U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

Aspects Of Light Rare Earth Element Resources, Bayan Obo, China

By

David M. Sutphin

and 

Lawrence J. Drew

Open-File Report 98-125

This report is preliminary and has not been reviewed for conformity with U.S. 

Geological Survey editorial standards (or North American Stratigraphic Code).
Aspects Of Light Rare Earth Element Resources, Bayan Obo, China

By David M. Sutphin and Lawrence J. Drew

Abstract

In this report, the light rare earth oxides are defined and their use in 

metallurgy, magnets, ceramics, electronics, chemicals, and other applications 

is discussed. Currently, almost the entire world's supply of these 

commodities are produced from only two deposits Bayan Obo in China and 

Mountain Pass in California. The essential characteristics of the Bayan Obo 

deposit are discussed in light of the larger family of deposits that compose 

Olympic Dam-type deposits, and specific geological factors are cited to explain 

its enormous size.

Introduction

As a result of recent economic developments, the world's supply of 

light rare earth elements comes from only two sources Bayan Obo in China 

and Mountain Pass in California. Environmental concerns, however, are 

threatening the closure of the only U.S. producer of these elements. The 

mineralogy of both deposits is such that the light rare earth elements are the 

most important of the rare earth elements. In this report, the light rare earth 

elements, some of the uses to which they have been applied, their mineralogy 

and geology, and their resources are briefly described.

The rare earth elements are the 15 chemically similar elements that 

range from lanthanum through lutetium. Often, yttrium, scandium, and 

thorium are considered as a part of the rare earth elements. This group is 

sometimes referred to as the "lanthanides", because lanthanum is the first in 

the series. The upper one half of this series is known as the "light" rare earth 

elements or "cerium" subgroup, and the lower one half is known as the 

"heavy" rare earth elements or "yttrium" subgroup. The eight light rare 

earth elements (table 1) include lanthanum (La), cerium (Ce), praseodymium 

(Pr), neodymium (Nd), promethium (Pm), samarium (Sa), europium (Eu), 

and gadolinium (Gd). Promethium does not have a stable isotope and is not 

found in nature.

Table 1 near here
Generally, the rare earth elements are not rare in the Earth's crust, and 

this is especially true for the light rare earth elements. In average crustal 

abundance, cerium, the most abundant of the rare earth elements, is more 

plentiful than nickel [58 parts per million (ppm)] and copper (47 ppm). 

Neodymium and lanthanum are more plentiful than cobalt (18 ppm) and lead 

(16 ppm). Praseodymium, samarium, and gadolinium, are more plentiful than 

tin (2.5 ppm). Europium, which is the scarcest of the naturally occurring of the 

rare earth elements, is about one half as abundant as tin (Jackson and 

Christiansen, 1993).

Uses of the light rare earth elements

The following discussion of the light rare earth elements in the form of 

rare earth oxides (REO) is based on summaries by Adams and Staatz (1973) 

and Jackson and Christiansen (1993). The light rare earth elements form the 

core of a small but increasingly important industry that began in 1883 when 

the incandescent gas mantle was developed. This mantle contained 1 percent 

ceria (CeO2) and 99 percent thoria (ThO2) and was in commercial production 

by 1891 (Neary and Highley, 1983). At the turn of the century, lighter flints 

that contained a complex alloy of rare earth metals, so-called "mischmetal," as 

the main ingredient were being manufactured. By the late 1940's, rare earth 

elements were an ingredient of ductile iron. In the late 1960's and early 

1970's, there was a major surge in demand for rare earth elements to be used 

in high-strength low-alloy steels and oil-refinery catalysts. In 1995, the 

approximate distribution of all rare earth production by end use was: 

automotive catalytic converters, 44 percent; petroleum-refining catalysts, 25 

percent; permanent magnets, 11 percent; glass polishing and ceramics, 9 

percent; metallurgical additives and alloys, 8 percent; phosphors, 3 percent; 

and miscellaneous, less than 1 percent (Hedrick, 1997).

