Evolution of Chakabeishan pegmatites and associated Be–Li mineralization in North Qaidam Terrane, China

Abstract

• Chakabeishan pegmatites probably underwent a prolonged (∼25 Ma) magmatic, magmatic–hydrothermal, and hydrothermal evolution process. • Tectonic (syn- to post-collisional) and magmatic–hydrothermal coupling controlled the Be and Li rare-metal mineralization. • Magmatic crystallization differentiation produced petrographic zonation sequence (Tur–Peg → Ms–Peg → Tur/Grt–Peg → Spd–Peg) with early Be (sodium beryl) and later Li (spodumene) enrichment. • Melt–fluid immiscibility and pervasive fluid metasomatism precipitated elbaite, eucryptite, zinnwaldite, and Li–Cs/Cs-rich beryl, enabling Li–Cs enrichment. The coupling mechanism of tectonic and pegmatite magmatic–hydrothermal evolution in controlling rare-metal mineralization within orogenic belts remains poorly understood. To address this issue, this study focused on the recently discovered Chakabeishan pegmatite-type Be and Li deposit in the North Qaidam terrane, Northern Tibetan Plateau. Integrating field geology, mineral assemblages, and rare-metal mineralization characteristics, the study conducted garnet–beryl chemistry and columbite–tantalite U–Pb dating. The mineral chemical characteristics of garnet (Fe–Mn) and beryl (Li–Cs) revealed that the Chakabeishan pegmatites underwent a progressive magmatic–hydrothermal evolutionary stage. Early magmatic crystallization differentiation led to initial Be (beryl) followed by Li (spodumene) enrichment during the magmatic to magmatic-hydrothermal stage. Fluid immiscibility resulted in extensive dissolution and metasomatism, further enriching Li and Cs during the hydrothermal stage, as evidenced by minerals such as elbaite, eucryptite, zinnwaldite, and Li‑/Cs‑rich beryl. Columbite–tantalite U–Pb dating yielded the ages of ∼ 240 Ma for the Be‑mineralized tourmaline–garnet pegmatite (magmatic stage) and ∼ 215 Ma for the Li‑mineralized elbaite–eucryptite pegmatite (hydrothermal stage). Combined with the tectonic evolution of the Zongwulong tectonic belt, this study proposes that the Chakabeishan pegmatite’s initial melt originated from the low-degree partial melting of the mid-lower crust during the syn -collisional orogeny, dominantly driven by shear heating. Pegmatite crystallization began at ∼ 240 Ma under confined high P–T conditions during syn -collisional compression, facilitating Be- and then Li-enrichment through fractional crystallization. However, the compressional-to-extensional tectonic shifts during the syn- to post-collisional transition stage triggered melt/fluid immiscibility, driving hydrothermal evolution and substantial Li–Cs (elbaite, eucryptite, and Li‑/Cs‑rich beryl) enrichment. This study highlighted that the distinct Be and Li enrichment in the Chakabeishan pegmatites might be governed by a prolonged (∼25 Ma), tectonically coupled magmatic–hydrothermal evolution.