In-Situ Geochronology and Geochemistry of Pitchblende from Xincun Uranium Deposit in Motianling Area, North Guangxi

To better determine the precise mineralization age and examine the genesis of the Xincun uranium deposit in Motianling granite, North Guangxi, it carried out detailed mineralogical observation, in-situ U-Pb isotope and chemical composition analysis on the pitchblende of U-silicified type mineralization from the Xincun deposit using scanning electron microscopy (SEM), electron probe (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The chemical age of pitchblende obtained by the electron probe gives a very wide range and thus cannot represent the true formation age of the Xincun uranium mineralization. The results of LA-ICP-MS U-Pb isotope indicate that the pitchblende of the Xincun deposit contains a certain amount of common Pb, and the Tera-Wasserburg diagram yields a lower intercept age of 57.61±0.34 Ma (MSWD=0.97), which can represent the exact timing of Xincun deposit. The major and trace element results show that the pitchblende of the Xincun deposit has high WO₃ and CaO contents but extremely low ThO content, indicating that the ore fluid may be enriched in U and W and has medium-low temperature. The chondrite-normalized patterns of rare earth elements in pitchblende are slightly rich in light-REE, and similar to the rare earth partition of the ore-hosting granite. It is thus speculated that the uranium is probably derived from the granitoid itself. The newly obtained mineralization age is consistent with the main metallogenic period (70-50 Ma) of the hydrothermal uranium deposits in northern Guangxi. Similar to most of the Mesozoic and Cenozoic hydrothermal uranium deposits in South China, the Xincun deposit was formed due to the extension and thinning of the Cretaceous-Paleogene lithosphere in South China.