Mineralogical Characteristics of Exsolved Spinel in the Panzhihua V-Ti Magnetite Deposit, Sichuan: Implications for the Mineralization Process

Spinel exsolution is widespread in titanomagnetite from the Fe-Ti oxide gabbro of the Panzhihua intrusion, Emeishan Large Igneous Province, SW China. However, little research has been conducted into the implications of patterns in the mineralogical characteristics of the spinel for spatial variation in the controls on the exsolution mechanism and, hence, the formation process of the ore deposit. This study selected the Lanjiahuoshan Ore Block in the Panzhihua V-Ti magnetite deposit to explore this issue, systematically studying exsolution textures in the titanomagnetite through petrographic observation and the integrated use of in-situ microanalysis. The results show that the exsolved spinel gradually becomes finer-grained and less abundant from the center to edge and the bottom to top of the ore bodies. Compositionally, there is an inverse correlation between the size of exsolved spinel grains and their Mg# value. In addition, there is compositional zonation in the spinel interiors, with a gradual increase in the Mg content and decrease in Fe content from the core to the rim. The analysis suggests that fractional crystallization of ferrotitanium magma with a high oxygen fugacity in a shallow magma chamber caused compositional differences in the primary magnetite solid solution in different parts of the Panzhihua intrusion. Additionally, the thermal evolution of the magnetite solid solution differed in different parts of orebody, bringing about variations in spinel development. Together, these effects resulted in spatial variation in the abundance, grain size, and morphology of spinel in different parts of the orebody and intrusion that follows an identifiable distribution law. Furthermore, the compositional zonation of exsolved spinels reflects the rapid growth of exsolution features in a high-temperature environment. Thus, the size, morphology, abundance, and composition of spinel exsolution features in titanomagnetite provide a valuable petrogenetic tool for estimating the maturity and formational environment of the deposit.