MINERALOGY AND GEOCHEMISTRY OF THE MUSHGAI KHUDAG REE DEPOSIT, MONGOLIA

The Mushgai Khudag REE deposit in southern Mongolia is composed of sheet-like magnetite-apatite ore bodies along the contact of the Jurassic syenite porphyry and the host Paleozoic sedimentary rocks. The ore bodies are locally accompanied by carbonatite dikes and veins of high-grade ores. Average REE grades depend on each ore body, ranging variably from 1.5 to 7.5%. The magnetite-apatite ores consist mainly of apatite, barian celestite, magnetite, hematite goethite, fluorite, gypsum, phlogopite, carbonates (mostly calcite), pyrite and monazite-(Ce). The carbonatite dike (and vein) ores are characterized by mineral assemblages of monazite-(Ce), apatite, carbonates, pyrite and magnetite. REE-bearing minerals are apatite (~2% REE) and monazite-(Ce) with small amounts of bastnasite, britholite and probably rhabdophane, a product of supergene alteration. Purple fluorite and sulfates contain trace amounts of REE. Apatite consists of pale green fluorapatite and reddish hydroxylapatite and the latter occurs commonly in high-grade ores. Monazite-(Ce) is the most dominant REE mineral, occurring along crystal boundary, crack and growth zone of apatite (mostly hydroxylapatite) crystals. Chondrite-normalized REE patterns of ores, consistent with the occurrences of apatite and monazite-(Ce), show the enrichment of LREE relative to HREE with small negative Eu anomalies. A positive correlation between P₂O₅ or Th and REE contents and high gamma-radiation dose rate suggest the occurrence of monazite-(Ce). In most of the high-grade ores, As content is higher (>100ppm) whereas Sr and Ba contents are relatively low (<1% Sr; <1500ppm Ba), suggesting that pyrite and barian selestite are indicative minerals high- and low-grade mineralization of REE. The geological, mineralogical and geochemical results and petrographic observation suggest that the REE mineralization of magnetite-apatite ores and high-grade carbonatite ores at the Mushgai Khudag resulted from exsolution from the syenite magmas during crystallization.