Separation of iron and copper in skarn deposits from the Yueshan ore field, eastern China: The control of magma physicochemical conditions

Iron and copper mineralization in the Yueshan ore field is intricately linked to the Cretaceous Yueshan diorite intrusions, yet they frequently materialize as distinct deposits. Despite this, the key factors dictating the segregation of these metals remain poorly understood. To gain insights into the role of magma physicochemical conditions in this segregation, we conduct a comparative analysis of the U-Pb dating, whole-rock geochemistry, and mineral chemistry of the newly uncovered Zhuchong skarn iron deposit (Fe:Cu ratio = 197) and the Anqing skarn copper deposit (Fe:Cu ratio = 35), both situated within the Yueshan ore field of the Middle Lower Yangtze River Valley Metallogenic Belt (MLYB) in eastern China. The garnet U-Pb dating of the Zhuchong and Anqing deposits are 138.3 Ma and 137.2 Ma, respectively, which are consistent with the zircon U-Pb dating (138.3 Ma) of diorites from the Zhuchong and Anqing deposit. The emplacement of diorite intrusions and Fe-Cu mineralization occurred at 137 – 139 Ma, consistent with the Cu-polymetallic mineralization epoch (145 – 137 Ma) in the MLYB. The diorite samples from the Yueshan intrusion and the Zhuchong and the Anqing deposits exhibit typical characteristics of adakites, such as high Sr/Y (28 – 174) and (La/Yb)_N (26 – 40), along with similar and enriched Sr-Nd (ε_Nd(t) =  − 7.95 ∼  − 7.33) and zircon Hf isotope (−14.9 ∼  − 6.4) compositions. This suggests that the magma source regions for the diorites from the three locations were all likely derived from the melting of sediment-rich subducted oceanic crust with the potential of copper mineralization. However, chemical compositions of zircon, apatite, and amphibole from diorites reveal that diorites in the Zhuchong and Anqing deposits were emplaced at different depth levels and physicochemical conditions. Specially, the diorite in the Zhuchong Fe deposit was emplaced at shallower levels (1.9 km), with higher oxygen fugacity (mean = ΔNNO + 1.9) and fluorine concentration (2096 ppm), water deficiency (3.3 ± 0.4 wt%), sulfur-poor conditions (avg = 494 ppm), lower pressure (49.2 MPa), temperature (746 ℃), and chlorine (945 ppm) concentration relative to the Anqing Cu deposit (Emplacement depth: 2.53 km; oxygen fugacity: ΔNNO + 1.5; F: 1720 ppm; H₂O_melt: 3.9 ± 0.4 wt%; S: 816 ppm; P: 66.9 MPa; T: 778 ℃; Cl: 945 ppm).

It is proposed that a shallower emplacement depth may have promoted extensive exsolution of magmatic fluids, leading to the outward migration of water, S, Cl, and Cu elements, which resulted in the high iron-copper ratio observed at the Zhuchong Fe deposit. This process could have led to the relative Cl-poor nature and smaller Cu size at the Zhuchong Fe deposit, as well as the widespread development of vein-type copper ore bodies in the region.