Multiple Sulfur Isotope Compositions in Mesoarchean Sulfide Deposits of the Karelian Craton: Implications for Determining the Sulfur Source, Biogeochemical Processes, and Deposit Genesis

In the present paper we demonstrate that most sulfides of the studied deposits of the Archean Sumozero-Kenozero greenstone belt within the Karelian Craton on the Fennoscandian Shield have nonzero Delta S-33 values. This indicates that proportions of seawater sulfate and elemental sulfur in Mesoarchean, included into the ores and resulting from UV photolysis, are different. Our results show that systematics of sulfur isotopes of sulfides generally reflects the mixing of mass-independently fractionated sulfur reservoirs with positive and negative Delta S-33 values. Pyrite is depleted in S-34 isotope, which was interpreted as evidence for microbial sulfate reduction. Variations in the positive Delta S-33 anomalies of the Leksa deposit and the general tendency for Delta S-33 sulfide content to increase with stratigraphic levels in certain boreholes most likely reflect the change in temperature and the fluid mixing throughout the life of the hydrothermal system. The presence of sulfides with strongly negative Delta S-33 anomalies suggests that atmospheric sulfur and seawater sulfate, rather than volcanic sulfur, were the prevailing source for mineral systems of the studied deposits. The presented data require the Mesoarchean seawater to contain sulfates at least locally.