A new indicator of ore-forming fluids source, N2-Ar-He compositions in fluid inclusions, including its principle and analytical method, has been introduced. A case study has been taken on two Chinese ore deposits: the Changkeng Au-Ag deposit and the Songxi Ag-Sb deposit situated in Guangdong Province, South China. The results show that the ore-forming fluids in the two deposits are mainly composed of formation water or sedimentary hot brines, which provides new evidence for the genesis of the two deposits.