Geology and fluid inclusions studies on Keketuohai No. 3 rare-element pegmatite, Xinjiang, Northwest China

The Keketuohai No. 3 pegmatite, the largest in China, located in Fuyun county, Xinjiang province, being mined for Li, Be, Nb and Ta contained spodumene, beryl, columbite, lepidolite and pollucite. The pegmatite occurs in a gabbro which lies above a biotite granite. It consists of 10 zones (in the probable sequences of formation): I) graphic zone with an assemblage of quartz, feldspar and beryl; II) coarse grained albite zone, the Be ore body; ID) massive microcline zone, with beryl; IV) muscovite and quartz zone, with Be, Nb, and Ta mineralization; V) cleavelandite and spodumene zone with Li, Nb and Ta mineralization; VI) quartz-spodumene zone rich in Li, Nb and Ta; VII) muscovite and albite zone with Ta and Cs mineralization; VIII) albite-lepidolite zone Li, Ta, Rb, Cs ore body; IX) quartz-pollucite zone with Cs mineralization; and X) massive microcline and quartz core, rich in Rb and Cs.

Fluid, fluid-melt and melt inclusions studies in spodumene, beryl, tourmaline and coexisting quartz from different zones show five types of inclusion: 1) melt inclusion, 2) fluid-melt inclusion, 3) H2O–CO2, 4) Halite bearing and 5) aqueous inclusions. Melt inclusions are found in zones I and III, and melt-fluid inclusions (consist of melt and fluids, the fluids are mostly H2_O and CO₂, in an inclusion ) occur in zones I, III and VI. Aqueous inclusions occur in all zones. Most H₂O–CO₂ inclusions are found from Zone VI to X. Halite bearing inclusions coexisting with aqueous inclusions occur mostly in zones IV to VII. Microthermometric data show that homogenization temperatures (Th) of melt inclusions ranges from 700 to 850°C. Th of melt-fluid inclusions ranges from 299 to 465°C, and that for aqueous, halite bearing and H₂O–CO₂ inclusions ranges from 220 to 404°C. The salinity of aqueous inclusions range from 5 to 9 wt.% NaCl. equiv., which is relatively low. The salinity of halite bearing and some fluid melt inclusions range from 28.50 to 32.00 wt.% NaCl equivalent. The Th H₂O–CO₂ inclusions range from 220 to 311°C and the pressure calculated from H₂O–CO₂ and CO₂ inclusions is from 1800 to 2500 bars.

The results indicate that the No.3 pegmatite was formed during the transition from magmatic to subsolidus hydrothermal fluid conditions, occurring probably in three stages. Stage 1 was magmatic; stage 2 required a processes of immisciblity to produce melts and fluids; the final stage was predominantly hydrothermal fluids. Li, Be, Nb, Ta and Cs mineralization occurred throughout this transition.