Geochronology and geochemistry of the Fogang granitic batholith and its implications for uranium mineralization

Peng Hu^1,2Weicheng Ding^1*Bin Liu^2,3*Wenfang Wei⁴Jiaming Qi²Liang Qiu⁵

• ¹College of Nuclear Technology and Automation Engineering, Chengdu University of Technology, Chengdu, China
• ²Research Institute No. 290, China National Nuclear Corporation, Shaoguan, China
• ³School of Earth Sciences and Engineering, Nanjing University, Nanjing, China
• ⁴Shaanxi Yanchang Petroleum (Group) Corp. Ltd, Xi'an, China
• ⁵School of Earth Sciences and Resources, China University of Geosciences, Beijing, Beijing Municipality, China

The E-W-striking Fogang granitic batholith represents the largest composite batholith in Guangdong province. The Pajiang uranium deposit has been identified in the southwestern portion of the Fogang granitic batholith. This deposit is hosted by the porphyritic biotite granite and the two-mica granites. In this study, zircon U-Pb dating, whole-rock geochemistry, and zircon trace element contents of porphyritic biotite granite and two-mica granites in the Pajiang area of the Fogang granitic batholith were systematically investigated. The results reveal that the geochronology of the porphyritic biotite granite, medium-to fine-grained two mica granite, and fine-grained two-mica granite are 156.7 ±0.7 Ma, 155.3 ±1.6 Ma, and 153.8 ±1.4 Ma, respectively. The porphyritic biotite granite is characterized by high SiO2, alkalis, and metaluminous to peraluminous (A/CNK = 0.95-1.21) but low rare earth contents (∑REE:107 ppm-465 ppm,) and negative Eu anomalies (δEu = 0.12-0.60), indicative of highly fractionated S-type granite. The porphyritic biotite granite, medium-to fine-grained two mica granite, and fine-grained two-mica granite all show high Rb/Sr values (1.52-41.7), indicating they originated from mature continental crust material. The porphyritic biotite granite has CaO/Na2O ratios of 0.06 -0.70, suggesting its source was a mix of pelitic and clastic rocks. The medium-to finegrained two mica granite, and fine-grained two-mica granite have lower CaO/Na2O ratios, indicating their source was pelitic rocks. The zircon trace element results indicate that the porphyritic biotite granite has a relatively higher oxygen fugacity (fO2 = -13.2), followed by the medium-to fine-grained two mica granite and the finegrained two-mica granite, which exhibit relatively lower oxygen fugacity (fO2 = -20.2, -17.3, respectively). Under low oxygen fugacity conditions, U tends to crystallize as independent uranium-bearing minerals during the magmatic stage. The crystallization processes of the medium-to fine-grained two-mica granite and the fine-grained two-mica granite are thus particularly favorable for the formation of uranium-bearing minerals such as uraninite and thorite. These minerals are susceptible to dissolution by hydrothermal fluids during subsequent uranium mineralization stages, thereby releasing U and providing a substantial source for mineralization. This further suggests that the two-mica granites serve as significant uranium source rocks for granite-type uranium deposits and hold favorable implications for mineral exploration.