Application of Electron Probe Microanalyzer to Study the Textures and Compositions of Alteration Coronas of Monazite from the Longhuashan Granite, Northern Guangdong Province
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摘要:
独居石是华南产铀花岗岩中常见的含铀副矿物。龙华山岩体是粤北诸广山复式岩体中一个重要的产铀花岗岩，该岩体的独居石具有蚀变晕圈现象。但是，该岩体中独居石蚀变晕圈的结构和成分特征以及对铀成矿的指示意义尚未开展研究。本文利用电子探针(EPMA)对龙华山岩体的独居石蚀变晕圈开展结构和成分研究。测试结果表明：独居石蚀变晕圈是从内到外由独居石、磷灰石(包括富钍矿物)和褐帘石-绿帘石构成的同心环带。质量平衡计算表明独居石在蚀变过程中，轻稀土(LREE)、Y、Th和U等元素发生活化迁移，流体带入Ca、Fe、Al、F等元素，形成了由磷灰石、褐帘石、绿帘石和富钍矿物组成的蚀变晕圈。EPMA面扫描图像显示独居石蚀变导致了铀发生活化，但铀主要在蚀变晕圈中富集。研究数据显示龙华山岩体中仅3.7%的铀赋存于独居石中，而80%以上的铀赋存在晶质铀矿中。本研究表明独居石对区域铀矿化贡献的成矿物质有限，晶质铀矿是龙华山岩体最重要的铀源矿物。
Abstract:BACKGROUND Monazite is a common uranium-bearing accessory mineral in granite-related uranium deposits in South China. The Longhuashan pluton is an important U-bearing granite in the Zhuguangshan batholith, Northern Guangdong. Distinct alteration coronas of monazite were observed in the pluton. However, textures and compositions of alteration coronas of monazite and their implications for uranium mineralization are still poorly understood.
OBJECTIVES To investigate the detailed textural and compositional evolution of monazite in granites during alteration, and to provide insights into uranium mobilization and enrichment in granite-related uranium deposits.
METHODS Textures and chemical compositions of alteration coronas of monazite were investigated using electron probe microanalyzer (EPMA).
RESULTS Alteration coronas of monazite in the Longhuashan granite consisted of newly formed apatite, allanite, epidote, and Th-rich phases. The monazite alteration coronas area was a concentric zone composed of monazite, apatite (including thorium-rich minerals), and monazite-epidote from the inside to the outside. Mass balance calculations showed that during the alteration of monazite, light rare earth (LREE), Y, Th, and U elements were remobilized and migrated, and the fluid brought in elements such as Ca, Fe, Al, and F to form alteration coronas composed of apatite, epidote, and thorium-rich minerals. The EPMA mapping showed that monazite alteration led to the remobilization of uranium, but the uranium was mainly enriched in the altered coronas. In this pluton, 3.7% U was located in monazite, and more than 80% U was hosted by uraninite.
CONCLUSIONS Monazite only contributes limited uranium to regional uranium mineralization, and uraninite is the most important host for uranium in the Longhuashan granite.
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Key words:
- monazite /
- alteration coronas /
- electron probe microanalyzer /
- Longhuashan granite /
- northern Guangdong
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表 1 龙华山花岗岩中独居石蚀变晕圈矿物(包括独居石、磷灰石、绿帘石、褐帘石)和晶质铀矿代表性电子探针分析结果
Table 1. Representative EPMA chemical compositions of monazite alteration coronas (including monazite, apatite, epidote, and allanite) and uraninite from the Longhuashan granite
