rice et al-2005

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IntroductionCerro Rico de Potosi is the worlds largest silver deposit.

Since 1544 production has exceeded 2 billion ounces (Goz) ofsilver and 100,000 tons (t) of tin. It has been estimated that afurther 440 Mt of disseminated silver ore still remain withinthe oxide cap (Bernstein, 1989). Cerro Rico is one of manydeposits carrying Ag and Sn which occur mainly within theEastern Cordillera of Bolivia and collectively form the majorpart of the central Andean tin belt (Ahlfeld and Schneider-Scherbina, 1964; Turneaure, 1971; Fig. 1). The basic geologyhas been described by Lingren and Creveling (1928), Jaskol-ski (1933), Turneaure and Marvin (1947), Turneaure (1960),and Steele (1996). More recent works have concentrated ongenetic aspects of the deposit. These have included studies offluid inclusions (Sugaki et al., 1988), geochronology (seebelow), stable isotopes (Steele, 1996), and the geologic set-ting (Frances et al., 1981; Schneider, 1985). Comprehensivegenetic models have been proposed by Sillitoe et al. (1975),Cunningham et al. (1991), and Steele (1996).

Cerro Rico is foremost a vein-hosted mineral deposit, al-though other styles of mineralization are present. It is bestclassified as a Bolivian polymetallic vein deposit (Ludingtonet al., 1992). The disseminated and bonanza vein silver oreswere deposited in an advanced argillic lithocap in the upperpart of the deposit (Steele, 1996), whereas the tin is found inthe veins at depth. The deposit is centered on a rhyodacitedome located on the western boundary of the early MioceneKari Kari caldera complex (Fig. 2). The dome intrudes a base-ment of Paleozoic marine sediments and an overlyingMiocene sequence of ignimbrites, volcanic breccias, tuffs,and epiclastic sediments, some of which represent outflowdeposits from the caldera. The dome has the form of a broadinverted cone and is composed of a homogeneous porphyriticrhyodacite with quartz, sanidine, and biotite phenocrysts,

which is typically, but not always, pervasively altered tosericite. The dome is unconformably overlain on the south-west side by the Huakajchi Ignimbrite Formation (Figs. 2, 3),a sequence of relatively fresh rhyolitic lapilli crystal-rich tuffsup to 300 m thick. The source of the ignimbrite is a vent nowoccupied by the nearby Huakajchi Dome Complex, which is

DURATION OF MAGMATIC, HYDROTHERMAL, AND SUPERGENE ACTIVITY AT CERRO RICO DE POTOSI, BOLIVIA

C. M. RICE, G. B. STEELE,*Department of Geology and Petroleum Geology, University of Aberdeen AB24 3UE, Scotland

D. N. BARFOD, A. J. BOYCE, AND M. S. PRINGLEScottish Universities Environmental Research Centre, East Kilbride, Glasgow G75 0QF, Scotland

AbstractNew high precision 40Ar/39Ar dating of sanidine and biotite from two rhyolitic domes and an ignimbrite,

combined with existing fission-track data and a hydrothermal sericite age, suggests that the world-class Agdeposit at Cerro Rico was emplaced during a protracted period of magma-related hydrothermal activitybeginning at 13.77 0.03 Ma and continuing for at least 0.2 m.y. This may have been sustained by a largesingle injection or repeated injections of fractionated Ag-enriched magma into a high-level magma cham-ber. K-Ar dating of alunite indicates that supergene oxidation had begun by about 13.5 Ma, soon afterdome emplacement, and progressed semicontinuously for at least 7.5 m.y. This oxidation, while not lead-ing to significant enrichment, has significantly enhanced the economic viability of the disseminated partof the orebody.

Corresponding author: e-mail, [email protected]*Present address: Rio Tinto Mining and Exploration Ltd., S. American

Region, El Bosque Sur 130, Piso 7, Las Condes, Santiago, Chile.

