Baron Von Groddeck, Chief Mining Councillor

of the Hartz Mining Districts and Director

of the Royal Prussian Academy of mines at Clausthal,

Germany.

 Translated by G. Thureau, F.G.S., Govornment 

Geologist and Inspector of Mines, from the Special Imprint of

the Journal of the German Geological Society of 1884.

 The Royal Academy of Mines at Clausthal was some

time ago placed in possession of a very fine collection of

Australian ores. That collection was presented to our

Academy by M. Wagenknecht, of Aachen (a Fellow of the

Royal  Society of Tasmania).

Amongst the samples were found a number of specimens.

of Tin Ore, together with the rocks and the minerals said to

be associated with same, from Mt. Bischoff, Tasmania. The

series interested me, particularly on account of a piece of

supposed Quartz-Porphyry, which rock, it was represented,

is associated (according to the description of S. H. Wintle*

and Geo. H. P. Ulrich) with those Tin Ores, and also because

of some peculiar, dense, greyish-blue coloured masses

of mineral  which most frequently are found to enclose

those  Tin Ores.

* S. H. Wintle ;  Stanniferous Deposits of Tasmania. Trans. Royal

Society of New South Wales, 1875, vol. ix., page 87.

 Geo. H. F. Ulrich. Written communication ; New York Book for

Mineralogy, etc., 1877, page 494

Thureau, G

Von Groddeck, (Baron)

