Metal sources in stratabound ore deposits in the Andes (Andean Cycle) - Lead isotopic constraints

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Reference

Metal sources in stratabound ore deposits in the Andes (Andean Cycle) - Lead isotopic constraints

FONTBOTÉ, Lluís, GUNNESCH, Klaus A., BAUMANN, A.

FONTBOTÉ, Lluís, GUNNESCH, Klaus A., BAUMANN, A. Metal sources in stratabound ore deposits in the Andes (Andean Cycle) - Lead isotopic constraints. In: Fontboté, L., Amstutz, G.C., Cardozo, M., Cedillo, E. & Frutos, J. Stratabound ore deposits in the Andes . Berlin : Springer, 1990. p. 559-573

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http://archive-ouverte.unige.ch/unige:78122

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1 / 1

Metal Sources in Stratabound Ore Deposits in the Andes (Andean Cycle) - Lead Isotopic Constraints

L. FONTBor:E1•2, K.A. GUNNESCH1•3 and A. BAUMANN⁴

1 Introduction

The scope of this paper is to discuss the major constraints posed by lead isotopic ratios with respect to possible metal sources in stratabound ore deposits of the Andean Cycle. This is based on a compilation of all available lead isotopic ratios of ore minerals from stratabound ore deposits. A few additional new analyses are also presented. In the present chapter only main patterns observed on a large re- gional scale will be dealt with; the reader is referred to the cited publications on the different ore deposits for additional details. Since information on lead isoto- pic ratios on host rock samples is fragmentary and, is in many cases lacking, the discussion will refer mainly to ore lead data.

Ore lead isotopic ratios from 19 ore deposits are considered. These data repre- sent a fairly good coverage for the main types of stratabound ore deposits of the Andean Cycle (Mesozoic-Recent) in the Central Andes. Thble 1 and Fig. 1 indicate the metallogenetic stage to which the considered ore deposits belong, according to the scheme presented by Fontbote (this Vol. a), which is based on the geotec- tonic position of the host rock. It can be seen that lead isotopic data from most geotectonic positions exist already.

2 Available Information on Lead Isotopic Ratios

The ore deposits dealt with in this chapter are described in other parts of this Vol- ume (see references in Table 1). Figure 2 is a compilation of all available lead iso- topic data of ore minerals of stratabound ore deposits in the Andean Cycle. It in- cludes data of Gunnesch and Baumann (1984, 1986}, Gunnesch (1986}, and Gun- nesch and Baumann (1990) from the ore deposits San Vicente, Huaripampa, Carahuacra, Machcan (stratiform part}, El Extraiio, Cercapuquio, Los leas, Raul, Condestable, and l..eonila-Graciela; of Mukasa (1984) from Raul and l..eonila- Graciela; of Puig (1988 and this Vol.) from Thlcuna, Maria Cristina, El Plomo, Triunfo-Carola; and El Soldado of Flint et al. (1986) from Caleta Coloso; and

1 Mineralogisch-Petrographisches Institut der Universitllt Heidelberg, INF 236, D-6900 Heidelberg, FRG

2 Present address: Dep. de Mineralogie, 13, rue des Maraichers, CH-1211 Genl:ve 4, Switzerland 3 Present address: U.A.N.L., Facultad de Ciencias de Ia Tierra, Ap. 104, 67700 Linares, NL, Mexico

4 Zentrallaboratorium ftlr Geochronologie, Institut fiir Mineralogie, Correnstr. 24, D-4400 Miinster, FRG

Stratabound Ore Deposits in the Andes L. Font bote, G. C. Amstutz, M. Cardozo, E. Cedillo, J. Frutos (Eds.)

© Springer-Verlag Berlin Heidelberg 1990

760 L. Fontbote et al.

Table 1. Metallogenetic stages (according to Fontbote this Vol. a) of the ore deposits with available lead isotopic data. References of isotope data in text. MVT: Mississippi Valley-type

Stage

Central Andes (5°- 42 •s)

Ore deposit with lead data

References (description of ore deposit)

Stage I (Triassic-Liassic). Ore deposits in a carbonate platform without apparent relation to a pair magmatic arc-back-arc basin (Pucara basin, Peru)

Ia) Volcanic-associated polymetallic Carahuacra, deposits (in part massive sulfides) Huaripampa,

Machcan Ib) MVT deposits at the base of the Shalipayco

carbonate sequence

Ic) MVT deposits within the carbonate San Vicente sequence

Dalheimer this Vol.

Dahlheimer this Vol.

Hirdes this Vol.

Fontbote this Vol. b Gorzawski et al. this Vol.

Stage II (Liassic-Albian). Ore deposits in the ensialic paleogeographic pair magmatic arc-back-arc basin, and in the platform at the continent edge

IIa) In volcanic sequences at the magmatic arc

lib) In volcaniclastic basins near the Caleta Coloso Flint and Turner this Vol.

volcanic arc (in part developed as Talcuna intra-arc basins)

Camus this Vol.

lie) In volcano-sedimentary sequences Los leas, Raul Wauschkuhn 1979; Cardozo this Vol.

in back-arc basins (in part de- Leonila-Graciela Vidal 1987; Fontbote this Vol. a veloped as marginal basins)

lid) Ore deposits in marine sedimen-

tary sequences in back-arc posi- Triunfo-Carola Diaz this Vol. b tion

lie) In platform sediments attached to the foreland

El Plomo, Jaula, Diaz this Vol. a Las Caiias Fontbote this Vol. a

El Extraiio Samaniego 1982; Fontbote this Vol. a Huanzala Carrascal and Saez this Vol.