2005 Society of Economic Geologists, Inc.Economic Geology, v. 100, pp. 16471656

FIG. 1. Location of Cerro Rico within the central Andean tin belt and dis-tribution of Tertiary Bolivian polymetallic ore deposits (after Ludington etal., 1992).

composed of intrusive, flow-banded rhyolite and associatedbreccias (Steele 1996). Small exposures of pale-gray volcanicash, which postdate the Huakajchi ignimbrite, are found onthe western and northeastern flanks of the Cerro Rico dome.At both outcrops the ash is subhorizontally bedded, overliesoxidized paleoscree deposits, and is in turn overlain by morerecent scree. Both exposures are believed to be remnants ofthe same air-fall event.

Central to understanding the genesis of this ore deposit is adetailed and accurate relative chronology of magmatic, hy-drothermal, and supergene events. Previous dating studieshave established the broad outline of magmatic and hy-drothermal activity, as discussed below. In this study we in-vestigate the relative age of key events by new high-precision

40Ar/39Ar and K-Ar determinations of previously dated andalso some undated rocks, in conjunction with new field ob-servations.

Sampling and Analytical Methods

Samples

The earliest possible mineralization would have been coin-cident with the emplacement of the Cerro Rico dome. Toconstrain this age limit we directly dated sanidine phenocrystsfrom the dome. The dome has been affected by pervasive,sericitic, hydrothermal alteration, but a pocket of weak propy-litic alteration adjacent to the Tajo Polo vein preserves smallexposures in which biotite and sanidine phenocrysts are es-sentially unaltered (Fig. 4). One field sample (CR-0420) wastaken from this pocket and 15 sanidine phenocrysts were se-lected for dating (Table 1). The sanidine appears to have crys-tallized relatively late in the cooling history.

The Huakajchi ignimbrite that overlies the flanks of thedome contains in the 25 de Mayo mine working partially ox-idized clasts of altered and mineralized porphyritic dacite(Figs. 2, 4). Therefore, its age not only constrains the maxi-mum duration of hypogene mineralization but also provides aminimum age for the beginning of supergene oxidation. Theignimbrite contains crystals of K-feldspar, quartz, plagioclase,and biotite. The biotite is fresh and aligned parallel to flow.Coarse biotite crystals were selected from one field sample(CR-0410).

Dating the Huakajchi dome which intrudes the Huakajchiignimbrite provides an independent check on the age of theignimbrite. Flow-banded, rhyolite porphyry from the Huaka-jchi dome contains phenocrysts of quartz, sanidine, biotite,and plagioclase. A combined population of 24 single phe-nocrysts of fresh sanidine were selected for irradiation fromfive different field samples (CR-0430, CR-0431, CR-0437,

1648 SCIENTIFIC COMMUNICATIONS

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FIG. 2. Geology of the area around Cerro Rico and sample locations in thisand previous studies. A-B is the line of section in Figure 3. C-D is the line ofsection in Figure 4. The volcanic ash deposits are too small to show, and sam-ple points only are indicated.

FIG. 3. Schematic cross section from Cerro Huakajchi to Cerro Rico,showing relative chronological relationships of volcanic units. Line of sectionin Figure 2.

CR-0438, and CR-0634). Of those, 11 were analyzed and nineof the 11 were used for age information (Table 3).

The age of unaltered volcanic ash horizons which overliethick weathered scree deposits on the flanks of Cerro Ricoprovides a minimum age for deep supergene oxidation of thedeposit. The ash consists of fine sand-sized feldspar, quartz,and biotite crystals. Sanidine and biotite separates were madefrom two samples, CR-0683 from the west flank and CR-0619from the north-east flank of Cerro Rico.

Dating of supergene alunite gives the ages of specificweathering events. We confirm the supergene origin of thesealunites by supporting stable isotope studies. Two samples ofalunite were collected from the Pailiviri mine. These are

creamy-white chalky alunite (sample CR-0215), which fillsthe Caracoles fault at level 1 and white porcellaneous alunite(sample CR-0589), forming a matrix within the San Antoniobreccia pipe on plan 70 (Figs. 2, 4). The latter alunite containsminor bands of somewhat corroded pyrite, indicating that thealunite postdates the pyrite.

