ANKERITE ROCKS AND THE GENESIS OF SIDERITE DEPOSITS IN THE EASTERN GRAUWACKEN ZONE, AUSTRIA

Investigations of ankerite rocks from the Steirische Erzberg region by microprobe and ore microscopy have revealed three main ankerite rock types: 1. fine grained ankerite rock with low Fe content and variable Mg/Fe ratio in the individual grains; 2. fine-grained Fe-rich ankerite rock with uniform composition; 3. sparry Fe-rich ankerite rock of uniform composition occurring essentially as veins. Within a stratigraphically uniform, tectonically undisturbed bed the fine-grained Fe-rich ankerite rocks lie either between fine-grained Fe-poor ankerite rocks and siderites or between limestones and siderites. The sparry Fe-rich ankerite rocks occur between the fine-grained Fe-rich ankerite rocks and siderites as well as in veins and nodules within these rocks. They also cut into and through the basal breccia; this suggests a Permosky thian age as the lower limit for their formation. The widely varying composition of individual ankerite grains in the fine-grained Fe-poor ankerite rocks is regraded as an indication of a primary sedimentary formation. The fine-grained Fe-rich ankerite rocks represent reaction products of sedimentary limestone, ankerite and siderite rocks during the Variscan orogenesis. The formation of the sparry Fe-rich ankerite rocks occurred during the alpine orogenic phase. Application of Fe-rich ankerites as a geothermometer results in model metamorphic temperatures of about 400°C at pressures from 2 to 3 kbar. The ankerite rocks from the Gollrader Bucht and the Eastern end of the Grauwacken zone which were examined for purposes of comparison lead one to expect formation temperatures of about 350°C. © 1979 Springer-Verlag.