Trace element geochemistry in apatite from the Gabo pegmatite type lithium deposit, Tethyan Himalaya, China: Implications for petrogenesis and metallogenesis

The Gabo pegmatite-type lithium deposit is located at the northeastern end of the Kulagangri dome in the eastern segment of the Himalayan metallogenic belt in Xizang and represents a recently discovered rare metal deposit. The ore-bearing pegmatites are primarily hosted within the marble of the Kulagangri dome detachment system, with lithium as the main ore-forming element, accompanied by rare metals such as beryllium, niobium, and tantalum. This study conducted detailed mineral geochemical research on apatite from the spodumene pegmatite and tourmaline granite in the Gabo mining area. The analytical results reveal that the apatite in the spodumene pegmatite of the Gabo lithium deposit exhibits high rare earth element (REE) contents. The total REE content in apatite from coarse-grained spodumene pegmatite ranges from 3085 to 9297 ppm, with TE_1,3 values of 1.4–1.6, Eu* values of 0.001–0.006, and Sr contents of 4.9–15.83 ppm. In fine-grained spodumene pegmatite, the total REE content ranges from 3899 to 7680 ppm, with TE_1,3 values of 1.4–1.5, Eu* values of 0.002–0.006, and Sr contents of 7.22–34.06 ppm. The apatite in spodumene pegmatite also shows low Zr/Hf and Nb/Ta ratios but high Y/Ho ratios (>28). In contrast, the tourmaline granite has a total REE content ranging from 848 to 3285 ppm, TE_1,3 values of 1.1–1.2, Eu* values of 0.041–0.214, and Sr contents of 38.75–72.14 ppm. These results indicate that the apatite in both the tourmaline granite and spodumene pegmatite of the Gabo lithium deposit exhibits high ∑REE, low Sr, a pronounced M−type tetrad effect, high TE_1,3 values, and significant negative Eu anomalies, reflecting highly fractionated characteristics. The spodumene pegmatite, in particular, shows ultra-high fractionation. These features suggest that the mineralization of the Gabo lithium deposit occurred during the magmatic-hydrothermal transition stage, where extensive fluid exsolution from the melt led to the significant enrichment and mineralization of lithium and other rare metal elements. Based on these findings, four mineralogical indicators of apatite in pegmatite—∑REE > 2500 ppm, high tetrad effect (TE_1,3 > 1.3), extremely low Eu* (<0.01), and low Sr (<35 ppm)—provide theoretical guidance for future exploration of new pegmatite-type lithium and other rare metal deposits in the Himalayan metallogenic belt.