The Mineralogical Characteristics And Its Genetic Significance Of Mengya'a Skarn Lead-Zinc Deposite In Tibet

Mengya'a lead-zinc deposit in Tibet locates in the east of the section of Longge'er-Niangqing tanggula Mesozoic islands chain.It is a typical skarn deposit of Gangdise north zone.Predecessors studied the geological characteristics,the source of ore-forming fluid,metallogenic material source and diagenetic and metallogenetic age of the deposit,but not detailed analysis have been carried out to the mineral characteristics of it.However,mineral compositions,mineral assemblage and alteration metasomatic relations have great significance to understand the genesis of the deposit and its environmental changes in the process of formation.Therefore,based on the detailed information of the ore deposit geological characteristics,knowledge of petrology,ore,ore deposit and rock geochemistry,mineralogy,this thesis studied the mineralsthe paragenetic association of Mengya'a deposit,divided the metallogenic period of time,analyzed the alteration zoning characteristics and mineralization characteristics.We analyzed the environment changes during the formation of ore deposit and provide the basis of occurrence for the precious metal elements Au,Ag and Sn which founded in orebody after the determination of mineral composition of skarn and metal sulfide.We finally discussed the formation of ore deposit.Lai gu group?C₂P₁l?is the main ore-hosting strata.The ore bodies are broadly distributed inlayered or stratoid,which obviously controlled by fracture structure.Fracture structures are given priority to with east-west and south-north.Such as Pb-14orebody,Pb-20 orebody,Pb-12 orebody are originated from the east-west fracture and Pb-21 orebody is occurred in the south-north bedding fault in the skarn zone between the layers.The nonmetallic minerals of mengya'a deposit are mainly garnet,pyroxene,wollastonite,actinolite,tremolite,epidote,chlorite,biotite,calcite,quartz,fluorite,etc.Metal mineral and their formation order is magnetite?pyrrhotite?pyrite?sphalerite?chalcopyrite??chalcopyrite?stannite??galena at the first mineralization,the second phase of mineralization is pyrrhotite?chalcopyrite.The mining area wall rock alteration is obvious:horizontally,it overall performance for alteration quartz porphyry,hornfel,garnet skarn,diopside,garnet skarn,marble,silicified limestone,imestone from rock mass to outside.Vertically,it performances for calcium limestone,quartz porphyry,sericite slate,epidotization limestone,diopside,garnet skarn,wollastonite marble,garnet skarn,actinolite skarn,silicide marble.Vertically,mineralization elements from down to up were pyritization?lead-zinc mineralization?pyrrhotite-magnetite mineralization.The whole rock analysis of rocks and ores shows that rock mass has high silica,medium-high aluminum and low titanium.Garnet-skarn in addition to a small number of redundant components of CaO,SiO₂ may exist in calcite,quartz,mostly are garnet?andradite,Ca₃Fe₂[SiO₄]₃?.Along with the depth deepening,the content of TFe,SiO₂,MgO has a negative correlation between the content of CaO in magnetite ores,it indicates that magnetite mineralization continuously strengthen as iron,magnesium,calcium metasomatism of silicon,aluminum,potassium.Along with the depth deepening,the content of lead and zinc has a negative correlation with the content of SiO₂ in lead-zinc ores,it shows that the process of lead-zinc mineralization is lead,zinc,iron,manganese of silicon,aluminum,potassium metasomatism process in fact.The characteristics of rare earth elements suggest that samples present to the right shelving,negative Ce anomaly distribution characteristics,which consistent with the rock mass.The characteristics of rare earth distribution of skarn and magnetite ore is negative anomaly of Ce,positive anomaly of Eu,light rare earth concentration,heavy rare earth loss,indicating that they are formed in high temperature oxidation environment.The rare earth distribution characteristics of lead-zinc ore inherited siltstone,its negative anomaly of Eu contraries to the skarn and magnetite ore,which suggestes that the ore forming environment from the high temperature oxidation environment to a low temperature deoxidizing environment from skarn stage to quartz-sulfide stage.The electron probe analysis shows that the skarn mineral composition is andradite+diopside-hedenbergite+wollastonite in mengya'a deposite,which is high temperature oxidation environment product.The value of Zn/Cd and Ga/In of sphalerite indicates it formed in the medium and low temperature environment.The value of Sb/Bi of galena indicates it formed in the low temperature and one of the value approachs stratabound galena's.Pyrrhotite belongs to monoclinic system which formed in the low temperature,and its Ni-Co relationship also indicates the skarn formation.The elements of chalcopyrite and elements also hint that they formed in low temperature environment.The composition characteristics of sulphide show that they formed in the medium-low temperature and weak acid environment,and they belong to skarn formation,and mengya'a deposite is one of stratabound skarn deposits.In conclusion,this article deems that the Mengya'a deposit is one of stratabound skarn lead-zinc deposits.In the time domain,the deposits went through the early stage to the late stage of skarn,quartz-sulfide stage,carbonate stage and supergene oxidation stage.From the skarn stage to quartz-sulfide phase,the deposite formation environment changes from high temperature oxidation environment to low temperature deoxidizing environment.In the space domain,its characteristics of alteration zoning andmineralizationis significantly,mainly displays in mineral composition,chemical composition and mineralization types of rock and ore which have different distances to rock mass.