Nb – Ta – Ti Oxides in Topaz Granites of the Geyersberg Granite Stock ( Erzgebirge Mts . , Germany )

Oxide minerals (Nb–Ta-rich rutile, columbite-group minerals and W-bearing ixiolite) represent the most common host for Nb, Ta and Ti in high-F, high-P2O5 Li-mica granites and related rocks from the Geyersberg granite stock in the Krušné Hory/Erzgebirge Mts. batholith. This body forms a pipe like granite stock composed of fineto middle-grained, porphyritic to equigranular high-F, high-P2O5 Li-mica granites, which contain up to 6 vol. % of topaz. Intrusive breccia’s on the NW margin of the granite stock are composed of mica schists and muscovite gneiss fragments enclosed in fine-grained aplitic and also topazand Li-mica-bearing granite. Columbite group minerals occur usually as euhedral to subhedral grains that display irregular or patched zoning. These minerals are represented by columbite-(Fe) with Mn/(Mn + Fe) ratio ranging from 0.07 to 0.15. The rare Fe-rich W-bearing ixiolite occurs as small needle-like crystals. The ixiolite is Fe-rich with relatively low Mn/(Mn + Fe) and Ta/(Ta + Nb) values (0.10–0.15 and 0.06–0.20, respectively). Owing to the high W content (19.8–34.9 wt. % WO3, 0.11–0.20 apfu), the sum of Nb + Ta in the ixiolite does not exceed 0.43 apfu. The Ti content is 1.7–5.7 wt. % TiO2 and Sn content is relatively low (0.3–4.1 wt. % SnO2).


