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Amphibolitic Cu-Fe Skarn Deposits in the Central Coast of Peru

CÉSAR E. VIDAL C.,

Perubar, S.A., Juan de Arona 830—Oficina 901, Lima 27, Peru

JORGE INJOQUE-ESPINOZA,

Chevron Mineral Corporation of Chile, Avenida El Golf 183, Santiago, Chile

GARY B. SIDDER,

U. S. Geological Survey, Denver Federal Center, Box 25046, Mail Stop 905, Denver, Colorado 80225

AND SAMUEL B. MUKASA

Department of Ceological Sciences, University of Michigan, 1006 C.C. Little Building, Ann Arbor, Michigan 48109-1063

Abstract

Monterrosas, Eliana, Raúl, and Condestable are the most important copper mines in the central coast of Peru, between lat 12°30' and 14°30' S. Their related geologic setting, close age relation, and similar mineralogy and geochemistry argue in favor of their grouping together as amphibolitic Cu-Fe skarn deposits. In the Lower Cretaceous continental margin, 100 to 120 Ma, incipient rifting accompanied by batholithic gabbrodiorite intrusions at depth gane rise to an elongated trough in which submarine volcanic and sedimentary rocks accumulated. Skarn deposits developed in association with the intrusive activity are characterized by chal-copyrite concentrations with low Ti magnetite and/or pyrite in actinolite gangue; minor con-stituents are amphiboles of hastingsitic and tschermakitic varieties, apatite, pyrrhotite, and sodic scapolite. Copper ores are fracture controlled and hosted by Albian gabbrodiorite plutons at Monterrosas and Eliana. Volcanogenic strata-bound mantos, as •ell as disseminated sulfide stringers and minor veins, occur at Raúl and Condestable in Hauterivian to Albian volcano-sedimentar). rocks. Geochronologic K-Ar data identify a Lower Cretaceous episode of metal-logenesis for Raúl, Condestable, and Eliana. Geologically related iron deposits of Hierro Acarí and Marcona are dated as Lower Cretaceous and Upper Jurassic, respectively.

Petrography and major element chemistry of the gabbrodiorite suite at Monterrosas and Eliana indicate a low K tholeiitic character with calc-alkaline affinities and high Na 20/K20 ratios. Late magmatic and hydrothermal activity developed in the roof zones of the plutons and formed amphibole-magnetite-chalcopyrite deposits. Mineralization at Eliana involved in-troduction of Cu, Fe, and P along fractures and contacts to form ore shoots. The scapolite alteration halo is characterized by Si, Na, and Ca enrichment. The hydrothermal deposits of Raúl formed mainly at subvolcanic levels with a dominant seawater component (Ripley and Ohmoto, 1977, 1979). Geologic correlation with Eliana and Monterrosas indicates that both magmatic and seawater components coincided to form amphibolitic Cu - Fe skarn deposits, which span the plutonio and volcanic environments.

Lead isotope compositions of chalcopyrite, pyrite, and galena define a primitive trend for these skarn deposits compared to that of porphyry Cu deposits in southern Peru and Chile and Sn deposits in Bolivia. The isotopic contrast is interpreted as a result of rifting and extreme crustal thinning during the Lower Cretaceous along the continental margin of central and southern Peru. This tectonic and metallogenetic environment continued into northern and central Chile •here amphibolitic Fe and Cu deposits of the same age range are known.

In troduction and Eliana together with the volcanogenic skarn de- posits of Raúl and Condestable (Fig. 1). The latter are

Al silicates and form by igneous and hydrothermal metasomatic processes at temperature ranges between 300° and 700°C. Min-eralizing fluids evolve from predominantly magmatic to predom-inantly meteoric. Skarn rocks are also formed by water-rock in-teraction in a variety of environments unrelated or distal to caus-ative plutons (Meinert et al., 1980).

1447

THE coast of Peru is a well-known copper province with major porphyry Cu-Mo deposits of Paleocene-Eocene age such as Cerro Verde, Cuajone, Quella-veco, and Toquepala. Second in production after these are the plutonio related skarn' deposits of Monterrosas

Following the definition of Einaudi and Burt (1982), skarn ore deposits occur in a gangue of coarse-grained, Fe-rich, Ca-Mg-

Econonde Ceotogy Vol. 85,1990, pp. 1447-1461

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Fic. 1. Simplified geologic map of the central coast of Peru showing the location of Cu-Fe skarn deposits (triangles) and ku-roko-type deposits (round-top squares). Cross section AA' shown in Figure 6.

briefly described in the present paper with special emphasis on their Lower Cretaceous geologic frame-work and their peculiar metasomatic nature. Similar deposits are known from Chile and the Soviet Union (Zharikov, 1970; Frutos, 1982). In Peru previous at-tempts to define the ore-forming agents have been directed at individual deposits. Evidence here pre-sented identifies a similar metal content, tight K-Ar age correlation, common geotectonic framework, and a distinct Pb isotope signature for Monterrosas, Eliana, Raúl, and Condestable.

This contribution is based on recent geologic, geo-chemical, and geochronologic research carried out by the authors in the central coast of Peru (Vidal, 1980; Mukasa, 1984; Sidder, 1984; Injoque, 1985). Pre-vious scientific work was nonexistent for Monterrosas, superficial concerning Eliana (Hudson, 1974; Agar, 1978; Ponzoni and Vidal, 1982), and controversial regarding the deposits of Raúl-Condestable (Ripley and Ohmoto, 1977, 1979; Wauschkuhn, 1979; In-joque et al., 1982; Cardozo, 1983). Atkin et al. (1985) discussed the genetic coherente of all these cupri-

ferous deposits with the Upper Jurassic to Lower Cretaceous iron deposits at Marcona and Hierro Acarí.

The Monterrosas Deposit

The Monterrosas vein deposit is located at the foothills of the Western Cordillera, 15 km northeast of lea, at elevations ranging from 1,000 to 1,200 m (Fig. 1).

Geologic setting

Copper ore is hosted by gabbros and diorites of the Patap superunit within the Arequipa segment of the Coastal batholith (Moore, 1984). Diorites and gabbros in the Monterrosas area consist of abundant andesine-labradorite and diopsidic augite, with minor amounts of magnetite, ilmenite, hornblende, sphene, apatite, and zircon. Zones of late magmatic to hydro-thermal alteration in the Patap intrusions commonly replace magmatic clinopyroxene and plagioclase feldspar by hypogene actinolite and chalcopyrite with variable amounts of chlorite, epidote, scapolite, tour-maline, sphene, apatite, magnetite, and traces of alkali feldspar, calcite, and milite mica.

Linga granodiorites and quartz monzodiorites in-truded the ore-bearing Patap diorites in the Monter-rosas area about 92 Ma and provide a mínimum age for mineralization (Sidder, 1984). Both rock tupes are in turn cut by east-west sinistral wrench faults. Frac-ture fillings within the younger Linga superunit con-sist of quartz, calcite, epidote, and potassium feldspar veinlets with traces of pyrite and chalcopyrite. Similar veinlets cut the dioritic Patap host rocks and the ac-tinolitic chalcopyrite ore.

The ore zone

Reserve estimates at the Monterrosas mine have fluctuated from 0.4 to 1.9 million tons with 2.0 to 1.1 wt percent Cu (Todd, 1983, 1984). The orebody is a fracture-controlled vein deposit within a complex fault belt characterized by en echelon fractures ori-ented about N 70° W and dipping almost vertically. The main ore shoot measures 430 m in length, 3 to 20 m in width, and 150 m in depth (Fig. 2A).

