Fluorite-bastnaesite rocks of the Ulan-Ude rare earth occurrence as a new type of carbonatites: Western Transbaikalia, Russia

The aim of the study is to show evidence confirm the magmatic nature of the bastnaesite-fluorite rocks of the Ulan-Ude occurrence that represent a specific type of carbonatites. The minerals have been studied using electron microscopy, thermometry, and analysis of stable isotopes. In many cases, the content of both fluorite and bastnaesite in the studied rocks reaches 50%. The rocks formation time (134.2 ± 2.6 Ma) is close to that of the carbonatites in the Western Transbaikalia province. The temperature of bastnaesite and fluorite formation in the rocks exceeds 500 C. The studied rocks contain phenocrysts of fluorite, bastnaesite, and tetraferriflogopite. Their crystallization began in the intermediate focus. The fine-grain fluorite matrix with small bastnaesite scales was formed in near-to-the-surface conditions and fixed by the hardening of the rocks. The carbonatites feature melt “dryness”, which determines an extremely low occurrence of the hydrothermal processes, changes in the xenoliths and previously formed minerals. The low fluid saturation level is confirmed by the absence of phlogopite water in position A, filled with fluorine only. The carbonatites are characterized by a higher content of sulfates represented by tenardite, glauberite, plumbojarosite, and corkite that are present in the composition of the salt melts released at the later stage of the melt crystallization.

fluorite, bastnaesite, carbonatite, glauberite, plumboyarozite