Granitoid genesis and related rare metal mineralization of the Shihuiyao Rb-Ta-Nb deposit in the Southern Great Xing'an Range, NE China

The Shihuiyao deposit, located in the southern Great Xing'an Range (SGXR) in China, is an important Rb-Ta-Nb polymetallic deposit hosted by Li-F-bearing granites in the eastern portion of the Central Asian Orogenic Belt. However, its emplacement history and petrogenesis, and mechanisms of rare metal enrichment remain controversial. This study presents a systemic petrographic, geochronological, mineralogical, and geochemical study on mineralized amazonite-bearing albitized granite, albitized granite, and greisen, as well as barren biotite granite in the Shihuiyao deposit. Uranium-Pb geochronological results for zircon, columbite-group minerals, and cassiterite demonstrate that both emplacement and mineralization occurred between ca. 145-140 Ma. The high TE1,3 values, and low Zr/Hf and Nb/Ta ratios of the biotite granite suggest that it crystallized from a highly evolved magma. Based on the late-stage crystallization of Fe-rich biotite, the depleted epsilon Hf(t) values of zircon (+6.5 to + 8.3), and the elevated bulk-rock F and alkali contents, it is inferred that the parental magma to the biotite granite originated from the partial melting of residual granulite-facies rocks within the lower crust. The distinct Zr/Hf, K/Rb, and Y/Ho ratios of barren and fertile granitoids suggest that they evolved independently as these ratios typically vary continuously within a magmatic system. The presence of snowball quartz, fluorite, and topaz suggest that the mineralized granites formed within a magmatic-hydrothermal system enriched in Na, F, and H2O. Such a Na-, F-, and volatile-rich melt (represented by the magma parental to the ore-bearing granitoids) would have separated from a conventional silicate melt (represented by the magma parental to the biotite granite), generating a scenario in which two immiscible silicate melts were present (melt-melt immiscibility). The increasing bulk-rock concentration of Rb, Ta, and Nb from the barren biotite granite to the fertile (amazonite-bearing) albitized granite indicates that melt-melt immiscible processes led to a significant enrichment of rare metals.