Britholite ores of the Sakharjok Zr–Y–REE deposit, Kola Peninsula: Geochemistry, mineralogy, and formation stages

Abstract

Britholite ores in the complex Sakharjok Zr–Y–REE deposit (Kola Peninsula) form linear bodies in nepheline syenite and contain britholite-group minerals and zircon as main ore minerals. Geochemical data indicate that the formation of the britholite ores of the Sakharjok Massif was mainly controlled by magmatic differentiation and lateto post-magmatic reworking of the rocks by alkaline and F, CO2-bearing fluids. The elevated content of ore components in magma is caused by its derivation from enriched mantle source. It was established that crystallization of britholite occurred at the late and post-magmatic stages of the massif formation and was assisted by fluids with different physicochemical properties. The widest spread fluorbritholite-(Ce) typical of the trachytoid nepheline syenite crystallized mainly during albitization from highly alkaline, F-rich and CO2-bearing fluids/solutions. Britholite-(Ce) and fluorbritholite-(Y) found in the most recrystallized porphyritic nepheline syenite were formed at the later hydrothermal stage from F-bearing water-rich (metamorphic?) solutions. Fluorcalciobritholite crystallized from high-temperature pegmatite melt/solution at high CO2 activity. Postcrystallization alterations of the britholite-group minerals from the Sakharjok deposit resulted in the formation of altered zones within crystals and rims around them. The composition of overgrowth rims indicates the removal of F, Ce, and La from britholite.