Geologic scenario from granitic sheet to Li-rich pegmatite uncovered by Scientific Drilling at the Jiajika lithium deposit in eastern Tibetan Plateau
Zhiqin Xu, Bihai Zheng, Wenbin Zhu, Yanjing Chen, Guangwei Li, Jianguo Gao, Xudong Che, Rongqing Zhang, Hai–Zhen Wei, Weiqiang Li, Guo‐Guang Wang, Guang‐Yi Wei, Haoyu Yan
Abstract
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 (0-900m) 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 (D2). The calc-silicate unit below 900 m only experienced earlier orogenic deformation (D1). 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 ∼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.