Gallium isotope variation in the Anning carbonate-hosted clay-type lithium deposit, Central Yunnan Province, China: Implications for the Li enrichment mechanism

Abstract

Lithium deposits are mainly divided into three types: pegmatite-type, brine-type, and clay-type. Compared with traditional pegmatite-type and brine-type lithium deposits, clay-type lithium deposits have attracted increasing attention due to their large scale and stable distribution. We investigated the Ga isotope composition of three drill cores (ZK401, ZK406, and ZK408) in the Anning carbonate-hosted clay-type lithium deposit in central Yunnan Province, China. The δ⁷¹Ga_NIST994 values and Ga contents of the claystone sample from drill core ZK401 are 0.97–1.33 ‰ and 1840–4070 ppm, respectively, with a negative correlation between the Ga content and δ⁷¹Ga_NIST994 value. For drill core ZK406, the δ⁷¹Ga_NIST994 values and Ga contents of the claystone sample are 1.26–1.48 ‰ and 29.8–5950 ppm, respectively, with a positive correlation between the Ga content and δ⁷¹Ga_NIST994 values. For drill core ZK408, the δ⁷¹Ga_NIST994 values and Ga contents of the claystone sample are 1.04–1.22 ‰ and 2.70–4990 ppm, respectively, with no significant correlation observed. Previous studies have suggested that smectite is the precursor of Li-bearing minerals. However, the X-ray diffraction analysis showed that the clay minerals in the three drill cores were primarily illite, clinochlore, kaolinite, and diaspore/boehmite, indicating that these clay minerals underwent mutual transformation during the diagenesis process. The Ga isotope data suggested that during the mineral conversion process, solid-state transformation was the dominant reaction mechanism of the strata in drill core ZK401, while dissolution-crystallization was the dominant reaction mechanism of the strata in drill core ZK406. The cancellation of both mechanisms' effects may explain the lack of a clear correlation between the Ga contents and δ⁷¹Ga_NIST994 values in drill core ZK408. Lithium is more likely to be preserved via the solid-state transformation mechanism during the conversion of the parent mineral (such as smectite) into daughter minerals (such as illite and clinochlore), while the dissolution-crystallization mechanism is more likely to lead to the redistribution of Li.