Within the city of Ulan-Ude, several sites of bastnaesite–fluorite rocks and calcite-containing rocks were found. They are confined to the exposures of Paleozoic schists and quartzites. The rocks have an age of 134.2 ± 2.6 Ma. They are brecciated lenticular and vein-like bodies cemented mainly with bastnaesite–fluorite aggregate. The content of fluorite in the rocks is several tens of percent, and the content of bastnaesite-(Ce) is 20–30%, often reaching 50%. Among the secondary minerals, there are monazite-(Ce), albite, and K-feldspar, and the accessory minerals are zircon, Nb-containing rutile, and manganilmenite. Light lanthanides are predominant among REE in the rocks. Bastnaesite and fluorite contain brine–melt fluid inclusions with homogenization temperatures of 490–520 °C. The salts of these inclusions are composed of predominant Na and Ca sulphates and subordinate Ca and REE carbonates, and the gas phase contains CO2. Gas inclusions and part of water–salt inclusions homogenized at 150–200, 290–350, and 430–450 °C. The salts of late fluids are composed of Ca and REE carbonates, K and/or Na chlorides, Ca, Mg, and Fe hydrosulphates, and Ca and Na hydrocarbonates, and the gas phase contains CO2 ± H2. The isotopic compositions of carbon (–5.9 to –8.3‰ δ13CV-PDB) and oxygen (4.3 to 8.3‰ δ18OV-SMOW) in bastnaesite and calcite fall in the PIC square specific to unaltered intrusive carbonatites. The primary strontium isotope ratios in fluorite and bastnaesite are equal to 0.70559–0.70568. The proximal location, close ages, and mineral and geochemical features indicate a genetic relationship of the studied rocks with the late Mesozoic carbonatites of southwestern Transbaikalia. The finding of this rock occurrence indicates a existence of one more carbonatite-bearing area and expands the distribution area of such rocks, which makes southwestern Transbaikalia promising for REE mineralization.