紫金山金铜矿明矾石的矿物学特征

Mineralogical Characteristics of Alunite from Zijinshan Gold-copper Deposit

紫金山金铜矿中火山机构的不同部位具有不同特征的明矾石。本文在野外地质调查工作的基础上,通过岩相学、矿相学、扫描电镜、能谱分析、电子探针、粉晶衍射、热差分析等手段,对紫金山金铜矿中的脉石矿物明矾石的矿物学特征进行了系统研究。明矾石分为岩浆热液、岩浆蒸汽、蒸汽加热三种成因类型。岩浆热液型明矾石呈鳞片状分布明矾石化花岗岩中或呈柱、粒状分布于矿体中;岩浆蒸汽型明矾石多呈脉状分布于矿体上部,晶体呈页片状;蒸汽加热型明矾石呈粉末状分布在潜水面附近。明矾石有多种颜色,与铜矿石矿物共生的明矾石多呈玫瑰色,而蚀变花岗岩中或潜水面附近粉末状的明矾石多呈粉红色、肉红色或灰白色。电子探针显示明矾石化学成分相对理论值具有富Al,贫K、SO3,富Na、Ca、Ba、Ti等微量元素,且火山口附近明矾石含钠较高,而远离火山口的北西矿带及深部含钠较低。粉晶衍射显示北西矿带中明矾石的晶胞参数从火山口由近到远,从浅部到深部,晶胞参数增大。热差分析显示火山口处明矾石的吸热谷较深。这些特征显示火山口附近明矾石形成温度较高,远离火山口的西北矿带及深部明矾石形成温度较低,其形成与潜火山活动有关。

Alunite from Zijinshan gold-copper deposit is of different mineral characteristics in different parts of the volcanic mechanism.Mineralogical characteristics of alunite with different shapes and in different parts of the volcanic mechanism were studied by petrography,ore microscopy,scanning electron microscope,energy spectrum analysis,electron probe,X-ray power diffraction analysis and differential thermal analysis.Alunite was divided into three genetic types,which are the magmatic hydrothermal type,the magma steam type and the steam heating type.Alunite of different genetic types has different shapes.In the magmatic hydrothermal type,it is in scaly in alunite alterated granitoid rocks or in tabular,columnar intergrowth-with-Cu-sulphide in the ore body.In the magma steam type,it is in the shape of leaf blade in the upper part of the ore body.In the steam heating type,it is in powder near the water table.Alunite has many kinds of color,it is in rose color when intergrowth with Cu sulphides,and pink,red flesh and gray white in other types.Data of electron microprobe indicate that the chemical composition of alunite from Zijin gold-copper deposit is of characteristics with extremely low K and S and high Al in major elements and high Na,Ca,Ba,and Ti in trace elements compared with theoretical values,and high Na near the volcanic crater and low Na in the north-west ore belt.Cell parameters of alunite in different positions of the volcanic mechanism change regularly in north-west ore belt.Crystal cell parameters of alunite in different positions of the volcanic mechanism increase regularly along the north-west ore belt from where near the crater to far from it.Differential thermal analysis shows that endothermic valley of alunite near the crater is deeper.These characteristics indicate that alunite near the crater was formed at higher temperature,while far from the crater was formed at lower temperature.Its formation was related with latent volcanic activities.