Present and potential applications of the light rare earth elements 

include metallurgy, magnets, ceramics, electronics, chemical, optical, medical, 

nuclear, and other uses. Lanthanum is used primarily in ceramic glazes and 

optical glass, such as camera lenses. Cerium, the chief constituent of 

mischmetal, is used in production of steel and other metals, where it 

improves the strength and ductility of aluminum alloys and raises the hightemperature properties of magnesium alloys. It is used also as radiation 

shielding in cathode-ray tube face plates, petroleum refinery catalysts, and 

ceramic capacitors and in heat-resistant alloys in jet engines. Praseodymium 

is a constituent of ceramic tiles, capacitors for electronics, and permanent 

magnets. It is mixed with neodymium for use in glassmaker's goggles. 

Europium, is used in colored lamps and cathode-ray tubes, where its oxide 

provides the red phosphor for color television. Europium is an essential 

constituent of control rods in nuclear reactors because it absorbs neutrons. 

Samarium is used mainly in permanent magnets and in glass lasers. 

Gadolinium is an essential component of solid-state lasers and computer 

memory chips. New uses for the light and heavy rare earth elements will
undoubtedly follow as technology advances and some present uses become 

obsolete.

Mineralogy and geology

The rare earth elements are mined and treated as oxides. Bastnaesite 

(CeFCOs), monazite (Ce,Th,Y) PO4, and xenotime (YPO4) are the three chief 

REO-bearing minerals. Bastnaesite is found in vein deposits, contact 

metamorphic zones, and pegmatites and as a rare accessory mineral in igneous 

rocks, most importantly in carbonatites and related veins. It contains about 70 

percent REO, mostly of the light rare earth elements, and is mined in China 

and California as a primary ore. Exploitable deposits of bastnaesite have been 

found as veins and disseminations in a complex of carbonate-silicate rocks, 

occurring with and related to alkaline intrusives (as in California, for example), 

in quartz veins cutting micaceous schists and quartzite, and in fluorite-bearing 

veins and breccia fillings in sandstone. Bastnaesite, which is a primary source 

of light rare earth elements at Bayan Obo and at Mountain Pass, is susceptible to 

chemical weathering, which causes the REO to dissolve and combine with 

available phosphates. Monazite and xenotime are found in primary deposits 

but are recovered principally from titanium or tin placers where they occur as 

accessory minerals.

The distribution of rare earth elements contained in each of the three 

chief REO-bearing minerals is variable. Typically, cerium accounts for almost 

50 percent of the contained rare earth elements in most bastnaesite and 

monazite. Lanthanum and neodymium make up most of the remaining light 

rare earth elements. Bastnaesite commonly contains less than 1 percent heavy 

rare earth elements, and monazite typically contains less than 3 percent. In 

xenotime, the light rare earth elements account for about 15 percent of total 

rare earth elements compared with 85 percent for the heavy rare earth 

elements. Individual rare earth element percentages vary for the same mineral 

from different locations.

Jackson and Christiansen (1993) explained that the proportions of rare 

earth elements contained in bastnaesite, monazite, xenotime, or other REObearing minerals are crucial to the success of a mineral-producing operation. 

Bastnaesite, which contains essentially all light rare earth elements, cannot 

supply heavy rare earth elements to industry. Xenotime, which contains a 

majority of heavy rare earth elements, can furnish few of the light rare earth 

elements. Monazite has a slightly better balance of light and heavy rare earth 

elements, but mining it for a few specific rare earth elements requires 

production of undesirable REO which then must be disposed.

Production and resources

It is difficult to refine the rare earth elements to a pure form because of 

their reactivity, and it is even more difficult to separate them into the 

individual elements or compounds owing to their chemical similarity. In 1996, 

world production of light and heavy REO was 79,600 tonnes (t) (Hedrick, 1997). 

