Amphiboles of the Rare Earth Element-Rich Baerzhe Peralkaline Granite, Northeast China: Recorders of Fluoride-Silicate Melt Immiscibility and Rare Metal Enrichment

Fluoride-silicate melt immiscibility has been proposed as an important control on light rare earth element (LREE) and Y enrichment in a peralkaline granite (e.g., the Strange Lake pluton in Canada). It remains uncertain, however, whether this immiscibility is a requirement for ore formation in mineralized peralkaline granites globally. In this contribution, we present an integrated mineralogical and chemical study of the REE-Zr-Nb-Be–mineralized Baerzhe peralkaline granite pluton, northeast China. We focus on the mineralogy and chemistry of amphibole and the bulk-rock geochemistry to investigate whether the metal enrichment at Baerzhe was facilitated by fluoride-silicate melt immiscibility or simply resulted from fractional crystallization. The Baerzhe pluton comprises, from early to late, a fine-grained hypersolvus granite, a porphyritic hypersolvus granite, and a transsolvus granite that evolved to pegmatite and a comb-textured amphibole unit. Hypersolvus granite enclave(s) can also be found within the transsolvus granite. The amphibole in these granitic units is arfvedsonite, providing evidence of fractional crystallization, magma mingling, fluoride melt immiscibility, fluid exsolution, and degassing. The presence of fluoride melt inclusions in arfvedsonite indicates that fluoride-silicate melt immiscibility took place in all units of the pluton except for the transsolvus granite, where rare metal mineralization occurs, and in pegmatite. The immiscibility was particularly intense in the porphyritic hypersolvus granite but very weak in other rock units, as evidenced by the abundance of fluoride melt inclusions in the former and lack of them in the latter. Most fluoride melt inclusions are rich in Ca and are enriched in LREEs, middle REEs (MREEs), and Y relative to heavy REEs (HREEs). The estimated volume of fluoride melt in the porphyritic hypersolvus granite was less than 1 wt %, storing about 2 to 6 wt % LREEs and up to 8 to 15 wt % MREEs and Y. The exsolution of fluoride melt, however, does not appear to have affected the trends of increasing concentrations of REEs and high field strength elements (HFSEs) in the coexisting silicate melt, which were driven by intense fractional crystallization of alkali feldspar, quartz, and arfvedsonite. Furthermore, the absence of fluoride melt inclusions in the transsolvus granite and pegmatite, together with their scarcity in the comb arfvedsonite unit, suggests that the early formed immiscible fluoride melt did not accumulate in the residual silicate melt. Fluoride melts appear to have played a limited role in the concentration of LREEs and Y to form the Baerzhe deposit. Instead, fractional crystallization of silicate minerals exerted a dominant control on the enrichment of REEs and HFSEs by factors of five to 20.