• 标题：The source, host minerals, and enrichment mechanism of lithium in the Xinmin bauxite deposit, northern Guizhou, China: Constraints from lithium isotopes
• 关键词：Li isotopes;Li source;Host minerals;Enrichment mechanism;Bauxite
• 作者：Bo Tang, Yong Fu, Shuang Yan, Pei-Wen Chen, Cheng Cao, Chuan Guo, Pang Wu, Zhen Long, Ke-Shu Long, Tian-Sun Wang, Yang Liu, Ying Yang

全文下载

Most bauxites in China can be classified as the paleoweathering crust sedimentary type (PCS type) and are normally enriched with lithium (Li). However, the source, host minerals, and enrichment mechanism of Li in bauxites are poorly understood. This study takes the Xinmin bauxite deposit, a typical PCS type in northern Guizhou, China, as a case to trace the Li source and its enrichment using Li isotopes. The Xinmin bauxite deposit is a typical PCS type deposit with high Li contents (average = 756 × 10?6) and heavy Li isotopic compositions (δ7Li) (average = 0.83‰). The Hanjiadian Formation’s silty shale (S1–2hj) and the Huanglong Formation’s limestone (C2h) are the parent rocks of the Xinmin bauxite deposit. The Dazhuyuan Formation (P1d), as bauxite-bearing rocks of the Xinmin bauxite deposit, was formed via diagenesis from the parent rocks’ weathering products. According to the Li isotope fractionation mechanism under chemical weathering, the Dazhuyuan Formation (P1d) should have a lighter δ7Li than parent rocks. However, the δ7Li of the Dazhuyuan Formation (P1d) is heavier than that of parent rocks, indicating that the Dazhuyuan Formation’s original δ7Li was altered by adding external-source Li characterized by heavy δ7Li. Considering the sedimentary environment and Li isotopes, this study proposes that the Li carried by surface runoff characterized by heavy δ7Li is the main source of Li in the Xinmin bauxite deposit. The correlations between clay minerals vs. Li contents and δ7Li show that illite and kaolinite may be the major host minerals of Li in the Xinmin bauxite deposit. Furthermore, the positive correlations of Li vs. Na2O and MgO indicate that smectite is another possible Li host mineral. Combined with the formation process of a PCS type bauxite deposit, this study proposes that Li enrichment occurs during the primary bauxitic material formation stage. During this stage, the Li carried by surface runoff converged into the coastal restricted basin where the primary bauxitic materials were deposited. After small-scale transgression, retention water formed in the basin with the rise in the groundwater table. The retained water evaporated under a hot and dry climate, and Li carried by surface runoff accumulated in the basin and was finally adsorbed by clay minerals such as illite, kaolinite, and smectite in the primary bauxitic materials. This mechanism can significantly increase Li contents and δ7Li in the PCS type bauxite deposit. Additionally, this mechanism implies that the migration process of Li in surface runoff during the formation of primary bauxitic materials determined the present Li contents and δ7Li of a PCS type bauxite deposit, mainly controlled by the sedimentary environment and hydrologic and climatic conditions at that time.