Abstract

Fractures in the Eye–Dashwa pluton near Atikokan, Ontario can be subdivided on the basis of their filling materials. These materials include aplite, epidote, chlorite, and gypsum–carbonate–clay, listed in order of decreasing age established from crosscutting relations. Textures indicate that infilling occurred during fracture growth. Continuous cooling of the pluton during fracturing is inferred from the expected crystallization temperatures of fillings. Fracturing began before the pluton was completely solidified (650–600 °C) and continued to temperatures below 100 °C. Many fractures appear to have been sealed by the filling materials after initiation but were subsequently sheared and filled by lower temperature materials. Apparently the majority of fractures formed during or immediately after pluton solidification and new fractures became smaller and more restricted in location as cooling progressed. Fractures and filling materials are seen as important features in assessing the possibility of movement of radionuclides in aqueous solutions away from a nuclear fuel waste repository.

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