Mineralogical, petrological and (isotope-) geochemical studies on the Felbertal scheelite deposit, Austria: Development of an epigenetic model for W mineralization in the context of Variscan magmatism in the Tauern Window

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Abstract

This thesis addresses the formation of the Felbertal scheelite deposit (Austria) and considers the question whether W mineralization is exclusively related to the intrusion of a highly fractionated granite (K1-K3 orthogneiss). Petrographic studies revealed two distinct types of K1-K3 orthogneiss, a dark-colored biotite-rich and a light-colored microcline-rich variety, respectively. Additionally, leucocratic aplite gneisses occur within the deposit. Whole rock geochemical analyses revealed that compatible elements (e.g., CaO, FeO, TiO2, P2O5, Ba, Sr) as well as element ratios (Zr/Hf, Nb/Ta) systematically decrease from the dark- to the light-colored K1-K3 orthogneiss variety and to the aplite gneiss. These trends indicate that fractional crystallization controlled the evolution of the granitic melts during magmatic differentiation. The new results demonstrate that the K1-K3 orthogneiss represents a metamorphosed series of granitic melts characterized by an increasing degree of magmatic differentiation. In-situ LA-ICP-MS U-Pb and Lu-Hf isotope analyses were performed on zircon from the K1-K3 orthogneiss and two samples from the Zentralgneise (Felbertauern augengneiss, Granatspitz gneiss). The U-Pb concordia ages from the K1-K3 orthogneiss and the aplite gneiss confine the emplacement period of the granitic melts between 341 Ma and 336 Ma. The corresponding apparent εHft values range from -7.6 to -4.9 indicating a continental crustal source. The isotopic data of zircon from the Felbertauern augengneiss are similar, yielding a U-Pb concordia age of 338.5 ± 1.3 Ma and apparent εHft values between -6.8 and -5.3. Zircons from the Granatspitz gneiss are significantly younger and show apparent εHft values between -3.1 and +2.5. These results suggest that the Felbertauern augengneiss and the dark-colored K1-K3 orthogneiss derived from the same/similar protolith and preclude any relationship of the K1-K3 orthogneiss with the Granatspitz gneiss. Additionally, trace element concentrations of zircon from the K1-K3 orthogneisses were analyzed by LA-ICP-MS. This revealed the occurrence of magmatic and hydrothermal zircon. The latter is enriched in W, U, Nb, Ta, and B compared to magmatic zircon and indicates the presence of a metal- and volatile-rich magmatic-hydrothermal fluid/vapor phase. Besides, the Sr isotope compositions of apatite from the K1-K3 orthogneiss and scheelite from various host lithologies within the deposit were analyzed by in-situ LA-ICP-MS. In the primary scheelite generations (Scheelite 1 and 2) the 87Sr/86Sr ratios range from 0.7208 to 0.7642 and from 0.7072 to 0.7683, respectively. Apatite from the K1-K3 orthogneiss revealed 87Sr/86Sr of 0.7204-0.7451. The Lu-Hf isotope data infer that entrainment of pre-existing scheelite into the melt via assimilation of ore-bearing metabasic host rocks during the emplacement of the K1-K3 granite cannot explain scheelite associated with the granite. Consequently, a primary magmatic W enrichment of the granitic K1-K3 melt is favored, whereby W and other incompatible trace elements (Nb, Ta, U) were concentrated in the residual melts via fractional crystallization. During the transition from the magmatic to the hydrothermal stage, these elements were transferred from the melt into hydrothermal fluids, which infiltrated the Early Paleozoic country rocks at Felbertal where they caused incongruent release of 87Sr from Rb-rich minerals (e.g. mica) during fluid-rock interaction and crystallization of scheelite and Nb-Ta minerals in a stockwork of quartz-scheelite veins. 87Sr/86Sr ratios of magmatic apatite from the K1-K3 gneiss are too radiogenic to be magmatic but are comparable to those of Scheelite 1. The Sr isotopic composition of apatite was therefore equally modified during the hydrothermal mineralization processes, therefore supporting the single-stage genetic model in which W mineralization is associated with the

Details

Translated title of the contributionMineralogische, petrologische und (isotopen-) geochemische Untersuchungen der Felbertal Scheelit Lagerstätte, Österreich: Entwicklung eines epigenetischen Modells für die W Vererzung im Zuge der variszischen magmatischen Aktivität im Tauern Fenster
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Raith, Johann, Assessor A (internal)
  • Finger, Fritz, Assessor B (external), External person
Publication statusPublished - 2015