1990 Mukasa, S, Vidal, C.E. and Injoque, J

Economic Geology Vol. 85, 1990, pp. 1438-1446

Pb Isotope Bearing on the Metallogenesis of Sulfide Ore Deposits in Central and Southern Peru

SAMUEL B. MUKASA,

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

Cf•SAR E. VIDAL C.,

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

AND JORGE INJOQUE-ESPINOZA

Cheeron Mineral Corporatioa of Chile, Avertida El Golf 183, Santiago, Chile

Abstract

Twenty-five Pb isotope determinations are presented on galena, chalcopyrite, pyrite, chalcocite, and cerussite from 15 mineral deposits in the copper, iron, and polymetallic provinces of central and southern Peru. The results are evaluated in comparison with the Pb isotope signatures of potential source materials which include plutonic rocks of the Coastal batholith, Precambrian basement rocks of the Arequipa massif, as well as midocean ridge basalts (MORB) of the Nazca plate, and Pacific Ocean pelagic-metalliferous sediments subducted at the Peru- Chile trench.

Several types of mineral deposits are recognized in the copper province which roughly coincides areally with exposures of the Early Jurassic to Paleogene Coastal batholith. Am- phibolitic Cu-Fe skarns with the magnetite-pyrite-chalcopyrite mineral association define an array with a Pb isochron age of 3.2 Ga in the 2ø7pb/2ø4pb versus 2ø6pb/2ø4pb covariation diagram. Mineral deposits of the copper veins in granitoids association at the Cata-Cahete and Cobre- Acarl districts plot along the same array. The isochron age of 3.2 Ga is too old to have chronological significance. The array of points is therefore interpreted as the product of mixing between two end members. Enriched subcontinental mantle (produced by mixing in the subduction zone of pelagic and/or metalliferous sediments and material from a MORB-like source) and supracrustal sedimentary materials are the most likely end members.

The Pb isotope compositions of disseminated porphyry Cu-Mo deposits are virtually identical to those of the batholithic rocks with which they are genetically linked. Thus the E1 Molino prospect has high Pb ratios, typical of plutons near the Lima-Arequipa segment boundary in the Coastal batholith (Mukasa, 1986a). The Pb content of the plutons is chiefly from mixtures of a depleted, MORB-like component and subducted pelagic-metalliferous sediments. The Cerro Verde deposit, like its host rocks, has low 2ø6pb/2ø4pb values most easily explained by mixing of materials derived from the Precambrian basement with those from the subduction- generated enriched subcontinental mantle keel.

Deposits of the auriferous vein association and the unclassified Unturos-Chacaya galena and pyrite deposit show a wide Pb isotope scatter in the Pacific Ocean sediment field, as well as both above and below it. Similarly, galena and chalcopyrite from the polymetallic province show major Pb isotope heterogeneities.

Extensive metal scavenging from local supracrustal materials occurred demonstrably. Ac- cordingly, metal sulfides along Andean-type plate margins do not have a unique source.

Introduction and Geologic Setting

THE Western and Coastal Cordilleras of the Peruvian

Andes consist of several lithotectonic units ranging in age from Proterozoic to Tertiary. Precambrian basement rocks, metamorphosed to granulite facies at 2.0 Ga and referred to as the Arequipa massif, crop out along the continental margin in central and southern Peru (Fig. 1). Paleozoic granitoids of the San Nicolfis batholith cut through the westernmost exposures of the massif, defining a linear belt. Early Jurassic to Pa-

leogene granitoids of the Coastal batholith intrude its medial and easternmost exposures, also defining a linear belt. In central and northern Peru, where Pre- cambrian basement rocks are not exposed, plutons of the Coastal batholith are emplaced in basaltic andesitc to andesitic volcaniclastics and flows of the Casma

Formation, a sequence deposited in elongate marginal basins of the West Peruvian trough between the Late Jurassic and the mid-Cretaceous (Pitcher and Bussell, 1977; Atherton et al., 1983). Tertiary volcanic rocks and stocks, chiefly of silicic composition, crop out in

1438

SULFIDE ORE DEPOSITS, CENTRAL AND SOUTHERN PERU 1439

a linear belt immediately to the east of the Coastal batholith (Fig. 1).

