ZIRCON AS INDICATOR OF MAGMATIC-HYDROTHERMAL TRANSITION IN THE EVOLUTION OF RARE-METAL PEGMATITES (ON THE EXAMPLE OF THE KOLMOZERO AND POLMOSTUNDRA LITHIUM DEPOSITS, KOLA PENINSULA, RUSSIA)
E.V. Levashova1, D.R. Zozulya2, L.N. Morozova2,3, S.G. Skublov1,4, P.A. Serov3
1Institute of Precambrian Geology and Geochronology, Russian Academy of Sciences, St. Petersburg, Russia
2В Geological Institute of the Kola Science Center, Russian Academy of Sciences, Apatity, Russia
3All-Russian Research Institute of Mineral Resources named after N.M. Fedorovsky, Moscow, Russia
4Saint Petersburg Mining University named after Empress Catherine II, Saint Petersburg, Russia
Keywords: zircon, geochemistry of trace elements, geochemistry of rare-earth elements, rare-metal pegmatites, Kolmozo deposit, Polmostundra deposit
Abstract
A detailed mineralogical and geochemical study (using EPMA and SIMS methods) of zircons from rare metal pegmatites of the Polmostundra and Kolmozero lithium deposits of global significance (Kola Peninsula, Arctic zone, Russia) has been conducted. The distinctive characteristics of the studied zircons are: (1) anomalously high lithium content (up to 327 ppm Li according to SIMS data); (2) high hafnium content (up to 39.21 wt.% HfO2 according to EPMA data); (3) low Zr/Hf ratios
(ranging from 0.74 to 4.70); (4) high volatile component content (up to 5.98 wt.%
– water, up to 0.65 wt.% – F, up to 0.12 wt.% – Cl according to SIMS data); (5)
high content of trace elements, the main of which are Th, Ta, U, Nb, Ca, P. The
results obtained indicate the rare metal specificity of the composition of the
highly fractionated pegmatite melt of the Polmostundra and Kolmozero deposits,
from which the zircons crystallized, and indicate a high degree of its fluid
saturation. The extremely low values of the Zr/Hf ratio for zircons can be
explained by the combined effect of fractionation of the original granite melt
with lithium accumulation in the residual melt during the final stage of
pegmatite evolution and the role of fluids saturated with volatile elements.
Zircons crystallization occurred during the magmatic stage of pegmatite
formation, while its alteration took place during the hydrothermal stage. The
distribution of REE in zircons from the Polmostundra and Kolmozero deposits
indicates the formation of core zones of grains during the magmatic stage and
rim zones during the hydrothermal stage
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