The Sedimentary Carbonate-Hostjed Giant Bayan Obo 

REE-Fe-Nb Ore Deposit of Inner ~vtongolia, China: 

A Cornerstone Example for Giant Polymetallic 

Ore Deposits of Hydrothermal Origin 

The work in this report was made possible b_v the 

U.S.!People 's Republic of China Earth Sciences 

Protocol between the Academy of Geological 

Sciences of the Ministry of Geology and Mineral 

Resources of China and the U.S. Geological 

Suney, Department of the Interior of the 

United States of America 

BULLETIN 2143 
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VA 20192. 
The Sedimentary Carbonate-Hosted Giant Bay an Obo 

REE-Fe-Nb Ore Deposit of Inner Mongolia, China: 

A Cornerstone Example for Giant Polymetallic 

Ore Deposits of Hydrothermal Origin 

By E.C.T. Chao, J.M. Back, J.A. Minkin, M. Tatsumoto, Wang Junwen, 

J.E. Conrad, and E.H. McKee, 

U.S. Geological Survey, U.S.A., 

Hou Zonglin and Meng Qingrun, 

Tianjin Geological Research Academy, Ministry of Metallurgical Industry, China, 

and Huang Shengguang, 

Baotou Steel and Rare Earth Corporation, Inner Mongolia, China 

U.S. GEOLOGICAL SURVEY BULLETIN 2143 

The work in this report was made possible by the 

U.S./People :s- Republic of China Earth Sciences 

Protocol between the Academy of Geological 

Sciences of the Ministry of Geology and Mineral 

Resources of China and the U.S. GeologiCal 

Survey, Department of the Interior of the 

United States of America 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1997 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

GORDON P. EATON, Director 

For sale by U.S. Geological Survey, Information Services 

Box 25286, Federal Center, Denver, CO 80225 

Any use of trade, product, or flrm names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government. 

Published in the Eastern Region, Reston, Va. 

Manuscript approved for publication June 28, 1995. 

Library of Congress Cataloging in Publication Data 

The sedimentary carbonate-hosted giant Bayan Obo REE-Fe-Nb ore deposit of Inner 

Mongolia, China : a cornerstone example for giant polymetallic ore deposits of 

hydrothermal origin I by E.C.T. Chao ... [et al.]. 

p. cm.-(U.S. Geological Survey bulletin ; 2143) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3:2143 

1. Hydrothermal deposits-China-Inner Mongolia. 2. Earths, Rare-China-Inner 

Mongolia. 3. Iron ores-China-Inner Mongolia. 4. Niobium ores-China-Inner 

Mongolia. 5. Geology-China-Inner Mongolia. 6. Geology, StratigraphicProterozoic. I. Chao, E.C.T. II. Series. III. Series: U.S. Geological Survey bulletin; 

2143. 

QE75.B9 no. 2143 

[QE390.5] 

557.3 s---dc20 

[553.4'94'095177] 95-24696 

CIP 
CONTENTS 

Abstract........................................................................................................................... 1 

Introduction..................................................................................................................... 2 

Acknowledgments................................................................................................... 3 

Part I. The Bayan Obo Ore Deposit............................................................................... 5 

Geologic and Tectonic Setting................................................................................. 5 

Field and Laboratory Studies................................................................................... 5 

Host Rocks and Cap Rocks of the Bayan Obo Ore Deposit.................................... 11 

Mineralogy of the Bay an Obo Orebodies ...... ................. ................. ... ... .. ................ 16 

Classification and Distribution of Ore 'JYpes .................... ... .. ............ ............ ......... 17 

Chemical Compositions of Ores.............................................................................. 24 

Textural Evidence for the Epigenetic, Hydrothermal, Metasomatic Origin of the 

Bay an Obo Ore Deposit . .. . .. ...... .... ........ ..... ...... ............. .... ... . .. ................. .... ... 28 

Radiometric Ages of Episodes of REE Mineralization and Chemical 

Compositions of Various Generations of REE Minerals................................. 31 

Fe Mineralization..................................................................................................... 36 

Nb Mineralization.................................................................................................... 50 

Radiometric Ages of Alkali Amphiboles as a Key to Determining Periods of 

Regional Metamorphism and Hydrothermal Activities .... .... .. ........................ 50 

Periods of Regional Metamorphism .. ...... ...... ... .. ...... .......... ....... ...... .............. ...... .... 50 

Periods of Hydrothermal Activities......................................................................... 52 

Remobilization and Metamorphic Effects on Mineral Ages ........................ ........... 52 

Mineral Paragenetic Sequence................................................................................. 53 

Part IT. Caledonian Subduction as a Possible Cause and Mechanism of Repeated 

Heating, Fracturing, and Hydrothermal Mineralization.................................. 55 

Subduction of the Mongolian Plate . .. .. .. .. ...... .......................................................... 55 

Generation of the Caledonian Hejao Plutonic Rocks and the Sources of MetalBearing Hydrothermal Solutions..................................................................... 57 

Part ill. Bayan Obo as a Cornerstone Example for Studying Giant Polymetallic Ore 

Deposits of Hydrothermal Origin.................................................................... 59 

Sources of the Ores.................................................................................................. 59 

Tectonic Setting ...................... ................. ...... ........... ...... .. . .................... ... .. .... .. ... ... . 59 

Mechanism for Concentrating the Ores in Source Reservoirs ................................. 59 

Mineral Paragenetic Sequence and Geochemical Evolution of the Bayan Obo 

Ore Deposit...................................................................................................... 60 

Mineral Assemblages and the Transport of REE's in Hydrothermal Fluids .... ....... 60 

Age or Ages of Episodes of Mineralization............................................................. 60 

Lithology and Age of Reservoir Rocks Favorable for Hosting the Ores................. 61 

Preservation of the Ores (Cap Rocks)...................................................................... 61 

Bayan Obo and the Porphyry Cu-Mo Ore Deposits of Peru and Chile ................... 61 

Part IV. Summary and Conclusions and References...................................................... 63 

Summary and Conclusions ..... .. . . . .. .. .. ... . .. .. . . .. ..... .. .... .. .. .. ... . ..... .. .... .. . ... .. .. . .. .... .. .. . . .. . 63 

References Cited...................................................................................................... 63 

III 
N CONTENTS 

FIGURES 

1. Geologic sketch map of the Bayan Obo region....................................................................................................... 6 

2. Generalized geologic map of the Bay an Obo study area .... .. ...................... .... .. ...... ...... .. .... ...... .. .. .. .. ...... ................ 7 

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 .................... ...................................................... .............. 8 

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......................................... 8 

5. Map showing localities of samples taken in and near the Main and East Orebodies ofBayan Obo ...................... 10 

6, 7. Photomicrographs of6. A fine-grained dolostone marble (sample 7B34), showing banding parallel to bedding and lenses of 

scattered detrital quartz grains......................................................................................................................... 12 

7. A coarse-grained dolostone marble (sample 7B34--16) containing rounded grains of detrital apatite and 

showing triple-junction grain boundary texture.............................................................................................. 13 

8. Geologic sketch map of the Main and the East Orebodies of Bayan Obo .............................................................. 18 

9-14. Photographs of9. Hand specimen (sample 7B30-1) of typical microfolded banded ore consisting of irregular to lenticular 

layers of monazite, granular hematite, and fluorite........................................................................................ 20 

10. Large block (sample 8B13) of banded ore consisting of layers of aegirine augite and magnetite.................. 21 

11. Block (sample 9B3i) showing both fluorite and aegirine augite as major gangue minerals........................... 22 

12. Block (sample 9B 13) of banded ore consisting of granular apatite, granular hematite, and monazite........... 22 

13. Outcrop on bench 1582 m of the East Orebody, showing bodies of massive magnetite and hematite ores... 23 

14. Block of H8 dolostone marble partly replaced by disseminated magnetite.................................................... 29 

15-17. Photomicrographs of doubly polished thin sections of15. The host dolostone marble (sample 7B36-'90) showing granulation texture and interstitial replacement 

of granulated grains of dolomite by very fine grained monazite .. .. .. .. .... .................... ........ .............. .... .......... 30 

16. Banded fluorite dolostone marble (sample 7B30-2) showing fluorite replacement of dolomite at triplejunction grain boundary................................................................................................................................... 30 

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

augite that partially replaced dolomite............................................................................................................ 32 

18-22. Diagrams showing18. Internal isochron showing mineral age of two aliquots of monazite from sample 796-48,278 ........... .......... 33 

19. Internal isochrons of monazite from two samples ( 490,330 and 34--24) of Bay an Obo ores showing 

ages and 208Pbf204Pb initial ratios..................................................................................................................... 34 

20. Internal isochrons of monazite from two samples (7B36-6 and 8B35-2) of Bayan Obo ores that show 

nearly identical 208Pbf204Pb initial ratios indicate different internal isochron ages, suggesting a common 

source reservoir that was tapped at different times ................. .... ...... ... ........ ......... ..... .. .. ................. ........... ..... 34 

21. Internal isochrons of monazite from two samples (7B31-2 and 7B31-9) ofBayan Obo ores that have 

essentially the same internal isochron mineral ages and 208Pbf204Pb initial ratios ............ .............................. 35 

22. Internal isochrons ofbastnaesite from sample 85-10,21-4 and monazite from sample 7B30-20 having 

different internal isochron mineral ages but high radiogenic 208Pbf204Pb initial ratios .. ............................ ..... 35 

23-30. Photomicrographs of doubly polished thin sections of23. Sample 8B2-2 showing euhedral martitized magnetite crystals blocking the continuous formation of 

later fine-grained hematite............................................................................................................................... 37 

24. Sample 7B37-1 showing euhedral martite crystals embedded in fine granular hematite............................... 38 

25. Sample 7B30-20 showing bands of granular monazite replaced interstitially by fine granular hematite...... 39 

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

granular hematite............................................................................................................................................. 40 

27. Sample 7B31-6 showing a magnetite veinlet cutting a banded monazite-bearing granular hematite ore...... 41 

28. Sample 9B15-1L showing a banded ore consisting of monazite-granular hematite-fluorite cut by 

late-stage, coarse-grained fresh magnetite aggregates .. .... .... ...... .. .. .. ...... ...... .. ...................... .......... ...... .. ........ 42 

29. Sample 796-48,517 showing relict banded ore consisting of folded monazite and granular hematite 

surrounded and replaced by late-stage, fresh magnetite.................................................................................. 44 

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 .. .. .... .. .. .. .......... ............ ...... .. ...... .. .. .. .. .. .. .. .... .. ......................... 46 
CONTENTS v 

31, 32. Diagrams showing radiometric ages of minerals in31. Sample 796-48,277. ............. .. ............. ... .. . .......... ... . . . ........ ... ........... .................. ... . . . .. . . . ..... . ...................... ...... 48 

32. Sample 8B7 .................................................................................................................................................... 49 

33. Summary diagram of K-Ar and 40 Arf39 Ar minimum mineral ages of alkali amphiboles of both metasomatic 

and metamorphic origin from samples of the Bay an Obo REE-Fe-Nb ore deposit, Inner Mongolia, China........ 51 

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...................................................................................................... 56 

35. Diagram showing the 39 Ar release pattern of incremental heating of hornblende from a quartz monzonite 

rock, sample 8B53-l, collected near Hejao, showing a plateau 40Arf39Ar Caledonian age of 455.0±3.4 Ma ...... 56 

TABLES 

1. Principal minerals of the Bay an Obo ore deposit................................................................................................... 11 

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

2. Ferroan dolomite in H8 dolostone marble .... .................................................... .......... ................................... 14 

3. Essentially unmineralized H8 dolostone marble............................................................................................ 14 

4. REB-mineralized H8 dolostone marble ......................................................................................................... 15 

5. Huanghoite, apatite, and aegirine augite........................................................................................................ 17 

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

6. Disseminated monazite ore in dolostone marble and monazite-hematite-apatite banded ores...................... 25 

7. Selected monazite(±bastnaesite)-fluorite (low-Fe, high REE) banded ores.................................................. 25 

8. Bastnaesite-apatite-aegirine augite-fluorite±martite banded ores with apatite±barite................................... 26 

9. High-Fe, REE, and fluorite banded ores........................................................................................................ 26 

10. High-REE, low-Fe, and low-F banded ores................................................................................................... 26 

11. Marble-hosted alkali amphibole-magnetite banded ores ............................................................................... 27 

12. Quartzite-hosted ores..................................................................................................................................... 27 

13. Generalized mineral paragenetic sequence of Bayan Obo..................................................................................... 29 

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

Bayan Obo.............................................................................................................................................................. 33 

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

of Bay an Obo ........................................................ ................................................................................................. 43 

Multiply 

micrometer (J.lm) 

centimeter (em) 

kilometer (km) 

metric ton 

degree Celsius (°C) 

CONVERSION FACTORS 

By 

0.000039 

0.3937 

0.6214 

1.102 

1.8 and add 32 

To obtain 

inch 

inch 

mile 

ton 

degree Fahrenheit 
VI CONTENTS 

ABBREVIATIONS, ACRONYMS, AND SYMBOLS 

Ma 

Ga 

m.y. 

wt. percent 

%o 

ppm 

ppb 

EPMA 

ICP-AES 

ICP-MS 

INAA 

ISE 

SEM-EDX 

SXRF 

WDXRF 

OPTS 

MDL 

USGS 

TIME 

mega-annum, 106 years ago 

gig a-annum, 109 years ago 

million (106) years 

CONCENTRATION 

weight percent 

per mil, parts per thousand ( 1 03) 

parts per million ( 1 Q6) 

parts per billion ( 1 09) 

RARE EARTH ELEMENTS AND OXIDES 

heavy rare earth element 

light rare earth element 

rare earth element 

rare earth oxides 

ANALYTICAL METHODS 

electron microprobe analysis 

inductively coupled plasma-atomic emission spectrometry 

inductively coupled plasma-mass spectrometry 

instrumental neutron activation analysis 

ion-selective electrode 

scanning electron microscopy with energy-dispersive X-ray 

analysis 

synchrotron X-ray fluorescence analysis 

wavelength-dispersive X-ray fluorescence spectrometry 

MISCELLANEOUS 

doubly polished thin section 

minimum detection level 

U.S. Geological Survey 
The Sedimentary Carbonate-Hosted Giant Bayan Obo REE-Fe-Nb Ore 

Deposit of Inner Mongolia, China: A Cornerstone Example for 

Giant Polymetallic Ore Deposits of Hydrothermal Origin 

By E.C.T. Chao, J.M. Back, J.A. Minkin, M. Tatsumoto, 

Wang Junwen, J .E. Conrad, and E.H. McKee, 

U.S. Geological Survey, U.S.A., 

Hou Zonglin and Meng Qingrun, 

Tianjin Geological Research Academy, Ministry of Metallurgical Industry, China, 

and Huang Shengguang, 

Baotou Steel and Rare Earth Corporation, Inner Mongolia, China 

ABSTRACT 

Detailed, integrative field and laboratory studies of the 

textures, structures, chemical characteristics, and isotopically determined ages and signatures of mineralization of 

the Bayan Obo deposit provided evidence for the origin and 

characteristics favorable for its formation and parameters 

necessary for defining giant polymetallic deposits of hydrothermal origin. Bayan Obo is an epigenetic, metasomatic, 

hydrothermal rare earth element (REE)-Fe-Nb ore deposit 

that is hosted in the metasedimentary H8 dolostone marble 

of the Middle Proterozoic Bayan Obo Group. The metasedimentary sequence was deposited on the northern continental 

slope of the North China craton. The mine area is about 100 

km south of the suture marking Caledonian subduction of 

the Mongolian oceanic plate from the north beneath the 

North China craton. 

The mineralogy of the deposit is very complex, consisting of more than 120 different minerals, some of which 

are epigenetic minerals introduced by hydrothermal solutions, and some of which are primary and secondary metamorphic minerals. The major REE minerals are monazite 

and bastnaesite, whereas magnetite and hematite are the 

dominant Fe-ore minerals, and columbite is the most abundant Nb mineral. Dolomite, alkali amphibole, fluorite, barite, aegirine augite, apatite, phlogopite, albite, and microcline are the most widespread gangue minerals. 

Three general types of ores occur at Bayan Obo: disseminated, banded, and massive ores. Broad zoning of these 

ore types occurs in the Main and East Orebodies. Disseminated ores are in the outermost zone, banded ores are in the 

intermediate zone, and massive ores are in the cores of the 

orebodies. On the basis of field relations, host rocks, textures, structures, and mineral assemblages, many varieties 

of these three types of ores have been recognized and 

mapped. 

Isotopic dating of monazite, bastnaesite, aeschynite, 

and metamorphic and metasomatic alkali amphiboles associated with the deposit provides constraints on the ages of 

mineralization and the history of the deposit. Textural relations, differences in chemical composition, and 232Thf2°8

Pb 

internal isochron ages of monazite and bastnaesite samples 

indicate that many episodes of REE mineralization occurred 

at Bayan Obo, ranging from about 555 Ma to about 398 Ma. 

Initial 208Pbf204Pb ratios suggest different sources of REE's 

for different generations of REE minerals. Relative ages of 

Fe mineralization were deduced from textural relationships 

of Fe minerals with other, dated mineral phases in the 

deposit. Most Nb mineralization was in the area of the West 

Orebodies and resulted in disseminated ore. Aeschynite, an 

early stage of Nb mineralization (438.2±25.1 Ma), occurs 

with huanghoite and alkali amphiboles in veins. 

The 40 Arf39 Ar ages of amphiboles, as well as petrographic textures, were used to distinguish three periods 

of regional metamorphism in the Bayan Obo mine area: 

(1) Late Proterozoic, about 890 Ma, which recrystallized H8 

carbonate to marble and crystallized lineated alkali amphiboles along foliation planes in the marble; (2) Caledonian, 

about 425-395 Ma, which resulted in metamorphic and 

metasomatic-metamorphic alkali amphiboles; and (3) Hercynian, about 300 Ma, based on biotite 40 Arf39 Ar analyses 

from biotite schist and folded banded ores. The 40 Arf39 Ar 

ages of metasomatic alkali amphiboles also place time constraints on the hydrothermal history of the ore deposit. 

Metasomatic amphiboles represent periods of intense 

hydrothermal activity, which began as early as 1.26 Ga; that 
2 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

date is based on the age of amphibole from a vein that 

crosscuts the H6 quartzite that underlies the H8 dolostone 

marble. Although much of the metasomatic amphibole 

formed during periods that overlapped the peak period of 

REE mineralization of banded ores, REE and alkali amp hibole phases generally occur in different mineral assemblages or are of very different ages in the same assemblage 

and, therefore, may have been derived from unrelated fluids. The ages of amphiboles in veins that crosscut banded 

ores indicate that the last stage of banded ores had formed 

by about 396 Ma. 

Two intense periods of subduction of the Mongolian 

oceanic plate beneath the North China craton during the 

Paleozoic era, and an intervening period of extension, may 

account for the repeated heating, fracturing (extensional 

tectonics), hydrothermal mineralization, and regional metamorphism (compressional tectonics) at Bay an Obo. Caledonian Hejao granitic plutonic rocks intruded the Bayan 

Obo Group along a major fault zone and were subsequently 

intruded by Hercynian granitic rocks about 50 km south of 

the mine area. Although the Hejao granitic rocks are low in 

REB's and therefore probably have a different crustal 

source than the REB-mineralizing hydrothermal solutions 

that formed the REB's deposit, both probably resulted from 

Caledonian subduction. 

On the basis of this study, we have selected eight 

parameters necessary to evaluate and understand the conditions needed for the formation of giant polymetallic deposits of hydrothermal origin, regardless of the type of ore 

deposit: (1) sources of the ores, (2) tectonic setting, (3) 

mechanism for concentrating and transporting the ore-bearing fluids, (4) mineral paragenetic sequence and geochemical evolution of the ore deposit, (5) mineral assemblages 

and transport of ore by hydrothermal fluids, (6) age or ages 

of episodes of mineralization, (7) lithology of reservoir 

rocks favorable for hosting ores, and (8) preservation of 

ores. Although of different ore type than Bayan Obo, the 

Andean giant and super giant porphyry Cu-Mo ore deposits 

that occur from Peru to Chile can be compared with Bayan 

Obo in order to study the importance of tectonic setting and 

duration of mineralization on the formation of giant deposits. Both Bayan Obo and the Andean porphyry Cu-Mo 

deposits are related to subduction of oceanic plates, and 

both were formed over long periods of time. In Bayan Obo, 

REE mineralization occurred over a period of more than 

150 million years (555 Ma to 398 Ma). Mineralization of 

many different porphyry Cu-Mo ore deposits occurred over 

a period of about 95 million years (100 to 5 Ma) (Clark, 

1993), and Clark (1993) summarized the migration of porphyry Cu mineralization in the Andes from north to south 

over a period of 50 million years. 

INTRODUCTION 

Bayan Obo is a giant polymetallic rare earth element 

(REE)-Fe-Nb ore deposit of hydrothermal origin in Inner 

Mongolia, China. Earlier studies were reviewed by Chao 

and others (1990, 1992, 1993). Detailed corroborative field 

and laboratory studies of textural and structural relations, 

chemical compositions of various generations of gangue 

and ore minerals, and isotopic and radiometric data on 

gangue and ore minerals were obtained during our research 

in order to describe and interpret the characteristics and ori.;. 

gin of this deposit. On the basis of the results of this study, 

we consider Bayan Obo a cornerstone example for giant 

polymetallic deposits of hydrothermal origin. In spite of the 

unique polymetallic ore types, very complex mineralogy, 

and history of regional metamorphism and mineralization, 

Bayan Obo can be compared with other,· different giant 

polymetallic deposits of hydrothermal origin. 

The Bayan Obo deposit was frrst discovered as an Fe 

deposit in 1927, and later was recognized as a polymetallic 

deposit of Fe, REB's, and Nb. The estimated total reserve of 

Fe (average grade Fe=35 weight (wt.) percent) for the 

Bayan Obo deposit is at least 1,500 million metric tons 

(Drew and others, 1990). On the basis of reported estimates 

of a total reserve of 48 million metric tons (average grade 6 

wt. percent REz03, Drew and others, 1990) to as much as 

100 million metric tons of RE20 3 of unspecified average 

grade (unofficial estimate from Chinese colleagues, oral 

commun., 1987), Bayan Obo is the world's largest known 

REE ore deposit. Recently, on the basis of drilling data, the 

Bayan Obo ore deposit was also acknowledged as the largest Nb deposit in China. Although the size has not been disclosed in the Chinese literature, Drew and others (1990) 

estimated the total reserve of Nb from Bayan Obo as 1 million metric tons of average grade of 0.13 wt. percent Nb. 

In this paper, part I describes the overall characteristics 

of the Bayan Obo ore deposit on the basis of updated field 

and laboratory findings, and data from previous work by 

other investigators that are pertinent to this study. Part II 

presents data available on the Caledonian subduction of the 

oceanic plate from the north under the North China craton. 

This continued subduction may have provided a mechanism 

of repeated activation of heating and fracturing of the lower 

crust, a mechanism that possibly produced the Caledonian 

plutonic rocks of Hejao, 50 km south of the mine region, 

and may have also generated acid hydrothermal solutions 

that scavenged and concentrated the sources of ore for the 

Bayan Obo deposit. Part m examines Bayan Obo as a cornerstone example of giant ore deposits of hydrothermal origin, with respect to eight parameters of importance in the 
INTRODUCTION 3 

formation of these deposits. Part IV presents a summary and 

conclusions and references. 

