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<p>Carbonatite-related rare earth elements (REE) deposits are rare with about 30 global occurrences with established reserves, and even fewer hosting significant Fe resources. Determining the genetic link between Fe- and REE-bearing carbonatites, and the mechanisms governing it, is fundamental to deciphering ore formation in carbonatitic systems. The Bayan Obo deposit, exceptionally enriched in both Fe and REE, provides a key opportunity to examine their genetic relationship and the processes controlling mineralization. Although the REE and Fe ores are mainly hosted by the <i>ca.</i> 1320&#xa0;Ma H8 unit, most REE is genetically linked to the newly recognized ~ 430&#xa0;Ma carbonatites. Our study is the first to show that Fe is temporally and genetically related to REE formation. Magnetite-associated monazite grains from massive and aegirine-type ores yield U–Pb ages of 425 ± 18&#xa0;Ma (~13 ppm U), 435 ± 41&#xa0;Ma (~0.4 ppm U), and 414 ± 46&#xa0;Ma (~0.6 ppm U), respectively, which is coeval with the ~ 430&#xa0;Ma REE-bearing carbonatites. The zoned distribution of Fe ores and magnetite crystallization temperatures of ~ 350–550 ℃ indicate a hydrothermal origin for Fe mineralization. We suggest that the progressive decrease in temperature and fluorine activity, and increase in oxygen fugacity during brine/fluid evolution are critical for Fe and REE mineralization at Bayan Obo. Our results highlight the importance of highly evolved carbonatitic systems in forming Fe-rich REE deposits.</p>

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A 430 Ma hydrothermal origin for magnetite at Bayan Obo

  • 期刊论文

  • 发表日期：

    2025年12月2日

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  • Lan Yang,
  • Yang Li,
  • Yan Yu,
  • Li-Guang Wu,
  • Fang-Yue Wang,
  • Xian-Hua Li

摘要

Carbonatite-related rare earth elements (REE) deposits are rare with about 30 global occurrences with established reserves, and even fewer hosting significant Fe resources. Determining the genetic link between Fe- and REE-bearing carbonatites, and the mechanisms governing it, is fundamental to deciphering ore formation in carbonatitic systems. The Bayan Obo deposit, exceptionally enriched in both Fe and REE, provides a key opportunity to examine their genetic relationship and the processes controlling mineralization. Although the REE and Fe ores are mainly hosted by the ca. 1320 Ma H8 unit, most REE is genetically linked to the newly recognized ~ 430 Ma carbonatites. Our study is the first to show that Fe is temporally and genetically related to REE formation. Magnetite-associated monazite grains from massive and aegirine-type ores yield U–Pb ages of 425 ± 18 Ma (~13 ppm U), 435 ± 41 Ma (~0.4 ppm U), and 414 ± 46 Ma (~0.6 ppm U), respectively, which is coeval with the ~ 430 Ma REE-bearing carbonatites. The zoned distribution of Fe ores and magnetite crystallization temperatures of ~ 350–550 ℃ indicate a hydrothermal origin for Fe mineralization. We suggest that the progressive decrease in temperature and fluorine activity, and increase in oxygen fugacity during brine/fluid evolution are critical for Fe and REE mineralization at Bayan Obo. Our results highlight the importance of highly evolved carbonatitic systems in forming Fe-rich REE deposits.