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Porphyry-Style Alteration and Mineralization of the Middle Eocene toEarly Oligocene Andahuaylas-Yauri Belt, Cuzco Region, Peru

JOS PERELL,

Antofagasta Minerals S.A. Ahumada 11, Oficina 602, Santiago, Chile

VCTOR CARLOTTO,

Departamento de Geologa, Universidad Nacional San Antonio Abad del Cuzco. Avenida de la Cultura, Cuzco, Per

ALBERTO ZRATE, PEDRO RAMOS, HCTOR POSSO, CARLOS NEYRA, ALBERTO CABALLERO,

Minera Anaconda Per S.A. Avenida Paseo de la Repblica 3245, Piso 3, San Isidro, Lima 27, Per

NICOLS FUSTER, AND RICARDO MUHR

Antofagasta Minerals S.A. Ahumada 11, Oficina 602, Santiago, Chile

Abstract

Originally known for its Fe-Cu skarn mineralization, the Andahuaylas-Yauri belt of southeastern Peru israpidly emerging as an important porphyry copper province. Field work by the authors confirms that mineral-ization in the belt is spatially and temporally associated with the middle Eocene to early Oligocene (~4832Ma), calc-alkaline Andahuaylas-Yauri batholith, a composite body with an areal extent of ~300 130 km em-placed into clastic and carbonate strata (e.g., Yura Group and Ferrobamba Formation) of Jurassic to Creta-ceous age. Batholith emplacement included early-stage, mafic, cumulate gabbro and diorite between ~48 and43 Ma, followed by pulses of granodiorite and quartz monzodiorite at ~40 to 32 Ma. Coeval volcanic rocksmake up the middle Eocene to early Oligocene Anta Formation, a sequence of >1,000 m of andesite lava flowsand dacite pyroclastic flows with interbedded volcaniclastic conglomerate. Sedimentary rocks include the redbeds of the Eocene to early Oligocene San Jernimo Group and the postmineralization late Oligocene toMiocene Punacancha and Paruro formations. Eocene and Oligocene volcanic and sedimentary rocks are in-terpreted to have accumulated largely in both transtensional and contractional synorogenic basins. New andpreviously published K-Ar and Re-Os ages show that much of the porphyry-style alteration and mineralizationalong the belt took place during the middle Eocene to early Oligocene (~4230 Ma). Thus, batholithic magmaemplacement, volcanism, and sedimentation are inferred to have accompanied a period of intense deforma-

tion, crustal shortening, and regional surface uplift broadly synchronous with the Incaic orogeny. Supergenemineralization is inferred to have been active since the Pliocene on the basis of geomorphologic evidence anda single K-Ar determination (3.3 0.2 Ma) on supergene alunite.

The belt is defined by 31 systems with porphyry-style alteration and mineralization, including 19 systemsgrouped in 5 main clusters plus 12 separate centers, and by hundreds of occurrences of magnetite-rich, skarn-type Fe-Cu mineralization. Porphyry copper stocks are dominated by calc-alkaline, biotite- and amphibole-bearing intrusions of granodioritic composition, but monzogranitic, monzonitic, quartz-monzonitic, and mon-zodioritic stocks occur locally. Hydrothermal alteration includes sericite-clay-chlorite, and potassic,quartz-sericitic, and propylitic assemblages. Calcic-potassic and advanced argillic alteration associations are lo-cally represented, and calc-silicate assemblages with skarn-type mineralization occur where carbonate countryrocks predominate.

Porphyry copper deposits and prospects of the belt range from gold-rich, molybdenum-poor examples(Cotabambas), through deposits carrying both gold and molybdenum (Tintaya, Los Chancas), to relativelymolybdenum-rich, gold-poor end members (Lahuani). Gold-only porphyry systems are also represented (Mo-rosayhuas). Gold-rich porphyry copper systems are rich in hydrothermal magnetite and display a positive cor-relation between Cu and Au in potassic alteration. The bulk of the hypogene Cu (-Au, -Mo) mineralization oc-curs in the form of chalcopyrite and bornite, in intimate association with early-stage potassic alteration which,in many deposits and prospects, is variably overprinted by copper-depleting sericite-clay-chlorite alteration.

Most porphyry copper systems of the belt lack economically significant zones of supergene chalcocite en-richment. This is due primarily to their relatively low pyrite contents, the restricted development of quartz-sericitic alteration, and the high neutralization capacities of both potassic alteration zones and carbonate coun-try rocks as well as geomorphologic factors. Leached cappings are irregular, typically goethitic, and containcopper oxide minerals developed by in situ oxidation of low-pyrite, chalcopyrite (-bornite) mineralization. Por-phyry copper-bearing stocks emplaced in the clastic strata of the Yura Group and certain phases of the An-dahuaylas-Yauri batholith may develop appreciable supergene chalcocite enrichment in structurally and litho-logically favorable zones.

Economic GeologyVol. 98, 2003, pp. 15751605

Corresponding author: e-mail, jperello@aminerals.cl

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Introduction

THE ANDAHUAYLAS-YAURI belt (Bellido et al., 1972; SantaCruz et al., 1979; Noble et al., 1984) covers an area of ap-proximately 25,000 km2 in southern Peru and extends forabout 300 km between the localities of Andahuaylas in thenorthwest and Yauri in the southeast (Fig. 1a). Until the late1980s, the Andahuaylas-Yauri belt had received only limitedgeologic scrutiny and was mainly known for its copper-bear-ing, magnetite skarn deposits (Terrones, 1958; Bellido et al.,1972; Sillitoe, 1976, 1990; Santa Cruz et al., 1979; Einaudi etal., 1981; Aizawa and Tomizawa, 1986), best exemplified byTintaya, Atalaya, Las Bambas, Katanga, and Quechua. Formost researchers, these occurrences were considered to becopper skarns associated with barren intrusions (e.g., Einaudiet al., 1981; Noble et al., 1984), although potassic alteration inhost porphyritic stocks had been described and characterizedas such (Yoshikawa et al., 1976; MMAJ, 1983; Noble et al.,1984). During the late 1980s, regional work complementedby detailed geologic studies at Tintaya and Katanga (Carlier etal., 1989), followed by grass-roots exploration in the regionduring the 1990s, confirmed the presence of porphyry-stylealteration and mineralization (e.g., Fierro et al., 1997) and re-

sulted in the discovery of additional, potentially economicporphyry copper deposits (Table 1) at Antapaccay (Jones etal., 2000), Los Chancas (Corrales, 2001), and Cotabambas(Perell et al., 2002), as well as porphyry-skarn mineralizationat Coroccohuayco (BHP Company Limited, 1999). Zinc-rich,Mississippi Valley-type mineralization was also discovered inthe region (Carman et al., 2000) adding to the metallogenicdiversity of the belt.

This paper describes the salient geologic features of anumber of porphyry Cu (-Au, -Mo) deposits and prospects ofthe Andahuaylas-Yauri belt that help to define this region as anew porphyry copper province. It also provides newgeochronologic data to constrain the age of the porphyry-style

alteration and mineralization in the belt and establishes re-gional correlations and comparisons with nearby porphyrycopper provinces. However, the paper is not designed tocover in full the complex geology of this still poorly under-stood region. Detailed geologic descriptions can be found inMarocco (1978) and Carlotto (1998) for the area under study,and in Clark et al. (1990) and Sandeman et al. (1995) fornearby southeastern Peru transects. The paper focuses onsystems for which the bulk of the mineralization is of por-phyry type and excludes those deposits in which skarn-typemineralization is the dominant style. Descriptions of the lat-ter can be found elsewhere (Terrones, 1958; Santa Cruz et al.,1979; Aizawa and Tomizawa, 1986; Fierro et al., 1997; Zweng

et al., 1997). Following a short review of the regional geologicsetting of the Andahuaylas-Yauri belt, the main geologic fea-tures of several deposits and prospects are described. Thepaper concludes with a section in which regional metallogenicaspects are reviewed.

Methods

Except for those deposits and prospects with published de-scriptions (e.g., Tintaya, Antapaccay, Los Chancas), much of

the work represents the product of more than three years ofexploration by the authors, including both regional (1:25,000scale) and detailed (1:5,000 scale) mapping. Field work wascomplemented by thin section petrographic studies to charac-terize rock types, alteration assemblages, and dominant veinstyles at each prospect. Rock names for the main batholith in-trusions and porphyry copper-bearing stocks follow thenomenclature of Streckeisen (1976, 1978) and are based onpoint counts (1,500 points) for modal proportions of key sili-cates. Unless otherwise stated, the K-Ar ages reported here

were determined at the geochronology laboratory of the Geo-logical Survey of Chile, Santiago and followed standard proce-dures and techniques (e.g., Dalrymple and Lamphere, 1966;

Steiger and Jaeger, 1977; Baksi, 1982). All ages are referred tothe geological time table of Haq and van Eysinga (1987).

Regional Setting

The Andahuaylas-Yauri belt is located at a distance of ~250to 300 km inland from the present-day Peru-Chile trench(Fig. 1). The region is underlain by thick sialic crust (50 to 60km; James, 1971), and straddles the transition zone betweenthe southern, normal subduction regime of southern Peruand northern Chile and the northern, flat subduction zone ofcentral and northern Peru (Cahill and Isacks, 1992). It is lo-cated immediately southeast of the Abancay Deflection(Marocco, 1978). The region encompasses parts of the inter-

montane depressions between the Eastern and WesternCordilleras and the northern extremity of the Altiplano (Fig.1b; Carlier et al., 1996; Chvez et al., 1996). The western partof the belt is characterized by a rugged, mountainous topog-raphy where ranges and snow-capped peaks above 4,500 mare incised by deep (>2,000 m), steep-sided canyons. Thesecanyons constitute the main drainage system of the regionand include the Santo Toms, Urubamba, Apurmac, Vil-cabamba, Mollebamba, and Antabamba rivers, all of whichdrain toward the Amazon basin. The eastern and southernparts of the region are characterized by the gently undulatingtopography of the ~4,000 m-high plateaus that extends intothe Altiplano of Bolivia (Fig. 1b).

