Granite In The Huichang Area, Southern Jiangxi Province:Genesis, Mineral Chemistry And Relationship With Uranium Mineralization

The Fucheng granite and the Zhulanbu granite we studied are located to the east and the west of the Huichang basin, respectively, in southern Jiangxi province. There are several uranium deposits, the Hecaokeng uranium deposit for example, in the western part of the Fucheng granite which is in contact with shoshonite series volcanic rocks. Based on electron microprobe analysis data and theoretic calculation results of biotites and chlorites from the west part of the Fucheng granite, this paper has studied the mineral chemistry of biotites and chlorites to discuss the relationship between the chloritization of biotites and uranium mineralization. Results show that most of the biotites belong to siderophyllite. The Fe-Mg compositions of biotites suggest that the Fucheng granite is a S-type granite derived from crust material.The oxygen fugacity (1g(fO2)) of the magma of the Fucheng granite is estimated relatively low (-15.0～-14.3),which means that the source rock was strongly reduced, which was beneficial to preliminary enrichment of uranium. The U content of the Fucheng granite is above 13 ppm, far higher than the average uranium content (3.5 ppm) of granite in the world. The F content in biotite is 1.41～2.01 wt%, showing that the granitic magma is rich in F. As U is easy to form complex with F in magma, this is probably one of the reasons that the Fucheng granite is rich in uranium. Chlorite is mainly derived from biotite by alteration, so it shows in shape of biotite. Chemical analysis results show that the chlorites mostly belong to chamosite and prochlorite, both of which are rich in Fe element and mainly formed in reductive environment. The formation temperatures of chlorite are between 246～ 307 "C (with average of 276℃).Furthermore, through the comparative analysis under the microscope, we find that the greenish alteration zone in deeper position is mainly caused by chloritization, while the reddish alteration zone in upper position is a result of hematitization occurring in later period.The U contents of the samples from the upper reddish alteration zone varies from 3.5 ppm to 9.4 ppm (with an average of 6.6 ppm), which is significantly lower than U contents of the samples from the lower greenish alteration zone (7.7-23.1 ppm, an average of 13.9 ppm). The Th contents of the samples from the reddish alteration and the greenish alteration zone are basically similar, with an average value of 35.7 ppm and 36.5 ppm, respectively. The U in the granite was activated by the large-volume greenish alteration of the granite in early stage, leached out by oxidized fluids and hematitization occurring in later stage, and mineralized when such U-rich fluids, in which the U6+ was in the stage of UO2F42-, UO2F3- and UO2F2, came across strongly reduced environment. Because of the redox character is totally different from U, Th was not involved in this mineralization process. The Fucheng granite has a high activity uranium leaching rate (52%). But the greenish alteration zone has a lower rate with 30%, and the reddish alteration zone has a lowest rate with 8.6%. It suggests that the Fucheng granite can be an important source of mineralization materials. The uranium in the reddish alteration zone has migrated mostly and partly in the reddish alteration zone. The migration of uranium may be the basis of uranium mineralization.The Zhulanbu granite is another important granitic pluton in Huichang area. LA-ICPMS zircon U-Pb dating results imply that the Zhulanbu granite was emplaced at 225～228Ma The Zhulanbu granite have geochemical characteristics of S-type granite. The samples are peraluminous, with high contents of Si and Al, high total alkalis and high K2O/Na2O ratios (higher than 1.2), low contents of Ca and Mg. All the samples show characteristic negative anomalies in Ba, Sr and Eu and positive anomalies of Rb, Th and U. Furthermore, the samples have low Ga/Al ratios and Zr+Nb+Ce+Y contents. Isotopically, they have low εNd(t) values (-7.9～-9.8) with whole-rock two-stage Nd isotopic model ages of 1.65～1.80Ga, and low εHf(t) values (-7.7～-12.9) with zircon Hf isotopic model ages of 1.74～2.06Ga. According to these data, we suggest that the Zhulanbu pluton was derived from partial melting of the Paleoproterozoic crustal rocks which was rich in pelite. The U contents of the Zhulanbu granite are 10.9 ppm, higher than the average uranium content (3.5 ppm) of granite in the world, but lower than the Fucheng granite (22.5 ppm) obviously. Furthermore, the Zhulanbu granite has a low activity uranium leaching rate, showing that it is not conducive to uranium mineralization.