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ECONOMIC GEOLOGYVo.. XXVI. MAY, I93I No. 3

THE TIN ORES OF UNCIA-LLALLAGUA, BOLIVIAYFRIEDRICH AHLFELD.

INTRODUCTION.

THE town of Uncia lies at an elevationof 3,815 meters, aboutlOO km. southeastof Oruro, in the heart of the Bolivian EasternAndes. Its latitude is 18 27 58 South and its longitude60 37' 41 west of Greenwich. The ore deposits re in thevery centerof the Bolivian metalliferousprovince. Toward thenorth and toward he south he intensityof mineralization rad-ually decreases. Uncia lies at the junction of the N.-S. axis ofthe Andes of Southern Bolivia and the NW.-SE. axis of theAndes of northern Bolivia, that is, Uncia lies at the bend in therange. The intensive peningup of the sedimentary hellat thisbend s apparentlyhe cause f the intrusion f unusuallyargequantitiesof igneous ock. It is the reason or the strikingwideningof the rangeand the numerousgneousntrusionsnthe EasternAndesbetweenatitudes18 S. and 19 S. Thebend n the axis of the range s also he cause f the abundanceof ore depositswithin his area--Oruro, Negro Pabellon,Moro-cocala,Poopo,Avicaya,Antequera,Huanuni,Challapacheta,hePujru-VilaApacheta roup,Colquechaca,nd manyothers.

ROCK FORMATIONS.

Paleozoic,probably Devonian, unfossiliferous andstones,quartzites and slateswith NW.-SE. strike form an anticline west Translated rom Germanmanuscript y JosephT. Singewald, r.

16 241

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242 FRIEDRICH alHLFELD,of Uncia in the crest of which the little stock 2 occurs that containsthe world's largest tin ore deposit. Immediately east o.f Unciaa small mass of Cretaceous Puca sandstone is folded into the oldersediments. In addition to the Uncia stock, volcanic rocks arerepresented y two small rhyo-andesite ccurrences ortheastofUncia and extensive uffaceousrhyolitic flows that extend fromNegro Pabellon o a little westof Catavi. They are younger hanthe ore deposition.Northeastof Uncia and at Catavi are hot springs. Lindgrenhasdescribedhe latter. They havedepositedargesinter erracesconsisting hieflyof aragonite,but includesiliceous ortions hatcontainopaland dark portionscarryingmanganese ydroxidegelwith an appreciableWO content. These springsare evidenceof the very young--probablyPliocene--ageof the ore deposition.The Uncia igneous ntrusion has the form of an ellipsewithaxes2,ooom.. ongparallelo thestrikeof therangeandx,7oom.long at right angles hereto. The mine workings ndicate t ex-tendsvertical n depth. It is a single ntrusion. The rock is sodeeplyaltereddown to the lowestmine workings (775 m. belowthe surface) that it cannotbe classified ith certainty. Feldsparsare completely ecomposednd are only recognizablehrough heiroutlines. Biotite has altered o aggregates f minute sericitescales. Only the frequently crusheddihexahedralquartzes areunchanged. I am indebted o Dr. Brendlerof Hamburg for ananalysisof a specimenrom the Siglo XX level. It is compared

2 Throughout the manuscript the author uses the term Lakkolith for theU'ncia intrusion and other similar bodies of igneous rock in the Bolivian Andes.These intrusions do not conform to the generally accepted shape and geologicrelations of a laccolith. On questioning the author on this point he suggestedusing Steinmann's term stehende Lakkolith, and showed by a sketch an intrusivemass suddenly tapering at the bottom into a small feeder such as usually shown inan ideal section of a laccolith. The main mass intrudes the strata and is not forcedbetween them, and the vertical extent is much greater than the horizontal. Atranslation of the term stehende Lakkolith would give so misleading a conceptto the English reader that it appears to me better to use the word stock, thereader keeping in mind that the stock is believed by the author to diminish incross-section uite abruptly in depth and not maintain or even increase ts diameterwith increasing depth. In support of this idea the author states that the mineworkings at Potosi have clearly demonstrated such a shape for that intrusion.--J. T. S. Jr.

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TIN ORES OF UNCIA-LLALLAGUA, BOLIVIA. 243belowwith an analysisof a fresh dacitefrom Oruro given byKittl. 8

Llallagua (Brendler)

SiO ...............TiO ..............AIO3 ..............

