Implications of U-Pb SHRIMP zircon data on the age and evolution of the Felbertal tungsten deposit (Tauern Window, Austria)

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 preAlpine 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 (547B27 Ma). Ensialic back-arc extension provided pathways for gabbroic and pyroxenitic melts as well as normal "I-type" granitoids (minimum crystallization age of 529B18 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 finegrained 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 Permomesozoic Schist Cover Habach Group Stubach Group Old Gneiss Series 1 := Eastern ore field 2 := Western ore field Scheelite deposit Felbertal: Variscan Volcanics: Carboniferous to Permian Early-Carboniferous Late-Carboniferous Permian Variscan Granitoids:lasting Variscan (amphibolite facies) metamorphic conditions till 282B2 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).

Early Palaeozoic tectonothermal events in basement complexes of the eastern Greywacke Zone (Eastern Alps): evidence from U-Pb zircon data

“…Similar magmatic ages of granitoid and gabbro formation were reported from the Sivretta and the Ötztal regions in the western Austroalpine units and in Penninic units exposed in the Tauern window (Müller et al 1995;Poller 1997;Schaltegger et al 1997;Eichhorn et al 1995Eichhorn et al , 1999. Similar ca.…”

“…On the other hand, geochronological data suggesting (possible back arc) ophiolite formation during the same 520-485 Ma time interval are reported from other Alpine regions (Pin and Carme 1987;Menot et al 1988;Vavra and Hansen 1991;von Quadt 1992;Eichhorn et al 1995Eichhorn et al , 1999Miller and Thöni 1995). These data suggest the existence of an orogenic environment with subduction and/or collision in the Late Cambrian/Early Ordovician.…”

The Tauern Window (Eastern Alps, Austria): a new tectonic map, with cross-sections and a tectonometamorphic synthesis

“…This series consists of Pre-Cambrian to Ordovician amphibolite, biotite gneiss and micaschist. These contain younger migmatitic layers and anatectic granites that formed during the Variscan event, i.e., in Early Carboniferous time (Schermaier 1991;Eichhorn et al 1999Eichhorn et al , 2000. Elongate layers of strongly deformed Early Carboniferous granitoids (e.g., the Felbertauern Gneiss Lamellae; Eichhorn 2000) form the base of a thrust sheet of this ''Serie der Alten Gneise'', also described as Riffl Nappe in the literature (Frisch 1980a).…”

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

“…These Early Ordovician ages were interpreted as magmatic and comparable to a hornblendite sample (LMS) that yielded a U-Pb zircon upper intercept age of 496±2 Ma (von Quadt 1992). Somewhat older Cambrian SHRIMP U-Pb zircon magmatic ages were reported from the fine-grained amphibolite (547±27 Ma) in the LMS and an I-type granite-a precursor to the Older K2 gneiss clast (529±18 Ma) and the Younger K2 gneiss (519±14 Ma, Eichhorn et al 1999). Eichhorn et al (2000) suggested an interruption of the magmatic activity from Lower Ordovician to Middle Devonian until the intrusion of a medium-to high-K I-type granodiorite (374±10 Ma) into the preVariscan units of the Stubach Group.…”

“…During the last decade several geochronological investigations have been conducted in the TW basement units (e.g. Cliff 1977Cliff , 1981So¨llner et al 1991;Vavra and Hansen 1991;von Quadt 1992;Peindl and Ho¨ck 1993;Eichhorn et al 1995Eichhorn et al , 1999Eichhorn et al , 2000Loth et al 1997). However, the distinction between the pre-Variscan and Variscan components of the basement rocks of the TW is still far from complete.…”