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Economic GeologyVol. 74, 1979, pp. 838-851

K-Ar Agesof IgneousRocks nd Mineralizationn Partof the Bolivian Tin BeltJ. NIGELGRANT,CHRISTOPHERALLS,WALDOAVILASALINAS,

ANDNORMANJ. SNELLINGAbstract

The Bolivian tin belt extends for about 800 km down the Eastern Cordillera of theCentral Andes. In the northern part the ore deposits are mainly tin-tungsten veinswhich are associatedwith granite-granodioritebatholiths. Previous work has shown thatthe batholithscan be divided into a northern group (Cordillera Real) which gives Meso-zoic K-Ar ages and a southern group (Illimani, Quimsa Cruz, and Santa Vera Cruz)which gives ower Miocene ages.In central and southern Bolivia the tin deposits are complex vein systems associatedwith near-surface stocksand subvolcaniceruptive complexes. The ages of these fall intotwo groups. A northern group, extending as far south as the Kari-Kari batholith nearPotosi, ranges in age from 23 to 20 m.y. Several of the major tin depositsof Bolivia,including the Catavi depositat Llallagua, fall within this group, which is apparently partof a larger lower Miocene igneousprovince that includes he plutons of Illimani and theCordillerasQuimsa Cruz and Santa Vera Cruz. A southerngroup includes he Cerro Ricostock at Potosi and the mineralized eruptive complexesof the (uechisla region whichgive upper Miocene ages ranging from 17 to 12 m.y. There are also extensivepost-mineral ignimbrite sheetswhich overlie rocks of both the above groups.The data show hat igneous ctivity within the tin belt took placeas a seriesof rela-tively discretepulses,with a general southwardshift with time. The north to southzonationof the metalscontained n the ore deposits, rom tungsten n the north, throughtin, to basemetals with tin in the south, s not consistently ependenton the age of theassociatedgneous ocksand appears o be primarily related o the depth at which thedepositswere formed and hence o the regional erosion evel, rather than to changes nthe geochemistry f the magmaswith time or space.The Miocene-Pliocene gneous ocksof the tin belt fit rather well into the consistentpattern of Andean magmatic evolutionwhich is becoming recognizable hroughout the Central Andes as a whole.

Geology of the Tin BeltTHE Eastern Cordillera of the Central Andes is anelongatedmassif,madeup mainly of a great thicknessof marineclasticsedimentary ocksof lower Paleozoicage. The sedimentary basin in which they wereformed extended across the width of the Andes almostto the present coastal region. It was underlain byPrecambrianmetamorphicocks, he extension f theBrazilian and Argentina cratons, and these are nowexposed n the coastal egionsof southernPeru andnorthern Chile (Cobbing and Pitcher, 1972) and inthe easternCordillera n Peru andnorthernArgentina(Audebaud et al., 1976; Turner, 1970). The pre-Andean tectonichistory of the regionhas beenpartlyobscuredby later events, but there were importantperiods of regional folding, and there are graniticplutons n the easternCordillera n Peru and Argen-tina which give lower and upper Paleozoic ages(Stewart et al., 1974; Clark et al., 1976).The Cordilleraowes ts morphotectonicdentity tothe Andean orogeny, which began in the Triassic

(James, 1971). Throughoutmost of the orogenyit was emergent, bounded on the west by theAltiplano molasse basin and on the east by thebasin whose olded sedimentary ocks now form thesub-Andean foothill zone. Within the Cordillera inBolivia, there are some remnants of epicontinentalsedimentary ormationsof Cretaceous ge which onceprobably covered the region but were largely re-moved by subsequent rosion (Schlatter and Neder-lof, 1966). The remainder of the Cordillera is madeup of shales nd sandstonesf Cambrian o Devonianage and igneous ocksof the Andean orogeny. Thesediments re strongly folded and faulted, but meta-norphism s generally weak or absent. The igneousrocks are acid to intermediatecalc-alkalineplutons,high-level stocks,and eruptive complexes. The tindepositsare associatedwith these. There are alsoextensivepostmineral gnimbritesheets. The majorigneousprovinces f the easternCordillera are shownin Figure 1, togetherwith the relevantagedata.

838

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K-Ar AGES OF IGNEOUS ROCKS AND THE BOLIVIAN TIN BELT 839

I I7/.. 700 660

200 kmI I

LAKETITICACLA PAZ\--

+

18 ; +. r- ORURO, /r--13 18CENTRALNDES ARICA

qBATHOLITHSF TRIASSIC O I ' mJ J JURASSICGE J J TARIJA 0 +RUPTIED LOWERIOCENEJATED V / BOLIVIA ' I ++ +' 25YI/ COMPLEXESlTHJ d m I " fill./(CuPORPHYRYIOF.MIOCENEGE + .

IFIG. 1. Major igneousprovincesof the Eastern Cordillera of the Central Andes,with locationsof relevant age data.Peru, data in Lancelot et al., 1976: 1, San Ramon, 252 4- 18 m.y. (Rb:Sr); 2, Villa Azul, 251m.y. (K-At); 3, Machupicchu, 46 4- 10 m.y. (Rb:Sr); 4, Quillabamba,257 4- 5 m.y. (U:Pb);and 5, Coasa, 238 4- 11 m.y. (U:Pb).Bolivia, the northern in belt, K-At data reported n Clark and Farrat (1973) and Everndenet al.(1977): 6, Sorata, 180 m.y.; 7, Zongo, 150 to 211 m.y.; 8, Taquesi, 199 m.y.; 9, Illimani, 26 m.y.;10, Quimsa Cruz, 26 m.y. and 23.8 4- 1.6 m.y.; and 11, Santa Vera Cruz, 22.8 4- 0.7 m.y.Bolivia, the southern in belt, K-At data reported n this paper (using he constantsof Steigerand Jiger,1977): 12, San Pablo-Morococala, 3.3 4- 0.4 m.y.; 13, Morococala gnimbrite,6.3 4- 0.1m.y.; 14, Llallagua,20.9 4- 0.4 m.y.; 15, Colquechaca, 2.6 4- 0.4 m.y.; 16, Los Frailes gnimbrite,7.5 4- 0.2 m.y.; 17, Karl-Karl batholith, 21.4 m.y.; 18, Cerro Rico stock, Potosi, 13.8 4- 0.3 m.y.;19, Tasna, 16.4 4- 0.3 m.y.; 20, Chorolque, 16.2 4- 0.3 m.y.; 21, Chocaya, 13.8 4- 0.2 m.y.; and22, Tatasi, 15.6 4- 0.3 m.y.Argentina,data reported n Sillitoe (1976, 1977): 23, PanchoArias, 15.4 4- 0.3 m.y. (K-Ar);24, Inca Viejo, 15.0 4- 0.2 m.y. (K-At); and 25, Aquilar, 123-133 m.y. (Rb:Sr).

