Hydrothermal reequilibration of igneous magnetite in altered granitic plutons and its implications for magnetite classification schemes: Insights from the Handan-Xingtai iron district, North China Craton

Hydrothermal reequilibration of igneous magnetite has led to progressive chemical purification, during which trace elements such as Ti, Al, Mg, Zn, and Cr contents decrease dramatically (up to 2-3 orders of magnitude different), coupled with significant increase in iron concentrations from less than 64 wt.% to higher than 70 wt.%. Results presented here show that magnetite is much more susceptible to textural and compositional reequilibration than previously thought. The reequilibrated magnetite has geochemical patterns that may be distinctively different from its precursor, making existing discrimination plots questionable when applied to genetic interpretation. Based on textural characterization and high-resolution in situ compositional analyses, we propose that the Fe versus V/Ti diagram can be more confidently used to discriminate between pristine igneous magnetite, reequilibrated magnetite, and hydrothermal magnetite.