As-P-REE-bearing zircon in granitic pegmatites of the axial zone of the Polar Urals

Research subject. Zircon with high contents of P, Y, REE, and As from altered granitic pegmatites forming veins cutting through the amphibolites of the Kharbey metamorphic complex (Polar Urals). Aim. To study the morphological features, internal structure, and chemical composition of zircon, as well as to establish the mechanism of its formation.

Methods. The study of zircon was carried out under binoculars, electron microscopes, and a Raman spectrometer. The internal structure of the mineral was analyzed using images obtained in the BSE and CL modes.

Results. In granite pegmatites, i.e., biotite-quartz-oligoclase and biotite-microcline-quartz-oligoclase rocks with a high content of Na2O (about 6 wt %), two morphological types of zircon were identified – prismatic pink and long prismatic brown. Prismatic pink varieties have an internal structure and composition characteristic of “classical zircon”, having crystallized from a magmatic melt at temperatures of 700–750°C. In individual cases, such crystals are overgrown with a thin rim of zircon, which has a dark color in CL images with an increased content of Ca, Al, Fe, Na, P, Y, REE, and As. Brown zircons are characterized by growth areas and those with uneven blocky, mosaic, and porous structures that appear dark in CL images. The darkest areas of the mineral (in images in CL and BSE modes) show increased concentrations of P2O5 (up to 6 wt %), Y2O3 (up to 9 wt %), UO2 (up to 4 wt %), ThO2 (up to 3 wt %), REE, FeO (up to 3 wt %), Al2O3 (up to 3 wt %), CaO (up to 3 wt %), and Na2O (up to 1 wt %), with the degree of disorder of the mineral structure (metamictity) increasing. The above elements, as well as, apparently, the hydroxyl group, are included in the structure of zircon according to complex substitution patterns. Crystallization of this type of zircon and the mineral that forms rims around zircon of the first type occurred at the post-magmatic stage of transformation of granites from hydrothermal fluid of high alkalinity at temperatures of 550–600°C. Zircon was subjected to repeated changes under the influence of solutions according to the principle of dissolution–redeposition, which occurred under a decrease in temperatures down to 240–330°C. As a result, zircon acquired a spongy structure, in the pores of which hydrothermal minerals were formed – arsenic pyrite, quartz, monazite, xenotime, chernovite, ankerite, albite, etc.

Conclusions. In the granitic pegmatites that form synmetamorphic veins in the amphibolites of the central area of the Kharbei metamorphic complex, three types of zircon are observed: magmatic (zircon of the first type), hydrothermal, and hydrothermally altered (zircon of the second type), differing in morphological features, internal structure, and composition. Judging by the chemical composition of hydrothermal minerals in the rocks, post-magmatic solutions were enriched in Na, P, As, and REE.

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