Metallogenic mechanism of alkalinesyenite in Dazhuang Nb-REE deposit, Fangcheng, Henan Province
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摘要:研究目的
河南方城大庄是新探明的中型铌-稀土矿床（Nb2O3资源量16245 t；伴生TRE2O3资源量30147 t），矿体主要赋存在碱性正长岩内。
研究方法本文在野外地质调查基础上，对含矿碱性正长岩与无矿碱性正长岩开展了岩石学、岩相学和地球化学的对比研究。
研究结果研究结果表明，含矿与无矿碱性正长岩均富碱、富铝，为典型的A型花岗岩，两类岩石的碱金属含量无明显差别，但（Na2O+K2O）/CaO、FeO*/MgO、K2O/MgO等参数明显不同。含矿碱性正长岩高场强元素Nb、Ta、Ce、U、Th、Zr、Y明显富集，Ba、Sr、P、Eu明显亏损；无矿碱性正长岩大离子亲石元素Rb、高场强元素Nb、Ta、Y和Th元素明显富集，而Ba、U、Sr、P、Ti和重稀土元素明显亏损，但亏损程度低于含矿碱性正长岩。
结论方城大庄含矿碱性正长岩不是无矿碱性正长岩热液蚀变（钠长石化）的结果，二者应是同一岩浆体系不同演化阶段溶体固结的产物，含矿碱性正长岩的分异演化程度明显高于无矿化的碱性正长岩。方城大庄稀有稀土元素的富集与岩浆高演化、分异过程密切相关。这一研究可为区域找矿勘查提供一定的地质依据。
创新点：岩石学和地球化学对比研究显示大庄Nb-REE矿中含矿碱性正长岩具有比无矿岩体更高的演化程度，表明成矿元素的富集与岩浆高分异演化密切相关，为找矿勘查提供了一定的岩石学证据。
Abstract:This paper is the result of mineral exploration engineering.
ObjectiveDazhuang deposit is a newly discovered medium-sized Nb-REE deposit (Nb2O3=16245t; Associated TRE2O3=30147 t)in Fangcheng, Henan Province and its ore body occurs within alkaline syenite.
MethodsIn this paper, the authors compared the differences between ore-bearing and barren alkaline syenites in terms of petrology, petrography and geochemistry based on detailed field observation.
ResultsThe results show that both ore-bearing and barren alkaline syenites belong to typical A-type granite which is rich in alkaline and aluminium and there is no significant differences in alkali metal contents between them, whereas the ratios of (Na2O+K2O) /CaO, FeO*/MgO, K2O/MgO are obviously different. Moreover, the ore-bearing alkaline syenite has high contents of high field strength elements such as Nb, Ta, Ce, U, Th, Zr and Y, but is depleted in Ba, Sr, P and Eu. The barren alkaline syenite displays high content of large ion lithophile elements of Rb and some high field strength elements such as Nb, Ta, Y and is depleted in Ba, U, Sr, P, Ti and heavy rare earth elements however, its depleted degree is lower than that of ore-bearing alkaline syenite.
ConclusionsThe comprehensive analysis implies that the ore-bearing alkaline syenite in Dazhuang deposit was not the product generated by hydrothermal alteration (albitization) of barren alkaline syenite, but formed from the same magmatic system at different evolution stages. The fractionation degree of the ore-bearing alkaline syenite is obviously higher than that of barren syenite and the enrichment of rare elements and rare earth elements in Fangcheng Dazhuang deposit is closely related to the high evolution process of A-type magma. These results may provide some petrological evidences for the REE prospecting in Fangcheng district.
