Characteristics of the unique Y-HREE-F-rich pegmatite system revealed by zircon geochemistry: a case study from mt. Ploskaya amazonite deposit, Kola Peninsula

An amazonite-quartz-albite pegmatite body of the Mt. Ploskaya intrudes a metavolcanic complex of the Keivy Terrane (Kola Peninsula) and is unique in its diversity and an assemblage of ore and accessory Y-Yb-Ta-Nb-Be-Pb-F mineralization. The studied zircon grains have a heterogeneous structure associated with the presence of relics of primary zircon (lighter in BSE regime) in a matrix of altered mineral (darker in BSE regime). The composition of relics and matrix is characterized by a decreased content of Zr (0.67–0.81 apfu) and increased content of Hf (0.13–0.15 apfu) and Yb (0.01–0.02 apfu). The composition of the matrix has a defcit of the analytical sum, which can vary 3 to 5 wt. %, and a higher content of non-formula elements: CaO and UO2 (>1 wt. %) and Na2O and ThO2 (>0.2 wt. %). The average total rare earth element (REE) content of the relict zones and the matrix is 19400 ppm and 27400 ppm, respectively. The average contents of Y and some heavy REEs (HREE: Yb and Lu) have a similar distribution: 19370 (15420 and 2430) ppm in relicts compared to 27390 (21740 and 3140) ppm in the matrix. An increased Ce/Ce* ratio of the zircon matrix indicates the phase crystallization under more oxidizing conditions compared to relics. The content of volatile components (H2O, F, and Cl) of the zircon matrix is 5–8 times higher relative to the relics. In this case, a H2O:F:Cl ratio of zircon varies from 35:5:1 in relics of igneous zircon to 20:5:1 in products of its hydrothermal alteration. The study of zircon of the Mt. Ploskaya pegmatite shows a selective accumulation of a number of trace elements and volatiles in residual fuids during the magmatic-hydrothermal evolution of the system and the unique HREE enrichment as a consequence of metasomatic alteration of the protolith.