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840 GRINT, HILLS, SALINAS, IND SNELLINGThe tin belt extends hroughoutBolivia, from nearthe Peruvian border to northern Argentina. To thewest, there is a well-definedprovinceof polymetallicdeposits n the Altiplano which are associatedwithMiocene-Pliocenehigh-level dacitic and andesiticigneous ocks (Ahlfeld, 1967). In the northernpart

of the tin belt (Sorata to Santa Vera Cruz plutons)the deposits re mainly tin and tungsten einsassoci-ated with granite-granodioriteatholiths. The batho-liths range n age rom Triassic o Miocene Cordani,1967; Clark et al., 1976; Evernden et al., 1977).Miocene ntrusionsoccur in the southernpart of thissectionand are the most highly mineralized. Thedepositsare numerous,but with one or two excep-tions they are individually small.In central and southern Bolivia the depositsareassociated ith acid eruptivecomplexes f Tertiaryage. They are complexvein systemscarrying tin,tin-silver, or tin with base metals and silver. Insome cases here is also widespreaddisseminatedmineralization n the altered igneousrocks. Thereare alsoa few importantdepositsormedby vein sys-tems n sedimentary ockswith which there is littleor no direct evidence or igneous activity (e.g.,Huanuni and Colquiri). Theseare probably elatedto high-levelstockswhich failed to reach he surfaceandhence re geneticallyimilar o the deposits ithineruptive omplexes.The subvolcaniceposits f thecentraland southern art of the tin belt are fewerbut,in general, re individuallyarger han those ssociatedwith the batholiths of the north and account for thegreater proportionof Bolivian tin production.The mineralizederuptives ange in character romvolcanics with no associated intrusive rocks otherthan dikes, to small stocklikebodiesof intrusive por-phyry and breccia. The essentially olcaniccom-plexessuchas Chocaya, atasi, and Colquechacaon-sist of acid lavas, ava domes,pyroclastics, nd vol-canicbrecciaswhich rest unconformably n the oldersedimentary ocks. The rocksare generallyquartz-porphyries anging n compositionrom rhyolite todacite. They all have an extensive entral zone ofhydrothermal alteration which may reach severalkilometers in diameter and within which the mineral-ized veins occur. At Chocayaand Tatasi the majorveins extend downward into the sedimentary ocksbeneath he volcanics. Pervasivehydrothermalalter-ation at thesecenterspostdates ll eruptiveactivity.At other centers,such as Chorolque,Oruro, andPotosi, volcanic rocks surround, and are in contactwith, a central stocklikebody of intrusive porphyryand breccia. Erosion has partly removed the over-lying volcanic superstructureat these centers. Thestocksare usually about one kilometer in diameter at

thesurfacendconicaln vertical ection,arrowingwith depth.At somecenters, uchas Llallaguaand San Pablo(Morococala), rosion as completelyemoved nyvolcanic uperstructurehichmayhavebeen resent,leavingonly a porphyrystock n contactwith thePaleozoic edimentaryocks.

Previous GeochronologicalDataThe geochronologyf the batholiths f the northerntin belt s currently nder nvestigationRobertson,1974: Clark et al., 1976). Reviewsof the availabledatahavebeen iven y ClarkandFarrar (1973) andby Avila (1975). The K-Ar dateswhich have beenreported range from Triassic to lower Miocene inage.The batholithsf the Cordillera ealgivemainlyTriassic o Jurassic geswhile the more southerlyIllimani,QuimsaCruz,andSantaVera Cruzplutons

give lower Miocenedates, he youngest eing 22.8m.y. for a sample from the Santa Vera Cruz(Cordani, 1967).In the southernin belt t has ongbeen ecognizedthatmanyof theeruptive enters re of Tertiaryage.Volcanic rocks unconformablyoverlie Cretaceoussediments t a number of localities,and the CerroRicostock t Potosicuts uff bedscontaining fossilfloraclassifieds ate Pliocene Berry, 1937) or Mio-cene Steinmann, 922). From this evidence gen-eral Mioceneor Plioceneage has previously eenappliedo all the igneousocksof the region Turne-aure, 1960).Prior to this study,only a limited numberof radio-metric age determinationshad been carried out onigneous ocks of the southernpart of the tin belt(Evernden, 1961; Clark and Farrar, 1973; Everndenet al., 1977). Although the detailsare not given,it is probablehat thesewerecalculated singslightlydifferentdecayconstants nd potassiumsotopic bun-dancevalues o thosecurrently n use (Steiger andJ//ger, 1977). Recalculation would result in aslight ncrease n the ages (approx. 3% for Mioceneages). These provided irm data on the age of theKari-Kari batholith and Agua Dulce lavas of thePotosi region (20-21 m.y.), and indicated a lateMiocene (7.3 m.y.) age for the post-mineralgnim-brites of the los Frailes Formation. They providelittle informationon the age of the mineralized rup-tive complexes, lthougha date of 9.4 m.y. whichwasreported for the Salvadora stock at Llallagua hasparticular significance. As noted above, he southernplutonsof the northern in belt havegiven ower Mio-ceneages, he youngest f which s 22.8 m.y. Thus atime break of over 10 m.y. was ndicatedbetween heemplacement f the igneous ocks and associatedmin-eralization of the southern end of the northern tin belt

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K-.4r AGES OF IGNEOUS ROCKS AND THE BOLIVIAN TIN BELT 841and the emplacement f the mineralizederuptivecomplexat Llallagua,near the northernend of thesouthern tin belt.

These data were used by Ahlfeld (1967) to pro-pose hat three episodes f mineralization ormed thetin belt as a whole. He recognized Mesozoicstageassociated with the batholiths of the Cordillera Real;a lower Miocene stageassociatedwith the southern-most plutonsof the northern in belt, in which heincluded the minor mineralization associated with theKari-Kari batholith near Potosi; and he ascribed allof the subvolcanic mineralization of the central andsouthern artsof thebelt o a distinct ounger upperMiocene-Pliocene)stage.The new agesdo not supportAhlfeld'ssubdivision.In particular it has not been possible o confirmEvernden's (1961) upper Miocene age of 9.4 m.y.for the Salvadorastockat Llallaguaand a mucholder,lower Miocene,age is preferred, he implications fwhich are discussed below.

