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Economic eologyVol. 83, 1988, pp. 657-663

OLIGOCENE MAGMATIC ACTIVITY AND ASSOCIATED MINERALIZATIONIN THE POLYMETALLIC BELT OF CENTRAL PERU

' PIERRE SOLERInstitut rancaiseRecherchecientifiqueour e DdveloppementnCoopdration-ORSTOM,13 rueLafayette,75480 ParisCedex10, France,and C.N.R.S.,U.A. 384, "Pdtrologie t Mdtallogdnie"

AND MICHEL G. BONHOMMEInstitutDolomieu,Universitd cientifiquet Mddicalede Grenoble nd C.N.R.S.,U.A. 69 "GologieAlpine,"15 rue Maurice Gignoux,38031 Grenoble,France

IntroductionAmong the most striking characteristics f theCenozoicevolutionof the centralAndesare the ig-neousactivity, both effusiveand intrusive,and theformation of very numerousassociated ase metal

ore deposits.The most important Pb-Zn-(Ag)-(Cu)metallogenetic rovince s the polymetallicbelt ofcentral Peru (Petersen, 1965; Bellido et al., 1969;Ponzoni, 1982; Soler, 1986; Soler et al., 1986).Althoughmineral depositsof Paleogeneage arecommon n southernPeru (e.g., Noble et al., 1984)and n the southernpart of the centralAndes e.g.,Sillitoe, 1981), they have not heretofore been rec-ognized in the cordillera of central Peru. In thisgeneralarea, a considerable umber of K-Ar deter-minations of middle and late Miocene have beenobtained on intrusive stocks, commonly subvol-canic,which are associated eographically nd veryprobablygeneticallywith theseore deposits, ndonalteration and gangue minerals of the actual de-posit.North to south,K-Ar radiochronologicalatafrom the depositsof Hualgayoc (Borredon, 1982),Antamina McKee et al., 1979), Raura D. C. Noble,unpub. data),Cerro de Pasco Silberman ndNoble,1977), Colquijirca (Vidal et al., 1984), Huaron(Thouvenin,1984; J. C. Baubronand J. M. Thou-venin, unpub. data), Rio Pallanga (Farrar andNoble, 1976), MorocochaEyzaguirre t al., 1975),Colqui (Kamilli and Ohmoto, 1977), Yauricocha(Giletti and Day, 1968), Huachocolpa McKee etal., 1975), Julcani (Noble and Silberman, 1984),and one fission-track determination for Pasto Bueno(Landis ndRye, 1974) gaveages n the rangeof 15to 7 m.y. The late Eocene-early Oligocene mag-maticbelt of centralPeruhaspreviously eenchar-acterized by the lack of associatedmineralization(Noble et al., 1984).Various volcanic and intrusive rocks were datedat the InstitutDolomieu Grenoble,France)aspartof a researchprogramdealingwith the spaceandtime evolution of magmatic and metallogeneticprocesses long a transectacross he central Peru-vian Andes.The presentcommunication ealswiththe part of this new data, which has alreadybeen

partly published Solerand Bonhomme, 988), thatrefers o metallogenetic spects;t allowsus o con-clude that there have been at least wo distinctpe-riods of polymetallic mineralization n central peruduring . e Oligoceneand Mioceneepochs.Data on two polymetallicdistr.icts re presented(Fig. 1). The economicallymportantMilpo-Ataco-cha district, located a few kilometers northeast ofCerro de Pasconear to the eastern edge of thepolymetallic elt, is shown o be of middle Oligo-ceneageand he noweconomically arginal hun-gar district, located in the middle part of the beltsouthwest f Cerro de Pasconear the top of thewestern cordillera, appears o be of normal middleMiocene age.

K-Ar GeochronologicDataAnalytical proceduresAll ageshavebeenobtainedby conventional -Ar

methods.The K content of the varioussampleshasbeen obtainedusingX-ray fluorescenceor K20 > 1wt percent and usingatomicabsorption or K20 < 1wt percentor in caseswhere only smallamountsof asamplewere available.The Ar content hasbeen de-

FIG. 1. Map of the northernpart of centralPeru showing helocation of areas referred to in the text and areas covered byFigures 2, 3, and 4. The dashed ines indicate the easternandwestern imitsof the polymetallic rovinceof centralPeru (fromSoleret al., 1986). 1 -- Cerro de Pasco-Atacocharea seeFig. 2),2 = Chungar-Huaron rea (seeFigs.3 and 4), Uc = Uchucchacuadeposit.

