华北南缘石炭纪仁村大型喀斯特型铝土矿物质来源与成矿过程研究

华北克拉通在中奥陶世至晚石炭世经历了强烈的风化和喀斯特化作用, 并在晚石炭世形成大规模喀斯特型铝土矿, 但是其物质来源及成矿过程目前仍存争议。本文选取华北南缘仁村大型喀斯特型铝土矿床, 在矿床地质剖析基础上, 对两个钻孔岩心进行矿物学、地球化学、碳-氧同位素分析, 剖析了成矿物质来源和成矿环境条件, 总结了铝土矿形成过程。仁村铝土矿含矿岩系赋存于奥陶系灰岩风化面之上的石炭系本溪组中, 含矿岩系自下而上包括铁质粘土岩、铝土矿和粘土岩。X衍射和扫描电镜-能谱分析显示铁质粘土岩主要矿物为菱铁矿和伊利石, 铝土矿主要矿物为硬水铝石、黄铁矿、菱铁矿、锐钛矿、伊利石和高岭石, 而粘土岩主要矿物为高岭石和勃姆石。铁质粘土岩元素组成以SiO₂、FeO、Al₂O₃为主, 铝土矿以Al₂O₃、SiO₂、FeO、TiO₂为主, 而粘土岩主要为SiO₂。微量元素Zr、Hf、Nb、Ta、Th和U等在含矿岩系呈现整体富集, 稀土元素主要在铝土矿层底部富集。硬水铝石-黄铁矿-菱铁矿矿物集合体、铝土矿层中Ce异常、La/Y和(La/Yb)_N比值以及菱铁矿C-O同位素组成(δ¹³C: -11.35‰~-7.63‰; δ¹⁸O: -9.26‰~-5.93‰)揭示铝土矿主要形成于地表碱性-还原的喀斯特洼地环境, 微生物广泛参与成矿过程。微量-稀土元素组成及稳定元素比率显示含矿岩系成矿物源存在明显垂向变化。其中, 底部铁质粘土岩主体为底板碳酸盐岩原地风化形成, 而顶部铝土矿和粘土岩为异地来源。综合前人研究成果, 提出了华北南缘石炭系本溪组铝粘土矿三阶段形成过程: 早期风化阶段(450~320Ma)形成铁质粘土岩或铁质风化壳; 物源输送阶段(320~310Ma)堆积大规模成矿物质; 成矿及后生改造阶段(< 310Ma)形成大规模铝粘土矿。

Source and metallogenic process of the Carboniferous Rencun large karstic bauxite deposit in the southern margin of North China Craton

The North China Craton (NCC) experienced strong weathering and karstification from the Middle Ordovician to the Late Carboniferous, and a large-scale karstic bauxite was formed in the Late Carboniferous, but its material source and metallogenic process are still controversial. In this paper, a large karstic bauxite deposit in Rencun, southern margin of NCC was selected to solve this problem. Based on the geological analysis of the deposit, analyses on mineralogical, geochemical and carbon-oxygen isotopic compositions were carried out on two drill cores to study the sources of ore-forming materials and ore-forming conditions, as well as the forming process of the bauxite. The Rencun ore-bearing rock series is distributed in the Carboniferous Benxi Formation on the weathering surface of Ordovician limestone, including Fe-bearing claystone, bauxite, and claystone from the bottom up. X-ray diffraction (XRD) and scanning electron microscope and energy dispersive spectroscopy (SEM-EDS) analysis show that the Fe-bearing claystone mainly consists of siderite and illite; the bauxite ore is composed of diaspore, pyrite, siderite, anatase, illite, and kaolinite; and the claystone is mostly kaolinite and boehmite. The Fe-bearing claystone is dominate in SiO2, FeO and Al2O3; the bauxite is rich in Al2O3, SiO2, FeO and TiO2; and the claystone is mainly SiO2. The entire ore-bearing rock series is rich in trace elements Zr, Hf, Nb, Ta, Th, and U, while rare earth elements (REE) are mainly enriched in the bottom of bauxite layer. The mineral aggregate of diaspore, pyrite and siderite, the Ce anomaly, together with the La/Y and (La/Yb)N ratio in the bauxite layer, and the C-O isotopic compositions of siderite (δ13C: -11.35‰ to -7.63‰; δ18O: -9.26‰ to -5.93‰) reveal that the bauxite was mainly formed in the alkaline and reducing karstic depression environment, with microorganisms widely involved in the mineralization process. The trace-rare earth element compositions and the stable elements ratios show that there are obvious vertical changes in provenance of ore-bearing rock series. The Fe-bearing claystone in the bottom of the series is mainly formed by autochthonous weathering of underlying carbonates, while the bauxite and claystone in the top layer are allochthonous. Combined with previous research results, we proposed a three-stage formation model of the Carboniferous Benxi Formation bauxite and claystone in southern NCC: the early weathering stage (450~320Ma) when an extensive Fe-bearing claystone or iron weathering crust were formed; the subsequent provenance transport stage (320~310Ma) when a large-scale ore-forming materials were accumulated; and the mineralization and later transformation stage (< 310Ma) when a large-scale bauxite and claystone were deposited.