Analysis67.75

0.2418.45

MolecularPer Cent

75.367o.2ol

12.1o8

Analysis

FeO .............FeO ...............CaO ...............MgO ..............KO ...............NaO ..............PO ...............MnO ..............-- HO ............+ HO ............

0.952.040.841.395.672.46

0.32

0.3951.9111.oo62.3114.038.663

64-7314.o4

Oruro (KittO

0.632-334-031.522.434.630.540.08o.313.53

MolecularPer Cent

73.4229.3970.2665e.1484.9112.5781.7655.1o90.259o.o75

For the Oruro rock the valuesof A, C, and F after Osann are'A (alkali feldspar) ............................. 6.874 mois.C (lime feldspar) .............................. 4.9IXF (dark constituents) ........................... 4.992

According o the terminology f Osann he rock is a dacite.Because f its great alteration the Llallagua rock can not bedefinitely etermined. t differsfrom the Oruro rockespeciallyin its lower lime content. On this. basis it would have to beclassedwith the quartz porphyries. Alteration, however,com-monly results n a decrease n lime and an increase n alumina.4It is very ikely, herefore, hat the freshLlallagua ock s richer nlime and similar to the dacite of Oruro and other Bolivian locali-ties.

The Llallagua ockhasbeen lterednot onlythroughweather-ing but alsoby pneumatolysis. neumatolyticlteration--evi-dencedby the occurrence f tourmaline--is restricted o certainportions, speciallyo the vicinityof the veinsandcountry ocka Erwin Kittl, Analisis de Roeas eruptivas de Bolivia, Revista Minera deBolivia, I927, Afio 2, p. 5I.4Waldemar Lindgren, Replacement n the Tin-bearing Veins of Caracoles,Bolivia, EcoN. Gv. t.., vol. 2i, p. 142, i926.

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244 FRIEDRICH AHLFELD.

inclusions. Such rock has a dark color causedby minute micro-scopic tourmaline needles. The pneumatolyticalteration is socompleten places hat almostnothing s left of the original rock.Thin sections how much secondaryquartz and pseudomorphsafter mica and hornblende.Although the rock often has a tuffaceousappearancen thinsection,physical-chemicalonsiderations uchas the high tempera-

ture of ore formation lead me to the conclusion that the intrusionis not a neck but a stock which had a thin sedimentaryshell.That no tuffs or glasses ave been found points o the samecon-clusion.

The contactaction of the magma on the intruded rock variesgreatly at the differentexposures ut is generallyslight. On the600 m. level at Uncia, the countryrock of the easterncontact schanged o a hard hornfels, indicating that the intensity of thecontactaction increaseswith depth. The condensation f thegases nd solutionshat carried he ores ook placemainly withinthe stock. Only a few veins, he contactvein for example,ex-tend for a shortdistancento the country ock. This is dueto thegreat absorptive apacityof the porousquartz porphyry for thesolutions. They condensed ainlynear the apexof the intrusion.

THE VEINS.

There are two. types of veins. One type are fillings of truefaults with occasionallyconsiderabledisplacement. They arepersistent nd regular in. strike and dip. To this group belongsthe importantSan Fermin-San Josdvein (Fig. ) which lies nearthe westcontact nd dips o ihe east. It is the real trunk veinand is joined in depth by a number of smaller veins. Other areForastera, Contacto,Reggis-Inka, and Salvadora. They are upto m. in width, and the San Fermin locally, even 8 m. Theyhave well-definedwalls and commonlya drusy and brecciatedfilling. The strike rangesbetweenN. 3o E. and N. 50 E.Between he veins of the precedingype are countless mallerveins representing eplacement f the wall rock along minutefractures. They are generallyparallel to. the large veins, but

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TIN ORES OF UNCL4-LLALLAGUA, BOLIVIA. 245some strike at right angles to them (cruzeras). They are oflimited extent along the strike and are characterizedby irregu-larity and non-persistencen their ore content. Their usualwidthis 5 to 30 cm. They becomeeaner n depth. The filling is com-pact and drusesare rare.A characteristichenomenons that someof the large veinsdo

Fro. x. Sketch of the Uncia Llallagua intrusion with the veins andfaults. The broken line is the boundary of the stock on the surface.not extend to the surface. An explanationof this will be sug-gested urther on. In Uncia-Llallagua9 main veins and about3 smallveins havebeenworked (Fig. ).