University of Tasmania Open Access Repository

388liEMAEKS ON TIN ORB DEPOSITS AT MODNTBISCHOFF, TASMANIA.By Baron Von Gtroddeck, Chief Mtning CouncillorOF THE Hartz Mining Districts and DirectorOF THE EOTAL PRUSSIAN AcADEMY OF MiNES AT CeAUS-THAL, GtERMANY.Translated by Gr. Thureau, F.Gr.S., from the Special Imprint-of tJie Journal of the German Geological Society of 1884.Read September 7th, 1885.The Royal Academy of Mines at Claustlial was sometime ago placed in possession of a very fine collection ofAustralian ores. That collection was presented to ourAcademy by M. Wagenknecht, of Aachen (a Fellow of the!Royal Society of Tasmania).Amongst the samples were found a number of specimens.of Tin Ore, together with the rocks and the minerals said to"be associated with same, from Mt. Bischoff, Tasmania. Theseries interested me, particularly on account of a piece ofsupposed Quartz-Porphyry, which rock, it was represented,is associated (according to the description of S. H. Wintle*and Geo. H. P. Uirichf) with those Tin Ores, and also becauseof some peculiar, dense, greyish-blue coloured massesof mineral^ which most frequently are found to enclosethose Tin Ores.The chemical and microscopical tests gave the unexpectedand interesting result, proving this Quartz-Porphyry to be aland of Topaz rock of porj^hyritic structure, the white orlight-coloured portions of same consisting of dense Topaz,whilst the greyish-blue were j)rincipally formed of denseTourmalines.As this appears to be—so far as I am aware—a totallyunknown occurrence of Topazes with Tourmalines carryingTin Ores, a description of same may be, under the circum-stances, justified. As to the value of this discovery, from ageological point of view, I am not able to form an opinion,and such can be only ascertained j^roperly after a careful* S. H. Wintle ; Stanniferous Deposits of Tasmania. Trans. EoyalSociety of New South Wales, 1875, vol. ix., page 87.+ Geo. H. F. Ulrich. Written communication ; New York Book forMineralogy, etc., 1877, page 494.J These dense masses of minerals at ftrst led to the belief, from thepeculiar strucbure of same, that they were the results of transmuted Quartz-Porphyries. An analogy certainly exists in the well-known transmutationof Granites, carrying Tin Ores, into " Greisen " or a felspar—less rock.These possibilities gave rise to the now detailed reseai-ches ; and this paperwas read by me at a meeting of the German Geological Society at Hanoverin September, 1884. At that time I was not aware of the real character ofthe supposed Quartz-Porphyry. The special examinations of same wereinitiated in October of the same year.—V.G,389examination of tlie several features observable in the respectivelocalities. To initiate and induce sucb examinations the fol-lowing may deserve attention :—1. PoBPHYRiTic Topaz Eock fkom Mount Bischoff.This resembles, to a considerable degree, the Quartz-Por-phyries, vsrhether such is examined by the naked or unarmedeye or by means of powerful lenses. It consists of a lightgrey-coloured, dense, hornstone-like base, in which numeroustransj)arent Quartz-crystals can be observed up to 3 mm. insize. One imagines, likewise, of being able to distinguish, inthe white coloured cross fractures of this rock, crystals offelspar. It carries also Iron Pyrites in both large and smallcrystal or in crystalline aggregations.A preliminary chemical examination showed that, after theremoval of the metalliferous ores, the pulverised rock wouldcompletely fuse with soda, but that hydro-chloric acid hadno effect except the deposition of a white precipitate, composedof Silicious Acid and Alumina. Distinct Fluor reactions wereoljserved with traces of Lime and Magnesia, but alkalis werenot present.The special chemical analysis of this sample of rock, freedfrom iron pyrites through nitric acid, as carried out by Dr.Sommerlad in the lioyai Academy's Laboratories, gave thefollowing results :—Silicious Acid 76-68LimeFluorAluminaMagnesiaPhosphoric Acid1196-4819-99TracesTracesTotal 104-34Specific Gravity 3-014Two separate tests were made, according to Rose's method,,for ascertaining the percentage of Fluor. The high percentageof Fluor, and the total absence of alkalis, are therefore con-vincing proofs of this rock not belonging to the Quartz-Porphyries, and that it can now be stated that it consists of,in round numbers, 35 per cent, of Topaz and 65 per cent, ofQuartz. To what form the 1*19 per cent, of Lime in thisrock can be assigned is not quite clear ; but it is quite pos-sible that some very minute crystals of " Titanite "—to bedescribed lower down—contain this Lime. In the samemanner, it is doubtful also how the Phosphoric Acid occui's,except with " Apatite." The solution which was obtained bytreating the ores with Nitric Acid in order to secure theirextraction was found to contain a considerable percentage ofG390Lime, owiug to some Calcites which the microscope had pre-viously discovered.On placing the pulverised rock into a " Thoulet's " solutionof 3'202 specific gravity, the ore was gradually precij)itated,and on analysis of that precipitate, the pi'esence of IronPyrites was detected with slight traces of Antimony, Copper,and Zinc. The chemical and microscopical examinationswhich followed established the composition of the rock asfollows :—Quartz, Topaz, Iron Pyrites, Calcite, Titanites, andApatite (?) of which the first two form the leading, and theremainder the less important constituents. The base of theTopaz, as ascertained by analysis, appears, in the microscopicslides under their proper magnifier, as an aggregation ofcolourless and irregularly formed crystalline grains not above0'02 min. measurement. Every now and