Salinity and temperature data from fluid inclusions in Cyprus and Kuroko massive sulfide deposits indicate that the ore solutions were buoyant on entering the sea water. They probably rose from the sea floor like axisymmetric conical plumes derived from a finite source or as line plumes from a linear source. The height to which they rose depended on the magnitude of the density gradients in the ambient sea water. However, in the region immediately above the outlet pipe, the effect of interference between the many vents making up the pipe and the effect of the anticipated very low flow rate probably caused necking of the plume and a drastic retardation of the normal loss of vertical velocity and buoyancy caused by entrainment of sea water.Solubility data for copper sulfides indicate that virtually all the copper was precipitated by quenching immediately above the vent. Precipitation of sphalerite (+ or - galena) took place higher in the plume and the very fine grained particles of these phases were probably entrained in the plume, carried to considerable heights, and in many cases dispersed. During the inevitable slowing of the flow rate and cooling of the hydrothermal system, sphalerite (+ or - galena) was precipitated immediately above the vent. This was probably the most important way of forming vertically zoned Cyprus and Kuroko deposits.Thin, sheetlike, well-banded deposits like Rosebery and many in the Bathurst area of Canada, may have a different form because they developed in relatively shallow water. In this situation the plume, stagnating at the water surface, spreads laterally for considerable distances, raining sphalerite and galena to the sea floor. This hypothesis predicts such deposits will have greater metal contents than other massive sulfides, higher Zn/Cu ratios, a lack of sulfur isotope equilibration between minerals, and wide geochemical halos.