EPISODIC ALTERATION IN PROTEROZOIC HIGH GRADE GNEISSES AT SPENT NUCLEAR FUEL REPOSITORY SITE, OLKILUOTO, SW FINLAND

The Olkiluoto Island is the final disposal site of the spent Finnish nuclear fuel. The ongoing construction and designing of the underground investigation facility and the future repository is aided by 3-D modelling. The geologic spatial data includes surface data and studies on 55 deep drill holes and cores (100–1100 m long) with a combined length of more than 32 500 m.

The bedrock at Olkiluoto site comprises mainly of supracrustal high-grade metamorphic rocks, which provide evidence of polyphase deformation. Igneous rocks include pegmatitic granites and sporadic diabase dykes. The Olkiluoto bedrock (~ 1.9 Ga old) was subjected to regional metamorphism and retrogressive stage which produced ubiquitous chloritization and pinitization of mafic minerals. Later alteration events are more localized and include minor greisenization and more significant zonal hydrothermal alteration. In addition, small restricted areas show local weathered potholes, some tens of meters deep.

The hydrothermal alteration consists of pervasive, fracture-controlled and fault-related processes. Several episodic long-term alteration events, which yielded metasomatic mineral associations in which, illite and kaolinite, as well as sulphides and calcite appear as major alteration products.

Rock types in Olkiluoto represent a wide range of overlapping compositional types, thus whole-rock geochemical alteration signatures are complex to identify. Altered and fresh samples show that the altered rocks have undergone K metasomatism, K gain and Na+Ca loss. The overall trend of alteration is the replacement of framework silicates by sheet silicates. Based on the textural observations and fluid inclusion data, the aqueous fluids were enriched in CO2 at the final stage of each cycle of hydrothermal alteration. Calcite effectively accommodates the Ca-ions released during decomposition reactions, and due to this process, calcite is common in inter- and intragranular positions and in calcite stockworks. The altered greisen samples show strongly elevated contents of some minor elements compared to background samples, e.g. Sn, F, W, Zn, Pb and Au.

Variation and anomalies in petrophysical values reflect mineralogical changes, amount of porosity and microfracturing of the rocks which increase with increasing intensity of alteration.