白云鄂博碳酸盐矿物的矿物化学成分标型特征

TYPOMORPHIC CHARACTERISTICS OF CARBONATE MINERALS FROM BAYAN OBO, INNER MONGOLIA, CHINA

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。

Carbonate rocks from Bayan Obo, Inner Mongolia, can be divided into sedimentary limestone and dolostone (H8 s), coarse grained dolomite marble (H8 c), fine grained dolomite marble (H8 f) and carbonatite dykes (D) based on their textures and occurrences. H8 s which shows very weak metamorphism and deformation is distributed to the north of the Kuangou fault zone, occurring together with quartz sandstone, sandstone and shale to form the Bayan Obo Group of middle Proterzoic age of normal sedimentary origin. In contrast to H8 s, however, H8 c and H8 f are distributed to he south of the Kuangou fault zone and have distinctive petrologicl features, both of which are strongly deformed and sheared and metamorphosed to dolomite marble. H8 c which hosts ores is mainly consisted of dolomite, apatite, magnetite, and pyroclore etc. These minerals show strong lineation which is the result of numerous clusters of euhedral magnetite and apatite drawn out into granular schlieren.The lineation resulted from shearing stretching and deformation is interpreted to be related to the regional thrusting and folding H8 f is close to the ores and is composed of dolomite, magnetite, monazite, bastnaesite and parisite etc. The fine grained monazites are lined to fill fractures in dolomites. Similar to H8 c, the minerals in H8 f also show strong lineation. Electronic microprobe is used to analyze the chemical compositions of carbonate minerals from these rocks at Bayan Obo. Results show that the main rock forming minerals—dolomite or ankerite in ore hosted rock from giant Bayan Obo REE Nb Fe ore deposit are sinmilar to those from carbonatite dykes (D), which occur 0 to 3 5 km northeast to the Bayan Obo deposit, and cut the Archean granitic gneiss and migmatite of the Wutai Group and the Proterozoic detrital rocks of the Bayan Obo Group. All these minerals contain MnO higher than 0 50 wt% and SrO greater than 0 15 wt%, which are identical to the typical signatures of the mantle derived carbonatite elsewhere in the world. However, they differ completely in chemical compositions from calcite and dolomite, with MnO and SrO content less than 0 1 wt%, in typical sedimentary limestone and dolostone (H8 s) north to the Kuangou fault. The variation of MnO and SrO content in dolomite from ore hosted dolomite can be interpreted by fractional crystallization of a carbonatitic magma origining from the mantle, which results in REE enrichement in the evolved residual magma. MnO and SrO content in carbonate minerals can be used as an important typomorphic characteristics to identify the genesis of minerals. It is suggested that the value of MnO and SrO content in carbonate minerals higher than 0 15 wt% is an indicator of crbonatite from sedimentary carbonate rocks.