EconomicGeology Vol. 85, 1990, pp. 1604-1628

GeomorphologicEnvironment and Age of SupergeneEnrichment of the Cuajone,Quellaveco, and Toquepala Porphyry Copper Deposits, SoutheasternPeru

ALAN H. CLARK, Departmentof GeologicalSciences, Queen's University, Kingston, Ontario, Canada K7L 3N6

RICHARD M. TOSDAL, U.S. GeologicalSurvey, 345 MiddlefieldRoad, Mail Stop901, MenloPark, California 94025

EDWARD FARRAR, Departmentof GeologicalSciences, Queen's University, Kingston, Ontario, Canada K7L 3N6

AND ARMANDO PLAZOLLES V.

SouthernPeru CopperCorporation, Casilla 303, Tacna,Peru*

Abstract Copperore grades of the Cuajone,Quellaveco, and Toquepala porphyry Cu(-Mo) deposits, situatedat 3,000 to 4,000 m a.s.1.on the Pacificslope of the CordilleraOccidental of south- ernmostPeru (lats17ø02'-17ø15 ' S), havebeen markedly increased by supergenesulfide enrichment.Following the emplacementof the hypogenemineralization in the earlyEocene (52-57 Ma) asthe terminalstage in the developmentof the ToquepalaGroup continental volcano-plutonicterrane, this Andean transect was gradually reduced by erosionin a semiarid climateto a low altitudetopography; unroofing of the depositshad taken place by the mid- Oligocene.The initiation ofcordilleran uplift at ca.25 to 26 Ma, accompaniedby theepisodic eruptionof felsicash-flow tuffs at the oceanwardfront of thevolcanic arc, led to morerapid erosionand the conversionof the mid-Tertiarylandscape into the subplanarAltos de Camilaca surface.This regionally extensive pediplain constitutes the majorlandform component of the presentprecordillera surrounding the porphyrycenters; its finalconfiguration was attained at ca. 18 to 19 Ma. Ash-flowtuff eruptionwas widespread and frequent at thistime. Supergeneenrichment began in the late Oligoceneduring the progressivelowering of topographyand continued through the moreabrupt water-table depression resulting from the latestOligocene to early Mioceneuplift. However,the distributionof chalcocite,sensu lato, in the three porphyrydeposits and the local postmineralizationlandforms and volcanic historiesdiffered significantly during the latter interval.At Toquepalaand, to a lesserextent, Quellaveco,the landformregimes were dominatedby openvalleys and ignimbrite blanketing was short-lived,whereas the exposedCuajone deposit was both the site of aggressiveearly Miocenevalley incision and the depositionof an unusuallythick, in part welded,ash-flow tuff at 22.8 ñ 0.7 Ma. As a result, the enrichmentblanket at Cuajoneremained relatively thin and waseven partially eroded in the early Miocene,whereas the blanketsat Quellavecoand Toquepalawere thickened. Continuedstrong uplift in the mid-Miocene(ca. 8-15 Ma) generatedthe arrayof apron andterrace pediments of the MultiplePediment stage which dominates the lowercordilleran slopesand, in the mineralizedareas, led to the deepeningof existingvalleys and the devel- opmentof newfluvial channels. Ignimbrite eruption persisted throughout the Miocene. Again, localregimes of erosionand volcanism proved inimical to supergeneactivity at Cuajone,but enrichmentcontinued at Quellavecoand, particularly,Toquepala, where there is no record of laterMiocene ash-flow accumulation. Uplift in thisperiod was apparently also most extensive in thevicinity of theToquepala deposit, which experienced the formation of a deepchalcocite blanket,while the extantenrichment zone at Quellavecowas thickened. The developmentof enrichedassemblages containing "massive" chalcocite had clearly terminatedby 13.1 ___0.4 Ma at Cuajoneand by 9.5 ___0.5 Ma at Quellaveco;the timingof laterenrichment at Toquepalais less constrained, but a mid-Mioceneage is probable. Supergene

* Presentaddress: MMaga Grenet 316, Umacollo,Arequipa, Peru.

1604 CUAJONE,QUELLA VECO, AND TOQUEPALADEPOSITS, SE PERU 1605

upgradingof the three Peruviandeposits was contemporaneous with that of copperminer- alizationin northernmostand northernChile, as at Chuquicamataand La Escondida,and in the Copiap6mining district, in all of whichareas a late Oligoceneto mid-Mioceneage has been inferred for supergenealteration. This major and regionallydeveloped metallogenic episodetook place during and between two majorperiods of cordilleranuplift, undersemiarid climaticconditions. The terminationof intenseenrichment along this 2,000-km stretch of the Cordillera Occidental in the late Miocene was a direct result of marked climatic desiccation, whichnot onlyreduced the overallsupergene activity but focusedfluvial erosion, leading to the incisionof steep-walledcanyons less favorable for supergenealteration than the earlier subplanarlandforms.

Introduction work of Segerstrom(1963), defined empiricalcor- SUPERGENE oxidation and attendant sulfide enrich- relationsbetween episodes of supergeneenrichment ment in the later Tertiary have contributedto the of copperand silver and a sequenceof regionallyex- developmentof high coppergrades in manyof the tensivesubplanar erosional landforms, the agesof Cretaceousand Paleogene porphyry copper deposits which were delimited by K-Ar geochronologyof in the central Andes of Peru and Chile, as in other overlyingignimbrite flows.Following Hollingworth comparable mineralized regions. However, the (1964), the erosionsurfaces in thisdesertic area, close weatheringof thesemajor deposits has received little to the southern limit of the Atacama Desert, were concertedstudy from the perspectiveof regionaland interpretedas pediments or, where regionallyexten- local geomorphology,despite the widely accepted sive,pediplains, which were inferredto haveformed importanceof landformsin the developmentof su- rapidlyin responseto episodesof abruptepeirogenic pergeneprofiles. This paper addressesthe physio- uplift of the oceanwardflank of the presentCordillera graphicenvironment and age of supergenealteration Principal.Two distinctperiods of metal enrichment in the Eocene porphyry Cu(-Mo) subprovinceof were recognized,each ascribed to a specificepisode southernmostPeru (Fig. 1), with particularreference of landformdevelopment. It wasassumed that super- to the Cuajone(17ø02 ' S-70ø42' W) andToquepala genesulfide formation was most active during the pe- (17ø15' S-70ø3T W) mines of the SouthernPeru CopperCorporation (SPCC) and the nearbyQuella- vecoprospect (17ø06 ' S-70ø37' w) of Mineroper6. Each depositis situatedat elevationsbetween 3,000 and 4,000 m a.s.1.on the Pacificslopes of the Cor- dilleraOccidental. The mineralizedarea now expe- riencesan arid climate(Meigs, 1953; InstitutoNa- cionalde Planificaci6n,1965) andis borderedto the BOLIVIA southwestby the hyperaridPeruvian Coastal Desert, the northern extension of the Chilean Atacama Desert (di Castri and Hajek, 1976). However, it has been widelyinferred on geomorphologic and other grounds (e.g.,Mortimer, 1969, 1973) that semiaridconditions CHUQUICAMATA©• / prevailedin this regionuntil the onsetof drasticcli- matedesiccation in thelate Miocene (e.g., Alpers and ßLA ES(•ONDIDA

Brimhall, 1988). ß25 ø / The time scaleof Berggrenet al. (1985) is used .. here, although the term mid-Miocene rather than ß EL SALVADOR middleMiocene is employedinformally for intervals COPlAP6 DISTRICT•;. significantlytransgressing the boundariesof the latter / time-stratigraphicunit, and mid-Oligoceneis usedto ./! , ARGENTINA referbroadly to the middlepart of thatepoch. Previousresearch on supergeneenrichment in the Andean deposits :1 / o 200 500 Detailed investigations(Clark et al., 1967a, b; Sil- :,.•i• k ' * -- km litoe et al., 1968; Mortimer, 1969, 1973; Sillitoe, i • 7øøw 85ø 1969) of Mesozoicto Paleogenevein systemsin the FIG. 1. ' Map showingthe Cuajone,Quellaveco, and Toquepala broaderCopiap6 mining district of north-centralChile porphyry copper depositsin southen•Peru, and other mineralized (ca.lat 26ø-29ø S; Fig. 1),building on the pioneering areas cited in the text. 1606 CLARK,TOSDAL, FARRAR, AND PLAZOLLES

