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IntroductionSOME OF the worlds richest and largest known Cenozoic cop-per, gold, and silver deposits occur in the Andean orogen as-sociated with continental arc magmatism. Some of these de-posits have been studied extensively, but very little ispublished about others, and although regional-scale varia-tions in metal sources have been documented, our under-standing of metal source processes remains incomplete. Onecommon method of determining the source of metals in anore deposit is to use lead isotope ratios to evaluate the sourcesof lead in associated igneous rocks, assuming that the igneousrocks represent the predominant source of metals in the ores.This approach is facilitated by extensive literature on thepetrology of continental arc magmas in general and centralAndean magmas in particular (e.g., James, 1982; Harmon etal., 1984; Hildreth and Moorbath, 1988; Aitcheson and For-

rest, 1994; Aitcheson et. al, 1995; Kay et al., 1999). Whilethese comparisons can provide clues to the source of ore met-als in many deposits, they are not entirely satisfactory on theirown. Most Andean metal deposits are formed by hydrother-mal activity that postdated any significant isotopic evolutionof the magma, and the processes that create the orebodies aredominantly hydrothermal. Even when ore metals are derivedfrom associated magmatic rocks, other elements in the ores,including strontium, oxygen, and sulfur, may come from othersources (e.g., Macfarlane et al., 1994). Other isotopic tracerssuch as Nd, Sr, and Os are only of indirect use in determiningpotential ore metal sources.

Macfarlane et al. (1990) and Petersen et al. (1993) summa-rized new and published lead isotope data for the ore depositsin the central Andes and defined three large-scale lead iso-tope provinces (Fig. 1, inset). Province I comprises thecoastal volcanic arc of Per and Chile, province II includesthe Jurassic and Cretaceous miogeosynclinal sedimentary belt

Sources of Lead in the San Cristobal, Pulacayo, and Potos Mining Districts, Bolivia, and a Reevaluation of Regional Ore Lead Isotope Provinces

GEORGE KAMENOV,Department of Geological Sciences, 24 Williamson Hall, University of Florida, Gainesville, Florida 32611

ANDREW W. MACFARLANE,

Department of Earth Sciences, Florida International University, Miami, Florida 33199

AND LEE RICIPUTIInorganic Geochemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831

AbstractNew lead isotope data on ores, crustal rocks, and leachates of crustal rocks, combined with data in the liter-

ature, provide important new constraints on the sources of ore metals in southwest to south-central Bolivia, in-cluding the very large recently discovered silver-zinc deposit at San Cristobal, the Pulacayo polymetallic dis-trict, and the giant Potos silver-tin-base metal deposit.

Lead isotope ratios of ores and igneous rocks from the San Cristobal deposit and from Paleozoic and Creta-ceous sedimentary rocks are compared with published data on high-grade Middle Proterozoic metamorphicbasement rocks. These data constrain the major source of lead, and by inference of other ore metals, at SanCristobal to be the metamorphic basement rocks. Leaching experiments on samples of Paleozoic and Creta-ceous sedimentary rocks show that the easily leachable lead from these rocks is much less radiogenic than thewhole-rock compositions. However, lead isotope ratios of both whole rocks and leachates of these upper crustalrocks are too radiogenic for them to be major sources of ore lead at San Cristobal.

Lead isotope ratios of ores from Pulacayo and Potos are similar to each other and lie within the range of Pa-leozoic and Cretaceous sedimentary whole-rock compositions. Leaching of Pb from the sedimentary rocks can-not explain the isotopic compositions of the Pulacayo and Potos ores, and the isotopic homogeneity of the Po-tos ores also argues against mixing of lead from diverse sources in the hydrothermal system. Lead from thesedimentary rocks may have been incorporated by magmatic assimilation followed by extraction of ore metalsfrom the resulting magma.

Lead isotope ratios of San Cristobal ores are different from those of Pulacayo, Potos, and other deposits tothe east, but resemble the compositions of ores and volcanic rocks in western Bolivia. On this basis we identifya new ore lead isotope province extending from San Cristobal northward across the eastern Altiplano and intosouthern Per. This province is coincident with but smaller than the extent of the proposed Arequipa-Anto-falla metamorphic basement craton. The degree of incorporation of ore metals from the metamorphic base-ment appears to depend on the timing and/or location of the mineralizing event. Ore deposits in the northernpart of province IV formed before the thickening of Andean crust, beginning around 20 Ma, and incorporatedminor amounts of metals from the metamorphic basement. Younger deposits farther to the south contain majorto dominant components of basement lead.

Economic GeologyVol. 97, 2002, pp. 573592

Corresponding author: e-mail, macfarla@fiu.edu

that dominates the geology of the high Andes of Per, andprovince III comprises the Eastern Cordillera of southeasternPer, central Bolivia, and northwestern Argentina, includingthe Bolivian tin belt. While the existence of these provincesand their general characteristics are well established, impor-tant details of their distribution and the roles of shallowcrustal rocks in their formation remain unclear. Studies basedmainly on lead isotope compositions of late Tertiary volcanicrocks, or combining volcanic rock data with ore deposit data,have documented extensive incorporation of basement leadinto late Tertiary magmas (Wrner et al., 1992; Aitcheson etal., 1995). A major component of relatively nonradiogenicmetamorphic basement lead was also documented in oresand volcanic rocks from the Berenguela district in westernBolivia (Tosdal et al., 1993; Tosdal, 1996). These effortsdemonstrate the need for a revision of the lead isotope

province scheme of Petersen et al. (1993) and an evaluationof the potential role of upper crustal Paleozoic and Creta-ceous-Tertiary sedimentary rocks in creating the isotopic sig-natures of province II and III ores.

The purpose of this study is to provide a clearer view of thederivation of ore metals in major deposits of the centralAndes, and of the relationship between the shallow crustalhost-rock sequences and the ore deposits. We are especiallyinterested in whether the ore metals represent new additionsto the shallow crust from an enriched mantle source region atdepth, or, alternatively, whether they represent remobiliza-tions of metals already enriched in the upper crustal rocks.We examine the lead isotope characteristics of crustal rocks insouth-central Bolivia and compare them with the ores at Po-tos, Pulacayo, and San Cristobal to determine the immediatesources of their ore metals, and we consider briefly the

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ChileArgentina

Bolivia

Pacific Ocean

Province I

ProvinceIII

19S

20S

21S

22S66W67W68W

SALAR DE UYUNI

UYUNI

TODOSSANTOS

TATASI

0 km 50km

100km

Quaternary alluvium

studyarea

PULACAYO

POTOSI

SAN CRISTOBAL

Tertiary volcanic rocks, chieflyandesitic to daciticTertiary ignimbrites, dacitic

Tertiary nonmarine conglomerates,sandstones, shales and mudstonesCretaceous marine and nonmarinesandstones, shales, marls and limestonesPaleozoic sedimentary rocks, dominantlymarine shale and sandstoneScale

Peru

Sample location

Province II

98M01398M014

98M01798M018

FIG. 1. Geologic map of the study area, simplified from Perez (1996). Inset shows the location of the study area and theapproximate boundaries of the lead isotope provinces proposed by Petersen et al. (1993).

sources of magmatic lead in the San Cristobal district. Wealso redefine the Andean lead isotope province map and ob-tain new insights into the source processes that create thoseprovinces.

Metal Provenance for Hydrothermal Ores

Hydrothermal leaching of ore metals

Within a hydrothermal system, the potential sources ofmetals can be evaluated by comparing the lead isotope ratiosof ores in the district with associated magmatic rocks, andwith host rocks in the system that may vary widely in age,lithology, and isotopic characteristics. If the host rocks andmagmatic rocks have different Pb isotope ratios, such com-parisons can suggest or rule out major source rocks and revealmixing among metal sources. In other geologic settings, hy-drothermal leaching of metals from sedimentary and meta-morphic rocks is well documented, including the UnitedStates Mississippi Valley-type ores (e.g., Crocetti et al., 1988),the major Irish base metal deposits (Dixon et al., 1991), andthe Wairakei magmatic-hydrothermal geothermal system(Hedenquist and Gulson, 1992). However, few studies havebeen made of Andean magmatic-hydrothermal systems. Suchstudies would require a knowledge both of the bulk lead iso-tope composition of the host rocks and of the lead that couldbe hydrothermally leached from them. A very small numberof lead isotope ratios of Andean crustal rocks have been pub-lished, and only a handful of compositions of leachates.

Sources of magmatic metals

Sorting out metal sources in continental arc magmas ismuch more complicated than determining where metalscame from in a given hydrothermal system, because many dif-ferent processes and sources are involved at different times.Potential metal sources for continental arc magmas includethe subcontinental mantle, the subducted oceanic plate, thecontinental crust through which the magmas rise, and sub-ducted sediments of various kinds that may be incorporatedinto the arc magma at its source. Subducted sediments con-tain hydrothermal components from the ocean crust, pelagicand continentally derived clastic sediments, and chemical andbiochemical components from the seawater (Lin, 1992;Peucker-Ehrenbrink et al., 1994). Budgets of various metals,as well as petrologic tracers like Sr, Nd, and Os, will varywidely among these potential source components, so thatmodeling of metal contributions from them is only approxi-mate at best.

