华北石炭纪艾雨头大型喀斯特铝土矿成因机制研究

Genetic mechanism of the large Carboniferous Aiyutou karstic bauxite in North China Craton

喀斯特型铝土矿中硬水铝石的成因机制对准确认识该矿床的成因至关重要,也是业界长期争论的焦点。早期普遍认为风化作用阶段形成的三水铝石经历后期压实变质作用脱水形成硬水铝石,然而近期大量研究显示自然界铝土矿中硬水铝石多为地表喀斯特型环境下结晶形成。华北晚石炭世发育大规模硬水铝石型喀斯特铝土矿,本文选取华北北部艾雨头大型铝土矿床为研究对象,在翔实的矿床地质研究基础上,借助X射线粉末衍射(XRD)、扫描电镜-能谱分析(SEM-EDS)及激光剥蚀电感耦合等离子体质谱(LA-ICP-MS)技术,探索硬水铝石的地表结晶条件和铝土矿的形成过程。艾雨头铝土矿含矿岩系剖面自下而上依次包括铁质粘土岩、铝土矿和粘土岩,其中:铁质粘土岩矿物组成主要为伊利石、高岭石、针铁矿、赤铁矿、硬水铝石和锐钛矿;铝土矿主要为硬水铝石、高岭石、赤铁矿、针铁矿和锐钛矿;粘土岩为高岭石、针铁矿、赤铁矿和锐钛矿。艾雨头喀斯特铝土矿顶底板岩石未发生变质作用,而且原位分析显示硬水铝石中含有较高含量的Si、Fe、Ti、Cr、V、Pb等元素,指示硬水铝石为表生沉淀成因。矿石中发现硬水铝石与赤铁矿和针铁矿密切共生,表明硬水铝石可在氧化条件下形成(Eh>0.2)。硬水铝石中氧化还原敏感元素间良好的线性关系(包括V/Mn-Cr/Mn、V/Fe-Cr/Fe、V/Fe-U/Fe和U-Cr)以及V、Cr、U元素的共同富集,指示硬水铝石沉淀时氧化还原阈值为T_(1)-T_(3)(即低氧-还原环境(-0.4<Eh<0.4))。硬水铝石中较高含量的Fe元素以及与硬水铝石共生的赤铁矿和针铁矿中较高含量的Al元素反映成矿前期已形成了大量的铁铝固溶体(Al_(x)Fe_(1-x)(OH_(y))^(3-y))。这些不稳定的铁铝固溶体在成岩阶段转变为含铁硬水铝石(Fe-AlOOH)和含铝针铁矿、赤铁矿(Al-FeOOH、Al-Fe_(2)O_(3));含铁硬水铝石在后期压实成岩过程中进一步转变为硬水铝石。该研究结果解释了艾雨头铝土矿中硬水铝石地表大规模结晶的基本条件和过程,对全面认识华北喀斯特铝土矿具有重要意义。

The genetic mechanism of diaspore in karstic bauxite is crucial to the accurate understanding of its genesis,and is also the focus of a long-term debate.Previous researchers generally believed that gibbsite formed in the weathering stage was dehydrated to form diaspore in the later compaction and metamorphism stage.However,a large number of recent studies have shown that diaspore in natural bauxite was mainly formed under the superficial karstic environment.Large-scale diaspore-type karstic bauxites were formed in the North China Craton(NCC)in the Carboniferous.On the basis of detailed geological research of the deposit,the crystallization conditions of diaspore and the formation process of bauxite are studied by methods of X-ray diffraction(XRD),scanning electron microscopy-energy dispersive analysis(SEM-EDS),and in situ laser-ablation inductively-coupled-plasma mass spectrometry(LA-ICP-MS).The ore-bearing rock series of Aiyutou bauxite deposit are Fe-bearing claystone,bauxite,and claystone from bottom to top.The mineral composition of the Fe-bearing claystone is mainly composed of illite,kaolinite,goethite,hematite,diaspore and anatase;the bauxite is mainly composed of diaspore,kaolinite,hematite,goethite and anatase;and the claystone is kaolinite,goethite,hematite and anatase.The roof and floor rocks of Aiyutou karstic bauxite have not undergone metamorphism,and the in-situ analysis result shows that the contents of Si,Fe,Ti,Cr,V,Pb and other elements in the diaspore are high,indicating a supergenetic precipitate origin.Diaspore in the ores was found in close association with hematite and goethite,indicating that diaspore can be formed under oxidation conditions(Eh>0.2).Good linear relationship between REDOX sensitive elements in diaspore(including V/Mn-Cr/Mn,V/Fe-Cr/Fe,V/Fe-U/Fe and U-Cr)and the co-enrichment of V,Cr and U elements indicated that the REDOX threshold of diaspore precipitation is T_(1)-T_(3)(i.e.,hypoxic-reductive environment:-0.4<Eh<0.4).The high content of Fe in diaspore and Al in hematite and goethite associated with diaspore indicate that a large number of iron-aluminum solid solutions(Al_(x)Fe_(1-x)(OH_(y))^(3-y))have been formed in the early stage of mineralization.These unstable iron-aluminum solid solutions are transformed into iron-bearing diaspore(Fe-AlOOH)and aluminum-bearing goethite/hematite(Al-FeOOH/Al-Fe_(2)O_(3))in the diagenetic stage.The iron-bearing diaspore was further transformed into diaspore during the later compaction diagenesis.The results of this study explained the basic conditions and process of large-scale crystallization of diaspore in Aiyutou karstic bauxite,which is of great significance for the comprehensive understanding of NCC karstic bauxite.