Composition and Structure of Zircon from Hydrothermal Uranium Occurrences of the Litsa Ore Area (Kola Region, Russia)

Tatiana Kaulina,Vadim Kamenetsky,Lyudmila Lyalina,Vadim Il’chenko,Maria Gannibal,Vladimir Bocharov

doi:10.3390/geosciences10080278

Abstract

Zircon grains from various metagranitoids (plagio- and monzo-granites, gneisses, metasomatic rocks, and pegmatoid veins) from the Skal’noe and Dikoe sites of the Litsa uranium ore area (Kola Region, Russia) were studied in order to reconstruct the sequence and timing of events in the area and to observe effects of hydrothermal process related to uranium mineralization on structure and composition of zircon. Individual zircon grains were studied by means of laser ablation inductively coupled plasma mass spectrometry (LA–ICPMS), ion microprobe and Raman spectroscopy. Isotopic LA–ICPMS data for the Skal’noye and Dikoe ore occurrences suggest the following age sequence of events in the area: intrusion of plagiogranites—2829 ± 12 Ma, formation of magmatic protolith of gneisses—2781 ± 17 Ma, metamorphism of plagiogranites—2636 ± 34 Ma; intrusion of monzogranites and pegmatoid veins—2549–2526 Ma, hydrothermal event with uranium input—2276 ± 21 Ma, last metamorphism of plagio- and monzo-granites—1892–1696 Ma. Ore-bearing rocks in the area are pegmatoid veins and quartz–feldspar metasomatites which contain uraninite. During a 2.3 Ga hydrothermal process, newly formed zircon rims grew simultaneously with the precipitation of uraninite in the veins and metasomatites. These rims are characterized by high U and rare earth elements (REE) contents (up to 6560 and 8760 ppm, respectively), dark cathodoluminescence, low Th-U ratios (0.1–0.007) and a flat LREE-enriched pattern, in some cases inherited from minerals, dissolved during a hydrothermal event (magmatic plagioclase and probably monazite). Hydrothermal zircon rims grew with partial dissolution of the magmatic zircon, as evidenced by the rounded and curved shapes of zircon cores. The degree of alteration caused by hydrothermal events depends on the uranium content in the pre-existing zircon. The effects of zircon alteration and newly formed zircon composition reflect the redistribution of uranium in rocks.

Highlights

Zircon is a widely used geochronometer due to its stability under many geological processes and its ability to retain radiogenic lead
The aim of the present paper was to show the influence of hydrothermal processes on zircon in uranium occurrences and the relationship between zircon alteration and the geochemistry of uranium
The back-scattered electron (BSE) and cathodoluminescence (CL) images, as well as the qualitative composition of inclusions, were studied using a LEO-1450 scanning electron microscope equipped with an XFlash-5010 Bruker Nano GmbH energy dispersive spectrometer with QUANTAX 200 software at the Geological Institute, Kola Science Center of Russian

Summary

Introduction

Zircon is a widely used geochronometer due to its stability under many geological processes and its ability to retain radiogenic lead (see, e.g., [1,2]). The zircon susceptibility to alteration increases in crystals with structure disordered due to metamictization ([13] and references therein), fracturing [14,15], or plastic deformation [16,17]. Zircon grains with high concentrations of U (and Th) are generally more susceptible to alteration, since the radioactive decay of these elements leads to the progressive damage of crystal structure with time ([13] and references therein). Zircon recrystallization associated with low-temperature fluid influence was confirmed by experimental zircon data [5] and geological observations [8,18,19,20]

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Conclusion