0361-0128/88/812/657-752.50 657

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658 SCIENTIFIC COMMUNICATIONSTABLE . K-Ar Radiochronologicata--Milpo-AtacochaArea

Location o. Field Analyzed K20 4OArtad Age(Fig.2) number Location fraction (%) 4OArtad%)/4Artors (tA/g) (m.y. __r)Yanamatei CP 66 76 14' 02" W Viii 3.74 76.9 1.844 15.2 _+ 0.4

10 42' 23" S P1 1.20 53.2 0.509 13.1 _+ 1.1$unkullo2 CP 54 76 08' 28" W Bi 8.48 92.3 8.58 30.9 _+0.510 39' 21" S P1 0.73 79.8 0.579 24.4 _+ 0.9Mariac 3 CP 55 76 07' 58" W Bi 8.59 92.4 8.62 31.1 _+0.410 40' 44" S P1 0.75 62.3 0.586 24.1 _+ 1.9Milpo-Socorro4 MI 15 76 12' 24" W WR 4.03 82.4 3.65 27.8 _+ 0.6

10 35' 42" S5 MI 106 76 12' 31" W H 0.91 37.5 0.883 29.8 _+ 2.510 35' 25" S P1 2.67 87.9 2.59 29.8 _+ 1.4Atacocha-San Gerardo6 AT 43 76 12' 55" W WR 3.66 76.9 3.49 29.3 _+0.5

10 34' 37 S7 AT 47 76 12' 55" W P1 0.71 62.3 0.599 25.9 ___ .510 34' 28" S WR 2.98 87.0 2.60 26.3 _+ 0.4 Bi -- biotite,H -- hornblende, 1 - plagioclase, R -- whole ock

terminedusing sotope ilutionof aSArn a 6-cm-radiusmass pectrometer.he spikewascalibratedagainsthe G1-O standardOdin, 1982). The meanvalue of this standard measured in Grenoble is24.92 nl/g ascomparedo the nternationalalueof24.82 nl/g.All calculationsse he constantsecom-mendedby $teiger and J/iger 1977).Most of the sampleshave been analyzedusingvariousmineral ractions luswhole-rockmaterial.The resultsare given n Tables and 2.Milpo-Atacochadistrict

Milpo andAtacocha re skarnandvein-typebasemetal depositsemplacedmostly (skarns nd veins)

in platform imestones f the Late Triassic ndLias-sic Pucar5Group and partly (veins) n sandstonesand orthoquartzitesof the NeocomianGoyllariz-quizga Group (Delgado, 1979; Soler, 1986). Theore deposits ppeargeometrically ssociated ith aseriesof small (

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SCIENTIFICCOMMUNICATIONS 659

10

? ] ? 3km

FIG. 2. Geologicmap of the Cerro de Pasco-Atacocharea (compiled rom Mgard, 1978, andunpub.data;Delgado,1979; Soler,unpub.data),with ocation f samples to 7. Definitionof patterns:i -- lower Paleozoic, -- Eo-Hercynian ranite SolerandBonhomme, npub.data),3 -- Mitu Group,4= Upper Triassic ndLiassicPucaraGroup (limestones), -- NeocomianGoyllarizquizga roup (sand-stones ndorthoquartzites), -- Middle to Upper Cretaceous edimentaryormations, = uppermostCretaceous nd Paleogene ed beds, 8 -- Oligocene ntrusive ocks,9 -- Miocene ntrusive ocks,10-- Quaternarydeposits, nd 11 -- polymetallicore deposits.which are located about 12 km east of Cerro dePasco;and for the Yanamate sampleCP 66) stockwhich is located about 4 km southeast of Cerro dePasco.The data are presented n Table 1.

