Geochemical and Sr-Nd isotopic data are presented for Precambrian (Late Paleozoic) leucocratic gneisses,
amphibolites and hornblendites (with very sparse diopsidic pyroxene relics), as well as for different mica fractions (biotite, muscovite, phengite) from the Felbertal scheelite deposit (Central Alps; Austria). These provide information on both primary magmatic processes and fluid contamination.
LILE and HFSE enrichments, positive eNd-values up to 7.6 (typical for rocks at active continental margin settings), anomalously high Rb and Cs concentrations, and unusually high 87Sr/86Sr-initial ratios (up to 0.80 in the hornblendites) are apparent in all analyzed rock lithologies (Fig. 1). An unaltered clinopyroxene-relic has an initial 87Sr/86Sr-ratio of 0.7067, which we interpret to represent the original pyroxenitic precursor of the hornblendite. The observed shift to high Sr-ratios can be related to an intense alteration process, caused by deep-seated metamorphic-magmatic fluids. These fluids probably derived from mica-breakdown reactions in the transition zone between upper and lower continental crust, releasing Rb, Cs and 87Sr into the fluid phase (Fig. 2). The fluid volume necessary to produce the anomalous Sr-ratios in the rocks of the Felbertal scheelite deposit can be estimated. Assuming a fluid composition dominated by biotite/phlogopite-chemistry (eNd = -7.6, 87Sr/86Sr = 1.54, Rb × 700 ppm, Cs × 440 ppm), first the resulting W/R ratio of about 2 gives an idea of the volumes and W concentrations involved in ore-forming processes. Second, the unusual high alkali enrichments in the Precambrian rocks of the Felbertal tungsten deposit could be explained.
To account for the primary tungsten enrichment in a magma chamber in deep crustal levels, biotite/phlogopite with concentrations up to tens of ppm of W obviously provides a suitable source. Dehydration reactions of micas due to granulite facies conditions supposedly release their tungsten and H2O (± F, ± Cl) content. Due to very high
fluorine concentrations in gneisses from the ore deposit, a tungsten transport as fluoride complexes in aqueous solutions seems most likely. In contact to a melt phase, tungsten in fluoride solutions partitions in favour of the melt, which may be an explanation for the primary W mineralization processes. Assuming that the total amount of fluid derived from mica-breakdown in an average granodioritc crustal rock, the source rock volume necessary for the estimated total 200,000 tons of W in the Felbertal scheelite deposit has to be a factor of 1000 greater than the mineralized rock volume.
Fig. 1: 87Sr/86Srt - *t Nd - diagram. The rock samples of the ore deposit can be arranged on a mixing line between the VAB-field and a fluid-composition dominated by a biotite(bi)-chemistry.Fig. 2: Rb/Cs ratio vs Cs (ppm) concentrations. The rock samples of the ore deposit can be arranged on a mixing line between the VAB-field and the biotite (bi)-composition.