Mineralogical, geochemical, fluid inclusion and isotope study of Hohentauern/Sunk sparry magnesite deposit (Eastern Alps/Austria): implications for a metasomatic genetic model

Abstract

The magnesite deposit of Hohentauern/Sunk, hosted in Visean carbonate rocks, is one of the best-exposed examples of “Veitsch type” sparry magnesite deposits, which are located in the Veitsch nappe of the Austroalpine Greywacke zone (Eastern Alps/Austria). The sparry magnesite is stratiform and stratabound within the Visean Steilbachgraben Formation and displays distinct metasomatic features and textures. The sparry magnesite of the Hohentauern/Sunk deposit is characterized by pinolites, rosettes and banded (zebra) textures. Due to microinclusions of dolomite and redolomitzation, the CaO/MgO of magnesite is elevated. Concentrations of selected major and trace elements (Fe-Mn, Sr-Ba, Cr-Ni) in sparry magnesite indicate formation in a marine/evaporitic environment and by Mg2+-metasomatism. The REE patterns of magnesite are characterized by low LREE/HREE, depletion of LREE and a negative Ce anomaly. The δ18O (9.59 to 12.32 ‰ SMOW) and δ13C (−2.23 to −0.02 ‰ PDB) values of magnesite overlap with those published for magnesite formed by metasomatic replacement of dolomite. Fluid inclusions in the sparry magnesite indicate a high salinity (22.4 mass% NaCl equivalent) of the Mg-rich fluid; it is of marine/evaporitic origin. All geological, petrographical and geochemical features support a diagenetic dolomitization of the carbonate host rocks followed by magnesite formation via metasomatic replacement and redolomitization. Sm-Nd geochronology indicates a Late Carboniferous to Early Permian age for magnesite formation triggered by intraformational circulation of fluids derived from buried Carboniferous evaporites.