Petrographic textures in the host diorites and the ore indicate that metasomatic replacement was the principal mode of ore formation with minor deposition by filling of fractures (Fig. 3). Criteria for replacement origin include eutaxial overgrowths and pseudo-morphism of actinolite after pyroxenes and horn-blende; scapolite veinlets are widespread and tend to replace plagioclase crystals with various stages of pseudo-morphism at a microscopic scale. The pre-dominant ore assemblage consists of chalcopyrite, magnetite, pyrite, and actinolite in variable propor-tions; smaller amounts of sodic scapolite, tourmaline, quartz, pyrrhotite, and cubanite are common.

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AMPHIBOLITIC CU-FE SKARN, CENTRAL COAST PERU

1449

ization and veinlets of scapolite. The outermost por- tions of the amphibolitized diorite pass inward to a zone of crystalline actinolite, which in turn grades

Underground mapping reveals a transitional zo-nation from dioritic wall rocks inward to the orebody. Contacts are indistinct due to widespread amphibol-

•

1450

VIDAL C., INJOQUE-ESPINOZA, SIDDER, AND MUKASA

FIG. 3. Microphotographic evídence of mineralogy and texture. A. Monterrosas: intergrowth of magnetite (mg) and actinolite (ac). B. Monterrosas: supergene martitization of magnetite (shades of gray) replaced by pyrite (py). C. Eliana: apatite (ap) and actinolite (ac) embayed by chalcopyrite (cp) and veined by pyrite (py). D. Eliana: enlarged detail of C. E. Condestable: actinolite (ac) intergrown with chalcopyrite (opaques), apatite (ap), and quartz (qz). F. Condestable: zoned amphibole associated with ore, white actinolite cores rimmed by black hastingsite rims. Length of bars is 1 mm.

into a zone of magnetite predominante (Fig. 3A). Stringers and blebs ofsulfides increase axially and de-fine a central zone of malsive to nearly massive chal-copyrite ore. Silicate, oxide, and sulfide zones pinch

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and swell along strike and may differ significantly in size on either side of the axial ore. Supergene effects include replacement of chalcopyrite by covellite and martitization of magnetite (Fig. 3B).

• 1451

Alteration

Alteration of the Patap diorites and gabbros is widespread and more intense toward the mineralized zone. Hydrous ferromagnesian minerals such as ac-* tinolite, epidote, and chlorite predominate in wall rocks adjacent to the ore zone; they replace augite in association with magnetite and chalcopyrite. Pseu-domorphic replacements of amphibole after augite are most common nearest sulfide-bearing rocks. Pla-gioclase feldspars show resorption along grain boundaries; they are riddled with sodic scapolite of dipyre variety (Mes ;), epidote, clinozoisite, and cal-cite. Penninite, sphene, magnetite, alkali feldspar, quartz, and white mica are minor constituents of the alteration zone. Scapolite, in particular, has a system-atic distribution with respect to the ore zone in that diorites from wall rocks adjacent to ore contain up to 65 percent scapolite.

A comparison of whole-rock geochemistry between fresh and altered diorites and gabbros at Monterrosas reflects general trends of metasomatic changes as dis-cussed by Sidder (1984, 19S7). Total iron increases toward the ore zone, whereas sili•a is antipathetic to this trend. N1etals such as copper and cobalt are at background levels in the dioritic wall rocks; anoma-bous concentrations of these elements plus gold, silver, and molybdenum characterize the actinolite and sul-fide zones.

Fluid inclusion and sulfur isotope studies in quartz and pyrite-chalcopyrite, respectively, indicate tem-perature ranges for hydrothermal alteration and ore deposition of about 300° to 500°C, salinities of 30 to over 50 equiv wt percent NaC1, and OS values of 1.6 to 3.3 per mil. High-temperature, salive, mag-matic fluids exsolved from the Patap gabbrodiorites are advocated to account for the origin, transport, and deposition—by decreases of temperature and acidity—of amphibolitic Cu-Fe ore at Monterrosas (Sidder, 1984).

The Eliana Deposit

The Eliana deposit is located 50 km east of Pisco and 230 km south of Lima, at an elevation of 1,200 m in a mountainous desert terrain (Fig. 1).

Ceologic setting

A gently folded volcanic sequence of Albian age is intruded by early gabbrodiorites of the Coastal batholith (Fig. 2B). The volcanic rocks consist of an-desitic lavas and agglomerates at the base with a cen-tral intercalation of silicic tuffs and calcareous sedi-ments, which in turn are overlain by andesitic lavas (Hudson, 1967). The entire sequence is metamor-phosed to assemblages of lower greenschist facies and has been folded into a northwest-trending open sync-

line with subordinate east-west folds in the western limb.

The eastern limb of the syncline has been intruded by the Eliana gabbrodiorite; younger monzonitic plu-tons of the Linga superunit intrude both the gabbro-diorite and the volcanic rocks. In the vicinity of the Eliana deposit, the Linga plutons are generally fresh; hydrothermal alteration is sparse and consists of quartz and K feldspar veinlets with sericite selvages.

Texturally, the gabbrodiorite is variable with areas of coarse crystallization and fine-grained chilled mar-gins. Net veins and patches of hydrothermal alteration are widespread and consist of secondary amphiboles with chlorite, calcite, and sphene. One such alteration zone lies aboye and to the east of the San Martín manto. Here, the otherwise steep-sided pluton be-comes a sill which is situated aboye the central inter-calation of tuffs and sedimenta (Ponzoni and Vidal, 1982).

The San Martín manto

The San Martín manto is located at the contact be-tween the Eliana gabbrodiorite sill and the volcanic sequence in the form of a flat-lying and moderately folded tabular structure (Fig. 2B). This manto has been mined for Cu ores at various times in the past; over 400,000 tons averaging 2.7 percent Cu were extractad during the period 1967 to 1972 from this deposit (E. Ponzoni, pers. cominun.). The richest concentrations of chalcopyrite ore occur within coarse-grained amphibolitic gangue and are located at antiform hinges of the manto below the gabbro-diorite sill.

Ac•ording to Ponzoni and Vidal (19S2), the out-crops of the San Martín manto are characterized by several lenticular misses which define a single, gently folded horizon. Thicknesses vary from 0.5 to as much as 12 m. Small discordant veins are present in the hanging wall; their mineralogy resembles that of the stratiform deposits.

The footwall contacts are in most places gradational from banded amphibolites to unaltered tuffs and sed-iments. The hanging wall is generally outlined by ir-regular masses of breccia, where fragments of ore and wall rock are cemented by comminuted material and calcite. These breccias grade laterally into mylonites and display wedge-shaped roots which cut across the ore-bearing structure. Sulfides tend to be more abun-dant toward the west, whereas magnetite predomi-nates eastward in the gabbrodiorite host (Fig. 2B). Actinolite and magnetite are the predominant min-erals in the gabbro-hosted Mojador deposit, 2 km southeast of the San Martín manto.

Alteration

The footwall contacts between the San Martín manto and the enclosing rocks are in most cases tran-

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sitional (Fig. 2B). Toward the west, a vertical zonation pattern shows massive chalcopyrite ore becoming finer grained and disseminated at depth and gradually changíng into pyritic amphibolites and cherts over a distante of 4 to 8 m. Disseminations, pods, and vein-lets and banded pyrite and chalcopyrite occur in the footwall. Unaltered tuffs, shales, and cherts are several meters away from the chalcopyrite zone. Zoning pat-terns of ore and wall-rock alteration have been traced up to 10 m beneath the San Martín manto.

