Mineralogical investigations of fractures

Optical microscopy, X-ray diffraction, and chemical analyses have been used to identify fracture filling minerals from areas at Finnsjoen, Uppland and Sternoe, Blekinge. These areas were selected, as investigation areas in order to study geological conditions within potential sites for radioactive waste disposal. Chemical analyses of minerals including stable isotopes and fluid inclusion studies have been made. Mineralization and dip of sealed and open fractures respectively were compared. The following results have been obtained: Dominating fracture filling minerals in Finnsjoen are calcite, prehnite and quartz. Chlorite, dolomite, wairakite, stilbite, laumontite, epidote, pyrite and clay minerals have also been identified. Microbreccias as well as mylonites are common phenomena. Calcite has been deposited as filling material at several occasions and at different physical conditions. Most of the silicates identified as filling material were probably deposited at hydrothermal conditions. Calcite analysed from Finnsjoen was deposited from water different from the present groundwater or deposited at a temperature which probably was more than 100 deg C. When the analysed calcite was deposited the temperature gradient was higher than at present. The filling mineral distribution is different in open and sealed fractures respectively. Dips of fractures are different for open and sealed fractures respectively in the upper part of the boreholes Fi 7 (Finnsjoen) and Ka 4 (Sternoe). Most of the gently dipping fractures are open and are suspected to be relatively young. A temperature of approximately 200 deg C prevailed during deposition of the calcite studied at Stripa. (author)