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246 FRIEDRICH AHLFELD.Major tectonicmovementsccurred ithin he stock ubsequentto ore deposition. The northeasternportion with the veinsCarnevalitoand Nueva sank along the Stanton and Diaz faultsThe faults are barren of ore and filled only with clayeycom-minuted material.

PARAGENESIS OF MINERALS.

The mineral illing all the veins s quite similardespitemanyvariations. The followingmineralswere recognized hich aredescribedn parageneticequencefrom near magmatic o moreremote magmatic affiliations:Hypogene Minerals.

Tourmaline.--Abundantn the country ock of the veinsandin certainbarrenshearzones. Occurs rregularlydistributednquartz n smallmicroscopiceedles nd shows'weak leochroism(Fig. 2). 5 Almost neverabsentwhen cassiterite ccurs, ut de-creasesn 'quantitypwards. he greisenccompanyingheSan Ferminveinon the deepestevelof the mine (775 m. belowthe surface)consistsnlyof tourmaline, uartz,anda little cassit-erite. Mica and topaz are absent. In part the tourmalinere-sembleschroite nd s withoutpleochroism.Quartz.--Associatedith the tourmalines much ine-grainedquartz as a pneumatolytic lteration of the country rock. Be-cause f this he silicacontent f some f the rock s very high.The frequentlywell-developedrapezohedral rystalsaccom-panying assiteriten druses re high-temperatureuartz formedunder 56o. It is contemporaneousith the cassiterite r older..Theabundant halcedonys probably upergene.

Cassiterite.--Occurslmostexclusivelyn twins of the Zinn-waldtype. They rarelyattain o a sizeof x cm.,and are gen-erallymuchsmaller.Transparentrystals re rare; they arecommonly lack. An analysisby G. Greene of a cassiteritecrystalyielded 4.o6 per cent SnO2and 4.3o per centFe. The5 The microphotographs f this paper were made with the kind aid 'of Mr.Ehrenberg.0 G. Greene, Solubility of Tin Minerals, Eng. and Min. ]our., vol. x22, p. 47,026.

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TIN ORES OF UNCII-LL/-1LL.dGU.4, BOLIVL4. 247crystalsare made up of zonal layers of various degreesof colorintensity. The most abundant orm of twin is after (o), andis usually n polysynthetic-cyclicggregates. In brecciated eins,especiallyn the San Fermin and Salvadora,cassiterite rystals

, .,'5:'' .., : . '..

. - . .

.-i . '.: . .,'., ? .

Fx. . F. 3-Fx. . Tourma]ine-greisen, country rock. San Fermin vein, SigloXX level. Thin sealion, X 74-Fx. 3- Cassiterite crystal (high relief) surroundedby stannite withunmixing partic]es of chalcopyrite. The cassiterite crystal is stronglycorroded. San Fermin vein, 330 m. below surface. Polished section,X47.

fillcavitiesetweenuartzrystalsnd oat ountryk frag-ments.n the large ore shoots, s thoseof the San Fermin vein, cassit-erite is the principalconstituent f the ore and often curs as-sociated ith wolframite,bismuthinite,nd pyrrhotite,ess re-quentlywith stannite nd apatite,withoutgangueminera]s.In oneof my specimensrom the 6o m. levelof the Salvadorashaft, n addition o the usual winnedcassiterite,re sphericalaggregatesof faintly colored needles. inkelmann describes

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248. FRIEDRICH AHLFELD.cassiteritewith gel structur.en rhyolite from Uncia. It wouldappear that cassiteritepresumably ormed from sols is moreabundant near the surface. I have not observed such in thedeeper ortionsof the deposit..4rsenopyrite.--Occurssolated n depth,as in the deepestevelof the San Fermin, in veinlets n greisen,apparentlyof pneuma-tolytic origin. Since it doesnot occur in associationwith otherminerals, ts geneticpositioncan not be definitelydetermined.14/olframite.--Wolframite of normal composition requentlyaccompaniesassiterite, specially n the Uncia side. Occasion-ally its crystalsare coveredby cassiteritecrystals. Manganese-rich varieties (hiibnerite) are formed under hydrothermalcon-ditions.