Cercapuquio Cedillo this Vol.

Hualgayoc Canchaya this Vol.

Stage III (Upper Cretaceous-Cenozoic). Ore deposits in continental intra-arc and foreland basins Ilia) Fluvial-lacustrine basins in inter-

mediate to acid volcanic environ- ment

IIIb) Molasse sequences in inter- S. Bartolo montane basins

Illc) Deposits related to Cenozoic alkaline volcanics

Southern Andes (south of 42° S)

SIIc) Ore deposits in Mesozoic back- El Toqui arc basins

Flint this Vol.

Wellmer and Reeve this Vol.

of Flint (1986) from San Bartolo; of MacFarlane (1989) from Hualgayoc of Soler and Rotach-Toulhoat and Soler (pers. commun.) from Huanzala; from Shali- payco, Jaula and Las Canas (this Chap). All previous data are of ore deposits oc- curring in the Central Andes. The only results from an ore deposit outside the central Andes are those from the El Toqui Zn-Pb deposit, located in the southern Andes (Puig this Vol.). With the exception of the data from Caleta Coloso, San

Metal Sources in Stratabound Ore Deposits in the Andes (Andean Cycle)

w

STRATABOUND ORE DEPOSITS IN THE CENTRAL ANDES (ANDEAN CYCLE)

ISTAGE

ml

r-IDa--!

Cu • Pb · Zn - Ag [I)

r---';..._-m b-=---i m c Cu - (U, V)

ISTAGE II I

[S]

m

1-II c---; r--n e ____,

Cu , Zn - Ba· Cu Zn - Pb-(Cu - Ag) , A LG EX HU CO

~ ·.···

CA (Th) LB -I)

El El

L. Cretac .

E

Sub- Andean b.

Cu(U.V)Zn - Pb

I ·-·II a --1 II b 1-II d---1 1-lie- - l Ba - Sr( Pb)

-

T CC MA JA

CA [B-Ij

"

7;:: '/,J.'.',~#i~@

M. Juras. - L. Cre tac.

ISTAGE I

1----I a --1

m

~ Pb - Zn(Mn , Fe ,Cu , Ba) Zn - Pb(Ba - F)

~ VA SV

761

Fig. 1. Schematic W-E cross-sections indicating geotectonic position of selected ore deposits with available lead isotope compositions in the Central Andes. Additional information on location and geologic background of the cross-sections is given in Fig. 2 of Fontbote (this Vol. a). CC Caleta Col- oso; CQ Cercapuquio; EX El Extrafio; HU Huanzahi; JA Jaula-Bellavista and Las Canas;

LG Leonila-Graciela; MA Triunfo-Carola and El Plomo; RA Raul; SB San Bartolo; SH Shalipayco;

SV San Vicente; TA Thlcuna; YA Carahuacra and Huaripampa

762

39 .5 3 9.0 38 .5

lb Shalopayco IMVT central

Pucara I

18.4 18.6

Ia Volcantc -associ"ted western Pucara

I

Ill

18 .8

I c San Vic. en e I VT eastern Pucara)

t

Stages Upper Triassic - Liassic (Pucara-G.) II Liassic- Lower

Cretaceous III Upper Cretaceous -

Tertiar

Sl 1

s.s-r--- - - -- - - ---.

.,. 0..

..

N

~ 15.7 .... 0..

..

15.6

18 .4 18.6

o Cale a Coloso

[I Talcuna

• Tr iunfo-Carola, El Plomo.

Jaula. Las Caiia s

v El Toqui

<) los leas

18 .8 19.0

~ Raul o Leo nita-Grac i ~ Ia

~ E I Sold ado

• Shalopayco

"' S. Bartolo

-+ Carahuacra- Huar1pampa

19.2

+error bars

o f l Extraiio o Cercapuquio

t> Hualgayoc

x Machc.in

*

L. Fontbote et a!.

Fig. 2. Compilation of lead isotopic compositions of ore minerals from stratabound ore deposits in the Central Andes (references in the text). Evolution curves of Doe and Zartman (1979)

Bartolo, and one analysis from Raul which correspond to copper sulfides, all oth- er plotted points are of galena analyses.

Information on pre-Andean stratabound ore deposits is available only from Aguilar (Brown 1962) and from Helvecia (Brodtkorb and Brodtkorb 1982) and will not be considered further.

