The origin of the Bayan Obo ore deposit, the largest REE deposit in the world, has long been deba... more The origin of the Bayan Obo ore deposit, the largest REE deposit in the world, has long been debated and various hypotheses have been proposed. Among them is that the Bayan Obo ore deposit is correlated with and has the same origin as the Sailinhudong micrite mound in the southern limb of the Bayan Obo synclinorium. To test this model, the Bayan Obo ore deposit and the Sailinhudong micrite mound are systematically compared for their geological features, elemental geochemistry, and C, O, and Mg isotopic geochemistry. We show that the Bayan Obo ore deposit and the Sailinhudong micrite mound are both calcareous, lens-like in shape, lack bedding features, and are both hosted in a sedimentary formation that consists of clastic sediments and carbonates, unconformably overlying the Archaean–Palaeoproterozoic crystalline basement. However, their geochemical characteristics differ markedly. Compared with the Sailinhudong micrite carbonates, the Bayan Obo ore-hosting dolomite marbles are strongly enriched in LREEs, Ba, Th, Nb, Pb, and Sr, and have very different (PAAS)-normalized REE patterns. Sailinhudong micrite carbonates have higher δ13CPDB and δ18OSMOW values, falling into the typical sedimentary field, but the Bayan Obo ore-hosting dolomites are isotopically intermediate between primary igneous carbonatite and typical sedimentary limestone. The δ26 Mg values of the Sailinhudong micrite carbonates are lighter than those of normal Mesoproterozoic sedimentary dolostone, while those of the Bayan Obo ore-hosting dolomite marble are isotopically heavier, similar to δ26 Mg of mantle xenoliths and Bayan Obo intrusive carbonatite. We conclude that the Bayan Obo ore deposit is not correlated with the Sailinhudong micrite mound; it is neither a micrite mound nor an altered micrite mound.
Sm-Nd isotopic measurements were undertaken to constrain the chronology of REE mineralization eve... more Sm-Nd isotopic measurements were undertaken to constrain the chronology of REE mineralization events (REE = rare-earth elements) at the Bayan Obo REE-Nb-Fe deposit in northern China. The earliest REE mineralization event was dated precisely using samples of coarse-grained dolomite from the Bayan Obo Orebodies and the carbonatite dikes in their vicinity, which yielded a Sm-Nd isochron age of 1286 ± 27 Ma. The Sm-Nd data indicate that during this early event, the rare-earth elements were sourced from the mantle. A significant thermal event at ca. 0.4 Ga resulted in the formation of late-stage veins with coarse crystals of REE minerals. REE mineralization developed during this event resulted from REE remobilization within the ore-bodies with minimal contribution from external sources. A series of ages between 1.3 and 0.4 Ga reported for Bayan Obo ores in some previous studies resulted from thermal disturbance and do not imply the existence of multiple events.
Some elements normally occur at trace levels while the majority of natural geological materials m... more Some elements normally occur at trace levels while the majority of natural geological materials may be exceedingly enriched in some special cases, such as the Bayan Obo ore deposit where REE and Nb are extremely enriched. These elements may not be removed completely during purification. Therefore, matrix effects will be caused during stable isotope ratio measurement in the MC-ICP-MS. Experiments have shown that the established methods of chromatographic separation of Cu, Fe, and Zn using AG MP-1 Anion Exchange Resin cannot make effective separation of Nb, W, and Cu from Fe using 20 mL 6 M HCl. It is also observed that the elution curves of W and Cu overlap at working conditions and thus W is present in measurable amounts in some sample solutions. Matrix effects in the MC-ICP-MS induced by Nb, W, and Cu during Fe isotope ratio measurements and by W during Cu isotope ratio measurements were thus investigated by examining their changes in delta values between doped and undoped standards. The results show that the effects of the matrix elements Nb, W, or Cu on Fe isotope ratio measurements are minimal in the case of m(Nb)/m(Fe)<0.005, m(W)/m(Fe)<0.01, or m(Cu)/m(Fe)<0.6. This finding, combined with the extremely low levels of W and Cu, and the fact that nearly 90% of Nb can be removed during purification, demonstrates that the methods of chromatographic separation of Fe established before are suitable for Bayan Obo ore samples and that the methods can be simplified when Cu elution is unnecessary. The effects of the matrix element W on Cu isotope determinations are minimal in the case of m(W)/m(Cu)<0.7. Therefore, W exerts no significant effect on the measurements of Cu isotopes for the majority of natural geological materials.
