The origin of lithium in hydrothermally overprinted Li-rich strata in the Mesoproterozoic Wumishan Formation: Insights from radiogenic and stable isotopes

Abstract

Significant lithium (Li) enrichment has been observed in specific strata of the Mesoproterozoic Wumishan Formation in North China, yet the mechanisms and origins of this enrichment remain contentious. Our previous mineralogical microscopic studies suggested that post-depositional hydrothermal fluids are crucial for Li enrichment; however, a lack of macroscale geological evidence, such as metasomatic textures, necessitating further isotopic investigation. This study compares the geochemical characteristics of Li-rich and Li-poor samples, utilizing Sr-Nd-Pb and Li isotopes, along with major and trace elements, to elucidate the enrichment mechanisms and sources of Li. Nd isotopic data reveal that the primary sedimentary sources of the Wumishan Formation strata are predominantly felsic components (ε_Nd(0): –15.64 to –24.83), with an contributions from basic weathering products (ε_Nd(t): –8.13 to 2.63). However, the Li-rich samples exhibit anomalously high enrichments of Sr (⁸⁷Sr/⁸⁶Sr: 0.7340–0.7844) and Pb isotopes (²⁰⁶Pb/²⁰⁴Pb: 18.939–32.520; ²⁰⁷Pb/²⁰⁴Pb: 15.822–17.087; ²⁰⁸Pb/²⁰⁴Pb: 38.311–49.381), along with homogeneous positive δ⁷Li values (+5.59 ‰ to + 6.93 ‰). The pronounced enrichment of radiogenic Sr and Pb isotopes, coupled with the absence of Li isotopic fractionation in clay minerals, strongly support the hypothesis that Li enrichment is associated with post-depositional hydrothermal alteration. Additionally, positive correlations between Li and trace elements (e.g., F and Rb), further reinforce this association rather than suggesting a link to chemical weathering. High ⁸⁷Sr/⁸⁶Sr and ²⁰⁸Pb/²⁰⁴Pb ratios, along with positive δ⁷Li values, imply that Li likely originates from the leaching of surrounding rocks. Conversely, differences in Sr, Nd, Pb, and Li isotopes between diorite and diabase confirm minimal contribution from magmatic sources. This study provides new insights into the genesis of Li-rich strata in the Wumishan Formation, further supporting our previous mineralogical evidence and offering significant geological implications for their exploration and exploitation.