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A model for the region suggests that the calc-alkaline magmas of the Andahuaylas-Yauri batholith and sub-sequent porphyry-style mineralization were generated during an event of subduction flattening which triggeredthe crustal shortening, tectonism, and uplift assigned to the Incaic orogeny. Shortening of the upper crustwould have impeded rapid magma ascent favoring storage of fluid in large chambers which, at the appropriatedepth in the uppermost crust, would have promoted large-scale porphyry copper emplacement. Geodynamicreconstructions of the late Eocene to early Oligocene period of flat subduction in the central Andes suggestthat emplacement of the Andahuaylas-Yauri batholith took place at an inflection corridor in the subductionzone broadly coincident with the position of the present-day Abancay deflection. Similarly, evidence from

southeastern Peru suggests that the Andahuaylas-Yauri belt may be continuous with the late Eocene to earlyOligocene porphyry copper belt of northern Chile and that the process of subduction flattening in southernPeru also may have taken place in northern Chile between ~45 and 35 Ma.

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FIG. 1. Sketch maps showing the location of the study area in the context of main geologic, geophysical, topographic, andphysiographic features of the Central Andes. a. Area with average elevation >3,000 m and depth contours of the subductedslab after Cahill and Isacks (1992). Oceanic features from Jaillard et al. (2000). b. The study area relative to main regionalphysiographic provinces (Jaillard et al. 2000), contours of crustal thickness (James, 1971), and main Precambrian basementunits (Ramos and Aleman, 2000).

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Precambrian and Paleozoic basement

Precambrian gneisses at Ro Pichari, ~130 km northwest ofCuzco (Carlotto, 1998) are probable extensions of theMaran massif exposed farther north and are interpreted byRamos and Aleman (2000) to constitute remnants ofperigondwanan terranes attached to the Amazonian Craton inthe Early Paleozoic (Fig. 1b). Paleozoic rocks in the region in-clude >10,000 m of volcanosedimentary, marine, and conti-nental rocks of Cambrian(?) to Early Permian age (Marocco,1978; Carlotto et al., 1996a; Carlotto et al., 1997). The upperpart of the pre-Andean basement is dominated by >1,000 mof volcanic and clastic rocks of the Mitu Group (Permian toEarly Triassic; Fig. 2).

Mesozoic and Cenozoic stratigraphy

The Mesozoic and Cenozoic stratigraphy of the region ischiefly made up of Jurassic and Cretaceous sedimentary se-

quences deposited in a paleogeographic setting dominated bytwo main basins (Western and Eastern Peruvian basins) sep-arated by the Cuzco-Puno basement high (Fig. 3; Carlotto etal., 1993; Jaillard and Soler, 1996). The Western basin, alsoknown as the Arequipa basin (Vicente et al., 1982), corre-sponds to the present-day Western Cordillera. It contains asedimentary pile (Middle Jurassic to Late Cretaceous) in ex-cess of 4,500 m thick with a lower part dominated by tur-bidites, a middle part with quartz arenite, and an upper part

with abundant limestone (Vicente et al., 1982; Jaillard andSantander, 1992). The northeastern edge of this basin, coinci-dent with the Andahuaylas-Yauri region, includes the Lagu-nillas and Yura groups (Marocco, 1978), made up of EarlyJurassic limestone and Middle to Late Jurassic quartz areniteand shale, with a total thickness of approximately 800 m (Fig.3). The top of the sequence contains the massive micriticlimestone, black shale, and nodular chert of the FerrobambaFormation (Marocco, 1978; Pecho, 1981). The Cuzco-Punohigh includes ~900 m of terrigenous red beds interbedded

with shale, limestone, and gypsum (Carlotto et al., 1993; Jail-lard et al., 1994). The age of these rocks is Late Jurassic to Pa-leocene (Fig. 3). The Eastern basin, also known as Putinabasin (Jaillard, 1994), is made up of several sequences of LateCretaceous marine clastic and carbonate rocks, with a totalthickness of ~ 2,600 m (Jaillard et al., 1993; Jaillard, 1994;Crdenas et al., 1997).

Eocene to early Oligocene stratigraphy

Two main units characterize the Eocene to early Oligocenestratigraphy of the region, including the sedimentary SanJernimo Group and the dominantly volcanic Anta Formation(Figs. 2 and 4). These units unconformably overlie the Meso-zoic and early Cenozoic sequences described above. The SanJernimo Group (Eocene to early Oligocene) consists of twomain formations (Kayra and Soncco; Fig. 4), with a totalthickness of ~4,500 m, made up of red bed terrigenous (sand-stone, shale, pelitic sandstone, and volcanic microconglomer-ate) strata interbedded with tuffaceous horizons near the top.The age of the San Jernimo Group is constrained by strati-graphic relations (it unconformably overlies strata with plantfossils of Paleocene to early Eocene age) and on K-Ar and Ar-Ar ages of 29.9 1.4 Ma and 30.84 0.83 Ma, respectively,from the upper tuffaceous horizons of the Soncco Formation(Fig. 4; Carlotto, 1998; Fornari et al., 2002). Sedimentation is

interpreted to have taken place initially in a fluvial environ-ment that progressed into structurally controlled, pull-apartbasins (Crdova, 1986; Noblet et al., 1987; Marocco and No-blet, 1990; Chvez et al., 1996). Between Cuzco and Sicuani,basal sandstone of the Soncco Formation includes horizons ofstratiform copper mineralization, up to several meters thick,

with hypogene chalcocite and bornite, and supergene copperoxides (Crdenas et al., 1999), which have similarities to thered bed deposits from the Bolivian Altiplano (e.g., Corocoro;Sillitoe, 1989) and northern Chile (San Bartolo; Travisany,1979). The San Jernimo Group is equivalent to the PunoGroup of the Peruvian Altiplano southeast of the study region(Fig. 4), where it is overlain by the volcanic horizons of theTacaza Group (Klinck et al., 1986; Clark et al., 1990; Jaillardand Santander, 1992). Farther south, sedimentary, conglom-eratic, red bed sequences are known in the Altiplano of Bo-livia (e.g., the lower horizons of the Tiwanaku Formation andthe Berenguela and Turco formations; Hrail et al., 1993), inthe Puna of northwestern Argentina (Geste and Quioa for-mations; Alonso, 1992; Kraemer et al., 1999; Coutand et al.,2001), and in the Salar de Atacama area of northern Chile(upper Purilactis Group; Mpodozis et al., 1999).

The Anta Formation is a >1,000 m sequence character-ized by a lower member with andesite lava flows and dacitepyroclastic flows locally interbedded with alluvial conglom-erate, and an upper member of fluvial conglomerate with

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TABLE 1. Geologic Resources for Main Deposits of the Andahuaylas-Yauri Belt

Deposit Tonnage ( 106) Cu (%) Au (g/t) Mo (%) Main reference

Tintaya districtAntapaccay 383 0.89 0.16 n.a. Jones et al. (2000); Fierro et al. (2002)Coroccohuayco 155 1.57 0.16 n.a. BHP (1999)Ccatun Pucara 24 1.44 n.a. n.a. BHP (1999)Quechua 300 0.68 n.a. n.a. E. Tejada (pers. commun., 2003)Tintaya 139 1.39 0.23 n.a. BHP Billiton (2003)1

Cotabambas AreaAzulccacca 24 0.42 0.39

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interbedded andesite and basaltic andesite flows (Fig. 4). Itsage is constrained to middle Eocene to early Oligocene bystratigraphic relations and K-Ar geochronology (Carlier et al.,1996; Carlotto, 1998). Southwest of Cuzco, two biotite-richdacitic flows from the middle part of the formation have re-turned K-Ar ages of 38.4 1.5 and 37.9 1.4 Ma, and abasaltic horizon from the upper part of the unit yields a K-Ar

whole rock age of 29.9 1.1 Ma (Carlotto, 1998). The AntaFormation andesites and conglomerates are interpreted to bestratigraphic equivalents of the San Jernimo Group red beds(Fig. 4), with the erosion products of the Anta Formation

feeding the San Jernimo basin located to the northeast. Thecoarsening-upward characteristics of the sequence, with allu-vial and fluvial conglomerates dominated by volcanic and plu-tonic clasts at the top, are interpreted to reflect topographicrejuvenation of the source regions in response to increasingregional tectonic uplift, with sedimentation in a piggy-backstyle basin environment (Carlotto, 1998).

Late Oligocene to Miocene stratigraphy

The late Oligocene to Miocene sedimentary deposits of theregion include the Punacancha (1,5005,000 m thick) andParuro (>1,100 m-thick) formations (Fig. 4). They are domi-nated by coarsening-upward red shale and sandstone, with

gypsum and conglomerate being characteristic in the upperparts of the sequences. Sedimentation is interpreted to havetaken place in a fluvial environment with braided rivers, floodplains and alluvial fans in structurally controlled basins (Car-lotto et al., 1996a, 1997; Jaimes et al., 1997; Romero et al.,1997). The age of these sequences is based on stratigraphicrelations and fossil flora, as well as a K-Ar age of 10.1 0.5Ma for a tuffaceous horizon near the base of the Paruro For-mation (Carlotto et al., 1997).