Eastward, where the manto is floored by the gab-brodiorite pluton, there are patchy disseminations and stockworks of amphibole, apatite, and magnetite. Within this aureole scapolite of marialite variety (Me 14) and subordinate Cu-Fe sulfides also occur. A terminal stage of fracture-controlled breccias infilled by calcite cuts the zonation described aboye. Late-stage crystallization of the Eliana gabbrodiorite over-lapped with the paragenetic sequence recognized for the San Martín manto (Figs. 3C and D and 4A).

The Raúl-Condestable Deposit

The mines of Raúl and Condestable are located 90 km southeast of Lima at elevations from 120 to 300 m (Fig. 1).

Geologic sctting

Stacked lenses, disseminations, and stringer ore of strata-bound chalcopyrite occur over 600 ni of the Hauterivian-Barremian Chilca Formation in the Raúl and Condestable mines (Fig. 2C). The Chilca succes-sion intercalates submarine andesitic lavas and vol-canoclastic rocks with fossiliferous sandy limestones and shales. The entire sequence forms a southwest-

FIC. 4. Comparative mineralogy and paragenetic sequence of the Eliana (A) and Raúl-Condestable (B) deposits. Bar thickness indicates relative abundante.

dipping monocline of variable thickness underlain conformably by the Neocomian Morro Solar shale, sandstone, and quartzite formations. The volcanic component of the Chilca Formation diminishes out-ward and especially to the north into the Lima region, where the succession is predominantly calcareous (Rivera et al., 1975). A shallow marine environment is indicated by the sedimentologic studies and strati-graphic correlations of Injoque et al. (1982) and Os-terman et al. (1983). Regional low-grade metamor-phism affects the Chilca Formation and the overlying .Casma Formation (Aguirre and Offler, 1985). Uplift and block tectonics along northeast-trending faults characterize the structural geology of the area.

Dacitic porphyry stocks intrude the aboye succes-sion defining elongated bodies, 1 to 5 km long. Minor sills and dikes of the same composition are also com-mon in the mine area. Plagioclase (An2 0_ 40), horn-blende, and quartz phenocrysts occur embedded in a fine-grained matrix of similar mineralogy. Argillic and sericitic alteration is widespread with sericite flakes in plagioclase and chlorite-calcite aggregates after hornblende. Postore intrusions are represented by a tonalitic stock and by a diabase clike swarm (Fig. 2C).

Copper mineralization is present mainly as strata-bound mantos, stringer zones, and disseminated ore-bodies with subordinate veins (Ripley and Ohmoto, 1977). Individual mantos are tabular to lensoidal structures up to 250 nn along strike and 200 m along dip. Their mineralogy is dominated by actinolite with grossular andradite garnet and diopsidic pyroxene in the lower parts of the Condestable succession (In-joque et al., 1982; Fig. 5); common accessories are apatite, scapolite, magnetite, pyrite, and chalcopyrite (Fig. 3E).

Alteration

Throughout the mining district, metasomatic rocks and associated mineralization forro irregular zones of marble and calc-silicate hornfelses. At outcrop scale the alteration is zoned showing amphibolitic, pyrox-ene, and/or garnet-rich cores which are surrounded by marble and oligoclase-andesine hornfelses. Mag-netita and sulfides represent stages of metallic min-eralization superimposed on the silicate stage (Figs. 3E and 4B). Common textural features include re-placement of amphiboles by magnetite and multiple veinlets of chalcopyrite and pyrite in both amphiboles and magnetite.

Amphiboles are major mineralogic constituents of the ore zones. Their composition is variable and zoned (Fig. 3F). Tremolite, actinolite, actinolitic horn-blende, ferrohornblende, and ferrotschermakite va-rieties have been determined using microprobe anal-ysis (Ripley and Ohmoto, 1979; Injoque, 1985). Am-phibolitic skarn comprises up to 90 percent of the

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1452 VIDAL C., INJOQUE-ESPINOZA, SIDDER, AND MUKASA

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AMPHIBOLITIC CU -FE SKARN, CENTRAL COAST PERU 1453

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gangue material; granoblastic textures and fabrics such as eutaxitic overgrowth predominate.

Calcareous and volcanic rocks were the protoliths replaced by amphiboles. The amphibolites of calcar-eous origin still retain patterns of original bedding with abundant synsedimentary faults, load casts, and in some areas pyritized fossils. They are banded, tex-turally homogeneous, and free of sphene. Amphibo-lites of volcanic origin occur as irregular layers with gradational margins; they have accessory sphene and their texture is moderately heterogeneous showing relics of either pyroclastic or porphyritic textures.

Veins and breccia zones showing metasomatic ef-fects similar to those described in the Chilca For-mation are rarely present in the intrusions. However, the dacitic dikes and sills are strongly altered in certain areas. A zoning pattern with brown actinolite-ferro-hornblende in the ore zone and actinolite-epidote-scapolite halos adjacent to the dacitic stocks has been described in the Raúl deposit (Cardozo, 1983).

Mineralization

Metallic mineralization is superimposed on the sil-icate alteration described aboye. The metallic min-erais show disseminated, infiliing, and replacement textures within a silicate-oxide-sulfide sequence typ-ical of skarn formation and subsequent hydrothermal alteration (Fig..4B).

Two distinet metallic associations can be recog-nized within a variety of host-rock types, namely Cu-Fe and Pb-Zn. Economic concentrations of the Cu-Fe association define strata-bound mantos, dissemi-nated and stringer orebodies, and minor veins. This association is dorninated by magnetite, pyrite, chal-copyrite, and pyrrhotite; accessory minerals are il-menite, molybdenite, bornite, mackinawite, vallerite, electrum, and marcasite. Copper is by far the most important economic element with gold and silver re-covered as by-products. The Pb-Zn association occurs in late-stage veinlets of galena, with traces of sphal-erite, chalcopyrite, pyrite, tetrahedrite, melnicovite, rutile, and gold in calcite gangue. It is volumetrically insignificant and warrants no separate mining.

Data presented for the Raúl deposit by Ripley and Ohmoto (1977, 1979) demonstrate quartz-magnetite isotope temperatures of 380° to 414°C, fluid inclu-sion filling temperatures of 320° to 360°C, and stratigraphically variable OS values suggestive of sea-water derivation. Modified or evolved seawater is ad-vocated to explain the sulfur, oxygen, and hydrogen isotope signatures of this particular deposit and the relatively high salinity character of its Huid inclusion population. According to the same authors precipi-tation of ore minerals occurred as a result of a decrease in temperature accompanied by increases in oxidation state and pH of the fluid.

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Metallogenic Correlation • Geologie framework

Crystalline basement of the Arequipa massif is ex-posed southeast of Pisco along the Coastal Cordillera of central and southern Peru (Fig. 1). It consists of granulites and gneisses of Precambrian age intruded by granitic plutons of the Paleozoic San Nicolás batholith. Northward continuation of this metamor-phic basement has been confirmed by offshore drilling and a combination of geophysical methods (Kulm et al., 1981). During Jurassic and Lower Cretaceous time the massif was emergent and controlled the westward extension of an inland sea where the predominantly volcanic basinal sequence accumulated. Gabbros and granites of the Coastal batholith were emplaced along the axis of the basin mainly during middle and Upper Cretaceous time.