Bismuthinite.--Commonly ccompanieshe cassiterite,espe-cially n the Llallaguasection,where t is found n nearlyall veins.Its lancet-like rystals,attaining a length of to cm., never showfree terminations ut are alwaysembeddedn cassiterite r pyr-rhotite. Occasionallyts crystals re coatedwith cassiterite. Insomeveins the bismuthinite ontentvaries proportionallywiththat of cassiterite.Acicularbismuthinite, imilar o jamesonite,was seen n drusesin arsenopyrite.Native Bismuth.--Occasionallybundant nd n masses eigh-ing up to several ilograms San Fermin vein). It is never ncrystal orm, but togetherwith bismuthiniteonstituteshe fillingbetween uartzcrystals. Since t occurso a depthof 4oo m., Ido not agree with Kittl 7 in ascribing ts formation from bis-muthinitehrough processf reductionn thezone foxidation.I believehat t wasdepositedirectlyrom solutionsnderpneu-

matolyticconditions nd that the predominancef nativehismuchnearthe surface nd of bismuthiniten depth,as frequently b-servedn Bolivia,s a phenomenonf primarydifferencen depth.Pyrrhotite is the most important mineral associatedwithcassiteriten most' f theveins. It formsmassiveompact assesin which cassitrite,bismuthiniteand wolframite are embedded7 Kittl, Algunas observacionesobre la formation de Bismuto nativo, Rev.Minera, Soc.Argentinade Min. y Geol.Afio 2, 93o, PP. -3-

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TIN ORES OF UNCL4-LL.4LL.4GU.4, BOLIVIA. 249frequentlywith idiomorphic oundaries.The abundancef pyr-rhotitehasescapedhe attentionof most observers ecauset hasbeen almost entirely converted o pyrite and marcasite. Onlyoccasionally,speciallywithin the sediments, oes one encounterfresh remnantsor remnants n process f alteration..4patite is the only hypogenephosphate hat occurs n ap-preciablequantity, even though mainly microscopic. It is con-fined to a few veins (San Fermin). Occasionallyt is found indrusesas rose-coloredabular crystals,but its usual mode of oc-currence s in blue-greenmasses ntergrown with cassiteriteandpyrrhotite. It is of pneumato.lyticrigin.

Monazite.--This is also a pneumatolyticmineral which wasfound onceon the 60 m. level of the Salvadoravein as small, ightrose-colored rystals ntergrown with cassiterite nd nacrite.Sta.nnite.--Stannite s more common han generally ecognizedbut s confinedo certain eins. As a mineral f thehigh-tem-peraturehydrothermalphase, t is more abundantnearer the sur-face. According o the former superintendentf the Uncia Mine,Engineer Hans Block, someparallel stringersof the Salvadoravein consisted lmostsolelyof stannite. The mineral was foundin suchquantitynear the surfaceat Uncia that the erectionof asilver eachingplant was seriously onsideredn t9o5 The min-eral .isespecially bundantmicroscopicallyn the ores of the Polovorin and San Fermin veins. In the latter vein it is in part atleastolder than bismuthinite ut alwaysyounger han cassiterite,the strongly orroded rystals f which t enclosesFig. 3). Itincludes nmixingparticles f chalcopyrite.The embedding fcassiterite rystals n stannite,which is so frequentlyseen nBoli.vian res,canbe explainedn two ways. On the one hand,stannitemay belong o a later phase f ore depositionnd mayhavebeendeposited y alkalinesolutionshat dissolved assiterite.On the otherhand, t mayhavebeendepositedy the same olu-tions that precipitatedassiterite,ismuthinite,nd pyrrhotite,and its depositionccompaniedy resorptionf the previouslyprecipitatedassiterite.On thebasis f my observationsn otherdeposits,ammore nclinedo hold o the atterexplanation.The stannite suallyoccursmassive. However,on the 2o m.

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25o FRIEDRICH AHLFELD.

level of the Salvadora ein, magnificentwinnedcrystalsup to 4cm. in sizewere encountered n crystalsof cassiterite nd coveredwith crystalsof hfibnerite.In depth,below4oo m., stannitedisappearsompletely.3'phaleriteappears o. be limited to the upper portion of thedeposits s a hydrothermalmineral. No material was availableto me for investigation. The sphalerite s said to be rich incadmiumand to have been found in the Contactovein togetherwith franckcite.

