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Economic GeologyVol. 86, 1991, pp. 415-421

A MODEL OF VOLCANIC DOME-HOSTED PRECIOUS METAL DEPOSITS IN BOLIVIA

CHARLES G. CUNNINGHAM,U.S. GeologicalSurvey,959 National Center,Reston,Virginia 22092

JAMESMCNAMEE,CompaKiaMinera del Sur S.A. Edificio Petrolero,Piso10, Av. 16 de Julio 1616, El Prado, La Paz, Bolivia

JosfPINTOVSQUEZ,United NationsDepartmentof TechnicalCooperationor Development DTCD), Casilla 2096, SantaCruz, Bolivia

AND GEORGE E. ERICKSENU.S. GeologicalSurvey,954 National Center, Reston,Virginia 22092

Introduction

Many, if not most,of the preciousmetal and poly-metallic tin deposits n the Andean highlandsofsouthwestern Bolivia are associated with volcanicdomes. hesedomes how ypicalextrusiveeatures,suchas laringbases, xplosion reccias, ndbeddedtuff rings,and are well exposedn the preciousmetaldistrictsof Carangas nd TodosSantosn the Cordil-lera Occidental near the western border of Bolivia,and he world-famous ulacayo ilverdeposit nd heCerro Rico de Potosi in-silver deposit n the Cordil-lera Oriental (Fig. 1). We suggest hat many otherpreciousmetalandpolymetallic in depositsn south-westernBoliviaalsoare hostedby volcanicdomes ndthat informationaboutknown deposits an be usedto constructa model which will serveas a guide forexplorationand resourceassessment.t shouldbenoted that these domes, which tend to be about akilometer in diameter, are generally the surfaceexpression f larger magmaticsystems t depth thatwere the sourcesof much of the heat, metals, and atleastpart of the hydrothermal luids hat formed hedome-hostedmetalliferous eposits.This report is an outgrowth of an InternationalUnion of GeologicalSciences/United ationsEdu-cational,Scientific, nd CulturalOrganization IUGS/UNESCO) DepositModelingProgramworkshoponvolcanic-hosted,pithermalpreciousmetal depositsthat was held in Bolivia in September1988. Theworkshopwashostedby the UN/DTCD-sponsoredMining InvestmentProgramand the ServicioGeo-16gico e Bolivia.ParticipantsncludedBolivian ndinternational epresentativesrommining,academic,and governmental ntities.The combination f fieldstudiesand workshopdiscussionsed to the recog~nition and definition of the volcanic dome model dis-cussed n this report.The Andeanhighlands f Boliviacomprise hreemajor physiographic rovinces.The Altiplano is abroad ongitudinalectonic asin hat is filled with

predominantly ertiary continental ediments nd sflankedon the eastby the CordilleraOrientalandonthe westby the CordilleraOccidental. he CordilleraOriental s underlainchieflyby a thick sequence fintensely olded ower Paleozoicmarineclasticsed-imentary ocksoverlainocallyby similarly eformedCretaceous-lowerTertiary continental sedimentaryrocks,undeformedate Tertiary unconsolidatedon-tinental sediments, nd late Oligocene o Pliocenevolcanic rocks. The Cordillera Occidental consistsmostlyof andesitic tratovolcanosnd rhyoliticash-flow tuffs hat overlie a basementof Jurassic nd Cre-taceous edimentary nd volcanic ocks.The Cordillera Oriental is the site of most of themetalliferousmineraldeposits f Boliviaand ncludesthe Bolivianin belt,whichextendshroughout oliviafrom northernmostArgentina o southernmost eru,and flankingbeltswhere the principaldeposits aveeither antimonyor lead and zinc as he predominantmetals.The southernpart of the tin belt is character-ized by polymetallicin deposits,mostof whicharesilver ich andall of whichhavea greatvarietyof tin,silver,andbasemetalmineralsgenetically elated osmall (1-2 km2) quartz latitic or dacitic porphyrydomesor stocksof early to middle Miocene age.Ear-lier reportshave suggestedhat the tin deposits reassociated ith a varietyof gneouseatures,ncludingsmall stratovolcanos,stocks, and domes (Ahlfeld,1967; Kelly andTurneaure,1970; Sillitoeet al., 1975;Grant et al., 1979; Francis et al., 1981; Sillitoe andBonham, 1984; Ericksen et al., 1987). Some of themajor tin-silverdistricts, uchas Colquechaca, ho-caya, and Tatasi,are associated ith sequencesfrhyolitic o daciticuffs, reccias,nd avas onsideredto be part of former stratovolcanosGrant et al.,1980), however, Chocaya s now recognized o behosted n volcanicdomes PintoVtsquez, 988). Thetin-silver veins at Potosi are centered on a dome thatforms the conical Cerro Rico de Potosi, and those atOruro are in a complexof interpenetrating omes ncoeval hyolitic volcanic ocks Chase,1948; Francis

