The Alkaline Rocks of Almunge (Central Sweden): Age, Petrogenesis and REE-potential

Sammanfattning

Rare Earth Elements (REE) have become a hot topic in the last few decades and are often considered to play a key role in the green transition but are also the source of international disputes. The REE’s consist of the 15 elements 57La–71Lu from the lanthanide series plus 21scandium and 39yttrium which all share similar geochemical and physical features. A combination of different REE-metals is used in much of the electronic we use in our daily lives, from cerium and lanthanum in LED-screens to neodymium and praseodymium in turbines used for electric vehicles and wind turbines; thus, the use of the elements is also expected to rise as the market expands. Today the global leader of extraction and production of REE-elements and technology is China which has caused the European union to classify REE-elements as critical raw materials with the aim ofemphasizing the importance of having domestic exploration, extraction and production as to not be dependent on international trade. Today there are no operating REE-mines in Europe with the exception for a few localities on the Kola peninsula in Russia, but both Europe and Sweden are home to many alkaline intrusions such as Norra Kärr, Alnö, Illímaussaq (Greenland) and Almunge outside of Uppsala. These rare intrusions are of interest as the presence of alkaline rocks are closely linked to the presence of REE hosted in exotic minerals such as eudialyte. The Almunge alkaline complex outside Uppsala is an intrusion which measures c. 5 x 3 kilometers in diameter and was first explored in 1914 by P. Quensel who detected cerium (Ce2O3) in the area, Gorbatschev built upon Quensel´s findings in the 1960´s but all in all, little geochemical analysis has been done in the area.This study has combined field work with optical microscopy and geochemical analysis such as electron microprobe to evaluate the REE-potential of the Almunge intrusion with focus on the four bodies of foid-bearing intrusive rocks within. The resulting analysis of trace elements has concluded that the concentrations of REE are low but with an emphasis on concentrations of light rare earth elements over heavy rare earth elements. Additionally, the radiometric dating of biotite mica and apatitefrom the intrusion has yielded an age of c. 1.9 Ga thus making Almunge the oldest alkaline intrusion in Sweden; and a result of syn-orogenetic magmatism from the Svecokarelian orogeny rather than aseparate, later intrusion. The results from the radiometric dating are coeval with those from previous studies of the area. Lastly, two new outcrops of nepheline-syenite have been found and additional data from the geochemical analysis has resulted in greater knowledge of the area.