Genetic linkage between parent granite and zoned rare metal pegmatite in the Renli-Chuanziyuan granite-pegmatite system, South China

Elucidating the causal relationship between parent granite and pegmatite and tracing the magmatic-hydrothermal evolution and rare metal mineralization are intriguing scientific issues that remain unsolved. The Renli-Chuanziyuan granite-pegmatite field, hosted in and adjacent to the Mufushan batholith, represents a significant rare metal ore district in southern China. The field reveals well-defined regional zonation, encompassing biotite, two-mica, and muscovite monzogranites at the core, which transition outward to microcline (K-pegmatite), microcline-albite (K-Na-pegmatite), albite (Na-pegmatite), and albite-spodumene pegmatites (Na-Li-pegmatite) at the southwestern margin of the Mufushan batholith. The U-Pb ages of the columbite-tantalite (coltan) grains from the Na-Li-pegmatite samples from the Chuanziyuan deposits are 139.1 ± 1.2 Ma and 138.6 ± 1.1 Ma, which are consistent with the ages of the coltan grains from the Renli deposit, indicating contemporaneous mineralization. Chemical analyses of muscovite, garnet, and coltan from various pegmatite zones reveal a gradual increase in the degree of differentiation and volatile components of the Early Cretaceous magmatic system from north to south. Particularly noteworthy is the significant increase in the abundance of Na-Li-pegmatites to the south, signifying the transition from an early melt stage with a minor fluid component to a melt-fluid stage with a substantial volume of unmixed fluid. This evolution led to the enrichment and precipitation of Nb and Ta in the Renli deposit and of Li in the Chuanziyuan deposit, which were facilitated by the separation and exsolution of flux-rich (e.g., F, H₂O) fluid. The highly evolved ca. 140 Ma muscovite monzogranite exhibits a well-defined spatial and temporal relationship with rare metal pegmatites, providing strong evidence of genetic associations. Furthermore, the geochemical characteristics of muscovite and garnet from the muscovite monzogranite overlap with those of mineralized pegmatites, further suggesting (together with the simulation study) that the muscovite monzogranite served as the parental granite of the mineralized pegmatites. These findings underscore the importance of understanding the evolution of the granite-pegmatite system, particularly in the context of rare metal exploration, and contribute to a deeper understanding of the ore-forming mechanisms of zoned rare metal pegmatites.