LAUSR

Abstract The Bleiberg Pb-Zn deposit in the Drau Range is the type locality of Alpine-type carbonate-hosted Pb-Zn deposits. Its origin has been the subject of on-going controversy with two contrasting genetic models proposed: (1) the SEDEX model, with ore forming contemporaneously with sedimentation of the Triassic host rocks at about 220 Ma vs. (2) the epigenetic MVT model, with ores forming after host rock sedimentation at about 200 Ma or later. Both models assume that, on a deposit or even district scale, a fixed paragenetic sequence of ore minerals can be established. The results of our detailed petrographic, chemical and sulfur isotope study of two key ore-samples from two major ore horizons in the Wetterstein Formation at Bleiberg (EHK02 Erzkalk horizon and Blb17 Maxer Banke horizon) demonstrate that there is no fixed paragenetic sequence of ore minerals. Small-scale non-systematic variations are recorded in textures, sphalerite chemistry and δ 34 S. In each sample, texturally different sphalerite types (colloform schalenblende, fine- and coarse-grained crystalline sphalerite) co-occur on a millimeter to centimeter scale. These sphalerites represent multiple mineralization stages/pulses since they differ in their trace element inventory and in their δ 34 S. Nonetheless, there is some correspondence of sphalerite micro-textures, sulfur isotope and chemical composition between the two samples, with microcrystalline colloform schalenblende being Fe-rich, having high Fe/Cd (15 and 9, respectively) and a light sulfur isotope composition (δ 34 S −26.0 to −16.2‰). Cadmium-rich and Fe-poor sphalerite in both samples has relatively heavier sulfur isotope composition: in sample EHK02 this sphalerite has Fe/Cd of ∼0.5 and δ 34 S from −6.6 to −4.6‰; in sample Blb17 Fe/Cd is ∼0.1 and δ 34 S ranges from −15.0 to −1.5‰. Barite, which is restricted to sample EHK02, has δ 34 S ≈ 17‰. The large variations in δ 34 S recorded on the mm to cm-scale is consistent with variable contributions of reduced sulfur from two different sulfur reservoirs. The dominant reservoir with δ 34 S values 34 S about −5‰ suggests a hydrothermal source likely linked with thermochemical sulfate reduction (TSR). Based on this small- to micro-scale study, no simple, deposit-wide paragenetic and sulfur isotope evolution with time can be established. In the Erzkalk ore (sample EHK02) an earlier Pb-Zn-Ba stage, characterized by heavy sulfur isotope values, is succeeded by a light δ 34 S-dominated Zn-Pb-F stage. In contrast, the several mineralization pulses identified in the stratiform Zn-Pb-F Maxer Banke ore (sample Blb17) define a broad trend to heavier sulfur isotope values with time. The interaction documented in these samples between two sulfur reservoirs is considered a key mechanism of ore formation.