钠交代作用对碱性花岗岩体系中稀土富集成矿的贡献

碱性岩是全球稀土资源的主要来源之一, 特别是中-重稀土资源。针对碱性花岗岩体系中稀土(REE)和部分高场强元素(HFSE)超常富集机制及其控制因素等关键科学问题, 目前尚缺乏深入系统的研究工作。本文重点关注碱性花岗岩成矿体系中的钠交代作用, 对常见的钠长石化、钠铁闪石化和钠辉石化等热液交代过程中的矿物交代反应过程及其稀有稀土元素再活化-迁移过程开展分析和讨论。在碱性岩中, 碱性角闪石和辉石被认为是REE和HFSE的重要载体, 同时也是岩浆分异演化的有效示踪剂。以中国黑龙江碾子山碱性花岗岩为例, 岩浆晚期的热液交代会导致碱性角闪石和辉石从富钙的矿物相向富钠的矿物相转变, 在这个相转变过程中会导致REE和HFSE的活化和迁移。特别是岩浆晚期热液富钠霓辉石的形成过程中REE和HFSE会大规模释放到交代流体中, 可能是导致这些关键金属元素在岩浆晚期超常富集和成矿的关键控制因素。因此, 碱性角闪石和辉石的钠指数Na^#=Na/(Na+Ca)摩尔比]可能是一个潜在的矿物地球化学指标, 或将为碱性岩型稀土资源勘查提供新的方法。

Na-metasomatism and its contributions to REE enrichment in alkaline granite systems

Alkaline igneous rock is one of the main sources of rare earth resources in the world, especially heavy rare earth resources. Until now, there is a lack of comprehensive and systematic research on key scientific questions such as the super-enrichment mechanism of rare earth (REE) and some high field strength elements (HFSE) in alkaline granite systems and their controlling factors. This paper focuses on Na-metasomatism in the alkaline magmatic-hydrothermal systems, and discusses the mineral replacement reaction processes and the remobilization and migration processes of rare earth elements in the hydrothermal replacement processes, such as albitization, arfvedsonization and aegirinization. In alkaline systems, alkali amphiboles and pyroxenes are considered to be the significant carriers of REE and HFSE, and are also effective tracers of magmatic differentiation and evolution. Taking the Nianzishan alkaline granite in NE China as an example, the late magmatic-hydrothermal metasomatism will lead to the evolution of alkali amphibole and pyroxene from calcium-rich minerals to sodium-rich mineral phases, which will lead to the remobilization and transferring of REE and HFSE in the late hydrothermal stage. In particular, REE and HFSE would be released into the metasomatic fluids, which may be the key controlling factors leading to the super-enrichment and mineralization of these key metal elements in the late hydrothermal stage. Therefore, the sodium index Na#=Na/(Na+Ca) molar ratio] of the alkali amphiboles and pyroxenes may be a potential mineral geochemical index, which will provide a new proxy for REE exploration in the alkaline-peralkaline plutons.