Oligocene and Miocene volcanic rocks in the region andnearby areas are largely dominated by the calc-alkaline se-

quences of the Western Cordillera (Inner-Western Cordilleraof Sandeman et al., 1995) and Altiplano, and include theTacaza (Oligocene) and Sillapaca (Miocene) groups. In addi-tion to these, a series of scattered, small shoshonitic volcaniccenters of Pliocene to Quaternary age occur in the region(Figs. 2 and 4; Wasteneys, 1990; Carlier et al., 1996; Carlotto,1998). The Tacaza Group consists dominantly of trachyan-desite, andesite, and rhyolite tuff (Klinck et al., 1986;

Wasteneys, 1990; Carlotto, 1998), with shoshonitic rocksbeing important in the Santa Luca area, southeast of Yauri(Clark et al., 1990; Sandeman et al., 1995). Shoshonitic vol-canism in the Santa Luca area took place between ~32 and24 Ma (Fig. 4; Clark et al., 1990; Sandeman et al., 1995),

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FM

ANTA

F

M

ATA

SPACA

PLU

TONS

Age(Ma)

PALEOCENE

EOCENE

EARLYEARLYEARLY MIDDLEMIDDLE LATELATELATE

OLIGOCENEMIOCENE

PLIOCENE

PLEISTOCENE

LA

TEST

TO

QUEPALA

PLUTONISM/

VO

LCANISM

PICOTANI

GP

SANJERONIMO

GP

PARUROFM

PUNACANCHAFM

KAYRAFM

Re

dbe

dCopper

SONCCOFM

EARLYTERTIARY

REDBEDS

ANDAHUAYLAS

-

YAURI

BATHOLITH

ARCO

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CUZCO

-SICUANI

VOLCANOES

SANTOTOMAS

IGNIMBRITES

TACAZAGP

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D

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C

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ALTIPLANO

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20

30

40

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nca

lc-a

lka

line

intrus

ions

Dom

inantlyma

ficcumu

late

ca

lc-a

lkaline

intrus

ions

FIG.4.Summarystrat

igraphiccolumnsforrepresentativeEocenetopresent-dayvolcanic,sedimentary,andintrusiveunitsofthestudyareaandnearbysouth

eastern

Perutransects.Columns

A,B,andDsimplifiedafterSandemaneta

l.(1995)andreferencesthereinandA.H.Clark(pers.commun.,2002).ColumnCforth

estudy

areacompiledafterCarlotto(1998),withadditionsafterCarlieretal.(1989,1996)andthisstudy.IncolumnC,

notethespatialandtemporalrelationshipsb

etween

batholithicplutons,volcanicrocksoftheAntaFormation,andthesed

imentaryredbedsequencesoftheSanJernimoGroup.

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whereas farther south, along the arc front, Tacaza-equivalentpyroclastic flows intercalated with the molassic MoqueguaFormation commenced at ~26 Ma (A. H. Clark, pers. com-mun., 2002). In the Andahuaylas-Yauri region, similar age(~29 Ma) shoshonitic rocks have been interpreted to be partof uppermost Anta Formation (see above and Carlotto, 1998),thereby implying some degree of temporal overlap with

Tacaza rocks (Fig. 4). Sillapaca Group rocks include mainlydacite flows with subordinate andesite in the southeasternpart of the study region (Carlotto, 1998) and subvolcanicdacite plugs and ash-flow tuff in the Santa Luca area (Fig. 4;Clark et al., 1990). Rocks from Santa Luca yield ages of be-tween ~22 and 14 Ma (Clark et al., 1990; Sandeman et al.,1995); elsewhere in the Puno region a second effusive event,also assigned to the Sillapaca Group, returns ages of between~14 to 12 Ma (Klinck et al., 1986).

If the correlations above are accepted (Fig. 4), it may bespeculated that the temporal overlap of the Oligocene toMiocene volcanism of the Western Cordillera, Altiplano, andEastern Cordillera (Sandeman et al., 1995) would also applyto the Andahuaylas-Yauri region. This interpretation is con-sistent with the suggestion by Sandeman et al. (1995) that a>350-km-wide arc was episodically active throughout south-ern Peru during late Oligocene and Miocene times. In theAndahuaylas-Yauri region, however, volcanism seems to havebeen intermittently active since the middle Eocene (Carlieret al., 1996, 2000; Carlotto, 1998; Carlotto et al., 1999).

The Andahuaylas-Yauri batholith

The northeastern border of the Western Cordillera in thestudy area is underlain by large bodies of intrusive rocks col-lectively known as the Andahuaylas-Yauri batholith (Carlier etal., 1989; Bonhomme and Carlier, 1990). It is also known lo-cally as the Abancay (Marocco, 1975, 1978) or Apurmac

batholith (Pecho, 1981; Mendvil and Dvila, 1994). Thename Andahuaylas-Yauri batholith is used in this paper, fol-lowing Bonhomme and Carlier (1990). The batholith is com-posed of a multitude of intrusions that crop out discontinu-ously for >300 km between the towns of Andahuaylas in thenorthwest and Yauri in the southeast. Its width varies be-tween ~25 km in the Tintaya area and ~130 km along theChalhuanca-Abancay transect (Fig. 5a).

In general terms, the batholith includes early-stage intru-sions of cumulates (gabbro, troctolite, olivine gabbro, gabbro-diorite, and diorite) followed by rocks of intermediate com-position (monzodiorite, quartz diorite, quartz monzodiorite,and granodiorite) (Fig. 5c; Carlier et al., 1989; Bonhomme

and Carlier, 1990; Carlotto, 1998). Subvolcanic rocks of dom-inantly granodioritic/dacitic composition, locally associatedwith porphyry-style mineralization, represent the terminalstage (see below). Early-stage cumulate rocks are exposedmainly along the northern border of the batholith (Fig. 5) be-tween Curahuasi and Limatambo (Carlier et al., 1989; Lig-arda et al., 1993), where petrologic work by Carlier et al.(1989, 1996) determined that they constitute typical calc-alkaline cumulates crystallized at the bottoms of shallow magmachambers, with temperatures of emplacement of ~1,000Cand pressure conditions of ~2 to 3 kbars. The intrusions of theintermediate stage are lighter gray in color, display medium-to coarse-grained, equigranular to slightly porphyritic textures,

and possess amphibole > biotite as the dominant ferromag-nesian phases, with local pyroxene in the more mafic mem-bers. They are regularly distributed throughout the regionand constitute the main mass of the batholith. Contact aure-oles within country rocks are extremely irregular in shape,size, and composition, although garnet skarn is typicallyformed in calcareous rocks (e.g., Ferrobamba Formation) and

biotite and cordierite hornfels are developed where the morepelitic facies of the Mesozoic formations are present (Car-lotto, 1998).

The age of the batholith is constrained by regional strati-graphic relations and geochronologic data (Table 2; Fig. 5b).Batholith rocks intrude mostly Mesozoic and early Cenozoicmarine and continental strata as well as the middle Eocene toearly Oligocene Anta Formation (Fig. 4). In addition, severalK-Ar ages reported by Carlier et al. (1996), Carlotto (1998),and Perell et al. (2002), together with a number of ages ob-tained during the course of the present study, confirm a mid-dle Eocene to early Oligocene age (~48-32 Ma) for the bulkof the batholith (Fig. 5). The geochronologic data support theinference by Bonhomme and Carlier (1990) that cumulaterocks are older (~48-43 Ma) and that intermediate composi-tion rocks are younger (~40-32 Ma), thereby corroboratingthe concept that batholith emplacement took place in at leasttwo main stages. The data also suggest, however, that consid-erable time overlap existed between the more mafic and themore felsic intrusions of the younger group (Fig. 5b).

Other intrusions

Post-batholith intrusive activity in the region is character-ized by a series of small syenitic stocks that have yielded K-Arages of ~28 Ma in the Curahuasi area (Carlotto, 1998). Theseintrusions are part of a larger alkalic magmatic province thatalso includes the basanites, phonotephrites, and trachytes of

the Ayaviri region, with ages between 29 and 26 Ma (Carlieret al., 1996, 2000).

Structural geology

The structure of the region is, in general terms, poorly con-strained and understood. Although some exceptions exist(Marocco, 1975; Pecho, 1981; Cabrera et al., 1991; Carlottoet al., 1996b; Carlotto, 1998), regional maps lack the detailedstructural data that would help to understand the regionaltectonics as a whole. The northeastern border of the WesternCordillera is dominated by Mesozoic to Cenozoic sequencesthat have been moderately to intensely deformed in large,northwest-trending folds with dominantly northerly vergence

(Fig. 2). Intense folding in the region typically involves car-bonate and shaly sequences (Ferrobamba Formation andequivalent units) that wrap around cores of quartz arenite ofthe Yura Group. Low- and high-angle thrusts locally accom-pany the most intense deformation and folding, particularly inthe southern quadrangles of the region (Pecho, 1981), withmost of the mapped thrusts displaying northerly vergence.This style has similarities to thin-skinned fold-thrust beltselsewhere (e.g., Benavides-Cceres, 1999), as no involvementof pre-Mesozoic basement is apparent.

The limit between the Western Cordillera and the Alti-plano is characterized by two main northwest-trending faultsystems (Limatambo-Ayaviri and Abancay-Yauri) with exposed

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7300

7330

50km

1400

1400

1430

1430

7230

7200

7130

43

.31

.9(1)

35

.13

.1(1)

39

.81

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10Quar

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12

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14

15

16

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Tintaya

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bas

Co

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mbas

STRATIGRAPHY

ANDAHUAYLAS

-YAURI

BATHOLITH

GRANODIORITE

QUARTZ

MONZODIORITE

MONZODIORITE

DIORITE

DIORITE/GABBRO(CUMULATES)

K-Ar(Carlotto,

1998)

a

b

c

BatholithPlutons

(1)

(1)

(1)

(1)

(1)

(1)

(1)(

1)

(1

)

20

24

28

.5

30

34

37 4

050

Ma

PUNACANCHAFORMATION

EOCENEOLIGOCENEMIOCENE

EARLYMIDDLELATEEARLYLATEEARLY

ANTAFmSANJERONIMOGp

??

