Petrology, Geochemistry and Mineralogy of Greisens Associated with Tin-Tungsten Mineralisation: Hub Stock Deposit at Krásno–Horní Slavkov Ore District,

The greisens evolved in the apical part of the Hub stock, formed by weakly greisenised topaz granites, are predominantly represented by Li-mica-topaz and topaz-Li-mica grei-sens. These greisens, relative to weakly greisenised topaz granites, are enriched in Ca, F, Fe, Li, Si, Sn and W and depleted in Al, K, Mg, Na, Ti, Y, Zr and ΣREE. Weakly grei - senised topaz granites show convex tetrads in the normalised REE patterns. Compared to topaz granites, the greisens display lower ΣREE concentrations, partly higher nega - tive Eu anomaly, high Y/Ho and low Zr/Hf ratios. Li-micas occurring in greisens are represented by zinnwaldite. Chemical composition of cassiterite is near to ideal SnO 2 (>99 wt.% SnO 2 ). The wolframite is represented by manganoan ferberite.


Introduction
Greisenisation is one of the most significant wall rock alterations, which occurs in graniterelated tin-tungsten ore deposits [1][2][3][4][5][6][7][8][9].Historically, the term "greisen" has been used firstly by miners from the Krušné Hory/Erzgebirge Mts. to describe wall rocks consisting of quartz, mica and topaz surrounding the Sn-W mineralisation [10].This area hosts a number of Sn-W deposits (e.g., Cínovec/Zinnwald, Altenberg, Ehrenfriedersdorf, Krásno-Horní Slavkov) bound to greisenised stocks of the Variscan granitic bodies.These granites represent highly fractionated granites of the Krušné Hory/Erzgebirge batholith [11][12][13].Besides the Cornwall ore fields in England, these deposits were an important European source of tin from the Bronze Age until 1991.The largest Sn-W-Li-Nb-Ta ore deposit in the Krásno-Horní Slavkov ore district is part of the Hub and Schnöd granite stocks, where Sn-W mineralisation of the greisen type was exploited from the 1200s to 1991.The total historical production of the Krásno-Horní Slavkov ore district is estimated at 52,000 tons of tin [14,15].
This study was carried out to characterise petrological, mineralogical and geochemical features of the Hub stock greisens.The presented data provide a basis for definition of the alteration zoning and also basis for estimating the magnitude of mass transfer and the integrated fluid flux involved in the formation of the stock greisen system.

Geological setting
The Krásno-Horní Slavkov ore district comprises mineralised topaz granite stocks along the SE margin of the Krudum granite body in the Slavkovský les Mts.area (Figure 1).The Krudum granite body is part of the Western Erzgebirge pluton [16][17][18][19][20].The inner structure of the granite stocks in the Krásno-Horní Slavkov ore district is remarkably stratified, comprising greisens, weakly greisenised topaz alkali-feldspar granites, and layers of alkali-feldspar syenites (Figure 2).According to the most widely accepted genetic models, the topaz alkali-feldspar granite stocks represent the apical parts of highly fractionated granite bodies [12,18,19].Greisens with Sn-W mineralisation are developed predominantly in the upper part of the Hub stock (Figures 2 and 3).The greisens are represented according to modal classification [2] predominantly by Li-mica-topaz and topaz-Li-mica greisens.The Li-mica and quartz greisens are less abundant.Very rare topaz greisens occur as small lenses in topaz-Li-mica greisens.Greisens occur in two structural types [19,21].The first and dominant one (greisens I) forms irregular bodies and lenses of various sizes.The uppermost part of the Hub stock is filled by massive greisen body, alternating deeper in the stock centres with weakly or intensively altered topaz granites.The lenticular greisen bodies are locally oriented parallel to the elongation of the stock; however, their form is usually irregular without sharp contact with weakly altered granites (Figure 3).The second type (greisen II) forms fracture-controlled bodies, sometimes occurring together with quartz veins (Figure 4a).
The occurrence of Sn-W mineralisation is controlled by the contact between granites and gneissic country rock.The highest ore concentrations occur, with some exceptions, in greisens and less frequently in the highly greisenised and argillitised topaz granites.The Sn-W mineralisation can be subdivided into (1) disseminated-type mineralisation, (2) ore domains, and (3) quartz veins.The disseminated mineralisation found in those greisens has typical content of 0.2-0.3wt.% Sn.The ore domains are globular or even irregular bodies with tens of centimetres in size, with a very high proportion of cassiterite (Figure 4b).Quartz, Li-mica, muscovite, and clay minerals (dickite, kaolinite, smectite and very rare cookeite and tosudite) are the accompanying minerals of these domains.Muscovite together with clay minerals forms a younger filling in vugs occurring in the ore domains [22,23].The NE-SW trending quartz veins are developed mainly in exocontact of the granite stock and less in greisen bodies, where they do not usually exceed 15 cm in thickness.In the uppermost part of the Hub stock occurs also quartz bodies with size more tens meters.Besides quartz, the quartz veins contain cassiterite and wolframite, sometimes Li-mica, apatite, and fluorite with highly variable amounts of Fe-Cu-As-Zn-Sn sulphides.The high enrichment in these sulphides especially in arsenopyrite and chalkopyrite occurs in the uppermost part of the Hub stock.

