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GEOLOGICAL SURVEY Origin and Ages of Mineralization of Bayan Obo , the World ' s Largest Rare Earth Ore Deposit , Inner Mongolia , China by
| Content Provider | Semantic Scholar |
|---|---|
| Author | Chao, Edward C. T. Tatsumoto, Mitsunobu Erickson, Ralph L. Jean, A. Minkin Back, Judith M. Zonglin Qingrun, Meng Wei-Jun, Sun Mckee, E. H. Brent Xi-Bin, Li Edwards, C. A. |
| Copyright Year | 2010 |
| Abstract | 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 Group, which is 1,880 m thick and consists of a sequence of quartzite, slate and shale, and crystalline dolomite (H1-H9, Fig. 1). The H9 black slate and shale south of the fault zone is in the trough of a syncline, and is underlain and flanked on both sides by the highly mineralized H8 dolomite. The striking differences in lithology, structural pattern, and mineralization north and south of the fault zone indicate a postoreformation age for the Kuanggou fault. Both REE ores and Fe-REE ores are stratabound within the H8 dolomite (Fig. 1). The dolomite ranges from 240 to 540 m in thickness, and is overlain by the relatively impervious, 340-360 m thick, H9 black slate and shale (Fig. 1), which may have acted as a caprock for the mineralizing solutions. The dolomite strikes E-W for about 16 km and is approximately 1-2 km in width in the N-S direction. The Main Ore Body and East Ore Body (Fig. 1) are located where the H8 dolomite is thickest. In the field, the ores were not found to be associated with any plutonic or volcanic rocks, and there has been no observed occurrence of alkalic granitic or subsilicic alkalic rocks in and adjacent to the mine region. Field and laboratory evidence indicates that the ore-bearing rocks have suffered both preand post-ore regional metamorphism. Mineral paragenetic sequence and ages of mineralization. Four independent methods were used to determine the mineral paragenetic sequence and ages of mineralization of the deposit: (a) field observations, (b) detailed petrographic textural analysis, (c) radioisotopic determination of mineral ages, and (d) chemical analysis of minerals determined by the preceding three methods to be of different generations in the mineral paragenetic sequence. Field and laboratory evidence indicates that the H8 host dolomite is of sedimentary origin (4, 7, 8), and not a carbonatite as previously suggested (3, 9, 10). This evidence includes: (a) conformable contacts of the dolomite with the overlying H9 black slate and shale and the underlying H7 mica schist and dolomite; (b) general massive appearance of the H8 dolomite, with occasional interbeds and lenses of quartzite, scattered detrital quartz, and rounded detrital apatite grains in planes parallel to the bedding; (c) presence, although rare, of algae microfossils in the dolomite (7); and (d) nature of the dolomite: ferroan (FeO 3-7 wt %), dominantly very fine-grained and well recrystallized (11). The 0 values for the H8 dolomite range from +12 to +16 0/00 (SMOW), and presumably were induced by interaction with hydrothermal solutions of light 0 isotopic composition. The mineralogy of the Bayan Obo ores is extremely complex, as shown by the fact that more than 100 minerals have been described from this deposit (12). The most important REE ore minerals of Bayan Obo are monazite ((Ce,La,Nd)PO^), bastnaesite ((La,Ce)(C03 )F), and huanghoite (BaCe(C03)2F); the most important Fe minerals are magnetite (Fe^O^) and hematite ^620^); and the most important Nb ore minerals appear to be fergusonite (YNbO^), aeschynite ((Ce,Ca,Fe,Th)Nb2 (0,OH) 6 ), and columbite (FeNb206 ) (see (12)). The two major types of dolomite-hosted REE and REE-Fe ores are: (a) disseminated REE ore, and (b) finely laminated and banded high-grade REE and REE-Fe ore that occurs in pods and lenses (Fig. 2). In addition, massive Fe ores with very low REE content occur in the central part of the major ore bodies and in the western part of the mine region. Mineral zoning is not apparent in the ore bodies (13). RE203 contents in the disseminated ores range from 2 to 6 wt %. In the banded ores and some massive Fe ores, ore grades vary from 2 to 25 wt % RE20-J and from 20 to 55 wt % Fe (averaging about 34 wt % Fe) (13). Replacement textures are widespread and are observable both megascopically and microscopically. The earliest introduction of stringers and sheaths of very fine-grained monazite aggregates, which preceded the introduction of magnetite in the mineral paragenetic sequence, came after recrystallization of the dolomite into a marble. For example, in the field, yellow streaks and irregular lacy networks of very fine-grained monazite were seen in essentially pure dolomite. Irregular patches of magnetite within dolomite and irregular boundaries between magnetite-rich areas and the dolomite host were also widely observed. Microscopically, fine-grained granular monazite occurs interstitially along dolomite triple-junction grain boundaries, destroying the host fabric by replacing the dolomite crystals (Figs. 3A and 3B). Where magnetite occurs in H8 dolomite that contains early monazite, the magnetite is interstitial between grains of dolomite and monazite (Fig. 4A). Where stongly twinned dolomite crystals occur with magnetite, the irregularly shaped relict dolomite remains in continuous optical orientation surrounded by the replacing magnetite (Fig. 4B). Similar destruction of the triple-junction grain boundary fabric of the host dolomitic marble by other minerals, such as magnesio-arfvedsonite, aegirine, phlogopite, fluorite, barite, apatite, and hematite, is also widespread. Banded ores are cut by veins of younger aegirine, with or without magnesio-arfvedsonite, and contain large huanghoite and aeschynite crystals. No evidence indicating a syngenetic origin for the ores, such as oolitic textures in Fe ores, was found. On the basis of field and microscopic textural analysis of disseminated and banded ores and radioisotopic dating of minerals, the generalized principal mineral paragenetic sequence (8) was determined to be: (a) magnesio-arfvedsonite, 802 + 19 Ma (1, Table 1); (b and c) disseminated monazite, earlier than magnesio-arfvedsonite with an age of 628 + 15 Ma (2, Table 1); (d) disseminated monazite, 594 + 4 Ma (5, Table 1); (e) magnesio-arfvedsonite, 440 + 11 Ma (3, Table 1); (f) early magnetite; (g) granular hematite; (h) magnesio-arfvedsonite, monazite, and bastnaesite, 425 + 10 Ma (4, 6, and 7, Table 1); (i) late magnetite; (j) vein huanghoite and aeschynite, 438 + 25 Ma (see below); and (k) Hercynian late-stage mineral assemblages including sulfides, microcline, albite, phlogopite, fluorite, barite, quartz, calcite, fergusonite, rare Ba and Sr REE fluorocarbonates, and Ba and Sr carbonates. Episodes of REE mineralization appear to be separate in time from those of Fe mineralization. Niobium mineralization occurred during and after the second stage of magnetite mineralization (stages i, j, and k). In general, radioisotopic ages (Table 1) are consistent with the texturally determined mineral paragenetic sequence. Mineral ages listed in Table 1 are considered to be minimum ages because later heating events may have resulted in a slight reduction of the radioisotopic ageS> 40 39 It is important to note that the K/Ar and Ar/ Ar ages of the four magnesio-arfvedsonites (Table 1) cover a span of about 400 million years and that these samples have distinctly different chemical characteristics, varying from MgOFeO to MgCKFeO, coupled with MnC^ content ranging from 0.06 to 5.68 wt %. Such distinct compositions suggest that each genera |
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| Alternate Webpage(s) | https://pubs.usgs.gov/of/1990/0538/report.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