COTABAMBAS

POMACANCHIS

LASBAMBAS

KATANGA

TINTAYA

ABANCAY

CURAHUASI

CUZCO

SICUANI

SANTOTOMAS

YAURI

MACHUP

ICHU

ANDAHUAYLAS

CHALHUANCA

LIVITACA

FIG.5.DistributionandageoftheAndahuaylas-Yauribatholithinthestudyarea.a.Displaysthemainbodyofthebatholith(Fig.2)andthelocationoftheK

-Arage

datafromthepresentstudy(Table2)andCarlotto(1998).b.AvailableK-AragedatarelativetovolcanismoftheAntaFormationandsedimentationoftheSanJern-

imoGroupasincolumn

CofFigure4.c.Compositionofthemainp

hasesoftheAndahuaylas-Yauribatholithon

aQAPdiagram(Streckeisen,1976),basedp

rimarily

onworkbythewritersw

ithadditionsafterPecho(1981),Carlieretal.(1989),andCarlotto(1998).Mainlocalities

studiedareidentifiedforbettercomprehens

ion(see

textfordescriptions).

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lengths of >300 km (Fig. 2). Both are made up of several seg-ments or smaller faults with individual continuous runs of >50km that display high-angle reverse and strike-slip movements.In the vicinity of the Abancay deflection (Marocco, 1978),these structures transpose Paleozoic plutonic rocks over

younger cover sequences. Farther east, near Curahuasi, theyplace deep cumulate facies of the Andahuaylas-Yauribatholith on top of either younger intrusions of the samebatholith or over volcanic horizons of the Anta Formation(Carlotto, 1998). Farther southeast, in the area of SantaLuca, high-angle reverse structures belonging to the south-eastern extension of the Abancay-Yauri fault are interpretedto have been associated with a major fold-thrust deformationevent (Jaillard and Santander, 1992). The ~300-km-long, 10to 50-km-wide corridor defined by the Limatambo-Ayaviriand Abancay-Yauri fault systems is occupied by the synoro-genic rocks of the Anta Formation and the San JernimoGroup. The two main fault systems are inferred to have been

active during Mesozoic time and to have largely controlledthe shape and extension of the Cuzco-Puno high in the region(Carlotto, 1998); they would therefore constitute structuresreactivated during Andean deformation (Jaillard and San-tander, 1992; Benavides-Cceres, 1999).

The Altiplano is characterized by the synorogenic se-quences that filled the basins of the San Jernimo Group andthe Punacancha and Paruro formations. These sequences dis-play intense synsedimentary deformation structures includingtight folding and fault-controlled progressive unconformities(Carlotto, 1998).

Tectono-Magmatic Synthesis

The main part of the region under consideration seems tohave been affected by several Late Cretaceous to Pliocenetectonic events (Marocco, 1975; Pecho, 1981; Cabrera et al.,1991; Carlotto et al., 1996b) of which the Eocene to earlyOligocene (Incaic) and Oligocene to Miocene (Quechua)pulses are the most important. Important sedimentary, tec-tonic, and magmatic activity occurred in the Eocene andOligocene. The red beds of the San Jernimo Group were de-posited in structurally controlled, northeast-trending synoro-genic basins localized at the boundary between the Easternand Western Cordillera. Fluvial sedimentation is thought tohave progressed from south to north (Fig. 6). The presence ofseveral progressive unconformities in the sedimentary se-

quences is interpreted to indicate that successive compressiveevents were modifying the original pull-apart transtensionalarchitecture into contractional basins (Crdova, 1986; Nobletet al., 1987; Marocco and Noblet, 1990; Chvez et al., 1996;Carlotto, 1998). Locally, important volcanism (Anta Forma-tion) accompanied deposition of the San Jernimo red bedsequences.

The bulk of the deformation, interpreted to have begun at~42 Ma (Carlotto, 1998) and thus broadly synchronous withthe Incaic orogeny of central Peru (Noble et al., 1974, 1979;Mgard et al., 1984; Mgard, 1987; Farrar et al., 1988;Sbrier et al., 1988; Sbrier and Soler 1991), is also thoughtto have been the most important event of compressive defor-mation in the region. Paleogeographic reconstructions (Fig.6) suggest that this northeast-directed deformation was re-sponsible for the development of the basins that accommo-dated middle Eocene to early Oligocene volcanism and sedi-mentation. Reactivation of older, basin-bounding structures

(e.g., Cuzco-Puno high) into major high-angle reverse faultsthat favored the uplift of the Andahuaylas-Yauri batholith,also took place during late Eocene to early Oligocene time(~4032 Ma; Carlier et al., 1996; Carlotto, 1998; see below).

This summary suggests that the Incaic orogeny in the studyregion constitutes a long-lived period of semicontinuous de-formation of ~20 to 15 m.y., between the middle Eocene andthe early Oligocene. Several distinct deformation events andassociated shortening and uplift are, however, apparent andmay have accompanied emplacement of the various phases ofthe Andahuaylas-Yauri batholith in at least two main events,at ~48 to 43 and ~40 to 32 Ma (Bonhomme and Carlier, 1990;Carlier et al., 1996). Evidence from outside the study region(Portugal, 1974; Jaillard and Santander, 1992; Carlotto et al.,1996b; Carlotto, 1998) strongly suggests that magmatism,folding, uplift, and erosion were all integral components ofthe Incaic orogeny in the Western Cordillera and Altiplano, acontrasting view to that of Clark et al. (1990) and Sandemanet al. (1995) for a contiguous transect (1320S) farthersoutheast.

Porphyry Copper Geology

Distribution

The Andahuaylas-Yauri belt extends for ~300 km and isdefined by 31 prospects and deposits with porphyry-style

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TABLE 2. K-Ar Ages of Various Intrusive Phases of the Andahuaylas-Yauri Batholith

Sample no. Latitude Longitude Mineral K (%) Radiogenic Ar (nl/g) Ar (at. %) Age 2

LIVIKAR 02 1418'58" 7144'10" Biotite 7.041 11.155 31 40.3 1.0PORKAR 02 1429'34" 7156'02" Biotite 7.212 10.100 25 35.7 0.9KATKAR 05 1426'38" 7154'30" Biotite 5.066 6.278 13 31.6 0.8COTKAR 01 1345'03" 7221'23" Biotite 7.556 1.335 17 43.2 1.1COTKAR 02 1341'11" 7221'14" Amphibole1 0.812 1.271 34 39.8 1.5PROGKAR 01 1406'02" 7228'35" Biotite1 6.833 9.171 26 34.2 0.9PROGKAR 02 1400'54" 7228'28" Amphibole 0.331 0.516 53 39.7 1.9LAHUKAR 01 1425'08" 7300'45" Biotite 7.493 10.544 27 35.8 0.9CHALCOKAR 02 1403'37" 7218'21" Amphibole 0.504 0.751 36 37.9 1.4

Constants: = 4.962 10-10y1; = 0.581 1010y1; 40Ar/36Ar = 295.5; 40K = 0.01167 at. percentSee Figure 5 for sample location1 Some degree of alteration to chlorite present

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alteration and mineralization, including 19 systems groupedin 5 main clusters plus 12 separate porphyry centers (Fig. 7a).Of these, 3 systems (Leonor, Leticia, and Aceropata; Fig. 7a)are excluded from this review due to a lack of data. The belthas a known maximum width of 130 km in a northeast-south-

west direction, with deposits exposed at elevations between3,400 and 4,700 m (Fig. 7b). However, beyond the limits of

the belt in the Eastern Cordillera, porphyry Cu-Mo mineral-ization is exposed at elevations as low as 2,800 m, as at Aurora(Fig. 7b).

A salient feature of the belt is the spatial distribution ofporphyry copper stocks around the edges of the main intru-sions that make up the Andahuaylas-Yauri batholith, as ex-emplified by the Katanga, Morosayhuas, and Las Bambasclusters and by the Pea Alta, Lahuani, Alicia, Leonor, and

Winicocha systems. Only at Panchita, Portada, Leticia,Cotabambas, and Antapaccay was the earlier plutonic phaseeffectively penetrated by the later porphyry stocks. Isolatedsystems, such as those of the Tintaya cluster and Trapiche,Los Chancas, and Chaccaro, far from the main batholithic

intrusions, are nevertheless related to smaller plutons andoutliers of the same batholith.

Host rocks

Country rocks for selected porphyry systems of the belt areindicated in Figure 8 and Table 3. Wall rocks to the Las Bam-bas, Katanga, and Tintaya clusters include either intrusive

batholithic rocks or sedimentary rocks preserved as roof-pen-dants in the batholith. Within the Mesozoic units, it is appar-ent that lower Ferrobamba Formation horizons, near theircontact with the Mara Formation, are the preferred hostrocks (Tintaya deposits; Fierro et al., 1997; Zweng et al., 1997;Jones et al., 2000), whereas the Chuquibambilla and Sorayaformations constitute common host rocks where the YuraGroup predominates (Fig. 8), in the southwestern part of thebelt (Los Chancas; Corrales, 2001). The Cotabambas clusteris restricted to diorite and granodiorite of the Andahuaylas-Yauri batholith (Perell et al., 2002), whereas both batholithintrusions and Anta Formation volcanic horizons make up thehost rocks of the Morosayhuas cluster farther north (Fig. 8).

PORPHYRY-STYLE ALTERATION AND MINERALIZATION, ANDAHUAYLAS-YAURI BELT, PERU 1585

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RioAp

urimac

Fault

Yau

r

i

F

a

u

lt

Pom

c

acan

his

Fault

Cotabam

basFault

AbancayFault

AyaviriF

ault

L imat amboF au lt

50 km

ABANCAY

ANDAHUAYLAS

CHALHUANCA

YAURI

SICUANI

SANTO TOMAS

LIVITACA

URCOS

CUZCO

MACHU PICHU

SAN JERONIMO GROUPBASINS

ANTA FORMATIO Na: CONGLOMERATES b: VOLCANIC ROCKSa

b

MAIN PLUTONS OFANDAHUAYLAS-YAURI BATHOLITH

MESOZOIC ROCKS

PALEOZOIC BASEMENT

SEDIMENT PROVENANCE

73

73

71

71

1414

FIG. 6. Schematic paleogeographic reconstruction of the study area during late Eocene to early Oligocene time. Note theintimate spatial relationship between batholithic plutons, volcanic rocks of the Anta Formation, and the sedimentary basinsof the San Jernimo Group in this reconstruction. Also shown are the main fault systems and high-angle thrusts that are in-terpreted to have controlled both uplift of the batholith at the deformation front south of Cuzco (the Cuzco-Puno high) andthe San Jernimo basins. Main localities are shown for reference. Modified after Carlotto (1998).

