Multistage Evolution of the Aue-Niederschlema Uranium Vein Deposit (Erzgebirge, Germany): Evidence from Pitchblende Dating

B. Förster Institut für Geowissenschaften und Lithosphärenforschung, Senckenbergstraße 3, 35390 Gießen, Germany

U. Haack Institut für Geowissenschaften und Lithosphärenforschung, Senckenbergstraße 3, 35390 Gießen, Germany

The U district of Aue in the Erzgebirge is located at the intersection of the Gera-Jachymov fault zone and the Lößnitz-Zwönitz syncline. The latter consists of late Ordovician to Devonian sediments and volcanics and is intruded by late Variscan peraluminous granites with contact aureoles. These, in turn, are cut by hydrothermal veins with three different assemblages of pitchblende, sulfides, selenides, arsenides, quartz, carbonates and ± fluorite. U occurs only in veins within the contact aureoles where the wall rocks are either C-rich black shales or interlayers of geochemically different Ca-Mg-rich volcanics and

The pitchblendes were studied by the microscope and by microprobe before 66 grains from 9 localities were selected for isotope analysis. Only remnants of the oldest pitchblende persist and at least two generations of younger pitchblendes were formed by remobilization of older ones. Most pitchblendes contain very much initial Pb, the secondary pitchblende a large inherited radiogenic component. Therefore, the definition of the initial Pb used for corrections becomes very important. For this reason, cogenetic Pb-bearing minerals were also analyzed. The Pb isotope ratios of these minerals are the same as the initial ratios of U-Pb isochrons. In some cases the inherited radiogenic Pb component was only estimated from U-Pb isochrons.

The following results were obtained: The U was introduced into the hydrothermal vein systems (quartz-pitchblende-calcite (* kku) association) 271 ± 6 Ma ago. Remobilizations of the first U generation resulted in the formation of dolomite-pitchblende-selenide (* mgu) associations in consequence of two discrete hydrothermal processes 190 ± 4 Ma and 120 ± 6 Ma ago. The latter event often caused cataclastic alteration and remobilization of older pitchblende generations associated with the
precipitation of Bi-Co-Ni arsenides (biconi association) and/or Ag-Cu-Pb sulfides or sulfoselenides (ags association). Reopening of the U-Pb isotope systems of pitchblendes by loss of Pb - in some cases combined with diffusion processes of
radiogenic daughter elements - occured 80+8-10 Ma and again 0 - 30 Ma ago. Some pitchblendes lost Pb recently.

Thus, the evolution of the U deposits in the Western Erzgebirge is not a continous but a multistage process which
reactivated older structures. The same age marks as in the Erzgebirge recur all over Europe. They are not associated with
magmatic events but with periods of crustal extension, i.e. : 270 Ma ago with the collapse of the Variscan orogen, 190 Ma ago with the Jurassic subsidence, 120 Ma ago with the opening of the Northern Atlantic Ocean and in the late Cretaceous and Tertiary as a response to Alpine orogenesis.