Ar-Ar method

All Ar-Ar data were obtained at the Natural EnvironmentResearch Council Argon Isotope Facility at the Scottish Uni-versities Environmental Research Centre. Individual sanidineand biotite crystals were liberated by handpicking fromcrushed samples of the dome and ignimbrite. Surface and

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FIG. 4. Cross section of Cerro Rico, showing vein systems and mine levels sampled in this and previous studies. Line ofsection in Figure 2.

TABLE 1. Samples for Geochronology and Stable Isotope Studies

Grid referenceSample no. E N Description Analyzed material

CR0410 4166 3717 Huakajchi ignimbrite, lithic-crystal tuff with quartz, feldspar, biotite Coarse biotite crystalscrystal-rich matrix

CR0420 5510 5020 Cerro Rico dome, sanidine-quartz-biotite porphyry with weak propylitic Sanidinealteration and late barite overprint (15 different phenocrysts)

CR0430 3690 2898 Huakajchi dome, porphyritic rhyolite with quartz, sanidine megacrysts, Sanidine biotite, and plagioclase





CR0683 4563 5012 Volcanic ash overlying weathered paleoscree; ash comprises biotite, Sanidine and feldspar, and quartz crystals biotite separates

CR0619 5570 5499 Volcanic ash overlying weathered paleoscree; ash comprises biotite, Sanidine and feldspar, and quartz crystals biotite separates

CR0215 5146 5511 Creamy white, chalky alunite in Caracoles fault Alunite separateCR0589 5275 5543 Breccia pipe matrix, minor disseminated pyrite Alunite separate

Notes: The grid reference refers to a local grid devised by COMIBOL; the center coincides with the summit of Cerro Rico, which is intersected by gridlines 5000E and 5000N

fracture contamination in sanidine crystals was removedusing dilute HF (8%) acid etching in an ultrasonic bath for 10min, followed by thorough rinsing with deionized water. Bi-otite samples were washed with deionized water in an ultra-sonic bath for 30 min. Individual crystals of sanidine and bi-otite were then selected under a binocular microscope andpackaged for irradiation.

Samples, along with duplicates and neutron flux monitors,were irradiated in the Oregon State University Triga reactor,Cd-shielded CLICIT facility. Taylor Creek Rhyolite (USGSStandard 85G003, 27.92 Ma) was used to monitor 39Ar pro-duction and establish neutron flux values for the samples. Gaswas extracted from samples using either an Nd-YAG continu-ous-wave laser or an all-metal resistively heated furnace at theScottish Universities Environmental Research Centre. Liber-ated argon was then purified of active gases using two gettersat 25 and 400C. Argon isotope ratios were measured on aMAP-215 single collector, static noble gas mass spectrometer.The total system blank for laser extraction was 2 1016 mol40Ar, 2 1017 mol 39Ar, 1 1017 mol 36Ar and for furnaceextractions, 4 1016 mol 40Ar, 4 1017 mol 39Ar, 2 1017mol 36Ar.

Plateau acceptance criteria follow those outlined in Pringle(1993). We accept an apparent 40Ar/39Ar age as an accurate es-timate of the cooling of a volcanic rock only if (1) a well-de-fined, high-temperature age spectrum plateau is formed by atleast three concordant (2), contiguous steps representing atleast 50 percent of the 39Ar released; (2) an isochron exists forthe plateau points that satisfies the F distribution test; (3) theplateau and isochron ages are concordant at the 95 percentconfidence level; and (4) the isochron 40Ar/36Ar intercept is at-mospheric in composition at the 95 percent confidence level.

