Role of fluids in Fe–Ti–P mineralization of the Proterozoic Damiao anorthosite complex, China: Insights from baddeleyite–zircon relationships in ore and altered anorthosite

The Damiao Fe–Ti–P deposit offers a rare opportunity for studying late-stage Fe–Ti ore-forming processes in Proterozoic anorthosites. The orebodies are hosted in anorthosite and commonly show chlorite-dominated alteration in the contact zone on both sides, but the nature and origin of fluids in Fe-Ti-P mineralization remains contentious. Baddeleyite is a common accessory mineral in anorthosites and Fe–Ti–P orebodies, and typically occurs as blebs and lamellae in primary ilmenite reflecting decreasing solubility of Zr in ilmenite during slow cooling and consequent exsolution of ZrO₂. Two types of zircon are identified in the Fe–Ti–P orebodies and altered anorthosite at Damiao, and both are related to hydrothermal replacement of baddeleyite by Si-rich fluids. The type-I zircon shows subhedral to anhedral shapes with variable sizes (5–50 μm) in Fe–Ti–P orebodies, and coexists with magnetite–rutile symplectite formed by ilmenite breakdown. In contrast, the type-II zircon typically occurs as tiny aggregates in chlorite–quartz–titanite replacement fronts of altered anorthosite, indicative of a hydrothermal origin. The type-I zircon yielded an age of 1739 ± 16 Ma, similar to the age of baddeleyite previously reported for the orebodies. Formation of the type-I zircon is related to the replacement of baddeleyite in the presence of Si-enriched hydrothermal fluids evolved from magma. Ti-in-zircon geothermometry indicates a fluid temperature of >700 °C for the formation of the type-I zircon. However, homogenization temperature of fluid inclusions in co-precipitated apatite suggests that the type-II zircon in altered anorthosite may have formed by later hydrothermal fluids at temperature of ~350 °C. Our results indicate that the Fe–Ti–P mineralization of the Damiao anorthosite complex involved hydrous melts and magmatic–hydrothermal processes, with the Fe–Ti oxides being formed at the magmatic stage and apatite at the hydrothermal stage.