ISSN:
1437-3262
Keywords:
Key words Tauern Window
;
Felbertal
;
Zircon geochronology
;
Tungsten mineralization
;
Scheelite
Source:
Springer Online Journal Archives 1860-2000
Topics:
Geosciences
Notes:
Abstract U–Pb SHRIMP analyses of zircons from various lithologies and ore bodies of the Felbertal scheelite deposit (western and eastern ore field) and neighbouring areas allow the reconstruction of the pre-Alpine magmatic and metamorphic processes responsible for the tungsten mineralization. The ore deposit belongs to the Magmatic Rock Formation, which is tectonically squeezed between the Habach Phyllite Formation and the Basal Schist Formation (all members of the Habach Group). In both the eastern and western ore field, the pre-mineralization geological processes are marked by the emplacement of basalts (547±27 Ma). Ensialic back-arc extension provided pathways for gabbroic and pyroxenitic melts as well as normal "I-type" granitoids (minimum crystallization age of 529±18 Ma). The rock assemblage forms a magmatic arc on an approximately 2 Ga continental Gondwana (?) margin. Post-emplacement tectonism and metamorphism have converted the basalts to fine-grained amphibolites, the gabbroic and pyroxenitic rocks to coarse-grained amphibolites and hornblendites, and the granitoids to leucocratic orthogneisses, respectively. Tungsten mineralization is intimately related to small patches and dikes of differentiated granitoids in the eastern ore field and the K2 ore body in the western ore field. The granitic melts have supposedly been generated by ongoing differentiation of calcalkaline magmas. They cut the older lithologies and intruded along the same pathways as the earlier melts. Fluids have been carried up along a major line in the eastern ore field. They caused the formation of an elongate ore body with a scheelite-quartz stockwork zone (scheelite-bearing quartz veinlets and veins) and an overlying, likewise elongate, 900-m-long, scheelite-rich quartzite lens. In the western ore field, accompanying fluids produced the K2 ore body. In this ore body, an eruption breccia occurs above a mineralized quartzite. The breccia (younger than 529±18 Ma) contains mineralized quartzite clasts as well as barren fine-grained amphibolite clasts and leucocratic orthogneiss-clasts that are similar to the surrounding host rock equivalents. The quartzite, which represents the main mineralization stage of the K2 ore body, is unsuitable for dating. However, the scheelite-rich quartzite lens of the eastern ore field is probably coeval. This lens locally lies on top of a differentiated and strongly mineralized gneiss. The crystallization age of this gneiss is 529±17 Ma, and marks the peak of tungsten input in the eastern ore field. Small, differentiated granitic dikes, which cut both the K2 eruption breccia and the K2 quartzite in the western ore field, contain only minor scheelite and mark a decrease in mineralization at 519±14 Ma. Thus, a period between 530 and 520 Ma and a setting between magmatic arc and (ensialic) back-arc may properly explain the likely scenario for the primary tungsten input (stage-1 scheelite) by differentiated granitic melts of calcalkaline character. Surprisingly, a second stage-2 scheelite formation was induced in the western ore field by a Variscan granite intrusion (K1–K3 gneiss; 336±19 Ma), the emplacement time of which is pre-dated by a cross-cutting dacitic dike of 340±5 Ma. This mineralization, which occurs in small quartz veins and within a quartz aureole atop the intrusion as well as an even younger mineralization in shear zones (yellowish-fluorescent stage-2 scheelite porphyroblasts), is bracketed between 355 Ma (the upper age limit of the K1–K3 gneiss precursor) and 335 Ma (the lower age limit of the dacitic dike, which is stage-2 scheelite free). Supposedly, long-lasting Variscan (amphibolite facies) metamorphic conditions till 282±2 Ma extended the scheelite remobilization. They caused a further dispersion of scheelite and induced the growth of individual grains and of rims around older grains (bluish-fluorescent stage-3 scheelite). The Alpine metamorphism of lower amphibolite to upper greenschist facies conditions caused a further, minor scheelite remobilization, especially along some faults and quartz veins, including sparse, but large, whitish-bluish-fluorescent crystals (stage-4 scheelite).
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/s005310050281
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