Large-scale liquid immiscibility in the Hongge layered intrusion hosting a giant Fe-Ti oxide deposit in SW China

Abstract

The Hongge layered intrusion hosts the largest magmatic Fe-Ti oxide ore deposit in China and provides a valuable opportunity for improving the current understanding of magma differentiation and associated ore-forming processes. In this study, we present geochemical data for apatite- and plagioclase-hosted inclusions in gabbros from the Hongge Upper Zone. The apatite-hosted melt inclusions have a large compositional range with 27.7–60.4 wt% SiO2 and 1.31–25.7 wt% FeOt, which is difficult to be achieved through fractional crystallization but can be well explained by an unmixing process and trapping of immiscible melts along the binodal of a two-liquid field. Only Fe-rich melt inclusions (with 23.8–33.1 wt% SiO2 and 17.7–35.8 wt% FeOt) are observed in plagioclase, which can be attributed to the preferential trapping of immiscible Fe-rich melts by crystallizing plagioclase. The cumulus clinopyroxene and plagioclase display complex zonations with different compositional domains, indicating that disequilibrium crystallization might have happened. We argue that efficient separation of immiscible liquids occurred in the magma chamber and contributed to the concentration of the ore-forming elements Fe and Ti in the remaining immiscible Fe-rich melt. Large amounts of Fe-Ti oxides crystallized from the collected Fe-rich melt forming the major Fe-Ti oxide ore layers of the Hongge intrusion.