贵州太平洞金矿床载金黄铁矿的矿物学特征及原位微区硫同位素分析

Mineralogical Characteristics and in situ Sulfur Isotopic Compositions of Au-bearing Pyrites in the Taipingdong Gold Deposit, Guizhou Province

太平洞金矿床是我国黔西南地区典型的卡林型金矿床之一。然而,由于发现时间短,其勘探和研究程度较低,对矿床成因模式和成矿机制的认识有待深入。本文在野外调查的基础上,详细分析了太平洞金矿床矿石的显微组构特征,对主要载金矿物——黄铁矿进行了岩相学鉴定、电子探针成分分析(EPMA)以及Nano-SIMS微量元素面扫描(Mapping)和原位微区硫同位素分析。显微镜下观察发现,矿石中的黄铁矿按形态分为草莓状黄铁矿、半自形?自形黄铁矿以及不规则状黄铁矿。电子探针成分分析表明黄铁矿中含有As(0～8.482%)、Au(0～0.092%)、Co(0.059%～0.195%)、Cu(0～0.407%)、Cr(0～0.083%)、Zn(0～0.016%)等多种微量元素。元素相关性分析证实As与S呈负相关关系。背散射电子图像(BSE)显示载金黄铁矿普遍发育环带结构,根据As元素的分布特征将环带黄铁矿细分可为PyⅠ(核部贫As的黄铁矿)和PyⅡ(核部富As的黄铁矿)两类。载金环带黄铁矿的元素面扫描及原位微区硫同位素分析表明:PyⅠ核部富Zn、Cr、Ni元素,贫As,δ^(34)S值多分布在1.1‰～5.9‰,环带则相对富集As、Au、Cu、Mn等元素,δ^(34)S值为–0.7‰～4.7‰;PyⅡ包括成矿期形成的富As核部(δ^(34)S值分别为0.6‰、4.7‰)和宽度不等的增生环带(δ^(34)S值为–1.6‰、3.7‰)。通过对比国内外卡林型金矿载金黄铁矿原位硫同位素特征,推断贵州太平洞金矿床中的硫来源于深部岩浆硫。同时,Nano-SIMS元素面扫描分析对BSE图像显示的黄铁矿环带中元素的分布进行了高分辨率的呈现,揭示了环带尺度的元素分带性——成矿过程中流体活动的"脉冲"效应,即增生环带往往是由更次一级的生长环带所组成的。高分辨率的元素面扫描结合原位微区硫同位素分析表明,黔西南地区的卡林型金矿的形成可能源于深部就位的岩浆分异出的载金气?液流体沿断裂系统经长距离运移到达地表,并充填?交代围岩成矿。

The Taipingdong gold deposit is one of the most important Carlin-type gold deposits in the Southwest Guizhou province.This paper reports a detailed research on the mineralogical features and in situ sulfur isotopic compositions of Au-bearing pyrites by using microscope,electron probe micro-analysis(EPMA)and nanometer-scale secondary ion mass spectroscopy(Nano-SIMS).The pyrites from the Taipingdong gold deposit have several morphological types,such as strawberry pyrites,euhedral pyrites and irregular pyrites.The EPMA results show that the Au-bearing pyrites also contain many other trace elements,including As(0– 8.482%),Au(0– 0.092%),Co(0.059%– 0.195%),Cu(0– 0.407%),Cr(0– 0.083%),Zn(0– 0.016%),etc.The negative correlation between As and S,indicating that As substitutes S in the pyrites.Meanwhile,BSE reflects the Au-rich pyrites usually have zonal textures.According to the distribution of As,the zonal pyrites in the ores can be divided into PyⅠand PyⅡ.PyⅠis the major type of the Au-bearing pyrites in this deposit,which has higher Au and As contents in the rims and lower in the cores.The analyses of PyⅠand PyⅡby EPMA and Nano-SIMS demonstrate that the cores of PyⅠare enriched in Zn,Cr,Ni,and relatively depleted in As,whileδ^34S values range from 1.1‰to 5.9‰.The rims of PyⅠare enriched in trace elements such as As,Au,Cu,and Mn,withδ^34S values varying from–0.7‰to 4.7‰.PyⅡconsists of As-rich cores formed during the mineralization and post-mineralization rims.δ^34S values of the Au-bearing pyrites from the Taipingdong gold deposit indicate that sulfur is mainly magmatic origin,which is in strike contrasting with the other Carlin-type deposits all over the world.Moreover,the zonation features of the pyrites as revealed by Nano-SIMS analyses suggested a pulse nature of the ore-forming fluids.We,therefore,infer that the Au-rich fluids from a deep magmatic system upwelled into the shallow levels along tectonic fractures,which interacted with carbonate rocks and formed the Carlin-type gold deposits in the southwestern Guizhou province.