2013

High- technology   metals potential of  alkaline  magmatism ( Russia)

 Kogarko L.N.

 Vernadsky Institute of Russian Academy of Sciences

   High- technology ( strategic) metals such as Zr, Hf,  Y, Nd, Ta, REE are mostly enriched in alkaline and carbonatitic rocks. Russia takes the first place on the Globe in the number of alkaline and carbonatites massifs .Russian resources of REE are on the second place after China .Nb and tantalum resources of Russia are enormous and largest in the world. Almoust all strategic   rare-metal deposits are related  to alkaline and carbonatitic massifs in Russia. At present time only two rare metal deposits are in operation-Lovozero and Khibina situated in Kola Peninsula-North-West of Russia.

   The Khibina alkaline massif (Kola Peninsula,Russia) hosts the world’s largest  and economically most important apatite deposit.The Khibina massif is a complex multiphase body built up from a number of ring-like and conical intrusions. The apatite bearing intrusion is ring-like  and represented by a layered body  of ijolitic composition with a thickness of about 1 -2 km. The upper zone is represented by different types of apatite ores.This rocks consists of 60-90% euhedral  very small (tenths of mm)apatite crystals.The lower zone is mostly  ijolitic composition. The lower zone grades into underlying massive urtite  consisting of 75-90% large (several mm) euhedral nepheline.Our experimental studies of systems with apatite demonstrated the near-eutectic nature of the apatite-bearing intrusion, resulting in practically simultaneous crystallization of nepheline, apatite and pyroxene. 

The mathematical model of the formation of the layered apatite-bearing intrusion based on the processes of sedimentation under the conditions of steady state convection  taking account of  crystal sizes is proposed.Under the conditions of steady-state convection large crystals of nepheline continuously had been settling forming massive underlying urtite  when smaller crystals of pyroxenes, nepheline and apatite had been stirred in the convecting  melt. During the cooling the intensity of convection decreased causing a settling  of smaller crystals of nepheline and pyroxene and later very small crystalls of apatite in the upper part of alkaline magma chamber.

 The Lovozero massif, the largest of the Globe layered peralkaline intrusion, comprises super-large rare-metal (Nb, Ta, REE) deposit. The main ore mineral is loparite (Na, Ce, Ca)2 (Ti, Nb)2O6 which was mined during many years. The composition of cumulus loparite changed systematically upward through the intrusion with an increase in Na, Sr, Nb, Th, Nb/Ta, U/Th and decrease in REE, Zr, V, Zn, Ba and Ti. Our investigation indicates that the formation of loparite ore was the result of several factors including the chemical evolution of high alkaline magmatic system and mechanical accumulation of loparite at the base of convecting unit.