Stable isotope composition of mineral proxies as a record of fluid evolution in fractured bedrock, the Olkiluoto site, Finland

Fluid evolution in fractured bedrock at the Olkiluoto site in Finland was investigated by examining the stable oxygen and carbon isotope composition of secondary calcite and sulfur isotope composition of secondary sulfide minerals, analyzed from bulk material by traditional techniques and in situ by secondary ion mass spectrometry (SIMS). The application of in situ analyses allowed for great spatial resolution to be obtained for the stable isotopic variation in the samples. The results of this study clearly demonstrated a high variability in isotopic composition of mineral grains within a few tens of micrometers. The study site is located on the western coast of Finland, hosts the Olkiluoto nuclear power plant and is the planned site for the geological disposal of spent nuclear fuel. Due to these reasons, the evolution of the deep bedrock groundwaters at the site is of special interest. The aim of this work was to characterize the latest fracture fillings and increase our knowledge of the hydrogeochemical evolution in deep fractured bedrock. In order to achieve this, drill core material was collected from water conducting fractures, concentrating on hydraulically active zones along which the infiltration of surface waters is most active. Calcite bearing fracture fillings were selected from numerous drill cores available from the site. Fracture filling calcites were grouped according to calcite morphology, mineral associations and their relative position in the fracture sequence into five groups. The latest fracture fillings were identified from the surfaces of open fractures as clear, platy or euhedral fillings (Group 1) which occasionally contained minor amount of silicate inclusions (Group 2). A low formation temperature of the late-stage fillings was confirmed by fluid inclusion studies. Massive, older calcite fillings were separated into three groups all representing high temperature fluid circulation. The massive fillings were broadly comparable by their morphology, mineral associations and stable oxygen and carbon isotope composition to the results obtained in earlier studies, which characterized hydrothermal circulation at the site. Group 3 fillings preceded the late-stage calcite fillings and are interpreted to be associated with a Paleozoic (541-252 Ma) thermal event caused by the uplift and erosion of the Caledonian nappes, affecting large parts of the Fennoscandian Shield. Analyses of cogenetic pyrite grains by SIMS showed highly variable sulfur isotope compositions indicative of bacterial sulfate reduction (BSR) in the fracture fillings of Groups 1, 2 and 3. Detailed analysis of the carbon isotope composition of the Group 3 calcite indicated degradation of organic matter as the electron source in BSR. Group 5 and Group 4 fillings represent the earliest hydrothermal events in the bedrock, likely related to the rapakivi granite and diabase intrusions of Mesoproterozoic (1600-1000 Ma) age. Variations in the oxygen and carbon isotope compositions of Group 4-5 fillings indicate evolution of the hydrothermal system and possible re-equilibration of the calcite fillings with later fluid flow.