Geochemistry And Genesis Of The Songliang Pb-Zn Deposit In Northeast Yunnan Province

The Songliang Pb-Zn deposit is located at the east bank of Jinsha River,intersection of northeast Yunnan and southeast Sichuan,where is the core of SichuanYunnan-Guizhou Pb-Zn metallogenic province(SYG).The Songliang Pb-Zn deposit is a lead-zinc deposit that rarely produces both veined(cut bed)and layered(bedding)orebodies in SYG.Previous studies on the geological characteristics,tectonic geochemistry and prospecting prediction.But limited by analysis methods and research means,there are few researches on the properties and evolution of ore-forming fluids,sources of ore-forming materials and enrichment of scattered elements,which restricts the determination of the genesis and characterization of the ore-forming process in Songliang Pb-Zn deposit.In this paper,the layered and vein orebody of Songliang PbZn deposit were studied.Then,advanced analytical techniques such as microthermometry、individual inclusion composition by LA-ICP-MS、in situ trace elements、in situ S-Pb isotope and non-traditional stable isotope(Zn isotope)were used.The source of ore-forming materials、the properties and evolution of ore-forming fluids during the formation of layered and vein orebody,as well as the enrichment characteristics of scattered elements of sphalerite were identified.The genesis of Songliang Pb-Zn deposit was determined,and the ore-forming process was described in detail.The main insights gained are as follows:①The properties and evolution process of ore-forming fluid in Songliang Pb-Zn deposit were identified.The ore-forming fluid properties are medium low temperature(average 168.9℃)and medium salinity(average 14.7wt%).The inclusion of sphalerite contains H2S、N2 and other volatile components.The concentration trend of alkali and alkaline earth metals is Na>Ca>K>Mg>Sr>Li≥Ba>Rb.The homogenization temperature of cal cite inclusions in veined orebody(average 231℃)is higher than that of sphalerite inclusions in layered orebody(average 134℃),and the salinity of both is similar.The ore-forming fluid gradually cooled and reserved during the formation of vein ore body→layered ore body.Correspondingly,the color of sphalerite changes from dark to light,in the early sub-stage to late sub-stage of sphalerite,ore-forming fluid cooling and salinity decrease.②The enrichment and replacement mechanism of critical metals in sphalerite from Songliang Pb-Zn deposit was identified.Sphalerite is associated with critical metals such as Cd、Ge and Ga in Songliang Pb-Zn deposit,and there are enrichment regularities characterized by selective enrichment and differential enrichment.The alternative mechanism of Ge is 2Cu++Ge4+(?)3Zn2+.The replacement mechanisms of In、Sn and other elements are mainly Sn3++In2++□(?)3Zn2+.③The source of ore-forming materials in Songliang Pb-Zn deposit was identified.Sulfide from Songliang Pb-Zn deposit is characterized by heavy sulfur enrichment,and reduced sulfur(δ34S=+4.0‰～+19.6‰)is derived from marine sulfate in Sinian Dengying Formation.And the mechanism of reducing sulfur is thermochemical sulfate reduction(TSR).The Pb isotope ratios of sulfide are 206Pb/204Pb=17.523～18.858、207Pb/204Pb=15.039～16.292、208Pb/204Pb=36.76～39.613,which indicate that the oreforming material is crust-derived lead.Ore-forming material originates from the mixture of basement and Sinian Dengying Formation.The δ66Zn of sphalerite is in the range of-0.126‰ to+0.082‰,which indicates that the ore-forming materials are derived from the mixture of the Sinian Dengying Formation carbonate rocks and basement rocks in the Kunyang and Huili Group.④Based on the geological characteristics,mineralogy,fluid inclusions,trace elements and isotopes of Songliang Pb-Zn deposit,it is considered that Songliang PbZn deposit can be analogous to MVT lead-zinc deposit.Besides,the metallogenic model of Songliang Pb-Zn deposit has preliminarily summarized.Metal-bearing fluid derived from basement rocks migrated upward along the NW tensile fault and mixed with reduced sulfur in Dengying Formation.The vein ore body was reserved in the fluid channel fault.Then the fluid overflowed to the surrounding siliceous banded dolomite and reserved layered ore body.