In volcanic terrains, igneous plutons are emplaced beneath topographic highs resulting in two distinct groundwater flow systems: topography- and buoyancy-driven. The interface between these systems is a potential site of ore deposition. This study investigates the interaction between topographic and hydrothermal flow systems using a 2D time-transient solution to the heat, fluid, and 18O/16O mass-transport equations. The goal of this study is to explore factors controlling the geometry, lateral extent, and stability of this interface. Results show that the interface geometry and stability (temporal and spatial) are most influenced by pluton depth and lateral extent of the topographic high. More laterally extensive topography produces a near horizontal interface whose location is stationary in space (¿500 m) and nearly constant in temperature (≥200¿C) for times >40,000 years. Less-extensive topography produces spatially and temporally stable interfaces, but at high angles to the surface. Shallow pluton emplacement results in a transient mixing zone. Many authors have suggested that ore formation occurs first and then stabilizes the interface by formation of a caprock. However, this study shows that under certain conditions, the interface can be stabilized first and that this may then lead to a caprock formation. An 18O/16O transport simulation shows that steep chemical gradients can be maintained nearly coincident to this interface for 10,000s of years. The spatial and temporal stability of the mixing interface, and maintaining steep chemical gradients along that interface, are three criteria necessary for mineral (ore) deposition involving fluid mixing at this hydrologic interface.