Nb-Ta-Ti-W-Sn-oxide minerals as indicators of a peraluminous P- and F-rich granitic system evolution: Podlesí, Czech Republic

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1 Mineralogy and Petrology (2007) DOI /s Printed in The Netherlands Nb-Ta-Ti-W-Sn-oxide minerals as indicators of a peraluminous P- and F-rich granitic system evolution: Podlesí, Czech Republic K. Breiter 1;2,R.Škoda 3, and P. Uher 4 1 Czech Geological Survey, Praha, Czech Republic 2 Academy of Sciences of the Czech Republic, Institute of Geology, v.v.i., Praha, Czech Republic 3 Institute of Geological Sciences, Masaryk University, Brno, Czech Republic 4 Department of Mineral Deposits, Comenius University, Bratislava, Slovakia Received February 6, 2007; revised version accepted April 11, 2007 Published online August 21, 2007; # Springer-Verlag 2007 Editorial handling: J. G. Raith Summary The strongly peraluminous, P- and F-rich granitic system at Podlesí in the Krusne Hory Mountains, Czech Republic, resembles the zonation of rare element pegmatites in its magmatic evolution (biotite! protolithionite! zinnwaldite granites). All granite types contain disseminated Nb-Ta-Ti-W-Sn minerals that crystallized in the following succession: rutile þ cassiterite (in biotite granite), rutile þ cassiterite! ferrocolumbite (in protolithionite granite) and ferrocolumbite!ixiolite!ferberite (in zinnwaldite granite). Textural features of Nb-Ta-Ti-W minerals indicate a predominantly magmatic origin with only minor post-magmatic replacement phenomena. HFSE remained in the residual melt during the fractionation of the biotite granite. An effective separation of Nb þ Ta into the melt and Sn into fluid took place during subsequent fractionation of the protolithionite granite, and the tin-bearing fluid escaped into the exocontact. To the contrast, W contents are similar in both protolithionite and zinnwaldite granites. Although the system was F-rich, only limited Mn-Fe and Ta-Nb fractionation appeared. Enrichment of Mn and Ta was suppressed due to foregoing crystallization of Mn-rich apatite and relatively low Li content, respectively. The content of W in columbite increases during fractionation and enrichment in P and F in the melt. Ixiolite (up to 1 apfu W) instead of columbite crystallized from the most fluxes-enriched portions of the melt (unidirectional solidification textures, late breccia). 69

2 K. Breiter et al. Introduction Nb, Ta, W, and Sn 2þ (often called rare metals) are chemical elements with high charge-to-ionic-radius ratios (HFSE). These elements do not substitute into common rock-forming minerals and thus behave incompatibly during the crystallization of silicate melts (Linnen and Cuney, 2005). HFSE are concentrated mainly by a high degree of fractional crystallization of peraluminous (Nb also peralkaline) granitic melt enriched in fluxing agents like F, B, P, Li, and water. Their experimentally verifiable high degree of solubility in F- and Li-rich melts enables accumulation of all these metals in late, low-temperature residual portions of melt. Such melts are geologically represented by rare-element pegmatites of LCT class (¼ Li, Cs, Ta-bearing) ( Cerny, 1991) and rare metal granites (Pollard, 1989). The varying influences of changing temperature, pressure, water content, and amount of fluxing agents during magmatic fractionation caused changes in composition of crystallized Nb, Ta, W, and Sn hosting minerals. The major hosts of Nb, Ta, W, and Sn in the less evolved granites and simple pegmatites are the Ti-oxides (rutile, ilmenite) and silicates (titanite). In more evolved granites and pegmatites, these elements primarily form simple or complex oxides like cassiterite, columbite, ixiolite, wodginite, microlite, wolframite, and others. The significance of the chemical composition of Nb-Ta minerals for description and interpretation of the evolution of fractionated magmatic systems, namely pegmatites, was recognized a long time ago and widely studied (Cerny, 1991 and references therein). For example, it has been recognized that columbite-group minerals generally evolve from ferrocolumbite to manganotantalite as a result of fractional crystallization in closed systems ( Cerny et al., 1985, 1986). However, under particular conditions the columbite evolution may differ from this ideal presumption (Lahti, 2000; Novák and Cerny, 2001; Novák et al., 2003). Linnen (1998) reported how Li influences the solubility of Ta, Nb and W in a melt, which may cause decoupling of these elements during crystallization of late magmatic Li-silicates. Rare metal granites and complex pegmatites represent two evolutionary paths of a specialized granitic melt. Comparing whole rock chemical composition and total volume of pegmatites like Tanco (Stilling and Cerny, 2006) with evolved peraluminous granites like Beauvoir (Raimbault et al., 1995), the similarity is obvious. Nevertheless, differences in mineral textures and overall zonality are enormous. Detailed deciphering of Nb-Ta-Ti-W-Sn minerals may help to specify P-T-X paths for both types of silicate systems and consequently explain their differences. While chemical and structural composition and evolutionary trends of Nb-Ta-Ti- W-Sn minerals from rare element granitic pegmatites has been the object of numerous exhaustive studies (e.g., Raimbault, 1998; Tindle and Breaks, 1998, 2000; Van Lichtervelde et al., 2006) comparative data from granites are rare. Descriptive mineral data from simple granite intrusions have been published relatively frequently (Ohnenstetter and Piantone, 1987;Abdala et al., 1998; Scott et al., 1998), but only a few studies about the evolution of Nb-Ta-Ti-W phases in whole fractionated granite suites have been performed (e.g., Yichun granite, China, Belkasmi et al., 2000; Huang, 2002; Cínovec, Czech Republic, Johan and Johan, 1994, 2005). The Podlesí granite system is comprised of biotite, protolithionite and zinnwaldite granites, marginal pegmatite (stockscheider), early and late breccias and several 70

