8/12/2019 Herzenberg-1936

1/6

COLLOIDAL TIN ORE DEPOSITS.

ROBERT HERZENBERG.

PEGM^TITIC pneumatolytic processes nd also high' temperaturehydrothermal processes annot entirely account or the featuresobserved n some of the tin deposits f Bolivia, especially n thedistricts of Oruro and Potosi. Such processes o not accountfor the fact that the cassiterite as formed at very low tempera-tures, that' it was formed at every stage of the ore deposition,and that in places t was the latest mineral formed, even ater thankaolin, aIunite, and siderite. Neither do they explain he collo-form structure of wood tin nor the extremely ine grains of tinminerals, which makes the beneficiation of Bolivian tin ores sucha difficult ask. For example, ailings rom in ores ontain pto 2 per cent of tin and extraction rom low grade ores is only

about 20 per cent. Further, such processes o not explain heextreme rregularity of the distribution of the ore shoots and thevariability f the tin content, ince long with rich parts guias)low grade ores are also found. Finally such processes o notaccount for the local enrichments of tin within the mines.

The basis f the pfieumatolytic heory was he classical xperi-ments of Daubrde, 2 who showed that tin fluoride reacted withsteam, yielding crystals of cassiterite, and that the liberatedhydrofluoric cid resulted n the formation of fluorine-bearing

minerals uch as fluorite, opaz, and apatite. The scarcity fthese minerals n the tin deposits f Oruro and Potosi has ledto the belief hat in Bolivia he pneumatolytic ction wag accom-panied by boron and not by fluorine because f the commonnessof tourmaline. However, he theory of boron pneumatolyticaction acks experimental vidence. Neither the borite of tin nor

x One tin mine in Negro Pabellon near Oruro has a chalky-like ore containingabout 5o per cent tin, but every attempt to concentrate t has failed.

2 Daubre, A.: Annal. d. Mines, 4th ser., vol. x6, p. 29, x849.

76x

8/12/2019 Herzenberg-1936

2/6

762 ROBERT HERZENBERG.

its reactions are known. It is assumed that the borite of tin

would be a carbide-like ubstance--hard ith a high fusing pointand only slightly volatile. No boron-bearing in minerals havebeen eported rom Bolivia, also there is no established unctionalconnection between the tin deposits and the tourmalinisation.Tourmaline rocks occur without tin, and tin without tourmaline.The coincidence ust be only casual.

Ahlfeld a supposes hat deep-seated ydrothermal olutions avetransported he cassiterite pward:

Through deep fractures formed after the intrusion of batholiths and

of near-surface masses, he solutions accumulated under high pressurewere suddenly ejected to near the surface, and there, under rapidly de-creasing emperature nd pressure, nstantly yielded heir metallic content.Therefore, in had no opportunity o precipitate completely rom the solu-tions at high temperatures ear the batholiths, but remained n part insolutions n labile equilibrium. The components hat originally existedas ions in solution went through a phase of sols and precipitated asmixed colloids which later unmixed and crystallized.

Ahlfeld's conception s not quite clear to me. If the solutionsinstantly yielded their metallic content, what is there to remainin solution n labile equilibrium? The zonal structure of mostcassiterite rystals shows hat they were not formed instantane-ously but developed y slow growth under changing onditions.

The experiments f Thugutt 4 in connection ith the possi-bility of the transportation f cassiterite n hot solutions re in-teresting. He dissolved assiterite n distilled water in platinumcontainers at 2 to 25 C. in oo hours. The solution con-tained only 0.00026 per cent SnO2. Quartz treated under thesame conditions ielded 0.062 per cent SiO2 or 240 times morethan cassiterite. His most interesting esult s that the resultingsolution s typically colloidal. It contained o ions and evapora-tion of the solution n a water bath yielded a tin oxide gel con-taining 62 per cent water. Consequently, ot solutions romdepth can contain only a trifling amount of tin oxide, which uponinstant precipitation an yield only a coagulated el of stannicacid.

a Ahtfeld, Friedrich: The Bolivian tin belt. EcoN. GEo,,., vol. 3 , p. 71-73, I936,4 Thugutt, S. J.: Archiwum Mineralogiczne, Warszawa, vol. 8, p. 8, I93.

8/12/2019 Herzenberg-1936

3/6

8/12/2019 Herzenberg-1936

4/6

764 ROBERT HERZENBERG.

during his experiments arried out under different conditions.

