U–Pb zircon, geochemical and Sr–Nd–Hf–O isotopic constraints on age and origin of the ore-bearing intrusions from the Nurkazgan porphyry Cu–Au deposit in Kazakhstan

Abstract

Nurkazgan, located in northeastern Kazakhstan, is a super-large porphyry Cu–Au deposit with 3.9Mt metal copper and 229tonnage gold. We report in situ zircon U–Pb age and Hf–O isotope data, whole rock geochemical and Sr–Nd isotopic data for the ore-bearing intrusions from the Nurkazgan deposit. The ore-bearing intrusions include the granodiorite porphyry, quartz diorite porphyry, quartz diorite, and diorite.Secondary ion mass spectrometry (SIMS) zircon U–Pb dating indicates that the granodiorite porphyry and quartz diorite porphyry emplaced at 440±3Ma and 437±3Ma, respectively. All host rocks have low initial 87Sr/86Sr ratios (0.70338–0.70439), high whole-rock εNd(t) values (+5.9 to +6.3) and very high zircon εHf(t) values (+13.4 to +16.5), young whole-rock Nd and zircon Hf model ages, and consistent and slightly high zircon O values (+5.7 to +6.7), indicating that the ore-bearing magmas derived from the mantle without old continental crust involvement and without marked sediment contamination during magma emplacement. The granodiorite porphyry and quartz diorite porphyry are enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) and depleted in high-field strength elements (HFSE), Eu, Ba, Nb, Sr, P and Ti. The diorite and quartz diorite have also LILE and LREE enrichment and HFSE, Nb and Ti depletion, but have not negative Eu, Ba, Sr, and P anomalies. These features suggest that the parental magma of the granodiorite porphyry and quartz diorite porphyry originated from melting of a lithospheric mantle and experienced fractional crystallization, whereas the diorite and quartz diorite has a relatively deeper lithospheric mantle source region and has not experienced strong fractional crystallization. Based on these, together with the coeval ophiolites in the area, we propose that a subduction of the Balkhash–Junggar oceanic plate took place during the Early Silurian and the ore-bearing intrusions and associated Nurkazgan porphyry Cu–Au deposit occurred in an intra-oceanic arc setting.