K-Ar method

All K-Ar data were obtained at the Natural EnvironmentResearch Council Argon Isotope Facility at the Scottish Uni-versities Environmental Research Centre. Alunite sampleswere gently disaggregated, dried overnight at 105C, andquantitatively divided into splits for K and Ar analysis to en-sure that the aliquots were representative of one another interms of composition. Potassium was measured by inductivelycoupled plasma optical emission spectrometry (ICP-OES) atthe University of Greenwich on duplicate splits. The abun-dance of 40Ar was determined in duplicate, using a 38Ar spike,and corrected for atmospheric contamination assuming all36Ar is air derived. Peak heights were measured using a Fara-day collector on a MAP-215 mass spectrometer. Sampleanalyses were interspersed with measurements of both inter-national mineral standards (HD-B1, 24.7 0.3 Ma: Fuhr-mann et al., 1987), in-house mineral standards (BS-133, 1020 13 Ma: J. G. Mitchell, pers. commun.) and air standards.

Stable isotope methods

All stable isotope data were obtained at the Isotope Com-munity Support Facility at the Scottish Universities Environ-mental Research Centre. Sulfur isotope data were obtained bystandard techniques of Coleman and Moore (1978), with re-producibility around 0.4 per mil based on repeat analyses ofNBS-123, NBS-127, CP-1 (internal standard). Sulfate oxygenisotope data were obtained following the method described inHall et al. (1991), following reprecipitation of alunite as BaSO4by standard techniques (Wasserman et al., 1992). Repeatanalyses of NBS-127 gave a mean 18O result of 9.7 per mil,with a reproducibility of around 0.4 per mil. The 18OOH datawere obtained following a total fluorination method of Wasser-man et al. (1992). A mean value of 9.6 per mil was obtainedfrom NBS-28 during fluorination analyses, with reproducibil-ity better than 0.5 per mil. Hydrogen isotope analyses wereperformed by the standard technique, modified by Wasser-man et al. (1992), in which alunites were carefully step heatedto 560C in vacuo to suppress H2SO4 production. Vienna Stan-dard Mean Ocean Water (V-SMOW), Vienna Standard LightAntarctic Precipitation (V-SLAP), and internal lab standardLST gave reproducibility better than 2 per mil, but we ex-pect the mineral reproducibility to be worse than the data forthese pure waters but still better than 5 per mil.

Chronology Results

Cerro Rico dome

Previous age determinations at Cerro Rico have focused ondating dome emplacement and constraining the age and


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FIG. 5. Hydrothermally altered and weathered clast (arrow) of Cerro Ricodome contained within relatively fresh Huakajchi ignimbrite. This field rela-tionship demonstrates that hydrothermal activity at Cerro Rico had effec-tively ceased and supergene oxidation had begun by the time of eruption ofthe Huakajchi ignimbrite.

duration of hydrothermal alteration and mineralization. Themost important contributions in this respect have beenmade by Grant et al. (1979), Schneider (1985), Zartman andCunningham (1995), and Cunningham et al. (1996). Perti-nent data are presented in Table 2. We have obtained a com-bined weighted mean age of 13.77 0.03 Ma from sanidinephenocrysts, which is equivalent to the age of dome em-placement (Table 3). The presence of sanidine (as opposedto perthite) testifies to the rapid cooling of the intrusion.The 40Ar/ 39Ar sanidine age is the most precise age determi-nation yet obtained from the Cerro Rico dome. The datingof unaltered, paragenetically late sanidines avoided the ef-fects of both hydrothermal alteration and potential inheritedmaterial as found in the study by Zartman and Cunningham(1995). This age is statistically indistinguishable from boththe U-Th-Pb zircon age of 13.8 0.2 Ma obtained by Zart-man and Cunningham (1995) and the 40Ar/ 39Ar age of 13.76 0.10 Ma for hydrothermal sericite reported by Cunning-ham et al. (1996). The sericite age is directly comparable tothe age of dome emplacement reported here, since bothage determinations employed the same radiometric methodand the same flux monitor. On the basis of these dates

hydrothermal activity had commenced almost immediatelyafter dome emplacement.