Location and Description of SamplesWe have dated42 rock samplesrom the followingareas: the San Pablo (Morococala) stock and Japoand Santa F mines; the Salvadora stock, Llallagua;the Colquechacaruptivecomplex; he Potosiregion,including he Cerro Rica stock; and the mineralizederuptive complexesof Chocaya,Tatasi, Chorolque,and Tasna (Fig. 1).The samples an be divided nto two groups:(a) The first consists f intrusive and volcanicunits directly associated ith mineralization. Theseare generallyquartz-latiteporphyriesmade up ofquartz, sanidine,plagioclase, nd biotite phenocrystsin a microcrystalliner glassymatrix. Many of theseunits have been hydrothermallyaltered; the commonalteration ypes are tourmalinization, ericitization,argillization, nd chloritization. Where possiblewehave dated both unaltered and altered samples romthe same unit.(b) The second onsists f volcanic nd intrusiverockswhichare not directlyassociated ith importantmineralization.These nclude ostmineralgnimbrite

formationsand the Kari-Kari pluton and somevol-canic formations of the Potosi region.Experimental Methods

We have dated biotite and sanidine concentratesfrom the unaltered rocks. In the hydrothermallyaltered rocks the new minerals which formed aremainly sericite (very fine grainedmuscovite)andtourmaline,with a little quartz. Since tourmalineis not considered o be reliable for K-At dating, wehaveprepared ericite oncentratesrom rockswith asubstantial tourmaline content. The whole rock was

used or dating ntensely ltered sericitized) amplescontainingittle or no tourmaline.Mineral concentrates ere preparedusing magneticand heavy liquid separationmethods. ConventionalK-Ar analytical techniqueswere used (Dalrympleand Lanphere, 1969). Analysis for potassiumwasdone by flame photomerryusing a lithium internalstandard. Argon analysiswas done by the isotopedilutionmethod n an AEI MS 10 massspectrometerusinga SAr tracer.Errors are quotedat m one standarddeviationandincorporate he statisticallydetermined rrors in thepotassiumdetermination, racer volume, and isotoperatio measurements, lus the usual correction foratmospheric Ar.

ResultsThe analytical resultsare summarized n Table 1.Considerationof the errors on the ages indicate atypicalvalue of -*2 percent, ncreasingo 2.7 percentin somecases. Using thesewe can calculate riticalvalueswhich must be exceededf we are to say with95 percentconfidencehat there is a real differencebetveenhe calculated gesof any two sampleswithan agedifference - R2). We haveused he equa-tion of Mcintyre (1963), as suggested y Dalrympleand Lanphere (1969). In terms of the agesof theanalyzedsamples, he critical values or comparingsingleage determinations re as follows (in m.y.)

Age ,, = 2% ,, = 2.7%20 1.1 1.516 0.9 1.21,3 0.7 1.06 0.3 0.5

San Pablo-Morococala egionThe region lies to the southeast f Oruro. It isan elevated plateau made up of folded Silurian-Devonian sedimentary ocks. During the Tertiarythese were intruded by small porphyry bodies andlater overlainby an extensivesheetof dacitic gnim-brite (MorococalaFormation) which makesup much

of the present and surface. The geologyof the SanPablo-Morococala rea is shown in Figure 2. Tinmineralization is found at a number of localities,mostly in veins in the Paleozoic sedimentary ocks,though in close associationwith the porphyries.The most important mines are Santa F andMorococala, where quartz-porphyry dikes are ex-posedunderground, nd Japo.The San Pablo stock s composed f quartz latiteporphyrywhich has undergonentensehydrothermalalteration (sericitization nd tourmalinization). Somepatchesof relatively fresh rock were preserved n a

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842 GRANT, HALLS, SALINAS, AND SNELLINGTABLE 1. Potassium-ArgonAge Data

% Vol Arrdnl CalculatedSampleno. Rock ype Material dated % K Artmo, gm age (m.y.):San Pablo-Morococala egion

NG1 dacite (ignimbrite) biotite 7.16 35 1.75 6.3 + 0.11Repeat 50 1.72 6.2 + 0.13NG2 dacite (ignimbrite) biotite 6.61 58 1.65 6.4 + 0.13NG6 quartz latite porph. sericite 3.98 42 3.86 24.8 + 0.44(breccia); alteredNG27 quartz latite porph.; sericite 4.51 39 3.56 20.2 q- 0.35alteredNG31 quartz latite porph. K-feldspar 10.98 14 10.02 23.3 q- 0.40(sanidine)NG21 altered porphyry whole rock 4.03 68 3.24 20.6 q- 0.44Repeat dyke (Japo mine) 61 3.48 22.1 q- 0.43NG14 Altered porphyry whole rock 3.36 56 2.63 20.0 q- 0.38Repeat dike (Santa Fe) 30 2.70 20.6 q- 0.36

Llallagua, the Salvadora stockNG487 quartz latite 36 4.85 21.6 q- 0.38Repeat porphyry biotite 5.73 40 4.65 20.8 q- 0.38NG38 altered porphyry whole rock 3.35 35 2.70 20.6 q- 0.36NG45 quartz latite K-feldspar 10.77 18 8.69 20.6 q- 0.35porphyry (sanidine)NG44 altered porphyry whole rock 2.58 38 2.13 21.1 q- 0.38

ColquechacaNG35A quartz latite biotite 6.20 46 5.49 22.6 q- 0.42(CH)NG54 quartz latite biotite 6.75 36 5.67 21.5 q- 0.35lavaNG62 altered lava whole rock 4.51 28 3.83 21.7 q- 0.37

Potosi regionNG68 dacite ignimbrite biotite 6.96 53 2.02 7.4 q- 0.23Repeat (Los Frailes Fm.) 58 2.03 7.5 q- 0.17NG71 rhyolite biotite 7.38 52 3.44 12.0 q- 0.22(Huakachi)NG93 altered porphyry whole rock 3.58 68 1.85 13.2 q- 0.32Repeat (Cerro Rico) 63 1.95 14.0 q- 0.33NG94 altered porphyry whole rock 3.30 47 1.81 14.1 q- 0.26Repeat (Cerro Rico) 50 1.82 14.1 q- 0.30NG73 rhyodacite porphyry biotite 5.69 39 4.59 20.6 q- 0.34(Karl-Karl)NG75 rhyodacite porphyry biotite 6.78 27 5.61 21.1 q- 0.33(Kari-Kari)NG75A rhyodacite porphyry biotite 6.42 35 5.60 22.3 q- 0.36Repeat (Kari-Kari) 38 5.37 21.4 q- 0.38NG75B rhyodacite porphyry biotite 6.97 37 5.91 21.7 q- 0.36(Kari-Kari)NG80 rhyodacite porphyry biotite 4.31 51 3.59 21.3 q- 0.39(Kari-Kari)NG83 granodiorite biotite 6.03 32 5.01 21.2 q- 0.34(Kari-Kari)

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K-,4r AGES OF IGNEOUS ROCKS AND THE BOLIVIAN TIN BELTTABLE 1-- (Continued)

843

% Vol lArr,(lnl CalculatedSample no. Rock type Material dated % K 4Ar,t... gm t age (m.y.) NG78 rhyodacite tuff biotite 5.61 36 4.61 21.0 - 0.34Repeat (Agua Dulce Fro.) 28 4.67 21.3 4- 0.36NG79 rhyodacite tuff biotite 7.22 24 5.91 20.9 - 0.32(Agua Dulce Fro.)NG96 rhyodacite tuff biotite 7.12 39 6.11 21.9 - 0.36Repeat (Canteria Fm.) 36 6.04 21.7 4- 0.38NG97 rhyodacite tuff biotite 7.40 24 6.11 21.1 - 0.33(Canteria Fm.)NG97A rhyodacite tuff biotite 7.10 38 6.05 21.8 - 0.36(Canteria Fm.)NG98 rhyodacite tuff biotite 7.42 35 6.19 21.3 - 0.34(Canteria Fm.)