Acknowledgments 

This joint research was made possible under the U.S./ 

People's Republic of China Earth Sciences Protocol 

between the Academy of Geological Sciences of the Ministry of Geology and Mineral Resources of China and the 

U.S. Geological Survey (USGS), Department of the Interior 

of the United States of America. Geologists of the Tianjin 

Geological Research Academy, Ministry of Metallurgical 

Industry, played a major role as joint research participants. 

We thank chief engineer Professor Yao Peihui and Ms. 

Hu Pinmei of the Geological Bureau of the Ministry of Metallurgical Industry for support of this joint research and for 

providing the necessary clearance for us to conduct field 

research and collect samples in the Bayan Obo mine region. 

We are grateful to Zhang Guozhong, general manager of the 

Baotou Steel and Rare Earth Corporation, and Jiang 

Guojen, director of the Bayan Obo mine, for their assistance 

in providing access to and use of the facilities at the mine. 

We thank mine geologist Wu Janjiang for guidance to critical outcrops both inside and outside of the Main and East 

Orebodies. We also thank Professor Reo Yingchen and 

other members of the Tianjin Geological Research Academy for providing additional Bayan Obo samples from their 

collections for this joint research. 

We thank the following USGS colleagues for their 

managerial support: Tom Ovenshine, Glen Alcott, John 

De Young, Jr., Bruce Lipin, and Klaus Schulz. We thank our 

colleagues J.E. Taggart, A. Bartel, D. Siems, P.H. Briggs, 

and James Crock for their contributions to the analyses of 

unmineralized and weakly mineralized marble samples and 

partial chemical analyses of ores. We also thank our colleagues George Ericksen and Bruce Doe for their technical 

reviews of this manuscript. 

PART I. THE BAYAN OBO ORE DEPOSIT 

GEOLOGIC AND TECTONIC SETTING 

Bayan Obo is a stratabound ore deposit hosted by Proterozoic marble of the Bayan Obo Group. The regional distributions of the Proterozoic Bay an Obo Group, the Archean 

metamorphic rocks of the Wutai Group, which unconformably underlies the Bayan Obo Group, and granitic rocks of 

Archean, Caledonian (early to late Paleozoic), and Hercynian (Permian) ages in the region are shown in figure 1. 

Alkaline plutonic or volcanic rocks have not been reported 

in this region and evidently are not present. The Bayan Obo 

mine area (fig. 2) is separated from the area to the north by a 

major fault zone, the Kuanggou fault. The differences of 

lithology and degree of metamorphism and mineralization 

on either side of the fault indicate that the age of the Kuanggou fault is probably post-late Caledonian (younger than 

400 Ma; Chao and others, 1992). The geology of the region 

north of the Kuanggou fault, which is broken by many 

extensional and thrust faults, is distinctly different from that 

of the mine area, and the stratigraphic sequence there has 

been designated as an undifferentiated unit of Proterozoic to 

Paleozoic sedimentary rocks. 

According to Bai and Yuan (1985), Li Yuying subdivided the Bayan Obo Group into nine lithologic units; a 

detailed stratigraphic section measured in the vicinity of the 

mine by Meng (1982) also distinguished nine lithologic 

units. This section was subsequently described in greater 

detail by Chao and others (1992). The lower part of the 

group (Hu, fig. 2) consists of arkosic quartzite (H1), fine- to 

medium-grained quartzite (H2), dark carbonaceous slate 

(H3), arkosic quartzite and slate (H4), slate with intercalated silty sandstone and quartzite (H5), a ridge-forming 

quartzite (H6), and transitional beds of dolostone marble, 

arkosic sandstone, and calcareous slate (H7). Overlying this 

lower sequence is the dominant ore-bearing unit, H8, which 

ranges from 240 to 540 m in thickness, the thickest part 

being near the Main and the East Orebodies. The H8 unit 

consists of dolostone marble and subordinate limestone 

marble, intercalated with quartzite in the upper and lower 

parts. Small amounts of porphyroblastic albite biotite schist 

and biotite-rich minette occur as lenses within this unit. The 

uppermost lithologic unit, H9, 340-350 m thick, consists of 

a sequence of interbedded gray to black shale, slate, and 

biotite schist that conformably overlies the H8 unit. 

A small swarm of 11 calcitic carbonatite dikes, mostly 

1-2m wide and up to 150m long, cut across an anticlinal 

structure in the Archean Wutai granitic gneisses and migmatites, approximately 1.5 km northeast of the mine area. 

The largest of these dikes was studied in detail by Le Bas 

and others (1992). Another meter-wide, dolomitic carbonate 

dike that cuts H1 or H4 arkosic quartzite northeast of the 

mine was recently discovered. The alkali amphibole in the 

fenitized contact zone of this carbonatite gave an 40 Arf39 Ar 

plateau age of 1.28 Ga (Ren Yingchen, oral commun., 

1993). 

South of the Kuanggou fault zone, the Bayan Obo 

Group crops out in a broad syncline in which the H8 and H9 

units form the central part. The north limb of the syncline 

dips generally 50°-70° s. The south limb is steeper, generally dipping 70°-80° N., but locally it is vertical or is 

slightly overturned (Meng, 1982). This syncline has many 

small chevron folds, meter-wide shear zones, and thrust 

faults, all trending east-west. Many of the more strongly 

mineralized zones in the orebodies follow the east-trending 

shear zones. 

On the basis of sequential stratigraphic analysis, Qiao 

and others (1991) correlated the Bayan Obo Group with the 

Chartai Group and interpreted both groups as having been 

deposited on the North China Platform or craton (fig. 3). 

The Chartai Group was deposited on the continental shelf, 

whereas the Bayan Obo Group was deposited on the continental slope of the craton. The Bayan Obo mine area is 

about 100 km south of the suture marking Caledonian subduction (fig. 4; Wang, 1985) that took place at the northern 

edge of the North China Platform or craton. A small granitic 

intrusion of Caledonian age occurs in the Hejao area, south 

of Bayan Obo (fig. 1). No direct evidence of rifting involving the Bayan Obo Group in the mine region was observed, 

although rifting was cited by Wang and others (1991) on the 

basis of geophysical evidence and the reported occurrence 

of volcanic rocks in the Chartai and Bayan Obo Groups. 

FIELD AND LABORATORY STUDIES 

Field studies consisted of identifying and sampling the 

many rock and ore types in the Bayan Obo mine area and 

determining the geographical extent and relations of the ore 

types. About 830 samples, including samples from three 

drill cores, were collected in the summers of 1987, 1988, 

and 1990. Sampling localities in and near the Main and East 

Orebodies are shown in figure 5. Locations of other samples 

provided by colleagues of the Tianjin Geological Research 

Academy, which are not known, are not given in figure 5. 

5 
6 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

~ Wutai Group (Archean) 

~ Bayan Obo Group 

(Middle Proterozoic) 

rn Ultrabasic rocks 

Fault-Dashed where 

inferred 

++-++ Railroad 

0 20 

EXPLANATION 

0 Quaternary sediments 

SEDIMENTARY ROCKS 

0 Tertiary 

0 Jurassic 

[0 Carboniferous 

CO Silurian 

40 KILOMETERS 

GRANITIC ROCKS 

0 Yenshan Granite (Jurassic) 

0 Hercynian Granite 

I ~/ Y~ I Caledonian Granite 

~ Wutai Granite (Archean) 

Figure 1. Geologic sketch map of the Bayan Obo region (modified from Institute of Geochemistry, 1988). 
~ 

IHeu I 

FIELD AND LABORATORY STUDIES 

EXPLANATION 

(The unit symbols for the Bayan Obo Group are used in 

accordance with standard Chinese geologic symbols 

for Middle Proterozoic) 

Bayan Obo Group (Middle Proterozoic) 

Sedimentary rocks, undivided (Tertiary) [BD 

Diorite (Permian) ~ Granite rocks (Permian) 

Sedimentary 

(Proterozoic 

rocks, 

and Cambrian) 

undivided 

~ 

SYMBOLS 

Contact 

Fault (f)-Ticks show direction of dip, where known 

Other faults--Arrows show relative movement 

Shale, slate, and schist 

Dolostone and limestone marble 

Sedimentary rocks (Hl- H7), undivided 

Wutai Group (Archean)-Gneisses 

Anticline 

Syncline 

+-+-++ Railroad 

+ 

+ 

1·~1 Orebodies--M, Main Orebody; 

E, East Orebody 

Figure 2. Generalized geologic map of the Bayan Obo study area (modified from Chao and others, 1992). 

7 
8 

A 

B 

GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

North China Platform 

Continental Shelf 

Chartai Group 

(Continental Crust) 

(Continental Crust) 

Continental Margin 

Continental Slope 

Sea Level 

Bayan Obo Group 

Oceanic Crust 

Sea Level 

-------- Bayan Obo Group 

Oceanic Crust 

Figure 3. Deposit relation of the Chartai and Bayan Obo Groups as cover beds deposited on the 

continental slope of the North China Platform or craton (modified from Qiao and others, 1991). 

A, Advance of the sea at an earlier stage, when the Chartai Group was deposited. B, Retreat of the 

sea at a later stage, when the Bayan Obo Group was deposited. 

Figure 4. Paleotectonic map of northern China in the Cale- ~ 

donian period (modified from Wang, 1985), showing locations 

of the Caledonian subduction, the Hercynian subduction, and 

the Caledonian Hejao anorogenic granites. 

ii lt~&* 5$~:f:t.l!ft 

TRANSITIONAL & OCEANIC OF CRUST 

~t:: 

so)tf ))lf,R, :iti~~, :kf5.·U.>tfJ§ , 

i&.ftf*?t 

S.O continuous, marginal seas, conti -

nental slopes & sea troughs 

$~ 

oceanic crust 

OTHERS 

Q Q Q I 

I I I 

o2 ~JHi$~ .f.R 

02 coorse clastic deposits 

if.~ :IJ U~ $ AA (col ~19i 1MU!E~ 

~* E. & M. Co ledonian ( 8 0) metamorphic & migmatite zones 

{tf,lj:fHH~A , 1£ , !f-1JU~$AA 

( > 4.41lif: ); , t:;, ~1Jo~:tt; M 

( 4.4-3.91lif:) 

granitic· intrusives, left: E. Caledonian 

( > 440 m.y.), right: l. Caledonian 

(440-390 m.y.) 

~?JK *tl.Jm M.E! , lL: B0,-~: 05 

island ore volcano-sedimentary ass., 

·teft: eo , right: os 
FIELD AND LABORATORY STUDIES 

:k~~:!fu~ CONTINENTAL TYPE OF CRUST 

~,JE IZ Stobie terrains 

Figure 4. Continued. 

*Wllli~ . € Fo¥;,$:..t7tlli~ 

long term positive region, uplifted after £ 

M;,W;H}~~M_ ~l!lP$~jJi:~1J.:£. ~:.lt O? § ..t 

7t. 1~T-f8. s, Fo ..t7t 

epicontinental, carbonate to fine clastic mainly, N. 

Chino uplifted after 02 , Yangtze region uplifted 

after s, 

±fu~ Jt~m . 4":.lt 03 Fo..t7t . 1~-TIZiffl~ o. 

!§ ..t7t. t~T- fR;f-$ s, !§ ..t7t introplotform subsiding zone, N. China uplifted 

after 0 l . Yangtze region western port uplifted 

after G, , eastern port uplifted after s1 

BJE!fu±R 7t;7r 

_ stable massifs undifferentiated 

5!iiJJ IZ Mobile terrains 

1111/l/1111 

J II II 

/Jill 

Pt; *lli~BX.!F1lO.!!LJ1; (~im,WJ) ~~ * 

uplifted end of Pt 3 or E. Caledonian (Xingkaion) 

fold zone 

"P1lo~3t-1 *. itl~itit~~=t<~1~~Jt~11 ±: M. Caledonian fold zone, marginal seas mainly 

uncompensated 

"PtJo~ 3t-~eH*, itl~~ &, ;1ij :ff t+~:fOA 1J ±: M. Caledonian fold zone, marginal sea & troughs 

mainly compensated 

Bfe1J0~3t-:f<Ei'm. ~.fPiH1J, ss3t±JU 7t 

L. Caledonian fold zone, residual sea trough, filled 

& uplifted after S 

9 
10 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

e 8832 

e 885 

e 884 

s 398-46 

e 8849 

e B821 

e7834 

• 881 

e 8847 

e 7835 

MAIN OREBODY 

e 881 7 e 881 8 

984 

EAST OREBODY 

e 781 0 

e 8848 

e 887 981 9 • e 7829 

s 796-48 

7812 

• 

e 7811 

- - 41 °46"30' 

e 781 

e 782 

e 783 

e784 

EXPLANATION e785 

N e 786 

r 

• Sample location with less than 10 samples e 787 

o- -o Sample location with more than 10 samples 

500 METERS s Location of drill hole - 110° e 788 

Figure 5. Localities of samples taken in and near the Main and East Orebodies of Bayan Obo. 

A few samples from the West Orebodies (16 small orebodies in an area extending about 8 km west of the Main Orebody) are also not shown in figure 5. In the summers of 

1988 and 1990, the preliminary mineral paragenetic 

sequences established petrographically from doubly polished thin sections (DPfS's) were checked in the field to 

confirm consistencies with field relations. Samples collected in 1987, 1988, and 1990 are numbered with a prefix 

7B, 8B, and 9B, respectively, followed by the sample location number. For example, samples from 7B30 were collected in 1987 at sample location number 30. Samples 

7B30-1, 7B30-20, and 7B30-30 are three separate samples from sample location 7B30. Except for drill cores, 

samples without a prefix 7B, 8B, or 9B were provided by 

colleagues from Tianjin. 

In this paper, we use the following grain-size designations: coarse, >500 ~m; medium, 250-500 J..lm; fine, 125-

250 ~m; very fine, 62-125 ~m; and extremely fine, <62J..lm. 

Because of the extremely fine grain size of most of the ore 

and gangue minerals in the Bayan Obo area, interference 

microscopy was used for identification of the minerals in 

DPfS's (Chao and Xie, 1990). Routine identification of 

very and extremely fine grained minerals was accomplished 

by preliminary optical microscopy and verified by scanning 

electron microscopy with energy-dispersive X-ray analysis 

(SEM-EDX). The optical and SEM-EDX studies were iterative because SEM-EDX usually examined smaller areas 

than optical microscopy. X-ray diffraction techniques were 

rarely used because of the difficulty of obtaining pure mineral concentrates from the extremely fine grained ores. Both 

optical microscopy and SEM were used for textural analysis 

of mineral replacement relations. 

Most mineral compositions were obtained by electron 

microprobe analysis (EPMA), using carefully selected and 

calibrated standards. Other techniques were also used to 

determine compositions of individual minerals. For example, monazites and bastnaesites were analyzed by isotope 

dilution techniques (Wang and others, 1994 ), by instrumental neutron activation analysis (INAA), and by synchrotron 

X-ray fluorescence analysis (SXRF) in a few cases (Chen 
HOST ROCKS AND CAP ROCKS OF THE BA YAN OBO ORE DEPOSIT 11 

Table 1. Principal minerals of the Bayan Obo ore deposit. 

[REE's, rare earth elements] 

Mineral category Principal minerals 

Host rock minerals ferroan dolomite 

Sr- and Mn-calcite 

quartz 

illite 

biotite 

apatite 

albite 

REE ore minerals monazite, (Ce,La,Nd)P04 

bastnaesite, (Ce,La,Nd)(C03)F 

huanghoite, Ba(Ce,La,Nd)(C03)2F 

parisite, (Ce,La,NdhCa(C03) 3F2 

cebaite, Ba3CeiC03) 5F2 

Nb (± REE's) columbite, FeNb20 6 

minerals. aeschynite, (Ce,Ca,Fe,Th)(Ti,Nbh(O,OH)6 

fergusonite, YNb04 

fersmite, (Ca,Ce,Na)(Nb,Tih(O,OH,F)6 

pyrochlore, (Ca,NahNb20 6(0H,F) 

Nb-rutile 

Fe ore minerals magnetite 

hematite 

martite 

goethite 

ilmenite 

and others, 1993). Wavelength-dispersive X-ray fluorescence spectrometry (WDXRF) was used for analyzing 

unmineralized and weakly mineralized dolostone marble. 

Because of the high contents of combined F, JFe, and REE's, 

inductively coupled plasma-atomic emission spectrometry 

(ICP-AES) was used for partial analysis of most of the 

disseminated and banded ores. BaO, SrO, Ce20 3, La20 3, 

Nd20 3, and Nb20 5 contents of unmineralized and REB-mineralized H8 dolostone marble were determined by inductively coupled plasma-mass spectrometry (ICP-MS). 

Fluorine and chlorine contents of the ores were determined 

by the ion-selective electrode (ISE) method (Jackson and 

others, 1987). C02 content was determined by using coulometric titration (Jackson and others, 1987). The WDXRF 

and ICP-AES methods of analysis were described by 

Taggart and others (1987) and Lichte and others (1987), 

respectively. 

Mineral separations for radiometric-isotopic analysis 

and age determination were made in the laboratories of the 

Tianjin Geological Research Academy, in Tianjin, and in 

USGS laboratories in Reston, Va. K-Ar and 40Arf39Ar ages 

were determined in USGS laboratories in Menlo Park, 

Calif. Precise internal isochron mineral ages of monazites 

and bastnaesites were determined by the 232ThJ2°8Pb method 

in USGS laboratories in Denver, Colo. 

Table 1. Continued. 

Mineral category 

Other carbonate 

minerals. 

Principal minerals 

benstonite, (Ca,Sr) B~Mg(C0 13 norsethite, BaMg(C03h 

barytocalcite, BaCa(C03)2 

strontianite, SrC03 

Common gangue and fluorite 

other minerals. barite 

Sulfide gangue 

minerals. 

aegirine 

magnesio-arfvedsonite 

magnesio-riebeckite 

apatite 

phlogopite 

microcline 

albite 

bafertisite, Ba(Fe,Mn)2 TiSi20 7(0,0H)2 

cerianite, (Ce,Th)02 

pyrite 

pyrrhotite 

sphalerite 

galena 

chalcopyrite 

HOST ROCKS AND CAP ROCKS OF THE 

BAYAN OBO ORE DEPOSIT 

The dominant host rock replaced by the Bayan Obo 

ores is H8 marble, followed by interbedded quartzite within 

the H8 unit, and overlying H9 biotite schist and albitized 

biotite schist. The H8 unit consists chiefly of dolostone 

marble but has subordinate limestone marble; interbeds of 

quartzite and feldspathic quartzite are present throughout 

the unit but are most prevalent in the upper and lower parts. 

Sparse layers of albitized biotite schist and biotite-rich 

(minette) lenses also are present. The H8 unit consists of a 

simple suite of rock-forming minerals (table 1), of which 

deformed and stretched ferroan dolomite is dominant in 

dolostone marble, calcite in limestone marble, and quartz in 

quartzite. Detrital mineral grains that are present include 

quartz, apatite, and albite in dolostone marble, apatite being 

abundant in coarse-grained dolostone marble. Detrital albite 

grains also occur in feldspathic quartzite, and albite porphyroblasts are found in albitized biotite schist. Scattered prismatic alkali amphiboles of metamorphic origin are present 

in the dolostone marble, where they are most abundant 

along foliation planes. 

The distribution, sedimentary structures and textures, 

and recent stable oxygen and carbon isotope data all indi-
12 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Figure 6. Photomicrograph of sample 7B34, a fine-grained dolostone marble, showing banding parallel to bedding and lenses of scattered detrital quartz grains (Q). (Transmitted light, crossed nicols.) See data for sample 7B34-14 (table 3) for chemical composition of the 

fine-grained dolostone marble. 

cate a sedimentary origin for the H8 marble unit. Furthermore, the H8 unit crops out over an area 16 km long and 2-

3 km wide, an outcrop area much greater than that of any 

known igneous carbonatite. Although metamorphism has 

caused intense mylonitization and recrystallization of H8 

carbonate, alternating thick and thin beds of the original 

sedimentary unit are still evident at many places (Chao and 

others, 1992). Original sedimentary features such as ripple 

marks, although rare because of the metamorphic effects, 

have been observed locally (Meng, 1982), and relict oolitic 

textures have been reported (Meng and Drew, 1992). Detrital quartz grains are present in the thin-bedded, more finegrained marble (fig. 6), and rounded detrital apatite grains 

are present in the coarser grained marble (fig. 7). In addition, fossilized blue-green algae and fungal material have 

been found in the mine region (Meng, 1982). Meng and 

Drew (1992) reported new data on stable oxygen and carbon isotopes from six selected samples of relict micrite 

(very fine grained carbonate rock) from the H8 dolostone 

marble. One of these samples has a 8180 value of 13.9 and a 

813C value of -9.27. The other five samples have 8180 values ranging from 16.5 to 21.0 and 813C values from -2.42 to 

-4.92. These authors concluded that the relict micrite component of the dolostone samples is of sedimentary origin. 

Thin beds of fine-grained to very fine grained highpurity quartzite are present in the H8 marble unit and 

locally contain sparse grains of detrital albite feldspar. In 

thin section, quartzite shows triple-junction grain boundaries; in highly sheared parts, the grains are stretched, having a width-to-length ratio of from 1:5 to more than 1:10. 

Thin sections also show mortar texture wherein granulated 

fine-grained quartz occurs between large quartz grains. 

Wang and others (1990, 1992) and Liu and Wang 

(1990) reported the presence of small shelly fauna of Early 

Cambrian age (the Qinghezhen fauna) in the Bayan Obo 

Group both north and south of the Bayan Obo region. This 
HOST ROCKS AND CAP ROCKS OF THE BAY AN OBO ORE DEPOSIT 13 

Figure 7. Photomicrograph of sample 7B34-16, a coarse-grained dolostone marble containing rounded grains of detrital apatite (ap) and 

showing triple-junction grain boundary texture. (Transmitted light, crossed nicols.) For chemical composition, see table 3. 

finding implies that the Bayan Obo Group may not be of 

Precambrian age. However, Li and Zhang (1993) and Zhang 

( 1994) refuted this finding when they reinvestigated the 

Qinghezhen small shelly fauna and found them to be artificially produced when dilute hydrofluoric acid (HF) was 

used to extract tubular structures from the carbonate rocks. 

A Middle Proterozoic age was attributed to the H1-H9 

Bayan Obo Group on the basis of the presence of fossilized 

blue-green algae and fungal material (Meng, 1982) and on 

the basis of sequence stratigraphic analysis (Qiao and others, 1991). The significance is questionable of the Pb-Pb 

and Sm-Nd model ages of 1400-1500 Ma that were used by 

previous investigators to estimate the age of the H8 (Chao 

and others, 1992; Conrad and McKee, 1992; Wang and 

others, 1994). The minimum age of the Bayan Obo Group is 

constrained by a 40 Arf39 Ar age spectrum analysis of alkali 

amphibole, which is interpreted to reflect an age of 1.26 Ga, 

from a vein cutting the H6 quartzite (Conrad and McKee, 

1992; see also sample Drew 1 in table 15). Alkali amphibole 

from another vein cutting the H6 quartzite in the western 

part of the mine area gave a 40 Arf39 Ar plateau age of about 

820 Ma (Conrad and McKee, 1992; see also sample 7B 13C 

in table 15). A minimum age of the H8 marble also is indicated by the 40 Arf39 Ar ages of alkali amphiboles occurring 

along foliation planes in the marble, which in spite of varying degrees of argon loss indicated by age spectrum and 

total fusion analyses (fig. 33), are as old as 890 Ma (samples 

40-14 and 8B24-1, table 15). If the dolostone marble is 

similar in age to the H6 quartzite, the minimum age of H8, 

as well as the entire Bayan Obo Group, is about 1.26 Ga, or 

Middle Proterozoic. By contrast, the earliest age of REE 

mineralization is 555 Ma (Wang and others, 1994), more 

than 70 million years (m.y.) younger than the H8 unit. 