The Lower Cretaceous marginal basin in the region formed by incipient rifting and a terminal gabbroic phase which created new mafic crust within an ex-tensional ensialic setting (Atherton et al., 1983, 1985). Submarine volcanism, sedimentation, and burial-tupe metamorphism coincided in the marginal basin which rested on thinned crust and prompted geothermal systems of high heat flow (Aguirre and Ofiler, 1985). Present-day crustal configuration preserves an anom-alous high-density core flanked to the west by the Arequipa massif and to the east by the Coastal batho-lith (Fig. 6 ; Couch et al., 1981).

The gabbro precursor of the batholith had a com-plex evolution: cumulate crystallization was the dom-inant process at depth with variable degrees of de-formation at high temperatures and, in many cases, late-stage pervasive amphibolization. This led Regan (1985) to postulate that final emplacement was ac-

complished by tectonic transport and that reconsti-tution occurred in subvolcanic environments due to volatile introduction from external sources. Dry gra-nitic magmas of the Coastal batholith were subse-quently emplaced and linked to compressive tectonic events. The greater part of the batholith emplacement corresponds to a well-recognized period of rapid sea-floor spreading and subduction of the Nazca plate be-tween 108 and 80 Ma (Moore, 1984). Subaerial vol-canic rocks of Paleocene to Miocene age cover the uplifted Mesozoic record of basinal volcanics and batholithic intrusions.

Igneous petrology

Five samples of the Eliana gabbrodiorite and two of the Linga monzogranites have been studied petro-graphically and geochemically (Table 1). Gabbro varieties consist of plagioclase (An 8 2_47), olivine (Fo60-64), and hypersthene; accessory constituents are ortho- and clinopyroxene, biotite, and apatite. Fresh diorites consist of plagioclase (An 50 _36), hornblende, and minor quartz; plagioclase shows protoclasis and normal zoning. Hornblende is commonly replaced by actinolite pseudomorphs altered by combinations of hastingsite, epidote, tourmaline, magnetite, and il-menite. Quartz is myrmekitic and hosts hvpersaline (luid in•lusions. Wall-rock alteration halos are seap-()lite rich; here plagioclase is veined, brecciated, and replaced by sodio scapolite in association with abun-dant actinolite and apatite. Seapolitized samples (e.g., 25213, Table 1) show Si, Ca, and Na enrichment with depletion of Fe, P, and Cu when compared to fresh diorites and gabbro. Patap sample 25229 has a po-tassic character due to the alteration imposed by the younger Linga plutons.

1454 VIDAL C., INjOQUE-ESPINOZA, SIDDER, AND MUKASA

Major e

Si02 Al203 -

TiO2 . Fe 203

MgO CaO Na20 K20 Mn0 P2O5

L013 FeO

H 2O Cu

Total

CIPIV ni

Quartz Corundi Orthock Albite A nort hit Diopside Hyperstr Magnetir Ilrnenite Apatite Nephelir Olivine

3 Pata: 2 Linc 3 LOL

A

50

Km o

A' DENSITY SE ISMIC

PISCO

HUANCAYO

VE LOCITY

mq/m 3 Km/s

-r • • • • • • . •

°

oo ...._

° \*..... • .. : • .* : .• ". z

. . . • ° • • • . • . • . • . • • . . • • . • ....... • • 3 . O

O o O O o

" 2.6

o o 3.3

• .• .•*.•,. ". • :

— — 2.8 •. : : . • : . . . . o o o o o—

The (A

xe n35 is va-- replace

eties co quartz: magnes

Pata

of the 2 versus clear th of high 6.2 to 1 trast wi terize t iones. plagioé tionatia

4

1.53

6.7

7.9 1 00000

2 /Á 3

4

+ +l 5 i v y v 1 6

O Km 100

1 -7-- 1

I x x x

Linga Patap

AMPHIBOLITIC CU -FE SKARN, CENTRAL COAST PERU

TABLE 1. Composition of Patap and Linga Intrusions in the Eliana Mine Area

1455

'onsti-Ate to y gra-,ubse-ctonic ment

id sea-".e be-al vol--r the :s and

id two )etro-

;abbro 1ivine

nts are Fresh ¡ende,

, is and sed by ions of .nd il-rsaline

scap-:d, atad

u n-s (e.g., 1. with

) fresh - a po-oy the

Cabro 25206

Diorite 25189

Diorite l 25213

Diorite 2 25229

Diorite 25230

Granodiorite 25207

Monzodiorite 25227

Major element chemical composition in wt percent, Cu in ppm

Si02 47.12 56.11 60.23 57.78 56.51 61.00 57.44 Al203 19.57 16.87 16.43 16.66 16.86 15.34 16.62 TiO2 0.94 1.32 0.91 0.81 1.09 0.87 0.95 Fe203 1.19 3.22 0.49 1.94 4.50 4.33 3.72 MgO 9.26 2.45 3.32 3.23 3.05 2.12 2.55 CaO 7.78 7.85 9.38 6.92 7.74 5.05 5.81 Na20 3.19 4.78 5.68 3.51 4.47 4.05 4.14 K20 0.51 0.61 0.33 1.82 0.39 2.31 2.73 MnO 0.23 0.05 0.09 0.16 0.05 0.07 0.12

P205 0.20 0.61 0.05 0.17 0.33 0.23 0.32 L013 0.11 1.24 0.52 0.21 0.64 0.97 0.51 FeO 9.03 4.32 2.35 6.17 4.28 3.08 4.82 H2O 0.11 0.88 0.47 0.21 0.64 0.60 0.51 Cu 43 26 6 71 32 17 61

Total 100.13 99.91 100.05 100.07 100.38 99.77 100.28

CIPW normative mineralogic composition

Quartz 8.40 5.86 9.20 10.18 16.36 7.26 Corundu m 0.11 Orthoclase 3.01 3.61 1.95 10.76 2.31 13.65 16.14 Albite 26.99 40.44 48.06 29.70 37.82 34.27 35.03 Anorthite 37.29 2.1.78 18.37 24.33 24.79 16.86 18.71 Diopside 9.87 22.28 7.37 ' 9.17 5.29 6.56 Hypersthene 1.13 4.50 0.19 13.07 5.64 3.56 7.61 Magnetite 1.73 4.67 0.71 2.81 6.52 6.28 5.39 Ihnenite 1.79 2.51 1.73 1.54 2.07 1.65 1.80 Apatite 0.47 1.44 0.12 0.40 0.78 0.54 0.75 Nepheline Olivine 26.51

1 Patap scapolite alteration. sample 25213 2 Linga potassic alteration, sample 25229 3 LOI = loss on ignition

The Linga monzodiorite consists of plagioclase (An 35_ 25), orthoclase, biotite, and quartz. Plagioclase is weakly altered to sericite and K feldspar; biotite is replaced by chlorite and sericite. Granodiorite vari-eties consist of plagioclase (An 45-26), hornblende, and quartz; hornblende is weakly altered to actinolite, magnesian hornblende, and edenite.

Patap and Linga intrusions from Eliana and Mon-terrosas plot predominantly in the cale-alkaline field of the AFM diagram (Fig. 7). However, on the K 20 versus Si0 2 diagram the Patap gabbrodiorites show a clear tholeiitic affinity; the monzonitic Linga suite is of high K character (Fig. 7). Na 20/K20 ratios from 6.2 to 17.1 for Patap gabbrodiorites are in sharp con-trast with the low values of 1.5 to 1.9 which charac-terize the Linga monzogranites and their alteration zones. REE patterns suggest that the Patap rocks are plagioclase cumulates with some clinopyroxene frac-tionation; the Linga rocks instead evolved by feldspar,

hornblende, and/or pyroxene fractionation (Agar and Le Bel, 1985; Injoque, 1985).

