ALTERATION AND FRACTURE FILLING SYSTEMS IN PROTEROZOIC HIGH GRADE MIGMATITES AT SPENT NUCLEAR FUEL REPOSITORY SITE, OLKILUOTO, SW FINLAND

The Olkiluoto Island in SW Finland is selected as a site for geologic disposal of spent nuclear fuel. The rocks of Olkiluoto are divided into supracrustal high-grade metamorphic rocks of polyphase deformation and igneous rocks including pegmatitic granites and sporadic diabase dykes. The Olkiluoto bedrock is mainly formed ~1.9 Ga ago and was later subjected to alteration processes: retrogressive stage of regional metamorphism, hydrothermal alteration related to younger magmatism, burial metamorphism and later surface weathering. Volumetrically the most significant, the hydrothermal alteration, consists of pervasive, fracture-controlled and fault-related processes. Alteration products have been studied from ca. 35 km of drillcore. Several episodic long-term alteration events have been identified including clay mineral formation, illitization and kaolinization, as well as sulphidization and calcite formation.

The aim of the investigation on fracture mineralogy is to typify the fracture system and to distinguish the fracture derived chemical and mineralogical alteration scheme. Based on the detailed mapping of ca. 65.000 fractures in drill cores, four main phases of fracture filling system are determined: Phase M and Phases I, II, and III, where the forming temperature of fracture fillings decline respectively. Typical members in Phase M fracture system comprises biotite, chlorite, sericite, calcite and epidote. These form cohesive fillings and clearly pre-date the hydrothermally derived incohesive fillings. The Phase I represents hydrothermally derived fracture system and fillings are mainly varying clay mineral assemblages. They include the largest quantity of fractures in Olkiluoto, and in places several locations the alteration has advanced also to pervasive stage.

Fractures of Phase II system have usually tensile appearance, and are characterized by rough surfaces and thick calcite coatings and veinlets. Phase III system is a compound the latest fractures. Typically these fractures are products of brittle deformation and they can contain the same fracture minerals as the hydrothermally derived fractures mixed with various amounts of suspension load, mechanically transported detritus and other weathering-derived phases.