These experiments ere repeated y St. Claire Deville but he didnot discover any orthorhombic rystals, and this form of tindioxide was therefore considered y Groth 6 to be dubious. Ifthe I-I202 solution s added drop by drop to the tin sulphide re-cipitate ntil all the sulphide s transformed nto sulphate, r untilthe solution becomes ompletely lear, he gel of colloidal stannicacid will coagulate apidly. It may be noted that in I86I St.Claire Deville 7 obtained tetragonal crystals of tin oxide, alsotwins, out of metastannic acid.

CONCLUSIONS.

These preliminary experiments, hich still leave much roomfor investigation, uggest mode of formation of colloidal inore deposits. First consideration ust be given to the state nwhich the tin was transported rom depth. This could have beenonly in alkaline solutions which in depth are sulphidized, ivingrise to solutions f alkali-sulphides hich in turn readily dissolve

cassiterite nd tin sulphide. Consequently in can be transportedin relatively concentrated olutions t relatively ow temperaturesin the form of sulpho-stannates, nd the most probable one, aswill be shown below, s potassium ulpho-stannate. Such sulpho-stannate olutions, n rising upward would circulate along fissures.Whether hey precipitate irectly ulpho-stannate inerals uchas teallite, franckeite, or cylindrite, or whether these minerals areformed by metasomatic rocesses y the action of sulpho-stannatesolutions pon galena and lead-antimony res s not known, butit does not affect this case. These problems ffer an interestingfield of investigation n experimental ineralogy.

The circulating ulpho-stannate olutions ay encounter cidwaters which neutralize hem, causing precipitation f tin sul-phide. Possibly he kolbeckite f the Maria-Teresa inehathis origin.

6 Groth, P.: Chem. Krystall., vol. x, p. 96, x9o6.7 Deville, St. Claire H.: Cornpt. Rend., vol. 53, P. 6, 86.

8/12/2019 Herzenberg-1936

5/6

COLLOIDAL TIN ORE DEPOSITS. 765

The precipitated or.transformed sulpho-stannate inerals, in

sulphides, r the original sulpho-stannate olutions ill eventuallyencounter descending oxygen-bearing atmospheric waters. Asindicated y the experiments ited, he resulting eaction s alwaysthe same .in that the tin is transformed into a colloidal solutionor a colloidal form of stannic acid. This solution in low con-

centration s present as sols. Along fissures t may give rise tothe formation of various sizes of cassiterite crystals, of crusts,or of stalactites, but in most cases he colloidal stannic acid willnot remain n solution, but will coagulate. Where larger massesof stannic acid gel occur, they will crystallize n the form ofwood-tin. For the most part, the coagulated material will de-scend s a turbid liquid. The rock will act as a filter and everypore and every small racture will become illed with the coagula-tions. As has been mentioned, he deposited r filtered stannicacid gel rapidly and easily changes nto cassiterite. Only thelarger accumulations f gel preserve he wood-tin structure hatdenotes heir colloidal origin.

Gels of iron, zinc, and antimony are also in part precipitated.The iron is precipitated as colloidal FeS, which later is trans-formed into pyrite or marcasite. Zinc gel is transformed ntowurtzite or blende, and antimony gel into metastibnite.

A common associate of colloidal stannic acid in the other de-

posits s alunite and in places, arosite. Ramdohr 8 says, Alu-nite is a product of disintegration f trachytic and similar rocksby mutual action of ascending olutions of alkali sulphides withoxidizing waters of the surface. The oxidation by means of

which the various forms of tin sulphide were changed rom theunstable ulphate of tin into the colloidal stannic acid also trans-formed he potassium ulphide f the assumed otassium ulpho-stannate nto potassium ulphate. This with the aluminum sul-phate of the rocks formed alunite, and with the iron sulphateformed jarosite. Alunite is difficult to observe n the surfacedeposits nd n many places as been overlooked, eing described

s Klockmann-Ramdohr: Lenrb. d. Mineralogie, I936, p. 447.

8/12/2019 Herzenberg-1936

6/6

766 ROBERT HERZENBERG.

as steatite or hornfels where admixed with the ayatullu (dead-

bone) of Oruro, which is pure alunite.The theory of colloidal origin of some of the tin ore deposits

of Bolivia offers the best explanation or the observations madein the studies of the ore deposits uring the development f themines. Possibly urther investigation may prove the existenceof other colloidal in ore deposits n other parts of the world.

ORURO, 30I, VlA,October, I936.