Nano-Micron Exsolved Spinels in Titanomagnetite and Their Implications for the Formation of the Panzhihua Fe–Ti–V Oxide Deposit, Southwest China

Abstract

The nano-micron exsolved spinels with various mineralogical characteristics in titanomagnetite from Fe–Ti oxide gabbros in the Panzhihua Fe–Ti–V oxide deposit, SW China, have been studied by field emission scanning electron microscopy (FE-SEM) and electron probe microanalysis (EPMA) based on comparisons of physical and chemical conditions at different stratigraphic heights to investigate the compositional inheritance between titanomagnetite and exsolved spinel and further explore the relationship between the morphology and growth of exsolved spinels. Restored chemical data for titanomagnetite combined with evidence from petrography and whole-rock geochemistry imply fractional crystallization of the Panzhihua Fe–Ti–V oxide deposit, where the titanomagnetite of thick massive oxides at the bottom of the No. VIII orebody represents the early crystallizing phase characterized by high temperature and oxygen fugacity. The chemical variation in the exsolved spinel, which has the same trend as the restored composition of titanomagnetite, represents inheritance from the parent rock within the Panzhihua deposit. Exsolved spinel continuously adjusts morphology and grain size to decrease the total energy of the manganate-spinel system from fine-grained spinels parallel to the {100} plane of titanomagnetite to spinels with complex stellate morphology to bulky granular spinels with high degrees of idiomorphism. The unusual multiple magma replenishment during the mineralizing process and at different stratigraphic heights in the Panzhihua intrusion had an important influence on the thermal evolution history of the orebody, resulting in the identifiable spatial distribution patterns of spinel morphology and grain size. Using spinel exsolution as a discriminator for the provenance of magmatic ore deposits may provide intuitive and easy mineralogical evidence to qualitatively discuss the evolution of the metallogenetic environment and the ore-forming conditions for similar large mafic intrusions.