It is well understood that Andean magmas generally, andAltiplano igneous rocks specifically, are dominated by crustallead (Hildreth and Moorbath, 1988; Stern, 1991; Aitchesonand Forrest, 1994; Aitcheson et al., 1995; Davidson and deSilva, 1995). This lead is derived from subducted marine sed-iments, from abraded and subducted continental crust, andfrom the continental crust through which the magma laterrises, or from a combination of those sources. Because deepmarine sediments typically contain about 25 ppm Pb and themantle probably less than 0.1 ppm, the addition of a smallamount of sediment will rapidly swamp the Pb isotope ratiosof magmas generated in a subduction-related mantle wedge

(Othman et al., 1989; Aitcheson and Forrest, 1994). Since iso-topic differences between the mantle and sediments aremuch greater for 207Pb/204Pb than for 206Pb/204Pb, the primaryeffect of mixing will be to increase 207Pb/204Pb, which is thecase for many island arc magmas (e.g., Kay et al., 1978; Milleret al., 1994).

Unlike subducted sediments, which are isotopically ho-mogenized to some degree by the sedimentary processes thatcreate them and then probably further mixed within the man-tle wedge, the continental crust through which magmas rise isoften strongly heterogeneous. Some Andean upper crustalrocks that experienced high-grade metamorphism during thePrecambrian (e.g., the Arequipa-Antofalla craton) containless radiogenic lead owing to depletion of U and sometimesTh during metamorphism (Tilton and Barreiro, 1980; Tosdal,1996). Other Precambrian terranes such as the Guyana andAmazon cratons have been enriched in U and contain highlyradiogenic lead (Montgomery and Hurley, 1978; Macfarlane,1999) In either case, lead isotope ratios of Proterozoic andArchean upper crustal rocks exposed at the surface, and sed-imentary rocks derived from them, are often distinct fromthose of lead in subduction-related igneous rocks, either pre-dicted by models (Zartman and Doe, 1981) or observed(Mukasa and Tilton, 1985; Macfarlane et al., 1990). This het-erogeneity can sometimes be used to distinguish betweencontributions of lead from the upper crust and from sub-ducted sediments (e.g., Mukasa and Tilton, 1985; Macfarlaneand Petersen, 1990).

Geology of the study area

Our study area extends from the Potos deposit southwestacross the Pulacayo and San Cristobal deposits (Fig. 1). Thestudy area lies in a very poorly constrained area of the isotopicprovince map of Petersen et al. (1993), and crosses their ten-tative province II-III boundary (inset, Fig. 1). The southernBolivian Andes consist of three morphotectonic provinces;from west to east, these are the Cordillera Occidental, the Al-tiplano plateau and the Cordillera Oriental. The CordilleraOccidental in the study area consists of late Miocene to Re-cent volcanic rocks primarily of andesitic to dacitic composi-tion, overlying Jurassic and Cretaceous sedimentary and vol-canic rocks (Cunningham et al., 1991; Richter et al., 1991).The Cordillera Oriental is a fold-and-thrust belt consistingmainly of Paleozoic deep marine and platform sedimentaryrocks and Cretaceous marine and nonmarine sedimentaryrocks. These rocks were deposited mostly on Precambrianbasement and were subsequently deformed and folded dur-ing at least three orogenic cycles: Caledonian (Ordovician),Hercynian (Devonian to Triassic), and Andean (Cretaceous toCenozoic) (Sempere et al., 1990; Sempere, 1995). The sourceof the Paleozoic sediments was the Precambrian Brazilianshield to the northeast, which is composed of granulitic,gneissic, and metasedimentary rocks, and the 2 Ga-old Pro-terozoic Arequipa massif of PerChile to the west andsouthwest (Litherland et al., 1989). The Altiplano plateau wasproduced mainly by crustal shortening and magmatic addi-tion (Isacks, 1988; Lamb and Hoke, 1997) and represents aseries of intermontane basins filled with thick deposits of con-tinental sediments of Cretaceous and Tertiary age (Kennan et

Pb SOURCES IN SAN CRISTOBAL, PULACAYO, AND POTOS MINING DISTRICTS, BOLIVIA 575

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al., 1995; Sempere et al., 1997). These continental sedimen-tary rocks unconformably overlie the folded Paleozoic sedi-mentary rocks (Gagnier et al., 1996) and locally Precambrianmetamorphic rocks (Lehmann, 1978). Many major and minorvolcanic centers, mainly with andesitic to dacitic composi-tions, are found in the Altiplano (Davidson and de Silva,1992). The San Cristobal and Pulacayo mining districts areclosely associated with such volcanic centers (Fig. 1).

Ore geology of the Potos, San Cristobal, and Pulacayo deposits

The famous Cerro Rico de Potos has been mined sinceprecolonial times and is considered to be the worlds largestknown silver deposit. Ores occur in veins and disseminationsrelated to north- and northeast-trending fracture systemswhich cut a dacitic dome complex and the surrounding hostrocks (Cunningham et al., 1991). The dome was emplacedthrough the thick, deformed Ordovician black shales of theCordillera Oriental. Early volcanic activity produced surge

and breccia deposits surrounding the vent, and the dome wasextruded onto those earlier deposits, plugging the vent. Thedome was emplaced at 13.8 0.2 Ma (U-Pb dating of zircon;Zartman and Cunningham, 1995). Mineralization in theupper part of the deposit was dominantly silver rich, withbase-metal ores (cassiterite, sphalerite, and galena) predomi-nating at lower levels.

Silver was discovered in the San Cristobal district in the1630s, and intermittent, small-scale mining operations havecontinued since that time (Fig. 2; Jacobson et al, 1969). Be-ginning in 1995, exploration by Apex Silver Mines Ltd. delin-eated one of the worlds largest open pit reserves of silver,comprising 240 million tonnes (Mt) of ore grading 2.0 oz/t sil-ver, 1.67 percent zinc and 0.58 percent lead (Apex SilverMines Ltd. 1999 Annual report). Two large open pits arebeing developed, centered roughly on the Tesorera andHedionda mine areas (Fig. 2). The deposits in this area areclustered near the center of a deeply eroded Tertiary an-desitic to dacitic intrusive-extrusive complex (Richter et al.,

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N

Animas mine

Toldos mine

San Cristobal

Hedionda Mine

Tesorera Mine

Scale (m)0 500 1000

Potoco Fm. (Eocene-Oligocene)

Quehua Fm. (Oligocene-Miocene)

Intrusive dacite porphyry(late Miocene)

Intrusive dacite porphyry breccia

Extrusive dacite porphyry

Colonmine

Bertha Mine

Trapiche Mine

Intrusive andesite

2106'

6712

'

Mine sites

Sample locations

99SC0999SC04

99SC01

91BSL016

91BSL015

91BSL022

99SC0399SC08

FIG. 2. Schematic surface geology of the San Cristobal area, modified after Jacobson et al. (1969). Sample locations areapproximate; latitude and longitude of district estimated from locations in Long (1991).

1991). Two major types of subvolcanic intrusive rocks are rec-ognized: andesite porphyry consisting of plagioclase and alteredhornblende phenocrysts in a microcrystalline groundmass;and dacite porphyry consisting of plagioclase, hornblende, bi-otite, and occasional quartz phenocrysts in a micro- to cryp-tocrystalline groundmass (Richter et al., 1991). Potassium-argon ages of 8.0 0.1 Ma and 8.5 0.3 Ma have beenreported on biotite from dacite samples (Ludington et al.,1992). The igneous rocks were emplaced into continental redbeds (sandstone, shale, and conglomerate) of the earlyEocene to early Oligocene Potoco Formation and intoOligocene to Miocene volcaniclastic rocks of the Upper Que-hua Formation. Galena, sphalerite, pyrite, and chalcopyriteoccur in veins, stockworks, and disseminations in hydrother-mally altered igneous rocks and breccias and in some placesin sedimentary and volcanoclastic rocks. Silver occurs as na-tive Ag or argentiferous galena. Mineralization in the Toldosarea consists of northeast-striking, steeply dipping veins con-taining hematite, siderite, barite, quartz, native Ag,stromeyerite, and tetrahedrite, with interspersed stockworksof silver-bearing pyrite veinlets (Jacobson et al., 1969; Richteret al., 1991).

The large silver-rich polymetallic Pulacayo deposit islocated in the western margin of the Cordillera Oriental. Pu-lacayo was discovered in 1833 and mined until 1958 (Cun-ningham et al., 1991) and is being explored for possible rede-velopment as a disseminated gold mine. The deposit isassociated with a dacitic volcanic dome intruded into Silurianshales and sandstones and overlain by Tertiary continentalsedimentary and pyroclastic rocks (Lyons, 1963). Ore miner-alization occurs in veins and veinlets hosted by the dome, ex-cept the large Tajo vein which extends into the Tertiary sedi-mentary rocks (Cunningham et al., 1991). Ore minerals arechiefly sphalerite, tetrahedrite, freibergite, argentiferousgalena, and chalcopyrite associated with abundant pyrite,quartz and barite (Pinto-Vasquez, 1993).

