Geochemical features of lithium–rich bauxite from the Benxi Formation in Qinyuan County, Shanxi, China: Insights into their depositional environment and lithium enrichment

Abstract

Most bauxites in China are the karst type, which usually enriches lithium (Li) resources. Therefore, Li mineralization theory must be further developed to realize the comprehensive utilization of bauxite resources. The Qinyuan Li–rich bauxite is formed in the Benxi Epoch of the Late Carboniferous and its parent rock is the underlying Ordovician limestone. In this study, the methods of stepwise extraction and element geochemical are utilized to discuss the occurrence state and sedimentary environment of Li in Qinyuan bauxite, respectively. Based on the differences in element composition, the Qinyuan Li–rich bauxite sequence can be divided into clay rocks, bauxitic clays, iron–rich clays, and bauxite from the bottom to the top. The Li content of the Qinyuan bauxite samples is highest in clay rocks (averaging 431 µg/g), followed by bauxitic clays (averaging 288 µg/g), bauxite (averaging 252 µg/g), and iron–rich clays (averaging 202 µg/g). The main mineral compositions of these samples are diaspore, calcite, anatase, pyrite, siderite, kaolinite, and illite. Among them, the Li content is positively correlated with clay mineral contents (R2 = 0.71), indicating that clay minerals are the main Li–rich minerals. More specifically, the clay mineral in bauxitic clays, bauxite, and iron–rich clays are kaolinite, and the positive correlation between Li and kaolinite (R2 = 0.46) indicates that kaolinite is the main Li–rich mineral. Nevertheless, the clay minerals in clay rocks are kaolinite and illite, and the correlation between Li and illite (R2 = 0.54) is higher than kaolinite (R2 = 0.10), indicating that Li is preferentially enriched in illite when kaolinite and illite coexist. It was found that the majority (higher than 99 %) of Li is hosted in the vacant sites of Si–O tetrahedral layers and the Al–O dioctahedral layers of illite and kaolinite to compensate the positive charge deficit caused by replacement of Si4+ and Al3+ in the tetrahedral and dioctahedral layers of clay minerals, respectively. Furthermore, the content ratios between typical elements with varied migration–deposition properties can indicate the depositional environment of Li–rich bauxite. Among them, MgO/Al2O3 and Rb/K (range from1.3 × 10-3 to 7.36 × 10-2 and 3 × 10-4 to 3.44 × 10-2 in samples, respectively) can indicate sedimentary environment. Th/U, V/Cr, and V/(V + Ni) (range from 0.45 to 3.73, 0.85 to 3.54, and 0.27 to 0.89 in samples, respectively) can indicate the paleo–redox conditions. Sr/Cu, CIA, and Cvalue (range from3.53 to 48.2, 93.8 to 99.9, and 0.43 to 7.29 in samples, respectively) can indicate paleoclimate. The results show that the Qinyuan bauxite possibly formed in the hot, humid (Sr/Cu > 5, CIA = 85–100, and Cvalue > 0.8), and dysoxic–anoxic (Th/U = 2–7, V/Cr = 2–4.25, and V/(V + Ni) = 0.46–0.84) marine–continental transitional (MgO/Al2O3 = 0.01–0.1 and Rb/K = 4 × 10-3–6 × 10-3) environment.