LONG-TERM HYDROGEOLOGICAL STABILITY OF CRYSTALLINE BEDROCK IN GLACIATED TERRAINS: EVIDENCE FROM THE PALMOTTU NATURAL ANALOGUE RESEARCH SITE, FINLAND

A study of the geochemistry of fracture filling calcite from the Palmottu research site, a Uranium-Thorium deposit located about 100 km northwest of Helsinki, Finland, was undertaken to investigate a natural analogue to a crystalline rock nuclear waste repository. Fracture filling minerals act as records of the paleohydrological and chemical conditions and therefore provide insight into magmatic processes, ore deposits and long term, deep disposal of radioactive waste in the crystalline rock environment.

An earlier investigation of uranium minerals identified a low temperature, recent origin for these calcites, evidence that relatively recent glacial water had penetrated to repository depth. A fluid inclusion study, combined with the isotope geochemistry of several generations of fracture calcite, was undertaken to better understand the thermal and fluid history in the crystalline rock environment. The study revealed that at least three fluids were recorded by fracture calcites. A crystalline calcite (with low salinity inclusions) precipitated at 139 to 238 ° C from a low salinity, Na-Cl fluid of hydrothermal origin. Another crystalline calcite (with high salinity inclusions) indicates another fluid was present. This calcite was identical in appearance to the other crystalline calcite, but appears to have precipitated at about 200 ° C from a high salinity, Ca-Na-Cl fluid of hydrothermal origin. A massive calcite precipitated at 43 to 286 ° C from a low salinity, Na-Cl fluid of hydrothermal origin that underwent equilibrium fractionation during cooling in a rock dominated system. The preservation of high temperature infillings and the cooling trend, indicating isotopic re-equilibration of water due to a very low water/rock ratio, demonstrate that the downward percolation of geologically recent waters is limited in the study area. This strongly supports the concept of long-term hydrogeological stability of the crystalline bedrock even in glaciated terrains.