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The Daye district represents one of the largest concentrations of skarn deposits in eastern China. There are two major types of skarn deposits (Fe skarn vs. Cu skarn) within this district, both genetically related to late Mesozoic high-K calc-alkaline granitoids. In this study, we present in situ compositional and Sr–Nd isotopic investigations of apatite from granitoids associated with Fe skarn and Cu skarn deposits to put constraints on the magma source, evolution, and volatile composition, which provide significant new insights into the genesis of the two contrasting mineralization styles in the Daye district. Apatite from granitoids related to Cu skarns and Lingxiang Fe skarn has εNd(t) values of − 8 to − 4, similar to that of the contemporaneous mafic rocks in the Daye district, which were dominantly derived from an enriched lithospheric mantle source. Apatite in granitoids associated with the Chengchao Fe skarn has lower εNd(t) values of − 15 to − 9, suggesting larger degrees of contaminations from ancient lower crust materials. Fractionation models based on apatite Sr/Y and Eu/Eu* ratios, suggest that magmas related with the Cu skarns have experienced amphibole-dominated fractionation under high pressure and hydrous conditions, whereas those associated with Fe skarns have undergone plagioclase-dominated fractionation at relatively low pressure and dry conditions. Based on results of apatite compositional analysis and apatite-melt partitioning data, the estimated magma Cl contents for the Fe skarn range from 3260 to 13,940 ppm, significantly higher than those for the Cu skarn (Cl = 430–5990 ppm). Apatite from Fe skarn-related intrusions has (87Sr/86Sr)t ranging from 0.7073 to 0.7082, whereas the variety from Cu skarn-related intrusions displays lower (87Sr/86Sr)t of 0.7054–0.7061. These Sr isotope data, combined with whole-rock S isotopes of this study and previous investigations suggest that the Fe skarn-related intrusions have assimilated larger amount of evaporite-bearing carbonate during magma ascent and emplacement. The assimilation process may have not only promoted magmatic water exsolution but also provided sufficient amount of Cl− and SO 4 2 - , which facilitated effective complexation and transportation of Fe2+ and subsequent oxidization of ferrous Fe to precipitate magnetite, respectively. We suggest that evaporite assimilation into the granitoid magmas has played a crucial role in the large-scaled Fe skarn mineralization in the Daye district. This study highlights that apatite is a sensitive petrogenetic-metallogenic indicator for granitoids and thus can be useful in mineral exploration. [ABSTRACT FROM AUTHOR]

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. • Magmatic oxidation state increases from Fe through Cu-Au-Fe to Cu-Mo skarn deposits. • Mo mineralization is promoted by increasing degrees of magmatic fractionation. • Zircon Eu/Eu* can be used for classification of skarn-related intrusions. • Zircon chemistry as an indicator for nature and metallogeny of intrusions. [ABSTRACT FROM AUTHOR]