riods of relative tectonic quiescenceand reduced and Toquepala deposits subsequentto hypogene erosionalrate interveningbetween the incisionsof mineralization and to correlate those histories with the associatederosion surfaces. The earlierepisode the distributionof supergeneore mineralsat eachsite. of enrichmentwas found to be asold as early Eocene, K-Ar dating of felsic ash-flowtuff units permitsthe whereasthe later enrichment period was less precisely delimitationof the agesof landformswhich underlie assignedto the late Oligoceneand early (or middle) or truncate them in the mine areas. The occurrence Miocene interval. The prevalenceof an arid or semi~ andnature of supergeneminerals have received only arid climatein thisregion throughout the Paleogene reconnaissancestudy (Horlick et al., 1981; Satchwell, is implied both by the nature of the dominantland- 1983; Clark, in prep.). Both "massive"and "sooty" formsand by thewidespread occurrence of evaporites supergenesulfide assemblages occur in the three de- in the continentalsupracrustal cover. All significant posits.As Sillitoe and Clark (1969) have shown,the sulfideenrichment occurred prior to the middleand initial stages,or lower limits, of enrichmentzones are late Miocene(•12.9-9.8 Ma; agesrecalculated with characterizedby the developmentof powderyaggre- the decayconstants of Steigerand J•iger, 1977) de- gatesof coppersulfides, whereas more extensivere- velopmentof the very extensiveAtacama pediplain, placementof hypogeneminerals results in coarser which widely truncatedpreexisting supergene pro- grained, sectile massesof the supergenesulfides. files.Renewed tectonic uplift, and inferredclimatic• Sooty assemblagesare, however, also developed in desiccationduring the latestMiocene to Pliocenein- the early stagesof oxidativeconversion of chalcocite terval (ca. 9-2 Ma), wasaccompanied by the termi- anddjurleite to moresulfur rich coppersulfides (e.g., nationof pedimentationand the restrictionof major anilite; Clark and Sillitoe, 1971). At Cuajone and erosion to the channels of the main exorheic rivers Quellaveco,the singleenrichment blankets, or chal- (Mortimer,1973). Only limitedoxidation and super- cocitezones, are defined largely on the basisofmega- ficialsulfide enrichment of the veinsystems occurred scopic mineralogicalrelationships and copper ore at this time (Segerstrom,1963; Sillitoeet al., 1968). grades.At Toquepala,at leasttwo spatiallyseparated Supergeneprocesses are all but inactive under the enrichmentzones have long been recognized (Richard hyperaridclimatic conditions now prevailingin the and Courtright, 1958), but the irregulardistribution Chileannorte chico below ca. 3,000 m a.s.1.(di Castri of supergenechalcocite (sensu lato) hasnot been in- and Hajek, 1976; UNESCO, 1980). vestigatedin detail.Regardless of thesedeficiencies, In the E1Salvador porphyry copper deposit (Fig. we considerthat the enrichedzones in eachdeposit 1), the intensesupergene enrichment of the upper are sufficientlydelimited to permit their broad cor- Eocenestockwork mineralization may not havebeen relation with stagesin local landformevolution. contemporaneouswith that which affected the vein It shouldbe emphasizedthat our methodologydif- systemsof the nearbyCopiap6 district. On the basis fers radically from that employedby Gustafsonand of K-Ar datingof supergenealunite, Gustafson and Hunt (1975) and Alperset al. (1984) and Alpersand Hunt (1975) concludedthat supergeneupgrading at Brimhall (1988) in their analysesof the age and for- E1 Salvadorbegan at ca. 37 Ma, within 5 Ma of the mationalconditions of supergeneenrichment zones emplacementof the hypogeneores. Mortimer et al. at E1Salvador and La Escondida,Chile, respectively. (1977), however, have shownthat oxidationand en- This is in part becausecareful searchhas so far failed richmentof the ca. 29-Ma Chuquicamataporphyry to revealthe occurrenceof supergenealunite suitable deposit,located 450 km to the north (lat 22o19'-20' for K-Ar dating. More critically, the undoubtedly S; Fig. 1), and the concomitantformation of the con- complexgeomorphologic evolution of the mineralized tiguousEx6tica (Mina S6r) chrysocolla mineralization, areas during the Neogene would introduce unac- occurredin the late Oligoceneto early Mioceneand ceptableuncertainties to the massbalance calculations wasthus essentially coeval with the later enrichment centralto the model developedby Alpers and Brim- in the Copiap6district. Alpers et al. (1984)and Alpers hall at La Escondida,where a muchsimpler sequence andBrimhall (1988) morerecently presented middle of landformevents attended the supergeneprocesses. MioceneK-Ar datesfor supergenealunite associated The physiographicdevelopment of the precordillera with the intense sulfide enrichment at the La Escon- of southernmostPeru has played a key role in pro- didaporphyry prospect at 24o16' S (Fig. 1). It was motingand restricting the enrichmentof the porphyry concludedthat supergenesulfide enrichment at La copper depositsand is thus critical to our analysis. Escondidaoccurred in the mid-Tertiary, and prior to Our interpretationsof the interrelationsof super- the late Miocene,as in the Coplap6and Chuquicamata genemineral zones and geomorphology are basedon districts. the standardsupergene profile first definedin detail by Emmons(1917): a land surfacegenetically asso- Methodologyof the presentstudy ciatedwith alterationis underlainsuccessively by a Our aimsin the presentresearch were to establish leached(lixiviated) zone, an oxidizedzone, a zone of the geologic-geomorphologicevolution of the areas supergenesulfide enrichment, and the protore. The immediatelysurrounding the Cuajone, Quellaveco, transitionfrom oxide to sulfideassemblages isassumed CUAJONE,QUELLA VECO, AND TOQUEPALA DEPOSITS, SE PERU 1607 to correspondapproximately to the water table extant A simplifiedchronology of mid- to late Tertiary at the time of alteration.Because pyrite-poor hypo- geologicand geomorphologic events in southernmost gene mineral assemblagesare not developedin the Peru is providedin Table 1, basedlargely on Tosdal upper parts of the three depositsunder discussion, et al. (1981, 1984). In outline, four major physio- the broad configurationsof the upper and lower graphicterrains constitute the present-daytopogra- boundaries of the sulfide enrichment zones are in- phy (Fig. 2a): (1) the Cordillerade la Costa,a dis- ferred essentiallyto reflectthose of the superjacent continuousbelt of mountainsattaining elevations of landforms (cf. Ambrus, 1977; Gullbert and Park, ca. 1,000 to 1,800 m a.s.1.;(2) the LlanurasCosta- 1986, p. 800-802). neras,a slopingfaceted terrain risingnortheastward from ca. 350 m a.s.1. at the inner foot of the coastal Geologyand Geomorphology mountainsto ca. 3,000 m a.s.1.;(3) the precordillera of Southernmost Peru (informalterm), a subplanarca. 4,000-m benchwith Selectedfeatures of the present-daytopographic a steepsouthwestern interface with the Llanuras;and relationships,the post-Eocenegeologic units, and the (4) the main Cordillera Occidental,which at this lat- majorNeogene landform domains of the areaof study itude is dominatedby stratovolcanoes,in part gla- areoutlined in Figure2a-c. The regionincorporates ciated, and reachinga maximumelevation of 5,815 a transectof the PeruvianCoastal Desert, itself part m a.s.1.at VolcftnTutupaca (Fig. 2a). of the AtacamaDesert, and conforms physiographi- The porphyrycopper deposits under consideration callyto thebasin and range or mountainand piedmont were emplacedin the early Eocene (52-57 Ma, Es- desertsof, e.g., Cookeand Warren (1973). The land- trada, 1975; McBride, 1977; Zweng, 1984; Beckinsale scapeis dominatedby an assemblageof steepmoun- et al., 1985; Clark et al., 1990) asthe terminalstage tain slopes,rock-cut alluviated pediments or pedi- in the developmentof an Upper Cretaceousto Pa- plains,and alluvial plains, traversed by widelyspaced leogenesubaerial volcano-plutonic arc comprising the exorheicriver systems,the Rios Moqueguaand Lo- Toquepala Group and a segmentof the Peruvian cumbaand their majortributaries, fed by snowmelt Coastal batholith (Bellido, 1979; Beckinsaleet al., in the high cordillera. 1985).The geologyof the Toquepaladeposit has been

D Post- mid-Miocene Valley-and-TerraceTerrain ] huntacalaFormation Multiple-PedimentTerrain (PL = Pampa Lagunas Pediment) I HuaylillasFormation Altosde Camilaca Surface D MoqueguaFormation '•,•'D-Basement- !D Pre- Oligocenerocks FIG. 2. The study area on the Pacific slope of the Cordillera Occidental, southernPeru. a. The major physiographicunits and locationsof the Cuajoneand Toquepalamines and the Quellaveco prospect.b. Simplifiedgeologic map of area(after Bellido and Landa, 1965; modifiedby Tosdalet al., 1981). c. Geomorphologicmap, showing distribution of Tertiarylandform domains (simplified after Tosdalet al., 1984). Abbreviations:C -- Cuajone,Q -- Quellaveco,T = Toquepala. 1608 CLARK,TOSDAL, FARRAR, AND PLAZOLLES

TAI•LE1. CenozoicGeologic and Physiographic Evolution of the PacificSlope of the CordilleraOccidental of SouthernmostPeru (after Bellido, 1979; Tosdalet al., 1981, 1984; Clark et al., 1990).

Geologicevents Major geomorphologicevents

Pliocene Developmentof large andesitic-daciticstratovolcanoes of Barroso Valley and Terrace stage:continued Group (-•5.3 Ma) episodicuplift, with incisionof Capillune Formation sedimentationand minor volcanism steep-walledvalleys in precordilleraand upper Llanuras Costaneras(ca. 8.5 Ma to present) Miocene "SenccaFormation" ignimbrite eruption (ca. 6.5 ___0.3 Ma) Multiple Pediment stage:renewed, "Maure Formation" sedimentationin basinsof high cordillera episodicuplift, causing Chuntacala Formation felsic volcanism and associated clastic developmentof apronand terrace sedimentation(14.2 ___0.4-8.9 ___0.6 Ma) pedimentsin LlanurasCostaneras Eruption of youngestignimbrite assignedto HuaylillasFormation (9-15 Ma) and of broad,open (18.6 _ 0.3-18.3 _ 0.3 Ma) valleysin evolvingprecordillera Emplacementof major early ignimbritesof Huaylillas Formation Final configurationof Altos de (-•22.8 ___0.7 Ma) Camilacasurface (ca. 18-19 Ma) Oligocene Eruptionof ignimbriteflows (-•29 - 0.2 Ma) and depositionof Initiation of stronguplift and rapid conglomeratichorizons in uppermostMoquegua Formation erosion,tilting and modifying Accumulationof MoqueguaFormation fine clasticsand evaporites mid-Tertiary landscape (? _•45-50 Ma) Development of subdued,low- altitudelandscape through slow erosion of Paleocene-Eocene arc

Eocene Hypogene mineralization:Toquepala, ca. 57 Ma; Quellaveco,ca. 56 Ma; Cuajone, ca. 52 Ma PMeoeene Final stagesof subaerialToquepala Group volcanism(to ca. 59 Ma) Intrusion of granitoidplutons (ca. 58.7-65.5 Ma)

describedby Richard and Courtright (1958), Zweng asthe Altos de Camilacasurface (Tosdal et al., 1984; (1984), and Zweng and Clark (1984, and in prep.), Table 1). A clear backscarpis preservedonly locally the Cuajonedeposit by Lacy (1958), Manrique and for this landform in the immediate study area (e.g., Plazolles (1975), and Satchwell (1983), and the Fig. 3d), but an extensivetopographic step defines Quellavecodeposit by Estrada(1975), Kihien (1975), muchof the upper limit of the surfacein the Desierto Guerrero and Candiotti (1979), Torpoco (1979), and de Clemesl north of the Rio Moquegua (Fig. 2); in Zimmermanand Kihien (1983). the area of the ore deposits,the Altos de Camilaca Followinghypogene hydrothermal activity, a pro- surfacehas, in part, the form of a pediment dome longed mid-Tertiary period of erosionensued, with (Mabbutt, 1955; Cooke and Warren, 1973, p. 196). the gradualreduction of the arc terrain to a topog- During the middle and earliestlate Miocene (ca. raphy of subduedrelief which, in the vicinity of the 15-8 Ma), the Altos de Camilacasurface was itself deposits,may not haveexceeded 500 to 1,000 m a.s.1. uplifted, variablytilted and eroded,with the forma- (Table1). The predominantlycontinental clastic strata tion of a successionof apronand terrace pediments of the Oligoceneto earliestMiocene Moquegua For- mation (Barfa, 1961; Bellido and Landa, 1965; Bel- (nomenclatureof Cookeand Warren, 1973), assigned to the Multiple Pedimentstage (Figs. 2c, 3b and c; lido, 1979; and Maroccoand Noblet, 1990) constitute the aggradationalfacies of this erosionalsystem (Fig. Table 1). Broad open valleysdeveloped at this time 2b). Beginningin the late Oligoceneand continuing acrossthe precordillera to the northeast.The later into the early Miocene,a successionof more abrupt Miocene uplift of the southernPeruvian transect of uplift episodesresulted in the developmentof a re- the Cordillera Occidentalwas probably greater than gionallyextensive, generally subplanar, largely deg- that which occurred at this time in other transects of radationallandscape, now preservedin the precor- the continentalmargin (Tosdal et al., 1984) and pre- dillera (Fig. 3a) and at the summitsof the Cordillera sumablyrepresents the topographicresponse to un- de la Costa(Fig. 2a and c). Relationshipsin the area usuallyabrupt crustalthickening. Pedimentation in upstreamfrom the town of Moqueguaillustrate that the Cordillera Occidental, at this asat other latitudes, significantrapid degradationof the precordillerahad ceasedin the late Miocene, contemporaneouswith takenplace prior to 25 Ma, generatingconglomerate inferred climatic desiccationat lower topographic horizonsin the uppermostMoquegua Formation to levels.Since that time, steep-walledvalleys and can- the southwest. The erosional surface attained its final yonshave been incisedinto the Pacificslope of the form immediatelyprior to 18 Ma and is referred to mountainsas a result of continued episodic uplift CUAIONE,QUELLA VECO, AND TOQUEPALA DEPOSITS, SE PERU 1609

