The evolution of immiscible silicate and fluoride melts: Implications for REE ore-genesis

Abstract

The Mid-Proterozoic peralkaline Strange Lake pluton (Québec-Labrador, Canada) exhibits extreme enrichment in high field strength elements (HFSE), including the rare earth elements (REE), particularly in pegmatites. On the basis of a study of melt inclusions, we proposed recently that fluoride–silicate melt immiscibility played an important and perhaps dominant role in concentrating the REE within the pluton. Here we present further evidence for silicate–fluoride immiscibility at Strange Lake from a sample of the hypersolvus granite, which contains an inclusion composed largely of REE and HFSE minerals.The inclusion (∼5cm in diameter) comprises a narrow rim containing chevkinite-(Ce) and zircon in a fluorite matrix, a core of fluorbritholite-(Ce) and bastnäsite-(Ce) and a transition zone between the rim and the core consisting of a fine-grained intergrowth of bastnäsite-(Ce), gagarinite-(Y) and fluorite. We propose that the inclusion formed as a result of silicate–fluoride immiscibility, which occurred early in the emplacement history of the Strange Lake pluton, and that it represents the fluoride melt. After separation of the two melts, the boundary between them acted as a locus of crystallisation, where crystals formed repeatedly due to heterogeneous (surface catalysed) nucleation. Zircon crystallised shortly after melt phase separation, and was followed by the growth of perthite together with arfvedsonite and quartz. As a result, the silicate melt surrounding the fluoride inclusion became enriched in volatiles that facilitated crystallisation of progressively larger crystals in the inclusion; large crystals of arfvedsonite and perthite were succeeded by even larger crystals of quartz. Massive crystallisation of chevkinite-(Ce) followed, forming the rim of the inclusion. The fluoride melt, which constituted the matrix to the silicate minerals and chevkinite-(Ce), crystallised after chevkinite-(Ce), forming fluorbritholite-(Ce) and fluorite.Aqueous fluid exsolved from the silicate melt and altered the inclusion, replacing fluorbritholite-(Ce) with fluocerite-(Ce) and then bastnäsite-(Ce). This was followed by the formation of a fine-grained intergrowth of bastnäsite-(Ce), gagarinite-(Y) and fluorite at the expense of the earlier bastnäsite-(Ce). Chevkinite-(Ce) was not affected. Zircon, however, was replaced by anhydrous zirconosilicates and, in turn, by hydrous zirconosilicates.The inclusion represents the first macroscopic example of silicate–fluoride immiscibility in nature. We propose that globules of the fluoride melt were initially dispersed within the silicate melt and preserved only rarely in unaltered hypersolvus granite. They accumulated in the residual melt (and therefore in pegmatites) scavenging REE, Ca and F from the silicate melt and, with rare exceptions, were later destroyed by fluids. The latter process contributed significantly to the enrichment of the host rocks in REE, Ca and F.