Analytical MethodsThe locations and brief descriptions of samples are found in

Table 1 and are indicated in Figures 1 and 2. Most rock sam-ples lack visible evidence of weathering or alteration of anykind. One igneous rock sample analyzed from the San Cristo-bal district (91BSL015) was hydrothermally altered. Whole-rock samples were sawn into slabs, and slabs having evidenceof pervasive weathering, secondary fractures, or veinlets werediscarded. Saw marks were ground away using silicon-carbidesandpaper, and the slabs were cleaned at least three times for5 min each in an ultrasonic cleaner with deionized water.Cleaned slabs were then broken into small chips, powderedin an alumina-lined shatterbox vessel; powders were stored inacid-leached polyethylene screw-top bottles.

Leaching experiments

Elements in crystalline rocks are distributed heteroge-neously among major, minor, and accessory minerals, or areadsorbed onto mineral surfaces. Ion exchange at such sur-faces and dissolution of surface layers supply most of the ele-ments mobilized at the beginning of fluid-rock interaction(Mller and Giese, 1997). Tessier et al. (1979) found thattrace metals (Cd, Co, Cu, Ni, Pb, Zn, Fe, and Mn) could be

grouped into five fractions: exchangeable, bound to carbon-ates, bound to Fe-Mn oxides, bound to organic matter, andresidual, and that Fe-Mn oxides and organic matter phasesscavenge trace metals far out of proportion to their own con-centration. We used NaCl and HCl solutions to extract themost easily leached lead from our sedimentary rock samples.

To determine which conditions leached the most lead(NaCl and HCl concentrations, time, and fluid/rock ratio)preliminary experiments were performed with sample98M013 (Table 2). The concentration of lead in the leachateincreased with increasing NaCl and HCl concentrations,fluid/rock ratio and time. Subsequent leaching runs were con-ducted on 200-mg samples of each of five sedimentary rocksamples using leachates of 2 ml 15 percent NaCl solution and2 ml 0.5N HCl. The 15 percent NaCl solution was preparedfrom Spex ultrapure NaCl (lot 09981 E) dissolved in ultra-pure water. Rock powders and leaching solutions were placedin Teflon-lined Parr acid digestion bombs, and the bombswere tightly closed and placed in a gravity convection oven at200C (the bombs were found to leak above 200C). Leach-ing of sample 98M018 by 0.5N HCl was conducted at 100C.NaCl leachates were heated for 24 h, and 0.5N HCl leachateswere heated for 7 h. Bombs were allowed to cool to roomtemperature before being opened. There was no visible lossof solution in any of the reported experiments. One ml ofeach leachate was carefully separated with micropipette, andhalf of the separated leachate was transferred to a cleanTeflon beaker, dried down twice with 0.5N HBr, and pre-pared for Pb isotope ratio measurement. The other half of theleachate was spiked with 99.86 percent 208Pb and 99.91 per-cent 235U and diluted to 10 ml with 0.5N HNO3. Isotope di-lutions were measured on a HP 4500 ICP-MS in the Depart-ment of Chemistry at Florida International University.

Lead isotope ratio measurements

Chemical preparation of mineral separates and whole rocksfor Pb isotope analysis were carried out in the Radiogenic Iso-tope clean laboratory of the Department of Earth Sciences atFlorida International University. Lead blanks in water, 0.5NHNO3 and 0.5N HBr prepared in the lab are uniformly 2g/gram, and total chemistry blanks are typically 500 g.Sample powders were dissolved in 3:2 HF-HBr mixture andevaporated in laminar flow boxes filtered at 0.3m. Sampleswere redissolved in 0.5N HBr and dried down twice. Pyritesamples were dissolved in 1:1 HNO3 and diluted in 0.5N HBr.Lead was then separated and purified with cation exchangecolumns and an HBr medium, following the procedure ofManhes et al. (1978). Hand-picked galena grains were dis-solved in warm 2N HNO3. Pb was deposited electrolyticallyon clean Pt electrodes and then redissolved in 2N HNO3. Aquantity of solution corresponding to approximately 1 gmPb was analyzed.

All Pb samples were loaded in phosphoric acid on single Refilaments with silica gel and run on a VG-354 multicollectormass spectrometer in the Transuranium Research Laboratoryat Oak Ridge National Lab. In-run precisions of 204Pb/208Pbvalues are better than 0.05 percent, and those of 206Pb/208Pband 207Pb/208Pb are better than 0.01 percent, except forleachates of sample 97H022. Normalization of 7 analyses ofthe lead isotope ratio standard SRM-981 against the average

Pb SOURCES IN SAN CRISTOBAL, PULACAYO, AND POTOS MINING DISTRICTS, BOLIVIA 577

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of values published for that standard by Todt et al. (1984) andHamelin et al. (1985) yielded correction factors of 0.149 per-cent/amu on 204Pb/208Pb, 0.079 percent on 206Pb/208Pb, and0.102 percent on 207Pb/208Pb. Standard deviations of SRM-981 analyses are 0.06 percent on 206Pb/204Pb, 0.12 percent on207Pb/204Pb, and 0.16 percent on 208Pb/204Pb; overall uncer-tainties of 0.05 percent per mass unit are cited for all ratios.

Results

Lead isotope measurements

Isotopic analyses of rock and ore samples from the studyarea are summarized in Table 3. Seven igneous rock samplesfrom the San Cristobal mining district were analyzed. Fiveigneous rocks show low lead isotope ratios relative to other

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TABLE 1. Description of Samples

Sample Latitude Longitude Area Description

Igneous rocks91BSL013 2107'06" 6713'03" San Cristobal Dacite dike, fine-grained gray matrix containing small feldspar and biotite (mm size)

crystals91BSL015 2106'12" 6713'45" San Cristobal Strongly hydrothermally altered white to yellow fine-grained aggregate containing

small quartz crystals; altered dacite? 91BSL016 2106'30" 6712' San Cristobal Dacite intrusion, light-colored fine-grained matrix containing small amphibole and

altered feldspar crystals (mm size)91BSL018 2104'52" 6711'39" San Cristobal Dacite, gray, fine-grained matrix containing some feldspar and biotite crystals91BSL020 2110'04" 6716'51" San Cristobal Basalt, black, fine-grained matrix, no visible crystals91BSL021 2102'06" 6724'09" San Cristobal Andesite dike, fine-grained dark gray matrix containing few euhedral amphibole

crystals up to 5mm 91BSL022 210607" 6712'04 San Cristobal Andesitic pluton, reddish fine-grained matrix containing altered feldspars and biotite

Sedimentary rocks97H016 1619'54" 6802'39" La Paz Fine-grained dark (black) shale, no visible weathering or alteration197H017 1619'54" 6802'39" La Paz Fine-grained dark (black) shale, no visible weathering or alteration197H018 1619'54" 6802'39" La Paz Fine- grained dark (black) shale layers alternating with thin gray to yellow siltstone

layers and some thin Fe oxide veinlets197H019 1842' 6535' Near Ocuri Red siltstone consisting of alternating thin red (finer-grained) and white (coarser-

grained) layers 97H020 1842' 6535' Near Ocuri Red siltstone consisting of alternating thin red (finer-grained) and white (coarser-

grained) layers97H022 1842' 6535' Near Ocuri Red siltstone consisting of alternating thin dark red (finer-grained) and light red

(coarser-grained) layers97H014 1856'50" 6522'46" Sucre Fine-grained dark (black) shale layers alternating with thin gray to yellow siltstone

layers and some thin Fe oxide veinlets197H015 1856'50" 6522'46" Sucre Fine-grained dark (black) shale, no visible weathering or alteration198M013 1938'45" 6548'09" Near Potos Fine-grained dark (black) shale, no visible weathering or alteration198M014 1936'55" 6547'04" Near Potos Fine-grained dark (black) shale, no visible weathering or alteration198M017 1952'49" 6542'09" Near Potos Fine-grained dark (black) shale, no visible weathering or alteration198M018 1952'49" 6542'09" Near Potos Fine-grained dark (black) shale, no visible weathering or alteration198M022 2034'04" 6538'84" East of Pulacayo Fine-grained dark (black) shale layers alternating with thin gray siltstone layers1

Ore samples98M024C 2025' 6642' Pulacayo Galena crystals from coarse-grained pyrite aggregate containing some galena, spha-

lerite, and quartz98M024A 2025' 6642' Pulacayo Pyrite crystals from coarse-grained pyrite aggregate containing some galena, spha-

lerite, and quartz98M024B 2025' 6642' Pulacayo Pyrite crystals from coarse-grained pyrite aggregate containing some galena, spha-

lerite, and quartz98M024D 2025' 6642' Pulacayo Pyrite crystals from coarse-grained pyrite aggregate containing some galena, spha-

lerite, and quartz99SC01 2105'42" 6712'37" San Cristobal Galena, fine-grained aggregate; core sample from 70.0 m99SC02 2105'55" 6712'29" San Cristobal Galena crystals from a small galena vein hosted in hydrothermally altered volcanic

rock; core sample from 118.8 m99SC03 2105'50" 6712'26" San Cristobal Galena crystals from ore aggregate containing galena, sphalerite, barite, and kaolin;

core sample from 283.6 m99SC04 2105'19" 6712'44" San Cristobal Galena aggregate;core sample from 155.4 m99SC08 2105'59" 6712'16" San Cristobal Galena crystals from a vein associated with chalcopyrite, sphalerite, and barite;

Trapiche mine adit99SC09 2105'22" 6712'11" San Cristobal Galena crystals from coarse-grained aggregate consisting of galena and bariteTD 07 2106'42" 6712'06" Toldos Toldos mine aditTD 09 2106'42" 6712'06" Toldos Toldos mine adit

1 Samples left a black carbon residue after dissolution

volcanic rocks in the central Andes, with 206Pb/204Pb = 17.579to 17.966, 207Pb/204Pb = 15.557 to 15.615, and 208Pb/204Pb =37.804 to 38.209. Sample 91BSL015 yielded an elevated207Pb/204Pb ratio. Sample 91BSL021 has higher 206Pb/204Pbthan the other igneous rock samples.

