Carbonatitic melts in olivine and magnetite from rare-metal carbonatite of the Belaya Zima alkaline carbonatite complex (East Sayan, Russia)

Abstract

The problem of the origin of carbonatite is still a priority in Russian and foreign publications. Attention to these rocks is mostly explained by their raremetal specialization and links of Nb, Ta, Zr, Sr, Ba, and REE deposits to them. In addition to the hypothesis of the magmatic origin of carbonatites, currently some authors consider these rocks as products of hydrother� mal-metasomatic activity. This contradiction may be solved by study of the inclusions of mineralforming media in minerals, which provide direct evidence for natural melts or fluids captured in host minerals. In spite of the active investigation of melt inclu� sions in minerals from rocks of the alkaline carbonatite complexes over the last decade, there are still some data including the quantitative characteristics of the chemical composition of melts for carbonatites, as well as on other associated nonsilicate (fluorite, apa� tite, etc.) rocks (1-6). Only a few publications con� cern carbonatites properly (3, 5, 6). However, estima� tion of the role of melts and their compositions is of great importance for understanding of the origin of alkaline carbonatite complexes including the pro� cesses and conditions of accumulation of commercial minerals in them. In this paper, we report the results of the study of the melt and crystalline inclusions coexisting in min� erals of orebearing forsterite-calcite carbonatite from the Belaya Zima massif (East Sayan), which allowed us to distinguish the carbonatitic melts responsible for the formation of these rocks for the first time. A number of complex (Nb, Ta, REE, U, Pb, Zn, and P) deposits including the largest Belaya Zima nio� bium deposit related to the carbonatite massif of the same name are distinguished within the East Sayan province of ultrabasic alkaline rocks and carbonatites. Based on the data of geochronological U-Pb study of zircons, the age of this deposit is 643 Ma (7). The Belaya Zima (Nizhnesayanskii) massif is located in the northern foothill of East Sayan, in the upper reaches of the Belaya Zima River. The massif occurs in the zone of large faults of northwestern orientation, in the area of the junction of the Siberian Platform and folded complexes of East Sayan. In plan, it has a pearlike shape and spreads to the northwest for 8.5 km includ� ing a branch elongated in the same direction (8). The central part is composed of carbonatites of various types. Ultrabasic alkaline rocks form a semiring zone surrounding the stock along the periphery.