then—especially inthe vicinity of the crystals of iron pyrites, the basic Topazpartakes of a more fibrous texture, gaining thereby anappearance which very closely assimilates with the denserwhitish Topaz referred to below. The very small crystals,already alluded to, as having been observed in the fracturesof the rock itself, sometimes enclose such fibrous Topaz, andthey are frequently coated by a whitish mineral, which onaccount of its granular composition renders the inner orenclosed crystals necessarily less clear, but opaque. It hasbeen impossible for me, at present, to classify these smallcrystals under any distinct system. It is likewise not possibleto declare same as " pseudo-morpMc " as there are no groundsfor such an assumption. On the other hand, they may beaccepted as imperfectly formed, minimised crystals of Topaz—microscopic Pycnite (?)—as composed of very minute fibrouscrystals, and that their want of clearness is due to those in-crustations. It remains now only to be observed, that withinthose minute crystals, grains or specks occur, which exhibitvividly coloured polarising colours (also of Topaz) as well asirregularly formed particles of iron pyrites.The Quartz occurs in the basic mass of the rock in the formof crystalline grains or aggregations from 0'06 to severalmins. in measurement. To judge from the very regularly-developed form—six sided—of some of these crystals, andtheir behaviour, when crossing each other's positions, the well-known Dihexadron would most likely be their form underthat system, if accomj)anied by the narrow column sideswhich are so frequently observed when embedded in Quartz-Porj)hyries. On magnifying these transparent and polarisingorystalline aggregations 400 times their original size, theyexhibit frequently those well-known vesicular openingsfilled with fluids. Fluids, because, they do not appearto consist of carbonic acid, as is sometimes the case,391on account of tlieir immobility and I have not, so far,been able to observe any really distinct changes in theirposition. Various granular and crystalline-foliated mineralscould not likewise be defined to satisfaction. Very fine,needle-like crystals occasionally occur, and they may, inall i^robability, hereafter be recognised as Apatite, whichwould explain the presence of phosphoric acid as shownby the analysis, which acid has not been found in any otherform in the rock. The aggregations of iron-pyrites exhibitin the thicker slides under the microscoi^e some very interest-ing features, as one is enabled to recognise cubes from 0*05to 0.15 min. in length, edgewise. These crystals are some-times enclosed for over half tbeir length in quartz crystals,the remaining half being embedded in the basic Topaz.Other very peculiar features are also observable with somevery long fibrous forms, measui'ing from to O'l mm. inwidth by 0.3 to 0.7 m.m. in length, also partly or rdiollyencrusted by a very thin coating of Calcite. These arevery probably lengthened cubes, w^iich have, by some occultmeans or another, developed in one direction more than inanother. These cubes are in-egularly formed crystals of ironpyrites. Besides these, there also occur rectangular, nearlycubical, but otherwise irregular crystals of ii'on pyrites,measuring from 0"35 to 2 mm. in size ; impregnated as theyare sometimes by crystals of quartz and Calcites ; these latterpredominate sometimes to such a degree over those sulphurets,which however retain their crystalline form, as to make samequite subordinate so far as proportions or percentage areconcerned.The Calcites occupy various forms and positions withinthese iron pyrites, and they are confined sometimes bypeculiar minute botryoidal and oblong crystals, brownishred in colour, which, from their general appearance, theirdistinct pleochroism, and the absence of the former fibrousforms, and being accompanied by a very vivid rainbow-likepolarism, cannot be mistaken for any other but " Titauite."The Calcites of this rock, so easily distinguished by theirvivid iridescent properties, occur most frequently in the foi-mof impregnations of the iron sulphides, also in distinct, butsmaller particles. These crystals of quartz and iron pyritesare evidently of a remoter origin than the basic mass of Topaz.Could it be assumed, for instance, that the latter consisted,whilst in the course of formation, or partook of the gelatinouscompound ?2. Dense White Topaz of Mt. Bischoff.This rock exhibits in its peculiar uneven and splinteryfractures a trace crystalline.392When heated, in a solution of Cobalt, it turns blue, andgives distinct reactions witb Fluor, After roasting, the factof Hydrofluoric, or Muriatic acids, having no effect is adistinguishing feature.Dr. Sommerlad's analysis gave the following results :—Silicious Acid ... ... ... .. 33"34Aluminous Acid 37-02Lime 0-83Fluor 17-64Total 108-73Specific Gravity... 3-456The alterations from the normal composition of Topaz(5 Al Si 05 + Al Si FIIO) are very insignificant, indeed, andthey may be attributed to these impurities, the existence ofwhich have been demonstrated when under the microscope,but the percentage of Lime is very remarkable as such cannotbelong to the Topaz proper, but is due to other mineralsinterspersed through the rock. The " slides " of this densewhite Topaz exhibit an aggregate of irregularly depositedstreaky or oblong groups of needle-like prisms from O'Ol to0-05 min. in width, to 0-04 to 0"3 min. in length, showing also,across their diameter vivid rainbow-like colours, which, how-ever, disappear entirely with their longer axis. Only veryrarely—as quite natural—are the rhombic cross-sectionsobservable in the directions just alluded to. The terminalapices of these prisms resemble those, with crystallized Topaz,customary pyramidal and domatic points.