Databank

Databank

Posiva publishes Working Reports and Posiva Reports. From the year 2006 nearly all the reports have been published on our webpage and they can be found in the databank. In the databank you can also find our Annual Reviews and some other publications as well. You can also find print-quality pictures and useful links in the databank.

Recent publications


Workreport 2020-16

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Name:

Investigations of Low Temperature Mineral Precipitates at Kyläniemi KYL-KR1, KYL-KR2 and Olkiluoto OL-KR46, OL-KR58

Writer:

Seitsamo-Ryynänen, M., Karhu, J.

Language:

English

Page count:

76

Summary:

Ten fracture mineral samples from Kyläniemi and 17 samples from Olkiluoto were collected for geochemical, isotope geochemical and mineralogic investigations. The samples were collected from fractures with water conductivity or their immediate vicinity. The sites taken for the study were selected to have a different environmental history during the last 11 700 years from that characterizing the Olkiluoto island area. During this time, the Kyläniemi region was first below stagnant glacier edge several hundreds of years and has never been overlain the Baltic Sea, excluding the possible effects of marine water intrusions during the last glacial period. At Olkiluoto, the drillcore sampled has been drilled under the Baltic Sea, to an area expected to be outside the effects of meteoric water circulation after the glacial period. In addition to fracture mineral samples, a recent mineral precipitate linked to bacterial sulfate reduction on a drillhole instrumentation from the Olkiluoto Island was studied mineralogically and geochemically.

At Kyläniemi, the mineralogic studies of the fracture fillings indicate the presence of more than one calcite generation on fracture surfaces. Calcite in fracture fillings is compositionally nearly pure CaCO3, with minor amounts of Mn. The δ13C and δ18O values of fracture calcite at Kyläniemi range from -24.5 to 34.2 ‰ and -19.3 to -4.4 ‰, respectively. The isotopic composition of the most calcite fillings may be in a low temperature equilibrium with waters similar to the present-day precipitation in the area. One calcite filling at depth of 100 m contained a calcite generation highly enriched in both 13C and 18O. These values give indications of microbial methanogenesis in a low temperature environment, possibly associated with a marine water intrusion during one of the earlier interglacial periods.

 Due to shearing and tectonic movements at the Olkiluoto study site, calcite crystal morphology could not be determined from the fracture fillings, and, therefore, their relation to the previously defined morphological fracture calcite groups could not be identified. Calcite in the fracture fillings is compositionally nearly pure CaCO3, with a minor Mn content. The δ13C and δ18O values of fracture calcite at Olkiluoto range from -25.8 to 7.9 ‰ and -18.4 to -4.7 ‰, respectively.  An unusually large enrichment in 18O in the upper ~50 m of the bedrock provides strong evidence for calcite precipitation from a water type nearly equal to the composition inferred for the Littorina Sea derived water. The δ34S values of fracture pyrite in the uppermost ~60 m of the bedrock range from -37.0 to +27.9 ‰, suggesting bacterial sulfate reduction in a closed system environment.

The precipitate formed on the drillhole instrumentation consists mainly of calcite. In addition, trace amounts of nanocrystalline or microcrystalline pyrite was observed in the precipitate. The δ13C and δ18O values of calcite range from -13.2 to -9.7 ‰ and -9.1 to -7.4 ‰, respectively. Precipitation appears to have occurred in equilibrium with the water residing in the instrumentation drillhole. The δ34S values of pyrite range from -5.7 to 8.3 ‰. The relatively small variability of the δ34S values suggests sulfate reduction in an environment approaching an open system.

Keywords:

Paleohydrogeology fracture fillings, mineral precipitate, stable isotopes, carbon isotopes, oxygen isotopes, sulfur isotopes.

File(s):

WR 2020-16_web (pdf) (10.3 MB)


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