【摘要】 本文通过方解石和白云石的碳和氧同位素分析,对比研究了白云鄂博赋矿白云岩、黑脑包微晶丘、北京西山微晶丘、宽沟北正常沉积灰岩和白云鄂博碳酸岩墙,从而探讨了赋矿白云岩的成因及其与超大型Fe-Nb-REE矿床的成因关系。结果为:①黑脑包腮林忽洞群顶部微晶丘和北京西山寒武系顶部微晶丘碳酸盐的δ¹³C值都在0±2‰左右,δ¹⁸O值为18.3‰～25.1‰,均具有典型海相沉积碳酸盐岩的特点;②白云鄂博东矿采场δ¹³C值为-7.9‰～-1.1‰,δ¹⁸O值为9.1‰～20.9‰;矿区东西两端δ¹³C值-7.9‰～-0.6‰,δ¹⁸O值8.6‰～25.7‰;均介于地幔流体与典型沉积碳酸盐岩之间。部分赋矿白云岩样品中白云石与方解石之间的碳氧同位素分馏△¹³C和△¹⁸O值均小于0‰,表明其受到过次生蚀变作用,低δ¹⁸O值白云石样品所对应的负△¹⁸O值反映了地幔镁质流体对沉积碳酸盐岩的强烈交代作用;③矿区—富稀土碳酸岩墙的δ¹³C值为-7.2‰～-4.7‰,δ¹⁸O价值为11.9‰～16.4‰,表明其碳酸岩岩浆并非原始地幔来源,而可能与俯冲板块携带的沉积碳酸盐岩与地幔流体在深部的高温混合熔融有关。碳酸岩墙中白云石与方解石之间的碳和氧同位素分馏均小于0‰,说明该碳酸岩墙中的白云石与方解石并非同成因矿物,至少其中之一为更多还原

【Abstract】 To study the genesis of the orebearing dolostone, one of the key problems of the genesis of the famous Bayan Obo deposit, the writers determine and compare the carbon and oxygen isotope compositions of dolomite and calcite of the ore-bearing dolostone with those of two micrite mounds and a carbonatite dyke.The micrite mound at the top of the Cambro-Ordovician Sailinhuodong Group, , in Heinaobao, about 25 km southeast of Bayan Obo, and the micrite mound of the Upper Cambrian in Jiu Yuan, the Western Hills, Beijing, show the δ13C values of 0±2‰ and δ18O values of 18. 3‰ to 25. 1‰, indicating normal marine precipitation. Oxygen isotope fractionations be- tween dolomite and calcite in the micrite mounds from Heinaobao are in disequilibrium, indicating dolomitization of limestone during the diagenesis.Profile - sampling was carried out for carbonates in the Bayan Obo orefield. Carbon and oxygen isotope analyses show that the dolostones within the East Mine and at the eastern end and western end of the orebearing dolostone belt have similar δ13C and δ18O ranges. The dolostones within the East Mine have δ13C of -7. 9‰ to -1.1‰ and δ18O of 9. 1‰ to 20. 9‰; the dolostones at the two ends have δ13C of -7. 9‰ to -0. 6‰ and δ18O of 8. 6‰ to 25. 7‰. Carbon and oxygen isotope fractionations (Δ13C and Δ18O) between dolomite and calcite in the orebearing dolostone from a part of the samples have Δ13C and Δ18O values with both less than 0‰, indicating the influence of secondary alteration. A number of low δ18O dolomite also has negative Δ18O values, indicating the intensive alteration by mantle-derived magnesium-rich fluids to sedimentary carbonates.A carbonatite dyke northeast 3 km notheast of the East Mine, intruded in H2 sandstone, has a small range of isotopic variation: δ13C varies from - 7. 23‰ to - 4. 7‰ , being similar to the normal mantle; δ18O varies from 11. 9‰ to 16. 4‰, being significantly higher than the δ18O of the mantle (5. 7‰±1. 0‰), indicating that the sedimentary carbonate was carried into the mantle by plate subduction to form 18O-rich carbonatite magma via mixing and melting at high temperatures and pressures. Both carbon and oxygen isotope fractionations between dolomite and calcite from the carbonatite dyke are less than 0‰, being in conflict with the sequence of 13C- and 18O-enrichment in dolomite, relative to calcite at thermodynamic equilibrium. Thus, the disequilibrium dolomite-calcite pairs are not cogenetic, and at least one of them is of secondary origin. This implies that the carbonatite dyke underwent hydrothermal alteration subsequent to its emplacement.The present studies suggest that the dolostone in the Bayan Obo ore district would result from metasomatism by mantle-derived fluids of the sedimentary carbonates. It is concluded that the carbonates of the micrite mounds underwent the metasomatism by mantle-derived fluids which were extremely rich in REE and some other elements, resulting in the formation of the su-perlarge Fe-Nb-REE ore deposit in Bayan Obo. The large variation in both δ13C and δ18O values of the ore-bearing dolostone are ascribed to the heterogeneous metasomatism by the mantle-derived fluid of the sedimentary carbonates.更多还原