Huakajchi ignimbrite

A combined weighted mean age of 11.85 0.04 Ma was ob-tained from the analyses of biotite crystals (Table 3). This ageis interpreted as the age of eruption of the Huakajchi ign-imbrite and represents the time at which the ignimbritecooled below the closure temperature of biotite. The 40Ar/39Ar age is indistinguishable from the K-Ar biotite age of 12.0 0.22 Ma obtained by Grant et al. (1979).

Since the flow unit contains partially oxidized clasts of min-eralized and vuggy silicified dome material (Fig. 5), and yet isrelatively fresh, the age of the ignimbrite constrains the ces-sation of hydrothermal alteration and mineralization to before11.85 Ma. The 40Ar/ 39Ar biotite age is consistent with the fis-sion-track zircon ages of Schneider (1985) and Cunninghamet al. (1996), which indicate that the Cerro Rico dome hadcooled below 175C by about 12.0 Ma. The dome was evi-dently exposed and being oxidized and eroded by 11.85 Ma.

This suggests that the main period of hydrothermal ac-tivity ceased prior to 11.85 Ma. Cunningham et al. (1996)


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TABLE 2. Summary of Previous Geochronological Data Pertinent to the Timing of Dome Emplacement, Hydrothermal Alteration, Mineralization, and Supergene Alteration at Cerro Rico

Sample no. Vein or lithology and location Method and material Age (Ma) Reference

NG71 Huakajchi ignimbrite K-Ar biotite 12.0 0.2 Grant et al. (1979)

M1a Pailaviri level 0 K-Ar alunite 6.3 0.2 Cunningham et al. (1996)M1b Pailaviri level 0 K-Ar alunite 6.4 0.2 Cunningham et al. (1996)M2 Pailaviri level 0 K-Ar alunite 5.7 0.2 Cunningham et al. (1996)M3 Mine level 16 K-Ar alunite 8.3 0.5 Cunningham et al. (1996)

K-Ar alunite 7.5 1.2 Ueno and Sugaki (1984)Pot-3a-79 Utne vein, level 5 K-Ar alunite 13.0 0.7 Schneider (1985)

repeat 13.2 2.5Pot-8-79 San Miguel vein, level 1 K-Ar alunite 12.4 1.2 Schneider (1985)

repeat 10.4 1.0Pot-12-79 Utne vein, level 5 K-Ar alunite 11.2 2.8 Schneider (1985)

repeat 11.6 0.8Pot-13-79 Utne vein, level 5 K-Ar alunite 10.4 0.6 Schneider (1985)Pot-15-79 Utne vein, level 5 K-Ar alunite 13.5 0.7 Schneider (1985)Pot-18-79 Bolivar Nueva vein, level 8 K-Ar alunite 12.6 0.6 Schneider (1985)As-137a Don Mauricio vein, Pailaviri level 0 K-Ar alunite-hinsdalite intergrowth 11.4 0.5 Schneider (1985)As-147 Bolivar 1 vein, Caracoles 0 K-Ar alunite 18.7 0.9 Schneider (1985)As-148 Caracoles fault, Caracoles 0 K-Ar alunite 18.5 1.0 Schneider (1985)

M6a Sericitized dacite, Pailaviri 0 40Ar/ 39Ar sericite 13.8 0.1 Cunningham et al. (1996)M6a Sericitized dacite, Pailaviri 0 K-Ar sericite 13.4 0.4 Cunningham et al. (1996)M9 Sericitized dacite, Pailaviri 0 K-Ar sericite 10.5 0.4 Cunningham et al. (1996)M10 Sericitized dacite, level 4 K-Ar sericite 11.1 0.4 Cunningham et al. (1996)

K-Ar sericite 12.8 0.6 J.I.C.A. (1985)