3 Peraluminous P- and F-rich granitic system evolution types of greisens (Breiter et al., 2005), making it a good candidate for a study such as this. The biotite, protolithionite and zinnwaldite granites evolved through fractional crystallization and are easily comparable to wall and intermediate zones of complex pegmatites, e.g., Tanco. The late magmatic breccia and greisens evolved through crystal-residual melt-fluid interaction and may be compared with late replacement units of complex pegmatites. Geological background The Podlesí granite stock is part of the Nejdek-Eibenstock granite pluton, western Bohemia, Czech Republic. The late-variscan Nejdek-Eibenstock pluton ( Ma), in the Saxothuringian zone of the Bohemian Massif, is composed of texturally and geochemically distinct cogenetic intrusions and related greisens (Breiter et al., 1999). The earliest intrusions are comprised of albite-topaz-annite granites (0.1 wt.% F, 0.2 wt.% P 2 O 5 ), followed by fluorine-enriched albite-topazlithian siderophyllite granites ( wt.% F, wt.% P 2 O 5 ; biotite granite in Fig. 1). The most highly fractionated residual melts of these magmas gave rise to small intrusions of extremely F-, P-, and Li-rich granites making up the Podlesí stock (Breiter et al., 2005). The Podlesí granite suite forms a tongue-like intrusion of albite-topaz-protolithionite 1 granite (Fig. 1), the upper contact of which is rimmed by a marginal pegmatite (stockscheider). At depths of 40 to 100 m the albite-topaz-protolithionite granite is intercalated with flat dykes of albite-topazzinnwaldite granite. Already the protolithionite granite is strongly fractionated and Fig. 1. Simplified cross-section of the Podlesí stock (UST unidirectional solidification texture) 1 Protolithionite is the name of Li-mica and is not approved by IMA (Rieder et al., 1999), but this name is often used by petrologists in describing the evolution of complex plutons of fractionated rare-metal granites. In many plutons one of the successive intrusive phases contains mica with an intermediate chemical composition between lithian-annite (or siderophyllite) and zinnwaldite, traditionally called protolithionite (Weis et al., 1993). Intrusive phases with mica corresponding to annite (<0.5% Li 2 O), Li-siderophyllite (0.5 1% Li 2 O), protolithionite (2% Li 2 O) and zinnwaldite (4% Li 2 O) also appear in the Nejdek-Eibenstock pluton. Because the release of the term protolithionite would cause problems in petrographic terminology and in describing the geological structure of the pluton. 71

4 K. Breiter et al. rich in P (0.5 wt.% P 2 O 5 ), F ( wt.%), Ga, Rb, Li, Cs, Sn, Nb, Ta, and W. The zinnwaldite granite represents batches of highly fractionated residual melts that are even more enriched in P (1 wt.% P 2 O 5 ), F (1 1.5 wt.%), Rb, Li, Ga, Nb, Table 1. Studied mineral assemblages from the Podlesí granite stock and whole-rock contents of some trace elements Symbol Rock type A Biotite granite 111, 278 B Greisenised biotite 68, 70, granite 2322 C Early breccia and 3610, stockscheider 3673 D Upper part of 130, the albite- 2754, protolithionite topaz granite E F G H I J K L Deeper part of the albiteprotolithionitetopaz granite Dyke of quartztopaz rock within the protolithionite granite Kfs-dominated UST and laminated rock near the upper contact of the dyke of zinnwaldite granite Qtz þ Zidominated UST near the upper contact of the dyke of zinnwaldite granite Inner part of the dyke of albitezinnwaldite-topaz granite Zinnwaldite granite, near the lower contact of the dyke Fragments in late breccia Matrix of late breccia Sample Nb-Ta-Ti- W-Sn minerals Other accessory minerals WR Nb (ppm) WR Ta (ppm) WR W (ppm) WR Sn (ppm) rutile, ilmenite topaz, fluorapatite, monazite-(ce) ? rutile, ferberite, topaz, fluorapatite not analyzed cassiterite rutile topaz, fluorapatite rutile, ilmenite, ferberite, cassiterite 2686 rutile, ilmenite, cassiterite 3389 ferrocolumbite, ferberite 132, 2756, 3518D, 134, , 4012, , 133, 3916, 3920 ilmenite, ferrocolumbite, ixiolite, ferberite, cassiterite ferrocolumbite, ixiolite, ferberite, cassiterite ferrocolumbite, ferberite, cassiterite 3663 rutile, ferberite, cassiterite 3747K 3747L ferrocolumbite, ixiolite, ferberite ferrocolumbite, ixiolite topaz, fluorapatite, monazite-(ce), xenotime-(y), uraninite topaz, fluorapatite, monazite-(ce), xenotime-(y), uraninite topaz, monazite- (Ce), bismuthinite, bismuth topaz, amblygonite, bismuthinite topaz, amblygonite, bismuthininite topaz, monazite-(ce) topaz, fluorite, xenotime-(y), cheralite-(ce) topaz, amblygonite topaz, amblygonite not analyzed

