Columbite-tantalite minerals from Nanping granitic pegmatites, South China: compositional trends and genetic implications

The No31 pegmatite group is the largest pegmatite cluster of the Nanping pegmatite field and represents one of the most important Ta-Nb deposits in China. The columbite-tantalite occurring in No31 pegmatites showed faint oscillatory zonation and complex patchy zonation patterns in back-scattered electron images. Microprobe analyses revealed that columbite-tantalite in the pegmatites has a wide range of composition. Neither Mn/(Mn + Fe) nor Ta/(Ta + Nb) atomic ratio has obvious trend of compositional variation along the profile of a single pegmatite dike. However, in the quadrilateral diagrams the compositional trend in columbite-tantalite varies front ferrocolumbite to tantocolumbite, then to manganotantalite. The features of zonation and composition in columbite-tantalite imply that the formation of Nanping pegmatites underwent original differentiation stage and later metasomatism stage.