FIGURE 12.8 Massive sulfide deposits (A) form when hydrothermal fluids discharge from mid-oceanic ridges and then mix with cool seawater. Here metallic ions bond with sulfur, forming sulfide minerals that eventually accumulate on the seafloor. Note how heat convection pulls cold seawater into the ridge, where it reacts chemically with basalt to form hydrothermal fluids. Photo (B) shows sulfide minerals precipitating as hydrothermal fluids escape from vents on the seafloor.