STUDIES OF SILICATE MELT INCLUSIONS IN QUARTZ AND CRYSTAL-RICH INCLUSIONS IN SPODUMENE FROM JIAJIKA GRANITIC PEGMATITE RARE-METAL DEPOSIT IN CHINA

Jiajika granitic pegmatite rare-metal deposit in Sichuan, China is the largest lithium deposit in Asia, where pegmatite dikes surround the granite body in both horizontal and vertical directions. The quartz in the granite hosts many silicate melt inclusions (SMIs), which are mainly composed of daughter minerals, aqueous, and vapor phases. In pegmatite dikes, crystal-rich inclusions (CIs), the dominant type of inclusions in spodumene, are composed of carbonic aqueous fluid and daughter minerals. In this study, we used: (1) a JY Horiba LabRam HR800 Raman system to identify the phases within SMIs and CIs, and (2) a hydrothermal diamond-anvil cell to apply external pressures to these inclusions during in situ observations of the melting processes of daughter mineral(s) and the total homogenization.

Raman spectroscopic analyses of the SMIs showed that muscovite was the most common daughter mineral, and the vapor phase commonly contained H₂O, N₂, CH₄, and occasionally H₂ with associated graphite, and possibly a species of (CH₄)_n(H₂)_m. The CIs in pegmatitic spodumene commonly contain cristobalite and zabuyelite crystals, and occasionally with additional calcite, spodumene, and possibly cookeite. They also contain a CO₂-H₂O-(NaCl) fluid phase, which occasionally was observed to coexist with CH₄ and/or graphite. The aqueous and melt phases in SMIs in quartz in granite homogenized into the melt phase mainly between 700 and 800 °C, and those in CIs in spodumene in pegmatite homogenized into the aqueous phase mainly between 500 and 700 °C.

The mechanisms for the occurrence of H₂ in SMIs are unknown. However, the retention of H₂ in these SMIs may be due to the low diffusion rate of H₂ in quartz at low temperatures and also the low H₂ gradient between SMIs and their surrounding environments of the host minerals. The formation of cristobalite within CIs may be ascribed to the extension of the stability field of cristobalite to low temperatures possibly due to the presence of lithium and/or H₂O. However, the pressure drop in CIs, resulted from the consumption of CO₂ during crystallization of zabuyelite and, to the minor extent, calcite, may also enhance the crystallization of cristobalite. This implies that the CIs were primary inclusions; they were kept enclosed after the entrapment of the fluid, from which the host spodumene crystals grew.