Chocaya volcanic complexNG134 rhyodacite lava biotite 7.36 31 3.98 13.9 - 0.22Repeat 34 4.13 14.4 - 0.26NG131 rhyodacite lava biotite 5.05 44 2.70 13.7 - 0.24NG136 rhyodacite lava biotite 5.82 41 3.02 13.3 4- 0.23Repeat 29 3.09 13.6 4- 0.23NG126 rhyodacite lava whole rock 3.60 49 1.75 12.5 -t- 0.24(altered)

ChorolqueNG123 rhyodacite whole rock 4.34 22 2.71 16.0 4- 0.27Repeat lava (altered) 40 2.76 16.3 4- 0.30NG279 rhyodacite whole rock 1.91 83 1.37 18.4 -t- 0.65lava (altered)

TatasiNG283 rhyodacite lava biotite 7.73 29 4.64 15.4 4- 0.24Repeat 41 4.83 16.0 4- 0.29NG150 rhyodacite lava biotite 7.51 30 4.57 15.6 -t- 0.25Repeat 38 4.50 15.4 4- 0.27

TasnaNG281 quartz porphyry whole rock 1.94 51 1.23 16.2 4- 0.31Repeat dike, altered 52 1.25 16.5 -t- 0.31

Others (Atocha tuff unit)NG99 rhyodacite uff, biotite 7.29 33 4.79 16.8 4- 0.27Repeat Chorolque road 26 4.77 16.8 -t- 0.31NG139 rhyodacite tuff, biotite 6.49 41 4.36 17.2 4- 0.32Atocha-Chocaya roadConstants used: 3 = 4.962 X 10 yr-; 3, = 0.581 X 10 yr-l; 4K/K = 0.01167 atom % Mean of duplicate analyses2 Errors at one standard deviation

narrow zone at the northern contact. The southernpart of the stock consistsof hydrothermal brecciamade up of fragments of altered igneousand sedi-mentary rocks in a fine, granular tourmalinizedma-trix. The porphyry dikes at the Santa Fg and Japomines are completelysericitized. All these alteredrocks contain disseminated sulfides and cassiterite.The sanidine ate of 23.3 -----.4 m.y. (NG31) is per-

haps the best estimateof the minimum age of em-placement f the San Pablo stock. The greater agegiven by sample NG6 should be regarded withsuspicion s the rock s a polymictbrecciawhichmaycontain older material. Sericitized porphyry(NG27) gavean ageof 20.2 -----.35 m.y., suggestinga resolvable ime differencebetween he emplacementof the stockand the pervasive lteration. This view

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844 GRANT,HALLS,SALINAS,AND SNELLINGLEGEND

PABLO TERTIARY

i? IGNIMBRITE+: SANABLOtockQuartz latite porphyry)

LOWER DEVONIAN

- PAMPAormationShale)SILURIAN TRONCHIR,ormationQuartzite)

' CANCANIRIormationGreywacke)' Mine K/Ar Sample Location

.---. Road

0 1800 3600mi i i , i i i , i.............. SC ALE^^^^^^^^^^^^^^

^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

^^^^^^^

^^

^^^

GEOLOGY BY F S TURNEAURE

FIG. 2. San Pablo-Morococala rea: geologyand sample ocations.is supported y the unweighted eanage (21.35 ----0.4 m.y.) for the altereddike in the Japomine ad-jacent to the stock. Since the mineralized veins atthe Japomine formedat the same ime, or slightlylater than the main stage of alteration of the stockand dikes, here appears o be a resolvable ime dif-ferenceof over 1 m.y. between mplacement f thestock and the mineralization in the veins.

The age of the altered dike in the Santa Fd mine(NG14) indicateshat igneous ctivityand mineral-izationwas essentially ontemporaneousith that ofthe San Pablo-Japoarea.

The 6 m.y. age of the Morococalagnimbrite ndi-cates he long time spanwhich separatedhe intru-sionof the mineralized orphyriesrom the eruptionof the ignimbritesheetwhich coversmuch of the area.Llallagua. the Salvadora stock

The Salvadora tock s an isolated odyof about1 km in diameter which intrudes the Silurian-Devoniansedimentary ocks ust to the southof theMorococala lateau. It probablywas the vent of avolcanowhose superstructure as been completelyerodedaway.

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K-Ar AGES OF IGNEOUS ROCKS AND THE BOLI?IAN TIN BELT 845It is madeup of intensely lteredporphyry acedwith irregular patchesand dikes of hydrothermalbreccia. Relativelyunaltered uartz-latite orphyrywas preserved at the contact of the stock in someareas. The alteration productsare sericite, ourma-line, quartz, and clay minerals, with disseminated

sulfidesndcassiterite.The stock s cut by a systemof rich tin-bearing eins,someof whichextendoutinto the surroundingedimentaryocks. They havemade his oneof the mostproductivein deposits fthe world (Turneaure, 1935).We have dated four samples f the stock fromdifferent ocations.Two are whole ock samples fsericitized orphyry--NG38 is from level 383, Salva-dora sectionof the mine, and NG44 is from the sur-face. One (NG45) is a sanidine oncentraterom arelatively unaltereddikelike off-shootof the stock atthe surface nd one s a biotiteconcentrateNG487)from almostunaltered orphyrynear the marginofthe stockon the 650 level, Siglo XX sectionof themine.

The agesof thesesamples iffer by less han onemillionyearsand give an unweightedmeanage of20.9 0.4 m.y. There is no resolvabledifferencebetween he ages of altered and unalteredsamples.The concordancyf the agesof the four samplesndi-cates hat a date of about21 m.y. is a reliableesti-mate of the minimum age of emplacement f thestockand is a goodestimate f the age of alterationand mineral deposition t Llallagua. This date ispreferred o the previously ublishedge of 9.4 m.y.(Evernden,1961; Evernden t al., 1977), althoughthe reason or the lack of agreements not known.Colquechaca