3 Lead Isotopic Composition and Geotectonic Setting

The classification of the stratabound ore deposits based on their geotectonic and paleogeographic position summarized in Table 1 is remarkably coherent with the lead isotopic ratios represented in Fig. 2. The lead isotopic data correlate closely with the geotectonic position of the correspondent ore deposit. There is a consis-

Table 2. Location, short description, and lead isotopic composition of analyzed samples Mine Coordinates Sample Analyzed mineral 206pb;204pb 207pb;204pb 208pb;204pb Shalipayco 10°50'S/75°58'W FYA-56 Galena as cement in basal conglomerate 18.560±0.011 15.637 ±0.012 38.536 ± O.o38 FSH-19 Galena in coarsely-grained dolomite, lower manto 18.564±0.014 15.655 ±0.015 38.595 ± 0.044 Las Caii.as 28°51'S/70°52'W FX-66 Galena in a barite diagenetic crystallization rhythmite 18.442 ±0.010 15.582 ± 0.011 38.239 ± 0.032 FX-67 Galena as cement of a volcaniclastic breccia 18.445±0.013 15.578 ± 0.013 38.228 ± 0.040 Jaula 28°12'S/70°38'W FX-80 Galena in carbonate sample at lower part of the manto 18.392±0.009 15.574±0.010 38.176 ± 0.033 Analyses carried out at the Zentrallaboratorium fiir Geochronologie, Inst. fiir Mineralogie, Munster, with a Teledyne NBS-type 12"90 solid source (thermal ionization) mass spectrometer. Additional analytical procedures can be found in Gunnesch and Baumann (1990). Errors according to the calculation suggested by Brevart et al. (1981)

I

::r i g s.

§ ~

"'

764 L. Fontbote et al.

tent trend towards more radiogenic Ph-isotopic ratios from west to east, i.e., from ore deposits located near the magmatic arc to ore deposits in carbonate platforms attached to the foreland. This is particularly clear for ²⁰⁶Pb/²⁰⁴Pb ratios but it is also recognized for the ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios. The trend does not depend on the age of the ore deposits which, with the exception of San Bartolo, comprises a relatively narrow age span of about 100 million years (Liassic-Al- bian). The following groups which, in general terms, become progressively more radiogenic moving from the magmatic arc to basins at the continent edge can be distinguished (Fig. 2).

Magmatic Arc-Back-Arc (Stages lib, //d). The ore deposits of northern Chile in Lower Cretaceous rocks in intra-arc position (stage lib, Caleta Coloso, and Thlcuna) and in the back-arc Atacama basin (stage lid, El Plomo, Thiunfo-Carola, Maria Cristina, Jaula y Las Canas) plot in a distinct field which represents the least radiogenic ratios of all stratabound ore deposits of the Central Andes. All values plot in a noteworthy tight cluster except for a few analyses from Caleta Col- oso. The homogeneity of the lead isotopic ratios of these deposits, which occur over a north-south distance of about 600 km, should be emphasized.

Marginal Basin (Stage 1/c). Lead isotopic ratios from ore deposits in the Lower Cretaceous aborted marginal basin of central Peru (stage lie, Los leas, Raul, Leonila-Graciela) show more spread and their ²⁰⁶Pb/²⁰⁴Pb ratios are distinctly more radiogenic than those in the Chilean intra-arc and back-arc basins. Two sub- fields are recognized. The ratios of the copper deposits of Raul-Condestable and of Los leas are less radiogenic than those of the Zn-Ba deposit of Leonila- Graciela. There is good agreement between the data of Gunnesch and Baumann (1986) and those of Mukasa (1984) for the respective ore deposits. The only value available of El Soldado plots also in this field.

Shalipayco (Stage /b) plots in a position similar to Raul.

Foreland Platform (Stage //e) and Central Pucara (Stage I a). The ore deposits of stage lie in the Santa Metallotect (El Extrafio, Huanzala), in Hualgayoc and in Cercapuquio, and the volcanic-associated deposits in the Pucara Group (stage Ia, Huaripampa, Carahuacra, stratiform part of Machcan) plot in fields character- ized by ²⁰⁶Pb/²⁰⁴Pb ratios slightly more radiogenic than the previous groups. The shape of these fields suggests mixing lines.

San Bartolo (Intra-Arc Basin in Stage III) falls in a field with similar ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios, but with distinctly lower ²⁰⁷Pb/²⁰⁴Pb ratios than the ore deposits in the foreland platform of stage lie.

Carbonate Platform at the Margin of the Brazilian Shield (Stage /c). San Vicente forms a completely separate population characterized by highly radiogenic ratios.

It can be seen that the lead isotope data are coherent with the geotectonic posi- tion of the host rock. The metallogenetic stages presented before correspond to clear fields of lead isotopic ratios. The first conclusion which can be drawn from

Metal Sources in Stratabound Ore Deposits in the Andes (Andean Cycle) 765

these results is that the lead isotopic composition varies according to different lead sources and does not depend on the age of formation of the ore deposits.

This is illustrated, for example, by the San Vicente data (stage Ic), which are much more radiogenic than younger deposits in stages II and III, and also by comparing the available lead isotopic ratios of ore deposits in central Peru of stages lie and lie. Although all these latter deposits are hosted by Lower Cretaceous rocks, the lead isotopic ratios are different, depending on their geotectonic position.

