Distinguished multiple events of fracture mineralisation related to far-field orogenic effects in Paleoproterozoic crystalline rocks, Simpevarp area, SE Sweden

Detailed investigations of fracture minerals have been carried out in the Simpevarp area, SE Sweden, in order to reveal the low-temperature evolution of the site and to discern different events of deformation, especially in the brittle regime. The study is part of the site characterisation programme for a potential deep-seated repository for spent nuclear fuel. The bedrock in the area is composed of generally un-metamorphosed 1.8 Ga crystalline rocks. Detailed SEM investigations of cross-cutting fracture mineralisations have revealed several generations of fillings formed at gradually lower temperatures during repeated events of fracturing and reactivation from the Paleoproterozoic until the Quaternary. 40Ar/39Ar geochronology and fracture orientation analyses have been used to relate each fracture filling generation to geological events, such as far-field effects from orogenies, as well as more local effects related to intrusions. Fracturing and formation of different fracture mineral parageneses during at least four different orogenies have been indicated: the waning stages of the Svecokarelian orogeny (> 1.75 Ga), the Danopolonian orogeny (~ 1.47–1.44 Ga), the Sveconorwegian orogeny (~ 1.1–0.9 Ga) and the Caledonian orogeny (~ 0.5–0.4 Ga). The fracture minerals and greisen related to the Danopolonian orogeny mainly formed in relation to the intrusion of two indicatively Danopolonian-related granites nearby. Furthermore, periods of extension are indicated in the area by the presence of late-Sveconorwegian dolerites. Fractures filled with lower Cambrian sandstone are plausibly formed in relation to far-field extensional effects of the opening of the Iapetus Ocean. Fracture minerals of Paleozoic age or younger show influence of loading and unloading cycles. The youngest fracture minerals identified indicate precipitation from waters of similar composition as the present groundwater.