CARBONATITE BRECCIAS OF THE TOMTOR ALKALINE MASSIF: STRUCTURAL, MINERALOGICAL, GEOCHEMICAL, AND (C, O, Sr) ISOTOPE FEATURES

Carbonatite breccias were discovered by borehole G2 in the north of the Burannyi site in the Tomtor massif (complex) (TC) hosting the largest Sc–Y–Nb–REE ore deposit. The breccias are petrographically composite rocks consisting of fragments of dolomite, dolomite–ankerite, ankerite, and calcite carbonatites with significant amounts of F–REE carbonates, pyrite, and fluorite. They show signs of fragmentation of carbonatites and calcite rocks, transition of these fragments by younger melt–brine enriched in CO2, F, S, and REE, and its interaction with breccia fragments. The studied rocks are classified as magmatic-fluidogenic and fluidogenic breccias, which are a unique source of information about the rock composition and deep-level processes within the TC. The trace element spidergrams of the carbonatite breccias are similar to those of carbonatites and unique high-grade Sc–Y–Nb–REE ores of the upper ore horizon of the TC. The spidergrams show enrichment in Th, Nb, La, Ce, and Nd and depletion in U, K, Sr, Zr, Hf, and Ti, as in other carbonatite complexes of the world (including those with breccias). The REE pattern shows enrichment in MREE and HREE. The formation of carbonatite breccias is accompanied by the crystallization of unique REE minerals: synchysite-(Ce), parisite-(Ce), and/or bastnaesite-(Ce), cebaite-(Ce), and burbankite. Сrystallization of REE-fluorocarbonates is associated with dolomite replacement by ankerite. The maximum amount of these minerals, as well as cebaite-(Ce) and burbankite, is found in calcite rocks in association with fluorite and pyrite. It is shown that the δ18O and δ13C (‰) composition points in carbonatite breccia fragments and in the interfragmentary space form a trend with high correlation, which is similar to the trend of mixing of C and O isotopes of carbonatites and sedimentary carbonates. However, it contradicts the data on the increasing contents of REE, Nb, P, and other elements typomorphic for carbonatites with increasing δ18O and δ13C (‰) values and is the result of the transformation of carbonates by a low-temperature deuteric fluid. The first obtained comprehensive data on carbonatite breccias give grounds to consider them a new type of mineralization in the TC.