Accurate Determination Of Stable Silver Isotopes And Its Application In Polymetallic Deposits In China

In the last decade,the understanding of the fractionation of nontraditional silver isotope has become a powerful tool in the earth and planetary sciences.As one of ore body elements,silver isotope geochemistry has potential significance to explore the formation and evolution of the precious metal ore deposits,compared with other traditional stable isotope approaches.In Au-Ag ore-deposits,silver minerals have complex paragenetic relationships with other minerals(e.g.chalcopyrite,sphalerite,galena,and pyrite etc.).It is difficult to remove such abundant impurities completely because the other metals are tens to thousands of times richer than silver.Both quantitative evaluation of matrix effects and modification of chemical chromatography were carried out to deal with the problems.Isobaric inferences(e.g.65Cu40Ar+ to 105Pd,208Pb2+ to 104Pd,and 67Zn40Ar+ to 107Ag+)and space charge effects dramatically shift the measured ?109Ag values.The selection of alternative Pd isotope-pairs is effective in eliminating spectral matrix effects,so as to ensure accurate analysis under the largest possible ranges for metal impurities,which are Cu/Ag:? 50:1,Fe/Ag ? 600:1,Pb/Ag?10:1,Zn/Ag? 1:1 respectively.Acceptable separation/purification of Ag from base-metal matrices can be achieved by a two-step ion-exchange procedure that reduces the ratios of metal/Ag to less than 0.15 mol/mol,permitting precise and accurate silver isotope analysis in ore deposits.On this basis of laboratory modification,the silver isotope geochemistry in the polymetallic deposits in the Chinese region has been investigated,including the Dazhuangzi gold deposit,Qixiashan lead-zinc-silver polymetallic deposit,the skarn-type deposit in southeastern Hubei and the Zijinshan low-temperature hydrothermal deposit.The silver isotopes varied from-0.043 ‰ to +1.017 ‰ and a systemic summary of ?109Ag variations in rocks,ore-deposits,and environmental materials in nature revealed that implies significant mass dependent silver isotope fractionation in geological processes(e.g.magmatic exsolution and hydrothermal alteration)and in physico-chemical and biological processes,including redox reaction,photo-reduction,adsorption-dissolution,and biological cycling etc.The silver isotopes in the Dazhuangzi gold deposit are near zero,showing the characteristics of the crust source.The silver isotopes of the other three types of skarn-type deposit,the Qixiashan lead zinc silver ore and the Zijinshan epithermal deposit are the most positive ?109Ag compared with the silver isotopes in the bulk silicate earth(BSE),reflecting the ore sources might be mantle derived or mantle mixed.