国内首次发现的硅稀土石、氟镧矿矿物学特征及成因研究

Taipingzhen deposit is a rare earth element deposit discovered recently in the west of Henan province. In our prophase research, several rare earth minerals were found in the deposit, among which törnebohmite and fluocerite-(La) were first discovered in China. Meanwhile, another two categories of rare earth minerals are not yet determined. Many rare minerals found in one deposit must be closely related with its special geological genesis. The research will focus on törnebohmite, fluocerite-(La) and other rare earth minerals. Based on the field geological investigation and petrographic observation, the research will further confirm the characteristics of mineral paragenesis and clarifie occurrence , and the distribution of these rare earth minerals by using back scatter imagery(BSE) and secondary electron imagery(SEI). Systematic study on mineralogical characteristics will be performed through electron probe microscopic analyzer(EPMA), X-Ray diffraction(XRD), differential thermal analysis (DTA), infrared spectrum(IRS), and laser raman spectroscopy(LRS) analysis. The research will also figure out the genesis of these rare earth minerals through the study of P-T condition, composition of fluid inclusions and the usage of C, H, O, S stable isotope tracer. This research not only complements the mineralogical data of these rare earth minerals, facilitating the future discovery of similar new minerals, but also reveals genetic mineralogy of rare earth minerals in Taipingzhen and plays a great role in guiding the prospecting and exploration of rare earth element deposits in the west of Henan Province.

太平镇稀土矿是豫西地区新发现的一座轻稀土矿床。课题组在对该矿床的前期研究过程中发现了多种罕见的稀土矿物，其中硅稀土石和氟镧矿在我国属首次发现，另有两种稀土矿物尚未确定种类。同一矿床产出众多的罕见矿物，可能与其独特的成因环境密切相关。本课题将以硅稀土石、氟镧矿等稀土矿物为研究对象，在野外地质调查和岩相学研究的基础上，进一步使用背散射图像及二次电子图像分析确认矿物的共生关系，厘清矿物的赋存状态与分布规律；利用电子探针成分、X射线衍射、差热分析、红外光谱、激光拉曼光谱等分析方法开展系统的矿物学特征研究；结合流体包裹体温压条件与成分分析、C-H-O-S稳定同位素示踪等方法进行综合研究，探讨矿物成因。课题的研究意义在于不仅补充完善上述稀有矿物的研究资料并为发现类似新矿物提供工作条件，更能从矿物学角度揭示太平镇稀土矿的矿床成因，从而对豫西地区进一步找矿工作有着积极的指导作用。

我国首次发现的氟镧矿在手标本上为乳白至浅黄色，呈半透明，玻璃光泽，贝壳-不规则断口。不完全解理，白色条痕，性脆，摩氏硬度为4-5，计算密度6.07g/cm3。偏光显微镜下显示无色透明，粒状-板状，一轴晶，负光性，ω = 1.611，ε = 1.605。电子探针分析得出矿物的经验化学式（La0.48Ce0.42Nd0.07Pr0.03）F2.97。X射线单晶衍射分析表明，该矿物属于六方晶系，晶胞参数：a=7.1400(4) Å, c=7.2980(5) Å, Z=6, 空间群P63cm(No.185)，激光拉曼光谱特征峰主要包括 228, 308, 370 cm-1，由La-F振动引起。. 羟硅铈矿粒径一般小于0.1mm，为自形-半自形，粒状-板状。该矿物呈浅绿色至橄榄绿色，透明，玻璃光泽，不规则断口，摩氏硬度约为4.5，计算密度5.08g/cm3。光学性质：二轴晶，正光性，多色性明显，单偏光下呈绿色，正交偏光下具有鲜艳的高级干涉色。电子探针分析得出该矿物的经验化学式：(Ce1.02La0.49Nd0.27Pr0.10 Sm0.02Gd0.01Th0.01Mg0.06 Ca0.01)Σ2.00 (Al0.99Fe0.03)Σ1.02(SiO4) Σ2.00(OH0.81F0.19)Σ1.00。X射线单晶衍射分析表明，该矿物属于单斜晶系，晶胞参数：a= 7.4382(3)Å, b= 5.6730(2)Å, c= 16.9819(8)Å, β=118.84(0)°, Z=4, V= 655.13(5)Å3, 空间群为P21/c。矿物的激光拉曼光谱特征峰主要为241, 362, 862, 895，954，3722 cm-1等。. LA-MC-ICP-MS获得氟碳铈矿结晶年龄409±16Ma，与ID-TIMS获得的407.8±3.3Ma在误差范围内一致。与蚀变岩中热液锆石的U-Pb年龄399.4±3.5Ma共同记录了该矿床的热液-成矿事件。H-O-S-Pb-Hf同位素特征表明，新元古代新生基性下地壳的HP-UHP榴辉岩的部分熔融为成矿提供了物质来源，太平镇稀土矿床形成于北秦岭晚古生代与碰撞后伸展作用有关的减压退变质过程。