In volcanic terrains, igneous plutons are often emplaced beneath topographic highs (e.g. caldera rims or volcanoes) resulting in two distinct groundwater flow systems: topography- and buoyancy-driven. The interface between these two flow systems has been proposed as a potential site of ore deposition. The development of a sizeable ore body due to fluid mixing between two such systems requires that the mixing interface remain spatially stable for significant periods of time. The interaction between topographic and hydrothermal flow systems is investigated using a 2-D time transient finite element model describing heat, fluid and ¹⁸O/¹⁶O mass-transport. Our aim is to explore the factors that control the geometry, lateral extent and stability of the interface between these flow systems. Our results show that the geometry and stability (temporal and spatial) of the mixing interface are most influenced by depth of pluton emplacement and lateral extent of the topographic high relative to the pluton width. More laterally extensive topography produces a near horizontal interface whose location is stationary in space (±500 m in elevation) and nearly constant in temperature (³ 200 °C) for times in excess of 40,000 years. Less extensive topographic highs also produce spatially and temporally stable interfaces, but they tend to be at high angles to the surface. Interface stability is favored by deeper pluton emplacement. Shallow pluton emplacement results in a very transient mixing zone in both time and space. A simulation with ¹⁸O/¹⁶O mass-transport shows that steep chemical gradients can also be maintained nearly coincident to the mixing interface for several 10,000’s of years. Finally, it has been proposed that the presence of a caprock is required to decouple the two flow systems, thus stabilizing the mixing interface. Our investigation, however, indicates that a stabilization of the mixing interface can occur without a caprock. It may be that the caprock then forms due to stabilization of the interface and associated mineral deposition.