U-Pb dating of fluid interactions: Hidden tectonic events in the Telemark domain (southern Norway)

Regional tectonic events are difficult to detect in a stable continental crust, especially in the absence of major tectonic structures. Tectonic events may, however, trigger crustal fluid migration, leading locally to the formation of alterations zones in uraninite and uranium-bearing minerals. Accordingly, these events may be assessed, for example, by U-Th-Pb dating.

The Evje-Iveland pegmatites intruded at the end of the Sveconorwegian orogeny into the metamorphic basement of the Telemark tectonic domain of southern Norway. After magmatic crystallization, several fluid interactions lead to the alteration of igneous minerals and to the formation of discrete secondary mineral assemblages. We investigated uraninite and secondary uranium-phases from a range of Evje-Iveland pegmatite localities.

Pegmatites in the Landsverk area have been subjected to a prominent hydrothermal brecciation event, leading to the crystallization of hydrothermal minerals, including quartz, albite, epidote and titanite assemblages. A broad age group around 500 Ma is also obtained from hydrothermally altered domains of the Einerkilen uraninite. In addition, secondary uranium phases uranophane-β and uranophane yield chemical ages corresponding to the Early Cretaceous and Paleogene. The post-Sveconorwegian mineralisation can tentatively be related to the Caledonian orogeny, opening of the North Sea rift system, and Scandinavian uplift, respectively.

Distinct age populations from hydrothermally overprinted uranium ores can be correlated with regional tectonic events, not otherwise detected in a stable continental crust.