Fe-Cu separation in skarn deposits: Insights from magmatic and hydrothermal titanite

Iron (Fe) and copper (Cu) mineralization often coexist in skarn deposits, but they can also occur as independent deposits. Skarn Fe and Cu mineralization in the Yueshan ore field (Eastern China) is associated with the Cretaceous diorite intrusions. Notably, while these deposits exhibit comparable iron reserves, their copper endowments vary greatly. However, the main factors controlling the Fe-Cu separation remain poorly understood. This study investigates U-Pb geochronology and chemistry of magmatic and hydrothermal titanite from the Anqing Cu-Fe (Fe: 39 Mt, @45.19%; Cu: 0.5 Mt, @1.29%) and Zhuchong Fe-(Cu) (Fe: 51 Mt, @50.10%; Cu: 0.05 Mt, @1.15%) deposits in the ore field. The U-Pb ages of the magmatic titanite from the Zhuchong and Anqing deposits were 139.6 ± 2.2 Ma (1σ, n = 30, MSWD = 0.7) and 139.6 ± 0.6 Ma (1σ, n = 35, MSWD = 1.4), respectively. The hydrothermal titanite U-Pb ages were 137.0 ± 0.5 Ma (1σ, n = 29, MSWD = 2.7) for Zhuchong and 137.9 ± 1.1 Ma (1σ, n = 26, MSWD = 2.1) for Anqing, showing consistent ages across both deposits. The in-situ Nd isotope (ε_Nd(t)) of titanite from Zhuchong Fe deposit decreased from -7.6 in magmatic to -8.0 in hydrothermal stages, suggesting some contamination of evaporite-rich wall rocks during the hydrothermal process. Nevertheless, the mean titanite ε_Nd(t) from the Anqing deposit remains constant at -7.0 from magmatic to hydrothermal stages and was higher than that from the Zhuchong deposit, implying less crustal material involvement during the emplacement of the Anqing pluton. The magmatic titanite from the Zhuchong deposit has higher levels of Sn, Ga, Fe, lower δEu, Sr/Y, and La/Yb, and similar F and Cu contents compared to those from the Anqing deposit. This suggests that the Zhuchong magma had a higher oxygen fugacity, was relatively water-poor, and exhibited a similar Cu ore-forming potential to that of the skarn Cu deposit. Compared to the Anqing hydrothermal titanite, the Zhuchong hydrothermal titanite had significantly higher F, high field strength elements (HFSE), and extremely high Nb/Ta (∼115) but relatively lower Cu content and La/Yb. The high F and low Cu in Zhuchong hydrothermal titanite suggest that the hydrothermal fluids in Zhuchong may have experienced extensive fluid exsolution and migration of CuCl^- components, ultimately resulting in the formation of shallow vein-type copper ore bodies surrounding the Yueshan pluton. Our findings suggest that Fe-Cu fractionation in skarn deposits could be attributed to fluid exsolution during magma emplacement. Titanite proves to be a valuable tool for elucidating magmatic-hydrothermal processes.