The Chemical Composition of Helvine-Group Minerals and Implications for Ore Genesis: Case Studies from the Dawan, Qidushan, and Taoxikeng Deposits

Abstract

Abstract Helvine-group minerals have characteristic chemical compositions and textures and occur in different environments within magmatic–hydrothermal deposits. However, their origin is still unclear. There are several quartz-vein beryllium deposits with different element assemblages existing in southeastern China; these include the Dawan Be-Mo, Qidushan Be-Zn-Sn, and Taoxikeng Be-W deposits. We present the chemical composition of helvine-group minerals analyzed by electron probe microanalysis and laser ablation inductively coupled plasma mass spectrometry, in addition to zircon sensitive high-mass-resolution ion microprobe U-Pb and cassiterite laser ablation inductively coupled plasma mass spectrometry U-Pb ages. The zircon and cassiterite U-Pb data indicate that the Qidushan deposit formed during the Late Cretaceous (127–132 Ma). The electron probe microanalysis data show that the helvine-group minerals from the three deposits are helvine and/or danalite. A wide variety of different trace elements display high concentrations in the different deposits [e.g., helvine-group minerals from the Dawan deposit are rich in Li, B, Sc, V, Ga, Y, Sn, Cs, Th, and U; those from the Qidushan deposit are rich in Sc, Cd, and Sn; whereas those from the Taoxikeng deposit are rich in Y, Cd, Sn, Ta, and total rare-earth elements (ΣREE). The material analyzed from the Dawan deposit has the highest concentrations of Li (28.03–134.96 ppm), Sn (126.01–709.24 ppm), Sc (108.10–287.64 ppm), Cs (0–37.24 ppm), Th (4.78–112.63 ppm), and U (0.79–4.13 ppm). The material analyzed from the Taoxikeng deposit has the highest concentrations of Y (594.95–899.95 ppm), Cd (22.63–25.21 ppm), Ta (0.41–0.86 ppm), and ΣREE (281.69–451.79 ppm). However, those from the Qidushan deposit have the lowest concentrations of Y (10.21–37.70 ppm), Sn (0–26.88 ppm), U (0–19.06 ppm), Eu (0–0.14 ppm), and Cs (being below the detection limit). All of them are rich in heavy REEs but poor in light REEs, with distinctive REE patterns. Our results reveal that both crystallographic substitution and fluid composition control the enrichment of trace elements in helvine-group minerals. The textures and trace element compositions of helvine-group minerals can represent various parageneses and ore-forming fluid compositions. We propose that the δEu values and (La/Yb)N ratios can be used as markers to discriminate the genesis of helvine-group mineral precipitated from different environments based on the chemical compositions and the origin of ore-forming fluids.