Fluoride-silicate melt immiscibility and its role in REE ore formation: Evidence from the Strange Lake rare metal deposit, Québec-Labrador, Canada

We propose that during or soon after emplacement, the early granitic magma exsolved a calcium fluoride melt into which rare earth elements (REE) partitioned preferentially. The conjugate silicate melt was consequently depleted in Ca and the REE, and enriched in Zr, Nb and Ti. Crystallization of the fluoride melt occurred late in the crystallization history of the silicate magma allowing it to accumulate in the volatile-rich residual magma that formed the pegmatites. This played a major role in the extreme enrichment of the pegmatites in Ca, F and REE. Crystallization of the pegmatites proceeded inwards from an outer zone in which feldspars, quartz, arfvedsonite and zirconosilicate minerals dominate to a core where the immiscible fluoride liquid collected and crystallized fluorite and REE minerals. Fluid exsolved from the silicate melt reacted with the products of both the silicate and fluoride melts, remobilizing most of the HFSE including the REE. This study reports a rare example of silicate-fluoride melt immiscibility and the first in which such immiscibility played a role in concentrating the REE to potentially economic levels.