These belts along the Peruvian continental margin comprise one of the most heavily mineralized regions in the world. Mineralization occurs as distinct metal

assemblages, defining parallel belts named metallo- genie provinces by Bellido and De Montreuil (1972). Three such provinces are recognized within the study area. The iron metallogenic province occurs in the Arequipa massif and spatially associated Paleozoic granites of the San Nicolfis batholith, whereas the copper and polymetallic provinces occur in the Coastal batholith and Tertiary stocks, respectively (Fig. 1). Nineteen of the 25 samples analyzed are from the copper province which is therefore described in greatest detail. The remaining six samples are from the other two provinces which are described only briefly. All sample locations are plotted on the map in Figure 1, and their coordinates are given in Ta- ble 1.

There are variations in the mineralization intensity within the copper province. The sector corresponding to the southern exposures of the Coastal batholith (Arequipa and Toquepala segments) is richly mineralized, whereas that corresponding to the central and northern exposures of the batholith (Lima and Trujillo segments) is mineralized only to a subeconomic extent (Bellido and De Montreuil, 1972; Cobbing et al.,

1981). The richly mineralized southern part of the copper province has several types of mineral deposits (Vidal, 1980), and a brief description of those sampled for this study follows.

Mineral Associations

Amphibolitic Cu-Fe skarn deposits

Mineral deposits at Eliana and Monterrosas (9 and 10, respectively, Fig. 1) are amphibolitic magnetite- pyrite-chalcopyrite skarns hosted by gabbros and diorites (Patap superunit) of the Coastal batholith. (Cobbing et al., 1981; Cobbing and Pitcher, 1983; and Pitcher et al., 1985, provide complete descrip- tions of superunits in the study area). Those at Ra61 and Marcona (8 and 11, respectively, Fig. 1) have a similar mineralogy but occur as strata-bound deposits in volcano-sedimentary piles of Neocomian and Pa- leozoic to Jurassic age, respectively. All four deposits formed during development of the Peruvian marginal basin in the Late Jurassic to the mid-Cretaceous.

Disseminated porphyry Cu-Mo deposits

Disseminated porphyry Cu-Mo deposits typically consist of stockwork mineralization with pyrite, chalcopyrite, and molybdenite as the principal hypogene sulfide phases; pyrrhotite, marcasite, covelite, and enargite are subordinate. These deposits are genetically associated with the most siliceous superunits in

E X Pr•NenAoT:oOicN volcanic fields • and associated stocks •"• Coastal Batholith

• Basinal volcanics and volcaniclastics Mesozoic

of

[-'• Platform clastics West Peruvian

• Thin cover of Mesozic Trough volcaniclastics & flows

• requipa Massif, with Paleozoic granites

...... Outer Shelf High

v M

V M V

ß V

km

Cuzco

M

300 '-.

'..

FIG. 1. Lithotectonic map of the Peruvian Western Cordillera showing the locations of prospects and mines sampled for this study in and adjacent to the Coastal batholith. The iron, copper, and polymetallic provinces occur in the belts labeled Paleozoie granites, Coastal batholith, and Cenozoic volcanic fields and associated stocks, respectively. Numbered open circles on the map mark the locations of the prospects and mines as follows: 1 = Cerro de Pasco, 2 = Sureo, 3 = Huaehoe, 4 = Leonila, 5 = Pueara, 6 = E! Molino, 7 = Cata-Cafiete, 8 = Ra61, 9 = E!iana, 10 = Monterrosas, 11 = Mareona, 12 = Agripina, 13 = Cobre-Aearl, 14 = Cerro Verde, and 15 = Unturos-Chaeaya.