Colquechacaas escribedyAhlfeldndSchnei-der-Scherbina1964) as a dacitestock,however, thasnot beengeologically appedn detail. Our ob-servationsndicatehat it is a volcanic omplex, rob-ably made up entirely of extrusive rocks. Thesecover an area of more than 100 km 2 and rest on anirregularbasement f foldedPaleozoic nd possiblyCretaceoussedimentary ocks. There is an exten-sive zone of pervasivesericite-tourmaline lterationin the centralpart of the complexwhich is cut bysmall in-bearingveins. There are a number of muchlarger sulfide-rich eins which cut the surroundingvolcanics. They have well-definedenvelopesofsericite-chlorite-carbonate alteration. Much silverwas mined rom someof these n the past, and basemetals are still being produced. Throughout heremainderof the complex here is widespread hlori-tic alteration.We have dated hree samples f quartz atite fromthe central part of the complex. Two of these are

biotite concentrates--NG35 CH is from virtuallyunalteredporphyry at the surface,whereasNG54 isfrom a slightlyaltered ock (chloritized, ericitized)from within he mine. The third sample NG62) isintenselyaltered (sericitized) whole rock. The threegive a spreadof agesof just over onemillionyears,from 21.5 0.35 to 22.6-----0.41m.y. The age dif-ferencebetween he unalteredbiotitesample nd thepervasivelysericitized ample s about 1 m.y., whichis less than the critical value required o resolvethem.The Potosi region

The general geology and sample locationsareshown n Figure 3. Folded Paleozoicand Cretace-ous sedimentary ocks form the basementof theregion. They are unconformablyverlainby:1. The Chalviri group, which ncludes he AguaDulce and Canteria volcanic formations. These restunconformablyn the basementndagesof approxi-mately20 m.y. havepreviously eenobtained or theAgua DulcevolcanicsEverndenet al., 1977).2. The Cerro Rico Formation, which is restrictedto the imemdiate icinityof the Cerro Rico stock. Itconsistsf sedimentsnd uffs containing Tertiaryfossil lora, restingunconformablyn the Paleozoicbasement. It has not been dated by radiometricmethods.3. The Los Frailes Ignimbrite Formation,which

unconformably verliesboth the basement nd rocksof the Chalvirigroup,and hasbeenpreviously atedat approximately m.y. (Everndenet al., 1977).4. The Huakachi rhyolitic ignimbrite; this is notin contact with the Los Frailes Formation. It over-lies rocks of the Cerro Rico Formation and containsmineralized orphyry ragments rom the Cerro Ricostock Turneaure, 1960).Intrusive units are the large Kari-Kari pluton,which has some minor tin and lead-zinc veins associ-ated with it, and the small Cerro Rico stock withwhich the great tin-silver deposit of Potosi is as-sociated. The Kari-Kari pluton is made up of rhyo-dacite and dacite porphyry and granodiorite,and itintrudes Agua Dulce lavas on its western flank.Everndenet al. (1977) have shown hat the plutonand the Agua Dulce volcanicsare of closelysimilarage, and our data are in agreementwith this. TheCerro Rico stock cuts the Cerro Rico Formation andis clearly older than the Huakachi rhyolite. Evern-den et al. (1977) suggestedhat the Cerro RicoFormation s equivalent o the basalAgua Dulce andthat the stock (which they did not date) is penecon-temporaneouswith these volcanicsand, hence, withthe Kari-Kari pluton.They follow Turneaure (1960)

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846 GRANT, HALLS, SALINAS, AND SNELLING

NG98',. /,

G93,94

vvvvvvvvvvvv......... NG 78 79vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv

vvvvvvvvvvvv--

4-4-

4-4-+

CERRO&KARl- KARl4-4' 4'

NG73,75

CHALVlRI4'

4' 4' 4-

4- 4-

4- 4-

4- 4.

4- 4-

4' 4' NG80 4-

Based on GEOBOL 1:100,000 Sheet. Potoil.

Fro. 3. Potosi area- geologyand sample ocations.

LEGENDQUATERNARYMoraine and AlluviumTERTIARYHuakachi Rhyolite

Cerro Rico PorphyryCaracoles Formation(tuffs and sediments)Kari-Kari BatholithKumurana Phase(granodiorite)Keri-Karl BatholithUdifferentiated{acid- intermediate intru$1ve)Agua Dulce Formatio(acid lavas)Canterie Formalio(acid I:)yroclastics)CRETACEOUSTorotoro Formation(red beds)PALAEOZOICOrdovican & SHu, ianSedimentary Rocks(mainly shales & quartzite)Geological BoundaryMain RoadK/Ar SamDie LocationMine

0 4,000 8,000 mI I , ISCALE

in assuminghat the Huakachi hyolite s an outlierof the Los Frailes Formation.

Our data contradict he above interpretationsofthe agesof the CerroRico stock nd Huakachi hyo-lite. The K-Ar ages or the Karl-Karl pluton (in-cluding he Kumuranagranodiorite t the southernend) and the Agua Dulceand Canteria olcarricsreindistinguishable. he unweightedmean age of 12sampless 21.4 -- 0.4 m.y. However, he age of theCerro Rico stock is considerablyyounger. Two

samples f intenselysericitized orphyry give an un-weighted mean age of 13.8--+0.3 m.y. Althoughthis is an alterationage, consideringhe relationshipselsewhere n the southern in belt it is unlikely thatemplacement f the stock ook place much more than1 m.y. before the completion f pervasivealteration.The Huakachi hyolitegives biotitedate (NG71)of 12.0- 0.2 m.y. This is consistentwith the geo-logical evidence hat it is younger than the CerroRico stock and shows hat it is not part of the Los

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K-Ar AGES OF IGNEOUS ROCKS AND THE BOLIVIAN TIN BELT 847Frailes Formation but is a distinctlyolder eruption.Our date for a sample NG68) of the Los Frailesignimbrite,25 km north of Potosi, s 7.5 -+-0.2 m.y.The mineralized i 7neouscenters of the Quechislar e 7ion

These include Tasna, Chocaya, Chorolque, andTatasi, which we have dated, and a number of othersmall centers with minor mineralization which havenot been dated.

The region s underlainby folded ower Paleozoicshales and sandstone. These were eroded to an ir-regularplateaubefore he beginning f igneous ctiv-ity in the region. The earliest gneous ormation san extensive acid crystal tuff unit which blanketsmuch of the land surface in the Atocha-Telemayuregion. Two samples of this tuff (NG99 andNG139) give an unweightedmean age of 16.9 -----.3m.y. The rocks of the mineralized gneouscenterswere intruded into the Paleozoic sediments or restunconformably n them. The eastern edge of theChocayaVolcanic Complex overlies he crystal tuffunit, but none of the other complexes re in contactwith it or with each other.

At Tasna, the Bi-Cu-Sn veins occur within aseveral km area of hornfelsic sedimentary rocks.There are somequartz-porphyrydikes, but no largeigneous ody s exposed t the surfaceor in the mineworkings. We have dated one sample rom a smalldike on the southernside. The rock was originallya quartz-feldspar-biotiteorphyry, probably quartzlatite, which has been ntenselysericitized.The wholerock analyses ive an unweightedmean age of 16.4-+-0.3 m.y. This is the minimumage for the altera-tion of the dike and surrounding edimentary ocks,and probablycloselyapproacheshe age of the min-eralization.

