Niobium, Critical Metal, and Progeny of the Mantle
Niobium, Critical Metal, and Progeny of the Mantle
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铌、重要金属和地幔的后代
DOI：
10.5382/econgeo.4994
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发表时间：
2022
期刊：
影响因子：
5.8
通讯作者：
O. Vasyukova
中科院分区：
文献类型：
--
作者：
A. Williams;O. Vasyukova
由于技术进步，以及超过90%的产量由一个国家(巴西)生产的事实造成的供应风险，Nb是一种需求量很大的关键金属。在本文中，我们回顾了目前正在开采的矿床和其他潜在的经济矿床的地质情况，并建立了它们的成因模式。除了Lovozero矿床(俄罗斯)由层状二氧化硅-不饱和碱性火成岩杂岩赋存外，目前正在开采的所有Nb矿床都赋存于碳酸岩中，而且大多数具有经济潜力的矿床也赋存于这些岩石中。由于其高度不相容的性质，以及产生相应岩浆所需的地幔部分熔融程度较小，Nb在碳酸盐岩和碱性硅酸盐岩石中的聚集是由于它的高度不相容。将Nb在碱性硅酸盐岩浆中浓缩到经济水平的主要控制是分离结晶，部分在侵位之前，但主要在侵位之后。在二氧化硅不饱和岩浆的情况下，最终残留物饱和在真透析岩和钙钛矿等矿物中，在从这些岩浆结晶的层状杂岩中形成富Nb的岩层。在二氧化硅饱和岩浆的情况下，最终的残留物结晶了伟晶岩，这些伟晶岩是经济Nb矿化的宿主，通常以焦绿石的形式存在。相反，碳酸盐岩浆在侵位前很少或根本没有分离结晶作用。此外，就位时的分级结晶对Nb的浓缩到经济水平的影响很小。相反，我们认为碳酸盐岩岩浆与其宿主的交代作用形成了像金云母(闪锌矿)这样的岩石，消耗了大量的岩浆，留下了磷珊瑚残留物，焦绿石结晶的数量足以形成经济矿床。尽管许多Nb矿床显示出强烈的热液蚀变证据，在此期间，Nb矿物学可能会发生重大变化，但由于Nb在高温下在水溶液中的溶解度极低，因此无法通过热液流体对金属进行显著的活化和富集。然而，碳酸盐岩赋存的Nb矿床的风化导致表生富集(主要是由于碳酸盐矿物的溶解)，这可以使Nb品位增加一倍，使次经济矿床变得经济。焦绿石是这些红土型矿床中的主要Nb矿物，尽管其组成与原生矿化有很大不同。本文评价了导致Nb矿床成因的各种过程，并提供了一个框架，我们希望它将指导未来对Nb矿床的深入研究，并为其成功的勘探和开发提供更有效的战略。
Niobium is a critical metal in high demand because of technological advances and the supply risk created by the fact that over 90% of its production is by a single country (Brazil). In this paper, we review the geology of the deposits that are currently being mined and other potentially economic deposits as well as develop models for their genesis. With the exception of the Lovozero deposit (Russia), which is hosted by a layered silica-undersaturated alkaline igneous complex, all the deposits that are currently being mined for niobium are hosted by carbonatites, and most of the deposits with economic potential are also hosted by these rocks. Niobium owes its concentration in carbonatites and alkaline silicate rocks to its highly incompatible nature and the small degree of partial melting of the mantle required to generate the corresponding magmas. The primary control on the concentration of niobium to economic levels in alkaline silicate magmas is fractional crystallization, partly prior to but mainly after emplacement. In the case of silica-undersaturated magmas, the final residue saturates in minerals like eudialyte and loparite to form niobium-rich horizons in the layered complexes that crystallize from these magmas. The final residue, in the case of silica-saturated magmas, crystallizes the pegmatites that are the hosts to the economic niobium mineralization, which commonly takes the form of pyrochlore. In contrast, carbonatitic magmas undergo little to no fractional crystallization prior to emplacement. Moreover, fractional crystallization on emplacement has minimal impact on the concentration of niobium to economic levels. Instead, we propose that the metasomatic interaction of the carbonatitic magmas with their hosts to form rocks like phlogopitite (glimmerite) consumes much of the magma, leaving behind a phoscoritic residue from which pyrochlore crystallizes in amounts sufficient to form economic deposits. Although many niobium deposits display evidence of intense hydrothermal alteration, during which there can be major changes in the niobium mineralogy, the extremely low solubility of niobium in aqueous fluids at elevated temperature precludes significant mobilization and, thus, enrichment of the metal by hydrothermal fluids. However, weathering of carbonatite-hosted niobium deposits leads to supergene enrichment (due largely to the dissolution of the carbonate minerals) that can double the niobium grade and make subeconomic deposits economic. Pyrochlore is the principal niobium mineral in these laterite-hosted deposits, although its composition differs considerably from that in the primary mineralization. This paper evaluates the processes that appear to be responsible for the genesis of niobium ores and provides a framework that we hope will guide future in-depth studies of niobium deposits and lead to more effective strategies for their successful exploration and exploitation.