b

c d

FIG. 3. Landformsof the Cuajone-Quellaveco-Toquepaladistrict. a. Characteristiclandscape of the precordillera: dissectedarea of the lower Miocene Altos de Camilacasurface, near Larampahuane (10 km north-northeastof Toquepala).View looking east-southeast.Accordant summits (ca. 4,100- 4,200 m a.s.l.) representremnants of the pediplain,whereas the southwest-dippingslopes are defined by the surfacesof flow units of the Upper Cretaceousor PaleoceneSerie Alta Voltanits. b. Thinly alluviatedPampa Lagunas apron pediment (11-14 Ma), the dominantlocal landformof the middle to late Miocene Multiple Pediment stagein the upper LlanurasCostaneras; Pampa is some 20 km west- southwestof Toquepala.Steep slopes in backgroundrepresent the back scarpof the pedimentand the lower slopesof the precordillera.The valleys(Quebrada Cucule in foreground)dissecting the pediment formedmainly in the PlioceneValley andTerrace stage.c. View to the southwestfrom Cerro Botifiaca, at the southwestmargin of the precordillera, acrossthe LlanurasCostaneras and toward the Cordillera de la Costa.Cerro Bafil, the mesaat right, is undedainby sedimentsof the Upper MoqueguaFormation and cappedby a 25.3 q- 0.8-Ma tuff. The courseof the Rio Tumilaca(left) is flankedby upper Miocene terrace pedimentsof the later Multiple Pedimentstage. Another tuff, correlativewith that overlying the Cuajone deposit(Tosdal et al., 1981), is interbeddedwith the MoquequaFormation and underlies the apron pedimenton the distant(left) skyline.d. View to the northeastacross the upper valley of Rio Asana,northeast of Quellaveco,in the inner precordillera.Major morainesare drapedacross the dissectedAltos de Camilacasurface. Cerro Condoriquifia(5,085 m), on the skyline,is probably undedain by the Upper CretaceousToquepala Group Voltanits, and thus constitutesthe local back scarp,albeit subsequentlyglaciated, of the pediplain. duringthe still activeValley and Terracestage of initial late Oligocenephases of rapid uplift. Several landformdevelopment (Table 1), causingconsider- tuffs are intercalated in the coarse elastic sediments abledegradation of the earlierplanate or gentlyin- of the uppermostMoquegua Formation (Fig. 3c; Bel- cisedlandforms (Figs. 2c and 3). lido, 1979; Tosdalet al., 1981). Subsequently,felsic In the early Neogene, the mineralizedarea was pyroelasticeruptions accompanied the development situated,as it is today,at the oceanwardfront of the of both the Altos de Camilaca surface and the land- volcanicare in the precordillera.The eruptionof formsof the MultiplePediment stage, resulting in the rhyodaciticash-flow tuffs apparently began locally at mantlingof wide areasof the precordilleranslope of ca. 29 Ma. (R. J. Langridge,pers. commun.,1989), the CordilleraOccidental (Tosdal et al., 1981, 1984). but becamemore intense at 25 to 26 Ma duringthe The widespreadtuffs contemporary with incisionof 1610 CLARK,TOSDAL, FARRAR, AND PLAZOLLES the Altosde Camilacasurface are assignedto the relief, with numerousinselbergs standing up to 30 to HuaylillasFormation. In contrast,tuffs that are pre- 60 m above the plain, and interspersedwith gravel- dominantlyrestricted to valleyswere erupted during filled depressions,the remnants of paleodrainage the Multiple Pedimentstage and are groupedas the channels.The altitudeof thisold surfacein the vicinity ChuntacalaFormation (Tosdal et al., 1981, 1984). of the mines rangesfrom 3,500 to 4,000 m a.s.1.To This distinction,first recognized at Cuajoneby Man- the northeast, it constitutes the foundation for the rique andPlazolles (1975), is criticalto the interpre- youngstratovolcanoes, the flanksof which extendto tation of the landform evolution of the area. Because within 10 km of the deposits.To the southwest,a the accumulationof even thin ignimbriteshas been short distancefrom the mines,an abrupt decreasein shownelsewhere to disruptground-water systems and altitude marksthe erosionalbackscarp and the upper terminatesupergene activity beneath subjacent sur- extensionsof the PampaLagunas apron pediment, the faces(e.g., Sillitoe et al., 1968), definitionof the time- oldestof the middle Miocene erosionsurfaces (Figs. spacerelationships of the pyroclasticunits and the 2c and 3b). Thus, the porphyry copper depositsare associatedlandforms is clearly of potentialimportance intersectedby erosionallandforms of differentorigins in the prediction of the extent of local enrichment whichextend back in time to the Paleogene-Neogene horizons.In the studyarea, the basaltuff overlying boundary(ca. 23-24 Ma). Moreover,it is evidentthat the erodedCuajone deposit is unusually thick (_< 180 the Cuajoneand Quellavecodeposits, and probably m) andincludes a majorwelded zone containing es- alsothe Toquepalaarea, were blanketed for sometime sentiallyundevitrified shards. Similarly the upper by Mioceneignimbrite sheets. Miocenetuff overlyingthe Quellavecodeposit in the Local GeomorphologicSettings Rio AsanaValley exhibitsrelics of flattenedshards, and Enrichment Zones suggestiveof originalwelding. In general,most tuffs of the HuaylillasFormation show only weak devitri- Cuajone deposit ficationand hydrothermalalteration. Those of the The Cuajonedeposit occurs on the south(left) flank ChuntacalaFormation are extensively devitrified but of the 500-m-deepcanyon of the R•o Torata (Fig. 4) are otherwiseunaltered. We conclude,therefore, that and more extensivelyin its tributary, the Quebrada the ash-flowtuffs emplaced in the vicinityof the ore Chuntacala,the westerlycourse of which transected depositspossessed low permeabilitiesand were thus the central part of the mineralizedzone (Manrique likely to act asaquicludes. andPlazolles, 1975). Lacy (1958) firstdrew attention Duringthe Pliocene,a localtransition from ignim- to the inversionof fluvialrelief whichtook place dur- brite eruptionto predominantlyandesitic-dacitic vol- ing the recent geologichistory of the area;thus, the canismculminated in the developmentof the stra- presentinterfiuve between the steep-walledTorata tovolcanoesof the BarrosoGroup (Table 1). Radical and ChuntacalaValleys is essentiallycoaxial with a coolingof the climate in the Pleistoceneled to the paleovalleywith gently dipping margins,partially formationof alpineglaciers and to the development preservedthrough infilling by volcanicflows. of extensivemoraines, which are well preserved The subplanar(Fig. 5a) northernand southernin- northeastof Cuajoneand Quellaveco(Fig. 3d) but terfiuvesof the Torata-Chuntacalavalley system, lying not in the immediatevicinity of the deposits. at ca. 3,800 to 4,200 m a.s.1.and dipping at ca. 4 ø to Around the three depositsunder discussion,the the west-southwest,are largelydefined by the upper precordilleranlandscape (Figs. 2a and 3a) is domi- surfacesof ash-flowtuffs of the HuaylillasFormation, natedby a high,southwest-sloping plain, representing which locally comprises up to 280 m of predominantly both the degradationalportion of the Altosde Cam- rhyodaciticash-flow tuffs with minorconglomerates ilacasurface and, over wide areas, the gentlydipping and agglomerates(Manrique and Plazolles,1975). surfacesof ignimbriteflows broadly coeval with that In the immediate Cuajone mine area, the basal landform.The plainhas been variably dissected by memberof the HuaylillasFormation is a thickwelded west- to southwest-trendingvalley systems.Some tuff (Fig. 5b; the "trachyte" of Manriqueand Pla- valleysoriginally possessed broad shallow cross sec- zolles,1975, p.), 22.8 +__0.7 Ma in age (Tosdalet al., tions and representedthe uppermostextensions of 1981). The tuff overliesan irregular,southwesterly the middle and upper Miocene pediments,which inclinedtopography with localrelief up to 120 m (ca. constitute much of the Llanuras Costaneras at lower 3,580-3,700 m a.s.1.).This flow is also coeval with altitudes.They were subsequentlyextensively mod- an unweldedtuff intercalatedin the uppermostMo- ified duringthe Valleyand Terrace stage, whereas queguaFormation (Fig. 3c) 30 km to the southwest othervalleys were initiatedduring the latter stage, (Tosdalet al., 1981). Ash-flowtuffs, therefore, prob- anddisplay simple V-shaped profiles. Prior to mining, ably coveredmuch or all of the Cuajonearea at this the porphyrycopper deposits were exposedin com- time. We considerthe undulatingerosional topog- positepolystage valleys. raphybeneath the HuaylillasFormation to represent The Altos de Camilaca surface of the immediate an early stage in the developmentof the Altos de areais characterized by a significantdegree of primary Camilaca surface. CUAJONE,QUELLA VECO, AND TOQUEPALA DEPOSITS, SE PERU 1611

588,000 E

8e.ooo N

FIG. 4. Simplifiedpremining physiographic map of the Cuajonemine area. Contour interval is 33.3 m. Shadedarea, covering much of the interfiuves between the Cocotea(south of maparea), Chuntacala, and Torata valleys,indicates approximate extent of subplanarO!igocene and Miocenelandscape, a confiationof Huayli!!asFormation ash-flow tops and the erosionalAltos de Camilacasurface. Area of Figure 6, centeredover the Cuajonemine, is outlined.

In QuebradaCocotea, the activedrainage system well (pers.commun., 1986) concluded that these units immediatelyto the south of QuebradaChuntacala arepart of the HuaylillasFormation, with therhyolitic (Fig. 4), the basaltuff attainsa maximumthickness of conglomerateprobably representing its localbasal 180 m, displaysa completewelded tuff profile (Tos- memberand the agglomeratepostdating the basal tuff. dal, 1978), andis underlainby a ca. 100-m-thickcon- The agglomerateis in partunderlain by thinmudflows glomerate (Manrique and Plazolles, 1975). These that containclasts identical to partsof the tuff. featuresstrongly suggest that a significantvalley ex- The conglomerate,the lower contactof whichhas isted in that area early in the evolutionof the Altos been intersected at elevations of ca. 3,565 to 3,670 de Camfiacasurface. Consolidation of thismajor ash- m (Fig. 6), probablyaccumulated in a valleyof mod- flow tuff wouldprobably have displaced the mainlocal erate depth in the pre-HuaylfilasFormation topog- drainageto the southand north, in the latter caseto raphy, perhapscontinuous with the palcovalleyun- the presentposition of QuebradaChuntacala. derlyingthe presentQuebrada Cocotea. The grayag- Detailedmapping of the Cuajoneopen pit in recent glomeratefilled in a relativelydeep and probably years (Satchwell,1983; F. B. Stevenson,writ. com- moresteep-walled channel, the floorof whichlay at mun., 1983) hasoutlined several postore stratigraphic elevations of ca. 3,475 and 3,430 m in the east and units (Figs. 5c and 6) not clearly identifiedon the westquadrants, respectively, of the presentopen pit preminesurface. These include a rhyolite clastcon- (Fig. 6). Thus, significantvalley development,ap- glomerate,exposed at the easternextremity of the proximatelyparallel to the more recent Quebrada mineralizedzone, and a younger"gray agglomerate" Chuntacala,is inferred to have occurred in the im- (a local informal term), exposedin the western and mediatevicinity of the Cuajoneorebody prior to and eastern sectorsof the pit. Manrique and Plazolles alsoat somelater stageduring the localdeposition of (1975, fig. 2) mappedthe westernarea of the latter the HuaylillasFormation. Because the youngestlocal unit as part of the ChuntacalaFormation. Satchwell ash-flow member of the formation is 18.6 _ 0.3 Ma (1983) originallyinterpreted both unitsto be older (recalculatedfrom date reported by Tosdal et al., thanthe basalignimbrite of the HuaylillasFormation 1981), the later of theseerosional episodes probably andassigned them a lateOligocene age. Subsequently, took placeat ca. 19 to 21 Ma. The gray agglomerate L. J. France(pers. commun., 1984) andP. C. $atch- is overlainunconformably by conglomerate,thereby 1612 CLARK, TOSDAL, FARRAR, AND PLAZOLLES