5 Peraluminous P- and F-rich granitic system evolution and Ta. Prominent layering and unidirectional solidification textures (UST, e.g., Shanon et al., 1982) are indicative of extreme enrichment of fluxing elements and rapid crystal growth of the crystallized melt (London, 1996, 2005). The strongest enrichment of lithophile elements is related to the apical parts of the zinnwaldite granite dykes, where concentrations are up to 4 wt.% F, 1 wt.% Li 2 O, and 1.5 wt.% P 2 O 5. These portions of the dykes were brecciated and cemented with quartz and albite during the late magmatic evolution. Mineralogically (quartz þ zinnwaldite þ topaz) these domains resemble topazites (Eadington and Nashar, 1978) or magmatic greisens (in the sense of Xiong, 1999). Metasomatic greisens are subordinate. Locally, thin, steep stringers of dark, P- and F-rich, but Li-poor greisens composed of quartz, topaz, siderophyllite, and abundant apatite with accessory cassiterite developed in the protolithionite granite. One of the zinnwaldite granite dykes was transformed into a light, P-, F- and Li-rich quartz-topaz greisen containing accessory zinnwaldite, wolframite, columbite, phosphates, and various Bi minerals. Chlorite-sericite mica-schists around the granite stock are metamorphosed to fine-grained quartz-topaz-tourmaline rocks and weakly mineralised with disseminated cassiterite. In order to describe the granite system in detail and to evaluate changes in mineral chemistry we distinguished 12 successive mineral assemblages (labelled A through L), beginning with the oldest granite intrusion (biotite granite, assemblage A) and finishing with the late breccia (assemblages K and L). Brief descriptions of all assemblages are provided in Table 1. Analytical methods Approximately 450 quantitative electron-microprobe analyses of Nb-Ta-Ti-W-Sn phases were conducted using 27 representative samples from the Nejdek-Eibenstock granite pluton. Minerals were analyzed in polished thin sections to obtain information about the genetic position of the individual grains in the rock. Back-scattered electron images (BSE) were taken prior to analysis to study the internal structure of individual mineral grains (Figs. 2 and 5) and care was taken to ensure that the point analyses were made in areas that appeared homogeneous. Complete wavelengthdispersive spectra were collected fro\m several crystals of each mineral to identify the complete spectrum of elements detectable with the electron microprobe used. Element abundances of Al, Bi, Ca, F, Fe, In, Mg, Mn, Mo, Na, Nb, P, Pb, Sc, Si, Ta, Th, Ti, U, W, Y, and Zr in oxide minerals were determined using a CAMECA SX100 electron microprobe (Masaryk University and Czech Geological Survey, Brno) equipped with five wavelength dispersive spectrometers. The accelerating voltage and beam currents were 15 kv and 20 or 40 na, respectively, with beam diameters from 1 to 5 mm. Different conditions were used to avoid decomposition of analyzed areas. The raw data were converted into element concentrations using appropriate PAP matrix corrections (Pouchou and Pichoir, 1985). The following standards and X-ray lines were used: K lines: Si on augite, Al on almandine, P on apatite, Ca and Fe on andradite, Mn on rhodonite, Ti on TiO, F on topaz, Sc on ScP 5 O 14 ;L lines: Zr on zircon, Y on YAG, Nb on columbite, W on scheelite, Mo on metallic Mo, In on InAs, Ga on GaP; M lines: Th on ThO 2, Pb on vanadinite, Bi on metallic Bi, Ta on Ta 2 O 5, and M lines: U on metallic U. WDS scans of 73

6 K. Breiter et al. Fig. 2. BSE microphotographs of rutile and cassiterite: a rutile and cassiterite (white) in biotite, biotite granite, sample 111, assemblage A; b rutile and cassiterite in quartz, greisen of biotite granite, sample 2322, assemblage B; c zoned rutile crystal, protolithionite granite, sample 2755, assemblage D; d patchy zoned rutile, protolithionite granite, sample 2687, assemblage E; e irregularly zoned rutile, zinnwaldite granite, sample 2758, assemblage I; f irregularly zoned automorph rutile crystals and cassiterite (white), zinnwaldite granite, sample 3663 (assemblage J). Numbers on microphotographs refer to microprobe analyses in Tables 2 and 6. Scale bar is 100 mm standards or samples and simulated WDS spectra were used to search for overlapping peaks and to accurately determine background positions. Pulse Height Analyses (PHA) in differential mode were used to eliminate the interference from reflections of other, unwanted elements. Major elements were measured for 20 s at the peak and for 10 s for each background. The counting times for minor and trace 74

