Sulphur isotope characteristics of recrystallisation, remobilisation and reaction processes: a case study from the Ramsbeck Pb-Zn deposit, Germany

Syntectonic hydrothermal Pb-Zn vein systems of the Ramsbeck deposit. German y, have been extensively overprinted by late-stage fluids responsible for f issure vein mineralisation. This has caused remobilisation of vein componen ts, notably of sphalerite and galena, as well as the formation of various S b sulphosalt minerals. including boulangerite, semseyite, tetrahedrite and bournonite. A detailed sulphur isotope study of sphalerite, galena and sulp hosalts related to different stages of recrystallisation. remobilisation an d reaction has been carried out using an insitu laser combustion technique. Primary sphalerite-galena ores from flat-lying thrust zones are characteri sed by homogeneous positive delta S-34 values in the range + 6.5 to + 7.7 p arts per thousand. In contrast, recrystallised and remobilised sphalerite a nd galena from both overprinted thrust zones and extensional fissure veins show delta S-34 values in the range -1.1 to + 2.9 parts per thousand and -0 .7 to +8.7 parts per thousand respectively. Sulphosalt minerals show a simi lar variation in their sulphur isotope compositions, which range between -1 .7 and +7.7 parts per thousand. Fissure vein hosted sphalerite and sulphosa lts display significantly more negative delta S-34 values than the primary Pb-Zn ores. Sphalerite which recrystallised in situ and boulangerite which formed by reactive replacement of primary galena show slightly more positiv e delta S-34 values than fissure vein hosted minerals. The compositional ra nge of remobilised mineral phases can be best explained by varying contribu tions (variable mixing) of sulphur derived from (1) the primary sphalerite- galena ores of the flat thrust systems, and (2) the hydrothermal fluid resp onsible for the fissure vein mineralisation. Equilibrium fractionation temp eratures calculated for sphalerite-galena pairs contrast strongly with flui d inclusion data. Isotopic fractionation related to the remobilisation has to be interpreted in terms of kinetic processes.