Samples and methods
The samples used in this study include archive material from underground mine workings and from underground boreholes performed in the area of the Hub stock.The studied greisens were sampled during the last exploration phase of this ore deposit (1987)(1988)(1989)(1990)(1991).Approximately, 80 greisen and 30 topaz granite samples were used for this study.Major elements and selected trace elements (Ba, Rb, Sr, Y, Zr, Nb, Ga, and Sn) were determined by X-ray fluorescence spectrometry using the Philips PW 1410 spectrometer at chemical laboratory of the DIAMO Ltd., Stráž pod Ralskem, Czech Republic.The FeO content was measured by titration, whereas the loss on ignition (LOI) was determined gravimetrically in the same laboratory.The F content was determined using an ion-selective electrode, also at chemical laboratory of the DIAMO Ltd.For selected 10 samples, the trace elements, inclusive REE, were determined by inductively coupled plasma mass spectrometry (ICP MS) using a Perkin Elmer Sciex ELAN 6100 ICP mass spectrometer at Activation Laboratories Ltd., Ancaster, Canada.The decomposition of the rock samples for ICP-MS analysis involved lithium metaborate/ tetraborate fusion.Since the analytical procedure for ICP MS involves lithium metaborate/ tetraborate flux fusion, the Li concentration was analysed separately by atomic absorption spectrometry on a Varian 220 spectrometer at the Analytical laboratory of the Institute of Rock Structure and Mechanics, v.v.i.(Academy of Sciences of the Czech Republic).The density of these selected samples was determined pycnometrically at the same laboratory.Approximately 55 quantitative electron-microprobe analyses of selected minerals (Li-mica, cassiterite, and wolframite) were performed using four representative polished thin sections of greisens.Minerals were analysed in polished thin sections to obtain information about mineral zoning in examined rocks.Back-scattered electron images (BSE) were acquired to study the compositional variation of individual mineral grains.The elemental abundances of Al, Ba, Ca, Cl, F, Fe, In, K, Mg, Mn, Na, Nb, P, Rb, Sc, Si, Sn, Ta, Ti, U, W, Y, and Zr were determined using a CAMECA SX 100 electron microprobe operated in wavelength-dispersive mode at the Institute of Geological Sciences, Masaryk University in Brno.The accelerating voltage and beam currents were 15 kV and 20 or 40 nA, respectively, with a beam diameter ranging from 1 to 5 μm.Peak count-time was 20 s and background time 10 s for major elements, whereas for trace elements, 40-60 s and 20-30 s, respectively, were used.The raw data were corrected using the PAP matrix corrections [24].The detection limits were approximately 450 ppm for Nb, 720 ppm for Ta, 600-700 ppm for U, 400-500 ppm for Y, and 600 ppm for Zr.The concentration of Li 2 O in analysed micas was estimated using the following empirical equation

Whole-rock chemistry
The weakly greisenised topaz alkali-feldspar granites are a highly peraluminous rocks with an aluminium saturation index (ASI = mol.%Al
Table 1.Representative whole-rock chemical analyses of weakly greisenised topaz granites and greisens from the hub stock, Krásno-Horní Slavkov ore district.

Contributions to Mineralization
The normalised REE patterns indicate the decreases in the LREE and HREE in greisens compared to weakly greisenised topaz granites and steep slopes from La to Sm but almost horizontal patterns between Gd and Lu.Both rock types show prominent negative europium anomaly.An additional feature that influences the distribution of the REEs in both rock types is the lanthanide tetrad effect.Quantification of the tetrad effect sizes was carried out using method proposed by Irber [27].Weakly greisenised topaz granites show convex tetrads in the normalised REE patterns (Table 2 and Figure 7).Normalising values according to Ref. [28].

