Problems of the genesis of giant apatite and rare metal deposits of the Kola Peninsula, Russia

Geochemical models were developed for the genesis of giant rare metal and apatite deposits of the Kola Peninsula on the basis of isotopic data, investigations of inclusions in minerals, experimental phase diagrams of systems containing ore minerals, and distribution of elements in rare metal (loparite) ores. The world's largest alkaline complexes (Khibiny and Lovozero) and related rare metal mineral deposits were derived from a depleted mantle reservoir (Sr-87/Sr-86 = 0.7035-0.7038 and epsilon(Nd) = 4.5-5), whereas crustal material was not involved in their genesis. The enormous enrichment of the alkaline rocks in alkalis, rare earth elements, niobium, zirconium, and phosphorus was related to the large-scale processes of mantle metasomatism, which occurred within a narrow time interval (371-361 Ma, the Late Devonian). The subsequent localization of ore matter took place in magma chambers during early stages through crystal fractionation accompanied by convection. The proposed geochemical model is consistent with new data on the distribution of Nb, Ti, Ca, Sr, REE, and Th in ore minerals within the vertical section of the Lovozero Massif. The compositional evolution of loparite in the Lovozero Massif during the differentiation of peralkaline magma is surveyed on the basis of more than 500 microprobe analyses of the mineral. From bottom to top in the vertical section of the massif, loparite shows increases in SrO (from 0.84 to 7.68%), Nb2O5 (from 7.40 to 13.35%), Ta2O5 (from 0.54 to 0.77%), ThO2 (from 0.5 to 1.0%), Sm (from 0.84 to 1.19%), and Na2O (from 8.6 to 10.0%) and decreases in CaO (from 4.35 to 3.08%), FeO (from 0.56 to 0.09%), TiO2 (from 40.04 to 36.62%), Ce2O3 (from 20.38 to 14.76%), La2O3 (from 10.05 to 7.8%), Nd2O3 (from 4.6 to 3.3%), and REE total (from 36.6 to 27.6%). The investigations revealed the presence of cryptic layering with respect to loparite composition on the scale of the whole massif and suggested that crystal fractionation under closed conditions in a giant magma chamber was responsible for the loparite evolution. The principles of cotectic saturation of alkaline magmas as a necessary condition for the formation of an ore deposit was formulated and applied as a basis for the assessment of rare metal mineral potential of igneous provinces.