Isotopic signatures of precent-day calcite and pyrite in low-temperature crystalline bedrock, Olkiluoto, SW Finland

Geochemical characteristics of precipitated fracture filling calcite and pyrite can provide much useful information about the deep bedrock environment at time their deposition. However, it has been difficult to identify coatings from present-day groundwater system. The aim this study was evaluate relationship between coexisting pyrite, present precipitation. Here we investigated fine-grained mineral precipitate deposited over a four-year period on surface monitoring equipment inserted into drillhole 530 m below sea level, Olkiluoto, which is planned site for final repository spent nuclear fuel. experimental setting also artificial in sense that drillholes have possibly affected circulation foreign object drillhole. Combining elemental isotope geochemical composition with previously published compositional data evidence microbial communities site, offered possibility get new insight precipitation fractionation processes taking place crystalline bedrock. concentration redox sensitive manganese gives supporting influx overlying units. ?13C (n = 13) ?18O 15) values vary ?13.2 ?9.7‰ ?9.1 ?7.4‰, respectively. Comparison respective local indicated near isotopic equilibrium its respect carbon oxygen. potential ultimate source DIC likely old coatings. ?34S 9) show relatively small variation ?5.7 8.3‰. This differs greatly huge span ?50 80‰ pyrites reported latest fillings Olkiluoto. restricted range interpreted result open system conditions during precipitation, dissolved sulfate entering large brackish SO4-type unit above. sulfur sulfide estimated be 25 ± 10‰, agreement results laboratory experiments bacterial reduction.