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ECONOMIC GEOLOGYAND THE

BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

VOL. 89, SEPTEMBER-OCTOBER, 1987 NO. 6

Kuroko-Type Deposits n the Middle CretaceousMarginal Basin of Central Peru

CISAR E. VIDAL C.*

Buenaventura ngenieros . A., Larrabure y Unanue 46, Lima 1, Perth, nd Depa-tamento e Geolog[a,Universidad ational de b, geniea, Tpac Amaru s/n, Lima 31, Pert

Abstract

Barite, massive sulfide, and siliceous stockwork deposits of Kuroko type in the Limaregion are associated with the Casma Group, a seqnence of submarine olcanic ocks ofMiddle Cretaceous age. These deposits were formed in an ensialic marginal basin withpredominantly asaltic o andesitic ill. Volcanic-hosted eposits ccur u the entire region;sediment-hosted eposits re restricted o the eastern Casma volcanic acies, which inter-calate with limestones and shales deposited on a shelf platform adjacent to the marginalbasin. In most cases, mineralization is spatially associated with dacitic domes and tuffbreccias with zones of quartz-serieite lteration; he latter have ocally been dated at 116 to106 m.y. by the K/Ar method. Strata-bound deposits of bedded barite, pyrite, sphalerite,and pyrrhotite overlie the fbeder zones.

The most mportant deposits of this kind are Leonila-Graciela and Juanira, with 4 milliontons of produced barite and 2.5 million tons of production plus reserves f massive ulfideore. They are located n a roof pendant of folded strata ntrnded by two plutons of theCoastal batholith. Contact metamorphism of hornblende-hornfels facies affects both oredeposits and wall rocks. K-Ar ages on hornblende-biotite pairs from the granitic rocksindicate that they were eraplaced 82 and 65 m.y. ago. Whole-rock ages on postmetamor-phic dikes vary between 31 and 39 m.y.

P-T conditions or contact metamorphism of hornblende-hornfels acies at Leonila-Gra-ciela are estimated at 2.1 to 2.6 kb and 300 to 500C on the basis of sphalerite geobaro-metry, stratigraphic econstruction, metamorphic mineralogy, and interpretation of discor-dant K/Ar age patterns. Mole percent FeS in sphalerites ncreases n a prograde sense romthe actinolite zone at Juanira o the biotite-mnscovite zone at Gracicla. In massive ulfidespecimens t varies correspondingly rom 15.4 _ 0.2 to 17.6 _4- .7. Sphalerites rom sili-ceous stockworks show the same trend with 14.7 _4- .4 mole percent FeS and 17.6 1.1mole percent FeS. Metmnorphic equilibration was reached only in the biotite-muscovitezone at Graciela. This is demonstrated y the hmnogeneity of high mole percent FeS valuesdetected n sphalerite, which coexists n nmtual contact with pyrite and hexagonal yrrho-tire.

Introduction

IN the southern part of the coastal egion of PeruCu, Mo, Au, aud Fe have long been mined h'omvein-, replacement- aud porphyry-type deposits(Bellido and De Montreuil, 1972). In contrast, herehas been little mining and exploration n the centraland northern coastal regions. Nevertheless, as aconsequence f intensilzing etroleum exploration,barite deposits were discovered during the early1950s. Discoveries in the Lima and Piura areaswere given special ttention because f their conve-

* Present address: Perubar S. A., Juan de Arona 830 go, Lima27, Perfl.

nient geographical ocation near he Callao port and

the Talara oil fields, respectively. Barite mining inseveral of the properties surrounding Lima has re-vealed massive sulfide zones with Zn-(Pb-Ag) ores,such as n Leonila-Graciela Fig. 1).

A cluster of barite +_ massive sulfide depositsoccurs within a semicircle of a 50-km radius cen-tered on Liana, losted by submarine Cretaceouswlc, nic rocks. It is the purpose of this paper todescribe he geologic setting and nature of thesedeposits. heir genesis nd subsequent volution sdiscussed n the light of recent geologic studies(Vidal, 1980), coupled with K-At dating and micro-

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probe nalyses f sphalerites. second luster fsimilar occurrences east of Piura is linked to Upper

Jurassic-Cretaceous olcanic ocks n northernPeru. The Tambo Grande deposit s the most m-portant epresentative f his egion Injoque t al.,1979; Llosa, 1979; Fig. 1). It was drilled out at aprefeasibility tage y the Bureau e RecherchesG(ologiques t MiniSres n 1978-1980. Drill-indi-cated reserves are 40 million tons of pyritic massivesulfide ore with high-grade concentrations ofCu-Zn-(Ag) ores. The near-surface, trata-boundcharacter nd the lensoid shape of this sulfide massare its main structural features. Barite zones, sili-ceous sulfide ore, and hematite chert beds have also

been eported romTambo Grande nd he nearbyprospects f Potrobayo, otoral, nd Morrop6n(Fig. 1). These geologic eatures nd he verticalzoning n the deposits re very similar o those fthe Japanese uroko deposits.

The northernmost egion where deposits f thiskind have been discovered ies southwest of Quitoin Ecuador. Unpublished nformation rom C. W.Farrell 1978) was available o the author concern-ing he Kuroko-type eposits t La Plata and Macu-chi. The La Plata deposit was evaluated n the 1950sby Sotopaxi Exploration Company and in1961-1965 by Duncan Derry Exploration. From1975 to 1982, Cia. Minera Toachi S.A., a joint ven-ture of Ecuadorian claim owners with OutokumpuOy, Metallgesellschaft .G., and Cia. de MinasBuenaventura S.A., operated the property on asmall scale. Reserves otaled 200,000 tons with 5.7wt percent u, 4.8 wt percent n, 3.6 ppm Au, and

44.8 ppm Ag. The deposit s characterized ystrata-bound lenticular orebodies of bedded mas-sive sulfides nd barite, with underlying, ow-gradedisseminations. he former are located along thecontact between two distinct volcanic successionsof Cretaceous ge. The basal equence onsists fvariably ilicified yroclastic ocks; he overlyingsequence onsists f basaltic avaswith minor he-matite breccia and uffs. The sequence s folded andfaulted, making he orebodies iscontinuous.

The regional xtent of volcanogenic assive ul-fide and barite deposits f Kuroko ype has not beenrecognized reviously n the Mesozoic ecord f he

central Andes. They represent an important groupof polymetallic re deposits mitted n the most e-cent metallogenetic ynthesis roposed or the re-gion (Clark et al., 1976; Ericksen, 1976; Putzer,1976; Sillitoe, 1976; Amstutz, 1978; Petersen,1979; Frutos, 1982).

Exploration and Mining HistoryBarite was irst explored nd mined n the coastal

region of central Peru in 1948 by the PeruvianChemical ndustry Company later renamed heBarmine Company nd Minera Barmine S.A.). Dur-ing he next 20 years bout 500,000 ons of bariteore was mined by underground methods rom theLeonila-Graciela orebody (Fig. 1). The Gracielaclaim was controlled hroughout hose years by theInternational Petroleum Company and was latersold to National Lead Industries, Inc.

From 1968 through 1980, barite production wasprogressively ncreased y means of an open-pitoperation. rom 1976 to 1979, t reached maxi-mum of 1,000 tons per day, representing ne of thelargest roducers n the world Martino, 981). Thebarite ore had a high specific ravity 4.2-4.4 g/cc)

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KUROKO-TYPE DEPOSITS, CENTRAL PERU 1411

and barium sulfate contents of over 80 wt percent;soluble salts varied from 60 ppm in Leonila to 300ppm in Graciela. The prospects f Balducho, Palma,Mara Teresa, and Elenita were discovered duringthis period (Fig. 2). In Graciela, massive ulfide oreswith high-grade concentrations of Zn were discov-

ered and evaluated by drilling. Similar discoveriesof massive ulfides were made at Juanita y PerubarS.A., a subsidiary of National Lead Industries, nc.Geologic mapping of the region, the Graciela openpit, and from the Elenita and Santa Cecilia mineshas been carried out by the author intermittentlysince 1975.