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SCIENTIFIC OMMUNICATIONS 417CARANGAS

Volcanoclasticediments, Breccia pipetuffand ocal locks .B ... iapipe ipst... d._ ?:...l;/.V ....centerfhill 1-' 'hO,, %knlne.X1 oLayered ... ia parall l / /--- I I fto idefhill / d'' Light-grayuffdips... d/__ h' -- -- Light-grayuffipsutwardq ///' Massivehyolite A ouHetrolithologic Alterationoned utwardilicabreccia clast size in to propylific from top of hill

ORA Brecciaipesuttingarapaceeins cutting manfie brecciaLocal volcanoclastic sedimentsTODOS SANTOS

Rhyoliteontortedflowbanding,/..i" Idilanan/OSsidian,anded/--Tuff,ipsnward i'""tBT;';;ecY;aORES Beddinglanesf uff earontactithndesitcRadialand concentric einscutting uff

CERRO RICOGas veslculesSilicifiedap---.o 'Alunite,aolinite I-- Lx,. einsSilicified J t I I I' .Heterolithicwaterlaid I t I f'lcanictuffairfall_uf_____ I

C OREeins cutting dome and wall rocksFIG. 2. Diagrammatic crosssections of volcanic dome-hostedpreciousmetal deposits u Bolivia.A. Cerro Espirito Santo,Car-augas.B. Todos Santos.C. Cerro Rico de Potosi.

Todos Santos districtMines of the Todos Santos district, also workedduringcolonialimes,are at the baseof the prominenthill adjacent o the town of TodosSantos. he hill is

a volcanicdome consisting f white, contorted low-bandedrhyolite overlyinga layered, flat- to inward-dipping tuff ring, which in turn overliesa basalex-plosion recciaandandesitic asementock (Fig. 2B).The tuff ringhasbeen ntrudedby a late rhyoliteplug.The baseof the flow-banded hyolite consists f a 9.-m-thick ayer of obsidian Fig. 2B) that formedwhenthe rhyolite was quenchedas it was extruded overthe tuff ring. Ore is present n the permeablebasalbrecciaoverlying he andesite asement nd n radialand concentric fractures and brecciated zones in thetuff ring that formed during emplacementof thedome.Pulacayodistrict

The famous ulacayo ilvermine (Fig. 1) exploitedone of the richestand mostproductivesilverveins nthe Andes. Unlike many deposits n the BolivianAndes,Pulacayowasnot discovered ntil 1833, wellafter Spanish olonial imes. Although a number ofveinsare present n the districtand are mainlyhostedby a dacitic dome, nearly all of the large silver pro-duction,which may haveexceeded5,000 metric tonsof fine silver, came from the 2.7-km-longTajo vein.This blind vein wasexploited o depthsof asmuch as1,100 m below the surface Ahlfeld and Schneider-Scherbina, 964). The vein averages little more hana meter in width, but at placeswidens o as much as6 m. It consists f silver-richbasemetal ore in a pre-dominately uartzgangue. in minerals re notpres-ent. The vein wasminedout and he operationclosedin 1958, but individual miners have since reworkedold waste dumps and mine fill to extract and hand-concentrate silver ore.The Tajo vein occurs n Tertiary continentalsed-imentary ocksmarginal o the upward-flaring aciticporphyry dome, which s about 4 km long and 9.kmwide. The vein dips nward oward he dome,parallelor subparallelo the dome's nward-dipping ontact.This relationship s reminiscentof veins ocalized ncone fractures developed during dome extrusion.However, the vein is too long and too straight o belimited to such a cone fracture. The vein structuremay be part of a larger fault that in part coincideswith the ring fracturezoneof the Pulacayo omeandis related o either a larger gneous ody at depthora regional ectonic eature.Cerro Rico de Potosi

