Geological and geochemical characteristics of karst bauxite-bearing sequences in Xiabu area, Central Shanxi Province, North China

Abstract

Karst bauxite resources are widespread in the Shanxi Province, accounting for the richest bauxite resources in China. The Shanxi bauxite deposits are hosted in the lower member of the Carboniferous Benxi Formation, which consists of karst bauxite-bearing sequences. In addition to being main economic source of aluminum, there are many critical metals in the karst bauxite-bearing sequences.The spidergram pattern, REE pattern, immobile element tracing, and CrNi bivariate plots, as well as the previous studies on detrital zircons of North China Craton (NCC) bauxite ores, show that iron-rich clays were mainly weathered from the underlying carbonates, whereas the bauxite ore and top clay have allochthonous protoliths. Based on field investigations and geochemical characteristics of five typical drillcore sections in the Xiabu area, as well as previous researches, we concluded four sedimentary successions and we perfected the geological model of the karst bauxite-bearing sequences in central NCC. The Ordovician carbonate bedrocks developed many paleokarst terrain on the NCC, which experienced 150 Myr of karstification and denudation from the Middle Ordovician. Paleokarst terrain determines the distribution and thickness of the iron-rich clay layer. And the iron-rich clay above or below the groundwater table are separated into red Shanxi-type iron or clay (Fe3+-bearing minerals) and grey clay (Fe2+-bearing minerals). Until the Late Carboniferous, the sedimentation suddenly occurred in the NCC. New sediments were predominantly source materials for bauxite deposits, which were primarily controlled by the landform of the discontinuous weathered iron-rich clay. Furthermore, the humid and warm climate conditions reinforce the intensity of chemical weathering and microorganism participation, which also contributes to the formation of bauxite ore layer and top clay layer.The bauxite-bearing sequences in the Xiabu area contain high contents of critical elements such as U, Th, REEs, and Sc. In this study, high-level natural radioactivity was detected in the bauxite ore layer through gamma-ray logging of vertical sections. The correlation analysis revealed that the highly elevated U and Th contents in the bauxite ore layer are positively correlated with the natural radioactivity. Therefore, utilizing the gamma-ray logging method is effective in prospecting concealed karstic bauxite deposits. The REEs are also highly enriched in the bauxite ore layer and are positively correlated with P and Sr, indicating that the REEs are mainly hosted by phosphate minerals. The Sc contents of red clay are extremely high (up to 206.7 μg/g; average 118 μg/g), reaching industry standards levels. The integrated utilization of critical metals of the bauxite-bearing sequences is an important strategy for facing the significant supply crisis.