1987 *vidal, c.e

ECONOMIC GEOLOGY AND THE

BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

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

Kuroko-Type Deposits in the Middle Cretaceous Marginal Basin of Central Peru

CI•SAR E. VIDAL C.*

Buenaventura Ingenieros S. A., Larrabure y Unanue 146, Lima 1, Perth, and Depa•-tamento de Geolog[a, Universidad National de b, genie•a, T•pac Amaru s/n, Lima 31, Pert•

Abstract

Barite, massive sulfide, and siliceous stockwork deposits of Kuroko type in the Lima region are associated with the Casma Group, a seqnence of submarine volcanic rocks of Middle Cretaceous age. These deposits were formed in an ensialic marginal basin with predominantly basaltic to andesitic fill. Volcanic-hosted deposits occur iu the entire region; sediment-hosted deposits are restricted to the eastern Casma volcanic facies, which inter- calate with limestones and shales deposited on a shelf platform adjacent to the marginal basin. In most cases, mineralization is spatially associated with dacitic domes and tuff breccias with zones of quartz-serieite alteration; the latter have locally been dated at 116 to 106 m.y. by the K/Ar method. Strata-bound deposits of bedded barite, pyrite, sphalerite, and pyrrhotite overlie the fbeder zones.

The most important deposits of this kind are Leonila-Graciela and Juanira, with 4 million tons of produced barite and 2.5 million tons of production plus reserves of massive sulfide ore. They are located in a roof pendant of folded strata intrnded by two plutons of the Coastal batholith. Contact metamorphism of hornblende-hornfels facies affects both ore deposits and wall rocks. K-Ar ages on hornblende-biotite pairs from the granitic rocks indicate that they were eraplaced 82 and 65 m.y. ago. Whole-rock ages on postmetamorphic dikes vary between 31 and 39 m.y.

P-T conditions for contact metamorphism of hornblende-hornfels facies at Leonila-Gra- ciela are estimated at 2.1 to 2.6 kb and 300 ø to 500øC on the basis of sphalerite geobarometry, stratigraphic reconstruction, metamorphic mineralogy, and interpretation of discordant K/Ar age patterns. Mole percent FeS in sphalerites increases in a prograde sense from the actinolite zone at Juanira to the biotite-mnscovite zone at Gracicla. In massive sulfide specimens it varies correspondingly from 15.4 _ñ 0.2 to 17.6 _4- 0.7. Sphalerites from siliceous stockworks show the same trend with 14.7 _4- 0.4 mole percent FeS and 17.6 ñ 1.1 mole percent FeS. Metmnorphic equilibration was reached only in the biotite-muscovite zone at Graciela. This is demonstrated by the hmnogeneity of high mole percent FeS values detected in sphalerite, which coexists in nmtual contact with pyrite and hexagonal pyrrho- tire.

Introduction

IN the southern part of the coastal region of Peru Cu, Mo, Au, aud Fe have long been mined h'om vein-, replacement- aud porphyry-type deposits (Bellido and De Montreuil, 1972). In contrast, there has been little mining and exploration in the central and northern coastal regions. Nevertheless, as a consequence of intensil•zing petroleum exploration, barite deposits were discovered during the early 1950s. Discoveries in the Lima and Piura areas

were given special attention because of their conve- * Present address: Perubar S. A., Juan de Arona 830 go, Lima

27, Perfl.

nient geographical location near the Callao port and the Talara oil fields, respectively. Barite mining in several of the properties surrounding Lima has re- vealed massive sulfide zones with Zn-(Pb-Ag) ores, such as in Leonila-Graciela (Fig. 1).

A cluster of barite +_ massive sulfide deposits occurs within a semicircle of a 50-km radius cen-

tered on Liana, l•osted by submarine Cretaceous w•lc, anic rocks. It is the purpose of this paper to describe the geologic setting and nature of these deposits. Their genesis and subsequent evolution is discussed in the light of recent geologic studies (Vidal, 1980), coupled with K-At dating and micro-

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probe analyses of sphalerites. A second cluster of similar occurrences east of Piura is linked to Upper Jurassic-Cretaceous volcanic rocks in northern Peru. The Tambo Grande deposit is the most important representative of this region (Injoque et al., 1979; Llosa, 1979; Fig. 1). It was drilled out at a prefeasibility stage by the Bureau de Recherches G(•ologiques et MiniSres in 1978-1980. Drill-indicated reserves are 40 million tons of pyritic massive sulfide ore with high-grade concentrations of Cu-Zn-(Ag) ores. The near-surface, strata-bound character and the lensoid shape of this sulfide mass are its main structural features. Barite zones, siliceous sulfide ore, and hematite chert beds have also

been reported from Tambo Grande and the nearby prospects of Potrobayo, Totoral, and Morrop6n (Fig. 1). These geologic features and the vertical zoning in the deposits are very similar to those of the Japanese Kuroko deposits.

The northernmost region where deposits of this kind have been discovered lies southwest of Quito in Ecuador. Unpublished information from C. W. Farrell (1978) was available to the author concern- ing the Kuroko-type deposits at La Plata and Macu- chi. The La Plata deposit was evaluated in the 1950s by Sotopaxi Exploration Company and in 1961-1965 by Duncan Derry Exploration. From 1975 to 1982, Cia. Minera Toachi S.A., a joint ven- ture of Ecuadorian claim owners with Outokumpu Oy, Metallgesellschaft A.G., and Cia. de Minas Buenaventura S.A., operated the property on a small scale. Reserves totaled 200,000 tons with 5.7 wt percent Cu, 4.8 wt percent Zn, 3.6 ppm Au, and 44.8 ppm Ag. The deposit is characterized by strata-bound lenticular orebodies of bedded massive sulfides and barite, with underlying, low-grade disseminations. The former are located along the contact between two distinct volcanic successions of Cretaceous age. The basal sequence consists of variably silicified pyroclastic rocks; the overlying sequence consists of basaltic lavas with minor hematite breccia and tuffs. The sequence is folded and faulted, making the orebodies discontinuous.

The regional extent of volcanogenic massive sulfide and barite deposits of Kuroko type has not been recognized previously in the Mesozoic record of the central Andes. They represent an important group of polymetallic ore deposits omitted in the most recent metallogenetic synthesis proposed for the region (Clark et al., 1976; Ericksen, 1976; Putzer, 1976; Sillitoe, 1976; Amstutz, 1978; Petersen, 1979; Frutos, 1982).

Exploration and Mining History Barite was first explored and mined in the coastal

region of central Peru in 1948 by the Peruvian Chemical Industry Company (later renamed the Barmine Company and Minera Barmine S.A.). Dur- ing the next 20 years about 500,000 tons of barite ore was mined by underground methods from the Leonila-Graciela orebody (Fig. 1). The Graciela claim was controlled throughout those years by the International Petroleum Company and was later sold to National Lead Industries, Inc.

From 1968 through 1980, barite production was progressively increased by means of an open-pit operation. From 1976 to 1979, it reached a maximum of 1,000 tons per day, representing one of the largest producers in the world (Martino, 1981). The barite ore had a high specific gravity (4.2-4.4 g/cc)

KUROKO-TYPE DEPOSITS, CENTRAL PERU 1411

and barium sulfate contents of over 80 wt percent; soluble salts varied from 60 ppm in Leonila to 300 ppm in Graciela. The prospects of Balducho, Palma, Mar•a Teresa, and Elenita were discovered during this period (Fig. 2). In Graciela, massive sulfide ores with high-grade concentrations of Zn were discovered and evaluated by drilling. Similar discoveries of massive sulfides were made at Juanita by Perubar S.A., a subsidiary of National Lead Industries, Inc. Geologic mapping of the region, the Graciela open pit, and from the Elenita and Santa Cecilia mines has been carried out by the author intermittently since 1975.

Flotation plants for Zn and Pb-Ag concentrates were put into operation in 1980 and 1983, coincid- ing with a sharp decline in the production of barite. Average head grades in 1985 vary from 10 to 14 wt percent Zn, 0.5 to 1 wt percent Pb, and 15 to 45 ppm 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 of high-grade sulfides are estimated at 2.5 million tons. Lower grade stockwork zones are not included in the latter estimate.