CerroRico de Potosi,a conicalhill adjacent o thecity of Potosi, s the world's argestknown silverde-posit. Production,since ts discovery n 1545, is es-timated to be between 30,000 and 60,000 metric tonsof fine silver Bernstein, 988). Cerro Rico consistsof a cone-shaped, pward-flaring ome (Fig. 2C;Franciset al., 1981) that extends veran areaof 1,200by 1,700 m at the surfaceand narrows o a 100-m-


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418 SCIENTIFICOMMUNICATIONSwide feeder dike at a depth of 800 m below he peakof CerroRico Suttill,1988). The outward-flaringaseof the dome overridesdacitic tuffs and underlyingbrecciacontainingragments f daciteandOrdovicianshale,which we interpret to be an explosionbreccia.Mostof the ore produced ame rom a 1,200-m-long,300-m-wide zone of north- and northeast-trending,closelyspaced,parallel veins hat cuts he dome andthe surroundinghost rock. Whole-rock radiometricdates K/Ar) indicate hat the domedatesat about 14Ma (Grant et al., 1979). The veins show horizontaland vertical zoning,with a high-temperature, eep-level cassiterite-rich ore surroundedby zones nwhichsilverandbasemetalsulfides redominate. hedepositwas initially mined for the very rich silverores in the upper part of Cerro Rico, but since helate 1800s hasbeen mined chiefly for tin. It is cur-rently being evaluatedasa low-grade,bulk-mineablesilver deposit.Oruro district

The Oruro district (ServicioGeo10gico e Bolivia,1979), in a seriesof low hills at the westernside ofthe cityof Oruro, soneof the major in-silver epositsof Bolivia hat hasbeen mined almostcontinuouslysinceearly Spanish olonial imes. Silver-rich in andbasemetal veinsoccur n a complexof quartz atiticdomes, which were extruded over lower Paleozoicrocks.Somedomes n the complexhave uff rings hatwere depositedon older domes.The igneous ockshavebeendated adiometrically t 16.1 Ma (Redwoodand Macintyre, 1989). Explosivebrecciasconsistoffragmentsof igneous and sedimentary ock (Mc-Namee, 1988), tend to occur at the igneous-sedi-mentaryrock contacts Chase,1948), and probablyformedduring eraplacement f the domes.The veins occur in clustersand cut the igneousrocks,breccias, ndsedimentaryocks,althoughmostof the veins are in the domes. A detailed structuralstudyby Chase (1948) showed hat the mineralizedfractures t Oruro formedchieflyby domingandre-lated subsidence.La Joya district

The La Joyadistrict,which s n a rangeof low hillswithin the easternpart of the Altiplanoabout50 kmnorthwestof Oruro (Fig. 1), has ow-grade,oxidized,preciousmetal ores that are currently being bulkmined and treated by heap eaching.The district n-cludesseveralmineralizedquartz latitic domes hatare near the margin of a caldera (Redwood,1987).Radiometricdating shows he domes o be 15 Ma(Redwoodand Macintire, 1989), thus making hemabout10 m.y. older han he caldera. he mineralizeddomes and some associated tocksare intensely al-tered, havingquartz, sericite,and abundantpyrite,andare cut by stockwork einletsof quartzandAu-

and Ag-bearingbasemetal sulfidesand pyrite (An-zoleaga,1988). Minor amountsof tungstenand bis-muth minerals are present in the primary ore, andalunite occurs n the oxide ores. Current mining op-erations re restricted o exploitation f the oxideorein Cerro Khoricollo,which contains n averageof 1.4g/metric ton gold and 20 g/metric ton silver and isamenable o heap eaching.At least wo other domesin this district show evidence of mineralization, haveold mineworkings, ndare currentlybeingexplored.Laurani district

