Isotopic signatures of precent-day calcite and pyrite in low-temperature crystalline bedrock, Olkiluoto, SW Finland

Abstract

Geochemical characteristics of precipitated fracture filling calcite and pyrite can provide much useful information about the deep bedrock environment at the time of their deposition. However, it has been difficult to identify fracture coatings precipitated from the present-day groundwater system. The aim of this study was to evaluate the relationship between the coexisting calcite and pyrite, and the groundwater present at the time of precipitation. Here we investigated fine-grained mineral precipitate deposited over a four-year period on the surface of groundwater monitoring equipment inserted into a drillhole at 530 m below sea level, at Olkiluoto, which is the planned site for a final repository of spent nuclear fuel. The experimental setting is also artificial in the sense that the drillholes have possibly affected groundwater circulation and a foreign object has been inserted into the drillhole. Combining the elemental and isotope geochemical composition of the precipitated calcite and pyrite with previously published compositional data on groundwater and evidence for microbial communities on this site, offered a possibility to get new insight of the precipitation and isotope fractionation processes taking place in deep crystalline bedrock. The concentration of the redox sensitive manganese in the precipitate gives supporting evidence for the influx of groundwater from overlying groundwater units. The δ13C (n = 13) and δ18O (n = 15) values of calcite vary from −13.2 to −9.7‰ and from −9.1 to −7.4‰, respectively. Comparison to the respective values in the local groundwater indicated that the precipitated calcite is in near isotopic equilibrium with its environment with respect to carbon and oxygen. The potential ultimate source of the carbon in the DIC and in the precipitate is likely in old fracture calcite coatings. The δ34S values of pyrite (n = 9) show relatively small variation from −5.7 to 8.3‰. This differs greatly from the huge span of δ34S values from −50 to 80‰ in fracture pyrites reported for the latest calcite fillings at Olkiluoto. The restricted range of δ34S values is interpreted to result from open system conditions during precipitation, with new dissolved sulfate entering from the large brackish SO4-type groundwater unit above. The isotopic fractionation of sulfur between dissolved sulfate and sulfide is estimated to be 25 ± 10‰, which is in agreement with the results reported in laboratory experiments for bacterial sulfate reduction.