Geochronologic investigation

Sarnples, analyses, and results: K-Ar age determi-nations were made using •hole-rock samples or 95 percent pure samples of alteration minerals frotn the Raúl, Condestable, Eliana, Hierro Acarí, and Marcona deposits. Whole-rock samples were all fresh, unal-tered igneous specimens; hypogene alteration min-erals used were actinolite, hastingsite, phlogopite, and sericite. Samples were all collected in underground workings where no supergene alteration is visible. They were ground and sieved to -60, +120 ASTM mesh. An aliquot of 0.5 gm of each sample was used for the determination of potassium by XRF spectrom-etry; duplicates of each sample were analyzed rou-tinely. Analyses were carried out at the Department of Geology, University of Nottingham, using a Philips

respec dle Cr At Hit a dacit teratic

Lead i

Lea the Mc and LE larger eral dt \N'estet jective relatiot of cent Details NIukasz here.

Tabl for six deposit Leonila eludes t and tres ore dep Cu-Fe s fine a 1 0.9517 isochro: age is tc thought more so '''Pb di There a chalcop Leonila establisl of meta'

The taceous the re is

Ore depo

Leonila Monterro Mont erro Eliana Raúl Raúl Marcona

TA BLE 2. K-Ar Data on the Amphibolitic Cu-Fe Skarn Deposits of the Peruvian Central Coast and Associated Igneous Rocks

Localit y Sample no. Rock tepe Material

dated K (%) Ar radioz•nic

(nlig) Age (?cía)

Raúl-Condestable 24627 Monzodiorite porphyry Whole rock 3.600 6.313 104 ± 2.9 Raúl-Condestable 24431 Trachyandesite porphyry 1Vhole rock 0.960 4.111 107 ± 2.8 Raúl-Condestable 24415 Trachyandesite porphyry Whole rock 0.600 2.458 102 ± 2.7 Raúl-Condestable 24644 Postore monzodiorite Mole rock 3.600 18.030 124 ± 3.0 Raúl-Condestable 24549 Postore diabase dike Whole rock 1.585 6.009 95 ± 2.6 Raúl-Condestable 24527 Hydrothermal alteration Hastingsite 0.860 4.434 128 ± 3.3 Raúl-Condestable 24572 H ydrot he r mal alteration Hastingsite 0.760 3.8S7 127 ± 3.1 Eliana 25178 Amphibolitic gangue Hastingsite 1.590 7.186 113 ± 3.0 Eliana 25195 Amphibolitic gangue A cti nolit e 0.120 0.551 115 ± 5.0 Marcona 21721 Postore latite dike Whole rock 3.735 20.730 137 ± 3.0 Marcona 24740 Postore shoshonite dike Whole rock 1.390 7,656 136 ± 3.0 Marcona 24687 Postore aplosyenite dike Whole rock 6.690 31.660 118 ± 3.0 Marcona 24748 Hydrothermal alteration Phlogopite 7.390 46.200 160 ± 4.0 Marcona 24804 Hydrothermal alteration Sericite 8.095 52.810 154 ± 4.0 Hierro-Acail 21306 Preore dacite Whole rock 0.250 1.092 109 ± 4.0

DécaY constan" , b = 4.962 10-1°a- I; e = 0.581 10-1 °a-1 ; 40 k/k (at) = 1.167 10-mole/mole; errors for calculated ages quoted 2 sigma level

•

1456

SK20

H1 gh K

Colc- alicaline 1.0

50 55 60 %5i02

FIG. 7. AFN1 and K 20 vs. Si0 2 diagrama for Patap gabbro-diorites (solid squares and cir•les) and Litiga monzogranites (open squares and circles) from the deposits of Eliana (cir•les) and Mon-terrosas (squares).

P\V 1400 spectroineter with a rhodium X-ray tubo. Isotope analyses were undcrtaken at the British Ceo-logical Survey; argon was determined by isotope di-lution using an MS 10 mass spe•trometer in static mode. Errors applied to the calculated ages take into account uncertainties in the potassium determina-

tions, the spike calibration, and the isotope ratio mea-surements. For details see appendix 5 in Injoque (1985); K-Ar data plus calculated ages and errors are given in Table 2.

Data interpretation: The ages obtained for the Marcona and Hierro Acarí deposits (Fig. 1) are in-cluded here to enlarge the data base for discussion. At Marcona, two distinct events are recognized; coarse-grained phlogopite and sericite gangue asso-ciated with the strata-bound magnetite ore served to date mineralization at 154 and 160 Ma. The dike swarm data confirm postore emplacement between 118 and 137 Ma. Thus, Marcona formed during the Upper Jurassic in association ‘vith the Río Grande volcanic event; the Tunga dikes were emplaced later in the Cretaceous.

Amphibole samples dated from the Raúl and Condestable deposits identify a Neocomian age of al-teration and mineralization at 127 and 128 Ma (Table 2). There is a close time correlation between this finding and the age assigned by independent paleon-tologic studies to the Chilca Formation, which hosts the amphibolitic copper ores (Rivera et al., 1975). Four samples of monzodioritic to trachyandesitic stocks and diabase dikes reveal a group of postmin-eralization ages that varies from 95 to 107 Ma. Postore igneous activity is concentrated during the Albian pe-riod. However, monzodiorite whole-rock saniple 24644 (Table 2) gives an age of 124 Ma that is indis-tinguishable, within analvtical error, from the Neo-coniian hydrothermal alteration dates. It is interpretad that at least some of the stocks emplaced in the Raúl-Condestable mine arcas overlap in time with hydro-thermal alteration and ore deposition.

Act inolite and hastingsite gangue samples from the Eliana deposit were dated 115 ± 5 and 113 ± 3 Ma,

20

VIDAL C., INJOQUE-ESPINOZA, SIDDER, AND MUKASA

TABLE 3. Pb Isotope Data from Ore Deposits in the Central Coast of Peru

Ore deposit Mineral sample

Age (Ma) 2o6pbro4pb

207Pb/204Pb 208pb/204pb

Leonila L (gn) 106-116 18.734 15.669 38.481 Monterrosas M (cp) 97-107 18.513 15.583 38.492 Monterrosas M (py) 97-107 18.555 15.584 38.471 Eliana E (cp) 113-115 18.683 15.640 38.604 Raúl R (cp) 124-128 18.639 15.633 38.598 Raúl R (gn) 124-128 18.561 15.615 38.463 Marcona Ma (py) 150-164 18.208 15.567 38.381

Abbreviations: cp = chalcopyrite, gn = galena, py = pyrite; approximate age ranges estimated from Moore (1984), Vidal (1987), and Table 2

AMPHIBOLITIC CU-FE SKARN, CENTRAL COAST PERU

1457

mea-Injoque -rors are

for the are in-

cussion. ¿nized; ue asso-

-ved to he dike 'tween

ring the grande ed later

,úl and of al-

_ (Table >en this ,aleon-

:h hosts 1975).

ndesitic stmin-

Postore -;an pe-sample is indis-

Neo-rpreted

Raúl-hydro-

-om the 3 Ata,

_.ocks

.ge (Ma)

± 2.9 )7 ± 2.8

+ 2.7 ..4 ± 3.0

± 2.6 ± 3.3

27 ± 3.1 1 ± 3.0

± 5.0 i7 ± 3.0

± 3.0 ± 3.0

10 ± 4.0 ± 4.0

19 ± 4.0

quoted

respectively. This concordancy is evidente for a mid-die Cretaceous, Aptian-Albian, age of ore formation. At Hierro Acarí the age of 109 ± 4 Ma obtained for a dacitic dike represents only a maximum age for al-teration and mineralization (Atkin et al., 1985).

