The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits
The origin and composition of carbonatite-derived carbonate-bearing fluorapatite deposits
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DOI：
10.1007/s00126-020-01010-7
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发表时间：
2020-08
影响因子：
4.8
通讯作者：
S. Broom-Fendley;P. Siegfried;F. Wall;Mary O’Neill;R. Brooker;Emily K. Fallon;J. Pickles;D. Banks
中科院分区：
文献类型：
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作者：
S. Broom-Fendley;P. Siegfried;F. Wall;Mary O’Neill;R. Brooker;Emily K. Fallon;J. Pickles;D. Banks
含碳酸盐的氟磷灰石岩石出现在全球分布的30多个碳酸盐岩复合体中，代表了化肥工业的磷的大量潜在供应。然而，在某些碳酸岩中形成含碳酸盐氟磷灰石的过程仍然不明确，推断为热液和风化机制。在这篇文章中，我们比较了Kovdor，Sokli，Bukusu，Catalão I和Glenover碳酸盐岩中含碳酸盐氟磷灰石岩石的共生和微量元素含量，以进一步了解它们的起源，以及评论可能对肥料生产有害的元素浓度。每个存款的磷灰石共生是大致相同的，包括残留的岩浆颗粒过度生长的几个不同阶段的碳酸盐氟磷灰石。第一种是在残余岩浆颗粒上形成外延生长，随后形成块状磷灰石，而块状磷灰石又被晚期自形磷灰石和胶状磷灰石世代横切。从成分上看，共生序列对应于稀土元素（REE）、Sr、Na和Th浓度大幅下降，U和Cd浓度增加。含碳酸盐的氟磷灰石表现出负Ce异常，归因于氧化条件下的survival环境，并结合纹理和成分的共性，支持这些岩石的风化起源。含碳酸盐的氟磷灰石具有Th含量，其比岩浆磷灰石颗粒低几个数量级，潜在地使这种磷灰石成为肥料工业的更具环境吸引力的原料。含碳酸盐氟磷灰石中铀和镉的含量高于岩浆碳酸盐磷灰石，但远低于大多数海相磷块岩。
Carbonate-bearing fluorapatite rocks occur at over 30 globally distributed carbonatite complexes and represent a substantial potential supply of phosphorus for the fertiliser industry. However, the process(es) involved in forming carbonate-bearing fluorapatite at some carbonatites remain equivocal, with both hydrothermal and weathering mechanisms inferred. In this contribution, we compare the paragenesis and trace element contents of carbonate-bearing fluorapatite rocks from the Kovdor, Sokli, Bukusu, Catalão I and Glenover carbonatites in order to further understand their origin, as well as to comment upon the concentration of elements that may be deleterious to fertiliser production. The paragenesis of apatite from each deposit is broadly equivalent, comprising residual magmatic grains overgrown by several different stages of carbonate-bearing fluorapatite. The first forms epitactic overgrowths on residual magmatic grains, followed by the formation of massive apatite which, in turn, is cross-cut by late euhedral and colloform apatite generations. Compositionally, the paragenetic sequence corresponds to a substantial decrease in the concentration of rare earth elements (REE), Sr, Na and Th, with an increase in U and Cd. The carbonate-bearing fluorapatite exhibits a negative Ce anomaly, attributed to oxic conditions in a surficial environment and, in combination with the textural and compositional commonality, supports a weathering origin for these rocks. Carbonate-bearing fluorapatite has Th contents which are several orders of magnitude lower than magmatic apatite grains, potentially making such apatite a more environmentally attractive feedstock for the fertiliser industry. Uranium and cadmium contents are higher in carbonate-bearing fluorapatite than magmatic carbonatite apatite, but are much lower than most marine phosphorites.