Home – Home – Jornals – Russian Geology and Geophysics 2019 number 12

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 ºC. 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 ºC. 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‰ d¹³C_V-PDB) and oxygen (4.3 to 8.3‰ d¹⁸O_V-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 the existence of one more carbonatite-bearing area and expands the distribution area of such rocks, which makes southwestern Transbaikalia promising for REE mineralization.