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The present study shows how detailed microscopy complemented by 40Ar/39Ar, 87Sr/86Sr and stable isotope data from fracture minerals increase the understanding of the low-T geological evolution in an area. At Forsmark, central Sweden, deep boreholes have been drilled through the crystalline bedrock as part of the site investigations conducted by the Swedish Nuclear Fuel and Waste Management Company (SKB).
A sequence of several events of fluid circulation and mineral formation has been distinguished, based on e.g. cross-cutting relations between different parageneses and stable isotope data. 40Ar/39Ar dating has been carried out to obtain absolute time constraints. Four main events of fracture mineral precipitation have been distinguished:
Generation 1 consists of epidote, quartz and chlorite, and brittle-ductile to brittle cataclasite sealed with these minerals (T 300°C). They are conspicuous in sub-horizontal and gently dipping fractures or in steep, WNW-ESE to NW-SE fractures. However, they are also present along fractures in other steeply dipping sets. This generation formed between 1.8 and 1.1 Ga, and is possibly related to the late Svecokarelian and/or Gothian tectonothermal events.
Generation 2 consists of a sequence of hydrothermal fracture minerals (T∼150-280°C) dominated by adularia, albite, prehnite, laumontite, calcite, chlorite and hematite. Generation 2 minerals are particularly common along steep, ENE-WSW to NNE-SSW and NNW-SSE fractures. 40Ar/39Ar ages from adularia indicate that this mineral either formed or the isotope system was reset in connection with early Sveconorwegian tectonothermal activity at 1.1 to 1.0 Ga. Formation of new fractures and breccias are inferred during this period and dissolution of fracture minerals occurred before the formation of generation 3 minerals.
Generation 3 consists of minerals precipitated under low-T conditions during the Paleozoic. The most abundant minerals are calcite, quartz, pyrite, corrensite and asphaltite. Stable isotopes in calcite and the presence of asphaltite indicate that the formation fluid was influenced by organic material, which may have emanated from an overlying sedimentary cover. The orientation of fractures with generation 3 minerals suggests reactivation of fractures filled with older minerals.
Generation 4 is dominated by chlorite/clay minerals and thin precipitates of calcite in predominantly hydraulically conductive fractures and fracture zones. These minerals are prominent along sub-horizontal and gently dipping fractures, but also in different sets of steeply dipping fractures. It is inferred that the hydraulically conductive fractures are ancient structures and that mineral precipitation has most likely occurred during a long period of time (after the Paleozoic?). However, some of the near-surface, sub-horizontal fractures, which include sheet joints formed in connection with stress release, may be Quaternary in age.
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