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Genesis of the altered Early-Middle Triassic oceanic tuff from western Sichuan: Constraints from the elevated δ7Li values
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DOI：10.1016/j.gexplo.2026.108043.png)
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Abstract
En 中文
Lithium is a critical metal, and recent exploration has revealed that clay-type lithium deposits hold considerable resource potential. However, our current understanding of these deposits remains limited. In western Sichuan, China, marine-altered volcanic ashes from the Early to Middle Triassic-characterized by a grayish-green colour and the presence of pea-sized, SiO2-rich granules-are locally referred to as Green Bean Rocks (GBR). These rocks are highly enriched in lithium, with concentrations reaching up to similar to 1000 ppm and averaging similar to 400 ppm, making them an ideal target for studying the diagenesis of Li-rich claystones. In this study, we conducted a comprehensive analysis of GBR, including mineralogical characterization (XRD and SEM), lithium leaching experiments using ammonium acetate (CH3COONH4), major and trace element geochemistry, and lithium isotope analysis. The results indicate that GBR primarily consists of quartz, illite, and illite-smectite mixed-layer minerals. Lithium ion leaching efficiency was approximately 3%, and lithium content shows a positive correlation with MgO and K2O concentrations. Whole-rock lithium isotopes in the GBR display unusually heavy and homogeneous signatures, with delta Li-7 values ranging from 15.26 +/- 0.27 parts per thousand to 14.06 +/- 0.18 parts per thousand and an average of 14.72 +/- 0.24 parts per thousand. The delta Li-7 values of the leaching residues (15.48 +/- 0.14 parts per thousand and 15.18 +/- 0.18 parts per thousand) are nearly identical to those of the whole-rock samples, whereas the leachates exhibit significantly heavier isotopic compositions, with delta Li-7 values of 23.24 +/- 0.01 parts per thousand and 23.16 +/- 0.21 parts per thousand. By integrating previously published TOF-SIMS data on Li-rich claystones with current understanding of lithium isotope geochemistry, we infer that lithium in the GBR is primarily hosted within the smectite layers of illite-smectite mixed-layer minerals, with lithium predominantly incorporated into the smectite structure. The lithium enrichment in the Green Bean Rock (GBR) is predominantly derived from Triassic seawater, with only a minor contribution from the alteration of primary volcanic ash. Although lithium concentrations in seawater are generally low (modern seawater contains only similar to 0.18 ppm Li), previous studies have indicated that the GBR was deposited in a restricted, highly evaporative marine environment. Such a setting facilitated the formation of smectite-phase clay minerals, during which lithium in seawater was progressively concentrated and ultimately adsorbed and incorporated into the clay phases, forming Li-rich claystones. These findings provide new constraints on the metallogenic conditions of Li-rich claystones developed in various geological settings.
Keywords:
Clay-type lithium deposits
Green-bean rocks (GBR)
Lithium isotopes
Illite-smectite mixed-layer minerals
Marine-altered volcanic ash
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