Study On Metallogenic Mechanism Of The Zhaojinggou Nb-Ta Deposit In North China Craton,Inner Mongolia,China

The Zhaojinggou Nb-Ta deposit,located in eastern part of the Daqingshan area in the northern margin of the North China Craton,Inner Mongolia,China,is mainly hosted in the mineralized dikes around the Hutoushan pluton and the residual pegmatite shell at the top of the pluton.This study completed an integrated investigation of the field geology,chronology,zircon Lu-Hf isotopes,mineralogy and geochemistry research to trace the magmatic and magmatic-hydrothermal process of the Zhaojinggou deposit and to clarify the factors that control the Nb-Ta concentration and mineralization.Geochemically,all samples from the Hutoushan pluton and the mineralized dikes are characterized by high contents of SiO₂?73.3%to 84.3%?,total alkalis?5.24%to9.60%?and Al₂O₃?7.94%to 13.8%?,and moderate A/CNK values?with an average of1.04?which is classified as a weakly peraluminous affinity.The trace element patterns are characterized by Rb,Nb,Ta,Zr and Hf enrichment and markedly negative Ba,Sr,P,Eu and Ti anomalies.Zircon U-Pb dating reveals that the emplacement of Hutoushan pluton and the Nb-Ta mineralization almost coevally occurred in the Early Cretaceous?126±1 Ma and127±2 Ma,respectively?.The studied samples show strong negative?Hf?t?values?-7.47 to-12.95?with crustal model ages ranging from1.65 to 2.00 Ga,indicating the parental magma of the Hutoushan pluton was derived from the partial melting of Palaeoproterozoic crust.Variations in bulk-rock geochemistry?K/Rb,Zr/Hf?and mineral composition?mica and columbite?are consistent with a magmatic evolution trend from the Hutoushan alkali-feldspar granite?HTAG?,through the Hutoushan pegmatite shell?HTPS?,to the mineralized dikes?MD?.Considering the significant higher bulk contents of Nb,Ta and F in the HTPS and the MD than in the HTAG as well as the similar contents of Nb and Ta in the biotite from the HTPS and the HTAG,we proposed that a residual Nb-Ta-riched brine-melt phase segregated from the crystal mush at the late stage of crystallization,which is also supported by the high Nb-Ta concentrations in the residual melt equilibrated with the biotite in the HTAG.Progressive concentration of F in the melt increases the solubility of Nb and Ta in granitic melts and decreases the viscosity by depolymerizing the melt,which finally contribute to the segregation and ascending of Nb-Ta-riched brine-melt phase.The Nb-Ta-riched brine-melt phase gathered at the top of the pluton,formed the pegmatite shell and intruded along cracks that provided spaces for the intruding mineralized dikes.Further crystallization of melt with decreasing temperature resulted in the saturation of Nb and random crystallization of columbite in the HTPS and the MD.Post-magmatic fluid that formed in the magmatic-hydrothermal transitional stage causes the M-type"tetrad effect"and further mineralization in the MD by fluid-rock interaction.Therefore,the enrichment and mineralization of the Zhaojinggou Nb-Ta deposit mainly occurred in the magmatic stage,and further alteration and mineralization occurred under the influence of magmatic-hydrothermal fluid.