Zr Mineralization of Alkali Rhyolites of the Pechalny Ore Field, Northeast Russia

The mineral forms of Zr and rare earth elements (REEs) in trachyrhyolites of the Pechalny ore field are considered. It is found that the Zr and REE minerals (zircon and monazite) form unusual polymineral microgranular isometric aggregates of globular structure. The size of the mineral particles in these aggregates varies from <0.01 to 1 µm. The aggregates are characterized by a high REE content and varying chemical composition. Zircon, which is dominant in these aggregates, is enriched in REEs and contains a hydroxyl group. The polymineral aggregates of the final stage of the formation are characterized by a spherical morphology, in which the peripheral part (rim) contains the maximum amount of Zr and REEs, whereas the central part contains the hydroxyl group and molecular water based on Raman spectroscopic data. The zircon microcrystals are dispersed in the entire rock volume, whereas the larger aggregates of spherical morphology are associated with Fe hydroxides or occur at the contact of quartz–feldspar aggregates with Fe-enriched zones. Joint fine intergrowths of zircon and REE minerals, the globular morphology of polymineral aggregates, and the heterogeneous structure and composition of globules indicate their formation during the postmagmatic process.