Geologic scenario from granitic sheet to Li-rich pegmatite uncovered by Scientific Drilling at the Jiajika lithium deposit in eastern Tibetan Plateau

The Jiajika pegmatite-type lithium deposit emplaced in Upper Triassic turbidites is located in the southeastern Songpan-Ganze terrane of the eastern Tibetan Plateau and is the largest hard-rock lithium deposit in China. The Jiajika Scientific Drilling Project (JSD) consists of the JSD-1 borehole, which is 3211 m deep, and two 1000 m boreholes, JSD-2 and JSD-3. The JSD-1 borehole is composed of the upper pelitic metasediment unit (0-900 m) and the lower are calc-silicate unit (900-3211 m). Pelitic metasediments at the depth of 0-400 m experienced Barrovian-Buchan metamorphism, while metasediments at the depth of 0-900 m were associated with domal deformation (D-2). The calc-silicate unit below 900 m only experienced earlier orogenic deformation (D-1). The mineralization of Li-Be and Nb-Ta occurs at a depth of 0-1200 m and 0-1800 m, respectively, while the Rb enrichment occurs throughout the whole borehole. Analyses of K-Fe-Ba-B-Li-Rb isotopes suggest that the Jiajika lithium deposit resulted from high-degree fractionation of the parent granitic magma and strong fluid immiscibility. The pegmatites may have crystallized from volatile-rich melts during two-stage magmatic-hydrothermal events at 205-210 Ma and 192-193 Ma. The JSD-1, JSD-2 and JSD-3 indicate that S-type granite related to the Jiajika lithium deposit was emplaced as multi-layer domal sheets, rather than as a deeply rooted pluton. The domal granitic sheets are surrounded by pegmatites and meta-sedimentary rocks. Therefore, we propose a domal granitic sheet model to interpret the geologic scenario from granite sheet to Li-rich pegmatite in the Jiajika ore field. The Upper Triassic turbidites in the Songpan-Ganze terrane underwent significant orogenic decollement at similar to 230 Ma, followed by deep anatexis, basement doming, magma rising and emplacement of granitic sheets. Finally, the rare-metal-rich pegmatites intruded under domal decompression condition, forming a multi-layer architecture intercrossed by the granitic sheets and the meta-sediments.

Automated summary

The Jiajika pegmatite-type lithium deposit in the southeastern Songpan-Ganze terrane of the eastern Tibetan Plateau is the largest hard-rock lithium deposit in China. The deposit is formed through high-degree fractionation of parent granite magma and strong fluid immiscibility, with two-stage magmatic-hydrothermal events. The S-type granite related to the deposit is emplaced as multi-layer domal sheets and surrounded by pegmatites and meta-sedimentary rocks.