Nb-Ta-Ti oxides in topaz granites of the Geyer granite stock (Erzgebirge Mts., Germany)

Abstract

Nb-Ta-Ti-bearing oxide minerals (Nb-Ta-bearing rutile, columbite-group minerals, W-bearing ixiolite) represent the most common host in high-F, high-P Li-mica granites and related rocks from the Geyersberg granite stock in the Krušné hory/Erzgebirge Mts. batholith. The Geyersberg granite stocks forms a pipe like granite stock composed of fine- to middle-grained, porphyritic to equigranular topaz- Li mica granites, containing up to 6 vol. % of topaz. Intrusive breccias on the NW range of the granite stock are composed of mica schist- and muscovite-gneiss fragments enclosed in fine-grained aplitic granite. Granites are partly greisenised, mainly along of steeply dipping NW-SE, and NE-SW trending faults. Quartz-Li-mica-topaz greisens are mineralised by cassiterite, arsenopyrite, wolframite and molybdenite. The high-F, high-P Li-mica Geyersberg granites, which represent the youngest granite intrusions in the Western and Middle Krušné Hory/Erzgebirge plutons, are highly fractionated S-type granites (ASI = 1.2–1.5) with Nb/Ta ratio = 0.74–1.12 and depletion in high-field-strength elements (HFS). Columbite group minerals occur usually as anhedral grains that display patchy zoning. These minerals are represented by columbite-(Fe) with Mn/(Mn+Fe) ratio ranging from 0.07 to 0.32. 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)