Laser-ablation ICP-MS analyses of rutile from the Bamble region, S. Norway
The Bamble Sector (Arendal-Bamble region) of the Fennoscandian Shield in S. Norway is affected by metasomatic processes related to large-scale fluid migration during Sveconorwegian (Grenvillian) tectonothermal events. In some areas such as at Ødegårdens verk, pervasive fluid infiltration has transformed gabbroic amphibolites into a characteristic scapolite-hornblende rock (ødegårdite). Ilmenite in the gabbroic protolith breaks down to rutile during the metasomatic processes while iron is removed from the system by the fluids. Thus, the metasomatism creates rutile ores. The region has been regarded to have a potential for economic rutile deposits. However, a disadvantage is a relatively high content of uranium in the rutile. In order to obtain additional information on the character of rutile in various parts of the region and the U contents, rutile from stream sediment heavy fractions have been studied by SEM and analysed for U by laser-ablation ICP-MS. Mixed mineral grains with ilmenite, rutile and titanite are common in the stream sediments indicating that rutile-forming geological processes in the respective areas have been incomplete; some stream sediment samples have "good rutile" without ilmenite or titanite intergrowths. Based on the laser-ablation ICP-MS, analysed rutile from the Arendal-Bamble region contains 30-80 ppm U in general, although values up to 180 ppm U have been identified. Values below 10 ppm U are common in ilmenite-rutile-titanite intergrowths representing incomplete rutile transformation. The potential for economically attractive rutile deposits in the Arendal-Bamble region is probably small due to the overall high uranium content in rutile.