Critical metal enrichment in Upper Carboniferous karst bauxite of North China Craton

Bauxite is a prominent constituent of the paleoweathering crust that formed during a similar to 150-million-year-long interval between the Ordovician and Late Carboniferous on the North China Craton, representing one of the largest karst bauxite deposits globally. Although critical metal-bearing minerals are found in this layer regionally, the mechanisms of their concentration are poorly known. In this study, samples of karst bauxite from five sites were studied using petrography, X-ray diffractometry (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), and inductively coupled plasma mass spectrometry (ICP-MS) in order to explore the distribution of critical metals and their enrichment mechanisms. The karst bauxite consists of three lithologic components: (1) ferritic claystone, having total Fe2O3 content > 50% (main minerals: hematite and goethite); (2) clayey bauxite, having Al2O3 > 35% and clay content < 50% (main minerals: diaspore and boehmite); and (3) bauxitic clay, having Al2O3 > 35% and clay content > 50% (main minerals: clay minerals, with lesser amounts of diaspore). The mean contents of Li (892 ppm), B (327 ppm), and Sigma REE (584 ppm) are relatively high, indicating industrial mining potential. Lithium is mainly in lithium-bearing chlorite (cookeite) and enriched in bauxitic clay, whereas B and REE are present mainly as adsorbed ions on the surfaces of diaspore, boehmite and clay minerals, although LREE also might exist as independent mineral phases (< 3 mu m diam.). Processes of mineral evolution linked to weathering played an important role in the enrichment of these elements. Ferritic claystone is relatively enriched in HREE, clayey bauxite in LREE, and bauxitic clay in B and Li. This pattern of critical metal enrichment reflects a three-stage genesis of bauxite linked to in situ weathering and bauxitization, which provide insights into the weathering history of other bauxite deposits globally.