1440 MUKASA, VIDAL C., AND INJOQUE-ESPINOZA

TABLE 1. A Summary of the Ore Deposits Studied Showing Their Names, Host Rocks, Locations (Longitudes and Latitudes), and the Numbers Corresponding to the Deposits on the Map in Figure 1

Map no. Deposit Host rocks • Location (long and lat)

i Cerro de Pasco Miocene volcanic center 10 ø 38' S, 76 ø 17' W 2 Surco Surco stock 11 ø 53' S, 76 ø 27' W 3 Huachoc Parat superunit 11 ø 40' S, 77 ø 02'W 4 Leonila Casma volcanics 12 ø 02' S, 76 ø 35' W 5 Pucar5 Casma volcanics and Tiabaya- 12 ø 06' S, 76 ø 37' W

Patap superunits 6 El Molino Tiabaya superunit 12 ø 15' S, 76 ø 35' W 7 Cata-Cafiete Incahuasi superunit 12 ø 35' S, 76 ø 24' W 8 Ra61 Casma volcanics 12 ø 40' S, 76 ø 40' W 9 Eliana Patap superunit 13 ø 47' S, 75 ø 47' W

10 Monterrosas Patap superunit 14 ø 00' S, 75 ø 37' W 11 Marcona Marcona Formation 15 ø 14' S, 75 ø 08' W 12 Agripina Incahuasi superunit 15 ø 09' S, 74 ø 31' W 13 Cobre-Acarl Linga superunit 15 ø 16' S, 75 ø 20' W 14 Cerro Verde Linga superunit 16 ø 35' S, 71 ø 40' W 15 Unturos-Chacaya Santa Rosa superunit 11 o 48' S, 76 o 22' W

• Most of the deposits studied are associated with magmatic series of superunits in the Jurassic to Pale%ene Coastal batholith; Cobbing et al. (1981), Cobbing and Pitcher (1983), and Pitcher et al. (1985) provide details about distributions, compositional variations, and degrees of mineralization for the superunits

the mineralized segments of the Coastal batholith. Two such deposits (El Molino (6) and Cerro Verde (14) in Fig. 1) are considered in this study. The E1 Molino deposit is hosted by the Upper Cretaceous Tiabaya granodiorite, and the Cerro Verde deposit by an unnamed porphyritic quartz monzonite stock spatially and temporally associated with the Linga and Yarabamba monzonitic suites of the Coastal batholith

(Le Bel, 1979; Cobbing and Pitcher, 1983). The Cerro Verde deposit has undergone supergene enrichment near its top as indicated by assemblages of chalcocite, cerussite, wulfenite, and bornitc. The zircon U-Pb ages on the porphyritic quartz monzonite are con- cordant at 61 Ma (Mukasa, 1986b), indicating that the stock represents the youngest plutonism in the Arequipa region.

Auriferous veins

Mineral deposits at Huachoc, Pucara, and Agripina (3, 5, and 12, respectively, Fig. 1) occur as auriferous veins. The veins chiefly consist of quartz, pyrite, calcite, chalcopyrite, galena, sphalerite, and arsenopyrite. Small amounts of gold may be present in quartz, or as inclusions in sulfides, or even in solid solution series with pyrite and/or chalcopyrite. Most of the auriferous veins are hosted by granodioritic suites of the Tiabaya and Incahuasi superunits of the Coastal batholith and by the adjacent country rocks. The Huachoc vein deposit is emplaced in Albian-age gabbros of the Patap superunit which are preserved as roof pendants on top of younger tonalites. At Pucar•, the Francesita Linda vein is hosted by a granodiorite of the Tiabaya superunit, a gabbro of the Patap superunit, and volcaniclastics of the Casma Formation

which the plutons intrude. Genetically, however, the vein is probably related to the Tiabaya superunit. Fi- nally, the Agripina veins occur in diorites and tonalites of the Incahuasi superunit to which they are akin.

