Mineralogical constraints on the genesis of W-Nb-Ta mineralization in the Laiziling granite (Xianghualing district, south China)

The Laiziling granitic pluton, located in the Xianghualing district, Hunan Province, China, comprises typical rare-metal granites with a pronounced vertical lithological zonation. This pluton includes three granitic phases comprising, from base to top, medium-grained (MG) alkali feldspar granite, fine-grained (FG) alkali feldspar granite and albite granite. Here we report the results of textural and chemical analyses of W-Nb-Ta oxide minerals from the Laiziling granitic pluton. The chemical compositions of whole-rock samples, rock-forming minerals and zircon have allowed us to identify the magmatic-hydrothermal processes that operated during the formation and rare-metal mineralization of the Laiziling pluton. The granites are slightly peraluminous to peraluminous, highly evolved and fractionated, as inferred from whole-rock geochemical and mineralogical (micas and zircon) signatures. All the micas in different granitic phases are classified into the zinnwaldite group. Li concentration in the primary micas (similar to 2.6-4 wt% Li2O) and Hf concentration (similar to 1-8 wt% HfO2) in the zircon increase for the base to the top granitic phases. Both alkali feldspar granites contain oxide mineral aggregates including columbite-group minerals, wolframoixiolite, qitianlingite, wolframite and cassiterite. The columbite-group minerals, wolframite and cassiterite also occur as isolated grains within the albite granite. The presence of the oxide minerals suggests that the W-Nb-Ta mineralization developed during the latter stages of the magmatic evolution, related to fluids that were compositionally evolved from the granites. The composition of the fluids controls the aggregate of the oxide minerals. The wolframite and cassiterite that precipitated within the Laiziling skarn-type deposit are chemically different from those within the granite, indicating a hydrothermal origin for the oxide minerals that formed as a result of pervasive alteration.