4 Discussion

4.1 Potential Metal Sources

In Fig. 3 the reviewed data are compared with the modern fields for lead isotopic ratios of MORB, primitive and mature arc, upper crust, lower crust, and pelagic sediments. These fields cover a wide isotopic range: between compositions for up- per crust and compositions of primitive island arc environments. Possible sources for the lead in Andean ore deposits and magmatic rocks have been discussed by McNutt et al. (1979}, James (1981}, Tilton et al. (1981}, Barreiro (1984}, Sillitoe and Hart (1984}, Puig (1988), and Gunnesch (1986). The main theoretical sources are the reservoirs proposed by Zartman and Doe ( 1981 ): ( 1) mantle reservoir, (2) upper old crust, (3) lower old crust, and (4) orogene reservoir (Fig. 4); in addition Sillitoe (1972) suggested that (5) subducted pelagic sediments could also contrib- ute as a significant lead source.

4.2 Metal Sources in the Different Geotectonic Positions

In the case of the stratabound ore deposits reviewed here, a reasonable hypothesis is based on the mixing of the following three "end members":

1. "Mantle" component represented mainly by the basic to intermediate magmat- ic arc in stage II. The homogeneity of the values observed in the Chilean ore deposits in arc and back-arc position can only be explained by a common reser- voir. Whether the slightly more radiogenic values of these deposits relative to MORB is a function of enriched mantle source (Mukasa and Tilton 1985}, or to mixing with crustal components, is not relevant for the present discussion.

The latter is more likely, as enriched mantle requires ocean island-type basalt - not feasible in this case.

2. "Orogene" component, i.e., recycled upper and lower continental crust, ex- pressed by moderately radiogenic values of numerous deposits of stages Ia, lie, and III.

3. "Upper continental crust" component mainly expressed by the highly radio- genic ²⁰⁶Pb/²⁰⁴Pb ratios of San Vicente.

The fourth theoretical reservoir proposed by Zartman and Doe (1981}, i.e., lower continental crust, is not thought to play a significant role in the investigated ore deposit because the lack of clearly low radiogenic ratios. It should be men-

766

LOWER ... --._

/

I X I

I +o /

\ ...._

__

M O RB

L. Fontbote et al.

lc

Ia

II

lla,b,d

37 ':":::---'-_L_---:-::-I--1,_---,-;;-'-;::---'----,~.,..--.L_--,~

16.50 17 .50 18.50 19.50 20.50

206 r b/2o • r b

CRUST

B

20 ,50

Fig. 3. Main fields of stratabound ore deposits in the Central Andes compared to the modern field for mantle, upper crust, lower crust, orogene, and pelagic sediments according to Zartman and Doe (1981). A mantle; B orogene; C upper crust; D lower crust

tioned, however, that near Arequipa the influence of lower continental crust in rock leads has been traced (Mukasa and Tilton 1985).

The predominance of one or the other reservoir determines the large-scale dis- tribution pattern of the lead isotopic ratios shown in Fig. 2 which largely corre- lates with the geotectonic position of the ore deposit or district. Considering the results more in detail, the mixed character of the lead isotope composition is clearly demonstrated in several districts (e.g., Cercapuquio, Shalipayco, San Vicente). The lead isotopic data show comparatively larger scatter of the

207Pb/²⁰⁴Pb values with respect to smaller variations in the ²⁰⁶Pb/²⁰⁴Pb ratios (Fig. 2). The resulting narrow steep fields can be interpreted by mixing between components with different 1.1 values. In some cases the available host rock data make it possible to discuss which local sources are involved. This discussion is be-

Metal Sources in Stratabound Ore Deposits in the Andes (Andean Cycle)

WSW

- 50 km

POS SIBLE ME TAL SOURCES I ideal cross sot ion ce ntral Peru Lower Cre aceous )

ENE

Upper cr ust (Precambr ian Bra zilian Shield l

IZI B rsJ [1] 12JB

less radiogen ic more (l ead l

767

Fig. 4. Potential metal sources in an ideal cross-section trough central Peru during Lower Cretaceous

yond the scope of the present paper and the reader is referred to more detailed publications (e.g., Gunnesch and Baumann 1990, Puig 1988).

In the following the major lead sources in the different metallogenetic stages will be discussed. For a better comprehension it will be begun with stage lL As mentioned above, stage II is characterized by the existence of a clear polarity from west to east between a subduction-related volcanic arc and a back-arc basin which is reflected by a consistent trend towards more radiogenic lead isotopic ratios moving from the magmatic arc to ore deposits on the platform at the edge of the continent.

The ore deposits in the northern Chilean magmatic arc and back-arc (stages lib and lid) plot in a tight cluster which represents the least radiogenic ratios of all stratabound ore deposits in the Central Andes. They plot in the field of mod- ern primitive volcanic arcs. Even the ore deposits originated without direct partic- ipation of volcanic processes such as, for instance, the small Mississippi Valley- type (MVT) Pb-Zn mine of Las Canas, show the same lead isotopic signature. It appears that the mainly basic volcanism at the magmatic arc is the predominant reservoir for these deposits.