M4 Altered porphyry, Mina Brac K-Ar whole-rock 7.3 0.6 Cunningham et al. (1996)NG93 Cerro Rico dome K-Ar whole-rock 13.2 0.3 Grant et al. (1979)

repeat 14.0 0.3NG94 Cerro Rico dome K-Ar whole-rock 14.1 0.3 Grant et al. (1979)

repeat 14.1 0.3

Pot-5-79 Cerro Rico dome, Pailaviri 0 Fission-track zircon 12.0 0.4 Schneider (1985)M6a Sericitized dacite, Pailaviri 0 Fission-track zircon 12.5 1.1 Cunningham et al. (1996)M6a Sericitized dacite, Pailaviri 0 U-Th-Pb zircon 13.8 0.2 Zartman and Cunningham (1995)

Notes: The mineralization at Cerro Rico is found within a number of major veins and vein systems such as the Utne, Bolivar, San Miguel, and Don Mauri-cio; these are accessed at various levels by a number of mines which include the Pailiviri (levels 013), Caracoles and Brac; some levels are common to morethan one mine; see figure 2 in Cunningham et al. (1996) and map 1 and cross sections in Steele (1996)


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TAB

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interpreted sericite ages of 10.5 and 11.1 Ma as representingrelatively later pulses of hydrothermal activity. A reassess-ment of their data indicates systematic relationships betweenage and K content in these samples (the latter being signifi-cantly lower than expected of a pure separate); the lower theage, the lower the K content. The systematic nature of this re-lationship is suggestive of mixing with younger formed claysor partial resetting during clay formation.

Huakajchi dome

A combined weighted mean age of 11.66 0.04 Ma was ob-tained from the analyses of sanidine phenocrysts from theHuakajchi dome. This high-precision date is the first age de-termination from the Huakajchi dome. The date is inter-preted as the age of intrusion, and it postdates eruption of theHuakajchi ignimbrite by about 0.2 m.y. Rapid cooling is evi-denced by the preservation of sanidine. The age of the domestatistically overlaps with the end of hydrothermal activity.

The 40Ar/ 39Ar ages of both the Huakajchi dome and theHuakajchi ignimbrite are consistent with field relationships.The age of dome emplacement places the timing of volcanicactivity at the Huakajchi center within the Upper Cebadillasvolcanic episode (Schneider, 1985; Schneider and Halls,1985). The Cebadillas volcanic episode (1017 Ma) is eco-nomically important and was responsible not only for CerroRico but also Bolivias largest historical Zn producer at Porcoand other significant mineralization at Cerro Tollojchi andChachacomiri.

Volcanic ash

Incremental heating experiments were performed on twobiotite separates (CR-0619 and CR-0683) and on one sani-dine separate (CR-0619) prepared from samples of volcanicash collected from the flanks of Cerro Rico (Table 3). Allthree experiments, particularly the biotites, failed to meet the

minimum criteria for reliable incremental heating ages (Lan-phere and Dalrymple, 1978; Pringle, 1993). However, 11 con-tiguous steps representing 85.4 percent of the total 39Arevolved from the sanidine separate yielded a weighted meanplateau age of 0.374 0.022 Ma. This age has the lowestMSWD, an 40Ar/36Ar intercept nearest to the atmosphericratio of 295.5, and yields the oldest measured age. We tenta-tively consider this date to be the best estimate for the age oferuption of the volcanic ash, but further work is required toresolve the age accurately.

Supergene oxidation

We know from the presence of weathered clasts in theHuakajchi ignimbrite that the Cerro Rico dome was being ox-idized and eroded by 11.85 Ma, but the duration of this oxi-dation is poorly constrained. The alunite dated in this studyhelps to constrain the duration of supergene oxidation. Weargue that the alunite we have dated is supergene on the basisof stable isotope compositions (Table 4, Fig.6).