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Geometry

Porphyry copper-bearing stocks of the Andahuaylas-Yauribelt are centered on multiple-pulse porphyritic intrusions(Table 3) that commonly display both dike- and cylinder-likegeometries. In plan view, the stocks generally range from~0.25 to 0.6 km2, but also include the much smaller examplesof the Morosayhuas cluster where the stocks can be as smallas 150 50m, as at Qenqo. In general, the form of the stocks

is difficult to determine, owing to (1) post-mineralizationmoraine cover (Katanga), (2) late mineral dikes (Cotabam-bas and Tintaya; Fig. 9), (3) syn- to post-mineralizationfaults (Cotabambas and Antapaccay; Fig. 9), and (4) texturaland compositional similarities between porphyry copper-bearing stocks and wall rocks (Winicocha, Morosayhuas,Chalcobamba, and Panchita). Moreover, other systems seemto have developed complex geometries from the outset,such as the rootless nature of the stocks at Chabuca and

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73 72 71

A

13

14

15

SECTION

FORI

NSET

LOSCHANCAS(18)PEA ALTA(24)

LEONOR(19)

PANCHITA(25)

LAHUANI(17)

TRAPICHE(32)

CHALCOBAMBA(9)FERROBAMBA(14)

SULFOBAMBA (29)

CRISTODELOSANDES(6)

50km

ALICIA (2)

PORTADA(2 6)

MONTERO JO(23)

KATANGA (16)

WINICOCHA(33)

SANJOSE(30)

COROCCOHUAYCO(1 3)

QUECHUA(28)

aANTAPACCAY(3)

TINTAYA(3 1)

HUACLLE(15)

LETICIA(2 0)

ACEROPATA(1)

CCALLA(7)

AZULCCACCA(5)CCARAYOC(8)

CHACCARO(10)

LLOCLLACSA (21)

QENCO(27)

MAKI(22)

CHILCACCASA(12)

CHA-CHA(11)

MOROSAYHUASCLUSTER

AURORA (4)

LASBAMBASCLUSTER

COTABAMBASCLUSTER

KATANGACLUSTER

TINTAYACLUSTER

A

1725

32

24

1918

3

1328

31

26

6

29 9

3023

14 16

2

338

120

510

7

15

12 21

4

5000

4500

4000

3500

3000

2500

Elevationmeters a.s.l.

1127

22

50 km

A A

b

FIG. 7. Distribution of the porphyry copper deposits and prospects referred to in this study. a. Illustrates the location ofthe main clusters at Morosayhuas, Katanga, Cotabambas, Las Bambas, and Tintaya, together with other separate deposits

and prospects. Numbers in parentheses are keyed to the section of Figure 7b. The Aurora prospect is also shown for refer-ence. b. Simplified section A-A displaying the distribution of the systems relative to present-day elevation above sea level.

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Coroccohuaycco (Fierro et al., 1997; Zweng et al., 1997) andthe bedding-parallel attitude of the multiple sills at Quechua(J. Perell, unpub. data, 2003), in the Tintaya cluster.

Systems dominated by tabular, dike-like geometries includethe Cotabambas (Fig. 9c), Las Bambas, and Morosayhuasclusters, and the Pea Alta and Quechua composite stocks.Typical cylindrical host intrusions include Los Chancas (Cor-rales, 2001), Cristo de los Andes, Chaccaro, and Alicia (Fig.9a), with plan dimensions of between 300 and 600 m.

Composition

Porphyry copper-bearing stocks of the belt are, like themajor intrusions of the Andahuaylas-Yauri batholith, typically

calc-alkaline in composition (Carlier et al., 1989; Bonhommeand Carlier, 1990). Although it is difficult to assign precisepetrographic names to altered and mineralized intrusiverocks, most of the porphyry-related intrusions studied possessan intermediate composition, with dacite and/or granodioritepredominanting (Fig. 10c). Exceptions include some quartzmonzodioritic stocks at Cotabambas (Perell et al., 2002; Fig.10c), the monzogranitic composition of Panchita, the mon-zonitic nature of some stocks in the Katanga cluster (MMAJ,1983), and the dominantly quartz monzonitic to monzoniticcomposition of the Tintaya cluster, including Quechua(Fierro et al., 1997; Zweng et al., 1997; Jones et al., 2000;Fierro et al., 2002).

Biotite and amphibole are by far the most abundant ferro-magnesian phenocrysts in all the porphyries studied, althoughpyroxene is also present locally at Katanga. Proportions of bi-otite to amphibole vary greatly among the different systems,

with biotite being more abundant at Alicia, Los Chancas,Pea Alta, Panchita, Trapiche, and in the Las Bambas andKatanga clusters. Amphibole dominates at Cristo de los

Andes, Portada, Chaccaro, and in the Cotabambas and Tin-taya clusters. Other important phenocryst populations arelargely dominated by plagioclase (30 to 80 vol %) and subor-dinate quartz eyes and orthoclase (~10 vol % each) (Fig.10c). Groundmass mineralogy is dominated by quartz, plagio-clase and orthoclase in microfelsitic aggregates, which locallycontain interstitial biotite.

In most deposits, the bulk of the mineralization seems to begenetically associated with one single phase of intrusion, as atAlicia, Cristo de los Andes, Pea Alta, Portada, and the Mo-rosayhuas and Cotabambas clusters (Table 3). Two phases areapparent at Los Chancas and Lahuani, and up to six phaseshave been described at Antapaccay (Jones et al., 2000). Simi-larly, inter- to late-mineral porphyry intrusions constitute in-tegral parts of all systems in the belt. These later intrusions

vary from one central phase at Alicia (Fig. 9), through two atCcalla (Cotabambas) and Chabuca (Tintaya), to at least threeat Antapaccay. Earliest inter-mineral porphyries exhibit simi-lar textures, compositions, and alteration products to themain intrusions, making distinction between them difficult.However, they tend to possess weaker versions of the same al-teration and mineralization types. Later inter-mineral and

younger phases display different compositions and textures,and are characterized by much weaker alteration, in additionto lacking significant hydrofracturing. Inter-mineral dikes ofroughly the same composition as that of the main stocks arepresent at Cotabambas, Alicia, Chaccaro, Antapaccay,

Quechua, Katanga, Lahuani, Las Bambas, Morosayhuas, andTintaya, whereas younger, compositionally and texturally dis-tinct phases occur at Tintaya, Antapaccay, Los Chancas,Katanga, Lahuani, and Pea Alta. These younger phases,

which may or may not include postmineral intrusions, arecommonly dominated by andesitic, dacitic, and microdioriticdikes. Postmineral mafic dikes are common at Tintaya (Fierroet al., 1997; Zweng et al., 1997).

The size, shape and location of inter- to late-mineral intru-sions, with respect to the main stock, exert a marked influ-ence on the geometry of both the main stocks and the min-eralized zones in porphyry deposits of the belt. In most cases,inter- to late-mineral phases are dike-like in form, as at

Lahuani and Pea Alta, and in the Tintaya, Katanga, andCotabambas clusters, but are cylindrical in shape at Aliciaand irregular at Morosayhuas and Chaccaro. In some de-posits, as at Ccalla at Cotabambas, the earliest inter-mineralphases are centrally located with respect to the main-phasestock, whereas later-mineral intrusions occupy peripheralpositions in association with late-stage dome and dikeswarms of dacitic composition (Fig. 9c). At Alicia, however,the cylinder-like, inter-mineral intrusion occupies a centralposition (Fig. 9a) and at Tintaya (e.g., the Chabuca deposit)the dikes cut the early-stage porphyry stock and related min-eralization almost at a 90-degree angle and extend far be-

yond mineralized zones (Zweng et al., 1997). Modification of

PORPHYRY-STYLE ALTERATION AND MINERALIZATION, ANDAHUAYLAS-YAURI BELT, PERU 1587

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PELITE

SANDSTONE

QUARTZITE

LIMESTONE

CONGLOMERATE

GYPSUM

ANDESITE

LOSCH

ANCAS

ANTAPACCA

Y

TINTAYA

QUECHUA

ALICIA

CHACCARO

KATANGA

MOROSAYHUAS

LAHUANI

PE

AAL

TA

C

RIST

ODE

LOSANDES

LASBAMBAS

WINICOCHA

COTABAMBAS

PUNACANCHAFm

ANTA/KAYRA/

SONCCOFms

SANJERONIMO

Gp

BAJOCIAN

-T

ITHONIAN

NEOCOMIAN

-TURONIAN

MAASTRICHTIAN

PALEOCENE

EOCENE

-EARLY

OLIGOCENE

ANDAHUAYLAS - YAURIBATHOLITH

YURAG

p

CHILCA/QUILQUE

Fms

PUQUIN/ANTA-ANTA

Fms

FERROBAMBA/ARCURQUINA/

AYAVACASFms

SORAYA/HUALHUANI

Fms

LABRA/CHUQUIBAMBI-

LLAFms

CACHIOS/PISTE Fms

LAGUNILLASGp

HUAMBO FmsMURCO/MARA/

FIG. 8. Schematic diagram illustrating the tentative location of selectedporphyry systems of the belt relative to the main Mesozoic to Cenozoic strati-graphic units of the region and the Andahuaylas-Yauri batholith.

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7159

1405

1344

7222

Late-mineral Diatreme

Late Porphyry

Main Porphyry

Pre-mineral Diorite

Skarn

Zone of IntenseStockworkVeining

1 km Late Dome and Dikes

Porphyry-relatedIntrusions

Andahuaylas-Yauri Batholitha: Dioriteb: Granodiorite

Skarn

a

b

200 m

Late Porphyry

Main PorphyrySkarn

Reverse Fault

Anticline

ANTAPACCAYALICIA

COTABAMBAS SAN JOSEc

a b

d

ANTAPACCAY NORTH

ANTAPACCAYSOUTH

500 m

AZULCCACCACCARAYOC

CCALLA

HUACLLE

7120

1500

4400m

4300m

4200m

?

