Monomineralic magnetite layers in the Bushveld Upper Zone formed by sinking Fe-rich immiscible melts

Monomineralic magnetite layers (MMLs) in mafic layered intrusions are a key global source of iron (Fe) and the primary suppliers of titanium (Ti) and vanadium (V). However, their formation mechanisms remain poorly understood. In this study, we examined drill core samples of Magnetite Layer 21 from the Upper Zone of the Bushveld Complex through detailed textural and mineralogical analyses. Petrographic observations reveal abundant plagioclase xenoliths and narrow plagioclase-rich layers within the magnetite layers. At the contacts between magnetite and plagioclase-rich layers, various symplectites (comprising olivine, plagioclase, amphibole, and biotite) and overgrowth rims (olivine and amphibole) rooted in Fe-Ti oxide grains are observed. Microprobe analyses indicate that the composition of primo plagioclase remains nearly constant (~An#54–56), while plagioclase in symplectites is significantly more anorthitic (~An#70–90). The Mg# of olivine and amphibole decreases from the rims toward the symplectites along the growth direction. Mass balance calculations suggest that these symplectites formed through reactions between primo plagioclase and Fe-rich immiscible melts. The presence of such melts is further supported by chilled margins within Magnetite Layer 21 and at its base, where fine-grained primo plagioclase with extremely anorthitic compositions (~An#90), extremely Fe-rich olivine (~Mg#30–35), and euhedral apatite assemblages are found. Based on mineral textures and compositions, we propose a new formation model for MMLs, in which Magnetite Layer 21 formed from Fe-rich immiscible melts that sank following the onset of immiscibility.