Lithium mineralization during evolution of a magmatic–hydrothermal system: Mineralogical evidence from Li-mineralized pegmatites in Altai, NW China

Abstract

Pegmatite lithium (Li) deposits are usually associated with high fractionation and complex evolution of the magmatic–hydrothermal system. However, the mechanism that accounts for the evolution of the magmatic–hydrothermal system and Li mineralization remains unclear. The Li-mineralized pegmatites in the Altai orogenic belt in northwest of China, including the Koktokay No.3 pegmatite, Xiaohusite No.91 pegmatite, and Talati No.1 pegmatite used in this study, show different rock-formation ages and ore deposit scales but similar magmatic–hydrothermal evolution and ore mineral assemblage. This study focuses on the relative contributions of magmatic and hydrothermal processes to the formation of Li deposits. Therefore, the texture and compositions of Li-rich minerals (micas, spodumene, montebrasite, and holmquistite) were systematically analyzed in these three pegmatites. From the outer to the inner zones of the studied pegmatites, micas varied from muscovite to Li-bearing muscovite and Li-bearing phengite to zinnwaldite to lepidolite, with decreasing K/Rb ratios (27.4–5.3, 27.0–4.1, and 15.5–5.0, respectively, in Koktokay No.3 pegmatite, Xiaohusite No.91 pegmatite and Talati No.1 pegmatite) and increasing Li (545–12540, 652–35519, and 627–2599 ppm), Rb (3388–16820, 3447–20970, and 5930–17096 ppm), Cs, Ta, and F concentrations, showing that fractional crystallization is the most important factor controlling Li mineralization in the magmatic stage. However, primary muscovite, spodumene, and montebrasite were altered by subsequent hydrothermal fluids, forming F, Li-rich mica rims, secondary montebrasite, secondary spodumene, and other secondary minerals. The occurrence of holmquistite indicated that Li-rich hydrothermal fluids entered into wall rock. These processes indicate that Li reactivation and migration during the hydrothermal stage would destroy Li mineralization in the pegmatite.