Regional Geology and Tectonic Setting

In the central coast of Peru marine sedimentary and volcanic strata of predominantly Lower Creta- ceous age were intruded during Upper Cretaceous to Paleocene times by the Coastal batholith (Fig. 2). Following the main stages of batholithic eraplace- ment the region was uplifted, peneplained, and cov- ered by a thick sequence of subaerial volcanic rocks known as the Calipuy Group. Deposition of the marine sedimentary and volcanic rocks took place within the eugeosynclinal zone of the West Peru- vian trough (Wilson, 1963; Cobbing, 1976), which has been recently interpreted as an ensialic marginal basin (Atherton et al., 1983). Three main tectono-stratigraphic units have been recognized in the latter sequence: the Morro Solar Group, the Pamplona and Atocongo Formations, and the Casma Group. A brief account of this stratigraphic succession follows.

Stratigraphy

Morro Solar Group: The Puente Piedra Forma- tion, consisting of 2,000 m of basaltic pillow lavas and water-lain tuffs intercalated with fossiliferous marly shales and limestone lenses of Berriasian age, forms the lowermost part of the Morro Solar Group. Thinning of individual lava flows toward the east indicates that their feeders lay to the west (Rivera, 1951). The main outcrop of these units lies in the Chil16n River area along the coast to the north of Lima; correlatable sequences are known from the

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

Conformably overlying the Puente Piedra For- mation are sandstones and shales of the Salto del

Fraile, Herradura, and Marcavilca Formations. More than 500 m of deltaic and fiuviatile clastic

strata alternate from the predominantly shaly base to the quartzite top of these formations (Fern/mdez Concha, 1948). Rosenzweig (1953) and Wilson (1963) have concluded that these clastic strata were deposited within a closed basin isolated from the ocean by positive lands to the west, from which the sediments were largely derived.

Pamplona and Atocongo Formations: Thinly bedded, fossiliferous marls and limestone beds, which become progressively thicker higher up in the sequence, characterize the 1,200-m-thick lithologic succession of the Pamplona and Atocongo Forma- tions. The basal contact with the Marcavilca quart- zites is transitional and consists of sandy limestone beds. These formations represent a transgressive sequence of carbonate strata underlying the volcanic rocks of the Casma Group. Their fossil assemblages are devoid of Tethyan key fossils. Both the Morro Solar clastics and the Pamplona-Atocongo carbonates are characterized by provincial faunas of Valanginian to Aptian age (Rivera et al., 1975). The peculiar fauna is a further indication of the isolated character of the basin.

Casma Group: On a regional scale, the Casma Group, as originally proposed by Myers (1974), en- compasses 6,000 to 9,000 m of submarine volcanic and interbedded sedimentary rocks that have locally been subdivided into several formations. In the coastal region of central Peru, Albian ammon- ites have been found at the base of the sequence (Wilson, 1963) and Cenomanian fossils have tenta- tively been identified from higher up in the succession (Guevara, 1978). Two individual basins have been delineated along the entire 1,000-km length of this volcanic belt; these are the Huarmey and Rio Ca•ete basins of Cobbing (1978), which had their main periods of subsidence during Albian and Neo- comian to Albian times, respectively (Fig. 1). The region between the Rhnac and Lur•n valleys is sup- posed to represent the area of interconnection between the two basins (Fig. 2); however, no detailed account of the transition is available.

A well-defined west to east facies change charac- terizes the Casma Group in the central coast of Peru. The western facies consists of basaltic to an-

desitic lavas, tuffs, and hyaloclastic breccias plus sporadic sedimentary intercalations with measured thicknesses on the order of 2,500 m. The eastern facies, as exposed in several roof pendants and valleys east of the batholith, is characterized by a mixed succession of andesitic to dacitic lavas, tuffs,

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and volcaniclastic sandstones intercalated with lenses of dark calcareous mudstone, shale, and im- pure limestone. The volcanic rocks of the Casma Group are coeval with a sequence of shelf limestones that crop out to the east of the area shown in Figure 2; this lithological polarity has been interpreted as representing a classic pair of eugeosynclinal and miogeosynclinal troughs.

Structural and igneous history

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

Mala areas (Fig. 2). Open anticlines with tighter upright synclines and subhorizontal fold axes are also 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 during Middle Cretaceous times. This folding event correlates with the sub-Hercynian phase of deformations defined in northern Chile and is currently referred to as the Mochica phase (Cobbing, 1985).

Belts of tighter folds and overall stronger deformation occur immediately to the east of the batholith in the Rimac, Lurln, and Mala valleys. Both the lithological spectrum and the deformation style of the eastern Casma Group facies are similar to the Huayllapampa Group of Myers (1974). These features can be explained in terms of sedimentation, volcanism, and folding, both during Middle Creta- ceous and Paleocene Incaic phases, being controlled by a hinge line. In fact, regional Andean and Andean-normal faults to the east of the Coastal

batholith (Fig. 2) were probably generated along such a deep basement structure. Major northeast- southwest-trending dextral wrench faults are known from the eastern sections of the Rimac and

Omas valleys. An important vertical reactivation of Tertiary age has been recorded for the Agua Salada fault in the Rimac valley (Fig. 4); this fault drops the Casma-Calipuy unconformity a minimum of 800 m to the east.

The Coastal batholith is a multiple and composite belt of plutons with an overall trend from gabbro and diorite to tonalitc and granodiorite. Figure 2 depicts only the general outcrop pattern of this complex batholith which has been subdivided into the Lima and Arequipa segments to the north and south of the Lurln River, respectively (Pitcher, 1978; Pitcher et al., 1985). Radiometric dating of various plutons in the region has given an age spectrum of 104 to 62 m.y. (Beckinsale et al., 1985; this paper). Plutons of the batholith contact metamor- phose the deposits at Leonila-Graciela, Cantera, and Balducho (Fig. 2).

Post-Incaic volcanism was subaerial and gave rise to the dacitic and rhyolitic ignimbrites of the Cali- puy Group. Felsic lava flows, agglomerates, and la- pilli tuffs are present toward the base of this 2,000- m-thick volcanic pile in the R•mac and LurCh sections. Minor intercalations of basaltic flows and subaqueous sediments also occur. The base of the Calipuy Group in the region has been dated at 41 m.y. by Noble et al. (1979).

Tectonic setting

The paleontology and sedimentology of the Lower Cretaceous sequence show that the basins were developed within an isolated inland sea along the continental margin. The source areas of the shallow-water sedimentary and volcanic rocks were located predominantly to the west. Myers (1975) suggested that the Precambrian to Paleozoic Are- quipa massif has a northward extension that was relatively uplifted during this time.

In contrast, during Albian to Cenomanian times the Casma volcanics were extruded into rapidly subsiding basins. Volcanism consisted of fissure eruptions of basalt in the western basin (Atherton et al., 1985; Pitcher and Bussell, 1985). To the east, the Casma volcanics interdigitate with progressively increasing amounts of sedimentary rocks. Volcanic centers in the form of felsic lava domes and

tuffbreccias have been recognized both in the western and eastern facies (Vidal, 1980). The entire setting is one of a marginal basin, in which a belt of new crust was generated by submarine basaltic volcanism presumably during a period of crustal extension and low rates of sea-floor spreading. Confir- mation for this tectonic regime comes from the studies of burial metamorphism, which indicate the presence of high geothermal gradients (Aguirre and Offier, 1985).

Furthermore, the gravimetric and seismic profiles presented by Jones (1981), Couch et al. (1981), and Bussell and Wilson (198.5) show a mass of high density and low velocity rocks beneath the volcanic basins. This arclike crustal anomaly disap- pears north of the Huarmey basin and south of the Rio Cafiete basin. It is interpreted to consist of mafic rocks in the form of upthrusted oceanic crust or basic intrusion complexes (Wilson, 1985). The latter interpretation is favored considering the abundance of gabbro putohs along the entire belt, which are partly coeval with the Casma volcanics (Regan, 1985). Isotopic signatures of the gabbros and later granitic rocks of the Coastal batholith are indicative of a mantle provenance with contamina- tion by Precambrian crust only south of the LurCh River (Mukasa and Tilton, 1985; Fig. 2).

