ORE-FORMING POTENTIAL OF ALKALINE MAGMAS

Geochemical models of the genesis of the largest rare-metal deposits, connected with agpaitic nepheline syenites, are discussed in this paper. Thermodynamic calculations demonstrate that high alkalinity prevents the separation of volatile components into the gas phase. The rare-earth deposits have been formed at the magmatic stage during the process of crystal differentiation, jointly with convection in the magmatic system. The saturation of alkaline magma in respect of ore minerals at early stages of crystallization is considered to be the necessary condition for the formation of cumulative magmatic deposits. These considerations are proved by data on micro-inclusions.