The Bayan Obo REE-Fe-Nb ore deposit in Inner Mongolia is world-famous for its extreme enrichment ... more The Bayan Obo REE-Fe-Nb ore deposit in Inner Mongolia is world-famous for its extreme enrichment of light rare earth elements; nevertheless, its origin remains controversial. This is partly due to the lack of consensus on the ore-forming epoch and episodes of REE mineralization. This paper made a detailed review on geochronological data and a re-assessment of Sm-Nd isotopic results in literatures. It is shown that the REE mineralization of Bayan Obo ore deposit occurred at ca. 1.3 Ga, consistent with the epoch of carbonatite dyke intrusion in this region, and REE in both the ore deposit and the carbonatite dykes were derived from the mantle. A significant thermal event occurred at ca. 0.44 Ga, resulting in the formation of some veins with coarse crystals of REE minerals such as huanghoite. However, the enrichment of REE during this event resulted from REE remobilization within the ore body itself, and the contribution from external source was minimal. A series of ages between ca. 1.3 Ga and ca. 0.44 Ga resulting from partial disturbance of the isotopic system had no specific meaning for REE mineralization. In short, REE in the Bayan Obo ore deposit were enriched by a single episode of mineralization that occurred in Mesoproterozoic and derived from the mantle only.
The giant polymetallic Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world. D... more The giant polymetallic Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world. Despite the fact that a great number of works have been done on the deposit, its origin remains controversial. Various genetic models have been proposed, including sedimentary origin, magmatic origin, hydrothermal origin or origin with multiple processes. Here the Fe isotope compositions of different types of rocks from the deposit and related geological formations, such as carbonatites, mafic dykes, and Mesoproterozoic sedimentary iron formation and carbonates, are systematically investigated. For the ore deposit, the average ı 56 Fe values are −0.03 ± 0.16‰ (2SD, n = 14), 0.01‰ ± 0.24‰ (2SD, n = 6), −0.07 ± 0.24‰ (2SD, n = 19) for bulk samples of fine-grained iron ores, gangue rocks and ore-hosting dolomite marble, respectively; and 0.01 ± 0.14 (2SD, n = 14), 0.08 ± 0.18 (2SD, n = 3), −0.21 for the magnetite, hematite and dolomite, the main Fe oxide and carbonate minerals in the deposit. The narrow range of the near-zero ı 56 Fe values of fine-grained iron ores and Fe oxide minerals are consistent with those of magmatic products such as igneous rocks and magmatic iron ores, but different from those of sedimentary or hydrothermal products like Precambrian sedimentary iron formations and hydrothermal iron ores reported previously. The slightly negative Fe isotope values of the orehosting dolomite marble are consistent with those of the carbonatite dykes in Bayan Obo area and the typical carbonatites worldwide, but different from those of Mesoproterozoic sedimentary carbonates. The small Fe isotope fractionations between magnetite and dolomite ( 56 Fe magnetite-dolomite ≈ 0.23‰), and between hematite and magnetite ( 56 Fe hematite-magnetite ≈ 0.07‰), indicate that the ore deposit experienced a very high temperature. Overall, the Fe isotope compositions are inconsistent with either sedimentary or hydrothermal origin, but support a magmatic origin for the Bayan Obo deposit.