7 Peraluminous P- and F-rich granitic system evolution elements were 40 and 60 s, respectively, and half time on each background. When only one background and a calculated background slope were applied, the background counting time corresponded to the peak counting time. Calculation of cation contents was based on the formula unit (apfu) and was constrained by normalizing to fixed numbers of O atoms. Microlite was normalised to a fully occupied B-site (Nb þ Ta þ Ti þ W) ¼ 2 and (OH) was calculated by charge-balancing to the anion total of 7. Results Rutile, ferrocolumbite, ixiolite, ferberite and cassiterite were identified and analyzed. Analyses are listed in Tables 2 through 6 and relationships among important chemical elements are shown in Figs. 3, 4, and 6 8. Early rutile, usually enclosed in biotite, with small rare inclusions of ferrocolumbite is the only Nb-Ta bearing mineral in the biotite granite. The upper, rapidly crystallized part of the protolithionite granite contains only rutile while the deeper fluid-enriched and more slowly crystallizing part of the stock contains rutile and cassiterite. The zinnwaldite granite dykes contain rutile, ferrocolumbite, cassiterite and ferberite. In domains with the highest concentration of fluids, such as the UST layers and the late breccia, ixiolite is also present. Ferberite postdates ferrocolumbite and ixiolite in all cases and cassiterite is formed after rutile (based on textural studies). Rutile Rutile is the most common Nb-Ta bearing mineral in the whole magmatic system at Podlesí (Table 2, Figs. 2 4) and occurs in all evolutionary stages from the biotite granite (assemblage A, Fig. 2a) to the homogeneous parts of the zinnwaldite granite dykes (assemblages I, J, Fig. 2e and f). Rutile is the exclusive host of Nb and Ta in the biotite and protolithionite granites as well as the greisens. Ferrocolumbite only appears in the zinnwaldite granite (assemblages G, H, and I) followed by ixiolite in the most evolved domains (assemblages G, J, K, and L). Rutile occurs mostly as inclusions in mica flakes. It occasionally forms wellevolved enhedral crystals (Fig. 2c and f), but irregular grains and aggregates are more common (Fig. 2b and d). The BSE images document high inhomogeneity of individual rutile grains, with most of the grains displaying irregular and patchy zonation (Fig. 2b, d, e) while some grains display regular zonation (Fig. 2c). This chemical inhomogeneity is caused primarily by the variation of Ta-contents. Distribution of other elements (Nb, Fe, Mn, and Sn) is much more regular. TiO 2 content in analyzed rutile varies from nearly the ideal 100 wt.% (some grains in assemblages A, C, and E) to less than 50 wt.% (minimum 29.9 wt.% TiO 2, 0.45 apfu Ti in assemblage I). Nb and Ta contents in rutile do not correlate with those in the whole rock. In the biotite granite (assemblage A) rutile is relative scarce (Ti is preferentially hosted in biotite) and thus is Nb- and Ta-rich (Fig. 3a). In the protolithionite granite and greisen and the biotite greisen (assemblages B, C, E, and F) Nb varies from 0 to 0.1 apfu and Ta is negligible, while in more evolved assem- 75

8 Table 2. Representative electron-microprobe analyses and structural formulae of rutile. Analyses spots in bold are shown on Fig. 2 Assemblage A A B B C D D D E E F I I I J J Sample A Point WO Nb 2 O Ta 2 O SnO ZrO TiO SiO Bi 2 O Y 2 O Sc 2 O Al 2 O FeO MnO MgO CaO Total Ti Sn Nb Ta Fe 2þ tot Mn W Sc Zr Si Bi Y Al Mg Ca Sum Cat

9 K. Breiter et al.: Peraluminous P- and F-rich granitic system evolution Fig. 3. Compositional diagrams of rutile: a Nb vs. Ta (apfu); b Mn=(Fe þ Mn) vs. Ta= (Nb þ Ta) (apfu) blages (D and J) Nb, and later Ta, are substantially enriched (Fig. 3a, Nb 2 O 5 up to 17 wt.%, Ta 2 O 5 up to 40 wt.%, up to 0.33 apfu Nb þ Ta). Rutile from assemblage I, in spite of associated columbite, is also Nb- and Ta-rich. Areas with different Ta contents either define regular growth zones (Fig. 2c) or are irregular (Fig. 2d, e and f). Iron content reaches a maximum of 11 wt.% FeO (0.18 apfu, in assemblages D, I, and J), while Mn and Mg contents are generally negligible (Table 2, Fig. 3b). Tin content usually ranges from 2 to 3 wt.% SnO 2 with the exception of assemblage A (biotite granite) and E (early breccia) where it is below 1 wt.% (Table 2). Tungsten content is usually lower than Sn content. The highest W contents (2 8 wt.% WO 3 ) were detected in the relatively older phases of the protolithionite Fig. 4. Ternary plot substitution) (Ti þ Sn þ W)-(Nb þ Ta)-(Fe þ Mn) (rutile-columbite-wolframite 77

10 K. Breiter et al. Table 3. Representative electron-microprobe analyses and structural formulae of ferrocolumbite and ixiolite. Analyses spots in bold are shown on Fig. 5 Assemblage G G G G H H H H I J J J K K L L Sample D K 3747K 3747L 3747L Point WO Nb2O Ta 2 O UO SnO ZrO TiO SiO Bi 2 O Y 2 O Sc 2 O Al 2 O FeO MnO MgO CaO Total W Nb Ta U Sn Zr Ti Si Bi Y Sc Al Fe Mn Mg Ca Sum Cat

