Through systematic microscope identification and scanning electron microscope observation,pyrites were divided into 6 types (i.e.,cuboidal,framboidal,fine-grained,coarse-grained,zoned and line-like pyrite) according to their topographic characteristics in the Nibao gold deposit,Pu'an,southwestern Guizhou.Electron microprobe analysis (EMPA) results for pyrites showed that gold-bearing minerals were mainly arsenian pyrite (zoned pyrite and fine-graned pyrite) and arsenopyrite.The core and zone of zoned pyrite were formed in different diagenetic and metallogenic stages,respectively.The core,formed in the diagenetic stage (partially formed in early hydrothermal stage),is poor in Au and As and is rich in Fe and S.The zone,formed in the main metallogenic stage,is rich in Au and As and is poor in Fe and S.There is a positive corresponding relationship between Au and As in the zone of zoned pyrite,and they have a positive correlation in a certain wedge-shaped space;the high Au content generally corresponds to the intermediate As content (2％ ～6％).In contrast,there is clearly a negative correlation between As and S,thus indicating that the As-rich zone was generated because As replaced S to enter into the lattice of the pyrite.Fine-grained pyrite has high Au and As contents and low Fe and S contents,and its characteristics are similar to those of the zone of zoned pyrite.As a result,it is inferred that the As-rich zone and the fine-grained pyrite are both formed in the main metallogenic stage.Arsenopyrite frequently interpenetrates or distributes along the rim of arsenian pyrite,which suggests that it was formed later than the arsenian pyrite in the hydrothermal stage.Therefore,the crystallization order of gold-bearing minerals in the Nibao gold deposit is roughly as follows:As-poor sedimentogenic pyrite (core) → zone of As-rich pyrite and fine-grained pyrite →arsenopyrite.Point analysis by EMPA and the scanning of the surface wave spectrum showed that the distribution of Au in gold-bearing minerals (i.e.,arsenian pyrite and arsenopyrite) is not uniform.According to the solubility limit of Au (Au/As 0.02) and the lgw (As)—lgw (Au) diagram,this paper inferred that Au mainly occurs in arsenian pyrite and arsenopyrite as an "invisible" solid solution Au + 1) and probably also as small amounts of nanooarticles native gold (Au0).%通过系统的显微镜鉴定及扫描电镜观察,将黔西南普安泥堡金矿床中的黄铁矿按形貌特征划分为6种类型,即立方体状、草莓状、细粒状、粗粒状、环带状及长条状黄铁矿.黄铁矿电子探针分析结果表明,主要载金矿物为含砷黄铁矿(环带状黄铁矿、细粒状黄铁矿)和毒砂.环带状黄铁矿核部和环带形成于不同的成岩、成矿阶段,其核部贫Au、As,富Fe、S,属成岩期的产物;环带富Au、As,贫Fe、S,形成于主成矿期.黄铁矿环带中Au与As具有正对应关系,在一定的楔形空间呈正相关,高Au含量往往与中等As含量(2％～6％)相对应;而As与S呈明显负相关,说明富砷环带是As取代S而进入黄铁矿晶格所致.细粒状黄铁矿具有高Au、As,低Fe、S的特点,类似于环带状黄铁矿环带部位的特征,推测富As环带与细粒状黄铁矿同属主成矿阶段的产物.毒砂常穿插或沿含砷黄铁矿边缘分布,表明其形成晚于热液期含砷黄铁矿.因此,泥堡金矿床中载金矿物的结晶顺序大致为:贫砷沉积成因黄铁矿(核部)—富砷黄铁矿环带和细粒状黄铁矿→毒砂.电子探针点分析和面波普扫描显示,Au在载金矿物含砷黄铁矿和毒砂中具有不均匀分布的特征,根据Au的溶解度极限(Au/As O.02)和lgw(As) —lgw(Au)图解,推测Au主要以“不可见”固溶体(Au+1)形式赋存于含砷黄铁矿和毒砂中,极少量可能为纳米级自然金(Auo).
展开▼
