The genetic model of the Hohentauern/Sunk sparry magnesite deposit (Eastern Alps/Austria)
Research output: Thesis › Doctoral Thesis
The Carboniferous of the Veitsch nappe in the Graywacke Zone (Eastern Alps/Austria) is the type region of the Veitsch type sparry magnesite. The numerous deposits/occurrences of sparry magnesite and talc are known as the Veitsch nappe magnesite-talc district in the Metallogenic Map of Austria. The Hohentauern/Sunk deposit, at the structural base of the Veitsch nappe, is one of the best exposed magnesite deposits in this district. The stratiform magnesite displays distinct metasomatic textures at the contact to the carbonate host rocks with distinct sedimentary and diagentic features. Dolomitization and redolomitization processes lasted from the diagenetic stage until late orogenic stages. Sparry magnesite of the Hohentauern/Sunk deposit is characterized by pinolites, rosettes and banded (zebra) textures. The magnesite contains irregularly distributed mikroinclusions of fine grained dolomite and evaporite minerals (sylvinite). On the other hand magnesite formation is followed by several generations of late dolomite. The sparry magnesite of the deposit has lower MgO and higher CaO contents in comparison to stoechiometric magnesite crystals. The elevated CaO/MgO ratio derives from relics of the dolomite microinclusions and various types of late dolomite formed by redolomitization of the magnesite. The concentrations of some trace elements illustrate a marine/evaporitic and a Mg2+-metasomatic origin of the magnesite mineralization. In the Hohentauern/Sunk deposit the sparry magnesite displays REE patterns with low LREE/HREE, depletion of LREE and a negative Ce anomaly. The stable isotopes (18O and 13C) data of the sparry magnesite overlap with those typical for magnesite formed by metasomatic dolomite replacement. Strongly scattered 87Sr/86Sr ratios are affected by several processes. Therefore they give no significant hint for the magnesite formation. However Nd-Sr isotopes show that the mineralizing fluid percolated through crustal rocks. Fluid inclusion studies on the sparry magnesite indicate a high salinity of the Mg-rich fluid which transformed the dolomite protolites. Sm-Nd geochronology of the sparry magnesite shows a range of Late Carboniferous to Early Permian. All geological, petrographical and geochemical features of the Hohentauern/Sunk deposit support a diagenetic dolomitization of carbonate host rocks followed by an epigenetic formation of the magnesite deposit, via metasomatic replacement of a dolomitic protolith by Mg-rich solutions. The age of mineralization, however, is still the subject of intensive discussion.