Origin of the post-collisional carboniferous granitoids associated with the Azhahada Cu-Bi deposit in Inner Mongolia, Northeast China and implications for regional metallogeny

Abstract

The Azhahada quartz vein-type Cu-Bi deposit is located in the Erenhot-East Ujimqin polymetallic metallogenic belt, on the western part of the Great Xing’an Range, Inner Mongolia, northeast China. The Cu-Bi veins are hosted by volcanic and metasedimentary rocks in the Devonian Niqiuhe Formation and Carboniferous felsic intrusions (monzogranite, syenogranite, and granite). Three hydrothermal ore stages (I-III) for Cu-Bi formation at Azhahada have been identified. Chalcopyrite and bismuthinite are the dominant ore minerals and mainly occur in vein stage II (quartz–pyrite ± chalcopyrite ± bismuthinite veins). New LA-ICP-MS zircon U-Pb dating indicates the monzogranite, syenogranite, and granite emplaced between 313.9 ± 1.7 and 308.4 ± 1.5 Ma. The monzogranite, syenogranite, and granite show the same geochemical affinity with I-type granites, which are characterized by high SiO2, K2O, and Al2O3 contents, low MgO, TiO2, and P2O5 contents, negative Eu anomalies (EuN/EuN* = 0.04–0.57), and strong enrichment in Rb, K, Pb, and Nd. Moreover, these granitic intrusions have low Mg# values of 22 to 39, low (87Sr/86Sr)i ratios (0.7023 to 0.7053), εNd(t) values between −0.8 and +0.4, positive εHf(t) values of +1.6 to +10.4, suggesting that their parental magmas were most likely derived from partial melts from the lower crust involving some mantle materials in a post-collisional tectonic setting. Sulfide samples from Azhahada have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios similar to those of granitic intrusions. The silicon isotope compositions of the quartz samples (−0.3 to −0.1‰) are highly consistent with values of the granitic intrusions (−0.4 to −0.1‰), indicating that the silicon was derived from the granitic magma. Combined our whole-rock geochemical and Sr–Nd–Hf–Pb–Si isotopic data with previous investigation of regional tectonic evolutions, we suggest that the Azhahada granitoids genetically associated with the Cu-Bi mineralization were generated by the postorogenic lithospheric extension and magmatic underplating during the Late Carboniferous. A post-collisional extensional event coupled with the rising of hot mantle derived melts triggered partial melting of the lower crust, as well as provided metals (Cu and Bi).