In contrast to the prevailing opinion that the hydrothermal siderite vein mineralization of the Siegerland developed in the course of the main deformation phase of the Variscan orogenesis, Rb-Sr-isotope analyses on ore and country rock samples indicate a much earlier formation. This is shown by Sr-isotope analyses on the most important ore types of the Siegerland, Siderite and by Rb/Sr-isotope analyses on country rocks of the lower Devonian Siegen-formation, hosting most of the mineralizations. In addition Pb isotope analyses on galenas and U/Pb studies on country rocks were performed. These results indicate that two different processes led to the development of siderite on the one hand and of sulfides on the other hand.
It could be demonstrated that the Sr-isotope signature of siderites from all over the Siegerland as well as within individual deposits is almost homogeneous whereas the Pb-isotopic signatures in Pb-sulfides vary greatly.
The Sr-concentration in siderite (0,4 - 4,4 ppm Sr), is low compared to that of the rocks (30 - 70 ppm Sr). The large mass of isotopically almost homogeneous siderite (more than 220 Mio. tons) indicates a large hydrothermal system extending over an area of 3000 km2. By contrast, Pb sulfides were precipitated from much smaller, locally confined hydrothermal systems.
Rb/Sr-isotope analyses on country rocks demonstrate that approx. 400 million years ago the shales and greywackes of the Siegen formation underwent a process, which resulted in a homogenization of the Sr-isotopes. This process, which clearly took place before the main folding phase of the Variscan orogenesis, established an almost homogeneous 87Sr/88Sr isotope ratio of approx. 0,716 in the whole region. This value -within ± 0,001- is identical with that of the siderites. Therefore, we argue that the extensive homogenization of the Sr-isotopes in the rocks of the Siegen formation is directly connected with the siderite mineralization.
U/Pb isotope analyses on rocks of the Siegen formation show that they might have issued the lead of the sulfides at various times.