白云鄂博富稀土元素碳酸岩墙的碳和氧同位素特征

Carbon and Oxygen Isotope Characteristics of the REE-Rich Carbonatite Dyke from Bayan Obo, Inner Mongolia, North China

重点解剖了一条距白云鄂博超大型REE Nb Fe矿床东矿北东方向 2km、切割白云鄂博群H1及H3岩性段的细粒方解石碳酸岩岩墙的碳和氧同位素地球化学特征。结果表明 ,碳酸岩的碳同位素组成变化范围较小 (δ13C值为 - 6 .6‰～ - 4 .6‰ ) ,与正常地幔碳δ13C值 - 5± 2‰一致 ;而氧同位素组成变化范围较大 (δ18O值为 11.9‰～ 17.7‰ ) ,显著高于地幔的δ18O值 5.7± 1.0‰ ,表明碳酸岩浆在结晶过程中或之后曾与低温热液流体发生了同位素交换。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‰ ,处于不平衡状态 ,说明该碳酸岩墙中的白云石与方解石并非同成因矿物 ,白云石可能为次生成因的。

This contribution analyses the geochemical features of car bo n and oxygen isotope compositions for a REE-rich carbonatite dyke which is loca ted at Dulahala and lies 2 km NNE of the eastern ore body of Bayun Obo giant RE E-Nb-Fe min eral deposit. The dyke cuts across H1 coarse grained quartz sandstone and H3 sha les of the Proterozoic Bayun Obo Group. The analytical results indicate that car bon isotope compositions of the carbonatites vary in a narrow range, with δ- 1 3C values between -6.6‰ and -4.7‰ (PDB), which are within the mantle value s ( -5±2‰); while oxygen isotope compositions δ- 18O have a large range of v ariation from 11.9‰ to 17.7‰ (SMOW) which are apparently higher than mantl e values (5.7±1.0‰), It requires that the carbonatite dyke is more li kely to be related to exchange with low temperature hydrothermal fluids during m agma crystallization or at some later stage after carbonatite magmatism. Both ca rbon and oxygen isotope fractionations between dolomite and calcite from the dyk e is less than 0‰, being in conflict with the sequence of - 13C - and - 18O-enrichment in dolomite relative to calcite at thermodynamic equi librium. The disequilibrium dolomite-calcite paires are not cogenetic, and dolomite might b e of secondary origin, which could be related to the extensive dolomitization in the Bayun Obo area.