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DISCUSSION AND COMMUNICATIONS

THE TIN DEPOSITS OF LL./tLL./tGU./t, BOLIVI./t.Sir:--Dr. Turneaure's aper on the tin deposits f Llallaguais a valuable ontributiono the genesis f theseuniquedeposits.

I take the libertyof presentingmy opinion,baseduponmy largeexperience ith Bolivian in deposits,egarding ome oints n hispaper.I agreewith the author n the essential oints,but my micro-scopic xaminations f ores revealedsomedifferent results e-spectinghe paragenesisf minerals. (See Turneaure's ig. o,graphical epresentationf mineral sequence.) The formationof tourmalineand cassiterite verlaps lightly,as is shownby theoccurrenceof tourmaline needleson cassiterite crystals. Cas-siteriteand wolframitesimilarlyoverlap. The early formationof bismuthinite,before wolframite and cassiterite,mentionedbythe author, is unusual; however, I observedsimilar cases n otherBolivian deposits,e.7. Ce?ro de Potosi and Chorolque. At.Llallagua herealso s later bismuthinite eplacing tannite. Ar-senopyrite ccursalways with early sulphides s veinletsor crys-tals n pyrrhotiteor as massive ein fillings. Replacementf thepyrrhotiteby marcasite-pyrite as left residualsof arsenopyritein marcasite-pyrite.

I agreewith the author egardinghe replacementf pyrrhotiteby marcasite-pyriteuring the last stageof hypogenemineraliza-tion, and alsoas to the fact that this process xtends o all zones.It was produced hrough a kind of autohydration of the in-trusiveby residualaqueous olutions, robablyalkalic,containingx Turneaure, F, S.: The Tin 'Deposits of Llallagua, Bolivia. Ecoa. GlgoL., vol.

3o, pp. I4-6o, x7o-9o, 935.2 Ahlfeld, F.: Ueber Zinnkies. Neues Jahrb. Min. B.B. 68, Abt. A, x934, pl. 8,

Fig. 7.219

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220 DISCUSSION iND COMMUNIClTIONS.

much HaS. These affected principally the pyrrhotite, whosecleavage along oooI), invisible n fresh pyrrhotite,appeared ypartial solution. The spaces etween he pyrrhotite plates werefilled by franckcite,which to a great extent has beenreplaced ylater pyrite. According to my observations,he franckcite ofLlallagua belongs o the late phaseof replacementof pyrrho-rite. Turneaure'sFig. 2 doesnot prove hat franckcite s olderthan pyrrhotite and replacedby it. I have never seen resh pyr-rhotite with franckeke. Siderite likewise.was formed by re-placement f pyrrhotite,and many samples ontain nclusions ffranckcite in siderite of the same generation. The age of thefranckeke t Llallgua corresponds,herefore,with that offranckeke n other'Bolivian eposits.The latest hypogeneminerals were rhodochrosite, phaleritewith colloform texture, chalcopyrite, rystallizedmarcasiteandwavellite, all in small amounts.As to the zonal distributionof minerals, he author speaksofa high-grade assiterite ore and a low-gradesulphide hell.Basedupon physico-chemicalonditions,which below the richcassiteritezone must have been other than above it, a mineraliza-tion in the vicinity of the magma different from that near thesurfacemust be expected. This proved o be true? At the out-cropsof many of the veins, mineralsof a mesothermal nd epi-thermal type were found. I had at my disposala number ofspecimens,ollected y Mr. H. Block,managerof the Uncia minein i9o8 from the upper2o metersof the deposit. Among thesethereare manysamples f purestannite, f tefrahedrite,ranckekein compact ein fillings,wurtzite, alsomanganese-rich olframite(hiibnerite), which s so characteristicor.the epithermalstibniteveins n Bolivia? Gmehlingo gives the following description f

s Moritz, H.: Beltrag zur Kenntnis der Sulfostannate Boliviens, Tell II. NeuesJahrb. Min. B.B. 66, Abt. A, pp. 191-212 , 1933.

4Ahlfeld, F.: The Tin Ores of Uncia-Llallagua, Bolivia. EcoN. Gv.ot.., vol. 26,pp. 241-257, I93 I.

Ahlfeld, F..' Ueber die Verteilung des Wolframs in der bolivianischen Zinn~provinz. Chemie der Erde, vol. 7, PP. i21-129, I932.

6Stelzner, A.: Die Silber-Zinnerzlagerstitten Boliviens. Zeit. deutsch. geol.Ges., vol. 49, P. I29, 1897.

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DISCUSSION AND COMMUNIC.,'t TIONS. 221

the outcrops f the Uncia-Llallagua eposit: In tieferenRegionenimmtderGehalt n Zinnerzenmit wenigen usnah-men)mehrundmehr b,deran Sitbererzenu; in derRegel indletztereAntimonverbindungen.Certainlyn otherveinsmuchcassiteritend marcasite-pyrite,formedby the oxidation f pyrrhotite,werealready o be foundnear the surface.

As to the deepestone, he epi-andmesothermal ineralsmen-tionedabove re missing, lso he bismuthminerals. Apart fromscanty assiterite, uchmarcasite-pyrite,nd smallquantities ffranckeite, formed in the way describedabove, arsenopyrite,pyriteandblacksphalerite ccur, ll formedat high temperatures.The change f veinsrich in cassiterite bove, nto sulphidic einswith sphalerite nd pyrrhotite n depth, s well known n Bolivia.The occurrence f marcasite-pyritend franckeite n the deeperlevels s to be explained y the fact that the last phaseof min-eralization asequally ffected ll zones. As to the formationoftourmaline,heauthorsrightwhen esayshat he ourmaliniza-tion has affected the whole intrusive rock, but in the near-surfacezone there is no tourmaline,or scarcely ny, in the veins, and inthe deepestevelsmany of the veinsare strongly ourmalinized.I believe hat, according o the evidencegiven above, he threevertical zonesdescribed y myself from Llallagua are well estab-lished. I also found the same zonal distribution of minerals atAvicaya and at Chorolque,n depositswhich have a certainsimi-larity with thoseof Llallagua.The lateral zoning at Llallagua can be studiedvery well in theperipheral onesof the Salvadoraneck. Many veins n the sedi-mentsare partly associated ith pyrite, blacksphalerite nd smallquantitiesof cassiteriten the form of needle in, [s is charac-teristic n mesothermal nd epithermalBolivian deposits. Thus,in the southeast art of the Uncia mine and nearly one kilometerdistant from the contact,Mr. Oropezaworks two veins,one withpyrite, sphalerite nd needle in, the otherwith tetrahedrite.

FRIEDRICH AHLFELD.LA PAz, BOLIVIA,Dec. 6, 935.