Columbite-group minerals and mica of peraluminous granite record the magmatic-hydrothermal processes that formed the Zhaojinggou Ta[sbnd]Nb deposit in the North China Craton

Abstract

The newly discovered Zhaojinggou TaNb deposit in the northern margin of the North China Craton contains ore reserves of more than 38 Mt. at an average grade of 0.013 wt% Ta2O5 and 0.012 wt% Nb2O5. The TaNb mineralization is mainly hosted in a peraluminous, albite granite body, which is about 400 m long and 20 to 100 m wide, with a surface exposure of ~0.05 km2. Granitic pegmatites and greisen veins that range from several centimeters to meters in width, occur in the margin of the granite body, and wolframite-quartz veins intrude the wall rocks up to tens of meters away from the granite contact. LA-ICP-MS UPb dating for columbite of the granite yields the lower intercept ages between 131.8 ± 1.0 Ma and 134.6 ± 2.1 Ma on the Tera-Wasserburg concordia diagram, indicating that the mineralization occurred in the early Cretaceous, coeval with the peak stage of lithospheric thinning and destruction of the North China Craton. The columbite grains in the granite are commonly euhedral, and are surrounded or penetrated by irregular tantalite patches/rims containing Ta2O5 as high as 59.5 wt%. The Ta concentrations within single columbite crystals commonly increase slightly from the center outwards, consistent with a systematic decrease of Nb/Ta caused by crystallization of columbite from the silicic magma. Mica in the granite shows disordered bright and dark domains in their BSE images; bright domains are F-rich zinnwaldite crystallized from highly evolved melts that were relatively enriched in Fe, Li, Rb, Nb and Ta, whereas the dark domains are F-poor muscovite crystallized from later hydrothermal fluids that were relatively enriched in Al, Mn, Ba, Sn and W. It is thus likely that the tantalite patches/rims around the columbite grains crystallized from a Ta- and F-rich hydrosilicate melt in a magmatic-hydrothermal transitional stage. The wolframite-quartz veins adjacent to the granite body likely formed from a W-rich hydrothermal fluid.