Fra.nckeite.--Thismineralwas not determined nalytically, utin polished urfaces y comparison ith franckeite rom Huanuni.It occursas a very thin coatingand in tabular crystalson pyritederived rom pyrrhotite. It was not seenwith fresh pyrrhotite.At a depthof about300 m. on the Contacto ein, I observed 4cm. stringerwith cassiterite n both walls and franckeite,pyritederivedfrom pyrrhotite,and siderite n the middle. A polishedsectionshowedcassiteriteilling the spaces etween he franckeitecrystals. This may representcontemporaneousepositionoffranckeite nd cassiterite. At greaterdepths ranckcitewas notobserved.

Hiibnerite.-It occurs ear the surface n separate einletsofmoreremotemagmatic ffiliation. It possesseshe typicalbrowncolor and radiatingprismatic tructure. Excellentcrystals refound in druses. In the Contacto vein it is associated withcassiterite. In a specimen rom the Salvadora vein cassiteritecrystals ncrust iibnerite rystals,hat is, are younger han thehfibnerite.

3'iderite. Occurs paringly ith pyrrhotite.Pyrite.--Appearso playa subordinateoleasa hypogenein-eral. Occasionalt coats assiterite.Most of the pyritehasbeenderived rom pyrrhotite.Rarehypogeneineralsrechalcopyritendgalena.

3'upergeneMinerals.Since heoxidation onehas ongsince eenworkedout, wasunableo studyhemineralsn place utwasdependentn speci-mens romoldercollections. t theoutcrop f mostveinswere

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TIN ORES OF UNCIA-LLALLAGUA, BOLIVIA. :251lage quantitiesof cavernousimonite derived from pyrrhotite.Davy mentionsnative arsenicand realgar derived from arseno-pyrite. Greenockite s a yellow, pulverulent oatingon pyriteis said to have beenquite common,especially'n the San Ferminvein.

Concerningupergeneismuthminerals, 'observedismuthochre in cassiteritepseudomorphic fter bismuthinite. Amongthe specimens f bismuthinite rom the upper levelswere somewith a'weaker metallic ustre. On polished ectionst was seenthat hebroad rystalsf bismuthiniteereopeningp nto eavesalongcleavage lanesand were bent into ce crystaland featheryforms. They possessed darker color than the fresh bismu-thinitc and lower hardness. The mineral s an ox3/sulphidefbismuthand apparently denticalwith ka.relinite,Bi4 SOs (Fig.4). A separation f this mineral for analysis s impossible naccountof its intimate intergrowthwith bismuthinite. The min-eralmusthavebeen uiteabundantn theupper evels t LlallaguaWith a limitedsupply f oxygen, tannite ecomposedo chal-copyrite,chalcocite, ornitc and corellite. In the presence fmoreabundant xygen he latter mineralswere oxidized. Coppersulphatemigrated downward, eactedwith fresh sulphides ndgave rise to copper-richcementationzones with native copper.The tin content of the stannite in the form of metastannic acidappearsnot to have migrated,but to have beendepositedn situas cassiterite. In the specimens f stannite rom near the surfacethat were available o me, I could ind no supergene assiterite.Suchmay havebeenpresent, ut overlookedn mining. Koeber-lin 8 states hat large quantities f stannitewere oxidized n theSan Fermin' vein. He describes beneath the limonitic zone aIom. thick zone o.f rich cassiterite, marcasite, and chalcanthite.The latter, in the almostcomplete bsence f other coppermin-erals, must have originated rom the weatheringof stannite.The phosphorus ontent of the primary ores and the readysolubilityof the apatitegave rise to the formationof numerousand in places arge quantitiesof supergeneron and aluminum

s Koeberlln, Geologic Features o Bolivia's Tin-bearing Veins, Eng. and Min.]our.' Press,vol. sex, pp. 636-642, x926.

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252 FRIEDRICH ZtHLFELD.