The 18OOH value of sample CR0589 is greater than the18OSO4 value and indicates disequilibrium, typical of the su-pergene environment (Rye et al. 1992). The 34SSO4, 18OOH,and D values of alunite plot within the fields of supergenealunite (Rye et al., 1992). The fine grain size, massive texture,and porcellaneous appearance of most alunite at Cerro Rico


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TABLE 4. Stable Isotope Data for Two Dated Alunite Samples from Cerro Rico

Sample 34S 18OSO4 18OOH Dno. (V-CDT) (V-SMOW) (V-SMOW) (V-SMOW)

CR 0215 4.9 7.2 2.2 69CR 0589 2.0 7.5 9.8 48

FIG. 6. Plot of alunite D vs. 18OOH and 18OSO4. Supergene alunite sulfate field (SASF) and supergene alunite OH field(SAOZ) after Rye et al. (1992). Primary magmatic water field (PMW) after Sheppard (1986). All data plot within the fieldsof supergene alunite.

are attributes generally considered diagnostic of a supergeneorigin (Sillitoe and McKee, 1996). Furthermore, where theparagenetic position can be determined the alunite is typi-cally late (Steele,1996). We have shown that supergeneweathering predates the eruption at 374 Ka of volcanic ash(Table 3), which overlies thick scree deposits of pervasivelyweathered dome material. The youngest supergene alunite is6 Ma, and our two ages for supergene alunite (9.7 0.3 and6.0 0.5 Ma: Table 5) further indicate that weathering con-tinued for at least 6 m.y. from emplacement of the Huakajchiignimbrite at 11.85 to 6.0 Ma.

Discussion and Conclusions

Duration of hydrothermal activity

Fission-track zircon ages from the Cerro Rico dome of 12.0 0.4 and 12.5 1.1 Ma have been obtained by Schneider (1985)and Cunningham et al. (1996), respectively. The data of Schnei-der (1985) underestimates the errors associated with standards,whereas the data of Cunningham et al. (1996) are considered torepresent a more realistic error estimate. These data fix the pe-riod in the thermal history of the dome when temperatures fellbelow the closing temperature of zircon for fission tracks(175C). They indicate that there were no significant heatingepisodes ( i.e., >240C for 104 yrs) after 12 Ma (Cunningham etal., 1996) and provide the best estimate for the collapse of thehydrothermal system at Cerro Rico. Whereas this date providesa lower limit for the end of main-stage Sn-Ag vein mineraliza-tion, which occurred at temperatures above 175C (Steele,1996), it is possible that disseminated silver, as acanthite, con-tinued to be deposited in the lithocap.

Thus, significant hydrothermal activity began immediatelyafter dome emplacement at 13.77 Ma and may have contin-ued until about 12.5 Ma when temperatures fell below theclosing temperature of zircon (Cunningham et al. 1996), giv-ing a minimum total duration of about 0.2 m.y. (Fig. 7). Ther-mal modeling indicates that small single bodies of magmacould sustain hydrothermal activity for a few tens of thou-sands of years and a large single body under exceptional cir-cumstances for nearly 1 m.y. (Cathles et al., 1997). Porphyrycopper systems in the Potrerillos district of Chile have a max-imum duration of about 0.2 m.y. (Marsh et al., 1997). Hy-drothermal activity in the large epithermal Au deposit atRound Mountain lasted for about 0.1 m.y. (Henry et al.,

1997). Thus, the observed minimum duration of hydrother-mal activity at Cerro Rico appears to be at the high end of thatnormally generated by a small high-level intrusion. If the du-ration of hydrothermal activity was closer to the mean valueof the fission track age (12.5 Ma), then either a large single in-trusion or multiple intrusions may have been required to sus-tain it. The beginning and end of hydrothermal activity aremarked by distinct magmatic events, the Cerro Rico domeand Huakajchi ignimbrite, respectively, and it is possible thatfurther intrusions were emplaced between these two into ahigh-level magma chamber below Cerro Rico.

