Implications of sulfur isotope fractionation in fracture-filling sulfides in crystalline bedrock, Olkiluoto, Finland

Abstract

A detailed investigation of 34S/32S ratios in fracture-bound pyrite and marcasite was conducted at Olkiluoto, western Finland, in order to gain information on paleohydrogeochemical conditions in a deep groundwater (0–400m b.s.l.) environment. The bedrock at Olkiluoto is being considered for a final repository of spent nuclear fuel. The 34S/32S ratios were measured in situ on individual sulfide grains, using secondary ion mass spectrometry (SIMS) supplemented by analyses from bulk sulfide material. The δ34S values of pyrite and marcasite show considerable variations, from −50‰ to +82‰ VCDT. Using published records and new data from fracture filling minerals, the pyrite fillings were classified into groups representing distinct episodes of fracture mineral formation. The δ34S values of these groups record a transition from hydrothermal activity to a low temperature groundwater environment. The δ34S values of hydrothermal pyrite vary between −18‰ and +34‰. However, hydrothermal pyrite that definitively precipitated at 128–245°C, evidenced by fluid inclusion data from co-genetic calcite, is characterized by a more limited variation in δ34S from −8.6‰ to +3.9‰. The high variability of δ34S within the hydrothermal pyrite precipitates indicates that biogenic fractionation effects cannot be completely excluded. Hydrothermal pyrite precipitation is likely related to hydrothermal systems produced by Mesoproterozoic rapakivi granite and diabase intrusions. Subsequent pyrite generations show a wider spread of δ34S values. Pyrite types representing formation temperatures from ca. 50 to 90°C have δ34S values ranging from −40‰ to +82‰, and the latest low temperature (<80°C) pyrite precipitates are characterized by δ34S from −50‰ to +78‰. The wide range of δ34S values is attributed to bacterial SO42- reduction, affected by Rayleigh fractionation under restricted SO42- input. It can be calculated that the highest δ34S values in pyrite represent consumption of ca. 90% of the original SO42- in the deep groundwater in a fracture section. Furthermore, in situ variations of the δ34S values in pyrite grains indicate that pulses of fresh SO42- with relatively non modified S-isotopic composition have periodically entered the fractures. The latest low temperature pyrite formation event most likely occurred during the Holocene period when brackish, SO42--rich waters of the Litorina Sea infiltrated the fracture network at 8000–4000 BP, providing substrate for SO42--reducing bacteria. Due to the complex Quaternary history of the site and low temperature conditions existing for millions of years, latest fracture filling calcite and pyrite may represent a compilation of several low temperature precipitation events. The records of δ13C in calcite and δ34S in coexisting pyrite indicate that paleogroundwaters have been influenced by the migration of a redox front between methanic and sulfidic environments. These findings can be used to consider the possible geochemical changes that may affect the materials used to contain spent nuclear fuel.