Contribution of pegmatite shells to magmatic evolution and rare metal mineralization: Insights from the Shihuiyao deposit, Inner Mongolia, Northeast China

Abstract

Highly evolved granite associated with pegmatite shells exhibits significant potential for rare metal mineralization; however, the mechanisms through which these pegmatite shells contribute to magmatic evolution and rare metal enrichment remain poorly understood. The Late Jurassic Shihuiyao Nb–Ta–(Rb–Be–Li) deposit is one of the largest rare-metal deposits in the Southern Great Xing’an Range (SGXR), Northeast China. Exploratory trenches expose distinct layered zones from top to bottom: alternating microcline pegmatite and aplite layers (zone I), topaz lepidolite albite granite and lepidolite amazonite pegmatite (zone II), and muscovite albite granite (zone III). We conducted U–Pb dating of cassiterite, monazite, and Nb–Ta oxide, monazite Nd isotopes, and whole-rock and mineral geochemistry for the three zones. Multi-mineral U–Pb ages indicate that the three zones formed during the Late Jurassic–Early Cretaceous (147–142 Ma). Geochemical analyses of whole-rock, mica, and microcline suggest an evolutionary sequence from zone I to zone III, and finally to zone II. The Zr/Hf, Nb/Ta, Y/Ho, and K/Rb ratios combined with the rare earth element (REE) tetrad effects suggest higher degree of differentiation and fluid–melt interaction of the Shihuiyao leucogranite without a pegmatite shell compared to coeval barren granites from both Shihuiyao and the SGXR. A progressive increase in the degree of evolution is evident from the leucogranite without a pegmatite shell to the leucogranite with a discontinuous shell, and ultimately to the leucogranite with a continuous shell. The pegmatite shell acted as a geochemical barrier that facilitated the accumulation of Li and F in the underlying magma, which played a crucial role in lowering the solidus temperature of the granitic magma. This process prolonged the crystallization duration while reducing melt viscosity and density, thereby creating favorable conditions for magma differentiation and fluid–melt interaction. Rapid crystallization of the earlier water- and Be-rich melt led to the Be mineralization in the pegmatite shell. Moreover, the formation of this shell served as a barrier for Li mineralization in the underlying topaz lepidolite albite granite. This study enhances our understanding of the critical contribution of pegmatite shells to magmatic evolution and rare-metal mineralization.