Isotopic Geochemistry And Evolution Of Ore-forming Fluids Of The Aerhada Pb-Zn-Ag Deposit,Inner Mongolia,Ne China

The newly discovered Arehada deposit is a large-scale Pb-Zn-Ag in the Dong Ujimqin,Inner Mongolia,located in the western segment of the Great Hinggan Range Ag-Pb-Zn-Cu-Mo-Au-Fe metallogenic belt in NE China.Ore bodies mainly occur as vein type and are hosted by sandstone and siliceous slate.Field observation and mineragraphy research shows that the major mineralization can be divided into three stages: arsenopyrite-pyrite-quartzstage?stage??,galena-sphalerite-pyrite-silverminerals-quartz stage?stage ??and sphalerite-pyritecalcite-fluoritestage?stages??.Microscope identification and electron analysis show that metallic minerals of this deposit mainly consist of galena,sphalerite,a rsenopyrite,pyrite,chalcopyrite,pyrrhotite,stannite and silver-bearing minerals.The dominant silver-bearing minerals are freibergite,pyrargyrite and canfieldite,followed by polybasite and hocartite.Silver minerals mainly embedded in sulfide cleavages and intercrystalline or replace sulfide minerals.Lattice silver fill microfractures in mineral mainly and sub-micron inclusion silver were found in side chalcopyrite,sphalerite,galena.The study of fluid inclusion shows that,four types of fluid inclusions have been identified in ore-bearing quartz,calcite and fluorite veins,i.e.,liquid-rich,gas-rich,three phase CO₂ aqueous inclusions,and pure gas or liquid aqueous inclusions.Microthermometric investigations on fluid inclusions reveal that ho mogenization temperatures for early to late stages range from 252.9 to 430.1??194.8 to 93.8??133.1 to 207.3?,respectively.Fluid salinities show a range from 4.65 to 14.04 wt.%NaCleqv?2.90 to 13.18 wt.%NaCleqv,3.71 to 10.24 wt.%Na Cleqv,respectively.The fluid compositions,as determined by laser Raman on fluid inclusions,are dominated by H₂O,CO₂ and CH₄,with minor proportions of N₂.The ore-forming fluid is medium to low in temperature,low in salinity and low in density.The fluid ?¹⁸OH₂O and ?DH₂O values vary from +1.6 to +9.3‰ and-121.7 to-56.0‰,respectively,mostly reflecting a magmatic fluid and a meteoric fluid dominant hydrothermal system for early and late stage mineralization,respect ively.The calculated ?³⁴SH₂S values of hydrothermal fluids in equilibrium with s ulfides range from +5.2 to +7.1‰,suggesting a sulfur source from mixtures of local magmatic and sedimentary rocks.The Pb isotope compositions of sulfides are similar to those of the local magmatic and sedimentary rocks,suggestingPb possibly from both sources.The noble gas isotope compositions of fluid inclusions hosted in sulfide demonstrate that the ore-forming fluids were domina ntly derived from the deep mantle.The ore-forming fluids and materials have the characteristics of mixed sou rces.Fluid mixing is inferred as the dominant mechanism for ore deposition,and the deposit can be classified as a medium to low temperature hydrothermal vein type deposit.