Genetic relationship of high-Mg dioritic pluton to iron mineralization: A case study from the Jinling skarn-type iron deposit in the North China Craton

• Interaction between delaminated ancient lower crust and mantle peridotite.
• High oxygen fugacity of primary magma.
• The complex is later leached by Fe-enriched magmatic hydrothermal fluids.
• The high Fe content in magmatic hydrothermal fluids is due to high chlorinity.

The Jinling complex is spatially and temporally associated with the Jinling skarn-type iron deposit. The complex is composed of biotite diorite, hornblende diorite, monzonite and quartz diorite. U–Pb dating of zircons from the biotite diorite and monzonite using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields ages of 126 ± 1.9 Ma and 128 ± 1.4 Ma, respectively. The unaltered rocks in the complex are characterized by variable contents of SiO2 (54.6–65.3 wt.%), MgO (2.7–9.2 wt.%), total FeO (3.5–8.8 wt.%), Na2O + K2O (5.2–8.9 wt.%), high Mg# values (73–88), Cr (103–452 ppm) and Ni (49–212 ppm) contents. The altered monzonite has lower MgO (2.1–3.7 wt.%), total FeO (1.2–2.6 wt.%) and higher Na2O + K2O (8.5–9.9 wt.%) contents. The initial (87Sr/86Sr)t ranges from 0.70450 to 0.70555 and εNd(t) shows a range of −3.0 to −8.0. The geochemical characteristics suggest that the primary magma witnessed the interaction between the partial melts of relatively oxidized delaminated ancient crust and mantle peridotite. Fractional crystallization and crustal contamination during the magmatic ascent and emplacement are also indicated. The Jinling skarn-type Fe deposit is of hydrothermal origin and the Fe enrichment can be ascribed to multiple factors. The delaminated ancient crustal source contributed to the high oxygen fugacity of the primary magma. Two-stage Fe-enrichment process involving fractional crystallization of the primary magma giving rise to high Cl and Fe contents in the magmatic hydrothermal fluid and later Fe-leaching process, accounts for the high-grade ore bodies.