Critical metal minerals are significant to the national economy and security, but whether their enrichment is controlled by the fractional crystallization process or by the magmatic- hydrothermal interaction remains controversial. Traditional geochemical methods can only indirectly restrict the source of ore- forming materials, and it isnt easy to distinguish the effects of fractional crystallization and magmatic- hydrothermal processes on mineralization. The isotope systems of fluid- active metals such as Rb, Ba, Sr, and U have different responses to these two mechanisms. We have studied the Rb and Ba isotopes of granites collected from the Himalayas and South China. Our research shows that the Ba isotope in the residual melt becomes heavier and the Rb isotope remains unchanged during mineral crystallization, while the Ba isotope in the granite becomes lighter and the Rb isotope becomes heavier during the magmatic- hydrothermal interaction. The hydrothermal fluid exsolved from deep magma brings abundant critical metals that are easy to migrate with fluid. Our study shows that stable metal isotopes, especially fluid- active metal isotopes, are very effective in tracing magmatic- hydrothermal processes and ore- forming fluid sources.