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- Title
Strontium isotope systematics of scheelite and apatite from the Felbertal tungsten deposit, Austria - results of in-situ LA-MC-ICP-MS analysis.
- Authors
Kozlik, Michael; Gerdes, Axel; Raith, Johann
- Abstract
The in-situ Sr isotopic systematics of scheelite and apatite from the Felbertal W deposit and a few regional Variscan orthogneisses ('Zentralgneise') have been determined by LA-MC-ICP-MS. The Sr/Sr ratios of scheelite and apatite from the deposit are highly radiogenic and remarkably scattering. In the early magmatic-hydrothermal scheelite generations (Scheelite 1 and 2) the Sr/Sr ratios range from 0.72078 to 0.76417 and from 0.70724 to 0.76832, respectively. Metamorphic Scheelite 3, formed by recrystallisation and local mobilisation of older scheelite, is characterised by even higher Sr/Sr values between 0.74331 and 0.80689. Statistics allows discriminating the three scheelite generations although there is considerable overlap between Scheelite 1 and 2; they could be mixtures of the same isotopic reservoirs. The heterogeneous and scattering Sr/Sr ratios of the two primary scheelite generations suggest modification of the Sr isotope system due to fluid-rock interaction and isotopic disequilibrium. Incongruent release of Sr from micas in the Early Palaeozoic host rocks of the Habach Complex contributed to the solute budget of the hydrothermal fluids and may explain the radiogenic Sr isotope signature of scheelite. Spatially resolved analyses revealed isotopic disequilibrium even on a sub-mm scale within zoned Scheelite 2 crystals indicating scheelite growth in an isotopic dynamical hydrothermal system. Zoned apatite from the W mineralised Early Carboniferous K1-K3 orthogneiss in the western ore field yielded Sr/Sr of 0.72044-0.74514 for the cores and 0.74535-0.77937 for the rims. Values of magmatic apatite cores from the K1-K3 orthogneiss are comparable to those of primary Scheelite 1; they are too radiogenic to be magmatic. The Sr isotopic composition of apatite cores was therefore equally modified during the hydrothermal mineralisation processes, therefore supporting the single-stage genetic model in which W mineralisation is associated with the intrusion of the K1-K3 metagranitoid at Felbertal. The subsequent regional metamorphic overprint of the deposit caused redistribution of Sr as a consequence of metamorphic reactions involving Rb and Sr-bearing minerals. Metamorphic Scheelite 3 and apatite rims (e.g., in the K1-K3 orthogneiss) generally became more radiogenic during this process. However, local recrystallisation of primary scheelite under closed conditions (without addition of Sr by the metamorphic fluid) is also documented. The latter process resulted in a homogenisation of the isotope composition of Scheelite 3. Further increase in Sr/Sr ratios in Scheelite 3 and apatite rims is attributed to Late Alpine (?) metamorphic recrystallisation and redistribution of Sr by metamorphic fluids.
- Subjects
STRONTIUM isotopes; SCHEELITE; APATITE; TUNGSTEN; SEDIMENTATION & deposition; HYDROTHERMAL deposits
- Publication
Mineralogy & Petrology, 2016, Vol 110, Issue 1, p11
- ISSN
0930-0708
- Publication type
Academic Journal
- DOI
10.1007/s00710-015-0416-0