Lead isotope compositions

Lead isotope compositions of sulfide samples from the Monterrosas, Eliana, Raúl, Condestable, Marcona, and Leonila ores have been determined as part of a larger study that has isotopically characterized min-eral deposits and associated igneous activity in the Western Cordillera of Peru (Mukasa, 1984). The ob-jective has been primarily to test metallogenetic cor-relations and to evaluate the geotectonic framework of central and southern Peru during the Cretaceous. Details on the analytical procedures are provided by Mukasa et al. (1990) and are, therefore, not repeated here.

Table 3 summarizes the Pb isotope ratios obtained for six sulfide samples of amphibolitic Cu-Fe skarn deposits and one sample from the coeval kuroko-type Leonila-Graciela deposit (Vidal, 1987). Figure 8 in-eludes the fields of known lithotectonic units in Peru and trends detected for these Cu-Fe skarns and other ore deposit groups in the central Andean region. The

207p O 206p 0 Cu-Fe skarn IV' 4Pb versus b/2 4Pb data de- fine a linear trend with a correlation coefficient of 0.9517 and slope of 0.2511, corresponding to a Pb isochron age of 3.2 Ga (Mukasa et al., 1990). This age is too old to have chronologic significante and is dought to reflect mixing processes between two or

• more source rocks. On the 2081,1v ^ "4Pb versus 206Pb/ 204Pb diagram the mixing array is not as well defined. There are considerable departures from the line by chalcopyrite from Monterrosas and galena from Leonila. K-Ar dating of the Cu-Fe skarn deposits has established a Lower Cretaceous (128-109 Ma) epoch of metallogenesis.

The Pb isotope mixing array for the Lower Cre-taceous amphibolitic Cu-Fe skarn deposits shows that there is a clear difference between these deposits and

those in the Cenozoic polymetallic districts of central Peru, Sn deposits in Bolivia, and porphyry Cu deposits in Chile and Colombia (Tilton et al., 1981; Gunnesch and Baumann, 1984; Sillitoe and Hart, 1984). A primitive, less radiogenic source is apparent for the Peruvian skarn deposits superseded only by some of the Colombian porphyry Cu deposits. Source mate-rials for the Pb seem to have been dominated by the "enriched" upper mantle in the sense of Tilton and Barreiro (1980). This upper mantle material is in itself probably the product of interaction between depleted mantle and subducted Pacific sediments (Sillitoe and Hart, 1984). The Precambrian basement rocks do not appear to have exerted much influence on these par-ticular deposits.

Concluding Remarks

Classic Cu skarn deposits such as Antamina in Peru and Bingham in the United States (Petersen, 1965; Einaudi and Burt, 1982) are different, in terms of the geologic and tectonic evolution of their ore-gener-ating systems, to the amphibolitic Cu-Fe skarn de-posits here discussed. Granodiorite to quartz mon-zonite stocks emplaced in compressive continental settings, limestone host rocks, and garnet-diopside gangue minerals typify classic Cu skarns. Amphibolitic skarns in Peru are related to parent dioritic plutons emplaced in a tensional marginal basin; volcanoclastic host rocks are predominant and amphibole is their main gangue. Classical Cu skarns are often related to porphyry Cu-(ufo) deposits and zone outward into major concentrations of Zn and Pb surrouncling their chalcopyrite core. Amphibolitic Cu-Fe skarn deposits instead are more specialized and show no polymetallic zonation. Both types have similar paragenetic rela-tionships with magnetite followed by chalcopyrite, in turn followed by sphalerite and galena.

Veins and contact metasomatic lodes of amphiboli-tic Cu-Fe skarn at Monterrosas and Eliana are hosted by gabbrodiorite plutons of the Cretaceous Patap unit. Actinolite and scapolite typify the silicate alteration stage, which produced metasomatic halos of Si, Na,

40

39

208 P 204Pb

38

185 19 206Pb/ 204Pb

17 17.5 18

and C_ (Siddt stages turn a: pyrite evolvt- magin_ iron (Meint subvoi drati ► : trigget Peru. stituen vasive inant _ 198:V graphr.

The compi, Monte each physic 1977 net-1)y formei. olite rock IN, contro: Lime,t• localls tionaL. (19S:2 prefer,- clasts fractm stocks. dant 1» Riplev lated tc by Ost(- ge ► th•: the lin;, Monter

Isot o-fluids a: compoi and gil Patap 1, started suggest, potheti( for the t. the case 1; Table geometr thermal .

15.8 -

15.7

207 Pt/

204 Pb

15.6

15.5

206Pb/204 p b

FIG. 8. Correlation diagram for Pb isotopes of ore deposits in the central coast of Peru. Isotopic data and abbreviations as in Table 2. Fields of Charcani gneisses (CC) after Tilton and Barreiro (1980) and Nazca plate metalliferous sediments (NPMS) after Unruh and Tatsumoto (1976) and Dasch (1981). Regression lines defined are Bolivian Triassic-Cretaceous ores (B), Chilean Paleocene-Eocene ores (Ch), and oceanic basalts (MORB) after Tilton et al. (1981); central Peru Oligocene-Miocene ores (Pe; Soler and Bonhomme, 1987) after Gunnesch and Baumann (1984); and Upper jurassic to Cretaceous ore deposits (P) described in this paper.

17 1

17.5 18 1

18.5 1

19

1459 AMPHIBOLITIC CU-FE SKARN, CENTRAL AL COAST PERU

and Ca enrichment and depletion of Cu, Fe, and P (Sidder, 1984; Injoque, 1985). Intermediate oxide stages are represented by low Ti magnetite, which in turn are followed by pyrite, pyrrhotite, and chalco-pyrite during the sulfide stage. Skarn mineralization evolved from tholeiitic to low K calc-alkaline parent magmas with high Na 20/K20 ratios comparable to iron skarn-related intrusions in British Columbia (Meinert, 1984). Interaction of these magmas at their subvolcanic emplacement level with basinal dehy-dration prompted by burial metamorphism may have triggered the mineralizing hydrothermal activity in Peru. On a regional scale, actinolite is a major con-stituent of the greenshist facies assemblage, the per-vasive alteration of the Patap gabbros, and the dom-inant gangue in the Cu-Fe skarn deposits (Regan, 19S5). Geothermal gradients were high and geo-graphically widespread (Aguirre and Offier, 1985).

The geologic evolution of Raúl and Condestable is complex and indirectly linked to that of Eliana and N1onterrosas. Skarn formation and ore deposition in each stratigraphic unit occurred under different physicochemical conditions (Ripley and Ohmoto, 1977). Within individual hydrothermal events, gar-net-pyroxene skarn and albite-epidote horufelses formed first and were followed by amphibole-scap-olite alteration, magnetite, and chalcopyrite. Host-rock permeability and composition represent the main controlling factors of strata-bound ore deposition. Limestones and shales are thoroughly amphibolitized ;

locally they preserve calcic skarn minerals composi-tionally akin to the Cu-Fe group of Einaudi and Burt (1982). Volcanoclastic sandstones and breccias show preferential replacement of their matrix with relic clasts and phenocrysts. Hydrothermal effects are fracture controlled in lava flows and porphyritic stocks. Recurrent circulation of seawater and atten-dant hydrothermal metamorphism as proposed by Ripley and Ohmoto (1977, 1979) were probably re-lated to the shallow-marine volcanic center evidenced by Osterman et al. (1983). Both findings fit into the geothermal scheme of Bird et al. (1984) and provide the link with the plutonic related skarns of Eliana and .

