Sr isotope geochemical studies (the⁸⁷Sr/⁸⁶Sr and δ¹⁸O-⁸⁷Sr/⁸⁶Sr systems) on the wall rocks and ores from the Lemachang independent Ag deposit in northeastern Yunnan provide strong evidence that the ore-forming fluids had flown through radiogenetically Sr-enriched rocks or strata prior to their entry into the locus of ore precipitation, and water-rock interaction is the main mechanism of Ag ore precipitation. The radiogenetically Sr-enriched source region may be the Proterozoic basement (the Kunyang and Hekou groups). Moreover, the theoretical modeling of the Sr isotopic system indicates that the ore-forming fluids contain as much as 3×10^-6 Sr with isotopic composition of Sr being 0.750 and that of oxygen 7.0‰ The ore-forming temperatures were estimated at 150–250°C for the carbonate rock-type ores and at 200–260°C for the clastic rock-type.