Sulfide Re-Os Isochron And Discussions In Pyrites From Meiling Cu Deposit In The Eastern Tianshan

The eastern Tianshan in Xinjiang,NW China,is located in the south margin of the Central Asian Orogenic Belt,spans the two major plates of Kazakhstan and Tarim,and is the most important copper producer in China.The eastern Tianshan in Xinjiang,NW China,The mineralization in it is very complicated.Meiling deposit in Kalatage is located in the Paleozoic Kalatage uplift and Late Paleozoic Dananhu-Tousuquan arc.The Ore body is hosted by basic-neutral-acid volcanic,volcanic clastic and subvolcanic rocks.In this paper,the Pyrite in Meiling deposit are chosen for study,combined with field works,elemental geochemistry and Re-Os isotopic geochronology.Re-Os isotopic system has been widely used in tracing and dating of sulfide deposit,crust-mantle activities and sedimentary rocks that full of organic matter.Especially in sulfide deposit,the kind of sample is very important.This thesis reviews the characteristics of Re-Os isotopic system and its applications,the techniques used in its analysis including chemical and mass spectrometer methods.Re–Os dating of pyrites in the disseminated and vein ores from Meiling Cu deposit in eastern Tianshan,NW China indicates that the disseminated and massive ores show large variations in both Os concentrations and isotopic ratios.Our isotopic data of the two kind ores have defined good Re-Os correlations and their respect isochron ages are 523±59 Ma and 707±99 Ma.The two isochron ages are both significantly older than the stratigraphic age of the Meiling Cu deposit.Together with the good correlations between ¹⁸⁷Os/¹⁸⁸Os ratio and the reciprocal of the common Os(i.e.,1/¹⁹²Os,R² is 0.9973 and 0.9945 for the disseminated and vein ores,respectively),we suggest that the initial Os isotopic compositions of these samples are heterogeneous and interpret the two ages as mixed isochron ages.Our mathematical deduction demonstrates that the observed Re–Os isochron correlations are the result of binary mixing without complete isotopic equilibrium at the time of formation,primarily due to limited diffusional exchange of Os isotopes between refractory Os-bearing inclusions and silicate minerals.The initial Os isotopic compositions may approach the value of continental crust.Crustal contamination during the formation of the deposit was responsible for the initial Os isotopic heterogeneity in the metallic sulfide system.Therefore,whether there is a good Re–Os isochron correlation for samples can be used to constrain the geological meaning of Re–Os isochron ages when we use the Re–Os isotopic system of metallic sulfides to date the formation age of metallic deposit.