Mineralogy and geochemistry of pyrochlore from the Shaxiongdong Nb-REE deposit, northwestern Hubei Province: Implications for the niobium enrichment mechanism in carbonatites
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摘要:
位于南秦岭武当地区的杀熊洞铌-稀土矿床是我国典型的碳酸岩-碱性岩型铌-稀土矿床, 虽已有不少研究, 但对于该矿床中铌的富集、结晶及后期流体改造矿物学方面的研究仍较为薄弱, 制约了铌富集机理的精细刻画。采用偏光显微镜、光学阴极发光和背散射电子进行了详细的岩相学观察, 查明了矿物的结构、组分和共生关系, 并用电子探针分析了2类烧绿石的成分, 探讨了铌富集机理。研究表明: 杀熊洞碳酸岩-碱性岩杂岩体发育有一套完整的蚀变辉石岩-正长岩-碳酸岩岩性组合, 三者在空间上紧密共生。杀熊洞杂岩体中铌矿物主要为烧绿石, 主要在岩浆演化最晚期的碳酸岩相中结晶, 为碱性岩浆演化晚期富集的产物。烧绿石包括原生(Pcl1)和蚀变(Pcl2) 两种类型, 前者呈自形、粒度大、发育震荡环带, 被认为是直接从碳酸岩熔体中结晶的岩浆烧绿石; 而后者则为原生烧绿石被热液交代后形成, 其形状不规则, 在背散射电子(BSE)图像中表现为不均匀的亮白色。利用电子探针对两类烧绿石进行了原位成分分析, 两类烧绿石的B、Y位置分别以Nb、F元素为主导, 而A位置以Ca为主导, Na次之, 因此将大部分烧绿石定名为氟钙烧绿石, 少数定名为氟钠烧绿石。两类烧绿石的Nb与F均存在较好的线性正相关关系, 说明氟作为挥发分, 可能对Nb在碱性岩浆-热液体系中的富集起到了关键的作用, 与前人实验模拟结果一致。另一方面, 两者也存在一定成分差别, 与原生烧绿石相比, 蚀变烧绿石具有明显较低的Nb2O5、CaO、Na2O、F等以及较高的UO2、Ta2O5、SrO、SiO2等含量, 这些差异说明交代烧绿石的流体具有更富Sr、Si和U但贫Na和Ca的特征。此外, 在交代过程中, 局部存在明显的铌活化与丢失。
Abstract:Objective The Shaxiongdong carbonatite-alkaline related Nb-REE deposit is located in the Wudang area of the south Qinling Belt, but the studies on Nb mineralization in this deposit are scarce.
Methods Detailed petrographic observations were made by using polarizing microscope, optical cathodoluminescence and backscattered electron testing. The structure, composition and symbiotic relationship of the minerals were identified. The composition of two types of pyrochlore was analyzed by electron probe and the mechanism of niobium enrichment was discussed.
Results The Shaxiongdong complex consists of three types of rocks that are spatially closely related: Meta-pyroxenite, syenite and carbonatite, of which the carbonatites are the major hosts of Nb mineralization characterized by main pyrochlore. This implies that Nb was concentrated to economic values in the carbonatite stage after extensive fractionation of the parental carbonated silicate magma. The pyrochlore in the deposit can be classified into two types on the basis of micro-textures: ① The primary ones (Pcl1) are euhedral to subhedral and display oscillatory zoning, implying their direct crystallization from carbonatite melts; ② The altered grains (Pcl2) exhibit patchy zonation under BSE images due to hydrothermal alteration. EPMA analytical results indicate that both types are rich in Nb and F at the B and Y sites, respectively, and at the A site, they are similarly mostly rich in Ca, thus named F-Ca-pyrochlore, with minor Na, thus named F-Na-pyrochlore.
Conclusion The good positive correlations between F and Nb in both types of pyrochlore indicate that F plays a key role in the enrichment of niobium during magma differentiation. On the other hand, compared to the primary pyrochlore, the altered ones have relatively low Nb2O5, CaO, Na2O and F but high UO2, Ta2O5, SrO and SiO2, indicating that the fluids responsible for pyrochlore alteration are rich in Sr, Si and U, and the alteration tends to mobilize Nb in early pyrochlore, a process that is able to lower the Nb budgets of the deposit.
