图1 鲁西地区地质简图(a) (据
纸质出版日期:2023-05-25,
网络出版日期:2023-02-27,
收稿日期:2022-09-10,
录用日期:2022-10-14
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郗山矿床是山东最重要的稀土矿床,也是除内蒙古白云鄂博和四川牦牛坪以外的第三大稀土矿床。本文对控制该矿床稀土矿化的石英正长岩的元素地球化学特征进行了详细研究,限定了该岩石的源区特征和形成的构造背景;并通过与四川牦牛坪稀土矿床石英正长岩的对比,探讨了该矿床的成矿潜力。研究结果显示,郗山稀土矿床石英正长岩是典型的橄榄粗安岩系岩石,它与牦牛坪石英正长岩具有相似的地幔源区特征,但其地幔源区流体作用的强度/规模不如后者,这可能是造成郗山稀土矿化规模较小的原因之一。元素地球化学特征还显示,与超大型牦牛坪稀土矿床的石英正长岩一样,郗山石英正长岩的地幔源区受到碳酸盐化的影响,同样具有形成超大型碳酸岩型稀土矿矿床的潜力。
The Xishan REE deposit is located in the western Shandong Province, which is the third largest REE deposit in China. The deposit is genetically associated with quartz syenite. In this study, we presented detailed geochemical studies on the quartz syenite to constrain its source, geodynamic setting, and metallogenetic potentials. The quartz syenite belongs to the shoshonitic series rocks and might be generated by the partial melting of a carbonate-metasomatized mantle source during the destruction of the North China craton. Compared with the quartz syenite from the Miaoniuping REE deposit in Sichuan Province, the Xishan quartz syenite is less hydrous, which might be one of the reasons that the Xishan REE deposit has a smaller mineralization scale. Considering that both the mantle sources of the Xishan and Maoniuping REE deposits were carbonate-metasomatized, combined with the carbonatite-related REE mineralization being significant in the Maoniuping REE deposit, it is worth exploring carbonatite-related mineralization in the Xishan REE deposit further.
郗山矿床是除内蒙古白云鄂博和四川牦牛坪以外的我国第三大轻稀土矿床,稀土矿化与中生代石英正长岩密切相关,矿体受NW向和NE向断裂控制并穿插于新太古代片麻岩中(
前人对郗山稀土矿床形成时代、成矿流体性质和成矿物质来源开展了研究,
华北克拉通是我国最古老的克拉通,由东西两个块体在早元古代沿着中央造山带拼合而成(
郗山稀土矿床位于山东省微山县城东南20km的郗山村,它产于鲁西地块南部的峄城凸起。区内岩浆活动较为发育,主要为新太古代、中元古代和中生代的侵入岩,其中中生代郗山碱性岩杂岩体控制了稀土矿床的形成(
图1 鲁西地区地质简图(a) (据
Fig.1 Geological maps of the western Shandong area (a) and the Xishan REE deposit (b)
(a) :1-中新生代火山-沉积盆地,2-以上古生代为底的断陷沉积盆地, 3-白垩纪断陷盆地,
;4-下古生界, 5-前寒武纪基底, 6-断层, 7-中生代侵入岩体, 8-微山郗山稀土矿床;
(b) :1-第四系, 2-新太古代侵入岩, 3-中生代燕山晚期郗山碱性杂岩体, 4-含稀土石英重晶石碳酸岩脉型矿体, 5-细脉浸染型矿体, 6-勘查线及编号, 7-断层及编号。
图2 郗山稀土矿床石英正长手标本(a)和显微(b)照片
Fig.2 Hand specimen image (a) and microscope images (b) of the quartz syenite from the Xishan REE deposit
Or-正长石, Ab-钠长石, Q-石英, Cal-方解石。
郗山稀土矿床的矿体大致呈118°方向展布,倾向SW,倾角40°~60°。已有的勘探工程显示,郗山稀土矿床的矿体走向最大长度为756 m,倾向最大延伸为852 m。郗山稀土矿床存在石英重晶石碳酸盐脉型和细脉浸染型两类型稀土矿化,前者主要分布在石英正长岩与围岩的接触带及附近,而后者主要分布在石英正长岩体内部。石英重晶石碳酸盐脉型矿体主要由重晶石、碳酸盐(方解石)、石英、萤石和硫化物组成,矿石矿物主要为氟碳铈矿和氟碳钙铈矿,该类矿体的稀土氧化物品位介于1.56%~7.46%之间;细脉浸染型矿体的主体为石英正长岩,含有萤石、绿泥石和硫化物(黄铁矿、黄铜矿、闪锌矿和方铅矿)等矿物,矿石矿物也为氟碳铈矿和氟碳钙铈矿,该类矿体的稀土氧化物品位为0.50%~4.37%,低于石英重晶石碳酸盐脉型矿体。