矿物 独居石(%) 磷灰石(%) 绿帘石(%) 褐帘石(%) 晶质铀矿(%) 点1 点2 点3 点1 点2 点1 点2 点3 点1 点2 点3 点1 点2 点3 CaO 1.38 0.47 2.25 42.29 46.07 16.38 16.78 15.61 10.89 11.83 12.76 0.00 0.01 0.00 P2O5 29.50 29.10 30.60 32.06 36.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 ThO2 9.23 8.53 10.00 12.13 8.35 0.00 0.10 0.00 0.18 0.14 0.14 1.01 1.16 1.06 La2O3 12.03 11.23 10.67 0.26 0.05 1.76 1.50 2.13 3.76 5.12 5.65 0.00 0.00 0.02 Ce2O3 24.32 26.02 22.78 0.62 0.47 6.25 7.75 7.41 12.75 11.36 11.09 0.11 0.00 0.00 Nd2O3 7.50 7.72 6.97 0.29 0.27 0.97 0.88 1.08 2.89 2.68 1.36 0.00 0.02 0.00 Pr2O3 7.17 8.66 7.27 0.73 0.48 1.55 1.12 1.42 2.93 2.56 2.51 0.00 0.02 0.06 Sm2O3 3.42 1.90 1.21 0.10 0.01 0.74 0.62 0.69 1.01 0.92 0.89 0.24 0.00 0.00 Dy2O3 2.41 1.51 2.58 0.37 0.00 0.47 0.81 0.54 0.71 0.58 0.81 0.00 0.00 0.00 Lu2O3 0.34 0.00 0.30 0.38 0.14 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 Y2O3 0.93 1.03 1.84 0.05 0.01 0.70 0.97 1.08 0.39 0.27 0.34 0.09 0.16 0.42 UO2 0.06 0.05 0.47 0.49 0.16 0.00 0.00 0.02 0.08 0.02 0.03 92.25 91.66 95.09 F 0.62 0.80 0.63 3.82 3.89 0.00 0.00 0.02 0.10 0.29 0.00 0.00 0.00 0.00 SiO2 0.73 1.03 0.40 3.63 2.54 33.57 32.88 33.77 31.68 31.87 32.05 0.00 0.00 0.04 Al2O3 0.00 0.00 0.00 0.00 0.00 21.76 21.63 21.76 18.05 19.31 19.68 0.00 0.00 0.00 FeO 0.00 0.00 0.00 0.00 0.00 9.40 8.83 9.49 11.74 10.83 10.14 0.09 0.00 0.00 MgO 0.00 0.00 0.00 0.00 0.00 0.04 0.06 0.06 0.08 0.10 0.06 0.01 0.00 0.00 PbO 0.03 0.00 0.05 0.08 0.07 0.00 0.00 0.00 0.00 0.00 0.00 2.78 2.78 2.85 总和 99.65 98.04 98.03 97.31 98.52 93.60 93.94 95.06 97.22 97.89 97.50 96.57 95.81 99.54 年龄(Ma) - - - - - - - - - - - 222 223 221 注：“-”表示未进行化学年龄计算。 
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表 2 龙华山花岗岩中独居石蚀变晕圈质量平衡计算结果
Table 2. Results of mass balance calculations of alteration coronas of monazite from the Longhuashan granite
元素 本文(%) 文献[13](%) 1 2 3 4 5 6 7 8 P2O5 9.16 9.16 31.70 29.90 7.96 7.96 30.37 28.45 SiO2 25.07 - - 0.70 27.26 - - 0.43 La2O3 2.98 2.98 10.32 11.45 3.24 3.24 12.37 12.66 Ce2O3 7.88 7.88 27.27 24.80 6.86 6.86 26.18 26.70 Pr2O3 1.80 1.80 6.23 7.68 0.98 0.98 3.74 3.67 Nd2O3 1.60 1.60 5.55 7.76 2.95 2.95 11.24 11.93 Sm2O3 0.65 0.65 2.24 2.44 0.66 0.66 2.51 2.42 Dy2O3 0.56 0.56 1.95 1.77 - - - - Lu2O3 0.07 0.07 0.24 0.19 - - - - Y2O3 0.37 0.37 1.28 1.47 0.68 0.68 2.61 2.24 ThO2 2.85 2.85 9.86 8.07 1.94 1.94 7.40 7.85 UO2 0.11 0.11 0.38 0.19 0.15 0.15 0.58 0.42 Al2O3 14.78 - - - 16.26 - - - FeO 8.04 - - - 7.13 - - - CaO 21.57 - - 1.21 21.39 - - 1.44 MgO 0.09 - - - 0.32 - - - F 1.14 - - 0.68 - - - 总和 98.72 28.02 97.00 98.31 97.78 25.43 97.00 98.20 注：第1列表示根据磷灰石、褐帘石、绿帘石所占体积和平均成分计算得到的这三个矿物的混合成分；第2列是指第1列元素成分减去SiO2、Al2O3、FeO、CaO、MgO、F的元素含量；第3列是指把第2列元素含量归一化为97%的标准化含量；第4列是指测试所得的独居石元素平均含量。第5、6、7、8列所代表的含义分别同第1、2、3、4列。“-”表示低于检出限。
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