The origin of the Bayan Obo ore deposit, the largest REE deposit in the world, has long been deba... more The origin of the Bayan Obo ore deposit, the largest REE deposit in the world, has long been debated and various hypotheses have been proposed. Among them is that the Bayan Obo ore deposit is correlated with and has the same origin as the Sailinhudong micrite mound in the southern limb of the Bayan Obo synclinorium. To test this model, the Bayan Obo ore deposit and the Sailinhudong micrite mound are systematically compared for their geological features, elemental geochemistry, and C, O, and Mg isotopic geochemistry. We show that the Bayan Obo ore deposit and the Sailinhudong micrite mound are both calcareous, lens-like in shape, lack bedding features, and are both hosted in a sedimentary formation that consists of clastic sediments and carbonates, unconformably overlying the Archaean–Palaeoproterozoic crystalline basement. However, their geochemical characteristics differ markedly. Compared with the Sailinhudong micrite carbonates, the Bayan Obo ore-hosting dolomite marbles are strongly enriched in LREEs, Ba, Th, Nb, Pb, and Sr, and have very different (PAAS)-normalized REE patterns. Sailinhudong micrite carbonates have higher δ13CPDB and δ18OSMOW values, falling into the typical sedimentary field, but the Bayan Obo ore-hosting dolomites are isotopically intermediate between primary igneous carbonatite and typical sedimentary limestone. The δ26 Mg values of the Sailinhudong micrite carbonates are lighter than those of normal Mesoproterozoic sedimentary dolostone, while those of the Bayan Obo ore-hosting dolomite marble are isotopically heavier, similar to δ26 Mg of mantle xenoliths and Bayan Obo intrusive carbonatite. We conclude that the Bayan Obo ore deposit is not correlated with the Sailinhudong micrite mound; it is neither a micrite mound nor an altered micrite mound.
Sm-Nd isotopic measurements were undertaken to constrain the chronology of REE mineralization eve... more Sm-Nd isotopic measurements were undertaken to constrain the chronology of REE mineralization events (REE = rare-earth elements) at the Bayan Obo REE-Nb-Fe deposit in northern China. The earliest REE mineralization event was dated precisely using samples of coarse-grained dolomite from the Bayan Obo Orebodies and the carbonatite dikes in their vicinity, which yielded a Sm-Nd isochron age of 1286 ± 27 Ma. The Sm-Nd data indicate that during this early event, the rare-earth elements were sourced from the mantle. A significant thermal event at ca. 0.4 Ga resulted in the formation of late-stage veins with coarse crystals of REE minerals. REE mineralization developed during this event resulted from REE remobilization within the ore-bodies with minimal contribution from external sources. A series of ages between 1.3 and 0.4 Ga reported for Bayan Obo ores in some previous studies resulted from thermal disturbance and do not imply the existence of multiple events.
Some elements normally occur at trace levels while the majority of natural geological materials m... more Some elements normally occur at trace levels while the majority of natural geological materials may be exceedingly enriched in some special cases, such as the Bayan Obo ore deposit where REE and Nb are extremely enriched. These elements may not be removed completely during purification. Therefore, matrix effects will be caused during stable isotope ratio measurement in the MC-ICP-MS. Experiments have shown that the established methods of chromatographic separation of Cu, Fe, and Zn using AG MP-1 Anion Exchange Resin cannot make effective separation of Nb, W, and Cu from Fe using 20 mL 6 M HCl. It is also observed that the elution curves of W and Cu overlap at working conditions and thus W is present in measurable amounts in some sample solutions. Matrix effects in the MC-ICP-MS induced by Nb, W, and Cu during Fe isotope ratio measurements and by W during Cu isotope ratio measurements were thus investigated by examining their changes in delta values between doped and undoped standards. The results show that the effects of the matrix elements Nb, W, or Cu on Fe isotope ratio measurements are minimal in the case of m(Nb)/m(Fe)<0.005, m(W)/m(Fe)<0.01, or m(Cu)/m(Fe)<0.6. This finding, combined with the extremely low levels of W and Cu, and the fact that nearly 90% of Nb can be removed during purification, demonstrates that the methods of chromatographic separation of Fe established before are suitable for Bayan Obo ore samples and that the methods can be simplified when Cu elution is unnecessary. The effects of the matrix element W on Cu isotope determinations are minimal in the case of m(W)/m(Cu)<0.7. Therefore, W exerts no significant effect on the measurements of Cu isotopes for the majority of natural geological materials.
