Using Multiphase Solid Inclusions to Constrain the Origin of the Baima Fe–Ti–(V) Oxide Deposit, SW China

Abstract

Multiphase solid inclusions within cumulus silicates, particularly olivine, in Fe–Ti oxide ores from the Lower Zone of the Baima intrusion, Emeishan large igneous province, SW China, have been identified for the first time using 2-D scanning electron microscope and 3-D high-resolution X-ray computed tomography. These inclusions are spherical to subspherical and range from 100 to 300 µm in diameter. They are composed dominantly of titanomagnetite and ilmenite with minor apatite, hornblende, phlogopite and pyrrhotite. The titanomagnetite in the inclusions has a low Cr content (<700 ppm) similar to the interstitial titanomagnetite, suggesting that these inclusions cannot be early crystallized mineral aggregates. In contrast, the spherical shape of these inclusions provides evidence of early trapped liquids from which these minerals crystallized. Based on the composition and modal proportions of the daughter mineral phases within the inclusions, the trapped liquids are estimated to have 82·1–59·6 wt % FeOT, 11·4–18·5 wt % TiO2, 2·69–6·12 wt % Al2O3, 1·40–4·47 wt % MgO, 0·87–4·93 wt % SiO2 and ∼1 wt % volatiles including F, S, Cl, P and H2O. Such a liquid composition deviates substantially from that of the slightly evolved ferrobasaltic magmas inferred to be parental to the Fe–Ti–(V) oxide-bearing mafic–ultramafic intrusions of the Emeishan large igneous province. It is thus speculated that these trapped liquids are immiscible Fe–Ti-rich melts formed upon cooling of ferrobasaltic magma. The net-textured and disseminated oxide ores have titanomagnetite compositions similar to those in the inclusions, suggesting that the oxide ores of the Baima intrusion also formed from Fe–Ti-rich melts immiscibly separated from ferrobasaltic magmas. We propose that immiscible Fe–Ti-rich liquids with high density percolated down through crystal-bearing silicate magma and crystallized an interconnected Fe–Ti oxide network interstitial to olivine, plagioclase and clinopyroxene. This study highlights that immiscible separation of Fe–Ti-rich liquids from ferrobasaltic magmas is an important mechanism in the formation of magmatic Fe–Ti–(V) oxide deposits hosted in mafic–ultramafic layered intrusions.