日本地球惑星科学連合2018年大会

講演情報

[EJ] ポスター発表

セッション記号 S (固体地球科学) » S-RD 資源・鉱床・資源探査

[S-RD33] 資源地質学

2018年5月23日(水) 10:45 〜 12:15 ポスター会場 (幕張メッセ国際展示場 7ホール)

コンビーナ:大竹 翼(北海道大学大学院工学研究院 環境循環システム部門)、荒岡 大輔(産業技術総合研究所地圏資源環境研究部門)、高橋 亮平(秋田大学大学院国際資源学研究科、共同)、野崎 達生(海洋研究開発機構海底資源研究開発センター)

[SRD33-P08] Petrography of hydrothermal magnetite ore and host rock of the Chandmani Uul deposit, southeast Mongolia

*ルブサンニャム オユンジャルガル1林 謙一郎1ルサンチュルテム ジャルガル2 (1.生命環境科学研究科 筑波大学、2.モンゴル国立大学)

キーワード:チャンドマニウウル、モンゴル、磁鉄鉱鉱床、IOCG

Hydrothermal type magnetite deposit at the Chandmani Uul locates in Dornogovi province, southeastern Mongolia. Ore reserves of this deposit are estimated as 25 Mt of Fe with 0.1 Mt of Cu and 3 t of Au. Tectonically, the deposit locates in passive continental margin that develops north of the Undurshi fault zone, the main Mongolian lineament. Magnetite ores are hosted in Shar Zeeg formation (Neoproterozoic to Lower-Cambrian) as >50 lenticular ore bodies of 100-200 m wide with 10-30 m in thickness. Lithology of host rocks is metamorphosed dacite, dacitic tuff and andesitic tuff. Middle- to Upper-Cambrian intrusive bodies of mixed granodiorite, diorite and diorite-porphyry intruded into Shar Zeeg formation. The intrusive bodies are believed to supply ore-forming fluid. We have collected ~200 ore and rock samples from drilling cores for petrographic examination.

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.