Zircon U–Pb geochronology and Sr–Nd–Pb–Hf isotopic constraints on the timing and origin of the Early Cretaceous igneous rocks in the Yongxin gold deposit in the Lesser Xing'an Range, NE China

Abstract

The Lesser Xing'an Range in NE China is an important gold district. However, the age and genesis of Early Cretaceous magmatic rocks that are coeval with gold mineralization remain obscure. A suite of Early Cretaceous igneous rocks in a newly discovered gold deposit at Yongxin was conducted zircon U–Pb dating and Hf isotope and bulk‐rock geochemistry and Sr–Nd–Pb isotope analyses. A dacite of the Guanghua Formation and an andesite of the Longjiang Formation have zircon U–Pb ages of 111.7 ± 1.5 Ma and 119.7 ± 1.9 Ma, respectively. The dacite has zircon εHf(t) values of 5.4 to 12.0 and depleted bulk‐rock Sr–Nd–Pb isotopes ((87Sr/86Sr)i = 0.7049 to 0.7056, εNd(t) = 1.2 to 2.5, (206Pb/204Pb)i = 18.202 to 18.320). The andesite has zircon εHf(t) values of 5.6 to 13.6, (87Sr/86Sr)i of 0.7049 to 0.7051, εNd(t) of 1.0 to 1.6, and (206Pb/204Pb)i of 18.199 to 18.297. The ~120 Ma diorite porphyry and granite porphyry have zircon εHf(t) values of 3.7 to 10.0 and 3.6 to 10.2, respectively. These two units show large variations of 87Rb/86Sr ratios (<1) and similar Sr–Nd–Pb isotopes compositions. All rock units are enriched in light rare earth elements and large‐ion lithophile elements and depleted in high‐field‐strength elements. The formation of Yongxin Early Cretaceous igneous rocks was related to variable degrees of partial melting from Jurassic basalt in the lower crust with significant contribution from ancient continental materials. The low Sr/Y ratios (2–24) and negative correlations between Sr, Eu/Eu, and SiO2 indicate that magmas of the Early Cretaceous igneous rocks at Yongxin have low water content and thus have low potentiality for porphyry copper mineralization.