Copper veins in granitoids

The copper vein association was designated for those veins related to plutons in the Coastal batholith but are not auriferous. From this group Pb isotope data are presented on the Cata-Cafiete and Cobre- Acarl mines (Fig. 1). Veins of the Cata-Cafiete deposit cut diorites and tonalites of the Incahuasi superunit as well as volcaniclastic rocks of Early Cretaceous age. Principal minerals in the Cata-Ca•ete veins are quartz, chlorite, pyrite, and chalcopyrite. Magnetite, sphalerite, arsenopyrite, bornitc, galena, ilmenite, molyb- denitc, and pyrrhotite are present only in minor amounts.

Monzonites of the Linga superunit host the Cobre- Acari copper veins. Pyrite, chalcopyrite, magnetite, specularitc, and tremolite are the most abundant phases in the veins. Apatite, calcite, quartz, galena, sphalerite, and chlorite are present only locally, and even then in modest abundances. Tourmaline, potas- sium feldspar, chlorite, and epidote dominate the al- tered zones surrounding the veins.

Polymetallic sulfides related to Tertiary stocks We have studied two deposits in the polymetallic

province: one at Surco and the other at Cerro de Pasco (2 and 1, respectively, Fig. 1). The Surco deposit occurs in a skarn composed dominantly of galena, chalcopyrite, molybdenite, and sphalerite, with stibnite and gold in smaller amounts. Mendoza and Chevarrin


(1983) have shown that genetically the deposit is related to hypabyssal stocks of Tertiary age, dated at 21 ___ 0.5 Ma by Mukasa (unpub. data) using the zircon U-Pb method.

The Cerro de Pasco deposit is associated with a volcanic center of Miocene age (Silberman and Noble, 1977), and it consists of two generations of mineralization. The earlier mineralization is dominated by pyrrhotite pipes that occur centrally and huge galena- sphalerite replacement bodies that surround the east- ern flank of the volcanic center. Enargite-luzonite- pyrite veins represent the second stage of mineralization and are emplaced principally within and around the volcanic center (Einaudi, 1977).

Anomalous samples

Three of the 25 samples in the group cannot be categorized as easily as the others. They include galena and pyrite from the Unturos-Chacaya deposit, which is a Cu-Pb-Zn skarn in tonalites of the Santa

Rosa superunit, and galena from the Leonila deposit (15 and 4, respectively, Fig. 1).

Analytic Techniques

Nearly pure, crushed, and handpicked samples of chalcopyrite, pyrite, chalcocite, and cerussite weighing 10 to 20, 50 to 100, 10 to 20, and 1 mg, respectively, were analyzed. The samples were dissolved in 3 ml of 7 N HNOa in PFA teflon beakers on hot plates set at 80øC. They were then heated to dryness, redissolved in 2 ml of 6 N HC1, and reevaporated to dryness. The residues were dissolved in 0.2 ml of 3 N HC1 and 3 ml of 1 N HBr before column chemistry.

Sample solutions were loaded onto 0.5-ml ion exchange columns filled with precleaned AG 1 X 8 100- to 200-mesh resin. Pb was purified by rinsing the columns with 8 col. vol of 1 N HBr and 5 col. vol of 3 N HC1. Pb was removed from the columns with 6 col.

vol of 6 N HC1. After evaporation to dryness, the residues were redissolved in 0.1 ml of 3 N HC1 and 1 ml of I N HBr. The solutions were loaded onto 0.2-ml

cleanup columns and further purification of Pb was accomplished using the same column volumes of re- agents as specified above. Upon evaporation to dryness, the samples were ready for mass spectrometry.

Galena samples weighing less than I mg were dissolved in 2 ml of 3 N HC1. Only 0.1 ml of the solutions were loaded onto 0.5-ml ion exchange columns con- taining the same cleaned resin as above, but equili- brated with 3 N HC1. The columns were washed with

5 col. vol of 3 N HC1, and Pb was removed using 6 col. vol of 6 N HC1. After evaporation to dryness, the residues were redissolved in 1 ml of 3 N HC1. Then

0.2-ml cleanup columns were used following the same procedures as those for the first column. Two blank analyses processed with these ore samples contained

0.6 and 0.7 ng of Pb, amounts which are negligible in corrections on Pb isotope compositions.

