内蒙古白云鄂博矿床巨量稀土的堆积及再活化历史:来自矿物微区Sm-Nd同位素的制约  

The REE mineralization and remobilization history of the giant Bayan Obo deposit,Inner Mongolia,China:Constraint from in-situ Sm-Nd isotopes of REE minerals

作　　者：李晓春 展云翔 范宏瑞[1,2] 杨奎锋 LI XiaoChun;ZHAN YunXiang;FAN HongRui;YANG KuiFeng(Key Laboratory of Mineral Resources,Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China;College of Earth and Planetary Science,University of Chinese Academy of Sciences,Beijing 100049,China)

机构地区：[1]中国科学院地质与地球物理研究所矿产资源研究重点实验室,北京100029 [2]中国科学院大学地球与行星科学学院,北京100049

出　　处：《岩石学报》2022年第10期2920-2932,共13页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41930430、92162323);中国科学院地质与地球物理研究所重点部署项目(IGGCAS-201901)联合资助。

摘　　要：内蒙古白云鄂博矿是世界著名的超大型稀土-铌-铁矿床,但该矿床的同位素定年结果复杂,成矿时代备受争议。为限定白云鄂博矿床的形成-演化历史,本文对矿床中的富稀土矿物——独居石、氟碳铈矿及磷灰石开展了微区Sm-Nd同位素研究。结果显示,独居石、氟碳铈矿及变形磷灰石的Sm-Nd同位素组成沿着13亿年的等时线分布,其ε_(Nd)(t=1300Ma)值和矿床中火成碳酸岩脉的ε_(Nd)(t=1300Ma)值十分接近。因此,白云鄂博矿床的稀土矿物主要形成于中元古代,成矿和幔源碳酸岩浆活动具有密切的成因关系。矿床中还发育未变形的磷灰石。多数未变形磷灰石的Sm-Nd同位素组成沿着4.5亿年的等时线分布,说明其形成于早古生代。值得注意的是,早古生代磷灰石的ε_(Nd)(t=450Ma)值和矿石中独居石及氟碳铈矿演化至4.5亿年的ε_(Nd)值高度一致,说明磷灰石生长所需要的稀土主要来自中元古代矿石中的物质再活化。此外,少量未变形的磷灰石形成于晚古生代(2.7亿年),其ε_(Nd)(t=270Ma)值和矿石中独居石及氟碳铈矿演化至2.7亿年的ε_(Nd)值非常接近,同样说明新生磷灰石中的稀土来自此前矿石中的物质再活化。总之,该研究表明白云鄂博矿床的巨量金属堆积主要发生在13亿年左右;矿床形成后分别在早古生代和晚古生代经历了两次改造作用,矿床改造过程没有明显提升资源量,但导致成矿元素的活化及部分矿物的新生,可能改变了矿石的品位及工艺性质。The Bayan Obo deposit in Inner Mongolia,China is well known for highly anomalous enrichments in REE.Isotopic dating studies of this deposit have reported complicated age dates,and thus debate persists over the age of mineralization.To better define the formation and evolution history of the deposit,this study conducts in-situ Sm-Nd isotopic analysis on REE-rich minerals(including monazite,bastnasite and apatite)of the deposit.The analyses show that the Sm-Nd isotope ratios of monazite,bastnasite and deformed apatite are aligned along the reference isochron of 1300Ma.In addition,theε_(Nd)(t=1300Ma)values of these minerals highly approach those of carbonatite dykes around the deposit.Therefore,it is proposed that the primary REE mineralization occurred at~1300Ma,and had a close genetic association with mantle-derived carbonatite.Quite a few undeformed apatite grains are present in the deposit.Most undeformed apatite grains have Sm-Nd isotope ratios aligned along the reference isochron of 450Ma,indicating that they were generated in the Early Paleozoic time.It is notable that these apatite grains haveε_(Nd)(t=450Ma)values highly approaching those of monazite and bastn site,implying that the REE in newly formed apatite were mainly sourced from primary REE minerals.We also identify a small portion of undeformed apatite grains that were formed at~270Ma,and theirε_(Nd)(t=270Ma)values highly approaching those of monazite and bastn site at the corresponding time.Similarly,the REE in the newly formed apatite were remobilized from previous REE minerals.In general,the primary REE mineralization mainly occur at~1300Ma in the Bayan Obo deposit.The deposit experienced two episodes of overprinting at Early Paleozoic and Late Paleozoic,respectively.The overprinting events did not obviously increase the metal tenor of the deposit,but induced REE remobilization and may change the grade of the ores.

关 键 词：稀土矿物 SM-ND同位素 成矿时代 白云鄂博 

分 类 号：P597.3[天文地球—地球化学] P618.7[天文地球—地质学]