U–Pb, Lu–Hf and trace element characteristics of zircon from the Felbertal scheelite deposit (Austria): New constraints on timing and source of W mineralization

Abstract

Combined in-situ U–Pb, Lu–Hf and trace element LA–ICP–MS analyses were performed on zircons from the W-mineralized K1–K3 orthogneiss and associated aplite gneiss from the Felbertal scheelite deposit. The textural and trace element characteristics suggest the presence of magmatic and hydrothermal zircon. Magmatic zircons have low concentrations of W (0.108–3.77ppm), Nb (5.04–48ppm), U (691–3079ppm), B (0.26–6.57ppm), and REE+Y (640–4576ppm) and form euhedral CL-bright cores with distinct oscillatory zoning (zone a). Saturation of P in the granitic melt causing apatite crystallization explains the presence of two sub-types of CL-bright zircon cores distinguished by distinct P and Ca concentrations and variable positive Ce-anomalies. Higher trace element abundances and lower Zr/Hf ratios occur in CL-dark zircons with weak oscillatory zoning (zone b) overgrowing the zircon cores and in granular textured zircons restricted to the aplite gneiss. The increase in trace elements in zircon can be linked to the differentiation of the granitic melt and prolonged zircon crystallization during the transition from the late magmatic to the hydrothermal stage. CL-dark unzoned (texture-less) domains (zone b′) in K1–K3 orthogneiss zircons and granular textured zircons from the aplite gneiss have supra-chondritic Y/Ho ratios (29–62) indicating zircon growth in the presence of F-rich hydrothermal fluids exsolved from the evolved K1–K3 melt. Zone b′ zircons have the highest concentrations of W (4.3–11.4ppm), Nb (63–405ppm), B (7.3–135ppm), REE+Y (0.31–1.87 mass%) and U (1.23–5.56mass%); they are interpreted as hydrothermal and related to W-mineralizing fluids. Magmatic and hydrothermal zircon has identical U–Pb and Lu–Hf isotopic signature indicating the same source. The in-situ U–Pb concordia ages of zircons of four K1–K3 orthogneisses and one aplite gneiss confine the emplacement period of this granitic series between 341.0Ma and 336.2Ma. The initial ɛHft values ranging from −7.6 to −4.3 indicate a crustal protolith. The Felbertauern augengneiss is similar; it yielded a U–Pb concordia age of 338.5±1.3Ma and initial ɛHft between −6.8 and −5.3. On the contrary, zircons from the Granatspitz gneiss yielded discordant U–Pb ages (Late Carboniferous to Early Permian) and the ɛHft values scatter between −3.1 and +2.5 excluding any relation with the K1–K3 orthogneiss. Our new LA–ICP–MS data corroborate the genetic relationship of the W-mineralization at Felbertal with the Early Carboniferous K1–K3 orthogneiss series. The K1–K3 orthogneiss was part of a larger magmatic suite that developed during the Variscan orogeny in the pre-Alpine basement of the Tauern Window in the Viséan.