The consistent trend observed in stage II in central Peru towards more radiogenic lead isotopic compositions in ore deposits close to the magmatic arc (west) to ore deposits in the foreland (east) is explained by the interaction of the two first components (Fig. 5). In central Peru the Lower Cretaceous back-arc is developed as an aborted marginal basin (see Fontbote this Vol. b). Spreading and subsidence have lead to a thinning of the continental crust. The fact that the two subfields of ore deposits in the central Peruvian marginal basin (stage lie) are slightly more radiogenic than those near the magmatic arc in northern Chile

768

Marginal Basin East Platform

WSW

, ... :· ^{~~! y • • w .· -=-}

~"'" ~\"'a, " ' ' / , ' '

~ ' ,,,, , ... , , ... .... ,:

~:;;fi; ·,;;::::-::}

(mantle contribution erosion and igneous activity at the marginal basin) at/through Paleoz.- PC

continental crust : ,...-.,. Erosion "orogene" source

0 Neocomian to Albian least trough!

~ Neocomian to Albian I west trough)

~ Triassic- Jurassic

~ Mitu IPm- T1 l

5Sl Paleo zoic and Precambrian basement

L. Fontbote et al.

Maran on Geanticline

Brazilian Shield

rn

El ENE San Vicente

Permian rift-related Mitu volcanics : source of low radio genic P b ?

leaching and erosion PC upper crust : source of radiogenic lead

- SO km

[SI[IJ!2JE3

less radiogenic more (lead)

Fig. 5. Geotectonic position of selected stratabound ore deposits in central Peru. ^Arrows indicate sche- matically lead isotope composition. Geology according to Megard (1987)

(stages lib and lid) could be attributed to segmentation factors, i.e., to systematic differences in the crust and/or upper mantle along the orogene. However, the con- siderable spread in the ²⁰⁶Pb/²⁰⁴Pb ratios of the Peruvian deposits makes it more reasonable to assume that the spread and the relatively increased ²⁰⁶Pb/²⁰⁴Pb ra- tios are the result of different amounts of upper crustal contamination of MORB material. This hypothesis seems to be supported by the internal differences in this group, with clearly more radiogenic lead isotopic ratios in Leonila-Graciela, oc- curring in the eastern Casma facies, compared to Raul and Los leas, which are located to the west. The west to east lithologic polarity observed in the marginal basin, with progressively increasing amounts of sedimentary rocks towards the east, is consistent with these isotopic differences. Interesting is also the fact that the Zn-Pb deposit El Toqui, which occurs in a back-arc position in the Southern Andes, plots in the same field as the massive sulfides in the central Peruvian mar- ginal basin. It appears that, despite a north-south distance of over 4000 km, a comparable geotectonic position results in similar lead isotope ratios, and proba- bly in similar metal sources.

A mantle contribution at the magmatic arc is in accord with geochemical in- vestigations on Mesozoic magmatic rocks in the Coastal Range in northern Chile (Berg and Breitkreuz 1983; Rogers and Hawkesworth 1989), and in the volcanic rocks in the Peruvian aborted marginal basin (Atherthon et al. 1983, 1985). Dif- ferent degrees of crustal contamination are assumed.

Metal Sources in Stratabound Ore Deposits in the Andes (Andean Cycle) 769 The ore deposits at the eastern platform in central Peru (stage lie) show slight- ly more radiogenic lead isotopic ratios than the previous groups. It is interesting to note that both ore deposits related to volcanogenic processes (El Extrafio, Huanzahi), and those without direct volcanic influence (Cercapuquio) display similar isotopic ratios. The fact that the lead isotopic ratios become less radiogenic moving towards the west can be explained by the increasing influence of the mantle reservoir with respect to the oro gene reservoir in the marginal basin.

A common feature of these deposits is that they are located in zones of thicker continental crust than the previous group and that none of the respective basin- fill sequences were sourced by the Jurassic-Lower Cretaceous magmatic arc.

Finally, let us discuss the results from ore deposits in stage I, that is, deposits hosted by rocks of the Upper-Triassic Peruvian Pucara basin. As displayed in Ta- ble 1, the Pucara basin formed essentially before the beginning of the paired mag- matic arc-back-arc basin which controlled the evolution of stage II. For this rea- son, a comparable west to east polarity is not present, and the distribution of met- al sources responds to other schemes.

The volcanic-associated ore deposits in the western Pucara (stage Ia, Carahuacra, Huaripampa, stratiform part of Machcan) show lead isotopic ratios very similar to those of the eastern foreland platform of stage lie. The data indi- cate mainly a contribution of the orogene reservoir. Leaching of the Paleozoic basement is probably the main source (Gunnesch and Baumann 1990).

The peculiar isotopic signature of the MVT deposit of Shalipayco (stage lib), which is much less radiogenic than the other ores of the Pucara Group, and plots in the same field as those of the Lower Cretaceous Peruvian aborted marginal ba- sin, can be explained by its paleogeographic position.

The orebodies in Shalipayco occur at the base of the Pucara transgressive se- quence, directly over a volcaniclastic conglomerate of the Mitu Group (Permian).

The Permian Mitu volcanism has alkaline and peralkaline characteristics and is interpreted in relation to an aborted tardi-Hercynian rifting process (Noble et al.

1978; Kontak et al. 1985). Leaching of Permian volcanic material could explain the lead isotopic signature of Shalipayco. This hypothesis is consistent with the analysis reported by Gunnesch (1986) of a Mitu group sample in the Atacocha district yielding similar ratios to those found in the ores of Shalipayco. Additional data would be necessary to confirm this lead source.

