Geochemical and Sr-Nd-Li isotopic constraints on the genesis of the Jiajika Li-rich pegmatite, eastern Tibetan Plateau: Implications for Li-mineralization

The origin of pegmatite and associated Li-mineralization mechanism remains controversial. The intimate temporal and spatial relations between the Jiajika Li-rich pegmatite (hosting the largest Li ore deposit in China) and the Majingzi granite pluton allow us to explore this issue by examining the trace elements and Sr-Nd-Li isotopes of the two rock units in eastern Tibetan plateau. The Jiajika Li-rich pegmatites show extremely low CaO, TFe2O3, MgO, Sr and Ba, and high Li and Rb when compared with the adjacent Majingzi two-mica granite, and their initial Sr isotopic ratios (0.7212 ~ 0.7249, obtained from apatite) are significantly higher than those of the granite (0.7087 ~ 0.7147) and the surrounding Xikang Group metapelites as well. Whole rock Li isotopes analyses yield 7Li values of +0.31 ~ +1.94 for the Li-rich pegmatite, -0.54 ~ -0.85 for the Majingzi two-mica granite, and -3.16 ~ +2.38 for the Xikang Group metapelites, respectively. Modeling studies on trace elements and Li isotopes consistently demonstrate that the Li-rich pegmatites are unlikely to have been originated from extreme differentiation of the Majingzi two-mica granite as traditionally thought. Instead, they could be directly generated by low degrees (< 20%) of muscovite-dehydration melting of a mixed source dominated by a Li-rich claystone and subordinate Xikang Group metapelites under amphibolite facies conditions. We suggest that the existence of Li-rich claystone interlayers (associated with evaporate) in the source is crucial to pegmatitic spodumene mineralization. This explains the general observation that Li-rich pegmatite always show Li isotopic compositions lighter than Li-poor pegmatite.