Abstract

The Mittersill scheelite deposit, Salzburg province, Austria, occurs in amphibolites and felsic gneisses of the polymetamorphic volcano-sedimentary Habach series (500 Ma). The latter forms part of the lower schist cover of the Tauern window. The deposit was discovered in 1967 by stream-sediment geochemistry, using a model of submarine W-Hg-Sb mineralization linked to mafic volcanism.The deposit consists of two parts, the Ostfeld open pit and the Westfeld underground mine. The scheelite-bearing metavolcanic sequence represents an imbricated stack of several thrusted hornblendite units and metavolcano-sedimentary units. The hornblendite units consist of hornblendites, coarse-grained amphibolites, and intercalations of rhyolitic to dacitic gneisses. The metavolcano-sedimentary units contain associated fine-grained amphibolites and gneisses. The entire sequence has undergone multiple metamorphism (up to 530 degrees C and 5-6 kbars) and deformation.The tungsten and its associated sulfide-sulfosalt mineralization occurs within the entire metavolcanic sequence but is of subeconomic importance (<0.3% WO 3 ). Mineable tungsten concentrations (1% WO 3 , max values up to 15% WO 3 ) occur exclusively in the hornblendite units and are hosted in both concordant felsic gneisses and fabric parallel as well as fabric crosscutting quartz veins.Isotopic evidence (U/Pb) shows conclusively that mafic and felsic rocks of the hornblendite units, as well as the mineralization, are cogenetic. Major, trace, and rare earth element data reveal that the ore-bearing hornblendite units are of boninitic parentage. Fine-grained amphibolites have been derived from island-arc tholeiites. It is proposed that preconcentration of tungsten took place in boninitic melts within a back-arc environment. Hornblendites, coarse-grained amphibolites, and cogenetic rhyolitic gneisses represent the host rocks for tungsten and its associated mineralization. The magmatic preconcentration was followed by intense mobilization and accumulation of tungsten into a complex system of quartz veins and lenses during two metamorphic events. The Mittersill scheelite deposit has thus to be considered as the result of metamorphic action on tungsten-bearing magmatic rocks.

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