| The Gangdese metallogenic belt has completely preserved and recorded the tectonic-magmatic-metallogenesis information since the Cenozoic Tethyan Oceanic subduction to India-Asia continental collision,and formed a series of Cu,Pb,Zn,Au,Ag,Mo and other polymetallic deposits related to intermediate acid volcanic-intrusive rocks in this belt.Numerous remarkable researches have been carried out on the geological characteristics,diagenetic metallogenic time,metallogenic mechanism,metallogenic model of porphyry-skarn deposits in the early stage,but little study focused on the epithermal deposit which associate with continental volcanic rock.The Sinongduo deposit was the first low sulfide epithermal Ag-Pb-Zn deposit in Gangdese metallogenic belt.In this paper,based on field investigation,geological logging,microscopic observation and the electron microprobe analyse,we mainly study the modes of occurrence of silver focusing on aiming at analyzing and discussing the mineralized elements precipitate and enrichment mechanism in the mining area,which can provide theoretical basis for ore mining,dressing and smelting,and improve the comprehensive economic value of deposits.The tectonic location of the Sinongduo epithermal Ag-Pb-Zn polymetallic deposit is located on the arc fault uplift belt in the southern margin of the western segment of the central Gangdese metallogenic belt,Tibet.The exposed strata in the mining area are Paleogene porphyry,volcanic breccia,crystal tuff of Dianzhong Fomation and Quaternary which are simple as a whole,and the ore bearing strata are the volcanic rocks of Dianhong Formation.The structures in the mining area are mainly near south-north-trending fractures,ring structures and tensile fault systems related to the volcanic edifice.Magmatic rocks in the mining area are widely distributed,mainly including tuff(64 Ma),dacite(62 Ma),rhyolite(65 Ma)and other volcanic extrusive rocks and granite porphyry dyke(12 Ma).There are many types of ore bodies,such as Pb-Zn orebodies of cryptoexplosive breccia type and hydrothermal vein type,independent Ag orebodies,and the shapes of orebodies are mainly irregular,vein and lenticular.The independent Ag ore bodies are mainly developed on the hanging wall of the cryptoexplosive breccia type Pb-Zn orebodies,which are closely related to the minerals such as Fe-Mn carbonate and jasper.The ore structure mainly includes vein and mesh-vein,massive,breccia structure,and the ore texture mainly includes metasomatic,crystal and cataclastic texture.Metallic minerals mainly include sphalerite,galena,pyrite,chalcopyrite,hematite,siderite,rhoderite and independent silver minerals.Nonmetallic minerals are mainly sericite,chalcedony,illite,quartz,calcite,and adularia.The types of wall-rock alteration include illite,sericite,silicification and carbonation,and typical epithermal alteration mineral assemblages:sericite-adularia-illite,chalcedony-quartz-j asper,and Fe-Mn carbonate-calcite.According to metallogenic characteristics,the relationship between mineral assemblages and intercalated mineral,the mineralization can be divided into three metallogenic stages:(1)the metallogenic stage of sphalerite-chalcopyrite-pyrite-pyrite-quartz assemblages,(2)the metallogenic stage of galena-sphalerite-independent silver mineral assemblages and(3)the metallogenic stage of pyrite-quartz-calcite assemblages.The results show that the silver elements in the Sinongduo deposit are hosted in two forms:independent silver mineral and isomorphism.(1)The independent silver mineral phase.The independent silver minerals are mainly of argyrite,pearceite and a small amount of acanthite,pyrargyrite and silver.Among them,argyrite,pearceite,acanthite,pyrargyrite developed in the fracture of Fe-Mn carbonate and jasper particles.In addition,the little silver and argyrite in the form of wrapped silver developed in pearceite.(2)Isomorphism.Selver element replaced the copper element in the form of isomorphism in pearceite.The silver element is mainly formed in the main metallogenic stage of galena-sphalerite-independent silver minerals.The metallogenic fluid is characterized by low temperature and low salinity,and the metal ions such as Ag+ Pb2+,Zn2+,As+ migrated mainly in sulfur complex like S2-,(S203)-and HS-.With the evolution of the metallogenic stage and the fluid boiling,the physical and chemical conditions such as temperature,pressure,and the value of pH has been changed suddenly,resulting in the disintegration of the sulfur complex and the release of metal ions,such as Ag+.Pb2+ and Zn2+.In addition,the metal precipitated in favorable ore-forming parts and mineralized,and formed a variety of mineral facies. |
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