Mixing of "mantle" and "orogene" end members can explain the main distri- bution patterns of the lead isotope ratios of all stratabound ore deposits in the Central Andes except those of the San Vicente belt (stage Ic) which are character- ized by highly radiogenic values. These compositions can be compared to the J- lead range of the Mississippi Valley district, which are characterized by negative or "future" model ages. This behavior is typical of upper crustal leads in cratonic regions and of many Mississippi Valley-type ore deposits sourced from such envi- ronments (Doe and Zartman 1979). The highly radiogenic values in San Vicente are related to a third end member which represents an "extra-Andean" factor. This factor corresponds to the role played by weathering and erosion of ancient upper crust of the Brazilian Shield, and/or leaching of detrital sediments derived from it. This is consistent with the paleogeographic position of San Vicente at the west- ern margin of the Brazilian Shield, and explains the different metal source of the

770 L. Fontbote et al.

belt in the eastern Pucara with respect to the other Andean stratabound ore deposits.

In summary, in Pucara-hosted deposits contributions of three main reservoirs must be considered: (1) orogene reservoir (stage Ia, Carahuacra-Huaripampa, Machcan), (2) leaching of volcanic material related to a Permian rifting (stage lb, Shalipayco ), and (3) leaching of old upper crust of the Brazilian Shield (stage Ic, San Vicente).

4.3 General Aspects

It is remarkable that the data of ore deposits in stages Ia, lie (and the only one analyzed of lllb ), as well as numerous nonstratabound ore deposits, plot very closely. This requires a homogeneous source in parts of the Central Andes lasting from Liassic to Cenozoic times. This source matches the theoretical "orogene"

reservoir (Zartman and Doe 1981) calculated on the basis of recycling and mixing of upper and lower continental crust.

Subducted Pacific pelagic sediments are not considered here as a significant lead source. The lead isotopic composition of metalliferous pelagic sediments on the Nazca Plate show lead isotopic ratios very similar to those of numerous An- dean ore deposits. This fact was used to support the hypothesis of deriving metals from subducted pelagic sediments (Silltoe 1972; Sillitoe and Hart 1984). However, as seen in Fig. 3, the field of pelagic sediments coincides with the "orogene" reser- voir. This is, as also observed by Sillitoe and Hart (1984), because the lead of pelagic sediments and manganese nodules is derived mainly from continental crust. Therefore, actually, there is no need to invoke subducted metalliferous sedi- ments as a significative metal source. The hypothesis of deriving a substantial part of the metals required for the Andean hydrothermal ore deposits from metallifer- ous sediments on the Nazca Plate during subduction has been rejected by several authors (Burnham 1981; Tilton et al. 1981; see also discussion in Field and Dasch 1981), and a mixing model with significant contribution of sialic crust is favored.

The lead isotope data on stratabound ore deposits reviewed here gives more evi- dence to rule out mobilization of subducted pelagic sediments as a potential lead source. All results indicate that the lead isotope composition of the considered de- posits responds to large-scale distribution patterns consistent with the geotectonic position of the host rocks. Thus, ore deposits associated with volcanic activity and those formed by other processes such as diagenetic cementation in the same geotectonic environment often display similar isotopic compositions. It is very im- probable that discrete lead reservoirs could determine such regional patterns.

And, last but not least, the ore deposits linked to the subduction-related magmat- ic arc show a distinctly less radiogenic lead isotopic composition than pelagic sed- iments (Fig. 3).

A last point should be underlined. Tilton et al. (1981, p. 811), in their review of the lead isotopic composition of hydrothermal ore deposits in two transects in the Central Andes, note that "one of the most remarkable features of the Pb data is the similarity in isotope patterns from each of the study areas in spite of their greatly differing tectonic setting?' The present discussion shows definitely that in

Metal Sources in Stratabound Ore Deposits in the Andes (Andean Cycle) 771

the case of the stratabound ore deposits a clear correlation exists between lead iso- topic composition and geotectonic position and nature of the host rocks.

5 Conclusion

The lead isotopic investigations on stratabound ore deposits in the Andes indicate clearly that mixing of different sources was involved relative to the geotectonic po- sition. The observed lead isotopic ratios are consistent with a model based on the different degree of mixing of three "end members". They are a mantle component (ore deposits of stage II located near the magmatic arc), an "orogene" component (mainly represented in deposits in the platform basin in stage lie but also in in- tramontane basins of stage lila, and in stage Ia), and old upper continental crust (in deposits of stage lc in the carbonate platform at the margin of the Brazilian Shield). The participation of subducted pelagic sediments is not necessary to ex- plain the lead isotope ratios in the investigated stratabound ore deposits. These isotopic results are coherent with tectonic, paleogeographic, and paragenetic evi- dence, which indicates that ore genesis is directly related to immediate geologic environment. The latter may or may not be, in turn, a result of the subduction process.

Note added to proof. Ore lead isotope ratios from additional Andean stratabound ore deposits (Buena Esperanza, Carolina de Michilla, Cifuncho, El Jardin, Man- tos Blancos, Mantos de Catemu, Punta del Cobre, Santo Domingo) are given in Fontbote et al. (1990).

