, and Cuajone, the timing of mag-matic events at the three porphyry Cu centers is remarkablysimilar over a 30-km distance. Implicit in the similarity of agesis the need for the construction of upper crustal plutoniccomplexes that fed the three porphyry Cu systems (e.g., Can-dela, 1991; Dilles et al., 2000). The ~30-km distance betweenthe porphyry centers at Cuajone and Toquepala is too great toappeal to a single large and convecting batholithic chamber,as many studies have shown that the upper crustal magmachambers generally are no larger than 10 by 15 km in hori-zontal dimension (Dilles and Proffett, 1995; Stavast et al.,2008; Memeti et al., 2010). Even those batholiths are rarelycomposed of a single magma body, but rather are a compos-ite series of short-lived upper crustal magma chambers coa-lescing to form a larger body over 1 to 4 m.y. (e.g., Dilles andWright, 1988; Coleman et al., 2004; Paterson et al., 2011). In-stead, spatially separate but essentially contemporaneous, ox-idized and hydrous plutonic complexes must underlie por-phyry Cu complexes in southern Peru.

The Paleocene-Eocene metallogenic belt

The Cuajone-Quellaveco-Toquepala cluster represents alarge grouping of porphyry Cu deposits that constitute the

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Paleocene to Eocene metallogenic belt in the central Andes.Timing of porphyry Cu formation in the belt began in the Paleocene at about 62 to 63 Ma, continuing until ~52 Ma(summarized by Sillitoe and Perelló, 2005). The Cuajone-Quellaveco-Toquepala deposit cluster is of intermediate age.Thus, formation of porphyry Cu deposits in this part of theAndean Cordillera spanned approximately ~10 m.y. This timespan is comparable to others in the Andes, with the greatChilean porphyry Cu belts having formed in 8 to 10 m.y. (Sil-litoe and Perelló, 2005, and references therein). These rela-tively short time frames within the larger magmatic evolutionat a convergent plate margin emphasize the unique series ofevents that form porphyry Cu deposits.

ConclusionsThe 10 new zircon U-Pb SHIRMP-RG ages of intrusive

phases at the Cuajone and Toquepala together with publishedages from the nearby Quellaveco deposit constrain the timingof magmatism associated with formation of these porphyryCu-Mo deposits. As with many porphyry Cu districts aroundthe world, magmatic events are distinctly episodic, with themineralizing events being associated with distinct intrusivephases. At Cuajone and Toquepala, these occurred at approx-imately the same time, with the ages constrained to 55.5 to56.2 Ma at Cuajone and 56.2 to 56.8 Ma at Toquepala. Col-lectively, the magmatic history of these three giant porphyryCu deposits places critical constraints on the metallogenicevolution of this part of the Andean Cordillera.

AcknowledgmentsThis study forms a portion of the Ph.D. thesis of the senior

author. Southern Peru Copper Corporation is thanked for ac-cess to the Toquepala and Cuajone deposits. Anglo AmericanExploration Peru funded the study, and its support is grate-fully acknowledged. Additional financial support was pro-vided by a Natural Sciences and Engineering Research Coun-cil Discovery grant to RMT, and research grants from theSociety of Economic Geologists to ATS. Although they maydisagree with details of our interpretations, we thank RichardSillitoe, Anthony Harris, Jeremy Richards, and MassimoChiardia for comments and reviews of drafts of the manu-script. MDRU contribution no. 305.

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