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ECONOMIC GEOLOGYVOL. XIX. APRIL-MAY, I924. No. 3

THE TIN DEPOSITS OF CHACALTAYA, BOLIVIA.WALDEMAR LINDGREN.

Introduction.mThus far fluorite has not been recorded from thecassiterite einsof Bolivia. The account ere given describesdistrict n which this mineral forms an essential art of the veinfilling.It is interestingo recall hat tourmaline, ncesupposedo beabsent rom the Bolivianveins,has ately been ound n abund-ance n certaindistricts. Ap.atite, ong ago mentioned y Do-meykoromColquechaca,asbeen bservedy Davy in alteredrocksnear veins,and has lately beennotedby Hess2 as quitecommonn veins n granite from 'Caracolesnd Pacuni.Topaz,stillanother f theminerals haracteristicf the nor-mal typeof tin veins,hasbeen oundby Davys as part of themineralization f certaindistricts. Quite latelyGeorgeH. Hall 4has confirmed heseobservations nd recordedapatite and topazfrom several other localities.

Thus the evidencehas been gradually accumulatinghat atleastoneof the typesof the cassiteriteeinsof Boliviaconformsvery closelyo the deposits f Cornwalland Saxony.Locationand Geology.--Any one ntending o investigatehexDavy,W. ., OreDepositionn heBolivianin-Silvereposits,cono-ticGEOLOGY, ol. I5, X920, p. 489.

2 Hess, F. L., Some Unique Bolivian Tungsten Deposits, Eng. and Min. Jor.,vol. xx2, pp. 492--499, Sept. 24, 92.

4 Hall, G. M., Paragenesis of the Bolivian Silver-Tin Ores, Geological Societyof Amy.fica, December Meeting, 923.

s Op. cit.I5 223

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224 W.4LDEM.4R LINDGREN.

Bolivian in deposits ill probablymake La Paz his first station.Well known and easilyaccessibleeinsare found on the slopesof the snow-coveredeak of Huayna Potosi,at Milluni, aboutfifteen milesnorth of the capitol. Theseveinsoccur n slate,notfar from the contactwith granite. There is, however,a districtwhich lies still nearer to La Paz--about nine miles north of thiscity--and whichcanbe very conveniently isited n a day's rip.I have found no reference in the literature to this district exceptsomenotes n Stelzner's aper, evidently eferring to somegold-bearing veins unknown to me.

The city of La Paz lies n a valley I,ooo feet deep,scooped utby the headwaters .fthe La Paz River in the Quarternarysandsand gravelsof the High Plateauor Altiplano. One may as-cend n comfortby automobileo the rim of the altiplano, o thelighthouse uilt to guide the inhabitantsof the foggy uplandsto the capitalcity. Then the traveller follows he rim for a fewmiles,meanwhileenjoyinga panoramaof rare beauty, imitedon the eastby the giant Illimani chain, deeplycoveredby daz-zling snow fields. It now becomes ecessaryo change o mule-back, and a gradual ascentover slopeswith heavy morainalmaterial leads to an elevation of about 5,0oo feet where onelooks up to the main divide of the CordilleraReal across heheadwaters of one of the eastern tributaries to the La Paz River.In this depression nd on the slopesof the main range lies theChacaltayadistrict.The country rock is a black slate,probablyof Devonian age,with 'a constant outhwest ip of from 4 to 26 parallel o thegentle ascent o the divide, which here at a low passattains anelevation of about 6,ooo feet. Numerous granite blocks arescattered bout he vicinityand a shortdistanceo the north anarea of intrusivegranite appears; hether t is isol.ated r con-nectswith a larger area o the eastwas not investigated. f themain range at this placecontainsgranite this rock must occupythe easternslopeof the divide. The elevationat the mill wasgiven ytheofficersf thecompanys4,462m. (4,44o eet).

Zetschrfft der Deutschen Geol. Gesell., vol. 49, x897, pp. 5x-x42.

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DEPOSITS OF CHACALTAYA, BOLIVIA. 225The Veins.--Thedistrict s apparently xtensively ineralizedthough he veinsare remarkablemore for number han for pro-duction. The outputof tin is certainly mall,but a considerableamountof tungstenore hasbeenextracted.Twenty veinsare said to exist on the propertyof the Cia.Estafiifera de Chacaltaya. Most of them strike east and westand dip steeply orth or south. The width ranges rom a fewcen.imetersup to one meter; the lowestworkingsare 5oo feetabove he mill; the highest, ike thoseof the Veta Hierro and thefluoritevein, rangeup to 2,ooo feet above he officeof the com-

panysituated ear hebottomof the valley. The lowestveinhasbeen worked principally for wolframite but containedalso alittle cassiterite. It has beenopened or a vertical distanceof afew hundred eet by short adit tunnels, he workingsbeingnowinaccessible.

The present explorationsare confined o the Veta Hierro.This is openedby a tunnel driven by hand and a few hundredfeet long and other more superficial penings. The country ockis blackslatewith somebedsof quartzite. The narrow vein is atplaces ich n cassiterite.The ore is handsorted nd he intentionis to convey t to the small mill by a tramway. On the fluoriteveinwhichies arther orth ndat a higher l[ration oworkis doneat present.For hospitalityand much information I am indebted o themanagerand engineer,Messrs.H. G. Schubertand Otto Lohse.There are many other prospectsn the vicinity, but the veinsin generalare swellingand contracting,and the ore is spotty.I have it on the authority of Mr. Maximo Nava of Uncia Minethat there are many tin prospectsn the granite area to the northwhich carry tourmaline.The Ore.--As far as observedhe ore forms a filling of openfissures; n part it is massive, n part it has pronounced omb-structureby radial quartz crystalsup to 8 centimetersn length.There appearso be very little alteration n the adjoiningcountryrock.

