POSIVA Report 2018-3



Hydrothermal Alteration of Bedrock at Olkiluoto


Aaltonen, I., Front, K., Gehör, S., Sahlstedt, E.



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The central and southern parts of the Finnish bedrock are composed of Paleoproterozoic metamorphic and igneous rocks. Olkiluoto bedrock consists of the 1.9–1.8 Ga old gneisses of the southern Satakunta region. The Olkiluoto bedrock displays three different alteration episodes, which can be identified by their different products and their modes of occurrence.

Retrogressive metamorphism at Olkiluoto culminated with a voluminous migmatisation of pelitic gneisses at temperatures exceeding 660 °C and at relatively low pressure of about 3.5–4 kbar causing sericitisation and saussuritisation of feldspars, and chloritisation and pinitisation of mafic minerals throughout the bedrock. Hydrothermal alteration is a process that has caused the most significant changes in the Olkiluoto bedrock. It often seems to have intense local occurrences; also at the future repository level of ca. 420 m.b.s.l. Surface weathering represents the youngest alteration process at Olkiluoto.

This report focuses on hydrothermal processes and effects on rocks of Olkiluoto, especially their fractures. In the background, there are large and systematic sets of detailed drillcore mapping data, which include information on fracture filling mineral assemblages and alteration zones, and results from studies of selected calcite fillings, their classification and stable isotope composition.


Hydrothermal alteration, gneiss, migmatite, spent nuclear fuel disposal, Olkiluoto, Eurajoki, Finland


POSIVA 2018-03_web (pdf) (42.1 MB)
Appendix I_web (xlsx) (102.6 KB)
Appendix II_web (xls) (312.5 KB)


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