The Laurani gold-silver istrict,west of the mainhighwayabout halfwaybetweenLa Paz and Oruro(Fig. 1), is centeredon a complexof flow-bandedrhyoliticdomes.The domes eportedlypenetratedasequence f andesitico rhyolitic avas nterpretedbyRedwoodand Macintyre 1989) to be the baseof aneroded stratovolcano. hese authors eported an 8.4-Ma age for the dacitic lava. The Laurani district iszoned, with veins n the central part of the districtcontainingnative gold, enargite, and alunite, andthose n the periphery containing redominantly en-nantite,sphalerite, ndargentiferous alena Ahlfeldand Schneider-Scherbina, 964). This mineral assem-blageandadvanced rgillically ltered ocksare sim-ilar to the settingof the Summitville,Colorado,golddeposit Stoffregen, 987), Iwato, Japan, eposit Iz-awaandCunningham, 989), andRodalquilar, pain,golddeposit Aribaset al., 1989; Rytubaet al., 1989),indicatihg hat Laurani s an acid sulfate,preciousmetal depositasdefinedby Heald et al. (1987).

Volcanic Dome-Hosted ModelThe extrusionof viscous,low-banded, teep-sideddomes ollowingexplosive ischarge f phreaticandphreatomagmaticreccias ndpyroclastic jecta s acommonphenoma n intermediate o silicicvolcanicedifices. uchdomes re often ate-stageeatureshatintrude the cores of stratovolcanos r ring fracturezones of calderas. Volcanic domes around the world,someof whichhaveassociatedre deposits, avebeenwell described n the literature (for example,Mac-donald,1972; LipmanandMullineaux,1981; Rowley

et al., 1981; Burt and Sheridan, 1987; Fink et al.,1987; Halsor et al., 1988; Cunninghamet al., 1989;Rytuba,1989). Althoughdomes howgreatdiversityof lithologyand structure, hey alsohavemany ea-tures in common that show their evolution to be asystematic rocess, scanbe seen n Figure 3. Domeformation begins with the intrusion of a viscousmagma,commonly longa structuralweakness uchasa regional ault or ring fracturezoneof a caldera.Upwardmovement f the magmamaycauseormationof a conical-shapedcone fracture" zone hat flaresupward;at places,sucha fracturezonemaybe partlycontrolledby an earlier, more regional ault zone.


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SCIENTIFIC OMMUNICATIONS 419Structural control such as regionalfault, structural margin of caldera,or throat of volcano

neracture -'"'Vlatialoc;llectAirallAsDips,,tf Coii:alrd/',rde stConeracture

Brecciatedexpandingcarapace ' Radial ndff Flow-banded concentric_. / - ..... . fractures formedAir // / uu-. ,x ' .fall [/ (' - ,) ) during omet-fi /ement/ NExplosionbreccia"ConeractureFIG. 3. Diagram llustrating he successivetagesn the evo-lution of volcanicdomes.A. Intrusionalongstructure, ormationof cone fractures,volatiles collect at top. B. Phreatomagmaticexplosionsorm conical ent rimmedby beddedbreccias.Air-fallash s depositedover the breecias.C. Flow-bandeddome em-placed, accompanied y the formationof radial and concentricfractures.

Volatiles hat tend to collect at the top of the magmamay be released xplosivelyn responseo suddendecreasesn pressure s he magma ears he groundsurface. Phreatomagmatic xplosions,which occur