Flotation plants for Zn and Pb-Ag concentrateswere put into operation in 1980 and 1983, coincid-ing with a sharp decline n the production of barite.Average head grades n 1985 vary from 10 to 14wt percent Zn, 0.5 to 1 wt percent Pb, and 15 to 45ppm Ag. The existing mills at Graciela and Santa

Cecilia can produce about 150 tons/day of Zn con-centrate (Fig. 3A). Metal production and reserves ofhigh-grade sulfides are estimated at 2.5 milliontons. Lower grade stockwork ones are not includedin the latter estimate.

Regional Geology and Tectonic Setting

In the central coast of Peru marine sedimentaryand volcanic strata of predominantly ower Creta-ceous age were intruded during Upper Cretaceousto Paleocene imes by the Coastal atholith Fig. 2).Following he main stages f batholithic eraplace-ment he region was uplifted, peneplained, nd cov-ered by a thick sequence f subaerial olcanic ocksknown as the Calipuy Group. Deposition of themarine sedimentary nd volcanic ocks ook placewithin the eugeosynclinal one of the West Peru-vian trough (Wilson, 1963; Cobbing, 1976), whichhas been recently interpreted as an ensialic mar-ginal basin Atherton et al., 1983). Three main tec-tono-stratigraphic units have been recognized nthe latter sequence: he Morro Solar Group, thePamplona and Atocongo Formations, and the CasmaGroup. A brief account of this stratigraphic ucces-sion follows.

StratigraphyMorro Solar Group: The Puente Piedra Forma-

tion, consisting f 2,000 m of basaltic pillow lavasand water-lain tuffs intercalated with fossiliferousmarly shales nd imestone enses f Berriasian ge,forms he owermost art of the Morro SolarGroup.Thinning of individual lava flows toward the eastindicates hat their feeders ay to the west (Rivera,1951). The main outcrop of these units lies in theChil16n River area along he coast o the north ofLima; correlatable sequences re known from the

latitude of the Lurln River and from the Pucusanaand Mala areas (Rivera et al., 1975; Fig. 2).

Conformably overlying the Puente Piedra For-mation are sandstones and shales of the Salto delFraile, Herradura, and Marcavilca Formations.More than 500 m of deltaic and fiuviatile clastic

strata alternate from the predominantly shaly baseto the quartzite op of these ormations Fern/mdezConcha, 1948). Rosenzweig (1953) and Wilson(1963) have concluded hat these clastic strata weredeposited within a closed basin isolated from theocean by positive ands o the west, from which thesediments were largely derived.

Pamplona and Atocongo ormations: hinly bed-ded, fossiliferous marls and limestone beds, whichbecome progressively hicker higher up in the se-quence, characterize he 1,200-m-thick lithologicsuccession f the Pamplona and Atocongo Forma-tions. The basal contact with the Marcavilca quart-

zites is transitional and consists f sandy imestonebeds. These formations epresent a transgressivesequence of carbonate strata underlying the vol-canic rocks of the Casma Group. Their fossil assem-blages are devoid of Tethyan key fossils. Both theMorro Solar clastics and the Pamplona-Atocongocarbonates re characterized y provincial aunas ofValanginian o Aptian age (Rivera et al., 1975). Thepeculiar fauna s a further indication of the isolatedcharacter of the basin.

Casma Group: On a regional scale, the CasmaGroup, as originally proposed y Myers (1974), en-compasses ,000 to 9,000 m of submarine volcanic

and interbedded sedimentary ocks that have lo-cally been subdivided into several formations. nthe coastal egion of central Peru, Albian ammon-ites have been found at the base of the sequence(Wilson, 1963) and Cenomanian ossils have tenta-tively been identified rom higher up in the succes-sion (Guevara, 1978). Two individual basins havebeen delineated along the entire 1,000-km lengthof this volcanic elt; these are the Huarmey and RioCaete basins of Cobbing (1978), which had theirmain periods of subsidence uring Albian and Neo-comian o Albian times, respectively Fig. 1). Theregion between he Rhnac nd Lurn valleys s sup-posed to represent the area of interconnection be-tween the two basins Fig. 2); however, no detailedaccount of the transition is available.

A well-defined west o east acies change charac-terizes the Casma Group in the central coast ofPeru. The western facies consists of basaltic to an-desitic lavas, tuffs, and hyaloclastic breccias plussporadic sedimentary ntercalations with measuredthicknesses on the order of 2,500 m. The easternfacies, as exposed n several oof pendants nd val-leys east of the batholith, is characterized by amixed succession f andesitic o dacitic lavas, tuffs,

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and volcaniclastic sandstones intercalated withlenses of dark calcareous mudstone, shale, and im-pure limestone. The volcanic rocks of the CasmaGroup are coeval with a sequence of shelf lime-stones hat crop out to the east of the area shown nFigure 2; this lithological polarity has been inter-preted as representing a classic pair of eugeosyn-clinal and miogeosynclinal roughs.

Structural and igneous history

The Morro Solar Group crops out in the core ofnorthwest-southwest-trending open anticlines tothe west of the Coastal batholith in the Lima and

Mala areas (Fig. 2). Open anticlines with tighterupright synclines nd subhorizontal old axes arealso known farther north in the Huarmey region(Myers, 1974; Webb, 1976). These folds are trun-cated by the batholith of which the earliest units are

Albian, indicating that folding occurred duringMiddle Cretaceous imes. This folding event corre-lates with the sub-Hercynian hase of deformationsdefined in northern Chile and is currently referredto as the Mochica phase (Cobbing, 1985).

Belts of tighter folds and overall stronger defor-mation occur mmediately o the east of the batho-lith in the Rimac, Lurln, and Mala valleys. Both helithological spectrum and the deformation style ofthe eastern Casma Group facies are similar to theHuayllapampa Group of Myers (1974). These fea-tures can be explained in terms of sedimentation,volcanism, and folding, both during Middle Creta-

ceous and Paleocene Incaic phases, being con-trolled by a hinge line. In fact, regional Andean andAndean-normal faults to the east of the Coastal

batholith (Fig. 2) were probably generated alongsuch a deep basement structure. Major northeast-southwest-trending dextral wrench faults areknown from the eastern sections of the Rimac andOmas valleys. An important vertical reactivation ofTertiary age has been recorded or the Agua Saladafault n the Rimac valley Fig. 4); this ault drops heCasma-Calipuy nconformity a minimum of 800 mto the east.

The Coastal batholith is a multiple and composite

belt of plutons with an overall trend from gabbroand diorite to tonalitc and granodiorite. Figure 2depicts only the general outcrop pattern of thiscomplex batholith which has been subdivided ntothe Lima and Arequipa segments o the north andsouth of the Lurln River, respectively (Pitcher,1978; Pitcher et al., 1985). Radiometric dating ofvarious plutons n the region has given an age spec-trum of 104 to 62 m.y. (Beckinsale t al., 1985; thispaper). Plutons of the batholith contact metamor-phose the deposits at Leonila-Graciela, Cantera,and Balducho Fig. 2).

Post-Incaic volcanism was subaerial and gave riseto the dacitic and rhyolitic gnimbrites of the Cali-puy Group. Felsic ava lows, agglomerates, nd a-pilli tuffs are present oward he base of this 2,000-m-thick volcanic pile in the Rmac and LurCh ec-tions. Minor intercalations of basaltic flows and

subaqueous ediments lso occur. The base of theCalipuy Group n the region has been dated at 41m.y. by Noble et al. (1979).

Tectonic setting

The paleontology and sedimentology of theLower Cretaceous sequence show that the basinswere developed within an isolated nland sea alongthe continental margin. The source areas of theshallow-water edimentary nd volcanic ocks werelocated predominantly to the west. Myers (1975)suggested hat the Precambrian o Paleozoic Are-quipa massif as a northward extension hat was rel-atively uplifted during this time.

In contrast, during Albian to Cenomanian imesthe Casma volcanics were extruded into rapidlysubsiding basins. Volcanism consisted of fissureeruptions of basalt n the western basin Atherton etal., 1985; Pitcher and Bussell, 1985). To the east,the Casma volcanics interdigitate with progres-sively increasing amounts of sedimentary rocks.Volcanic centers in the form of felsic lava domes and

tuffbreccias have been recognized both in the west-ern and eastern acies Vidal, 1980). The entire set-ting is one of a marginal basin, n which a belt of

new crust was generated by submarine asaltic vol-canism presumably during a period of crustal ex-tension and ow rates of sea-floor preading. Confir-mation for this tectonic regime comes from thestudies of burial metamorphism, which indicate thepresence of high geothermal gradients Aguirre andOffier, 1985).