Vertical block tectonics and ensialic character are

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FIG. 4. Geology and structural profile AA' from the Cocachacra roof pendant with K-Ar sample location and mineral deposits of Kuroko type: Leonila-Graciela (1), Juanita (2), Santa Cecilia (3), Chamodada (4) and Elenita (5). Lithological units shown are the Ricardo Palma tonalitc (a), the Can- chacaylla monzogranite (b), unassigned minor stocks (c), submarine volcaniclastics and lavas (d), limestone key bed (e), tuff breccias (f), subaerial tuffs and agglomerates (g), and recent fluvio-aluvial deposits (h). Central inset shown in Figure 7.

the main regional features that dominated sedimentation, faunal developments, and volcanism within a submarine trough of extensional nature. Following

this Lower Cretaceous stage, batholithic emplacement, deformation, uplift and subaerial volcanism were controlled by major Andean trending faults

FIG. 3. Field and hand specimen views from the Leonila-Graciela deposit. A. Graciela open pit looking south from 1320 NE bench; in the background, Santa Cecilia mine and mill. B. Concordant contact between metavolcanic footwall and barite zone, 1260 bench looking north (at the bottom of hammer handle). C. Hand specimen showing banded barite with pyrite intercalatlons. D. Massive sulfide zone with near-vertical beds in the southwest side of the Graciela open pit, 1220 level. E. Massive sulfides from same locality as in D, showing lenses and delicate intercalation of barite (white). F. Hand specimen from massive sulfides. Note coarse-grained nature and banded barite. Sulfides shown are sphalerite (black) and pyrrhotite (gray). G. Banded hand specimen from massive sulfides, Palma prospect. Note delicate compositional banding, barite load cast, and fine grain. Sulfides present are sphalerite, pyrite, and pyrrhotite.


and lineaments as discussed by Myers (1974) and Pitcher and Cobbing (1985).

Mineral Deposits

The region shown in Figure 2 is characterized by three different types of metallic mineral deposits. These are: (1) the granite-hosted Cu veins at Cumias and Bosa Maria mines directly north of the Omas Biver, (2) the volcanic- and sediment-hosted amphibolitic Cu mantos and veins of the Bafil and Condestable deposits east of Mala, and (3) volcanichosted, strata-bound barite and Zn-Fe-(Pb-Ag) sulfide deposits of Kuroko type (Vidal, 1980). Deposits of the latter group are present through the entire region; their main representatives are the Leonila- Graeiela and Juanira deposits 50 km east of Lima. Taken as a group, they have most of the geologic elements that characterize the Japanese Kuroko- type deposits as will be described and discussed in subsequent parts of this paper.

Mar(a Teresa

This deposit is located 6 km west of Huaral (Fig. 2). It has been explored intermittently and mined for barite and Pb-Ag ores on a small scale since 1973. Two inclined adits, surface trenches, and dia- mond drill cores were available for study. Several strata-bound barite lenses as much as 12 m thick crop out for a distance of 250 m within beds of variably altered felsic tuff. These tuffs are underlain and partly disrupted by irregularly shaped and locally brecciated bodies of silicified rock. Abundant pyrite and traces of galena, sphalerite, and chalcopyrite are present. Primary sulfide bodies, on the order of 35,000 tons, average œ10 ppm Ag, 2.2 wt percent Pb, 0.1 wt percent Zn, and 0.03 wt percent Cu.

Barite occurs as monomineralic lenses and pods within argillically altered and partly silicified tuffs. They are typically massive or banded on a centime- ter scale. Banding reflects contrasts in grain size, recrystallization fabric, and features induced by weathering such as porosity or oxidation stains. Lenses of pyrite boxwork locally parallel the banded barite.

Zones of silicification are fracture controlled along their sides and base; however, their tops occur almost parallel to bedding and directly underneath the barite horizon. In places interlocking blocks of volcanic country rocks can be recognized; such breccia zones are conspicuously silicified by equigranular quartz with sericite and stockwork sulfides. Late swarms of veinlets of chalcedony with sericite, pyrite, jarosite, and chlorite are exposed in the underground workings.

The volcanic sequence that overlies the mineralized zone consists of amygdaloidal basaltic lavas and hyaloclastic breccias. Slight metamorphic recrystallization has taken place, as indicated by patches of biotite and quartz-epidote veinlets. Regional patterns of burial metamorphism might have con- curred with contact aureole effects related to younger granitic plutons of the Jecu/m type (Pitcher et al., 1985). JecuSn tonalites are known from œ km to the northwest of the mining area.

Aurora Augusta

The Aurora Augusta deposit is located 1.5 km west of Jicamarca gorge, about 20 km northwest of its confluence with the R•mac valley. From 1975, at least 150,000 tons of barite ore has been produced; polymetallic sulfide zones are currently under exploration. Both ore types are found in irregularly shaped upright bodies within a strongly silicified funnel in volcanic rocks of the Casma Group. The nearest granitic rocks are 2 km to the northwest and to the east of the deposit, respectively; they belong to the Santa Rosa superunit of the batholith and intrude unaltered and unmineralized Casma vol-

canic rocks (Fig. 2). Andesitic volcaniclastic rocks interbedded with

vesicular lavas dipping moderately to the southwest form the hanging wall of the mineralized zone. Finely bedded tuffs and limy shales occur sporadi- cally. Calc-silicate minerals are found in some of the latter limy horizons. The contact between the hanging-wall volcanics and the mineralized zone is controlled by bedding; it is grossly concordant and abrupt. Quartz-epidote veinlets and vug fillings are found directly above the silicified body, which is cut by postore andesitc dikes.

The mineralized complex is subcircular in plan, with an exposed diameter of about 80 m. Away from its roughly strata-bound and concordant roof, clear intrusive features are found toward the eastern side of the steep margin. Interlocking blocks of volcanic country rocks define breccia zones, within which postsilicification pebble-breccia dikes are found locally (Fig. 5G and H). The matrix is composed of finely comminuted volcanic material with irregular zones of intense silicification where tabular bodies of barite and/or sulfides appear. Tabular barite bodies occur throughout the complex but are con- centrated toward its top. Here, the largest ones reach 8 m across and 70 m along strike and have a known vertical extent on the order of 50 m. Barite ore is fine grained, equigranular, and essentially monomineralic; large megacrysts are present in only a few places. The transition to the zone of silicification is marked by increasing amounts of quartz, jarosite, and pyrite (Fig. 5F).


Embayed aggregates and single phenocrysts of plagioclase with recrystallized margins are commonly observed in thin sections in the zone of silicification. Phenocrysts and volcanic clasts are en- closed in a granoblastic polygonal groundmass of quartz and are partly replaced by sericite. Sericite and chlorite are also found in the siliceous groundmass with sulfide minerals (Fig. 6G and H). Sulfide- rich zones are invariably associated with intense silicification. An early generation of sphalerite and pyrite is veined and partly replaced by chalcopyrite-quartz-sericite assemblages (Fig. 6F, G, and H).

Leonila-Graciela

Leonila-Graciela, by far the most important mining district of its kind, is located 50 km to the east of Lima in the R•mac valley (Figs. 3 and 4). A detailed account of the geology and mineralogy of these deposits 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 recumbent anticline of Juanita. Past production and reserves are on the order of 4 million tons of barite and 2.5

million tons of high-grade Zn-(Pb-Ag) ore. Sporadic enrichments of Cu-(Au) have been detected in the so-far undeveloped stockwork zones. The eastward continuation of the Juanita orebody has been tec- tonically offset and disrupted along the dextral Corte de Ladrones fault; it is mined separately and referred to as the Santa Cecilia orebody (Fig. 7). Siliceous stockwork and breccia zones are also

known from the Chamodada and Elenita mines (Fig. 4); at Elenita, polymetallic sulfide ore averages 7.0 wt percent Zn, 1.5 to 3.0 wt percent Pb, and 100 to 130 ppm Ag.

All these mines and prospects are located in the Cocachacra roof pendant. Eastern Casma facies have a minimum thickness of 600 m; they consist of submarine volcaniclastics, lava flows, and tuff breccias with an intercalation of limestone and marl. The

sequence has been folded into relatively tight, An- dean-trending, and northwest-plunging anticlines and synclines. Structural analysis of the banded ores demonstrates a locally disharmonic attitude with overall congruence at regional scale (Vidal, 1980). Repeated intrusion by at least two separate units of the Upper Cretaceous Coastal batholith has been recorded. K-Ar dating indicates that the Ricardo Palma tonalitc was emplaced 82 m.y. ago and that the Canchacaylla monzogranite is 65 m.y. old (Fig. 4, Table 1). Contact metamorphic aureoles have been developed adjacent to these intrusions and 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 volcanic rocks and the Coastal batholith. In the Coca-

chacra area, the Calipuy Group is barren and consists of a monotonous sequence of agglomerate and ash-flow tuff about 1,200 m thick.

