Study On Genesis Of Mengxing Lead-zinc Deposit In Western Yunnan: LA-ICP-MS Trace Elements And S Isotope Evidence From Sphalerite

The Mengxing Pb-Zn deposit is located in the central and western part of Baoshan Block in the"Sanjiang"Tethyan metallogenic belt in southwest China.The proven Pb+Zn metal content is>1.02 Mt（average grade of Pb+Zn>12.125%）.It is the most representative and typical medium-low temperature hydrothermal lead-zinc deposit in the region.Due to the weak geological research work of the deposit,the origin of the deposit is controversial,and there are various views such as stratigraphy/sedimentation modification,SEDEX,MVT type,etc.In view of this,this paper carried out detailed field geological research and indoor mineralographic observation and analysis of the deposit,and used LA-ICP-MS analysis and testing technology to test the sphalerite S isotope and trace elements of different types,different stages,different stages and different colors in space,and carried out surface scanning treatment of sphalerite with ring zone,in order to reveal the source of mineralization and the evolution process of mineralization,and explore the origin and mineralization mechanism of the deposit.The geological characteristics of the ore body showed that the ore body was layered,stratigraphic,vein-vein-vein-like in the Silurian Shangrenhe Bridge Formation on the east flank of Mengxing,and the surrounding rock was mainly bioclastic limestone and crystalline limestone.According to the interspersed symbiotic combination relationship of minerals,the mineralization process is divided into three periods:sedimentary period,hydrothermal superposition transformation period and late metallogenic period.The sedimentary period has typical characteristics of early sedimentary mineralization,while the hydrothermal superposition modification period shows typical characteristics of late hydrothermal superposition mineralization,and a large number of gangue minerals（mainly calcite）are formed in the later stage of mineralization,interspersed with ore bodies in the metallogenic period.The geochemical characteristics of trace elements showed that the sphalerite in Mengxing lead-zinc deposit was rich in Ge,Cd,As and Sb elements,and was poor Fe,Mn,In,Sn and Co,showing a medium and low temperature mineralization environment.Sphalerite with annular band characteristics showed that the dark ring band was more enriched with Fe,Mn,Cu,Ge,As,Ag,Sb,Pb elements than the light ring band,and the depleted Ga,Cd,Sn elements were depleted,indicating that the formation temperature of the dark ring band was higher than that of the light ring band.Compared with the hydrothermal superposition transformation period,sphalerite in the sedimentary period is more enriched with Fe,Co,Ga,In and poor Mn,Cu,Ge,As,Ag,Cd,Sn,Sb,Pb elements.The temperature of hydrothermal superposition transformation period（209.78°C～240.18°C）was higher than that in the sedimentation period（174.74°C～203.18°C）,and the space of the deposit showed that the upper ore body was more rich in Ga,Ge low-temperature elements than the lower ore body,and poor Fe,Mn,Cu,Ag,Pb and other low-temperature elements.There are homogeneous substitution,coupling substitution and microinclusion forms of trace elements in sphalerite.The geochemical characteristics of S isotope showed that the distribution of S-isotope of sphalerite in Mengxing lead-zinc deposit was wide(δ³⁴S_VCDT=-6.45‰～37.06‰),and the sedimentary period(δ³⁴S_VCDT=13.29‰～37.06‰,mean27.97‰)and hydrothermal superposition period(δ³⁴S_VCDT=-6.45‰～31.57‰,mean value 9.01‰)were more differentiated.It is speculated that the early stage was mainly affected by the thermochemical sulfate reduction（TSR）in seawater sulfate,and the mineralization scale and grade of the original ore body were expanded due to the superposition of the mineralization hydrothermal fluid(δ³⁴S_VCDT=-5‰～5‰)and the early ore body in the early stage.Spatially the results of S-isotope test results of sphalerite showed that theδ³⁴S_VCDT value of the shallow ore body was higher than that of the deep ore body,suggesting that hydrothermal action had a greater influence on the lower ore body in the later stage.Mengxing lead-zinc deposits are similar to sedimentary lead-zinc deposits,and the early layered ore body was formed in the Caledonian period（Silurian period）,which is a product of seafloor jet-sedimentary genesis（SETEX type）,and the ore body formed in this period has a certain economic grade and scale;In the later stage of the ore deposit（Indo-China-Yanshan period）,the land-land collision and extrusion of the Baoshan massif and the Tengchong plot in this area led to frequent large-scale magmatic activities,which provided heat energy for the transport of mineralizing fluid in the lower part of the Mengxing mining area,and the mineralizing hydrothermal fluid surged up the structural channel and superimposed with the early ore body to form a rich large ore body,according to which it was believed that the Mengxing ore deposit should be a hot water sedimentation-hydrothermal superimposed modified lead-zinc deposit.