11 Peraluminous P- and F-rich granitic system evolution granite (early breccia and stockscheider, assemblage C). In more evolved assemblages where rutile is associated with ferberite, W content in rutile is wt.% WO 3. Aluminium contents of rutile are about 0.15 wt.% Al 2 O 3 ( apfu Al), though rutile from the biotite granite is substantially enriched in Al (about 1 wt.% Al 2 O 3, 0.02 apfu Al). The addition of Al into the rutile structure is probably charge-balanced with oxygen vacancies (cf. Hata et al., 2002) due to Al 2 œti 2 O 1 substitution mechanism. Nb, Ta, and Fe 2þ enter the rutile lattice according to columbite-type substitution ((Fe, Mn) (Nb, Ta) 2 Ti -3 ; mainly in assemblage D, I, and J; Fig. 4). In some rutile crystals from the biotite granite (assemblage A) iron is not accompanied by any penta- or hexavalent elements. This iron is probably in the form Fe 3þ and its substitution is, as with Al, compensated through the substitution: Fe 2 œti 2 O 1. Ferrocolumbite In the zinnwaldite granite, ferrocolumbite (Table 3, Figs. 4 6) is of irregular shape (Fig. 5a) or forms small, short prismatic to needle-like crystals, mm in size (Fig. 5e and f). It is enclosed in quartz or mica, but most frequently it is interstitial. Ferrocolumbite grains generally show irregular zoning and inhomogeneities in its chemical composition. No systematic core-rim evolution was observed. In mineral assemblages G and J columbite is often overgrown by ferberite (Fig. 5b d). In assemblage J the replacement zone of an unidentified Bi- and Ta-bearing mineral phase evolved between the columbite core and the ferberite rim (Fig. 5c and d). Ferrocolumbite is Fe-rich in all cases and the Mn=(Fe þ Mn) ratio generally ranges from 0.1 to 0.2. Only in the assemblage D the Mn=(Fe þ Mn) ratio decreases to less than 0.1, and in assemblage J it occasionally increases to 0.4. The Ta=(Nb þ Ta) ratio is around 0.1 (only occasionally up to 0.3) in less fractionated assemblages D, H, I, and J, and from 0.20 to 0.35 in more evolved assemblages G, K, and L (Fig. 6a). In addition, the ferrocolumbite from Podlesí is rich in tungsten. There is complete mixing between ferrocolumbite and ferberite endmembers (Fig. 4). In this article we use the term ferrocolumbite for mineral grains with less than 0.2 apfu W. Grains with more than 0.2 apfu W are described as ixiolite (see below). Titanium is another omnipresent constituent of ferrocolumbite, with a general range from 0.1 to 0.3 apfu and a maximum of 0.6 apfu in assemblage D (compare Fig. 4). Due to the substantial amount of W and Ti the sum of Nb þ Ta only reaches 1.6 to 1.8 apfu when normalization to 6 oxygens was used. Tin content correlates well with Ti, and the ratio of Sn to Ti varies from 0.1 to 0.2 (Fig. 6b). Ca and Mg contents are negligible. Ixiolite In this paper, the term ixiolite is used to describe minerals with columbite-like chemical composition and W content greater than 0.2 apfu. This likely corresponds with tungstenian ixiolite, though no information about the structural state of grains with transitional composition is available. Ixiolite was only observed in the most fluid-enriched parts of the zinnwaldite granite: as needle-like crystals in the UST- 79

12 K. Breiter et al. Fig. 5. BSE microphotographs of columbite and ixiolite: a irregular patchy zoned ferrocolumbite grain, laminated domain of the zinnwaldite granite, sample 2756, assemblage G; b ferrocolumbite crystal (dark core) with patchy zoned ferberite overgrowth, laminated domain of the zinnwaldite granite, sample 3518D, assemblage G; c partly resorbed and altered ferrocolumbite (core) with ferberite overgrowth, zinnwaldite granite, 3663, assemblage J; d altered ferrocolumbite (core) with overgrowth of un-identified Bi-Ta phase and ferberite rim, zinnwaldite granite, 3663, assemblage J; e ferrocolumbite crystals, brecciated zinnwaldite granite, sample 3747K, assemblage K; f needle-like crystal of ferrocolumbite-ferroixiolite growing on zinnwaldite layer, UST-domain of zinnwaldite granite, sample 4010, assemblage H. Numbers on microphotographs refer to microprobe analyses in Tables 3 and 4. Scale bar is 100 mm. On sample f note increase of the W-content along long axis of crystal (see Table 3) 80

13 Peraluminous P- and F-rich granitic system evolution Fig. 6. Compositional diagrams of ferrocolumbite (filled symbols) and ixiolite (open symbols); a Mn=(Fe þ Mn) vs. Ta=(Nb þ Ta); b Sn vs. Ti (apfu); compare different evolutionary trends of columbite from the zinnwaldite granite (1) and the UST and brecciated granite (2) and ixiolite from the UST and brecciated granite (3) layers (assemblages G and H, Table 3, Fig. 5f) and interstitial grains in the late breccia (assemblages K and L, Table 3). Ixiolite is generally Fe-rich but it has slightly more Mn than the associated ferrocolumbite (Fig. 6a; Mn=(Fe þ Mn) ¼ 0.15 to 0.35 in assemblages G, and K and in assemblages H and L). The Ta=(Nb þ Ta) ratio in ixiolite is slightly higher than that of associated ferrocolumbite. Due to high W contents the sum of Nb þ Ta only reaches apfu and decreased by up to 1 apfu in assemblage G. Titanium is negatively correlated with W and typically ranges from 1 to 2 wt.% TiO 2 ( apfu Ti), though amounts up to 4.8 wt.% TiO 2 (0.25 apfu Ti; Fig. 6b) were occasionally measured. Tin varies generally around 1 wt.% SnO 2 (0.05 apfu Sn) and reached a maximum of 5 wt.% SnO 2 (0.25 apfu Sn) in assemblage G (Kfs-dominated UST). Ferberite Ferberite (Table 4, Figs. 5 and 7) was found in two distinct generations. Hydrothermal ferberite from the biotite greisen (assemblage B) forms large grains and crystals (mm to cm in order) and is chemically near the ideal formula FeWO 4, containing more than 0.97 apfu W. Ferberite from the other assemblages (possibly of magmatic origin) forms small (hundreds of microns in order) disseminated needle-like Nb- and Ta-enriched and W-depleted crystals. Ferberite may be enclosed in silicates, but more frequently is interstitial. In assemblages G and J, ferberite commonly forms rims around older columbite crystals. Ferberite from the UST layer (assemblage G, Fig. 5b) is the most Nb- and Ta-enriched (up to 0.3 apfu Nb and 0.2 apfu Ta) and the most W-poor ( apfu W). The Mn=(Fe þ Mn) and Ta=(Nb þ Ta) ratios in ferberite are usually slightly higher than in the associated columbite or ixiolite (Fig. 7). Due to high content 81