China was the leading REO producer with 50,000 t and was followed by the
United States with 20,000 t; the former Soviet Union, 6,000 t; India 2,700 t; and 

other countries, including Brazil, Malaysia, Sri Lanka, and Zaire, less than 1,000 

t each. According to Jackson and Christiansen (1993), this output could be 

doubled if the operations that do not currently recover REO would do so. Of 

the world's resources in 1993, about 52 percent was in China; 22 percent, 

Namibia; 15 percent the United States; 6 percent, Australia; and 3 percent India 

(Jackson and Christiansen,1993). Jackson and Christiansen (1993) noted that 

rare earth element resources were more than sufficient to meet the demand for 

the predictable future, and Hedrick (1997) suggested that undiscovered rare 

earth element resources were very large relative to expected demand.

Model for Olympic Dam-type deposits

Discovery of the enormous Olympic Dam iron-copper-uranium-goldsilver-rare earth element deposit in Australia led to considerable speculation as 

to its origin and its relationship to other ore deposits. Several deposits, such as 

Great Bear Lake in Canada, Kiruna in Sweden, Bayan Obo, and others, have 

characteristics in common with the Olympic Dam deposit, and a descriptive 

model for the origin of this type of deposit was put forth by Oreskes and 

Hitzman (1993).

Olympic Dam-type deposits are only loosely associated with each other 

in some of their outward ore-body characteristics, such as morphology; that is, 

stratiform to cross cutting. They have no preferred host rocks and only a 

weak association with contemporaneous igneous activity. These deposits 

exhibit close association with respect to a shallow environment of formation, 

strong evidence of structural control, and intense hydrothermal alteration 

and replacement.

Oreskes and Hitzman (1993) argued that the magnetite- and hematitedominated systems in the Olympic Dam-type deposits were formed primarily 

by hydrothermal processes and argue that perhaps in only one of the deposits 

in this class (the Great Bear Lake district) is there evidence for a direct link 

between the mineralization and igneous activity. Although this class of 

deposits is only weakly linked to contemporaneous igneous activity, the 

evidence of structural control is strong. In general, structural control is 

evident whether along regional fault systems, in major folds, or bedding 

planes.

The alteration patterns associated with these deposits are usually 

intense iron metasomatites and replacement bodies are common, as are 

sodic and potassic alterations. Intense sodic and potassic alteration often 

occurs in the conversion of the wallrock to sodic and potassic metasomatites. 

Hitzman and others (1992) propose a general wallrock alteration model that 

progresses from sodic alteration at depth to potassic alteration at shallower 

levels in these ore bodies. Oreskes and Hitzman (1993) admitted that their 

thinking may have been biased by their experience with the Olympic Dam 

deposit itself and the data on the Bayan Obo deposit that was, at the time of 

their study, only starting to become available. At Bayan Obo, the pattern of
sodic and potassic alteration is spatially intermingled with much potassic 

alteration after sodic alteration.

Oreskes and Hitzman (1993) proposed a genetic spatial model for the 

occurrence of the Olympic Dam-type deposits, as shown in figure 1. In this 

figure, a postulated progression of motivating magmatic events spans 

locations where the subducting plate is far inboard in the continent (either 

back arc or pure rift) to other locations near the edge of the continent where 

the subducting plate has created igneous complexes associated with a volcanic 

arc. The span from Olympic Dam to the Bayan Obo deposits is located in the 

back arc and (or) rift environment and that from the Kiruna deposit and the 

Great Bear Lake deposit is more-closely associated with the volcanic arc 

environment.

Fig 1 near here

The Bayan Obo deposit

Bayan Obo is regarded as a hydrothermal carbonate replacement of a 

Middle Proterozoic dolomite (the Y8 dolomite; Drew and Meng, 1990,1995) 

and the tectonic setting is postulated to be that of a intracontinental rift (Drew 

and others, 1990). Proterozoic age has been confirmed by ^Ar/^Ar dating of 

Bayan Obo vein riebeckite at a minimum of 1.18 Ga (Conrad and McKee, 

1992). When additional data were considered, this date has been reestimated 

at 1.26 Ga (J.E. Conrad, U.S. Geological Survey, written commun., March 

1998). The hydrothermal system, which emplaced the ore, created an 

alteration halo in the dolomite and associated sedimentary rocks that cover 

about 50 km^. A variety of veins and (or) calcite dikes that have a carbonatitelike mineralogy, cut footwall rocks, and stockworks of veins is found at 

several locations in the footwall.

