钦-杭带南段坡仔营斑岩型钼矿中黄铁矿微组构及其标型意义

摘要

The Pozaiying deposit is one of most significant deposits in southwestern section of Qinzhou-Hangzhou Belt.The deposit is of the central planar alteration type and ore bodies are composed of three types' veins of A,B and D.In order to deepen the understanding of the mineralization process,some pyrite samples among ore veins from Pozaiying deposits were retrieved to characterize the signature of microfabrics with SEM,XRD and Raman spectral analysis for reshaping the metallogenic process of thermodynamics and kinetics.The results of SEM show three points as follows:(1) pyrite with droplets-shaped microcrystalline on crystal face among A vein is rare,hypautomorphic-xenomorphic,and about 100 ～ 200μm;(2) pyrite with oval-layered microcrystalline on crystal face among B vein is common,euhedral-hypautomorphic,and about 500 ～ lO00μm;(3) pyrite with angular and linear microcrystalline on cubic crystal face among D vein is about 10 ～ 20mm.Our analysis indicates that the size of pyrite particle increases and microcrystalline on crystal faces becomes more regular from A vein to B vein to D vein,which reveals that the crystallization rate of pyrite may be reduced and the crystallization environment tends to be stable from early to late period.The results of XRD shows an obvious difference that the main peak is 28.5° and the secondary peak is 37.1° in A vein,and respectively,are 37.1° and 33.1° in B vein,and are 33.1° and 56.3° D vein were detected,although the outline of the curves look similar between each sample of pyrite in different veins.All the above evidences indicate that pyrite in A vein may consist of an octahedron enclosed with (1 1 1) crystal plane and pentagonal dodecahedron enclosed with (2 1 0) crystal plane,by which,we speculate at its formation temperatures should be higher than 300℃.Samples in B vein should be made up of pentagonal dodecahedron enclosed with (2 1 0) crystal plane and cubic enclosed with (1 1 1) crystal plane,and its formation temperatures should be between 200 ～ 300℃ or higher.Samples in D vein mainly consist of cubic enclosed with (1 0 0) crystal plane and its formation temperatures should be less than 200℃.The Raman spectrμm reveals that all the parameter of samples in A vein vEg =348.0 ～ 350.7cm-1,vAg =385.2 ～ 386.5cm-1 and vTg =441.9 ～ 422.8cm-1,compare with them,the corresponding value of B vein and D vein were shifting to low frequencies by 2 ～6cm-1 and 5 ～ 14cm-1;the scattering intensity of sample in A vein Irg =388.8 ～～745.5,IAg =1532.8 ～～2071.8 and ITg =238.9 ～254.4,while the corresponding scattering intensity of B vein and D vein are enhanced significantly.Comparative analysis shows that the characteristics of Raman spectrum shift towards low-frequencies from A to B to D indicating that the growth pressure of the three types of pyrite decreases gradually and the scattering intensity increases from A to B to D also indicates that the formation temperatures between the three gradually decreases.Thus,our studies suggest that the formation temperature and pressure of the vein from early to late decrease in Pozaiying molybdenum deposit.%坡仔营钼矿是钦-杭成矿带南段最重要斑岩型矿床之一.该矿床发育典型“中心式面型蚀变”和A、B和D三种类型矿化脉.为了加深对其成矿过程的认识,本研究选取不同矿脉中黄铁矿,通过扫描电镜(SEM)、X射线粉晶衍射(XRD)和激光拉曼光谱(Raman),研究其微组构标型特征,以期反演成矿过程的热力学及动力学信息.SEM结果显示,A脉内黄铁矿较少,呈100～ 200μm半自形-他形,籽晶呈扁平乳滴状;B脉黄铁矿强烈发育,呈500～ 1000μm自形-半自形,籽晶呈近椭圆形层状;D脉内黄铁矿呈立方体产出,粒度10 ～ 20mm左右,三组相互垂直晶纹发育,籽晶棱角状线形展布.A脉→B脉→D脉中黄铁矿粒度逐渐变大,籽晶更加规则,指示成矿早期至晚期,黄铁矿结晶速度逐渐变缓,成生环境趋于稳定.XRD结果展示,各样品谱线均发育着多组黄铁矿特征峰,但其强峰发育存在差异.A脉最强峰为28.5°,次强峰37.1°;B脉和D脉相应峰位分别为37.1°、33.1 °和33.1°、56.3°.衍射峰型特征表明,A脉样品晶形应以(111)面组成的正八面体单形为主,其次为(210)晶面组成的五角十二面体,其成生温度应大于300度;B脉样品应以(2 1 0)组成的五角十二面体单形为主,其次为(100)晶面组成的立方体单形,形成温度应介于200 ～ 300℃之间或略大;D脉样品主要以(100)晶面组成的立方体为主,其形成温度应小于200℃.Raman光谱揭示,A脉样品的vEg=348.0 ～ 350.7cm-1、vAg =385.2～386.5cm-1、vTg=441.9 ～～422.8cm-1,与之相比,B脉和D脉的对应值分别向低频偏移2～6cm-1和5～ 14cm-1;A脉样品的散射强度IEg=388.8 ～ 745.5、IAg=1532.8～2071.8、ITg=238.9～254.4,而B及D脉样品的散射强度依次明显增强.自A脉→B脉→D脉,拉曼位移向低频偏移,散射强度依次增强,指示三者的成生压力依次降低.本研究认为,坡仔营斑岩型钼矿的成生早期为一个高温、高压岩浆热液活动为主阶段,随着成矿温度、压力的降低,成矿系统氧逸度逐渐降低,硫及金属元素逐步转变为金属硫化物形式而成矿.