On the basis of field and laboratory textural evidence, 

including an unpublished cathodoluminescence study by 

Meng Qingrun (unpub. data, 1988), the H8 carbonate rocks 

were probably deposited as a limestone unit, which was 

dolomitized after lithification by addition of large quantities 

of Mg and Fe to form the principal mineral, ferroan dolomite. Subsequent regional metamorphism caused myloniti-
14 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Table 2. Chemical composition (wt. percent) of ferroan dolomite from H8 dolostone marble of Bayan Obo. 

[Analysts: J.M. Back and J.A. Minkin. Analysis by electron microprobe analysis (EPMA). Suffixes of sample numbers: C, coarse-grained; F, fine-grained. 

<,less than. For each sample, values represent 5-14 analyses of a total of 5-8 grains. All concentrations were below minimum detection levels (fyiDL's) for 

BaO (MDL 0.09 percent), Nd20 3 (MDL 0.16 percent), and Ce20 3 (MDL 0.24 percent). Total Fe as FeO] 

7B25-1C 7B36-10C 7B36-10F 7B34-15C 7B34-15F 

Oxide 

Range Average Range Average Range Average Range Average Range Average 

CaO 28.58-29.19 28.88 27.06--27.76 27.43 27.66--28.19 27.92 28.19-29.13 28.69 28.60-29.80 29.02 

MgO 19.24-19.70 19.41 13.78-17.03 15.42 16.64-17.54 17.12 18.51-19.54 19.01 18.15-19.01 18.73 

MnO .45-.62 .55 1.67-2.48 2.11 1.76--2.46 2.11 .65-.87 .77 .70-.89 .80 

FeO 3.77-4.06 3.90 6.17-10.47 8.34 5.60-6.26 6.03 3.89-5.03 4.45 4.35-4.69 4.52 

SrO .58-.79 .68 <.12-.38 .19 <.12-.49 .20 <.11-.35 .18 .11-.25 .18 

Total 53.42 53.49 

Table 3. Chemical compos1t1on (wt. percent) of essentially 

unmineralized H8 dolostone marble of Bayan Obo. 

[Analysts: J.E. Taggart, A. Bartel, and D. Siems. Analysis by wavelengthdispersive X-ray fluorescence spectrometry (WDXRF) for Si02, AI20 3, 

total iron as Fe20 3, MgO, CaO, Na20, K20, Ti02, P20 5, and MnO. 

Analysts: E. Brandt, C. Papp, and J. Sharkey. Analysis by coulometric titration for C02 and by ion-selective electrode (ISE) for Cl and F. 

Analyst: P.H. Briggs. Analysis by inductively coupled plasma-mass spectrometry (ICP-MS) for BaO, SrO, Ce20 3, La20 3, Nd20 3, and Nb20 5• 

*, below minimum detection level (MDL); MDL for Al20 3=0.10, 

Na20=0.15, K20=0.02, TI02=0.02, Cl=O.Ol] 

Coarse-grained Fine-grained 

Constituent 

7B25-1 7B34-16 7B34-14 

Si02 0.57 0.46 5.81 

Al20 3 * * * 

Ife203 4.56 3.97 4.41 

MgO 17.5 17.2 17.1 

CaO 31.4 32.1 28.7 

Na20 * * * K20 * .04 * Ti02 * * * 

P20s 1.52 3.23 .31 

MnO .69 .62 .73 

C02 44.2 43.7 43.4 

Cl * * * 

F .10 .23 .03 

BaO .05 .01 .02 

SrO .83 .63 .22 

Ce20 3 .02 .04 .46 

La20 3 .01 .02 .26 

Nd20 3 .01 .02 .20 

Nb20 5 .06 .21 <.01 

Total 101.52 102.48 101.66 

-O=F 0.04 0.10 0.01 

Total 101.48 102.38 101.65 

Total RE20 3 0.04 0.08 0.92 

1 Total Fe as Fe20 3• 

53.38 53.10 53.25 

zation and recrystallization of this ferroan dolomite long 

before mineralization took place. 

EPMA of ferroan dolomite from three dolostone marble samples of the H8 unit is given in table 2. Of these, sample 7B25-1C is coarse-grained dolomite marble; 7B36-10C 

is coarse-grained ferroan dolomite embedded in a finegrained ferroan dolomite "matrix" (7B36-10F); and 7B34-

15C is coarse-grained ferroan dolomite surrounded by finegrained ferroan dolomite, 7B34-15F, of the same sample. 

The MgO and FeO contents of both the coarse-grained ferroan dolomite and the fine-grained dolomite are variable, 

ranging from 13.78 to 19.54 wt. percent MgO and from 3.77 

to 10.47 wt. percent FeO. Textural relations indicate that the 

fine-grained ferroan dolomite in some of the marble was 

formed by mylonitization or granulation during regional 

metamorphism. EPMA indicates that neither the coarse- nor 

the fine-grained ferroan dolomite was chemically homogenized after recrystallization. It is notable that in a carbonate 

environment, large quantities of Fe introduced during dolomitization did not precipitate as primary oolitic Fe ore of 

sedimentary origin. 

The chemical composition of essentially unmineralized 

dolostone marble (table 3) is an essential parameter for 

evaluation of a syngenetic versus epigenetic origin of the 

disseminated REE ores of Bayan Obo. A study ofDPTS's of 

unmineralized dolostone marble (samples 7B25-1 and 

7B34-16) from the Bayan Obo mine area shows that the 

marble contains neither monazite nor bastnaesite. The two 

samples have essentially the same bulk chemical compositions, and there is variation in the concentration of minor 

components such as RE20 3 and P20 5• The total ~03 and 

P20 5 contents of sample 7B34-16 are 0.08 wt. percent (800 

ppm) and 3.23 wt. percent, respectively, whereas the con-
HOST ROCKS AND CAP ROCKS OF THE BA YAN OBO ORE DEPOSIT 15 

Table 4. Chemical composition (wt. percent) of REB-mineralized H8 dolostone marble of Bayan Obo. 

[Samples are listed from left to right in order of increasing total R.Ez03 content. 

Analysts: J.E. Taggart, A. Bartel, and D. Siems. Analysis by wavelength-dispersive X-ray fluorescence spectrometry 

(WDXRF) for Si02, AL20 3, total iron as Fe20 3, MgO, CaO, Na20, K20, Ti02, P20 5, and MnO. 

Analysts: E. Brandt, C. Papp, and J. Sharkey. Analysis by coulometric titration for C02 and by ion-selective electrode 

(ISE) for Cl and F. 

Analyst: P.H. Briggs. Analysis by inductively coupled plasma-mass spectrometry (ICP-MS) for BaO, SrO, Ce203, 

La20 3, Nd20 3, and Nb20 5. 

*, below minimum detection level (MDL); MDL for Al20 3=0.10, Na20=0.15, K20=0.02, Ti02=0.02, Cl=O.Ol] 

Constituent 7B36-10 7B34-12 7B14-2 7B34-10 7B36-12 7B36-2 

Si02 

Al20 3 

Ife203 

MgO 

CaO 

Na20 

K20 

Ti02 

P20s 

MnO 

C02 

Cl 

F 

BaO 

SrO 

Ce20 3 

La203 

Nd20 3 

Nb20 5 

Total 

-O=F 

Total 

Total RE20 3 

* 

* 

7.60 

14.7 

30.8 

* 

* 

.90 

.49 

2.18 

42.3 

* 

2.15 

.36 

.20 

.55 

.28 

.22 

.21 

102.94 

0.90 

102.04 

1.05 

10.4 

.11 

4.97 

15.6 

26.6 

* 

.07 

* 

.88 

.83 

41.5 

* 

.01 

.06 

.26 

.76 

.59 

.21 

.02 

102.87 

102.87 

1.56 

tents of these constituents in sample 7B25-1 are 0.04 wt. 

percent ( 400 ppm) and 1.52 wt. percent. These values are 

several times higher than the averages for limestones elsewhere (Bai and Yuan, 1985). Petrographically, coarsegrained dolostone marble such as that of samples 7B25-1 

and 7B34-16 contains detrital apatite (fig. 7). It is probable 

that the small amount of REE's in unmineralized dolostone 

marble is contained chiefly in the detrital apatite, which 

possibly had its source in the Archean cratonallandmass. In 

contrast, sample 7B34-14, a very weakly mineralized dolostone marble that contains extremely fine grained monazite, 

has 0.76 wt. percent (7,600 ppm) RE20 3 and 5.81 wt. percent Si02• DPTS's show the presence of detrital quartz, 

which accounts for the high concentration of Sio2, and small 

elongate aggregates of fine-grained monazite, which 

account for the RE20 3 present (table 3). 

Throughout the outcrop area (fig. 2), most of the finegrained dolostone marbles of the H8 carbonate unit show at 

least minor evidence of REE mineralization. In hand specimens of fine-grained dolostone marble, pale yellow streaks 

indicate the presence of monazite or bastnaesite. Coarsegrained dolostone marble locally also contains disseminated 

.51 1.36 .20 .20 

* * * * 

7.38 

16.2 

26.8 

9.12 

16.8 

29.1 

8.30 

15.9 

28.0 

6.87 

15.5 

27.9 

* 

.02 

* 

* 

* 

* 

* 

* 

* 

* 

1.04 

1.14 

42.5 

1.09 

1.02 

46.1 

.39 

.98 

2.23 

43.7 

.18 

1.00 

1.23 

41.2 

* 

.01 

.27 

.38 

.91 

.68 

.24 

.01 

* 

.02 

.01 

.26 

1.01 

.82 

.26 

.02 

* 

.03 

.04 

.20 

1.02 

.78 

.31 

.10 

* 

1.51 

.94 

.25 

2.11 

1.52 

.57 

.03 

98.09 106.99 

0.01 

106.98 

2.09 

102.18 

0.01 

102.17 

2.11 

101.01 

0.64 

100.37 

4.20 

98.09 

1.83 

monazite or bastnaesite, but in general, the coarse-grained 

marble is less fractured, granulated, and mineralized than 

the fine-grained marble. Table 4 lists six samples of weakly 

mineralized dolostone marble in which the mineralization 

consists of increasing amounts of total REE's ranging from 

1.05 to 4.20 wt. percent total RE20 3• Therefore, we conclude that the REE's in unmineralized Bayan Obo dolostone 

marble are present only in detrital apatite and that RE20 3 

contents greater than about 800 ppm in fine-grained dolostone marble are due to weak epigenetic mineralization. 

Thus, our findings do not support Bai and Yuan's (1985) 

conclusion that the unusually high amount of REE's in 

Bayan Obo carbonate host rocks is of sedimentary-volcanogenic syngenetic origin. 

The uppermost H9 unit of the Bayan Obo Group, 

which is the cap rock of the Bayan Obo orebodies, consists 

of dark-gray to black shale, slate, biotite schist, and albitized biotite schist. It conformably overlies the H8 marble 

unit. The least metamorphosed parts of the H9 unit are black 

shales that contain detrital quartz and illite, whereas the 

more intensely metamorphosed H9 shales now are biotite 

and albitized biotite schists, which are much more dominant 
16 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

in the mine region than the relatively unmetamorphosed 

shales. Near the Bayan Obo orebodies, chevron folds are 

prevalent in the H9 schists, where intercalated boudinage 

lenses of gray limestone are present. Identification of this 

carbonate as limestone lenses differs from interpretations by 

previous workers (Liu, 1986), who considered them to be 

carbonatite dikes. Mineralized H9 biotite schist locally contains magnetite and REE minerals (Chao and others, 1992). 

MINERALOGY OF THE 

BAYAN OBO OREBODIES 

The Bayan Obo ores are complex, as is indicated by 

the number of minerals listed, which range from 120 to 174, 

according to various reports (Bai and Yuan, 1985; Zhang 

and Tao, 1986; Institute of Geochemistry, 1988; Chen and 

others, 1990). These minerals (table 1) consist of both primary and secondary metamorphic minerals of the host rocks 

and the epigenetic ore and gangue minerals introduced by 

hydrothermal solutions. Most of the Bayan Obo minerals, 

particularly in the banded ore, are very fine to extremely 

fine grained, late-stage minerals; the minerals in veins cutting the previously formed ores are coarser grained. Typically, Bayan Obo hand specimens show a variety of colors 

that are characteristic of some of the most abundant minerals. Among these are pale-yellow monazite, yellow to 

brownish-yellow bastnaesite, pale- to dark-blue alkali 

amphibole, yellow phlogopite, pale-green to green aegirine 

and aegirine augite, colorless to lavender-purple to darkpurple fluorite, bluish-gray to gray hematite and martite, 

and dark-gray to black magnetite. 

REE, Fe, and Nb ore minerals have been identified 

in the Bayan Obo ores. The most abundant REE minerals 

are very fine grained monazite and bastnaesite and coarser 

grained huanghoite (table 1). Fine- to medium-grained 

bastnaesite also occurs in late-stage ore dominated by 

aegirine and aegirine augite, where it is associated with 

apatite and barite. Huanghoite grains generally are larger, up 

to 2-3 em in diameter, and commonly are associated with 

medium- to coarse-grained aegirine-rich ores. Cordylite 

(Ba(Ce,Nd,La)z(C03)3F2) is similar to huanghoite in composition (table 1) but differs in usually being very fine 

grained. Less common REE minerals are parisite and 

cebaite (table 1). The monazites and bastnaesites of Bayan 

Obo ores are typically very low in U (less than 5 ppm) and 

low in Th (less than 7,000 ppm). Columbite is the most 

abundant Nb mineral, and aeschynite, fergusonite, fersmite, 

and Nb-rutile are common Nb-bearing minerals. Pyrochlore 

and other Nb- and REE-bearing minerals such as chevkinite 

((Ca,Ce,Th)iFe,Mg)2(Ti,Nb,Fe)3Si40 22) are rare. The most 

important Fe minerals in the ores are magnetite, primary 

fine granular hematite, and martite (pseudomorphs of hematite after euhedral magnetite). Goethite is present in altered 

zones associated with both primary magnetite- and bematite-rich ores. Ilmenite is present locally, particularly in the 

late-stage banded ores. 

Ba-bearing and Sr-bearing carbonate minerals such as 

benstonite, norsethite, barytocalcite, strontianite, and Sr-calcite (table 1) occur as late-stage minerals replacing carbonate host rocks along grain boundaries. The Ba-bearing 

carbonate minerals appear to be unrelated to barite, which is 

one of the most common gangue minerals. 

The most widespread gangue minerals are alkali 

amphiboles (predominantly magnesio-arfvedsonite), fluorite, barite, aegirine augite, phlogopite, apatite, albite, and 

microcline. Alkali amphiboles provide information about 

the episodes of hydrothermal activity and regional metamorphism. One of the most distinctive characteristics of 

alkali amphiboles in Bayan Obo is that they appear to be 

unrelated to REE mineralization, unlike the amphiboles in 

carbonatites. Several generations of magnesio-arfvedsonite, 

and its varied chemical contents of FeO, MgO, MnO, K20, 

and Na20, can be distinguished on the basis of textural evidence and 40 Arf39 Ar ages. Magnesio-riebeckite, also 

present, is more common in veins than magnesio-arfvedsonite and occurs as overgrowths on prismatic magnesioarfvedsonite cores, such as shown in sample 7B34-1. 

Fluorite, on the basis of textural relations, usually 

replaces REE ore minerals such as monazite and bastnaesite 

and Fe minerals such as martite and granular hematite. Fluorite and barite commonly replace aegirine in the banded 

assemblages. Late-stage fluorite and barite also occur as 

veinlets cutting banded ores. 

Aegirine augite replaces host rock dolostone and limestone marble, but more commonly, it occurs together with 

fluorite in banded ores. Although textural evidence indicates there may be more than one generation of aegirine and 

aegirine augite, the chemical composition of aegirine augite 

appears to be fairly uniform (see representative composition 

in table 5). Because aegirine augite and alkali amphibole 

usually do not occur in the same mineral assemblage, the 

time and space of formation of these two minerals were 

generally unrelated. 

Phlogopite, commonly associated with the various 

generations of alkali amphiboles, replaces host dolostone or 

limestone marble. Phlogopite and alkali amphiboles partially replaced the host marble as well as the fine-grained 

monazite and bastnaesite that had previously partially 

replaced this marble, and were subsequently partially 

replaced by late-stage unmartitized magnetite. 

Medium-grained detrital apatite occurs in coarsegrained H8 dolostone marble. Fine-grained to very fine 

grained granular apatite and porphyroblastic apatite of 

hydrothermal origin generally are associated with small 

amounts of monazite and granular hematite in banded lowgrade REE ores. The detrital apatite can be distinguished 

from very fine grained to fine-grained hydrothermal apatite 

by distribution, origin, and REE content. EPMA and SXRF 

analyses by Chen and others (1993) indicate that the light 
CLASSIFICATION AND DISTRIBUTION OF ORE TYPES 17 

Table 5. Chemical composition (wt. percent) of huanghoite, apatite, and aegirine augite of Bayan Obo. 

[Analysts: J.M. Back and J.A. Minkin. Analysis by electron microprobe 

analysis (EPMA). -, not analyzed. 

* below minimum detection level (MDL); MDL for Pr20 3=0.09, 

Yz03=0.07, P20 5=0.04, Ti02=0.04, Mn0=0.04, Mg0=0.06, and 

K20=0.0l] 

Constituent 

La20 3 

BaO 

Nd20 3 

Cez03 

Pr20 3 

Y z03 

CaO 

P205 

F 

SiO, 

TiO~ 

Al203 1

Fe20 3 

MnO 

MgO 

CaO 

Na20 

K20 

Total 

-O=F 

Total 

Huanghoite 

7840-3 

7.98 

37.05 

9.12 

23.14 

2.70 

.11 

.05 

* 

4.00 

84.15 

1.69 

82.46 

1 Total Fe as Fez03• 

Apatite 

7825- 1 

0.15 

.19 

.35 

* 

* 

53.08 

41.35 

4.87 

99.99 

2.05 

97.94 

Apatite 

7831-9 

0.52 

1.10 

1.46 

.25 

1.05 

49.79 

39.56 

93.73 

93.73 

Aegirine 

augite 

8813-12 

52.56 

* 

.45 

33.67 

* 

* 

.15 

13.51 

* 

100.34 

100.34 

rare earth element (LREE) concentrations in detrital apatite 

in coarse-grained dolostone marble sample 7B25-1 are 3-6 

times lower than those in hydrothermal apatite in banded 

ore sample 7B31-9 (table 5). 

Albite is widespread, especially as a replacement mineral in the H9 biotite schist, where albite usually preceded 

and was replaced by late-stage unmartitized magnetite. 

Albite also occurs in veinlets in some banded ores and in 

late-stage albite-hornblende veins cutting H9 pelitic host 

rocks. Late-stage K-feldspar, usually microcline, occurs as 

an isolated accessory gangue mineral in some of the banded 

ores and is a common metasomatic mineral replacing H9 

biotite schist (Drew and others, 1990). 

Bafertisite and cerianite are accessory minerals at 

Bayan Obo (table 1). Bafertisite typically occurs in aegirine 

magnetite ores as a prismatic, bright-orange, strongly pleochroic accessory mineral. Cerianite, an alteration product 

(Zhang and Tao, 1986), is bright red in hand specimen, is 

red and isotropic in DPTS, and is the only Bayan Obo mineral radioactive enough to require shielding and caution in 

handling of samples that contain it. 

Bayan Obo ores contain only sparse sulfide minerals, 

principally pyrite, pyrrhotite, sphalerite, galena, and chalcopyrite. Pyrite occurs as scattered disseminated grains in 

the H8 carbonate host, commonly associated with disseminated barite and trace amounts of fluorite. Pyrite also occurs 

as fine stringers and veinlets crosscutting banded ores. The 

other sulfides are late-stage minerals that occur in some disseminated ores and in small amounts in banded ores. 

CLASSIFICATION AND DISTRIBUTION 

OF ORE TYPES 

There are three broad categories of ore types: disseminated, banded, and massive. The classification of Bayan 

Obo ores used in this report is based on the classification 

developed in the early 1960's, which was published by the 

Institute of Geochemistry (1988) and is, generally, the classification currently used by mine geologists. The classification is based on three principal parameters: (1) field 

relations and the type of host rock replaced by the ore and 

gangue minerals, such as H8 dolostone or limestone marble, 

quartzite, and H9 biotite schist and albitized biotite schist; 

(2) textures and structures, whether the ore minerals are disseminated, banded, or massive; and (3) mineral assemblages; presence or absence of REE minerals, in particular, 

monazite and bastnaesite; and types of Fe-oxide minerals 

and dominant gangue minerals. The following accessory 

minerals, present only in minor amounts, are not included in 

the classification scheme: barite and Ba-bearing REE minerals such as huanghoite, cordylite, and cebaite; scattered 

apatite; ilmenite; bafertisite; microcline; pyrite and other 

sulfides, such as chalcopyrite, sphalerite, and pyrrhotite; 

Nb-bearing minerals such as columbite, Nb-bearing ilmenorutile, fersmite, aeschynite, and fergusonite; and other rare 

minerals such as barytocalcite, benstonite, and strontianite. 

Where relicts of host rock are not observed, the ore classification is based only on textural-structural criteria and mineral assemblages. 

The distribution of ore in the Main and the East Orebodies is shown in figure 8, the outlines of the orebodies 

being based on average Fe contents of at least 20 wt. percent. The disseminated dolomite-hosted REE ores, from 

less than 2 to more than 6 wt. percent total REz03, are distributed generally at or below the footwalls of the Main and 

East Orebodies and above the hanging wall of the East Orebody (fig. 8). Massive Fe ores, made up of various Fe-oxide 

minerals, occur in the central parts of both ore bodies and are 

surrounded by banded ores, which are the dominant type of 

ore in the orebodies. This broad zoning was recognized by 

mine geologists as early as the 1960's. 

The following classification scheme, which is used in 

this report, includes some ore types that are not mappable 

units and, therefore, are not delineated in figure 8. 
18 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

H9 

Main Orebody 

Lithologic Units 

Black shale slate, biotite schist, 

and albitized biotite schist 

Dolostone and limestone marble 

with lenses of quartzite 

EXPlANATION 

Disseminated, Banded, and Massive Ores 

Pf\TJ9 

~ 

Dolostone marble-hosted REE-martite + 

magnetite disseminated ore 

~ Dolostone marble-hosted low-grade Fe ore 

[;-J:A::-J REE-hematite + martite-fluorite banded ore 

~ Apatite-hematite-REE banded ore 

~ Granular hematite-martite massive ore 

H9 

0 500 m I 

- 110° 

Undifferentiated hematite-martite-magnetite 

massive ore 

Aegirine augite-magnetite banded ore 

Biotite schist-hosted biotite-magnetite ore 

Quartzite-hosted ore 

Limestone marble-hosted alkali amphibolemagnetite ore 

Dolerite dikes 

Boundary of 20 wt. percent Fe 

Approximate boundary between ore types 

Figure 8. Geologic sketch map of the Main and the East Orebodies of Bayan Obo (modified from Institute of Geochemistry, 1988). 
CLASSIFICATION AND DISTRIBUTION OF ORE TYPES 19 

A. Ores hosted by H8 dolostone and limestone marble: 

1. Disseminated ores: 

a. Marble with disseminated monazite only (sample 7B36-6; sample locations are shown in fig. 

