The Changjiang U ore field developed typical granite-related U mineralization in the Zhuguangshan complex, China. Pitchblende is the most important ore mineral in these mineralizations. In this study, the mineralogy and geochemistry of pitchblende were investigated by electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) to identify the genesis of the Changjiang U ore field. Pitchblende exhibits colloidal, fragmented, spherulitic and fine-grained crystals in U ores. Its geochemical compositions are similar to those of other granite-related U deposits in South China, which have elevated contents of U, Sr, As and W; low contents of Pb, Th, Zr, Nb, Ta, Hf, Co, Ni and rare earth elements (REEs); and variable amounts of Ca, Si, Bi, Y, V and Zn. These geochemical signatures suggest that mineralization occurred through hydrothermal genesis and that the hosting Youdong and Changjiang granites acted as the dominant U sources. The uraninite in these granites might be the major U source mineral. Uranium mineralization occurred under the following conditions: low temperature (<250°C), low oxygen fugacity (log fO2=-29.5 - -25.5), weakly acidic (pH=5.3-5.9), high CO32- and F- contents and a silicon-saturated solution. Rapid changes in the physicochemical conditions of the ore-forming fluid are responsible for the precipitation of pitchblende. Combined with previous studies, we propose that U-rich granites, Cretaceous-Tertiary crustal extensions, regional faults and hydrothermal alterations were the critical factors for U formation in the Changjiang ore field.

《地球科学》(ISSN 1000-2383;CN42-1874/P) (原刊名《地球科学——中国地质大学学报》ISSN 1000-2383;CN42-1233/P)，创刊于1957年，是由教育部主管、中国地质大学主办的综合性地学学术理论期刊，月刊，2015年改名为《地球科学》,她以反映中国地质大学教学与科研最新的、高水平的基础地质、应用地质、资源与环境地质及地学工程技术研究成果为主要任务。

The Mianhuakeng deposit, located within the Zhuguangshan batholith in the Nanling area, is currently recognized as the largest granite-related uranium deposit in China. A portion of the uranium ore bodies is spatially associated with NE-trending mafic veins within the granite. In this study, the field investigation, zircon U-Pb dating, S and Pb isotope analysis, and whole-rock geochemical analysis were conducted on these mafic veins to explore their crystallization age, petrogenesis, tectonic setting, and relationships with uranium mineralization. The weighted mean result of zircon U-Pb is 189 ± 3 Ma, suggesting that the mafic dyke was crystallized during the Early Jurassic. The whole-rock geochemistry and isotopes exhibit characteristics of intraplate basalts, suggesting that the mafic dykes originate from an enriched mantle source consisting of garnet–spinel lherzolite, with an estimated partial melting of 1%–5%. Mafic magmas underwent low-degree contamination from the lower crust during upwelling, induced by the extension of the lithosphere during the Early Jurassic. The analyses of pyrite sulfur isotopes in mafic samples vary between −2.9‰ and 1.8‰, significantly different from that of pyrite (−14.4‰ to −7.8‰) formed during the uranium mineralization. Furthermore, the ages of the pitchblende of 127–54 Ma are much younger than the crystallization ages of mafic dykes, indicating that the mafic magmas did not contribute to the uranium mineralization of Mianhuakeng deposit during magmatism. However, the abundant reducing minerals (e.g., pyrite, hornblende, and Fe²⁺-bearing minerals) in the mafic dykes can act as a redox barrier, reducing mobile U⁶⁺ to immobile U⁴⁺ during fluid–rock interaction, thereby facilitating uranium precipitation from the hydrothermal ore-forming fluids. The secondary fractures created by the intrusion of mafic magma probably provided favorable pathways for the movement of hydrothermal fluids.