The carbon and oxygen isotopic composition of Fe-carbonate ore and its calcitic to dolomitic Devonian host rocks at the Steirischer Erzberg siderite deposit (Greywacke zone, Upper Austroalpine Unit) were determined in order to constrain the source and nature of the Fe-rich mineralizing fluid. The 5tSO-values obtained for various Fe-carbonate generations and the carbonate host lie within a similar range between + 14.6 and + 21.6%o (V-SMOW). No good correlation exists between the relative ages of the carbonate phases and their O isotopic composition. The variation in 6180-values is due to metamorphic recrystallization with locally variable fluid/rock ratios. The average 613C-value of the carbonate host is + 0.5 + 1.2%o (PDB) which corresponds well to worldwide Phanerozoic marine carbonate values. The first Fecarbonate generation has slightly lower 6taC-values, on average -1.4 +__ 0.8%0 (PDB). Recrystallization of both the carbonate host minerals and the ankerite/siderite led to significantly lower 6t3C-values of -4.2 _ 0.6%0 and -4.7 +__ 0.7%0, respectively. Within the basal breccia of the post-Hercynian transgression series matrix calcite/ dolomite shows an average 3taC-value of -2.9 +__ 0.7%0, and matrix siderite/ankerite an average value of -4.1 + 0.4%0. These data, together with Sr isotope data published previously, strongly support a late-diagenetic or epigenetic first Fe-mineralization from convecting formation waters. They ascended along extension faults and were driven by an increased heat flow caused by crustal thinning during a Devonian rifting phase that initiated the separation of the Noric terrane from Africa. A potential source of the Fe could have been the underlying Ordovician acid volcanics. Regional metamorphism related to collision tectonics in the Late Carboniferous (Hercynian) and later during the Alpine orogeny, caused intensive recrystaUization and partial mobilization of the various carbonate phases.