Characteristics and geological significance of geochronologic results from the Bayan Obo deposit, Inner Mongolia
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摘要:
内蒙古白云鄂博矿床是世界罕见的特大型REE-Nb-Fe多金属共生矿床，由于经历了复杂的区域地质演化历史，至今仍对稀土成矿时代认识不一。基于前人的同位素年代学研究成果，结合矿床地质特征对稀土成矿时代进行讨论。研究表明，白云鄂博矿区经历了华北克拉通2.5~1.8 Ga的前寒武纪构造演化过程；哥伦比亚超大陆裂解的最后阶段，约1.3 Ga在白云鄂博裂谷形成了巨型稀土矿床，与同期碳酸岩墙形成时代一致；受后期华北克拉通北缘古亚洲洋俯冲碰撞造山的影响，白云鄂博地区引发热液交代活动，形成了晚期稀土矿脉和原有稀土矿物的重结晶，导致H8赋矿白云岩同位素系统受到扰动，呈现出多期次（1.0~0.2 Ga）的同位素年龄结果。总之，白云鄂博是一个在约1.3 Ga裂谷成矿并叠加了加里东和海西2期热扰动事件的巨型稀土矿床。
Abstract:Objective The Bayan Obo deposit in Inner Mongolia is a rare superlarge REE-Nb-Fe polymetallic deposit worldwide. There are still controversies on the timing of REE mineralization in the deposit due to the complex geological evolution in the region.
Methods Based on previous isotopic dating results, in combination with geological characteristics of the deposit, this paper re-evaluates potentially multiple episodes of REE mineralization.
Results The Bayan Obo deposit has experienced the Precambrian tectonic evolution of the North China Craton between 2.5-1.8 Ga. The major REE mineralization was formed in the Bayan Obo rift at ~1.3 Ga during the final stage of the breakup of the Columbia supercontinent, coeval with carbonate dike in the region. Hydrothermal metasomatism or overprints occurred throughout the Bayan Obo area, which is related to the late Paleo-Asian Ocean subduction-collision orogeny in the northern margin of the North China Craton, producing the late REE veins and recrystallization of original REE minerals. Such a hydrothermal modification event has variably disturbed the isotopic system of the H8 ore-bearing dolomite, resulting in multiple periods of isotopic ages (1.0-0.2 Ga) with peaks at 440 Ma and 270 Ma.
Conclusion In summary, the Bayan Obo deposit primarily formed from the ~1.3 Ga rift metallogenic event, superposed by Caledonian and Hercynian tectonothermal events.
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Key words:
- Bayan Obo deposit /
- isotopic age /
- rare earth metallogenic epoch /
- North China Craton
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图 1 内蒙古白云鄂博矿区地质图(据文献[7]略修改)
Figure 1. Geological map of Bayan Obo ore deposit, Inner Mongolia
图 2 白云鄂博矿区野外露头及岩心照片
a.白云鄂博东矿南部基底麻粒岩；b.H8白云岩钻井岩心；c.碳酸岩脉；d.Wu-dyke中霓长岩，含大量氟碳铈矿稀土矿物
Figure 2. Field photographs and core image of the Bayan Obo ore deposit
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