when the magma ntersects roundwater, are com-mon. These explosions lear a funnel-shaped entabove he magma nd deposit rudelybedded ingsof explosion reccia,which contain ragments f thebed rockand he solidifiedop of the magman a tuff-aceousmatrix, around he vent. Subsequent xplosiveactivity commonlyejects finer pyroclasticmaterialthat formsan air-fall uff ring over he breccias. hesetuffsdip outwardaround he outer perimeterof thebrecciaand dip inward at the angleof repose, ntothe vent. Viscous low-bandedmagma hen invadesthe vent and s extrudedasa dome Fig. 3). Not un-commonly, art of the previously epositeduff ringis displaced y the extrudingmagma,and a layer ofobsidianmay form at the contactof the tuff and heoverridingdome. The surfaces f the domescom-monlyaremantiedwithbrecciated ome ock ormedby breakingof the domecarapaceby the expandingmagmacore. The dome may alsobe cut by brecciapipes formedby explosivedegassing f the magmaandby late dikesandplugs.Somedomesmaybe de-stroyedexplosively y renewed ntrusionof magmainto the conduit.Mineral deposits re generally ormedafter domeemplacement nd tend to occur n permeablezonesor fracturesas llustrated n Figure 4. As previouslynoted, the main Tajo vein at Pulacayo occupiesastructure hat has n part the spatial elation o a conefracturebut which may, n part, be a reactivated e-gional fault. The heterogeneous xplosion recciasand associated bedded tuffs at the bases of domes arefavored loci of mineralization in some domes such asthat at Todos Santos. Radial and concentric fracturesand faults,which formedduring orceful ntrusionofthe domes, also are favored loci of mineralization.

MODEL OF VOLCANIC-DOME HOSTEDPRECIOUS-METAL DEPOSITS IN BOLIVIA

EXAMPLES MINERALIZATIONCONTROLCarangas..,recciaipesCerro ico,/..'...... .,..........-CrosscuttingeinsCarangas"'"j"''" Mantle recciaTodosan .....--**-..,--...,*.....*.,...*. :.i:.**:..>.,.*.,,.::.....:..:Airall./"':"':"'"""'"?...'...."i':: ::;? ''":'"'''...''.......?:.':'....u

Pulacayo ":i:; ili:? f 'ConeFracture

FIG. 4. Model ofvolcmdc ome-hosted reciousmet] depositsin Bolivia.


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420 SCIENTIFICOMMUNICATIONSBreccia ipes,whicharehighlypermeable tructures,commonly re ntenselymineralized.Brecciasormedby breakingof the solidifieddomecarapace ndvol-caniclastic edimentson domesmay alsobe favoredsites or mineralization, speciallywhere mineralizedfluidsvented at the dome surface o form hot springs.Through-goingracturesystems, uchas he parallelswarmof veins n the Cerro Rico de Potosidome, maybe related to regional aults hat controlleddome em-placementand which were subsequentlyeactivatedto open ractures o the mineralizing luids.The vein mineralogyand wall-rockalterationmin-eralsof mostdeposits tudied n Boliviaare similar othoseof the quartz-adularia epositmodeldescribedby Heald et al. (1987) and documented y Halsoretal. (1988) for the DeLamar, Idaho, preciousmetaldeposit.Even though the dome and its associatedmineraldeposits re spatially elated, the principalsourceofthe mineralizing luidsand the heat to drive the hy-drothermalsystemprobably was a deeper magmaticsystem hat was also the sourceof the magma hatformed the dome. Crosscutting elationsshow thatore is formedafter the eraplacement f the associatedigneous ocks,and in somecases suchas Potosi), tis highly mprobable hat the volumeof exposedockcouldprovide he volumeof knownore. Nevertheless,the closerelation between dome eraplacementandmineralization s clearly ndicatedby the systematiczonal arrangementof mineral speciesaccording otemperaturegradients, he similarzonal arrangementof fluid inclusion omogenizationemperatures ithrelatively high core temperatures nd ower periph-eral temperatures, nd fluid inclusion alinities,whichtend to be highest n vein minerals n the core of thedome and owest n the peripheryasdocumented ySugakiet al. (1988). The presenceof tin in thesede-positsseems o be chiefly a functionof deep-seatedsources nd processes.