4100m

200 m

Cerro Saiwa

Leached Capping

Supergene Chalcocite Blanketa: Intersectedby Drillingb: Projected

a

b

Late Andesite Dike

Hydrothermal Breccia

Main Porphyry

Drill Hole

FIG. 9. Main geologic attributes of selected porphyry copper systems of the Andahuaylas-Yauri belt. a. Displays the cylin-drical form of the porphyry copper-bearing stock at Alicia and the central location of the late-mineral porphyry dike, asmapped by the writers. b. Illustrates the structurally controlled nature of the porphyry copper systems at Antapaccay and thelarge postmineral diatreme breccia (simplified after Jones et al., 2000 and Fierro et al., 2002). c. Displays the cluster atCotabambas and the structurally controlled nature of the stocks at Ccalla and Azulccacca, together with the peripheral, late-mineral dome and its dike swarm (simplified after Perell et al., 2002). d. Schematic cross section through the San Jos por-phyry system at Katanga displaying the distribution of the main geologic units and the location of the supergene enrichmentzone. Based on data provided by the Metal and Mining Agency of Japan (1983) and mapping by the writers.

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7300

7330

7300

7330

50km

1330

1400

1430

7230

7200

7130

MACHUPICHU

ABANCAY

ANDAHUAYLAS

CHALHUANCA

CUZCO

SICUANI

LIVITACA

SANTOTOMAS

YAURI

MOROSAYHUAS

CLUSTER

COTABAMBAS

CLUSTER

LLOCLLACSA

QENCO

MAKI

CHILCACCASA

CHACCARO

ALIC

IA

WINICOCHA

KATA

NGA

SANJOSE

TINTAYA

ANTAPACCAY

COROCCOHU

AYCO

QUECHUA

PORTADA

MONTEROJO

CHA-C

HA

LETICIA

HUACLLE

CCALLA

AZULCCACCA

CCARAYOC

ACEROPATA

CHALCOBAMBA

FERROBAMBA

SULFOBAMBA

LOSCHANCAS

PEAALTA

LEONOR

PANCHITA L

AHUANI

TRAPIC

HE

CRISTODE

LOSANDES

LASBAMBAS

CLUSTER

KATANGA

CLUSTER

TINTAYA

CLUST

ER

1A

LICIA

2C

RISTODELOSANDES

3C

HACCARO

4K

ATANGACLUSTER

5P

ORTADA

6P

EAALTA

7P

ANCHITA

8W

INICOCHA

9L

ASBAMBASCLUSTER

10T

RAPICHE

11C

OTABAMBASCLUSTER

RHYOLITE

DACITE

A

P

90

90

60

60

20

10

35

65

9

0

20

5

1

2

3

4

5

6

7

89 1

0 11

Q

Skarn

Fe-(

Cu

,Au

)

(1)

(1)

(1)

(1)

(1)

(1)(1

)

(1)

20

24

28

.530

34

37 4

050

Ma

PUNACANCHAFORMATION

EOCENEOLIGOCENEMIOCENE

EARLYMIDDLELATEEARLYLATEEARLY

ANTAFmSANJERONIMOGp

TINTAYA(3)

WINICOCHAMONTEROJO

CRISTODEANDES

LOSCHANCASPANCHITA PEAALTA

LAHUANITRAPICHE

SULFOBAMBACHALCOBAMBAFERROBAMBA

ALICIACCALLACHILCACCASA

CHACCARO

PORTADA

SANJOSEKATANGA

TINTAYA(2)

QUECHUA

STRATIGRAPHY

ANDAH

UAYLAS-Y

AURI

B

ATHOLITH

PORPHYRYCOPPER

ALTERATION-M

INERALIZATIO

N

GRANODIORITE

QUARTZ

MONZODIORITE

MONZODIORITE

DIORITE

DIORITE/

GABBRO(CUMULATES)

K-A

R(Carlotto,

1998)

(1)

OTHERINTRUSION

INTER-L

ATEPHASEPORPHYR

Y

AND/ORALTERATION

MAINPHASEALTERATION

Re-Os(Mathuretal.

,2001)

(2)

K-Ar(Nobleetal.

,1984)

(3)

200km

WNW

SSE

?

?

a

b

c

FIG.10.a.Distributio

nofporphyrycopperclustersandsystemsre

lativetotheAndahuaylas-YauribatholithandFe-Cuskarnoccurrences.Notethepreferredloca-

tionoftheporphyryclustersalongtheedgesofmainbatholithicbodies.b.Agedistributionofselectedporphyryco

pperdepositsandprospectsofthebelt(Table4)rel-

ativetotheAndahuaylas-Yauribatholithandthevolcanicandsedimentarystratigraphyoftheregion.c.Domina

ntcompositionofselectedporphyrycopper-bearing

stocksofthebeltaccordingtotheirmodalmineralcontentsonaQAP

diagram(Streckeisen,1978).

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ore zones at Cotabambas, Tintaya, and Las Bambas by intru-sion of the inter- to late-mineral bodies is appreciable.

Hydrothermal alteration and mineralization

Six distinct types of alteration-mineralization are recogniz-able in porphyry systems of the Andahuaylas-Yauri belt.These make up the potassic, propylitic, sericitic (phyllic), ad-

vanced argillic, and calc-silicate types of Meyer and Hemley(1967) and subsequent investigators (Lowell and Guilbert,1970; Guilbert and Lowell, 1974), as well as an alteration typecharacterized by sericite, chlorite, and clays (illite-smectite).The latter was originally termed SCC-type by Sillitoe andGappe (1984) in the Philippines porphyry copper depositsand is now referred to as intermediate argillic alteration bySillitoe (2000). An additional, less widespread alteration-min-eralization type includes the mixed calcic-potassic assem-blages observed at several deposits and prospects.

Potassic alteration: With a few exceptions (Morosayhuas,Winicocha), potassic alteration is the principal alteration typedirectly associated with mineralization in Andahuaylas-Yauriporphyry systems (Table 3). In all cases, potassic alteration oc-curs early in the evolution of each system and consists ofquartz, biotite, and K-feldspar. Hydrothermal biotite replacesferromagnesian components, typically magmatic hornblendeand, less commonly, magmatic biotite. It also occurs in thegroundmass of porphyry stocks and in veinlets, either alone oraccompanied by other silicate phases. Early, typically barrenbiotite seams and veinlets occcur at several systems, includingPea Alta and Cotabambas, and can be compared with theearly biotite veins described by Gustafson and Quiroga (1995)at El Salvador, Chile. In most deposits and prospects, includ-ing the Cotabambas, Tintaya, Las Bambas, and Katanga clus-ters, and at Lahuani, Alicia, and Los Chancas, biotite is ac-companied by K-feldspar, which at Cotabambas and Tintaya

constitutes a volumetrically significant alteration mineral. Forexample, the most intense potassic alteration at Ccalla is dom-inated by aggregates of quartz and K-feldspar, with local de-

velopment of graphic textures and complete destruction oforiginal rock textures. K-feldspar also occurs in a variety of

veinlet types with quartz and biotite, within the veinlets or asalteration halos, and as partial replacements of original pla-gioclase sites. Calcite, apatite, anhydrite, and magnetite areadditional minerals in potassic alteration assemblages and arealso common constituents of veinlet assemblages. Conspicu-ous magnetite accompanies potassic alteration in gold-richporphyry systems of the belt, and at Cotabambas attains >5

vol percent (Perell et al., 2002).

Major quantities of quartz were introduced as either uni- ormultidirectional veinlets during potassic alteration in all de-posits and prospects, but most characteristically at Cotabam-bas, Antappaccay, San Jos, Ferrobamba, Chalcobamba, Maki,Llocllasca, and Winicocha. In at least five systems, includingMaki at Morosayhuas, San Jos at Katanga, Ccalla and Azulc-cacca at Cotabambas, and Winicocha, the quartz veinlets co-alesce to form massive bodies of nearly pure quartz. Whereoverprinting by either quartz-sericitic or sericite-clay-chloritealteration is intense, these massive bodies are the only rem-nants of the early-stage potassic alteration. In common withporphyry systems elsewhere (e.g., Gustafson and Hunt, 1975;Gustafson and Quiroga, 1995; Sillitoe, 2000), a variety of

quartz veinlets, introduced in several generations, character-ize potassic alteration in porphyry deposits and prospects ofthe Andahuaylas-Yauri belt. Typical assemblages and texturescompare closely with the A- and B-type veinlets described byGustafson and Hunt (1975) from El Salvador porphyry cop-per deposit, Chile. A-type veinlets carry significant mineral-ization in the form of chalcopyrite and/or bornite at a number

of deposits, including the Ccalla and Azulccacca centers atCotabambas, Pea Alta, Ferrobamba, and Chalcobamba atLas Bambas, and Antapaccay at Tintaya. At Alicia and Chal-cobamba, B-type veinlets are characterized by semicontinouscenterlines filled by millimeter- to centimeter-sized grains ofbornite and chalcopyrite, whereas at Pea Alta, Lahuani, LosChancas, and Quechua, they are dominated by chalcopyriteand molybdenite. A-veinlets also occur in copper-poor, gold-bearing porphyry systems, such as those from the Morosay-huas cluster (see below), where they contribute minoramounts of chalcopyrite. Gold-rich porphyry copper depositsof the belt, such as the Ccalla and Azulccacca centers atCotabambas, contain appreciable amounts of magnetite-bearing veinlets that are similar to the M-type veinlets ofClark and Arancibia (1995) and to the A- and C-type veinletsdescribed by Cox (1985) at Tanam, Puerto Rico.