a

c d

FIG. 5. Landformsand Neogeneunits in Cuajonemine area.a. View to the northwestfrom vicinity ofCerro Vifia Biancatoward the Cuajonemine area (see dumps). The west-southwest-slopingsubplanar ridgesare largely definedby the tuffs of the lower MioceneHuayli!!as Formation, but are locally degradationaland represent the Altosde Camilacasurface. The thickestdevelopment of earlyHuay!fllas Formationtuffs is exposedon the far (north)slopes of QuebradaCocotea, the majorvalley in the middle distance.b. South wall of the Cuajone open pit, June 1980 (note selectedbench elevations).Basal, 22.8 + 0.7-Ma, ashflow of the Huayli!!asFormation overlies an irregul.arunconformity (dashed line) incisedinto the Que!!avecoquartz porphyry of the ToquepalaGroup. The leached cappingextends from ca. 3,800 to ca. 3,495 m, overlyingthe tapered southernlimit of the enrichmentblanket. Lower part of the pit is in primary or "transitional,"i.e., weakly enriched,ore (seetext). c. Easternquadrant of the Cuajone open pit, lookingeast-northeast, June 1983. Premine courseof QuebradaChuntacala is clearly shown.Ridge on the right is underlainby Huayli!!asFormation ash flow (22.8 _+0.7 Ma) and thaton the left by ashflows and agglomerates of the middle and !ate Miocene Chuntacala Formation (--•13.1 + 0.4 Ma). Trough-shapedbody of gray agglomerate,herein assignedto the mid-Huayli!!as Formation,fills in the steep-walledalluvial channel (dashed), incised into enrichedores. The exposed baseof the agglomerateis at ca. 3,475 m.d. View to the west-southwestdown QuebradaChuntacala to upper levels of the Cuajone mine. Skyline is dominatedby slopingignimbrites of the Huayli!!as Formation.The ridge separatingthe Chuntacalaand Torata (right) valleysis underlainby the Chuntacala Formationand corresponds to the courseof an openmid-Miocene valley. Axesof earlier lower Miocene valleyslay along,or to the southof, the area of the open pit. suggestingthat east-westvalleys were incisedhere (Tosdalet al., 1981), this valley was filled in by a more than once in the later HuaylillasFormation in- thick (max 485 m) successionof ash-flowtuffs, ag- terval. glomerates,and laharsof the ChuntacalaFormation. With the transitionto the MultiplePediment stage The tuffs now underlie the interfiuve between the after about18 Ma, uplift andrenewed erosion led to Torata and ChuntacalaValleys (Figs. 5d and 6). This the developmentof a broadopen valley, the axisof palcovalleywas probablycoeval with the regionally which lay 600 to 800 m to the north of the earlier extensive middle Miocene (11.5-14.1 Ma) Pampa channelor channels.Beginning at 13.1 + 0.4 Ma Lagunasapron pediment (Tosdal et al., 1984), which CUAJONE,QUELLA VECO, AND TOQUEPALADEPOSITS, SE PERU 1613

541,200 E t • • ! ...... I ...... • ......

FIG.6. Simplifiedmap delineating the major postmineralizati.on geologic units in theCuajone mine area, includingthe HuaylillasFormation (<22.8 _ 0.7 Ma) and ChuntacalaFormation (<13.1 _ 0.4 Ma) volcanicson the southand north marginsof the open pit, respectively.The approximatelocations of the rhyolitic conglomerateand gray agglomerateare shown(after Satchwell,1983). Line A-B cor- respondsto the sectionin Figure 7. definesmuch of the upper parts of the Llanuras approximateboundaries of the supergenezones. Costanerasto the southwestof the Cuajone mine Cuajonewas originallya 470-million-metric-tonde- (Fig. 2c). posit, with an averagegrade of 1 percent Cu, and Intraformation unconformities within the Chun- containingca. 75 million metric tonsof enrichedsul- tacalaFormation (Manrique and Plazolles,1975) in- fide ores(avg grade > 1.5% Cu). The sulfideenrich- dicatethat the minearea experienced episodic middle ment,dominated by chalcocitesensu stricto, was ap- andlate Mioceneuplift, presumablyduring the suc- parently confined to a single blanketlike horizon, cessivedevelopments, to the southwest,of the Cerro some20 m in averagethickness, which dipped gently de lasChulpas, Pampa Sitana, and Cerro Sagolloter- to the west, and in the north-south cross section, race pediments(Tosdal et al., 1984), each of which showeda gentletroughing in its centralportions (Fig. recordsa discretestage in the middleand late Mio- 7). To the south,the enrichmentzone thinnedmark- cene uplift of the Cordillera Occidental.However, edly toward the limit of the mineralizedzone (Fig. erosionat that time apparentlydid not unroof the 5b). Partiallyoxidized ores with remnantsupergene orebody.Thereafter (probably post-6.5 Ma), renewed chalcocitewere locallyintersected by the (premine) uplift duringthe Valley and Terrace stageinitiated floor of Quebrada Chuntacala,and beneath the the developmentof the extantvalley of the Quebrada northerninterfiuve, were clearlyexposed on the floor Chuntacalaand the deepercanyon of the R;o Torata of the palcovalleyat the baseof the ChuntacalaFor- (Fig. 4). This involvedthe erosionof muchof the local mation; oxidized remnantsof chalcocitepersist to Huaylillasand ChuntacalaFormations volcanic cover within a few metersof this erosionsurface. Moreover, and, eventually,renewed incision into the orebody it is evidentthat in the easternarea of the pit ores alongthe axisof the ChuntacalaValley. containingboth coherent (massive)and powdery A crosssection (Fig. 7) illustratesthe premineto- (sooty)chalcocite occurred within a few metersof pographyin the immediateCuajone mine area, the the unconformity(palcovalley sides and bottom) un- broaddistribution of postoregeologic units, and the derlyingthe grayagglomerate. It isnot knownif chal- 1614 CLARK, TOSDAL,FARRAR, AND PLAZOLLES

A B SW NE • CHUNTAC 37oo-..

[] Leachedzone _ • Oxidezone Ou,ro, - • Su•erge•sulfidezone x / - 0 200 3•oo ' ; ' m

FIG. 7. Schematicsouthwest-northeast cross section through the Cuajonedeposit showing premine topography,the approximatelimit of ore-gradeCu mineralization,the major supergenezones, and the dispositionof Neogenevolcanic-sedimentary units. The smalloutlier of volcanicrocks close to the pit axisis mappedby Manrique and Plazo!!es(1975) as the ChuntacalaFormation, but we concurwith Satchwe]!(1983) in assigningit to the HuaylillasFormation (gray agglomerate).Note that there are other minordiscrepancies between the two descriptionsof the postoregeology. Simplified and modified from Manrique and P!azo!les(1975). cocite occurred immediately beneath the rhyolitic the lower part of the leachedzone formedthrough conglomerate.On the southernbenches of the open oxidationof sulfide-enriched(chalcocitic) ores, once pit, the taperedextremity of the enrichmentzone lay continuous with the extant enrichment zone. It is, approximately150 m belowan irregularunconform- therefore,inferred that sulfideenrichment may have ity, representingan early stagein the evolutionof the originallyoccurred as high asthe 3,550-m level, i.e., Altos de Camilacasurface (at ca. 3,500-3,640 m within 20 to 100 m of the latestOligocene or earliest a.s.l.), and here it is overlainby the major ash-flow Mioceneerosional topography, and that subsequent tuff of the lower HuaylillasFormation (Fig. 5b). By lowering of the water table causeda significant the early 1980s, muchof the mainenrichment zone downwardpenetration of oxidationand leaching.The had been mined out, and the lower part of the open transitionalore (seeabove) shows features character- pit lay in "transitional"ore (Satchwell,1983), char- isticof interfacesbetween supergene sulfide and pri- acterizedby largelysuperficial coatings of sootychal- mary ore zones(see Alpere and Brimhall, 1989) but cooitcon hypogenesulfides. The supergeneassem- probably representsa moderate redistributionof blagecomprises (Clark, in prep.) varyingproportions copperin responseto the fall in the water table. ofdjurleite,roxbyite ((Cu, Fe)l.SaS, Clark, 1972; (Cu, The observedrelationships at Cuajoneare in per- Fe)].74-1.82S, Mumme et el., 1988), and anilite. The missiveagreement with a modelin whichsulfide en- grade of the underlying primary ore averages0.8 richmenttook place beneath a subplanartopography percentCu (Satchwell,1983). ancestral to the Altos de Camilaca surface. If the The sulfide-enrichedzone passes upward into the leachedcapping developed through degradation of oxidezone (Fig. 7), whichhas an averagethickness previouslyenriched sulfide ores, then the upperzones of 15 m andan averagegrade of 1.3 percentCu. The of the depositmay bear a recordof a relativelypro- oxidezone is considerablythicker (to 40 m) in the longedperiod of supergenealteration beneath a sub- northern part of the deposit,i.e., in the area which duedlandscape undergoing gradual dissection; hence, lies beneaththe gentlyinclined southern side of the supergeneactivity may havebegun significantly be- valley which formsthe depositionalsurface for the fore the endof the Oligocene.In anycase, enrichment Chuntacala Formation. The oxide ores are dominated probablyceased at 22.8 _ 0.7 Ma wheneruption of by malachiteand chrysocolla, but remnantsof super- the basal(or near-basal)tuff of the HuaylillasFor- genesulfides are widespread.The oxidezone itself is mationmantied the deposit.The thick leachedzone overlainby a hematite-bearingleached zone (0.1- underlyingthat unit, and exposedon the southwall 0.2% Cu), with a maximum(preserved) thickness of of the openpit (Fig. 5b), probablydeveloped largely 120 m. The mineralogicand textural relationships prior to that volcanicepisode. (Stevenson,1972; Horlick et el., 1981; Satchwell, Subsequently,but before 18.6 _ 0.3 Ma (see 1983) suggestthat the entire oxide zone and at least above),further oxidationand leachingof the super- CUAJONE,QUELLA VECO, AND TOQUEPALADEPOSITS, SE PERU 1615

overliesthe orebody,and is now in contactwith ores % % originally moderatelyrich in chalcocite.Satchwell % \ (1983) ascribesthe "troughing"of the leachedzone andthe local development of extensivesupergene ka- olinitc to weatheringbeneath this valley. However, the steepconfiguration of the valley,and the proba- bility that it wasrapidly filled in by the agglomerate, suggeststhat supergeneprocesses would not have ASANA i beenvery effective at thisstage and, moreover, would haveagain been terminatedon the onsetof eruption outerlimit of the Chuntacala Formation at 13.1 _ 0.4 Ma. It remainspossible that the troughingof the enrichment blanket (Fig. 7), and/or the developmentof the tran- -78,000N sitionalenriched ores, may reflect valley incisionin mid-HuaylillasFormation times, but it is moreprob- able that they were controlledby the undulatingna- ture of the earliest Miocene landforms ancestral to D - Holocenealluvium the Altos de Camilaca surface. Supergeneoxidation was probablyrenewed fol- Neogenevolcanlcs and sediments lowingthe erosionof the ChutacalaFormation strata duringthe Valley andTerrace stage, but it is consid- D pre- Neogene eredunlikely that significantenrichment has occurred since the middle Miocene. FIG. 8. Geologicsketch map of the Quellavecoprospect area (simplifiedand modified after Estrada, 1975) showingdistribution Quellavecodeposit ofpostmineralizationNeogene volcanic and sedimentary units of the Huayli!las,Chuntacala, and "Seneca"Formations. The outline Copperore reserves at the Quellavecoprospect of the phyllic-argillicalteration approximately corresponds to the (Fig. 1) are variouslyreported as 200 millionmetric limit of ore-grade mineralization.The northwest-southeastline (C-D) indicateslocation of the crosssection in Figure 12 andthe tonsat 0.95 percentCu (Hollister,1974) or 405 mil- southeastpart of the crosssection in Figure 11. lion metrictons at 0.8 percentCu (Estrada,1975). The Quellavecoorebody is exposedon bothflanks of the deep westerlytrending valley of the Rio Asana gene chalcociteores may have resultedfrom their andis elongatedin a northwesterlydirection (Fig. 8). exposureduring the erosionof the deepvalley system The broadphysiographic relationships' (Fig. 9) are occupiedby the gray agglomerate,which directly- similarto thoseat Cuajone.The AsanaValley is in-

