Potassium-, Uranium- and Thorium-concentrations in bedrock. A study of the Sahlgrenska Anomaly and Änggårdsbergen, Gothenburg

Abstract

This study investigates the RA-granite (also known as Kärra Granite), which form a northsouth trending intrusive body traversing Gothenburg. The spatial distribution of the radioactive elements K, U and Th in RA-granite was measured in southern Gothenburg (Änggårdsbergen and Sahlgrenska University Hospital), Sweden. The 1311 Ma old intrusive RA-granite has high concentrations of U and Th. It is the dominating bedrock of Änggårdsbergen, Sahlgrenska University Hospital and parts of Medicinareberget. The area has been deformed during two major orogenies but the RA-granite has only been affected by the last orogeny, the Sweconorweigan orogeny. This study investigates concentrations in the areas mentioned above, with focus on Sahlgrenska University Hospital/Medicinareberget where an enrichment in Th and K has been found in previous studies, the so called Sahlgrenska anomaly. The aims of the report were to, via field studies, chemical and optical analysis, map the spatial distribution of K, U and Th – and to investigate in which minerals U and Th reside. The main focuses of the report were to discuss the possible extension of the Sahlgrenska anomaly, evidence of possible hydrothermal and tectonic events in this area. The spatial distribution of Th shows a local enrichment surrounding the Sahlgrenska anomaly and an increase of concentrations from east to west. Uranium has lower concentrations surrounding the anomaly. Potassium is enriched surrounding the Sahlgrenska anomaly but have normal granite values throughout the field area. The study has shown new normal values for the radioactive elements in RA-granite, 3.5-5 % K, 7-18 ppm U and 30-70 ppm Th. The U/Th ratios indicates a hydrothermal event both in parts of Änggårdsbergen but also surrounding the Sahlgrenska anomaly. The radioactive elements U and Th were found in accessory minerals zircon and titanite, both primary but also possibly secondary titanite. No biotite was found, but instead we found the typically hydrothermal mineral chlorite, so all biotite could have been chloritized. Allanite was found in previous studies but not in this study. The U/Th ratios indicates a hydrothermal event both in parts of Änggårdsbergen but also surrounding the Sahlgrenska anomaly. The conclusion of the report was that we now have normal values for the RA-granite, that hydrothermal events have taken place at the Sahlgrenska anomaly and tectonic implications were seen in the microscopy studies.