含铀副矿物的热液蚀变:来自华南鹿井矿田碎裂蚀变岩型铀矿床的矿物学研究

The hydrothermal alteration of U-bearing accessory minerals:an example from the mineralogical study of cataclastic alteration granite-type uranium deposit in the Lujing ore field, South China

花岗岩型铀矿中铀的来源问题,长期以来是铀矿床学研究的热点问题之一。大多数学者认为其成矿物质主要来源于花岗岩本身的含铀副矿物,然而对于含铀副矿物热液蚀变行为研究较少。鹿井铀矿田位于诸广山复式岩体的中部,是华南最主要花岗岩型铀矿田之一,碎裂蚀变岩型铀矿化在该矿田内占主导地位。小山铀矿床位于鹿井矿田中部,是近些年新发现的碎裂蚀变岩型矿床。本文以钻孔ZK1-1为研究对象,对热液蚀变带开展了精细矿物学研究。研究表明:蚀变带中发育有晶质铀矿、铀石—钍石、独居石、磷钇矿、锆石、磷灰石、金红石等含铀副矿物。晶质铀矿、铀石—钍石中铀含量高,热液蚀变条件不稳定,铀容易释放;独居石蚀变为直氟碳钙铈矿和磷钇矿蚀变为次生磷灰石过程中容易释放出铀;锆石因结构稳定,铀难以释放;磷灰石、金红石中铀含量较低,供铀能力差。综合分析认为花岗岩中晶质铀矿、铀石—钍石是主要铀源矿物,独居石、磷钇矿为次要铀源矿物。

The sources of granite-type uranium deposit in China have been widely mentioned in previous studies. It is generally accepted by most of the researchers that the uranium sources came from the U-bearing minerals in granite. However, the course of uranium evolution during the hydrothermal alteration of granite has not been paid much attention. The Lujing uranium ore field located in the middle part of the Zhuguangshan complex granite is one of the most important granite type uranium ore field in South China. This ore field is dominated by the uranium mineralization related cataclastic alteration. The Xiaoshan uranium deposit, a newly discovered deposit, is located in the central section of the Lujing ore field. In this study, the mineralogy of hydrothermal zones from drill hole ZK-1 in the Xiaoshan deposit has been carried out. The uraninite, uranite-thorium, monazite, xenotime, zircon, apatite and rutile have been found in the altered zones. The uraninite and coffinite-thorite host the highest uranium contents and uranium in these minerals is easily released during later alteration by oxidizing fluids. The monazite and xenotime have a moderate quantity of uranium that can potentially be liberated when they are altered to synchysite and apatite, respectively. Zircon shows stable crystal structure and does not release uranium during hydrothermal alteration. Apatite and rutile are not uranium-source minerals due to the negligible uranium content in these two minerals. Summarizing, uraninite and coffinite-thorite are the main uranium-source minerals for the Xiaoshan deposit, while Monazite and xenotime are secondary uranium-source minerals.