Massive sulfide zones have been found in the

Graciela open pit and in the underground workings at Juanita and Santa Cecilia (Fig. 7). Their internal structure is banded, showing sphalerite and pyrite as the main hypogene constituents. Minor phases are galena, tetrahedrite, chalcopyrite, pyrrhotite, and barite; traces of jamesonitc, bornitc, mackina- witc, molybdenite, and magnetite have also been observed. Grain size is coarse and textures are me-

tamorphic (Fig. 6). granoblastic to lepidoblastic in- tergrowths of sulfides and barite are common. Pyr- rhotite is markedly more abundant in the Leonila- Graciela deposit; X-ray diffractograms give single but moderately asymmetric (102) peaks, indicating its hexagonal character. Microprobe analysis and etching with saturated chromic acid confirm this finding (Fig. 6A and B).

Although obscured by metamorphic recrystallization, the paragenetic sequence involves early barite, sphalerite, and pyrite which are embayed, rimmed, and veined by quartz-chalcopyrite-galena +_ tetrahedrite (Fig. 6C and E). Most sphalerite grains contain blebs of chalcopyrite, and in the Gra- ciela specimen, uncommon pyrrhotite. The Juanita and Santa Cecilia sphalerites commonly lack exso- lution textures. Pyrite is commonly even grained and develops a granoblastic mozaic texture (Fig. 6D); inclusions of chalcopyrite, galena, and sphalerite are rare.

Massive sulfide zones are underlain by irregular masses of siliceous stockwork developed mainly as replacements of dacitic lavas. Such is the case of the stockwork zone southwest of Graciela and the zone

that rims the Juanita orebody in the 1,200-m level (Fig. 7). Two different types of stockwork zones have been recognized; the most common type consists of a quartz-sericite-chlorite matrix with veinlets and disseminations of pyrite, sphalerite, chalcopyrite, galena, tetrahedrite, and native Au (Fig. 6E). The second type consists of pyrite-rich tetrahedrite disseminations in siliceous microbreccias such as those found in the 1330 bench of the Leonila de-

posit. Barite beds typically overlie the massive sulfide

zones at Leonila-Graciela and Juanita; the Graciela barite zone also overlies metasedimentary and metavolcanic rocks (Fig. 3B). The barite beds, con- taining more than 80 percent barite forming a structureless granoblastic mozaic, are intercalated with lenses of calcite and pyrite-sphalerite; the barite zone is banded and has verv abrupt contacts with both the massive sulfides and the metasedimentary

1418 cgsAa E. VIDAL C.

*** ba

sp

.,..., 4 cm

KUROKO- TYPE DEPOSITS, CENTRAL PERU 1419

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

tercalated with porphyroblastic barite (Fig. 5C and D). The contact metamorphic overprinting of Leonila-Graciela will be described and discussed in more detail later.

Palma

The prospect of Palma is located 3 km south of the Lur•n valley in the Palma gorge. It has never been mined and is currently incompletely explored. A thin and layered barite sulfide lens is traceable for more than 100 m, reaching a maximum thickness of 3 m. It is zoned northward from a barite zone into a

massive sulfide bed (Fig. 3G). Preliminary assays indicate an average of 13 wt percent Zn, 2.4 wt percent Pb, and 45 ppm Ag. Black pyritic shales directly underlie the ore horizon. As in the Coca- chacra roof-pendant, at Palma the eastern facies of the Casma Group consists of limestone and shale intercalations in volcaniclastic sandstones, lavas, and breccias.

Although the folds in the host rocks have not been mapped, major NW-trending open folds are present. The ore horizon is located in the axial hinge zone of an anticline. The anticline core exposed consists of black shale with framboidal pyrite and delicate structures indicative of soft-sediment

deformations (Vidal, 1980). The western limb is made up of a bed with massive pyrite, sphalerite, pyrrhotite and chalcopyrite, which in turn is over- lain by argillaceous limestones. Barite lenses increase in size and number toward the hinge zone and the eastern limb of the anticline. Disharmonic contortions of the massive sulfide bed resemble those 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 intermittently in the past several years. Strata-bound lenses of barite and pyrite-sphalerite occur in upright position within a contact metamorphosed septa of spotted slate and hornfelsic graywacke. Maximum width of the septa is 300 m and, therefore, could not be shown in Fig- ure 2. Siliceous stockwork zones with chalcopyrite are adjacent to the strata-bound ores on the east.

Barite textures are granoblastic and locally cataclas- tic; the richest barite zone is located along the northeastern wall.

Local intrusives of the Tiabaya superunit (Pitcher et al., 1985) are tonalitic to granodioritic in composition and are accompanied by porphyritic felsic dikes. Contact metamorphic effects include the coarse granoblastic textures of the ores and the hornfelsic nature of the country rocks.

Cantera

The prospect of Cantera is located 8 km northeast of Mala (Fig. 2). A small and stratiform barite-pyrite-(calcite) mass is hosted by limy shales and sandstones that are intercalated within lava flows and

volcaniclastics toward the base of the Casma Group. A zone of incipient silicification is developed underneath the barite horizon. Barite is of very coarse grain and exhibits replacement structures in the surrounding shales and limy sandstones. The ore and alteration showings are relatively small and have not encouraged further exploration.

K-Ar Determinations

A K-Ar dating program was designed to unravel the time of formation of the Aurora Augusta deposit and the subsequent contact metamorphic overprint recorded at the Leonila-Graciela deposits. By means of thin section evaluation of supergene effects, nine out of twelve samples collected were determined to be suitable for K-Ar dating.

Samples AA2 and AA3 come from quartz-sericite-chlorite alteration halos that surround the

stockwork orebodies at Aurora Augusta (Fig. 2). Samples CHP, SRX, INC, and E2 represent fresh holocrystalline plutonic rocks from two different intrusions that contact metamorphosed the ores at Leonila-Graciela (Figs. 2 and 4). Three samples from postmetamorphic dikes--J1238, G10, and G2 from Leonila-Graciela--were also dated. All the

age determinations were carried out in the Isotope Geology Unit of the British Geological Survey in London.

Analytical procedures

Sericite-quartz concentrates were prepared from samples AA2 and AA3 by heavy liquid and hand-

FIG. 5. Microphotographs from thin sections: Graciela (A, B, C, D, and E) and Aurora Augusta (F, G, and H). A. Granoblastic barite. Parallel nicois. B. Lepidoblastic intergrowth of barite (ba) and sphalerite (sp). Crossed nicols. C. Calc-silicate band (gray to black) intercalated with barite (ba) and pyrite (opaque in barite). Parallel nicols. D. Enlargement of C showing garnets (gt) and epidote-diopside (shades of gray) in barite aggregate (ba). Crossed nicols. E. Quartz-sericite (qz-src) alteration in siliceous stockwork; relic plagioclase lath (pl). Parallel nicols. F. Barite (ba) zone with pyrite (py) and jarosite (jar), fine-grained siliceous matrix. Crossed nicols. G. Pebbles of quartz aggregates (qz) and minor amounts of pyrite (py) in comminuted matrix with quartz-sericite (qz-src). Crossed nicols. H. Ptygmatic veinlets and pebbles of quartz-pyrite-(barite) in matrix as in G. Crossed nicols.


•po

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; 8mm •

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! :

...

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... .. .

0. 2 mm

D

_I

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0.2 mm: :

•p

,.0.2 mm ... t

cpy •

U

sp

--¾

p¾

qz

].

'0.2 mm

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


picking methods. X-ray diffractograms were used to confirm the presence of sericite and to determine its percentage. Peak-height ratios of sericite (002) vs. quartz (100) reflections were 0.16 for sample AA2 and 1.23 for sample AA3.

High purity biotite and hornblende concentrates were prepared for samples SRX, CHP, and INC by standard electromagnetic and heavy liquid separa- tion techniques. Analyses of the remaining samples were performed on -60 to +120 mesh whole-rock powders.

Potassium analyses were carried out on an Instru- mentation Laboratories 543 flame photometer, using Li as an internal standard. Each sample was analyzed in duplicate; additional analyses were carried out only for samples that gave results with more than one percent difference. Average results were used for the age calculations. Argon determinations were performed by the isotope dilution technique on a Micromass 1200 mass spectrometer. Duplicate Ar analyses were performed only for the sericite concentrates. The decay and abundance 4øK/KTotal constants recommended by Steiger and JSger (1977) were used.

