A finite difference model of the cooling of an igneous intrusive of limited volume is developed and used to investigate the relation between igneous intrusion, the formation of liquid and vapor dominated geothermal systems, and the formation of porphyry-type ore deposits. The model takes into account the properties of pure water and accommodates the phenomena of boiling and condensation. Permeability, level of intrusion, and pluton volume are systematically varied. Pressure, temperature, and fluid velocity are computed as functions of time.It is found that a self-supported, vapor dominated steam zone is commonly (but briefly) formed above the intrusive. Condensed water bounds the steam zone above, and if the hydrothermal solutions are saline, a zone of boiling bounds the steam zone below. For pure water condensation is far more important than boiling--the solutions circulate around the critical point of water to become gaseous without boiling. Despite large temperature variations, convection causes fluid pressures throughout the whole uniform permeability system to be close to normal cold-water hydrostatic values. Thus, even in an active converting system with moderate permeability variations, fluid pressure will tend toward normal hydrostatic values. Fluid circulation appears easily sufficient to produce a typical porphyry copper ore shell, but base metal precipitation probably must be controlled by mechanisms other than simple temperature drop.