Tungsten Mineralization and Metamorphic Remobilization in the Felbertal Scheelite Deposit, Central Alps, Austria
Rudolf Höll, Roland Eichhorn, 1998. "Tungsten Mineralization and Metamorphic Remobilization in the Felbertal Scheelite Deposit, Central Alps, Austria", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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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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The types of mainly metallic mineralization found in metamorphic terranes are reviewed and an attempt is made to define the genetic relations between the mineralization and the metamorphic events.The terms metamorphosed, metamorphic, and metamorphogenic as applied to ores are also considered.The development of thought and the history of investigations on ores in metamorphic terranes aretraced from the early work in the second half of the nineteenth century onward. Early conceptions ofmetamorphism as an ore-forming process (metamorphogenesis) were seemingly not followed up by theiroriginators, contemporaries, or immediate successors and were neglected until comparatively recentyears. The idea of metamorphism as a modifier of preexisting, mainly sulfidic, but also oxidic, mineralizationwon more immediate and general acceptance in the early decades of the present century. InNorth America, emphasis seems to have been mainly on the deformational aspects of the metamorphism,whereas elsewhere, especially in Europe, the textural and mineralogical results of the metamorphic recrystallizationalso received considerable attention and metamorphism as an ore-forming process hadwon a certain degree of acceptance. This difference in emphasis may perhaps be referred to the differentviews held regarding the initial genesis of the ores in the two regions.The late 1940s and the 1950s witnessed a considerable revision of ideas on ore genesis, especially regardingstrata-bound massive sulfide ores. A parallel revival of interest in the role of metamorphism,probably not unrelated to the foregoing, began in the early 1950s, to begin with concerning metamorphosedores. However, new thoughts concerning metamorphogenesis related to granitization or ultrametamorphismas an ore-forming process began to be published.The following decades witnessed an almost explosive increase in the number of publications dealingwith the effects of metamorphism on ore mineralization of practically all types, but with a definite emphasison sulfide ores of the strata-bound type. One of the most significant breakthroughs in this respectconcerned the world-famous Broken Hill deposit, New South Wales, although the metamorphosed natureof ores in the Scandinavian Caledonides, the North American Appalachians, the Lachlan fold beltof eastern Australia, the Sanbagawa terrane of Japan, the Urals, and Proterozoic fold belts in southernAfrica, have all been thoroughly documented.In recent years, however, the interpretation of many massive sulfidic ores in metamorphic terranes asmetamorphosed has been increasingly questioned, and syntectonic, metamorphogenic, origins havebeen advocated. There is obviously a great need to be able to distinguish more