Origin of Fe–Ti Oxide Ores in Mafic Intrusions: Evidence from the Panzhihua Intrusion, SW China

Economic concentrations of Fe–Ti oxides occur as massive, conformable lenses or layers in the lower part of the Panzhihua intrusion, Emeishan Large Igneous Province, SW China. Mineral chemistry, textures and QUILF equilibria indicate that oxides in rocks of the intrusion were subjected to extensive subsolidus re-equilibration and exsolution. The primary oxide, reconstructed from compositions of titanomagnetite in the ores and associated intergrowths, is an aluminous titanomagnetite (Usp40) with 40 wt % FeO, 34 wt % Fe2O3, 16·5 wt % TiO2, 5·3 wt % Al2O3, 3·5 wt % MgO and 0·5 wt % MnO. This composition is similar to the bulk composition of the oxide ore, as inferred from whole-rock data. This similarity strongly suggests that the ores formed from accumulation of titanomagnetite crystals, not from immiscible oxide melt as proposed in earlier studies. The occurrence of oxide ores in the lower parts of the Panzhihua intrusion is best explained by settling and sorting of dense titanomagnetite in the ferrogabbroic parental magma. This magma must have crystallized Fe–Ti oxides relatively early and abundantly, and is likely to have been enriched in Fe and Ti but poor in SiO2. These features are consistent with fractionation of mantle-derived melts under relatively high pressures (∼10 kbar), followed by emplacement of the residual magma at ∼5 kbar. This study provides definitive field and geochemical evidence that Fe–Ti oxide ores can form by accumulation in ferrogabbro. We suggest that many other massive Fe–Ti oxide deposits may have formed in a similar fashion and that high concentrations of phosphorus or carbon, or periodic fluctuation of fO2 in the magma, are of secondary importance in ore formation.