Rare-Metal Rocks of the Ingursky Massif (Western Transbaikalia): Composition, Geochemistry, and Factors that Determined the Formation of Rare-Metal Mineralization

On the example of a massif—Ingursky, formed within the North Mongolian–West Transbaikal polychronic rift system—the conditions that contributed to the formation of rare-metal mineralization in it were considered. The basis was mineral-petrogeochemical studies of the ore-bearing Ingursky and nearby ore-free Sherbakhtinsky massifs. The totality of the compositions of these arrays forms a general sequence, which can be considered as an evolutionary series that arose during the differentiation of the magma of the syenite composition. The first of the two main factors that led to the formation of rare-metal (Be, Ta, Nb, Th, U, HREE) mineralization in the pegmatites of the Ingursky massif is the high fluid saturation of the alkaline-granitoid melts of the massif, which contributed to their deep differentiation with the accumulation of ore mineralization in the late derivatives of pegmatite and pneumatolytic processes. The second is a rather long, ~6 million year, thermal history of rocks of the massif at rather high temperatures, about 900–500°C, which contributed to the accumulation and redistribution of ore elements at the final stages of the magmatic process.