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Insights on magmatism and mineralization from micas in the Xihuashan granite, Jiangxi Province, South China

The Xihuashan tungsten deposit, as one of the most important deposits in South China, has close relationship with magmatic evolution of the Xihuashan granitic complex. Micas in granitic rocks associated with the Xihuashan tungsten deposit are highly variable in two coeval trends. The trioctahedron micas changed in the evolution sequence of "Fe-biotite → siderophyllite → protolithionite", and the dioctahedron micas evolved from muscovite/phengite to Li phengite. Li incorporates into the crystal lattice of mica through the substitution of"Fe2++Mg → AlⅥ+Li", which increases Li contents in the mica during the evolution. However, the mica's F contents are nearly invariable because the partition coefficient for F between mica and melt decreases with decreasing mica's Mg#(=100Mg/(Mg+Mn+Fe)) value. For the low partition coefficient, fluorine was reserved dominantly in the melt, which would promote the magmatic evolution and mineralization. Due to the fluid metasomatism, the zoning micas are ubiquitous in the Xihuashan W-bearing granites. Li and F contents in the rims as phengite are much lower than those in the cores of the zoning micas. Additionally, some calcite grains associated with phengite were formed by CO2-rich fluid metasomatism. It is suggested that some high-CO2but low-Li and F fluids have participated during evolution of the Xihuashan granites. Furthermore, the variation of micas reveals that oxygen fugacity in the Xihuashan granites had changed. The magmatic system had shifted to less oxidizing condition during evolution, which is important for the tungsten-mineralization.