WANG Hui-hui1, QI Dong-mei1*, MA Zhi-jie2, ZHANG Chao3, CHAI Feng-mei4, ZHANG Xue-bing1, LI Nuo5, LI Yong6, XIA Yong-qi1

(1. Xinjiang Key Laboratory for Geodynamic Processes and Metallogenic Prognosis of the Central Asian Orogenic Belt, Xinjiang University, Urumqi 830046, China; 

2. China Molybdenum Co., Ltd., Luanchuan 471500, China; 

3. Department of Geology, Northwest University, Xian 710069, China; 

4. School of Geology and Geomatics, Tianjin Chengjian University, Tianjin 300384, China; 

5. Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography, Chinese Academy of Sciences, Urumqi 830011, China; 

6. Institute of Geology and Mineral Exploration, Nonferrous Geological Exploration Bureau, Xinjiang Uygur Autonomous Region, Urumqi 830000, China)

Abstract: Beryllium as an important strategic metal is widely applied in many fields. This paper focuses on the enrichment of beryllium in the Chinese Altai orogenic belt, the beryl of the Dakalasu rare metal-bearing granitic pegmatite has been studied. The beryls were sampled from the coarse-grained wall zone ( Brl-I ), intermediate blocky zone ( Brl-II ), and fine-grained and miarolitic zone (Brl-III). Petrographic observations and in situ analyses of the major (EPMA) and trace elements (LA-ICP-MS) were carried out. The grain size of beryl from the coarse-grained and intermediate zone are 20 - 50 cm, whereas the beryls from the fine-grained zone are smaller than 1cm. Brl-I coexists with black tourmaline, alkaline feldspar, albite, garnet and apatite, Brl-II is associated with dark Ta-bearing minerals, albite, muscovite and hydrothermal zircon, and Brl-III grows with albite and muscovite. Elemental compositions of beryl show that these beryl are Na-Li beryl species, and the element substitution mechanisms are Na (Fe2+, Mg) □-1Al-1 for both Brl-I and II, NaLi□-1Be-1 for Brl-III. On the one hand, the Na/Cs and Mg/Fe ratios of Brl-I are greater than 20.46 and 0.31, respectively. On the other hand, Brl-II and III show apparently low values, i.e., Na/Cs and Mg/Fe ratios are lower than 20.44 and 0.20, respectively. In line with these evidences, the parental magma of the Dakalasu pegmatite is rich in volatile (B) and alkaline feldspar at the early stage, and subsequently it becomes enriched in albite due to progressive fractional crystallization. Together with the experimental petrology and the Rayleigh fractionation model, it can be concluded that fractional crystallization up to 95% can lead to Be saturation in the melt, followed by the crystallization of rare metal minerals such as beryl. This paper and previous research show that the enrichment of beryllium in the Dakalasu deposit is mainly due to crystallization fractionation and melt-melt-fluid immiscibility. The coexistence of beryl and black alkaline tourmaline can likely be used to beryllium exploration. 

Keywords: beryl; granitic pegmatite; crystallization differentiation; mineral chemistry; Chinese Altay

ACTA MINERALOGICA SINICA Vol. 43, No. 5, 2023, Page 594