Study On Genesis Of Bayan Obo REE-Nb-Fe Deposit, Inner Mongolia

The Bayan Obo REE-Nb-Fe deposit is the largest LREE deposit and the second largest Nb deposit in the world, and it is also a large Fe deposit. The genesis and forming age of the deposit have been debated for decades by geologists both of China and worldwide. The two most important problems are the mineralization age and how and when the H8dolomite was formed.In order to clarify the above questions, on the basis of a comprehensive summarizing and reviewing of existing results, a lot of works have been done. We try to solve some basic problems in the Bayan Obo area, and then analyze the REE mineralization age and its genesis.Through detailed strata research, it is found that strata in north and south Kuangou fault are the same, and the Bayan Obo Group is correlated with Sailinhudong Group in the Heinaobao, southeast Bayan Obo deposit. Zircons from basement rocks in the Heinaobao yield an age of～2.5Ga (two LA-ICP-MS results are2522±18Ma and2495±11Ma, respectively), they are formed at late New Archean, this age is similar with the age of basement rocks and zircons from carbonatite in the Bayan Obo. The chronology studies of basement rocks show that there is not only magmatic activity and intense metamorphic thermal event during early Proterozoic(1.9～2.0Ga), Archean rocks (2522±18Ma) of North China craton are normal in this area, the Bayan Obo deposit is indeed developed upon the North China Craton basement. The same basement age is indirect evidence for that stratum develop upon them (thus the Bayan Obo Group and Sailinhudong Group) are comparable.Basement in the Bayan Obo are peraluminous cal-alkaline series rocks, and their protolith is Archaean sedimentary rocks; while basement in the Heinaobao are aluminous high-K calc-alkaline series, whose protolith is probably K-rich alkaline volcanic rock. Tectonic study indicated an island arc environment, the formation of these basement rocks is related to the island arc related magmatism.Carbonaities in the Bayan Obo are formed in the age of1.3～1.4Ga, and there is no other carbonatite magmatism in this area in the long geologic period. In the Mesoproterozoic(1.20～1.70Ga), North China Craton broke away from Columbia supercontinent, carbonaitie magma intruded into basement and Bayan Obo Group in rift environment. Zircons from carbonatite gain two ages of2507±35Ma and1904±19Ma, respectively. Both ages are consistent with the age of basement rocks. Furthermore, inclusions in these zircons are silicate, indicating that zircons in the carbonatite are entrapped from the basement during carbonatitie magma intrusive process. Most zircons in the carbonatite have variableεHf(t) values of-20.4to1.7, with Paleoproterozoic to Neoarchean Hf model ages of2259Ma to3207Ma (with an average of2572±120Ma). The Hf model age is the same with the zircon age, indicating the formation of basement is closely related to cratonization of North China craton, indirectly supported that supracrustal sequences in the late New Archean of North China craton (including BIF) are in an island arc environment.Geochemical studies of carbonatite dykes in the Bayan Obo show obviously igneous carbonatite features, with extremely high REE contents. Moreover, mineral analysis indicate that dolomite has a stronger ability to hold rare earth elements than calcite, and rare earth elements in the carbonatite exist in the form of accessory minerals. Carbonatite magma and fluids have high ore-forming potential, there is no doubt that the Bayan Obo REE-Nb-Fe deposit is related to carbonatite.The207Pb-206Pb isochron age (158±100Ma) of the sedimentary limestone of the Bayan Obo Group in the north Kuangou fault may represent its deposition time, indicating the Bayan Obo Group was deposited in Mesoproterozoic. The207Pb-206Pb isochron age of sedimentary dolomite of the Sailinhudong Group in the Heinaobao is1425±420Ma, though it is a little younger than the age of sedimentary limestone in the Bayan Obo, similar age indicate that Bayan Obo Group is comparable with Sailinhudong Group, and comparative studies between H8dolomite and sedimentary carbonate rocks in the Heinaobao is reasonable. Zircon SIMS U-Pb age dating of K-rich slate show that they was form at1.65Ga, one Group of age concentrates in the Late Archean-Early Paleoproterozoic (2519.9Ma), consisting with the zircon ages of basement rocks in the Bayan Obo region, and the age of518.8Ma represent the Caledonian magmatic event. Chronology studies support that the Bayan Obo Group was