In situ observations of tungsten speciation and partitioning behavior during fluid exsolution from granitic melt

Most economically important tungsten (W) deposits are of magmatic-hydrothermal origin. The species and partitioning of W during fluid exsolution, considered to be the controlling factors for the formation of ore deposits, are thus of great significance to investigate. However, this issue has not been well addressed mainly due to the significant difference in reported partition coefficients (e.g., from strongly incompatible to strongly compatible) between fluid and melt (D_W^fluid/melt). Here, we used an in situ Raman spectroscopic approach to describe the W speciation, and to quantitatively determine the D^fluid/melt of individual and total W species in granite melts and coexisting Na₂WO₄ solutions at elevated temperatures (T; 700-800 °C) and pressures (P; 0.35-1.08 GPa). Results show that WO₄^2- and HWO₄^- are predominant W species, and the fractions of these two species are similar in melt and coexisting fluid. The partitioning behaviors of WO₄^2- and HWO₄^- are comparable, exhibiting strong enrichment in the fluid. The total D_W^fluid/melt ranges from 8.6 to 37.1. Specifically, D_W^fluid/melt decreases with rising T-P, indicating that shallow exsolution favors enrichment of W in evolved fluids. Furthermore, Rayleigh fractionation modeling based on the obtained D_W^fluid/melt data was used to describe the fluid exsolution processes. Our results strongly support that fluid exsolution can serve as an important mechanism to generate W-rich ore-forming fluids. This study also indicates that in situ approach can be used to further investigate the geochemical behavior of ore-forming elements during the magmatic-hydrothermal transition, especially for rare metals associated with granite and pegmatite.