Samples were loaded onto rhenium filaments using the silica gel-phosphoric acid method. All ratios were normalized for fractionation using replicate analyses of U.S. National Bureau of Standards (NBS) Pb sample SRM-981 (Catanzaro, 1967; Catanzaro et al., 1968). The normalization was 0.09 ___ 0.02 percent per mass unit for 2ø4pb/2ø6Pb and 2ø7pb/•ø6Pb, and 0.13 ___ 0.02 percent per mass unit for •øsPb/•øaPb. Precisions on Pb ratios are _0.1 percent or better, and for Pb concentrations they are ___0.25 percent or better.

Results

Pb isotope ratios for the metal sulfides are listed in Table 2; Pb concentrations measured for some of the samples are also included. The Pb ratios are plotted on the diagrams in Figure 2. These diagrams also include the isotopic fields for some potential Pb sources along an Andean-type plate margin. In Peru, the plotted potential Pb sources include subducted oceanic crust of the Nazca plate and both the subducted pelagic and metalliferous sediments of the Pa- cific Ocean basin. The fields for plutons in the southern segments of the Coastal batholith (Mukasa, 1986a) are included as well for comparison. Potential Pb sources not included on the diagrams in Figure 2 are (1) Precambrian basement rocks of the Arequipa massif where low •ø•Pb/2ø4pb ratios (all <17.20) would require a constricted scale on the diagrams; (2) enriched subcontinental mantle keel which must have

Pb ratios that lie between the fields for depleted mantle (Nazca plate MORB) and subducted Pacific Ocean sediments; and (3) supracrustal sediments where fields on the diagrams generally overlap with fields for Pa- cific Ocean pelagic sediments and Mn nodules.

The Pb isotope data for the amphibolitic Cu-Fe skarn deposits (open hexagon, solid triangles, and open upright triangles) define an array on the •ø7Pb/ •ø4pb versus •øaPb/2ø4pb covariation diagram (Fig. 2a) with a correlation coefficient of 0.9517 and a slope of 0.2511, corresponding to a Pb isochron age of 3.2 Ga. Mineral deposits of the copper veins in granitoids association at Cata-Cafiete (half-filled circle) and Cobre-Acari (open inverted triangle) plot along the same array. When these vein deposits are considered in the linear regression, the correlation coefficient becomes 0.9460, and the slope is reduced slightly to 0.2338, translating into a Pb isochron age of 3.1 Ga. These dates are too old to have chronological significance, and the data are therefore interpreted in terms of a mixing scheme. On the •øsPb/2ø4pb versus •ø•Pb/ •ø4pb diagram (Fig. 2b), the mixing array is not well defined since pyrite from Monterrosas (one of the solid triangles) is considerably below it. This pyrite sample, however, has only 0.659 ppm of Pb, compared to 5.372 ppm for the cogenetic chalcopyrite,


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2O6 Pb/204pb

FIC. œ. 2ø?Pb?ø4pb versus 2ø6pb?ø4Pb and •øsPb?ø4Pb versus •ø6Pb/•ø4Pb covariation diagrams and b, respectively) for metal sulfide samples from the Western Cordillera of Peru. The following symbols have been used for the prospects and mines that are numbered on the location map in Figure 1: open circles = (1) Cerro de Pasco; vertically half-filled square = (2) Surco; open square = (3) Huachoc; open diamond -- (4) Leonila; solid diamond -- (5) Pucar•t; half-filled diamond -- (6) El Molino; half-filled circle = (7) Cata-Cafiete; open upright triangles = (8) Ra(ll; open hexagon = (9) Eliana; solid triangles -- (10) Monterrosas; solid hexagon = (11) Marcona; diagonally half-filled square = (12) Agripina; open inverted triangle -- (13) Cobre-Acar{; solid circles -- (14) Cerro Verde; and solid squares = (15) Unturos-Chacaya. These data are compared to the fields for Nazca plate MORB, East Pacific metalliferous sediments, Pacific Ocean pelagic sediments, and southern Coastal batholith plutons. Also shown for reference is the two-stage growth curve for conformable crustal Pb of Stacey and Kramers (1975); data defining the fields on the diagrams are from Unruh and Tatsumoto (1976) for the Nazca plate MORB; Unruh and Tatsumoto (1976) and Dasch (1981) for East Pacific metalliferous sediments; Reynolds and Dasch (1971), Meijer (1976), Unruh and Tatsumoto (1976), Sun (1980), and Vidal and Clauer (1981) for Pacific Ocean pelagic sediments; and Mukasa (1986a) for southern Coastal batholith plutons.