There are two opposing opinions on the origin of the dolomitic host 

rock for this deposits Drew and others (1990,1995) and Le Bas and others 

(1997). According to Drew and Meng (1990,1995) who mapped the Bayan Obo 

mine and surrounding area, field evidence showing that the main carbonate 

unit which hosts the ore bodies is of sedimentary origin is indisputable. It is 

possible, almost certain, that when the carbonatite-derived hydrothermal 

system invaded the Y8 carbonate unit that chemical reactions were so violent 

in the vicinity of the high-grade ore bodies that it is impossible today to 

determine which blocks of rock are only altered or which are completely 

replaced. It is possible that carbonatite dikes intruded the sedimentary 

carbonate, but field investigations have never revealed any significant 

volume of calcite in this unit. In this regard, all the carbonatite dikes and 

veins in the footwall are calcite with an associated carbonatitic mineralization 

and wallrock fenitization. They did not find any field evidence to support the 

hypothesis that the Y8 unit as mapped was originally anything other than 

sedimentary rock deposited as part of a normal marine-shelf sequence. Le Bas

6
and others (1997), however, favored a totally magmatic origin (intrusive 

carbonatite) for the coarse-grained dolomite that is found in patches within 

the main fine-grained dolomite unit. The fine-grained unit is, in turn, also 

believed to have been a possible intrusive carbonatite that was recrystallized 

under the influence of later mineralizing fluids.

The carbonate-replacement mechanism is very efficient at trapping 

ores. At Bayan Obo, this mechanism was enhanced by the 350-meter-thick Y9 

shale that overlies the carbonate. The mineralizing system converted the 

shale to a mixture of sodic and potassic metasomatites, thereby producing an 

impervious, tightly sealing caprock. This combined interaction of the 

carbonate-replacement mechanism confined by such a seal produced the 

perfect situation for creating an ore body.

The chief rare earth element-bearing mineral at Bayan Obo is the 

hydrothermal-reaction mineral bastnaesite. Estimated resources at Bayan Obo 

are 1,500 million tons of iron (average grade, 35 percent), at least 48 million t 

of rare earth oxides (average grade, 6 percent), and about 1 million t of 

niobium (average grade, 0.13 percent) (Drew and others, 1990). The mine is 

operated by the Ministry of Metallurgy Industry as an iron mine for the 

production of ore that is shipped directly to a steel mill for the production of 

rail and wire. Rare earth ore is produced as a coproduct and either stockpiled 

or shipped for processing at the time of mining.

Conclusions

The light rare earth elements are essential ingredients in many of today's 

high-technology applications. The ores from which their oxides are produced 

come from two sources in the world. If environmental restrictions lead to the 

closing of the Mountain Pass deposit, only the Bayan Obo deposit would 

remain in operation. The Bayan Obo deposit is a carbonate-replacement ore 

body that attained its enormous size through the unusual coincidence of a 

number of geologic factors that include the juxtaposition of carbonate host rock 

under a thick overlying shale that functioned as an impervious, tightly sealing 

caprock, the location of the host rock and the caprock in a continental massif 

that was, in turn, rifted apart, thereby allowing the emplacement of an 

alkaline/carbonatite complex shallow in the Earth's crust.
REFERENCES CITED

Adams, J.W., and Staatz, M.H., 1973, Rare-earth elements, in Brobst, D. A., and 

Pratt, W.P., eds., United States mineral resources: U.S. Geological Survey 

Professional Paper 820, p. 547-556.

Conrad, J.E., and McKee, E.H., ^Ai/^Ai dating of vein amphibole from the 

Bayan Obo iron-rare-earth element-niobium deposit, Inner Mongolia, 

China Constraints on mineralization and deposition of the Bayan Obo 

Group: Economic Geology, v. 87, no. 1, p. 185-188.

