Rb enrichment processes in highly evolved granites: Insights from mica and K-feldspar from the Baishitouquan pluton, Eastern Tianshan

Highly evolved granites are closely related to rare metal mineralization. The mechanisms controlling rare metal enrichment, however, remain poorly constrained. The highly evolved Baishitouquan (BST) pluton in the Eastern Tianshan, which hosts the Zhangbaoshan Rb deposit, has been well characterized and provides an excellent opportunity to study Rb enrichment processes in granitic systems. The BST pluton has five lithological zones, which, from bottom to top, are leucogranite (Zone-a), amazonite-bearing granite (Zone-b), amazonite granite (Zone-c), topaz-bearing amazonite granite (Zone-d), and topaz albite granite (Zone-e). To investigate Rb enrichment processes in the BST pluton, compositional variations in K(–Rb)-rich minerals from the different lithological zones are characterized and interpreted in the context of textural variations. The main K- and Rb-bearing minerals in the pluton are mica and K-feldspar, which are mainly represented by phengite and microcline, respectively. Phengite in Zone-e occurs as euhedral grains with irregular boundaries, indicating that they crystalized from the Baishitouquan magma and were overprinted by hydrothermal fluids. Phengite is generally enriched in F (1.05–9.94 wt%), Li (780–10171 ppm), and Rb (3133–6657 ppm), and is characterized by low K/Rb and Nb/Ta ratios. Most microcline grains occur as elongate, euhedral–subhedral laths, with tartan and Carlsbad twinning. These microcline grains are enriched in Rb (1547–2927 ppm), Pb (15–120 ppm), and Cs (14–100 ppm), and have low K/Rb and Al/Ga ratios. In the BST pluton, the concentrations of Li, F, and Rb in phengite increase from Zone-a to Zone-c, then decrease in Zone-d and Zone-e. Conversely, the K/Rb and Nb/Ta ratios decrease from Zone-a to Zone-c, then increase in Zone-d and Zone-e. The Rb concentration and K/Rb ratio of microcline exhibit a similar trend. Considering the gradual changes in lithology among the zones, the variations of mineral chemical from Zone-a to Zone-c implies continuous evolution of the magma dominated by crystal fractionation. The occurrence of secondary mica, and the abrupt geochemical changes in phengite and microcline in Zone-d and Zone-e are indicative of hydrothermal activity. The gradual decrease in Al/Ga ratio of microcline from Zone-d to Zone-e and the presence of secondary mica that is compositionally similar to primary mica suggests that the fluids were magmatic in origin. During magmatic evolution, volatile-rich (e.g., F) melts have lower viscosities and solidus temperatures than volatile-poor melts, allowing them to undergo extreme degrees of fractional crystallization. This significantly enhances the solubility of Rb, leading to continuous enrichment of Rb in the residual melt. As the BST system evolved from one dominated by magmatic processes to one dominated by hydrothermal activity, the crystallization of topaz consumed F, reducing the solubility of Rb in the melt, and leading to a decrease in the concentration of Rb in phengite and microcline. Hydrothermal alteration of phengite would have liberated Rb into the fluids, resulting in lower Rb concentrations in secondary mica compared to primary mica. Therefore, the enrichment of Rb in the BST pluton was mainly controlled by extreme degrees of fractional crystallization of a F-rich granitic magma.