The relationship between petrogenesis and ore formation during the development of ultrabasics is discussed. It is emphasized that it is different for deep-seated pyrope peridotite and shallower spinel-enstatite peridotite facies. The influence of depth (pressure) on the composition of fluids which accompany ultrabasic magmatism is considered. It is inferred that the pressure is responsible for H2O and CO2 prevailing in fluid inclusions in diamonds. Regular compositional correlations are established for chrome-spinellids (chromites) and rock-forming chrome-spinellids, as well as general correlations between the compositions of chromite and sulphide mineralization and surrounding ultrabasics. These correlations indicate that liquid immiscibility was responsible for the separation of ore-forming magmas from ultramafic silicate melts of various compositions. The general specificity of orecontent in ultramafic complexes is explained by means of the hypothesis on basic-ultrabasic immiscibility in magmas under the influence of fluid components, with hydrogen and hydrogen sulphide playing an important role. Rhythmic layering in massifs is explained by repeated separation of the developed magmatic layers as well as by differentiation crystallization proceeding under the effect of temperature and magmatic composition gradients. Rhythmic layering reflects the departure of the magmatic systems from equilibrium on fast cooling, much in the same way as zoning in crystals.