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 (delta C-13: -11.35 parts per thousand to -7.63 parts per thousand; delta O-18: -9.26 parts per thousand to -5.93 parts per thousand) 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 (4500 similar to 320Ma) when an extensive Fe-bearing claystone or iron weathering crust were formed; the subsequent provenance transport stage (320 similar to 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.