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, Germany, have been extensively overprinted by late-stage fluids responsible for fissure vein mineralisation. This has caused remobilisation of vein components, notably of sphalerite and galena, as well as the formation of various Sb sulphosalt minerals, including boulangerite, semseyite, tetrahedrite and bournonite. A detailed sulphur isotope study of sphalerite, galena and sulphosalts related to different stages of recrystallisation, remobilisation and reaction has been carried out using an insitu laser combustion technique. Primary sphalerite-galena ores from flat-lying thrust zones are characterised by homogeneous positive '³⁴S values in the range +6.5 to +7.7?. In contrast, recrystallised and remobilised sphalerite and galena from both overprinted thrust zones and extensional fissure veins show '³⁴S values in the range -1.1 to +2.9? and -0.7 to +8.7? respectively. Sulphosalt minerals show a similar variation in their sulphur isotope compositions, which range between -1.7 and +7.7?. Fissure vein hosted sphalerite and sulphosalts display significantly more negative '³⁴S 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 positive '³⁴S values than fissure vein hosted minerals. The compositional range of remobilised mineral phases can be best explained by varying contributions (variable mixing) of sulphur derived from (1) the primary sphalerite-galena ores of the flat thrust systems, and (2) the hydrothermal fluid responsible for the fissure vein mineralisation. Equilibrium fractionation temperatures calculated for sphalerite-galena pairs contrast strongly with fluid inclusion data. Isotopic fractionation related to the remobilisation has to be interpreted in terms of kinetic processes.