The Mushgai khudag
volcanic-plutonic complex consists of four REE mineralization zones:
carbonatite zone, apatite zone, magnetite zone, and monazite zone. REE
mineralization occurs within peripheries of alkaline magmatic rocks which
consist of porphyritic syenite, microsyenite and quartz syenites. Three types
of LREE-rich apatite can be found in the carbonatite, apatite, and monazite
zones. Crystal-1 type of apatite exists as hexagonal prismatic shape and is
mostly found in the apatite zone, and in syenite. Crystal-2 type of apatite can
be exposed also at the apatite zone, and carbonatite zone as brecciated massive
crystalline aggregate. Crystal-3 type of apatite demonstrates the compositional
zoning texture with monazite as inter-zoning, and is only found in monazite
zone. The LREE-bearing apatites from the Mushgai khudag complex are mostly
fluorapatite to hydroxyl-bearing fluorapatite with variable REE content.
Apatites from the monazite zone present individual sulfur-rich monazite grain,
and are formed by comprehensive substitutions.
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