| Fracture mineral investigations are an important part of site characterization for radioactive waste disposal programs in crystalline rock. Fracture minerals act as "tape recorders" of past fluid chemistry, isotopic composition, pressure and temperature conditions during paleohydrogeological events, such as metamorphic or glacial episodes. Past performance of the rock mass during such events can help predict future water and rock conditions, a performance assessment requirement for potential radioactive or other disposal sites in crystalline rock.; Fracture mineral studies from Olkiluoto, Palmottu and Mantsala in Finland, and Stripa and Aspo in Sweden have revealed that several different fluids were involved in calcite precipitation including magmatically equilibrated water, old meteoric water (isotopically unlike the current water found at the site) that had undergone water/rock interaction, hydrothermally altered meteoric water, basinal brines and seawater. These were old calcite crystals that were not in isotopic equilibrium with the present-day fluids in the crystalline rock.; A comparison of fracture mineral studies (from the current study and in the literature) found that local variations of rock type and fluids were the major controls on fracture mineral type. Most study sites recorded the presence of late-stage hydrothermal calcite and calcite from old meteoric or basinal waters that had undergone varying degrees of water/rock interaction. Fluid inclusion evidence shows that two fluid types are common to all sites, an elevated temperature low salinity fluid of Na-Cl composition, and a lower temperature high salinity fluid of Ca-Na-Cl composition. A few sites, where hydraulic conductivities are high, show evidence of calcite dissolution and recent re-precipitation in equilibrium with the present-day waters. |
