Formation of low-Ti and high-V magnetite in iron oxide-apatite mineralization by fluid-aided alteration: insights from Baixiangshan deposit, Eastern China

Abstract Magnetite trace element chemistry (especially Ti) has been widely used to decipher the genesis of iron oxide-apatite (IOA) deposits for which several models have been proposed. These include crystallization from Fe-rich melts or precipitation from high-temperature magmatic saline to hypersaline fluids. However, the reasons behind the highly variable trace element chemistry of magnetite from these IOA deposits remain unclear. Here, we present textural and geochemical data for low-Ti and high-V magnetite sampled from the Baixiangshan IOA deposit in the Ningwu volcanic basin (Eastern China) to demonstrate its formation by hydrothermal alteration and re-equilibration of primary high-Ti (>1 wt.% TiO2) and high-V (>0.1 wt.% V2O3) magnetite. The magnetite grains are characterized by low Ti (0.15 wt.% TiO2 on average) and high V (up to 0.31 wt.% V2O3 on average), and commonly show hydrothermal reequilibrated textures. Ti-rich minerals (e.g., anatase and titanite) occur as interstitial fillings along the magnetite grain boundaries. These textures, coupled with the magnetite trace element data, are interpreted as evidence for the metasomatic alteration and fluid-aided recrystallization of the original primary Ti- and V-rich magnetite, which acted as the Ti source. It indicates that the Ti was relatively mobile during the fluid-aided, re-equilibration process. The conclusions from this study highlight how the low-Ti and high-V magnetite, commonly found in many IOA deposits worldwide, most likely represents the metasomatic alteration of a high-temperature, high-Ti, and high-V magnetite originally inherent to these deposits. It also highlights the localized but high mobility of Ti in the metasomatizing fluids relative to the immobility of V in the magnetite from IOA deposits and other high-temperature hydrothermal systems. The proposed new classification system based on Ti + V versus Fe of magnetite offers a new framework for distinguishing between primary and metasomatically re-equilibrated magnetite from IOA deposits and probably other high-temperature hydrothermal systems, which could have broader implications for understanding the genesis of magnetite in various magmatic and hydrothermal environments in general.