Fluid Evolution in the Iivaara Alkaline Complex (Finland): a Fluid Inclusion Study

Alkaline igneous rocks are known to host a variety of rare metal deposits, including HFSEs and REEs. The Iivaara alkaline complex (Finland) and the surrounding fenite aureole show Ti-dominated enrichment of HFSEs carried by titanite and apatite. Mineralogy and textural observation indicate a shallow intrusion level, fast cooling, and steep temperature gradients, as well as expulsion of different fluids. So far, the roles of different fluids in rare metal enrichment in such systems have not been fully understood. New results of a fluid inclusion (FI) study including FI petrography, microthermometry, phase identification using Raman spectroscopy, and quantification of elements by LA-ICP-MS reveal the fluid evolution in the Iivaara alkaline complex. Three stages can be distinguished: magmatic fluid, post-magmatic fluid, and late aqueous fluid. Magmatic fluid is characterized by relatively high salinity, with a dominance of methane in the vapor phase. This fluid type is responsible for the fenitization process as well as transporting some important elements and metals like HFSEs and REEs. The post-magmatic fluid is characterized by relatively low salinity with very low to no methane concentration in the vapor phase. This fluid is responsible for recrystallization of some minerals, especially apatite and the formation of cancrinite as a replacement mineral and as a vein. And the last fluid is an aqueous fluid which is very low in salinity, interpreted as meteoric water influx at the very late stage.