Six galena samples and one pyrrhotite from San Cristobalcontain relatively nonradiogenic Pb, with 206Pb/204Pb = 17.766to 17.890, 207Pb/204Pb = 15.555 to 15.631 and 208Pb/204Pb =37.954 to 38.206. Samples of sphalerite and hematite fromthe Toldos deposit, a small polymetallic deposit near SanCristobal, have slightly higher 206Pb/204Pb. Three pyrites andone galena from the Tajo vein in the Pulacayo mining districtvary from 18.600 to 18.746 in 206Pb/204Pb, 15.664 to 15.808 in207Pb/204Pb, and 38.850 to 39.212 in 208Pb/204Pb. These valuesare much more radiogenic than the values for San Cristobalores.

Lead isotope analyses were performed on whole-rock pow-ders of the most common sedimentary rocks in the region, 11Paleozoic black shales and 3 Mesozoic red siltstones. The sed-imentary rocks are more radiogenic than the San Cristobalvolcanic rocks, with 206Pb/204Pb = 18.337 to 19.226, 207Pb/204Pb= 15.640 to 15.814, and 208Pb/204Pb = 38.494 to 39.920.

Leaching experiments

Concentrations of Pb in NaCl leachate solutions increasedsharply after 24 h of leaching, up to 1,492 ppb (Table 3; Fig.3). A similar enrichment was observed after 7 hours with HClleaching. Lead concentrations of HCl leachates are slightlyhigher than NaCl leachates, with samples 91H017 and91H015 having very similar concentrations. Leachates of Pa-leozoic black shales contained more Pb (up to 2,704 ppb Pb)than the Cretaceous red siltstone leachates (up to 266 ppb).The red siltstone differed from the black shales in that littlePb was leached by NaCl solution, but almost all of the Pb con-tained in the rock powder was leached out in 0.5N HCl. Leadisotope ratios of leachates were consistently lower than thewhole-rock compositions, to extents well in excess of instru-mental error. Lead isotope ratios of leachates range in206Pb/204Pb from 18.131 to 18.623, in 207Pb/204Pb from 15.621to 15.768, and in 208Pb/204Pb from 38.018 to 39.180. Residuesof two leaching experiments were analyzed. The residue ofsample 98M018 was more radiogenic than the bulk rock com-position, but both leachate and residue analyses of sample97H022 were less radiogenic than the whole-rock analysis;this is difficult to explain unless the whole-rock powder wasnot well homogenized.

Uranium behaved differently from lead during the same ex-periments. The NaCl leachates of samples 98M013 and98M018 contained less U than the blank level, indicating thatU was scavenged by the rock powder. The NaCl leachate ofsample 91H017 contained U close to the blank level, and theNaCl leachate from sample 97H015 contained relatively highU. Differences in U mobility between these samples may bedue to the presence of U in different phases, or differing

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TABLE 2. Leaching Experiments with Sample 98M013 under Different Conditions

Duration Solution Temp. (C) NaCl (%) HCl (N) U (ppb) Th (ppb) Pb (ppb)

24 h 1 ml 200 5 5.04 13.92 2.424 h 1 ml 200 10 4.82 10.01 28.624 h 1 ml 200 15 4.6 11.07 12424 h 2 ml 200 15 14.4 1913 days 10 ml 200 3.5 4.6 6.15 2543 days 10 ml 200 10 4.7 5.26 59824 h 2 ml 200 1 36.8 145.6 4810

*All runs with 0.2 g rock powder

97H022*

10 100 1000Pb (ppb)

97H015**

97H017**

98M013**

98M018**

97H022*

0.1 1 10 100

U (ppb)

97H015**

97H017**

98M013**

98M018**

A

B

15% NaCl leachate0.5N HCl leachate

15%

NaC

l Bla

nk

6.2N

HCl

Bla

nk15

% N

aCl B

lank

* K-T red siltstone** Pz black shale

FIG. 3. Lead (A) and uranium (B) content in NaCl and HCl leachates. Pbblank of 6.2 N HCl is less than 10 ppb in (A).

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TABLE 3. Lead Isotope Composition of Rocks, Leachates, and Ores from the Study Area

Sample Material Pb U 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

San Cristobal ppm ppm

91BSL013 Volcanic rock 101 2.0 1.40 17.966 15.615 38.20991BSL015 Volcanic rock 55 1.02 1.31 18.050 15.711 38.51891BSL016 Volcanic rock 45.4 0.4 0.62 17.899 15.588 38.11591BSL018 Volcanic rock 14 0.8 4.04 17.579 15.557 37.80491BSL020 Volcanic rock 5.0 0.4 5.66 17.628 15.603 37.92191BSL021 Volcanic rock 24.7 1.8 5.15 18.528 15.655 38.66591BSL022 Volcanic rock 120 0.8 0.47 17.838 15.591 38.09699SC01 Galena 17.882 15.631 38.20699SC02 Galena 17.812 15.624 38.15799SC03 Galena 17.784 15.579 38.04999SC04 Galena 17.813 15.612 38.11899SC06 Pyrrhotite 17.890 15.577 38.06899SC08 Galena 17.772 15.588 38.04699SC09 Galena 17.766 15.555 37.954

Toldos99A07 Sphalerite 17.999 15.708 38.57399T11 Hematite 17.964 15.637 38.238

Pulacayo98M024A Pyrite 18.717 15.759 39.04198M024B Pyrite 18.746 15.808 39.21698M024C Galena 18.600 15.664 38.85098M024D Pyrite 18.620 15.698 38.913

Paleozoic and Cretaceous sedimentary rocks ppb ppb

97H013 Sedimentary whole rock 18.691 15.680 39.02497H014 Sedimentary whole rock 18.830 15.688 39.10097H015 Sedimentary whole rock 18.831 15.814 39.55597H015 NaCl leachate 1,492 30.8 1.46 18.510 15.690 38.94497H015 HCl leachate 1,518 28.7 1.34 18.482 15.659 38.88997H016 Sedimentary whole rock 18.739 15.664 39.31397H017 Sedimentary whole rock 18.821 15.700 39.33797H017 NaCl leachate 886 1.9 0.15 18.452 15.643 38.96197H017 HCl leachate 931 29.9 2.2 18.475 15.652 38.98897H018 Sedimentary whole rock 18.824 15.790 39.56597H019 Sedimentary whole rock 18.514 15.640 38.49497H020 Sedimentary whole rock 18.589 15.764 38.76497H020 NaCl leachate 18.131 15.646 38.01997H022 Sedimentary whole rock 13,000 2000 11.0 18.757 15.802 39.17297H022 NaCl leachate 162 45.3 19.6 18.250 15.621 38.32197H022 HCl leachate 266 51.1 13.7 18.282 15.622 38.36897H022 HCl residue 18.685 15.645 38.86598M013 Sedimentary whole rock 35,600 2200 4.4 18.712 15.802 39.32098M013 NaCl leachate 43.5 0.61 0.99 18.623 15.768 39.18098M013 HCl leachate 2,704 31.1 0.82 18.381 15.648 38.58498M014 Sedimentary whole rock 19.226 15.734 39.92098M017 Sedimentary whole rock 18.889 15.676 39.74698M018 Sedimentary whole rock 19.261 15.741 40.14698M018 NaCl leachate 165 0.43 0.18 18.596 15.671 39.17998M018 HCl leachate 470 30.5 4.59 18.549 15.643 39.05098M018 HCl residue 20.137 15.778 42.16398M022 Sedimentary whole rock 18.337 15.743 38.610

Blank NaCl 25 0.9 2.55 22.138 17.056 42.811Blank 6.2N HCl 0.48 0.4 56.7

amounts of reduced carbon in the sediments, which couldhave allowed U scavenging by the more reducing powders.However, C content of sample powders was not measured.Sample 97H022, a red siltstone considered to be free of or-ganic matter, yielded the highest U content in the NaClleachate.

In contrast to experiments with 15 percent NaCl, the 0.5NHCl solutions leached about 10 to 15 percent of U from theshales and about 25 percent from the red siltstone. Shaleleachates have very similar U concentrations, about 30 ppb;U in the red siltstone leachate was 51 ppb. Whole-rock pow-ders of shale (98M013) and red siltstone (97H022) have verysimilar U content, about 2 ppm (Table 3). Different U con-tent in leachates of shales and siltstone probably reflect dif-ferences in the mineralogical siting of U between the tworock types.