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致谢: 防灾科技学院李真真副教授、河南理工大学高迪博士、河南省地质调查院裴玉华高级工程师在成文过程中给予了建设性意见和建议；审稿专家及编辑部老师对论文提出了宝贵的修改意见，在此一并致以诚挚的谢意！
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图 1 大庄一带区域地质简图
1—第四系；2—华北陆块盖层；3—中秦岭盖层；4—南秦岭盖层；5—下古生界；6—中—新元古界；7—古元古界；8—太古界；9—花岗岩；10—超镁铁岩；11—大庄矿区位置；12—主要断裂及编号；①洛南—栾川断裂；②瓦穴子—小罗沟断裂；③朱阳关—夏馆断裂；④商—丹断裂；⑤木家垭—西峡断裂；⑥小陡岭—田关断裂；⑦尖坏寨—黄风垭断裂；⑧城口—房县断裂
Figure 1. Regional geological map of Dazhuang deposit
1-Quaternary; 2-Caprock of North China plate; 3-Caprock of mid-Qinling; 4-Caprock of Southern Qinling; 5-Lower Paleozoic; 6-Meso-Neoproterozoic; 7-Paleoproterozoic; 8-Archaeozoic; 9-Granite; 10-Ultramafic rock; 11-Mining area location of Dazhuang; 12-Main faults andits serial number: ①Luonan-Luanchuan fault; ②Wachangzi -Xiaoluogou fault; ③Zhuyangguan-Xiaguan fault; ④Shang-Dan fault; ⑤Mujiaya-Xixia fault; ⑥Xiaodouling -Tianguan fault; ⑦Jianlouzhai-Huangfengya fault; ⑧Chengkou-Fangxian fault
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图 2 大庄矿区地质简图
1—全新统冲积层；2—下更新统鲁山组；3—中细粒黑云母正长岩；4—中细粒黑云角闪正长岩；5—中细粒角闪霞石正长岩；6—片麻理产状；7—构造蚀变岩；8—矿区边界；9—断层及编号；10—地质界线；11—低品位铌矿体及编号；12—工业铌矿体及编号
Figure 2. Simplified geological map of Dazhuang deposit
1-Holocene alluvium; 2-Lushan formation of lower Pleistocene; 3-Medium-fine biotite syenite; 4-Medium-fine biotite hornblende syenite; 5-Medium-fine hornblende nepheline syenite; 6-Gneissose occurrence; 7-Structural altered rock; 8-Mine field border; 9-Fault and its serial number; 10-Geological boundary; 11-Low grade niobium orebody and its serial number; 12-Industrial niobium orebody and its serial number
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图 3 碱性正长岩样品野外照片
a—角闪霞石正长岩；b—黑云母正长岩；c—霞石正长伟晶岩；d—碱性正长岩中晶洞；e—角闪霞石正长岩中萤石；f—黑云母正长岩和角闪霞石正长岩呈渐变过渡关系
Figure 3. Field and Microscope photographs of alkalinesyenite samples
a-Hornblende nepheline syenite; b-Biotite syenite; c-Nepheline syeuite pegmatite; d-Miarolitic inalkaline syenite; e-Fluorites in hornblende nepheline syenite; f-Biotite syenite and hornblende nepheline syenite tend to show gradational relationships
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图 4 碱性正长岩样品镜下照片
a—角闪霞石正长岩中交代形成的钠长石；b—角闪霞石正长岩中后期萤石晶体充填分布于霞石晶体裂隙；c—褐钇铌矿的背散射电子图像; d—烧绿石的背散射电子图像
Figure 4. Microscope photographs of alkaline syenite samples
a-Albite formed by metasomatism in hornblende nepheline syenite; b-Fluorite crystal filling and distribution in nepheline crystal cracks in the middle and late period of hornblende nepheline syenite; c-Back-scattered electron images of fergusonite; d-Back-scattered electron images of pyrochlore
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图 5 大庄碱性正长岩（Ga/Al）-（Na2O+K2O）/CaO、（Ga/Al）-FeO/MgO、（Ga/Al）-K2O/MgO图解
（据Whalen et al., 1987; 方块代表含矿碱性正长岩，三角代表无矿碱性正长岩）
Figure 5. (Ga/Al)vs.(Na2O+K2O)/CaO, (Ga/Al) vs. FeO/MgO, (Ga/Al) vs. K2O/MgO diagrams for the Dazhuang alkaline syenite
(after Whalen et al., 1987; Square for ore-bearing alkalinesyenite, triangle for non-mineral alkaline syenite)
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图 6 含矿（a）与无矿化碱性正长岩（b）的稀土元素分配模式
（据Sun and McDonough, 1989; 样号同表 1）
Figure 6. REE distribution pattern of ore-bearing (a) and non-mineral(b) alkaline syenite
(after Sun and McDonough, 1989; Sample code are same as table 1)
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图 7 含矿（a）与无矿碱性正长岩（b）原始地幔标准化微量元素蛛网图
（据Sun and McDonough, 1989; 样号同表 1）
Figure 7. Spidergram of ore-bearing (a) and non-mineral(b) alkaline syenite
(after Sun and McDonough, 1989; Sample code are same as table 1)
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图 8 大庄碱性正长岩δEu-δCe、SiO2/%-δEu、La/10-6-La/Sm图解
(据杨武斌，2009; 方块代表含矿碱性正长岩，三角代表无矿碱性正长岩）
Figure 8. δEu vs. δCe, SiO2/% vs. δEu, La/10-6 vs. La/Sm diagrams for Dazhuang alkaline syenite
(afterYANG Wubin, 1989; square for ore-bearing alkaline syenite, triangle for non-mineral alkaline syenite)
表 1 大庄碱性正长岩主量元素（%）与微量元素（10-6）含量
Table 1 The major(%)and trace elements (10-6)contents of the Dazhuang alkaline syenite
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