,•.oo..s4,..o.ooE4•,o,, • ,...o, FIG.9. Simplifiedphysiographic map of the Quellavecoprospect area (of. Fig. 4). Notethe courses of the RiosAsana and CharaqueValleys and the interveningunnamed quebrada. The shadedarea indicatesthe approximateextent of the subplanarOligo-Miocene landscape, but the smoothlower (southwesterly)part of the interfiuvebetween the Asanaand unnamed valleys is definedby gently dippingflows of the "SenecaFormation" (see Fig. 10b).The areaof Figure8 is outlined. 1616 CLARK,TOSDAL, FARRAR, AND PLAZOLLES

cisedinto a modifiedAltos de Camilacasurface, rep- resentedhere (Fig. 10a)by a gentlyrolling high plain ca. 3,800 to 4,100 m a.s.1.;the valley floor at the Quellavecocamp is 3,500 to 3,600 m. Discontinuous remnantsof Huaylillas and ChuntacalaFormation ignimbritesoccur on higherinterfiuves. One suchash- flow tuff, exposedca. 7 km to the southwestof the deposit,has yielded a K-Ar age of 18.4 ñ 0.6 Ma (Tosdalet al., 1981), similar to that of the younger of the dated HuaylillasFormation flows at Cuajone. On the southeastedge of the AsanaValley, a small erosionalremnant of a 13.1 ñ 0.7-Ma ignimbriteas- signedto the ChuntacalaFormation (Tosdal et al., 1981) is preservedin a swaleon the Altosde Camilaca surface.To the northwestof the deposit,a similar middleMiocene age of 12.5 ñ 0.6 Ma is reportedfor b a thick sequenceof alternatingash-flow tuffs and la- harswhich is exposedin the valley of the Rio Cha- raque, a right bank tributary of the Rio Asana(Fig. 8). The tuff sequenceis lithologicallysimilar to the Chuntacala Formation-type section at Cuajone. However,the uppermostpreserved tuff in the Cha- raqueValley hasan ageof 6.5 ñ 0.6 Ma anddisplays violet quartz phenocrystsconsidered characteristic of the Pliocene ash flows of the Seneca Formation in the region (Tosdal, 1978; Tosdal et al., 1981). This as- signmentis, however,probably incorrect (R. J. Lan- gridge,pers. commun., 1989). The interfiuvesbetween the valleys of the Rio Charaqueand R.ioAsana have quasiplanartops and dip gently to the west-southwest(Figs. 9 and 10b). The lower slopesin bothvalleys are underlainby vol- canicrocks of the Upper Cretaceous-PaleoceneTo- quepalaGroup and intrudedby broadlycoeval plu- tons(Bellido, 1979). The smoothinterfiuve between the Asanaand Charaque Valleys is the slightlyeroded depositionalsurface of an ash-flowtuff that is tenta- tively correlatedeither with the SenecaFormation in the CharaqueValley, or with an underlyingflow of the upper part of the ChuntacalaFormation. Other tuffs assignedto the ChuntacalaFormation are ex- posedon the northernslopes of the upperAsana Val- ley at an approximateelevation of 3,750 to 3,775 m a.s.1.Thus, the Quellavecodeposit was buried at dif- ferenttimes by volcanicrocks of the middleand upper FIG. 10. Landformsand Neogene units in the Quellaveco Miocene Chuntacala and uppermost Miocene and prospectarea. a. View to east-southeastfrom the slopesof Cerro PlioceneSeneca Formations. Undoubtedly the deposit Charaquetoward the area of the Quellavecoprospect (note drill accessroads on S slopeof the AsanaValley). Unnamedquebrada was alsoat one time coveredby the HuaylillasFor- visiblein middledistance. The skylineis modifiedAltos de Cam- mation. ilacasurface capped locally by flowsof the Huaylillasand Chun- A conspicuousfeature of the rightbank of the Asana tacala Formations. b. View to the northwest across the Rio Asana Valley for a considerabledistance upstream from the Valleyand Quellavecomineralized area. West-southwest-dipping Quellaveco deposit (Fig. 10c)is a thick,white-weath- interfiuve is capped by a tuff tentatively assignedto the upper Miocene-PlioceneSeneca Formation, underlain by flows of the ering, unweldedash-flow tuff underlainby a discon- middle MioceneChuntacala Formation. c. View northeastup the tinuous fluvial conglomerate,together constituting Rio AsanaValley from the southernexposures of Quellavecode- the Quellavecobreccias and conglomerates of Estrada posit. White tuff (9.5 ___0.5 Ma) of the uppermostChuntacala (1975). The flowis 9.5 ñ 0.5 Ma (Tosdalet al., 1981) Formationis evidentalong the north bankof the valley,where it lies on a rock cut terrace incised into the Chuntacala Formation and lacksthe violet quartzeyes of the upperunit in and underlyingToquepala Group. Smoothsurfaces in distance the CharaqueValley. The tuff cannotbe tracedalong are morainesoverlying the Altosde Camilacasurface. the northern slopesof the AsanaValley to the west CUAJONE,QUELLA VECO, AND TOQUEPALADEPOSITS, SE PERU 1617 of the deposit.Upstream, a topographicbench incised erosionin the area of the presentCharaque Valley into volcanicflows and intrusionsof the Toquepala and in the approximateposition of the presentRio Group and the overlyingChuntacala Formation vol- Asana,where these valleysare narrower and with canics(above) locally takes the placeof the whitetuff, steeperslopes than the earliersystems; (6) deposition and we concludethat the latter was depositedin a of fluvialconglomerates and the 9.5 _+0.5-Ma ignim- valley incisedinto the older rocksto a level closeto brite, which wasassigned to the uppermostChunta- the Paleogene-Neogeneunconformity. Similar strati- ealaFormation (Tosdal et al., 1981); (7) eruptionof graphicand geomorphologicrelationships probably the upper Miocene and PlioeeneSeneca Formation occuron the southslope of the presentAsana Valley, ignimbrites,now preservedonly aboveca. 3,800 m but the tuff andthe underlyingbench are lessexten- a.s.l.;and (8) incisionof the presentdeep canyonof sivelypreserved. Moreover, the erosionalsurface un- the Rio Asana,the valley of the Rio Charaque,and derlyingthe conglomerateand young tuff liesca. 75 the interveningquebradas. The age of the faulting m higher on the southside of the valley, probably episodealong the axisof the Rio Asanacannot be resultingfrom post-9.5-Mafaulting along the present preciselydefined, but it probablycoincided with ei- AsanaValley (Estrada,1974). ther stages(6) or (7) above.The approximaterela- On the basisof the aboverelationships, the follow- tionshipsof the abovefeatures are illustratedin the ing sequenceof Neogeneevents in the area is pro- sketchcross section of Figure 11, alongthe north- posed:(1) finalstages in the developmentof the Altos westerlytrending axis of the Quellaveeoorebody (el. de Camilacasurface at ca. 19 to 25 Ma; (2) eruption Figs. 8, 9, and 12). Stage(3) is correlatedwith the of the youngerHuaylillas Formation ignimbrites at developmentof the PampaLagunas pediment in the about 18.4 Ma; (3) incisionof a broadopen valley, LlanurasCostaneras and stage(5) with the Cerro Sa- the axisof which probablycoincided approximately gollo pediment,respectively the earliestand latest with the present interfluve between the Asanaand episodesin the Multiple Pedimentstage (Tosdal et CharaqueValleys; (4) depositionof a thicksuccession al., 1984).The post-9.5-Madeepening of the Asana of ChuntacalaFormation ignimbrites and lahars, be- and other valleysoccurred during the subsequent ginningat ca. 13.1 _+0.7 Ma andfilling in the palco- Valley and Terrace stage. valley and locallyspilling over its southernrim onto Estrada(1974, 1975) has shownthat a singlesu- the Altosde Camilacasurface; (5) focusingof fluvial pergenesulfide enrichment blanket is developedat

NW SE

4100 Baseof ChuntacalaFro. Aolxoximate0o 's•tio• ofAltos de Cam•aca surface I •'ø•• I _....•e ...... t./'/' / 13.1*Ma Se•ccaFm.' (?6.5 Ma) ..__,. ,,? 3900 12.5 Ma'" • •, •, •, •, , o.c, ' ' ,oo

•3500 fo m a.s.t R•

LEGEND • ...... AolxoxJ•ate Limitof Quelaveco Deposit

• To•)ogr.•y

•,•_• OosiltonsExlx)•ed of(R) volcanicand projected flow (L) 0.5

---- ? -' Tentativeextrapolation of flow

-""'"- •ferredOosltion of I:)aleosurface

f.,. / Fault

! 0 I km

FIG. 11. Schematicnorthwest-southeast cross section along axis of the Quellavecoorebody, illus- trating interrelationshipsof presenttopography, Neogene ignimbrite flows (some undated), inferred paleolandforms,and the Que!lavecoenrichment zone. The sectionis approximatelyorthogonal to the extantriver valleys.The last episodeof faultingalong the axisof the Asanavalley is assumedto have occurredafter eruptionof the 9.5 ___0.5-Ma ignimbrite.Vertical scale is exaggerated. 1618 CLARK,TOSDAL, FARRAR, AND PLAZOLLES

-3coo c D (SE)

--3300

SUPERGENE ZONES [] Alluvium I [] Leached(andoxidized) zone --3300 [] Neogenevolcanice .r•]moderately-to-strongly enriched zone [] 'Quellavecocon91omerale' [] weakly-enrichedzone -- 0 2oo 4oo [] pre-Neogenerocks m •m....I.

FIG. 12. Crosssection (looking northeast) of the Quellavecosupergene zones, after Estrada(1974; sect. 13). Note the different distributionof postoresedimentary and volcanicunits on the southand north flanksof the AsanaValley. The volcanicsare undifferentiated,but in this sectionthey probably comprisemainly the 9.5 _+0.5-Ma white tuff (seeFig. 10c) of the upper ChuntacalaFormation. Note the intersectionof the enrichmentzone by the erosionsurface (valley bench) underlying the fluvial conglomerateand the displacementof the supergeneprofile by the easterlytrending fault.

