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Fritz Ebner, University of Leoben (Austria)
Amir Morteza Azim-Zadeh, University of Leoben (Austria)
Shao-Yong Jiang, State Key Laboratory for Mineral Deposit Research (China)
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Sparry magnesite of Veitsch type was described for the first time from the Graywacke zone in the Eastern Alps/Austria. The abandoned Hohentauern/Sunk deposit is one of the best known magnesite deposits in this region (Ebner et al. 2004). It is hosted in Early Carboniferous shallow water carbonate rocks metamorphosed within the greenschsist facies during the Cretaceous orogeny. Magnesite and host rocks exhibit the following isotope patterns:
34S values of drilled gypsum (+17,6 ± 0,2 ‰ CDT) and anhydrite (+17,2 ± 0,2 ‰ CDT) intercalated within clastic sediments close to the magnesite may indicate a Carboniferous age.
δ87Sr/86Sr ratios of the sparry magnesite and their dolomite and limestone host rocks have been analysed in two different ways: 1) hand picked minerals by HR-ICP-MS and 2) LA-ICP-MS along selected profiles. The ratios measured by HR-ICP-MS from limestones (0.708256-0,708735) correspond with Late Carboniferous sea water; the rations of dolomite (0.708256-0.709338) and magnesite without signs of redolomitization (0.70918-0.70881) are increased in respect to Carboniferous seawater. However, the LA-ICP-MS values along the profiles differ from the analysis of selected crystals and indicate strong inhomogeneities between the individual crystals and even inside the crystals. A crustal origin of the mineralizing fluid is indicated in the δ Nd vs. 87Sr/86Sr diagram.
A Sm/Nd isochrone from coarse grained (pinolit type) magnesite indicates a 285 Ma age which geological relevance is actually difficult to interpret. Nevertheless it is ± contemporaneous with HT/LP metamorphism (peak around 270 ± 30 Ma), magmatism and extensional tectonics observed in other segments of the Austroalpine crystalline basement.
The δ18O values of magnesite range from 9.59 ‰ to 12.32 ‰ SMOW (mean: 10.67 ‰; n=9), those of δ13C from -2.23 ‰ to -0.02 ‰ DPB (mean: -1,52 ‰; n=9). They are depleted relative to the carbonate host rocks and overlap with values reported for magnesite formed by metasomatic dolomite replacement. Phases of late dolomite and redolomitisized magnesite are enriched in δ18O. In summary the δ18O and δ13C values record three major geologic events of unknown absolute age: (1) dolomitization of limestone, (2) low temperature metasomatic replacement of dolomite by magnesite, and (3) redolomitization of magnesite during hydrothermal processes.
Reference:
Ebner, F., Prochaska, W., Troby, J. & Azim Zadeh, A.M.: Carbonate hosted sparry magnesite of the Greywacke zone, Austria/Eastern Alps.- Acta Petrol. Sinica., 20: 791-802, Beijing 2004.
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