Carbonatitic Magma Fractionation and Contamination Generate Rare Earth Element Enrichment and Mineralization in the Maoniuping Giant REE Deposit, SW China
Carbonatitic Magma Fractionation and Contamination Generate Rare Earth Element Enrichment and Mineralization in the Maoniuping Giant REE Deposit, SW China
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牦牛坪巨型稀土矿床碳酸岩浆分异和污染导致稀土元素富集和矿化
DOI：
10.1093/petrology/egad037
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发表时间：
2023
影响因子：
3.9
通讯作者：
Zheng X
中科院分区：
文献类型：
--
作者：
Zheng X
碳酸岩侵入体蕴藏着世界上最重要的轻稀土元素 (LREE) 矿床，其形成通常需要非凡的肥沃资源、岩浆演化和热液事件。然而，碳酸盐岩浆演化，尤其是硅酸盐岩的分异结晶和污染在稀土元素富集中的作用，仍然是个谜。位于中国西南部的牦牛坪世界级稀土矿床是破译岩浆演化和相关稀土元素富集的理想目标，因为它显示出从深层的中粒到粗粒方解石碳酸岩（碳酸岩 I）到浅层渐进伟晶状方解石碳酸岩（碳酸岩 II）的连续演化。在这两种类型的方解石碳酸盐岩中，根据岩相和地球化学特征可将方解石分类为四代。早期结晶方解石（Cal-I 和 Cal-II）存在于碳酸岩 I 中，并表现出等粒和多边形镶嵌结构，而碳酸岩 II 中的晚期方解石（Cal-III 和 Cal-IV）是大尺寸卵晶（长度 >0.5 毫米），具有应变诱导的波状消光和弯曲孪晶片层。所有这些世代的方解石都产生相似的、接近球粒状的 Y/Ho 比值 (26.6–28.1)，并被推断为岩浆起源。值得注意的是，MgO、FeO 和 MnO 从 Cal-I 到 Cal-IV 的逐渐富集伴随着 REE 含量的显着增加（~800 至 2000 ppm），具有富含 LREE 和平缓至陡峭的球粒陨石归一化 REE 模式（分别为 (La/Yb)N= 3.1–26.8 和 (La/Sm)N= 0.9–3.9）。如此显着的稀土元素富集归因于岩浆分异结晶作用的延长，最初的分异结晶程度较低（熔体再采矿分数（F） = ~0.95），通过形成丰富的方解石堆积而发展到后期（F = 0.5-0.6）。岩浆演化过程中轻稀土和重稀土行为的差异很大程度上取决于金云母、角闪石和单斜辉石从碳酸盐岩熔体中的分离，这由逐渐升高的 (La/Yb)N 比率从 3.1 到 26.8 表示。四代方解石的C、Sr同位素组成差异显着，δ13CV-PDB从-3.28下降到-9.97‰，87Sr/86Sr从0.70613上升到0.70670。根据空间关系和岩相观察，δ13C的相对富集和Cal-I和Cal-II的87Sr/86Sr比值的贫化表现出继承自初始碳酸岩浆的主要同位素特征。相比之下，Cal-III 和 Cal-IV 的可变 Sr 和 C 同位素组成被解释为硅酸盐围岩成分污染和脱碳反应损失 CO2 的结果。为了模拟此类污染过程，引入了罗利挥发和蒙特卡罗模拟，模型结果表明碳酸盐岩熔体-围岩相互作用需要硅酸盐岩石中 40% 的放射性 Sr 污染和碳酸盐岩熔体中 35% 的 CO2 脱气。此外，δ13C值下降与稀土元素含量增加之间的正相关性，以及氟碳铈矿（Ce）沉淀，表明污染过程中稀土元素进一步积累。总之，除了富含稀土元素的岩浆源之外，碳酸岩岩浆演化过程中的分异结晶和污染程度被认为是全球碳酸岩相关稀土矿床中稀土元素富集和矿化的重要机制。
Carbonatite intrusions host the world’s most important light rare earth element (LREE) deposits, and their formation generally requires extraordinary fertile sources, magmatic evolution, and hydrothermal events. However, carbonatitic magma evolution, particularly the role of fractional crystallization and contamination from silicate rocks in REE enrichment, remains enigmatic. The Maoniuping world-class REE deposit in southwestern China, is an ideal target to decipher magmatic evolution and related REE enrichment as it shows continuous textual evolution from medium- to coarse-grained calcite carbonatite (carbonatite I) at depth, to progressively pegmatoidal calcite carbonatite (carbonatite II) at shallow levels. In both types of calcite carbonatites, four generations of calcite can be classified according to petrographic and geochemical characteristics. Early-crystalizing calcite (Cal-I and Cal-II) are found in carbonatite I and exhibit equigranular and a polygonal mosaic textures, while late calcites (Cal-III and Cal-IV) in carbonatite II are large-size oikocrysts (>0.5 mm in length) with strain-induced undulatory extinction and bent twinning lamellae. All these generations of calcite yield similar, near-chondritic, Y/Ho ratios (26.6–28.1) and are inferred to be of magmatic origin. Remarkably, gradual enrichment of MgO, FeO and MnO from Cal-I to Cal-IV is coupled with a significant increase in REE contents (~800 to 2000 ppm), with LREE-rich and gentle-to-steep chondrite-normalized REE patterns ((La/Yb)N= 3.1–26.8 and (La/Sm)N= 0.9–3.9, respectively). Such significant REE enrichment is ascribed to protracted magma fractional crystallization with initial low degree of fractional crystallization (fraction of melt remining (F) = ~0.95) evolving to late stage (F = 0.5–0.6) by formation of abundant calcite cumulates. Differential LREE and HREE behavior during magma evolution largely depend on separation of phlogopite, amphibole, and clinopyroxene from the carbonatitic melt, which is indicated by progressively elevated (La/Yb)Nratios ranging from 3.1 to 26.8. The four generations of calcite have significantly different C and Sr isotopic compositions with δ13CV-PDBdecreasing from −3.28 to −9.97‰ and87Sr/86Sr increasing from 0.70613 to 0.70670. According to spatial relations and petrographic observations, the relative enrichment of δ13C and depletion in87Sr/86Sr ratios of Cal-I and Cal-II show primary isotopic characteristics inherited from initial carbonatitic magma. By contrast, the variable Sr and C isotopic compositions of Cal-III and Cal-IV are interpreted as the results of contamination by components derived from silicate wall rocks and loss of CO2by decarbonation reactions. To model such contamination processes, Raleigh volatilization and Monte Carlo simulation have been invoked and the model results reveal that carbonatitic melt-wall rock interaction requires 40% radiogenic Sr contamination from silicate rocks and 35% CO2degassing from carbonatitic melt. Moreover, positive correlations between decreasing δ13C values and increasing REE contents, together with bastnäsite-(Ce) precipitation, indicate further REE accumulation during the contamination processes. In summary, alongside REE-rich magma sources, the extent of fractional crystallization and contamination during carbonatitic magma evolution are inferred to be important mechanisms in terms of REE enrichment and mineralization in carbonatite-related REE deposits worldwide.