Mineralogical and geochemical investigations of the Li-rich clay strata from Central Yunnan, Southwest China

Abstract

• Chlorite is the primary host mineral for Li in the Li-rich clay strata. • The primary source of the Li-rich clay strata is carbonate basement. • Alkaline dysoxic freshwater conditions favor the formation of Li-rich clay strata. • The late-stage modification of the fluid contributed to the redistribution of Li. The recently identified lithium (Li)-rich clay strata in the southwestern Yangtze Block represent a potentially significant global sedimentary Li resource, with estimated Li 2 O reserves exceeding 2 million tons. However, the genesis and metallogenic process of these Li-rich clay strata remain unclear. This study systematically investigates 251 samples from 18 drill cores of the early Permian Daoshitou Formation in center Yunnan, southwestern Yangtze Block, using inductively coupled plasma mass spectrometry (ICP-MS), X-ray diffractometry (XRD), scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS), TESCAN Integrated Mineral Analyser (TIMA), and laser-ablation ICP-MS (LA-ICP-MS) to elucidate the provenance and deposition pattern of Li-rich clay strata. The Li concentration of Li-rich clay strata from the Daoshitou Formation ranges from 2.7 to 6790 ppm, showing a strong correlation with Al 2 O 3 , SiO 2 and A/S, indicating a dominant aluminosilicate affinity. Illite, chlorite, and kaolinite dominate the aluminosilicate minerals in Li-rich clay strata. The strong positive correlation (R 2 = 0.85) between chlorite and Li concentration indicates that chlorite is the main host mineral for Li, with LA-ICP-MS results showing Li concentration in chlorite up to 12,000 ppm. A high chemical index of alteration (CIA) and the abundant presence of kaolinite, boehmite, and diaspore in the Li-rich clay strata indicate intense supergene weathering. The PAAS-normalized trace elements diagrams and UCC-normalized REE patterns reveal similarities between the Daoshitou Formation’s Li-rich clay strata and the carbonate rocks from the Weining Formation, demonstrating that these carbonate rocks are the primary sources. The weakly alkaline environment created by the weathering of carbonate rocks facilitates the formation of clay minerals, primarily montmorillonite, which is conducive to the adsorption and retention of Li during the weathering stage. However, the presence of Li-rich chlorite in large quantities, an unusual product of supergene weathering, suggests that the Li-rich clay strata underwent significant reformation during the burial stage. TIMA and SEM analyses reveal the distribution characteristics of dissolved elements and minerals suggest this late-stage reformation is related to groundwater influence. The geochemical indicators (Sr/Ba: 0.003 to 1.85, avg. 0.28; Ni/Co: 1.05 to 31.66, avg. 5.99; V/ (V + Ni): 0.42 to 0.99, avg. 0.77) indicate that the depositional environments of the Li-rich clay strata were terrestrial freshwater settings under oxic-suboxic conditions, which might be influenced by both sedimentation and later groundwater reconstruction. Overall, the formation of the Li-rich clay strata appears to be controlled by a combination of weathering-deposition, burial and late reformation. The findings presented in this study provide insights into the formation mechanisms and prospecting strategies for sedimentary Li resources.

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