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Key words:
- pyrochlore /
- Nb-REE mineralization /
- carbonatite /
- Shaxiongdong
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图 1 南秦岭早志留世碱性岩浆岩分布图(据文献[19]修改)
Figure 1. Distribution of Early Silurian alkaline magmatic rocks in the South Qinling Belt
图 2 杀熊洞碳酸岩-碱性岩型铌-稀土矿床矿区地质图(据文献[18]修改)
Figure 2. Geological map of the Shaxiongdong carbonatite-alkaline related Nb-REE deposit
图 3 蚀变辉石岩镜下照片
a.榍石与角闪石共生，部分角闪石发生了绿泥石化作用；b.蚀变辉石岩中大规模的绿泥石化作用；c, d.蚀变辉石岩主要矿物组合；Amp.角闪石; Chl.绿泥石; Ttn.榍石
Figure 3. Photograph of meta-pyroxenite rock
图 4 正长岩镜下照片
a.正长岩中的锆石、黑云母与碱性长石共生；b.正长岩中的独居石生长在黑云母条带周围；c.正长岩中典型的矿物组合特征；d.方解石呈细脉状和弥散状分布在正长岩中；Ae.霓石；Afs.碱性长石; Bt.黑云母; Cal.方解石；Mnz.独居石；Py.黄铁矿；Zr.锆石
Figure 4. Photograph of syenite
图 5 碳酸岩显微照片
a.烧绿石、霓石和磷灰石在碳酸岩中共生；b.碳酸岩中的霓石和烧绿石；c.碳酸岩中发育的稀土矿物；d.碳酸岩中磷灰石和烧绿石的阴极发光图像；Ae.霓石；Afs.碱性长石; Ap.磷灰石；Cal.方解石；Mnz.独居石; Pcl.烧绿石；Bt.黑云母
Figure 5. Micrograph of carbonatite
图 6 杀熊洞碱性杂岩体中烧绿石BSE镜下照片
a, b.具有明显震荡环带，且未经历明显后期改造的烧绿石；c, d.原生烧绿石受到了流体改造作用而呈现不均匀的亮白色；Pcl1.原生烧绿石；Pcl2.蚀变烧绿石
Figure 6. BSE images of pyrochlore in the Shaxiongdong complex
图 7 杀熊洞碳酸岩-碱性岩杂岩体中烧绿石元素协变图解
Figure 7. Compositional variations (in atoms per formula unit) of pyrochlore from the Shaxiongdong carbonatite-alkaline complex
图 8 杀熊洞碳酸岩中烧绿石A、B位置主量元素图解
a.2种类型的烧绿石B位置均以Nb为主，落在烧绿石族的范围；b.2种类型的烧绿石在A位置上以Ca元素为主；X.烧绿石A位置除Ca、Na以外的其他元素总和
Figure 8. A and B site of pyrochlore in Shaxiongdong carbonatite
表 1 杀熊洞碳酸岩中烧绿石主微量元素分析结果(均值)
Table 1. Major element compositions of pyrochlore from the Shaxiongdong carbonatite(average)
岩性 碳酸岩 Pcl1 Pcl2 N=21 N=5 F 4.43 1.42 Na2O 5.82 1.26 Al2O3 0.01 0.02 TiO2 4.70 5.70 CaO 13.45 8.02 UO2 0.49 1.40 ThO2 0.03 0.02 ZrO2 0.25 0.24 Y2O3 0.05 0.02 FeO wB/% 0.16 1.78 SiO2 0.55 1.96 MgO 0.00 0.01 MnO 2.32 3.51 PbO 0.33 0.35 Nb2O5 64.25 60.37 SrO 2.78 8.10 Ta2O5 0.35 0.70 Nd2O3 0.18 0.27 Ce2O3 0.65 1.15 La2O3 0.13 0.21 总计 100.94 96.50 
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