Mineralogical characteristics and mineralization implication of Li and Be-bearing minerals from Zhawulong pegmatite-type rare-element deposit, western Sichuan, China

The Zhawulong granitic-pegmatite type rare-element deposit is located in the junction area of Sichuan and Qinghai provinces, whose tectonic position belongs to the Songpan-Ganze orogenic belt in northwestern China. This deposit includes an estimated 1. 75 million tons of Li2O, and its main ore minerals are beryl and spodumene. Previous studies have focused on the reconstruction of its magmatic-hydrothermal evolution through analysis on the multi-stage fluid inclusions. However, the evolution degree of the magma and crystallization conditions from magmatic to hydrothermal stages remain poorly constrained because of the absence of mineralogical and isotopic evidence. The No. 14 albite-spodumene pegmatite is one of the largest pegmatite dikes with a greatest lithium reserve in the Zhawulong deposit. This pegmatite dike could be classified into four centric zones from border to core: the quartz-muscovite zone (zone I), the microcline-albite zone (zone II), the albite-spodumene zone (zone III) and the spodumene-quartz zone (zone IV). Two Li-Be mineralization stages have been identified based on beryl and spodumene mineralogical characteristics. The stage I is characterized by crystallization of magama, forming primary beryl and spodumene with simple textures (homogenous and progressive zoning). The stage II is characterized by hydrothermal alteration, forming secondary Cs- or Na- enriched beryl veinlets together with secondary muscovite. The relatively low Cs20 contents (0. 71% similar to 2. 28%) in beryl suggest the No. 14 pegmatite has experienced lower degrees of fractional crystallization than many other Li-Cs-Ta pegmatites in the world. Primary spodumenes from zone III and IV show a variable delta(7) Li values (-1. 70 parts per thousand similar to 1.67 parts per thousand), indicating both of them are crystallized from a highly evolved aqueous melt immisibile with exsolving fluids. delta(7) Li values (-1. 90 parts per thousand similar to -0.03 parts per thousand) of secondary spodumenes are slightly lower than those in primary ones, suggesting the crystal-fluid interaction only produces a restricted Li isotope fractionation in the closed system. The slightly increasing Cs2O contents of the secondary beryls compared to the primary ones without deposition of highly differentiated minerals (including pollucite and lepidolite) indicate that the hydrothermal fluids have a limited effect on the rare elements metallogeny. In summary, the No. 14 pegmatite in Zhawulong deposit experienced a relatively lower degree of magmatic differentiation and a limited Cs-fluid exsolution compared to the highly differentiated rare-element pegmatites such as Tanco and Koktoghay No. 3 pegmatite.