Mineralization age of the Xiangshan uranium ore field, South China redefined by hydrothermal apatite U-Pb geochronology

The Xiangshan volcanic-related uranium ore field, located in the Ganzhou-Hangzhou Uranium-Polymetallic Metallogenic Belt (GHMB) in South China, has been the country’s largest historic uranium producer. Although this ore field has been intensely studied for decades, the absolute age of uranium ore formation was still debated. Conventional U-Pb dates on U-minerals (i.e., pitchblende, brannerite) and Ar-Ar dates on associated alteration minerals (i.e., illite) from different uranium deposits yield a wide range of ages (122 to 85 Ma). The uncertainty regarding the exact timing of uranium mineralization at Xiangshan has limited the development of accurate ore genesis models and precluded a better understanding of the links between metallogenesis and regional-scale geodynamics during the Cretaceous-Tertiary period in South China. In this work, we focus on applying in-situ LA-ICP-MS U-Pb geochronology to magmatic and hydrothermal apatite from two typical uranium ore deposits at Xiangshan, intending to indirectly constrain the exact timing of uranium mineralization and reconstruct the magmatic-hydrothermal history. U-Pb age determinations on magmatic zircon and apatite from the three types of ore-hosting volcanic rocks yielded precise crystallization ages of 133 to 136 Ma and 135 to 140 Ma, respectively. These results are identical within uncertainty to previously reported U-Pb dates obtained in the study area, and thereby tightly constrain the emplacement age of the Xiangshan volcanic complex that hosts uranium deposits. Moreover, the potassium feldspar-biotite Ar-Ar dating for the major volcanic rocks demonstrates that these host rocks were cooled and completely solidified through the biotite/potassium feldspar closure temperature (below 300 °C) at 133–131 Ma. Subhedral or anhedral aggregates of hydrothermal apatite ubiquitously formed in the high-grade uranium ores at the Shannan and Zoujiashan deposits and are commonly intergrown with fluorite, illite and uranium minerals. Relative to magmatic apatite from the host rocks, the hydrothermal apatite is typically characterized by much higher U and Th contents, depletion of LREEs, enrichment in HREEs and a much more pronounced negative Eu anomaly. As such, the hydrothermal apatite is unequivocal of hydrothermal origin and formed synchronously with uranium mineralization. LA-ICP-MS U-Pb dating of these hydrothermal apatite crystals yielded an absolute 206Pb/238U age of 126.7 ± 0.6 Ma for the Shannan deposit and 131.3 ± 7.2 Ma for the Zoujiashan deposit. The new ages are much older (>10 Ma) than those previously reported ages on ore or gangue minerals such as pitchblende and illite. Thus, one unidentified mineralization event likely occurred at ca. 131–127 Ma, soon after solidification and subsequent brittle deformation or faulting within the volcanic caldera. We, therefore, proposed that volcanic magmatism was highly likely to be the ‘heat engine’ driving hydrothermal fluid convection and possibly provide the essential conditions (i.e., magmatic fluids, U sources) for the formation of the U mineralization. Another implication of this study is that dating hydrothermal apatite with high U concentrations associated with uranium mineralization could be considered a complementary geochronological tool in the study of analogous hydrothermal uranium deposits worldwide.