Extremely thick Fe-Ti-V oxide layers are hosted in mafic-ultramafic intrusions of the Emeishan large igneous province (LIP) in the Pan-Xi district of southwest China, accounting for a quarter of the world’s Ti and V resource. It is unclear why these small intrusions contain such huge ore reserves that form world-class Fe-Ti-V oxide deposits. We find that the Hongge intrusion contains 35% Fe-Ti-V oxides, which is twice the typical content in mafic-ultramafic intrusions worldwide and the experimentally determined cotectic proportion in natural ferrobasaltic magma systems. The V content is almost constant in titanomagnetite across the entire Hongge intrusion in the Emeishan LIP, indicating a small (10–20%) proportion of cotectic Fe-Ti-V oxide during fractional crystallization. The bulk composition of the intrusion indicates an open magma system at the time of its formation. Clinopyroxene phenocrysts from overlying basalts contain Fe-Ti-V oxide inclusions, indicating that the phenocrysts crystallized at depth from magma saturated in Fe-Ti-V oxide and were then transported to the surface. We suggest that these intrusions were feeder conduits to the overlying basalts, where the silicates were cotectic with Fe-Ti-V oxides which were then extracted from the underlying intrusion as phenocrysts. Such a fundamental process is key to increase the proportion of oxide minerals in the residual assemblage, thereby upgrading the barren oxide-bearing rocks to world-class Fe-Ti-V oxide deposits in the small intrusions of the Emeishan LIP. A similar process might have occurred in LIPs elsewhere, meaning that intrusions formed as conduit-like open systems to the basalts in LIPs are good exploration targets for giant high-grade Fe-Ti-V oxide deposits.