内生碳酸岩型稀土矿床中磷酸盐作用研究综述

A review on the role of phosphate in endogenetic carbonatite-hosted REE deposits

碳酸岩型稀土矿床是全球稀土最主要的来源。磷酸盐是该成矿体系中常见的组分,但对其在碳酸岩稀土成矿过程中的具体作用仍缺少系统的认识。笔者等综述了磷酸盐在碳酸岩岩浆形成、演化和稀土富集成矿过程中作用,并提出现存问题和研究展望。磷在碳酸盐熔体中具有很高的溶解度。磷的存在有利于稀土在地幔极低程度部分熔融过程和碳酸盐—硅酸盐液态不混溶过程中优先进入碳酸盐熔体,形成初始碳酸盐熔体中稀土的预富集。碳酸岩岩浆演化过程中,稀土将优先进入到不混溶的磷酸盐熔体或独居石和磷灰石等磷酸盐矿物中,这些熔体和矿物的行为很可能是控制体系中稀土行为和成矿潜力的关键因素。岩浆作用过程中形成的富稀土磷酸盐矿物还可为热液阶段稀土矿化提供成矿物质来源。磷酸盐还是热液过程中稀土的有效沉淀剂,有利于轻重稀土矿物在流体晚期阶段成矿。未来工作应更多关注磷酸盐在碳酸岩岩浆演化过程中的作用及其中重稀土的富集机理研究,针对具体成岩成矿过程开展实验岩石学和熔体包裹体研究。

Carbonatite-type rare earth elements(REE)deposits are the primary source of REE in the world.Phosphate is a common component in this mineralization system,but there is still a lack of systematic understanding of its role in the REE mineralization process of carbonatite.This article reviews the role of phosphate in the origin,evolution,and REE enrichment of the carbonatites and proposes current problems and research prospects.Phosphorus has a very high solubility in the carbonate melt.The presence of phosphorus is conducive to the preferential partitioning of REE into carbonate melt during the extremely low-degree mantle partial melting process and the carbonate—silicate liquid immiscibility process,therefore forming a pre-enrichment of REE in the initial carbonatitic melt.During the evolution of carbonatite magma,REE will preferentially enter immiscible phosphate melts or phosphate minerals such as monazite and apatite.The behavior of these melts and minerals is likely to be a key factor controlling REE's behavior and mineralization potential in the system.The REE-rich phosphate mineral formed during the magmatic stage can also be a source of REE mineralization in hydrothermal processes.Phosphate mineral is also an effective precipitator for REE in the hydrothermal process,which is conducive to mineralizing light and heavy REE in the late hydrothermal fluid stage.In the future,more attention should be paid to the role of phosphate in the evolution of the carbonatitic magmatic stage and the enrichment mechanism of heavy REE in this system,and experimental petrology and melt inclusion studies should be carried out for specific mineralization processes.