and could therefore have been affected more pro- foundly by Precambrian basement rocks in the area.

The two disseminated porphyry Cu-Mo deposits analyzed (El Molino and Cerro Verde represented by a half-filled diamond and solid circles, respectively, in Fig. 2) have markedly different Pb isotope ratios. The El Molino deposit situated near the Lima-Are-

quipa segmental boundary of the Coastal batholith lies within a small area where the fields for enriched

subcontinental mantle keel and sediments overlap. In contrast, samples from the Cerro Verde deposit define a cluster between the fields of enriched mantle and/ or Pacific Ocean sediments and Precambrian base-

ment rocks. Moreover, among the five Cerro Verde


data points, there is a pronounced spread in the 2ø7pb/ 2ø4pb ratios for the measured •øaPb/2ø4pb ratios.

Three samples were analyzed from the auriferous vein deposits. Pyrite was the only mineral obtained from the deposits at Agripina (diagonally half-filled square) and Pucar• (solid diamond). Because these samples are extremely poor in Pb (<0.5 ppm), the significance of their ratios is difficult to ascertain. The third sample in this group is chalcopyrite from Huachoc (open square), which on both Pb covariation diagrams (Fig. 2) lies within the fields for Pacific Ocean pelagic and metalliferous sediments.

The polymetallic belt was sampled at Cerro de Pasco and Surco (Fig. 1). Four galenas from Cerro de Pasco (open circles) show a very limited range in •ø6Pb/•ø4pb ratios, but a large one in the •ø7pb/2ø4pb and •øspb/•ø4pb ratios. In Figure 2a, one of the four samples overlaps with the pelagic sediment field; the rest lie above it. In Figure 2b, three of the four samples plot within the pelagic sediment field. A chalcopyrite sample from Surco (vertically half-filled square) in the same mineral belt has notably different ratios. In Figure 2a, the Surco ratios plot between the fields for MORB and Pacific Ocean metalliferous sed-

iments, whereas in Figure 2b they plot within the fields for Pacific Ocean pelagic and metalliferous sediments.

Galena and pyrite from the Unturos-Chacaya skarn deposit (solid squares) in tonalites of the Santa Rosa superunit yield ratios that place them between the fields for Nazca plate MORB and Pacific Ocean pelagic sediment on the •ø7pb/•ø4pb versus 2ø6Pb/•ø4pb diagram, but within the field for pelagic sediments on the •øsPb/•ø4pb versus •øaPb/2ø4pb correlation diagram.

Lastly, pyrite from the Marcona deposit (solid hexagon) has the lowest •øaPb/•ø4pb ratio of all the samples studied and plots closest to the Pb isotope field for the Precambrian basement rocks.

Discussion

Pb isotope data on the amphibolitic Cu-Fe skarn deposits at Rafil, Eliana, and Monterrosas (open upright triangles, open hexagon, and solid triangles, respectively) define a line where the slope corresponds with a Pb isochron age of 3.2 Ga (Fig. 2a). Because none of the potential end members are of that age in Peru, this date is interpreted as having no chronological meaning. More likely, the line is a mixing array where two end members have low and high •ø7pb/ •ø4pb ratios, respectively. The nature of these end members is not certain, but they are least likely to be a MORB-like source and Precambrian basement rocks

alone since these would produce mixing arrays with a negative slope. If the enriched mantle cluster defined by rocks from central Chile (Tilton and Barreiro,

1980) is conceptually time-adjusted to the Albian age of the Eliana and Monterrosas deposits using the Sta- cey-Kramers (1975) curve for an average crustal 2ssU/ •ø4pb value, then enriched subcontinental mantle keel becomes a viable Pb source for these deposits. The high •ø?Pb/•ø4pb end member is probably supracrustal sedimentary rocks or Pacific Ocean pelagic sediments.

