| 对白云鄂博矿床大地构造环境的几点认识 |
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| 关键词:geotectonic environments REE rift belt Bayun Obo |
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| Geotectonic Setting of the Bayan Obo Deposit, Inner Mongolia |
| Located on nothern margin of the North China platform, the supper large Bayan Obo Fe-Nb-REE deposit is hosted by an ore-bearing formation composed of dolomite, iron bed and potassium-rich slate, which is 50 1300m thick and 18km long, and is concordant and synchronously folded with its wallrocks, Several small scale REE-bearing cabonatite and syenits veins intrude the strata. Studies by different methods have demonstrated that the rare earth mineralization occurred in the Mid-Proterozoic epoch, and S, Sr, Nd, H, Si, C and O isotope compositions have indicated that the ore-forming materials were derived from the mantle.
The ore-bearing stratum belongs to the upper portion of Jianshan Formation, Bayan Obo Group which extends more than 500km in west-east length and 20 to 50km in north-south width with a thickness of about 10000m. The bayan Obo Group is dominated by clastic rocks. Its lower part is molasse formation and the middle and upper parts are flysch formations. Carbonate rocks take account for about one tenths of the Group. The aubhors indicated in 1977 that the LREE-rich ore-bearing system was a marine volcanic-sedimenta-ry carbonatite and tyachyte formation. In 1982 Li Jiliang demonstrated that 320m thick u-ralite-epidosite was metamorphosed alkaline basaltic rocks. In addition, serpentinite olis-tolith and Bayinjiaolake cumulus gabbros were also recognized in the Bilute and Jianshan Formations, respectively. These sub-alkaline mafic-ultramafic and intermediate-acidic rocks of the Bayan Obo Group possess 2 to 10 times higher REE cONTENTS AND 3 to 32 times higher LREE/HREE ratios than those of similar rocks in Middle Asia-Monogloia plate of transitional and oceanic crust located on the northern side of the Sino-Korean plate.
The elongated distribution pattern of the Bayan Obo Group with its clastic rock-domi-nated thick sedimentary sequence and subalkaline-alkaline extrusive and cumulus rocks in-dicate that it was formed in a rift environment of which the underneath anomalous upper mantle was the only possible source to provide abundant CO2,F,REE,Nb Fe Ca, Mg, K Na Sr Ba Pand other incompatible elements and mantle-characterized istopic com-positions. After the death of Bayan Obo rift system, gabbros and granites were emplaced in the area in Caledonian and Hercynian periods. These rocks were anatectic proudcts of the crust, and had REE contents and LREE/HREE ratios much lower than those of the corresponding rocks formed in rifting period. They show unlikely any relations to the for-mation of Bayan Obo deposit with its destinctive element association and isotopic composi-tions.
It is suggested that the origin of the Bayan Obo deposit is closely seated to the forma-tion of the Bayan Obo rift belt. |
| Bai Ge,Yuan Zhongxin,Wu Chengyu.1996.Geotectonic Setting of the Bayan Obo Deposit, Inner Mongolia[J].Acta Geoscientica Sinica,17(z1):1-8. |
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