This thesis deals with the genesis of the siderite mineralizations of the Steirische Erzberg ore deposit. Main fields of interest are to examine thin sections under the microscope, and to analyse fluid inclusions and rock samples. The presence of idiomorphic quartz in the carbonate phases indicates authigenic, hydrothermale origin. The fluid chemistry was determined by mechanical release of the fluids (crush-and-leach method) and chemical analysis (IC). The rock samples provide material for the analysis of Rare Earth Elements (REE). The analysis was carried out by ICP-MS (inductively-coupled-plasma mass-spectrometry). The fluid chemistry of all samples is very similar and the composition fits best with basinal brines (oilfield brines), characterized by the elements Cl, Br, and I. Analyses of REE provide a possibility to solve genetic problems. All samples taken from the Erzberg and Grillenberg show negative Eu-anomalies, which indicates a sedimentary source of iron. The genesis of the ore deposit involved different processes. For sure, remobilisation took place. Epigenetic formation of the siderite ore deposit of the Steirische Erzberg, assisted by hydrothermal fluids of high salinity, is quite probable due to the results from fluid inclusions and microscopy. However, the analysis of REE contradicts that result. Both opposing genetic models are quite probable. Future analysis of isotopes and absolute age determination are recommended.