14 K. Breiter et al. Table 4. Representative electron-microprobe analyses and structural formulae of ferberite. Analyses spots in bold are shown on Fig. 5 Assemblage B F G H I J J J J Sample A 3518D Point WO Nb 2 O Ta 2 O SnO ZrO TiO SiO Bi 2 O Sc 2 O FeO MnO MgO CaO Total W Nb Ta Sn Ti Sc Fe Mn Mg Ca Si Zr Bi Sum Cat Table 5. Representative electronmicroprobe analyses and structural formulae of uranmicrolite WO Nb 2 O Ta 2 O UO ThO SnO TiO Bi 2 O Sc 2 O FeO MnO CaO Na 2 O F Total W Nb Ta Ti U Th Sn Bi Sc Fe Mn Ca Na F Sum Ox Sum Cat

15 Peraluminous P- and F-rich granitic system evolution Fig. 7. Compositional diagram of ferberite: Mn=(Fe þ Mn) vs. Ta=(Nb þ Ta) (apfu) of tetra- and pentavalent-elements, the sum of Fe þ Mn decreased to as low as 0.8 apfu. Contents Sn and Ti are well correlated, mostly in the range of wt.% of SnO 2 and TiO 2. Uranmicrolite Only one homogeneous grain of uranmicrolite (Table 5) was found, located in the feldspar-dominated UST layer (assemblage G). This is the most Ta-enriched phase in Podlesí, where the Ta=(Nb þ Ta) ratio reached 0.8. Full occupancy of the B-site (Nb þ Ta þ Ti þ W ¼ 2) was used for normalization. In the A-site microlite shows a high amount of U ( apfu; wt.% UO 2 ), Na ( apfu; wt.% Na 2 O), Ca ( apfu; wt.% CaO), and vacancies ( late). The cations Th, Sc, Y, REE, Fe, Mn, and Bi do not exceed apfu in the A-site. In the B-site Ta ( apfu) prevails over Nb ( apfu) and Ti ( apfu). Only small amounts of W and Sn were recorded. According to the valid classification (Hogarth, 1977), this mineral corresponds to uranmicrolite. Cassiterite The distribution of cassiterite is independent of the distribution of the Nb-Ta-Ti-Woxides. Chemically, two distinct types of cassiterite can be distinguished (Table 6, Fig. 8). Cassiterite from the fluid-rich assemblages (G, J, and L) from the zinnwaldite granite forms small homogeneous disseminated interstitial grains, and is significantly Nb- and Ta-enriched with concentrations of Nb 2 O 5 up to 3 wt.% (0.03 apfu) and Ta 2 O 5 up to 10 wt.% (0.07 apfu). Nb and Ta are positively correlated, while Ta=(Nb þ Ta) fluctuated around 0.3 in the zinnwaldite granite dyke (assemblage J) and around 0.6 in the matrix of the late breccia (assemblage L). Both elements enter the cassiterite lattice according to columbite substitution Fe 2þ (Nb, Ta) 2 Sn -3. The other, clearly epigenetic (hydrothermal, assemblage B) cassiterite appears in the biotite greisen and is associated with rutile (Fig. 2a and b). Cassiterite grains ( mm) are inhomogeneous in BSE, but their chemical composition is near 83

16 K. Breiter et al. Table 6. Representative electron-microprobe analyses and structural formulae of cassiterite. Analyses spots in bold are shown on Fig. 2 Assemblage B E F G J J L L Sample A K 3747L Point WO Nb 2 O Ta 2 O SnO TiO SiO Bi 2 O FeO Total Sn Nb Ta Ti Fe 2þ Fe 3þ W Si Bi Fig. 8. Compositional diagram of cassiterite: (Nb þ Ta) vs. Fe (apfu) ideal SnO 2 (>99 wt.% SnO 2 ). The only important trace element causing the BSE pattern is Fe. Contents of Ta and Nb are very low, with Ta>Nb. Disseminated cassiterite in the protolithionite granite is interpreted as magmatic according to its textural position, but is Nb- and Ta-poor, similar to the cassiterite from greisen. Contents of Mn,MgandCaincassiteritearebelowdetection limits. 84

