铜陵矿集区层状硫化物矿床成因--来自胶状黄铁矿-菱铁矿型矿石矿物学制约

Mineralogical Characteristics of Colloform Pyrite—Siderite Ore from the Xinqiao Deposit and Its Role in Mineralization of the Stratabound Sulfide Deposit in Tongling Ore District,Eastern China

新桥硫铁矿床是铜陵矿集区重要的硫(铁、铜、金)多金属矿床,长期以来对其成矿作用存在争议。采用光学显微镜、粉晶X射线衍射(XRD)、场发射扫描电镜(FE-SEM)和高分辨透射电镜(HR-TEM)等矿物学研究手段,对新桥硫铁矿床层状矿体中胶状黄铁矿—菱铁矿型矿石矿物学进行了研究,结果表明该矿石主要由黄铁矿和菱铁矿组成(含量分别为74.3%和24.4%),含少量石英和伊利石;黄铁矿主要呈现纳米—亚微米粒径的自形、半自形立方体形态,菱铁矿以20~50μm的半自形菱面体产出,石英和伊利石呈现碎屑颗粒形态;偏光显微镜下胶状黄铁矿和菱铁矿呈现共生关系,FE-SEM下观察到微米粗粒菱铁矿与纳米—亚微米黄铁矿之间呈现镶嵌关系,可观察到菱铁矿表面存在黄铁矿的印模。FE-SEM和HR-TEM观察和能谱分析确证胶状黄铁矿—菱铁矿型矿石中仍然保存有少量的微生物残体和有机质。这些综合信息反映新桥胶状黄铁矿非岩浆热液成因,而是生物化学沉积成因。该层状铁硫化物—碳酸盐建造由于具有较高的地球化学活性,对燕山期岩浆活动驱动热液形成层状矿体发挥了重要的控制作用。

Objectives： As a representative stratiform iron sulfide deposit in Tongling metallization cluster in the Middle—Lower Yangtze Metallogenic Belt, the mineralization and mechanism of the Xinqiao iron sulfide deposit remians a topic of debate. The Colloform pyrite ore is an important constituents of the stratabound sulfide orebodies in the Xinqiao deposit, and its formation and genesis are still controversial which on the contrary limits identification of the stratabound sulfide orebody. Methods：The microstructure characteristics of colloform pyrite—siderite ore in Xinqiao deposit were carried out using powder X ray diffraction （XRD）, optical microscope, scanning electron microscopy （SEM） and high resolution transmission electron microscopy （HR TEM） to investigate mineral compositions, morphology, further discuss genesis and mechanism of the iron sulfide deposit. Results：Pyrite and siderite （74.3% and 24.4% in the total mass, respectively） are the two dominant minerals with rare quartz and illite （less than 5% in the total mass） in the ore. The grain sizes of pyrite range from several nanometer to hundreds of nanometers and the granular pyrites are dominated by euhedral—subhedral cubic morphology with minor spheroid and irregular microcrystals. While siderite is presented as euhedral rhombohedron with a size of 20 μm ~30 μm. In the ore xenomorphic clasts quartz and illite were discriminated by TEM. Optical microscopic observation indicated that pyrite and siderite formed mosaic texture. It is confirmed that organic matter fragments were also residual in this ore by the analysis of FE SEM、HR TEM and EDS. Conclusions： The colloform pyrite—siderite ore in the Xinqiao iron sulfide deposit was direct precipitation through biogeochemistry mechanism in the sub closed marine basin instead of magmatic hydrothermal process. This colloform pyrite layer may also act as geochemical barrier or precipitant for the Mesozoic copper mineralization.