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MINERALIZATION AND PARAGENESIS OF THE HUANZALA MINE, CENTRAL PERU--A DISCUSSIONPIERRE SOLER,

InstitutFranqais e Recherche cientifiqueour e Ddveloppementn Coopdration, RSTOM,213 rue Lafayette,75010 Paris,France,and C.N.R.S.,U.A. 384 Pdtrologie t Mdtallogdnie

ROLANDOCARRASCAL,ND JORGE AEZDepartamentode Geologia,UniversidadNacionalde Ingeneria,Avenida TupacAmaru Km. 4, Lima, Peru

Sir: In a recent paper Imai et al. (1985) presentedmineralogical nd petrologicaldata on the Huanzalapolymetallicore deposit,centralPeru. This paper isan importantstep n the understanding f physicaland chemicalconditions f ore formation n Huanzala,but the geneticnterpretationheyproposesnotveryconvincing ecausemacroscopicndmicroscopice-lationsbetween the ore assemblagesnd wall rock(e.g., Cretaceoussedimentary ormationsand Mio-cenecalc-alkaline ranodioritic ikes)are not consid-ered in detail.To begin with, it is of some mportance o pointout that the Huanzala ore occurs as strata-boundlensesmantos), 0 to 200 m long,70 to 250 m high,and 2 to 15 m wide, which are repeateddiscontin-uously long km but alwaysn the same tratigraphiclevels (so-called etas--V1, V2, V3, V4, and V5),wedgedup by characteristicutitic key beds.MantosV1 to V4 are located n the upper part of the Valan-ginian limestoneSantaFormation and manto V5 is atthe baseof the overlying, lternating late-sandstoneCarhuaz Formation. No mineralization is known inthe limestones f the lower part of the SantaFor-mation analternating late-sandstone-limestonenitforminga progressiveransgressionetween he un-derlyingdeltaicChimuFormation nd he overlyingplatformimestone, pperSanta ormation), lthoughthe granodioriticdikes ntersect his ower SantaFor-mation as well.The stratigraphic ontrolof ore distributions rig-orous and the mantosare displacedby thrust andwrench faults, not the granodioriticdikes that cutthese aultsandare obviously ost-tectonic. urther-more, the distribution of ore and the associated al-teration,especiallyhe skarn ype, doesnot showanyclosegeometric elationshipwith the intrusive ocks.Moreover, he intrusion-limestoneontacts enerallylack any metasomaticr hydrothermalmanifestationand are nearly always acking n ore. On the otherhand, no mineralization ppears n the fault planes,contrary o what onewouldexpect or a post-tectonichydrothermal re deposit.The observationsointedout here make very unlikely a post-tectonic, ndtherefore, a Miocene hydrothermalorigin for theHuanzalaore deposit Carrascal t al., 1983; Carras-cal, 1984).

Furthermore, the ore presentsa general macro-scopicand microscopic oncordancewith the strataand shows eculiarstructures nd extures,suchasapyrite geopetal exture, framboidalpyrite, a ribbonstructureof pyrite and sphaleriteand/or galena,mi-cro-synsedimentaryr syndiageneticaults,andcon-volutions nvolvingore minerals.A detailed study of ore distribution n mantoV3lenses Carrascal,1984) shows hat in each lens thezonation is generally concentric with a higher Zncontent owardthe rims and a higher Pb content o-ward the center of the lenses. n addition, Carrascal(1984) notesa migrationof mantoV3 lenses o thenorthwestaccompaniedy a weak ncrease f Zn andPb contents nda weakdecrease f Cu content)whenascendingn the stratigraphic equence.No verticalzonationof Zn, Pb, andCu contents ppears. inally,on a larger scale, he distributionof mineral assem-blages showsa stratigraphicevolution (Carrascal,1984), from black,Fe-poor,chalcopyrite-rich phal-erite which dominates n mantosV1 and V2, to red,Fe-rich, chalcopyrite-poor phaleritewhich domi-nates in mantos V4 and V5. All these observationsmakevery probable synsedimentarynd/orsyndi-ageneticdepositionof the greater part of the min-eralization.The ore and ganguemineralogy,as describedbyCarrascalet al. (1983), Tsuchiyaet al. (1983), Car-rascal 1984), and n even more detail by Imai et al.(1985) implies ntroduction f elementsby hydro-thermalsolutionst high emperaturesapproximately300C). The only sourceof the introducedelements,according o the Andean geologiccontext and the

pretectonicand probablysyndiagenetic haracterofore formationat Huanzala,has to be volcanogenic.In fact, the existence f volcanic ctivitycontempo-raneous ith sedimentationsprovedby the presenceof tuffaceous limestones and some tuffs interbeddedin the upper SantaFormation. Moreover, Huanzalais not an isolated case and the Santa Formation hasbeen recognized s an importantmetallotectof thecentral and northern Peruvian Andes (Samaniego,1981), with numerous, robablyvolcanogenic,trata-boundore deposits f proximal El Extrato) or moredistal character Malaquita,Pueblo Libre, Pacclon-Llamac,Pachapaqui).

