江西牛形坝-柳木坑金银铅锌多金属矿床铋矿物类型及指示意义

The Type of Bismuth Minerals in the Niuxingba-Liumukeng Au-Ag-Pb-Zn Polymetallic Deposit,Jiangxi Province,and Its Geological Significance

江西银坑矿田的牛形坝-柳木坑矿床是赣南于都-赣县矿集区典型的金银铅锌多金属矿床。根据野外调研、系统矿相学研究和电子探针成分分析,表明铋在牛形坝-柳木坑矿床中是一种重要的伴生矿化金属元素,呈独立矿物或呈微细矿物包体分布在早期硫化物内部或边缘。铋矿物种类丰富,主要包括块硫铋银矿、硫铋银矿、板硫铋铜铅矿、针硫铋铅矿、硫铜铋矿、库辉铋铜铅矿、辉碲铋矿等。与同一矿集区石英脉型盘古山钨铋矿床相比,两矿床铋矿物种类均出现Pb-Ag-Bi-S、Te-Bi-S元素组合,赋存形式和含量有所不同,Cu-Pb-Bi-S元素组合仅在牛形坝-柳木坑矿床出现。矿床属于Au-Bi-(低Te)-S体系,铋盐矿物形成于中低温、中高密度、低盐度的成矿环境。从岩浆中分馏出来的成矿流体发生减压沸腾迫使大量蒸汽相逸散,硫逸度增加,铋主要以硫络合物形式运移,随着黄铁矿等硫化物沉淀,硫被大量消耗形成金矿物和碲化物(碲银矿)。成矿流体演化过程中由于温度和硫逸度进一步降低导致在成矿后期沉淀出大量铋硫盐矿物。成矿过程中的Bi促进金的迁移富集。铋矿物在两矿床中的赋存特征、化学成分、形成环境以及沉淀机制等表现出差异,说明铋具有成矿选择性,起到对成矿物质示踪的作用。

The Niuxingba-Liumukeng Au-Ag-Pb-Zn polymetallic deposit in the Yinkeng ore field is located in the Yudu-Ganxian ore cluster area,south Jiangxi Province.Field investigation,systematic mineralographical observation,and electronic probe composition analysis indicate that bismuth is one important associated element in the Niuxingba-Liumukeng Au-Ag-Pb-Zn polymetallic deposit,and occurs as an independent mineral or micro-particles in internal or marginal parts of early formed sulfide.Bi-bearing minerals are versatile,including pavonite,matildite,gustavite,berryite,aikinite,emplectite,krupkaite and tetradymite.Compared with Pangushan quartz vein-type W-Bi deposit in the same ore cluster area,two ore deposits contain two Bi-bearing mineral associations (Pb-Ag-Bi-S and Te-Bi-S),but vary in mineral occurrence and contents.Cu-Pb-Bi element association is found only in the Niuxingba-Liumukeng deposit. The presence of bismuth minerals suggest the deposit belongs to Au-Bi-(low Te)-S system,and formed in the geological environment with medium and low temperature,medium high density and low salinity. Bismuth migrated in the form of sulfide complexes after massive gases escaping and sulfur fugacity increasing which was compelled by boiling ore-forming fluid derived from magma.With precipitation of sulfide (e.g.pyrite),a large amount of sulphur were transferred into gold-bearing minerals and telluride. Further decreasing of temperature and sulfur ~ugacity during the evolution of ore-forming fluid resulted in deposition of bismuth sulfosalt minerals at the late stage of mineralization.Bi promoted gold migration and enrichment during the process of mineralization.Bi-bearing minerals at the two deposits present some differences in occurrence state,chemical components,forming environment,and precipitation mechanism, suggesting Bi’s preference in ore-forming process and thus playing a significant role in tracing ore-forming material source.