白云鄂博赋矿白云岩与微晶丘和碳酸岩墙的碳氧同位素对比研究  

Carbon and Oxygen Isotope Compositions of Ore-bearing Dolostone in the Bayan Obo Deposit as Well as Two Typical Micrite Mounds and a Carbonatite Dyke

作　　者：杨晓勇[1] 章雨旭[2] 郑永飞[1] 杨学明[1] 徐宝龙[1] 赵彦冰 彭阳[2] 郝梓国[2] 

机构地区：[1]中国科学技术大学地球与空间科学系,合肥230026 [2]中国地质科学院地质研究所,北京100037

出　　处：《地质学报》2000年第2期169-180,共12页Acta Geologica Sinica

基　　金：科技部国家重点基础研究发展规划项目(编号G1999043204);国家自然科学基金(编号49772093;49872032);地质行业科学技术发展基金(编号959622)

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δ^(13)C值都在0±2‰左右,δ^(18)O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δ^(13)C值为-7.9‰～-1.1‰,δ^(18)O值为9.1‰～20.9‰;矿区东西两端δ^(13)C值-7.9‰～-0.6‰,δ^(18)O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△^(13)C和△^(18)O值均小于0‰,表明其受到过次生蚀变作用,低δ^(18)O值白云石样品所对应的负△^(18)O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δ^(13)C值为-7.2‰～-4.7‰,δ^(18)O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。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 sedi

关 键 词：白云岩 微晶丘 碳酸岩墙 氧同位素 碳同位素 

分 类 号：P618.31[天文地球—矿床学]