LAUSR

Among the rich REE–Nb ores of the upper ore horizon of the Tomtor deposit, under the Jurassic deposits (borehole 324, interval 47–62 m), ultrapotassiс rocks (feldspar porphyrite), consisting of more than 90% potassium feldspar (K-feldspar), are found. The rocks contain titanium–niobium oxide and rare grains of fersmite, Ce-bastnaesite, Ce-monazite, calcite, barite, and strontianite. K-feldspar is unevenly replaced by muscovite. By the type of REE and element distribution on the multidiagrams, the rock correlates well with the rich REE–Nb ores from the Burannyi site at the lower levels of contents. The Ar–Ar age of K-feldspar corresponds to 323.4 Ma and is correlated with the geological time of the formation of the rich REE–Nb ores from the Tomtor deposit, as well as the time of occurrence of magmatic and hydrothermal activity in this part of the region during this period.