Two-stage, U-mineralization of A-type granites from the Huangmeijian complex, eastern China

Abstract

Parts of A-type granites are closely related to some critical mineral resources. The study of origin and geochemical properties of those A-type granites are of great significance in revealing the metallogenic potential. The Early Cretaceous Huangmeijian (HMJ) complex is located in the Lower Yangtze River Belt (LYRB), eastern China, which is proposed to be associated with uranium deposits. Here, we present whole-rock major and trace elements, and zircon U–Pb geochronology to reveal the formation of the two-stage, uranium mineralization of A-type granites. The high Zr + Nb + Ce + Y concentrations (598−1181 ppm) and Ga/Al ratios (2.7−4.2) suggest an A-type geochemical affinity. On the ternary discriminant diagrams, they can be further divided into A₁-type. In-situ LA-ICP-MS U–Pb dating on the zircon grains from the HMJ complex yielded weighted mean ages of 129.7 ± 1.6 Ma (quartz alkali feldspar syenite), 131.3 ± 1.0 Ma (quartz alkali feldspar syenite), and 118.7 ± 0.7 Ma (alkali feldspar granite), respectively. The former two age results are consistent within error, and similar to other A-type granites in the LYRB. The latter one is newly identified, and is obviously younger than most A-type granites developed in the LYRB. Geochemical data suggest that these A-type granites are formed under high temperature and low oxygen fugacity conditions in an extensional setting. The presence of huttonite favors for the hydrothermal event and the uranium mineralization. It is important to note that, compared to the ca. 130 Ma quartz alkali feldspar syenites, the ca. 118 Ma alkali feldspar granites have obviously higher U (21.3–79.2 ppm) and Th (81.2–104 ppm) concentrations, which are also higher than those of other A-type granites developed in the LYRB. Moreover, the U concentration is even higher than some uranium mineralized granites of South China. Thus, besides the previously reported ca. 130 Ma A-type granite, the ca. 118 Ma A-type granite is of great probability to be a main source of uranium for the formation of uranium deposits in the LYRB, eastern China.