Magnetite chemistry has been widely used to distinguish igneous versus hydrothermal origins for a range of magnetite-bearing mineral deposits. However, the origin of Ti-rich magnetite with ilmenite lamellae from iron oxide-apatite (IOA) deposits and other magmatic-hydrothermal systems remains highly debated. In this study, we present petrographic, textural, and elemental data for magnetite in disseminated, brecciated, massive, and vein ores from the Washan and Taocun IOA deposits of Eastern China to constrain the physicochemical parameters (i.e., temperature, oxygen fugacity, and coexisting fluid compositions) of magnetite formation and evolution in magmatic-hydrothermal systems.
Two types of magnetite, primary high-Ti magnetite (1.49–4.89 wt % Ti) and secondary low-Ti magnetite (mostly <1 wt % Ti), have been widely identified in different types of ores. Primary high-Ti magnetite contains abundant, well-developed ilmenite lamellae that formed during oxy-exsolution processes at temperatures >550°C. Field and microscopic evidence suggest that this magnetite was coeval with or slightly later than a hydrothermal mineral assemblage of albite, fluorapatite, and actinolite. This type of magnetite has a minor/trace element (e.g., Ti, Al, V, Cr, Ni, and Co) composition that is distinct from typical igneous magnetite. We propose that the primary high-Ti magnetite crystallized from high-temperature, Fe-bearing hydrosaline liquids and present new criteria for discriminating high-Ti hydrothermal magnetite from igneous magnetite based on Cr, Co + Ni, and Ti + V + Al systematics (as determined by laser ablation-inductively coupled plasma-mass spectrometry [LA-ICP-MS]). The high-Ti magnetite grains have subsequently undergone variable degrees of coupled dissolution-reprecipitation to produce trace element-deficient, low-Ti magnetite and newly formed titanite and rutile. This study highlights that high-Ti magnetite can precipitate from high-temperature hydrosaline liquids, which has implications in the evolution of IOA deposits and other magmatic-hydrothermal systems.