Mass changes during greisenisation
The origin of greisens is essentially connected to the chemical losses and gains from the alteration of the parent weakly greisenised topaz granites.For the detailed investigation of losses and gains during greisenisation, the isocon method developed by Grant [29] was applied.The scattering of the elements in the isocon plot for selected samples of Li-micatopaz greisen suggests that the major and trace elements were mobile to variable extent (Figure 8).Origin of Li-mica-topaz greisen was connected with enrichment of Ca, F, Fe, Li, Mn, Si, Sn and W and loss of Al, Ba, K, Mg, Na, P, Rb Th, Y and Zr.Elements hosted by accessory zircon, monazite and xenotime (such as Zr, Th and Y) become partly depleted during greisenisation.The oxides as K 2 O and Na 2 O, together with Rb and Ba, were removed due to feldspar dissolution.

Mineral chemistry 6.1. Lithian mica
Lithian mica in greisens according classification of Tischendorf et al. [25] is represented by Fe-rich polylithionite (Figure 9 and Table 3).Grains of zinnwaldite are chemically homogeneous, without   internal zoning.In many places, they contain small zircon and rarely uraninite inclusions.Estimated lithium contents in analysed lithian micas from greisens reach up to 1.55 apfu (atoms per formula unit).Fluorine contents in these mica reach up to 1.40 apfu.

Sn-W minerals
Cassiterite occurs usually as subhedral to anhedral chemically homogenous grains in the BSE (backscatter electron) mode.Its chemical composition is near ideal SnO

Mineral and chemical changes during greisenisation
The greisenisation is a typical hydrothermal alteration accompanying the origin of raremetal granites and associated Sn-W-Li-Mo mineralisation [1,7,[30][31][32][33].Greisens of the Hub stock are formed in the uppermost part of a weakly greisenised topaz granite cupola.In a similar position occur greisen bodies in the Altenberg, Cínovec/Zinnwald, Sadisdorf and Ehrenfriedersdorf Sn-W ore deposits, representing the most significant Sn-W deposits in the Krušné Hory/Erzgebirge ore district [34].
In the Cínovec/Zinnwald granite cupola in the Eastern Erzgebirge greisen bodies consist of two structural types: (i) flat thin greisen zones and quartz veins with wall-rock greisens, (ii) irregular greisen bodies several tens of metres in size, following the morphology of the granite cupola contact [35].Similar greisen bodies, formed in the Nejdek-Eibenstock and Horní Blatná plutons in the Western Erzgebirge, show a pattern of wall-rock alteration, the intensity of which decreases outward.All these structural types of greisen bodies display varying mineralogical composition with predominance of quartz-, topaz-, Li-mica-topaz and Li-mica greisens in the German part of the Western Erzgebirge and with predominance of quartz-, topaz-Li-mica-and Li-mica greisens in the German part of the Eastern Erzgebirge [2].Therefore, the mineralogical compositions of greisens (I) found in the Hub stock are similar to those of the Western Erzgebirge.

Contributions to Mineralization
Chemical changes during greisenisation of topaz granites are strongly dependent on the prior state of the rock and the intensity of alteration.Silica in topaz greisens is lost, whereas in Li-mica-topaz and quartz greisens, SiO 2 is gained.Alumina is gained in topaz-Li-mica and in topaz greisens.Due to the decomposition of albite and alkali feldspars, there is a significant loss of Na and K.In contrast, additions of Li reflect the growth of Li-mica during greisenisation, especially Li-mica greisens.Similar gains and losses were found in selected greisens from the German part of the Krušné Hory/Erzgebirge Mts [2].
The REE plots indicate a similar course of REE patterns in weakly greisenised topaz granite and both greisen varieties, suggesting a partial dissolution of the refractory accessory minerals, but not their complete removal.An additional feature that influences the distribution of the REE in weakly greisenised topaz granites and greisens from the Hub stock is the occurrence of the convex lanthanide tetrad effect.Older studies of the tetrad effect in fractionated granites from the Smrčiny/Fichtelgebirge and Krušné Hory/Erzgebirge Mts. proposed that processes of fluid-melt interaction caused the development of a convex tetrad effect during crystallisation of the silicate melt [27,31].However, more recent studies [36,37] from the Zinnwald ore deposit in the Eastern Krušné Hory/Erzgebirge Mts.demonstrated that tetrad effect was developed prior to greisenisation in stage of sub-solidus albitisation of topaz granites.The lower ΣREE concentrations in greisens according to Ref. [37] suggested that the greisenisation resulted in remobilisation of the REEs.The depletion of the ΣREE concentrations in greisens from the Hub stock is accompanied by changes in the tetrad effect, Y/Ho, Eu/Eu* and Zr/Hf ratios (Figure 10).The partly increasing Y/Ho ratio in greisens could be explained by migration of fluorine-rich hydrothermal fluids [38,39].The more negative Eu anomaly in the greisens relative to weakly greisenised topaz granites is behaving as a divalent species during greisenisation, as predicted by Sverjensky [40] and Wood [41].Similar increases of the negative Eu anomaly in the greisens were found in the True Hill granite greisens of southwestern New Brunswick, Canada [7].The decreasing of Zr/Hf ratio in the greisens relative to weakly greisenised topaz granites is, according to Irber [27], affected by strong hydrothermal alteration.

