褐帘石稀土活化对离子吸附型稀土矿床成矿的指示  

REE remobilization of allanite:Implications for the formation of ion-adsorption REE deposits,South China

作　　者：冯雨周 陈华勇[1,2,3] 肖兵 初高彬[1] 郑惠 沈灿 Yuzhou Feng;Huayong Chen;Bing Xiao;Gaobin Chu;Hui Zheng;Can Shen(CAS Key Laboratory of Mineralogy and Metallogeny,Guangzhou Institute of Geochemistry,Chinese Academy of Sciences,Guangzhou 510640,China;University of Chinese Academy of Sciences,Beijing 100049,China;Guangdong Provincial Key Laboratory of Mineral Physics and Materials,Guangzhou 510640,China;Key Laboratory of Metallogenic Prediction of Nonferrous Metals and Geological Environment Monitoring,School of Geoscience and Info-Physics,Central South University,Changsha 410083,China)

机构地区：[1]中国科学院广州地球化学研究所,中国科学院矿物学与成矿学重点实验室,广州510640 [2]中国科学院大学,北京100049 [3]广东省矿物物理与材料研究开发重点实验室,广州510640 [4]中南大学有色金属成矿预测与地质环境监测教育部重点实验室,长沙410083

出　　处：《科学通报》2023年第10期1217-1229,共13页Chinese Science Bulletin

基　　金：广东省基础与应用基础研究重大项目(2019B030302013);国家创新研究群体科学基金(41921003);国家自然科学基金(42103058,41725009,42173065);广东省重点实验室项目(2020B1212060055)资助。

摘　　要：古城矿床为华南地区新发现的一个同时富含轻重稀土的离子吸附型稀土矿床,矿体位于粗粒黑云母花岗岩风化壳中.褐帘石是粗粒黑云母花岗岩中一种重要的副矿物(占比约为0.09%(vol)),U-Pb年龄分析结果显示其结晶年龄为102.8±4.3 Ma.化学成分分析结果显示原生褐帘石具有富轻稀土的特征,其轻稀土、重稀土和轻稀土/重稀土比值分别为179778~189429、2165~4595 ppm(1 ppm=10^(–6)m^(3)/m^(3))和41~84.这些原生褐帘石轻稀土含量约占全岩轻稀土总量的97%,为古城矿区轻稀土元素的主要来源.褐帘石结构与成分分析指示,稀土元素的存在会导致其晶体结构中稀土元素与氧原子构成的化学键键长增加而键强减弱.在被岩浆-热液流体交代时,这些化学键发生破裂,进而导致轻稀土元素会被淋滤出来并进入到热液流体中,并在随后主要进入了氟碳钙铈矿中.综上,褐帘石被交代后形成氟碳钙铈矿的过程中轻稀土元素发生了活化、迁移和富集,且在该过程中轻稀土元素之间发生了分馏.本研究表明,原生褐帘石被热液流体交代有利于轻稀土元素的富集成矿,但对重稀土矿化的贡献较小.The Gucheng deposit in South China is a newly discovered ion-adsorption-type rare earth element (REE) deposit with bothLREE and HREE mineralization, of which theΣLREE andΣHREE oxides account for about 45% and 55%, respectively.The orebody is hosted in the weathered coarse-grained biotite granite (CGBG). Allanite is a key accessory mineral (0.09%(vol)) in the CGBG. Allanite fs-LA-ICP-MS U-Pb dating shows that the magmatic allanite crystallized at 102.8 ± 4.3 Ma,representing the initial LREE enrichment in the magma evolution. Geochemical data show that the primary magmaticallanite is characterized by LREE enrichments, containing ΣLREE = 195744 to 211588 ppm, ΣHREE = 2365 to5059 ppm, and LREE/HREE = 40.8 to 83.6. LREEs of the primary magmatic allanite account for ~97% of the total LREEsof the CGBG, representing the most important LREE source at Gucheng. SEM-EDS (Scanning electron microscope-Energy Dispersive Spectrometer) analysis and BSE (Back-scatterred Electron) imaging show that the primary allanite hasbeen replaced by hydrothermal synchysite-(Ce) via dissolution-reprecipitation. The formation of synchysite-(Ce) indicatesthat this hydrothermal fluid is rich in CO_(3)^(2–) and F^(–), and was likely sourced from magmatic-hydrothermal processesassociated with the CGBG emplacement. Single-crystal diffraction data show that the polyhedral volume (Ca and LREE atA2 occur with adjacent O atoms in the primary allanite) increases with increasing bond distance of A2–O3, A2–O7, A2–O8and A2–O10, comparing with the typical epidote. These structural variations of primary allanite may have resulted from theREE incorporation.Meanwhile, the REE incorporation in allanite can weaken the bond strength of A2–O3, A2–O7, A2–O8 and A2–O10. Asa result, the alteration of primary allanite by hydrothermal fluid would release LREEs and Ca from the A2 site. Our alteredallanite samples have higher ΣHREE (145697 to 162278 ppm) but lower ΣLREE (150440 to 169564 ppm) and LREE/HREE (14.5 to 50.5) than the primary allanit

关 键 词：褐帘石 稀土活化 离子吸附型稀土矿床 华南地区 

分 类 号：P618.7[天文地球—矿床学]