REE compositions of ores, altered rocks and related unaltered rocks have been determined. Comparative studies of REE geochemical characteristics of these geologic bodies indicate that REE patterns of ore-forming fluids in the Zhenyuan and Mojiang gold deposits show marked changes, while that of the Daping gold deposit remains relatively unchanged. Therefore, the REE pattern (strong LREE enrichment, weak negative Eu anomaly and strong Ce anomaly) of ore-forming fluids of the Daping gold deposit can represent the REE pattern of primary ore-forming fluids. Studies indicate that the ore-forming fluids with such a REE pattern might be CO2-riched mantle fluids produced by mantle degassing.