Acknowledgments. This investigation was supported by the Deutsche Forschungsgemeinschaft. Previ- ous versions of this manuscript have benefited from critical reading by E. Cedillo (Univ. of Guana- juato, Mexico), and St. Flint (Shell Research, Netherlands). The first author wish to thank especially A. Puig (Sernageomin, Chile) for making available unpublished lead isotope data during the prepara- tion of the manuscript. The figures have been drafted by H. SchOnfelder.

References

Atherton MP, Pitcher WS, Warden V (1983) The Mesozoic marginal basin of central Peru. Nature (Lond) 305:303-306

Atherton MP, Sanderson LM, Warden V, McCourt WJ (1985) The volcanic cover: chemical composi- tion and the origin of the magmas of the Calipuy Group. In: Pitcher WS, Atherton MP, Cobbing EJ, Beckinsale RD (eds) Magmatism at a plate edge: The Peruvian Andes. Blackie, Glasgow, pp 273-284

Barreiro BA (1984) Lead isotopes and Andean magmagenesis. In: Harmon RS, Barreiro BA (eds) An- dean magmatism: chemical and isotopic constraints. Shiva Cheshire, pp 21-30

Berg K, Breitkreuz C (1983) Mesozoische Plutone in der Nordchilenischen Kustenkordillere: Petroge- nese, Geochronologie, Geochemie, und Geodynamik mantelbetonter Magmatite. Geotekt Forsch 66, 107 p

Brevart 0, Dupre B, Allegre CA (1981) Metallogenesis at spreading centers: Lead isotopic systematics for sulfides, manganese-rich crusts, basalts, and sediments from the Cyamex and Alvin areas (East Pacific Rise). Econ Geol 76:1205-1210

Brodtkorb MK, Brodtkorb A (1982) Datos isot6picos de plomo de Ia mineralizaci6n de Ia mina La Helvecia, prov. de Ia Rioja. Rev Assoc Geol Argentina 37:358-361

772 L. Fontbote et a!.

Brown JS (1962) Ore leads and isotopes. Econ Geol 57:673-720

Burnham CW (1981) Convergence and mineralization - is there a relation? In: Kulm LD, Dymond J, Dasch EJ, Hussong DM (eds) Nazca Plate: crustal formation and Andean convergence. Geol Soc Am Memoir 154:761-768

Camus F Geological characteristics of stratabound deposits associated with lacustrine sediments, cen- tral Chile. This Vol, pp 449-462

Cardozo M The Copara metallotect in central Peru: Geological evolution and ore formation. This Vol, pp 395-412

Carrascal R, Saez J Stratabound polymetallic ore deposits of the Santa Metallotect in the Huanzala and Pachapaqui mining areas in central Peru. This Vol, pp 555-568

Dalheimer M The Zn-Pb-(Ag) deposit Huaripampa and Carahuacra in the mining district of San Crist6bal, Central Peru This Vol, pp 279-291

Diaz LL (a) The Bellavista Zn-Ag Mine, Copiap6 Region, Chile. This Vol, pp 513-522 Diaz LL (b) The Mamiiia Mine, Atacama Region Chile. This Vol, pp 523-536

Doe BR, Zartman RE (1979) Plumbotectonics. The Phanerozoic. In: Barnes HL (ed) Geochemistry of hydrothermal ore deposits. Wiley, New York, pp 22-70

Field CW, Dasch EW (1981) Epilogue: Geostill reconsidered. In: Kulm LD, Dymond J, Dasch EJ, Hussong DM (eds) Nazca Plate: Crustal formation and Andean convergence. Geol Soc Am Mem 154:817-824

FlintS (1986) Sedimentary and diagenesis controls on red-red ore genesis. The Middle Tertiary San Bartolo Copper Deposit, Antofagasta Province, Chile. Econ Geol 81:761-778

FlintS, Clemmey H, Thrner P (1986) Conglomerate-hosted copper mineralization in Cretaceous An- dean Molasse: the Coloso Formation of northern Chile. Geol Mag 123:525-536

Font bote L (a) Stratabound ore deposits in the Andes: A review and a classification according to their geotectonic setting. This Vol, pp 79-110

Fontbote L (b) Stratabound ore deposits in the Pucara Basin - an overview. This Vol, pp 253-266 Font bote L, Calvez JY, Pincheira M, WolfF (1990) Geotectonic position and metal sources of strata-

bound ores in the Central Andes - Lead isotopic constraints. Symp Int Geodynamique Andine, Grenoble may 1990, Colloques et Seminares, ORSTOM, Paris, pp 347-350

Gorzawski H, Fontbote L, Field CW, Tejada R Sulfur isotope studies in the zinc-lead Mine San Vicente, central Peru. This Vol, pp 305-312

Gunnesch KA (1986) Petrologie und Metallogenese des Atacocha-Distrikts im Rahmen der geologi- schen Entwicklung Zentral-Perus. Habilitationsschrift, Univ Heidelberg, 177 p (unpublished) Gunnesch KA, Baumann A (1984) The Atacocha District, Central Peru: Some metallogenic aspects.