Quellaveco,as at Cuajone (Fig. 12). Two zonesare humedAltos de Camilacasurface (Fig. 11), which is distinguished:an upper zone of moderateto strong at a significantlygreater relative depth than at Cua- enrichmentoverlying a lowergrade zone, presumably jone (seeabove). A verticalseparation of thisextent representinga downwardtransition to unalteredhy- pogeneores. The chalcocite-bearingzone is signifi- cantlythicker (50-60 m) and more irregularthan at Cuajone.It attainsdepths of œ50to 300 m below the presentsurface and hasa gentletroughlike form in verticalcross section, crudely reflecting the config- uration of the AsanaValley slopes(Estrada, 1975). The enrichedzone is generallyoverlain by a leached and/oroxidized zone, extending from surfaceto max- imum depthsof 80 m. An east-west-trendingfault closeto the presentaxis of the AsanaValley offsets the northern part of the enrichmentblanket down- wardabout 75 m (Fig. 12; Estrada,1975). Restoration of this offsetwould enhancethe subparallelrelation- ship of the presentvalley and the troughin the en- richmentblanket (Fig. 12). The enrichedzone at Quellavecois truncatedby the lower slopesof the presentAsana Valley. On the north flank of the valley, the unconformitybeneath the 9.5 + 0.5-Ma ignimbriteand the underlyingcon- glomerateintersects the sulfideenrichment zone as well asthe overlyingleached-oxidized zone (Figs.19. and 13); oxidizedremnants of chalcociteoccur within 50 cm of this old valley floor. Similarrelationships are apparentbetween the enrichmentblanket and the vestigiallypreserved outliers of tuff on the southern slopesof the valley(Fig. 12; seeEstrada, 1974, 1975). The relationshipsbetween supergene enrichment FIG. 13. View to the north-northeastacross the AsanaValley and the older topographicallyhigher landformsare incisedinto oxidized and enriched Que!!avecomineralization. lessclear. The highestexposures of the enrichedzone, White cliffs on the far slopeof tributary quebradaare outcrops of 9.5 ___0.5-Ma tuff, which immediatelyoverlies a ca. 8-m-thick at its southernextremity, lie at altitudesof ca. 3,750 conglomerate.Lower slopesare in the ore zone, with remnants m a.s.l. approximatelyœ50 m below the horizontally of supergenechalcocite persisting to within 50 cm of the uncon- extrapolatedposition (ca. 4,000 m a.s.l.) of the ex- formity. CUAJONE,QUELLA VECO, AND TOQUEPALA DEPOSITS, SEPERU 1619 between land surface and water table would not be Neogenetuffs in the immediatemine area. However, unusual in a mountainous semiarid environment. For an earlyMiocene K-Ar ageof 18.3 Ma hasbeen de- example,in physiographicallysimilar settings in the termined(Tosdal et al., 1981) for a tuff whichcaps a Copiap0district of Chile (Fig. 1), enrichmentassem- 40-m-thicksuccession of tuffsand conglomeratesof blagesare intenselydeveloped at depthsof ca. 200 the UpperMoquegua Formation some 20 km south- to 300 m belowthe presentsurface in the Chafiarcillo southeastof Toquepala.This tuff is lithologicallyand silverveins (Whitehead, 1919), and supergenechal- chronologicallysimilar to the 18.4 +_0.6-Ma ignim- cociteis abundantat depthsin excessof 275 m in the brite in the Quellavecoarea, some 15 km northof Dulcineade Llamposcopper mine, at a level approx- Toquepala(see above). Other tuffs of the Huaylillas imatelycoinciding with the preminewater table (Sil- Formationnear Cuajone and also some 50 km to the litoe et al., 1968; Sillitoe, 1969). Similarly, the south near the Peru-Chile border have similar K-Ar leachedzone overlying the enrichmentblanket at the agesof 18.6 ___0.6 and18.4 ___0.5 Ma, respectively La Escondidaporphyry deposit (Fig. 1) locallyattains (seeabove; Bel16n and Lef&vre,1976). The wide- depthsof 400 m belowsurface (Brimhall et al., 1985; spreadoccurrence of tuffsof similarage throughout Ojeda, 1986; Alpersand Brimhall, 1989). It is there- theregion renders it likelythat the Toquepala deposit fore possiblethat enrichmentat Quellavecocould wascovered by tuffsat thattime in the earlyMiocene. have occurred beneath the lower Miocene surface TheToquepala porphyry copper deposit originally prior to its mantlingby the 18.4-Ma flow. croppedout in a southerlytrending valley, Quebrada The openvalley inferred to havesubsequently de- Toquepala,and its shorttributary, Quebrada Huan- velopedat Quellavecowould (Figs.11 and 12) have canani,which were eroded to depths of ca. 300 m been incisedto within 75 to 100 m of the preserved into an erosion surface with a local mean altitude of enrichmentzone. If hydrologicconditions had been ca. 3,600 m a.s.1.and with a gentlesouth-southwest- suitablefor supergenealteration, it ishighly probable erly slope(Figs. 14 and 15a). As at Cuajoneand that enrichmentwould have takenplace at that time, Quellaveco,this landscape near Toquepala had sig- perhapsinvolving upgradingand deepeningof an nificantprimary relief but, fromthe widespreadoc- earlier formed blanket. The troughlikeform of the currenceof accordant,in part planatesummits (Fig. enriched zone may broadly reflect this valley. En- 15a),it maybe assignedto the Altosde Camilacasur- richment would again have been interrupted and face(Tosdal et al., 1984). Immediatelyto the south- probablyterminated by depositionof the thickChun- westof Toquepala,the surfaceslope steepens to ca. tacalaFormation succession, beginning at ca. 13.1 Ma. 7o (Fig.15b) before falling abruptly to analtitude of Our model, therefore, involves two or more su- ca. 2,600 m a.s.1.at the northeasternmargin of the perimposedenrichment phases resulting, not in the PampaLagunas pediment (Fig. 2). This landform developmentof verticallyseparate blankets but in the configurationpostdates all significantlocal movement thickeningand upgradingof a singlezone (see for on the major west-northwest-trendingIncapuquio example,Locke, 1926; Brimhall et al., 1985; Alpers fault (Wilsonand Garcia, 1962), whichtraverses the and Brimhall, 1989). The apparentlygreater devel- lowerprecordilleran slope (Bellido and Landa, 1965; opment(and/or preservation) of enrichedsulfide ores Tosdal,1978). A comparablysteep inclination to the at Quellavecothan at Cuajone may be in part the Altos de Camilacasurface is observedelsewhere only result of a lessintense regime of localvalley incision at the marginof an apparentMiocene caldera some into the Altos de Camilacasurface during the early 30 km southeastof Toquepala.A similar volcanic Mioceneover the intervalrepresented by the Huay- eventmay have occurrednorth or northeastof the lillas Formation.Although we infer that enrichment mine, althoughno circular structureis visible on at Quellavecowas more protracted than at Cuajone-- Landsatimages in thatarea, perhaps due to burialby possiblyextending from the later Oligoceneto the youngervolcanics. middle Miocene--it had similarlyterminated before Prior to mining,Quebrada Toquepala possessed a the closeof the Multiple Pedimentstage prior to 9.5 composite,goblet-shaped profile which, by analogy Ma. Oxidation, but little sulfide enrichment, accom- with othervalley profiles in thisregion, suggests that panied the more recent stagesin the deepeningof the gentlydipping upper slopesprobably developed the AsanaValley. The majorfault alongthe valleyhas during the Multiple Pedimentstage, and the more notfocused significant sulfide enrichment or oxidation steeplyincised lower slopes during the ensuingValley of the ores. and Terrace stage. Supergenesulfide enrichment in the Toquepala Toquepaladeposit depositextends over a muchgreater vertical interval Of the three porphyrycenters, the Toquepalade- than at Cuajoneor Quellaveco;secondary chalcocite positdisplays the mostextensive record of sulfideen- after chalcopyriteand bornite occurred between richment(Richard and Courtright,1958; Anderson, 3,525 and at least ca. 3,100 m a.s.1.(Richard and 1982), but paradoxically,this cannotbe precisely Courtright, 1958; Southern Peru Copper Corp., constrained in time due to the absence of datable unpub.repts.). Much of the enrichedore hasbeen 1620 CLARK,TOSDAL, FARRAR, AND PLAZOLLES

1982). The mostextensive enrichment zone, and the only one systematicallydelimited on the basisof ore grade by mine personnel(Fig. 17), has a broadly planar,but locally irregular, upper surfacewhich is subhorizontalin east-westprojection but which falls from an altitude of ca. 3,350 m at the northern limit of the depositto ca. 3,250 m at the southernlimit (a horizontal distanceof ca. 700 m). In contrast,the lower boundaryof the enrichmentzone is very in- dented, with deep roots resultingin enriched ore thicknessesin excessof 150 m. Althoughlocally such downwardextensions of chalcocitedevelopment lie beneaththe valley axis,in generalthere is no corre- lationbetween enrichment blanket configuration and/ or thicknessand the premine valley form. A local

b

FIG. 14. Preminephysiography of the Toquepalamine area. QuebradaToquepala and its tributariesare incisedinto a south- southwest-dippingplanate landform assigned to the Altos de FIG. 15. Landformsin the Toquepalamine area. a. View to Camilaca surface. the northeastfrom Cerro Toquepala,across the easternhalf of the Toquepalaopen pit (June 1982). In the distance,Cerro Con- doriquifiastands above the southwest-sloping,planate, rock-cut, mined,but its originaldistribution may be defined landform--the Altos de Camilacasurface. Upper coursesof the QuebradasToquepala (center) and Huacanani(right) incisethe approximately.Remnants of zonesaffected by en- pediplain.b. View to the east-southeastfrom BarrioAzul (5 km richmentare visible (Fig. 16a)in the openpit because southwestof the Toquepalaopen pit), along the southwestern of the associatedsupergene argillic alteration, which margin of the precoralii!era.The southwest-dippinglandform givesrise to a pale, bluish-graycoloration on broken representsa tilted andstrongly dissected remnant of the Altosde rock surfacesand, in the upperlevels, through the Camilacasurface, developed here in the Upper CretaceousTo- quepalaGroup volcanics and Paleocene granitoid intrusions. The developmentof reddish-brownhematite, an oxidation flat-toppedmesa rising above the surfacein the middleground is productof chalcocite-pyriteassociations (Anderson, a remnantof the Huayli!!asFormation (Be!!ido and Landa, 1969). CUA]OHE,QUELLA VECO, AND TOQUEPALADEPOSITS, SE PERU 1621

a

3400

SECTION

34

32O0

3

FIG. 17. Schematicwest to east crosssections through the lower main enrichmentblanket of the Toquepalamine, showing its relationsto the premineconfiguration of QuebradaToquepala. The sectionstraverse the southern(J), central (P), and northern (U) parts of the deposit.The central break in the enrichmentin the northernpart of the depositdelimits the postoreand essentially barren dacite agglomeratebreccia pipe. The enrichmentblanket shownis as delimited by premine drilling on the basisof copper grades(+0.45%: after unpub.data, Southern Peru CopperCorp.).

