Ore formation at the Washan iron oxide–apatite deposit in the Ningwu Ore District, eastern China: Insights from in situ LA-ICP-MS magnetite trace element geochemistry

Abstract

Iron oxide–apatite (IOA) deposits are an important type of iron deposit that may host large reserves of iron and other elements. The Washan deposit in the Ningwu Ore District is a classic giant IOA deposit hosted by porphyritic diorite. At Washan, four Fe mineralization stages have been identified with clear crosscutting relationships, forming four types of ores: disseminated ores, breccia ores, and magnetite–actinolite and magnetite–apatite–actinolite veins. Trace element concentrations and single-grain geochemical profiles of magnetite from the four Fe mineralization stages were obtained via laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). The geochemical profiles show consistent Ti, V, Mg and Al contents from cores to rims, indicating that magnetite might have crystallized quickly under relatively stable conditions in each stage. Compatible element (e.g., Co, V, Ni, and Cr) contents of the Washan magnetite are similar to those of magmatic Fe–Ti–V deposits, with high V and Ti contents, indicating a close magmatic link. Increasing Mg, Al, Mn, Ni and Ga contents and decreasing Cr content from early to late stages imply increasingly intense reactions between Fe-bearing fluids and country rocks (e.g., diorite and evaporites), which also suggests a magmatic-hydrothermal origin. We propose a new genetic model for ore formation at the Washan IOA deposit. First, an Fe-rich liquid, which may have formed via liquid immiscibility, ascended through the magma system, resulting in the disseminated ores and albite alteration in the porphyritic diorite. With cooling and crystallization of the magma, the intrusive rocks developed fractures and brecciation, which may have provided pathways for Fe-bearing fluids. After the breccia (or massive) ores formed, the pathways would have been quickly sealed by mineral precipitation. Continuous liquid immiscibility and magma crystallization may have produced similar fluids with more sedimentary material involved and its markers displayed. When the fluid pressure overcame the lithostatic pressure, the fractures that developed may have provided the necessary conduits for emplacement of the magnetite–actinolite and magnetite–actinolite–apatite veins. Subsequently, the magmatic-hydrothermal system might have remained active during cooling and differentiation, while pyrite–quartz and calcite veins formed.