Enrichment of heavy REE and Th in carbonatite-derived fenite breccia
Enrichment of heavy REE and Th in carbonatite-derived fenite breccia
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DOI：
10.1017/s0016756821000601
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发表时间：
2021-07
影响因子：
2.3
通讯作者：
S. Broom-Fendley;H. Elliott;C. Beard;F. Wall;Paul EB Armitage;A. Brady;E. Deady;W. Dawes
中科院分区：
文献类型：
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作者：
S. Broom-Fendley;H. Elliott;C. Beard;F. Wall;Paul EB Armitage;A. Brady;E. Deady;W. Dawes
由于碳酸岩的成因倾向于轻稀土元素，重稀土元素在碳酸岩中的富集是罕见的。我们描述重稀土元素富集在fenitized响岩角砾岩，专注于小卫星发生1-2公里的Songwe山碳酸盐岩，马拉维。在角砾岩基质中，含稀土元素的矿物组合包括磷钇矿、锆石、金红石/金红石和少量的硬硅钙石/钍石，以及萤石和磷灰石。含锶碳酸盐岩脉、碳酸盐岩捕虏体和广泛的钾化作用表明重稀土矿化与碳酸盐岩侵位之间存在成因联系。我们建议，重稀土元素被保留在热液流体是残余来自碳酸盐岩沉淀后的轻稀土矿物。角砾化作用为这种流体提供了一个集中的通道，使重稀土元素的运输和磷钇矿沉淀在长花岗岩。持续的流体-岩石相互作用导致含重稀土矿物的溶解，并进一步沉淀磷钇矿和膨润土/钍石。在最大Y含量为3100 µg g−1时，HREE浓度不足以构成矿石，但这些岩石的成分和结构与其他碳酸盐岩相关的HREE富集情况相似，表明它们都是通过与石化作用相关的共同机制形成的。重稀土元素矿物的沉淀只发生在预先存在的结构提供了一个集中的管道，为fenitizing流体，减少流体-国家-岩石的相互作用。富重稀土元素和钍的fenite角砾岩作为一个指标，流体从碳酸盐岩排出，并可能指示轻稀土矿化的源碳酸盐岩体内的存在，在深度。
Abstract Enrichment of the heavy rare earth elements (HREE) in carbonatites is rare as carbonatite petrogenesis favours the light (L)REE. We describe HREE enrichment in fenitized phonolite breccia, focusing on small satellite occurrences 1–2 km from the Songwe Hill carbonatite, Malawi. Within the breccia groundmass, a HREE-bearing mineral assemblage comprises xenotime, zircon, anatase/rutile and minor huttonite/thorite, as well as fluorite and apatite. A genetic link between HREE mineralization and carbonatite emplacement is indicated by the presence of Sr-bearing carbonate veins, carbonatite xenoliths and extensive fenitization. We propose that the HREE are retained in hydrothermal fluids which are residually derived from a carbonatite after precipitation of LREE minerals. Brecciation provides a focusing conduit for such fluids, enabling HREE transport and xenotime precipitation in the fenite. Continued fluid–rock interaction leads to dissolution of HREE-bearing minerals and further precipitation of xenotime and huttonite/thorite. At a maximum Y content of 3100 µg g−1, HREE concentrations in the presented example are not sufficient to constitute ore, but the similar composition and texture of these rocks to other cases of carbonatite-related HREE enrichment suggests that all form via a common mechanism linked to fenitization. Precipitation of HREE minerals only occurs where a pre-existing structure provides a focusing conduit for fenitizing fluids, reducing fluid – country-rock interaction. Enrichment of HREE and Th in fenite breccia serves as an indicator of fluid expulsion from a carbonatite, and may indicate the presence of LREE mineralization within the source carbonatite body at depth.