[SRD33-P08] Petrography of hydrothermal magnetite ore and host rock of the Chandmani Uul deposit, southeast Mongolia
Keywords:Chandmani Uul, Mongolia, magnetite deposit, IOCG
Volcanic rocks of Shar Zeeg formation have been intensely modified by hydrothermal alteration and deformation during intrusion of granitoids. The volcanic rocks are composed of phenocrysts (plagioclase, 10-15%) and groundmass (quartz-feldspar, 85-90%). Polysynthetic twinning of plagioclase (0.5-1.8mm) is bended during cataclase process. The groundmass shows microfelsitic texture, which is overgrown by sericite and calcite aggregates. Numerous veins of calcite, quartz and epidote cut host rocks. Fine-medium grained biotite granodiorite exhibits a massive texture and contains about 60% plagioclase, 20-25% quartz, 10-15% biotite. Pyrite, magnetite, sphene and apatite are found as accessory minerals.
Mineralization of this deposit developed to replace volcanic rocks of Shar Zeeg formation. Magnetite-hematite ore is most abundant in the deposit. Massive mushketovite (prismatic aggregate of magnetite with small amount of hematite at the rim of magnetite crystals) is the major mineral. Calcite-quartz veinlets usually cut magnetite-hematite ore. Magnetite-chalcopyrite ore is also abundant in the deposit, and appears relatively lower level of the mine. Under ore microscope, chalcopyrite with small amount of pyrite is filled by aggregate of small magnetite grains. Hematite accompanies along grain boundary of magnetite.
Previous research suggests that this deposit is skarn-type in origin, however no skarn mineral is included in ore except small amount of garnet in association with sulfides. Limestone crops out ~2 km south of ore bodies, however no mineralization is found in this limestone. Similarity of tectonic setting and mineral assemblage of the deposit with those of world IOCG (Iron Oxide Copper Gold) deposits suggests that the origin of this deposit is IOCG type.