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 origin 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 first discovered as an Fe deposit in 1927, and later was recognized as a polymetallic deposit of Fe, REE'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 RE2O3, Drew and others, 1990) to as much as 100 million metric tons of RE2O3 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 III examines Bayan Obo as a cornerstone example of giant ore deposits of hydrothermal origin, with respect to eight parameters of importance in the formation of these deposits. Part IV presents a summary and conclusions and references.
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 Ren 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 DeYoung, 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.
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Last updated 12.18.97