Sulphur-isotope variations in seafloor and sub-floor, Cyprus-type VMS deposits of the Northern Apennine ophiolites (Italy): preliminary results

Sulphur isotope analyses of sulphide minerals have been carried out for the first time on VMS deposits in Tethyan ophiolites of the Northern Apennines. The overall range of delta(34)S is between -2.9 parts per thousand and 11.4 parts per thousand. The average delta(34)S is 2.6 parts per thousand in seafloor stratiform deposits, 5.9 parts per thousand in stratabound ores within basalt, 9.6 parts per thousand, 8.9 parts per thousand and 5.8 parts per thousand in stockwork veins crosscutting basalt, gabbro and serpentinite, respectively. Inorganic, thermochemical reduction of seawater sulphate is assumed to be the dominant source of sulphur in the deposits. However, the lower, neagtive delta(34)S values recorded in stratiform ores indicate involvement of sulphate-reducing bacteria in the exhalative vent processes. In contrast, the lower delta(34)S values observed in serpentinite-hosted veins may indicate incorporation of mantle-derived sulphur in the hydrothermal fluids.