| 3701铀矿床成因的同位素地球化学研究 |
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| 引用本文： | 闵茂中,孔令福.3701铀矿床成因的同位素地球化学研究[J].沉积学报,1989,7(1):81-89. |
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| 作者姓名： | 闵茂中 孔令福 |
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| 作者单位： | 1.南京大学地质系; |
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| 基金项目： | 南京大学地质系和核工业部中南地勘局307大队的联合研究成果之一 |
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| 摘 要： | 3701铀矿床赋存于L花岗岩体外接触带泥盆系灰岩中.成矿时代属燕山晚期及喜山期.根据铀-铅同位素体系演化特征以及硫、氧、碳同位素组成资料表明,该矿床的成矿物质是多源的,它们来自成矿围岩及花岗岩;矿液水主要来自大气降水.成矿过程经历了围岩成岩阶段铀的预富集、花岗岩侵入时地层中铀的活化转移和增值,围岩吸咐从花岗岩中淋出的铀.以及与燕山期、喜山期构造运动有关的热水溶液改造成矿作用.因此,它属多源、热液改造的层控铀矿床.
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| 关 键 词： | 铀矿床 铀- 铅年龄 成矿物质来源 同位素组成 |
| 收稿时间： | 1986-10-31 |
STUDY ON ISOTOPIC GEOCHEMISTRY OF ORIGIN OF THE 3701 URANIUM DEPOSIT |
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| Institution: | 1.Department or Geology. Nanjing University;2.No.307 Geological Party, Bureau of Geology and Exploration or Center-Southern China |
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| Abstract: | The ore deposit occured in carbonate rocks located in an exocontact belt (O- 120m) of the L granite batholith (Fig. 1) .The deposit was hosted by argillaceous limestone of the Yingtang Formation, Middle Devonian.The L granite body with an area of 238km2has K-Ar age of 202-318Ma. U-Pb ages of pitchblende from the deposit are 65.0±0.6Ma, 59.2 ± 0.6Ma, 30.7 ± 0.6Ma.The time-gap between the age of granite and the time of ore deposition is large, which indicates there is not a direct relationship between the mineralization and intrusive activity of the L granite body. Sulfur isotopic compositions of 30 pyrites from the ores range from δ34S values of+0.8 to -39.81 per mil which are approximately similar to those of the host rocks (ranging from -8.05 to -20.49 per mil for pyrites (Table 2) . The 4 analyzed calcites from ores have δ13C values between -0.23 and -1.53 per mil (Table 4) . The 4 analyzed host rocks have δ13C values between +1.004 and -0.965 per mil.These data are very similar. U-Pb isotopic compositions of 3 K-feldspars from the granite, 3 galenas associated with pitchblende and 7 granitic rocks have been determined (Table 3, 5) .The calculative results suggest: (1) .All leads belong to an anomalous type low in radiogcnctic lcad. These data possess a good linear relation in 207Pb/ 204Pb-206Pb /204Pb plots (Fig.4), implying a genetic connection between the granite and the ore. (2) .Average loss of uranium in granitc has been 59.51% since the granite is formed, while average addition of uranium in granite near limestone has been 49.56%.It is an important uranium source for the mineralization. The δ18O values ( SMOW, 6 samples) of ore-forming fluid at 120°-281℃ in equilibrum with microcrystallinc quartzs and calcitcs range from -3.14 to+13.92 per mil (Table 6), which are higher than those of ancient meteoric water in the area.It may be due to the oxygen isotopic exchange between the meteoric water (δ13OH2o = -9~-10pcr mil) and limestone (δ18OH2o ≠ +12.68~+26.15 per mil) . There arc reasons to believe that in the ore constituents S, C were mainly derived from the host rocks (argillaceous limestone), and U, Pb mainly from the granite.The ore-forming fluid was dominated by meteoric water. It can be imagined that the surface water in the period of continental weathering and oxidation (Cretaceous) infiltrated downwards, leaching out large amounts of uranium from the granitic rocks to form infiltration solution. A part of uranium in solution was adsorpted by argillaceous limestones. The solution was heated at large depth and then moved upwards, during which it dissolved constantly uranium from the host rocks to form the deposit. Therefore, the 3701 uranium deposit is a stratabound one of the multi-source and later multi-superimposition of hydrothermal transformation. |
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