Volcanic ash contributes to the formation of clay-type Li deposits: Constraints from the late Permian claystone in the northern Upper Yangtze region, South China

Abstract

Clay-type Li deposits are one of the major types of Li resources globally and can be genetically categorized into carbonate-hosted and volcanic-hosted sub-types. Most recently, a series of clay-type Li deposits with Li₂O contents of 0.13–0.39 wt% have been discovered in the Wangpo claystone from the bottom of the Upper Permian, southern Shaanxi, northern Upper Yangtze, which have become key targets for commercial exploration. However, some key issues such as provenance, Li-hosting minerals and their origin, and Li enrichment mechanism, remain unclear to date, hampering a better understanding of Li mineralization in the Wangpo claystone. Herein, a comprehensive study of field geology, petrology, whole-rock geochemistry, and zircon U–Pb–Hf isotopes and trace element reveals that the Wangpo claystone was deposited on carbonate platforms in a littoral-neritic environment at 261–259 Ma, and underwent intense chemical weathering without involvement of hydrothermal alteration, as evidenced by the extremely high CIA values (98–99). Vitric fragments, coupled with the trace element compositions and ε_Hf(t) values (−8.8 to +5.6) of euhedral zircons with uniform ages (∼260 Ma), collectively suggest a volcanic ash origin for the Wangpo claystone, primarily derived from the Paleo-Tethys arc with a minor contribution from the Emeishan Large Igneous Province (ELIP). Whole-rock XRD and in-situ analyses (SEM-EDS, TIMA, and LA-ICP-MS) demonstrate that kaolinite (1096–1292 ppm Li) and cookeite (1390–1730 ppm Li) are the primary Li carriers in the Wangpo claystone. Mineral paragenesis and morphology suggest that the authigenic kaolinite was produced by chemical weathering of volcanic ash, which was dissolved and altered to release Li. Subsequent burial diagenesis facilitated partial transformation of kaolinite into cookeite under Li-rich conditions, driving Li enrichment. Accordingly, the Wangpo claystone may represent a unique clay-type Li deposit in South China, distinct from both carbonate-hosted and volcanic-hosted types. This study highlights that volcanic ash subjected to chemical weathering and diagenetic alteration plays a pivotal role in Li mineralization of claystone, which could provide valuable guidance for future exploration and development.