Petrogenesis of pegmatite and supernormal enrichment mechanism of lithium in Heyuan lithium deposit, southern Jiangxi Province: Constraints from zircon and apatite U-Pb dating and whole-rock geochemistry

Lithium plays an important role in national economic construction and national defense science and technolog, and is known as "the metal that moves the world forward". South China is an important producing area for granite-type niobium, tantalum, lithium and other rare earth minerals, among which pegmatite-type lithium deposits in Jiangxi Province are mainly distributed in the southern Jiangxi Province and are believed to be closely related to Huitong pluton. However, petrogenesis of pegmatite and supernormal enrichment mechanism of lithium have not been well understood and revealed. In this paper, Heyuan lithium deposit, a typical representative of pegmatite-type lithium deposit in Jiangxi Province, is taken as the research object. The diagenetic and metallogenic chronology, geochemistry and Hf-Nd isotopes are systematically carried out to clarify the petrogenesis of pegmatite, reveal the supernormal enrichment mechanism of lithium, establish the metallogenic model of pegmatite-type lithium deposit, and finally uncover the genetic problems of pegmatite-type lithium deposit in Jiangxi Province. The zircon U-Pb ages of fine-grained alkali feldspar granite and coarse-grained biotite granite obtained from the Huitong complex rock are 408.7 +/- 2.8Ma and 401.2 +/- 2.7Ma. The U-Pb age of apatite in the spodumene pegmatite is 387 +/- 10Ma, which is basically consistent with the zircon U-Pb age of the Huitong complex rock within the error range, showing that the mineralization of spodumene is genetically related to the Huitong complex rock. Coarse-grained biotite granite shows A-type granite characteristics: high content of rare earth elements (average Sigma REE is 276.57x10(-6)), high silicon (69.81%similar to 75.31%), moderately rich alkali (K2O: 3.96%similar to 6.16%, Na2O: 2.22%similar to 3.25%), relatively obvious negative Eu anomaly (delta Eu mean 0.34); 10000Ga/Al ratio and high field strength elements Zr+Nb+Ce+Y content range from 2.98 to 3.76 (>2.6) and 408.3x10(-6) to 544.1x10(-6) (>350x10(-6)), respectively. Fine-grained alkali feldspar granite shows aluminous highly differentiated S-type granite characteristics: low TiO2, Fe2O3, CaO, MgO, Sigma REE, Nb/Ta and Zr/Hf ratio, higher Li, Rb, Cs and fractionation effect (La/Yb)(N); the presence of muscovite mineral; all samples exhibit standard corundum molecules, with a mass fraction range of 2.87% to 3.56%, greater than 1%; aluminum saturation index (A/CNK) is greater than 1.1; Th and Y contents are low, which are negatively correlated with the content of Rb. The isotopic ratio of epsilon(Nd)(t) is -9.43 to -9.10 and -13.49 to -13.35, respectively. The Nd isotopic composition is very uniform, which can better reflect the characteristics of the source region. The coarse-grained biotite granites (A-type) should be derived from the partial melting of the lower crust, while the fine-grained alkali long granite (S-type) may be the partial melting product of the middle and lower crust. The Li content of coarse-grained biotite granite and fine-grained alkali feldspar granite in Huitang pluton is 35.5x10(-6)similar to 103x10(-6), but the Li content of metasedimentary rocks is 71.4x10(-6)similar to 1390x10(-6) (average value is 662.36x10(-6)). Rare metals such as pegmatitic lithium come from the remelting of Li-rich metasedimentary rocks. The large-scale, composite granite of the Huitong provided a persistent and sufficient heat source for the edge surrounding rock (metamorphic sedimentary rocks) undergoing high-temperature melting. Thus, the Li-rich metamorphic sedimentary rocks are partially melted to form Li-rich pegmatitic magma. Some prospecting implications can be obtained from the research: the area of abnormal distribution of lithium content in metamorphosed sedimentary rocks above the strata is the primary exploration target, and the formation of pegmatitic lithium deposits is the result of the coupling of tectonic, magmatic and mineral-bearing hydrothermal Li abnormal distribution.