High resolution mineral-chemical analysis of scheelite from the Felbertal tungsten deposit, Austria

The Felbertal tungsten deposit is situated in the Hohe Tauern range, near Mittersill, Austria. The ore is hosted in the volcano-sedimentary, polymetamorphic units of the Pre-Permian Habach Complex as disseminated and stockwork mineralization, mostly associated to quarz. An Eastern and a Western ore zone (EOZ and WOZ), which are spacially devided by the NS oriented valley, have been distinguished. The genetical relation of different postulated mineralization events of scheelite in the EOZ and WOZ has not yet been fully resolved. Previous studies reported four different generations, characterized mainly by different fluorescence colour and molybdenum (Mo) content. The first and second generation are characterised by yellow fluorecence and relatively high Mo contents, whereas the later third and fourth generation where observed to have whitish to blue fluorescence colour with low Mo concentration. Processes of remobilisation and recrystallization have been accepted to be responsible for different generations.(Höll, R., & Eichhorn, R., 2000)

Formly less well explored ore bodies have been targeted in the WOZ in the last years. Scheelite in this study was collected in recently worked districts of the mine, namely K8 ore body, K2 Brekzie and scheelite-dotted (SD)-gneiss. Associated scheelites display complex and diverse zonation under UV light.

To target this, new quantitative and qualitative analysis of scheelites have been made using electron probe micro analysis (EPMA). Qualitative molybdenum distribution maps, quantitative element profiles and cathodoluminescence (CL) images have been combined with microtextural analysis to reveal a new view into the mineralisation history.
Molybdenum distribution maps of scheelite grains between 0,5 and 300 mm are of multiphase character with often sharp, irregular edges between plateaus of even concentration, fractures and peripheral areas contain microscale Mo-sulfides. Profiles show a mostly fluctuating pattern between concentrations of 0,00 to 4,14 wt% ox. Mo. The element maps reveal a more complex microscale distribution of Mo, than can be visualized by UV-light.
Under the microscope scheelite shows ondulating extinction, complex fracturing and occasional recrystallization.

The formation of the scheelite apparently involved multiple dissolution and reprecipitation during a fluid-mineral interaction where relatively Mo-poor scheelite replaced Mo-rich scheelite. This was followed by predominantly brittle deformation but with occasional dynamic recrystallization of scheelite. These new observations will ultimately influence the genetical interpretation of the Felbertal tungsten deposit.

References:

Höll, R., & Eichhorn, R. (2000). Tungsten mineralization and metamorphic remobilization in the Felbertal scheelite deposit, Central Alps, Austria. Chapter, 11, 233-264.