Introduction
Niobium-and tantalum-bearing oxide minerals are common in topaz granites and related rocks (pegmatites, alkali-feldspar syenites).The most common Nb-Ta-bearing minerals in these parageneses are Nb-Ta-bearing rutile and columbite-group minerals.
In the Krušné Hory/Erzgebirge region, these minerals were described in the past by Johan and Johan [1], Rub et al. [2] and Breiter [3] from the Cínovec granite cupola, by Breiter et al. [4] from the Podlesí granite stock and by René and Škoda [5] from the Krásno-Horní Slavkov ore district in the Slavkovský Les Mts.In the German part of the Krušné Hory/Erzgebirge region, these minerals occur especially in the Sauberg, Vierung and Geyersberg granite stocks.The Sauberg and Vierung granite stocks are part of the Ehrenfriedersdorf Sn-W-Nb-Ta ore deposit.However, the Sn-W-Nb-Ta mineralisation from all of these granite stocks were only briefly described in the past, without a detailed description of the Nb-Ta mineralisation [6,7].The Ehrenfriedersdorf ore deposit has been closed since 1990 and Nb-Ta-Ti minerals can only be sampled from various mineralogical collections.The Geyersberg granite stock lies in the most significant area of old mining subsidence ("Pinge") which has recently been opened for geological excursions.Various fractionated granites were sampled during final operations before the closing down of the Sn-W mining in this area.This author also later studied the petrology, geochemistry and mineralogy of these granites.Thus, the aim of the presented paper is a detailed description of mineralised granites, including their mineralogy and the composition of Nb-Ta-Ti minerals.The composition of Nb-Ta-Ti minerals was further correlated with distinctly better described Nb-Ta-Ti minerals from the Czech part of the Krušné Hory/Erzgebirge region [1][2][3][4][5].The Krušné Hory/Erzgebirge batholith consists of three individual plutons: Western, Middle and Eastern, each representing an assembly of shallowly emplaced granite units about 6-10 km paleodepth, with a maximum preserved vertical thickness of the pluton 10-13 km below the present surface level [12,13].All these granites have been grouped in a number of ways in the past [14][15][16][17][18][19][20].Recently, the mostly used classification of these granites subdivides them into late collisional and The Krušné Hory/Erzgebirge batholith consists of three individual plutons: Western, Middle and Eastern, each representing an assembly of shallowly emplaced granite units about 6-10 km paleodepth, with a maximum preserved vertical thickness of the pluton 10-13 km below the present surface level [12,13].All these granites have been grouped in a number of ways in the past [14][15][16][17][18][19][20].Recently, the mostly used classification of these granites subdivides them into late collisional and post-collisional granites.The late collisional granites were divided into the following three groups: (1) low-F biotite granites, (2) low-F two-mica granites and (3) high-F, high-P 2 O 5 Li-mica granites.The post-collisional granites were divided into high-F, low-P 2 O 5 Li-mica granites and medium-F, low-P 2 O 5 biotite granites [8,20].
The Geyersberg granite stock is one of the shallow intrusive granite bodies in the middle section of the Krušné Hory/Erzgebirge batholith.These granite bodies (Vierung, Sauberg, Geyersberg, Ziegelberg and Greifenstein) are located near the old mining towns of Ehrenfriedersdorf and Geyer.The occurrence of granite stocks and ridge-shaped apical parts of the Middle Erzgebirge granite pluton is controlled by the NW-SE striking Geyer-Elterlein-Herold shear zone, which contains the Geyer-Schönfeld, Franzenhöhe Wilischthal and Wiesenbaden fault systems.The origin of these fault systems is contemporaneous with intrusions of microgranite, aplite and lamprophyre dykes.The generation of the subsurface morphology of the above-mentioned granite stocks was investigated by numerous exploration boreholes [6,7,21] and interpreted using regional gravity measurements [22] (Figure 2).post-collisional granites.The late collisional granites were divided into the following three groups: (1) low-F biotite granites, (2) low-F two-mica granites and (3) high-F, high-P2O5 Li-mica granites.The post-collisional granites were divided into high-F, low-P2O5 Li-mica granites and medium-F, low-P2O5 biotite granites [8,20].The Geyersberg granite stock is one of the shallow intrusive granite bodies in the middle section of the Krušné Hory/Erzgebirge batholith.These granite bodies (Vierung, Sauberg, Geyersberg, Ziegelberg and Greifenstein) are located near the old mining towns of Ehrenfriedersdorf and Geyer.The occurrence of granite stocks and ridge-shaped apical parts of the Middle Erzgebirge granite pluton is controlled by the NW-SE striking Geyer-Elterlein-Herold shear zone, which contains the Geyer-Schönfeld, Franzenhöhe Wilischthal and Wiesenbaden fault systems.The origin of these fault systems is contemporaneous with intrusions of microgranite, aplite and lamprophyre dykes.The generation of the subsurface morphology of the above-mentioned granite stocks was investigated by numerous exploration boreholes [6,7,21] and interpreted using regional gravity measurements [22] (Figure 2).The Geyersberg granite stock forms a pipe like granite stock composed of fine-to middlegrained, porphyritic to equigranular high-F, high P2O5 Li mica granites, containing 2-6 vol.% of topaz.Small bodies of intrusive breccias on its NW margin are composed of mica schists and muscovite gneiss fragments enclosed in fine-grained aplitic granite.All granite varieties are partly altered (greisenisation and albitisation), mainly along steeply dipping NW-SE and NE-SW trending The Geyersberg granite stock forms a pipe like granite stock composed of fine-to middle-grained, porphyritic to equigranular high-F, high P 2 O 5 Li mica granites, containing 2-6 vol.% of topaz.Small bodies of intrusive breccias on its NW margin are composed of mica schists and muscovite gneiss fragments enclosed in fine-grained aplitic granite.All granite varieties are partly altered (greisenisation and albitisation), mainly along steeply dipping NW-SE and NE-SW trending faults.In the western and eastern margins of the granite stock, small layers and lenses of coarse-grained marginal pegmatites (stockscheider) are present (Figure 3).faults.In the western and eastern margins of the granite stock, small layers and lenses of coarsegrained marginal pegmatites (stockscheider) are present (Figure 3).

