Occurrence and Geochemistry of REE Minerals From the Western Part of the Lugiin Gol Carbonatite Deposit, Southeast Mongolia

ABSTRACT The Lugiin Gol deposit is one of four REE deposits (Khalzan Burged, Mushigai Khudag, and Khotor) in Mongolia. It consists of a nepheline syenite stock, equivalent dike rocks, and more than 400 carbonatite veins within an area of approximately 13 km 2 . This study focuses mainly on the western part of the Lugiin Gol deposit. The western Lugiin Gol deposit consists of many carbonatites that fill NE‐trending fractures in sedimentary rock. The minerals in the carbonatites include calcite, dolomite, strontianite, kutnohorite, ankerite, fluorite, synchysite‐(Ce), bastnaesite‐(Ce), parisite‐(Ce), synchysite‐bastnaesite intergrowths, rutile, apatite, goyazite, quartz, K‐feldspar, muscovite, chlorite, Al‐Si mineral, Na‐Si mineral, pyrite, sphalerite, chalcopyrite, galena, Fe hydroxide, and graphite. Synchysite‐(Ce), bastnaesite‐(Ce), parisite‐(Ce), and synchysite‐bastnaesite intergrowths are REE fluorocarbonates. Synchysite‐(Ce), the most abundant REE fluorocarbonate, occurs as disseminated euhedral crystals in carbonates, Fe hydroxide, and K‐feldspar. It is LREE‐dominant, with La/Ce ratios ranging from 0.47 to 0.84, and its LREE abundance decreases in the order Ce > La > Nd > Gd > Sm > Eu. Bastnasesite‐(Ce), the second most abundant REE fluorocarbonate, occurs as granular crystals closely intergrown with synchysite‐(Ce). It is also LREE‐dominant, with La/Ce ratios ranging from 0.68 to 0.91, and LREE abundances in the order Ce > La > Nd > Gd > Eu > Sm. Parisite‐(Ce), the third most abundant REE fluorocarbonate, occurs as anhedral or granular crystals that are closely intergrown with or replaced by synchysite‐(Ce). It is LREE‐dominant, with La/Ce ratios ranging from 0.38 to 0.61, and LREE abundances in the order Ce > La > Nd > Gd > Sm > Eu. The synchysite‐bastnaesite intergrowths occur as granular crystals and are LREE‐dominant, with La/Ce ratios ranging from 0.81 to 0.97, and LREE abundances in the order Ce > La > Nd > Gd > Eu. The grain size and intergrowth textures of the REE minerals govern the grinding fineness required to achieve sufficient mineral liberation. REE mineral grains in this deposit range from fine (< 150 μm) to moderately coarse (< 400 μm), but they commonly occur locked with gangue minerals. This indicates that the ore must be ground sufficiently fine to break the intergrowths and liberate the REE minerals. Based on the observed REE mineral textures, REE mineralization was formed by the addition of Ca and a decrease in temperature in ore‐bearing fluids (from approximately 100°C to over 400°C) at relatively low pressures. Therefore, information on the occurrence and chemical composition of REE minerals can be used as basic data for understanding REE minerals genesis and improving their recovery rates.