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Tungsten Mineralization and Metamorphic Remobilization in the Felbertal Scheelite Deposit, Central Alps, Austria

By
Rudolf Höll
Rudolf Höll
Institut für Allgemeine und Angewandte Geologie, Luisenstrasse 37, D-80333 München, Germany
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Roland Eichhorn
Roland Eichhorn
2
Bayerisches Geologisches Landesamt, Heβstrasse 128, D-80797, München, Germany
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Published:
January 01, 1998

Abstract

The Felbertal scheelite deposit is located in the northern part of the central Hohe Tauern (Austria). It occurs in an up to 400-m-thick section of the Precambrian (?) to Cambrian Habach Group, consisting of fine- and coarse-grained amphibolites, hornblendites, I-type granites, and quartzites. In the present paper, a comprehensive review of the voluminous literature covering the geologic, mineralogical, petrographic, geochemical, microthermometric, tectonic, geochronological, and isotopic aspects of the deposit is given, and then an attempt is made to combine these various aspects into a unifying evolutionary model of ore genesis.

The premineralization history of both ore fields of the Felbertal scheelite deposit starts with fine-grained amphibolites, classified as volcanic arc basalts, at 547 ± 21 Ma. Sills of hydrothermally altered pyroxenites and gabbros, followed by I-type granites, intruded the basalts during Cambrian time. This rock suite may have been derived by differentiation and fractional crystallization of a calc-alkaline basaltic magma in an active continental margin setting.

Primary scheelite formation is linked to the subsequent emplacement of highly differentiated granitic rocks (with within-plate granite characteristics) some 515 m.y. ago, as indicated in the eastern ore field by a granite gneiss (522 ± 11 Ma) underlying a scheelite-rich quartzite (507 ± 29 Ma.). This elongate quartzite lens, the contiguous granite gneiss lens, and an underlying stockwork zone mark a single feeder system. The rhythmic, fine-grained, and thinly laminated quartzite is interpreted as having been produced by the consecutive filling, under high fluid pressure, of a cavity with quartz and the oldest detectable scheelite mineralization (stage 1 scheelite), periodically interrupted by detachment and sliding phenomena. The feeder system narrows downwards as its WO3 grade diminishes. The western ore field consists of several orebodies (K1 to K8). In the K2 orebody, the emplacement of an I-type granite (older K2 gneiss: 525 ± 14 Ma) was followed by the deposition of a bowl-shaped quartz mass and the formation of an eruption breccia (510 ± 36 Ma), intruded by a younger granite dike (younger K2 gneiss: 512 ± 10 Ma). Primary scheelite deposition was coeval with the formation of the quartz mass; it was strongly enhanced during the formation of the eruption breccia, accompanied by elevated F contents, but was only minor in the subsequent intrusion of the small, younger K2 granite.

Primary mineralizing fluids are not preserved in fluid inclusions of scheelite due to metamorphic overprints. However, they may still be characterized by isotopic data of scheelite, which reflect a crustal origin with 87Sr/86Sr = 0.72 to 0.74, negative εNd ratios, elevated 238U/204Pb (206Pb/204Pb, 207Pb/204Pb) ratios, δ18O values of 8 per mil, as well as enrichments in Rb and Cs. Significant contributions to the fluids were possibly released from micas due to breakdown, leaching, or restructuring reactions, which may occur in deeper sections of a thick continental crust at an active continental margin setting. Such fluids apparently infiltrated a magma chamber of the Habach Group rock suite and caused the subsolidus formation of tschermakitic hornblende (with 87Sr/86Sr ratios of 0.74) by replacing clinopyroxene (with 87Sr/86Sr ratios of 0.707). Eventually, they transferred a presumably dissolved W content into the melt.

There is no clear geologic record from the time interval between the early Paleozoic (Cambrian) and the emplacement of the late Paleozoic Habach intrusive rocks. One of the latter is represented in the western ore field of the Felbertal scheelite deposit by a horseshoe-shaped granite gneiss intrusion in the K1 and K3 orebodies, dated at 336 ± 19 Ma. The total mass of these K1-K3 gneisses is approximately 5 million tons. The gneisses display geochemical and isotopic characteristics of a differentiated, orogenic melt of mixed crust-mantle origin. This melt may have been produced by the injection of mantle-derived magmas into a (thick) continental crust, presumably during a period of crustal relaxation. There is evidence for a K1-K3 granite-related scheelite deposition in these granites, in apical quartz masses, and in their host rocks (mainly fine-grained amphibolites) in the form of quartz veins and small veinlets that are as much as tens of meters from the granite contact.

Variscan amphibolite facies metamorphism between 325 and 280 Ma induced a period of pervasive remobilization and caused an ubiquitous, low-grade dispersion of the preexisting early and late Paleozoic scheelite mineralization and the formation of some high-grade enrichments of scheelite porphyroblasts along shear zones (stage 2 scheelite). Several sets of crosscutting, scheelite-bearing quartz veins and veinlets were formed under protracted amphibolite-facies metamorphic conditions. The metamorphic fluid regime further induced an alteration of the geochemical patterns of preexisting rocks (e.g., enrichments of Rb and Cs as well as of U, and 207Pb, compared to Th, and 208Pb). Furthermore, elevated 87Sr/86Sr ratios, possibly released from micas, dominated the isotopic signature of the recrystallizing, metamorphic scheelites.

Small lamprophyric dikes were emplaced in Late Variscan time after the intrusion of the Granatspitze central gneiss protoliths, the emplacement of the K1-K3 granite, and the subsequent shear zone formation. A change of the previously prevailing oxidizing conditions occurred, presumably after the intrusion of the lamprophyres. Subsequently, strong reducing conditions are indicated by WS2-MoS2 exsolutions in the preexisting stage 1 and stage 2 “molybdoscheelites.”These scheelites were followed by the coexistence of MoS2 and of bluish, fluorescent, pure scheelites (stage 3 scheelite), the crystallization of which has been dated by Sm-Nd at 319 ± 34 Ma. Evidence for the reducing conditions during the Late Variscan tungsten remobilization period is provided by a methane component in the fluid inclusions within such scheelites and accompanying quartz.

Under Alpine lower amphibolite- to upper greenschist-facies metamorphic conditions, the scheelite remobilization was obviously less intense. It was locally focused along some faults and quartz veins, usually as sparse, but large, whitish-bluish fluorescent crystals with a Sm-Nd age of 29 ± 17 Ma (stage 4 scheelite).

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Contents

Reviews in Economic Geology

Metamorphic and Metamorphogenic Ore Deposits

Frank M. Vokes
Frank M. Vokes
Volume Editor
Department of Geology and Mineral Resource Engineering Norwegian University of Science and Technology Trondheim, Norway
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Brian Marshall
Brian Marshall
Volume Editor
Department of Applied Geology University of Technology Sydney NSW 2007 Australia
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Paul G. Spry
Paul G. Spry
Volume Editor
Department of Geological and Atmospheric Sciences Iowa State University Ames, Iowa 50011 USA
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Society of Economic Geologists
Volume
11
ISBN electronic:
9781629490182
Publication date:
January 01, 1998

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