AcknowledgmentsThe authors are indebted to the other leaders ofthe lUGS/UNESCODepositModelingWorkshop--Merwin Bernstein, Richard Henley, Andre Pante-leyev, Richard Sillitoe, and Alan Wallace whosethoughtful uggestionsndobservationsontributedto the field data and model development.We alsoappreciate he advice,guidance,and data suppliedby the workshoporganizersOscarAnzoleagaV., En-rique ArteagaR., Alberto Manrique,and FernandoUrquidi B., and reviewsby Tom Casadevall, amesRytuba,RobertKoeppen, ndan Economic eology

reviewer.

July 20, December12, 1990

REFERENCESAlhfeld,F., 1967, Metallogenetic pochs ndprovinces f Bolivia:Mineralium Deposita, v. 2, p. 291-311.Ahlfeld, F., and Schneider-Scherbina, ., 1964, Los yacimientosmineralesy de hidrocarburos e Bolivia: Bolivia Dept. Nac.Geologia Bol. 5 (especial), 388 p.AnzoleagaV., O., 1988, The La Joyacopper-gold-silver istrict,Bolivia, in Extended Abstracts,Yacimientosepithermales enambientes de volcanismo reciente: La Paz, Bolivia, ServicioGeol. Bolivia, UGS/UNESCO Deposit ModelingWorkshop,p.

11-12.Arribas, A., Jr., Rytuba, J. J., Rye, R. O., Cunningham, C. G.,Podwysocki,M. H., Kelly, W. C., Arribas, A., Sr., McKee,E. H., and Smith, J. G., 1989, Preliminary study of the oredeposits nd hydrothermal lteration n the Rodalquilar alderacomplex, southeasternSpain: U.S. Geol. Survey Open-FileRept. 89-327, 39 p.Bernstein,M., 1988, Ore guidesand exploration echniques orhydrothermalgold-silverdepositsn the BolivianAndes, n Ex-tended Abstracts,Yacimientosepithermales en ambientesdevolcanismo reciente: La Paz, Bolivia, Servicio Geol. Bolivia,IUGS/UNESCO Deposit Modeling Workshop, p. 13-30.Burt, D. M., and Sheridan,M. F., 1987, Types of mineralizationrelated to fluorine-rich silicic lava flows and domes: Geol. Soc.

America Spec. Paper 212, p. 103-109.Chace, F. M., 1948, Tin-silver veins of Oruro, Bolivia: ECON.GEOL.,v. 43, p. 333-383 and 435-470.Cunningham,C. G., Arribas, A., Jr., Rytuba, J. J., and Arribas,A., Sr., 1989, Evolution of the Los Frailes caldera, Cabo deGatavolcanicield,southeasternpain: .S. Geol.SurveyOpen-File Rept. 89-325, 21 p.Ericksen, G. E., Eyzaguirre, V. R., Urquidi, B. F., and SalasO., R., 1987, Neogene-Quaternary olcanism ndmineralizationin the central Andes:Circum-PacificEnergy and Mineral Re-sourcesConf., 4th Singapore,Aug. 17-27, 1986, Trans., p.537-550.Fink, J. H., ed., 1987, The emplacement f silicicdomesand avaflows:Geol. Soc.AmericaSpec.Paper 212, 145 p.Francis, P. W., Baker, M. C. W., and Halls, C., 1981, The KarlKarl caldera, Bolivia, and the Cerro Rico stock: Jour. Volca-nology Geother. Research,v. 10, p. 113-124.Grant, J. N., Halls, C., Avila Salinas,W., and Snelling,N.J., 1979,K-Ar agesof igneous ocks and mineralization n part of theBolivian in belt: ECON.GEOL.,v. 74, p. 838-851.Grant, J. N., Halls, C., Sheppard,S. M. F., and Avila, W., 1980,Evolutionof the porphyry in deposits f Bolivia:Soc.MiningGeologists apanSpec. ssue 8, p. 151-173.Halsor, S. P., Bornhorst,T. J., Beebe, M., Richardson,K., andStowd,W., 1988, Geologyof the DeLamarsilvermine, daho--a volcanic dome complex and genetically associated ydro-thermal system:ECON. GEOL., v. 83, p. 1159-1169.Heald, P., Foley, N. K., and Hayba,,D. O., 1987, Comparativeanatomy of volcanic-hosted pithermal deposits:Acid-sulfate