Calcic-potassic alteration: Calcic-potassic alteration is rep-resented at Cotabambas, Morosayhuas, and Pea Alta (Table3). The assemblage is characterized by veinlets of quartz, acti-nolite, and hornblende, with important K-feldspar, biotite,apatite and calcite, and volumetrically minor amounts ofclinopyroxene and epidote. In general, plagioclase is variablyaltered to K-feldspar, calcite, and/or epidote, whereas mag-matic biotite and amphibole are selectively replaced by nee-dles of actinolite, commonly intergrown with apatite. Mag-matic pyroxene is altered to aggregates of actinolite-apatiteand actinolite-biotite, and magmatic hornblende is converted

to mixtures of clinopyroxene, biotite, and hornblende as inseveral systems of the Morosayhuas cluster. Alteration halosto various veinlet sets include K-feldspar, actinolite, biotite,and chlorite. Magnetite is a common constituent, and chal-copyrite, as part of this association, gives rise to ore-grade Cu-Au mineralization.

Sericite-clay-chlorite alteration: Several deposits andprospects of the belt, including Alicia, Chaccaro, Pea Alta,Las Bambas, Morosayhuas, Cotabambas, and Tintaya, possesssignificant sericite-clay-chlorite alteration as part of their orezones (Table 3). This assemblage imparts a pale-green over-print to potassic alteration and gives a soft aspect to the rock(cf. Sillitoe and Gappe, 1984). It generally modifies, but with

some degree of preservation, original rock textures. Sericite-clay-chlorite alteration varies in both intensity and mineral-ogy, although assemblages defined for systems of the beltalways include one or more associations of sericite (fine-grained muscovite), illite, smectite, chlorite, calcite, quartz,and varied proportions of epidote, halloysite, and albite. Pla-gioclase (both phenocrysts and groundmass) is replaced by apale-green, greasy sericite assemblage which also includes il-lite and, locally, smectite. Amphibole and biotite, the latter ofmagmatic and/or hydrothermal origin, are characteristicallyreplaced by chlorite. Calcite is common as a replacement ofplagioclase, and in some deposits and prospects, includingCotabambas and Chaccaro, it is a major constituent of the

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assemblage. Albite is locally important as a replacement ofplagioclase, as at Morosayhuas and at the Chabuca Este de-posit at Tintaya (Zweng et al., 1997).

Quartz veinlets include various associations with chloriteand calcite, and halos of green sericite, halloysite, and mixed-layer illite-smectite are common (Cotabambas, Chaccaro).

Where quartz veining is intense and halos coalesce, the rock

is completely replaced by fine- to very fine-grained mosaics ofquartz, sericite, and mixed-layer illite-smectite that obliterateoriginal host rock textures, as at Ccalla, San Jos, Maki, Llo-cllacsa, and Winicocha. Chalcopyrite is locally present insome veinlet assemblages and may constitute monomineralic

veinlets with chlorite and quartz, but in general, chalcopyritecontents are lower than in earlier potassic alteration-mineral-ization. Bornite, if present in earlier assemblages, rarely sur-

vives sericite-clay chlorite alteration. Pyrite is typically pre-sent in the form of veinlets and disseminations (Chaccaro,Maki), and is volumetrically important at Cotabambas. Mag-netite is variably transformed to martite, and specularhematite is a characteristic constituent of the assemblage.

Quartz-sericitic alteration: Well-defined zones of quartz-sericitic alteration accompany ore in several systems of thebelt at San Jos, Cristo de Los Andes, Quechua and, possibly,at Winicocha and Chilcaccasa. Moderate amounts are alsopresent at Cotabambas (Ccalla), Los Chancas, Chaccaro,Lahuani, Pea Alta, and Morosayhuas (Table 3). The quartz-sericitic assemblages typically comprise white, texturally de-structive aggregates of quartz, sericite (fine-grained mus-covite), and illite, accompanied by several percent pyrite. Inall systems mentioned above, quartz-sericitic alteration typi-cally overprints earlier-formed potassic or, as at Cotabambas,sericite-clay-chlorite alteration. Broad quartz-sericitic alter-ation halos around potassic cores, common in many porphyryCu-Mo deposits worldwide (e.g., Lowell and Guilbert, 1970),

are not widely developed in systems of the Andahuaylas-Yauribelt, although they are inferred at Los Chancas (Corrales,2001). At the Ccalla deposit in Cotabambas, structurally con-trolled patches of quartz-sericitic alteration abut intermediateargillic assemblages in the upper parts of the system (Perellet al., 2002) and contributed to the formation of an irregularchalcocite blanket. A similar situation is also observed atCristo de los Andes, Quechua and at the San Jos deposit inthe Katanga cluster (MMAJ, 1983).

D-type veinlets (Gustafson and Hunt, 1975) are typicallyassociated with overprinting quartz-sericitic alteration inmost systems of the belt where this style of alteration occurs.D veinlets fill planar, continuous, centimeter-wide structures

with pyrite and quartz, which develop quartz-sericitic halos.Tourmaline, a common constituent of sericitic alteration inmany parts of the world, is rarely developed in Andahuaylas-Yauri porphyry systems, occurring only at Trapiche and Mo-rosayhuas.

Advanced argillic alteration: Hypogene advanced argillicalteration is not recognized as a common assemblage in por-phyry systems of the Andahuaylas-Yauri belt. However,sericite-rich alteration that also contains pyrophyllite andkaolinite-group minerals is present at San Jos, Winicocha,and Maki (Table 3). At San Jos, advanced argillic alterationis intimately associated with transgressive structures, whereasat Maki it is controlled by permeability contrasts at the

contact between quartz diorite and volcanosedimentary coun-try rocks. At Maki and San Jos, advanced argillic alteration issuperimposed on the porphyry stocks and associated potassicand sericite-clay-chlorite alteration, whereas at Winicocha itis developed at higher elevations and constitutes the roots ofa porphyry copper lithocap.

Propylitic alteration: Propylitic alteration in Andahuaylas-

Yauri belt porphyry systems (chlorite, epidote, and calcite) isfound mainly as part of the outer halo confined to noncar-bonate wall rocks. In other systems, as at Cotabambas, Chac-caro, Lahuani, and Las Bambas, propylitic alteration occurs

within porphyry copper ore zones in late-mineral stocks anddikes. In both cases, disseminated and veinlet pyrite, inamounts of ~1 vol percent, is common.

Calc-silicate alteration: Calc-silicate alteration is repre-sented in many deposits and prospects of the belt. Indeed, as-sociated mineralization has constituted the main source ofCu-Au ore at the Tintaya (Terrones, 1958; Santa Cruz et al.1979; Noble et al., 1984; Zweng et al., 1997) and Katanga(MMAJ, 1983) mines, and it is an important contributor tomineralization at the Las Bambas skarn-porphyry cluster andthe Quechua deposit (E. Tejada, pers. commun., 2003). Inaddition, proximal calc-silicate assemblages and associatedskarn-type mineralization are present in most systems of thebelt, excluding Cotabambas, Cristo de los Andes, Pea Alta,and Morosayhuas. However, at distances of ~3 km, all of thedeposits have distal skarn-type assemblages in roof-pendantsof Ferrobamba Formation and equivalent units.

Garnet, diopside, epidote, and actinolite are the character-istic calc-silicate assemblages (Terrones, 1958; Santa Cruz etal., 1979). At Tintaya (Fierro et al., 1997; Zweng et al., 1997);calc-silicate alteration and mineralization occur in endoskarnand exoskarn facies, and as products of prograde (anhydrous)and retrograde (hydrous) events (Table 3). The bulk of the Cu

(-Au, -Mo) mineralization at Tintaya and Las Bambas was in-troduced during prograde events, typically as chalcopyriteand, less commonly, bornite, whereas at the smaller Alicia sys-tem, bornite, with or without chalcopyrite, is the dominantCu and Au contributor. Distal skarn mineralization in por-phyry-centered systems of the belt is similar to that elsewhere(Einaudi et al., 1981), in that it is richer in Pb and Zn (e.g.,Morosayhuas). Another expression of the distal environmentis the structurally and lithologically controlled, yellow-brown

jasperoid developed in limestone beyond the skarn front atTintaya, Las Bambas, Katanga, and Lahuani, which at Tintayais reported to contain up to 1 ppm Au (Zweng et al., 1997).The presence of jasperoid in several deposits and prospects is

evidence that they constitute integral parts of porphyry-cen-tered systems in the region. Moreover, at Lahuani (Table 3),distal replacement of calcareous shale by structurally con-trolled, As-anomalous jasperoidal mantos resembles the Car-lin-style gold environment described around some porphyrycenters (Sillitoe and Bonham, 1990).

Hydrothermal breccias

Hydrothermal breccias are poorly documented in An-dahuaylas-Yauri porphyry systems (Table 3). During thisstudy, they were identified at most deposits and prospects, anobservation supported by descriptions of Antapaccay (Joneset al., 2000; Fierro et al., 2002) and Tintaya (Fierro et al.,

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1997; Zweng et al., 1997). Most of the breccias are volumet-rically small, with that at Antapaccay probably constitutingthe largest single mass in any porphyry system in the belt(Fig. 9). As at Antapaccay, all the observed breccias postdatemain-stage mineralization, although contact (igneous) brec-cias associated with the emplacement of early and intermin-eral porphyry stocks are clearly intermineral in timing. This is

particularly evident where the breccias are cut by mineralizedveinlets, as at Cotabambas and San Jos. Most mapped hy-drothermal breccias are either dike-like in form or occur asnarrow zones at intrusive contacts. Dikelike breccias possessstrong structural control and conform to pebble dikes as iscommon in porphyry systems worldwide. They typically con-sist of centimeter-sized, subrounded lithic clasts supported by

volumetrically important matrices of finely comminuted (rockflour) material. Illite, chlorite, and fine-grained (dusty) pyriteare typical constituents of the matrices. Larger expressions ofa similar style of brecciation, as at Winicocha, includerounded to subrounded, exfoliated clasts, tens of centimetersin size, in a sericitic matrix.

