Crystallization processes and genesis of scheelite in a quartz vein-type W deposit (Xianghuapu, South China)

• Published in: Chemical geology Vol. 613; p. 121142
• Main Authors: Liu, Biao, Li, Huan, Liu, Yu-Guo, Algeo, Thomas J., Luo, Xin-Yu, Girei, Musa Bala, Wu, Qian-Hong, Kong, Hua, Zhang, De-Xian, Jiang, Jiang-Bo
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 20.12.2022
• Subjects: Fluid inclusion; Fluid mixing; Fluid-rock interaction; Mapping analysis; Oscillatory zonation; Fluid-rock interaction; Fluid mixing; Mapping analysis; Fluid inclusion; Oscillatory zonation
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2022.121142

Scheelite is widely developed in quartz vein-type tungsten (W) polymetallic deposits, and CL images, REE patterns and SrO isotope distributions of this mineral provide important constraints regarding its precipitation mechanism. We undertook a systematic investigation of the geochemistry (using in-situ SrO isotopes, EPMA, and LA-(MC)-ICP-MS), CL images, and fluid inclusions along core-to-rim transects of single scheelite grains from the Xianghuapu W deposit (South China) to constrain the source of ore-forming materials and reconstruct the crystallization history of the deposit. The scheelite grains show heterogeneous signatures in CL images recording a three-stage (core–mantle–rim) crystallization history. The scheelites also show decreasing concentrations of REEs from core to rim (core mean = 20.1 ppm, SD = 13.2; mantle mean = 18.7 ppm, SD = 17.8; rim mean = 5.80 ppm, SD = 5.27), as well as contrasting REE patterns and MREE distributions. There are significant variations in concentration, from core to rim of the scheelite, of MREEs (core mean = 10.0 ppm, SD = 6.43; mantle mean = 4.89 ppm, SD = 5.33; rim mean = 0.79 ppm, SD = 0.68) and HREEs (core mean = 39.4 ppm, SD = 23.4; mantle mean = 22.3 ppm, SD = 20.3; rim mean = 4.74 ppm, SD = 4.07). The 87Sr/86Sr ratios and δ18Oscheelite values decrease from core (87Sr/86Sr = 0.740817–0.743807; mean 18Oscheelite = −6.2‰) and mantle (87Sr/86Sr = 0.740048–0.745267; mean δ18Oscheelite = −7.0 ‰) to rim (87Sr/86Sr = 0.733809–0.738398; mean δ18Oscheelite = −7.9 ‰). In addition, the homogenization temperatures of fluid inclusions from the scheelite grains also decrease from core (225–236 °C, mean = 230 °C, SD = 8.20) and mantle (184–217 °C, mean = 201 °C, SD = 11.2) to rim (153–178 °C, mean = 166 °C, SD = 7.04), but the salinity of fluid inclusions changes less (mean = 1.48 wt%, 0.84 wt%, and 0.75 wt%, respectively). The low salinity and δ18OH2O values of the fluid suggest a large amount of meteoric water was mixed into the metallogenic system, resulting in a pH increase and triggering scheelite precipitation. The high Sr contents and change in 87Sr/86Sr ratios reveal that W mineralization was mainly related to Jurassic granitic magmatism, and extensive fluid-rock interaction occurred during the scheelite crystallization process. From core to rim, the magmatic Sr decreased, while the proportion of Sr derived from the limestone wall rock increased. The inhomogeneous REE and SrO isotope distributions may be a ubiquitous feature of quartz vein-type W deposits in magmatic settings globally, and in situ LA-(MC)-ICP-MS analysis can assist in revealing the complex crystallization processes of scheelite in such deposits. [Display omitted]