and adularia-sericiteypes:ECON.GEOL.,v. 82, p. 1-26.Izawa, E., and Cunningham,C. G., 1989, Hydrothermal brecciapipes and gold mineralization n the Iwashita orebody, Iwatodeposit,Kyushu,Japan:ECON.GEOL.,v. 84, p. 715-724.Kelly, W. C., andTurneaure,F. S., 1970, Mineralogy,paragenesisand geothermometryof the tin and tungstendepositsof theeastern Andes, Bolivia: ECON. GEOL., v. 65, p. 609-680.Lipman, P. W., and Mullineax, D. L., eds., 1981, The 1980 erup-tionsof MountSt. Helens,Washington: .S. Geol.SurveyProf.Paper 1250, 844 p.Macdonald,G. A., 1972, Volcanoes: nglewoodCliffs,New Jersey,Prentice-Hall, Inc., 510 p.McNamee, J., 1988, Field guide, Yaimentos pithermales enambientes de volcanismo reciente: La Paz, Bolivia, ServicioGeol. Bolivia, lUGS/UNESCO Deposit Modeling Workshop,73 p.


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SCIENTIFIC OMMUNICATIONS 4 21Pinto Vtsquez,J., 1988, Modelo conceptualy evaluation del po-tetial mineral del yacimento de metales preciososChocaya,Potosi, Bolivia: U. N. Programapara el Desarrollo, UN/DTCDBol.-87-012, 48 p.Redwood, S. D., 1987, The Soledad caldera, Bolivia: A Miocenecaldera with associated pithermal Au-Ag-Cu-Pb-Zn mineral-ization: Geol. Soc. America Bull., v. 99, p. 395-404.Redwood,S. D., and Macintyre, R. M., 1989, K-Ar datingof Mio-cene magmatism nd related epithermal mineralizationof thenortheasternAltiplano of Bolivia:ECON.GEOL.,v. 84, p. 618-630.Rowley, P. D., Steven, T. A., and Mehnert, H. H., 1981, Originand structural implications of upper Miocene rhyolites inKingstonCanyon,Piute County,Utah: Geol. Soc.AmericaBull.,v. 92, p. 590-602.Rytuba,J. J., 1989, Epithermalpreciousmetal deposits ssociatedwith volcanic systems:Economic Social Comm. Asia Pacificworkshopon epithermalmineralization, sukuba,Japan,May9-12, 1989, Proc., p. 1-7.Rytuba, J. J., Arribas,A., Jr., Cunningham,C. G., McKee, E. H.,Smith, J. G., and Arribas, A., Sr., 1989, Evolution of the Ro-

dalquilar caldera complex and associated old-alunite deposits,Cabo de Gata volcanic field, southeasternSpain: U.S. Geol.Survey Open-File Rept. 89-326, 19 p.Servicio Geo10gicode Bolivia, 1979, Fajas mineralizadas de losAndes Bolivianos: La Paz, Scale 1:1,000,000.Sillitoe, R. H., and Bonham,H. F., Jr., 1984, Volcanic andformsand ore deposits:ECON. GEOL., v. 79, p. 1286-1298.Sillitoe, R. H., Halls, C., and Grant, J. N., 1975, Porphyry tindeposits n Bolivia: ECON. GEOL., v. 70, p. 913-927.Stoffregen, . E., 1987, Genesis f acidsulfatealterationandAu-Cu-Ag mineralization t Summitville,Colorado:ECON.GEOL.,v. 82, p. 1575-1591.Sugaki, A., Kojima, S., and Shimada,N., 1988, Fluid inclusionstudies f the polymetallic ydrothermal re depositsn Bolivia:Mineralium Deposita, v. 23, p. 9-15.Suttill, K. R., 1988, Cerro Rico de Potosi: Eng. Mining Jour.,March 1988, p. 50-53.YacimientosPetrolferosFiscalesBolivianos ServicioGeo16gicode Bolivia, 1978, Mapa Geo16gico e Bolivia:La Paz, Escala1:1,000,000.

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