The ChocayaComplex s made up almostentirelyof extrusive rocks. It now covers an area of over100 km and has been extensivelyeroded since itsformation. It is madeup of rhyodacitic yroclasticsand lavascut by a few dikes. There is a very exten-sive zone of strong hydrothermalalteration in thecenter of the complex (mainly sericite/carbonate ndargillic alteration) and the important in and silver-lead veins lie within this. We have dated both freshrocks from the outer part of the complex and analtered sample rom the center.The biotite ages of the three unaltered samplesgive an unweightedmeanof 13.8 +--0.2 m.y. which sa good estimateof the minimum age for the eruptionof much of the volcanic material. The whole rockage of the sericitized sample (NG126) from thepervasive lteration one s 12.5 + 0.2 m.y. This ageis significantly ounger han the biotite ages of theunaltered ocksand suggestshat, as at severalother

centers, here was a time span of at least 1 m.y. be-tween he eruptionof the volcanics nd the comple-tion of pervasive alteration.The Tatasi Volcanic Complex is similar toChocaya. It includes acid lavas, lava domes, andpyroclastics. Its present extent is over 100 km .Widespread hydrothermal alteration in the core ofthe complex s generallyargillic, and the mineralizedveinscarry silver and basemetalswith little tin. Twoseparatesamplesof unaltered rhyodacitic ava fromdifferent ocalities ear the edgeof the complexgivean unweightedmean age of 15.6-----0.3m.y. Wehave not dated any altered rocks from the centralregion.The ChorolqueComplexconsists f a central coreof breccia nd ntrusiveporphyry the volcanic ent),which is surroundedby acid lavas and pyroclasticswhich have largely been removed by erosion, ex-posing the Paleozoic sedimentsunderneath. Therocks are all altered. The central core is altered to aquartz-tourmaline ock, and the major tin-bearingveins are within this. It is surroundedby a halo ofsericiticalterationwhichgradesout to chloritization.Even in the volcanics farthest from the vent themafic minerals are chloritized, and the only rockssuitable or K-Ar dating are the sericitized olcanics.We have dated wo samplesrom the sericiticaltera-tion zone. NG279 givesa whole-rock ge of 18.4 -----0.6 m.y., but this sample roved o havea very hightourmaline content and is considered to be unreliable.A goodestimateof the age of the alteration s givenby NG123 whichgavea whole-rock ge of 16.2 + 0.3m.y. (unweightedmean of duplicateanalyses). Thisis considered o be a reliable minimum age for theformation of the ChorolqueComplex.

DiscussionGeneral considerations

The geochronology f igneousactivity in the tinbelt is by no means fully understood;however, anumber of distinctive features are evident which canbe consideredn termsof the chronologyf magmaticactivity in the Eastern Cordillera of the CentralAndes as a whole.1. No Paleozoicageshave beenmeasuredon anyof the igneous ockswithin the tin belt, although heEastern Cordillera in both Peru and Argentina hasundergone long and complexhistory of Paleozoicigneousactivity.2. The oldest known igneousrocks and ore de-positswithin the tin belt are the granite-granodioritebatholiths nd associatedin-tungsten eposits f theCordillera Real. Thesegive Triassic to lower Juras-sic K-Ar ages and were apparentlyemplaced, t adistance of over 300 km from the coast, at about the

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848 GRANT, HALLS, SALINAS, AND SNELLINGsame time that the initial Andean volcanic activitywas beginning in the coastal regions of southernPeru and northern Chile.

3. All later significant gneousactivity was Mio-ceneor younger n age.4. Within the tin belt, presently available agessuggest hat magmaticactivity took place n a seriesof relatively discretepulses. The geographical is-tribution of dates suggestshat each pulse was con-fined to a limited section of the tin belt, with littleoverlap xceptn the caseof the youngestvent thepostmineral gnimbrites). Further work may reveala more complexdistribution,particularly n the re-gion to the east of Lake Titicacawheremineralizedhigh-level ruptives avebeen ound,suggestinghatthe Miocene igneousprovince may extend north-ward parallelwith, but to the west of, the Mesozoicplutonics.

To the north of the tin belt, in Peru, the EasternCordilleracontains variety of plutonicand volcanicrockswhich have given Paleozoicagesranging fromOrdovician to Permian, and there are also PliocenevolcanicsStewart et al., 1974). A Permian gneousevent seems o have been the most widespreadandimportant,havingproduced cid and basicvolcanicsand graniticplutons. The southernmostf the majorplutons, he Coasabatholith,has given Permianzir-con U-Pb ages (238-----11 m.y., Lancelot et al.,1976). This suggestshat the northernmost lutonsof the tin belt, which give Triassic K-Ar ages,maybe distinctlyyounger than the Permian plutonsofPeru which contain no tin mineralization, and hencethat the northern termination of the tin belt cor-respondswith a significantbreak in the chronologyof igneous ctivity n this region. The possibility e-mains, however, hat the K-Ar agesof the Bolivianplutonsare too young,and they couldbe contempor-aneouswith the plutonsof southernPeru (Fig. 1).At its southern end, the tin belt terminates justsouthof the Bolivia-Argentinaborder. The EasternCordillera in northern Argentina has undergonealongand complex istoryof magmatic ctivitywhichis relativelywell documentedClark et al., 1976).There is a pre-Andeanpattern of granitic plutonicbelts, of Cambrian to Carboniferousage, orientednorthwest o southeastand becomingprogressivelyyounger oward the Chileanborder. Someof theseplutonshave tin-tungsten-bismuthineralization s-sociated with them. In the Permian there was vol-canicand high-level lutonic ctivity,with associatedcoppermineralization,n a belt close o the Chileanborder. SomeMesozoicages (Cretaceous)have alsobeen recorded (Sillitoe, 1977). A northward ex-tensionof theseolder gneous rovincesnto Boliviahas not been ecognized.There are however, ub-

volcanic eruptive complexesof Miocene age in theEastern Cordillera in northern Argentina. Theiragesoverlap with thoseof the southernmost roup oftin-mineralized igneous complexes of the Boliviantin belt, and they may representa southward ontin-uation of that igneousprovince. The Miocene gne-ous complexesn northern Argentina containcoppermineralization,ncludinga numberof porphyry cop-per deposits Sillitoe, 1977). thus the southern erm-ination of the Bolivian tin belt seems to be markedby an apparentlyabrupt change n the metal contentof the ore depositsxvithin a continuousmagmaticprovince rather than the termination of a belt ofigneous ocks of a particular type or age. However,the lack of age data and detailedgeological nvestiga-tions in the region of the Bolivia-Argentina borderleaves considerableuncertainty regarding the geo-graphical and geochronologicalelationshipsof themineral belts in this area.