Discussion

Lead isotope ratios of the metamorphic basement

The oldest known basement in the central Andes is the Are-quipa-Antofalla craton (Ramos, 1986). The northern part ofthis craton, called the Arequipa massif, is exposed in southernPer and has Early Proterozoic protolith ages (Dalmayrac etal., 1977). Lead isotope ratios of high-grade gneisses from theArequipa massif are shown in Figure 4. They have very low206Pb/204Pb (

1990; Wrner et al., 2000). It is also known from gneiss andgranofels clasts and xenoliths in Tertiary sedimentary and vol-canic rocks in western Bolivia, northwestern Argentina, andnorthern Chile (Tosdal, 1996). Rocks from the southern Are-quipa-Antofalla craton yield generally Middle Proterozoicprotolith ages (Damm et al., 1990). Although these rocks arenot as isotopically distinctive as the Arequipa massif, theyhave low present-day 206Pb/204Pb (17.2518.00) and elevated207Pb/204Pb and 208Pb/204Pb values (Fig. 4; Tosdal et al., 1993;Aitcheson et al., 1995; Tosdal, 1996). Samples from CerroUyarani yield Early Proterozoic protolith ages, but theirwhole-rock lead isotope ratios overlap with those from thesouthern Arequipa-Antofalla craton. A few have lower206Pb/204Pb, like the Arequipa massif (Wrner et al., 2000).

Tosdal (1996) proposed that the entire Arequipa-Antofallacraton was overprinted by Grenville age (ca 1.1 Ga) meta-morphism. Granulite facies metamorphism in the northernpart of the basement terrane produced depletion of U withrespect to Th, while lower grade metamorphism in the south-ern part of the craton produced less fractionation of U/Th andU/Pb values. The Belen complex exposed in northern Chilewas apparently not affected by Grenvillian metamorphismand instead shows two Paleozoic metamorphic events.Wrner et al. (2000) interpret these differences to reflect amajor terrane boundary between the Belen complex and therest of the Arequipa-Antofalla craton; however, their whole-rock lead isotope systematics are similar.

Lead isotope ratios of Paleozoic and Cretaceous sedimentary rocks

The early Paleozoic section sampled in the study area isdominated by a sequence of dark, deep-marine shales up to15 km thick, formed in a large foreland basin, possibly at apassive-type margin (Gagnier et al., 1996). Lead isotope ratiosof the Paleozoic shales are slightly more radiogenic on aver-age in 207Pb/204Pb and 208Pb/204Pb than the present-day esti-mate of the bulk earth from the Stacey-Kramers model (Fig.4), and are substantially more radiogenic than both the meta-morphic basement of the Arequipa massif (Tilton and Bar-reiro, 1980) and the Antofalla craton of northern Chile(Wrner et al., 1992; Aitcheson, et al., 1995; Tosdal, 1996).Lead isotope ratios of the Ordovician shales are fairly typicalof upper crustal continental sedimentary rock compositions,and do not appear to have been affected by U and Pb loss dueto metamorphism. The Arequipa-Antofalla craton is thereforeunlikely to be the source of sediments in the Paleozoic shalessampled for this study, which were most likely derived fromthe Brazil Shield to the east.

Cretaceous-Tertiary sedimentary rocks in the study area aremostly red beds. Compositions of nine K-T sedimentary rocksamples cover a broad range of lead isotope compositions, butcoincide generally with the compositions of the underlyingPaleozoic shales (Aitcheson et al., 1995; this study); theytherefore probably share the same general provenance as thePaleozoic sedimentary rocks.

Lead isotope ratios of San Cristobal district igneous rocks

San Cristobal district igneous rocks are mostly high K an-desites and dacites, except for one basalt and one sample thatshows extensive hydrothermal alteration (Kamenov, 2000).

The unaltered samples have low 206Pb/204Pb values for given207Pb/204Pb and 208Pb/204Pb and reflect the same source oflead (Table 3; Fig. 5). On the plot of 207Pb/204Pb versus 206Pb/204Pb, they lie well above the compositions of Nazca platebasalts and Nazca plate metalliferous and pelagic sediments(Unruh and Tatsumoto, 1976; Dasch, 1981; Hamelin et al.,1984), indicating a crustal lead isotope signature. The mostmafic sample (91BSL020; SiO2 = 48.3 wt %) contains leadthat is isotopically identical to that in the more felsic sam-ples(Kamenov, 2000). Lead isotope ratios of the igneous rocksdo not overlap with the K-T and Paleozoic sedimentary rocks.Instead, Pb isotope ratios of the igneous rocks resemble thoseof the Altiplano and the northern Chile metamorphic basement.

Sample 91BSL021, an andesitic dike located northwest ofthe San Cristobal district and intruded into the Potoco For-mation (Kamenov, 2000), contains more radiogenic lead thanthe other samples. The minor basaltic center, Chiar Kkollu,located north of San Cristobal at 1926' S and 6723' W, is iso-topically similar to sample 91BSL021, with 206Pb/204Pb =18.520, 207Pb/204Pb = 15.629, and 208Pb/204Pb = 38.67. David-son and de Silva (1992) suggest that these basalts representthe primitive magmas that fed the more felsic magmatism.The lead isotope ratios of the Chiar Kkollu basalt are alsonearly identical to the average of all Chilean ore deposits(Puig, 1988), which are thought to represent the compositionof the enriched sub-Andean mantle wedge (Macfarlane et al.,1990).

The isotopic compositions of lead in unaltered igneousrocks from the San Cristobal district lie near the radiogenicedge of the field of Altiplano and northern Chile basementrock compositions (Fig. 6). Igneous rocks in the San Cristobaldistrict may therefore have acquired their lead isotope signa-ture by mixing between a high 206Pb/204Pb parental magmawith the metamorphic basement. If the compositions of ChiarKkollu volcanic rocks and average province I ores approxi-mate the parental composition, the proportions of lead con-tributed by the parental magma and by the metamorphicbasement may be estimated. Averages of published isotopiccompositions for the two parts of the Arequipa-Antofalla cra-ton are shown in Figure 6, together with the average ofChilean ores (Puig, 1988). In this model, the ores of the SanCristobal deposit, with an average 206Pb/204Pb = 17.82, con-tain about 25 percent lead from the primary source and 75percent from the metamorphic basement.

Sample 91BSL015 has higher 207Pb/204Pb and slightlyhigher 206Pb/204Pb than the unaltered igneous rock samplesfrom San Cristobal. This sample shows evidence of strong hy-drothermal alteration, but its composition does not corre-spond to the compositions of hydrothermal ores in the districtor to the compositions of leachates from sedimentary hostrocks and remains enigmatic.

Hydrothermal leaching of lead

Uranium is incompatible relative to lead in metamorphicfeldspars and micas, so easily leachable parts of rocks such asgrain boundaries and crystal defects acquire much higher (238U/204Pb) than the bulk rock during metamorphism andproduce highly radiogenic lead over time. Leaching of old,high-grade metamorphic rocks produces much more radi-ogenic lead in solution than the whole-rock composition

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(Gray and Oversby, 1972). Leaching of younger, less stronglymetamorphosed rocks is less well understood. Figure 7 shows208Pb/204Pb versus 206Pb/204Pb for 15 percent NaCl and 0.5NHCl leachates of six samples of nearly unmetamorphosed Pa-leozoic and Cretaceous sedimentary rocks from the studyarea. Plots of 207Pb/204Pb versus 206Pb/204Pb show the same re-lationships. Lead isotope ratios of the easily leachable frac-tions of Paleozoic sedimentary rocks are much lower thanthose of bulk, HF-soluble samples, and the Pb isotope ratiosof the HCl and NaCl leachates of four samples are within an-alytical error of each other. The concentrations of lead in theHCl and NaCl leachates of each sample are also similar, es-pecially for samples 97H015 and 97H017 (Fig. 3). The effectof leaching appears to be fairly insensitive to the leachingmethodboth leachates extracted roughly the same solublefraction of the samples. The NaCl leachate of sample 98M013(Fig. 7b) is more radiogenic than the HCl leachate, nearlyidentical with the bulk sample, and lead concentrations inthese two leachates are substantially lower than in the othersamples (Table 3), suggesting that the NaCl and HCl

leachates attacked different lead-bearing materials in this Pa-leozoic black shale. Most trace elements in sedimentary rocksare contained in nonlithic particles such as Fe and Mn oxidesand hydroxides, amorphous aluminosilicates, carbonates, sul-fides, and organic matter (Martin et al., 1987). Evidently, theleachates of our samples reflect the nonlithic fraction, whilethe whole-rock data represent mixtures of the nonlithic andlithic fractions. The analysis of the HCl residue of sample98M018 supports this idea (Fig. 7a), indicating a proportionof about 60 percent lead in the leachable fraction and 40 per-cent in the residue.