17 Peraluminous P- and F-rich granitic system evolution Discussion Aluminium in rutile Although Al-doped rutile is an interesting high-technology material (Hatta et al., 1996; Gesenhues, 2001; Stevenson et al., 2002), data about Al content in natural rutile are scarce. Zack et al. (2002, 2004) published Al-contents in rutile from basic and felsic metamorphic rocks that generally vary between 50 and 400 ppm (maximum about 800 ppm) in UHP-gneiss from Seidenbach, Germany. Haggerty (1991) reported 0.03 to 0.27 wt.% Al 2 O 3 in rutile from kimberlite xenoliths. The incorporation of Al 3þ into industrial Al-doped rutile is charge-balanced with oxygen vacancies through the Al 2 œti 2 O 1 substitution mechanism (Hatta et al., 1996). Horng et al. (1999) found up to 9 wt.% Al 2 O 3 and substitution (Nb, Ta) 5þ Al 3þ Ti 4þ -2 in Nb- and Ta-rich rutile synthesized from peraluminous melt. However, these experiments were not conducted under natural geologic conditions; the parental melt contained only Si, Al, K, Ti, Nb and Ta, and no M 2þ. In the real geologic environment divalent cations (Fe and=or Mn) are present and columbite-like substitution ((Fe, Mn) 2þ (Nb, Ta) 2 Ti 3 ) is preferred. Data about Al content in rutile from granitic or pegmatitic environments is scarce, and only Yueqing and Wenying (1995) mention 0.14 wt.% Al 2 O 3 in rutile from Nanping pegmatite in China. In Podlesí, rutile from less fractionated biotite granite and its greisen accommodate Al in substantial amounts, ranging from 0.2 to 1.2 wt.% Al 2 O 3 (0.019 apfu Al). This is probably the highest Al value ever reported from rutile in any type of rock. Al in rutile was not detected in any of the other granite types in Podlesí. The Al-rich rutile in the Podlesí granite is typically poor in Nb, Ta, and Fe, and is slightly enriched in Si and Ca (Table 2). Mineral host of scandium Columbite and wolframite are usually the major hosts of Sc in pegmatites. High amounts of Sc were found mostly in Fe- and Nb-rich columbite (up to 2 4 wt.% Sc 2 O 3 Novák and Cerny, 1998). Johan and Johan (1994, 2005) found wt.% Sc 2 O 3 in rutile, wt.% in columbite and wt.% Sc 2 O 3 in zircon in Li- and F-rich granite from Cínovec, while Breiter and Fryda (1995) mentioned wt.% Sc in wolframite from the same deposit. At Podlesí however, Sc concentrates primarily in zircon (usually , occasionally up to 3.45 wt.% Sc 2 O 3 ), while rutile contains less than 0.05 wt.%, ferberite usually less than 0.1 wt.%, ixiolite and columbite wt.% with a maximum of 0.8 wt.% Sc 2 O 3 in two grains. Zircon in Podlesí is a late mineral, as are ferrocolumbite, ixiolite and ferberite, but no textural arguments for their relative ages are available. Evolution of the Mn=(Fe þ Mn) and Ta=(Nb þ Ta) ratios during fractionation Linnen and Keppler (1997) and Linnen (2004) investigated the solubilities of Mn(NbTa) 2 O 6 and Fe(NbTa) 2 O 6 minerals in water-saturated haplogranitic melt, respectively. They established that, near the Ni=NiO buffer, the solubility of Fe end-members (ferrocolumbite and ferrotapiolite) is one order of magnitude higher 85

18 K. Breiter et al. than those of the Mn end-members. Consequently, crystallization should proceed from manganocolumbite to ferrotantalite (or ferrotapiolite). This opposes the generally observed pegmatitic trend of Mn enrichment where the Mn=(Mn þ Fe) ratio generally increases with magmatic fractionation (e.g., Cerny et al., 1986). This means, that another crystallizing mineral phase is important in controlling the Mn-Fe fractionation. However, as the solubility of Fe-members will depend on the redox state, no experiments tracking this dependence were so far conducted. The primary Mn=(Mn þ Fe) vs. Ta=(Ta þ Nb) evolution trends of columbitetantalite and related Nb-Ta oxide minerals in rare element granitic pegmatites generally depends on the level of magmatic crystallization and concentration of fluxing elements, especially F. F- and Li-poor environments of less evolved beryl and some spodumene type pegmatites produced a trend with Ta=(Ta þ Nb) increasing with a low Mn=(Mn þ Fe) ratio in ferrocolumbite to ferrotantalite (locally with ferrotapiolite) compositions (Cerny et al., 1986; Novák et al., 2003). This is opposite to the contemporaneous increase of both Mn=(Mn þ Fe) and Ta=(Ta þ Nb) ratios in manganocolumbite to manganotantalite in F- and Li-rich environments in more evolved complex spodumene, petalite or lepidolite type pegmatites (Greer Lake, Manitoba, Canada, Cerny et al., 1986; Chedeville, France, Raimbault, 1998; Separation Rapids, Ontario, Canada, Tindle and Breaks, 1998, 2000; Eraj arvi, Finland, Lahti, 2000; Rozná, Czech Republic, Novák et al., 2001). Moreover, in boron-rich environments tourmaline (schorl) is said to control the Mn-Fe fractionation (Tanco pegmatite, Manitoba, Canada; Van Lichtervelde et al., 2006). This fractionation for rare-metal granites seems to be more complex. Granites in the Eastern Desert, Egypt, showed that Mn-rich columbite occurs in the most F- enriched granite varieties, like in the previously mentioned pegmatites (Abdalla et al., 1998). In the F-rich Beauvoir granite system of France magmatic manganocolumbite and manganotantalite prevail, but late columbites are Fe-rich. The later trend on the end of the system evolution was interpreted as an effect of post-magmatic fluids (Ohnenstetter and Piantone,1987;Raimbault, 1995). On the other hand, Kudrin et al. (1994) mentioned nearly pure manganocolumbite and manganotantalite from the F-poor spodumene-bearing granite in Altai, Kazakhstan ( wt.% Li 2 O, wt.% P 2 O 5, less than 0.1 wt.% F in bulk granite). To conclude, fluorine alone is probably not the only factor controlling Mn-Fe fractionation. At Podlesí the Mn=(Fe þ Mn) ratio in oxides increases from rutile (0.1), through ferrocolumbite (0.1 to 0.2) to ferberite (0.1 to 0.3) and ixiolite (0.15 to 0.35). Therefore, the Mn=(Fe þ Mn) ratio is relatively low, especially considering the high contents of fluorine in the rocks ( wt.%, Fig. 9a). Whole-rock Mn=(Fe þ Mn) ratios increased in Podlesí only slightly from about 0.02 to 0.03 in the biotite granite to about in the zinnwaldite granite. For comparison, at Beauvoir Mn=(Fe þ Mn) is (Raimbault, 1995), and at Tanco Mn= (Fe þ Mn) is approximately 0.53 (Stilling and Cerny, 2006). Crystallization of zinnwaldite with Mn=(Fe þ Mn) of to only slightly increased the Mn=Fe ratio in residual Podlesí melt, therefore Li has only a small influence on the evolution of the Mn=(Fe þ Mn) ratio in Podlesí (Fig. 9b). In contrast, simultaneous crystallization of abundant Mn-rich apatite (3 4 wt.% MnO, Mn= (Fe þ Mn)3) effectively depleted the melt in Mn and did not allow substantial increase of the Mn=(Fe þ Mn) ratio at Podlesí. 86