In: Wauschkuhn A, Kluth C, Zimmermann R (eds) Syngenesis and epigenesis in the formation of mineral deposits. Springer, Berlin Heidelberg New York Tokyo, pp 448-449

Gunnesch KA, Baumann A (1986) Bleiisotopie in Zentralperu. Berliner Geowiss Abh Reihe A Son- derbd (X Geowiss Lateinam-Kolloq, Berlin, Abstracts), pp 136-137

Gunnesch KA, Baumann A (1990) Lead isotopic variations across the Central Peruvian Andes. Econ Geol 85:5

Hirdes W Machcan mine: A Pb-Zn ore deposit in Pucara sediments of the Atacocha district/central Peru. This Vol, pp 293- 303

James DE (1981) Role of subducted continental material in the genesis of calc-alkaline volcanics of the Central Andes. In: Kulm LD, Dymond J, Dasch EJ, Hussong DM (eds) Nazca Plate: Crustal formation and Andean convergence. Geol Soc Am, Memoir 154, p 769-790

Kontak DJ, Clark AH, Farrar E, Strong DF (1985) The rift associated Permo-Triassic magmatism of the Eastern Cordillera: a precursor of the Andean orogeny. In: Pitcher WS, Atherton MP, Cobbing EJ, Beckinsale RD (eds) Magmatism at a plate edge: The Peruvian Andes. Blackie, Glasgow, pp 36-44

MacFarlane AW (1989) Lead, sulfur and strontium isotopes in the Hualgayoc area, Peru, and lead iso- tope provinces of the Central Andes. Ph D Thesis, Harvard Univ, Cambridge, Massachussetts, 370 p

McNutt RH, Clark AH, Zentilli M (1979) Lead isotopic compositions of Andean igneous rocks, lati- tudes 26° to 29°S: petrologic and metallogenetic implications. Econ Geol 74:827-837 Megard F (1987) Cordilleran Andes and marginal Andes: A review of Andean geology north of the

Arica Elbow (19°S). In: Monger JWH, Francheteau J (eds) Circum-Pacific orogenic belts and evo- lution of the Pacific Ocean basin. Am Geophys Union, Washington, pp 71-95

Metal Sources in Stratabound Ore Deposits in the Andes (Andean Cycle) 773 Mukasa S (1984) Comparative Pb isotope systematics and zircon U-Pb geochronology for the Coastal, San Nicolas and Cordillera Blanca batholiths, Peru. Ph D Thesis, Univ of California, Santa Bar- bara, pp 383

Mukasa SB, Tilton OR (1985) Pb-isotope systematics as a guide to crustal involvement in the genera- tions of the Coastal batholith, Peru. In: Pitcher WS, Atherton MP, Cobbing EJ, Beckinsale RD (eds) Magmatism at a plate edge: the Peruvian Andes. Blackie, Glasgow, pp 235-238 Noble DC, Silbermann ML, Megard F, Bowman f:JR (1978) Comendite (peralkaline rhyolite) in the

Mitu Group, Central Peru: Evidence of Permtan-'ll'iassic crustal extension in the Central Andes.

J Res US Geol Surv 6:453-457 r

Puig A (1988) The geologic and metallogenetic significance of lead isotope composition of lead in galena of the Chilean Andes. Econ Geol 83:843-858

Puig A Lead isotopes in Chilean ores. This Vol, pp 749-758

Rogers G, Hawkesworth CJ (1989) A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge. Earth Plan Sci Amsterdam 91:271-285

Samaniego A (1982) Correlation of strata-bound mineral deposits in the Early Cretaceous Santa Metalotect of North and Central Peru. In: Amstutz GC, El Goresy A, Frenzel G, Kluth C, Moh G, Wauschkuhn A, Zimmermann R (eds) Ore genesis, the state of the art. Springer, Berlin Heidel- berg New York Tokyo, pp 508-527

Sillitoe RH (1972) Relation of metal provinces in western America to subduction of oceanic lithosfere.

Geol Soc Am Bull 83:813-818

Sillitoe RH, Hart SR (1984) Lead-isotope signatures of porphyry copper deposits in oceanic and conti- nental settings, Colombian Andes. Geochem Cosmochim Acta 48:2135-2142

Tilton OR, Pollak RJ, Clark AH, Robertson RC (1981) Isotopic composition ofPb in Central Andean ore deposits. In: Kulm LD, Dymond J, Dasch EJ, Hussong DM (eds) Nazca Plate: Crustal forma- tion and Andean convergence. Geol Soc Am Mem 154:791-816

Vidal CE (1987) Kuroka 'IYPe deposits in the Middle Cretaceous marginal basin of Central Peru. Econ Geol 82:1409-1430

Wauschkuhn A (1979) Geology, mineralogy and geochemistry in stratabound Cu deposits in the Me- sozoic Coastal Belt of Peru. Bol Soc Geol Peru 62:99-143

Wellmer FW, Reeve EJ The Toqui zinc-lead-copper-silver deposits, Aysen province, Chile. This Vol, pp 473-484

Zartman RE, Doe BR (1981) Plumbotectonics - the model. Thctonophysics 75:135-162