REE-Nb-Zr MINERALIZATION OF ORE ZONE № 5 OF THE BURPALA INTRUSION (NORTHERN BAIKAL REGION)

Five ore zones have been identified within the Burpala intrusion, the most of them have metasomatic origin. The study established that rare-metal and rare-earth (Zr, REE, Nb, Th) accessory mineralization is not associated solely to metasomatic rocks. This assemblage has also been identified in syenites weakly affected by metasomatic processes, where a rare-metal minerals occur as interstitial aggregates.Ore zone № 5 is composed of fine- to medium-grained alkaline and nepheline syenites with an agpaitic index of 0.80– 1.42, in which, in comparison to the previously studied metasomatic ore zones № 1 and 2, the REE-Nb-Zr mineralization is late-magmatic. The main Nb, REE, and Zr minerals are loparite-(Ce), eudialyte, zircon, and minerals of the wöhlerite group (låvenite, rosenbuschite); chevkinite-(Ce), apatite, and catapleiite are present in subordinate amounts. The morphological characteristics of the minerals and the absence of clear signs of metasomatic processes point to their late-magmatic origin during the differentiation of alkaline magma: poikilitic loparite and eudialyte crystals formed at conditions of high melt alkalinity and supersaturation, whereas a subsequent decrease in agpaiticity led to the destabilization of zirconosilicates (eudialyte, låvenite) and the extensive crystallization of interstitial zircon, as well as the replacement of loparite by a titanite–pyrochlore assemblage. The transitional position of the ore-bearing Burpala syenites between typically miaskitic and agpaitic varieties led to the formation of a mixed type of ore mineralization.