Geochronology, Hf isotope, geochemical characteristics of the metallogenic rock mass in Nomint copper deposit, Northeastern Mongolia, and its constraint on metallogenic tectonic setting

The Nomint medium-scale copper deposit, located in the central part of the Mongolia-Okhotsk suture zone, is the largest known copper deposit in the northeastern part of Mongolia. To explore the relationship between the rock mass and mineralization, this paper conducts zircon U−Pb chronology, trace element, and Hf isotopic geochemical research on the ore-forming rock mass of this deposit, based on mineralogical studies. The results show that the biotite granodiorite yielded zircon U−Pb ages of 166.3±2.3 Ma and 162.6±1.6 Ma, respectively, representing the products of the late Middle Jurassic magmatism; In the geochemical composition, the metallogenic rock mass are SiO₂ (62.81% ~ 66.03%) > 56%, Al₂O₃ (15.01% ~ 15.77%) > 15%, MgO (2.06% ~ 2.72%) < 3%, K₂O/Na₂O < 1, trace elements are relatively enriched in LREE and relatively deficient in HREE. Eu anomaly was not obvious and Y (10.67×10⁻⁶ ~ 14.05×10⁻⁶, < 18×10⁻⁶) and Yb (1.20×10⁻⁶ ~ 1.65×10⁻⁶, < 1.9×10⁻⁶) were poor, and Sr enrichment (547.6×10⁻⁶ ~ 661.8×10⁻⁶, greater than 400×10⁻⁶). It indicates that the metallogenic rock mass has the geochemical characteristics of Adakite rock. In the meanwhile, the metallogenic rock mass has relatively high Mg^#, Cr and Ni contents, which is similar to Adakite formed from melting of crust under subsidence. In terms of Hf isotopic composition, the pluton has a low ε_Hf(t) value (0.3 ~ 4.3) and a Mesoproterozoic model age (T_DM2=1183 ~ 937 Ma), suggesting that some Mesoproterozoic basement rock components were mixed during the magmatic rise. The comprehensive research indicates that the metallogenic rock magma of the deposit originated from partial melting of the lower crustal delamination, formed in an intracontinental extensional environment following the closure of the Mongolia-Okhotsk Ocean in the late Middle Jurassic.