Muscovite as a potential tool for magmatic differentiation and identifying Li mineralization in the Jiajika regional pegmatites, West Sichuan

Jiajika is of great economic value as one of the super-large pegmatite lithium deposits in China. The Jiajika pegmatite exhibits spatial zoning around the two-mica granite, which from proximal to distal regions are (I) microcline pegmatite zone, (II) microcline-albite pegmatite zone, (III) albite pegmatite zone, (IV) spodumene pegmatite zone and (V) muscovite (lepidolite) pegmatite zone. In order to study the regional evolution of pegmatite and the enrichment mechanism of rare element, the muscovite in pegmatites have been studied by EPMA and LA-ICP-MS. Based on the internal structure and chemical composition, two-stage evolution of regional pegmatites could be recognized: homogeneous primary muscovite in zones I-IV and muscovite with compositional zoning in zone V. They show significant differences in the composition of Li, Rb, Cs, K/Rb and K/Cs, indicating a significant increase in the degree of evolution and an increase in the proportion of fluid components. This indicates that the system transitioned from a stage dominated by melt to a relatively unstable stage dominated by fluid. From zone I to zone IV primary muscovite shows a limited decreasing trend in K/Rb and Kb/Cs ratios but slightly relatively increases in Li, Rb, Cs and Ta contents, indicating regional pegmatites experience moderate degree differentiation. The oscillatory changes of muscovite in zone V are mainly controlled by the melt-fluid immiscibility. Overall, primary muscovite at Jiajika has high contents in Li, Cs and B, indicating that the initial melt has great ore-forming potential. The K/Rb ratio <= 20 or the Cs content >= 400 x 10(-6) in muscovite, which can be a geochemical indicator of lithium mineralization in Li-Cs-Ta (LCT) pegmatites.