Britholite-group minerals as sensitive indicators of changing fluid composition during pegmatite formation: evidence from the Keivy alkaline province, Kola peninsula, NW Russia

The Keivy alkaline province, Kola Peninsula, NW Russia, consists of vast alkali granite massifs and several dike-like nepheline syenite bodies. It contains numerous rare-metal occurrences, formed by a complex sequence of magmatic, late-magmatic and post-magmatic (including pegmatitic) processes. The Sakharjok nepheline syenite pegmatite contains a remarkably diverse number of britholite group minerals, pointing to different physico-chemical conditions in the fluid. REE and actinides distribution in the host rock indicates that the late-magmatic (and pegmatitic) fluids were alkaline, with significant amounts of F and CO. From REE and F variations of the britholite group minerals possible fluid compositions at different stages are suggested. The earliest fluorbritholite-(Ce) formed locally from a late magmatic, high temperature F-rich fluid. Fluorbritholite-(Y) presumably crystallized from a F-bearing and CO-rich fluid; marked F saturation resulted in precipitation of abundant fluorite due to a temperature drop. Variations in REE and F contents in the most abundant fluorcalciobritholite indicate a successive decrease of F in the fluid during its evolution. The relationship between intergrown fluorapatite and fluorcalciobritholite and the presence of zones with a REE-rich fluorapatite between them indicate a continuous to sudden crystallization in this mineral sequence. The сrystallization of the latest “calciobritholite” is related to the input into the fluid of CO and/or HO.