MESOPROTEROZOIC HYDROTHERMAL ALTERATION IN 1.9 GA OLD HIGH GRADE GNEISSES AT SPENT NUCLEAR FUEL REPOSITORY SITE, OLKILUOTO, SW FINLAND

The Olkiluoto Island was opted in 1999 for the final disposal site of the spent Finnish nuclear fuel. The total number of deep drill holes (100 - 1000 m long) is now 43 with a combined length of ca. 20 875 m. The rocks can be divided into supracrustal high-grade metamorphic rocks of polyphase deformation and igneous rocks including pegmatitic granites and sporadic diabase dykes.

In support of construction and design of the underground rock characterization facility and the future repository, the existing geological data of the study site is combined as a 3-D model. The geological and geophysical survey has generated an extensive data set, which is divided into spatial models that present lithology, ductile deformation, brittle deformation, hydrothermal alteration and various statistics of the underground rock mass.

Following the main stages of regional orogeny the Olkiluoto bedrock (~ 1.9 Ga old) was subjected to localized but extensive hydrothermal alteration events initiated by rapakivi granite magmatism 1.58 - 1.55 Ga ago. Estimated temperature interval is from slightly over 300ºC to less than 100ºC.

A direct association of alteration with rapakivi granites can be seen on the site as topaz- and fluorite-bearing late-magmatic greisen veins and networks although their volume is insignificant. The actual hydrothermal alteration consists of both pervasive and fracture-controlled processes. Several episodic long-term alteration events have been identified in the Olkiluoto bedrock. They include chloritization, silicification, epidotization and seritization in small volumes. The most significant alteration events are generated in low temperature. They include the separate clay mineral formations: illitization and kaolinization, as well as sulfidization and calcite formation.