Origins, closed system formation and preservation of calcites in glaciated crystalline bedrock: evidence from the Palmottu natural analogue site, Finland

Abstract

A study of the geochemistry of fracture filling calcite from the Palmottu research site, a U–Th deposit located about 100 km NW of Helsinki, Finland, was part of a comprehensive natural analogue research project considering the migration of U in the crystalline bedrock environment. Fracture filling minerals act as records of the paleohydrological and chemical conditions and therefore provide insight into magmatic processes, ore deposits and long-term stability of the crystalline bedrock environment in relation to deep disposal of radioactive waste. An earlier investigation of calcite and associated U minerals identified a recent origin for these fracture infillings. Uranium series disequilibrium data indicated that relatively recent, glacial water had penetrated to a depth of 62 m. A fluid inclusion study, combined with the isotope geochemistry of several generations of fracture calcite, was undertaken to further study this aspect and to better understand the thermal and fluid history in the crystalline rock environment in general. The study revealed that at least 3 fluids were recorded by fracture calcites: 1) A crystalline calcite precipitated at 139–238 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic origin, 2) A massive calcite (with high salinity fluid inclusions) precipitated at 136–141 °C from a high salinity, Ca–Na–Cl fluid of magmatic or metamorphic origin and 3) A massive calcite (with low salinity fluid inclusions) precipitated at 43–286 °C from a low salinity, Na–Cl fluid of magmatic or metamorphic 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, despite the reworking of highly soluble U compounds in the upper tens of metres of the bedrock that was shown by U series disequilibrium studies. The veins in which these calcites occur have not experienced a later phase of fluid activity that dissolved or altered the calcites. Therefore, it is likely that they have not been reactivated as transmissive fractures since that initial hydrothermal episode, even during the Quaternary history in which the formation was subjected to ice sheet loading and unloading.