5), with bastnaesite only (samples 34-24, not 

shown in fig. 5, and 8B7), or with monazite and 

small amounts of bastnaesite (sample 796-

48,278). Locally, fluorite is an abundant gangue 

mineral. 

b. Marble with disseminated monazite and magnetite or martitized magnetite (samples B494-

89-98, not shown in fig. 5, and 7B36-1B), with 

or without fluorite. 

c. Marble with disseminated granular hematite, 

with or without small amounts of magnetite 

(samples 7B36-12 and 9B4), and with or without gangue minerals such as fluorite and barite. 

d. Marble with disseminated monazite or bastnaesite and magnetite, and significant amounts 

of extremely fine grained granular aegirine augite, with or without fluorite (sample 796-48, 

459). 

e. H8 marble with disseminated monazite, bastnaesite, magnetite, and the gangue minerals 

phlogopite and alkali amphibole (mainly magnesio-arfvedsonite) (samples 796-48,338 and 

796-48,277). 

f. H8 limestone marble with disseminated monazite closely associated with significant amounts 

of extremely fine grained granular apatite, with 

or without bastnaesite, magnetite, and granular 

hematite (sample 398-46,305). 

2. Banded ores: 

a. Monazite (±bastnaesite)-granular hematite-fluorite banded ore (type FH, figs. 8 and 9). This 

type of banded ore is widespread and consists 

mainly of varying proportions of alternating 

submillimeter- to millimeter-thick discontinuous laminae and bands consisting principally of 

extremely fine grained monazite, extremely fine 

grained granular hematite, and fluorite. There 

are many varieties of monazite-hematite-fluorite banded ores. 

b. Aegirine augite-magnetite banded ores, with or 

without varying amounts of monazite or bastnaesite (type AeM, figs. 8 and 10). There are 

many varieties of this type of banded ore. 

Where aegirine augite is the dominant gangue 

mineral, apatite is common, bastnaesite is usually the dominant REE mineral, and martite or 

magnetite is the dominant Fe ore mineral. 

c. Mixture of types a and b, consisting of monazite, granular hematite, and magnetite, and both 

aegirine and fluorite as associated gangue minerals (fig. 11 ). Banded ore that contains both 

fluorite and aegirine gangue minerals is the 

most abundant and widespread type of banded 

ore at the Main and East Orebodies of Bayan 

Obo. Because this ore type does not occur as a 

mappable unit, it is incorporated in banded ore 

types 2a (FH) and 2b (AeM) shown in figure 8. 

d. Monazite-phlogopite-alkali amphibole-magnetite banded ore (type AmpM, fig. 8), with or 

without fluorite and aegirine augite. This type 

of banded ore is closely related to the disseminated ore that has a similar mineral assemblage 

(type 1e). 

e. Granular apatite-granular hematite-monazite 

banded ore (type Ap, figs. 8 and 12) that usually 

contains fluorite. This ore type, characterized 

by the presence of granular apatite, is closely 

related to disseminated ore that has a similar 

mineral assemblage (type lt). 

3. Massive Fe ores: 

Massive Fe ores consist principally of at least one 

of the Fe-oxide minerals granular hematite, martite, 

and magnetite (fig. 13). They represent more 

intense Fe mineralization than either disseminated 

or banded ores. Massive Fe ores occur in H8 carbonate and quartzite hosts and H9 schist; they also 

replace banded ores of known or unknown host 

rocks (samples 7B32-1 and 7B40-2). 

B. Ores hosted by H8 quartzite: 

The interbedded quartzite in the H8 carbonate unit 

makes up a very minor part of the H8 and is mineralized only in the Main and East Orebodies, particularly 

along the footwall of the East Orebody (fig. 8). In spite 

of the resistance of quartz to chemical alteration, the 

types of mineralization of quartzite-hosted ores appear 

similar to those hosted by the H8 carbonates. 

1. Disseminated ores: 

The most common quartzite-hosted disseminated 

ore contains extremely fine grained aegirine augite, 

bastnaesite, and barite, with or without varying 

amounts of magnetite (sample 9B 18-3c). Some of 

these ores also contain various amounts of fluorite. 

2. Banded ores: 

a. Bastnaesite-granular hematite-fluorite banded 

ore with or without minor granular aegirine 

(samples 7B30-39, 7B30-40, 7B30-41A, and 

7B30-41B). This type of banded ore occurs at 

the footwall in the northeastern part of the East 

Orebody (fig. 8). 

b. Aegirine-magnetite banded ore, with or without 

bastnaesite and monazite (sample 7B30-43). 

c. Aegirine-fluorite-magnetite ore, with a variety 

of Ba-bearing REE fluor-carbonate minerals, 

such as huanghoite and cebaite; barite is common (samples 7B30-47, 9B2-5-1, 9B13-2, 

398-46, 142). 
20 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

m 

Figure 9. Hand specimen (sample 7B30-1) of typical microfolded banded ore consisting of irregular to lenticular layers of monazite (m), 

granular hematite (he), and fluorite (fl). Scale is in centimeters. 
CLASSIFICATION AND DISTRIBUTION OF ORE TYPES 21 

Figure 10. Large block of banded ore consisting of layers of aegirine augite (aeg) and magnetite (mt). Sample 8B 13 excavated from 

bench level 1558 m, at the east end of the East Orebody. 
22 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 
CLASSIFICATION AND DISTRIBUTION OF ORE TYPES 

f-- Figure 11. Block (sample 9B3i) showing both fluorite (fl) 

and aegirine augite (aeg) as major gangue minerals. 

Figure 13. Outcrop on bench 1582 m of the East Orebody, showing bodies of massive magnetite (mt) and hematite (he) ores. 

f-- Figure 12. Block (sample 9B 13) of banded ore consisting of 

granular apatite (ap), granular hematite (he), and monazite (m). 

23 
24 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

d. Granular apatite-granular hematite banded ore, 

with varying amounts of monazite (samples 

9B12a, 8B35-8). 

3. Massive Fe ores: 

Quartzite-hosted massive martite ores with granular 

hematite contain relict quartz (samples 7B39-4, 

398-46,335). Quartzite-hosted massive granular 

hematite ore is also present, but apparently is not a 

major ore type. 

C. Ores hosted by H9 biotite schist and albitized biotite 

schist: 

The ores hosted in biotite schist occur only in the hanging wall of the Main Orebody (fig. 8). The most common type of ore in biotite schist is aegirine augitebiotite-magnetite disseminated ore (sample 9B5-2). 

Locally, small amounts of REE minerals such as 

cebaite occur in low-grade, disseminated magnetite 

ores dominated by aegirine augite. 

CHEMICAL COMPOSITIONS OF ORES 

Selection of ore types for chemical analysis was based 

on mineral assemblages. Two different billets of each 

banded ore sample to be analyzed, each approximately 2-3 

em wide, 4-6 em long, and 0.5 mm thick, were cut across 

the banded structure. A DPTS was made from one billet, 

and the other billet was submitted for chemical analysis 

(tables 6-12). Two DPTS's (parts A and B) were needed 

for large samples, which extended for a distance of more 

than 6 em across the bands. Sample locations are shown in 

figure 5. 

Because of the high contents of combined F, Fe, and 

REE's in many of the ore types, only partial analyses of ore 

samples could be obtained by the ICP-AES method, which 

did not analyze for Si02, Na20, C02, or in most cases, P20 5; 

therefore, low analytical totals are reported in tables 6-12. 

Total Fe is reported as Fe20 3 (tables 6-12), because ferric 

Fe is the dominant form of Fe in most of the minerals 

present. Each sample of ore from a quartzite host has a low 

total of constituents because Si02 content was not determined. Similarly, for a sample containing aegirine augite, 

the total is low because neither Si02 nor Na20 was determined. Because of the heterogeneity and erratic variation of 

mineral assemblages in bands in banded ores, the chemical 

composition of an ore is not expected to correlate with the 

modal mineral composition of the corresponding DPfS. 

Nevertheless, the general mineral assemblage of the DPfS 

does reflect the chemical composition qualitatively. 

Partial analyses of selected banded ores from Bayan 

Obo (tables 6-12) are divided on the basis of mineral 

assemblages from DPTS studies and concentrations of 

REE's and Fe and are discussed as follows: 

1. Dolostone marble having relatively high REE content, 

and monazite-hematite-apatite banded ores (table 6). 

Partial analyses of the chemical compositions of two 

fractions of a high-grade REB-disseminated ore sample, 

7B30-33 (7B30-33A and 7B30-33B, table 6), each 

having significant amounts of relict host carbonate, 

show contents of RE20 3 of 19.56 and 13.23 wt. percent, 

respectively. Sample 7B30-33A contains more REE's 

(monazite) and Fe (granular hematite) and has less fluorite than 7B30-33B. Partial chemical analyses of monazite-hematite-apatite banded ores are also shown in table 

6. The bands of granular apatite visible in the corresponding DPfS's are not reflected in the analyses 

because P20 5 concentrations were not determined. Sample 7B30-27 contains more Fe (total Fe20 3 is 18.6 wt. 

percent, occurring as granular hematite) and REE's 

(total RE20 3 is 8.28 wt. percent) than sample 7B37-3. 

The CaO present in both samples is due to apatite and 

fluorite. The host rock, probably dolostone marble, has 

been completely replaced by gangue and ore minerals. 

2. Monazite (±bastnaesite)-fluorite (low-Fe, high-REE) 

banded ores (table 7). 

The high content of fluorite in the fluorite-type banded 

ores is indicated by the high F and CaO contents in the 

analyses (table 7). ~03 contents of the seven samples 

analyzed range from 12.29 to 25.06 wt. percent. Samples 7B31-1A and 7B31-1B from successive bands of 

the same hand specimen are similar in composition, but 

the total RE20 3 content of 7B31-1B (25.06 wt. percent) 

is appreciably higher than that of7B31-1A (18.27 wt. 

percent). The presence of small amounts of MgO in 

these samples is probably due to the presence of phlogopite observed in the hand specimen, whereas Fe comes 

from fine granular aegirine augite and magnetite. 

3. Bastnaesite-apatite-aegirine augite-fluorite±martite 

banded ores (table 8). 

These ores are characterized by the presence of finely 

granular aegirine augite, in dominant bands, and variable amounts of fluorite. The principal REE mineral is 

very fine grained to medium-grained bastnaesite. Chemically, aegirine augite and local, sparse martite are 

reflected by the total Fe present (Fe20 3 ranges from 6.86 

to 20.0 wt. percent), whereas fluorite is reflected by the 

Ca and F contents (CaO ranges from 2.94 to 37.78 wt. 

percent and F ranges from 1.02 to 16.0 wt. percent). The 

total RE20 3 content ranges from 3.05 to 17.45 wt. percent (table 8). 

4. High-Fe, REE, and fluorite banded ores (table 9). 

Samples 7B30-28A and 7B30-28B are from the same 

hand specimen, which consists of bands containing mixtures of magnetite, monazite, bastnaesite, apatite, and 

fluorite. Magnetite accounts for the total Fe present 

(24.31-37.17 wt. percent Fe20 3) in this sample, and 

monazite and bastnaesite account for the total ~03 
CHEMICAL COMPOSITIONS OF ORES 

Table 6. Partial chemical composition (wt. percent) of disseminated monazite ore in dolostone marble and of monazite-hematiteapatite banded ores of Bayan Obo. 

[Analyst: P.H. Briggs. Analysis by inductively coupled plasma-atomic 

emission spectrometry (ICP-AES) for all oxides reported; Si02, Na20, 

P20 5, and C02 were not determined. 

Analysts: J. Sharkey and E. Brandt. Analysis by ion-selective electrode 

(ISE) for F. 

*, below minimum detection level (MDL); MDL for Al20 3=0.04, 

K20=0.24, and Ti02=0.03] 

Disseminated monazite ore Monazite-hematite-apatite 

Constituent in dolostone marble banded ores 

7B30-33A 7B30-33B 7B30-27 7B37-3 

Al20 3 0.13 0.06 * 0.32 

Ifez03 15.7 9.86 18.6 6.58 

MgO 5.47 5.80 .03 .05 

CaO 16.79 29.38 22.39 33.58 

K20 * * * * 

Ti02 * * .15 .05 

MnO 2.58 1.68 .02 .04 

F 2.18 7.98 4.38 8.98 

BaO .05 .05 .02 1.45 

Ce20 3 9.72 6.68 4.22 2.22 

La20 3 7.04 4.10 2.46 1.04 

Nd20 3 2.80 2.45 1.63 1.40 

NbzOs .02 .01 .05 .01 

SrO .08 .10 .10 .09 

Total 62.56 68.15 54.05 55.81 

-O=F 0.92 3.36 1.84 3.78 

Total 61.64 64.79 52.21 52.03 

Total RE20 3 19.56 13.23 8.31 4.66 

1 Total Fe as F~03. 

Table 7. Partial chemical composition (wt. percent) of selected monazite(± bastnaesite)-fluorite (low-Fe, 

high-REE) banded ores of Bayan Obo. 

[Analyst: P.H. Briggs. Analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES) for all oxides 

reported; Si02, Na20, P20 5, and C02 were not determined. 

Analysts: J. Sharkey and E. Brandt. Analysis by ion-selective electrode (ISE) for F. 

*,below minimum detection level (MDL); MDL for K20=0.24, Mg0=0.03] 

Constituent 7B30-1 7B30-11 7B30-20 7B30-32 7B31-1A 7B31-1B 7B38-3 

Al20 3 0.13 0.08 0.06 0.36 0.66 0.40 .19 

Ifez03 9.29 11.72 7.72 6.15 7.58 6.01 14.3 

MgO .23 .08 * .55 1.11 .68 .13 

CaO 4.57 37.78 27.98 25.18 3.78 29.38 39.18 

K20 * * * * .36 * * 

Ti02 .60 1.17 1.00 4.17 .18 .12 .08 

MnO .03 .02 .01 .23 .08 .06 .01 

F 28.0 17.6 17.0 12.8 16.8 1.8 17.2 

BaO .30 .09 .04 7.70 .11 .04 3.57 

Ce20 3 9.49 10.42 10.66 10.54 9.37 12.88 6.32 

La20 3 5.04 5.28 4.69 6.33 5.98 8.33 4.10 

Nd20 3 2.80 3.50 3.97 3.73 2.92 3.85 1.87 

Nb20 5 .30 .47 .36 .33 .11 .11 .02 

SrO .09 .06 .05 .18 .06 .07 .10 

Total 96.87 88.27 73.54 78.25 76.10 72.73 87.07 

-O=F 11.79 7.41 7.16 5.39 7.07 4.55 7.24 

Total 85.08 80.86 66.38 72.86 69.03 68.18 79.83 

Total RE20 3 17.33 19.20 19.32 20.60 18.27 25.06 12.29 

1 Total Fe as F~0 . 

25 
26 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Table 8. Partial chemical composition (wt. percent) of bastnaesite-apatite-aegirine augite-fluorite±martite banded ores 

with apatite±barite of Bayan Obo. 

[Analysts: J.E. Taggart, A. Bartel, and D. Siems. Analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES) for 

all oxides reported; Si02, Na20, Pp5, and C02 were not determined. 

Analysts: J. Sharkey and E. Brandt. Analysis by ion-selective electrode (ISE) for F. 

*, below minimum detection level (MDL); MDL for K20=0.24] 

Constituent 7B30-3 7B30-16A 7B30-16B 7B30-25 7B30-26 7B30-44 7B30-45A 7B30-45B 

Al20 3 0.26 0.13 0.11 0.17 0.53 0.26 0.25 1.57 1

Fe20 3 15.73 7.43 10.29 8.72 17.16 20.0 13.87 6.86 

MgO .08 .08 .03 .07 .17 .08 .07 .13 

CaO 10.07 37.78 32.18 27.98 2.94 18.19 27.98 33.58 

K20 * * * * * * * .96 

Ti02 .20 .10 .05 .17 .70 .12 .15 .13 

MnO .04 .04 .02 .02 .03 .03 .03 .01 

F 1.68 16.0 13.6 16.0 1.02 10.20 14.4 15.0 

BaO 3.24 3.01 3.13 4.35 5.69 2.01 3.80 6.48 

Ce20 3 8.90 4.45 6.44 4.45 6.44 1.64 5.04 3.28 

La203 5.98 2.35 4.93 1.99 3.40 .82 2.23 1.41 

Nd20 3 2.57 1.28 1.63 1.40 1.75 .59 1.40 .98 

Nb205 .29 .02 .02 .06 .54 .01 .02 .07 

SrO .28 .13 .13 .18 .17 .11 .10 .17 

Total 49.32 72.80 72.56 65.56 40.54 54.06 69.34 70.63 

-D=F 0.71 6.74 5.73 6.74 0.93 4.29 6.06 6.32 

Total 48.61 66.06 66.83 58.82 40.11 49.77 63.28 64.31 

Total ~03 17.45 8.08 13.00 7.84 11.59 3.05 8.67 5.67 

1 Total Fe as F~0 • 

Table 9. Partial chemical composition (wt. percent) of high-Fe, Table 10. Partial chemical composition (wt. percent) of highREE, and fluorite banded ores of Bayan Obo. REE, low-Fe, and low-F banded ores of Bayan Obo. 

[Analyst: P.H. Briggs. Analysis by inductively coupled plasma-atomic [Analyst: P.H. Briggs. Analysis by inductively coupled plasma-atomic 

emission spectrometry (ICP-AES) for all oxides reported; Si02, Na20, emission spectrometry (ICP-AES) for all oxides reported; Si02, Na20, 

P20 5, and C02 were not determined. P20 5, and C02 were not determined. 

Analysts: J. Sharkey and E. Brandt Analysis by ion-selective electrode Analysts: J. Sharkey and E. Brandt. Analysis by ion-selective electrode 

(ISE) for F. (ISE) for F. 

*,below minimum detection level (MDL); MDL for K20=0.24] *,below minimum detection level (MDL); MDL for K20=0.24; >,greater 

Constituent 7B30-28A 7B30-28B 7B30-37 7B31-14 

than] 

Constituent 7B31-10 7B31-19 7B31-23 7B37-l 

Al20 3 0.25 0.17 0.13 0.09 1

Fe20 3 24.31 37.17 45.75 35.7 Al20 3 0.26 0.30 0.24 0.36 

MgO .25 .25 .07 .10 1

Fe20 3 5.00 5.71 2.14 10.01 

CaO 8.40 3.92 1.32 7.70 MgO .12 .45 .26 .36 

K20 * * * * CaO 8.67 8.12 11.61 12.73 

Ti02 .47 .23 .07 .75 K20 * * * * MnO .04 .02 .02 .04 Ti02 .08 .15 .07 .12 

F 3.98 1.68 .54 4.38 MnO .01 .03 .02 .02 

BaO 4.24 2.79 3.91 .39 F 4.49 2.41 3.28 6.38 

Ce20 3 11.24 5.50 1.17 9.14 BaO 2.46 .54 .46 .36 

La20 3 8.09 3.87 .46 6.33 Ce20 3 21.08 >23.40 >23.40 18.74 

Nd20 3 3.03 1.52 .40 2.33 La20 3 15.25 16.42 15.25 12.90 

Nb205 .14 .05 .01 .18 Nd20 3 6.42 8.28 8.05 4.78 

SrO .15 .07 .03 .03 Nb20 5 .53 .14 .01 .13 

Total 64.59 57.24 53.88 67.16 SrO .09 .07 .08 .05 

-D=F 1.68 0.71 0.23 1.84 Total 64.46 >66.02 >64.87 66.94 

Total 62.91 56.53 53.65 65.32 -D=F 1.89 1.01 1.38 2.69 

Total RE20 3 22.36 10.89 2.03 17.80 Total 62.57 >65.01 >63.49 64.25 

Total R~03 42.75 >48.10 >46.40 36.42 

1 Total Fe as Fe20 3. 

1 Total Fe as Fe20 3. 
CHEMICAL COMPOSITIONS OF ORES 27 

Table 11. Partial chemical composition (wt. percent) of marblehosted alkali amphibole-magnetite banded ores of Bayan Obo. 

[Analyst: P.H. Briggs. Analysis by inductively coupled plasma-atomic 

emission spectrometry (ICP-AES) for all oxides reported; Si02, Na20, 

P20 5, and C02 were not determined. 

Analysts: J. Sharkey and E. Brandt. Analysis by ion-selective electrode 

(ISE) for F. 

*, below minimum detection level (MDL); MDL for Ti02=0.03] 

Constituent 7B33-5 7B33-6A 7B33-6B 7B33-7 

Al20 3 0.17 0.28 0.36 0.25 

Ife203 17.16 17.16 15.73 24.31 

MgO 4.64 4.97 5.31 4.81 

CaO 20.99 19.59 19.59 19.59 

K20 .48 .60 .72 .48 

Ti02 .03 * .03 .03 

MnO 3.49 4.26 5.42 3.87 

F 9.58 9.58 7.98 7.18 

BaO .02 .04 .04 .03 

Ce20 3 2.22 1.76 2.46 2.11 

La20 3 .96 .83 1.17 .88 

Nd20 3 .97 .76 .98 .86 

Nb20 5 .01 .01 .01 .01 

SrO .11 .13 .11 .13 

Total 60.83 59.97 59.91 64.54 

-O=F 4.03 4.03 3.36 3.02 

Total 56.80 55.94 56.55 61.52 

Total RE20 3 4.15 3.35 4.61 3.85 

1 Total Fe as Fe20 3• 

Table 12. Partial chemical composition (wt. percent) of quartzite-hosted ores of Bayan Obo. 

[Analyst: P.H. Briggs. Analysis by inductively coupled plasma-atomic emission spectrometry (ICP-AES) for all oxides reported; Si02, Na20, P20 5, and 

C02 were not determined. 

Analysts: J. Sharkey and E. Brandt. Analysis by ion-selective electrode (ISE) for F. 

