The ore forming processes and the timing of ore precipitation of the Pb-Zn deposits within the Drau Range have been controversially discussed for decades. A combination of conventional analytical techniques (TIMS Pb & Rb-Sr) combined with techniques that allow a high spatial resolution (e.g. EMP, LA-combustion) was applied in order to investigate the ore forming processes.
Electron microprobe analyses revealed a strong heterogeneity in the trace element composition of sphalerite. From the distribution patterns and the observed inter-element relations it is concluded that only a part of the measured trace/minor elements is directly incorporated into the sphalerite lattice (Fe, Cd, Ge). The remaining trace elements (Pb, As, Tl, Cu) are possibly present in nano-inclusions rather than in solid solution within the sphalerite lattice.
High-resolution sulfur isotope analyses displayed also very variable δ34S values (-30.4 to +1.8 ‰) of the investigated sulfides. More than 50% of the investigated sulfides have a very light sulfur isotope composition (< -20‰). This is interpreted as dominant input of reduced sulfur from a reservoir, where sulfate reduction was caused by bacterial metabolism (bacteriogenic sulfate reduction, BSR). In three samples from the Bleiberg deposit a temporal variability in δ34S of co-occurring sulfides is documented on the cm-scale. The chronological evolution of the δ34S values is not uniform within these three samples. The wide and non-uniform variation in sulfur isotope composition of the sulfides indicates that a further sulfur reservoir was involved into ore formation. Within this reservoir the reduction process caused a smaller sulfur isotope fractionation and sulfides derived from this second reservoir have heavier sulfur isotope compositions. It is assumed that within the second reservoir a thermochemical process caused sulfate reduction (thermochemical sulfate reduction, TSR).
In contrast to the observed heterogeneity in trace element and sulfur isotope composition, the Pb isotope composition of sphalerite and galena from the different studied deposits is rather homogeneous and indicates a crustal origin of the ore lead. Mostly coexisting sphalerite and galena show very similar Pb isotope compositions. Therefore it is assumed that both base metals were mainly derived from the same crustal source. No systematic difference in the Pb isotope composition of chemically or sulfur isotopically distinct sphalerite was observed. Furthermore, the geological context of the individual deposits had also no influence on the Pb isotope composition of the sulfides.
Due to the missing correlation between trace element composition and δ34S it is concluded that the metals and the reduced sulfur were transported in different fluids. The trace metals were presumably transported together with the base metals (Pb+Zn), which originated from a crustal reservoir. The observed trace element variations result presumably from the leaching of metals from the country rocks during the migration of the metal bearing fluid. Furthermore, the observed trace element variations are attributed to the prevailing physico-chemical conditions (e.g. redox conditions) at the depositional site during the ore precipitation.
Rb-Sr isotope measurements of sphalerite were performed to date the age of ore formation. In addition to the data of this study, Rb-Sr data of sphalerites (+ one pyrite) from the western Bleiberg mine, determined by J. Schneider (Melcher et al., 2010) were considered for the calculation of isochron ages. Three meaningful ages, which show a small uncertainty were calculated from differently divided subsets: (1) 204.2±3.2 Ma (n = 5); (2) 195.1±2.6 Ma (n = 7); (3) 225±2.1 Ma (n = 3). The majority of the data indicates an epigenetic ore formation at approximately 200 Ma (and younger). However, the possibility of an earlier probably syngenetic stage of ore formation at about 225 Ma cannot be excluded.