Various explanations for the isotopic difference betweenthe leachable and HF-soluble fractions of the sedimentaryrock powders deserve consideration. The Paleozoic samplesare deep-marine shales formed in a foreland basin, possiblyalong a passive margin (Gagnier et al., 1996). The sampleswere apparently not isotopically homogenized, because re-gressions through leachate and whole-rock analyses for indi-vidual samples yield ages of 1.2 to 4.2 Ga, far older than thesedimentation age. The leachable and HF-soluble fractions

Pb SOURCES IN SAN CRISTOBAL, PULACAYO, AND POTOS MINING DISTRICTS, BOLIVIA 583

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38

39

40

41

42

15.4

15.5

15.6

15.7

15.8

15.5 16.5 17.5 18.5 19.5 20.5

206Pb/204Pb

207 P

b/20

4 Pb

208 P

b/20

4 Pb

Potos oresSan Cristobal oresSan Cristobal igneous rocksPulacayo ores

Nazca Plate sedimentsNazca Plate MORBPz, K and T sedimentary rocks

Arequipa Massif basement

Toldos ores

Altiplano/N. Chile Mid-Proterozoic basement

FIG. 5. Lead isotope ratios of ores and igneous rocks from the San Cristobal, Potos, and Pulacayo mining districts of Bo-livia, compared with potential metal sources. Uncertainties of ore and igneous rock analyses are comparable to symbol sizes.Data sources include those listed for figure 4, and Unruh and Tatsumoto (1976), Dasch (1981), Tilton et al. (1981), Hamelinet al. (1984) and Macfarlane et al. (1990).

may have been deposited with different initial lead isotope ra-tios if hydrothermal activity in or near the basin contributedto the leachable fraction, although we are not aware of evi-dence for such activity. These rocks may also have been per-meated by diagenetic or low-temperature metamorphic fluidswith low values. If such fluids contained nonradiogenic Pbfrom the metamorphic basement, they could have producedthe observed isotopic systematics.

The nonradiogenic compositions of the shale leachates mayalso be explained by their low values (Table 3), and the ob-servation that some of the shales scavenged U from theleachate solution rather than the reverse. During diagenesisor low-grade metamorphism, pore fluids may have mobilizedlead into the leachable fraction of the rock, but U remainedinsoluble due to the reducing nature of the black shale. Thelow values in the leachable fractions would produce nonra-diogenic Pb in this part of the rock over time. The failure ofwhole-rock and leachate analyses to form sensible isochronssuggests that at least one component (probably the leachablefraction) was open to U and/or Pb exchange at various times.This explanation could be valid for clay and organic-rich sed-imentary rocks (i.e., the shales) but not for the red siltstones.Sample 97H022 is a red siltstone and the NaCl and HClleachates are also less radiogenic than the whole-rock sample,but the values are relatively high (Table 3).

Sources of metals in the Potos, Pulacayo, and San Cristobal deposits

Comparison of the lead isotope compositions of ore miner-als from San Cristobal with those of the related igneous rocks,

the regional Paleozoic and Cretaceous sedimentary rocks,and the underlying regional metamorphic basement yields apicture of the likely provenance of metals in this deposit (Fig.8). The similarity of ore lead isotopes with those in the ig-neous rocks in the district indicates that the proximal sourceof ore metals in the hydrothermal system was the igneous ac-tivity. Isotopic compositions of leachates of the Paleozoic andCretaceous sedimentary rocks are not consistent with those ofthe ore minerals, so hydrothermal leaching of these rockscannot have supplied a significant proportion of the oremetal. The igneous rocks themselves appear to have acquiredtheir lead mainly from the regional metamorphic basementwith a lesser component, probably a province I-type well-mixed source derived from the sub-Andean mantle wedge en-riched by subducted sediments. Ores from the Toldos depositin the San Cristobal district have higher 206Pb/204Pb than theother San Cristobal ores and igneous rocks. The significanceof this difference is not clear, but may reflect the presence ofan igneous body at depth with isotopic characteristics differ-ent from those sampled so far.

The Pulacayo and Potos deposits are both related to felsicdomes emplaced through and onto the Paleozoic sedimentarysequence. Some veins in each deposit extend into the subjacentsedimentary rocks. Hydrothermal circulation must have af-fected the sedimentary rocks, and lead isotope ratios of Potosores are known to resemble those of whole-rock Paleozoicshale samples (Macfarlane et al., 1990). Until now, however, ithas not been clear whether the lead was incorporated by mag-matic assimilation or derived from hydrothermal leaching ofthe Paleozoic sedimentary rocks. Analyses of ore minerals from

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15.5

15.6

15.7

16.5 17 17.5 18 18.5 19

San Cristobal oresBerenguela oresToquepala oresCerro Verde oresMarcona ores

Altiplano/N. Chile basement

25%

50%

75%

25%75%

Province Ia (Chile) oresArequipa Massif

Average Arequipa MassifEarly-Proterozoic basement

Average Province Ia

Average Altiplano/N. ChileMid-Proterozoic basement

206Pb/ 204Pb

207 P

b/20

4 Pb

Madrigal ores

FIG. 6. Comparison of lead isotopic compositions of the basement beneath province IV, and ores from province Ia andprovince IV. Data from Tilton and Barreiro (1980), Damm et al. (1990), Macfarlane et al. (1990), Aitcheson et al. (1995), Tos-dal (1996), and Wrner et al. (2000) and this study. Mixing trajectories are shown between the average composition of provinceIa lead and lead from the Arequipa massif metamorphic basement and from northern Chile metamorphic basement.

the Tajo vein in the Pulacayo mining district and from the Po-tos deposit are shown in Figure 8 with data for Paleozoic andCretaceous sedimentary rocks. Ore minerals from Pulacayoand Potos are isotopically very similar to each other, and bothresemble the whole-rock compositions of the regional Paleo-zoic and Cretaceous sedimentary rocks. The leachates havemuch lower 206Pb/204Pb than ores from the Potos and Pulacayodistricts (Fig. 8), which suggests that hydrothermal leaching of

the Paleozoic sedimentary rocks cannot have been a significantsource of metals in the Potos and Pulacayo ores. Instead, leadmust have been incorporated by magmatic assimilation or ana-texis of sedimentary rocks, followed by partitioning of thosemetals into hydrothermal fluids. Melting of the Paleozoic sedi-mentary rocks is supported by the presence of at least two agesof inherited zircon, one Mesozoic and one Proterozoic, in thePotos dacite volcanic dome (Zartman and Cunningham, 1995).

Pb SOURCES IN SAN CRISTOBAL, PULACAYO, AND POTOS MINING DISTRICTS, BOLIVIA 585

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38

40

42

44

18 19 2038

39

40

18 18.5 19

38

39

40

18 18.5 1938

39

40

18 18.5 19

38

39

40

18 18.5 1938

39

40

18 18.5 19

98M018Pz black shale

A B

C D

E F

206Pb/204Pb 206Pb/204Pb

208 P

b/20

4 Pb

208 P

b/20

4 Pb

208 P

b/20

4 Pb

98M013Pz black shale

97H015Pz black shale

97H020Pz black shale

97H017Pz black shale

97H022K-T red siltstone

FIG. 7. 208Pb/204Pb versus 206Pb/204Pb for Paleozoic and Cretaceous sedimentary rocks from the study area, compared toleachates and residua. Solid squares represent whole-rock analyses, open squares are 15 percent NaCl solution leachates,open diamonds are 0.5N HCl leachates and solid diamonds are residua of HCl leaching experiments. Uncertainties of leadisotope measurements are comparable to symbol sizes, except for leachates of 97H022, which yielded low-intensity runs.

Reevaluation of Andean lead isotope provinces

Although there are very well documented differences be-tween the predominant ore metals mined in different parts ofthe Andes, they appear to be largely independent of the orelead isotope provinces, and therefore the source regions ineach province. For example, both gold-rich and gold-poordeposits in province I have the same lead isotope characteris-tics, and in province III copper, silver, gold, and base-metaldeposits all possess the same general isotopic characteristics(Macfarlane et al., 1990). These isotopic provinces illustratethe importance of long-lived, isotopically distinct crustalreservoirs in providing metals to a variety of different types ofcontinental arc ore deposits.

Province I ores have a restricted range of lead isotopic ra-tios, with 206Pb/204Pb = 18.2 to 18.8, 207Pb/204Pb = 15.55 to15.69, and 208Pb/204Pb = 38.11 to 38.95 (Fig. 8). Macfarlane etal. (1990) proposed that the lead in province I was derivedfrom the sub-Andean mantle enriched by subducted sediments.The relatively lead-rich subducted sediments would rapidlydominate the lead isotope composition of the sub-Andeanmagma source region (Aitcheson and Forrest, 1994; Mac-farlane, 1999). Province III lead is much more isotopically

variable than province I lead, having 206Pb/204Pb = 17.9 to25.18, 207Pb/204Pb = 15.51 to 16.00, and 208Pb/204Pb = 37.7 to40.07 (Fig. 8). The Bolivian-Argentine section of province III(referred to here as province IIIa) define a trend with ele-vated 207Pb/204Pb and 208Pb/204Pb relative to 206Pb/204Pb, indi-cating dominantly continental sources. Several authors havesuggested that the Paleozoic sequence in the central Andescould be the source of radiogenic province III type lead(Tilton et al., 1981; Macfarlane et al., 1990; Tosdal et al., 1993).Province IIIa deposits show a progression from Ordoviciandeposits at the nonradiogenic end of the array, to Mesozoicdeposits in the middle of the array, to Tertiary deposits at theradiogenic end (Macfarlane et al., 1990). This progression isthought to reflect episodic melting of the thick Paleozoic se-quence having 238U/204Pb about 10 and 232Th/204Pb about 45(Schuiling, 1967; Tilton et al., 1981; Macfarlane et al., 1990).Derivation of late Cenozoic magmatism related to provinceIIIa ores primarily from melting of crustal materials is sup-ported by Nd values in the range of 2 to 10 for these rocks(Miller and Harris, 1989).

