Geochemical fingerprints of apatite from the Qiongjiagang pegmatite-type lithium deposit, Himalaya

Abstract

Lithium is a critical strategic metal with significant reserves in pegmatites, serving as the primary source for global Li production. Apatite has attracted increasing attention as an indicator in petrogenesis studies and for the exploration of ore deposits. In this study, we investigated the volatile compositions and major and trace elements of apatite from the Qiongjiagang pegmatite-type lithium deposit in Himalaya. Apatite derived from spodumene pegmatite exhibits relatively constant and high total rare earth element (ΣREE+Y) concentrations, ranging from 5899 to 8540 ppm. In contrast, apatite in barren pegmatite displays evidently lower (ΣREE+Y) concentrations, varying between 1345 and 3095 ppm. The REE patterns of apatite in spodumene pegmatite generally exhibit a flat shape [(La/Yb)_N = 1.55–2.15)], with distinctively negative Eu anomalies (Eu_N/Eu_N* = 0.14–0.22), slightly positive Ce anomalies (Ce_N/Ce_N* = 1.03–1.13), and low Y/Ho ratios (28–30). By contrast, apatite in barren pegmatite shows middle rare earth element (MREE)-depleted downward-convex patterns [(La/Yb)_N = 1.99–20.4)], strongly negative Eu anomalies (Eu_N/Eu_N* = 0.01–0.14), slightly positive Ce anomalies (Ce_N/Ce_N* = 1.10–1.24), and high Y/Ho ratios (30–55, with an average of 50). Overall, the high concentrations of ΣREE (and Y) and low Th/U and Y/Ho ratios can serve as diagnostic indicators to distinguish apatite in spodumene pegmatite from that in barren pegmatite. Furthermore, the flat REE pattern may represent a common feature of apatite from lithium deposits. Differences in the Ce and Eu anomalies between apatite from these two kinds of pegmatites likely reflect formation under different redox conditions. Consequently, based on calculations derived from apatite volatile compositions, the melt associated with spodumene pegmatite may contain higher water content compared to that of the barren one. Therefore, the mineralized pegmatite system may incorporate substantial amounts of H₂O-rich fluids, which play a crucial role in lithium mineralization.