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

The Olkiluoto Island was selected in 1999 as the final disposal site for spent Finnish nuclear fuel. Construction and design of the underground rock characterization facility and the future repository is supported by 3-D models of the existing geological data collected over twenty years. Due to the extensive amount of data and level of detail, the geological model has been divided into several submodels presenting e.g. lithology, ductile deformation, brittle deformation, hydrothermal alteration and various statistics of the underground rock mass. The total number of deep drill holes (100 - 1100 m long) is now 53 with a combined length of more than 30 000 m.

The rocks are divided into supracrustal high-grade metamorphic rocks of polyphase deformation and igneous rocks including pegmatitic granites and sporadic diabase dykes. Over the course of main stages of regional orogeny the Olkiluoto bedrock (~ 1.9 Ga ago) was subjected to more localized greisenization and hydrothermal alteration events initiated by younger rapakivi granite magmatism (1.57 - 1.54 Ga). The influence of rapakivi granite magmatism is distinguished on-site as topaz- and fluorite-bearing late-magmatic greisen veins and networks (T = 260 - 430°C) although their volume is insignificant.

Hydrothermal alteration is widespread and consists of pervasive, fracture-controlled and fault-related processes. Several episodic long-term alteration events have been identified. Spatially and volumetrically, the most significant alteration events show clay mineral formation, illitization and kaolinization, sulfidization, calcite formation, silicification, epidotization, and seritization. The fluid inclusion data indicate temperature range for the alteration events from slightly over 300ºC to less than 100ºC. The events of hydrothermal alteration have markedly modified the whole rock and mineral composition and the REE signature of the altered rocks. Along with greizenization the concentrations of Sn (588 ppm), F (> 5000 ppm), W (679 ppm), Zn and Pb (> 1%), and Au (1120 ppb) were elevated in comparison to the unaltered rocks. The porosity of the altered rocks locally exceeds 6 wt-%.