THE HYDROTHERMAL REE MINERALIZATION OF QUARTZ-FLUORITE VEINS IN THE LUGIIN GOL NEPHELINE SYENITE COMPLEX, SOUTH MONGOLIA

The Lugiin Gol nepheline-syenite complex intrudes Late Permian sedimentary rocks of the Lugiin Gol Formation, which are metamorphosed to hornfels. The main rock type of the Lugiin Gol complex is medium- to coarse-grained and fine-grained nepheline-bearing syenite. The nepheline-syenite commonly composed K-feldspar, nepheline, biotite, hornblende and clinopyroxene, with small amounts of apatite, zircon, fluorite, opaque minerals and rare earth minerals. Opaque minerals are composed of pyrite, galena, spharelite, hematite and thorianite. RE minerals are composed of synchysite-(Ce), parasite-(Ce) and thorn-britholite-(Ce).

The RE mineral bearing quartz-fluorite veins are distributed radially around the boundary of nepheline syenite and hornfels, and are 30 cm on average thickness. These veins are characterized by hydrothermal breccia texture and strong altered hallo. This RE mineral bearing breccia vein is composed of broken fragments of fluorite (high Sr-Y) and cemented by fluorite (Low Sr-Y), quartz, RE minerals and small amount of very fine-grained pyrite. Total REE concentrations of these veins range from 2.9 to 5.6 wt.%. RE minerals are composed of synchysite-(Ce) and parasite-(Ce) in brecciate texture with quartz and fluorite. Synchysite and parasite are commonly observed to be euhedral to subhedral coarce grained and lath- and radial-lath shaped. Fluid inclusions occur in fluorite and are dominated by aqueous H₂O–NaCl types.

Based on mineral assemblage and texture of quartz-fluorite-REE veins, it is inferred that a fluorine-bearing hydrothermal activity was associated with REE mineralization.