<p>Primary tin deposits are associated with felsic magmatic-hydrothermal systems. However, triggers of tin deposition from tin-bearing hydrothermal fluids remain debated. In particular, the relative roles of fluid mixing, water–rock interaction, and simple cooling are poorly constrained. Here we present in situ oxygen isotope (δ¹⁸O) analyses of cassiterite from 15 tin deposits spanning a wide spectrum of geographic settings and geological environments. The δ¹⁸O trajectories for most samples (13 out of 15) remain constant along profiles across zoned crystals and between different generations, indicating stable fluid-δ¹⁸O during crystal growth. The remaining two samples show deviations consistent with alteration-related modification. The deposit-specific yet internally invariant fluid signatures indicate that neither progressive fluid mixing nor ongoing water–rock interaction is necessarily required to trigger cassiterite precipitation. Instead, our results suggest that cassiterite precipitation is universally controlled by physicochemical changes such as cooling and/or redox variations, rather than by large-scale fluid mixing and/or water–rock interaction.</p>