The Yanamate granodioritic porphyric stockgives slightly discordantplagioclase-whole-rockages.The plagioclase henocrysts re partly serici-tized, so hat the ageof 15.2 m.y. has o be consid-

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660 SCIENTIFIC COMMUNICATIONS

ered as a minimumage for the emplacementof thisstock; t appears o be approximately ontempora-neouswith the Cerro de Pasco ntrusions Silber-manand Noble, 1977) and slightlyyounger han heMarcapunta tockat Colquijirca Vidal et al., 1984).The Sunkulloand Mariac granodioriticporphyricstocksgave discordant geson biotite and plagio-clase.The plagioclase ge probablycorrespondsoa partial reset; the plagioclase phenocrysts areslightly sericitized, but the biotite appearsunal-tered. Cobbinget al. (1981) obtained n ageof 29.5m.y. on plagioclaseor the Quinua stock ocatedseveral kilometers to the north in the same belt(Fig. 2). The K20 contentof the materialdatedbyWilson 2.06%) ndicates n mperfectseparation fthe plagioclase;he obtained ge s ntermediate e-tween the ageswe obtainedon biotite and plagio-clase.Thus he data set appears o be internallyco-

herent and an age of 31.0 _+0.5 m.y. maybe consid-ered as a good approximation for the age ofemplacementof the stocks f the Ticlacayan-Mariacbelt.The agesobtained or the intrusions ssociatedwith the Milpo and Atacochadepositsare discor-dant and scattered between 25.9 _+ 1.5 and 29.8_+2.5 m.y. The datedporphyricquartz dioritesandgranodiorites howa clear alterationof the plagio-clase (sericitization and carbonation), so that theplagioclase nd whole-rockageshave to be consid-ered as minimum ages. However, sample MI 106givesagesof 29.8 _+2.5 m.y. on hornblende nd29.8 _+1.4 m.y. on plagioclase;we may assume nage about 30 m.y. for the emplacement of theMilpo-Atacocha tocks.Moreover, he similarityofthe petrographic and chemical features of thestocksof the Milpo-Atacochadistrict and those of

0 I 2 3 4I I I I I

FIC. 3. Geologicmapof the Chungar-Huaron rea compiled rom Cobbing,1973, andThouvenin,1984), with location f sample . Definitionof patterns: = Chim6andSanta ormations, = CarhuazFormation,3 = Farrat, Pariahuanca, hulec,and Pariatambo ormations, = Jumasha nd CelendlnFormations, = Upper Cretaceous ndPaleogene ed beds,6 -- post-IncaicCalipuyvolcanic ocks,7-- Miocenentrusiveocks, = polymetallicre depositsndoccurrences,= Pliocenegnimbrites(Bosque e Piedra),and 10 = lakesandQuaternary eposits.

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SCIENTIFICCOMMICATIONS 661the Ticlacayan-Mariac elt allowsus o propose hatall these stocksbelong to a singleOligocenemag-matic ensemble,emplacedabout 31 m.y. ago.Chungatdistrict

The Chungarskarn-type asemetaldeposit s as-sociated oth spatiallyand geneticallywith a grani-tic stock (1,300 m NS-750 m EW), which is em-placed n Turonian platform limestonesof the Ju-mashaFormationalongan importantregional hrustfault. Ore consistsmostlyof sphalerite,galena,chal-copyrite, and pyrrhotite in an andradite-garnetskarngangue,which wasdeveloped n the westerncontact of the stock and in roof pendants.In this area (Fig. 3), age determinations havebeen performed on three igneousbodies: (1) thegranite associatedwith the Chungar ore deposit(samples U 31, CU 56, and CU 57; Fig. 4); (2) the

Chalhuacochagranodiorite located about 8 kmnorth of Chungar Fig. 3), with which the polyme-tallic prospect f Don Miguel (similar o Chungar) sassociatedsampleCH 28); and (3) an andesitic ill(sampleCU 40), intrusive nto the Jumasha orma-tion near the easterncontactof the Chungar granite(Fig. 4). The results re given n Table 2.SampleCU 31 of the Chungargranitegivescon-cordantbiotite-plagioclasegesof about 13.0 m.y.Both additional samplesgive biotite K-Ar ages nagreementwith this concordant ge. Both biotiteand plagioclase ppearcompletelyunalteredandthisage 13.0 +_ .5 m.y.) canbe regarded s he ageof emplacement f the Chungargranite.The Chalhuacochagranodiorite gives a biotiteage of 10.0 _+0.3 m.y.; the analyzedbiotite showsno alteration and this age may be considered s agoodapproximationor the age of emplacement fthis stock, which appears to be slightly youngerthan the Chungargranite. Both the ChungarandChalhuacocha tockswere emplacedalong he samethrust fault, probably inked with the "Incaic" tec-tonic phase Noble et al., 1979).Thouvenin 1984) givesan ageof 10.3 _ 0.2 m.y.for the secondstage of ore formation at Huaronwhich is located in the same general area (Fig. 3),and J. C. Baubron and J. M. Thouvenin (unpub.data; Thouvenin, writ. commun., 1985) have ob-tainedK-Ar agesof 15.6 _ I m.y. on plagioclaseorthe unaltered part of the Huaron stockand of 13.2_ 0.7 m.y. on chlorite or the first stageof the alter-ation of the stock.The CU 40 andesiticsill, which belongs o an in-trusive ensemblerecognizedfrom Oyon (Romani,1982) about 50 km to the north and stretching othe latitude of Chungar (Soler, in prep.), gives anage of 25.1 _ 1.6 m.y. on whole rock; this has o beregardedasa minimumage or the emplacement fthis sill. Conceivably, his age correspondso a par-tial reset due to a weak alteration (carbonation)ofthe rock and to the low-grademetamorphicepisodeof Miocene age, the existenceof which has beenrecently demonstrated Soler, 1987) in the westerncordillera of central Peru.

