Petrogenesis and metallogenesis of the Early Cretaceous Naoniushan Cu- dominated polymetallic deposit in the central Great Xing'an Range, NE China

摘要

The Naoniushan Cu-dominated polymetallic deposit in the central Great Xing’an Range is associated locally with Pb-Zn-Ag or Mo mineralization. Mineralization mainly occurs in the Lower Cretaceous continental volcanic rocks. Here, we present new LA-ICP-MS zircon U-Pb ages, whole-rock geochemical data, Sr-Nd-Hf isotopic data, and Re-Os ages for the Naoniushan deposit. The emplacement of the porphyry occurred at 137.8 ± 1.4 Ma and the mineralization at 134.3 ± 0.8 Ma. The granodiorite porphyry has high SiO2(67.84–73.70 wt%) and K2O + Na2O (7.67–8.30 wt%) contents and low contents of MgO (0.60–1.04 wt%, Mg#= 0.44–0.51), TFeO (1.29–2.16 wt%), CaO (1.67–3.66 wt%) and K2O/Na2O (0.75–0.87). The porphyry is rich in large ion lithophile elements (LILEs, e.g., Rb, Sr, Ba, and K) and light rare earth elements (LREEs), without significant Eu anomalies (δEu = 0.84–1.01), and depleted in heavy REE (HREEs) with LREE/HREE = 12.78–16.57 and (La/Yb)N = 15.37–20.68, high field strength elements (HFSEs, e.g., Nb, Ta, and Ti) and siderophile elements (e.g., Cr, Co, and Ni). These features are similar to those of adakitic rocks derived from thickened lower crust. The Naoniushan porphyry exhibits positive εNd(t) values ranging from +1.9 to +2.8 and relatively low initial87Sr/86Sr ratios, ranging from 0.70458 to 0.70497. Most in-situ zircon Hf isotopic analyses show variable positive εHf(t) values ranging from +3.1 to +9.4, corresponding to relatively young two-stage Hf model ages ranging from 994 to 591 Ma (excluding one spot). These isotopic features suggest that the primary magma of the Naoniushan granodiorite porphyry was mainly derived from the partial melting of Neoproterozoic juvenile lower crust (comprising a mixture of 80% juvenile basalt and 20% ancient lower crust). These elemental, isotopic, and geochronologic data, combined with the regional geology, deposit geology and fluid characteristics, suggest that the Naoniushan Cu-dominated polymetallic deposit formed in an evolving environment that decreased in compression over time due to slowing of the subduction of the Paleo-Pacific Plate beneath the Eurasian Plate, and the transition from a compressional to an extensional tectonic regime following the closure of the Mongol-Okhotsk Ocean.