Pb isotope data on the copper veins in granitoids at Cata-Cafiete and Cobre-Acari (half-filled circle and open inverted triangle, respectively) also lie between the two proposed end members. Accordingly, these vein deposits are also believed to be the products of mixing between materials derived from enriched mantle and supracrustal rocks or subducted pelagic sediments.

Pb isotope compositions of the disseminated porphyry Cu-Mo deposits are consistent with those of the batholithic rocks to which they are genetically linked. The E1 Molino deposit (half-filled diamond) near the Lima-Arequipa segmental boundary of the Coastal batholith has Pb isotope ratios that are nearly identical with those of the host Tiabaya granodiorites (Mukasa, 1986a). The ratios fall within a small area of overlap between the fields for enriched mantle keel and pelagic or supracrustal sediments. An origin of Pb from an enriched mantle keel for this deposit is preferred because the host rocks, believed to be derived from underplated mantle materials (Cobbing and Pitcher, 1983; Mukasa, 1986a), have similar ratios. Additionally, derivation of Pb from supracrustal materials and its concurrent distribution throughout the disseminated sulfides require that the host rocks possess enough heat to support such Pb mixing. A strong thermal impression would then have been re- corded by the supracrustal rocks, but none is noted.

By contrast, disseminated porphyry Cu-Mo deposits and their host rocks at Cerro Verde (solid circles) have considerably lower •øaPb/2ø4pb ratios. The ratios fall between fields for the Precambrian basement

rocks and sub-Andean enriched mantle; they are closer to the latter. This is generally the same pattern found for Pb in the nearby Arequipa and Barosso volcanic rocks (Tilton and Barreiro, 1980). Accordingly, Pb in the sulfide deposits at Cerro Verde is primarily of enriched mantle keel origin, but it has been sig- nificantly contaminated with Precambrian basement Pb. The five Cerro Verde samples show some differ- ences in 2ø7pb/•ø4pb values. Although these are en- closed by the mixing envelope between the Precam- brian basement and the enriched mantle, to some de- gree, they probably reflect recent modifications resulting from supergene enrichment. Three of the five samples (SOP-9, SOP-10, and SOP-11, Table 2) are chalcocite and cerussite collected from the zone

of supergene enrichment; the other two are chalcopyrite samples from the zone of primary sulfides.

Auriferous veins were sampled at Huachoc, Pucarft,


and Agripina and are represented by an open square, solid diamond, and diagonally half-filled square, respectively, in Figure 2. Interpretation of the results on the Pucar• and Agripina deposits is, unfortu- nately, complicated by the fact that the pyrites analyzed have very low Pb concentrations (<0.5 ppm). It is not certain, for example, whether the high 2ø?Pb/ 2ø4pb ratio in pyrite from Pucar•t is typical of all the other sulfides in the deposit, or whether it is simply an artifact of subsequent isotopic contamination of Pb-deficient phases in the supracrustal environment. Chalcopyrite from Huachoc has 1,136 ppm Pb. Its isotopic ratios lie within the sediment fields, but through extrapolation from other areas, those of its gabbroic host rocks in the Patap superunit of the Coastal batholith lie within the enriched mantle field

(Mukasa, 1986a). An origin involving the mixing of Pb derived from the enriched mantle and supracrustal materials is therefore indicated.