Methods
Approximately 60 quantitative electron-microprobe analyses of Nb-Ta-Ti oxides were performed using five representative samples from the Geyersberg stock.Minerals were analysed in polished thin sections to obtain information about mineral zoning in the examined rocks.Backscattered electron images (BSE) were acquired to study the compositional variation of individual mineral grains.The abundances of Al, Bi, Ca, Fe, Mg, Mn, Nb, Sc, Si, Sn, Ta, Ti, U, W, Y and Zr were determined using a CAMECA SX 100 electron microprobe operated in a wavelength-dispersive mode at the Institute of Geological Sciences, Masaryk University in Brno, Czech Republic.The accelerating voltage and beam currents were 15 kV and 20 nA or 40 nA, respectively, with a beam diameter ranging from 1 μm to 5 μm.
Whole-rock composition analysis of three representative granite samples was performed (Table 1).Major elements were determined by X-ray fluorescence spectrometry using the PANalytical Axios

Methods
Approximately 60 quantitative electron-microprobe analyses of Nb-Ta-Ti oxides were performed using five representative samples from the Geyersberg stock.Minerals were analysed in polished thin sections to obtain information about mineral zoning in the examined rocks.Back-scattered electron images (BSE) were acquired to study the compositional variation of individual mineral grains.The abundances of Al, Bi, Ca, Fe, Mg, Mn, Nb, Sc, Si, Sn, Ta, Ti, U, W, Y and Zr were determined using a CAMECA SX 100 electron microprobe operated in a wavelength-dispersive mode at the Institute of Geological Sciences, Masaryk University in Brno, Czech Republic.The accelerating voltage and beam currents were 15 kV and 20 nA or 40 nA, respectively, with a beam diameter ranging from 1 µm to 5 µm.
Whole-rock composition analysis of three representative granite samples was performed (Table 1).Major elements were determined by X-ray fluorescence spectrometry using the PANalytical Axios Advanced spectrometer at Activation Laboratories Ltd., Ancaster, ON, Canada.Trace elements 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, ON, Canada.The decomposition of the rock samples for ICP-MS analysis involved lithium metaborate/tetraborate fusion.In-lab standards or certified reference materials were used for quality control.The detection limits were approximately 1-3 ppm for Ba, Rb, Sr and Zr, 0.01-0.2ppm for Nb, Ta, Th and U and 0.002-0.05ppm for REE.The concentration of Li 2 O in analysed Li-micas was estimated using the following empirical equation: [Li 2 O = (0.289 × SiO 2 ) − 9.658], recommended by Tischendorf et al. [24].

Columbite-Group Minerals
Columbite-group minerals occur in both varieties of high-F, high-P 2 O 5 Li-mica granites, mostly as euhedral to subhedral grains.Some grains display irregular or patched zoning (Figure 5).No systematic core-rim compositional evolution was observed.The columbite-group minerals are represented predominantly by columbite-(Fe) with a Mn/(Mn + Fe) ratio varying from 0.07 to 0.15 and with relatively low Ta/(Ta + Nb) values (0.06-0.41) (Table 3).
Minerals 2019, 9, x FOR PEER REVIEW 9 of 14 In this paper, the term ixiolite is used to describe minerals of columbite-like chemical composition and W content greater than 0.10 apfu (O = 2).However, structural data confirming the identity of this mineral are not available.The W-bearing ixiolite was observed as needle-like subhedral crystals occurring in fine-grained aplitic granite from intrusive breccias (Figure 6).The ixiolite is Fe-rich with relatively low Mn/(Mn + Fe) and Ta/(Ta + Nb) values (0.10-0.15 and 0.06-0.20,respectively).Owing to the high W content (19.8-34.9wt.% WO3, 0.11-0.20 apfu), the sum of Nb + Ta in the ixiolite does not exceed 0.43 apfu.The Ti content is 1.7-5.7 wt.% TiO2 and Sn content is relatively low (0.3-4.1 wt.% SnO2) (Table 4).

