Pb and Sr isotope and geochemical data from the Pb-Zn deposit Bleiberg (Austria):: constraints on the age of mineralization

The timing of Zn-Pb mineralization hosted by early dolomitized lagoonal limestones (Crest facies) at Bleiberg (Carinthia, Austria) has been constrained using Sr-isotopes. This late stage Zn-Pb mineralization is a special feature of the Bleiberg deposit. Samples of the mineralized Crest facies are characterized by higher concentrations of minor and trace elements (except Ba and Sr) compared to samples from the weakly mineralized Wetterstein limestone of the lagoonal facies. The samples from the Crest facies indicate that a fluid with a minimum Sr-87/Sr-86 ratio of 0.7083 reacted at 210 +/- 30 Ma with carbonate rocks having Sr-87/Sr-86 ratios of approximately 0.7077 during a late stage of ore formation. The Sr-87/Sr-86 ratios correlate with the Mn and Cl concentrations. Lead isotope data of whole rock samples of Bleiberg yielded an isochron age of 180 +/- 40 Ma. They furthermore confirm the presence of two types of common lead; an isotopically distinct ore lead component is present within and close to the ore bodies. The other common Pb component is present in host rocks and in gangue minerals and is distinguished from the ore lead by lower Pb-207/Pb-204 and Pb-208/Pb-204 ratios. The Sr and the Pb ages are consistent with geological evidence indicating a Triassic age of Pb-Zn mineralization and support genetic models emphasizing the role of bacteriogenic sulfate reduction at low temperatures prior to subsidence and burial. Elevated Sr-87/Sr-86 values (>0.7080) of gangue minerals indicate an epigenetic origin of strontium. Our results are consistent with a genetic model postulating formation of the ore-bearing hydrothermal fluids "at depth" where they leached lead from pre-Upper Carboniferous basement rocks.