Occurrence characteristics and main control mechanism of trace elements in Early Permian coal in the southern margin of North China Plate
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摘要:
煤中关键金属是矿产资源勘探开发的新领域。目前对华北板块南缘早二叠世煤中微量元素赋存特征及主控机制研究较少, 制约了关于该区煤沉积物源、沉积环境的认识以及煤的清洁高效利用。基于X射线衍射(XRD)、X射线荧光光谱(XRF)、电感耦合等离子体质谱(ICP-MS)等分析方法, 对华北板块南缘早二叠世中煤矿物学与煤地球化学特征进行了研究。结果表明, 华北板块南缘早二叠世煤中矿物主要发育高岭石、伊利石、铵伊利石、绿泥石和方解石等, 主量元素以SiO2, Al2O3与CaO为主。煤层样品中Li元素相对富集, 质量分数为54.5×10-6~116×10-6(均值76.83×10-6); Zr、Th元素轻微富集, 质量分数分别为34.14×10-6~160.73×10-6(均值73.81×10-6)和3.22×10-6~17.79×10-6(均值7.85×10-6); 其他元素质量分数接近或低于世界硬煤平均值, 其中Co、Zn、Rb、Cd和Cs等元素明显贫化。通过相关性分析与地球化学解释得出以下结论: ①华北板块南缘早二叠世煤中Li元素主要赋存于高岭石等黏土矿物中, 其含量主要受控于陆源碎屑; ②受华力西构造运动影响, 北部阴山古陆中元古代钾长花岗岩与石炭系本溪组古风化壳铝土矿被抬升风化剥蚀, 并成为华北板块南缘早二叠世煤的主要供源区; ③泥炭沼泽水介质偏咸与缺氧还原的沉积环境有利于Li元素被高岭石等黏土矿物吸附并沉积聚集。
Abstract:Objective The study focuses on the development of critical elements in coal as a new area of mineral resource exploration.
Methods Various analytical methods including X-ray diffraction, X-ray fluorescence spectrum, and inductively coupled plasma-mass spectrometry were used to investigate the mineralogy and geochemical characteristics of Early Permian middling coal in the southern margin of the North China Plate.
Results The results indicate that the Early Permian coal in this region is mainly composed of minerals such as kaolinite, illite, tobelite, chlorite, and calcite, with SiO2, Al2O3, and CaO as the main elements. The coal seam samples also show enrichment of Li, with an average content of 76.83×10-6, as well as slight enrichment of Zr and Th. Other elements, such as Co, Zn, Rb, Cd, and Cs, are significantly depleted compared to the world average for hard coal.
Conclusion Through correlation analysis and geochemical interpretation, it was concluded that the Li element in the Early Permian coal is mainly associated with clay minerals like kaolinite and is controlled by terrigenous debris. The uplift, weathering, and denudation of Mesoproterozoic K-feldspar granite and Carboniferous Benxi Formation palaeoweathering crust bauxite in the northern Yinshan ancient land, influenced by Variscan tectonic movement, became the primary source area for the Early Permian coal in the southern margin of the North China Plate. The salty and anoxic reducing sedimentary environment of the peat water medium favors the adsorption and deposition of Li by clay minerals, particularly kaolinite.
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Key words:
- the southern margin of North China Plate /
- Li element /
- clay mineral /
- coal /
- sedimentary environment
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图 1 二叠纪聚煤前岩相古地理图(a)[24]与石炭系-二叠系地层综合柱状图(b) (HST.高位体系域;EST.海侵体系域; SQ.层序)
Figure 1. Lithofacies paleogeography map when coal was forming before Permian (a) and comprehensive histogram of Carboniferous-Permian strata (b)
表 1 山西组二1煤工业分析与元素分析
Table 1. Proximate and ultimate analysis of the No.21 coal from the Shanxi Formation
wB/% 样品名 MJ-1 MJ-2 MJ-3 MJ-4 MJ-5 MJ-6 MJ-7 MJ-8 MJ-9 MJ-10 平均值 Mad 1.70 0.71 0.82 0.65 0.80 1.02 1.31 0.93 1.17 0.43 0.95 Ad 16.39 14.88 13.36 14.39 11.75 8.86 8.29 7.69 8.85 11.00 11.55 Vdaf 11.99 11.39 12.01 11.12 11.80 10.92 10.96 10.95 10.58 10.77 11.25 S 0.70 0.09 0.38 1.56 0.41 0.50 0.82 0.12 0.48 0.28 0.53 H 2.99 3.46 4.45 5.53 3.63 3.69 4.61 2.66 3.14 3.13 3.73 C 79.98 93.27 73.04 71.75 82.18 79.57 70.78 96.22 76.55 80.57 80.39 N 1.03 1.28 1.12 1.07 1.13 1.14 0.97 1.56 1.17 1.46 1.19 注:Mad.水分;Ad.灰分;Vdaf.挥发分;S, H, C, N均为元素符号 表 2 山西组二1煤中矿物质量分数
