FLUORITE FROM RARE-METAL DEPOSITS OF SUSHCHANO-PERZHANSKA TECTONIC ZONE (UKRAINIAN SHIELD): CRYSTAL CHEMISTRY AND GENETIC FEATURES
Keywords:
fluorite, crystal chemistry, X-ray-luminescence, photoluminescence, in-frared spectra, optical active centres, rare metal mineralization, metasomatism, Sushchano-Perzhanska tectonic zone, Ukrainian shield.Abstract
Fluorite from two rare-metal objects of Sushchano-Perzhanska tectonic zone (Volyn block, Ukrainian shield) has been investigated: from Perzhanske Be deposit (genthelvite, phenacite) and Yastrubetskyi Zr-REE-Y ore occurrence (zircon, britholite, orthite etc.). The nature and, if necessary, annealed (1173 K, 20 min) crystals were stud-ied by means of X-ray luminescence (RL) and photoluminescence (PhL), IR-spect-roscopy and X-ray analysis. Fluorite of different colour and morphological features has been sampled from per-zhanski granites (including those with blue quartz) and granite-porphyries, genthelvite-bearing siderofillite-albite-microcline-perthite and albite-microcline-perthite metasoma-tites and greisens (Perzhanske deposit) and from leucocratic syenites (with ferroedenite hornblende and biotite) of the upper layered series and meso-, leucocratic syenites (am-phibole-biotitic and biotitic) of the main layered series, aegirine-riebeckite syenites of the central core of the Yastrubetskyi massif and also from exposed quartz blocks in the south-eastern part of the Yastrubetskyi ore occurrence. Fluorite crystals from Perha syenites and aegirine-riebeckite varieties of granites are almost identical to each other by the bands and lines of different optically active centres ratio intensity of in RL-spectra. The high concentration of Er3+·Na+, Dy3+·Na+, Dy3+·Fί⎯, Се3+ and Sm3+-centres are typomorphic for them, as like as for fluorites from aegirinearfvedsonite- annite syenites of Nb-Ta-Zr-REE Katuginske deposit (Russia). The ratio TR3+ Y/TR3+ Ce is 1.5–2.5. Fluorites from granite-porphyries and leucocratic syenites of the Yastrubetskyi massif are also similar to each other. They are characterized by less concentration of Sm3+ and lack of Се3+; TR3+ Y / TR3+ Ce = 2.6–3.6. Intensity of Dy3+·OH⎯ line (573 nm) in RL-spectra of fluorite from Perha granites and syenites is very weak. But it is more intensive in spectra of crystals from granite-porphyries. Its dominating intensity totally changes the spectra configuration of fluorite from veinlets in quartz blocks. In IR-spectra of this fluorite, the clear bands of stretching (3 265–3 280 cm–1) and banding (~ 1 740 cm–1) vibrations of structurally bounded water are observed. Tendency to accumulate Mn in the fluorite of late generations is inherent to both rare-metal objects. Decrease of the total impurity centres concentration (parameter Σimp.: Σdef) and increase of TR3+Y-ions content are also observed. Lines of U6+-ions in spectra of some annealed (1173 K, 20 min.) crystals in PhL-spectra were observed. Bright green radiation of ions U6+ in crystals can be continuous, distributed by zones or have dotted character. This indicates the superimposed character of uranium isomorphic entrance to the fluorite structure. The red gleam of visually dark purple crystals and high concentration of M-centres (2VF–) are the peculiarities of U-bearing fluorite. Transportation (migration) of uranium perhaps is due to some late rising deep fluor-carbonate fluids in zones of tectonic frac-turing. Increasing of a unit cell parameter directly correlates with the impurity ion con-tent and low values of the coherent scattering area L – with crystallinity. They are de-fined by the hydration degree and by the isomorphic features of impurity elements in-cluding uranium. The revealed variations in fluorite crystal chemistry features from the deposits of two different magmatic systems within Sushchano-Perzhanska tectonic zone indicates heterogeneity of rare and volatile components of their maternal sources and is the evi-dence of the many stages of this mineral crystallization conditions. For Perzhanske Be deposit they are connected with the processes of infiltration metasomatism and for Yas-trubetskyi Zr-REE-Y ore occurrence – with crystal differentiation of deep magma.
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