Study On Mineralization Of Lead-Zinc Deposits In Northeastern Yunnan And Northwestern Guizhou Province,China

The Northeast Yunnan and Northwest Guizhou district is located in the southwest margin of the Yangtze platform.Over 400 Pb-Zn deposits(occurrences)have been found in this region,which contain over 20Mt of Pb and Zn,making it an important Pb-Zn polymetallic industrial base in China.After many years of exploration and development,basic geological data is well documented,but there is still considerable controversy about the genetic research of these lead-zinc deposits,especially regarding the ore-forming age,ore-forming fluid source,metal sources and metallogenic mechanism of these deposits.In this paper,the Maoping lead-zinc deposit in Northeast Yunnan and Shanshulin,Shaojiwan,Yadu and Tianqiao lead-zinc deposits in Northwest Guizhou are taken as the research deposits.On the basis of comprehensive introduction of regional geology and geological characteristics of the deposits,sulfide LA-ICPMS analysis of sulfur isotope and trace elements,microthermometry of fluid inclusions,analysis of the fluid inclusion by LA-ICPMS and sphalerite Rb-Sr dating are carried out,which constrains the metallogenic age,ore-forming material and fluid source of the lead-zinc deposit in Northeast Yunnan Northwest Guizhou.On this basis,the metallogenic types of Pb-Zn deposits in the study area are classified,and combined with regional geological evolution,the coupling relationship between lead-zinc mineralization and major geological events in the study area is analyzed in detail,and a metallogenic model is established.Finally,according to the geological,geochemical characteristics,metallogenic background and ore-controlling factors,the metallogeny of the lead-zinc deposits in Northeastern Yunnan and Northwestern Guizhou is systematically summarized.The following are major findings:(1)The lead-zinc deposits are mainly hosted in Sinian to Permian carbonate strata,dominated by dolomite.Lead-zinc deposits in northeastern Yunnan are mainly controlled by NE-trending reverse faults,and the deposits in northwestern Guizhou are basically controlled by NW-trending reverse faults,the ore bodies are stratified,veins and lenticular,followed by breccia and network veins.The metallogenic period of the deposits are relatively unified,which can be divided into three stages:the first stage is pyrite stage,the second stage is lead-zinc sulfide stage which is the main metallogenic stage of the lead-zinc sulfide,and the third stage is carbonate stage.Metallic minerals are mainly sphalerite,pyrite and galena,and non-metallic minerals are mainly calcite,dolomite and quartz.Alteration is characterized by carbonation and pyritization.(2)The sulfide LA-ICPMS sulfur isotopic analysis constrains that the?³⁴S of these lead-zinc deposits are distributed in the range of 10?23‰.The enrichment of?³⁴S with the characteristics of pyrite>sphalerite>galena suggests equilibrium fractionation of sulfur isotopes.This finding constrains the total?³⁴S of the ore forming fluids of Maoping deposit within the range of 19?22‰,which is mainly from the thermochemical reduction of marine sulfate in the underlying Sinian Dengying Formation or Cambrian strata;the total?³⁴S of the ore-forming fluid of the deposits in Northwest Guizhou ranges from 13‰to 19‰,and the reducing sulfur of ore-forming fluid may have multiple sources,mainly from the underlying strata and brine sulfate.The analysis results of sulfide trace elements show similar characteristics to typical MVT lead-zinc deposits.The sulfide LA-ICPMS trace elements analysis shows that sphalerite is characterized by enrichment of Ge,Ga,Cd and Ag,and poor in Bi,Ni,Co,Ti and Tl;pyrite is relatively rich in Ni,Co and As,which shows the similar characteristics to the typical MVT lead-zinc deposits.(3)The temperature of the ore-forming fluid of these deposits is concentrated between 120?250?,and the salinity is concentrated between 7?14wt%,which suggests an ore-forming fluid with moderate salinity at low-moderate ore-forming temperatures.The composition of fluid inclusions is complex,the gas phase mainly consists of H₂O,CO₂ and a small amount of CH₄,and the liquid phase mainly contains Na,Ca,K,Mg,Cl,etc.LA-ICPMS fluid inclusion microanalysis shows that the contents of Li,Rb,Sr,Cs and Ba are also high in fluid inclusion.By comparing magmatic ore-forming fluid,MVT lead-zinc ore-forming fluid and basin brine data,it is found that the ore-forming fluids of the lead-zinc deposits in northeast Yunnan and northwest Guizhou are of basin brine origin.(4)The Rb-Sr geochronology of sphalerite shows that the Maoping Pb-Zn deposit has a metallogenic age of 202.5±8.5 Ma,which is in the Late Triassic to Early Jurassic period,later than the Devonian ore bearing strata.Combined with the previous research results in this area,the metallogenic age of the lead-zinc deposits in the Northeast Yunnan Northwest Guizhou area is further constrained,mainly between 191-228 Ma.Based on the characteristics and source of ore-forming fluid,the source of the metallogenetic material,the characteristics of the trace elements and the study of regional tectonic evolution,it is considered that the lead-zinc deposits in this region belong to MVT type lead-zinc deposits.At the same time,the mineralization of Zn-Pb deposits widely distributed in the study area has a good coupling characteristic with the evolution of Youjiang Basin in the Indosinian period,the large-scale fluid migration driven by the compression orogeny in Youjiang Basin resulted in the formation of large-scale Pb-Zn deposits in the study area.The evolution process of the foreland basin related to the formation of lead-zinc deposits involves two stages:the turbidite basin stage in the middle Triassic and the extinction stage from late Triassic to early Jurassic.The early turbidite deposition stage is the preparation period of relatively high temperature basin brine of 150?280?,with the highest temperature of 300?350?;During the Late Triassic to Early Jurassic,the Youjiang foreland basin contracted and uplifted from SE to NW under the action of NWW compression stress.Due to the uplift topography and structural expulsion of compression orogeny,a large scale of basin brine with high temperature was discharged from the basin.The basin brine migrated along Yadu-Ziyun fault and the unconformity between the basement and the sedimentary cover in the NWW direction,and extracted a large number of ore-forming elements from the basement and sedimentary strata.Finally,metal sulfide is precipitated in NW,NE trending reverse faults and folds to form Pb-Zn deposits in the study area.(5)The Pb-Zn deposits in the study area are jointly controlled by stratum,lithology,structures and lithofacies.First,the deposits are selectively hosted in the strata from Sinian to Permian,and the ore-bearing strata are getting older and older form southeast to northwest,but the lithology is always carbonate rock,mainly dolomite;Secondly,regional deep large faults,local large faults and secondary structures within the scope of ore deposits control the overall scope of metallogenic areas,the distribution of ore deposits and the occurrence feature of ore bodies,respectively.On the basis of comprehensive analyses of geochemistry,lithofacies palaeogeography,metallogenic model,metallogeny and metallogenic conditions,it is considered that the NW end of the Yadu-Mangdong fault zone in Northwest Guizhou is the most potential area in further exploration,and the area between Lianfeng-Qiaojia fault and Kuangshan-Jinniuchang fault in Northeast Yunnan may have better prospecting potential.