Monterrosas. Isotopic constraints for the seawater-dominated ore

fluids at Raúl infer the presence of a subdued igneous component (Ohmoto and Rye, 1979). Tectonic setting and gabbrodioritic plutonism akin to the mineralized Patap plutons at Eliana and Monterrosas could have started 10 m.y. before at Raúl and Condestable, as suggested by the K-Ar data. However, a distal or hy-pothetical plutonic source is not considered essential for the genesis of these volcanogenic deposits. As in the case of the Upper Jurassic Marcona deposits (Fig. 1; Table 2), amphibolitic Cu-Fe skarns of strata-bound geometry are interpreted to be closely related to geo-thermal systems developed as part of submarine vol-

canism. Subsea-floor water-rock interaction in this environment, as advocated to explain regional and lo-cal metamorphic patterns, represents the main source for the ore fluids that generated Raúl, Condestable, and Marcona. Ore deposition is envisaged essentially as an endogenous process with little or no hot spring-related exhalative sedimentary accumulations.

Marginal basin development, gabbrodiorite plu-tonism, and burial metamorphism are the vital tec-tonic, igneous, and hydrothermal links proposed for the genesis of the amphibolitic Cu-Fe skarn deposits in the central coast of Peru. Schematic positioning of these deposits in their geologic and geophysical set-ting is shown in Figure 6. This model is consistent with that proposed for the magnetite deposits of Acarí and Marcona in southern Peru (Atkin et al., 1985) and Romeral and others in northern Chile (Bookstrom, 1977). Amphibolitic skarn deposits define two main belts 400 km long southeast of Lima and 600 km long north of Santiago (Petersen, 1970; Frutos, 1982). They correspond to an episode of Upper Jurassic to Lower Cretaceous metallogenesis as evidenced by radiometric dating of the main representatives (Oyarzún and Frutos, 1984; this paper).

Recognition of the regional distribution, genetic coherence, and transition into copper- or apatite-rich varieties of major magnetite deposits in Chile and Peru was done by Park (1972). Nevertheless, he pro-posed a dominant crustal source for the iron which is not in agreement with the Pb isotope data here pre-sented. Further study is needed to elucidate in detail the geochemistry of these amphibolitic Cu -Fe skarn deposits and to confirm their paleotectonic signifi-canee as markers of marginal basin initiation along convergent plate boundaries of central Andean type.

Ackno•ledgments

The authors would like to thank all the staff mem-bers of Cía. Minera Pativilca, Cía. Minera Austria Du-vaz, Cía. Minera Cóndor, and Cía. Los Montes who have supported their research during the past ten years. Special thanks are due to the following geol-ogists and mining engineers: E. Ponzoni, G. Abele, V. R. Eyzaguirre, R. Ravello, A. Cossío, J. Zúíliga, and C. Ríos. Acknowledged also are J. Mendoza, C. Mir-anda, and P. Soler from the Instituto Geológico Mi-nero y Metalurgico (INGEMMET) and Office de la Recherche Scientifique et Technique Outre-Mer (O.R.S.T.O.M.).

J. Injoque acknowledges the encouragement and technical backing from P. Harvey and B. Atkin. G. Sidder is especially grateful to his thesis advisor Cyrus Field. S. Mukasa extends his gratitude to G. R. Tilton, whose research grants supported the early stages of the isotopic work. K -Ar dating was carried out in the British Geological Survey with direct support of N. J. Snelling. César E. Vidal thanks Wallace S. Pitcher

■-77.trriir"

Sillitoe. PorP; Color 2135

Soler, F époc del Lima.

Tilton. cale 1247

Tilton. 1981. Ceo!.

Todd. mine

1460 VIDAL C., INJOQUE-ESPINOZA, SIDDER, AND MUKASA

for guiding his research on the Coastal batholith me-tallogenesis and Norma, his wife, who patiently typed the present contribution.

Perubar, S.A., and Buenaventura Ingenieros, S.A., supported generously the preparation of the manu-script and illustrations. The manuscript was greatly improved by the efforts of Economic Geology review-ers, and the authors wish to give them credit for an oyeran improvement of the same.

REFEREN CES

Agar, R. A., 1978, The Peruvian Coastal batholith: Its monzonitic rocks and their related mineralization: Unpub. Ph.D. thesis, Univ. Liverpool, 261 p.

Agar, R. A., and Le Bel, L., 1985, The Linga super-unit: High-K diorites of the Arequipa segment, in Pitcher, W. S., Atherton, M. P., Cobbing, E. J., and Beckinsale, R. D., eds., Magmatism at a plate edge. The Peruvian Andes: Glasgow, Blackie and Son Ltd., p. 119-127.

Aguirre, L., and Offier, R., 1985, Burial metamorphism in the western Peruvian trough: Its relation to Andean magmatism and tectonics, in Pitcher, W. S., Atherton, M. P., Cobbing, E. J., and Beckinsale, R. D., eds., Magmatism at a plate edge. The Peruvian Andes: Glasgow, Blackie and Son Ltd., p. 59-71.

Atherton, M. P., Pitcher, W. S., and Warden, V., 1983, The Me-sozoic (Huarmey) marginal basin of central Peru: Nature, v. 305. p. 303-306.

Atherton, M. P., Warden, V., and Sanderson, L. M., 1985, The Mesozoic marginal basin of central Peru: A geochcmical study of within-plate-edge volcanism, in Pitcher, W. S., Atherton, M. P., Cobbing, E. J., and Beckinsale, R. D., eds., Magmatism at a plate edge. The Peruvian Andes: Glasgow, Blackie and Son Ltd., p. 47-58. •

Atkin, B. P., Injoque-Espinoza, J., and Harvey, P. K., 1985, Cu-Fe amphibole mineralization in the Arequipa segment, in Pitcher, W. S., Atherton, M. P., Cobbing, E. J., and Beckinsale, R. D., eds., Magmatism at a plate edge. The Peruvian Andes: Glasgow, Blackie and Son Ltd.. p. 261-270.

Bird, D. K., Schiffman, P., Elders, W. A., Williams, A. E., and McDowell, S. D., 1984, Calc-silicate mineralization in active geothermal systems: ECON. GEOL., v. 79, p. 671-695.

Bookstrom, A. A., 1977, The magnetite deposits of El Romeral, Chile: ECON. GEOL., v. 72. p. 1101-1130.

Cardozo, M., 1983, Raúl als Beispiel vulkanogener Kupferlager-statten im Copara Metallotekt, Zentralperu: Unpub. B.Sc. dis-sert., Germany, Univ. Heidelberg, 240 p.

Couch, R., Whitsett, R., Huehn, B., and Briceiio-Guarupe, C., 1981, Structures of the continental margin in Peru and Chile: Ceo!. Soc. America Mem. 154. p. 703-726.

Dasch, E. J., 1981, Lead isotopic composition of metalliferous sediments from the Nazca plate: Geol. Soc. America Mem. 154, p. 199-210.

Einaudi, M. T., and Burt, D. M., 1982, Introduction; terminology, classification, and composition of skarn deposits: ECON. GEOL., v. 77, p. 745-754.

