Textural and chemical characteristics of beryl from the Baishawo Be-Li-Nb-Ta pegmatite deposit, Jiangnan Orogen: Implication for rare metal pegmatite genesis

Abstract

The Baishawo pegmatite deposit is a newly discovered large Be-Li-Nb-Ta deposit, with estimated reserves of 16,160 t BeO @ 0.084 %, 11,280 t Li2O @ 0.85 %, and 3,450 t (Ta2O5 + Nb2O5) @ 0.201 %. The 3# pegmatite is internally zoned into border (contact against two-mica granite), wall, intermediate, and core zones. Beryl, the dominant economic mineral, occurs mainly in the intermediate and wall zones. In the intermediate zone, beryl coexists with feldspar and quartz, and in the wall zone, with quartz, tourmaline, garnet, and feldspar. Geological and petrographic observation, Electron-probe Microanalysis (EPMA), TESCAN Integrated Mineral Analyzer (TIMA), X-ray mapping, and Laser Ablation Inductive Coupled Plasma Mass Spectrometry (LA-ICP-MS) analysis were conducted to study the textural and compositional features of the beryl. The EPMA data show the concentrations of Al2O3, SiO2 and BeO in beryl are 17.61–18.32 wt.%, 64.01–65.23 wt.%, and 13.09–13.89 wt.% from the intermediate zone, respectively, and 17.57–18.52 wt.%, 64.33–65.15 wt.%, and 13.20–14.10 wt.% from the wall zone, respectively. Beryl from the intermediate zone contains higher concentrations of Ti (4.29–34.5 ppm), Mn (33.9–277 ppm), Rb (86.2–230 ppm), Cs (3333–4522 ppm) and Sn (0.571–28.7 ppm) than beryl from the wall zone (Ti 2.65–5.70 ppm, Mn 19.0–54.2 ppm, Rb 104–114 ppm, Cs 747–1242 ppm and Sn 0.085–1.26 ppm). Substitutions in beryl of the intermediate and wall zones are Li for Be, and Fe for Al. The distributions of alkali metals, Mg and Fe in beryl suggest that the pegmatite in the two beryl-bearing zones is the product of magmatic-hydrothermal interaction. Decreasing Mg content and Mg/Fe ratio with increasing Cs content and Cs/Na ratio suggest that beryl compositions reflect evolution of a highly differentiated two-mica granite magma, of which there is a residual fraction.