Evolution of accessory Nb–Ta–W–Sn–Ti–Fe oxide minerals from the Markersbach highly evolved aluminous A-type granite, Erzgebirge, Germany: Magmatic to hydrothermal processes and elemental mobility during F-rich fluid–rock interaction

The Markersbach granite (Erzgebirge/Krun 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.% WO 3 ; 0.2 apfu W), while cassiterite and mainly rutile are rich in Nb and Ta (18.5 wt.% Nb 2 O 5 ; 0.13 apfu Nb; 21.1 wt.% Ta 2 O 5 ; 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 latemagmatic 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.