19 Peraluminous P- and F-rich granitic system evolution Fig. 9. Relations among selected chemical elements in bulk granites during magmatic evolution of the Podlesí granite system; a F (wt.%) vs. Mn=(Fe þ Mn) (atomic proportion); b Li 2 O (wt.%) vs. Mn=(Fe þ Mn) (atomic proportion); c F (wt.%) vs. Ta=(Mn þ Ta) (atomic proportion); d Li 2 O (wt.%) vs. Ta=(Nb þ Ta) (atomic proportion); e Nb þ Ta vs. Sn (ppm); f Nb þ Ta vs. W (ppm) At Podlesí, the Ta=(Nb þ Ta) ratio in various minerals varies from 0.2 to 0.6, with large differences occurring among individual grains and in zoned grains. The biggest variations were found in rutile. Ixiolite is typically slightly enriched in Ta when compared with associated ferrocolumbite and ferberite. At Podlesí, the Taenrichment is limited and only for ixiolite it is coupled with slight enrichment of Mn. In ferberite and columbite, enrichment of Ta is coupled with a slight decrease in Mn. Linnen (1998) and Wolf and London (1993) found that high concentrations of Li (as well as F and P) in a melt increased the solubility of manganotantalite more than the solubility of manganocolumbite (no experiments with the Fe-endmembers were conducted). Podlesí granites are rich in F and P, but only moderately 87

20 K. Breiter et al. enriched in Li (0.1!0.3 wt.% Li 2 O) in comparison with the Beauvoir granite ( wt.% Li 2 O) or the Tanco pegmatite (1.2 wt.% Li 2 O). At Podlesí, increasing Li 2 O content increases the sum of Nb þ Ta from about 50 to about 150 ppm, but Nb-Ta fractionation is small (Fig. 9c and d). The highest concentrations of ferrocolumbite and ixiolite were found in the UST domains, where crystallization of zinnwaldite þ apatite layers suppressed Li, F, and P contents in the melt and decreased the solubility of Ta þ Nbandpromotedferrocolumbitecrystallization (Fig. 5f). Summarizing and utilizing the granitic pegmatite criteria, Podlesí columbite shows a rather small degree of Mn-Fe and Ta-Nb fractionation in comparison with chemically similar F-enriched amblygonite and lepidolite type pegmatites. One possible explanation could be the relatively low content of Li in Podlesí granites, only rarely exceeding 0.3 wt.% Li 2 O, which suppressed Ta-Nb fractionation. Moreover, ubiquitous crystallization of Mn-rich apatite restricted the Mn-Fe fractionation. Relationships among Nb, Ta, W, and Sn in evolved peraluminous melts The Podlesí granite system may be compared with other peraluminous moderately to highly P- and F-enriched granites, such as Beauvoir (Raimbault, 1995), Yichun (Huang et al., 2002) and those in Thailand (Pollard et al., 1995), and in a more limited manner with complex rare-element pegmatites of the LCT family, e.g., Tanco ( Cerny et al., 1985). The above-mentioned granitic systems differ significantly in the content of individual fluxing elements (F, P, B, and Li) and in the relative abundance of ore minerals and their chemical composition. One specific feature of Podlesí is the apparent Sn versus Ta ( Nb) antagonism (Fig. 9e). Ta and Nb effectively concentrated in residual melt during the fractionation of protolithionite granite, while Sn either crystallized as cassiterite (associated with rutile) or partitioned into the fluid phase and escaped into surrounding phyllite schists. The schists above and around the granite stock are tourmalinized and contain quartz topaz Li-mica cassiterite stringers. The residual melt (¼ zinnwaldite granite) is Ta- and Nb-rich but Sn-poor. This is a major difference from similar P-rich granites at Beauvoir (Raimbauld, 1995) and Yichun (Huang, 2002), as well as the F- and B-enriched, Li- and P-poor specialized granites in Thailand (Pollard et al., 1995) where Sn and Ta are positively correlated and both elements are enriched in the most fractionated residual melt. Another specific feature of Podlesí is the high W content in all granites (Fig. 9f), mineralogically expressed by the common occurrence of W-rich ferrocolumbite and ixiolite to niobian ferberite. However, similar Nb-Ta-W mineral assemblages are known from some other fractionated rare-metal granites, such as the Cínovec granite cupola, Czech Republic (Johan and Johan, 1994, 2005) or the Dlhá Valley granites, Slovakia (Malachovsky et al., 2000), as well as from some phosphate-rich rare-element granitic pegmatites such as Cyrilov and Dolní Bory in Moldanubicum, Czech Republic (M. Novák, personal. communications). Magmatic or hydrothermal origin of main ore minerals Linnen and Cuney (2005) presented a review of actual views of the origin of Tamineralization based on experimental work and experience from world-class Ta- 88