Skarn metallogeny through zircon record: An example from the Daye Cu-Au-Fe-Mo district, eastern China

Abstract

The Daye district in Eastern China hosts more than 50 Fe-Cu-Au-Mo porphyry-skarn deposits that are associated with intermediate to felsic rocks of Early Cretaceous ages. The origin of these ore-related granitoids has been intensively investigated, but the petrogenetic factors controlling variable metal associations (e.g., Fe, Cu–Mo or Cu-Au-Fe) in different skarn deposits remain not well understood. Here, we present Hf isotope and trace element data of zircons from ore-related intrusions of four representative skarn deposits (i.e., the Lingxiang and Chengchao Fe skarn, the Tonglushan Cu-Au-Fe skarn and the Tongshankou Cu–Mo porphyry-skarn deposits) to better understand controls of the source, oxidation state and fractionation degree of the magmas on the polymetallic metallogeny in the Daye district.Zircons from the ore-related granodiorite at Tongshankou, quartz diorite at Tonglushan and diorite at Lingxiang have comparable εHf(t) values ranging from −11.3 to −1.1, which are similar to values of the Early Cretaceous mafic rocks (εHf (t) = −11.9 to −1.2) in the Daye district and adjacent areas. This similarity suggests that these ore-related intrusions were largely derived from an enriched lithospheric mantle source. In comparison, zircons from the ore-related diorite and quartz monzonite associated with the Chengchao Fe skarn have much lower εHf(t) values (−11.6 to −25.3), indicating larger degrees of crustal contribution. On average, zircons from the Cu-related intrusions at Tongshankou and Tonglushan have higher Ce/Ce* (>1100; method of Zhong et al., 2019) and Ce4+/Ce3+ (>170; method of Ballard et al., 2002) ratios, which are interpreted reflecting higher oxidation state for the magmas. In contrast, zircons from Fe-related intrusions at Chengchao and Lingxiang have much lower average values of Ce/Ce* (<700) and Ce4+/Ce3+ (<90), suggesting relatively reduced melts. Zircons from the Tongshankou granodiorite have the highest Yb/Dy (averaging 7.02) ratios and Eu/Eu* (averaging 0.74), likely indicating the prevalence of early amphibole fractionation and the suppression of plagioclase crystallization and thus indicating more hydrous property of the Tongshankou ore-related magmas. In addition, the generally higher zircon Hf contents of the granodiorite associated with the Tongshankou Cu–Mo deposit is consistent with greater degree of magma fractionation than intrusions associated with the Fe and Cu-Au–Fe skarn deposits.Preliminary results presented here further reveal that zircon Eu/Eu* ratio can be a useful discrimination for intrusions associated with the various metal associations in the Daye district. Zircon Eu/Eu* of 0.45 broadly separates the Fe skarns and Cu-bearing skarns, while Eu/Eu* of 0.65 can further distinguish the Cu-Au-Fe skarn from the Cu–Mo variety. This study highlights zircon chemistry as a useful indicator for the nature, evolution and metallogeny of the ore-related intrusions.