H-O, C, S, N stable isotope systems are widely used to trace the sources of ore-forming fluids; nevertheless, with the steady accumulation of isotope-related data, the complexity and uncertainty of fluid tracing have become more and more distinct. First, stable isotopic compositions usually overlap various potential source reservoirs and thus result in uncertainty and multi-interpretation. Second, once leaving their sources, ore fluids might experience complex geological processes till ore precipitation. During these processes, various isotopic exchanges and fractionations occur, which dramatically change isotopic compositions of fluids. In addition, diverse post-mineralization alterations further complicate isotopic features of ores. Consequently, the isotopic data from ores should be interpreted very cautiously, especially when used to trace potential sources of ore-forming fluids. With examples of well-researched orogenic lode gold deposits, this paper points out that the interpretation of complex stable isotope data depends not only on potential source reservoirs but also on complicated ore-forming processes, which include isotope exchange during fluid-rock interaction along the pathways traveled by the auriferous fluid, isotopic fractionation at the depositional site, and possible post-ore resetting.Intense sulfur isotope fractionation can be easily driven by oxidation which generally occurs during fluid-rock interaction, phase separation due to fluid pressure fluctuation, and fluid mixing. Therefore, considerable variation of mean δ34S values can occur in quartz veins in different orientations in the same deposits, and even among sulfide crystals of the same stage in the same hand specimen. Wall-rock interaction, immiscibility of CH4 and CO2, and evolution of ore fluids inevitably result in very diverse carbon isotope data of hydrothermal carbonates (especially calcites), which overlap several potential source reservoirs. H-O isotopic data also vary significantly due to the exchange with wall rocks. δD data, commonly obtained from bulk fluid inclusion analysis or calculation according to hydrous mineral analysis using appropriate mineral-water fractionation equation, generally signifies involvement of meteoric water and post-ore resetting because of the failure to eliminate secondary fluid inclusions. In addition, N stable isotope compositions, recorded in N-bearing minerals, e.g., K-feldspar and mica, commonly overlap a variety of potential source reservoirs, particularly metamorphic and magmatic ones. It is therefore very difficult to constrain ore_forming fluid sources by means of stable isotope studies without a serious consideration of the influence of ore-forming processes. However, intensive isotopic researches based on spatial and temporal variation may unveil not only the evolution of ore-forming fluids but also their sources.