Frutos, J., 1982, Andean metallogeny related to the tectonic and petrologic evolution of the cordillera. Some remarkable points, in Amstutz, G. C., El Goresy, A., Frenzel, G., Kluth, C., Moh, G., Wauschkuhn, A., and Zimmerman, R. A., eds., Ore genesis. The state of the art: Berlin, Springer-Verlag, p. 493-507.

Cunnesch, K. A., and Baumann, A., 1984, The Atacocha district, central Peru: Some metallogenetic aspects, in Wauschkuhn, A., Kluth, C., and Zimmermann, R. A., eds., Syngenesis and epi-genesis in the formation of mineral deposits: Heidelberg, Springer-Verlag, p. 448-456.

Hudson, C., 1967, Geología de la mina Eliana, Río Seco: Unpub. Eng. thesis, Lima, Peru, Univ. Nac. Ingeniería, 160 p.

- 1974, Metallogenesis as related to crustal evolution in southwest central Peru: Unpub. Ph.D. thesis, Univ. Liverpool, 245 p.

Injoque, J., 1985, Geochemistry of the Cu-Fe amphibole skarn deposits of the Peruvian central coast: Unpub. Ph.D. thesis, Univ. Nottingharn, 357 p.

Injoque, J., Mendoza, J., Miranda, C., Aranda, A., Ramirez, L., and Andrade, R., 1982, Petrología y mineralogía del yacimiento Raúl-Condestable y reinterpretación de su génesis: Lima-Perú, Inst. Geológico Minero y Metalurgico unpub. rept., 134 p.

Kulm, L. D., Thornbury, T. D., Schrader, H., Masias, A., Reisig, J. M., and Johnson, L., 1981, Late Cenozoic carbonates on the Peru continental margin: Geol. Soc. America Mem. 154, p. 469-524.

Meinert, L. D., 1984, Mineralogy and petrology of iron skarns in western British Columbia, Canada: ECON. CEOL., v. 79, p. 869-882.

Meinert, L. S., Newberry, R., and Einaudi, M. T., 19S0, An over-view of tungsten, copper and zinc-bearing skarns in North America: U. S. Geol. Survey Open-File Rept. 81-355, p. 303-327.

Moore, N. D., 1984, Potassium-argon ages from the Arequipa segment of the Coastal batholith of Peru and their correlation with regional tectonic events: Geol. Soc. London Jour., v. 141, p. 511-519.

Mukasa, S. B., 1984, Comparative Pb isotope systematics and zir-con U-Pb geochronolog> for the Coastal, San Nicolas and Cor-dillera Blanca batholiths, Peru: Unpub. Ph.D. thesis, Santa Bar-bara, Unir. California. 362 p.

Mukasa, S. B., Vidal, C. E., and Injoque-Espinoza, J., 1990, Pb isotope bearing on the metallogenesis of sulfide ore deposits in central and southern Peru: ECOS. CEOL., y. 85. p.

Ohmoto, H., and Rye, R. 0., 1979, Isotopes of sulphur and carbon, in Barnes, H. L., ed., Ceochemistry of hy drothermal ore de-posits: New York, Wiley -Intersei., p. 509-567.

Osterman, C., Cardozo, M., and Wauschkuhn, A., 1983, Descrip-ción y correlación de los depósitos volcánico-sedimentarios del Cretásico inferior en la región Lima-Caiiete: Soc. Geol. Perú Bol., v. 70, p. 35-45.

Oyarzún, J., and Frutos, J., 1984, Tectonic and petrological frame of the Cretaceous iron deposits of north Chile: Nlining Ceology, y. 34, p. 21-31.

Park, C. F., Jr., 1972. The iron ore deposits of the Pacific basin: ECON. CEOL., v. 67. p. 339-349.

Petersen, U., 1965, Regional geology and major ore deposits of central Peru: ECON. CEOL., y. 60, p. 407-476.

- 1970, Nletallogenic provinces in South America: Geol. Rundschau, v. 59, p. 834-897.

Ponzoni, E., and Vidal, C., 1982, Copper mineralization and mag-matic hydrothermal brines in the Río Pisco section of the Pe-ruvian Coastal batholith: A discussion: ECON. GEOL., v. 77, p. 1951-1955.

Regan, P., 1985, The early basic intrusions, in Pitcher, W. S., Atherton, M. P., Cobbing, E. J., and Beckinsale, R. D., eds., Magmatism at a plate edge. The Peruvian Andes: Glasgow, Blackie and Son Ltd., p. 72-89.

Ripley, E. M., and Ohmoto, H., 1977, Mineralogic, sulfur isotope, and Huid inclusion studies of the strata-bound copper deposits at the Raúl mine, Peru: ECON. GEOL., v. 72, p. 1017-1041.

- 1979, Oxygen and hydrogen isotopic studies of ore depo- sition and metamorphism at the Raúl mine, Peru: Geochi ►n. et Cosmochim. Acta, v. 43, p. 1633-1643.

Rivera, R., Petersen, G., and Rivera, M., 1975, Estratigrafía de la costa de Lima: Soc. Geol. Peru Bol., v. 45, p. 159-186.

Sidder, G. B., 1984, Ore genesis at the Monterrosas deposit in the Coastal batholith of Ira, Peru: Unpub. Ph.D. thesis, Oregon State Univ. 221 p.

- 1987, Alteration and mineralization in the Monterrosas copper-iron deposit, Peru: U. S. Geo!. Survey Circ. 995, p. 65-66.

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Sillitoe, R. H., and Hart, S. R., 1984, Lead-isotopic signatures of porphyry copper deposits in oceanic and continental settings, Colombian Andes: Geochim. et Cosmochim. Acta, v. 48, p. 2135-2142.

Soler, P., and Bonhomme, M., 1987, Sobre la existencia de dos épocas metalogenéticas cenozoicas en la provincia polimetálica del Perú central [abs.): Congreso Peruano de Geología, 6th, Lima, 1987, Abstracts YM-4, p. 110.

Tilton, C. R., and Barreiro, B. A., 1980, Origin of lead in Andean calc-alkaline lavas, southern Peru: Science, v. 210, p. 1245-1247.

Tilton, G. R., Pollak, R. J., Clark, A. H., and Robertson, R. C. R., 1981, Isotopic composition of Pb in central Andean ore deposits: Geol. Soc. America Mem. 154, p. 791-816.

Todd, J. C., 1983, Vertical crater retreat at Peru's new copper mine proves successful: Eng. Mining Jour., v. 184, p. 28 -29.

— 1984, Low copper prices, poor grades imperil Monterrosas mine: Eng. Mining Jour., v. 185, p. 17-19.

Unruh, D. M., and Tatsumoto, M., 1976, Lead isotopic compo-sition and uranium, thorium and lead concentrations in sedi-mento and basalts from the Nazca plate: A preliminary report: Deep Sea Drilling Proj. Initial Repts., v. 34, p. 341-347.

Vidal, C., 1980, Mineral deposits associated with the Peruvian Coastal batholith and its volcanic country rocks: Unpub. Ph.D. thesis, Univ. Liverpool, 239 p.

— 1987, Kuroko-type deposits in the middle Cretaceous mar-ginal basin of central Peru: ECON. GEOL., v. 82, p. 1409-1430.

Wauschkuhn, A., 1979, Geology, mineralogy and geochemistry of the stratabound Cu deposits in the Mesozoic Coastal belt of Peru: Soc. Geol. Perú Bol., v. 62, p. 161-192.

Zharikov, V. A., 1970, Skarns: Internat. Geology Rev., v. 12, p. 541-559,619-647,760-775.