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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


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 232Th/208Pb 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 208Pb/204Pb 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+-25.1 Ma), occurs with huanghoite and alkali amphiboles in veins.

The 40Ar/39Ar 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 40Ar/39Ar analyses from biotite schist and folded banded ores. The 40Ar/39Ar 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 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 amphibole 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 Bayan 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 REE's and therefore probably have a different crustal source than the REE-mineralizing hydrothermal solutions that formed the REE'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.

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Last updated 12.18.97