Zircon as an Indicator of Magmatic-Hydrothermal Transition in the Evolution of Rare Metal Pegmatite (using the example of the Kolmozero and Polmostundra lithium deposits, Kola Peninsula, Russia)

Abstract

Abstract —This paper describes a detailed mineralogical and geochemical study (using electron probe microanalysis and secondary ion mass spectrometry) of zircon from rare metal pegmatites of the Polmostundra and Kolmozero lithium deposits (Kola Peninsula, Arctic zone, Russia). The zircons studied here have several distinguishing features. Firstly, anomalously high lithium content (up to 327 ppm Li according to secondary ion mass spectrometry data). Secondly, high hafnium content (up to 39.21 wt.% of HfO2 according to electron probe microanalysis data). Thirdly, low Zr/Hf ratios (from 0.74 to 4.70). Fourthly, high content of volatile components (up to 5.98 wt.% of H2O, up to 0.65 wt.% of F, and up to 0.12 wt.% of Cl according to secondary ion mass spectrometry data). Finally, higher content of impurity elements, the main of which are Th, Ta, U, Nb, Ca, and P. It is revealed by the results obtained that the rare metal specificity of the composition of highly fractionated pegmatite melts of the Polmostundra and Kolmozero deposits, from which zircons crystallized, are the key indicator of the high degree of its fluid saturation. The extremely low values of the Zr/Hf ratio for zircons can be explained by the combined influence of fractionation of the initial granitic melt with the accumulation of Li in the residual melt at the final stage of the evolution of pegmatites and the role of fluids saturated with volatile elements. Zircon crystallization occurred at the magmatic stage of pegmatite formation, and zircon alterations occurred at the hydrothermal stage. The nature of the rare earth element distribution in zircons from the Polmostundra and Kolmozero deposits indicates that the formation of the central zones and margins of grains took place during the magmatic and the hydrothermal stage, respectively.