Samples from Cerro de Pasco and Surco in the polymetallic belt show the highest and largest range in •ø?Pb/•ø4Pb ratios. These are similar to galena Pb isotope compositions at Casapalca and Pasto Bueno (Doe and Zartman, 1979) in the same mineral belt. In Figure 2a, chalcopyrite from the Surco deposit (vertically half-filled square) lies very close to the MORB field, and except for one sample, galena from Cerro de Pasco (open circles) defines a cluster above the sediment field. There is an extreme 0.9 percent difference in •ø?Pb/•ø4Pb ratios between chalcopyrite from Surco and sample SOP-21 (Table 2) from Cerro de Pasco in the same mineral belt. Even within the

same deposit at Cerro de Pasco, galena shows a wide range in 2ø?Pb/•ø•Pb ratios. These variations are suggestive of either shallow level, heterogeneous mixing that involved supracrustal sediments and an enriched mantle component or derivation of Pb from a single, isotopically heterogeneous sedimentary SO!lrce.

Galena and pyrite from the Unturos-Chacaya deposit (solid squares) are isotopically different from each other. In Figure 2a the data points lie between the fields for MORB and Pacific Ocean pelagic sediments, and in Figure 2b they both fall within the pelagic sediment field. Inasmuch as Pb isotope ratios of the Santa Rosa tonalites with which the sulfides are associated fall in the enriched mantle field, it seems that supracrustal mixing is required to explain their origin.

Finally, pyrite from the Marcona deposit (solid hexagon) yields the lowest •ø6Pb/•ø4pb ratios of all the sulfide samples measured. The Pb is apparently a mixture between that derived from the Precambrian basement rocks and that from either enriched mantle

(developed by the mixing of depleted mantle and pelagic sedimentary components) or supracrustal sediments. The Marcona deposit is most important for its

magnetite ore. That the Precambrian basement is a prominent metal source as suggested by Pb isotope data is corroborated by the high abundance of FegO• (* means total iron) in these rocks--as high as 15 wt percent in some areas (Barreiro, 1982).

Conclusions

The main conclusions can be summarized as follows:

1. Amphibolitic Cu-Fe deposits of mid-Cretaceous age and copper veins in granitoids of probable Late Cretaceous age define a line with an isochron age of 3.2 Ga on a 2ø?Pb/•ø4Pb versus •ø6Pb/•ø4Pb diagram. This age has no chronological significance, and the array of points is interpreted as a mixing line. Enriched mantle keel and supracrustal sedimentary rocks are the most likely end members for this mixing.

2. Low gøaPb/2ø4pb ratios indicating the presence of a Precambrian crustal component of Pb are shown by the hypogene sulfide and supergene carbonate samples from Cerro Verde and the hypogene pyrite sample from Marcona. Samples closest to the Coastal batholith segment boundary near Lima, by contrast, generally have the highest •ø•Pb/•ø4Pb ratios. These observations are broadly consistent with the conclusions reached by Mukasa (1986a) through a Pb isotope study of feldspars from the batholith. These feldspars show that the Precambrian basement is important as a source of materials incorporated into the batholith only in southern Peru. Seismic and gravity modeling have shown that along the axis of the batholith, the Precambrian basement is four to five times thicker in southern Peru than it is in northern Peru (Couch et al., 1981; Jones, 1981).

3. Generally high Pb ratios and pronounced het- erogeneity with respect to the zø7Pb/gø4Pb ratios for deposits in the polymetallic belt attest to the impor- tance of supracrustal materials as a source of lead and possibly other metals.

4. Mineral deposits with the same paragenetic associations did not necessarily have a common Pb source. It seems that, in addition to Pb mixing at the subcrustal level, a great deal of Pb (and by inference other metal) scavenging from local supracrustal materials occurs. Accordingly, metal sulfides along An- dean-type plate margins do not have a unique source.

Acknowledgments

Samples for this study were collected mainly by the authors with kind additions by J. Mendoza and C. Miranda. We are indebted to C. A. Hopson, A. P. LeHuray, U. Petersen, G. R. Tilton, and two Economic Geology reviewers for critical reviews of the original manuscript, and C. F. McQueen for drafting the diagrams and typing the manuscript.


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Documents

1990 Mukasa, S, Vidal, C.E. and Injoque, J