REE fractionation during crystallization and alteration of fergusonite-(Y) from Zr-REE-Nb-rich late- to post-magmatic products of the Keivy alkali granite complex, NW Russia

The Keivy alkali granite complex, Kola Peninsula, NW Russia, contains numerous Zr-REE-Y-Nb occurrences and deposits, formed by a complex sequence of pegmatitic, hydrothermal and metasomatic processes. The rare-metal rich lithologies have abundant fergusonite-(Y) mineralization. >90 EPMA analyses of fergusonite-(Y) from different occurrences are used to document REE behavior during crystallization and alteration of the mineral. Variations in Y/HREE ratio during growth of fergusonite-(Y) in a pegmatite are explained by a change of the fluorine content in the fluid. Loss of Y and HREE and addition of Ce during alteration are governed by two major processes: Ce accumulation in late derivatives and high F contents in the crystallization environment. The decreases of Ce and other REE in some occurrences resulted from a low F content. The extent of alteration possibly depends on the actinide content in fergusonite-(Y) and the resulting radiation damage. Positive Eu/Eu* anomalies in fergusonite-(Y) from a number of occurrences were due to external contributions of Eu into the fluid from country rocks after the breakdown of plagioclase. The specific REE distribution in fergusonite-(Y) indicates crystallization at low CO2/H2O and changing F activity. A F-rich fluid is suggested during fergusonite-(Y) formation in alkali granite pegmatite and quartzolite, and a low-F fluid is characteristic of minerals from a metasomatite and amazonite pegmatite located in country rocks of alkali granite massifs.