Results and interpretation

The results of the K-Ar analyses are presente d in Table 1. Discordant ages were obtained on sericite concentrates from the Aurora Augusta deposit, namely 106 _ 39 and 116 4- 18 m.y. for sample AA2, compared to 68 4- 2 and 63 4- 2 m.y. for sample AA3. Samples AA2 and AA3 were found to contain 2.93 wt percent and 5.77 wt percent K, respectively, thus indicating sericite concentrations of at least 35 and 70 percent. Large analytical uncertain- ties in the ages for sample AA2 are due to the high proportions of atmospheric 4øAr. Spontaneous Ar release was noted prior to fusion of this sample. Mass spectrometric scans found no indications of possible interferences from organic matter. The absence of zeolites and/or additional potassium-bearing phases which could also have interfered with the emission of Ar from the sericites was confirmed by X-ray diffraction methods. It is believed that fluid inclusions observed in the quartz intergrown

with sericite in sample AA2 were responsible for the anomalously high proportion of atmospheric 4øAr and its spontaneous evolution prior to fusion. However, Ar released from these fluid inclusions could also be partly of either radiogenic nature, formed from K + ions in solution, or excess argon-- that is, 4øAr without 36Ar in atmospheric proportion --derived from older rocks that were attacked by the hydrothermal solutions (D.C. Noble, pers. commun.).

Samples CHP and SRX from the northeastern margin and core, respectively, of the Ricardo Palma tonalitc also have discordant ages. Almost identical hornblende ages of 82 4- 2 m.y. were obtained on both samples; biotite ages of 61 4- 2 and 66 _ 2 m.y. were clearly younger. Hornblende and biotite concentrates from the Canchacaylla monzogranite, sample INC, gave concordant ages of 67 4- 2 and 64 4- 2 m.y., respectively. Note that both the biotite ages from the Ricardo Palma tonalitc and the younger sericite ages from the Aurora Augusta deposit are similar to the concordant hornblende-biotite ages obtained for the Canchacaylla monzogranite.

A whole-rock age of 39 4- 1 m.y. was obtained for sample E2 from an apophysis of the Canchacaylla monzogranite in the vicinity of the Leonila-Graciela deposit. Similar Cenozoic ages of 39 4- 1, 37 _ 1, and 31 4- 1 m.y. were obtained for the respective postmetamorphic dikes J1238, G10, and G2 in this deposit.

Discordant age patterns such as those here reported for the Aurora Augusta sericites and for the Ricardo Palma pluton hornblende and biotite pairs are common in the study region. Snelling (1981) relates this resetting to thermal disturbances induced by the Centered Complexes of the Coastal batholith, which were emplaced about 68 to 62 m.y. ago. The hornblende ages of samples CHP and SRX are in good agreement with the regional age range for the emplacement of the Santa Rosa superunit, to which the Ricardo Palma pluton has been assigned (Pitcher et al., 1985). In fact, a zircon U-Pb age of 86.4 m.y. has recently been obtained for this pluton (J. Cobbing, writ. commun., 1986). It is concluded that the hornblende ages of 82 m.y. represent the emplacement age of this pluton and that

FIG. 6. Microphotographs from polished sections: Graciela (A, B, C, D, and E) and Aurora Augusta deposits (F, G, and H). A. Iron sphalerite (sp) in mutual contact with pyrite (py) and hexagonal pyrrhotite (po) with barite (ba) gangue. B. Hexagonal pyrrhotite grain (po) etched with chromic acid showing lamellae of monoclinic pyrrhotite (darker gray). C. Iron sphalerite (sp) etched with chromic acid showing grain boundaries outlined by chalcopyrite, twinning, and occasional triple points. D. Equi- granular intergrowth of coarse pyrite showing triple points. E. Sphalerite (sp) veined by chalcopyrite (cpy) and quartz (qz). F. Sphalerite crystal (sp) outlined and veined by chalcopyrite (white) and quartz (qz). G. Sphalerite (sp) with chalcopyrite blebs and stringers associated with sericite (black flakes). H. Interlocking pyrite crystals (py) veined by quartz (qz), chalcopyrite (cpy), and sericite (black flakes).

1422 C•$Aa E. VIDAL C.

-- Leonila Graciela __

-5000 N--

Juanita

[• Post-metamorphic dyke • Biotite-muscovite hornfelses • Tremolite-actinolite hornfelses

•--• Barite ore • Massive pyrite-sphalerite ore :.:• Siliceous stockwork zone •, Banded ore

(•) K/Ar sample location

0 rn 200

anta Cecilia

ß 60

/ 1400

m

'•Central Highway -Juanita•• - • '

• • - •2oo.

B o m 200 I I

FIG. 7. Simplified geologic map from level 1200 and structural profile BB' from Leonila-Graciela, Juanita, and Santa Cecilia orebodies. See Table 2 for mineralogical description of individual samples in the metamorphic country rocks.

K UROKO-TYPE DEPOSITS, CENTRAL PERU 14 2 3

TABLE 1. Radiometric K-Ar Data and Location of Analyzed Samples

Sample Material K 4øARraa 4øARatmos Calculated age Latitude Longitude no. analyzed (%) (nl/g) (%) (m.y. ___ 2a) south west

Dikes (Leonila-Graciela) G2 • Whole rock 0.92 1.11 62 30.9 ___ 1.1 11o54.5 ' 76ø34 • G10 • Whole rock 4.82 6.93 24 36.6 __+ 1.0 11ø54.5 ' 76ø34 • J1238 • Whole rock 2.17 3.36 24 39.4 _ 1.1 11ø54.5 • 76ø34 '

Granites (Leonila-Graciela) E2 Whole rock 3.13 4.81 48 39.1 +_ 1.2 11ø54.4 ' 76034.2 ' INC Biotite 4.49 11.38 29 64.0 +_ 1.8 11ø53.4 ' 76034.5 '

Hornblende 0.55 1.46 41 66.7 - 2.0 SRX Biotite 6.12 15.98 6 65.9 - 1.8 11ø54.9 ' 76037 '

Hornblende 0.72 2.33 27 81.8 ___ 2.3 CHP Biotite 5.58 13.54 10 61.3 _ 1.7 11ø54.1 ' 76ø34.8 •

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 11ø59.5 • 76051 '

Sericite 5.77 15.53 10 68.0 +_ 1.9 AA2 Sericite 2.93 12.43 93 105.9 ___ 39.5 11059.5 ' 76051 '

Sericite 2.93 13.62 84 115.8 _ 17.9

Decay constants as in Steiger and Jiiger (1977) • Mine coordinates: 5230 N/10130 E (G2); 4895 N/10135 E (G10); 5265 N/10230 E (J1238); see Figure 7

the younger biotite ages were reset below the blocking temperature of hornblende.

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

TABLE 2. Protoliths and Metamorphic Assemblages of Samples Shown in Figure 7

Sample Metamorphic coordinates Protolith assemblages

Biotite-muscovite zone

5350 N, 10000 E 5330 N, 10150 E 5270 N, 10150 E 5240 N, 10250 E 5240 N, 10250 E 5220 N, 10200 E 5170 N, 10080 E 5125 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-qz Mudstone Qz-src-biot

Actinolite zone 5260N 10040E

5190N 10050E

5140N 10230E

5050N 10025E 5025N 9985E

4990 N 10030 E

4960N 9980E 4940 N 10000 E

4890 N 10035 E 4700N 10175E

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

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

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

Minerals shown in parentheses represent accessory constituents

m.y., conform to the above-mentioned pattern of thermal resetting. The high proportion of atmospheric argon causes the high degree of uncertainty of the ages of sample AA2, which could then lie anywhere between 66 and 146 m.y. or 98 and 134 m.y. The atmospheric argon is thought to have been derived mainly from fluid inclusions in quartz and not from the sericite, thus yielding ages that are appreciably older than the age of trapping (D. C. Noble, pers. commun.). However, it is tempting to interpret the 106- and 116-m.y. ages from sample AA2 as representing the true age of hydrothermal activity. Such an age range for the Kuroko-type mineralization at Aurora Augusta fits in well with the chronology of regional events, namely the Mid- dle Cretaceous age of the Casma Group and the Upper Cretaceous age of the granitic rocks of the Coastal batholith.

