A one-dimensional, physical model describing two-phase fluid flow is used to simulate the effect of boiling on silica precipitation in geothermal and epithermal precious metal systems. The extent to which decreasing temperature and fluid vaporization are responsible for quartz precipitation is dependent on three related factors - the temperature of the fluid entering the two-phase system, the change in fluid temperature with respect to distance of fluid travel, and the extent of fluid vaporization in regions of gradual temperature decline. Boiling contributes to significant quartz precipitation in systems with high-temperature basal fluids, and in deeper portions of systems in which extensive vaporization occurs. Temperature reduction is a dominant precipitation mechanism in near- surface regions where temperature reduction is rapid, and in systems with lower temperature fluids. Owing to the small difference in quartz solubility between the liquid and vapor phases at low temperatures, boiling does not contribute to significant quartz precipitation in low temperature, near-surface regions. Quartz precipitation is most intense in systems with high mass flux/permeability ratios and low initial fluid temperatures. Geothermal systems with high mass flux/permeability and moderately low initial fluid temperatures are most effective in producing epithermal systems with abundant gold. Numerical modeling indicates that sufficient copper can be partitioned from a "typical" calc-alkaline melt into an exsolving fluid to produce an economic porphyry copper deposit. Neither non-magmatic sources nor an additional hidden magma source are necessary to provide copper to the system and an elevated initial copper concentration in the melt is not necessary. Melts in shallow systems with initial water concentrations of at least 2.5 wt.% water and Cl/H₂O as low as 0.03 can produce economic deposits with volumes of 50 km³ or less, regardless of copper compatibility. In deeper systems deposits may be produced from melts of less than 30 km³ if copper behaves incompatibly prior to water saturation or if the initial melt is water-rich and requires only minor crystallization to achieve water saturation. If copper behaves compatibly prior to water saturation very large volumes of melt may be required. High salinity fluids may be produced directly from a crystallizing melt and immiscibility is not necessary to produce the high salinities observed in some systems. Depending on the temperature, pressure, initial water content, and the extent of crystallization of the melt, the bulk salinity of the aqueous fluids exsolved from a melt may vary from < 2.0 wt.% NaCl to saturation levels (84 wt.% NaCl at 700°C). Fluid evolution during the magmatic-hydrothermal transition and coincident molybdenite precipitation at Questa, New Mexico, has been traced using fluid inclusion microthermometry. The lack of cogenetic liquid- and vapor-rich inclusions, plus final homogenization of most saline, liquid-rich inclusions by halite dissolution indicate that high-salinity fluids were generated by a mechanism other than fluid immiscibility. Pressure fluctuations, responsible for the formation of a magmatic-hydrothermal breccia, are capable of producing the observed fluids and inclusion behavior. Solubility data indicate that the crystallizing aplite porphyry generated fluids with salinities as high as 57 wt.% NaCl equivalent. / Ph. D.