The Kalkstock, a barren limestone domain in the siderite deposit of the Styrian Erzberg, was geologically mapped between the Levels "Etage I" and "Hell". Using Surpac software and based on field and borehole data a 3-D model was created. Thereby it is possible to identify areas with insufficient information for a commercial surface working and further the volume of the barren domains were calculated. Beside structural mapping the Lower Devonian limestones (flaser-limestone, calcareous schist, massive limestone) were defined macro- and microscopically. They originated from micritic limestones which were transformed to micosparite. Due to the low grade metamorphic overprint they are marbles in terms of petrography. Carbonate hosted ore-limestone transitions are cloudy or concordant to the sedimentary bedding. Further iron mineralizations occur within the Permian Präbichl-Conglomerate and crosscutting veins. Devonian limestones were the precursor rock for the metasomatic ore formation by Fe rich fluids. Under the microscope the succession of ore minerals consists of two generations of siderite and three of ankerite. Mostly the ore paragenesis consists of siderite and ankerite, both of the first generations. Partly siderite (1) is substitued by ankerite (1) and ankerite (1,2) is followed by siderite (2). Lastly ankerite (3) was formed before the formation of calcite veins. Some components of zonar ankerite with relicts of dark cores occur within the fine clastic matrix of the breccias. Possibly they are former sedimentary grains of siderite at which Fe mobilized during pseudomorphism formed the growth rims. This implies a sedimentary input of pre-conglomeratic grains of siderite/ankerite. Nevertheless a post-Permian age is suggested for the massive metasomatic ore bodies. This is constrained by the lack of mineralized components in the barren limestone conglomerate and the existence of limestone components crossed by the metasomatic front in mineralized conglomerate. The chemistry of the individual minerals was analyzed by an electron micro probe. The ankerite shows its typical chemistry. The siderite plots in the rhodocrosite-magnesite-siderite diagram close beside the border of the stability field of siderite and sideroplesite. Thus siderite and ankerite coexist in crystals of the same sample. The calcite is relatively clean and includes only less Mg.