Furthermore, the gravimetric and seismic pro-files presented by Jones (1981), Couch et al.(1981), and Bussell and Wilson (198.5) show a massof high density and low velocity rocks beneath hevolcanic basins. This arclike crustal anomaly disap-pears north of the Huarmey basin and south of the

Rio Cafiete basin. It is interpreted to consist ofmafic rocks n the form of upthrusted oceanic crustor basic intrusion complexes Wilson, 1985). Thelatter interpretation is favored considering theabundance of gabbro putohs along the entire belt,which are partly coeval with the Casma volcanics(Regan, 1985). Isotopic signatures of the gabbrosand later granitic rocks of the Coastal batholith areindicative of a mantle provenance with contamina-tion by Precambrian rust only south of the LurChRiver (Mukasa and Tilton, 1985; Fig. 2).

Vertical block tectonics and ensialic character are

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and lineaments as discussed y Myers (1974) andPitcher and Cobbing (1985).

Mineral Deposits

The region hown n Figure 2 is characterized ythree different ypes of metallic mineral deposits.These are: (1) the granite-hosted Cu veins atCumias and Bosa Maria mines directly north of theOmas Biver, (2) the volcanic- nd sediment-hostedamphibolitic u mantos nd veins of the Bafil andCondestable eposits ast of Mala, and 3) volcanic-hosted, strata-bound arite and Zn-Fe-(Pb-Ag) ul-fide deposits f Kuroko ype (Vidal, 1980). Depositsof the latter group are present hrough he entireregion; heir main epresentatives re the Leonila-Graeiela and Juanira eposits 0 km east of Lima.Taken as a group, hey have most of the geologicelements that characterize the Japanese Kuroko-

type deposits swill be described nddiscussed nsubsequent arts of this paper.

Mar(a Teresa

This deposit s located 6 km west of Huaral (Fig.2). It has been explored ntermittently nd minedfor barite and Pb-Ag ores on a small scale since1973. Two inclined adits, surface renches, and dia-mond drill cores were available for study. Severalstrata-bound barite lenses as much as 12 m thickcrop out for a distance f 250 m within beds ofvariably ltered elsic uff. These uffs are underlain

and partly disrupted y irregularly shaped nd o-cally brecciated odies of silicified ock. Abundantpyrite and races of galena, phalerite, nd chalco-pyrite are present. Primary sulfide bodies, on theorder of 35,000 tons, average 10 ppm Ag, 2.2 wtpercent Pb, 0.1 wt percent Zn, and 0.03 wt per-cent Cu.

Barite occurs as monomineralic enses and podswithin argillically ltered and partly silicified uffs.They are typically massive r banded on a centime-ter scale. Banding reflects contrasts n grain size,recrystallization abric, and features nduced byweathering such as porosity or oxidation stains.

Lenses of pyrite boxwork locally parallel thebanded barite.Zones of silicification are fracture controlled

along their sides and base; however, their topsoccur almost parallel to bedding and directly un-derneath the barite horizon. In places nterlockingblocks of volcanic country ocks can be recognized;such breccia zones are conspicuously ilicified byequigranular quartz with sericite and stockworksulfides. Late swarms of veinlets of chalcedony withsericite, pyrite, arosite, and chlorite are exposed nthe underground workings.

The volcanic equence hat overlies he mineral-ized zone consists f amygdaloidal asaltic avas ndhyaloclastic reccias. light metamorphic ecrystal-lization has aken place, as ndicated by patches ofbiotite and quartz-epidote veinlets. Regional pat-terns of burial metamorphism might have con-curred with contact aureole effects related toyounger ranitic lutons f the Jecu/m ype Pitcheret al., 1985). JecuSn onalites re known rom kmto the northwest of the mining area.

Aurora Augusta

The Aurora Augusta deposit s located 1.5 kmwest of Jicamarca orge, about 20 km northwest ofits confluence with the Rmac valley. From 1975, atleast 150,000 tons of barite ore has been produced;polymetallic sulfide zones are currently under ex-ploration. Both ore types are found n irregularly

shaped upright bodies within a strongly silicifiedfunnel in volcanic rocks of the Casma Group. Thenearest ranitic ocks re 2 km to the northwest ndto the east of the deposit, espectively; hey belongto the Santa Rosa superunit of the batholith andintrude unaltered and unmineralized Casma vol-canic rocks (Fig. 2).

Andesitic volcaniclastic rocks interbedded withvesicular avas dipping moderately o the southwestform the hanging wall of the mineralized zone.Finely bedded uffs and imy shales ccur sporadi-cally. Calc-silicate minerals re found n some of thelatter limy horizons. The contact between the

hanging-wall olcanics nd the mineralized zone scontrolled by bedding; t is grossly oncordant ndabrupt. Quartz-epidote einlets and vug fillings arefound directly above the silicified body, which iscut by postore andesitc dikes.

The mineralized complex s subcircular n plan,with an exposed iameter f about 80 m. Away romits roughly trata-bound nd concordant oof, clearintrusive features are found toward the eastern sideof the steep margin. nterlocking locks f volcaniccountry ocks define breccia ones, within whichpostsilicification ebble-breccia ikes re ound o-cally (Fig. 5G and H). The matrix s composed f

finely comminuted olcanic material with rregularzones of intense silicification where tabular bodiesof barite and/or sulfides appear. Tabular baritebodies occur hroughout he complex ut are con-centrated toward its top. Here, the largest onesreach 8 m across nd 70 m along strike and have aknown vertical extent on the order of 50 m. Bariteore is fine grained, equigranular, nd essentiallymonomineralic; large megacrysts are present inonly a few places. he transition o the zone of si-licification is marked by increasing amounts ofquartz, arosite, nd pyrite (Fig. 5F).

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Embayed aggregates nd single phenocrysts fplagioclase with recrystallized margins are com-monly observed n thin sections n the zone of silici-fication. Phenocrysts and volcanic clasts are en-closed n a granoblastic olygonal groundmass fquartz and are partly replaced by sericite. Sericiteand chlorite are also found in the siliceous ground-mass with sulfide minerals Fig. 6G and H). Sulfide-rich zones are invariably associated ith intense si-licification. An early generation of sphalerite andpyrite is veined and partly replaced by chalcopy-rite-quartz-sericite ssemblages Fig. 6F, G, and H).Leonila-Graciela

Leonila-Graciela, by far the most mportant min-ing district of its kind, is ocated 50 km to the east ofLima in the Rmac valley (Figs. 3 and 4). A detailedaccount of the geology and mineralogy of these de-posits has been given by Vidal (1980). Individual

orebodies consist of bedded barite, massive sulfide,and siliceous stockwork zones. Folded and strata-bound lenses of barite overlie massive sulfide zones

in the Leonila-Graciela syncline and the recumbentanticline of Juanita. Past production and reservesare on the order of 4 million tons of barite and 2.5

million tons of high-grade Zn-(Pb-Ag) ore. Sporadicenrichments of Cu-(Au) have been detected n theso-far undeveloped stockwork zones. The eastwardcontinuation of the Juanita orebody has been tec-tonically offset and disrupted along the dextralCorte de Ladrones ault; it is mined separately andreferred to as the Santa Cecilia orebody (Fig. 7).Siliceous stockwork and breccia zones are alsoknown from the Chamodada and Elenita mines Fig.4); at Elenita, polymetallic sulfide ore averages .0wt percent Zn, 1.5 to 3.0 wt percent Pb, and 100 to130 ppm Ag.

