Geochronology and geochemistry of Cu-Ni mineralized mafic-ultramafic rocks in the Daaobaogou deposit of the Dunhuang Block in the southeastern margin of the Tarim Plate
The copper-nickel mineralized mafic-ultramafic rocks in the Daaobaogou deposit of the Dunhuang Block are located in the southeastern margin of the Tarim Plate.It is of great significance to study the diagenetic age and geochemical characteristics of those rocks for understanding the Cu-Ni mineralization and predictive prospecting the Cu-Ni resources in the Dunhuang Block.In this study,we have carried out analyses of the zircon U-Pb chronology,Sr-Nd-Hf isotopes,major and trace elements of those mafic-ultramafic rocks.The results show that the olivine pyroxenite of mafic-ultramafic rocks in the Daaobaogou deposit has zircon SHRIMP U-Pb age of(331.7±7.3)Ma(early Carboniferous epoch).The mafic-ultramafic rocks are characterized with relatively low Mg# values(33.45-71.55),enriched LREE(La N/Yb N=6.23-15.19)and LILE(Rb,Th,U,etc.),depleted HFSE(Nb,Ta,Ti,etc.),and relatively weak negative Eu anomalies(δEu=0.66-1.08).Based on the AFM and Ni/Cu-Pd/Ir diagrams,they belong to the tholeiitic series.Combined with studies on characteristics of regional geology,it is believed that they were formed in the post-collision extensional environment by the evolved magmas through the evolution towards the calc-alkaline one from the original high magnesium tholeiitic basaltic magma which was originated from the partial melting of the lithosphere mantle.In the evolution process of original magma,the fractional crystallization,crustal material mixing and segregation of Cu-Ni sulfides had occurred in the magma.
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