Cesium-rubidium mineralization in Himalayan leucogranites

Abstract

This paper presents a study of a newly discovered pollucite-lepidolite-albite granite (PLAG) in the Himalayan leucogranite belt, which marks the first occurrence of pollucite, a major cesium silicate mineral, in the Himalayan orogenic belt (China). The rock appears at the northern part of the Gyirong pluton, coexisting with the tourmaline-bearing two-mica granite (TMG). Primary rare-metal minerals include lepidolite (Li), spodumene (Li), pollucite (Cs), cassiterite (Sn), and microlite (Ta). Micas, mainly lithian muscovite to lepidolite, contain 4.07 wt.% Li₂O and 0.76 wt.% Rb₂O on average. The average Li₂O content of the spodumene is 7.95 wt.%. Pollucite not only has an average Cs₂O content of 34 wt.%, but also has an elevated Rb₂O content of about 0.16 wt.%. Notably, this granite attains industrial grades for rare metals, specifically with Li₂O, Rb₂O, and Cs₂O contents of 0.49–1.19 wt.%, 0.12–0.24 wt.%, and 0.69–2.33 wt.%, respectively. Dating results of magmatic accessory cassiterite and monazite indicated that the PLAG was formed at 19–18 Ma, slightly later than the TMG (22–20 Ma) of the Gyirong pluton. Thus, these two types of granites may form within the same magmatic system considering their pulsating intrusive contact, formation ages, and whole-rock and mineral chemical compositions. Furthermore, the abundant presence of pollucite suggests that the PLAG experienced high degrees of magmatic fractionation. In comparison to the Pusila spodumene pegmatite in the Himalaya and the Yashan topaz-lepidolite granite in Jiangxi, South China, the Gyirong PLAG exhibits different whole-rock and mineral compositions, resulting from differences in source materials and fractionation processes. Notably, the difference in fluorine (F) content may determine the degree of rare-metal element enrichment. The discovery of Gyirong PLAG highlights multiple stages and types of rare-metal mineralization in the Himalayan leucogranite belt, which is controlled by the South Tibetan Detachment System. The Cs-bearing geyserite deposit exposed along the Yarlung-Zangbo River, together with Himalayan leucogranites, constitutes two systems of rare-metal elements migration and enrichment. These two systems reflect the interaction among Earth systems across time and space, emphasizing how the Himalayan orogeny controls mineralization. As a result, the Himalayan leucogranite belt has considerable prospecting potential for cesium and rubidium resources and may be a crucial area for future exploration and resource utilization.