All these mines and prospects are located in theCocachacra roof pendant. Eastern Casma facieshave a minimum hickness f 600 m; they consist fsubmarine volcaniclastics, ava flows, and tuff brec-cias with an intercalation of limestone and marl. The

sequence has been folded into relatively tight, An-dean-trending, and northwest-plunging anticlinesand synclines. tructural analysis f the banded ores

demonstrates a locally disharmonic attitude withoverall congruence at regional scale Vidal, 1980).Repeated ntrusion by at least wo separate units ofthe Upper Cretaceous Coastal batholith has beenrecorded. K-Ar dating indicates that the RicardoPalma tonalitc was emplaced 82 m.y. ago and thatthe Canchacaylla monzogranite s 65 m.y. old (Fig.4, Table 1). Contact metamorphic aureoles havebeen developed adjacent o these ntrusions nd af-fect some of the orebodies. Following the Paleo-cene Incaic stage of folding, uplift, and denudation,the Calipuy Group of subaerial volcanic rocks was

extruded; it overlies discordantly the Casma vol-canic rocks and the Coastal batholith. In the Coca-chacra area, the Calipuy Group is barren and con-sists of a monotonous equence of agglomerate ndash-flow tuff about 1,200 m thick.

Massive sulfide zones have been found in the

Graciela open pit and n the underground workingsat Juanita and Santa Cecilia (Fig. 7). Their internalstructure s banded, showing sphalerite and pyriteas the main hypogene constituents. Minor phasesare galena, tetrahedrite, chalcopyrite, pyrrhotite,and barite; traces of jamesonitc, bornitc, mackina-witc, molybdenite, nd magnetite have also beenobserved. Grain size is coarse and textures are me-

tamorphic Fig. 6). granoblastic o lepidoblastic n-tergrowths of sulfides nd barite are common. Pyr-rhotite is markedly more abundant n the Leonila-Graciela deposit; X-ray diffractograms ive singlebut moderately symmetric 102) peaks, ndicatingits hexagonal character. Microprobe analysis ndetching with saturated chromic acid confirm thisfinding (Fig. 6A and B).

Although obscured by metamorphic ecrystalli-zation, the paragenetic sequence nvolves earlybarite, sphalerite, and pyrite which are embayed,rimmed, and veined by quartz-chalcopyrite-galena+_ etrahedrite (Fig. 6C and E). Most sphaleritegrains ontain blebs of chalcopyrite, nd n the Gra-ciela specimen, uncommon yrrhotite. The Juanitaand Santa Cecilia sphalerites commonly ack exso-lution textures. Pyrite is commonly even grainedand develops a granoblastic mozaic texture (Fig.

6D); inclusions f chalcopyrite, galena, and sphal-erite are rare.Massive sulfide zones are underlain by irregular

masses f siliceous stockwork developed mainly asreplacements f dacitic avas. Such s the case of thestockwork zone southwest of Graciela and the zone

that rims the Juanita orebody n the 1,200-m level(Fig. 7). Two different types of stockwork oneshave been recognized; he most common ype con-sists of a quartz-sericite-chlorite matrix with vein-lets and disseminations f pyrite, sphalerite, chalco-pyrite, galena, etrahedrite, and native Au (Fig. 6E).The second ype consists f pyrite-rich etrahedrite

disseminations in siliceous microbreccias such asthose found in the 1330 bench of the Leonila de-posit.

Barite beds typically overlie the massive ulfidezones at Leonila-Graciela and Juanita; he Gracielabarite zone also overlies metasedimentary andmetavolcanic ocks (Fig. 3B). The barite beds, con-taining more than 80 percent barite forming astructureless ranoblastic mozaic, are intercalatedwith lenses of calcite and pyrite-sphalerite; he bar-ite zone s banded and has verv abrupt contacts withboth the massive sulfides and the metasedimentary

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1418 cgsAa E. VIDAL C.

*** ba

sp

.,..., 4 cm

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KUROKO- TYPE DEPOSITS, CENTRAL PERU 1419

footwall (Fig. 3C). Metamorphic effects are seen nthe latter interface where calc-silicate bands are in-

tercalated with porphyroblastic arite (Fig. 5C andD). The contact metamorphic overprinting ofLeonila-Graciela will be described and discussed inmore detail later.

Palma

The prospect of Palma is located 3 km south ofthe Lurn valley in the Palma gorge. It has neverbeen mined and s currently ncompletely explored.A thin and ayered barite sulfide ens s traceable ormore than 100 m, reaching a maximum hickness of3 m. It is zoned northward from a barite zone into a

massive sulfide bed (Fig. 3G). Preliminary assaysindicate an average of 13 wt percent Zn, 2.4 wtpercent Pb, and 45 ppm Ag. Black pyritic shalesdirectly underlie the ore horizon. As in the Coca-chacra roof-pendant, at Palma the eastern acies ofthe Casma Group consists of limestone and shaleintercalations in volcaniclastic sandstones, lavas,and breccias.

Although he folds n the host ocks have not beenmapped, major NW-trending open folds arepresent. The ore horizon is located in the axialhinge zone of an anticline. The anticline core ex-posed consists f black shale with framboidal pyriteand delicate structures indicative of soft-sediment

deformations (Vidal, 1980). The western limb ismade up of a bed with massive pyrite, sphalerite,pyrrhotite and chalcopyrite, which in turn is over-lain by argillaceous limestones. Barite lenses in-crease n size and number toward the hinge zoneand the eastern limb of the anticline. Disharmoniccontortions of the massive sulfide bed resemblethose at Graciela.

Balducho

The Balducho deposit is located in the head-waters of the Rio Chilca, 40 km NE of Pucusana(Fig. 2). It has been mined ntermittently n the pastseveral years. Strata-bound enses of barite and py-rite-sphalerite occur in upright position within acontact metamorphosed epta of spotted slate andhornfelsic graywacke. Maximum width of the septa

is 300 m and, therefore, could not be shown n Fig-ure 2. Siliceous tockwork ones with chalcopyriteare adjacent to the strata-bound ores on the east.

Barite textures are granoblastic nd ocally cataclas-tic; the richest barite zone is located along thenortheastern wall.

Local ntrusives of the Tiabaya superunit Pitcheret al., 1985) are tonalitic to granodioritic n compo-sition and are accompanied by porphyritic felsicdikes. Contact metamorphic effects include thecoarse granoblastic textures of the ores and thehornfelsic nature of the country rocks.Cantera

The prospect of Cantera s ocated 8 km northeastof Mala (Fig. 2). A small and stratiform barite-py-rite-(calcite) mass s hosted by limy shales nd sand-stones that are intercalated within lava flows and

volcaniclastics oward he base of the Casma Group.A zone of incipient silicification s developed un-derneath he barite horizon. Barite is of very coarsegrain and exhibits replacement structures in thesurrounding shales and limy sandstones. he oreand alteration showings are relatively small andhave not encouraged urther exploration.

K-Ar Determinations

A K-Ar dating program was designed o unravelthe time of formation of the Aurora Augusta depositand the subsequent ontact metamorphic overprintrecorded at the Leonila-Graciela deposits. Bymeans of thin section evaluation of supergene ef-fects, nine out of twelve samples ollected were de-termined to be suitable or K-Ar dating.

Samples AA2 and AA3 come from quartz-seri-cite-chlorite alteration halos that surround thestockwork orebodies at Aurora Augusta (Fig. 2).Samples CHP, SRX, INC, and E2 represent freshholocrystalline plutonic rocks from two differentintrusions hat contact metamorphosed he ores atLeonila-Graciela (Figs. 2 and 4). Three samplesfrom postmetamorphic ikes--J1238, G10, and G2from Leonila-Graciela--were also dated. All the

age determinations were carried out in the IsotopeGeology Unit of the British Geological Survey inLondon.

Analytical proceduresSericite-quartz concentrates were prepared from

samples AA2 and AA3 by heavy liquid and hand-

FIG.5. Microphotographs rom hin sections: raciela A, B, C, D, and E) and Aurora Augusta F, G,and H). A. Granoblastic arite. Parallel icois. . Lepidoblastic ntergrowth f barite ba) and sphalerite(sp). Crossed icols. C. Calc-silicate and (gray to black) ntercalated with barite (ba) and pyrite(opaque n barite). Parallel nicols. D. Enlargement f C showing arnets gt) and epidote-diopside(shades f gray) n barite aggregate ba). Crossed icols. E. Quartz-sericite qz-src) lteration n sili-ceous tockwork; elic plagioclase ath (pl). Parallel nicols. . Barite ba) zone with pyrite (py) andjarosite jar), fine-grained iliceous atrix. Crossed icols. G. Pebbles f quartz aggregates qz) andminor amounts f pyrite (py) in comminuted matrix with quartz-sericite qz-src). Crossed icols. H.Ptygmatic einlets and pebbles of quartz-pyrite-(barite) n matrix as n G. Crossed icols.