formed in Mesoproterozoic.The Ar-Ar age of biotite from biotitite from Bayan Obo is289.1±1.8Ma, indicating it is the result of Hercynian magmatism, since Hercynian magmatism is generally accepted to have nothing to do with REE mineralization, the exist of biotitite should not be the evidence of H8dolomite is carbonatite. Riebeckite from riebeckite vein of banded type ore has Ar-Ar age of389.5±3.0Ma, represented the age of reworking process rather than second mineralization age. Sm-Nd isochron ages of H8dolomite in the Bayan Obo are concentrated in-1.20Ga, younger than the age of carbonatite, as Bayan Obo is an REE deposit, it is clear that the Sm-Nd age stand for mineralization age. Though many REE minerals have Th-Pb isochron ages of Caledonian, their Pb-Pb ages are similar to the Sm-Nd isochron age. This shows that Th-Pb system was reset in Caledonian, thus these ages are more likely the age of reworking process rather than another mineralization. Besides, the mineralization process has only occur once, about1.2Ga, after carbonatite intrusion.The whole rock geochemical studies of H8dolomite show that coarse-grained dolomite especially those with low rare earth content, have similar characteristics to sedimentary carbonate rocks, fine-grained dolomite have nearly the same features as igneous carbonatite, and using some trace elements can easily distinguish sedimentary carbonate rock and carbonatite. Though the whole rock geochemical features between coarse and fine grained dolomite are different, their carbonate minerals (dolomite) have similar geochemical characteristics, which are completely different from the carbonatite. Rare earth element content of dolomite mineral in the H8dolomite (both coarse and fine-grained dolomite) is low, with the highest total rare earth content is146ppm, much lower than the calcite or dolomite in carbonatite. Moreover, dolomite from coarse-grained dolomite retain signatures of sedimentary carbonates in the core, indicating that dolomite is the result of mineralization of sedimentary carbonate rocks by REE-rich fluids derived from carbonatite magma. Moreover, SrO content in carbonate minerals is a more sensitive indicator than MnO to distinguish carbonatite from sedimentary carbonate rocks.The sulfur isotopic composition of the intensively REE-mineralized dolomites and pyrite separated from those dolomites show two obvious peaks; one peak in δ34S values is around0‰, which is characteristics of a mantle-source; another is around+8‰, being significantly higher than the mantle sulfur source value and falling into the sedimentary carbonate field. The sulfur isotopic composition of barite samples indicates that they are richer in34S than the dolomites, with an average δ34S value of+12.50‰, this value is lower than+17‰that was suggested by Holser et al.(1996) to be the composition of sulfates precipitated from Precambrian seawater. The calculated834S∑s results using Ohmoto model are6.1‰and10.1‰, respectively, both values have characteristics of mantle source that exchanged and/or mixed with the sulfur source in the crust or seawater. Sulfur in the whole rock and sulfides came from two sources:the mantle and seawater. Metasomatism unevenly formed two sulfur isotope peaks in the mineralized dolomites.The sedimentary age of K-rich slate are about1.65Ga, if slate is volcanic rock or metavolcanic rock, there should be zircon with age of～1.65Ga, whereas typical magmatic zircon is formed at Hercynian, and there is no1.6Ga zircon. Geochemical studies of H9K-rich slate show that they normal metamorphic sedimentary rocks, and they also have suffered from carbonatite magma. In summary, mineralized dolomites in the Bayan Obo REE-Nb-Fe deposit are the product of sedimentary carbonate hydrothermally metasomatised by carbonatite magma and/or associated fluids, and coarse-grained dolomite marbles are the result of weak mineralization of sedimentary carbonate rocks, and fine-grained dolomite marbles are the product of intensive mineralization. The Bayan Obo rift mobilized at about1.3～1.4Ga, accompanied by extremely high REE carbonatite magmatism, H8sedimentary carbonate of the Bayan Obo Group was mineralization and metasomatised to form the whole deposit. In Caledonian era, the North China Block subducted toward the Siberian plate, and then collided each other and combined together. Meanwhile, the Bayan Obo deposit was strongly disturbed, later phase minerals, such as pyrite and riebeckite, were formed, and some isotopic systems were reset and show Caledonian apparent ages.