Abstract

The Tux-Lanersbach magnesite-scheelite deposit is one of several small strata-bound scheelite deposits (total ore reserves, 0.5 Mt at ca. 0.9% WO 3 ) situated in low-grade metamorphic lower Paleozoic sequences of the lower Austroalpine Innsbruck quartz phyllite zone. A polymetamorphic evolution of these volcano-sedimentary and sedimentary series is indicated by Ar-Ar dating. The dominant M 2 -D 2 event is of Eoalpine (Cretaceous) age and the retrograde M 3 -D 3 event, of late Alpine (Tertiary) age. A pre-Alpine (Variscan?) M 1 event is suggested from textural and isotope data.Scheelite was produced as a by-product of magnesite and is hosted by various chlorite-rich phyllites and black schist-rich metaclastic rocks and mainly by metacarbonate rocks (ferroan dolomite, magnesite). The scheelite mineralization was affected by both Alpine metamorphic and deformational events and the youngest scheelite (scheelite 3) generation crystallized late to post-D 3 . Coeval carbonate-quartz veins show increased radiogenic 87 Sr/ 86 Sr ratios up to 0.73449 Corresponding fluids (type 2 fluid inclusions) have a low salinity (H 2 O-NaCl system, 1-7 wt % NaCl equiv), an average T h (156 degrees + or - 16 degrees C), and show a meteoric fluid signature with a delta 18 O fluid value of ca. 2 per mil and a delta D fluid value of -80 to -90 per mil SMOW. Scheelite 2 quartz veins formed syn-D 3 at temperatures between 270 degrees and 295 degrees C. Corresponding H 2 O-NaCl-CaCl 2 -(MgCl 2 ) fluids (type la and b fluid inclusions) are moderately saline (10-22 wt % NaCl equiv, NaCl/CaCl 2 = 0.4), have a higher T h (150 degrees --240 degrees C), and exhibit metamorphic fluid characteristics (delta 18 O fluid = 8-9.5ppm; delta D fluid = ca. -45ppm SMOW). A dilution trend in type 1 fluid inclusions is best explained by fluid mixing, which is regarded as an important process for precipitation of tungsten in syn-D 3 metamorphic veins. The Sr isotope composition of scheelite 2 is strongly radiogenic ( 87 Sr/ 86 Sr up to 0.73320). Scheelite 1 mineralization, economically the most important type, occurs in phyllitic and carbonate rocks and is at least partly of re-Alpine age. It is characterized by the lowest observed 87 Sr/ 86 Sr values (ca. 0.715-0.719). Intense deformation of carbonate-hosted scheelite porphyroblasts produced mylonitic scheelite-dolomite marbles. The resulting banded scheelite ores are thus products of deformation and not relics of a syngenetic fabric.The C-O isotope composition of carbonate rocks allows the distinction of three major groups. delta 13 C values of scheelite-free carbonate rocks range between -0.5 to -1.2 per mil and delta 18 between 13.2 to 16.5 per mil. The delta 13 C values of scheelite-bearing ferroan dolomites cluster between -3.1 to -8.3 per mil and delta 18 O between 12 9 to 17.1 per mil. The delta 13 C of magnesites definin the third major rou ran e from -2 5 to -4.5 per mil and delta 18 O from 11.65 to 16.47 per rail (mean value: delta 13 C = -3.3 + or - 0.6ppm; delta 18 O = 13.2 + or - 1.6ppm). Although 13 C values of scheelite-free carbonate rocks still reflect sedimentary marine values, scheelite-bearing dolomites exhibit 13 C depletion. Oxygen isotopes indicate disequilibrium between magnesites and dolomitic host rocks and support an epigenetic model of spar magnesite formation. The present data do not support either a syn- or epigenetic concentration for the primary tungsten.

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