Sn-W minerals
The Sn in cassiterite is usually substituted by Fe, Mn, Ta, Nb and W [42].The enrichment of Ta and Nb in cassiterite is significant for those occurring in pegmatites, granites, and hightemperature quartz veins [43][44][45].The cassiterite analyses from granites and pegmatites fall along a linear array with Nb + Ta and Fe + Mn-concentrations varying while mostly maintaining a ratio of 2:1 corresponding to the coupled substitution 3 Sn 4+ ⇔ 2(Nb, Ta) 5+ + (Fe, Mn) 2+ .This relationship is common to cassiterite from highly fractionated granites and pegmatites worldwide and different to those from hydrothermal cassiterite-quartz-vein deposits and greisen deposits [44].Although cassiterite is main ore mineral in greisens, data about Nb, Ta, and W concentrations are rare [46][47][48][49][50]. Cassiterites from greisens are usually distinctly depleted in Fe, Nb, and Ta in comparison to cassiterites from granites, pegmatites, and hightemperature quartz veins.This depletion is also significant for cassiterites from examined greisens.The depletion in Nb, Ta, and Fe could be explained by lower crystallisation temperature of greisen cassiterites [48].
The Nb and Ta are concentrated in wolframites from rare-metal granites and pegmatites, with predominance of Nb about Ta [64].Wolframites from Sn-W deposits in the German part of the Krušné Hory/Erzgebirge ore district, including wolframites from the Hub stock, are ferberites with variable Mn/Fe ratios [65][66][67].From quartz veins in the upper part of the Hub stock was also described the purest known (end-member) hübnerite.Hübnerite occurs in small cavities in these quartz veins [14].

Conclusion
The greisens of the Krásno-Horní Slavkov Ore District evolved in the apical part of the weakly greisenised topaz granite Hub stock and are represented by Li-mica-topaz, topaz-Limica, quartz, Li-mica and topaz greisens.These greisens occur in two structural types.The first and dominant one (greisen I) forms massive greisen bodies in the uppermost part of the Hub stock.The second type (greisen II) forms fracture-controlled bodies, sometimes occurring together with quartz veins.

Figure 2 .
Figure 2. Schematic cross section of the south-eastern part of the Krudum granite body (after [19], modified by author).

Figure 3 .
Figure 3. Schematic cross section of the hub stock (after [19], modified by author).

Figure 10 .
Figure 10.Evolution of tetrad effect, Y/Ho, Eu/Eu* and Zr/Hf ratios in the Li-mica-topaz greisens from the Hub stock.

Table 2 .
Distribution of REE in weakly greisenised topaz granites and greisens from the Hub stock.

Table 3 .
Representative microprobe analyses and crystalochemical formulae of lithium micas from greisens, Hub stock.

Table 4 .
Representative microprobe analyses and structural formulae of cassiterite from greisens of the Hub stock (wt.%).
The disseminated Sn-W mineralisation occurring in greisens has typical content of 0.2-0.3wt.% Sn.The predominately Li-mica-topaz and topaz-Li mica greisens relative to weakly greisenised topaz granites are enriched in Si, F, Fe, Li, Si, Sn and W and depleted in Al, K, Mg, Na, Ti, Th, Y, Zr and ΣREE.Weakly greisenised topaz granites show convex tetrads in the normalised REE patterns.The greisens display lower ΣREE concentrations, high negative Eu anomaly, high Y/Ho, and low Zr/Hf ratios relative to the topaz granites.Li-micas occurring in greisens are represented by zinnwaldite.The chemical composition of cassiterite is near ideal SnO 2 (>99 wt.% SnO 2 ).The wolframite is represented by manganoan ferberite with 70-76 mol.%FeWO 4 .