Lead from province II ores have consistently higher208Pb/204Pb and 207Pb/204Pb than the province I ores (Fig. 8).Province II ores frequently show steep mixing arrays that

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15.5

15.6

15.7

18.2 18.4 18.6 18.8 19

Province I or

es

207 P

b/20

4 Pb

206Pb/204Pb

Province III Pz-Ksedimentarywhole-rocks

38.0

38.2

38.4

38.6

38.8

39.0

39.2

15.8

208 P

b/20

4 Pb

ColquiJulcaniCasapalca

OrcopampaMilpo/Atacocha

Potos oresPulacayo ores

Sedimentaryrock leachates

Provinc

e I ores

Province III Pz-Ksedimentarywhole-rocks

Province II ores Province III

Province IIIa

ores

Province

IIIa ore

s

FIG. 8. Comparison of lead isotope ratios in province I and IIIa ore deposits with those in province III sedimentary rocks,sedimentary rock leachates, and province II ore deposits. Data from compilations of Macfarlane et al. (1990) and sourcestherein and of Aitcheson et al. (1995) and sources therein, Macfarlane (1999) and this study. Province I includes ores fromcentral and northern Chile, and some coastal areas of Per. Province IIIa includes deposits in Bolivia and northwest Ar-gentina to the east and south of the central Altiplano (cf. Fig. 9a).

originate within the province I field and extend to composi-tions within the province III field. While steep arrays canform on lead isotope plots due to uncorrected instrumentalfractionation, and fractionation cannot be ruled out as a causeof some of the spread in the province II arrays, other factorsmitigate against their being mere artifacts of the analysis. Thearrays are steeper than fractionation arrays on 208Pb/204Pb206Pb/204Pb diagrams and span a larger range of compositionthan can be explained by typical analytical uncertainties. Thearrays occur only in analyses by various authors from a re-stricted area of the Peruvian cordillera, and not in data fromdeposits in provinces I and III. The arrays therefore probablyindicate mixing of province I lead with upper crustal compo-nents having higher 207Pb/204Pb and 208Pb/204Pb typical ofprovince III ores (Macfarlane, 1999). In principle, mixingcould take place either by magmatic assimilation or hy-drothermal scavenging of radiogenic lead from host Paleozoicand Mesozoic sections. However, leachates of Paleozoic andCretaceous sedimentary rock plot at 206Pb/204Pb values muchtoo low to serve as an acceptable radiogenic endmember forany of the province II arrays (Fig. 8). The province II mixingarrays are, therefore, more likely caused by assimilation of ra-diogenic province III-type material by relatively nonradi-ogenic, province I type magmas, followed by extraction ofmetals from magmas that are not completely homogenized.

The Potos, San Cristobal, and Pulacayo mining districtsspan the tentative boundary between province II and III pro-posed by Petersen et al. (1993). Only one analysis each hasbeen available for San Cristobal and Pulacayo, so their posi-tion within the lead isotope province scheme has not beenwell constrained. The Potos deposit was included in the orig-inal lead isotope province map of Macfarlane et al. (1990),and plotted within the range of the Paleozoic sedimentaryrock lead isotope ratios and clearly represents province IIImetal sources. The same is true of the Pulcayo deposit.

Late Miocene San Cristobal ores have 206Pb/204Pb valuesbetween 17.766 and 17.882, lower than the least radiogenic(Ordovician) ores from province III and far lower than anyTertiary province III ores (Fig. 9c, d). Miocene polymetallicvolcanic-hosted veins from the Berenguela district, north-northwest of San Cristobal in west-central Bolivia, also con-tain relatively nonradiogenic lead, interpreted by Tosdal et al.(1993) to reflect incorporation of Pb from the Proterozoicmetamorphic terrane underlying the area. Most of the vol-canic rocks in western Bolivia north of San Cristobal alsoshow relatively low lead isotope ratios, interpreted by manyauthors to reflect local basement influence (e.g., Tosdal et al.,1993; Aitcheson et al., 1995). Farther northwest in southernPer are the large Paleocene porphyry copper deposits atToquepala and Cerro Verde, and the undated Pb-Zn-Ag de-posit at Madrigal, and the late Jurassic volcanogenic mag-netite deposit at Marcona (Mukasa et al., 1990; Fig. 9a). Al-though these deposits lie within the coastal magmatic arcassigned to lead isotope province I by Macfarlane et al.(1990), their ore lead isotope data are all shifted toward lower206Pb/204Pb and higher 208Pb/204Pb values than ores of theChilean section of province I. Macfarlane et al (1990) attrib-uted this shift to incorporation of lead from the local meta-morphic basement, and divided province I into three sub-provinces as a result.

Based on these new and published data, we have defined anew ore lead isotope province in the central Andes, extendingfrom the coastal area of southern Per through southern Perand western Bolivia to San Cristobal (Fig. 9a). Province IV isdivided into subprovinces reflecting the relatively minorbasement imprint in the southern Per segment (IVa) and themuch stronger basement imprint in the Bolivian segment(IVb). The internal subdivision of province IV is only approx-imate and may well be gradational; although the southernpart of the Arequipa-Antofalla metamorphic basement hasgenerally similar lead isotope characteristics, it appears to beheterogeneous in age and may be a mosaic of smaller terranes(Aitcheson et al., 1995; Wrner et al., 2000). The eastern andsoutheastern boundary between province IV and provinceIII, which passes roughly north-south through the center ofthe Altiplano, is constrained by analyses of several small Ter-tiary polymetallic deposits (Maria Luisa, Almacen, Escala,and Todos Santos), and the Bi- and W-bearing Lipia-Galandeposit (Long, 1991; Aitcheson et al., 1995), and by analysesof copper ores from the Corocoro district south-southeast ofLake Titicaca (A.W. Macfarlane and H. Lechtman, unpub.data). Each of these deposits has a typical Tertiary provinceIII-type lead isotope signature and contrasts strongly withSan Cristobal and with the volcanic rocks in the northern Al-tiplano province of Aitcheson et al. (1995). The southwesternboundary (west and southwest of San Cristobal) is still poorlyconstrained due to a lack of ore deposit data. The westernboundary between province I and province IV is loosely de-fined by northern Chilean porphyry Cu deposits includingChuquicamata and Tignamar in far northern Chile (Zentilli etal., 1988; Puig, 1988).

Although province IVa and IVb ores both contain lead fromthe local metamorphic basement, the relative proportions ofbasement lead in their ores differ. In the San Cristobal de-posit, as we have seen, about 75 percent of the lead in the oreappears to be derived from the metamorphic basement (Fig.6). A similar estimate of mixing between the average Are-quipa massif metamorphic basement and a hypothetical pri-mary magma with a lead isotope composition like that ofprovince I yields about 10 percent basement-derived lead inthe Toquepala and Cerro Verde deposits, about 12 percent inMadrigal ores and 18 percent in Marcona ores (Fig. 6).Berenguela district ores have higher 208Pb/204Pb than the SanCristobal ores, suggesting that some of the Berenguela orelead was assimilated from higher-grade metamorphic base-ment with elevated Th/U like that of the Arequipa massif. Iso-topic compositions of ores from the Berenguela district arecompatible with incorporation of about 25 percent of high-grade metamorphic basement lead.

While these estimates are crude, the different proportionsof basement lead in ores from provinces IVa and IVb must besignificant. Deposits in the two parts of province IV differ inage. The Peruvian deposits in province IVa, which contain theleast basement lead, are late Paleocene, with Cerro Verdehaving formed about 58.9 2.0 Ma (K-Ar date on biotite;Estrada, 1975) and Toquepala about 58.7 1.9 Ma (K-Ar dateon biotite; Clark et al., 1990). Ore formation farther south inthe Berenguela district, which contains more basement lead,probably took place in mid- to late Miocene (Wallace et al.,1992). San Cristobal, which contains the most basement lead,

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Toquepala

22

70

0 400Scale (km)

Cerro VerdeMadrigalMarcona

Deposits with Pb isotope data

22

1818

70

1414

66

6674

Berenguela

San Cristobal

Tignamar

A

Orcopampa

Julcani

IVa

IVb

Approx. limitof Altiplano

7050

22

70

Approx. depth toMoho (km) (James , 1971)

22

1818

70

1414

66

6674

Approx. margin of Arequipa-Antofalla craton (Todsal, 1996)

IVb

B

60

Julcani

IVa

Extent of province IV

19.018.518.0206 204

Pb/

Pb

207

204

15.5

15.6

15.7

15.8

38.5

39.0

Pb/

Pb

208

204

39.5

Province IVb Province IVa

38.0

San Cristobal ores

Berenguela ores

Province IProvince IIProvince IIIaProvince IIIb

Pb/ Pb

Province IVb

Province IVa

Province IVa ores

Province IV

Toldos ores

C

D17.5

FIG. 9. Ore lead isotope provinces of the central Andes. (A) Geographic distribution of ore lead isotopic provinces; (B)position of province IV relative to the margins of the Arequipa-Antofalla terrane (Tosdal, 1996) and contours of crustal thick-ness (James, 1971); (C) and (D) lead isotope ratios of ores from each province. Black dots indicate locations of deposits withfor which lead isotope data are available. Data sources as in Figure 9, plus Tosdal et al. (1993).

formed about 8.5 Ma (Ludington et al., 1992). These secularchanges in the proportion of basement lead incorporationmay correlate with the history of crustal thickening in the Al-tiplano.

