Hydrothermal alteration of pyrochlore group minerals from the Miaoya carbonatite complex, central China and its implications for Nb mineralization

•An episode of metasomatic events of 235.4 ± 4.1 Ma overprinted the Nb mineralization of the Miaoya complex.•Pseudomorph after uranpyrochlore was successively replaced by ferrocolumbite, uraninite and Nb-bearing rutile.•Nb and U liberated from uranpyrochlore were transported by reducing, high-temperature, (Fe, Si, S, CO2)-bearing fluids.•Fluids responsible for uranpyrochlore alteration were concomitant with the hydrothermal reworking of REE mineralization.