The Logachev and Rainbow hydrothermal fields differ from any other known subsea hydrothermal fields by the fact that they are associated spacially and genetically with serpentinites, rather than with volcanic rocks. Some of these fields include Co and Co-Zn hydrothermal rocks. The most common sulfide minerals are pyrite, chalcopyrite, bornite, cubanite and sphalerite. Developed in these ore fields are small particles of Ni-Co minerals, such millerite and pentlandite. Abnormally high a few percent contents of cobalt have been found. The distinctive feature of these deposits is the poor spatial separations of copper- and zinc-bearing sulfides which are deposited in different thermal conditions. Moreover, these sulfide deposits often contain small magnetite and goethite grains. The hydrocarbons found in hydrothermal deposits differ in their composition from those studied in other known subsea hydrothermal fields. Most of them are of abiogenic origin. The specific composition and these hydrothermal solutions and deposits is associated with the anomalous composition of their primary hydrothermal solution that had originated as a result of water interaction with the ultrabasic rocks of the lower crust and upper mantle, and also with the substantial transformation of this material as it rises toward the ground surface phase separation. Because of this phase separation, the hydrothermal solutions are characterized by their rapidly changing temperature and salinity. This was responsible for the highly variable density and floatation of the hydrothermal fluids flowing to the ocean, which resulted in the formation of an unusual, multilayered hydrothermal plume with neutral buoyancy, which propagated from the floor up to a few hundred meters above it. This multilayer plume can be used as a searching criterion for hydrothermal fields associated with serpentinite. The hydrothermal deposits accumulating at the ocean-floor surface provide information both for the composition of the primary hydrothermal solution, and for the transformation of the latter during its rise toward the oceanic bottom.