Duration of supergene activity

Alunites from various levels in the Cerro Rico deposit havebeen dated by previous workers and ages range from about 18


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TABLE 5. Summary of K-Ar Results for Supergene Alunite from Cerro Rico

Sample no. K (wt %) Split Run % 40Ar* 40Ar* mol/g (1010) Age (Ma)

CR-0215 10.27 A 1 19.1 0.914 5.96 0.2310.75 2 19.5 0.913 6.03 0.23

10.51 0.34 B 1 21.3 0.917 6.00 0.222 22.1 0.910 6.05 0.22

6.01 0.45

CR-0589 10.76 A 1 41.2 1.513 9.70 0.1210.86 2 39.8 1.509 9.67 0.12

10.81 0.07 B 1 34.5 1.520 9.74 0.132 32.6 1.513 9.70 0.13

9.70 0.25

Notes: 40Ar* = the calculated % of radiogenic Ar (assuming no excess 40Ar after subtraction of 40Ar contributed by atmospheric gases)

FIG. 7. Summary of geochronological data pertinent to the timing of domeemplacement, hydrothermal activity, and supergene oxidation at Cerro Rico.Range and precision of alunite ages from previous studies arranged accord-ing to age. References: 1 = Zartman and Cunningham (1995), 2 = Cunning-ham et al. (1996), 3 = Schneider (1985), 4 = Ueno and Sugaki (1984), 5 = thisstudy.

to 6 Ma (Table 2). All, with the exception of those around 18Ma, have been attributed to magmatic events. The 18 Maages are spurious and due to excess argon (Schneider, 1985).The remaining alunites have physical characteristics that aresimilar to the supergene alunite in our study. Therefore, wesuggest that these alunites are also of supergene origin andrecord semicontinuous weathering from around 13.5 Ma,shortly after dome emplacement, to 6 Ma (Fig. 7).

The oldest alunite ages therefore overlap with hydrothermalactivity, indicating that supergene fluids may have coexistedand perhaps mixed with hydrothermal fluids within fractures.This could explain the occurrence of sulfide veins which cutand are crosscut by alunite veins (Cunningham et al. 1996).

The extended period of supergene activity seen at CerroRico occupies about half of the total range of supergene agesrecorded for mineral deposits elsewhere on the Bolivian Alti-plano by Columba and Cunningham (1993), Darke (1996),and Darke et al. (1997). However, there is no evidence thatthis has resulted in significant enrichment of Ag on the Alti-plano. At Cerro Rico the ubiquitous formation of secondarysilver halides, which are stable in acid weathering environ-ments, prevented the downward movement of Ag to the watertable to form an enrichment blanket (Steele, 1996). Neverthe-less, this protracted period of weathering had an importantbearing on the economic viability of the disseminated orebodyin that silver was released from refractory ore minerals and re-precipitated as cyanide leachable secondary silver minerals.

Size of the Cerro Rico orebody

The extraordinary richness of the Cerro Rico silver depositis not due to weathering processes as is the case with many ofthe Tertiary porphyry Cu deposits of northern Chile (Sillitoeand McKee, 1996; Ossandon et al., 2001). Instead the enrich-ment of Ag must be related to hypogene processes. Recentstudies of melt inclusions have shown that the magmas atCerro Rico were highly fractionated and enriched in incom-patible elements such as Ag and Sn (Dietrich et al., 1999,2000). However, fractionation alone is unlikely to account forthe unusual abundance of silver at Cerro Rico. We suggestthat the long duration of magma-related hydrothermal activityin this system was another important factor. This could havebeen sustained by a large single injection or multiple injec-tions of fractionated Ag-enriched magma from a deep reser-voir into a high-level magma chamber underlying Cerro Rico.

AcknowledgmentsGBS gratefully acknowledges a Natural Environment Re-

search Council (NERC) CASE award. EMICRUZ Ltda. (ajoint venture between Rio Tinto plc and COMSUR S.A.) isthanked for providing GBS with field support. We thankJ.B. Fulton for drafting the figures and technical staff atScottish Universities Environmental Research Centre(SUERC) for assistance. SUERC is funded by NERC andthe Scottish Universities. AJB is funded by NERC supportof the Isotope Community Support Facility at SUERC. DBand MSP are funded by NERC support of the Argon Iso-tope Facility at SUERC. We thank two referees for theirconstructive comments.

October 22, 2004; October 3, 2005

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