FIG. 4. Geologicmap of the Chungar granite (Cia. MineraChungar,unpub.data;Soler,unpub.data),with locationof sam-ples9 to 12. Definitionof patterns: = Chim6 Formation sand-stones nd orthoquartzites), -- Jumasha ormation limestones),3 -- Oligocenesills,4 -- Miocene ntrusive ocks,5 = Skarnandore, 6 -- lakesand Quaternarydeposits.

ConclusionsThe present K-Ar data for the Chungar granite,the Chalhuacocha ranodiorite,and the Yanamateporphyricgranodioriteconfirm he importanceofthe medium to late Miocene magmaticepisode nthe western cordillera and high plateausof centralPeru; the Chungarore depositand the Don Miguelprospectare two more polymetallicoccurrences s-sociatedwith this magmaticepisode.The early and middle Miocene magmaticarc isidentified from the medium elevations of the Pacific

slopeof the westerncordillera--the Rupay acolith

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662 SCIENTIFICCOMMUNICATIONSnear Cajatambo Soler and Bonhomme,1988) andthe East Churin stock n the Huaura River valley(Cobbinget al., 1981)--to the Amazonianslopeofthe eastern cordillera, where various alkaline stockshave been identified n the Oxapampa rea (Soler,unpub. data). Preliminary dating on three of thesestocks asgiven ages n the range of 21.0 +__.0 to12.8 +__.6 m.y. by K-Ar on plagioclase nd whole-rock material (Soler and Bonhomme, n prep.).The data obtained for the andesitic sill nearChungar and for the porphyric intrusionsof theMilpo-Atacochadistrict and the Ticlacayan-Mariacbelt are important from two pointsof view.First they confirmandeven ncreasehe volumet-ric importanceof the middle Oligocene magmaticepisode in central Peru. The middle Oligocenemagmatic rc is identified rom the mediumPacificslopeof the westerncordillera--i.e., from the WestChurin and Paccho Tingo stocks in the HuauraRiver valley (Cobbinget al., 1981; Soler, 1987)--to at least he limit between the high plateausandthe easterncordillera Ticlacayan-Mariac elt). Thehypothesis f an eastwardextensionof this arc inthe eastern cordillera cannotbe disregarded,butthe chronologyof magmaticactivity in this area isstill poorly documented.Second,hey demonstratehat the middleOligo-cene magmatic pisodeactuallyplaysa metalloge-netic role in the centralPeruvianpolymetallicbelt;this fact s totally new. The only previousndicationof possibleore formationduring Oligocene imeswas an age of 25 m.y. obtainedby D.C. Noble(unpub.data,quoted n Romani,1982) on the smalldaciticporphyrystockassociated ith the Uchuc-chacuaore deposit, located about 50 km north-northwestof Chungarat the crest of the westerncordillera Fig. 1). Thissingle gewasconsideredobe equivocal y Noble,who pointedout the possi-bility of excessadiogenic rgonwhichmighthavebeen extracted rom the underlyingPrecambrianbasement.Our data suggest hat no excess rgon spresent nd hat he ageobtained y Noble,consid-ering both the hydrothermalalterationand theMiocene egionalmetamorphismSoler,1987), is aminimumage for the emplacement f this stock.Consequently,he Uchucchacua tock appears obelong o the samemagmatic pisode s he sillsofthe Chungar reaand the stocks f the Milpo-Ata-cochadistrictand he Ticlacayan-Mariacelt. Thus,middle Oligocenemetallogeneticgneous ctivity sdocumented from the crest of the western cordil-lera at Uchucchacuao the easternpart of the highplateaus t Milpo-Atacocha.This metallogenetic gneous activity appears obe slightlyyounger han that associated ith theCu-Fe skarndeposits t Tintaya and Chalcobamba(Noble et al., 1984) which belong to the Anda-

