Magmatic-hydrothermal processes forming the Meishan iron oxide-apatite deposit in the Ningwu volcanic basin (Eastern China): Insights from trace element and Fe-O isotope data of magnetite

Abstract

Iron oxide-apatite (IOA) deposits are an important source for Fe that is variably accompanied by other elements such as P, Co, and REE. Still, their origin remains enigmatic, with existing models highlighting the role of magmatic, liquid immiscibility, magmatic hydrothermal processes, or a combination of those processes. Here we present new trace element and Fe-O isotope data of magnetite from the Meishan IOA deposit (338Mt iron-oxide ores) in Eastern China to further constrain the ore genesis. The Meishan deposit consists of disseminated, massive, and vein-type iron ores that are hosted within the Early Cretaceous andesitic rocks and, less significantly, in a coeval porphyritic gabbro-diorite intrusion. Petrographic and textural studies reveal six types of magnetite from the gabbro-diorite porphyry and various types of iron ores, and many of them show dissolution-reprecipitation textures. Trace element concentrations of magnetite, notably Ti, Al, Ga, and V, progressively decrease upwards, indicating a cooling trend likely from magmatic to hydrothermal conditions. A similar decreasing trend is also observed from magmatic magnetite in the ore-hosting rocks through massive and brecciated to the vein-type ores. Magnetite from the vein-type ores has δ⁵⁶Fe values of 0.14–0.41 ‰ and δ¹⁸O values of 4.38–5.53 ‰, which are similar to those for the gabbro-diorite porphyry (0.25–0.36 ‰ and 3.77–4.62 ‰) and for the massive ores (0.16–0.19 ‰ and 3.41–5.70 ‰), respectively. The above textural, elemental, and isotopic evidence indicate that the Meishan IOA deposit is of magmatic-hydrothermal origin and records repeated vents of hydrothermal fluids.