A variety of fluid flow phenomena involving fluids with thermal and compositional variations are reviewed, first as they are observed in simple laboratory experiments and then as they may apply to the formation of sulfide deposits resulting from exhalation of hot saline solutions from vents in the sea floor. Of particular interest is the case where the effluent is both very salty and hot, so that the two properties have opposing effects on the density difference between the exhaled fluid and its surroundings. This can lead to a very nonlinear density behavior during mixing, which makes it possible for initially light fluid to become heavier than sea water and for an oscillating flow to develop. Even more important are the "double-diffusive" effects which can occur because of the different molecular diffusion rates of the two properties. An outflow can separate into two parts, a hot, less concentrated plume which rises and a warm concentrated flow which spreads as a bottom current away from the source, maintaining a sharp boundary with the overlying sea water as it does so. If the hot salty fluid is injected into a density gradient, a situation which is typical of the ocean, a stratified lateral transport of the lighter fraction can result. For the heavier fraction, the effect of the combined processes is to maintain a stable boundary between the sea water and an exhaled hydrothermal ore solution, which might thus flow with minimal mixing along the sea floor over large distances to a distant depression before dumping its contained metals. A continuing inflow of dense fluid into such a depression produces a stable stratification, so that in a steady state the outflow spilling over the edge of the depression would be at a lower temperature and salinity and higher or lower f (sub O 2 ) . This condition provides a mechanism for localizing precipitation of sulfides within a small restricted depression from very large volumes of ore solution. Evidence of density stratification in the Bushveld Complex suggests the importance of related phenomena in the formation of layered igneous complexes. Analogous behavior in porous media is also indicated.