An unusual salt-rich (SR) and salt-poor (SP) fluid immiscibility (FFI) could occur in a variety of hydrothermal fluids. The segregation of SR fluid from SP fluid provides a potential mechanism for substantial mass enrichment during some ore forming processes. However, the physical properties of SR and SP fluids, which determine the hydrodynamic behaviors of unmixed fluids, have not been well constrained. This study adopted in situ optical observation, quantitative Raman analysis, and the mass conservation law to derive the density of the SR and SP fluids. On this basis, falling sphere viscometry was employed to determine the viscosity of the SR fluid. Results showed that SR fluid exhibited an obviously higher density but comparable viscosity to SP fluid, favoring its high mobility and strong tendency to segregate from SP fluid. Consequently, the FFI could contribute tremendously to mass enrichment during fluid evolution.