On the origin and age of hydrothermal thorium-enriched carbonate veins and breccias in the Møre-Trøndelag fault zone, central Norway.

Fault-controlled, thorium-enriched carbonate veins of the inner Trondheimsfjord area have been investigated mineralogically and palaeomagnetically in order to provide a time-frame for tectonic and hydrothermal activity within the Møre-Trøndelag Fault Zone (MTFZ). As determined by optical microscopy, SEM and XRD, the red veins and breccias have a mineralogy dominated by quartz, Kfeldspars, iron-carbonates, calcite, fluorite and barytes. The main thorium-containing phase is thorogummite (Th(SiO4)1-x(OH)4x. The hydrothermal fluids which altered host-rocks adjacent to faults and joints changed throughout time. The earliest hydrothermal phase was one of Na-metasomatism, later followed by precipitation of iron-carbonates. Subsequent phases of hydrothermal alteration include a dominant phase of K-metasomatism and a later quartz, calcite and fluorite hydrothermal flux. Locally, veins and breccias are highly thorium-enriched, the average thorium content of analysed samples is 760 ppm, the Th/U-ratio being 28/1. The red veins disclose three remanence components, AI, All and B , the first two being confined to highly radioactive, late-stage, thorium-enriched breccias. The B component is found both in host-rocks and hydrothermally altered veins. The results indicate magmatic and/or hydrothermal activity which we relate to tectonic events during Permian (B ), Late Jurassic/Early Cretaceous (All) and/or Late Cretaceous/Early Tertiary (AI) time.