The crust beneath the modern Altiplano is 70 to 75 kmthick (Fig. 9b); thickening is thought to have begun about 40m.y. ago and increased rapidly about 20 m.y. ago (Isacks,1988; Lamb and Hoke, 1997; James and Sacks, 1999). The 25Ma Chiar Kkollu basalts are not extensively contaminated bymetamorphic basement lead, and are thought to representthe primary magmas that feed the arc (Davidson and de Silva,1992). Crustal thickening was accompanied by increasing87Sr/86Sr in magmas erupted through the thickened crustwithin the last 20 m.y. and by assimilation of old, isotopicallyevolved metamorphic basement by late Cenozoic magmaserupted in and near the study area (Davidson et al., 1990;Feeley, 1993). Volcanic rocks in the western Altiplano andCordillera Occidental containing dominantly metamorphicbasement lead are younger than 20 Ma (Aitcheson et al.,1995). Major and trace element data for the San Cristobal dis-trict igneous rocks (Kamenov, 2000) indicate extensive horn-blende fractionation, consistent with combined assimilationand fractional crystallization under high total pressure andPH2O (Davidson and de Silva, 1995). The isotopic homogene-ity of lead in igneous rocks from San Cristobal indicates thatchemical differentiation of the magmas occurred after thecontamination with basement material, and that the magmaswere then emplaced into the upper crustal rocks without sig-nificant further assimilation. Incorporation of lead from themetamorphic basement by deep magmatic assimilation ap-pears to have been enhanced in province IVb by the thicken-ing of the Andean crust after 20 Ma.

The Arequipa-Antofalla metamorphic basement terrane isbelieved to have a much greater extent than the province wecan define based on ore lead isotope data (Fig. 9b; Tosdal,1996). The area of basement influence on volcanic rock com-positions is also wider and extends further to the south thanprovince IV (Wrner et al., 1992; Aitcheson et al., 1995). Thebasement influence zones defined by Aitcheson et al. (1995)occupy most of the width of the Altiplano in Bolivia, corre-sponding roughly to the 60 km crustal thickness contour onFig. 9b, though it also is not as wide as the proposed Are-quipa-Antofalla craton. Aitcheson et al. (1995) divided theirarea of basement influence into a northern Altiplano zonewhere the basement was nonradiogenic, a transition zonewhere the basement was of mixed radiogenic-nonradiogeniccharacter, and a southern Altiplano zone with radiogenic leadisotope signatures. Our province IV extends well into thetransition zone of Aitcheson et al. (1995), where San Cristo-bal contains the least radiogenic ore lead in the central Andes.

The significance of the much smaller area of province IVcompared to that of the metamorphic basement that providedits distinctive ore lead isotope signature, or to the isotopic do-mains defined from volcanic rock compositions, is not clear.Province IV is defined by a small number of samples com-pared to the basement domains outlined by scores of volcanicrock analyses (Aitcheson et al., 1995). Analyses of additionaldeposits will better define province IV and may expand itsboundaries. However, the eastern boundary of province IV isthe best constrained, and it indicates a much narrower zone

of influence than that reflected in volcanic rocks. The base-ment may be thinner in some areas than others, or it may notoccur at the depth where magmatic assimilation took place inall areas, and it probably contains isotopically heterogenousenclaves that create different isotopic imprints on assimilatingmagmas. None of these possibilities alone explain why vol-canic rocks and ores in the same or nearby areas should showstrong isotopic differences.

There is also an age bias between the sampling of volcanicrocks and ores. Most Altiplano ore deposits are of Mioceneage and have been exposed by some erosion; none of the leadisotope data are from ore deposits known to be younger.Many of the volcanic rocks analyzed by Aitcheson et al. (1995)are of Pleistocene and Holocene age. Because the crust be-neath the Altiplano is thought to have thickened significantlyin that interval (Lamb and Hoke, 1997), the Miocene oresmay reflect basement influence only over a small area wherethe crust was thickest at that time. The Pleistocene-Holocenevolcanic rocks may reflect extensive basement assimilationover a wider area of thickened crust.

ConclusionsLead isotope ratios of ores and igneous rocks from the San

Cristobal district, of regional sedimentary rock units andleachates of those sedimentary rocks, and of the regionalmetamorphic basement provide a clear picture of the sourceof metals in the San Cristobal Ag-Zn deposit. Ore Pb in thehydrothermal system, and probably geochemically similarmetals such as Cu, Zn, and Ag, were derived from the associ-ated igneous rocks. Those igneous rocks in turn appear tohave assimilated about 75 percent of their lead from the Mid-dle Proterozoic metamorphic basement (part of the Are-quipa-Antofalla craton) which underlies this area. The re-maining, more radiogenic component was probably derivedfrom greater depth. Ore lead in the Pulacayo and Potos de-posits was derived by magmatic assimilation of the thick EarlyPaleozoic sedimentary sequence that immediately underliesthose deposits, perhaps because the sedimentary rocks arethicker to the east beneath those deposits, or the Middle Pro-terozoic metamorphic basement may be absent there.

Lead isotope ratios of leachates of samples from the thickEarly Paleozoic and Cretaceous sedimentary sequences ofthe Eastern Cordillera and Altiplano indicate that hydrother-mal leaching of these rocks is probably not an importantmechanism for concentrating ore metals in the central Andesgenerally. Instead, most central Andean ore deposits inland ofthe coastal belt (province I) appear to contain a minor to dom-inant proportion of lead derived by magmatic assimilation ofthe crust. This implies that central Andean ores form at leastpartly by reconcentration of metals already resident in thecrust. Additions from a deeper, more radiogenic source, likeprovince I lead, make up the rest of the metal budget, andprobably represent new additions of metals to the crust. Thecentral Andean crust has probably therefore been progres-sively enriched in lead and perhaps other ore metals throughtime. This may partly explain the extraordinary wealth ofmagmatic-hydrothermal ores in the orogen. Although world-class deposits like Potos, San Cristobal, and Chuquicamata(which is dominated by province I type lead) can contain met-als from greatly different sources if the magmatic-hydrothermal

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processes are favorable, enrichment of the crust in metalsmay have played a role in the genesis of all of them. ProvinceI crust may have been enriched in metals over time, but if iso-topically distinct metamorphic basement is absent, magmaticassimilation will not be revealed by lead isotope analyses.

The new data in this paper and the review of published datahas allowed an extensive revision of the lead isotope provincemap of the central Andes and helped to define a new lead iso-tope province. The presence of isotopically distinctive Earlyand Middle Proterozoic crust (the Arequipa-Antofalla craton)beneath this province provides a clear indication of the extentof midcrustal magmatic assimilation by major ore-formingcontinental arc systems. Ores in the northern part of thisprovince, designated province IVa, contain a small to moder-ate component of lead from the highly nonradiogenic Are-quipa massif rocks that underlie the region. Ores in the south-ern part of this province, designated province IVb, containthe majority of lead extracted from the underlying metamor-phic basement. Province IVa ores are of Paleocene age andformed prior to the late Tertiary thickening of the Andeancrust, while later Miocene deposits to the south contain thelargest proportions of basement-derived lead. Thicker crustin Miocene time may have promoted ponding of magmas atintermediate levels in the crust and extensive assimilation ofmetamorphic basement in province IVb. Still younger Pleis-tocene-Holocene volcanic rocks in the Altiplano reflectstrong basement influence over a broader area, perhaps re-flecting a greater extent of thickened crust with time. Furtherstudies of the depth of the Arequipa-Antofalla metamorphicbasement combined with new measurements of the lead iso-tope ratios and chronology of Altiplano ores should provideimportant insights into the depth of assimilation and metalsource processes in continental arcs.

AcknowledgmentsThis work grew out of the MS thesis of the senior author at

Florida International University. We would like to thankRosemary Hickey-Vargas for advice and instruction duringthe thesis work, and Grenville Draper and K. Panneerselvamfor instructive comments. Yun Cai and Alberto Sabucedo ofthe FIU Department of Chemistry provided access to andsupport for our use of the ICP-MS laboratory. The researchwas partially sponsored by the Division of Chemical Sciences,Geosciences and Biosciences, Office of Basic Energy Sciences,U.S. Department of Energy, under contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managedand operated by UT-Battelle, LLC. Eddie McBay providedtechnical support for the lead isotope measurements made byAWM at ORNL. Johnny Delgado, Mark West, Carlos Lozano,and Magdalena Luna of Andean Silver helped us obtain sam-ples from San Cristobal and Pulacayo. Heather Lechtman ofMIT collected some of the samples of Paleozoic sedimentaryrocks and provided field support for the collection of the rest.Steve Ludington of the U.S. Geological Survey generouslyprovided igneous rock samples from San Cristobal and gavepermission to cite his unpublished report on that deposit.This manuscript was significantly improved by thoughtful andconstructive reviews from Richard Tosdal, Daniel Kontak,John Dilles, and Mark Hannington.July 1, 2001; January 21, 2002

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