Isotopic and Geochemical Features of High-Hafnium Zircons of the Vasin-Mylk LCT Pegmatite, Kola Peninsula: Compositional Zoning and Crystallization Conditions

A comprehensive investigation was conducted on high-hafnium zircons from the LCT (Li-Cs-Ta) pegmatites of the Vasin-Mylk rare-metal deposit within the Fennoscandian Shield. In situ analysis of trace element composition and oxygen isotope ratios were performed using secondary ion mass spectrometry (SIMS), complemented by internal structural examination via scanning electron microscopy (SEM). The research focuses on deciphering compositional zoning within zircon crystals and characterizing their geochemical signatures to constrain crystallization conditions. The study revealed anomalously high concentrations of Hf (up to 381,000 ppm) and Li (up to 152 ppm), paired with extremely low abundances of U (~10 ppm) and total rare earth elements (~35 ppm). Marked geochemical contrasts were identified between the central and rim domains of the zircons. Central zones display well-fractionated rare earth element (REE) patterns featuring positive Ce and negative Eu anomalies, while the high-Hf rims exhibit weakly differentiated spectra with variable Ce anomalies. The identified W-type tetrad effect suggests crystallization from a melt strongly influenced by coexisting fluids. The obtained δ18O values are consistent with a mantle source and suggest crystallization within a system closed to external fluids. The zircons from the Vasin-Mylk deposit crystallized during the transitional period between the late magmatic and early hydrothermal stages of a highly differentiated pegmatite system. These results contribute to a better understanding of ore genesis in LCT pegmatite systems.