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 craton2. The host H8 marble and quartzite are sedimentary rocks that were metamorphosed prior to mineralization3. 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 deposit5. 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 rocksIn 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 Mb 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 subduction 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.

The origin of Bayan Obo, the world's largest known rareearth-element (REE) ore deposit, has been a subject of geologic interest and speculation.Recent field and laboratory observations support the early conclusion that the Middle Proterozoic host rock of the Bayan Obo ores is not a carbonatite, but a metamorphosed, sedimentary dolomitized limestone.Petrographic textural analysis indicates that the ores are epigenetic, hydrothermal and metasomatic in origin.Radioisotopic data together with the textural analysis show that the principal episodes of mineralization for this unique deposit probably began about 800 Ma and continued to at least 425 Ma, a span of about 400 million years.Introduction.The use of rare earth elements (REEs) in high technology (for example high-strength ceramics, catalytic functions, permanent magnets, phosphors in video displays, and high-temperature superconductors) (1) makes the occurrence of these elements of increasing economic interest worldwide.Bayan Obo, the world's largest known REE deposit, with inferred reserves of more than 30 million metric tons of rare earth oxides (RE20^)(2), has until recently been of extremely limited access for study by the international scientific community.Five hypotheses for the origin of the Bayan Obo deposit have been proposed (3): a) high-temperature hydrothermal metasomatism related to Hercynian granitic rocks that are widespread in the mine region, b) and c) sabkha sedimentary syngenesis preceding regional metamorphism with or without additional hydrothermal metasomatism, d) magmatic carbonatite, and e) exhalative volcanogenic carbonatite sedimentation.Results reported below support a sixth hypothesis, a late Proterozoic to Caledonian age epigenetic, hydrothermal, metasomatic origin.Location and geologic setting.The Bayan Obo REE deposit was discovered in 1927 as a Fe deposit (3).Currently both Fe and REEs are mined; Nb ore is also present.The deposit is located in Inner Mongolia, at 41°45'N and 110°E.The mine is owned and operated by the Baotou Iron and Steel Corporation of the Ministry of Metallurgical Industry of the People's Republic of China.The rocks of the Middle Proterozoic Bayan Obo Group, which hosts the ore, represent a platform sequence on the northern flank of the Archean Sino-Korean craton.Sedimentary rocks of the sequence were probably deposited in an E-W-trending graben or trench, with the open sea to the north (4), and then uplifted and regionally metamorphosed at about 1.4 Ga (5).Geologically, the Bayan Obo region (Fig. 1) can be divided into two parts: (a) north of the Kuanggou fault zone is an unmineralized, weakly regionally metamorphosed limestone and shale sequence H1-H10 (Hu in Fig. 1) (6); and (b) south of the Kuanggou fault zone the mine region is more strongly regionally metamorphosed and includes the Bayan Obo