*,below minimum detection level (MDL); MDL for K20=0.24, Mg0=0.03, MnO=O.Ol, and Ti02=0.03] 

Constituent 7B30-38 7B30-39 7B30-41A 7B30-41B 7830-43 7B30-52A 7B30-52B 7B30-53A 7B30-53B 

Al20 3 0.06 0.04 0.08 0.11 0.60 0.21 1.57 0.43 0.42 

Ife203 4.15 9.15 12.2 20.02 13.73 6.15 7.86 3.29 7.01 

MgO * * * .07 .35 .08 .12 .10 .12 

CaO 26.58 22.38 4.34 4.76 4.20 23.79 25.19 32.18 34.98 

K20 * * * * .36 * 1.44 * * 

Ti02 .28 * .07 .10 .40 .23 .38 .15 .13 

MnO .02 * .09 .37 .04 .02 .03 .01 .01 

F 14.4 1.4 1.98 1.78 1.58 13.6 13.6 19.0 21.0 

BaO 3.80 1.23 5.36 2.79 2.34 4.35 2.01 6.03 6.81 

Ce20 3 4.80 1.87 .87 .94 1.99 5.86 3.98 7.96 5.39 

La20 3 2.81 1.04 .35 .77 .55 2.58 2.23 3.99 2.81 

Nd20 3 1.28 .44 .49 .30 .92 1.63 1.11 2.22 1.52 

Nb20s .02 .01 .02 .02 .04 .02 .02 .02 .01 

SrO .06 .04 .04 .04 .11 .22 .17 .15 .14 

Total 58.26 46.60 25.89 32.07 27.21 58.74 59.71 75.53 80.35 

-O=F 6.06 4.38 0.83 0.75 0.66 5.73 5.73 8.00 8.84 

Total 52.20 42.22 25.06 31.32 26.55 53.01 53.98 67.53 71.51 

Total RE20 3 7.89 3.35 1.71 2.01 3.46 10.07 7.32 14.17 9.72 

1 Total Fe as Fe20 3• 
28 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

present (10.89-22.36 wt. percent). The differences in Fe 

and REE content between 7B30-28A and 7B30-28B 

reflect differences in the abundance of Fe versus REE 

bands in the samples analyzed. Sample 7B30-37 is a 

dolostone marble replaced by granular hematite and 

trace amounts of monazite. Sample 7B31-14 is a banded 

Fe-REE ore sample containing bands of granular hematite, REE minerals (monazite and bastnaesite), and fluorite. It has 35.7 wt. percent Fe20 3 and 17.80 wt. percent 

total ~0 • 

5. High-REE and low-Fe and low-F banded ores (table 10). 

These banded ores consist of prominent bands containing abundant monazite and bastnaesite. Fluorite and 

other gangue minerals, such as aegirine in 7B31-1 0 and 

apatite in 7B31-23 and 7B37-1, may be present. The 

~03 contents of the four samples listed in table 10 

range from 36.42 to greater than 48.10 wt. percent. 

Although samples having such high REE content are not 

rare at Bayan Obo, most of the REE ores contain less 

than 20.0 wt. percent total R~03 (tables 6-9). 

6. Marble-hosted alkali amphibole-magnetite banded ores 

(table 11). 

This ore type occurs near or at the hanging wall of the 

East Orebody (figs. 5 and 8). The ores have rather low 

(3.35-4.61 wt. percent) total ~03 contents and are 

characterized by a rather high MnO content (3.49-5.42 

wt. percent) and high Fe content due to the presence of 

late-stage magnetite. Fluorite is generally present, as 

indicated by the 7.18-9.58 wt. percent Fin the samples. 

7. Quartzite-hosted ores (table 12). 

The samples in table 12 are from a mineralized bed of 

quartzite along the footwall of the northeastern part of 

the East Orebody (figs. 5 and 8). The total values in 

table 12 are low because Si02 contents of the host 

quartzite and silicate gangue minerals were not determined by the ICP-AES method of analysis. Aegirine 

augite and fluorite are generally present in varying 

amounts; alkali amphibole is typically absent from ores 

where aegirine augite is an essential component. The 

dominant REE mineral is bastnaesite, but lesser amounts 

of cebaite also are present. 

8. Massive Fe ores consisting of granular hematite and 

martite or of magnetite or of mixtures of Fe-oxide 

minerals. 

Although these three ores were not analyzed, according 

to mine geologists, massive Fe ores may contain as 

much as 55 wt. percent Fe. Most massive Fe ores contain less than 3 wt. percent R~0 • 

TEXTURAL EVIDENCE FOR THE 

EPIGENETIC, HYDROTHERMAL, 

METASOMATIC ORIGIN OF THE 

DAYAN OBO ORE DEPOSIT 

Multiple lines of evidence are needed in order to determine the origin and the mineral paragenetic sequence of the 

Bayan Obo ore deposit (Chao and others, 1993). Replacement textures observed in the field and petrographically 

from DPTS's are the most important criteria, and are aided 

by corroborative criteria such as chemical compositions and 

radiometric isotopic ages of various generations of ore and 

gangue minerals. An absolute mineral paragenetic sequence 

can be partially established on the basis of absolute ages of 

episodes of REE mineralization and the unique chemical 

compositions and isotopic ratios of the different generations 

of REE minerals. Without the age and compositional data of 

key ore and gangue minerals, only the relative mineral 

paragenetic sequence of complex mineral assemblages can 

be inferred. For this reason, episodes of REE mineralization 

as a whole are well established, whereas episodes of Fe and 

Nb mineralization are not. On the basis of widespread metasomatic replacement textures, and chemical compositions 

and radiometric isotopic ages of ore and gangue minerals, 

the generalized mineral paragenetic sequence of Bayan Obo 

was determined (table 13). 

Only a few examples of textural evidence of metasomatic replacement at Bayan Obo are illustrated here. These 

are typical or representative features of replacement of host 

rock (marble, quartzite, and biotite schist) by either an ore 

or a gangue mineral, as well as later replacement of an earlier replacement mineral. 

There is widespread evidence of host dolostone marble 

replacement by various ore minerals, such as monazite, 

bastnaesite, martitized magnetite, fine granular hematite, 

and fresh magnetite. In the field (fig. 14), a block of lightcolored H8 dolostone marble was partly replaced by disseminated gray magnetite. Typically, the Proterozoic host 

H8 dolostone marble was granulated and recrystallized by 

regional metamorphism before being replaced interstitially by extremely fine grained monazite of Caledonian age 

(fig. 15). 

Also widespread was the metasomatic replacement of 

host rocks by gangue minerals such as fluorite, aegirine 

augite, phlogopite, alkali amphibole, fine granular apatite, 

albite, and microcline. Hydrothermal replacement of marble 

by fluorite is observed from initial interstitial replacement 

to the development of bands of fluorite in the host rocks 

(fig. 16) to terminal stage, where the host dolomite occurs 

as scattered islands completely surrounded by fluorite. 
TEXTURAL EVIDENCE FOR THE ORIGIN OF THE BAY AN OBO ORE DEPOSIT 29 

Table 13. Generalized mineral paragenetic sequence of Bayan Obo. 

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

2. Occurrence of alkali amphibole veins crosscutting the underlying H6 quartzite. The minimum 40 Arf39 Ar age of the 

alkali amphibole is 1.26 Ga (Conrad and McKee, 1992). 

3. 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 40 Arf39 Ar 

modified minimum ages, ranges from 800 to 900 Ma. Possibly accompanying the regional metamorphism are hydrothermal alkali amphibole veins with similar 40 Arf39 Ar ages, 

cutting the underlying H6 quartzite. 

4. Early disseminated high-Nd monazite and bastnaesite mineralization of H8 marble (Th-Pb internal isochron ages of 

555 and 553 Ma). 

5. 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). 

6. Early martitized magnetite replacement of H8 dolostone marble with early monazite mineralization and with martitized 

magnetite replaced later by surrounding monazite. 

7. 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 

(40 Arf39 Ar age of 425 Ma). 

8. Granular primary fine-grained hematite replacing monazite in 

hematite-fluorite banded ores (monazite internal isochron 

age of 423 Ma). 

9. Aegirine augite-rich veins containing huanghoite and 

aeschynite (Th-Pb mineral-isochron age of 438±25 Ma). 

10. Fibrous alkali amphibole vein cutting banded ore (40 Arf39 Ar 

plateau age of 396±6 Ma). 

11. Late-stage magnetite mineralization replacing Mn-rich magnesio-arfvedsonite (4° Arf39 Ar age of 343±3 Ma). 

12. Hercynian age late-stage Nb mineralization. 

Figure 14. Block of H8 dolostone marble (d) partly replaced by disseminated magnetite (mt). Scale indicated by 1.8-cm coin. 
30 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 
RADIOMETRIC AGES OF EPISODES OF REE MINERALIZATION 31 

f- Figure 15. Photomicrograph of a doubly polished thin section (sample 7B36-'90) of the host dolostone marble showing 

granulation texture and interstitial replacement of granulated 

grains of dolomite (d) by very fine grained monazite (m). Similar to figure 6 in paper by Chao and others (1992). (Transmitted 

light, crossed nicols.) 

Extremely fine grained aegirine augite similarly replaced 

host dolomiW. Terminal-stage replacement of marble, where 

relict dolomite grains occur completely surrounded by 

extremely fine grained aegirine augite, is shown in figure 

17. 

RADIOl\,fETRIC AGES OF EPISODES OF 

REE MINERALIZATION AND CHEMICAL 

COMPOSITIONS OF VARIOUS 

GENERATIONS OF REE MINERALS 

Mter more than 2 years of testing, the 232ThP08Pb 

internal isochron method was adopted for dating REE 

minerals (Wang and others, 1994). This method is preferred 

over other methods because precise ages are obtained on the 

basis of actual measurements of Th and Pb, rather than by 

assuming initial Pb concentrations. A total of 14 monazite 

samples and 3 bastnaesite samples were separated from 

Bayan Obo disseminated and banded ores by the Tianjin 

Geological Research Academy laboratory in Tianjin and the 

USGS laboratory in Reston, Va. The purity of single samples of monazite or bastnaesite was then checked by using 

SEM-EDX in the Reston laboratory before submitting the 

samples to the USGS laboratory in Denver, Colo., for isotopic determination. Each of the single samples of the REE 

minerals was further separated into several fractions for isotopic analysis by using a Franz magnetic separator equipped 

with an alcohol chute (Wang and others, 1994). The results 

for these 17 samples were described by Wang and others 

(1994). Two pieces of evidence suggest that the 232ThP08 Pb 

data are reliable: (1) all of the monazite and bastnaesite 

samples submitted yielded good internal isochrons, so that 

none were rejected; and (2) the internal isochron ages of 

two splits of monazite sample 796-48,278, submitted 3 

months apart to the USGS Denver laboratory, are 422± 16 

and 415±8 lvta, respectively, their combined internal isochron age being 419±3.7 Ma (fig. 18). Each split of sample 

796-48,278 is represented by five data points. The isotope 

dilution analyses of the two splits are, respectively, 1,767 

and 1,770 ppb total U, 2,830 and 2,449 ppm total Th, and 

f- Figure 16.. Photomicrograph of a doubly polished thin section (sample 7B30-2) of banded fluorite dolostone marble 

showing fluorite (fl) replacement of dolomite (d) at triplejunction grain boundary. (Transmitted light.) 

54.26 and 48.96 ppm total Pb. A few internal isochrons are 

presented here to help illustrate that different generations of 

monazite and bastnaesite differ not only in their chemical 

1 . . .. 1 208pbf204pb t' composition and age, but a so m m1t1a ra tos, 

which probably represent different hydrothermal solutions. 

Chemical compositions of monazites and bastnaesites 

were obtained by EPMA, using various calibrated standards. EPMA data on LREE's of monazite and bastnaesite 

(table 14) are in general agreement with the published average chemical composition of 17 monazite and 22 bastnaesite samples from Bayan Obo, determined by wet chemical 

methods (Zhang and Tao, 1986, p. 99). EPMA data on REE 

content of apatites (table 5) are also in good agreement with 

data obtained by the SXRF method (Chen and others, 

1993). Although the 17 REE mineral samples that were 

dated were also analyzed using INAA for all REE's, including the heavy REE's (HREE's), the INAA data showed 

significantly lower amounts of LREE's when compared 

with the EPMA data. Research is in progress to account for 

this discrepancy. Because the amounts of HREE's are too 

low to be analyzed by the EPMA method, complete REE 

distribution curves are not available. EPMA of six of the 

monazites shows amounts of Y20 3 ranging from 0.12 to 

0.39 wt. percent. Further studies may indicate whether 

recovery of Y from this ore deposit is economical. 

Concentrations of LREE's (La, Ce, Nd, and Pr), U, Th, 

and Ph and initial 208Pbf204Pb ratios constitute the eight 

chemical parameters used to distinguish monazites and 

bastnaesites in this study. The chemical compositions and 

internal isochron ages of monazites and bastnaesites are 

given in table 14, arranged according to internal isochron 

ages. The earliest bastnaesite and monazite mineralization 

in disseminated marble ores occurred at 555±11 Ma (sample 

8B7) and 553±16 Ma (sample 7B36-6), respectively. The 

bastnaesite sample 8B7 (table 14) is characterized by a relatively low La20 3 content (22.92 wt. percent) and a high 

Nd20 3 content (10.53 wt. percent) compared with other 

dated bastnaesite samples analyzed. The bastnaesite of this 

generation is also characterized by low U, Th, and Ph concentrations, as well as a low initial 208Pbf204Pb ratio. The 

monazite in sample 7B36-6 is characterized by uniquely 

low La20 3 content (13.26 wt. percent), very high Nd203 

(11.27 wt. percent), high Pr20 3 (4.06 wt. percent), extremely 

high Ce20 3 ( 41.55 wt. percent, the highest reported), and a 

moderately high 208Pbf204Pb initial ratio (39.76±2.06). Disseminated monazite from dolostone marble sample 34-24 

(table 14, fig. 19), with an internal isochron age of 532±3.1 

Ma, is chemically characterized by a moderately low La203 

content (17 .63 wt. percent), a moderately high Nd20 3 content ( 10.80 wt. percent), and a low 208Pbf204Pb initial ratio 

(34.89±0.15). 

Bastnaesite from banded ore sample 796-48,280 (table 

14), with an internal isochron age of 496±13 Ma, is characterized by a high La20 3 content (28.64 wt. percent) and a 

low Nd20 3 content (7 .83 wt. percent). Monazite from 
32 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Figure 17. Photomicrograph of a doubly polished thin section (sample 796-48,449) of marble showing relict dolomite (d) grains surrounded by fme granular aegirine augite (aeg) that partially replaced dolomite. 

banded monazite-granular hematite-fluorite sample 8B35-

2, with an internal isochron age of 474±4.5 Ma, is characterized by low Nd20 3 content (8.37 wt. percent), low C~03 (33.84 wt. percent), relatively high total U (3,384 ppb), and 

a moderately high 208PbJ204Pb initial ratio ( 40.17±0. 70). Disseminated bastnaesite from dolostone marble sample 85-

10,21-4 (table 14), with an internal isochron mineral age of 

475±8 Ma, is chemically characterized by a moderate La20 3 

content (24.89 wt. percent), a high Nd20 3 content (10.90 wt. 

percent), and an unusually high 208PbJ204Pb initial ratio 

(59.83±5.40). Although the monazite from 8B35-2 and 

bastnaesite from 85-10,21-4 are nearly identical in their 

internal isochron age, they probably were not deposited 

from the same hydrothermal solution because their initial 208PbJ204Pb ratios are very different. 

Monazite samples from banded ores of diverse chemical compositions have internal isochron ages ranging from 

474±4.5 Ma (sample 8B35-2, table 14) to 404±14 Ma 

(sample 9B9-3, table 14); the peak of REE mineralization 

of the Bayan Obo ores occurred between 430 and 420 Ma. 

Monazite sample 9B9-3 (table 14), with an internal isochron age of 404±14 Ma (the youngest monazite from 

banded ores), has the highest total U content of 7,630 ppb. 

The youngest monazite from marble-hosted disseminated 

ore sample 12-6A, has an internal isochron age of 398±9.7 

Ma and the highest total Th content (6,071 ppm) of all REE 

samples analyzed. Only two samples, 7B31-2 and 7B31-9 

(table 14), are nearly identical with respect to all eight 

chemical parameters. Samples 796-48,270 and 796-48,278 

(table 14) are similar in LREE composition and age, but the 

large uncertainties regarding the age and 208Pbf204Pb initial 

ratio for sample 796-48,270 make comparison between 

these two samples difficult. 

The internal isochrons, in ascending order of 208Pb/ 204Pb initial ratios, for selected samples are shown in figures 

19-22. Monazite from sample 490,330 (fig. 19), with an 

internal isochron mineral age of 431±2.7 Ma, has the lowest 208PbJ204Pb initial ratio (32.47±0.17) and is the most primitive, least radiogenic sample. Monazite from sample 34-24, 

with an internal isochron mineral age of 532±3.1 Ma, 

is the next least radiogenic and has a 208Pb1204Pb initial ratio 

of 34.89±0.15. Monazite from sample 7B36-6 (fig. 20) 
RADIOMETRIC AGES OF EPISODES OF REE MINERALIZATION 

.0 

c... 

""" 0 

6,000 

:a 4,000 

c... 

00 

0 

N 

2,000 

100,000 200,000 300,000 400,000 

232Th/204pb 

Figure 18. Internal isochron showing mineral age of two aliquots of monazite from sample 796-48,278. The lower five data 

points are based on Th/Pb isotopic ratios of one aliquot, and the 

upper points are for the other aliquot. 

33 

Table 14. Partial chemical composition and internal isochron mineral ages of monazites (m) and bastnaesites (b) of Bayan Obo. 

[Ore type: d, disseminated ore; bd, banded ore. *,Age and isotope dilution analyses from Wang and others (1994); analysts: Wang Junwen and M. Tatsumoto. **, determinations by electron microprobe analysis (EPMA); analysts: J.M. Back and J.A. Minkin. RE oxides in weight percent; total U in parts per 

billion (ppb); Th and Pb concentrations in parts per million (ppm)] 

Sample no ................ 8B7 7B36-6 3~24 796-48,280 8835-2 85-10,21-4 983-2 796-48,277 490,330 

Mineral ..................... b m m b m b m m m 

Ore type .................... d d d bd bd d bd d bd 

Age (Ma)* ................ 555±11 553±16 532±3.1 496±13 474±4.5 475±8 437±9.4 432±8.5 431±2.7 

La203** 22.92 13.26 17.63 28.64 25.41 24.89 23.51 18.60 24.76 

Nd20 3** 10.53 11.27 10.80 7.83 8.37 10.90 8.22 10.03 7.45 

Ce20 3** 39.11 41.55 37.38 36.37 33.84 36.24 34.65 36.91 35.25 

Pr203** 3.87 4.06 3.84 2.96 2.72 3.48 2.94 3.40 2.80 

u (ppb)* 55.17 506.8 559.8 853.1 3,384 236.7 449.7 321.2 201.7 

Th (ppm)* 994.2 3,670 2,678 1,284 808.6 563.6 1,202 2,988 1,102 

Ph (ppm)* 27.23 142.7 115.2 29.60 29.06 15.61 68.37 60.98 32.42 

Initial 

2ospbJ204Pb* 36.58±14.7 39.76±2.06 34.89±0.15 36.7 4±4 7. 97 40.17±0. 70 59.83±5.40 49.21±1.75 61.47±30.26 32.47±0.17 

Sample no ................ 7831-23 7B31-2 7831-9 7B30-20 796-48,278 796-48,270 989-3 12-6A 

Mineral ..................... m m m m m m m m 

Ore type .................... bd bd bd bd d bd bd d 

Age (Ma)* ................ 429±11 424±5.6 418±11 421±3.3 419±3.7 415±17 404±14 398±9.7 

La203** 19.69 23.77 22.89 13.60 20.10 20.17 19.81 17.07 

Nd20 3** 10.98 9.13 9.54 13.61 10.31 9.63 10.88 12.85 

Ce20 3** 35.95 33.92 34.63 38.69 36.12 37.42 34.96 35.84 

Pr203** 3.58 3.20 2.90 4.32 3.45 3.32 3.18 3.98 

u (ppb)* 313.9 5,233 3,617 461.3 1,768 340.7 7,630 3,196 

Th (ppm)* 787.2 832.2 803.8 2,417 2,657 2,273 2,012 6,071 

Ph (ppm)* 28.34 139.2 34.95 55.54 51.86 43.44 50.82 194.8 

Initial 208PbP04Pb* 47.31±10.12 44.97±0.48 45.40±1.30 66.65±5.32 35.49±10.17 39.61±99.27 41.13±18.3 41.67±1.32 
34 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

490,330 

1,500 1,500 

.0 

a... 

'<:!" 

0 

~ 1,000 a... 

.0 

a... 

'<:!" 

0 

N 1,000 :0 

a... 00 

0 

N 

00 

0 

N 

500 500 

20,000 40,000 60,000 80,000 20,000 40,000 60,000 80,000 

232Th/204pb 232Th/204pb 

Figure 19. Internal isochrons of monazite from two samples (490,330 and 34-24) of Bayan Obo ores showing ages and 208PbP04Pb 

initial ratios. These monazites of different ages have the most primitive nonradiogenic 208PbP04Pb initial ratios of all Bayan Obo ores 

we have tested. 

250 

-5: 200 '<:!" 

0 

N 

:0 

a... 

00 

~ 150 

100 

2,500 5,000 

232Th/204pb 

7,500 10,000 

250 

iE 200 '<:!" 

0 

N 

:0 

a... 

00 

~ 150 

100 

50 ~--~--~--~--~--~--~----~~ 

2,000 4,000 6,000 

232rh;204pb 

8,000 10,000 

Figure 20. Internal isochrons of monazite from two samples (7B36-6 and 8B35-2) of Bayan Obo ores that show nearly identical 208Pbf204Pb initial ratios indicate different internal isochron ages, suggesting a common source reservoir that was tapped at different 

times. 
RADIOMETRIC AGES OF EPISODES OF REE MINERALIZATION 35 

Initial 208Pb;204Pb ratio=44.97±0.48 Initial 208Pb;2°4Pb ratio=45.40±1.30 

100 

150 .0 .0 

a.. a.. '<:!" '<:!" 0 0 

N N 

:n 80 :n a.. a.. 00 00 

0 0 

N N 

100 

60 

40 ~--~--L---~--~--~--~~--L---~ 

0 1 ,000 2,000 3,000 4,000 2,000 4,000 6,000 8,000 

232Th/204pb 232Th;204pb 

Figure 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 208PbP04Pb initial ratios. The monazites in these samples probably were deposited from the same 

hydrothermal solutions. 

500 

450 

.0 

'<:!"a.. 400 0 

N 

:n 

a.. 

00 

~ 350 

300 

250 

5,000 

Initial 208pb;2°4Pb ratio=59.83±5.40 

85-10,21-4 

475±8 Ma 

10,000 15,000 

232Th/204pb 

20,000 

1,200 

1,000 

ii: 800 '<:!" 

0 

N 

:n 

a.. 

00 

0 

N 600 

400 

200 

10,000 

Initial 208Pb;204Pb ratio=66.65±5.32 

20,000 30,000 40,000 50,000 

232Th/204pb 

Figure 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 208PbP04Pb initial ratios. 
36 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

has an internal mineral isochron age of 553±16 Ma and a 208Pbf204Pb initial ratio of 39.76±2.06 that is indistinguishable from the 208Pbf2°4Pb initial ratio of monazite sample 

8B35-2 (fig. 20), 40.17±0. 70. The internal isochron age of 

monazite sample 8B35-2 is 474±4.5 Ma, clearly younger 

than 553±16 Ma, and may indicate that the same or similar 

source reservoir for REE's was tapped and mobilized at different times. Monazites from samples 7B31-2 and 7B31-9 

are similar in REE chemical composition (table 14) and 

indistinguishable in internal isochron age and 208Pbf204Pb 

initial ratios (fig. 21). Therefore, it is probable that the monazites from 7B31-2 and 7B31-9 originated concurrently 

from hydrothermal solutions derived from the same source 

reservoir. These two samples were collected more than 50 

m apart, about 130m above the footwall of the East Orebody (fig. 5). 

Bastnaesite from sample 85-10,21-4 (table 14 and fig. 

