Electron Microprobe Analysis of Biotite with Reequilibration Texture in Altered Trachyte
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摘要: 黑云母的化学成分中蕴含着重要的成因信息，对具有交代结构的黑云母进行微区成分分析，能够精细反映交代蚀变过程中元素迁移情况，为解决矿床形成机制方面的科学问题提供矿物学证据。本文以北大巴山平利地区蚀变粗面岩中具有再平衡结构的黑云母斑晶为研究对象，进行电子探针面扫描和剖面成分分析。结果表明，斑晶内部为岩浆成因黑云母，边缘为"扩散-反应"过程所致的再平衡成因黑云母。斑晶边缘与内部相比，TiO2含量由平均5.78%降至2.22%，表明在交代蚀变过程中黑云母中Ti等高场强元素被淋滤出进入流体；而CaO含量由平均0.06%升至0.14%，F含量由平均0.60%升至0.80%，指示蚀变流体内富含Ca、F等组分。本研究揭示了粗面岩中铌矿床的形成与热液作用关系密切，成矿流体中Ca、F等组分在铌元素的迁移及富集过程中具有重要作用。Abstract:
BACKGROUNDThe chemical composition of biotite contains important genetic information. The in-situ analyses of biotite with metasomatic texture can reflect the migration of the elements during the metasomatic alteration, and provide mineralogical evidence for the ore-forming mechanism. OBJECTIVESTo investigate the migration of elements during trachyte alteration and reveal the Nb mineralization mechanism in altered trachyte. METHODSBiotite phenocryst with reequilibration texture in altered trachyte in the Pingli area of North Dabashan was studied, and the EMPA element mapping and line profile analyses were carried out. RESULTSThe results show that the phenocryst was magmatic in the core and the reequilibrated origin on the rim, caused by diffusion-reaction process. Compared with the core, the TiO2 content decreased from an average of 5.78% to 2.22% in the rim, indicating that Ti was leached out into the fluid during the exchange alteration. The CaO content increased from 0.06% to 0.14% on average, and the F content increased from 0.60% to 0.80% on average, suggesting that the alteration fluid is rich in Ca and F. CONCLUSIONSThe formation of Nb deposits in trachyte is closely related to the hydrothermal activity, and the components such as Ca and F in the ore-forming fluid play an important role in the migration and enrichment of Nb. -
Key words:
- biotite /
- electron microprobe analysis /
- reequilibration textures /
- Nb deposit /
- trachyte
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表 1 黑云母斑晶电子探针分析结果
Table 1. Representative electron microprobe analysis of biotite phenocrysts
成分 斑晶内部 斑晶边缘 TC-B3-20(7个分析点) TC-B3-30(6个分析点) 平均值
(%)TC-B3-20(6个分析点) TC-B3-30(4个分析点) 平均值
(%)最小值
(%)最大值
(%)平均值
(%)最小值
(%)最大值
(%)平均值
(%)最小值
(%)最大值
(%)平均值
(%)最小值
(%)最大值
(%)平均值
(%)SiO2 34.62 36.02 35.30 34.00 35.56 34.89 35.10 35.81 36.36 36.11 35.72 36.56 36.25 36.18 TiO2 4.72 7.39 5.93 4.65 7.14 5.62 5.78 1.98 2.19 2.11 2.23 2.44 2.33 2.22 Al2O3 12.27 13.30 12.63 12.26 13.09 12.60 12.62 13.90 14.59 14.33 13.53 13.86 13.71 14.02 FeOT 21.30 22.50 21.82 20.32 21.52 21.17 21.50 22.06 23.08 22.48 21.65 22.36 21.98 22.23 MnO 0.97 1.28 1.16 1.25 2.00 1.51 1.34 1.21 1.36 1.31 1.23 1.43 1.33 1.32 MgO 8.92 9.50 9.19 9.22 10.27 9.82 9.51 9.14 9.52 9.32 9.47 9.85 9.73 9.53 CaO 0.05 0.22 0.11 0.00 0.00 0.00 0.06 0.06 0.33 0.16 0.05 0.14 0.11 0.14 Na2O 0.13 0.26 0.21 0.20 0.31 0.25 0.23 0.27 0.47 0.35 0.22 0.33 0.28 0.32 K2O 8.78 9.23 8.99 8.71 9.35 9.11 9.05 8.85 9.32 9.12 9.18 9.59 9.42 9.27 F 0.33 0.68 0.48 0.68 0.73 0.71 0.60 0.62 0.83 0.70 0.82 0.98 0.90 0.80 总量 94.25 96.57 95.34 94.70 95.22 94.96 95.15 94.82 95.91 95.27 94.75 95.49 95.11 95.19 XFeO* 0.70 0.73 0.72 0.69 0.71 0.70 0.71 0.71 0.72 0.72 0.70 0.71 0.71 0.72 基于22个O原子计算阳离子数 成分 斑晶内部 斑晶边缘 TC-B3-20(7个分析点) TC-B3-30(6个分析点) 平均值
(%)TC-B3-20(6个分析点) TC-B3-30(4个分析点) 平均值
(%)最小值
(%)最大值
(%)平均值
(%)最小值
(%)最大值
(%)平均值
(%)最小值
(%)最大值
(%)平均值
(%)最小值
(%)最大值
(%)平均值
(%)Si 5.45 5.56 5.51 5.37 5.54 5.48 5.50 5.58 5.69 5.64 5.63 5.70 5.67 5.66 AlⅣ 2.22 2.45 2.33 2.25 2.44 2.33 2.33 2.31 2.42 2.36 2.30 2.37 2.33 2.35 AlⅥ 0.00 0.03 0.00 0.00 0.00 0.00 0.00 0.24 0.33 0.28 0.18 0.23 0.20 0.24 Ti 0.56 0.85 0.70 0.55 0.84 0.66 0.68 0.23 0.26 0.25 0.26 0.29 0.27 0.26 Fe 2.76 2.95 2.85 2.64 2.84 2.78 2.82 2.88 3.01 2.94 2.82 2.95 2.88 2.91 Mn 0.13 0.17 0.15 0.16 0.27 0.20 0.18 0.16 0.18 0.17 0.16 0.19 0.18 0.18 Mg 2.06 2.24 2.14 2.14 2.40 2.30 2.22 2.14 2.22 2.17 2.22 2.30 2.27 2.22 Ca 0.01 0.04 0.02 0.00 0.00 0.00 0.01 0.01 0.06 0.03 0.00 0.02 0.01 0.02 Na 0.04 0.08 0.06 0.06 0.09 0.08 0.07 0.08 0.14 0.10 0.07 0.10 0.08 0.09 K 1.74 1.84 1.79 1.75 1.87 1.82 1.81 1.77 1.85 1.82 1.84 1.90 1.88 1.85 XMg 0.41 0.44 0.43 0.44 0.46 0.45 0.44 0.42 0.43 0.42 0.43 0.45 0.44 0.43 T(℃) 737 792 766 738 792 763 765 601 622 614 625 645 635 625 注：FeOT表示电子探针测试的全铁含量；XFeO*=FeO*/(FeO*+MgO)；FeO*=FeOT+MnO；XMg=Mg/(Mg+Fe)。T(℃)计算据公式：T={[ln(Ti)-a-c×(XMg)3]/b}0.333[21]。 
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