本文样品采集于山东微山稀土矿区ZK13-1钻孔岩芯(
元素 | ZK13- | 元素 | ZK13- | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
F16 | F18 | F37 | F39 | F47 | F16 | F18 | F37 | F39 | F47 | |||
SiO2 | 69.27 | 68.79 | 64.50 | 64.58 | 69.29 | Mo | 2.97 | 29.9 | 41.3 | 1.88 | 78.7 | |
TiO2 | 0.16 | 0.11 | 0.12 | 0.20 | 0.20 | Ag | 0.34 | 0.15 | 0.49 | 0.42 | 0.32 | |
Al2O3 | 13.50 | 13.27 | 17.30 | 14.08 | 13.55 | Cd | 0.19 | 0.11 | 0.29 | 0.27 | 0.49 | |
TFe2O3 | 2.00 | 1.30 | 1.12 | 2.02 | 1.30 | In | 0.01 | 0.01 | 0.01 | 0.01 | 0.05 | |
MnO | 0.08 | 0.12 | 0.07 | 0.15 | 0.06 | Sn | 0.80 | 0.48 | 0.77 | 0.86 | 1.32 | |
MgO | 0.40 | 0.53 | 0.45 | 0.40 | 0.21 | Sb | 0.31 | 0.24 | 0.11 | 0.28 | 0.28 | |
CaO | 2.58 | 2.57 | 1.53 | 2.52 | 2.25 | Te | 0.02 | 0.03 | 0.03 | 0.04 | 0.05 | |
Na2O | 4.38 | 3.20 | 9.21 | 5.68 | 4.27 | Cs | 0.43 | 0.69 | 0.17 | 0.17 | 0.35 | |
K2O | 4.05 | 4.49 | 2.41 | 4.36 | 4.68 | Ba | 4 498 | 7 372 | 2 482 | 4 714 | 2 195 | |
P2O5 | 0.06 | 0.04 | 0.07 | 0.18 | 0.09 | La | 149 | 188 | 124 | 474 | 82 | |
SO3 | 0.53 | 0.43 | 0.91 | 1.51 | 0.89 | Ce | 230 | 304 | 191 | 785 | 140 | |
LOI | 2.86 | 3.40 | 1.91 | 3.85 | 2.82 | Pr | 23.5 | 30.8 | 20.3 | 77.7 | 14.7 | |
合计 | 99.85 | 98.25 | 99.61 | 99.51 | 99.62 | Nd | 71.4 | 90.8 | 61.1 | 239 | 47.4 | |
Li | 10.2 | 5.31 | 149 | 3.10 | 11.7 | Sm | 10.8 | 11.4 | 7.71 | 32.3 | 8.15 | |
Be | 2.57 | 1.91 | 7.25 | 3.35 | 1.70 | Eu | 3.31 | 3.94 | 2.17 | 7.28 | 2.15 | |
B | 3.60 | 6.06 | 1.19 | 2.83 | 2.27 | Gd | 8.39 | 9.20 | 5.87 | 23.0 | 5.81 | |
Sc | 3.09 | 1.64 | 3.01 | 1.21 | 1.87 | Tb | 0.77 | 0.79 | 0.47 | 2.01 | 0.58 | |
V | 23.0 | 18.8 | 30.8 | 21.6 | 21.0 | Dy | 3.30 | 3.31 | 1.85 | 8.05 | 2.54 | |
Cr | 6.52 | 4.60 | 6.00 | 4.02 | 7.00 | Ho | 0.60 | 0.59 | 0.33 | 1.34 | 0.43 | |
Mn | 560 | 903 | 479 | 1103 | 394 | Er | 1.42 | 1.42 | 0.78 | 3.17 | 1.00 | |
Co | 2.29 | 1.66 | 1.26 | 3.31 | 3.69 | Tm | 0.20 | 0.19 | 0.11 | 0.38 | 0.13 | |
Ni | 4.29 | 4.57 | 2.14 | 3.39 | 4.36 | Yb | 1.25 | 1.13 | 0.69 | 2.13 | 0.76 | |
Cu | 19.2 | 8.52 | 13.3 | 8.04 | 16.9 | Lu | 0.18 | 0.15 | 0.10 | 0.28 | 0.11 | |
Zn | 20.8 | 16.7 | 18.3 | 37.4 | 73.1 | Hf | 3.70 | 2.07 | 4.40 | 3.15 | 2.63 | |
Ga | 19.5 | 19.1 | 27.8 | 25.5 | 17.2 | Ta | 0.95 | 0.37 | 1.85 | 2.48 | 0.38 | |