The concordant ages of the hornblende-biotite pair from sample INC are interpreted as the age of emplacement of the Canchacaylla monzogranite. Similar Rb-Sr whole-rock isochron and zircon U-Pb

ages have been obtained for the nearby Santa Eula- lia pluton (Beckinsale et al., 1985; Mukasa and Til- ton, 1985). The Canchacaylla pluton seems to belong to the Puscao superunit of the batholith based on its monzogranitic petrography and the ages here reported. It is envisaged that the biotite resetting advocated for samples CHP and SRX was produced by this intrusion. Sample E2 from an apophysis of the Canchacaylla pluton gave a whole-rock age of 39 ___ 1 m.y.; this age is spuriously low, probably due to argon leakage from the potassium feldspars. It

1424 CI•SAR E. VIDAL C.

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

Ages ranging from 31 to 39 m.y. were obtained on three samples from the postmetamorphic dike swarm at Leonila-Graeiela. The age of this event correlates favorably with the post-Ineaie II onset of subaerial volcanism referred to as Calipuy Group in central Peru (Noble et al., 1979, 1985). Further- more, it provides a clue to the anomalously young age recorded for sample E2.

Contact Metamorphism at Leonila-Graeiela

The time of granite emplaeement into the Coea- ehacra roof pendant has been determined by the K-Ar dating presented above. According to the Cretaceous time scale of Harland et al. (1982), plutons of Campanian and Maestriehtian age cut the volcano-sedimentary eastern facies of the Casma Group. Undated dioritie to monzogranitie stocks in the region are thought to represent apophyses of underlying plutons. Figure 4 shows the location of these intrusions and the approximate limit of their composite contact aureoles. Only the southeastern corner of the Coeaehaera roof pendant has not been affected by contact metamorphism. The rest of it consists of metamorphic rocks of the hornblende- hornfels and albite-epidote facies.

Two metamorphic profiles were studied in preliminary fashion away from the Ricardo Palma tonalitc and the Canehaeaylla monzogranite. Garnet- diopside-(vesuvianite) marbles associated with biotite-muscovite metavoleanie rocks define an 80- to 100-m-wide aureole to the east of the Ricardo

Palma tonalitc; the same assemblage occurs in a wider zone, as much as 1,000 m southward from the Canehaeaylla monzogranite. Similar metamorphic assemblages were found in patehy zones along the core and eastern limb of the Chamodada syncline (Fig. 4). This distribution of hornblende-hornfels facies rocks indicates the presence of buried grani- toids beneath the latter syncline. The anomalously wide zone south of the Canehaeaylla pluton may also indicate buried apophyses and that at depth the main intrusive contact dips to the south.

Metamorphic mineral assemblages

Marly limestones and quartzofeldspathie voleani- elastic tuffs and lavas were sampled to investigate the contact metamorphic effects in the vicinity of the Leonila-Graeiela and Juanita deposits. Figure 7 shows the location of these samples and their distinction into biotite-muscovite hornfelses and aetin-

olite hornfelses, both of the hornblende-hornfels facies. Most of the Leonila-Graeiela deposit lies within the biotite-muscovite zone, whereas the Juanita and Santa Cecilia deposits lie within the aetinolite zone. Table 2 summarizes the available in-

formation on the metamorphic mineralogy. Map-

ping of the metamorphic zonation shows that the distribution of the garnet-diopside hornfelses coincides with that of the biotite-muscovite zone.

Biotite-muscovite hornfelses have formed from mudstones, voleanielastie tuffs, and daeitie lavas surrounding the Leonila-Graeiela deposit (Fig. 7). In mudstones and tuffs, biotite and muscovite zones occur as laminations parallel to bedding; inter- growths with granoblastie quartz and preferential orientation of micas are widespread. Daeitie lava protoliths preserve relict porphyritie texture and show reerystallized phenoerysts of plagioelase and K-feldspar. Granoblastie quartz associated with biotite is abundant in the matrix. Biotite is commonly found as pseudomorphie replacements of aetinolite and intimately associated with white micas; it ac- counts for 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 lenses of eale-silicate minerals are found interealated with barite in a

narrow zone at the contact (Fig. 5C and D). In some eases, blastoporphyritie barite crystals engulf eale- silicates. Accessory minerals are calcite, quartz, sphene, mixed layer clays, and pyrite.

Hornfelses of the aetinolite zone are developed predominantly in daeitie lavas and tuffs of the Jua- nita and Santa Cecilia deposits (Table 2; Fig. 7). Original textures and premetamorphie mineralogy are better preserved in this zone. Tremolite-aetinolite aggregates occur as matrix constituents, in veinlets, and as partial replacements of ferromagnesian phenoerysts; they can make up 30 percent of these rocks. Associated minerals are epidote, ehlorite, calcite, sphene, and various sulfide minerals. Marly limestones in this zone consist of metamorphic assemblages dominated by garnet-calcite-quartz with little or no diopside.

The metamorphic zonation described is coinci- dent with the appearance of hexagonal pyrrhotite in the biotite-muscovite zone at Leonila-Graeiela and

with its absence from the aetinolite zone at Juanita and Santa Cecilia. It also coincides well with a prograde increase in mole percent FeS in sphalerite.

Geothermometry

Evidence for the thermal history of the ore deposits during contact metamorphism derives from: (1) the predominantly hexagonal structure of pyrrhotite, (2) the transition in the metavoleanie wall rocks from aetinolite to biotite, (3) the distances to intrusions of known composition and diameter, and (4) the discordant patterns of K-Ar ages obtained on hornblende-biotite pairs.

As regards the distribution and nature of pyrrho- tire in the main ore deposits, it is important to recall


that only traces of this mineral are found in the Juanita and Santa Cecilia deposits. In contrast, pyrrhotite is a common constituent of both massive sulfide ores and siliceous stockwork zones at Leo-

nila-Graciela; in a few localities it can compose as much as 40 percent of the massive sulfides (Fig. 3F). Pyrrhotite concentrates from samples G16 and G7 from the Graciela deposit have been studied by means of X-ray powder diffractometry, observation of polished sections etched with saturated chromic acid, and microprobe methods (Table 3). It was found that the (102) reflection was a single and well-defined peak with slight asymmetry, a pattern indicative of a pedominantly hexagonal structure (Arnold, 1966). The relatively high atomic percent Fe values obtained by microprobe analyses also indicate the presence of high-temperature, Fe-rich varieties (Table 3). Visual confirmation for these findings was observed on polished sections, etched with chromic acid according to the procedure of Arnold (1966), where grains of hexagonal pyrrhotite show 5 to 10 percent of darker gray lamellae of monoclinic pyrrhotite (Fig. 6B).

Inversion temperatures for the monoclinic to hexagonal transition of pyrrhotites in equilibrium with pyrite are estimated at 308øC (Scott, 1974). The accessory nature of monoelinie pyrrhotite as lamellae within hexagonal pyrrhotite at Graciela thus in-

dicates a metamorphic event above 300øC, with relatively rapid cooling to prevent reequilibration.

Actinolite and biotite-muscovite hornfelses are

the most common metamorphic rocks in the vicinity of the Leonila-Graciela deposits (Fig. 7). The metamorphic transition from actinolite to biotite is ob- servable in many thin sections, and as discussed by Winkler (1979), indicates temperatures on the order of 420øC for pressures of about 2.6 kb. Evi- dence for this pressure estimate will be presented in the next section.

The Canchacaylla monzogranite was emplaced as a granitic magma and defined a pluton with diame- ters of 3 to 4 km. The Leonila-Graciela deposits are located 1,500 to 2,000 m south of the Canchacaylla pluton; considering these parameters, it is possible to infer that the mineral deposits were heated to temperatures on the order of 380 ø to 420øC (Winklet, 1979).

