Ore-Forming Process and Redox Zoning of Sandstone-Type U Deposits: Evidence from U Series Disequilibrium
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摘要: 铀系不平衡技术被用来研究新疆伊犁盆地库捷尔太砂岩型铀矿床的氧化还原分带和成矿过程.取自该矿床的26个样品的铀、钍含量以及234 U/ 238U, 230 Th/ 234 U和230 Th/ 238U活度比值分别用ICP -MS和α能谱仪进行了测量.不同氧化还原带岩石的铀、钍含量和钍/铀比明显不同: 强氧化带岩石的U、Th含量和Th/U比分别为12.4 μg/g, 4.5 μg/g和0.48; 弱氧化带分别为20.4 μg/g, 5.0 μg/g和0.38; 过渡带(矿化带)分别为169.7μg/g, 4.7μg/g和0.07; 还原带(未蚀变带)分别为6.8μg/g, 3.7μg/g和0.87.其同位素特征亦有明显差异: 氧化带岩石234 U/ 238U大多大于1, 过渡带(矿化带)岩石部分大于或等于1, 部分小于1, 还原带(未蚀变带)岩石大多大于1; 氧化带岩石230 Th/ 234 U和230 Th/ 238U大多大于1, 过渡带(矿化带)岩石大多小于或等于1, 还原带(未蚀变带)岩石大多大于1.这可作为砂岩型铀矿床矿体定位的指示剂.铀系不平衡特征还示踪了该矿床的成矿作用过程.Abstract: Uranium series disequilibrium techniques were used to provide information on the redox zoning and ore-forming processes of the Kujie'ertai uranium deposit, Yili basin, Xinjiang, NW China. The uranium and thorium contents and the activity ratios of 234U/238U, 230Th/234U and 230Th/238U for about 26 samples from the deposit were measured using ICP-MS and α-spectrometer respectively. The characteristics of uranium and thorium isotopes for different redox zones are significantly different, which can serve as indicators for locating ore-bodies of the sandstone-type uranium deposit. The U and Th concentrations and Th/U ratios of rocks are 12.4 μg/g, 4.5 μg/g and 0.48 for the strongly oxidized zone, 20.4 μg/g, (5.0 μg/g) and 0.38 for the weakly oxidized zone, 169.7 μg/g, 4.7 μg/g and 0.07 for the transitional zone (the mineralized zone), 6.8 μg/g, 3.7 μg/g and 0.87 for the reduced zone (the unaltered zone) respectively. These different redox zones also present different ratios of radioactive nuclides for various samples. The 234U/238U ratios are mostly greater than 1 for the strongly oxidized zone, partly greater or less than 1 and partly close to 1 for the redox front (the mineralized zone), and predominantly greater than unity for the reduced zone (the unaltered zone) respectively. While the ratios of 230Th/234U and 230Th/238U are greater than or close to unity for the oxidized zone, partly greater than 1 and partly less than or close to 1 for the redox front (the mineralized zone), and greater than or close to unity for the reduced zone (the mineralized zone). The uranium ore-formation processes in the deposit are traced effectively by U series disequilibrium.
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图 1 伊犁盆地地质略图
1. 第四系; 2. 第三系; 3. 侏罗系; 4. 三叠系; 5. 上古生界; 6. 下古生界; 7. 中上元古界; 8. 海西期花岗岩; 9. 逆断层; 10. 性质不明断层; 11. 推测断层; 12. 铀矿床; 13.铀矿点
Fig. 1. Simplified geological map of Yili basin
图 2 库捷尔太铀矿床不同分带U含量及Th/U比值
Fig. 2. Uranium concentrations and Th/U ratios of various redox zones, Kujie'ertai uranium deposit
图 3 库捷尔太铀矿床不同分带铀系同位素比值(图中样品序号与表 2中的一致)
Fig. 3. Ratios of U series isotopes in different redox zones of Kujie ' ertai uranium deposit
图 4 库捷尔太铀矿床Ⅰ-Ⅱ旋回砂岩234U/238U-230Th/238U比值
Fig. 4. 234U/238U versus 230Th/238U for rock samples in Kujie ' ertai uranium deposit
表 1 伊犁库捷尔太铀矿床Ⅰ-Ⅱ旋回含矿层铀钍含量
Table 1. Uranium and thorium concentrations of samples from layers Ⅰ-Ⅱ of Kujie'ertai uranium deposit, Yili basin
表 2 伊犁盆地库捷尔太铀矿床Ⅰ-Ⅱ旋回含矿层放射性同位素测试结果
Table 2. Radioactive isotopic ratios of samples from layers Ⅰ-Ⅱ of Kujie ' ertai uranium deposit, Yili basin
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