Silicate melt immiscibility as the cause of large-scale rare-metal mineralization in a peralkaline granite system: The case of the Baerzhe deposit in NE China

Abstract

The Early Cretaceous peralkaline Baerzhe pluton hosts a potentially large REE-Nb-Zr-Be deposit in inner Mongolia, northeastern China. The mechanism responsible for the extreme enrichment of rare metals and rare earth elements in the pluton is still ambiguous. This study presents new evidence for silicate melt immiscibility as the key mechanism at Baerzhe using amphibole-group minerals from spherulite granite, which contains spherulites with abundant REE- and HFSE-bearing minerals. The spherulite from the transsolvus granite is composed of two distinct zones, i.e., a dark-colored core consisting of arfvedsonite aggregates and a light-colored rim consisting mostly of quartz and feldspar, rare amphibole, and abundant HFSE- and REE-bearing minerals. Four types of amphibole (AmpI, Amp-IIa, Amp-IIb and Amp-III) from the transsolvus granite and one type (Amp-IV) from the subsolvus granite are recognized, and all of them are magmatic fluoro-arfvedsonite. The earliest phase consists of euhedral inclusions of Amp-I within quartz or feldspar. They show an enrichment in HREEs relative to LREEs and a depletion in medium REEs, consistent with the REE pattern controlled by the mineral lattice. This implies that Amp-I likely formed in an initial homogeneous mel relatively depleted in REEs; thus, the mineral structure played a dominant role in REE partitioning. Compared with other types of amphiboles, Amp-IIa and Amp-IIb from the spherulite phase display the highest REE contents, with flat LREEs and MREEs and a slight upward HREE pattern. Combined with the significant accumulation of REE- and HFSE-bearing minerals in the rim zone, differences in REE patterns among different amphibole types imply that the spherulite crystallized in a volatile-rich silicate melt and was probably the product of silicate melt immiscibility. Interstitial Amp-III with lower REE contents from the matrix phase crystallized in the separate volatile-poor silicate melt. The Amp-IV from the subsolvus granite has the lowest CaO content, with strong depletion in LREEs relative to HREEs, suggesting a more evolved melt composition. It is concluded that silicate melt immiscibility may serve as an important key mechanism that occurred in the early stage of magmatic evolution, resulting in the enrichment of REEs and HFSEs, which played a critical role in the formation of large to giant ore deposits such as the Baerzhe deposit.