The concordant and discordant K-Ar age patterns obtained on hornblende-biotite pairs from the Ri- cardo Palma and the Canchacaylla intrusions also enable geothermometric parameters to be derived. As discussed above, the hornblende ages from samples CHP and SRX reflect the time of emplacement, whereas the corresponding biotite ages were reset at the time of the Canchacaylla intrusion. The metamorphic peak recorded at Leonila-Graciela

TABLE 3. Selected Microprobe Analyses of Sphalerites and Pyrrhotites (in wt %)

Sample Mineral Analysis FeS P no. assemblage no. S Zn Fe Mn Cu Total (mole %) (kb)

G16

G7

j11

j1

G16

J1

Sphalerite

Sp-py-po 1-1 33.4 55.5 10.2 0.80 0.04 99.9 17.68 2.29 Sp-py-po 1-8 32.9 56.3 10.4 0.80 0.02 100.5 17.82 2.16 Sp-py-po 7-3 33.7 57.1 9.7 0.66 0.45 101.6 16.53 3.27

Sp-py 7-18 32.5 55.7 10.8 0.12 0.08 99.2 18.51 1.59 Sp-py-po 7-26 32.0 56.5 10.7 0.11 0.02 99.4 18.09 1.94 Sp-po 7-28 33.2 56.2 9.6 0.11 0.02 99.2 16.73 3.10

Sp-py 3 35.3 55.6 8,5 0.28 nd 99.7 15.24 4.44 Sp-py 11 33.1 58.4 8.7 0.31 nd 100.5 14.85 4.81 Sp-py 24 33.1 58.4 8.0 0.23 0.03 99.8 13.87 5.77

Sp-py 2 32.7 57.6 9.1 0.16 0.05 99.5 15.57 4.15 Sp-py 10 32.6 58.2 9.0 0.17 nd 100.3 15.31 4.39 Sp-py 21 33.1 57.8 9.1 0.19 0.03 100.4 15.61 4.11

Pyrrhotite

2-1 37.2 59.5 96.8 2-2 36.6 58.9 95.5 2-5 37.3 58.3 95.7 2-6 37.5 57.8 95.5

25 36.9 58.0 96.2 27 37.2 60.9 98.7 28 38.2 60.4 99.6

Trace elements measured in sphalerite and pyrrhotite are omitted from these lists but included in the totals; po = pyrrhotite, py = pyrite, sp = sphalerite, nd = not detected


was probably reached shortly after the emplacement of the Canchacaylla monzogranite; thus, blocking temperatures of hornblende and biotite would represent maximum and minimum estimates for temperature during metamorphism. Based on the work of Hart et al. (1968) and Kistler (1974), contact metamorphism and Ar release from the Ri- cardo Palma biotites occurred approximately between 300 ø and 500øC.

Geobarometry Barometric indicators for the contact metamor-

phism which affected the Leonila-Graciela deposits are found from: (1) mole percent FeS contents of sphalerites in mutual contact with pyrite and hexagonal pyrrhotite, (2) regional stratigraphic reconstruction, and (3) contact aureoles related to the Coastal batholith.

Figure 8 shows the results of 80 high quality microprobe analyses of sphalerites, coexisting with

Massive sulfide

G •e: 40% po [] • 17. e +0.7 wt % Mn 1

'mol• % FeS

Siliceous

stockwork

311: tr. po 14.7 + 0.4

1Z6_+ 1.1

•'4 •e ;e 'mol• % FeS

wt % Mn 1

FIG. 8. Mole percent FeS in sphalerite histograms and Cu-Mn abundances in sphalerites from the Graciela (samples G16 and G7) and Juanita deposits (samples J1 and J11). Top. Massive sulfide ore zone. Bottom. Siliceous stockwork zone. Hatchuring as in Figure 7. Means and standard deviations quoted.

pyrite and hexagonal pyrrhotite, in four representative polished sections from Graciela and Juanita. Pyrrhotite is present only at trace levels in the Juanita specimen. Equilibrium criteria for the sphalerite-pyrite-pyrrhotite assemblage in the Gra- ciela specimen are the mutual contacts and the coarse, polygonal-granoblastic textures of these minerals (Fig. 6A, C, and D). Analyses were performed on an ARL microprobe at 20 kV, using 10 sec and 4 sec counts for peaks and adjacent back- grounds, respectively. Synthetic sulfide standards were used for calibration. Sphalerites were analyzed for S, Zn, Fe, Mn, Cu, Cd, and Hg (see Table 3 for selected microprobe analyses).

Sphalerites from samples G16 and Jl--massive sulfide specimens from Graciela and Juanita, respectively--have means and standard deviations of 17.6 ___ 0.7 and 15.4 ___ 0.2 mole percent FeS. A similar trend of iron enrichment in the biotite-mu-

scovite zone was also found for samples G7 and J1 l, siliceous stockwork specimens, with 17.6 ___ 1.1 and 14.7 ___ 0.4 mole percent FeS. Cu and Mn never exceed 1 percent; these elements are slightly enriched in a metamorphic prograde sense as is the case for FeS. Spread of data is minimal for sphalerites from massive sulfide zones, indicating better metamorphic equilibration than sphalerites from siliceous stockwork zones (Fig. 8).

Mole percent FeS data for sphalerites in samples G16 and G7 have been plotted with mineralogical discrimination on histograms (Fig. 9A). Sample G16 is a massive sulfide specimen with 40 percent pyrrhotite, 20 percent sphalerite, 15 percent pyrite, traces of chalcopyrite, and 25 percent barite. G7 is a sample from siliceous stockwork zone with 10 percent sphalerite, 7 percent pyrrhotite, 5 percent pyrite, 2 percent chalcopyrite, and 75 percent of quartz-sericite gangue. Using equation (1) of Lusk and Ford (1978), the mole percent FeS data has been converted into pressure estimates (Table 3 and Fig. 9B). Estimates obtained for the metamorphic assemblage sphalerite-pyrite-hexagonal pyrrhotite in massive sulfide sample G16 are 2.6 ___ 0.5 kb. This is a preferred pressure estimate for contact metamorphism at Leonila-Graciela considering that the larger spread of analytical data obtained for the siliceous stockwork specimen G7 makes less reli- able results. As shown in Table 3, mole percent FeS in sphalerites from the Juanita specimen give anomalously high pressure estimates ranging from 4.1 to 5.8 kb. However, these estimates have no real value considering that pyrrhotite is present only as min- ute inclusions in chalcopyrite. No evidence of metamorphic equilibration such as major amounts of sphalerite, pyrite, and pyrrhotite, with abundant mutual contacts could be observed.

Estimates of lithostatic pressure can be calculated


G 16 Messi¾s sulfide

--I sp - py

I• sp - DO

I Sl:) - DY - po

G7 Siliceous sto(;kwork

16 17 18 19 2O

mole %

G16

G?

40 % po

20 % ep

15 % •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

I i i i

a Kbars

FIG. 9. A. Mole percent FeS in sphalerite histograms, Graciela deposit, with discrimination for mineral assemblages: sphalerite (sp), pyrite (py), pyrrhotite (po), chalcopyrite (cpy), barite (ba), and quartz, biotite, sericite, etc. (qz). B. Mean and standard de- viation (star and bars, respectively of previous mole percent FeS populations calculated into pressure (kb) according to equation (1) of Lusk and Ford (1978). Sp-py-po: sphalerites in mutual contact with pyrite and hexagonal pyrrhotite.

alternatively for reconstructed stratigraphic piles of the Casma Group. Average densities of 2.87 and 2.78 g/cc have been reported for rocks from the western and eastern Casma facies, respectively (Bussell and Wilson, 1985). To account for the average estimate of 2.6 kb obtained via sphalerite geobarometry, rock columns on the order of 9.2 to 9.5 km would be necessary. Thicknesses on the order of 9 km are maximum estimates for the Casma Group on a regional scale (Myers, 1974; Cobbing, 1978), therefore, the mean value of 2.6 kb obtained from the sphalerite compositions seems to be a maximum estimate.

Mineralogical pressure indicators in contact aureoles surrounding plutons from the Coastal batholith indicate typical pressures between 1 and 2 kb (Atherton and Brenchley, 1972).

Discussion of Results

Most of the salient features that characterize

Kuroko-type deposits in Japan (Horikoshi, 1969; Sato, 1974; Ohmoto and Skinner, 1983) are also present in the volcanogenic barite and base metal sulfide deposits of the central coast of Peru. Maria Teresa, Aurora Augusta, and Juanita are volcanichosted deposits; Elenita and Palma are sediment- hosted. Feeder zones of siliceous stockwork and breccia underlie strata-bound barite and massive

sulfide zones at Leonila-Graciela, Juanita (Fig. 7), and Mar{a Teresa. Feeder zones without overlying strata-bound ores are represented by the Aurora Augusta and Elenita deposits, which bear close re- semblance to deposits like Uwamuki 2 and 4 in Japan (Date and Tanimura, 1974; Bryndzia et al., 1983). No feeder zones are known so far underneath the strata-bound deposits at Palma (Fig. 2). Massive sulfide beds are compositionally banded and locally exhibit clastic textures, compaction structures, disruption of individual lenses, and small-scale folds indicative of soft sediment defor-

mation as described in analogous Japanese deposits (Ito et al., 1974; Hashiguchi, 1983); incompetent behavior during subsequent folding enhanced the resulting disharmonic structures (Vidal, 1980).

