Abstract
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 fine- to 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 topaz- and 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).
Highlights
The rare Fe-rich W-bearing ixiolite occurs as small needle-like crystals
Niobium- and tantalum-bearing oxide minerals are common in topaz granites and related rocks
The younger, metasomatic apatite occurs as micro grains (1–2 μm) enclosed in partly albitised plagioclases
Summary
Niobium- and tantalum-bearing oxide minerals are common in topaz granites and related rocks (pegmatites, alkali-feldspar syenites). 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. 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
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