Figures

1. Geologic sketch map of the Bayan Obo region

2. Generalized geologic map of the Bayan Obo study area

3. Diagrams showing the deposit relation of the Chartai and Bayan Obo Groups as cover beds deposited on the
continental slope of the North China Platform or craton

4. Paleotectonic map of northern China in the Caledonian period, showing locations of the Caledonian subduction, the Hercynian subduction, and the Caledonian Hejao anorogenic granites

5. Map showing localities of samples taken in and near the Main and East Orebodies of Bayan Obo

6, 7. Photomicrographs of--

6. A fine-grained dolostone marble (sample 7B34), showing banding parallel to bedding and lenses of scattered detrital quartz grains

7. A coarse-grained dolostone marble (sample 7B34-16) containing rounded grains of detrital apatite and showing triple-junction grain boundary texture

8. Geologic sketch map of the Main and the East Orebodies of Bayan Obo

9-14. Photographs of--

9. Hand specimen (sample 7B30-1) of typical microfolded banded ore consisting of irregular to lenticular layers of monazite, granular hematite, and fluorite

10. Large block (sample 8B13) of banded ore consisting of layers of aegirine augite and magnetite

11. Block (sample 9B3i) showing both fluorite and aegirine augite as major gangue minerals

12. Block (sample 9B13) of banded ore consisting of granular apatite, granular hematite, and monazite

13. Outcrop on bench 1582 m of the East Orebody, showing bodies of massive magnetite and hematite ores

14. Block of H8 dolostone marble partly replaced by disseminated magnetite

15-17. Photomicrographs of doubly polished thin sections of--

15. The host dolostone marble (sample 7B36-'90) showing granulation texture and interstitial replacement of granulated grains of dolomite by very fine grained monazite

16. Banded fluorite dolostone marble (sample 7B30-2) showing fluorite replacement of dolomite at triple-junction grain boundary

17. Marble (sample 796-48,449) showing relict dolomite grains surrounded by fine granular aegirine augite that partially replaced dolomite

18-22. Diagrams showing--

18. Internal isochron showing mineral age of two aliquots of monazite from sample 796-48,278

19. Internal isochrons of monazite from two samples (490,330 and 34-24) of Bayan Obo ores showing ages and 208Pb/204Pb initial ratios

20. Internal isochrons of monazite from two samples (7B36-6 and 8B35-2) of Bayan Obo ores that show nearly identical 208Pb/204Pb initial ratios indicate different internal isochron ages, suggesting a common source reservoir that was tapped at different times

21. Internal isochrons of monazite from two samples (7B31-2 and 7B31-9) of Bayan Obo ores that have essentially the same internal isochron mineral ages and 208Pb/204Pb initial ratios

22. Internal isochrons of bastnaesite from sample 85-10,21-4 and monazite from sample 7B30-20 having different internal isochron mineral ages but high radiogenic 208Pb/204Pb initial ratios

23-30. Photomicrographs of doubly polished thin sections of--

23. Sample 8B2-2 showing euhedral martitized magnetite crystals blocking the continuous formation of later fine-grained hematite

24. Sample 7B37-1 showing euhedral martite crystals embedded in fine granular hematite

25. Sample 7B30-20 showing bands of granular monazite replaced interstitially by fine granular hematite

26. Sample 7B30-20 showing strings of rounded monazite surrounded and replaced interstitially by fine granular hematite

27. Sample 7B31-6 showing a magnetite veinlet cutting a banded monazite-bearing granular hematite ore

28. Sample 9B15-1L showing a banded ore consisting of monazite-granular hematite-fluorite cut by late-stage, coarse-grained fresh magnetite aggregates

29. Sample 796-48,517 showing relict banded ore consisting of folded monazite and granular hematite surrounded and replaced by late-stage, fresh magnetite

30. Sample 796-48,277, a low-grade ore consisting of alkali amphibole, monazite, and magnetite, as well as relict grains of the limestone marble host rock

31, 32. Diagrams showing radiometric ages of minerals in--

31. Sample 796-48,277

32. Sample 8B7

33. Summary diagram of K-Ar and 40Ar/39Ar minimum mineral ages of alkali amphiboles of both metasomatic and metamorphic origin from samples of the Bayan Obo REE-Fe-Nb ore deposit, Inner Mongolia, China

34. Schematic diagram showing five stages of north to south subduction of the Siberian oceanic plate under the northern margin of the North China craton

35. Diagram showing the 39Ar release pattern of incremental heating of hornblende from a quartz monzonite rock, sample 8B53-1, collected near Hejao, showing a plateau 40Ar/39Ar Caledonian age of 455.0+-3.4 Ma

Tables

1. Principal minerals of the Bayan Obo ore deposit

2-5. Chemical composition of samples from Bayan Obo:

2. Ferroan dolomite in H8 dolostone marble

3. Essentially unmineralized H8 dolostone marble

4. REE-mineralized H8 dolostone marble

5. Huanghoite, apatite, and aegirine augite

6-12. Partial chemical composition of ores from Bayan Obo:

6. Disseminated monazite ore in dolostone marble and monazite-hematite-apatite banded ores

7. Selected monazite(bastnaesite)-fluorite (low-Fe, high REE) banded ores

8. Bastnaesite-apatite-aegirine augite-fluoritemartite banded ores with apatitebarite

9. High-Fe, REE, and fluorite banded ores

10. High-REE, low-Fe, and low-F banded ores

11. Marble-hosted alkali amphibole-magnetite banded ores

12. Quartzite-hosted ores

13. Generalized mineral paragenetic sequence of Bayan Obo

14. Partial chemical composition and internal isochron mineral ages of monazites and bastnaesites of Bayan Obo

15. Chemical composition and minimum mineral ages of various generations of selected alkali amphiboles of Bayan Obo

CONVERSION FACTORS

Multiply	By	To obtain
micrometer (mm)	0.000039	inch
centimeter (cm)	0.3937	inch
kilometer (km)	0.6214	mile
metric ton	1.102	ton
degree Celsius (°C)	1.8 and add 32	degree Fahrenheit

ABBREVIATIONS, ACRONYMS, AND SYMBOLS

TIME

Ma    mega-annum, 106 years ago

Ga    giga-annum, 109 years ago

m.y.    million (106 ) years

 

CONCENTRATION

wt. percent   weight percent

per mil    parts per thousand (103 )

ppm   parts per million (106 )

ppb    parts per billion (109 )

 

RARE EARTH ELEMENTS AND OXIDES

HREE    heavy rare earth element

LREE    light rare earth element

REE   rare earth element

RE2O3   rare earth oxides

 

ANALYTICAL METHODS

EPMA    electron microprobe analysis

ICP-AES    inductively coupled plasma-atomic emission spectrometry

ICP-MS    inductively coupled plasma-mass spectrometry

INAA    instrumental neutron activation analysis

ISE    ion-selective electrode

SEM-EDX    scanning electron microscopy with energy-dispersive X-ray analysis

SXRF    synchrotron X-ray fluorescence analysis

WDXRF    wavelength-dispersive X-ray fluorescence spectrometry

 

MISCELLANEOUS

DPTS    doubly polished thin section

MDL    minimum detection level

USGS    U.S. Geological Survey