Table 2. Mineral contents of the No.21 coal in the Shanxi Formation
wB/% 矿物 最大值 最小值 平均值 矿物 最大值 最小值 平均值 蒙脱石 2.16 0.43 锐钛矿 4.66 0.56 2.07 高岭石 56.59 33.02 46.91 黄钾铁矾 2.43 0.21 伊利石 40.46 11.89 27.15 磷灰石 1.94 0.49 铵伊利石 7.87 3.02 5.14 硬水铝石 7.05 0.22 绿泥石 16.77 1.89 7.25 黄铁矿 0.44 0.10 方解石 14.37 1.23 7.03 白云石 1.23 0.23 勃姆石 0.43 0.17 注: 无均值矿物表示部分煤层低于检出限 表 3 山西组二1煤常量元素氧化物质量分数
Table 3. Oxide contents of major elements of the No.21 coal in the Shanxi Formation
SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O MnO TiO2 P2O5 SiO2/Al2O3 wB/% 最大值 6.98 6.22 0.83 0.23 1.93 0.06 0.08 0.01 0.40 0.32 1.12 最小值 3.20 2.99 0.55 0.11 0.75 0.02 0.02 0.00 0.07 0.01 1.06 平均值 4.85 4.43 0.66 0.15 1.23 0.04 0.05 0.00 0.19 0.08 1.09 表 4 山西组二1煤微量元素质量分数
Table 4. Trace elements contents of the No.21 coal in the Shanxi Formation
最大值 最小值 平均值 世界煤平均值 最大值 最小值 平均值 世界煤平均值 Li 116.00 54.50 76.83 12.00 La 48.32 9.08 17.71 11.00 Be 0.84 0.38 0.61 1.60 Ce 71.68 16.26 29.14 23.00 Sc 7.74 1.98 3.54 3.90 Pr 6.52 1.71 2.93 3.50 V 32.65 9.62 14.94 25.00 Nd 19.02 6.13 9.43 12.00 Cr 30.35 5.58 12.28 16.00 Sm 3.87 1.13 1.79 2.00 Co 1.10 0.61 0.90 5.10 Eu 0.70 0.23 0.34 0.47 Ni 6.19 2.95 3.94 13.00 Gd 3.48 1.30 1.77 2.70 Cu 33.95 9.43 17.59 16.00 Tb 0.49 0.20 0.27 0.32 Zn 6.68 3.06 4.28 23.00 Dy 3.20 1.20 1.65 2.10 Ga wB/10-6 15.99 4.67 7.71 5.80 Ho wB/10-6 0.60 0.25 0.32 0.54 As 2.89 2.73 2.81 8.30 Er 1.64 0.65 0.90 0.93 Rb 3.00 0.63 1.75 14.00 Tm 0.24 0.10 0.14 0.31 Sr 382.52 88.03 198.27 110.00 Yb 1.58 0.60 0.87 1.00 Y 16.35 7.21 9.23 8.40 Lu 0.24 0.10 0.14 0.20 Zr 160.73 34.14 73.81 36.00 Hf 4.16 1.09 2.04 1.20 Nb 13.34 2.37 5.84 3.70 Ta 0.96 0.18 0.45 0.28 Mo 1.50 0.74 1.02 2.20 Tl 0.34 0.09 0.16 0.63 Cd ND 0.01 0.01 0.22 Pb 24.57 4.12 8.93 7.80 Sn 3.38 0.64 1.36 1.10 Th 17.79 3.22 7.85 3.30 Cs 0.17 0.06 0.12 1.00 U 5.00 0.97 1.96 2.40 Ba 81.10 17.69 41.65 150.00 V/Zn 4.80 2.10 3.40 1.08 V/(V+Ni) 0.85 0.68 0.78 0.66 表 5 稀土元素地球化学参数
Table 5. Geochemical parameters of rare earth elements
样品编号 REY LREY MREY HREY (La/Lu)N (La/Sm)N (Gd/Lu)N δEu δCe wB/10-6 MJ-1 177.93 149.41 24.23 4.29 2.16 1.87 1.23 0.95 0.89 MJ-2 116.50 97.88 15.76 2.86 1.73 1.63 1.14 0.83 0.92 MJ-3 71.61 58.97 10.70 1.94 1.83 1.88 1.04 1.03 0.86 MJ-4 85.08 67.45 14.90 2.73 1.44 1.74 1.03 1.04 0.88 MJ-5 54.00 40.25 11.58 2.17 0.83 1.09 0.99 0.96 0.96 MJ-6 50.18 38.19 10.23 1.75 1.07 1.05 1.15 0.88 0.93 MJ-7 51.75 39.44 10.53 1.78 1.06 1.28 1.13 0.99 0.93 MJ-8 46.33 34.31 10.24 1.79 0.87 1.21 0.99 1.05 0.94 MJ-9 57.23 41.09 13.70 2.44 0.76 0.96 0.95 1.11 0.93 MJ-10 55.80 43.16 10.69 1.96 1.06 1.20 1.01 1.00 0.95 平均值 76.64 61.01 13.26 2.37 1.37 1.48 1.08 0.97 0.91 注: (La/Lu)N, (La/Sm)N, (Gd/Lu)N为标准化后值; δEu=EuN/[(0.67×SmN)+(0.33×GdN)], δCe=CeN/(0.5×LaN+0.5×PrN) 表 6 煤中Li与常量元素氧化物含量及微量元素含量比值的Pearson相关系数
Table 6. Pearson correlation coefficients between Li and major element oxides content, trace element content ratio R
项目 SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O MnO TiO2 P2O5 V/Zn V/(V+Ni) Li 0.92 0.92 0.31 0.45 0.44 0.87 0.93 0.39 0.80 0.69 0.85 0.74 -
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