Ore zone geometryMost Andahuaylas-Yauri porphyry deposits and prospects

possess mineralization that is variably hosted by porphyrystocks and their immediate country rocks. The following ex-amples document the variety observed in the belt.

1. Mineralized skarns are present in country rocks whereporphyry stocks intrude carbonate rocks of the FerrobambaFormation and equivalent units, as at Tintaya, Alicia, andChalcobamba. Significant mineralization, however, is alsohosted by both porphyry stocks and wall rocks at Fer-robamba and San Jos, despite the fact that country rocksthere are dominated by carbonate horizons of the Fer-

robamba Formation.2. Porphyry stocks constitute the main host to ore wherecountry rocks are dominated by the terrigenous facies of theYura Group and equivalent units, as at Lahuani, Cristo de losAndes, Los Chancas and Quechua, or by volcaniclastic andred bed horizons of the Anta Formation, as in the Morosay-huas cluster.

3. Ore seems to be evenly distributed between porphyrystocks and wall rocks where intrusions of the Andahuaylas-Yauri batholith constitute the dominant country rock, as atCotabambas.

The Tintaya cluster further exemplifies the diversity of

mineralization styles and ore hosts, including (1) skarns at thevarious Chabuca deposits and Coroccohuayco, and associatedwith low-grade porphyry-style mineralization, (2) porphyrystocks, with minor skarn mineralization at Quechua, and (3)porphyry stocks and dioritic country rocks with small amountsof skarn at Antappaccay (Jones et al., 2000; Fierro et al.,2002).

Metal contents

Porphyry copper deposits and prospects of the Andahuay-las-Yauri belt range from gold-rich, molybdenum-poorexamples (Cotabambas), through deposits carrying bothgold and molybdenum (Tintaya, Los Chancas), to relatively

molybdenum-rich, gold-poor end-members (Lahuani). Gold-only porphyry systems, although poorly explored, are presentin at least two areas at Morosayhuas and Winicocha.

The Ccalla and Azulccacca centers at Cotabambas are thebest examples of the gold-rich category of porphyry copperdeposits (Perell et al., 2002) of the belt, with an average Augrade >0.3 ppm and appreciable volumes averaging >0.4

ppm. Molybdenum contents are low,

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dating was conducted on inter- to late-mineral dikes or hy-

drothermal alteration events. Amphibole was dated frominter- to late-mineral porphyry stocks and dikes at Trapiche,Katanga, and San Jos, whereas sericitic alteration was datedat Chilcaccasa and Winicocha. All K-Ar dates are consideredhere as minimum ages because (1) the K-Ar method recordscooling rather than crystallization of the dated silicates (e.g.,biotite) and (2) the K-Ar system offers no way of experimen-tally testing whether later Ar loss occurred to produce artifi-cially young ages (J. Dilles, pers. commun., 2003).

Overall, the data confirm the presence of a widespread, lateEocene to earliest late Oligocene porphyry copper event inthe belt (Noble et al., 1984), with ages ranging between ap-proximately 39.5 1.1 Ma at Pea Alta and 28.7 0.8 Ma at

Winicocha (Table 4; Fig. 10b). This age range can be ex-panded further into the middle Eocene when the Re-Os ageof 41.9 0.2 Ma for Tintaya (Mathur et al., 2001) is consid-ered. If, following Table 4, only those ages from unalteredsecondary biotite are taken into account, it can be inferredthat much of the main-stage potassic alteration in the beltformed between approximately 42 and 35 Ma, i.e., during themiddle to late Eocene. The K-Ar age of 35.2 0.9 Ma (Table4; Fig 10b) for magmatic biotite from a skarn-related intru-sion at Sulfobamba further confirms the age range of the beltfor both porphyry- and nonporphyry-related mineralization.

No sub-belts or age trends are apparent from Figures 7and 10, and porphyry-style alteration and mineralization are

inferred to have occurred almost simultaneously along a

~130-km-wide, 300-km-long belt. The Katanga cluster andnearby Winicocha system, however, consistently return theyoungest ages in the belt and seem to have been active at theend of the metallogenic episode responsible for the An-dahuaylas-Yauri belt. Although both biotite and amphiboleages from this cluster are modified by a chloritic overprintthat presumably caused Ar loss, the plutons of the area re-turn the youngest ages of the entire Andahuaylas-Yauribatholith. Thus, on the basis of available K-Ar data, it is sug-gested that a distinct, localized plutonic and porphyry copperevent took place in the Katanga area during the earlyOligocene, with minimum ages between approximately 35and 30 Ma.

Supergene EffectsThe depth of partial to complete oxidation of sulfides in

porphyry deposits and prospects of the belt is commonly 30to 50 m but, locally, extends to 150 m. As expected from therugged topography of much of the region, oxidation tends tobe thicker under ridge crests and nearly absent beneath val-ley floors. Most of the porphyry systems lack economicallysignificant zones of supergene enrichment, because of therelatively low pyrite contents, the poorly developed nature ofquartz-sericitic alteration, and the high neutralization capaci-ties of both potassic alteration zones and carbonate countryrocks. Consequently, most cappings are immature, typically

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TABLE 4. K-Ar Ages of Alteration Minerals from Selected Porphyry Systems of the Andahuaylas-Yauri Belt

Deposit or prospect(cluster) Mineral K (%) Radiogenic Ar (nl/g) Ar (at. %) Age 2

Chilcaccasa(Morosayhuas) Sericite2 7.537 10.410 6 35.2 0.9

Ccalla(Cotabambas) Secondary biotite 7.393 10.375 10 35.7 0.9

Monte Rojo(Katanga) Secondary biotite1 6.373 7.916 17 31.7 0.8

San Jos(Katanga) Amphibole2 0.753 0.982 68 33.2 1.9

Katanga Pit(Katanga) Amphibole2 1.068 1.231 31 29.4 1.0

Ferrobamba(Las Bambas) Secondary biotite1 6.465 9.271 18 36.5 1.0

Chalcobamba(Las Bambas) Secondary biotite1 6.434 9.002 13 35.6 0.9

Sulfobamba(Las Bambas) Magmatic biotite1 5.176 7.146 7 35.2 0.9

Chaccaro Amphibole2 0.743 0.985 38 33.8 1.2Los Chancas Secondary biotite2 7.664 9.608 23 32.0 0.8Alicia Secondary biotite 7.162 10.392 11 36.9 0.9Portada Secondary biotite1 6.742 9.515 21 35.9 0.9

Winicocha Whole rock (sericite2

) 5.055 5.692 29 28.7 0.8Lahuani Secondary biotite 7.442 10.482 19 35.9 0.9Trapiche Secondary biotite 7.374 8.747 27 30.3 0.8Pea Alta Secondary biotite 7.557 11.743 21 39.5 1.1Panchita Secondary biotite 7.328 10.367 30 36.0 1.0Cristo de los Andes Biotite 6.737 9.810 25 37.1 1.0

Constants: = 4.962 10-10y-1; = 0.581 10-10y-1; 40Ar/36Ar = 295.5; 40K = 0.01167at. %See Figures 7 and 10c for sample location and plots1Some degree of alteration to chlorite present2Mid- to late-mineral porphyry phase or hydrothermal alteration event

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goethitic in composition, with some containing appreciablecopper in the form of malachite, chrysocolla, neotocite, pitchlimonite, and associated copper oxide minerals. In general,these cappings have developed by in situ oxidation of low-pyrite, typically chalcopyrite-bornite mineralization with totalsulfide contents of

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WC: WESTERN CORDILLERA

A : ALTIPLANOEC : EASTERN CORDILLERA

ANDAHUAYLAS - YAURI BATHOLITH

a,b,c : EARLY, INTERMEDIATE,ANDLATE PHASES

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MAIN STAGE OF PORPHYRY COPPER EMPLACEMENTEND-STAGE ANTA ARC

( 50 - 45 Ma)

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INITIAL STAGE OFPORPHYRY COPPER EMPLACEMENT ANTA ARC

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FIG. 11. Schematic sequence of cross sections illustrating the formation of the Central Andes at the latitude of the An-dahuaylas-Yauri belt and contiguous southeastern Peru, between the Eocene and the earliest Oligocene. Geodynamic set-ting largely based on models after Clark et al. (1990), Sandeman et al. (1995), and James and Sacks (1999). Present-day po-sitions of the Western Cordillera, Altiplano, and Eastern Cordillera shown for reference. a. Illustrates the middle Eocene(5045 Ma) magmatic arc along the eastern edge of the Western Cordillera. b. Displays emplacement of the early phases ofthe batholith, followed by the intrusion of the intermediate-composition plutons (between 41 and 38 Ma) that make up muchof the present-day body of the batholith. Synchronous magma ascent and deformation of the uppermost crust (Incaicorogeny) in conjunction with slab flattening is implied. Volcanism of the Anta Formation (Anta Arc) is important at the West-ern Cordillera-Altiplano transition. Deformation along the Zongo-San Gabn zone, interpreted to have occurred at the cra-ton-orogen interface (Farrar et al., 1988; Clark, 1993; Sandeman et al., 1995), is shown for reference. c. Illustrates the mainstage of porphyry copper formation along the belt together with the terminal stages of Anta Formation volcanism (3832 Ma)(see Fig. 12 for details).

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~40 Ma is interpreted to have increased deformation andshortening of the upper crust (James and Sacks, 1999),thereby impeding magma ascent and favoring evolution oflarge magma chambers (e.g., Sillitoe, 1998; Kay et al., 1999).Thermal and mechanical gradients, associated with large-vol-ume magma emplacement, would have eventually given riseto the tectonic and gravitational conditions in the upper crust

(e.g., Kimbrough et al., 2001) that favored rapid tectonic (sur-face) uplift and denudation, and subsequent intrusion of themore differentiated, intermediate-stage phases of thebatholith, between ~40 and 35 Ma (Figs. 11b and c). In theproposed model, synchronous surface uplift and unroofing as-sisted with the ascent of thes