22) has an internal isochron mineral age of 475±8 Ma and a 

very high 208Pbf204Pb initial ratio of 59.83±5.40. Monazite 

from sample 7B30-20 (table 14 and fig. 22), with an age of 

421±3.3 Ma, has one of the lowest La20 3 contents and the 

highest Nd20 3 content, as well as one of the highest, most 

radiogenic 208Pbf204Pb initial ratios (66.65±5.32). These two 

REE samples came from different hydrothermal solutions, 

not only different from each other but also from the solutions giving rise to the rest of the analyzed REE samples. 

Spatial relations of REE mineralized samples are generally lacking. Four of the dated samples are however from 

the same drill core. They are 796-48,270, 796-48,277, 

796-48,278, and 796-48,280 (table 14). Except 796-48,280 

(depth 280m), where bastnaesite is the REE mineral dated 

with an internal isochron age of 496±13 Ma, the other three 

are monazite samples, from depths of 270, 277, and 278 m, 

respectively. Monazite from 796-48,270, 415±17 Ma, is 

from a banded ore hosted by dolomite marble and associated with granular aegirine, magnetite, phlogopite, and fluorite; monazite from sample 796-48,277, 432±8.5 Ma, is 

from a disseminated ore hosted by limestone marble and 

associated with abundant alkali amphibole, magnetite, and 

small amounts of aegirine, fluorite, and pyrite; and monazite from 796-48,278, 419±3.7 Ma, is also from a disseminated ore hosted by limestone marble and associated with 

alkali amphibole, magnetite, fluorite, and pyrite. None of 

these three monazites can be distinguished readily by age, 

chemical composition, or initial 208Pbf204Pb ratio (owing to 

large uncertainties). These three samples may be considered 

to be of similar age and deposited by the same hydrothermal 

source. 

The internal isochron mineral ages of episodes of monazite and bastnaesite mineralization are shown in table ·14 

and figures 18-22, 32, and 33. The episodes of REE mineralization recorded so far ranged from 555 to 398 Ma, covering a geologic time span of more than 150 million years. 

Chao and others (1992, 1993), on the basis of preliminary 

partial internal isochron and chemical data, concluded that 

the role of REE remobilization from carbonate host rocks 

was improbable, and that the effects of low-grade Caledonian and Hercynian regional metamorphism and Hercynian granitic intrusion on the ages of REE mineralization were minimal. The extensive and refined detailed data 

presented by Wang and others (1994) and in this paper, particularly the more complete chemical, isotopic, and initial 208Pbf204Pb ratios, fully substantiate the earlier conclusions. 

Most monazites with different internal isochron ages, chemical compositions, and initial 208Pbf204Pb ratios were probably brought in at different times and deposited by different 

hydrothermal solutions. 

Fe MINERALIZATION 

In general, there are three major Fe-oxide minerals 

associated with Fe mineralization, each perhaps occurring 

in more than one generation. These are ( 1) euhedral martite 

or martitized magnetite grains scattered in disseminated 

ores, in scattered aggregates in bands of banded ores, and in 

massive ores; (2) fine-grained to very fine grained primary 

granular hematite that forms disseminated patches and scattered lenses in dolostone marble disseminated ores and in 

bands in banded ores, and also occurs in massive ores; and 

(3) nonmartitized magnetite in disseminated, banded, and 

massive ores. Massive Fe ores commonly consist of a combination of the above three Fe-oxide minerals. Additional, 

but minor, occurrences of Fe minerals are very fine grained 

disseminated magnetite with rims of hematite, rare wormy 

textures in hematitic ores, and goethite in weathered Feoxide ores. The rimmed and wormy textures may be of 

metamorphic origin. 

Data on ages of Fe mineralization are fragmentary 

because the Fe-oxide minerals cannot be isotopically dated 

and we were not successful in identifying various generations of fresh magnetite in terms of trace-element contents. 

Hence, textural relations among Fe-oxide minerals and 

dated REE minerals and alkali amphiboles were used to 

help determine the relative paragenetic sequence for Fe 

mineralization. These data yielded age information regarding only the latest and not the earliest age of Fe mineralization, but indicated that euhedral martite and martitized 

magnetite formed before granular hematite in disseminated 

and banded ores, and fresh magnetite formed last in disseminated, banded, and massive ores. 

Euhedral martite and martitized magnetite are apparently the oldest Fe ore minerals in ores hosted by dolostone 

marble. Martite and martitized magnetite crystals are euhedral crystals of magnetite completely or partially replaced 

pseudomorphously along crystallographic directions of the 

original magnetite host. Thus, they record a two-phase process: the deposition of euhedral magnetite and later pseudomorphous replacement of magnetite by hematite. Where 

martite or martitized magnetite occurs with granular hema-
Fe MINERALIZATION 37 

100 ~m 

I I 

Figure 23. Photomicrograph of a doubly polished thin section of sample 8B2-2 showing euhedral martitized magnetite (mr) crystals 

blocking the continuous formation of later fine-grained hematite (he). Some martite crystals are completely surrounded by the granular 

hematite. (Reflected light, crossed nicols.) 

tite, such as in some of the banded REE-Fe ores, texturally, 

martite and martitized magnetite preceded the granular 

hematite. This relation is illustrated where preexisting martitized magnetic crystals blocked the formation of continuous bands of granular hematite (fig. 23) and where euhedral 

martite crystals are completely embedded in fine, granular 

hematite (fig. 24). We interpret these relations to indicate 

that euhedral magnetite crystals formed first and were 

pseudomorphously replaced by hematite along crystallographic directions. The martite locally blocked and was 

incorporated into the younger bands of granular hematite. 

Granular hematite represents the next type of Fe mineralization. Evidence for the earliest episodes of granular 

hematite mineralization is found in sample 8B35-2, where 

fine-grained hematite with associated Nb-rutile replaced 

monazite with an age of 474±4.5 Ma (table 14), and therefore crystallized after that date. Unfortunately, a minimum 

age for the hematite cannot be determined. Similarly, granular hematite progressively replaced dated monazite in sample 7B30-20 (figs. 25 and 26) and is, therefore, younger 

than 421±3.3 Ma, the radiometrically determined age of the 

monazite. The monazite of sample 7B30-20 has about equal 

concentrations of La20 3 (13.60 wt. percent) and Nd20 3 

(13.61 wt. percent) (table 14), which are low for La20 3 and 

uniquely high for Nd20 3 at Bayan Obo. Because of the 

unique chemical composition of this dated monazite, it can 

be used as a guidepost for estimating relative ages of minerals formed, based on textural evidence, before and after this 
38 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Figure 24. Photomicrograph of a doubly polished thin section of sample 7B37-1 showing euhedral martite (mr) crystals embedded in 

fine granular hematite (he). (Reflected light, crossed nicols.) 
Fe MINERALIZATION 39 

Figure 25. Photomicrograph of a doubly polished thin section of sample 7B30-20 showing bands of granular monazite (m) replaced 

interstitially by fme granular hematite (he). (Reflected light, crossed nicols.) 
40 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

100 ~m I I 

Figure 26. Photomicrograph of a doubly polished thin section of sample 7B30-20 showing strings of rounded monazite (m) surrounded 

and replaced interstitially by fine granular hematite (he). (Reflected light, crossed nicols.) 

monazite crystallized. Hematite bands in ores were cut by 

alkali amphibole veins (sample 7B30-30) with an 40 ArJ39 Ar 

plateau age of 395.8±6.2 Ma; hence, the age of mineralization of some of the primary granular hematite probably lies 

between 421 and 396 Ma. 

The occurrence of fresh magnetite in folded banded 

ore and in undeformed massive Fe ores indicates that at 

least two generations of fresh magnetite ores are present at 

Bayan Obo and apparently postdate the granular hematite. 

The textural relation between early granular hematite and 

late magnetite can be seen in sample 7B31-6, where a veinlet of fresh magnetite crosscuts bands of monazite and fine, 

granular hematite in banded REE-Fe ore (fig. 27); in sample 

9B 15-1L, where banded granular hematite-monazite-fluorite ore was replaced and cut by fresh massive magnetite 

(fig. 28); and in sample 796-48,517, where relict bands of 

granular hematite and monazite were disrupted and surrounded by fresh magnetite (figs. 29A and B). These relations also indicate that massive magnetite ores formed after 

banded ores and are supported by the apparent textural zoning in the Main and East Orebodies, where massive Fe ores 

occur in the center of the orebodies and are surrounded by 

banded ores. 

The 40 Arf39 Ar ages of coexisting alkali amphiboles can 

further refine the ages of Fe mineralization and also add 

information about the long period of time the mine area was 

subjected to hydrothermal activity. For example, in sample 

796-48,277, a drill hole sample of low-grade, disseminated 

monazite-magnetite ore that contains abundant alkali 

amphibole (figs. 30A and B), host limestone marble was 

replaced by aggregates of fine-grained monazite, which 

were in turn enclosed and replaced by alkali amphibole. 

Fresh magnetite replaced both monazite and alkali amphibole and is, therefore, younger than these phases. In this 
Fe MINERALIZATION 41 

100 1-Jm 

I I 

Figure 27. Photomicrograph of a doubly polished thin section of sample 7B31-6 showing a magnetite (mt) veinlet cutting a banded monazite (m)-bearing granular hematite (he) ore. (Reflected light, crossed nicols.) 

sample, the monazite was determined to be 432±8.5 Ma 

(table 14 and fig. 31A) by the 232Thf2°8Pb internal isochron 

method, and the alkali amphibole (magnesio-arfvedsonite) 

has a plateau age of 343.1±3.0 Ma (table 15 and fig. 318), 

which is considered to be a minimum age, determined by 

the 40 Arf39 Ar incremental heating method. The age and relatively flat spectrum for this metasomatic-metamorphic 

amphibole suggest that it has not been significantly reset by 

the younger Hercynian granitic intrusion or regional metamorphism. Because the magnetite replaces the alkali 

amphibole, the age of this magnetite is younger than the age 

of the alkali amphibole, about 343 Ma. Although the age 

difference between monazite and amphibole, about 90 m.y., 

is large, the ages are well documented and are consistent 

with the textural relations. In samples 8B7 and 34-24, alkali 

amphibole also replaced REE minerals, and the ages of the 

amphibole and REE phases have been determined. Bastnaesite in sample 8B7 is 555±11 Ma (fig. 32A), and alkali 

amphibole in the same sample is 395.5±4.4 Ma (table 15 

and fig. 32B). In sample 34-24, the monazite is 532±3.1 Ma 

(table 14), and the coexisting alkali amphibole, dated by the 

K-Ar method by the Tianjin laboratory, is 441±11 Ma. In 

samples 79~8.277 and 8B7, there is a possibility that the 

alkali amphibole may have been reset by the replacement 

and mineralization of late-stage fresh magnetite. If so, then 

the determined minimum alkali amphibole ages date the age 

of magnetite mineralization. In sample 34-24, magnetite is 

not present; hence, resetting of the alkali amphibole age by 

late-stage magnetite-precipitating hydrothermal solutions 

is unlikely. 

Text continues on p. 50. 
42 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

100 J.Jm 

I I 

Figure 28. Photomicrograph of a doubly polished thin section of sample 9B 15-lL showing a banded ore consisting of monazite (m)-

granular hematite (he)-fluorite (fl) cut by late-stage, coarse-grained fresh magnetite (mt) aggregates. (Transmitted light, crossed nicols.) 
Table 15. Chemical composition (wt. percent) and minimum mineral ages of various generations of selected alkali amphiboles of Bayan Obo. 

[Analysts: J.M. Back and J.A. Minkin. Analysis by electron microprobe (EPMA). *,below minimum detection level (MDL); MDL for Ti02=0.04. Minimum 40 ArP9 Ar mineral ages determined by J.E. Conrad, E.H. McKee, and B.D. Turrin; n.d., not determined] 

Sample no ...... Drew 1 7Bl3C 40-14 8824-1 7825-2 8842 7834-lP 7834-IF 796-48,252 796-48,366 7830-30 887 796-48,277 

Si02 55.07 56.01 56.55 55.31 57.19 55.96 57.54 57.24 55.98 53.53 55.90 55.75 53.85 

Ti02 .07 .25 .04 * .12 * * .05 .06 * * * .04 

Al 20 3 .55 .26 .59 .33 .46 .48 .37 .26 .66 .70 .58 .47 .44 1

Fe0 22.74 14.15 13.24 8.84 9.29 15.48 12.80 10.81 17.18 14.30 14.45 9.55 12.59 

MnO .30 .62 .13 .13 .04 2.82 .08 .13 .91 4.91 .57 .90 5.68 

MgO 10.28 14.95 15.96 17.96 18.55 12.88 17.12 18.34 14.01 11.84 15.44 19.30 12.47 

CaO .48 .65 .61 2.01 1.06 1.77 .72 4.15 2.33 2.56 2.69 3.28 1.92 

Na20 7.33 7.69 7.03 7.74 7.18 7.19 8.98 6.13 6.74 6.74 7.17 6.91 7.64 

K20 1.03 1.95 2.93 2.00 2.26 2.21 1.18 1.18 1.02 2.14 1.57 .67 1.90 

F .98 2.03 2.51 2.20 2.52 2.87 2.56 1.38 2.04 3.08 2.81 3.40 3.41 

Total 98.83 98.56 99.59 96.52 98.67 101.66 101.35 99.67 100.93 99.80 101.18 100.23 99.94 

-O=F 0.41 0.85 1.06 0.93 1.06 1.21 1.08 0.58 0.86 1.30 1.18 1.43 1.44 

Total 98.42 97.71 98.53 95.59 97.61 100.45 100.27 99.09 100.07 98.50 100.00 98.80 98.50 

Minimum 

mineral 

age (Ma) 1,260 820 890 890 n.d. n.d. 424.8±2.4 428.0±7.3 419.5±2.0 n.d. 395.8±6.2 395.5±4.4 343.1±3.0 

-

1 Total Fe as FeO. 

"T1 

~ 

~ 

z 

tTl 

:;tl 

~ 

r-' 

N 

...., 

~ 

0 

z 

~ 

(,;) 
44 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

100 J.Jm 

I I 

Figure 29. Photomicrographs of a doubly polished thin section of sample 796-48,517 showing relict banded ore consisting of folded 

monazite (m) and granular hematite (he) surrounded and replaced by late-stage, fresh magnetite (mt). A, Transmitted light; B, Reflected 

light. 
Fe MINERALIZATION 45 

Figure 29. Continued. 
46 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Figure 30. Photomicrographs of a doubly polished thin section of sample 796-48,277, a low-grade ore consisting of alkali amphibole 

(mga), monazite (m), and magnetite (mt), as well as relict grains of the limestone marble host rock. The alkali amphiboles contain inclusions of monazite. Monazite and alkali amphiboles are replaced and cut by fresh magnetite. A, Transmitted light; B, Reflected light. 
Fe MINERALIZATION 47 

Figure 30. Continued. 
48 

420 

~ 400 

(/) 

_J 

UJ 

u 

(/) 380 UJ 

UJ 

0: 

(.9 

~ 360 

UJ 

0: 

::J 

I- 340 <( 

0: 

UJ 

Cl.. 

725 

- ~ ----

UJ 320 ~ 

IGIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

3,500 .-----~-------.-------.---------., 

796-48,277 MONAZITE 

432±8.5 Ma 

3,000 

fi: 2,500 '<1" 

0 

N .......... 

...0 

Cl.. 

00 

~ 2,000 

1,500 

A 

1,000 L._ ___ __.__ ___ ____,L ____ __.__ ___ _ 

50,000 100,000 150,000 

232Thpo4Pb 

796-48,277 

AMPHIBOLE 

I 

I !+----------------- 343.1±3.0 ----------------~::: 

--_?_6_~---- 795 c---::::::~??:::::: :::: c-----::::~?~:::::: :::::"---------~Q9_ --------_f:::::~~?::::: [ ------ -----~-- --------------.r- ... ---------------------- -----------------" 

3oo~~--~--~--~'~--~'--~~--~--~~--~--~~~--~--~--~~--~--~~ 

0 10 20 30 40 50 60 70 80 90 100 

B 39Ar RELEASED, CUMULATIVE PERCENT 

Figure 31. Radiometric ages of minerals in sample 796-48,277: A, Internal isochron showing mineral age of the monazite (432±8.5 Ma), 

which replaced the host limestone marble and occurs as abundant inclusions in alkali amphibole; B, 40 Arf39 Ar plateau showing age 

(343.1±3.0 Ma) of the coexisting alkali amphibole that replaced the monazite. 
5: 1,500 

0 """ 

(',I 

-..0 

0... 

00 

~ 1,000 

500 

Fe MINERALIZATION 

887 BASTNAESITE 

555±11 Ma 

A 0 ~~~~--~---L--~--~--~--~--~~ 

0 40,000 

232Thpo4Pb 

80,000 

49 

430.-------------------------------------------------------------------~ 

en 

::> 

U5 

_J 410- LU 

u 

en 

LU 

LU 

a: 

~ 

~ 390-

LU 

a: 

::> 

~ a: 

~ 370 -XZ9 

~ 

LU 

I887 AMPHIBOLE 

~loooE;;:::....__ _______________ 395.5±4.4 ----------~::~ 

1070 1300 

88o -------r----

850 r------------------ 1 -----~-~Q______ 970 ·L ::::::::::::::::::::::::::::f:::::::::::::::::::::::::::::::t------------------1_--------------~~ ------- -----

350 ~~--~~--~~~--~--~~~--~~--~--~~~--~~--~--~~~--~~--~~~--~--~ 

0 10 20 30 40 50 60 70 80 90 100 

B 39Ar RELEASED, CUMULATIVE PERCENT 

Figure32. Radiometric ages of minerals in sample 8B7: A, Internal isochron showing mineral age of the bastnaesite (555±11 Ma), which 

replaced the host dolostone marble; B, 40ArP9Ar plateau showing age (395.5±4.4 Ma) of the coexisting alkali amphibole. 
50 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Nb MINERALIZATION 

Because of the very fine grained nature of most of the 

Nb minerals and our limited resources, only fragmentary 

data on the occurrence of Nb-bearing minerals are presented here. The most important Nb mineralization in the 

mine area is associated with the West Orebodies, which are 

west of the Main Orebody. Most of the Nb minerals occur 

as disseminated ores. Nb-rutile is closely associated with 

granular hematite in banded ores near the footwall of the 

Main Orebody, and columbite (table 1), the main Nb ore 

mineral, is closely associated with fresh magnetite in disseminated magnetite ores hosted by marble and dominated 

by alkali amphibole. Aeschynite (table 1) is the only coarsegrained Nb mineral that occurs in veins and is associated 

with huanghoite and alkali amphiboles. Pyrochlore (table 

1 ), which is the dominant and typical Nb mineral associated 

with igneous carbonatites, is not a common Nb mineral in 

Bayan Obo, except in marble in contact with the Hercynian 

granodioritic rocks in the eastern part of the mine area. Preliminary fragmentary data indicate that the 438.2±25.1 Ma 

age of vein aeschynite (Chao and others, 1993), a niobate 

mineral, may represent the earliest known age of Nb mineralization. The latest age of Nb mineralization is based on 

the occurrence of pyrochlore in skarn marble that is in contact with Hercynian biotite granodiorite, which occurs about 

2 km east of the East Orebody. 

RADIOMETRIC AGES OF 

ALKALI AMPHIBOLES AS A KEY TO 

DETERMINING PERIODS OF 

REGIONAL METAMORPHISM AND 

HYDROTHERMAL ACTIVITIES 

The regional metamorphic and hydrothermal histories 

and the evolution of the Bayan Obo ore deposit are very 

complex. A general representation of this development is 

presented schematically in figure 33. Data on episodes of 

REE and Fe mineralization have already been described. 

Alkali amphiboles were used to decipher the complex metamorphic and metasomatic episodes of Bayan Obo. Three 

types of alkali amphiboles, namely, metamorphic, metasomatic, and metasomatic-metamorphic, were dated. Alkali 

amphiboles of these three origins were differentiated on the 

basis of texture and abundance. Amphiboles of regional 

metamorphic origin occur as prismatic and fibrous or 

asbestiform crystals disseminated within or linearly aligned 

along foliation planes in the H8 marble host rocks, and usually show some sign of deformation, such as strong wavy 

extinction or bent asbestiform. Metasomatic alkali amp hiboles generally occur in ore hosted by H8 and locally may 

replace more than 10-50 volume percent of the host rock. 

The alkalis, Fe, and Si necessary to form the amphibole 

were derived from hydrothermal solutions. Metasomaticmetamorphic alkali amphiboles are represented by large 

quantities of linearly aligned alkali amphiboles within and 

replacing the host marbles. Many metasomatic-metamorphic alkali amphiboles have magnesio-arfvedsonite prismatic cores thinly rimmed with magnesio-riebeckite (for 

example, samples 7B34-1P and 7B34-1F, table 15). 

Samples representing different generations of these 

three types of alkali amphiboles vary in the FeO/MgO ratios 

and in amounts of MnO and alkalis. All Bayan Obo alkali 

amphiboles are characteristically low in Al20 3• Generally, 

the chemical compositions of metamorphic alkali amphiboles are richer in MgO than FeO, whereas the hydrothermal metasomatic alkali amphiboles are richer in FeO than 

MgO. 

PERIODS OF REGIONAL 

METAMORPHISM 

Previous and contemporary studies of Inner Mongolia 

indicate that several periods of regional metamorphism 

could have occurred in the Bayan Obo region. For central 

Inner Mongolia, Tang and Yan (1993) recognized four different periods of regional metamorphism-Middle Proterozoic, early Paleozoic (two stages), and late Paleozoic. The 

two stages of early Paleozoic metamorphism were ( 1) subduction zone metamorphism that resulted in paired metamorphic belts in the Ondor Sum (Wenduermiao) ophiolite 

(fig. 4) and the Bainaimiao island arc complex, and (2) orogenic metamorphism that occurred during the collision of 

an island arc with the continent (North China craton). 

According to Tang and Yan, the Middle Proterozoic and late 

Paleozoic metamorphic events were associated with rifting 

and were characterized by low-pressure metamorphic 

facies. Zhang and Tao (1986) recognized two stages of Hercynian granite rocks; biotite ages show the early stage of 

311-306 Ma and the later stage of 258-234 Ma. 

We recognize three stages of regional metamorphism 

(fig. 33) in the Bayan Obo mine region (Conrad and others, 

1991; Chao and others, 1995): Late Proterozoic (about 900 

Ma), Silurian to Mississippian (Caledonian, 425-343 Ma), 

and Pennsylvanian to Triassic (Hercynian, 310-220 Ma). 

On the basis of the presence of alkali amphiboles of metamorphic origin, the degree of metamorphism during the 

Late Proterozoic and Caledonian events was probably 

equivalent to low-grade amphibolite facies. For Hercynian 

rocks, the presence of metamorphic biotite indicates a 

greenschist facies of regional metamorphism. 

Late Proterozoic regional metamorphism converted 

H8 carbonate to marble and sandstone to quartzite. The 

marble and quartzite show poorly to well-developed triplejunction grain boundary textures, the result of mylonitization and recrystallization. Coarse-grained ferroan dolomite 

in H8 dolostone marble was strongly twinned and stretched, 
PERIODS OF REGIONAL METAMORPHISM 51 

CALEDONIAN DISTURBED AGES 

LATE 

PROTEROZOIC 

MIDDLE 

PROTEROZOIC 

~ ::J 

OJ 

<( 

OJ 

0 

a: 

a.. 

....... ....... 

--+-- ... 

