Liquid immiscibility in the Panzhihua intrusion, SW China: Evidence from ore textures and Fe–Ti oxide-rich globules in gabbros

Abstract

Abstract The Panzhihua intrusion hosts a large Fe–Ti oxide deposit in the Permian Emeishan large igneous province (ELIP), SW China. The mechanism of such massive Fe–Ti oxide accumulation in the layered intrusion is still a matter of debate. In this study, we suggest that the ore formation occurred via density-driven liquid segregation of immiscible Fe-rich liquid, instead of simple mineral sorting of Fe–Ti oxides. Interstitial oxides and kinked plagioclase crystals reveal that the Fe–Ti oxides were crystallized in-situ after their surrounding silicate minerals. Dissolution texture of primocryst plagioclase is ubiquitous in the ores, indicating that the primocryst plagioclase grains were in disequilibrium with the surrounding melt. Fe–Ti oxide-rich globules (~38 wt% FeO) are present in the ore-barren gabbro, and likely represent solidified pockets of the immiscible Fe-rich liquid. Olivine in the Fe–Ti oxide ores occurs as primocrysts (avg. Fo = 68.5) that crystalized before the interstitial Fe–Ti oxides, or as growth rims (avg. Fo = 77.5) that crystalized after interstitial Fe–Ti oxides. The primocryst olivine was likely in equilibrium with the Panzhihua parental magma, and its composition matches well with that of the basalt (Fe2O3T = ~16 wt%) exposed near the intrusion. The olivine rim was likely in equilibrium with the residual melt produced from ~33% crystallization of the Fe–Ti oxide-rich globule. This study confirms that the ore layers are formed by the sinking of dense immiscible Fe-rich liquid to squeeze out the original silicate melt in the crystal mush, forming the (semi-)massive Fe–Ti oxide layered ores.