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

one that characterized the Miocene Kuroko stage of Japan (Tanimura et al., 1983). However, on palco- geographic terms, the submarine environment in Peru was predominantly of shallow-water character throughout the Cretaceous whereas deep environ- ments are postulated for the formation of Kuroko- type deposits (Guber and Merrill, 1983). The failed rift hypothesis of Cathies et al. (1983) seems to be in agreement with the tectonic interpretation proposed for the Middle Cretaceous marginal basin of Peru by Atherton et al. (1983, 1985).

Differences noted between the Peruvian and Jap- anese Kuroko-type deposits involve the virtual absence of associated gypsum and ferruginous chert in the former. Pyritic gypsum masses are known only from the Chamodada prospect (Fig. 4) where they are rare and relatively small. Ferruginous chert beds have not yet been found directly overlying the ore deposits. Nevertheless, chert intercalations are present throughout the entire succession and are especially abundant in the limestone unit of Coca- chacra (Fig. 4). Taken together with the common presence of calcite in tuff breccias and in the barite ore, these facts indicate partial development and departure from the idealized Kuroko-type deposit of Japan (Eldridge et al., 1983). This could be the result of differences, at the time of ore deposition, in seawater and rock geochemistry (Ohmoto et al., 1983).

1428 Ct•$AR E. VIDAL C.

Contact metamorphism recrystallized the Baldu- cho and Leonila-Graciela ores. Similar thermal ef-

fects have been noted at Aurora Augusta and Maria Teresa, although no intrusions are found in the im- mediate vicinity. Palma and Elenita are unmetamorphosed. At least two periods of contact metamorphism affected Leonila-Graciela. Dating of the nearby plutons demonstrates their Campanian (82 m.y.) and Maestrichtian (65 m.y.) ages. Most deposits are found in the zone of hornblende-hornfels facies, within biotite-muscovite or actinolite hornfelses (Fig. 7). Several geothermometric parameters coincide to indicate peak metamorphic temperatures in the range 300 ø and 500øC. Pressure during metamorphism, as indicated by sphalerite geobarometry, was 2.6 _+ 0.5 kb. However, this estimate seems to represent only a maximum value. Prograde enrichment of iron in sphalerite coexisting with pyrite and hexagonal pyrrhotite as here reported (Fig. 8; Table 3) has been described for Canadian and Japanese deposits of Kuroko type (Urabe, 1974; Scott, 1976).

At Leonila-Graciela, Juanita, and Santa Cecilia (Fig. 7) mineral textures are clearly indicative of two main sulfide assemblages. An early and domi- nant assemblage of pyrite-sphalerite, with pyrrho- tire in the metamorphic biotite-muscovite zone at Graciela, is veined, rimmed, and partly replaced by subordinate chalcopyrite _+ galena-tetrahedrite. As- sociated gangue minerals are barite and quartz-sericite in the early and late generations, respectively (Fig. 7E-H). Evidently, contact metamorphism recrystallized the Leonila-Graciela ores producing hexagonal pyrrhotite and new metamorphic textures, such as porous pyrite crystals, inclusions of sphalerite-galena-chalcopyrite-pyrrhotite in poikil- litic pyrite, and oriented chalcopyrite blebs in sphalerite (Fig. 6A-D). However, the same paragenetic position of chalcopyrite was also observed in a specimen from the unmetamorphosed Palma prospect. These findings are suggestive of thermally intensifying regimes with late generations of chalcopyrite replacing earlier formed sphalerite, as described in Japanese Kuroko deposits (Eldridge et al., 1983).

Summary

1. Kuroko-type deposits were generated in asso- ciation with submarine volcanism of Cretaceous age in Peru and Ecuador. The geologic record has preserved three main clusters of deposits in the Lima, Piura, and Quito regions.

2. Ore deposits and prospects of Kuroko type are found in the Lima region associated with Albian- Cenomanian volcanism of the Casma Group. In accord with this stratigraphic age span, K-At sericite dates of 106 and 116 m.y. have been obtained for

hypogene alteration zones in the Aurora Augusta deposit. The tectono-magmatic setting for the Casma sequence is one of a 1,000-km-long marginal basin with predominantly basaltic to andesitic fill. The eastern facies of this volcanic belt is transitional

to a continental platform succession of limestones and dolomites.

3. Felsic feeder complexes including dacitic lava domes, tuff breccias, and zones of intense hydrothermal silicification are present in the lower parts of the deposits under question. Low-grade stockwork zones and high-grade strata-bound barite and massive sulfides are distinctly mappable ore types. In accord with the genesis proposed for the Mio- cene Kuroko deposits of Japan, it is concluded that the Peruvian ores were deposited both on the sea floor and directly underneath it.

4. Subsequent geologic evolution for most of these deposits involved burial, uplift, folding, and contact metamorphism during Upper Cretaceous time. Renewed folding, faulting, and dike intrusion followed in Paleocene to Oligocene times.

5. The largest and best known deposits in the region are Leonila-Graciela, Juanita, and Santa Ce- cilia; they are found 50 km east of Lima in the Co- cachacra roof pendant. Leonila-Graciela and Juanita represent two discrete centers of explosive volcanism and exhalative hydrothermal activity; Santa Ce- cilia is a faulted portion of the eastern barite orebody at Juanita. Past production and reserves for all three deposits are estimated to be 4 million tons barite and 2.5 million tons of high-grade Zn-(Pb-Ag) sulfides.

6. The Cocachacra ores were contact metamor-

phosed to hornblende-hornfels facies by the Ri- cardo Palma tonalite and the Canchacaylla monzogranite, which were emplaced about 82 and 65 m.y. ago. Estimates of pressure and temperature for the metamorphic peak are 2.6 _+ 0.5 kb on the basis of sphalerite geobarometry and 400 ø _+ 100øC as indicated by several lines of evidence. Postmetamor- phic dikes were emplaced between 39 m.y. and 31 m.y.

7. Geobarometric estimates computed from mole percent FeS data in sphalerites intergrown with hexagonal pyrrhotite and pyrite are relatively high compared to estimates derived from stratigraphic reconstruction and to contact aureoles else- where in the region. Thus, pressure ranges calculated by sphalerite geobarometry for the Graciela deposit are quoted only as maximum values.

Acknowledgments

The Alexander von Humboldt Foundation provided financial support for K-Ar dating in London and microprobe analysis in Heidelberg. I would like to thank Christian Amstutz who made it possible for


me to work during 1985 to 1986 at the Mineralogic and Petrographic Institute, Heidelberg University. Norman Snelling, of the British Geological Survey, allowed me to use the geochronological facilities; I am particularly grateful to him and to Chris Rundle for 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 at Heidelberg helped and advised in the mineral sepa- ration and microprobe analysis procedures. The present investigations are based on research carried out in the late 70s under a British Council scholar-

ship in the Geology Department at Liverpool Uni- versity. Wallace Pitcher deserves many thanks and credit for introducing me to the regional setting and for his careful supervision both in the field and labo- ratory stages.

Minera Barmine S. A., Minera Cecibar S. A., and Perubar S. A. authorized visits to the pertinent mines. Personal thanks are expressed to Baldomero Rodriguez, proxy manager and director, and Miguel Montestruque, mining manager of the latter company, for arranging logistic support and providing information.

Buenaventura Ingenieros S. A. provided a year's leave of absence. I particularly wish to thank Al- berto Benavides Q., who made the leave of absence possible, and who otherwise greatly facilitated the study. Back at the Universidad Nacional de Ingen- ieria, Maria Jesfs Ojeda has continually supported my research activities. Finally, I thank my dear wife, Norma Luz, who through the years has had to cope with our travels and my enthusiasm for geology. Her moral support and good humor will always provide comfort and inspiration.

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1987 *vidal, c.e

Documents

barite deposits

ore deposits

important deposits

volcanichosted deposits

kurokotype deposits

lima region

hornfels facies

hornblendebiotite pairs