Material And Ore-forming Fluid Sources Of The Mengxing Pb-Zn Deposit,West Yunnan Province,SW China

The Mengxing lead-zinc deposit is the most representative deposit of epithermal lead-zinc deposits in Baoshan Block,but the vulnerable points of the fluid source and evolution.As a result,there are two different views on the genesis of the deposit:SEDEX and MVT.In this paper,we systematically studied the geology,geochemical characteristics of C-H-O-S-Pb isotopes of the Mengxing deposit,in order to revealing the sources of ore-from fluids and materials,and discuss its ore genesis.The geological characteristics of the Mengxing deposite show that the ore bodies occurred as veined and lenticular in the laminar limestone and bioclastic limstone in the upper Renheqiao formation which exist in Mengxing secondary syncline.The mineralization period can be divided into sedimentary diagenetic stage dominated by ores Fe and hydrothermal stage dominated by ores of Fe,Pb and Zn.Ore minerals are dominated by pyrite,sphalerite,galena,quartz and carbonate minerals.The ore textures and structures are characterized by euhechal-subhechral granular texture,metasomatic dissolution-residual structure,vein structure and vein-reticular structure.The theoretical modeling fluid-rock interaction,CO₂degassing and mixing of two fluids show that the calcite precipitation is due to the fluid-rock interaction together with decreases in temperature,and the sea water also played an crucial role.The H₂CO₃is the dominate dissolved carbon species of ore-forming fluid in Mengxing deposit,the?¹³C and?¹⁸O data of stage?and stage?deduced to be 1.0‰,10.5‰and?0.8‰,7.5‰,respectively.The calcite precipitation temperatures of stage?and stage?range from 270?to 315?and from 182?to 230?,respectively.The?¹³C and?¹⁸O decreased in stage?shows that CO₂degassing and sea water occurred before fluid-rock interaction.Isotopic geochemical characteristics show that the value of?D_V-SOMWand value of?¹⁸OH₂O in different stages in range of?47.1‰to?90.1‰and 3.6‰to 10.9‰,which are similar to those of the Xiyi and Chipu deposits,and show that the H₂O in hydrothermal fluids is sourced from basinal brine.The research result indicates that?³⁴S values of the sulfides are characterized by the enrichment of?³⁴S,ranging from?0.84‰to 27.90‰,which average of 9.9‰,and similar to?³⁴S of Xiyi deposit indicating that the sulfur mainly from marine carbonatite,by Thermao-chemical Sulfate Reduction and Bacterial Sulfate Reduction.The Pb isotope compositions of the sulfides shows²⁰⁶Pb/²⁰⁴Pb,²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb ranges of 18.267 to 18.345,15.691 to 15.753 and 38.575 to 38.705,demonstrating that crust-derived nature obviously,and the result of research is consistent with Pb-Zn deposits in the Baoshan Block,and it is shown that the sources of ore-froming materials of the Mengxing deposit are mainly derived from the upper crust.The deposit shows many similarities with typical MVT Zn-Pb deposits include the tectonic setting,deposit geology,geochemistry of deposit,ore-forming fluid characteristics and the ore material sources.Thus,we propose that the Mengxing lead-zinc deposit should be classified as Mississippi Valley type.