At the latitude of northern Argentina, Andeanmagmatic activity began in the coastal region ofChile in Pertoo-Triassic ime. The focus of mag-matism migrated inland across he Western Cordil-lera, forming a succession f narrow longitudinaligneousbelts, parallel with the coast, and decreasingin age eastward. In the Miocene here was an abrupteastwardexpansion. Magmatism "broke out" of theWestern Cordillera, orming he mineralized ruptivecomplexes f the Eastern Cordillera mentionedabove(Clark et al., 1976).Farther north, in the latitude of the Bolivian tinbelt, he chronology f igneous ctivity n the WesternCordillera is less well known, but a similar patternapparently xists. The Miocene gneous ctivity ofthe Eastern Cordillera, which produced he minera-lized plutonsand subvolcanicruptivecomplexes fthe tin belt to the south of the Cordillera Real, waspart of the break-outevent which lasted nto thePliocene. In the upper Pliocene there was a con-tractionof magmatic ctivitywestward, ack nto theWestern Cordillera. This is recognizedboth in Bo-livia and n Argentina. Thus, he igneous eology fthe tin belt to the south of the Cordillera Real fitsrather well into a coherentpattern of Andean gne-ous activity which is recognizablehroughout hewhole region. The samecannotbe said of the plu-tons and associated in-tungsten deposits of theCordillera Real whose relationships to the largerpattern of magmaticand tectonicevolutionremainobscure.Geochronolo9y f the southern tin belt

Previouschronological ubdivisions f the tin belt,such as those of Schneider-Scherbina1962) orAhlfeld (1967), are not consistentwith the new age

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K-.4r .4GES OF IGNEOUS ROCKS AND THE BOLIFI.4N TIN BELT 849A = ALTERED

'J'Maracacalagnimbrite

LosFrailesgnimbrlteHuakachi ignimbrite

Pluton-QuimsaruzCordani1967 andRoberton,lg74)Pluton SantaVeraCruz (Cardani,1967)4-

San Pabla stock

Dyke, Santa Fe

-I I (AI)''Oyke,apa ine

A; A,in [. Llallagua, Salvadara stackAj A j Calquechaca

i[ II Iis [[ Kari-Kari PlutanCerra ico IIIStack ... AguaDulceandCanteria Valcanics

I IPast-mineral Ignlmbrltesi I I

ChoralqueTasnaChacaya- ,, ,

i[0 I

-H-TatallI J Atocha uffSouthern Upper Mlacene) Graul Northern ( Lawer Mlacene ) Graul

..... 'e ' ' ' ' ' '14 16 I 20 : 24AGE ( Ma )

FIG. 4. Summary of K' Ar age data for the central and southernpart of the Bolivian tin belt.data reportedhere. The latter proposedwo metallo-genic epochs or the southern in belt; a plutonicphaseof Mioceneage which ncluded he minor min-eralization of the Kari-Kari batholith, and an upperMiocene subvolcanic hase which included all theimportant in deposits nd whoseage was basedona 9.4 m.y. age for the Llallagua stock. Ahlfeld alsoproposed hat the Mesozoicbatholithsof the Cordil-lera Real with their associated mineralization werepost-tectonic lutons of a Variscan orogeny,whichpreceded the Andean cycle. Although few radio-metric ages were available,he recognized hat themore southerly plutons of the Cordilleras QuimsaCruz, Santa Vera Cruz, and possibly llimani, wereyounger than those farther north. He also notedthe similarity in metal content and certain morpho-logical eauresshownby the deposits ssociated iththese plutons and the subvolcanic epositsof thesouthern tin belt.

Our data are plotteddiagrammaticallyn Figure 4,which shows hat the agesof mineralizedsubvolcaniccenters of the southern tin belt fall into two distinctgroups. All the igneous ocks associated ith min-eralization which we have dated, as far south asColquechaca,iveagesbetween pproximately3 and20 m.y. This overlapswith the agesof the plutonsofQuimsaCruz and SantaVera Cruz. The mineralizedcenters outhof Colquechacaave ages angingbe-tween 17 and 12 m.y., although n the Potosiregion,

the igneousrocks of the Kari-Kari batholith andAgua Dulceand CanteriaFormationsall within theolder group. This suggestshat the Quimsa Cruz,SantaVera Cruz, and Illimani plutons,and the north-ern subvolcanic enters,all belong o a distinctprov-ince of lower Mioceneage. Although they have notbeendated n this study, t is probablehat the majorore deposits t Colquiri, Oruro, and Huanuni couldalso be included. If this is the case, hen this groupcontains ll of the most mportantpredominantlyin-producing eposits f the Bolivian in belt, while thesoutherngroup is characterized y deposits ich insilver and base metals (Potosi, Chocaya, Tatasi,Tasna). To the north, the Cordillera Real forms adistinct older (Mesozoic) provinceof tungsten-richdeposits. ts geochronologys complex nd,althoughmuch work has been done, the results have not beenpublished et (Robertson, 1974).The data show that there was a distinct southwardshift n high-levelgneousctivity n the southerninbelt between 0 and 17 m.y. ago,with overlapof theolderand younger ge groups n the Potosi egion.Consideringhe tin beltas a whole here s a markedoverall rend towardyounger ges n the south,withthe ocus f igneous ctivity pparentlymoving outh-ward in a seriesof discontinuous teps.There is an overall north to south change n themetalcontentof the ore deposits.Mineralization s-sociated ith the plutonics f the northern in belt is

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850 GRANT, HALLS, SALINAS, AND SNELLINGpredominantlyof tin-tungsten ype. As noted above,the two age groups in the southern in belt can bebroadly distinguishedby their differing types ofmineralization. The northerngroup (older than 20m.y.) is made up mainly of depositsassociatedwithhigh-level plutons or subvolcanic tocks,and the oresare of quartz-cassiteriteype with tin as the dominanteconomicmetal. Included here are the depositsofthe Cordillera Quimsa Cruz, the San Pablo stock,Llallagua, the Kari-Kari batholith, etc. There are,however,exceptions, uchas Colquechaca hich is avolcaniccomplex containingsulfide-richbase metaland silver mineralization with little tin. The Orurotin-silver depositalso probablybelongswith this agegroup. The southerngroup (younger han 17 m.y.)is dominatedby sulfide-richores containing mport-ant amounts of base metals and silver in addition totin, and the depositsare within eruptive complexesof essentiallyvolcanic character. They include theCerro Rico (Potosi), Chocaya,and Tatasi. Again,there are exceptions, ncluding Chorolquewhich isa brecciatedstock with quartz-cassiterite eins, andalso Isca-Iscawhich is a brecciapipe with tin-tung-sten veins. Thus the differences n the style ofmineralization and the metal content of the ore de-positswithin the tin belt are not consistently elatedto their age but primarily reflect he depth at whichthe deposits ormed, rather than changes n the geo-chemistryof the parent magmaswith time or space.This argumentcannot,however,be applied to thenorthern and southern terminations of the tin belt,which must reflect somemore fundamentalgeochemi-cal factors.The general rend from tungsten, hroughtin, to silver and base metals from north to southdown the tin belt parallels the decrease n erosionlevel southward. This does not imply that any in-dividual center of mineralization s, or was, verticallyzoned hroughout he full range of stylesand metalassociations,rom plutonic o volcanic,whichare dis-played laterally within the tin belt as a whole.In most of the mineralized subvolcaniccomplexesthere is little doubt that the pervasivehydrothermalalteration is younger than virtually all the igneousrocks. Thus the alteration ages are minimum agesfor the igneous ctivity. This shows hat the maxi-mum probable ime between he coolingof the earliestigneousrocks and the completionof the pervasivealterationat a given center s of the order of 1 m.y.