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po

cpy

t

sp

ba

; 8mm

sP'..

:

...

..

... .. .

0. mm

D

_I

:.- .

0.2 mm: :

p,.0.2 mm ... t

cpy

U

sp

--

p

qz

].

'0.2 mm

8 mm ----------

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picking methods. -ray diffractograms ere used oconfirm he presence of sericite and to determineits percentage. Peak-height atios of sericite (002)vs. quartz (100) reflections were 0.16 for sampleAA2 and 1.23 for sample AA3.

High purity biotite and hornblende concentrates

were prepared or samples RX, CHP, and NC bystandard lectromagnetic nd heavy iquid separa-tion techniques. nalyses f the remaining sampleswere performed on -60 to +120 mesh whole-rockpowders.

Potassium nalyses were carried out on an Instru-mentation Laboratories 543 flame photometer,using Li as an internal standard. Each sample wasanalyzed n duplicate; dditional nalyses ere car-ried out only for samples hat gave results withmore than one percent difference. Average esultswere used or the age calculations. rgon determi-nations were performed by the isotope dilution

technique on a Micromass 200 mass pectrometer.Duplicate Ar analyses were performed only for thesericite concentrates. The decay and abundance4K/KTotalonstants ecommended y Steiger andJSger 1977) were used.

Results nd interpretation

The results f the K-Ar analyses re presente inTable 1. Discordant ages were obtained on sericiteconcentrates rom the Aurora Augusta deposit,namely 106 _ 39 and 116 4- 18 m.y. for sample

AA2, compared o 68 4- 2 and 63 4- 2 m.y. for sam-ple AA3. Samples AA2 and AA3 were found o con-tain 2.93 wt percent and 5.77 wt percent K, respec-tively, thus indicating sericite concentrations f atleast 35 and 70 percent. Large analytical uncertain-ties n the ages or sample AA2 are due to the highproportions of atmospheric Ar. Spontaneous rrelease was noted prior to fusion of this sample.Mass spectrometric scans ound no indications ofpossible nterferences rom organic matter. The ab-sence of zeolites and/or additional potassium-bear-ing phases which could also have interfered withthe emission of Ar from the sericites was confirmedby X-ray diffraction methods. It is believed thatfluid inclusions bserved n the quartz ntergrown

with sericite n sample AA2 were responsible orthe anomalously igh proportion of atmospheric4Ar nd ts spontaneous volution rior to fusion.However, Ar released from these fluid inclusionscould also be partly of either radiogenic nature,formed rom K + ions n solution, r excess rgon--

that s, 4Arwithout 36Ar n atmospheric roportion--derived from older rocks hat were attacked bythe hydrothermal solutions (D.C. Noble, pers.commun.).

Samples CHP and SRX from the northeasternmargin and core, respectively, f the Ricardo Palmatonalitc also have discordant ges. Almost denticalhornblende ages of 82 4- 2 m.y. were obtained onboth samples; iotite ages of 61 4- 2 and 66 _ 2 m.y.were clearly younger. Hornblende and biotite con-centrates rom the Canchacaylla monzogranite,sample NC, gave concordant ges of 67 4- 2 and 644- 2 m.y., respectively. Note that both the biotite

ages from the Ricardo Palma tonalitc and theyounger sericite ages rom the Aurora Augusta de-posit are similar to the concordant hornblende-bio-tite ages obtained for the Canchacaylla monzo-granite.

A whole-rock age of 39 4- 1 m.y. was obtained orsample E2 from an apophysis f the Canchacayllamonzogranite n the vicinity of the Leonila-Gracieladeposit. Similar Cenozoic ages of 39 4- 1, 37 _ 1,and 31 4- 1 m.y. were obtained or the respectivepostmetamorphic ikes J1238, G10, and G2 in thisdeposit.

Discordant age patterns such as those here re-

ported for the Aurora Augusta ericites nd for theRicardo Palma pluton hornblende nd biotite pairsare common n the study region. Snelling 1981)relates this resetting to thermal disturbances n-duced by the Centered Complexes f the Coastalbatholith, which were emplaced about 68 to 62m.y. ago. The hornblende ges of samples HP andSRX are in good agreement with the regional agerange or the emplacement f the Santa Rosa uper-unit, to which the Ricardo Palma pluton has beenassigned Pitcher et al., 1985). In fact, a zircon U-Pbage of 86.4 m.y. has ecently been obtained or thispluton J. Cobbing, writ. commun., 1986). It is con-

cluded hat the hornblende ges of 82 m.y. repre-sent the emplacement age of this pluton and that

FIG. 6. Microphotographs rom polished ections: raciela A, B, C, D, and E) and Aurora Augustadeposits F, G, and H). A. Iron sphalerite sp) n mutual ontact ith pyrite py) and hexagonal yrrho-tite (po) with barite ba) gangue. . Hexagonal yrrhotite rain po) etched with chromic cid showinglamellae of monoclinic yrrhotite darker gray). C. Iron sphalerite sp) etched with chromic acidshowing rain boundaries utlined y chalcopyrite, winning, nd occasional riple points. D. Equi-granular ntergrowth f coarse yrite showing riple points. . Sphalerite sp) veined y chalcopyrite(cpy) and quartz qz). F. Sphalerite rystal sp) outlined nd veined y chalcopyrite white) and quartz(qz).G. Sphalerite sp)with chalcopyrite lebs nd stringers ssociated ith sericite black lakes). .Interlocking yrite crystals py) veined by quartz qz), chalcopyrite cpy), and sericite black lakes).

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1422 C$Aa E. VIDAL C.

-- Leonila Graciela __

-5000 N--

Juanita

[ Post-metamorphic yke Biotite-muscovite ornfelses Tremolite-actinolite hornfelses

-- Barite ore

Massive yrite-sphalerite re:.: Siliceous tockwork one, Banded ore

() K/Ar sample location

0 rn 200

anta Cecilia

60

/1400

m

'Centralighway-Juanita - '

- 2oo.

B o m 200IFIG. 7. Simplified eologic map rom evel 1200 and structural rofile BB' rom Leonila-Graciela,

Juanita, nd Santa Cecilia orebodies. ee Table 2 for mineralogical escription f individual samples nthe metamorphic ountry ocks.

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K UROKO-TYPE DEPOSITS, CENTRAL PERU 14 2 3

TABLE1. Radiometric K-Ar Data and Location of Analyzed Samples

Sample Material K 4ARraa 4ARatmos Calculated ge Latitude Longitudeno. analyzed (%) (nl/g) (%) (m.y. ___a) south west

Dikes (Leonila-Graciela)G2 Whole rock 0.92 1.11 62 30.9 ___ 1.1 11o54.5 ' 7634

G10 Whole rock 4.82 6.93 24 36.6 __+ 1.0 1154.5 ' 7634 J1238 Whole ock 2.17 3.36 24 39.4 _ 1.1 1154.5 7634

Granites Leonila-Graciela)E2 Whole rock 3.13 4.81 48 39.1 +_ 1.2 1154.4 ' 76034.2 'INC Biotite 4.49 11.38 29 64.0 +_ 1.8 1153.4 ' 76034.5 '

Hornblende 0.55 1.46 41 66.7 - 2.0SRX Biotite 6.12 15.98 6 65.9 - 1.8 1154.9 ' 76037 '

Hornblende 0.72 2.33 27 81.8 ___ 2.3CHP Biotite 5.58 13.54 10 61.3 _ 1.7 1154.1 ' 7634.8

Hornblende 0.75 2.45 33 82.0 +_ 2.3

Tuff breccias Aurora Augusta)AA3 Sericite 5.77 14.32 8 62.8 +_ 1.8 1159.5 76051 '

Sericite 5.77 15.53 10 68.0 +_ 1.9AA2 Sericite 2.93 12.43 93 105.9 ___ 9.5 11059.5 ' 76051 '

Sericite 2.93 13.62 84 115.8 _ 17.9

Decay constants s n Steiger and Jiiger 1977) Mine coordinates: 230 N/10130 E (G2); 4895 N/10135 E (G10); 5265 N/10230 E (J1238); see Figure 7

the younger biotite ages were reset below theblocking emperature f hornblende.

