Paleohydrogeological events recorded by stable isotopes, fluid inclusions and trace elements in fracture minerals in crystalline rock, Simpevarp area, SE Sweden

Abstract Fracture minerals calcite, pyrite, gypsum, barite and quartz, formed during several events have been analysed for δ 13 C, δ 18 O, δ 34 S, 87 Sr/ 86 Sr, trace element chemistry and fluid inclusions in order to gain knowledge of the paleohydrogeological evolution of the Simpevarp area, south-eastern Sweden. This area is dominated by Proterozoic crystalline rocks and is currently being investigated by the Swedish Nuclear Fuel and Waste Management Co. (SKB) in order to find a suitable location for a deep-seated repository for spent nuclear fuel. Knowledge of the paleohydrogeological evolution is essential to understand the stability or evolution of the groundwater system over a time scale relevant to the performance assessment for a spent nuclear fuel repository. The ages of the minerals analysed range from the Proterozoic to possibly the Quaternary. The Proterozoic calcite and pyrite show inorganic and hydrothermal-magmatic stable isotope signatures and were probably formed during a long time period as indicated by the large span in temperatures (c. 200–360 °C) and salinities (0–24 wt.% eq. CaCl 2 ), obtained from fluid inclusion analyses. The Paleozoic minerals were formed from organically influenced brine-type fluids at temperatures of 80–145 °C. The isotopic results indicate that low temperature calcite and pyrite may have formed during different events ranging in time possibly from the end of the Paleozoic until the Quaternary. Formation conditions ranging from fresh to brackish and saline waters have been distinguished based on calcite crystal morphologies. The combination of δ 18 O and crystal morphologies show that the fresh–saline water interface has changed considerably over time, and water similar to the present meteoric water and brackish seawater at the site, have most probably earlier been residing in the bedrock. Organic influence and closed system in situ microbial activity causing disequilibrium are indicated by extremely low δ 13 C (down to −99.7‰), extreme variation in δ 34 S (−42.5‰ to +60.8‰) and trace element compositions. The frequency of calcite low in δ 13 C and high in Mn, as well as pyrite with biogenically modified δ 34 S decreases with depth. Strontium isotopes have been useful to separate the different generations and the Sr isotope ratios in the groundwaters have been determined mainly by in situ water–rock interaction processes. The difficulty of separating late Paleozoic calcite from possibly recent calcite, and the fact that these calcites are usually found in the same fracture systems indicate that water conducting structures have been intermittently conductive from the Paleozoic and onwards. The methodology used has been successful in separating the different generations and characterising their formation conditions.