Uraninite is the main contributor to the bulk-rock uranium concentration in many U-rich granites and is the most important uranium source for granite-related uranium deposits. However, detailed textural and compositional evolution of magmatic uraninite in granites during alteration and associated uranium mobilization have not been well documented. In this study, textures and geochemistry of uraninites from the Zhuguangshan batholith (South China) were investigated by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The geochemical data indicate that the Longhuashan and Youdong plutons are peraluminous leucogranite, the Changjiang pluton is highly fractionated high-K calc-alkaline granite, and the Jiufeng pluton belongs to a high-K calc-alkaline association.Uraninites from the Longhuashan and Youdong granites have lower concentrations of ThO₂ (0.9–4.0 wt %) and rare earth elements (REE)₂O₃ (0.1–1.0 wt %) than those from the Changjiang and Jiufeng granites (ThO₂ = 4.4–7.6 wt %, REE₂O₃ = 0.7–5.1 wt %). Uraninites observed in the Longhuashan, Youdong, Changjiang, and Jiufeng granites yielded chemical ages of 223 ± 3, 222 ± 2, 157 ± 1, and 161 ± 2 Ma, respectively. The samples (including altered and unaltered) collected from the Longhuashan, Youdong, and Changjiang granites are characterized by highly variable whole-rock U concentrations of 6.9 to 44.7 ppm and Th/U ratios of 0.9 to 7.0, consistent with crystallization of uraninite in these granites being followed by uranium leaching during alteration. Alteration of uraninite, indicated by altered domains developing microcracks and appearing darker in backscattered electron (BSE) images compared to unaltered domains, results in the incorporation of Si and Ca and mobilization of U. In contrast, the least altered samples of the unmineralized Jiufeng granite have low U concentrations (5.3–16.4 ppm) and high ΣREE/U (13.6–49.4) and Th/U ratios (2.1–5.6), which inhibit crystallization of uraninite, as its crystallization occurs when the U concentration is high enough to exceed the substitution capacity of other U-bearing minerals. These results indicate that the Longhuashan, Youdong, and Changjiang granites were favorable uranium sources for the formation of uranium deposits in this area. This study highlights the potential of uraninite alteration and geochemistry to assist in deciphering uranium sources and enrichment processes of granite-related uranium deposits.

中文翻译：

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通过改变从花岗岩中的铀矿中释放铀：对花岗岩相关铀矿来源的影响

铀矿是许多富铀花岗岩中大块岩石铀浓度的主要贡献者，也是花岗岩相关铀矿床最重要的铀源。然而，花岗岩中岩浆铀矿在蚀变和相关铀移动过程中的详细结构和成分演化尚未得到很好的记录。在这项研究中，利用扫描电子显微镜 (SEM) 和电子探针微量分析 (EPMA) 研究了朱光山基岩（华南）铀矿的结构和地球化学。地球化学资料表明，龙华山和油东岩体为过铝白质花岗岩，长江岩体为高分异高钾钙碱性花岗岩，九峰岩体为高钾钙碱性组合。₂ (0.9–4.0 wt %) 和稀土元素 (REE) ₂ O ₃ (0.1–1.0 wt %) 比来自长江和九峰花岗岩的那些 (ThO ₂ = 4.4–7.6 wt %, REE ₂ O ₃= 0.7–5.1 重量％）。在龙华山、油东、长江和九峰花岗岩中观测到的铀矿化学年龄分别为 223±3、222±2、157±1 和 161±2 Ma。从龙华山、油东和长江花岗岩采集的样品（包括蚀变和未蚀变）的特征是全岩 U 浓度变化很大，为 6.9 至 44.7 ppm，Th/U 比为 0.9 至 7.0，与这些样品中的铀矿结晶一致花岗岩在蚀变过程中被铀浸出。铀矿的改变，表现为改变的畴产生微裂纹并在背散射电子 (BSE) 图像中与未改变的畴相比显得更暗，导致 Si 和 Ca 的掺入以及 U 的移动。 相反，未矿化九峰花岗岩中变化最小的样品具有低 U 浓度 (5.3–16.4 ppm) 和高 ΣREE/U (13.6–49.4) 和 Th/U 比 (2.1–5.6)，这抑制了铀矿的结晶，因为它会发生结晶当 U 浓度高到足以超过其他含 U 矿物的替代能力时。这些结果表明，龙华山、油东和长江花岗岩是该地区铀矿形成的有利铀源。这项研究强调了铀矿蚀变和地球化学在帮助破译花岗岩相关铀矿床的铀来源和富集过程方面的潜力。因为当U浓度高到足以超过其他含U矿物质的替代能力时，就会发生结晶。这些结果表明，龙华山、油东和长江花岗岩是该地区铀矿形成的有利铀源。这项研究强调了铀矿蚀变和地球化学在帮助破译花岗岩相关铀矿床的铀来源和富集过程方面的潜力。因为当 U 浓度高到足以超过其他含 U 矿物的替代能力时，就会发生结晶。这些结果表明，龙华山、油东和长江花岗岩是该地区铀矿形成的有利铀源。这项研究强调了铀矿的改变和地球化学的潜力，可以帮助破译铀源和花岗岩相关铀矿的富集过程。