伟晶质岩浆的同化混染与分离结晶(AFC)作用及铀成矿效应:以纳米比亚湖山铀矿为例  

Assimilation-fractional crystallization(AFC)of pegmatiticmagma and its implications for uranium mineralization:A case study of the Husab uranium deposit,Namibia

作　　者：黄冉笑 王果胜[1] 袁国礼[1] 邱坤峰 Hounkpe Jechonias BIDOSSESSI HUANG Ranxiao;WANG Guosheng;YUAN Guoli;QIU Kunfeng;Hounkpe Jechonias BIDOSSESSI(School of Earth Sciences and Resources,China University of Geosciences(Beijing),Beijing 100083,China;China National Nuclear Geology and Mining Technology Group Co.,Ltd,Beijing 100013,China;Department of Earth Sciences,Faculty of Sciences and Engineering,University of Abomey-Calavi,Cotonou 999105,Benin)

机构地区：[1]中国地质大学(北京)地球科学与资源学院,北京100083 [2]中核地矿科技集团有限公司,北京100013 [3]贝宁阿博梅卡拉维大学科学与工程学院地球科学系,科托努999105

出　　处：《地学前缘》2022年第1期377-402,共26页Earth Science Frontiers

基　　金：国家自然科学基金项目(41872100);中非高校20+20合作计划项目(2019);国家重点研发计划项目(2018YFC0604106)。

摘　　要：纳米比亚湖山铀矿位于达马拉造山带的中央南部地区,工业铀矿物为晶质铀矿,属于伟晶岩型铀矿床。关于不同矿石中铀元素的富集与沉淀机制还存在一定争议。为了揭示伟晶质岩浆演化与铀矿化作用的关系,本文对矿区内不同矿物组成的伟晶岩型矿石开展了岩石和矿物地球化学研究。野外及镜下鉴定结果显示,矿化伟晶岩可以分为“简单类型”矿体和“复杂类型”矿体。前者具有正常的花岗伟晶结构,晶质铀矿均匀分布于造岩矿物之间,矿化程度低到中等;后者表现出非均匀的结构特征,且矿化程度极高,晶质铀矿在成因上与大量黑云母团块有明显的空间联系。地球化学研究表明:在“简单类型”伟晶岩中,铀元素主要通过伟晶质岩浆的分离结晶作用富集;“复杂类型”伟晶质岩浆的演化则明显受控于同化混染作用,其铀矿化为岩浆同化混染与分离结晶(assimilation-fractional crystallization,AFC)作用产物。具体而言,外来基性组分(FeO,MgO,TiO_(2),MnO)的混入导致“复杂类型”熔体中矿物的结晶顺序发生改变,长石类矿物的“延后”结晶为黑云母提供了更加有利的结晶空间和条件,促使黑云母以团块状聚集的形式产出。黑云母的大量析出会引发残余岩浆中UF_(m)^(4-m)络合物的水解,导致晶质铀矿在团块黑云母内部或周围沉淀。因此,本文有关“简单类型”和“复杂类型”产铀伟晶岩的研究,有效地揭示了岩浆演化过程与铀矿化机制,丰富了伟晶岩型铀矿床理论,为后期勘查开发提供了科学依据。The Husab uranium deposit in the southern Central Zone of the Damara Orogenic Belt,Namibia,is a typical pegmatite-hosted ore deposit with uraninite as the main industrial uranium mineral.The mechanisms of uranium enrichment and precipitation in different ores are not clear.In order to reveal the relationship between pegmatitic magma evolution and uranium mineralization,we carried out whole-rock and mineral geochemical studies on pegmatite ores with different mineral compositions.Field and microscopic investigations showed the mineralized pegmatites can be divided into‘simple’and‘complex’types.The‘simple’pegmatite has normal granitic pegmatite texture,with a uniform distribution of uraninites among rock-forming minerals and low to medium level mineralization.The‘complex’pegmatite is characterized by heterogeneous texture with extremely high level mineralization and spatial association of uraninite with biotite agglomerates.Geochemical analysis showed that,in‘simple’pegmatite,uranium enrichment is mainly achieved through fractional crystallization of pegmatitic magma;while in‘complex’pegmatite,magmatic evolution is clearly controlled by assimilation and contamination,and uranium mineralization is the result of assimilation-fractional crystallization(AFC)of pegmatitic magma.The mixing of external mafic components(FeO,MgO,TiO_(2),MnO),specifically,led to change in the mineral crystallization sequence in‘complex’pegmatite melt,where“delayed”crystallization of feldspar minerals provided biotite with more favorable crystallization space/condition and led to massive biotite aggregation.Abundant biotite precipitation induced hydrolysis of UF_(m)^(4-m) complex in residual magama,which resulted in the precipitation of crystalline uraninites in and around biotite agglomerates.Thus,by studying the two types of uraniferous pegmatites one can better understand the magmatic evolutionary process and uranium mineralization mechanism,so as to enrich our knowledge about the pegmatite-type uranium depo

关 键 词：伟晶岩型铀矿 矿化过程 同化混染与分离结晶作用 达马拉造山带 纳米比亚 

分 类 号：P619.14[天文地球—矿床学] P611.12[天文地球—地质学]