白云鄂博矿床成因的Mg同位素制约

Magnesium isotopic constraints on the genesis of Bayan Obo ore deposit

白云鄂博Fe-REE-Nb矿床是世界著名的巨型多金属矿床,它的成因一直是个激烈争论的问题。近年来Mg同位素研究快速发展,在示踪幔源火成碳酸岩和沉积碳酸盐岩方面显示出一定的潜力,为白云鄂博矿床成因问题的究提供了新的途径。对白云鄂博矿床H8白云岩、碳酸岩墙白云岩,以及中元古代沉积白云岩,腮林忽洞微晶丘白云岩的Mg同位素进行了对比研究。研究结果表明:碳酸岩墙样品的δ26Mg-DSM3变化范围为-0.34‰～-0.14‰,平均值-0.24‰,落在地幔岩端元;中元古代沉积白云岩的δ26MgDSM3为-1.81‰～-1.53‰;H8白云岩的δ26MgDSM3变化范围为-1.13‰～-0.10‰,平均值-0.53‰,部分落在地幔岩范围,部分落在地幔岩和沉积白云岩之间;而腮林忽洞微晶丘白云岩的δ26MgDSM3最轻,为-1.99‰～-1.93‰。白云鄂博矿床赋矿白云岩的Mg同位素组成特征不支持正常白云岩沉积成因和微晶丘成因观点,更倾向于火成碳酸浆成因观点。

Bayan Obo Fe-REE-Nb ore deposit is the largest light REE deposit in the world.Its genesis,however,has been a subject of longstanding debate.Here we present Mg isotope data of ore-hosted rocks（H8 dolomite marble）,carbonatite dykes nearby,and related sedimentary carbonate rocks measured by multiple collectors inductively coupled plasma source mass spectrometry（MC-ICP-MS）.δ26MgDSM3 values of carbonatite dykes range from-0.34‰ to-0.14‰,consistent with those reported for mantle-sourced igneous rocks;mesoproterzoic sedimentary dolostone vary between-1.81‰ to-1.53‰ in δ26Mg;the δ26Mg values of H8 dolomite marble from ore bodies fall into the range from-1.13‰ to-0.10‰ with an average of-0.53‰,which are between those of igneous rocks and sedimentary dolostone.Sedimentary marine micrite from Heinaobao,ca.25km southeast of the Bayan Obo ore deposit show the lightest Mg isotope composition,with δ26MgDSM3 values varying from-1.99‰ to-1.93‰.Our Mg isotope data indicate that H8 dolomite marble of Bayan Obo ore deposit is not a micrite mound or normal sedimentary carbonate rocks.The genesis of Bayan Obo ore deposit can be better explained by a mantle-drived carbonatite origin.