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

Abstract

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 TE_1,3 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 ε_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.