Discussion
The high-F, high-P 2 O 5 Li-mica granites of the Geyersberg granite stock are very similar to the same granite varieties found in the Podlesí granite stocks or in the area of the Krásno-Horní Slavkov ore deposit [4,5] in the Czech part of the Erzgebirge/Krušné Hory region.All these granites represent highly fractionated peraluminous granitic rocks which contain variable contents of topaz and lithium micas.
Niobium and tantalum display similar geochemical behaviour during fractionation in granitic melts.During these processes, the elements are typically incompatible and may thus reach higher concentrations in highly fractionated granitic rocks.The magmatic fractionation leads to an enrichment of Ta relative to Nb and to significant decreases of the Nb/Ta ratio in these rocks [26][27][28].The subsequent fractionation of Nb and Ta and decrease of the Nb/Ta ratio is coupled with subsequent hydrothermal alteration (muscovitization, greisenisation) of granitic rocks.Peraluminous granitic rocks that show significant interaction with hydrothermal fluids systematically display Nb/Ta ratios <5.Therefore, the ratio Nb/Ta = 5 appears to be a good marker to discriminate mineralized from barren peraluminous granites [29] (Figure 7).Enrichment of Ta and consequently low whole-rock Nb/Ta values were previously found in highly peraluminous granites from the Beauvoir stock in France [30,31], the Yichun complex in China [32] and the Limu complex in China [33].In comparison with these localities, the high-F, high-P 2 O 5 Li-mica granites from the Geyersberg, Krásno-Horní Slavkov ore district [5] and this study, Podlesí [34] and the high-F, low P 2 O 5 Li-mica granites of the Cínovec granite cupola [2,35] of the eastern Krušné Hory/Erzgebirge batholith show a lower degree of granite fractionation and, consequently, a higher Nb/Ta ratio.The Nb enrichment of these granites probably reflects a slightly different nature of the protolith [36].
Columbite group minerals together with Nb-Ta-rich rutile are important accessory minerals in highly fractionated granitic rocks.The presence of Nb and Ta in rutile is commonly interpreted to indicate a solid solution between TiO 2 and (Fe, Mn)(Nb, Ta) 2 O 6 [37].The solubility of rutile in magmas varies significantly with the nature of the melt.However, its solubility in peraluminous granites is very low [38,39].
Beauvoir stock in France [30,31], the Yichun complex in China [32] and the Limu complex in China [33].In comparison with these localities, the high-F, high-P2O5 Li-mica granites from the Geyersberg, Krásno-Horní Slavkov ore district [5] and this study, Podlesí [34] and the high-F, low P2O5 Li-mica granites of the Cínovec granite cupola [2,35] of the eastern Krušné Hory/Erzgebirge batholith show a lower degree of granite fractionation and, consequently, a higher Nb/Ta ratio.The Nb enrichment of these granites probably reflects a slightly different nature of the protolith [36].Columbite group minerals together with Nb-Ta-rich rutile are important accessory minerals in highly fractionated granitic rocks.The presence of Nb and Ta in rutile is commonly interpreted to indicate a solid solution between TiO2 and (Fe, Mn)(Nb, Ta)2O6 [37].The solubility of rutile in magmas varies significantly with the nature of the melt.However, its solubility in peraluminous granites is very low [38,39].

Conclusions
The high-F, high-P 2 O 5 Li-mica granites found in the Geyersberg stock are highly fractionated S-type granitic rocks (ASI = 1.2-1.5)with low Nb/Ta ratio (0.9-1.3) which contain Nb-Ta rutile and columbite-group minerals as relatively rare accessories.These Nb-Ta accessories, together with W-bearing ixiolite, represent the most significant accessory minerals.The low Nb/Ta ratio in these rocks is coupled with their hydrothermal alteration, which is characterised mainly by albitisation of plagioclase and occurrence of a younger generation of apatite.Columbite-group minerals are represented by columbite-(Fe) with Mn/(Mn + Fe) ratio ranging from 0.07 to 0.15.The rare Fe-rich W-bearing ixiolite occurs as small needle-like crystals.The ixiolite is Fe-rich with relatively low Mn/(Mn + Fe) and Ta/(Ta + Nb) values (0.10-0.15) and (0.06-0.20), respectively.
Funding: The research for this paper was carried out thanks to the support of the long-term conceptual development research organisation RVO 67985891.

Figure 7 .
Figure 7. Ta versus Nb concentrations in high-F, high-P 2 O 5 Li-mica granites and related rocks.

Table 1 .
Chemical analyses of high-F, high-P 2 O 5 Li-mica granites of the Geyersberg granite stock.