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.

中文翻译：

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重稀土元素和钍在碳酸盐岩衍生的角砾岩中的富集

碳酸岩中重稀土元素 (HREE) 的富集是罕见的，因为碳酸岩的成因有利于轻 (L) REE。我们描述了 fenitized 音长岩角砾岩中的 HREE 富集，重点关注距离马拉维松圭山碳酸盐岩 1-2 公里的小型卫星矿点。在角砾岩地块中，含 HREE 的矿物组合包括磷钇矿、锆石、锐钛矿/金红石和次要的huttonite/thorite，以及萤石和磷灰石。含 Sr 的存在表明了 HREE 矿化和碳酸盐岩侵位之间的遗传联系碳酸盐岩脉、碳酸盐岩包体和广泛的分矿化。我们建议 HREE 保留在热液流体中，这些热液流体是在 LREE 矿物沉淀后残留从碳酸盐岩中提取的。角砾岩为这些流体提供了一个聚焦管道，使高稀土运输和磷矿中的钇钇沉淀。持续的流体-岩石相互作用导致含高稀土元素的矿物溶解，并进一步沉淀出磷钇矿和huttonite/thorite。最大Y含量为3100 µg g-1，本示例中的 HREE 浓度不足以构成矿石，但这些岩石的成分和质地与其他与碳酸盐相关的 HREE 富集情况相似，这表明它们都是通过与铁矿化相关的共同机制形成的。只有在预先存在的结构提供了一个集中管道以使流体进行泥化、减少流体-围岩相互作用的情况下，才会发生 HREE 矿物的沉淀。尖晶石角砾岩中 HREE 和 Th 的富集可作为流体从碳酸盐岩中排出的指标，并可能表明在深度源碳酸盐岩体内存在 LREE 矿化。