The Bayan Obo REE-Fe-Nb ore deposit in Inner Mongolia is world-famous for its extreme enrichment ... more The Bayan Obo REE-Fe-Nb ore deposit in Inner Mongolia is world-famous for its extreme enrichment of light rare earth elements; nevertheless, its origin remains controversial. This is partly due to the lack of consensus on the ore-forming epoch and episodes of REE mineralization. This paper made a detailed review on geochronological data and a re-assessment of Sm-Nd isotopic results in literatures. It is shown that the REE mineralization of Bayan Obo ore deposit occurred at ca. 1.3 Ga, consistent with the epoch of carbonatite dyke intrusion in this region, and REE in both the ore deposit and the carbonatite dykes were derived from the mantle. A significant thermal event occurred at ca. 0.44 Ga, resulting in the formation of some veins with coarse crystals of REE minerals such as huanghoite. However, the enrichment of REE during this event resulted from REE remobilization within the ore body itself, and the contribution from external source was minimal. A series of ages between ca. 1.3 Ga and ca. 0.44 Ga resulting from partial disturbance of the isotopic system had no specific meaning for REE mineralization. In short, REE in the Bayan Obo ore deposit were enriched by a single episode of mineralization that occurred in Mesoproterozoic and derived from the mantle only.
The giant polymetallic Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world. D... more The giant polymetallic Bayan Obo REE-Nb-Fe ore deposit is the largest REE deposit in the world. Despite the fact that a great number of works have been done on the deposit, its origin remains controversial. Various genetic models have been proposed, including sedimentary origin, magmatic origin, hydrothermal origin or origin with multiple processes. Here the Fe isotope compositions of different types of rocks from the deposit and related geological formations, such as carbonatites, mafic dykes, and Mesoproterozoic sedimentary iron formation and carbonates, are systematically investigated. For the ore deposit, the average ı 56 Fe values are −0.03 ± 0.16‰ (2SD, n = 14), 0.01‰ ± 0.24‰ (2SD, n = 6), −0.07 ± 0.24‰ (2SD, n = 19) for bulk samples of fine-grained iron ores, gangue rocks and ore-hosting dolomite marble, respectively; and 0.01 ± 0.14 (2SD, n = 14), 0.08 ± 0.18 (2SD, n = 3), −0.21 for the magnetite, hematite and dolomite, the main Fe oxide and carbonate minerals in the deposit. The narrow range of the near-zero ı 56 Fe values of fine-grained iron ores and Fe oxide minerals are consistent with those of magmatic products such as igneous rocks and magmatic iron ores, but different from those of sedimentary or hydrothermal products like Precambrian sedimentary iron formations and hydrothermal iron ores reported previously. The slightly negative Fe isotope values of the orehosting dolomite marble are consistent with those of the carbonatite dykes in Bayan Obo area and the typical carbonatites worldwide, but different from those of Mesoproterozoic sedimentary carbonates. The small Fe isotope fractionations between magnetite and dolomite ( 56 Fe magnetite-dolomite ≈ 0.23‰), and between hematite and magnetite ( 56 Fe hematite-magnetite ≈ 0.07‰), indicate that the ore deposit experienced a very high temperature. Overall, the Fe isotope compositions are inconsistent with either sedimentary or hydrothermal origin, but support a magmatic origin for the Bayan Obo deposit.
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