Drew, L.J., and Meng, Qingrun, 1990, Geologic map of the Bayan Obo area, 

Inner Mongolia, China: U.S. Geological Survey Map 1-2057, scale 

1:50,000.

 1995, Large-scale structural-geological setting of the Bayan Obo ironREE deposit, China: Global Tectonics and Metallogeny, v. 5, nos. 1 and 

2, p. 33-36.

Drew, L.J., Meng, Qingrun, and Sun, Weijun, 1990, The Bayan Obo iron-rare-earthsniobium deposits, Inner Mongolia, China, in Wooley, A.R., and Ross, 

Malcolm, eds., Alkaline igneous rocks and carbonatites: Lithos, v. 26, no. 1, p. 

43-65.

Hedrick, J. B., 1997, Rare earths: U.S. Geological Survey Mineral Commodity 

Summaries--1997, p. 134-135.

Hitzman, M.W., Oreskes, Naomi, and Einaudi, M.T., 1992, Geological characteristics 

and tectonic setting of Proterozoic iron oxide (Cu-U-Au-REE) deposits: 

Precambrian Research, v. 58, no. 1-4, p. 241-287.

Jackson, W. D., and Christiansen, Grey, 1993, International Strategic Minerals 

Inventory summary report Rare-earth oxides: U.S. Geological Survey 

Circular 930-N, 68 p.

Le Bas, M.J., Spiro, B., and Yang, Xueming, 1997, Oxygen, carbon and strontium

isotope study of the carbonatitic dolomite host of the Bayan Obo Fe-Nb-REE 

deposit, Inner Mongolia, N China: Mineralogical Magazine, v. 61, no. 4, p. 

531-541.

Neary, C.R., and Highley, D.E., 1983, The economic importance of the rare-earth 

elements, in Henderson, Paul, ed., Rare earth element geochemistry,-- 

Developments in geochemistry: Armsterdam, Netherlands, Elsevier 

Scientific Publishing Co., p. 423-466.

Oreskes, Naomi, and Hitzman, M.W., 1993, A model for the origin of Olympic Damtype deposits, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J.M., 

eds., Mineral deposit modeling: Geological Association of Canada Special 

Paper 40, p. 615-633.

Pincock, Alien and Holt, 1988, A worldwide study on rare earths: Pincock, Alien 

and Holt, 72 p.

Wellmer, F.W., Hannak, W., Krauss, Ulrich, and Thormann, A., 1988, Deposits of 

rare metals, in Kursten, Martin, ed., Raw materials for new technologies-- 

Proceedings of the Fifth International Symposium, Hannover, Germany: 

Hannover, Federal Institute for Geosciences and Natural Resources, p. 71-121.

8
Table 1. Light rare earths and their properties

[ppm, parts per million. Sources: Pincock, Alien, and Holt, 1988, p. 1.2; Wellmer 

and others, 1988, p. 116; Jackson and Christiansen, 1993, p. 4]

Element

Chemical 

svmbol

Atomic 

number Valence

Atomic 

weight

Average 

crustal

abundance 

(ppm) Oxides

Lanthanum...... La 57 3+ 138.92 29.00

Cerium............. Ce 58 3+,4+ 140.13 70.00

Praseodymium. Pr 59 3+,4+ 140.92 9.00

Neodymium..... Nd 60 3+ 144.27 37.00

Promethium..... Pm 61 3+ 145.00

Samarium........ Sm 62 2+,3+ 150.43 8.00

Europium......... Eu 63 2+,3+ 152.00 1.30

Gadolinium...... Gd 64 3+ 156.90 8.00

La2O3 

CeO2

none

Eu2O3 

Gd2O3
OLYMPIC 

DAM

WERNECKE 

REDBANK

BAYAN 

OBO

KIRUNA

S.E. MISSOURI

ACROPOLIS

GREAT

BEAR

LAKE

Tabular-concordant iron-oxide 

(Cu-U-LREE) mineralization

Breccia-hosted iron-oxide 

(Cu-U-LREE) mineralization

Figure 1. Mineral deposit model for the Olympic Dam-type deposits (Hitzman and others, 1992).