Fro. 4- Bismuthinite (white) altering to karelinite (gray). Theblack is bismuth ochre and limonite. Salvadora vein. Polished section,X 235.Fro. 5. Pyrite pseudomorphic fter pyrrhotite with basal cleavage(white) and volframite (gray). San Fernfin vein. Polished section,

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TIN ORES OF UNCIA-LLALLAGUA, BOLIVIA. :253phosphates.n the San Fermin vein,400 m. beneathhe surface,I observed avernousmassesof wavellite that in places illed theentire width of the vein. Wavellite is the most common of thephosphatesnd occursvery frequentlyas a coating on quartzcrystals nd cassiterite nd as pseudomorphsfter large tabularmarcasiterystals. n the wavellite ruses re occasionallyheiron-aluminumphosphates escribedby Gordon, vauxite (lightblue), metavauxite, nd paravauxite both colorless). Vivianite,which is especially haracteristic f the Bolivian deposits,occursin beautiful ransparent rystalsboth in druses n massive assit-erite and togetherwith phosphate-allophanend pyrite in separateveins. The abundantoccurrenceof vivianite ceases t about 36om. below the surface. Severalof the phosphateshat I observedhave not yet been named,as, for example,a hydrous ron phos-phate that togetherwith vauxite forms white silky tufts. Onheating t turns brown.In open fractures hat cut the Forasteravein, I observed t adepth of 45 m. besideswavellite an abundance f chalcedony,nthe outer ayersof which minutequantitiesof sulphides re visiblein polished ections. They includepyrite, wurtzite;a white iso-tropicore of the smaltite amily, a light greenmineral ntergrownwith the white that is perhaps etrahedrite, nd an unknownanis-'tropic rose-coloredcopper mineral. These chalcedonydepositsmay have been formed by either ascendingor descending olu-tions.

Before eaving he consideration f the minerals, he pyrite andmarcasite derived from pyrrhotite should be mentioned. Theyare widespreadand abundant in nearly all the veins. Pseudo-morphsof pyrite after pyrrhotite are found in all the Boliviandeposits. Polishedsections how that the transformation beginsalong the boundariesof pyrrhotite with other minerals and ad-vances nto the pyrrhotite individuals o form a network of pyritelathsenclosingresh pyrrhotitegrains (Fig. 6). Even when thetransformation s complete, he basalparting of the pyrrhotite iswell preserved Fig. 5)- A subtractionof substance ppears oaccompanyhe change,because ountless ruses re left lined withminute pyrite crystals. To explain the chemistryof this trans-

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254 FRIEDRICH A HLFELD.formation is difficult. Either a subtraction of iron or an addi-tion of sulphurmay have occurred. A subtraction f iron (assulphateor carbonate, s an oxidation process onfined o the zoneof oxidation. In Llallagua almost all pyrrhotite' s transformedto pyrite,evenat depths o whichoxidationprocesseso not reach.Fresh pyrrhotiteoccursonly in small nests. Even at depthsof4oo m. or as much as 600 m. there is little or no pyrrhotite. Inother Bolivian deposits,however, as at Tasna, there is muchpyrrhotite at depth; there the transformation does not seem tohave extendeddeep. Similar observationsre at hand for otherdepositsrom otherpartsof the world. A subtraction f iron canhaveoccurred nly as a double ransformationperhapshroughthe formationof acid ron carbonate). Newly formedcarbonatesor hydroxides ught hen o occur n association ith the pyrite.But suchs not thecase. At Llallagua ven he readilyoxidizablebismuthinitewithin the pyrite pseudomorphss preserved n-altered. Another explanationmust thereforebe sought. Thechangemusthavebeenbroughtabout hrougha directadditionof sulphurmolecules. his could akeplace hrough he actionon the pyrrhotitef thermalwaters arrying -OS /cco.ding othe reaction: FeS q- H_oS---FeS-o - H2.It is reasonableo assume omesolution nd transportationfpyrrhotitet thesameime. The formation f pyrite rompyr-rhotitewouldnot henbea processestrictedo thezoneof oxida-tion but would 'be restricted to the action of thermal waters con-taininghydrogenulphide.The processequiresurtherelucida-tion.

Summaryof Hypo#eneMineralization.The following eneral tatementsanbe madeconcerninghedistribution nd quantityof the hypogene ineralsn the Llal-lagua-Llnciadeposits.. In largequantity:pyriteand marcasite erived rom pyr-rhotite.

