Origin of carbonatite-related niobium deposits: Insights from pyrochlore geochemistry
Origin of carbonatite-related niobium deposits: Insights from pyrochlore geochemistry
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与碳酸岩有关的铌矿床的起源：烧绿石地球化学的见解
DOI：
10.1016/j.gca.2023.12.010
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发表时间：
2024
影响因子：
5
通讯作者：
Velásquez-Ruiz F
中科院分区：
文献类型：
--
作者：
Velásquez-Ruiz F
巴西中部Alto Paranaíba火成岩省（APIP）的碳酸盐岩相关铌矿床目前占全球铌产量的92%。在APIP中，辉绿石富集于磁铁矿-磷灰石-四铁云母±碳酸盐岩或磷灰石中，在下第三纪与碳酸盐岩呈互层状，在上第三纪与磷灰石和方解石碳酸岩呈滋养脉状，在上第三纪与磷灰石和方解石碳酸岩呈滋养脉状。光斑岩-碳酸盐岩组合的成因可以用三种假说来解释：(1)结晶分离，(2)液体不混溶，和/或(3)基底交代作用（磷化作用）后形成的光斑岩浆。然而，目前尚不清楚焦绿石的形成是否仅限于碳酸盐岩事件，碳热液事件，或两者兼有，这一知识差距在本工作中得到了解决。为了研究富焦绿石磷灰石的成因，对catal<s:1> o II杂岩Boa Vista Nb矿岩浆岩和碳热岩中的焦绿石晶体进行了阴极发光（CL）图像、化学图谱和LA-ICP-MS数据分析。在Boa Vista矿中，下部深埋带主要记录了振荡和斑片带状构造，而上部浅埋带则记录了次生溶蚀、骨架状和无带状构造。在Boa Vista矿，钙绿石是主要的铌相，只有两个钙绿石异常值。辉绿石CI球粒陨石归一化稀土元素分布与碳酸盐岩和磷灰岩的地球化学结果一致，表明辉绿石为岩浆成因，在碳热脉中存在辉绿石反晶。烧绿石的Sr/Y / La和Na / Ce图显示出连续分馏模式，其中有一些反晶和初生相的混合物。利用CL对积云间方解石进行的检查提供了在四铁云母磷光石岩脉中存在碳酸盐岩浆残留物的证据，并表明富含碱性碳酸盐的流体在运输重矿物（即磁铁矿、磷灰石、焦绿石）方面发挥了作用。因此，Boa Vista铌矿的结构和化学证据表明，富焦绿石磷矿的成因是碳酸盐岩浆中重矿物通过分馏结晶作用的物理分离，导致富焦绿石碳酸盐和磷石岩脉的侵位。catal<s:1> o II的启示可能会扩展到其他APIP碱碳酸盐岩复合体，因为它们具有共同的成因联系，并应激发对其他碳酸盐岩相关铌矿床的焦绿石地球化学的进一步研究，这将对提高我们对全球铌成矿作用的认识至关重要。
The carbonatite-related Nb deposits of the Alto Paranaíba Igneous Province (APIP) in central Brazil, currently account for ∼92 % of the global Nb production. In the APIP, pyrochlore is abundant in magnetite–apatite–tetraferriphlogopite ± carbonate rocks or phoscorites, occurring as interbedded layers with carbonatites in the lower hypogene zone, feeding dike swarms of phoscorite and calcite carbonatite, and late-stage carbothermal veins in the upper hypogene zone. The origin of the phoscorite-carbonatite association can be explained by three hypotheses: (1) crystal segregation from fractional crystallization, (2) liquid immiscibility, and/or (3) phoscoritic magma formation after basement metasomatism (fenitization). However, it is not well understood whether pyrochlore formation is limited to a carbonatitic event, carbohydrothermal, or both, and this gap of knowledge is addressed in this work. To investigate the petrogenesis of pyrochlore-rich phoscorite, cathodoluminescence (CL) images, chemical maps, and LA-ICP-MS data were acquired of pyrochlore crystals from magmatic and carbothermal rocks from the Boa Vista Nb mine, Catalão II Complex. In the Boa Vista mine, oscillatory and patchy zoning were identified as primary pyrochlore textures commonly recorded at the lower hypogene zone, while secondary dissolution, skeletal and zonation-free textures are registered at shallower depths in the upper hypogene zone. Calciopyrochlore is the dominant Nb phase at the Boa Vista mine, with only two kenopyrochlore outliers. The pyrochlore CI chondrite-normalized REE distribution is consistent with geochemical results of the carbonatite and phoscorite rocks, indicating a magmatic origin for pyrochlore and the presence of pyrochlore antecrysts in carbothermal veins. The Sr/Y vs La and Na vs Ce diagrams in pyrochlore indicate a continuous fractionation pattern, with some mixtures of antecrysts and primary phases. An examination of intercumulus calcite using CL provide evidence of carbonatitic magma residues within tetraferriphlogopite phoscorite dikes and suggests that alkaline–carbonate-rich fluids played a role in transporting heavy minerals (i.e., magnetite, apatite, pyrochlore). Consequently, the textural and chemical evidence in the Boa Vista Nb mine indicates that the origin of pyrochlore-rich phoscorites is the result of physical segregation of heavy minerals from a carbonatite magma by fractional crystallization, leading to the emplacement of pyrochlore-rich carbonatite and phoscorite dikes. The implications at Catalão II may extend to other APIP alkaline-carbonatite complexes, as they share a genetic connection, and should motivate further studies focusing on pyrochlore geochemistry in other carbonatite-related Nb deposits, which will be crucial for advancing our knowledge of global Nb metallogenesis.
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