The formation of the ore-bearing dolomite marble from the giant Bayan Obo REE-Nb-Fe deposit, Inner Mongolia: insights from micron-scale geochemical data
The formation of the ore-bearing dolomite marble from the giant Bayan Obo REE-Nb-Fe deposit, Inner Mongolia: insights from micron-scale geochemical data
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内蒙古白云鄂博巨型稀土铌铁矿床含矿白云石大理岩的形成：来自微米级地球化学数据的见解
DOI：
10.1007/s00126-019-00886-4
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发表时间：
2020
影响因子：
4.8
通讯作者：
Zhang W.
中科院分区：
文献类型：
--
作者：
Chen W;Liu H.Y.;Lu J.;Jiang S.Y.;Simonetti A.;Xu C.;Zhang W.
地球上最大的稀土元素（REE）存款，巴彦鄂博（中国），成因一直存在激烈的争论，特别是是否主机白云岩大理岩是沉积或岩浆成因。漫长的（中元古代到古生代）和复杂的（岩浆到交代）地质作用使地球化学解释复杂化。本文对巴彦鄂博大理岩中的容矿白云岩进行了全面的岩相学和原位高空间分辨率Sr-Pb同位素及地球化学研究。根据岩相学证据，将白云岩大理岩分为粗粒大理岩（CM）、细粒大理岩（FM）和非均质大理岩（HM）三种相。所有碳酸盐都是具有高SrO和MnO含量（> 0.15重量%）的铁白云石，与火成岩相吻合微量元素组成的这些辉长岩是高度可变的个体之间和内部的样品，与CM白云岩显示最强的轻稀土富集。白云岩的206 Pb/204 Pb和207 Pb/204 Pb比值与地幔值基本一致。然而，初始208 Pb/204 Pb比值定义了一个大范围从35.45到39.75，这可能是由于纳入放射性铅的分解独居石和/或氟碳铈矿在早古生代交代作用。白云岩的Sr同位素组成变化范围较大（87 Sr/86 Sr = 0.70292-0.71363）。CM白云岩的Sr同位素组成（~（87）Sr/~（86）Sr = 0.70295-0.70314）相对一致，是中元古代地幔的典型组成。FM和HM中白云岩的~（87）Sr/~（86）Sr比值和放射成因~（206）Pb/~（204）Pb比值的变化可能代表了早古生代交代作用中的重结晶作用，并有围岩放射成因Sr和Pb的贡献。因此，我们的原位地球化学数据支持中元古界岩浆成因的含矿白云岩大理石的巴彦鄂博存款，随后在早古生代进行了强烈的交代。
The genesis of Earth’s largest rare earth element (REE) deposit, Bayan Obo (China), has been intensely debated, in particular whether the host dolomite marble is of sedimentary or igneous origin. The protracted (Mesoproterozoic to Paleozoic) and intricate (magmatic to metasomatic) geological processes complicate geochemical interpretations. In this study, we present a comprehensive petrographic and in situ, high-spatial resolution Sr-Pb isotopic and geochemical investigation of the host dolomite from the Bayan Obo marble. Based on petrographic evidence, the dolomite marble is divided into three facies including coarse-grained (CM), fine-grained (FM), and heterogeneous marble (HM). All carbonates are ferroan dolomite with high SrO and MnO contents (> 0.15 wt.%), consistent with an igneous origin. Trace element compositions of these dolomites are highly variable both among and within individual samples, with CM dolomite displaying the strongest LREE enrichment. In situ206Pb/204Pb and207Pb/204Pb ratios of the dolomite are generally consistent with mantle values. However, initial208Pb/204Pb ratios define a large range from 35.45 to 39.75, which may result from the incorporation of radiogenic Pb released from decomposition of monazite and/or bastnäsite during Early Paleozoic metasomatism. Moreover, in situ Sr isotope compositions of dolomite indicate a large range (87Sr/86Sr = 0.70292–0.71363). CM dolomite is characterized by a relatively consistent, unradiogenic Sr isotope composition (87Sr/86Sr = 0.70295–0.70314), which is typical for Mesoproterozoic mantle. The variation of87Sr/86Sr ratios together with radiogenic206Pb/204Pb signatures for dolomite within FM and HM possibly represents recrystallization during Early Paleozoic metasomatism with the contribution of radiogenic Sr and Pb from surrounding host rocks. Therefore, our in situ geochemical data support a Mesoproterozoic igneous origin for the ore-bearing dolomite marble in the Bayan Obo deposit, which subsequently underwent intensive metasomatism during the Early Paleozoic.