Metallogeny of the large-scale Carboniferous karstic bauxite in the Sanmenxia area, southern part of the North China Craton, China

Abstract

Above two billion tons of bauxite were formed during the exposure of the North China Craton (NCC) in 445–300 Ma. Mineralogical, geochemical, and isotopic analyses of karstic bauxite deposits in the Sanmenxia area situated within southern NCC were conducted to explore the genetic processes of Carboniferous bauxite. The bauxite contains three layers: a lower Fe-bearing claystone layer, a middle bauxite ore layer, and an upper claystone layer. Fe-bearing claystone is dominated by illite and hematite in the uplift and illite, siderite, and pyrite in the depression. Bauxite ore is composed of diaspore, illite, pyrite, and anatase, and the upper claystone consists of illite, diaspore, and anatase. Distinct pyrite δ34S values (11.7–15.2 and −11.5 to 1.0), illite (87Sr/86Sr)i (0.711917–0.713588 and 0.708000–0.711057) and εNd(t) (−12.2 to −11.4 and −11.8 to −9.7) values, and the immobile elemental ratios were obtained between the lower Fe-bearing claystone and upper bauxite ore and claystone. Together with chemistry of detrital rutile, it is denoted the Fe-bearing claystone was formed from the underlying limestone, whereas the bauxite ore and claystone were derived from both igneous rocks along craton margin and regional Precambrian metamorphic rocks. The visible microorganisms and wide development of framboidal pyrite with negative δ34S within the bauxite ore provide evidences of microbial activity during bauxite formation. Bio-leaching process after the deposition of allochthonous source materials caused the downward migration of ions (Al3+, Fe2+, Ti4+, REE3+, etc.). These ions encountered an alkaline and reducing condition in the karstic depression, forming diaspore, pyrite, and anatase and REE enrichment at the bottom of bauxite ore. This large input of raw materials, warm and humid climate, and microorganisms led to large-scale bauxite formation in the NCC during late Carboniferous.