*To judge from the specimens now before me, dense Topaz,intermixed with dense greyish blue Tourmaline, can easily bediscerned with the naked eye, though the latter occurs informs unlike its usual character, maintaining, however, theordinary fibrous forms.Tourmaline is seen distinctly in the " slides," in the formof botryoidal nests and as filling cavities of the Topaz rock.3. TOUBMALINE FEOM Mt. BiSCHOFF.This occurs also in the Carbonates of Iron as needlesmeasuring 0-26 mm. thick, to 1 m. in length, from darkgreen to blackish colours. " Slides " exhibit the same like-wise, as a dirty violet nucleus, which is enveloped by agreenish coating. Their pleochroism is very strongly marked,and the colouring partakes of lighter and sti'onger tints,according to the manner and position in which the crystals* G. Tom Kath has already described Topaz crystals from the WaratahMine, Mount Bischoff, as from | to J min. in size. These crystals formedfine crystalline aggregations in which crystals of Cassiterite -were embedded.Also the occurrence of nests of radiating crystals of Pycnlte. Bonn, 1879.393are being held by the observer. These proportionally largerci'ystals of Tourmaline are gradually developed from agranular and indistinctly radiated mass which appears underthe microscope as finely radiated or irregularly fibrous. Themineral in question fuses, under the blow-pipe, with butlittle effect at the edges and it gives distinct reactions ofBoron, and very little of Fluorine.*Dr. Sommerlad has undoubtedly proved, from decimalanalysis, that these light greyish blue masses of minerals areTourmalines.Silicious Acid ... 36-86Aluminous Acid 3672Boracic Acid 10-56Peroxyde of Iron ... ... ... ... 5*66Peroxyde of Manganese ... ... ... 0-66Lime 0-34Magnesia 3-92Kali (Potassium)... ... ... ...1-11Natrium .. 3-57Water 116Fluor 0-61Total 101-17Specific Gravity ... 3-042.Closing Remarks.According to the descriptions given by Messrs. Wintle andGeo. H. F. TJlrich of Mt. Bischoff, it would appear that, onthe top of that mountain, there occurs a bold stockwerk-likemass of porphyry which has penetrated a non-fossiliferous, andvery probably, very old formation of slate, sandstone andquartzite. These latter exhibit, in the immediate neighbour-hood, or, within the contact planes of the porphyries, greatirregularities and considerable contortions in the bedding ofthe strata.Wintle speaks of tinlodes in this porphyry ; according toUlrich, however, the tin ores occur as impregnations of theporphyry only, and principally so within the planes ofcontact with the schists. Ulrich reports also that theWaratah Company at the south-eastern flanks of the mountainis engaged in the exploitation of a lode (?) of Quartz-Porphyrycarrying tin ore, which intersects independently the adjacentschistose (blue) formation.It is quite evident, from these descriptions, that the* These coarse needle-like aggregations of Tourmaline from Mt.Bischoff have that peculiar appearance, and remind strongly of those of^euxite.394Quartz-Porpliylies at Mt. Bischoff must be considered the" carriers " of tin ores.The specimen, I have been presented with, and which Ikave described as closely resembling Quartz-Porphyry, hadtherefoi'e to be recognised as representing the occurrence oftin ore at that mountain, and therefore as occupying a veryimportant position.If, however, it has now been unmistakably proved that thatspecimen is actually not Quartz-Porphyry, but, a porpliyriticTopaz BocJc, then the question arises, whether any such Quartz-PorjDhyry.occurs at Mt. Bischoff at all, and whether thewhole formation, assumed to be such rock, is very probably aTopaz Eiock, and in what manner, and under what conditionsthe latter occurs contiguously to the real Quartz-Porphyry.The Topaz Eock, as described, is certainly a very remarkableformation in its character, and it is to my knowledgeexceptional in its kind. I have named it Topaz Rock, becauseof its comj)osition—like the well-known so-called TopazE-ock, near Auerbach, in Saxony—of Quartz and Topaz,though there it exhibits not the slightest resemblance in itsmode of occurrence to that of Mt. Bischoff.A probably not unimportant fact, which tends to connectboth of these occurrences, otherwise so widely apart, shouldnot, at this stage, be omitted to be mentioned. M. Breith-aujet,* it should be borne in mind, has already proved thatthe Topaz Eock of Auerbach forms, at the surface, a hugewell-like mass, which in itself contains or forms an immenselode of Tin Ore.Topaz, it is well known, is a very frequently and widelydisseminated mineral in close conjunction with Tin Oredeposits ; but, to our knowledge, such an extraordinarilydeveloped formation as that at Mt. Bischoff in Tasmania isnot known to exist elsewhere. The occurrence of Tourmalinethere is also very different from any other known in con-nection with Tin Ore deposits. These two minerals—viz..Topaz and Tourhialine (which are, however, not even men-tioned in Messrs. Wintle and Ulrich's descriptions)—in theirdense forms apjDcar to play a very important and prominentpart in the celebrated Tasmanian Tin Ore deposit. It mightprove, in all probability, very interesting and valuable toascertain whether or not such dense—and therefore easilypassed over—masses of Tourmaline occur also Avith other TinOre deposits.A. Von Geoddeck.* New Year Book for Mineralogy, 1885, page 788.

Remarks on tin ore deposits at Mount

Bischoff, Tasmania

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Remarks on tin ore deposits at Mount Bischoff, Tasmania

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