Original paper

Geochemical evolution and late re-equilibration of NaCs-rich beryl from the Koktokay #3 pegmatite (Altai, NW China)

Wang, Ru; Cheng Che, Xu; Dong Zhang, Wen; Lan Zhang, Ai; Cheng Zhang, Hui


Geochemical variations of Na- and Cs-rich beryl were studied in the Koktokay #3 pegmatite, Altai district, China. Complex internal textures in individual crystals characterize two groups of beryl (primary and secondary). Complete analyses of the primary beryl reveal a broad range of alkali contents (mostly Na and Cs) and femic contents (mostly Fe) between outer (I-IV) and inner (V-VII) zones of the pegmatite. Primary beryl in outer zones contains 0.16-0.92 wt% FeO, 0.39-1.87 wt% Na2 O and 0-0.43 wt% Cs2 O, respectively, whereas that in inner zones contains negligible FeO but higher alkalis with 1.19-1.99 wt% Na2 O and 2.99- 3.95 wt% Cs2 O. Three types of secondary beryl can be distinguished. (1) The first is the Cs-enriched veinlets with up to 8 wt% Cs2 O and 2.2 wt% Na2 O in primary beryl from the zones I-IV, which resulted from precipitation of Cs-rich fluids along fractures in primary beryl crystals. (2) The second type is a mosaic-like texture consisting of a Cs-Na-poor domain and Na-Cs-rich veinlets within primary, blocky beryl crystals. Textural relationship indicates a two-step hydrothermal alteration. (3) The third is Na-enriched beryl that occurs as a marginal zone of primary beryl or as network of veinlets in primary beryl, and is simply related to hydrothermal replacement of primary beryl by Na-rich fluids.


berylcaesiumchemical analysiskoktokay #3 pegmatitealtaichina