2. In quantity: ourmaline, uartz,cassiterite.3- In appreciableuantity:bismuthinite, olframite, tannite.

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TIN ORES OF UNCI.4-LL.4LL.4GU.4, BOLIVI.4. :2554. Sparingly: arsenopyrite,native bismuth, pyrite, franckeite,hfibnerite,sphalerite?).5. Sporadically r in minutequantity: apatite,monazite,siderite,nacrite,chalcopyrite.

, ORIGIN OF THE ORES.

The sequence f minerals n the Uncia-Llallaguadepositcor-respondso the order hat hasbeenworkedout for the magmaticcycle from physical-chemical onsiderations. Three zones arerecognizedn ascendingrder,eachonecharacterizedy the pre-dominance f differentminerals. In the deepest ortions f thedeposit ourmaline s most abundant. It is not confined o theimmediatewall rock of the veins,but is widelyand irregularlydistributed hrough he countryrock. Higher up. it is morerestrictedn its distribution nd disappearsntirelynear the sur-face.The second one s characterizedy an abundancef heavymetals. Cassiteriteppearsirst. It is found n smallquantityas a constituentf thegreisenn the deepest ineworkingsinthe SanFerminveinat 775 m. below he surface). The lowerboundary f tin ore depositionn the Uncia sectioniesbetween55o m. and 6oo m. below he surface;n the Llallagua ection,which s nearer he periphery f the intrusion,t lies deeper.Arsenopyrites the oldestof' the sulphides.The formationoftourmaline eased hen he depositionf cassiterite as nitiated.It occursn thewallrockof thetin veins s an olderdeposition,but notwithin hecompactassiterite assesharacteristicf thesecondone. As a consequencef rapidlyalling emperature,cassiteriteasdepositederysuddenly. t occursn theformof

shootsbolsones)f unexampledichnessut of very imitedverticalxtent.These hootsre ound t greater epthsear heperipheryf thestock n heSanFermin ein, sa consequenceof themore apid oolingnderhe nfluencef theproximityof the ntrudedock, han n thecenter,s for examplen theSalvadoraein,whereheyextend loseo thepresenturface.Togetherithcassiteriteere epositedismuthinite,olframite,apatite,ndpyrrhotite.Quartz ontinuedobedepositedn con-

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256 FRIEDRICH AHLFELD.siderablequantities. The nature of the mineralizationsuggestsvery concentrated cid solutionswith strong gas phaseand tem-perature of 400 to 500. The wall rock of the ore shootscon-tains tourmalinebut has also been ntensivelysericitized.The third zone, arthest emoved rom the magma, s character-izedby ess assiteritend hepredominancef sulphidesridsul-phosaltsof tin, lead, copper, antimony, and zinc. These arelacking in the lower parts of the mine at depthsbelow 45 m.,and increase n abundanceupwards. Stannite appears irst andeven occurs in the rich cassiterite shoots. Then follow in orderfranckeite,hiibnerite,sphalerite, iderite,all mineralsof the hy-drothermal sequence. Their investigation s made difficult be-cause he uppermost art of the deposit asbeen eached y oxida-tion processes.Furthermore, he upper parts of the mine areworked out and no longeraccessible.Koeberlin'sstatement hat anumberof the most mporfantveinsdo not extend o the surface sperhaps o.be explainedby the fact that in their uppermost artsthey did not carry cassiterite ut easilydestroyed ulphides ndsulphosaltsnd that only limonite remains.The transformation of pyrrhotite into pyrite and marcasitedoubtless ccurred t the closeof the periodof mineralization.

CONCLUSIONS.

The formation of the Uncia-Llallagua deposit s consideredohave been a single continuousprocess. Differences n mineralcontentare not regardedas due to depositionby different solu-tions ascending t different times, but by one and the samesolu-tion which changedts compositionhrough fractionaldistillationand thereby gave rise to various primary differences n depth.This view can not be proven absolutely, ut is probable rom thestandpointof our understanding f the physical-chemicalrin-ciplesof ore deposition.Let us summarizeoncemore the resultsof the precedingcon-siderations. High tensionsolutions ich in volatile constituentspenetrated he top of a small stock, he tuff-like quartz porphyryof which was ideally suited for the deposition f ores. Mineral-

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