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Recent publications


Workreport 2016-35

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Name:

Mineralogical, Stable Isotope and Fluid Inclusion Studies of Fracture Filling Calcite at Hästholmen, Syyry and Olkiluoto

Writer:

Sahlstedt, E., Karhu, J.

Language:

English

Page count:

96

Summary:

Fracture mineral and fluid inclusion studies were conducted on samples from the Syyry, Hästholmen and Olkiluoto sites in order to investigate fluid evolution at sites with a different hydrogeologic history. The existing chemical and isotopic records from Olkiluoto were supplemented by analyses of the chemical composition and the C and O isotopic composition of fracture filling calcite from the Hästholmen site near Loviisa and Syyry site in Sievi. Fluid inclusions were studied and microthermometric measurements conducted on fracture calcite material from Syyry, Hästholmen and Olkiluoto. The report also presents results from a literature survey on fluids related to fracture filling calcite in Finland and Sweden. Finally, new data of the C and O isotopic composition of calcite in Olkiluoto till are presented in this report.  

Mineralogical studies of the fracture fillings from Hästholmen site indicate a sequence of mineral precipitation: 1) dolomite (±quartz, fluorite, hematite, sulfide, chlorite) followed by 2) replacement of dolomite with calcite (dedolomitization) and 3) precipitation of calcite (±quartz, kaolinite) in one or possibly two sequences. Fluid inclusion studies indicate that some of the calcite precipitated at low temperatures (<80°C). The δ13C and δ18O values of the calcite range from -14.6 ‰ to -11.6 ‰, and from -13.9 ‰ to -7.2 ‰ (VPDB), respectively. The isotopic composition of the calcite is not in equilibrium with present-day groundwaters at Hästholmen, indicating that it precipitated prior to the establishment of the current groundwater regime. Dedolomitization process observed in the samples indicates that after dolomite precipitation fluids with high Ca2+/Mg2+-ratio circulated in the fractures.

At Syyry, calcite occurs in fractures together with clays, host rock fragments, quartz, sulfides and feldspars, as tight, transluvent fillings or euhedral crystals. The δ13C and δ18O values of the calcite range from -21.5 ‰ to -7.4 ‰, and from -19.1 ‰ to -8.5 ‰ VPDB, respectively. The isotopic composition of the calcite is not in equilibrium with present-day groundwaters. Fluid inclusions studies indicate that the calcite formed at moderate temperatures of <90°C, from a high salinity, Ca-(Na)-Cl-type fluid. The fluid composition was calculated and the d13CHCO3 values range from -17 to -10 ‰ (VPDB) and the d18OH2O values from -2 to 0 ‰ (VSMOW). The isotopic composition of the fluids is comparable to the high salinity fluids from Swedish sites, interpreted to have a Paleozoic origin.

Results from fluid inclusion studies of Olkiluoto samples indicate that mixing of meteoric and magmatic waters occurred during the earliest hydrothermal episode. Later, the fractures reactivated for the circulation of high salinity, Ca-Na-Cl-type fluids (Group 3 calcite/fluid). The isotopic composition of the high salinity fluid is similar to that of the Palezoic fluids found at the Äspö and Forsmark sites.

Isotopic analyses of the matrix calcite and the limestone pebbles occurring in the Olkiluoto till indicate two distinct sources for these fragments: 1) matrix calcite in Mezoproterozoic (Jotnian) sandstone and 2) Phanerozoic marine carbonate, likely transported from the Baltic Sea bottom by glacial movement.

Keywords:

Fissure fillings, calcite, stable isotope, carbon isotopes, oxygen isotopes, fluid inclusions.

File(s):

Mineralogical, Stable Isotope and Fluid Inclusion Studies of Fracture Filling Calcite at Hästholmen, Syyry and Olkiluoto (pdf) (13.4 MB)


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