Early paleozoic Rb mineralization in South China: Geochronology and geochemistry of Xianglushan Rb-rich granite in the Yuechengling batholith, South China

Abstract

Rubidium (Rb) deposits in South China are typically associated with highly evolved Yanshanian or Indosinian granites, while Rb mineralization related to Caledonian granites remains poorly documented. The occurrence state and enrichment mechanism of Rb in Caledonian granite is still unclear. This study investigates a newly discovered Rb-rich granite at Xianglushan within the Yuechengling batholith, South China. The Xianglushan Rb-rich granite exhibits a magmatic-hydrothermal sequence that evolves from monzogranite through monzogranite (K-feldspar alteration) to albitized granite. LA-ICP-MS U-Pb dating of zircon and cassiterite from the Xianglushan Rb-rich granite yielded concordia ages of 416.6 ± 3.5 Ma and 415.9 ± 2.4 Ma, respectively. Zircon ε_Hf(t) values (−6.9 to −3.6) and T_DM2 model ages (1.65–1.90 Ga) indicate the magma was derived from the partial melting of Paleoproterozoic crustal material. Compared to barren Caledonian granites in Yuechengling, the Xianglushan Rb-rich granite is weakly to strongly peraluminous, with high SiO₂, DI, TE_1,3 values and non-CHARAC behavior in element pairs (Zr/Hf, Nb/Ta, Y/Ho, K/Rb). These characteristics indicate that they experienced highly fractionated evolution and melt-fluid interaction, which promoted the enrichment of Rb. Muscovite and K-feldspar are the primary host minerals for Rb resources. The Rb-rich magmatic (Ms-1) and hydrothermal muscovite (Ms-2 and Ms-3) mainly occur in muscovite monzogranite and altered granite, respectively. Ms-1 exhibits the highest Rb content, ranging from 7603 to 8249 ppm, while Rb contents gradually decrease from Ms-2 to Ms-3. This study reveals that the Caledonian granite could enriche Rb during both magmatic and hydrothermal stages. Early and extensive magmatic muscovite precipitation reduces the Rb content of the residual fluids.