The younger sericite ages rom the Aurora Au-gusta deposit obtained on sample AA3, 63 and 68

TABLE . Protoliths and Metamorphic Assemblagesof Samples Shown n Figure 7

Sample Metamorphiccoordinates Protolith assemblages

Biotite-muscovite zone

5350 N, 10000 E5330 N, 10150 E5270 N, 10150 E5240 N, 10250 E5240 N, 10250 E5220 N, 10200 E5170 N, 10080 E5125 N, 10225 E

Tuff Qz-biot-(chl)Tuff Biot-qz-(src-pl)Limestone Gt-diop-ep-(calc-ba-chl)Limestone Gt-diop-(calc-qz)Mudstone Qz-src-(biot-pl)Limestone Gt-diop-ep-(calc-chl)Dacitic lava Biot-src-qzMudstone Qz-src-biot

Actinolite zone5260N 10040E5190N 10050E

5140N 10230E5050N 10025E5025N 9985E4990 N 10030 E4960N 9980E4940 N 10000 E4890 N 10035 E4700N 10175E

Dacitic ava Qz-act-(biot)Tuff Act-(qz-ep-calc)

Tuff Qz-act-(biot)Dacitic ava Act-(chl-qz-biot)Lava Act

Dacitic lava Act-(chl-qz-biot)Dacitic lava Act-(ep-calc-qz)Tuff Act-(biot)Tuff Qz-actTuff Act-(biot)

Abbreviations: actinolite (act), biotite (biot), calcite (calc),chlorite (chl), diopside diop), epidote (ep), garnet (gt), plagio-clase pl), quartz (qz), sericite src)

Minerals shown n parentheses epresent accessory onstitu-ents

m.y., conform o the above-mentioned attern ofthermal resetting. The high proportion of atmo-spheric argon causes he high degree of uncertaintyof the ages of sample AA2, which could then lieanywhere etween 66 and 146 m.y. or 98 and 134m.y. The atmospheric rgon s thought o have beenderived mainly from fluid inclusions n quartz andnot from the sericite, thus yielding ages that are

appreciably older than the age of trapping (D. C.Noble, pers. commun.). However, it is tempting tointerpret the 106- and 116-m.y. ages rom sampleAA2 as representing he true age of hydrothermalactivity. Such an age range for the Kuroko-typemineralization at Aurora Augusta its in well withthe chronology of regional events, namely he Mid-dle Cretaceous age of the Casma Group and theUpper Cretaceous age of the granitic rocks of theCoastal batholith.

The concordant ages of the hornblende-biotitepair from sample NC are interpreted as he age ofemplacement of the Canchacaylla monzogranite.Similar Rb-Sr whole-rock isochron and zircon U-Pbages have been obtained or the nearby Santa Eula-lia pluton (Beckinsale t al., 1985; Mukasa and Til-ton, 1985). The Canchacaylla luton seems o be-long to the Puscao uperunit of the batholith basedon its monzogranitic etrography and the ages herereported. It is envisaged hat the biotite resettingadvocated or samples CHP and SRX was producedby this intrusion. Sample E2 from an apophysis fthe Canchacaylla luton gave a whole-rock age of39 ___ m.y.; this age s spuriously ow, probably dueto argon leakage from the potassium eldspars. t

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1424 CISAR E. VIDAL C.

could be indicating a minimum estimate or a Ceno-zoic resetting event.

Ages ranging from 31 to 39 m.y. were obtainedon three samples rom the postmetamorphic ikeswarm at Leonila-Graeiela. The age of this eventcorrelates avorably with the post-Ineaie I onset of

subaerial olcanism eferred to as Calipuy Group incentral Peru (Noble et al., 1979, 1985). Further-more, it provides a clue to the anomalously oungage recorded for sample E2.

Contact Metamorphism at Leonila-GraeielaThe time of granite emplaeement nto the Coea-

ehacra roof pendant has been determined by theK-Ar dating presented above. According to theCretaceous ime scale of Harland et al. (1982), plu-tons of Campanian and Maestriehtian age cut thevolcano-sedimentary eastern facies of the CasmaGroup. Undated dioritie to monzogranitie stocks n

the region are thought to represent apophyses ofunderlying plutons. Figure 4 shows he location ofthese intrusions and the approximate limit of theircomposite contact aureoles. Only the southeasterncorner of the Coeaehaera oof pendant has not beenaffected by contact metamorphism. The rest of itconsists of metamorphic rocks of the hornblende-hornfels and albite-epidote acies.

Two metamorphic profiles were studied in pre-liminary fashion away rom the Ricardo Palma ona-litc and the Canehaeaylla monzogranite. Garnet-diopside-(vesuvianite) marbles associated with bio-tite-muscovite metavoleanie rocks define an 80- to100-m-wide aureole to the east of the Ricardo

Palma tonalitc; the same assemblage occurs in awider zone, as much as 1,000 m southward from theCanehaeaylla monzogranite. Similar metamorphicassemblages ere found in patehy zones along thecore and eastern limb of the Chamodada syncline(Fig. 4). This distribution of hornblende-hornfelsfacies rocks ndicates he presence of buried grani-toids beneath the latter syncline. The anomalouslywide zone south of the Canehaeaylla pluton mayalso ndicate buried apophyses nd that at depth themain intrusive contact dips to the south.

Metamorphic mineral assemblagesMarly limestones nd quartzofeldspathie oleani-

elastic tuffs and lavas were sampled o investigatethe contact metamorphic effects n the vicinity ofthe Leonila-Graeiela and Juanita deposits. Figure 7shows he location of these samples and their dis-tinction into biotite-muscovite hornfelses and aetin-olite hornfelses, both of the hornblende-hornfelsfacies. Most of the Leonila-Graeiela deposit lieswithin the biotite-muscovite zone, whereas theJuanita and Santa Cecilia deposits ie within the ae-tinolite zone. Table 2 summarizes the available in-formation on the metamorphic mineralogy. Map-

ping of the metamorphic zonation shows hat thedistribution of the garnet-diopside ornfelses oin-cides with that of the biotite-muscovite zone.

Biotite-muscovite hornfelses have formed frommudstones, voleanielastie tuffs, and daeitie lavassurrounding he Leonila-Graeiela deposit Fig. 7).

In mudstones and tuffs, biotite and muscovite zonesoccur as laminations parallel to bedding; inter-growths with granoblastie uartz and preferentialorientation of micas are widespread. Daeitie lavaprotoliths preserve relict porphyritie texture andshow reerystallized phenoerysts f plagioelase ndK-feldspar. Granoblastie uartz associated ith bio-tite is abundant n the matrix. Biotite is commonlyfound as pseudomorphie eplacements of aetinoliteand intimately associated with white micas; t ac-counts or 10 to 40 percent of these rocks. Particu-larly relevant are the garnet-diopside-(epidote)hornfelses found at the footwall interface between

the Graeiela barite ore and the underlying recta-sedimentary rocks. Granoblastie enses of eale-sili-cate minerals are found interealated with barite in anarrow zone at the contact Fig. 5C and D). In someeases, blastoporphyritie arite crystals engulf eale-silicates. Accessory minerals are calcite, quartz,sphene, mixed ayer clays, and pyrite.

Hornfelses of the aetinolite zone are developedpredominantly n daeitie avas and tuffs of the Jua-nita and Santa Cecilia deposits Table 2; Fig. 7).Original textures and premetamorphie mineralogyare better preserved n this zone. Tremolite-aetino-lite aggregates ccur as matrix constituents, n vein-

lets, and as partial replacements of ferromagnesianphenoerysts; hey can make up 30 percent of theserocks. Associated minerals are epidote, ehlorite,calcite, sphene, and various sulfide minerals. Marlylimestones n this zone consist of metamorphic as-semblages ominated by garnet-calcite-quartz withlittle or no diopside.