huaylas-Yauri opperprovince Bellidoet al., 1969;Santa Cruz et al., 1979) of the south-centralseg-ment of the PeruvianAndes (Soler et al., 1986).June22, December21, 1987

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ore deposit, northern Peru: ECN. GEOL., v. 69, p.1025-1059.McKee, E. H., Noble, D. C., Petersen, U., Arenas, M., and Bena-vides, A., 1975, Chronologyof late Tertiary volcanism ndmineralization, Huachocolpa district, central Peru: ECON.GEOL., v. 70, p. 388-390.McKee, E. H., Noble, D. C., Sherkenbach, . A., Drexler, J. W.,Mendoza, J., and Eyzaguirre,V. R., 1979, Age of porphyryintrusion,potassic lteration,and related skarnmineralization,Antamina district, northern Peru: ECON. GEOL., v. 74, p.928-930.M6gard,F., 1978,Etudeg6ologique esAndesdu P6rouCentral:Office de la RechercheScientifiqueet Technique Outre-MerMem. 86, 310 p.Noble, D.C., McKee, E. H., andM6gard,F., 1979, Early Tertiary"Incaic" tectonism, plift andvolcanic ctivity,Andesof cen-

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Romani,M., 1982, Gologiede la rgion minire Uchucchacua-HaciendaOtuto, Prou: Unpub. Ph.D. thesis, nst. Dolomieu,Univ. Grenoble,176 p.Santa Cruz, S., Guerrero, T., Castilia, F., and Caro, E., 1979,Geologla e yacimientose cobre n "skarn" n a regi6nsur-orientaldel Per6:Geol.Soc.Per6 Bol.,v. 59, p. 153-176.Silberman, . L., andNoble,D.C., 1977,Ageof gneousctivityand mineralization, Cerro de Pasco, central Peru: ECON.GEOL., v. 72, p. 925-930.Sillitoe,R. H., 1981, Regional spects f the Andeanporphyrycopperbelt in Chile and Argentina: nst. Mining MetallurgyTrans., v. 90, sec.B., p. B15-B36.Soler,P., 1986, La provincepolymtallique esAndesdu Proucentral:Chron.RechercheMiniere, no. 482, p. 39-54.-- 1987, Sur l'existenced'un pisode de mtamorphismergional d'ge Miocene nfrieur dans a Cordill,re Occiden-tale des Andesdu Prou central:Acad. Sci. [Paris]ComptesRendus,v. 304, p. 911-916,

Soler,P., andBonhomme,M. G., 1988, New K-Ar agedetermina-tions of intrusive ocks rom westerncordilleraand high pla-teausof centralPeru.The identification f magmatic ulses ndepisodes f mineralization:our.SouthAmericanEarthSci.,v.1, no. 2, in press.Soler,P., Grandin,G., and Fornari, M., 1986, Essaide synthsesur a mtallogniedu Prou: Godynamique, . 1, p. 33-68.Steiger,R. H., and Jiger,E., 1977, Subcommissionn geochro-nology:Conventionon the use of decayconstantsn geo- andcosmochronology:Earth Planet. Sci. Letters, v. 36, p.359-362.Thouvenin,J.-M., 1984, Le gisementpolymtallique Zn-Pb-Cu-Ag de Huaron Prou): Description escorpsminraliss,tude paragntique, datationsK/Ar des alterationshydro-thermales, partition de l'argent:Chron.RechercheMiniSre,no. 477, p. 35-54.Vidal, C., Mayta, O., Noble, D.C., and McKee, E. H., 1984,Sobre la evoluci(n de soluciones hidrotermales desde el centrovolcnicoMarcapuntaen Colquijirca,Pasco:Geol. Soc.Per6,60th JubileeAnniv.Vol., no. 10, p. 1-14.