The Composition and Metallogenic Significance of Micas from Jiajika Two-Mica Granite, Sichuan Province

Jiajika super large lithium deposit is one of the most important lithium resources in China. The two-mica granite located in the southern of ore field is always regarded as mother rock of pegmatites. Systematical mineralogy study of the granite can not only provide basic data for mechanism of rare metals, but also has important meaning to the potential mineralization of rare metals. Based on systematical field work, the major and trace element compositions of micas from Jiajika two-mica granite were obtained by electron microprobe analysis (EMPA) and laser ablation plasma mass spectrometry (LA-ICP-MS), showing that the biotites are magmatic biotites. The biotites are extremely rich in Al and Fe, but poor in Mg. The biotites from Jiajika two-mica granite belong to siderophyllites (but close to protolithionite). Although muscovites from Jiajika two-mica granite show metasomatic reaction with biotites, they should still be regarded as magmatic muscovites after combining macroscopic characteristics of petrology, characteristics of chemistry and characteristics of petrographic. Muscovites are extremely rich in Al, poor in Fe and Mg. The muscovites from Jiajika two-mica granite belong to pure muscovites. According to test data, we make use of total-aluminium geobarmeter and titanium content geothermometry to calculate pressure and temperature of rock formation. The results show that the emplacement pressure varies from 430 MPa to 560 MPa, with the average pressure of 480 MPa；the crystallization temperature varies from 480 ℃ to 550 ℃, with the average temperature of 520 ℃. The calculated result of Si geobarmeter of muscovite is almost the same with that of biotite. In addition, according to the chemical composition of biotite, the oxygen fugacity of magma-hydrothermal system ranges from 10⁻¹⁷ to 10⁻¹⁸. The results also show that the oxygen fugacity increases from south to north. The study of major and trace elements composition of micas from Jiajika two-mica granite shows that rare metal elements are mainly hosted in micas. The content of rare metal elements in biotites is the highest. The geochemical analysis of biotites also shows that Jiajika two-mica granite belong to peraluminum S-type granite, and its formation is mainly related to the crustal magma. From the above, the study of composition of micas from two-mica granite suggests that the granite was formed in high pressure and oxygen-free environment. It provides significant instructions to prospecting of rare metals in Jiajika orefield.