The role of magmatic versus metasomatic processes in Ta-Nb mineralization at the Jiabusi Deposit, Inner Mongolia: Evidence from textural and chemical features of rare metal minerals and mica

The exact role magmatic versus metasomatic processes play in Ta -Nb mineralization is a question that has been debatable for several decades. The Jiabusi deposit is chosen to address this question, for its ample endowments of a variety of Ta -Nb oxides and mica in the various rock types that could provide insights into the whole evolutional history of ore-forming processes during magmatic-hydrothermal evolution. Textural and chemical evidence suggests that Fe -mica in the Fe -mica granite and pegmatitic portion is magmatic, indicating that the coprecipitated columbite-group minerals (CGM) and pyrochlore-supergroup minerals (PSM) are also magmatic. In the lepidolite granite and aplite, on the other hand, two stages of mineralization occured, represented by unzoned CGM that are in equilibrium with lepidolite, snowball quartz, and fluorite that all have a metasomatic origin and subsequent CGM/PSM overgrowths that co-precipitated with metasomatic Cs-lepidolite and saccharoidal albite. These CGM overgrowths have 1-2 time(s) higher Ta/(Nb + Ta) values than the preceded CGM, generating a clear compositional gap between them. Both stages of CGM and CGM/PSM precipitated from a hydrosilicate liquid (HSL), the partial migration of which results in the disequilibrium textures observed. The lepidolite granite is largely a metasomatic rock, the protolith of which has been obscured by intense metasomatism governed by the HSL. This HSL, however, is mainly constrained to the lepidolite granite but a few migrated into the Fe -mica granite. This is the main reason for the different mineralizing styles and intensities between the two rock types.