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Table 13. Generalized mineral paragenetic sequence of Bayan Obo.


Deposition of the sedimentary H8 carbonate rocks interbedded with quartzites, followed by dolomitization.


Occurrence of alkali amphibole veins crosscutting the underlying H6 quartzite. The minimum 40Ar/39Ar age of the alkali amphibole is 1.26 Ga (Conrad and McKee, 1992).


Mylonitization, granulation, and recrystallization of the H8 carbonate into marble with the development and growth of alkali amphiboles due to regional metamorphism. The age of this period of metamorphism, based on the 40Ar/39Ar modified minimum ages, ranges from 800 to 900 Ma. Possibly accompanying the regional metamorphism are hydrothermal alkali amphibole veins with similar 40Ar/39Ar ages, cutting the underlying H6 quartzite.


Early disseminated high-Nd monazite and bastnaesite mineralization of H8 marble (Th-Pb internal isochron ages of 555 and 553 Ma).


Later disseminated high-La, high-Nd bastnaesite and high-La, low-Nd monazite mineralization of H8 marble (Th-Pb internal isochron ages of 496, 476, and 474 Ma).


Early martitized magnetite replacement of H8 dolostone marble with early monazite mineralization and with martitized magnetite replaced later by surrounding monazite.


Monazite mineralization in banded ores associated with aegirine augite, fluorite, hematite, and apatite with Th-Pb internal isochron mineral ages between 430 and 420 Ma, accompanied by alkali amphiboles in sheared H8 marble (40Ar/39Ar age of 425 Ma).


Granular primary fine-grained hematite replacing monazite in hematite-fluorite banded ores (monazite internal isochron age of 423 Ma).


Aegirine augite-rich veins containing huanghoite and aeschynite (Th-Pb mineral-isochron age of 438+-25 Ma).


Fibrous alkali amphibole vein cutting banded ore (40Ar/39Ar plateau age of 396+-6 Ma).


Late-stage magnetite mineralization replacing Mn-rich magnesio-arfvedsonite (40Ar/39Ar age of 343+-3 Ma).


Hercynian age late-stage Nb mineralization.

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