Variscan metagranitoids in the central Tauern Window (Eastern Alps, Austria) and their role in the formation of the Felbertal scheelite deposit

Abstract

The W mineralised Early Carboniferous orthogneisses (K1 and K3 orthogneiss) in the Felbertal scheelite deposit represent a chemically evolved metagranitoid series. Some of its characteristics are high concentrations of F (<4438ppm), Nb (<86ppm), Ta (<13ppm), and U (<74ppm) and REE patterns with distinct negative Eu-anomalies (Eu/Eu*=0.24–0.48) and increasing HREE concentrations (LuN/HoN=1.93–2.81). The systematic chemical trends documented for a multitude of elements (e.g., SiO2, TiO2, P2O5, Ba, Nb, Ta) and their respective ratios (e.g., 1/TiO2, Nb/Ta, Zr/Hf) indicate that crystal-melt fractionation controlled the evolution of the granitic melts. The higher differentiated, peraluminous light-coloured K1-K3 variety (ASI=0.99–1.08, Nb/Ta=5–7, Zr/Hf=13–18) evolved from the less differentiated, metaluminous dark-coloured variety (ASI=0.93–1.03; Nb/Ta=6–10, Zr/Hf=18–24). Peraluminous holo-leucocratic aplite gneiss represents the most evolved member of the series (ASI=1.11–1.12, Nb/Ta=4, Zr/Hf=9–10). Modelling of magmatic differentiation assuming Rayleigh fractionation shows that c. 70–90% of the residual granitic magma had crystallised at the time of the emplacement of the aplites. When compared to barren metagranitoids in the central Tauern Window (“Zentralgneis”), the metaluminous dark-coloured K1-K3 orthogneiss shows some geochemical similarities with the peraluminous Felbertauern augengneiss, one of the regional orthogneisses exposed near the W deposit. Elevated concentrations of Nb (<36ppm), Ta (<5.3ppm) and U (<30ppm) distinguish it from other regional Zentralgneis types and illustrate its genetic relation with the K1-K3 orthogneiss. We propose that the Felbertauern augengneiss represents a peraluminous granitic melt, generated by melting of a source assemblage containing hydrous F-bearing minerals (i.e., biotite). Progressive dehydration melting of the same (or a similar source) at higher temperature involving a Ca-phase (i.e., hornblende) produced melt batches of metaluminous composition; i.e., dark-coloured K1-K3 orthogneiss. An alternative model for explaining the unusual chemical characteristics of the latter would be entrainment and separation of a peritectic and restitic accessory mineral assemblage co-existing with the granitic melt. Higher contents of fluxing elements such as F in the melt may have been responsible for lowering the solidus temperature of the granitic melts allowing concentration of W and other elements in the residual melts and later on in the exsolved magmatic hydrothermal fluids. These processes were a pre-requisite for the development of highly specialised W-rich granitic melts and formation of the world-class Felbertal scheelite deposit.