| 川西冕宁-德昌稀土矿带霓长岩的地球化学特征及地质意义 |
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| 引用本文： | 舒小超,刘琰,李德良,贾玉衡.川西冕宁-德昌稀土矿带霓长岩的地球化学特征及地质意义[J].岩石学报,2019,35(5):1372-1388. |
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| 作者姓名： | 舒小超 刘琰 李德良 贾玉衡 |
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| 作者单位： | 中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083;自然资源部深地动力学重点实验室, 中国地质科学院地质研究所, 北京 100037,自然资源部深地动力学重点实验室, 中国地质科学院地质研究所, 北京 100037,中国地质大学地质过程与矿产资源国家重点实验室, 北京 100083,桂林理工大学地球科学学院, 桂林 541004 |
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| 基金项目： | 本文受国家重点基础研究发展计划"973"项目（2015CB452600）、国家自然科学基金重大研究计划（91855217）和面上项目（41772044）、中国地质科学院基本科研业务费（YYWF201705）、自然资源部中国地质调查局地质调查项目（DD20190060）及高等学校学科创新引智计划（B07011）联合资助. |
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| 摘 要： | 霓长岩化作用是指碳酸岩(或碱性岩)流体对围岩的交代蚀变,它是碳酸岩型稀土(REE)矿床常见的蚀变类型,其所形成的岩石即为霓长岩。对霓长岩的深入研究可以鉴别碳酸岩体的存在,厘定碳酸岩岩浆(或流体)的地球化学性质及源区特征,这对于找寻碳酸岩相关的矿产资源(尤其是REE)以及剖析矿床成因机制有着重要的地质意义。川西冕宁-德昌稀土矿带是中国最重要的轻稀土矿带之一,包括牦牛坪超大型、大陆槽大型、木落寨和里庄中小型REE矿床以及一系列矿点。REE矿化与碳酸岩-碱性岩杂岩体密切相关,受一系列新生代走滑断裂的控制。该矿带广泛发育霓长岩化蚀变带,尤以大陆槽及里庄矿床为显著。岩相学分析表明,大陆槽和里庄霓长岩中的矿物多呈他形粒状结构,主要由长石、黑云母、霓辉石以及少量副矿物组成;主微量元素分析表明,霓长岩的碱质(K_2O+Na_2O)、MgO、Fe_2O_3T含量较高,且富集REE、Sr、Ba等微量元素;电子探针分析表明,霓长岩中的霓辉石Fe OT含量较高,长石Na_2O及K_2O含量较高,Ca O含量极低。An-Ab-Or三角图解显示长石主要为透长石和钠长石,属碱性长石系列;黑云母的地球化学成分图解表明云母的成因类型为交代型且具有相对富镁、贫铁等特征,属镁质黑云母。霓长岩化作用的交代流体含有较高的CO_2组分,且富含碱质、Mg、Fe及REE、Sr、Ba等元素。对比霓长岩与原岩的主微量元素发现:相比于正长岩原岩,在主量元素中,霓长岩的Fe、Mg、Ca等元素含量增加,Si、Al等元素含量降低;微量元素中,霓长岩的REE及Sr、Ba等元素显著增加。这意味着交代流体含有的Fe_2O_3T、MgO、CaO等组分在霓长岩化过程中被带进了围岩,而SiO_2和Al_2O_3等从围岩中被逐出。大陆槽及里庄矿区发育的角砾岩指示了矿区曾经历过频繁的角砾岩化事件,这提高了霓长岩作用的强度,并且为矿脉的穿插及REE矿物的沉淀提供了空间。在霓长岩化过程中,流体-围岩的组分交换反复发生,这削弱了REE络合物的稳定性,伴随多期次的热液活动及构造事件,最终完成REE活化→迁移→沉淀的过程。
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| 关 键 词： | 霓长岩|蚀变作用|组分迁移|REE矿化|冕宁-德昌REE矿带 |
| 收稿时间： | 2018/12/20 0:00:00 |
| 修稿时间： | 2019/3/10 0:00:00 |
Geochemical characteristics and geological significance of fenites in the Mianning-Dechang REE belt, western Sichuan Province |
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| SHU XiaoChao,LIU Yan,LI DeLiang and JIA YuHeng.Geochemical characteristics and geological significance of fenites in the Mianning-Dechang REE belt, western Sichuan Province[J].Acta Petrologica Sinica,2019,35(5):1372-1388. |
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| Authors: | SHU XiaoChao LIU Yan LI DeLiang and JIA YuHeng |
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| Institution: | State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China;MNR Key Laboratory of Deep-Earth Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China,MNR Key Laboratory of Deep-Earth Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China,State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China and School of Earth Sciences, Guilin University of Technology, Guilin 541004, China |
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| Abstract: | Fenitization is induced by the ffluids exsolved from carbonatites (or alkaline rocks); the ultimate product of fenitization is called fenite. A comprehensive study of fenites can identify the presence of carbonatite, determine the geochemical characteristics and source of carbonatite magmas (or fluids), which is of great geological significance for the exploration of carbonatite-related mineral resources (especially REE) and the analysis of the genesis of these deposits. The Mianning-Dechang REE belt in western Sichuan Province, one of the most significant LREE belt in China, is approximately 270km long and 15km wide, hosting one giant (Maoniuping), one large (Dalucao), two small-medium-sized deposits (Muluozhai and Lizhuang) and numerous other mineralized occurrences. The REE mineralization is associated with the carbonatite-alkaline complexes, controlled by a series of Cenozoic strike-slip faults. Fenitization belts are widely observed in this belt, especially in the Dalucao and Lizhuang deposits. Petrographic observations suggest that the fenites in the Dalucao and Lizhuang deposits usually contain calcite veins, showing obvious hydrothermal filling and metasomatism phenomenon. The minerals in fenites are mostly of xenomorphic granular structure, mainly composed of feldspar, biotite, augite and accessory minerals (e.g., pyrite). Major and trace element data indicate the fenites have high contents of alkalis (K2O+Na2O), MgO, Fe2O3T, and are rich in some trace elements such as REE, Sr and Ba. Electron microprobe analyses indicate that aegirine-augites in fenites have a high content of FeOT, and feldspars are rich in Na2O and K2O, whereas the CaO content in feldspars is extremely low. The An-Ab-Or diagram shows the feldspars are mainly sanidine and albite, belonging to alkaline feldspar. The geochemical composition diagrams suggest that the genetic type of biotites is metasomatism and is characterized by high Mg and low Fe contents, which belongs to magnesium biotite. Fenitizing fluids have a high CO2 content and are rich in alkalis, Mg, Fe, REE, Sr, Ba, etc. By the comparison of major and trace elements among fenite and syenite, the followings were obtained:in major elements, the contents of Fe, Mg and Ca in fenite have increased compared with syenite, whereas the loss of Si and Al occurred; in trace elements, the contents of REE, Sr, Ba have significantly increased. That means the components of Fe2O3T, MgO and CaO in fenitizing fluids were brought into country rocks during fenitization, whereas SiO2 and Al2O3 were expelled from country rocks. The breccias in the Dalucao and Lizhuang deposits indicate that the two mining areas have experienced frequent brecciation, which enhanced the strength of fenitization and provided the space for vein overcutting and REE precipitation. During the process of fenitization, component migration during fluid-rock reaction has occurred frequently, diminishing the stability of REE-bearing complexes. Fenitization was accompanied by multiple hydrothermal activities and tectonic events, and these processes of REE activation, migration and precipitation were finally completed. |
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| Keywords: | Fenite|Alteration|Component migration|REE mineralization|Mianning-Dechang REE belt |
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