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Therefore, his combination f field and aboratoryobservations leads us to conclude that Huanzala is agood candidate or a high-temperature retaceousvolcano-sedimentaryre depositwith somesubse-quent Mioceneremobilization.As pointedout in the presentdiscussion,he me-tallogenicnterpretationf thisdepositscomplicatedby the presenceof Miocenecalc-alkalinentrusivestocks nd dikeswhichmay modifysubstantiallyhepreviousmineralassemblages.hey may eventuallybe hostsof additionalore deposition y subsequenthydrothermalism,s s known rom other Santaoredeposits uch sTuco-ChiraSamaniego,981),Ven-turosa Diaz Bernal, 1984), and probablyOyon aswell.As shownby SolerandLara (1983) and Soler un-pub.data),centralPeruores romSanta trata-boundandMiocenehydrothermal eposits, othassociatedwith calc-alkalineAndean magmatism,show verysimilargeochemical atterns, o hat further deduc-tionsabout he respective artsplayedby Cretaceousvolcanogenic rocessesnd Miocenehydrothermalprocessesn Huanzala nd similarore deposits, ithobviousconsequencesor explorationand miningschemes,will require more detailed investigation.Work in progress oncerningare earthdistributionsin fluorites rom both Miocene metasomatic nd hy-drothermaland Santastrata-bound re deposits ndK-Ar datingof alterationmineralsof Huanzalawillprobablyprovidenew material or the answer.

AcknowledgmentsWe wish to thank Mitsui Mining and SmeltingCompany,Ltd., and CompaniaMinera SantaLuisaS. A. who enabled us to work in Huanzala and veryespecially iroshiSato,ChiefGeologist ndAssistantGeneralManager f C. M. Santa uisaS. A., andYo-

shihiroTsuchiya nd Mario Toledo, esidentgeolo-gists t heHuanzalamine, orhelpful ndenthusiasticdiscussions.June 14, 1985

REFERENCESCarrascal, ., 1984, E1yacimiento stratoligadoZn, Pb, Ag, Cu)de Huanzala,Huanuco,Peru: Unpub. Ph.D. thesis,UNI Univ.,Lima, Peru, 145 p.Carrascal,R., Saez, J., and Soler, P., 1983, El yacimientoestra-toligado Pb, Zn, Cu, Ag) de Huanzala Huanuco,Peru Central):Discusiongenetica:Soc. Geol. Peru. Bol., no. 71, p. 1-15.Diaz Bernal,N., 1984, El modelogeneticoHuarochiri:Soc.Geol.Peru Bol., JubileeVol., 60th Anniv., pt. 2, p. 1-22.Imai, H., Kawasaki,M., Yamaguchi,M., andTakahashi,M., 1985,Mineralization nd paragenesis f the Huanzalamine, centralPeru: ECON. GEOL., v. 80, p. 461-478.Samaniego, ., 1981, Stratabound b-Zn-(Ag-Cu) re occurrencesin Early Cretaceoussedimentsof north and central Peru. Acontributiono their metallogenesis:npub.Ph.D. thesis,Univ.Heidelberg, 210 p. Soler, P., and Lara, M. A., 1983, Metales menores y trazas aso-ciados l plomoy al zinc en los yacimientos olimetalicos elosAndesPeruanos: intesis egional:Soc.Geol. Peru Bol., no.72, p. 147-158.Tsuchiya,Y., Toledo, M., Mendoza,D., and Soto,R., 1983, Geo-logiadel yacimientominerode Huanzala: oc.Geol.PeruBol.,no. 71, p. 17-40.

MINERALIZATION AND PARAGENESIS OF THE HUANZALA MINE, CENTRAL PERU--A REPLYHIDEKI IMAI

CentralResearchaboratory, itsuiMiningand Smelting ompany, td., Haraichi,AgeoCity,Saitama refecture,apan 62Sir: n the Huanzalaminingarea, he mineralizedzone extends 6 to 8 km in the limestone of the SantaFormation; it has a width of 150 to 200 m. The ore-bodiesoccuras enslike ormsparallel o the hangingwalls and footwalls of the formation.Solersays hat mantos 1 throughV5 always xistin the same tratigraphicevels n the Santa nd heoverlyingCarhuazFormation.This statement s notadequaten the strictest ense. or convenience akeat the mine, he orebodies re divided nto five ayers,neverthelesshey are distributed poradicallyn theSantaFormation,as shownhere in Figure i and infigure3 of my paper (Imai et al., 1985).Somezinc and ead ores n this depositare closelyassociated ith skarnminerals,suchas garnet,diop-side,vesuvianite, pidote,etc. (Fig. 2 A, B, and C).

From this observation, classify he deposit as py-rometasomatic, or contact metasomatic. Also, theparagenesisf the sulfidemineralsn the deposit e-sembleshat of Cerro de Pasco.The depositat Cerrode Pasco s generally ecognized o have a hydro-thermal replacementorigin.I emphasizedhesepoints n the discussionf thegenesis f the Huanzaladepositand it continuesobe my basicviewpoint. find this deposit o be thesame sMorocochan Peru (Petersen, 965), Kamiokain Japan Imai, 1978a),andSangdongn Korea John,1963, 1978), and to be similar o Cerro de PasconPeru (Petersen,1965; Einaudi, 1977).Sheetlikedikes,of Miocene-age uartz porphyryoccur n the SantaFormation.Granodioriteporphyryis found n the Chimu Formation figs. and 2, Imai