Sr-Nd-Pb and zircon Hf isotopic constraints on the petrogenesis of two types of early Cretaceous intrusive rocks in the Tongling ore-cluster region: Implications for Cu-Au polymetallic mineralization

Abstract

Early Cretaceous magmatic activities in the Tongling ore-cluster region of eastern China produced a series of igneous rocks, which can be divided into early-stage ore-bearing intrusions associated with large-scale Cu polymetallic mineralization, and late-stage intrusions that are generally ore-barren. Petrogeneses of these intrusive rocks and their relationship to mineralization are still controversial. To clarify these scientific issues, early-stage ore-bearing intrusive rocks (granodiorite from the Jinziwei deposit and granodiorite porphyry from the Yaojialing deposit) and late-stage ore-barren granodiorite dykes from the Jinziwei deposit are studied. Zircons from early-stage ore-bearing intrusions and late-stage ore-barren rocks yield U–Pb ages of 141–139 Ma, and 129 Ma, respectively, indicating that the gap between the two-stage magmatic events was approximately 10 Ma. The SiO2 contents of the two types of rocks are similar (62.15–68.11 wt% and 64.02–65.91 wt%, respectively), whereas their Sr/Y values (49.26–140.54 and 17.90–21.24, respectively) and Y contents (7.4–16.2 ppm and 20.2–24.0 ppm, respectively) are significantly different, suggesting that the former is adakite and the latter is non-adakite. Whole-rock Sr-Nd-Pb and zircon Hf isotopic compositions indicate that the early-stage ore-bearing intrusions were derived from partial melting of the subducted slab (oceanic crust with marine sediments), followed by the interaction of lithospheric mantle peridotite and the later incorporation of Middle-Upper Yangtze continental crust materials during magma ascent/emplacement. The late-stage ore-barren dykes evolved from the early-stage intermediate-basic magma in the Tongling region, which was originated from the enriched lithospheric mantle metasomatized by slab fluids/melts followed by the addition of a small amount of crustal materials. The zircon and apatite geochemical results show that high oxygen fugacity of the early-stage intrusion is a key factor in regional Cu-Au mineralization. Our study reveals the fact that magma with F-rich or Cl-rich characteristics directly controls the type of mineralization. Furthermore, the F-rich magma is favorable for Cu-Mo deposits, and on the other hand, the Cl-rich magma is favorable for Cu-Au-Zn deposits.