MULTIPHASE GROUNDWATER-FLOW NEAR COOLING PLUTONS

We investigate groundwater flow near cooling plutons with a computer p rogram that can model multiphase flow, temperatures up to 1200 degrees C, thermal pressurization, and temperature-dependent rock properties. A series of experiments examines the effects of host-rock permeabilit y, size and depth of pluton emplacement, single versus multiple intrus ions, the influence of a caprock, and the impact of topographically dr iven groundwater flow. We also reproduce and evaluate some of the pion eering numerical experiments on flow around plutons. Host-rock permeab ility is the principal factor influencing fluid circulation and heat t ransfer in hydrothermal systems. The hottest and most steam-rich syste ms develop where permeability is of the order of 10(-15) m(2). Tempera tures and life spans of systems decrease with increasing permeability. Conduction-dominated systems, in which permeabilities are less than o r equal to 10(-16) m(2), persist longer but exhibit relatively modest increases in near-surface temperatures relative to ambient conditions. Pluton size, emplacement depth, and initial thermal conditions have l ess influence on hydrothermal circulation patterns but affect the exte nt of boiling and duration of hydrothermal systems. Topographically dr iven groundwater flow can significantly alter hydrothermal circulation ; however, a low-permeability caprock effectively decouples the topogr aphically and density-driven systems and stabilizes the mixing interfa ce between them thereby defining a likely ore-forming environment.