controlof the depth of enrichmentby postorefaults is apparentin someparts of the deposit.This does not, however,explain why the mainenrichment zone wasboth thicker (Fig. 17) and of higher meangrade in the western quadrantof the depositthan in the easternpart in manyeast-west sections. In the absence of evidence for major north-strikingfaults through the deposit,this asymmetricaldistribution of enrich- ment maybe inferred to reflect primary-gradevari- ations,or the preservationof thick impermeablerhy- olites(Serie Alta) of the premineralizationToquepala Group over the easternpart of the orebody(Richard and Courtright, 1958), which could have inhibited copperleaching and enrichment. At the northern and southernlimits of the open pit, the main enrichmentzone waslocally separated from the lower slopesof QuebradaToquepala by less FIG. 16. Distribution of enriched zones on west and northwest than 20 m (Fig. 17), and its formationtherefore cer- wallsof the Toquepalaopen pit. a. Stronglyforeshortened view lookingwest from the 3,145-mlevel (June1975). Dark (in part tainly predated at least the later stagesof valley in- shadowed)benches in foregroundare in transitionalore, with cision.The overall southerlydip (ca. 8o-9 ø) of its erraticdistribution of powdery(sooty) chalcocite on hypogene assemblages.In higherlevels, the sulfide-enrichedzones are whi- tish,due to the developmentof supergenekaolinRe, and the ox- strQnglyoxidized, extends to the marginof the pit and to within idized and leachedzones are darker.The lower (main)enriched 50 m of the dissectedAltos de Camilacasurface, here partially zone immediatelyoverlies the transitionalore and is essentially mantiedby dumps.The lower three visiblebenches are mainly continuousaround the westwall of the pit. The overlyingolder in transitionalore. Selectedbench elevations are shown.c. Oblique enrichedzones are lessregular in form.Selected bench elevations view of the westwall of the openpit, lookingsouthwest from the areshown (m.a.s.1.). b. Northwestquadrant of theopen pit (August 3,280-mbench (June 1982), Comparewith Figure17a--enriched 1975), showing(areas delimited by white lines) the main lower ores(pale) are nowexposed quasicontinuously from the shallowest enrichedzone and the irregular,discontinuous, distribution of levels (top-right) to the upper boundaryof the hypogeneore theupper enriched zones (view is essentially contiguous with that (lower 4-5 benches).The oxidizedzone on the upper benches in 17a).The highestdevelopment of chalcocite(at top-right),now correspondsto dark areas.Bottom level at 3,085 m a.s.i. 1622 CLARK,TOSDAL, FARRAR, AND PLAZOLLES upper surface,broadly paralleling that of the upper cocite zone to the Altos de Camilaca surface indicates precordilleranslope in the southernpart of the mine that enrichmentbegan prior to the final stagesin the area(Fig. 15b), implies a controlby a subplanarsouth- development of that landform, i.e., in the earliest dippingtopography. Whereas the upper and central Mioceneor before. Althoughno tuffscrop out in the parts of this enrichmentzone were dominatedby mine area,the widespreadpreservation of ca. 18-Ma massivecoherent chalcocite,its lower sectionand lat- ash-flowtuffs throughout the regionrenders it prob- eral peripheriesdisplay extensive development of able that the depositwas then buried, resultingin sootysupergene sulfides, giving rise to transitional interruptionof the supergeneprocess. Moreover, an ore, as at Cuajone. The lowermostenriched ores are earlier episodeof coveringby tuff may be recorded mineralogicallysimilar to thoseat Cuajone,compris- by the occurrenceof a 23.3 ___0.8-Ma tuff (Tosdalet ing blackto midnight-bluepowdery coatings of djur- al., 1981) in the MoqueguaFormation some 25 km leite, roxbyite,and anilite (Clark, in prep.). to the west of, andbroadly downslope from, Toque- The originallarge-scale distribution of chalcocite- pala.It cannotbe confirmedwhether any tuffs of the bearingores in the shallowerlevels of the openpit is ChuntacalaFormation were subsequentlyemplaced now difficultto reconstruct,largely because routine in the area. mineralogicobservations have not been carried out The higher altitude early enrichment,at least in duringmining, while, to the authors'knowledge, no part, clearlypredated the initial incisionof Quebrada suchstudies were made in theinitial drilling program. Toquepala.Indeed, the subplanarupper surfaceof It is evidentthat suchhigher altitude and presumably the lower (main)supergene sulfide zone, and its close olderenrichment was also focused in the western(or approachto the preminevalley bottom, implies that northwestern)quadrant of the deposit.Anderson it was similarlygenerated, largely or entirely, prior (1982,fig. 12.15) identified such a singlezone dipping to local valley development.However, the available fairly steeplyto the south.However, observationof data do not permit us reliably to assignan age to the thedistribution of relictchalcocite and supergene ka- importantuplift episodewhich we infer immediately olinite on the western and northwestern benches of precededthe mainenrichment period. Three possible the pit hasrevealed extremely irregular and varying modelsmay explainthe interrelationsof enrichment patternsas the pit wallswere displacedto the west andlandform development at Toquepala.In the first from 1975 to 1982. Whereasin 1975 to 1978 (Fig. model, all enrichment would be ascribed to weath- 16a, b), two, apparentlyseparate, supergene sulfide eringbeneath the evolvingAltos de Camilacasurface zoneswere evident,centered at ca. 3,330- and 3,440- in the late Oligoceneand early Miocene, the super- m elevations, in 1982 remnants of chalcocite were geneactivity being eventually terminated by eruption visible(Fig. 16c) on mostbenches between the 3,280- of the 18.3- to 18.4-Ma HuaylillasFormation ignim- and 3,490-m benchesand were locally apparently brite. Accordingto thismodel, the majoruplift which continuous with the lower main enrichment zone. precededthe main enrichmentwould be assignedto However, as pointed out by Anderson(1982), ores the latest stagesin the generationof the surface.In immediatelyoverlying the lowerenrichment zone are the secondmodel, the early upper chalcocitezones characterizedby extensivedevelopment of jarosite are inferredto haveformed during the finalstages in after hypogenechalcopyrite. The highestsurviving the developmentof the Altos de Camilacasurface, enrichmentobserved during the periodof study,at whereasthe deep zone may have developedin re- the 3,535-m level, lies only 40 to 75 m below the sponseto rapid uplift, tilting, andwater table descent accordantsummits assigned to the Altos de Camilaca causedby localcaldera formation, presumably in the surfaceadjacent to the northwestmargin of the pit earlyMiocene (at 18.3 Ma ?). Thismodel must remain (Figs.15a and 16b). hypotheticalin the absenceof evidencefor a caldera We concurwith Richardand Courtright (1958) and to the north of Toquepala.However, becauseash- Anderson(1982) that the unusuallythick enrichment flow eruptionwould normally be intimatelyassociated in theToquepala deposit resulted from periodic uplift with crustalflexure in a caldera setting (Smith and andthat the spatialisolation of the lower(main) zone Bailey,1968), the period immediatelyfollowing such probablyrecords an episodeof uplift of a sufficient an event would probablybe unpropitiousfor super- magnitudeto have depressedthe local water table gene activity. The third model would correlatethe belowthe level previously affected by supergenesul- early enrichmentwith the Altos de Camilacasurface fideformation. The irregularenrichment zones above and the later with the earlier part of the Multiple ca. 3,330 m mayreflect either two or moreperiods Pedimentstage and probably specifically with the in- of alterationseparated by significantuplift events or termediatestages in the developmentof the Pampa a singleprotracted period of enrichmentaccompa- Lagunasapron pediment, for which the tilted surface nyinga relativelyslow depression of the watertable. southof the mineconstitutes the originalbackscarp. As notedpreviously, no firm temporalconstraints The lower main enrichment would thus have devel- maybe placedon the supergenesulfide enrichment opedfollowing erosion of the 18,3 ___0.6-Ma ignim- at Toquepala.The proximityof the uppermostchal- brite and probably at ca. 13 Ma (see Tosdal et al., CUAJONE,QUELLA VECO, AND TOQUEPALADEPOSITS, SE PERU 1623

1984, fig. 5). In this model, the considerableextent the early Miocene;and (2) the localfrequency ofem- of the lower chalcocitezone would reflect (1) the placementand persistence of Miocenetuffs. Both pa- stronguplift andresulting downward displacement of rameterswere themselvesstrongly influenced by the the water table which characterizedthe Multiple local regime of fluvial channeldevelopment. Pedimentstage (Tosdal et al., 1984); (2) the absence, In passingfrom Cuajone to Toquepala,there was or rapid removal by erosion,of the ChuntacalaFor- evidently a progressivediminution in the intensity mationin the immediatearea; and (3) the lack of deep (i.e., extent and frequencyand/or rapidity) of valley incisionof QuebradaToquepala until the later Mio- developmentacross the evolvingsubplanar precor- cene. dilleran landscapeduring the earliestMiocene and a Whereas we consider it clear that enrichment of concomitantdecrease in blanketingby tuff flows.At the Toquepaladeposit had begun prior to the Multiple Cuajone,we infer that the supergenesulfide enrich- Pedimentstage, very probablyin the latestOligocene mentblanket is largelyrelict fromthe late Oligocene to earliest Miocene, we cannot decide between the and was formed in association with the evolution of abovemodels for the age of the later, more intense the subduedlandscape ancestral to the Altosde Cam- enrichmentstage. Nevertheless, we prefer the third ilaca surface. At that time, gradual erosion and modelin whichchalcocite development persisted into depressionof the water table permittedthe progres- the middle Miocene (ca. 11-18 Ma; see above). sive downwardextension of chalcocitedevelopment and the encroachment of the leached zone over earlier Synthesis enriched horizons. Since then, the local conditions The abovediscussion of the Tertiary geomorphol- havebeen largely unfavorable for supergeneactivity, ogic contextof supergeneprocesses in the area sur- due almostentirely to the presenceof extensivetuffs roundingthe Cuajone,Quellaveco, and Toquepala on top of the deposit.Valley incisionacross the tuffs mines demonstratesboth the potential of this ap- and subjacentorebody and potentiallyfavorable wa- proachin the elucidationof the controlson the eco- ter table lowering were of insufficientduration for nomicallycritical enrichmentand the uncertainties anysignificant enrichment, because the valleyswere inherent in the unravelingof complexcordilleran quicklyfilled by successivetuffs and other pyroclastic landformdevelopment. rocks of the Huaylillas and ChuntacalaFormations. The ore deposits were emplaced in the early It is clear in any casethat all significantsupergene Eocene(Clark et al., 1990) at shallowdepths (equiv- activitypredated consolidation of the basal(13.1-Ma) alent to confiningpressures of ca. 100-150 bars; ashflow of the ChuntacalaFormation. Thus, the major Zweng, 1984) but, given the considerablethickness episodicuplift associatedwith the Multiple Pediment of subaerialvolcanic strata constituting the Toquepala stage failed to generate important enrichment at Group, at a significantelevation (>2,000 m a.s.1.). Cuajone,although it is possiblethat somedevelop- Followingtheir unroofingin the Eoceneand Oligo- ment of the sooty chalcociteof the transitionalores cene during a prolongedperiod of comparativetec- may have persistedinto this period. Only minor ox- tonic quiescence,all three porphyry centersexperi- idationmay be assignedto the later Mioceneand sub- enceda historyof episodicuplift andmantling by tuffs sequentinterval. during the initial stagesin the developmentof the The 80-m-thick enrichmentblanket in the Quel- present-dayCordillerra Occidental at thislatitude. It lavecodeposit formed prior to the 9.5 ___0.5-Ma tuff is evident that supergeneactivity was most intense of the ChuntacalaFormation which directly overlies duringconversion of a regionallyextensive erosional oxidized chalcociteores. The markedly troughlike surfaceof low altitudeand relief into the faceted,po- form of the enrichedzone suggeststhat it developed, lyphase,and mountainouslandscape of the present at least in part, beneath an extensive open valley precordillera. which we infer to have been in existence in the mid- Despite the lacunae in our knowledge of local Miocene, between 18.4 and 13.1 Ma. However, we Neogene geologicevents, a comparisonof the en- stronglyfavor a modelin whichsuch enrichment dur- richmenthistories of thesethree depositsprovides a ing the Multiple Pedimentstage was superimposed basisfor an understandingof the factorswhich influ- upon an earlier supergeneprofile which was gener- ence supergeneprocesses. In particular, the pro- ated beneath the Altos de Camilaca surface in the gressiveincrease in the extent of chalcociteminer- earliestMiocene or latestOligocene, probably coeval alization,from the modestblanket at Cuajonethrough with the enrichment at Cuajone. Thus, despite the the thicker enrichedzone at Quellavecoto the mul- complexsuccession of Mioceneerosional and volcanic tiple blanketsat Toquepala,can be linked with dif- eventsrepresented in the Quellavecodistrict, the ep- ferencesin erosionalhistory and the extent of burial isodesof valleyincision were, in aggregate,favorable beneathNeogene pyroclastic rocks. It is apparentthat for the deepeningof the main chalcocitezone, and the critical parametersaffecting the formationand/ the blanketingby the Huaylillasand Chuntacala For- or preservationof enrichmentzones in thesedeposits mationtuffs was insufficientlyprotracted to impede were (1) the locationand rate of valleyincision during seriouslythe supergeneactivity. 1624 CLARK, TOSDAL,FARRAR, AND PLAZOLLES