Rb | 99.2 | 122 | 30.6 | 93.5 | 115 | W | 3.51 | 2.82 | 0.63 | 2.44 | 3.87 | |
Sr | 1 544 | 2 121 | 3 593 | 3 719 | 790 | Pb | 13.1 | 6.49 | 13.4 | 6.92 | 8.25 | |
Y | 18.3 | 18.6 | 9.88 | 41.0 | 13.1 | Bi | 0.19 | 0.34 | 0.19 | 0.26 | 0.40 | |
Zr | 186 | 83.0 | 261 | 197 | 105 | Th | 45.3 | 11.6 | 24.5 | 53.7 | 5.98 | |
Nb | 54.0 | 19.9 | 47.5 | 87.0 | 36.2 | U | 10.3 | 1.96 | 15.9 | 12.5 | 12.0 |
1)主量元素氧化物w/%,微量元素w/10-6 。
郗山石英正长岩过渡元素Cr、Ni、Co和V的质量分数相对较低,分别为4.02×10-6~5.99×10-6、2.14×10-6~4.57×10-6、1.26×10-6~3.69×10-6和18.8×10-6~30.8×10-6;大离子亲石元素Rb、Sr和Ba分别为30.6×10-6~115×10-6、790×10-6~3719×10-6和2 195×10-6~7 372×10-6,明显低于牦牛坪石英正长岩(其相应元素分别为121×10-6~204×10-6、6 138×10-6~11 865×10-6和4 105×10-6~9 660×10-6);高场强元素Zr、Hf、Nb和Ta分别为83.0×10-6~201×10-6、2.07×10-6~3.70×10-6、19.9×10-6~87.0×10-6和0.37×10-6~2.48×10-6,其中Zr、Hf和Ta与牦牛坪石英正长岩相当(后者分别为96.0×10-6~342×10-6、2.50×10-6~12.8×10-6和0.76×10-6~1.08×10-6),而Nb明显高于后者(5.09×10-6~26.7×10-6)。郗山石英正长岩稀土元素总量为305×10-6~1 655×10-6,总体上低于牦牛坪石英正长岩(723×10-6~1 543×10-6),但两者Y相近,分别为9.88×10-6~41.0×10-6和5.90×10-6~41.9×10-6。
在TAS图解中,郗山石英正长岩的投点落在石英二长岩-正长岩区(
图3 郗山石英正长岩地球化学分类图(Le Maitre et al.,2002;Peccerillo and Taylor, 1976;
Fig.3 The TAS, SiO2 vs.K2O, Na2O vs.K2O and Ta/Yb vs.Ce/Yb diagrams of Xishan quartz syenite
Nelsen(1992)和Muller & Groves(1992)将橄榄玄粗岩系岩石和超钾质岩石统称为钾质火成岩。人们普遍认为钾质火成岩是富集的陆下岩石圈地幔部分熔融的产物,其地幔源区富含角闪石和金云母等 富钾矿物(Prelevic et al.,2009;
在球粒陨石标准化的稀土元素配分模式图中,郗山石英正长岩富集轻稀土元素、亏损重稀土元素,轻重稀土分馏较强,无明显的Eu异常,与牦牛坪石英正长岩的稀土元素配分模式非常一致(
图4 郗山石英正长岩的稀土分布模式图(a)和微量元素蛛网图(b)
Fig.4 REE distribution patterns (a) and trace element spider diagrams (b) for the Xishan quartz syenite
在TiO2-Na2O+K2O判别图解中,郗山和牦牛坪石英正长岩的投影点均落在碳酸岩化地幔的区域内(
图5 郗山石英正长岩的TiO2 vs.Na2O+K2O(据
Fig.5 (a) TiO2 vs.Na2O+K2O,(b) K2O vs.Ba/La,(c) Rb/Yb vs.Ba/La and (d) Ba/Nb vs.Ba/La diagrams for the Xishan syenite
地幔源区含角闪石和金云母的脉体不但制约了钾质岩浆岩的形成,而且还控制了它的元素地球化学特征,这主要是由于角闪石与金云母本身具有不同的元素组成。已有的研究显示,角闪石具有相对较低的Rb和Th含量,但其K、Sr、Ba、LREE、HFSE等元素含量较高;而金云母则明显贫REE、HFSE和Th,但富K、Sr、Ba、Rb等元素(
郗山石英正长岩的Ba/La比值与K2O呈正相关,牦牛坪石英正长岩也显示出同样的变化特征(
钾质岩浆岩具有特定的构造指示意义,被作为地球动力学研究的重要“岩石探针”(
在Al2O3-TiO2和Zr-Y图解中,郗山和牦牛坪石英正长岩的投点均落在与弧相关的钾质岩浆岩区域内(
图6 郗山石英正长岩构造环境判别图(据
Fig.6 Discrimination diagrams for the tectonic settings of the Xishan quartz syenite
郗山稀土矿床成矿岩体石英正长岩是典型的橄榄粗安岩系岩石,其地幔源区特征与牦牛坪石英正长岩的地幔源区特征相似,但它的地幔源区流体作用强度/规模不如后者,这可能是造成郗山稀土矿床矿化规模相对小的原因之一。与超大型牦牛坪稀土矿床的石英正长岩一样,郗山石英正长岩的地幔源区也受到碳酸盐化的影响,具有形成超大型碳酸岩型稀土矿矿床的潜力。
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