The metamorphic zonation described is coinci-dent with the appearance f hexagonal yrrhotite nthe biotite-muscovite zone at Leonila-Graeiela andwith its absence rom the aetinolite zone at Juanitaand Santa Cecilia. t also coincides well with a pro-grade ncrease n mole percent FeS in sphalerite.

Geothermometry

Evidence for the thermal history of the ore de-posits during contact metamorphism derives from:(1) the predominantly hexagonal structure of pyr-rhotite, (2) the transition in the metavoleanie wallrocks rom aetinolite o biotite, (3) the distances ointrusions of known composition nd diameter, and(4) the discordant atterns of K-Ar ages obtained onhornblende-biotite pairs.

As regards he distribution and nature of pyrrho-tire in the main ore deposits, t is important o recall

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was probably reached shortly after the emplace-ment of the Canchacaylla monzogranite; thus,blocking temperatures of hornblende and biotitewould represent maximum and minimum estimatesfor temperature during metamorphism. Based onthe work of Hart et al. (1968) and Kistler (1974),contact metamorphism nd Ar release rom the Ri-cardo Palma biotites occurred approximately be-tween 300 and 500C.

GeobarometryBarometric indicators for the contact metamor-

phism which affected he Leonila-Graciela depositsare found from: (1) mole percent FeS contents ofsphalerites n mutual contact with pyrite and hexag-onal pyrrhotite, (2) regional stratigraphic econ-struction, and (3) contact aureoles related to theCoastal batholith.

Figure 8 shows he results of 80 high quality mi-

croprobe analyses of sphalerites, coexisting with

Massiveulfide

Ge:0%o [] 17. +0.7 wt % Mn 1

'mol % FeS

Siliceousstockwork

311: tr. po14.7 + 0.4

1Z6_+.1

'4 e ;e 'mol % FeS

wt % Mn 1

FIG. 8. Mole percent FeS n sphalerite histograms nd Cu-Mnabundances n sphalerites rom the Graciela samples 16 andG7) and Juanita deposits samples 1 and J11). Top. Massive sul-fide ore zone. Bottom. Siliceous tockwork one. Hatchuring as nFigure 7. Means and standard deviations quoted.

pyrite and hexagonal yrrhotite, in four representa-tive polished sections from Graciela and Juanita.Pyrrhotite is present only at trace levels in theJuanita specimen. Equilibrium criteria for thesphalerite-pyrite-pyrrhotite assemblage n the Gra-ciela specimen are the mutual contacts and thecoarse, polygonal-granoblastic extures of theseminerals (Fig. 6A, C, and D). Analyses were per-formed on an ARL microprobe at 20 kV, using 10sec and 4 sec counts for peaks and adjacent back-grounds, espectively. Synthetic sulfide standardswere used for calibration. Sphalerites were ana-lyzed for S, Zn, Fe, Mn, Cu, Cd, and Hg (see Table 3for selected microprobe analyses).

Sphalerites rom samples G16 and Jl--massivesulfide specimens from Graciela and Juanita, re-spectively--have means and standard deviations f17.6 ___ .7 and 15.4 ___ .2 mole percent FeS. Asimilar trend of iron enrichment in the biotite-mu-

scovite zone was also ound for samples G7 and J1 ,siliceous tockwork specimens, with 17.6 ___ .1 and14.7 ___ .4 mole percent FeS. Cu and Mn neverexceed 1 percent; these elements are slightlyenriched n a metamorphic prograde sense as s thecase or FeS. Spread of data s minimal or sphaler-ites from massive sulfide zones, indicating bettermetamorphic quilibration han sphalerites rom si-liceous stockwork zones (Fig. 8).

Mole percent FeS data for sphalerites n samplesG16 and G7 have been plotted with mineralogicaldiscrimination n histograms Fig. 9A). Sample G16is a massive ulfide specimen with 40 percent pyr-

rhotite, 20 percent sphalerite, 15 percent pyrite,traces of chalcopyrite, nd 25 percent barite. G7 isa sample from siliceous stockwork zone with 10percent sphalerite, 7 percent pyrrhotite, 5 percentpyrite, 2 percent chalcopyrite, nd 75 percent ofquartz-sericite gangue. Using equation (1) of Luskand Ford (1978), the mole percent FeS data hasbeen converted into pressure estimates Table 3and Fig. 9B). Estimates btained or the metamor-phic assemblage phalerite-pyrite-hexagonal yr-rhotite in massive sulfide sample G16 are 2.6 ___ .5kb. This is a preferred pressure stimate or contactmetamorphism t Leonila-Graciela considering hat

the larger spread of analytical data obtained or thesiliceous stockwork specimen G7 makes ess reli-able results. As shown n Table 3, mole percent FeSin sphalerites rom the Juanita specimen give anom-alously igh pressure stimates anging rom 4.1 to5.8 kb. However, these estimates have no real valueconsidering hat pyrrhotite is present only as min-ute inclusions n chalcopyrite. No evidence of me-tamorphic equilibration uch as major amounts fsphalerite, pyrite, and pyrrhotite, with abundantmutual contacts could be observed.

Estimates of lithostatic pressure can be calculated

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G 16Messis sulfide

--I sp py

I sp DO

I Sl:)DYpo

G7Siliceous sto(;kwork

16 17 18 19 2O

mole %

G16

G?

40 % po

20 % ep15 % y

tr cpy

26 % be

10 % sp

7 % DO

% py

2 % cpy

75% QZ

Ik ep-py-po

sp-py

sp-py

I i

SD-DY-DO

Ii i

a Kbars

FIG. 9. A. Mole percent FeS n sphalerite istograms, racieladeposit, with discrimination or mineral assemblages: phalerite(sp), pyrite py), pyrrhotite po), chalcopyrite cpy), barite ba),and quartz, biotite, sericite, etc. (qz). B. Mean and standard e-viation star and bars, espectively f previous mole percent FeSpopulations alculated nto pressure kb) according o equation(1) of Lusk and Ford (1978). Sp-py-po: phalerites n mutualcontact with pyrite and hexagonal yrrhotite.

alternatively or reconstructed tratigraphic iles ofthe Casma Group. Average densities of 2.87 and2.78 g/cc have been reported for rocks from thewestern and eastern Casma facies, respectively(Bussell and Wilson, 1985). To account for the

average estimate of 2.6 kb obtained via sphaleritegeobarometry, ock columns on the order of 9.2 to9.5 km would be necessary. Thicknesses on theorder of 9 km are maximum estimates for the CasmaGroup on a regional scale Myers, 1974; Cobbing,1978), therefore, the mean value of 2.6 kb obtainedfrom the sphalerite compositions eems o be amaximum estimate.

Mineralogical pressure ndicators n contact au-reoles surrounding lutons rom the Coastal batho-lith indicate typical pressures etween 1 and 2 kb(Atherton and Brenchley, 1972).

Discussion of Results

Most of the salient features that characterize

Kuroko-type deposits n Japan (Horikoshi, 1969;Sato, 1974; Ohmoto and Skinner, 1983) are alsopresent in the volcanogenic barite and base metalsulfide deposits of the central coast of Peru. MariaTeresa, Aurora Augusta, and Juanita are volcanic-hosted deposits; Elenita and Palma are sediment-hosted. Feeder zones of siliceous stockwork andbreccia underlie strata-bound barite and massive

sulfide zones at Leonila-Graciela, Juanita (Fig. 7),and Mar{a Teresa. Feeder zones without overlyingstrata-bound ores are represented by the AuroraAugusta and Elenita deposits, which bear close re-semblance to deposits like Uwamuki 2 and 4 inJapan (Date and Tanimura, 1974; Bryndzia et al.,1983). No feeder zones are known so far under-neath the strata-bound deposits at Palma (Fig. 2).Massive sulfide beds are compositionally andedand locally exhibit clastic textures, compactionstructures, disruption of individual lenses, andsmall-scale folds indicative of soft sediment defor-mation as described n analogous apanese eposits(Ito et al., 1974; Hashiguchi, 1983); incompetentbehavior during subsequent olding enhanced heresulting disharmonic tructures Vidal, 1980).

