Chemical compositions of carbonate minerals from Bayan Obo, Inner Mongolia, North China, and their petrogenetic implications

THE origin of the giant Bayan Obo REE-Nb-Fe ore deposit has been under much debate since its discoveryas an iron deposit by Prof. Ding Daoheng in 1927. The main dispute is concentrated on the genesis of theore-hosted dolomite marbles of the deposit, Some authors considered them to be sedimentary in origin,while others pointed out that they could be carbonatite. However, there are very few researches concerning the compositions of carbonate minerals which may provide a direct clue to their petrogenesis. Herewe show the variations of compositions in carbonate minerals from Bayan Obo. Carbonate rocks from Bayan Obo, Inner Mongolia, could be divided into sedimentary limestone anddolostone (H8_s), coarse-grained dolomite marble (H8_c), fine-grained dolomite marble (H8_f) and carbonatite dykes (D) based on their texture, structure, mineral progenesis and occurrences. H8_s thatshows weak metamorphism and deformation is distributed to the north of the Kuangou fault, occurring

白云鄂博碳酸盐矿物的矿物化学成分标型特征

对白云鄂博地区不同产状的碳酸盐矿物进行了系统的电子探针分析，其结果表明白云鄂博ＲＥＥ－Ｎｂ－Ｆｅ矿床的赋矿白云岩中，无论是粗粒结构的，还是细粒结构的白云岩，其主要造岩矿物白云石或者铁白云石均具高锰（ＭｎＯ＞０．５ｗｔ％）和高锶（ＳｒＯ＞１．５ｗｔ％）的特征，与矿床北东方向０～３．５ｋｍ范围内切割五台群花岗片麻岩－混合岩及白云鄂博群碎屑岩的火成碳酸岩岩墙中的碳酸盐矿物非常相似。它们完全不同于宽沟断裂以北典型沉积石灰岩及白云质石灰岩中的方解石和白云石（ＭｎＯ＜０．１ｗｔ％，ＳｒＯ＜０．１ｗｔ％）。赋矿白云岩中白云石的锶和锰含量系统变化反映碳酸岩浆发生了分离结晶作用，其结果可以导致残余岩浆中ＲＥＥ高度富集。本文认为碳酸盐矿物的锶和锰含量可以作为识别其成因的重要标型特征，其氧化物ＭｎＯ＞０．１５ｗｔ％和ＳｒＯ＞０．１５ｗｔ％为火成碳酸岩的标志。