AcknowledgmentsOf the many peoplewho assistedn this investiga-tion we wish to thank in particular the following:Ing. Jose Torrez, formerly Director of Geobol; theManagement nd staff of the CorporacionMinera deBolivia for permission o visit the minesand for pro-

viding accommodationnd geological ssistance;Mr.C. Rundle, Institute of GeologicalSciences, ondon,for assistancevith the analyticalwork; and Messrs.S. Le Chevalier,A. Stevens, nd I. Ferriday (Im-perial College),for assistance ith sampleprepara-tion. J. N. G. and C. H. were financed by theNatural Environment Research Council, London.J. N. G.BILLITON INTERNATIONAL METALS

LOUIS COUPERUSPLEIN 192514 HP THE HAGUE, NETHERLANDS

C.H.DEPARTMENT OF GEOLOGY

ROYAL SCHOOL OF MINESIMPERIAL COLLEGE OF SCIENCE AND TECHNOLOGY

SOUTH KENSINGTON, ONDONSW7 2AZ,ENGLAND

w. A. S.SERVICIO GEOLOGICO DE BOLIVIA

FEDERICO ZUAZO No. 1673LA PAZ BOLIVIAN.J. S.INSTITUTE OF GEOLOGICAL SCIENCES

GEOCHEMICAL IVISION,64 GRAY'S NN ROAD,LONDON WC1X 8NG, ENGLAND.4pril 20, November 0, 1978REFERENCES

Ahlfeld, F., 1967, Metallogenic epochs and provinces ofBolivia: Mineralium Deposita, v. 2, p. 291-311.Ahlfeld, F., and Schneider-Scherbina, A., 1964, Los yaci-mientos minerales y de hidrocarburos de Bolivia: BoliviaDept. Nac. Geologia, Bol. 5, 388 p.Audebaud, E., Laubacher, G., and Marocco, R., 1976, Coupegologique des Andes du sud du Prou de l'Ocan Pacifiqueau bouclier Brsilien: Geol. Rundschau, v. 65, p. 223-264.Avila, W., 1975, Un modelo de tectonica de placas para elorigin del cinturon estanifero Boliviano: Soc. Geol. Bo-liviana Bol., no. 21.Berry, E. W., 1937, The fossil flora of Potosi, Bolivia: JohnHopkins Univ., Studies in Geology, no. 13, p. 9-67.Clark, A., and Farcar, E., 1973, The Bolivian tin province:notes on the available geochronological data: ECON.GF.OL., v. 68, p. 102--106.Clark, A., Farcar, E., Caelles, J., Haynes, S., Lortie, R.,McBride, S., Quirt, G., Robertson, R., and Zentilli M.,1976, Longitudinal variations in the metallogenetic evolu-tion of the central Andes: A progressreport: Geol. Assoc.Canada, Spec. Paper 14, p. 23-58.Cobbing, E. J., and Pitcher, W. S., 1972, Plate tectonicsandthe Peruvian Andes: Nature, v. 240, p. 51-53.Cordani, U., 1967, Determinaciones de edades radiometricasde rocas Bolivianas: Arch. Centro de Documentacion,Servicio Geol. Bolivia, La Paz (unpub. rept.).Dalrymple, G. B., and Lanphere, M. A., 1969, Potassiumargon dating: San Francisco, Freeman and Co., 258 p.Evernden, J. F., 1961, Edades absolutas de algunas rocasigneas en Bolivia, por el metodo potasio-argon: Soc.Geol. Boliviana, Noticiero 2, p. 3.Evernden, J. F., Kriz, S. J., and Cherroni, M., C., 1977,Potassium-argon ages of some Bolivian rocks: ECON.GEOL.,v. 72, p. 1042-1061.James, D. E., 1971, A plate tectonic model for the evolutionof the central Andes: Geol. Soc. America Bull., v. 82, p.3325-3346.

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K-Ar AGES OF IGNEOUS ROCKS AND THE BOLIVIAN TIN BELT 851Lancelot, J., Laubacher, G., Marocco, R., and Renaud, J.,1976, U/Pb radiochronology f two granitic plutons fromthe eastern Cordillera (Peru): Extent of Permian mag-matic activity and consequences:Montpellier, France,Centre Geol. Geophys.Contr. no. 222, 5 p.Mcintyre, D. B., 1963, Precision and resolution in geo-chronometry, in Albritton, C. C., ed., The fabric ofgeology: Reading, Mass., Addison-Wesley, p. 112-134.Robertson, R., 1974, Notas sobre el metodo del K-Ar dedatacion de rocas e interpretacion de edades obtenidashasta ahora: Proy. Plutonismo, Servicio Geol. Bolivia,La Paz, Inf. Prelim. No. 2, (unpub. rept.).Schlatter, L. E., and Nederlof, M. H., 1966, Bosquejode lageologia y paleogeografica e Bolivia: Servicio Geol. Bo-livia, Bol. 8, 49 p.Schneider-Scherbina, A., 1962. fber metallogenetischeepochen Boliviens und den hybriden charakter der soge-nannten zinn-silber-formation: Geol. Jahrb., v. 81, p. 157-170.Sillitoe, R. H., 1976, Andean mineralization: a model for the

metallogeny of convergent plate margins: Geol. Assoc.Canada, Spec. Paper 14, p. 59-100.1977, Permo-Carboniferous, Upper Cretaceous, andMiocene porphyry copper-typemineralization n the Argen-tinJan Andes: Ecoa. GEOL., . 72, p. 99-103.Steiger, R. H., and JSger, E., 1977, Subcommission n geo-chronology: conventionon the use of decay constants ngeo- and cosmochronology: Earth Planet. Sci. Leters, v.36, p. 359-362.Steinmann, G., 1922, Uber die junge Hebung der KordilleraSfidamerikas: Geol. Rundschau,v. 13, p. 1-9.Stewart, J. W., Evernden, J. F., and Snelling, N.J., 1974,Age determinations from Andean Peru: a reconnaissancesurvey: Geol. Soc. America Bull., v. 85, p. 1107-1116.Turner, J., 1970, The Andes of northwestern Argentina:Geol. Rundschau,v. 59, p. 1028-1063.Turneaure, F. S., 1935, The tin depositsof Llallagua, Bo-livia: Ecoa. GEOL.,v. 30, p. 14-60, 170-190.-- 1960, A comparativestudy of the major ore depositsofcentral Bolivia: EcoN. GzoL., v. 55, p. 217-254, 574-606.