In our opinion,the considerablevertical extent and the supergeneenrichment was presumably related to apparentlypolystage nature of the Toquepalaenrich- an early stagein the developmentof that landform. ment is a direct reflectionof the ephemeralnature of In the smallLluta (Cercana)Cu(-Ag, Pb) district Miocene ignimbritecover in the mine area and of (V•trgas,1975), situated120 km southof Toquepala, unusuallystrong uplift and tilting, probably in the veins were exploited in which massivesupergene mid-Miocene.The dispositionof the highestaltitude chalcocitewas extensivelyconverted to malachite. residual chalcocite zones showsclearly that they The enrichedveins crop out on a rock-cutbench of formed beneath the Altos de Camilaca surface or its the QuebradaLluta. This bench is interpreted as a late Oligoceneantecedents. A brief interruptionof restrictedterrace pediment, and its elevationstrongly supergeneactivity at ca. 18.3 Ma probablyoccurred suggeststhat it is equivalentin age to the Multiple in responseto burialbeneath tuffs. The deepestmain Pedimentstage of the Toquepala-Cuajonearea. Thus, enrichmentzone was generated following a significant enrichmenthere is at least middle Miocenein age episodeof uplift and oceanwardtilting of the pre- and may have begundevelopment in the early Mio- cordillerain the Toquepalatransect. This event cannot cene. be dated, but we favor an origin during the earlier Elsewhere in southern Peru, 110 km west-north- part of the Multiple Pediment stage,i.e., at ca. 13 west of Cuajone, the large Cerro Verde-SantaRosa Ma, rather than prior to the probableemplacement porphyry copper center (16ø32' S), broadly coeval of the 18.3-Ma flow. Thus, developmentof massive with the depositsunder discussion(Estrada, 1975), chalcociteores may have persistedsomewhat later displaysan extensivesupergene profile (Kihien, 1975; thanat Quellavecoand it is implicitthat no significant Mineroper6,unpub. data). Mining operationsby Mi- burial by the ChuntacalaFormation ignimbrites took neroper• in the Cerro Verde sectorhave been based place. on high-gradeoxidized ores, dominated by brochant- It is evident that both physiographicand climatic ite, which overlie a zone of sulfideenrichment up to conditionswere broadlyfavorable for supergeneac- 300 m in thickness,attaining a depthof 465 m below tivity in the late Oligoceneto middleMiocene interval the 2,903-m-a.s.1. summit of Cerro Verde. In the ad- in southernPeru. Economicallyimportant enrichment jacent SantaRosa sector, 1.5 km to the southeast,a certainly occurredin the early Miocene (ca. 23-18 muchthinner (30-50 m), but laterallyextensive, sub- Ma) but had ceasedby the earliestlate Miocene.The horizontal enrichment zone lies ca. 50 m beneath an correspondencebetween the uppermostpreserved interconnected series of small intermontane basins. zonesof supergenechalcocite and the Altosde Cam- The surroundingsummits are accordantand, in many ilacasurface strongly implies that enrichmentbegan cases,are flat topped.They aretentatively interpreted as early as approximately25 to 30 Ma, probably as the remnants of the landform which controlled the shortlyfollowing exhumation of the deposits.Sub- enrichmentprocess, the "Caldera surface"of ]enks sequently,whereas climatic conditions favorable for (1948). Both the accordantsummits and the inter- enrichment clearly persisted into, and perhaps montanebasins are consideredto representpediments throughout,the middle MioceneMultiple Pediment that are asyet undated.However, on the basisof long- stage,the extentof new enrichmentand preservation distance correlations of landforms, we tentatively of extant supergenezones was entirely dependent suggestthat the oldersummit surface is equivalentin upon the local configurationof valley development age to the Altos de Camilacasurface, whereas the and the presenceor absenceof Miocene tuffs over basinfloors, representing the upper extensionsof the the deposits.Thus, the aggressivedevelopment of very extensivealluviated pediplain which constitutes cordilleranrelief in the Miocenecaused the upgrading the PampasCostaneras (Pampa de La ]oya) ca. 15 to of an earlier formedchalcocite blanket at Quellaveco 20 km southwestof the minearea, are probablycoeval and generatednew enrichmentzones at Toquepala, with the Multiple Pediment stage.If this model is but led to oxidationand erosion of supergeneprofiles correct, the enrichment of the Cerro Verde-Santa at Cuajone. Rosadeposit would be inferred to be, at leastin its earlier stages,of late Oligoceneor earliestMiocene RegionalCorrelations age. The great depthsattained by the enrichment Supergene enrichment has upgraded numerous processbeneath Cerro Verde itself may reflect the smallcopper vein systemsexposed on the Pacificslope permeabilityof the breccia bodieswhich host the of the Cordillera Occidental elsewhere in southern- mineralization in this area and may record a pro- most Peru. The Norviii mine, 13 km southeastof To- tracted historyof supergeneactivity extendinginto quepala(V•trgas, 1975), worked oresin which chal- the Multiple Pediment stage. cocite,after chalcopyrite,had been oxidizedto mal- The age and physiographicsetting of supergene achiteand chrysocolla(Clark, unpub.data). The de- enrichmentin northernChile are remarkablysimilar positcrops out on the southwesterlytilted segment to thoseestablished in southernPeru. Supergeneen- of the Altosde Camilacasurface (see Fig. 15b), and richment of the Chuquicamataporphyry systemin CUAJONE,QUELLAVECO, AND TOQUEPALADEPOSITS, SE PERU 1625 northern Chile (Fig. 1) occurredin the interval 16.5 mentsand open valleys. That both supergeneactivity to 26 Ma (Mortimeret al., 1977) beneatha regionally andpedimentation have essentially terminated in this developedplanate landscape broadly correlative with regionhas long been ascribedto the onsetin the later the Altos de Camilaca surface. To the south, the Mioceneof anarid andeven hyperarid climatic regime youngerof the two enrichmentepisodes recognized (e.g., Mortimer, 1969; Mortimer and Sari•, 1975; in the Copiap6 district (e.g., Sillitoe et al., 1968; Mortimer et al., 1977), itself a productof the north- Mortimer, 1973), thoughnot preciselydelimited, had ward propagationof a coldlittoral current.This aspect terminatedprior to the developmentof the Atacama of the later historyof the Andeshas been analyzed pediplain,before 12.9 Ma (Clark et al., 1967a; Sillitoe in detailby Alpersand Brimhall (1988), who identify et al., 1968). Thisepisode followed a lateEocene (39- the essential causative factors to be the establishment to 43-Ma) episodeof copper mineralization(Farrar of the circum-Antarcticcurrent and the subsequent et al., 1970; Clark et al., 1976). A late Oligoceneand formation of the Antarctic ice cap at 13 to 15 Ma. early Miocene age for enrichment in that area of They alsoemphasize the key importanceof the cli- northernChile may thereforebe inferred. The atten- matic desiccation and resultant decrease in erosion dant regional erosionallandform, the Sierra Checo rates in the preservationof earlier formed supergene del Cobre surface(Mortimer, 1973), thusprobably zones in what is now the Atacama Desert. In this con- formed at the same time as the Altos de Camilaca text, it shouldbe emphasizedthat economicallysig- surfacein the studyarea. Finally, Alpers and Brimhall nificantenrichment has also affected the very young (1988) report early and middle MioceneK-Ar ages Rio Blanco-Disputada(_•7.4 Ma; Warnaarset al., for supergenealunite within the unusuallyextensive 1985) andE1 Teniente (Howell and Molloy, 1960; and high-grade chalcocite blanket and associated _•4.7 Ma; Clark et al., 1983) depositsof centralChile, leachedcapping of theLa Escondidaporphyry deposit whichhave experienced a pluvial cold-temperate, or (Fig. 1). even periglacial,climate throughout much of their It is therefore apparentthat the late Oligoceneto brief exposurehistory. early Miocene interval proved favorablefor intense Although there is someevidence that major en- supergeneupgrading of copperdeposits along a ca. richmentmay have locally persisted to a slightlylater 2,000-km stretchof the Pacificslope of the evolving period in southernPeru than in northern Chile, the CordilleraOccidental (-Principal), from southern Peru availableage constraintsare insufficientto define a to north-central Chile. The essential simultaneous significantnortherly change in the time of enrichment enrichmentdefines a discretemetallogenic episode, cessation.Instead, in comparingthe two broad tran- or perhapstwo closelyspaced episodes, of great im- sects,we would emphasizethe greater extent, both portance, directly correlated with major pulsesof absoluteand relative, of the late Oligoceneto middle uplift of the cordillerain the latest Oligocene(ca. Mioceneuplift eventsin southernPeru. Thus, whereas 18-25 Ma) andearly middle Miocene (ca. 12-16 Ma). muchof northernChile had alreadyachieved consid- In thiscontext, the ca. 37-Ma ageof supergenealunite erablealtitudes by the Eocene(e.g., Mortimer, 1973), in the E1Salvador deposit recorded by Gustafsonand southernPeru lay closeto sealevel until the late Oli- Hunt (1975) is anomalous.For this depositthe geo- gocene(Tosdal et al., 1984). Thereafter, uplift and morphologyonly constrainsenrichment to havepre- crustalthickening were far more abrupt in this area dated the local formationof the Atacamapediplain, than farther to the south.This would favor deep en- prior to 12.9 Ma. Lacking any evidenceto the con- richment of the ore deposits.However, the region trary, we canonly conclude that climaticand landform surroundingthe Cuajone,Quellaveco, and Toquepala conditionsat E1 Salvadorwere suitablefor major en- porphyry centersexperienced more intensevolcanic richmentin the earlyOligocene, although no regional activityduring the late Oligoceneand early Miocene erosionalsurface or enrichmentepisode of this age interval than did geomorphologicallycomparable hasbeen recognized in the region(Mortimer, 1973). areasin northern Chile. The closelyspaced eruption Conclusions of thicktuffs at thiscritical juncture in southernPeru musthave played a key role in limiting the extent of Supergene enrichment processesin the central enrichmentin the depositsunder studyand particu- Andescontext achieved their maximum development larly in the caseof Cuajone.In contrast,there is no in the late Oligoceneand early to middleMiocene, evidencefor localvolcanism in the areasurrounding stimulatedby anoptimal conjunction ofepisodic uplift La Escondida,Chile, during the early and middle of theCordillera Occidental and a prevailingsemiarid Miocene, thus permitting essentiallyuninterrupted climate.The moderateprecipitation (perhaps ex- developmentof an enrichmentblanket both consid- ceeding10 cm/annum)inferred for this periodnot erably thicker and of higher grade than thosepre- onlypermitted significant circulation of meteoricwa- servedin southernPeru. Volcanismof late Oligocene ter throughthe upper partsof the exposedore de- and early Miocene age (ca. 25-15 Ma) affectedwide positsbut favoredthe formationof subplanarpedi- areasof northernChile, extendingfrom the Peruvian 16 2 6 CLARK,TOSDAL, FARRAR, AND PLAZOLLES borderto at leastlatitude 26 ø S (e.g.,Zentilli, 1974; Andersondisplayed considerable patience in the typ- Coiraet al., 1982). However,the scatteredgeochro- ing of the penultimateand final manuscripts. nologicdata suggestthat major mid-Tertiarytuffs Revisionof the manuscriptwas considerably aided were erupted lessfrequently southof ca. 21ø S in by the detailed,sympathetic, and constructive reviews northern and north-central Chile where the Paleocene of two EconomicGeology reviewers. to mid-Oligoceneporphyry copper deposits occur, This paper is a contributionto the Queen'sUni- than in northernmost Chile and southernmost Peru versity Central Andean Metallogenetic Project (Mortimer et al., 1974; Tosdalet al., 1981; Lahsen, (CAMP). 1982). In particular,ignimbrites of thisage are prob- REFERENCES ablyless extensively developed on the lower altitude Alpers, C. N., and Brimhall, G. H., 1988, Middle Miocene climatic slopesof the cordillera.The lack of significanten- change in the AtacamaDesert, northern Chile: Evidence from richment in some porphyry depositsin northern supergene mineralization at La Escondida:Geol. 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