The tectonic setting of a marginal basin duringMiddle Cretaceous times in Peru is similar to the

one that characterized he Miocene Kuroko stage ofJapan Tanimura et al., 1983). However, on palco-geographic terms, the submarine environment inPeru was predominantly f shallow-water haracterthroughout the Cretaceous whereas deep environ-ments are postulated or the formation of Kuroko-type deposits Guber and Merrill, 1983). The failedrift hypothesis of Cathies et al. (1983) seems o bein agreement with the tectonic interpretation pro-posed or the Middle Cretaceous marginal basin ofPeru by Atherton et al. (1983, 1985).

Differences noted between the Peruvian and Jap-anese Kuroko-type deposits nvolve the virtual ab-sence of associated ypsum nd erruginous hert nthe former. Pyritic gypsum masses re known onlyfrom the Chamodada prospect Fig. 4) where theyare rare and relatively small. Ferruginous chert

beds have not yet been found directly overlying heore deposits. Nevertheless, chert intercalations arepresent throughout the entire succession nd areespecially abundant n the limestone unit of Coca-chacra (Fig. 4). Taken together with the commonpresence of calcite in tuff breccias and in the bariteore, these facts indicate partial development anddeparture from the idealized Kuroko-type depositof Japan Eldridge et al., 1983). This could be theresult of differences, at the time of ore deposition,in seawater and rock geochemistry Ohmoto et al.,1983).

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1428 Ct$AR E. VIDAL C.

Contact metamorphism ecrystallized he Baldu-cho and Leonila-Graciela ores. Similar thermal ef-fects have been noted at Aurora Augusta nd MariaTeresa, although no intrusions are found in the im-mediate vicinity. Palma and Elenita are unmeta-morphosed. At least two periods of contact meta-

morphism affected Leonila-Graciela. Dating of thenearby plutons demonstrates heir Campanian 82m.y.) and Maestrichtian (65 m.y.) ages. Most de-posits are found in the zone of hornblende-hornfelsfacies, within biotite-muscovite or actinolite horn-felses Fig. 7). Several geothermometric arameterscoincide to indicate peak metamorphic tempera-tures n the range 300 and 500C. Pressure duringmetamorphism, s ndicated by sphalerite geobaro-metry, was 2.6 _+ 0.5 kb. However, this estimateseems o represent only a maximum value. Progradeenrichment of iron in sphalerite coexisting withpyrite and hexagonal pyrrhotite as here reported

(Fig. 8; Table 3) has been described or Canadianand Japanese eposits f Kuroko ype (Urabe, 1974;Scott, 1976).

At Leonila-Graciela, Juanita, and Santa Cecilia(Fig. 7) mineral textures are clearly indicative oftwo main sulfide assemblages. n early and domi-nant assemblage f pyrite-sphalerite, with pyrrho-tire in the metamorphic biotite-muscovite zone atGraciela, is veined, rimmed, and partly replaced bysubordinate chalcopyrite _+galena-tetrahedrite. As-sociated gangue minerals are barite and quartz-ser-icite in the early and late generations, espectively(Fig. 7E-H). Evidently, contact metamorphism e-

crystallized the Leonila-Graciela ores producinghexagonal pyrrhotite and new metamorphic tex-tures, such as porous pyrite crystals, nclusions ofsphalerite-galena-chalcopyrite-pyrrhotite n poikil-litic pyrite, and oriented chalcopyrite blebs insphalerite (Fig. 6A-D). However, the same para-genetic position of chalcopyrite was also observedin a specimen from the unmetamorphosed Palmaprospect. These indings are suggestive f thermallyintensifying egimes with late generations of chal-copyrite replacing earlier formed sphalerite, as de-scribed in Japanese Kuroko deposits (Eldridge etal., 1983).

Summary1. Kuroko-type deposits were generated n asso-

ciation with submarine volcanism of Cretaceous agein Peru and Ecuador. The geologic ecord has pre-served three main clusters of deposits n the Lima,Piura, and Quito regions.

2. Ore deposits nd prospects f Kuroko ype arefound in the Lima region associated with Albian-Cenomanian volcanism of the Casma Group. In ac-cord with this stratigraphic age span, K-At sericitedates of 106 and 116 m.y. have been obtained for

hypogene alteration zones n the Aurora Augustadeposit. The tectono-magmatic setting for theCasma sequence s one of a 1,000-km-long marginalbasin with predominantly basaltic o andesitic ill.The eastern facies of this volcanic belt is transitionalto a continental platform succession f limestonesand dolomites.

3. Felsic feeder complexes ncluding dacitic lavadomes, tuff breccias, and zones of intense hydro-thermal silicification are present in the lower partsof the deposits under question. Low-grade stock-work zones and high-grade strata-bound barite andmassive sulfides are distinctly mappable ore types.In accord with the genesis proposed or the Mio-cene Kuroko deposits of Japan, t is concluded hatthe Peruvian ores were deposited both on the seafloor and directly underneath t.

4. Subsequent geologic evolution for most ofthese deposits nvolved burial, uplift, folding, and

contact metamorphism during Upper Cretaceoustime. Renewed folding, faulting, and dike intrusionfollowed n Paleocene o Oligocene imes.

5. The largest and best known deposits n theregion are Leonila-Graciela, Juanita, and Santa Ce-cilia; they are found 50 km east of Lima in the Co-cachacra oof pendant. Leonila-Graciela and Juanitarepresent wo discrete centers of explosive volcan-ism and exhalative hydrothermal activity; Santa Ce-cilia is a faulted portion of the eastern barite ore-body at Juanita. Past production and reserves or allthree deposits are estimated to be 4 million tonsbarite and 2.5 million tons of high-grade Zn-(Pb-Ag)

sulfides.6. The Cocachacra ores were contact metamor-

phosed o hornblende-hornfels acies by the Ri-cardo Palma tonalite and the Canchacaylla monzo-granite, which were emplaced about 82 and 65 m.y.ago. Estimates of pressure nd temperature or themetamorphic peak are 2.6 _+ 0.5 kb on the basis ofsphalerite geobarometry and 400 _+ 100C as in-dicated by several ines of evidence. Postmetamor-phic dikes were emplaced between 39 m.y. and31 m.y.

7. Geobarometric estimates computed frommole percent FeS data in sphalerites intergrown

with hexagonal pyrrhotite and pyrite are relativelyhigh compared to estimates derived from strati-graphic econstruction nd o contact aureoles else-where in the region. Thus, pressure ranges calcu-lated by sphalerite geobarometry or the Gracieladeposit are quoted only as maximum values.

AcknowledgmentsThe Alexander von Humboldt Foundation pro-

vided financial support for K-Ar dating in Londonand microprobe analysis n Heidelberg. would iketo thank Christian Amstutz who made t possible or

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me to work during 1985 to 1986 at the Mineralogicand Petrographic nstitute, Heidelberg University.Norman Snelling, of the British Geological Survey,allowed me to use he geochronological acilities;am particularly grateful to him and to Chris Rundlefor their time and effort in introducing me to the

methods of K-Ar dating.Colleagues A. Wauschkuhn, K. and M. Gunnesch,L. Fontbot6, S. Schmidt, and W. Zimmerninck atHeidelberg helped and advised n the mineral sepa-ration and microprobe analysis procedures. Thepresent nvestigations re based on research carriedout in the late 70s under a British Council scholar-ship n the Geology Department at Liverpool Uni-versity. Wallace Pitcher deserves many thanks andcredit for introducing me to the regional setting andfor his careful supervision oth in the field and abo-ratory stages.

Minera Barmine S. A., Minera Cecibar S. A., and

Perubar S. A. authorized visits to the pertinentmines. Personal hanks are expressed o BaldomeroRodriguez, proxy manager nd director, and MiguelMontestruque, mining manager of the latter com-pany, for arranging ogistic support and providinginformation.

Buenaventura ngenieros S. A. provided a year'sleave of absence. particularly wish to thank Al-berto Benavides Q., who made the leave of absencepossible, and who otherwise greatly facilitated thestudy. Back at the Universidad Nacional de Ingen-ieria, Maria Jesfs Ojeda has continually upportedmy research activities. Finally, I thank my dearwife, Norma Luz, who through he years has had tocope with our travels and my enthusiasm or geol-ogy. Her moral support and good humor will alwaysprovide comfort and inspiration.

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