Distribution of radioactive elements in the RA-granite bedrock northeast of Säve, Sweden A comparison with the Änggårdsbergen area of Central Gothenburg

Abstract

This study aims to compare data regarding the spatial distribution of radioactive elements K, U and Th in two exposed areas of the RA-granite, a 1311 ±8 Ma granitic unit unevenly enriched in these elements. Several previous studies have been carried out in the Änggårdsbergen area of Gothenburg, and the results of these were compared with data and physical samples collected from an RA-granite ridge northeast of Säve, roughly 16 km north of Änggårdsbergen but part of the same unit. The spatial relationships observed in the Säve ridge were found to be different from those observed in Änggårdsbergen. Whereas in Änggårdsbergen both U and Th increased to the western part of the area, and no spatial relationship could be detected in the K concentration, in the Säve ridge the K concentration increased markedly in the western part of the area, the U concentration was highest in the middle and east, and the Th concentration was highest in the east. Magnetic susceptibility was also measured and compared, and found to be greatest in the western part of the Säve ridge, in contrast with the central band of increased susceptibility found in Änggårdsbergen. Similarly to studies of Änggårdsbergen, no correlation between magnetic susceptibility and concentration of K, U or Th could be found. However, an increase in U concentration in the rock of the Säve ridge was generally accompanied by an increase in Th concentration. Analysis of physical samples taken from the Säve ridge showed that U seems to mainly reside in zircon and probably in allanite, whereas Th mainly resides in allanite, monazite and xenotime. Potassium is believed to mainly reside in potassium feldspars, which become less common in areas of lower potassium concentrations. Based on the mineralogy found in analysis of thin sections, the degree of hydrothermal alteration in the rock increases from west to east, with significant breakdown of biotite, iron oxide and titanite to form chlorite and ilmenite.