胶东三山岛金矿床黄铁矿显微结构和微量元素特征:对金富集机制的指示

Microstructure and Trace Elements of Pyrite from Sanshandao Gold Deposit in Jiaodong District:Implications for Mechanism of Gold Enrichment

三山岛金矿床是位于胶东金矿集区西北部的超大型破碎带蚀变岩型金矿床.该矿床细致的矿相学及元素地球化学研究尚有不足,限制了对其金富集机制及过程的理解.基于野外地质调查和室内矿相学研究将该矿床划分为4个成矿阶段:石英-绢云母-黄铁矿阶段()、石英-金-黄铁矿阶段(Ⅱ)、石英-金-多金属硫化物阶段(Ⅲ)和碳酸盐-石英阶段(Ⅳ).黄铁矿是该矿床主要载金矿物,其中第Ⅰ阶段黄铁矿(Py1)呈自形且基本无变形,As、Au含量较低;第Ⅱ阶段黄铁矿(Py2)可分为未变形或弱变形的Py2a和强烈变形的Py2b两种类型,Au和As在Py2a中含量高,而在Py2b中含量降低;第Ⅲ阶段黄铁矿(Py3)分为与石英共生的细粒黄铁矿Py3a和与多金属硫化物共生的Py3b两个亚世代,均变形较弱,Au、As含量中等.黄铁矿原位微量元素分析指示Co、Ni和Ag以固溶体形式进入黄铁矿晶格,而Pb、Zn、Cu主要以硫化物包裹体形式存在.黄铁矿裂隙和粒间发育大量以银金矿为主的可见金,是该矿床中金的主要赋存状态.不可见金主要为黄铁矿中晶格金,且其与As关系密切:As;替代黄铁矿内的S;使得黄铁矿晶格发生畸变,并促使Au^(+)进入到黄铁矿晶格中.第Ⅱ阶段成矿流体减压沸腾导致金沉淀并以包体金、晶隙金的形式与Py2a共生.受成矿期构造活动影响,第Ⅱ阶段Py2b发生的位错蠕变、晶格旋转等应变行为可促进晶内形成一种"快速通路",通过晶内扩散或流体介导将黄铁矿内不可见金活化,并在黄铁矿颗粒的微裂缝或晶隙中再富集为可见金.

The Sanshandao gold deposit,located in the northwest of Jiaodong district,is a super-large altered-rock type gold deposit.The lack of detailed studies of mineralogy and element geochemistry in this deposit limits the understanding of Au enrichment mechanism and process.Based on field work and mineralogical observation in the deposit,the Au mineralization is divided into four stages:the quartz-sericite-pyrite stage(Ⅰ),quartz-gold-pyrite stage(Ⅱ),quartz-gold-polymetallic sulfide stage(Ⅲ) and carbonate-quartz stage(Ⅳ).The main gold-bearing mineral is pyrite.Py1 in stage Ⅰ is characterized by euhedral grains with no deformation and low contents of As and Au.In stage Ⅱ,the Py2 can be divided into two sub-generations:coarse Py2a with no deformation and Py2b with massive plastic and brittle deformation.The contents of Au and As are high in Py2a and low in Py2b.In stage Ⅲ,the Py3 includes fine-grained euhedral Py3a coexisting with quartz and Py3b coexisting with polymetallic sulfide,both with weak deformation and medium Au and As contents.In-situ trace element analysis of pyrite indicates Co,Ni and Ag entering pyrite lattice by forming solid solution,whereas Pb,Zn and Cu mainly presenting as sulfide inclusions.The main occurrence state of gold is visible Au that occurs in the cracks and intergranular space of pyrite as electrum.The invisible Au in pyrite is lattice gold,and its enrichment is closely related to As.To be specific,As-replacing S-entered into pyrite,resulting in lattice distortion of pyrite,which prompted the Au^(+) into the pyrite lattice.The decompression boiling of ore-forming fluids in stage Ⅱ resulted in Au precipitation and intergrowth with Py2a as visible inclusions and intergranular gold.The plastic deformation of Py2b in stage Ⅱ induced by syn-activation of ore-controlling fault,such as dislocation creep,lattice rotation,promoted the formation of intracrystalline "fast pathways",and further remobilized invisible Au through intragrain diffusion or fluid-mediated liberation.The remobilized Au re-concentrated as visible gold in microfractures and boundaries of pyrite grains,which contributes to the formation of high-grade gold ores in Sanshandao.