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

Ye Qiu; Xiaolin Wang; Jianjun Lu; I-Ming Chou; Ye Wan; Rongqing Zhang; Wenlan Zhang; Rui Sun

doi:10.1016/j.scib.2022.10.024

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

Sci Bull (Beijing). 2022 Nov 30;67(22):2358-2368. doi: 10.1016/j.scib.2022.10.024. Epub 2022 Oct 29.

Authors

Ye Qiu¹, Xiaolin Wang², Jianjun Lu¹, I-Ming Chou³, Ye Wan³, Rongqing Zhang⁴, Wenlan Zhang¹, Rui Sun⁵

Affiliations

¹ State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China.
² State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China; Frontiers Science Center for Critical Earth Material Cycling, Nanjing University, Nanjing 210023, China. Electronic address: xlinwang@nju.edu.cn.
³ Key Laboratory of Experimental Study under Deep-sea Extreme Conditions, Institute of Deep-sea Science and Engineering, Chinese Academy of Sciences, Sanya 572000, China.
⁴ State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China; Frontiers Science Center for Critical Earth Material Cycling, Nanjing University, Nanjing 210023, China.
⁵ Department of Geology, Northwest University, Xi'an 710069, China.

PMID: 36546225
DOI: 10.1016/j.scib.2022.10.024

Abstract

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.

Keywords: Fluid exsolution; In situ observation; Partition coefficient; Species; Tungsten.