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Free keywords:
nioboixiolite, Nb-Ta minerals, A-type granite, Markersbach, Erzgebirge
Abstract:
The Markersbach granite (Erzgebirge/Krušné Hory, Germany) is a late Variscan, post-collisional, highly evolved and autometasomatically altered, high-F and low-P aluminous A-type leucogranite. The diversity of Nb–Ta–W–Sn–Ti–Fe oxide minerals within the Markersbach granite was studied here by electron probe microanalysis to unveil the chemical evolution of the system during magmatic to hydrothermal stages. The rock-forming minerals of the granite comprise quartz, perthitic K-feldspar, albite, and Li-rich annite to Li-rich siderophyllite (zinnwaldite). Accessory minerals are zircon, thorite, and rarely monazite-(Ce), xenotime-(Y), and chernovite-(Y), topaz, fluorite, REE–Ca–Th fluorocarbonates and fluorides–oxyfluorides, and the Nb–Ta–W–Sn–Ti–Fe oxide phases. Common W-rich columbite supergroup minerals (nioboixiolite series, columbite-(Fe), qitianlingite) and minor Nb–Ta-rich rutile, cassiterite, minerals of the pyrochlore supergroup and Nb-rich ferric oxide (goethite?) were identified in the granite. Oscillatory-zoned nioboixiolite I, recrystallized patchy nioboixiolite II and columbite-(Fe) are rich in W (≤38.8 wt. % WO3; ≤0.2 apfu W), while cassiterite and mainly rutile are rich in Nb and Ta (≤18.5 wt. % Nb2O5; ≤0.13 apfu Nb; ≤21.1 wt. % Ta2O5; ≤0.1 apfu Ta). Minerals of the pyrochlore supergroup commonly replace columbite supergroup minerals and show a wide compositional variability for U, Th, REE, Pb, W and Si. At the scale of individual minerals, the evolution of the Mn/(Fe+Mn) and Ta/(Nb+Ta) ratios differs and reflects complex fluid–melt interactions. These compositional variations and textural observations suggest the late-magmatic crystallization of oscillatory zoned nioboixiolite I and small disseminated, compositionally homogenous, columbite-(Fe). The marginal domains of nioboixiolite II, Nb–Ta rich porous cassiterite, rutile and qitianlingite replacing columbite-(Fe) are probably of late-magmatic to early post-magmatic origin. This late evolution reflects a F-rich character of the system, evolving from a melt-dominated to a more fluid-driven regime, under flux-rich magmatic conditions. Lower temperature fluids induced the partial dissolution and in-situ metasomatic replacement of primary ore minerals combined with a minor remobilization of rare metals, giving rise to pyrochlore supergroup minerals and Nb-rich goethite-like mineral. This ferric oxide represents the latest rare-metal bearing phase in the Markersbach granite, as well as a unique secondary host of, geochemically less mobile, Nb.
