Brittle tectonothermal evolution in the Forsmark area, central Fennoscandian Shield, recorded by paragenesis, orientation and Ar-40/Ar-39 geochronology of fracture minerals

In this paper, we show how studies of fracture mineral paragenesis, their orientation and Ar-40/Ar-39 geochronology can be applied in order to recognise the brittle tectonothermal evolution in an area. Samples were selected from nearly 18 km of drill cores from the upper I km of the Fennoscandian Shield obtained during the site investigation for a repository of spent nuclear fuel in Forsmark, central Sweden. Four major events of fracturing and/or reactivation of fractures associated with fracture mineralisation have been distinguished. The first event was characterised by precipitation of epidote, quartz and chlorite, along preferably sub-horizontal and gently-dipping fractures or steep, WNW-ESE to NW-SE fractures. Precipitation occurred between 1.8 and 1.1 Ga, possibly during the late stage of the Svecokarelian orogeny close to 1.8-1.7 Ga. The second event is associated with precipitation of hematite-stained adularia and albite, prehnite, laumontite, calcite and chlorite, preferably along steep, ENE-WSW to NNE-SSW and NNW-SSE fractures. Precipitation occurred around 1107 +/- 7 to 1034 +/- 3 Ma, probably due to effects from the Sveconorwegian orogeny. This event was followed by a period with dissolution of fracture minerals and subsequent precipitation of mainly calcite, quartz, pyrite and asphaltite during the Palaeozoic. The formation fluid emanated from an overlying organic-rich sedimentary cover. Precipitation occurred during reactivation of Proterozoic fractures, but formation of new fractures is also inferred, possibly due to far-field effects of the Caledonian orogeny, or elevated hydrostatic pressure due to its foreland basin. The latest event is dominated by clay minerals, chlorite and calcite along hydraulically conductive fractures. These minerals are prominent along sub-horizontal and gently-dipping fractures, but also occur in sets of steeply-dipping fractures. It is inferred that the hydraulically conductive fractures are Proterozoic structures in which mineral precipitation has occurred during a long period. However, some of the near-surface, sub-horizontal fractures may be Quaternary in age. (C) 2009 Elsevier B.V. All rights reserved