The fluorite vein carries massive, white, or iron-stained ore

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226 WALDEMAR LINDGREN.

with occasionalmall drusy cavities n which quartz crystalsproject. This ore consists f a ground work of milky quartzcrystals p to 2 centimetersn length o which are attached mallcrystalsor crystallinemasses f dark brown cassiterite. Theprincipaland distinctly ater ganguemineral s a white fluorite,in large crystallinemasses,he individuals eingoften two centi-meters in width. The fluorite fills in the interstices and cementsthe earliercrystals o a massive re. The latestmineral s pyrite,which occurs n small cubesor irregular masses eplacing theearlier mineralsand particularly he fluorite.Needlesof tour.maline re found in the pyrite and in the cas-siterite,morerarely in the quartz. The successions thus: ()quartz; (2) cassiterite; 3) fluorite; (4) pyrite, while thedepositionf tourmalinen small uantitiesroceededt leastduring he ater and argerpart of the mineralizing eriod.The ore from the Veta de Hierro is more varied in composi-tion. It is richer in cassiterite, ontainsmore tourmaline and lessfluorite than the material described above.

One specimenrom the upper tunnel on this vein showsnextto the wall a few centimeters f drusy quartz, on which is de-veloped quartzcomb, he individualcrystals eaching6 centi-meters n length. Between hem and somewhat ater are crystalsof almost black cassiterite, on which are rosettes of a white micacorrespondingo lepidolite,except that it containsno lithium.It is presumedhat the lithium has been eached s the specimenshowsmuch imonitebetween he quartz crystals.Anotherspecimen hows t the basea thin crustof quartzwitha few tourmaline needles. Above this lies five centimeters ofgranular quartz, brownishor greenish n places rom admixedfine tourmalineneedles. There are also a few grains of pyriteand chalcopyrite,he exact place of which in the paragenesissnot certain. A little fluorite is present n this layer. The topcrust, three centimeters hick, is composedof fibrous browntourmaline n slender,straight fibers. With this tourmaline area few quartzcrystals nd specks f pyrite.A third specimen howsat the base wo centimeters f coarse

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DEPOSITS OF CHACALTAYA, BOLIVIA. 227crystalline uartz,surmounted y a layer of greenmassive uartz,aboutone centimeter hick. The green color is caused y fineslender ourmalineneedles f greencolor,embeddedn the quartz.Abovethis follows severalcentimeters f massivequartz mixedwith aggregates f dark brown cassiterite. Near the top thiscontainssmall aggregatesof iron black, granular stannite,andit is coveredby a last crust of quartz, only a few millimeters nthickness.

A fourth specimenonsists f a drusymassof white quartzand green, ourmaline-bearinguartz, irregularlymixed, furthersmallcrystals f cassiteritendmuch nterstitialluorite. The/'eare many solutioncavitiescoatedwith delicatecrustsof lightgreen ourmaline eedles. In largepart thesecavities esult romthe solution of the fluorite which leaves the slender needles oftourmalineattached o or projecting from the quartz crystals.Needlesof tourmaline re found n quartz, n fluoriteand inpyrite.The following successions clearly established:

PARAGENESIS OF MINERALS.

Early Late(uartzTourmalineCassiterkeLepidoliteStanniteFluoritePyrite

Accordingo Beck the general uccessionn tin-bearing einsis as follows' Lel)idolke(uartz, Topaz,Casskerke, rsenopy-rite, Fluorite.In the samevolume (p. 289) the successiont Ehrenfrieders-dorf, Saxony is as follows' Topaz, Wolframite, Cassiterite,Arsenopyrite,Quartz I, Apatite,Chlorite,Pyrite, Quartz II.

6 Beck, R., Lehre yon den Lagerst/itten, vol. x, x9o9, p. 255.

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2$ W.4LDEM.4R LINDGREN.

Davy 7determineshe general uccessionn Boliviaas follows:Tourmaline ndTopaz,Pyrite,Arsenopyrite, uartz,Cassiterite,Bismuth, Bismuthinite,Cylindrite and Franckeite, Stannite,Sphalerite, halcopyrite,etrahedrite, amesonire,uby silver.l-Iall 8 gives he followingsuccessionor Bolivia: Tourmaline,Topaz,Apatite,Quartz,Pyrite, Arsenopyrite, assiterite,Wolf-ramire, Chalcopyrite,Sphalerite,Complexsulphides.Many of the differencesre of course aused y the over-lapping of the periodsof formation of many minerals. Thepresent ase greesn the main with the first tablegivenby Beck,and seemso differ from the successionivenby Davy and Hall(I) in the fact that quartzhere s the earliestmineral;(2) in thevery long period of formationof tourmaline; 3) in the factthat pyritehere s oneof the youngestmineralswhereast usuallyin Bolivia precedes assiterite. Davy's table indicates,however,that bothpyriteand quartzcontinuedo be deposited,n smalleramounts,beyond he closeof the cassiterite eriod.

LAB01ATORY F ECONOMIC GEOLOGYMt, ss. INSTXTUT- 0V T-CNOL0CY.

Op. cit.80p. cit.