....... ....... 

H 

....... 

....... -+- ....... 

....... 

-+-

....... 

....... 

....... ....... 

--+--....... 

--+-- ....... 

....... 

....... 

....... ....... 

--...-

....... 

....... 

)I( )I 

.............. 

.............. 

....... ....... ....... ....... .............. 

....... ....... --+-- ....... 

I( )I 

-+-

-+-

EXPLANATION 

....... K-Ar age 

1400 

....... 

....... ....... ....... ....... -+-

.. ....... --...-

....... ....... ....... ....... 40 ArP9 Ar total-fusion age 

....... 40 ArP9 Ar plateau age 

....... ....... ....... ....... ....... ....... 

....... 

Figure 33. K-Ar and 40 ArJ39 Ar 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. 

and prismatic alkali amphiboles crystallized parallel to the 

direction of carbonate stretching within and along foliation 

planes in the marble. The minimum age of this event of 

regional metamorphism, based on single-grain, laser totalfusion 40 Arf39 Ar analyses of lineated magnesio-arfvedsonite 

prismatic crystals in coarse-grained H8 dolostone marble 

(samples 40-14 and 8B24-1, table 15) is about 890 Ma. 

The chemical compositions of these magnesio-arfvedsonites from coarse-grained H8 dolostone marble are characterized by MgO>FeO, about 2-3 wt. percent K20, and about 

2.2-2.5 wt. percent F. 

The Caledonian regional metamorphic event began at 

about 425 Ma and continued until at least 395 Ma, based on 40 Arf39 Ar ages of amphiboles. The 425 Ma age is based on 

two types of metasomatic-metamorphic alkali amphiboles 

that occur in fine-grained to very fine grained, distinctly 

foliated, dolostone marble (samples 7B34-1F and 7B34-1P, 

table 15). One type of amphibole occurs as fine- to mediumgrained prismatic magnesio-arfvedsonite crystals (7B34-

1P) and the other as extremely fine grained asbestiform 

magnesio-riebeckite crystals (7B34-1F). The prismatic 

crystals are zoned or rimmed with magnesio-riebeckite that 

has a composition identical to the coexisting asbestiform 

magnesio-riebeckite. The 40 Arf39 Ar plateau age of the prismatic crystals analyzed with the rim intact is 424.8±2.4 Ma, 

and the 40 Arf39 Ar plateau age of the asbestiform magnesioriebeckite is 428.0±7.3 Ma, which is indistinguishable from 

that of the prismatic amphibole. Because of the large 

amounts of alkali amphiboles present, particularly the 

asbestiform variety, we interpret this mineral occurrence to 

be of metasomatic-metamorphic origin. The chemical composition of the prismatic amphibole (sample 7B34-1P) is 

much more sodic than the asbestiform variety, which is distinctly more calcic; both varieties of amphibole are higher 

in MgO than FeO (table 15). Caledonian metamorphism 

continued to at least about 395 Ma, based on the 40 Arf39 Ar 

plateau age of coarse, bent, asbestiform blue-green amphibole in a 2- to 3-cm-wide vein that crosscuts banded ore 

(sample 7B30-30, table 15). This amphibole is oriented 

normal to the wall of the vein, has a 40 Arf39 Ar plateau age of 

395.8±6.2 Ma, and is characterized by a MgO content 

slightly higher than that of FeO. On the basis of the age of 
52 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

this alkali amphibole, the Bayan Obo banded ores are interpreted to have formed before 400 Ma. 

In addition to the widespread occurrence of Hercynian 

biotite granites and granodiorites in the Bayan Obo region, 

evidence for the Hercynian regional metamorphism is based 

on 40 Arf39 Ar single-grain total-fusion analyses of biotite 

from H9 biotite schist, which was reset and yielded ages 

ranging from 309.8±5.8 to 297.1±5.2 Ma. The 40Arf39Ar 

plateau age of the Hercynian granodiorite, based on a single 

biotite sample, is 259.0±3.1 Ma. A sample of K-feldspar 

from a pegmatite dike crosscutting H9 biotite schist gave a 

K-Ar age of 225±7 Ma. 

The occurrence of post-Caledonian or late Caledonian 

or early Hercynian low-grade regional metamorphism was 

also needed to account for the widespread presence of 

microchevron folds in banded ores and the recrystallized 

triple-junction grain boundary textures observed in some of 

the very fine grained REE and gangue minerals (apatite, 

barite, and fluorite) within bands of the banded ores. This 

period of low-grade regional metamorphism probably 

occurred before the main stage of fresh magnetite mineralization because fresh magnetite replaces folded banded 

ores, as shown in figure 29. Hercynian regional metamorphism also accounts for the isoclinal and chevron folds that 

are widespread in the H9 biotite schists with limestone boudinage. Clearly, the episodes of magnetite and Nb mineralization and the range of Hercynian regional metamorphism 

are complex and require more detailed study. 

PERIODS OF HYDROTHERMAL 

ACTIVITIES 

Magnesio-riebeckite from a vein crosscutting H6 

quartzite (sample Drew 1, table 15) gave a disturbed 40Ar/ 39 Ar spectrum inferred to reflect an age of 1.26 Ga (Conrad 

and McKee, 1992) and is the earliest evidence of hydrothermal activity in the Bayan Obo region. This magnesioriebeckite is high in FeO (22.74 wt. percent) and low in 

MgO (10.28 wt. percent) and K20 (1.03 wt. percent). The 

next period of hydrothermal activity in the mine area that 

we were able to document is indicated by magnesio-arfvedsonite from a vein (sample 7Bl3C, table 15) that also crosscuts the H6 quartzite in the West Orebodies region, about 

5.2 km west-southwest of the west end of the Main Orebody. Prismatic alkali amphibole from this vein gave a disturbed 40 Arf39 Ar plateau age of 820 Ma. The disturbance 

suggests that the age should be considered a minimum age 

of vein formation. The chemical composition of this magnesio-arfvedsonite has nearly equal amounts of FeO and 

MgO (14.15 and 14.95 wt. percent, respectively), 1.95 wt. 

percent K20, and 2.03 wt. percent F. 

Based on textural evidence and abundance, alkali 

amphibole of hydrothermal metasomatic origin occurs in 

marble-hosted disseminated Fe ore samples 8B42; 796-

48,252; 796-48,366; 8B7; and 796-48,277. The disseminated ores contain relicts of the marble host, small amounts 

of monazite or bastnaesite, and abundant alkali amphibole 

and magnetite. Apatite occurs locally. Chemically, these 

alkali amphiboles are generally slightly richer in FeO than 

MgO and contain from 1.77 to 3.28 wt. percent CaO. Alkali 

amphibole from sample 8B42 contains 2.82 wt. percent 

MnO (table 15), whereas alkali amphiboles from samples 

79~8,366 and 79~8,277 are uniquely high in MnO 

(4.91 and 5.68 wt. percent, respectively, table 15). Alkali 

amphibole from sample 8B42 has a K-Ar age of 442±11 

Ma, determined by the Tianj in laboratory. The 40 Arf39 Ar 

plateau ages of alkali amphibole from samples 79~8,252, 

8B7, and 79~8,277, obtained by Conrad and McKee, are 

419.5±2.0, 395.5±4.4, and 343.1±3.0 Ma, respectively 

(table 15). The high MnO content of metasomatic alkali 

amphiboles generally seems to be characteristic of latestage Fe mineralization. On the basis of textural relations 

for all these samples of marble-hosted disseminated ore, 

alkali amphiboles replaced small amounts of monazite, and 

the amphibole, in turn, was replaced by magnetite. Therefore, the magnetite mineralization may be younger than the 

youngest (343.1±3.0 Ma) metasomatic alkali amphiboles 

(sample 79~8,277). 

The ages of metasomatic alkali amphiboles correspond 

to episodes of intense hydrothermal activity in the Bayan 

Obo mine area, when most of the REE mineralization of 

banded ores took place. However, because the episodes are 

represented by different mineral assemblages, the mineralizing fluids for the metasomatic amphiboles and banded 

ores may have been unrelated. Many of the banded ores 

having high RE20 3 contents do not contain alkali amphiboles. As previously mentioned, where REE minerals coexist with alkali amphiboles in marble-hosted disseminated 

ores, the monazite and bastnaesite are usually much older 

than the alkali amphiboles, for example, in samples 8B7 

and 79~8,277 (tables 14 and 15). Hornblende from a plagioclase-hornblende vein that crosscuts banded ore has a 40 Arf39 Ar plateau age of 355.1±4.8 Ma, which together with 

the age of alkali amphibole from a vein that crosscuts 

banded ore (sample 7B30-30), indicates that all banded 

ores in the Main and the East Orebodies were formed before 

about 396 Ma, the metamorphic age of the older amphibole. 

REMOBILIZATION AND METAMORPHIC 

EFFECTS ON MINERAL AGES 

Chao and others (1993) determined that remobilization 

of monazite was unlikely because any metal-transporting 

acid hydrothermal solutions entering along microfractures 

and grain boundaries within the carbonate host rock would 

tend to dissolve and remove the carbonate host, become 

neutralized, and precipitate the metals and other mineral 

components from such solutions. To a very minor extent, 
MINERAL PARAGENETIC SEQUENCE 53 

bands of fluorite in banded ores do show slight resorption of 

early minerals such as REE, Fe oxide, and gangue minerals, 

but resorption is limited to within the band itself. Therefore, remobilization and redistribution of early REE minerals and alkali amphibole are likely to have been minimal or 

insignificant. 

The effect of metamorphism on mineral ages was estimated by Chao and others (1992, 1993). Metamorphism did 

not cause resetting of the ages of various generations of 

REE minerals because closure temperatures of monazite are 

higher than the temperatures reached during the low-grade 

Caledonian and Hercynian thermal events. Harland and others (1990) stated that the U-Th-Pb system closure temperature for monazite is in the range 600-500°C for cooling 

rates of 10-1 °C/m.y., and Parrish (1990) cited a 725±25°C 

closure temperature for the U-Pb system in monazite. The 232ThP08Pb internal isochrons of monazites and bastnaesites 

(figs. 18-22, 31A, and 32A; Wang and others, 1994) indicate no resetting of mineral ages. 

According to Harrison ( 1981 ), the closure temperatures of hornblende are 578-490°C for cooling rates in the 

range 500-5°C/m.y. The ages of alkali amphiboles from 

samples Drew 1 and 7B 13C have been slightly disturbed, 

probably by the latest Caledonian events. Conrad and 

McKee (1992) found a 10-percent decrease of the age of 

Drew 1 due to 40 Ar loss. Multiple analyses of alkali amphibole subsamples from a vein (sample 7B13C, table 15) have 

disturbed 40 Arf39 Ar spectra, the apparent ages ranging from 

about 820 to 640 Ma (fig. 33). The age of 820 Ma is considered the minimum age of this alkali amphibole. Because of 

evidence of disturbance of pre-Caledonian alkali amphiboles due to 40 Ar loss, mineral ages of various generations 

of alkali amphiboles are considered minimum ages. However, the Caledonian ages for alkali amphiboles of various 

origins generally show good to excellent 40 Arf39 Ar plateau 

release patterns (figs. 31B and 32B; Chao and others, 1993). 

Because of the good to excellent plateau release patterns of 

Caledonian-age alkali amphiboles, these alkali amphiboles 

do not appear to have been seriously affected by thermal 

events associated with the intrusion of granitic rocks or 

regional metamorphic events of Hercynian age. 

In comparison with hornblende, biotite has a much 

lower estimated closure temperature, about 345-280°C for 

cooling rates in the range 100-1 °C/m.y. (Harrison and others, 1985). The biotite in biotite and albitized-biotite schists 

of the H9 lithologic unit has been reset and has 40 Arf39Ar 

single-crystal total-fusion ages ranging from 309.8±5.8 Ma 

to 297 .1±5.2 Ma. It is likely that the Hercynian granitic 

intrusion and regional metamorphic events have reset all 

biotite ages in the Bayan Obo mine region. 

MINERAL PARAGENETIC SEQUENCE 

The key evidence for the epigenetic origin of the 

Bayan Obo ore deposit, in addition to the mineral textural 

relations, lies in the precise ages of episodes of mineralization of various generations of monazites and bastnaesites, 

and the minimum mineral ages of alkali amphiboles (fig. 

33). The more than 100 carefully chosen doubly polished 

thin sections that were studied in detail provided the textural relations of both gangue and ore minerals replacing 

not only the host rocks but also ore and gangue minerals in 

various assemblages of disseminated and banded ores. A 

few of the selected replacement textures are shown in figures 14-17. Such textural data are corroborated with chemical compositions and isotopic mineral ages of the various 

generations of both alkali amphiboles and REE minerals. 

The resulting generalized mineral paragenetic sequence, 

updated from Chao and others (1993), is given in table 13. 

Part II. CALEDONIAN SUBDUCTION AS A POSSIBLE CAUSE AND 

MECHANISM OF REPEATED HEATING, FRACTURING, AND 

HYDROTHERMAL MINERALIZATION 

SUBDUCTION OF THE 

MONGOLIAN PLATE 

Subduction of the Mongolian oceanic plate during 

Caledonian time (about 555-396 Ma) may have been a 

mechanism for repeated heating, fracturing, and hydrothermal mineralization at Bayan Obo (Chao and others, 1995). 

The tectonic setting of the region that includes the Bayan 

Obo mine area was discussed by Zhao and Li ( 1987), Wang 

and others (1991), and Tang and Yan (1993). These three 

reports do not fully agree in either description or available 

isotopic age data. As a consequence, the history and ages of 

tectonic development and regional metamorphism of Inner 

Mongolia remain obscure. Zhao and Li ( 1987) outlined a 

five-stage plate tectonic model for the North China cratonal 

margin in Inner Mongolia during the Paleozoic (fig. 34). As 

a result of oceanic spreading, the Mongolian oceanic plate 

moved southward during the Paleozoic, causing two episodes of subduction of the Mongolian oceanic plate underneath the North China cratonal margin. The earliest 

subduction began at the end of the Xin Kai stage of the late 

Early Cambrian and possibly extended to the early Caledonian. This period of subduction shortened the eugeosynclinal sedimentary sequence of the Xin Kai stage and was 

accompanied by continental granitic intrusions. The second 

phase of subduction was near the end of the Middle Silurian 

and consisted of high-angle rapid subduction of the Mongolian plate, resulting in formation of the Bainaimiao island 

arc and accompanying island-arc granitic rocks. 

Wang and others (1991) described, as resulting from 

subduction, a 1 ,800-m-thick ophiolite unit that extends 

east-west for a distance of about 100 km at Ondor Sum 

(Wenduermiao), about 275 km east-northeast of Bayan Obo 

(fig. 4). This ophiolite unit consists, in ascending order, of 

200 m of metamorphosed harzburgite, 400 m of cumulate 

gabbros, 200 m of sheeted diabase dikes, 500 m of basic pillow lavas, and 500 m of abyssal sedimentary rocks. Wang 

and others (1991) reported that fossils found in the abyssal 

sedimentary rocks are late Paleozoic to Cambrian in age, the 

K-Ar age of amphibole in metagabbro is 632-626 Ma, and 

the metamorphic age of blueschists in the ophiolite series is 

435±61 Ma. The ophiolite sequence is unconformably overlain by Upper Silurian beds containing coral fossils. 

Two types of granitic rocks are associated with the 

gabbroic rocks of the ophiolite sequence found in the Ondor 

Sum region between Tulinkai and Deyenqimiao. One type 

consists of small bodies of grayish-white, very fine grained 

plagioclase granite, containing about 60 volume percent of 

oligoclase-andesine plagioclase feldspars, about 30 volume 

percent quartz, 5 percent K-feldspars, 1 percent biotite, and 

accessory amounts of zircon and magnetite (Zhao and Li, 

1987). The other type consists of rare exposures of granophyre associated with pillow lavas in the ophiolite suite. 

The granophyres are red and fine grained, having intergrowths of K-feldspar and quartz (15-20 volume percent), 

plagioclase with Carlsbad twins, and about 5-7 volume percent small scattered grains of quartz. The granophyre also 

contains 5-10 volume percent of dark minerals, mainly epidote and chlorite, and the accessory minerals magnetite, 

hematite, apatite, zircon, and rutile. 

Tang and Yan (1993) compiled geologic and tectonic 

sketch maps of central Inner Mongolia and a model for the 

Proterozoic to middle Paleozoic plate tectonic evolution of 

the Inner Mongolian suture zone. They also described the 

ophiolite suite of the Ondor Sum (Wenduermiao) region and 

interpreted it to be a residual fragment of early Paleozoic 

oceanic crust. According to their description, the ophiolite 

consists of a lower (> 700-m-thick) gabbro and diabase, a 

middle sequence of sheeted dike swarms and pillow lavas, 

and an upper sequence of tuffaceous sediments and siliceous-argillaceous rocks. Tang and Yan further reported the 

metabasalt as having a Rb-Sr (whole-rock isochron?) age of 

509±40 Ma. However, east of Ondor Sum, in the Kedanshan-Wudaoshimen district on the northern side of the Xar 

Moron River, they found many ophiolite blocks containing 

chert with Ordovician microfossils. 

Although more isotopic age data are needed, the simple model proposed by Zhao and Li (1987), slightly modified and reproduced in figure 34, provides a hypothetical 

mechanism for repeated heating, fracturing, and hydrothermal mineralization that accounts for the episodes of REE 

mineralization of the giant Bayan Obo REE-Fe-Nb ore 

deposit. The Caledonian subduction can be traced for an 

east-west distance of a little more than 10° longitude, or 

approximately 850 km. The extent of Caledonian subduction is of interest with regard to other possible giant polymetallic ore deposits that are known or reported to occur 

along the northern margin of the North China craton. The 

more extensive Hercynian subduction occurred north of the 

trace of Caledonian subduction (fig. 4) and was accom55 
56 GIANT BA YAN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

---------- .. , ...... ~ 

, ~ - . 

Land mass 

• 

===~- Initial ocean spreading 

stage 

Early subduction stage 

Late subduction stage 

Terminal subduction stage 

Figure 34. Schematic diagram proposed by Zhao and Li (1987), showing five stages of north 

to south subduction of the Siberian oceanic plate under the northern margin of the North China 

craton. 

500.-------------------------------------------------------------------~ 

(/) 

2 480- (/) 

__J 

w 

u 

(/) 

w 

8853-1 

90Y0075 

11-'E::------------------- 1- 455.0±3.4 -----------------..:::;;..!-

~ 460 - 840 880 -r---~-~Q ___ t 1030 _f _______ ,_~Q9 _______ _ 

~ ~ ~~~~~~~~~~~~========================================= __________ [~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -------------------

w 

0:: 440 - ::> 

I-

<( 

0:: - w 

a_ 

~ 420 - w 

I400 I I I I I I I I I 

0 10 20 30 40 50 60 70 80 90 100 

39Ar RELEASED, CUMULATIVE PERCENT 

Figure 35. The 39 Ar release pattern of incremental heating of hornblende from a quartz monzonite rock, sample 8B53-1, collected near 

Hejao, showing a plateau 40 Arf39 Ar Caledonian age of 455.0±3.4 Ma (J.E. Conrad and E. H. McKee, unpub. data, 1990). 
GENERATION OF THE CALEDONIAN HEJAO PLUTONIC ROCKS 57 

panied by widespread granitic intrusion. The importance of 

the Hercynian subduction with regard to metallogenic ore 

deposits has not been demonstrated. 

The history of intermittent, episodic REE mineralization, based on the general paragenetic sequence and ages of 

mineralization, would require repeated and alternating periods of fracturing (tensional tectonics) and compression 

(regional metamorphism). Compression was required to 

produce heating and generation of hydrothermal solutions 

to scavenge and concentrate REE's from the deep crustal 

source area. Fracturing was required to provide channels for 

the hydrothermal solutions to ascend to near the surface to 

produce the giant Bayan Obo REE deposit. 

GENERATION OF THE 

CALEDONIAN HEJAO PLUTONIC 

ROCKS AND THE SOURCES OF METALBEARING HYDROTHERMAL SOLUTIONS 

An east-trending belt of granitic plutonic rocks about 

20-30 km wide and at least 120 km long, about 50 km south 

of Bayan Obo (fig. 1) has a tentative Paleozoic (Caledonian) age based on the age of similar granitic rocks overlain by Silurian sedimentary rocks north of the Kuanggou 

fault. According to Zhao and Li (1987), these granitic rocks 

follow a deep major fault-fracture zone. They intruded 

metamorphic rocks of the Bayan Obo Group and were, in 

tum, intruded by Hercynian granitic rocks. The only petrographic description available for the rocks in this belt is that 

of Zhao and Li ( 1987), which shows the rocks to consist 

mainly of granodiorite, monzonitic granite, and quartz 

biotite diorite. The plutonic rocks are chiefly grayish to pale 

yellowish, fine to medium grained, and foliated and have 

poorly developed granular texture. Mineral compositions 

vary widely; the rocks consist of 45-65 volume percent plagioclase (oligoclase-andesine), 0-30 volume percent Kfeldspar (perthite), a few to 30 volume percent quartz, and 

2-10 volume percent mafic minerals (biotite or hornblende). 

Accessory minerals are magnetite, ilmenite, zircon, apatite, 

garnet, and tourmaline. 

We collected several samples of pink, medium-grained 

quartz monzonite or monzonitic granite (for example, sample 8B53-1) from outcrops near Hejao (fig. 1). They contain fresh black hornblende 0.5-1 em across and scattered 

small zircon crystals. The 39 Ar incremental heating release 

pattern of hornblende from sample 8B53-1 shows a plateau 

age of 455.0±3.4 Ma (fig. 35; J.E. Conrad and E.H. McKee, 

unpub. data, 1990). Thirteen small zircon crystals were also 

separated from this sample. The best estimated U-Pb age of 

the zircon, 451 ± 17 Ma (see details in Wang and others, 

1994 ), is in good agreement with the 40 ArP9 Ar age of 

Ordovician (Caledonian) hornblende. 

Zhao and Li ( 1987, table 1) also published several 

chemical analyses of the granitic rocks of Hejao. Among 

them are two analyses with high total alkalis (values in wt. 

percent): 

Si02 ......................... . 

Ti02 .......................... . 

Al20 3 ...................... .. 

Fe20 3 ...................... .. 

FeO ........................ . 

MnO ..................... .. 

MgO ..................... .. 

CaO ...................... .. 

Na2o ......................... . K2o .......................... .. 

P20s ......................... . 

Loss on ignition .... . 

Sample number 

513 415 

77.31 

.05 

11.47 

.37 

.41 

.02 

.35 

.19 

1.82 

7.27 

.00 

.24 

75.72 

.16 

13.26 

.15 

.45 

.02 

.33 

.51 

1.88 

6.60 

.00 

.55 

According to Eby (1990), A-type (denoting either anorogenic or anhydrous) granitoids are characterized by their 

relatively high alkali contents, low CaO contents (at 

Si02>70 percent; Na20+K20=7-11 percent, and Ca0<1.8 

percent), and high FeO/MgO values. A-type granitic rocks 

were emplaced into non-orogenic settings, both within 

crustal plates and along plate margins, during the waning 

stages of subduction-zone-related magmatism. The above 

chemical compositions of Hejao granite samples 513 and 

415 meet the criteria of high total alkalis at Si02 greater 

than 70 percent, and low CaO content, but they do not meet 

the criterion of high FeO/MgO values. Zhao and Li ( 1987) 

classified three other granitic rocks as S-type granites. On 

the basis of the average o180 value of 10.55 per mil (%o) for 

whole-rock analyses of three Hejao types of granitic rocks, 

Zhao and Li concluded that they are characteristic of continental crustal origin. The Hejao granitic rocks, however, 

contain a total REE content ranging from 36 to 58 ppm 

(Zhao and Li, 1987) and are unrelated to the hydrothermal 

sources rich in REE 's for the REE mineralization of the 

Bayan Obo ore deposit. 

The Hejao granitic plutonic complex should be studied 

in greater detail with regard to field occurrences, petrology, 

geochemistry, and isotopic geochronology. We suggest that 

the Hejao granitic plutonic rocks, an age of 455±3.4 Ma, 

and the hydrothermal solutions that were enriched in REB's 

could have been related to Caledonian subduction of the 

Mongolian-Siberian oceanic plate beneath the North China 

craton that occurred at 555-395 Ma, as outlined by Zhao 

and Li (1987) and Tang and Yan (1993). The crustal sources 

for the Hejao granitic rocks and the REE hydrothermal 

solutions may have been quite different because the Hejao 

granitic rocks are about 50 km south of the Bay an Obo mine 

region. 

Part III. BAYAN OBO AS A CORNERSTONE EXAMPLE FOR STUDYING 

GIANT POLYMETALLIC ORE DEPOSITS OF HYDROTHERMAL ORIGIN 

Doe (1993) examined giant ore deposits using criteria 

parallel to evaluation of giant oil fields with respect to 

sources of the ore. Parameters related to the occurrence of 

giant ore deposits were also the subject of a workshop 

(Whiting and others, 1992). 

In order to investigate, evaluate, and understand the 

characteristics favorable to the formation of giant and super 

giant polymetallic ore deposits of hydrothermal origin, 

regardless of the types of ores present, it is necessary to 

obtain pertinent data for the following parameters: 

(1) sources of the ores, (2) tectonic setting, (3) mechanism 

for concentrating and transporting the ore-bearing hydrothermal fluids, ( 4) mineral paragenetic sequence and 

geochemical evolution of the deposit, (5) mineral assemblages and the transport of ore by hydrothermal fluids, 

(6) age or ages of episodes of mineralization, (7) lithology 

of reservoir rocks favorable for hosting the ores, and (8) 

preservation of the ores. This spectrum of data may not be 

available for many giant or super giant ore deposits. Here, 

we focus the discussion of the above parameters on the 

basis of findings from the Bayan Obo giant ore deposit. We 

then cite for possible comparison the giant and super giant 

porphyry Cu-Mo deposits of Peru and Chile in South America. Although these are different types of deposits, they 

share some characteristics with Bayan Obo. 

SOURCES OF THE ORES 

Both the geologic setting and isotopic signatures of the 

REE ore minerals suggest a crustal source for REE. The formations underlying the Proterozoic Bayan Obo Group consist of Archean and Early Proterozoic granitic and 

migmatitic gneiss intruded by pegmatitic dikes and stocks. 

According to the Institute of Geochemistry (1988, p. 505), 

the Early Proterozoic granitic rocks and the widespread 

Early Proterozoic pegmatites contain relatively high contents of RE20 3, which for the granitic rocks range from 800 

to 1,308 ppm, notably higher than the average of about 290 

ppm REz03 for granitic rocks elsewhere. The granite-pegmatites in the area of Hayehhutung locally contain more 

than 10 volume percent allanites, some of which occur as 

round or lenticular bodies exceptionally high in LREE's 

(Zhao Zhenhua, oral commun., 1993). Some of the pegmatitic rocks contain Nb-Ta minerals. The only direct evidence 

we found with regard to REz03 sources was the occurrence 

of above-normal concentrations of RE20 3 in detrital apatite 

(see sample 7B25-1, table 5) from coarse-grained H8 dolostone marble. 

The abundance of allanite in Early Proterozoic rocks in 

the Bayan Obo area, and the widespread low- to minablegrade monazite disseminated in ores throughout the 16 km 

by 2-3 km extent of the mine region, led investigators of the 

Institute of Geochemistry ( 1988) to favor a syngenetic origin for the Bayan Obo REE ores. This interpretation is not 

supported by our studies (Chao and others, 1992, 1993; 

Wang and others, 1994; this paper). 

Pb and Nd isotopes from Bayan Obo ore and gangue 

minerals, analyzed by Wang and others (1994), indicate a 

crustal origin for the REE's at Bayan Obo. Typically, the 

206Pbf204Pb ratios are low and the 208Pbf204Pb ratios are high 

for the REE-bearing and REE minerals. In the same suite of 

REE minerals, U is strongly depleted with respect to Th, 

and monazite, bastnaesite, REE-bearing apatite, and aegirine augite have high negative eNd values ( -16 to -19). These 

isotopic signatures clearly indicate a crustal source for 

Bayan Obo REE ores. 

TECTONIC SETTING 

Caledonian subduction, related to REE mineralization 

at Bayan Obo, has been discussed in parts I and IT (figs. 4 

and 34). 

MECHANISM FOR CONCENTRATING 

THE ORES IN SOURCE RESERVOIRS 

We propose that the REE's and Fe were scavenged and 

concentrated from the underlying crust by acidic hydrothermal solutions activated by Caledonian subduction. In addition, repeated fracturing and shearing during the Paleozoic 

Era produced the channels for the hydrothermal solutions to 

bring the REE's and Fe from deep crustal source areas and 

deposit them mostly in the H8 host marble. On the basis of 

the experimental studies of Remley and others ( 1992), we 

suppose that such long-distance travel by hydrothermal 

solutions enriched with scavenged REE's and Fe is feasible 

and probable. 

59 
60 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

MINERAL PARAGENETIC SEQUENCE 

AND GEOCHEMICAL EVOLUTION OF 

THE DAYAN OBO ORE DEPOSIT 

The geochemical evolution of the Bayan Obo ore 

deposit can be deduced from the paragenetic relations of the 

hydrothermal minerals (table 13). REB's evidently were the 

first ore elements introduced, at about 555 Ma, along with P 

and F. The solutions containing these elements had random 

spatial and time distribution and produced disseminated 

ores containing monazite or bastnaesite or mixtures of these 

two major REE minerals. These disseminated REE ores 

were followed by the formation of REE banded ores, and 

later by Fe20 3 (hematite) ores, which together with martite 

and magnetite formed the massive Fe cores of the Main and 

East Orebodies. 

During the peak periods of ore formation, REE's, Fe, P 

(to form monazite and apatite), Na, Si (to form aegirine 

augite), F (to form bastnaesite and fluorite), and Ba (to form 

barite) were brought in by different hydrothermal solutions 

in order to produce the wide variety of banded ores. Na, K, 

and Si (to form alkali amphiboles) were also introduced 

during the peak and waning phases of REE mineralization, 

but deposited in different parts of the orebodies than the 

above assemblage. During this period, U and S were 

uniquely missing as geochemical components. 

During late-stage hydrothermal activities, Nb took on 

increasing importance. Nb accompanied and followed Fe 

mineralization, and was accompanied and followed by 

minorS activity (to form gangue sulfides). The last stage of 

hydrothermal activity consisted of Ba and K metasomatism, 

which was widespread in the H9 biotite schist (Drew and 

others, 1990). 

Geochemically, the Bayan Obo ore deposit was 

enriched by ore metals in the sequence REE's, Fe, and Nb; 

and the gangue elements P, F, and Ba. Bayan Obo is characterized by the scarcity or lack of U, S, Ta, AI, the precious 

metals Au and Ag, and base metals Cu, Pb, and Zn. 

MINERAL ASSEMBLAGES AND THE 

TRANSPORT OF REE'S IN 

HYDROTHERMAL FLUIDS 

Humphries (1984) stated that transport of REE's in 

hydrothermal fluids for any great distance is probably 

accomplished by the formation of carbonate, fluoride, or 

sulfate complexes. In addition, experimental studies cited 

by Humphries (1984) indicate that for most REE's, 

RE(Cl,F)2 + has been identified as the most stable complex 

in acidic solutions rich in CI- or p- ions, but RE(Cl,F)3 is 

most stable for La. The presence of fluoride significantly 

increases the mobility of the REE's. 

For the early stage of disseminated monazite in ores 

hosted by H8 marble, REE's clearly were associated with 

only P anions, and therefore the acidic REE hydrothermal 

solutions were enriched in P ions only. In the case of the 

early-stage precipitation of disseminated bastnaesite, F was 

introduced, so that the most stable complexes could have 

been RE(Cl,F)2 + and RE(Cl,Fh. 

Monazite and bastnaesite in banded ores could also 

have resulted from precipitation of complex ions of carbonates or fluorides from the hydrothermal fluids; the bands of 

monazite and bastnaesite are probably unrelated to the associated fluorite bands, which are in general later in the 

paragenetic sequence of the assemblage. The early stage of 

disseminated REE mineralization and the peak period of 

REE mineralization in banded ores were apparently not 

related to any alkali cations because aegirine augite and 

alkali amphiboles, if present, always belong to later stages 

in the paragenetic sequence of those assemblages. 

Although casual inspection of the complex Bayan Obo 

deposit may lead to the conclusion that the associations of 

alkali amphiboles, aegirine augite, and REE minerals are 

related to a carbonatite magma, the experimental data from 

Humphries ( 1984) and the younger ages of the amphiboles 

and augite with respect to associated REE minerals refute 

this. Multiple episodes of REE mineralization span a period 

of about 150 million years at Bayan Obo (Wang and others, 

1994), and, therefore, invoking a carbonatite source for 

REE's would also require evidence to indicate carbonatite 

magmatic activity lasting 150 million years. Bayan Obo, 

thus, is a prime and valuable example of the importance of 

paragenetic sequence, rather than mineral assemblages, in 

discerning the history and origin of a deposit, as well as the 

fact that associations of REE minerals, alkali amphiboles, 

and aegirine augite need not invoke a carbonatite source. 

We were unable to obtain any information on the 

nature of the fluids from fluid inclusions. We found no fluid 

inclusions in any of the REE minerals we have studied. 

Fluid inclusions are most abundant in late-stage barite, 

which is essentially unrelated to specific episodes of REE 

mineralization. Minute fluid inclusions are also present in 

fluorite. Limited data on such inclusions were published by 

the Institute of Geochemistry (1988). 

AGE OR AGES OF EPISODES OF 

MINERALIZATION 

Bayan Obo is uniquely informative with respect to precise ages of REE mineralization and relative sequence of Fe 

and Nb mineralization. Episodes of REE mineralization 

began about 555 Ma and lasted until about 400 Ma, a period 

of about 150 m.y. Fe mineralization began about 430 Ma 

and lasted until after about 343 Ma. Bayan Obo is also 

unique in that the complex metamorphic and metasomatic 

histories can be documented by minimum mineral ages of 
BAY AN OBO AND THE PORPHYRY Cu-Mo ORE DEPOSITS OF PERU AND CHILE 61 

the alkali amphiboles present, which cover a range from 

about 1.26 Ga to about 300 Ma. 

LITHOLOGY AND AGE OF 

RESERVOIR ROCKS FAVORABLE 

FOR HOSTING THE ORES 

Although the H8 marble is not the only rock type that 

hosts Bayan Obo ores, it is clearly the dominant host rock. 

As marble, the reservoir host rock was particularly reactive 

with acid hydrothermal solutions, producing C02 when the 

carbonate was dissolved by the acid solutions. Contemporaneously, the metals precipitated from the solution because 

of cooling and change of pH when the acid solution was 

neutralized. Clearly, carbonate host rocks rank as the most 

favorable reservoir host rocks for these ores. 

Besides lithology, permeability of the host rock is also 

an important property to be considered. Textural evidence 

from Bayan Obo ores suggests that replacement of the host 

rock minerals was predominantly along grain boundaries 

and along microfractures. When extensive metasomatic 

replacement occurred, the host rock became completely 

replaced, as shown by some of the banded ores. 

The age of the host rock is apparently also important. 

H8 is of Middle Proterozoic age, which existed prior to 

Caledonian episodes of mineralization. Any carbonate host 

rocks younger than the Caledonian would not have been 

mineralized. Hence, Proterozoic carbonate host rocks overlying Archean and Early Proterozoic source rocks, in conjunction with younger tectonic processes such as 

subduction or rifting, represent a good combination for the 

formation of these ore deposits. 

PRESERVATION OF THE ORES 

(CAP ROCKS) 

Another favorable condition for the formation of the 

Bayan Obo ores results from the presence of H9 biotite and 

albitized biotite schist, which is pelitic and less permeable 

than the underlying H8 host rocks. Although not perfectly 

impermeable, the H9 biotite schist acted as a cap rock and 

slowed down or stopped the escape of the incoming acid 

hydrothermal solutions. The ores precipitated by the solutions were therefore confined and preserved. 

BAYAN OBO AND THE PORPHYRY Cu-Mo 

ORE DEPOSITS OF PERU AND CHILE 

In order to gain further insight regarding giant and 

super giant ore deposits of hydrothermal origin, it may be 

useful to compare the Bayan Obo deposit with the Andean 

giant and super giant porphyry Cu-Mo ore deposits of South 

America from Peru to Chile, although they are different ore 

types. Whether the key factor of giant ore deposits is the 

tectonic setting that sustains the long duration of mineralization should be carefully examined and tested. 

Using the National Academy of Science size classification of ore deposits, Clark (1993) listed two Andean 

behemothian (>31 million metric tons of Cu) porphyry Cu 

ore deposits (El Tiniente and Chuquicomata) and seven 

super giant (>10 million metric tons ofCu) porphyry Cu ore 

deposits (El Abra, Chuqui Norte, Los Pelambres--El 

Pachon, Rio Blanco-Los Bronces, La Escondita, Collahuasi-Rosario, and Mansa Mina). However, from the economic geology point of view, the best studied Andean 

porphyry Cu-Mo deposit is the giant El Salvador, which is 

used as a standard for this type of ore deposit (Gustafson 

and Hunt, 1975). Of particular interest is the long period 

over which these deposits formed, which, on the basis of the 

ages of the porphyries, lasted approximately 95 m.y. (100-5 

Ma) (Clark, 1993, fig. 10). 

On the basis of the study of El Salvador, most investigators favor the interpretation that the subvolcanic porphyries with which these giant, super giant, and behemothian 

Cu-Mo ore deposits are associated were related to the subduction of the Nazca (Farallon) oceanic plate (Clark, 1993, 

fig. 12). Hydrothermal solutions that deposited Cu-sulfide 

veins were directly related to the mantle-derived melt that 

resulted in crystallization of the porphyry stocks close to the 

surface. Clark, using a study by Pardo-Casas and Molnar of 

1987 (cited in Clark, 1993), summarized the timing of mineralization with respect to the displacement of the Nazca 

(Farallon) plate relative to South America as follows: (a) 

before and during early Eocene (62-52 Ma), porphyry copper emplacement in southern Peru and northernmost Chile; 

(b) before and during late Eocene and early Oligocene (42-

31 Ma), emplacement in northern Chile; and (c) since 10.59 

Ma, emplacement of central Chilean porphyry Cu deposits. 

Thus, regionally the formation of Andean porphyry ore 

deposits migrated from north to south within about 50 million years. 

The Andean porphyry Cu-Mo ore deposits are no 

doubt of hydrothermal origin because the principal ore metals occur mostly in two of three types of veins in the marginal zones of porphyries, according to Gustafson and Hunt 

( 1975). The most important and the earliest Cu mineralization at El Salvador formed the A veins disseminated with 

chalcopyrite-bornite and locally, traces of molybdenite. B 

veins crosscut A veins and were clearly later. B veins characteristically contain molybdenite-chalcopyrite and traces 

of bornite in some of the veins. D veins, poor in quartz, 

crosscut both A and B veins and are the youngest veins. 

Pyrite is usually predominant in D veins, and chalcopyrite, 

bornite, enargite, tennantite, sphalerite, and galena are also 

present. Minor molybdenite and many other sulfides occur 

locally. 
62 GIANT BAY AN OBO REE-Fe-Nb ORE DEPOSIT OF INNER MONGOLIA, CHINA 

Gustafson and Hunt (1975) concluded that, based on 

isotopic age data, the period of mineralization in the veins 

and associated porphyry is essentially less than a million 

years, not resolvable by the K-Ar method of dating available at the time of their study. Redating the veins and porphyries using the 40 Arf39 Ar and Re-Os methods may 

provide additional information about the mineralization history at El Salvador. The porphyry stocks that intruded the 

volcanic-covered terrain required time to cool before fracturing could occur to provide openings for the deposition of 

Cu ore in veins. If one assumes the cooling rate of a porphyry stock to be 50-10°C/m.y. and if the original meltmush was at a temperature above 800°C, it would require 

several million years to cool before fracturing could occur. 

Therefore, dating the porphyries with the 40 Arf39 Ar method 

or with Rb/Sr mineral isochrons and dating the age of the 

veins directly using the Re-Os method on Cu-sulfides and 

molybdenite may provide new insights about the duration 

of mineralization at El Salvador. 

The basic question of interest is whether the hydrothermal solutions necessarily are related to the generation of the 

porphyry melt by subduction of the Nazca (Farallon) plate. 

Subduction-related heating could easily have generated the 

porphyry melt and the hydrothermal solutions enriched in 

sulfides at essentially the same time, or the hydrothermal 

solutions may have been generated several million years 

later than and independent of the melt. 

Bayan Obo appears to have some aspects in common 

with the porphyry Cu-Mo ore deposits of the Andes: (1) 

they both lie along the margin of a craton affected by subduction of an oceanic plate; (2) they both have a long duration of formation of ore, covering a span of more than 50 

m.y.; and (3) they are both of hydrothermal origin. The 

principal differences are ( 1) they are different deposit types; 

(2) Bayan Obo episodes of mineralization were restricted to 

a single deposit, whereas the Andean porphyry Cu-Mo 

deposits, representing a span of more than 90 m.y., consist 

of many separate giant and super giant deposits of similar 

types; and (3) evidence for Caledonian subduction in the 

Bayan Obo area is restricted to an 850-km trace, whereas 

according to Clark ( 1993 ), the location of subduction of the 

Nazca (Farallon) plate under the Andean crust moved from 

north under Peru, to south under southern Chile, to east 

under northern Chile. This movement would account for the 

sequence and locations of the porphyry Cu-Mo deposits of 

the Andean belt. 

In the case of Bayan Obo, there is no evidence of any 

igneous melt related to the hydrothermal solutions that 

deposited the REE's, Fe, and Nb of that deposit. The key 

lies in the source area, where the subduction-related heat 

generation caused scavenging of ore metals from the lower 

crust and enrichment of ore metals in the hydrothermal 

solutions. Within the mine region (16 km by 2-3 km) at 

Bayan Obo, multiple episodes of REE mineralization, 

hypothetically generated by Caledonian subduction, lasted 

more than 150 m.y. With regard to the Andean porphyry 

Cu-Mo deposits, episodic subduction (from before 60 Ma to 

about 5 Ma) activated the mantle-crust source region to produce the world's largest porphyry Cu-Mo ore deposits. 

One may also wish to compare Bayan Obo and the 

Andean porphyry Cu-Mo ore deposits with the Sn granite 

ore deposits of Bolivia, central South America, or other 

areas of the world, to see if activities in certain tectonic settings, such as rifting or subduction, can cause repeated generation of either ore-related igneous melts or ore-related 

hydrothermal solutions. Similarities of types of ore deposits 

are not necessarily required as a key to the origin of giant 

ore deposits in favorable tectonic settings. 
PART IV. SUMMARY AND CONCLUSIONS AND REFERENCES 

SUMMARY AND CONCLUSIONS 

In part I, we described the characteristics and paragenesis of the giant Bayan Obo REE-Fe-Nb ore deposit: 

1. Bayan Obo is on the northern edge of the North China 

craton, near the suture of Caledonian subduction of the 

Mongolian plate beneath the craton. 

2. The host H8 marble and quartzite are sedimentary rocks 

that were metamorphosed prior to mineralization. 

3. The deposit is mineralogically very complex, and ore 

classification is based on host rocks, ore type (disseminated, banded, or massive), and mineral assemblages. 

The Main and the East Orebodies are surrounded by disseminated ores· and have intermediate zones of banded 

ores and massive Fe cores. 

4. Metamorphic and replacement textures provide evidence 

for the epigenetic, hydrothermal, metasomatic origin of 

the Bayan Obo ore deposit. 

5. Precise ages of episodes of REE mineralization, supported by chemical compositions of various generations 

of monazites and bastnaesites, indicate that the episodes 

of REE mineralization lasted from about 555 to 395 Ma, 

a duration of about 150 m.y. 

6. Textural relations among the three main Fe ore minerals, 

precise ages of REE minerals, and minimum ages of 

associated alkali amphiboles of metasomatic origin indicate that most Fe mineralization occurred between about 

430 and 390 Ma. Most Nb mineralization resulted in disseminated ores associated with the West Orebodies. 

7. Textural relations, mineral ages, and chemical composition of various generations of REE's and alkali amphiboles indicate three periods of regional metamorphism: 

Late Proterozoic, early Paleozoic (Caledonian), and late 

Paleozoic (Hercynian). These data also provide evidence 

of hydrothermal activities ranging from 1.26 Ga to 

beyond 343 Ma. 

8. The complex history of the Bayan Obo deposit, from the 

deposition of the H8 carbonates (>1.26 Ga) to the end of 

Hercynian (Permian) granitic intrusion (about 260 Ma), 

is illustrated by the generalized mineral paragenetic 

sequence (table 13). 

In part II, we described and discussed evidence for 

Caledonian subduction, origin of the Caledonian Hejao granitic rocks 50 km south of the Bayan Obo mine region, and 

the probability that Caledonian subduction and regional 

metamorphism provided a repeated activation mechanism 

and heat source that helped to generate not only REE- and 

Fe-rich hydrothermal ore fluids of Bayan Obo, but also the 

A-type and S-type anorogenic Hejao granitic rocks. 

In part III, we discussed the giant Bayan Obo REE-FeNb ore deposit as a cornerstone example of a giant ore 

deposit in terms of the following parameters: (1) the Early 

Proterozoic and crustal sources of REE's, Fe, and Nb on the 

basis of (a) occurrence of large amounts of LREE-rich 

allanites in Early Proterozoic gneisses and pegmatites, and 

(b) isotopic indicators of large negative eNd values; (2) the 

favorable tectonic setting for mineralization during the 

Caledonian orogeny; (3) the probable mechanism for concentrating ore metals in the hydrothermal solutions in 

source reservoirs; (4) the geochemical evolution, based on 

the mineral paragenetic sequence; (5) the mode of transport 

of REE complexes from the lower sialic crust to the H8 unit 

of the Bayan Obo Group; (6) the ages of episodes of mineralization; (7) the Middle Proterozoic H8 marble as the 

favorable reservoir rocks hosting the Bayan Obo ores; and 

(8) the less permeable H9 pelitic biotite and albitized biotite 

schist as cap rocks that probably preserved the ore within 

the upper part of the H8 marble host rocks. 

Bayan Obo and the Andean giant and super giant porphyry Cu-Mo ore deposits were both formed in subduc1tion 

zone environments over long periods of time. Although the 

ore types differ, comparison of these deposits may help to 

increase our understanding of the paragenesis of giant polymetallic deposits. 

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9 "'~~~~'II~~ IJ~ ~~Ill