Hydrothermal reworking as a mechanism forming high-grade Fe-Ti ores in layered intrusions

Abstract

The genesis of high-grade massive Fe-Ti oxide ores (up to >90 vol.%) in layered intrusions remains highly debated. Based on in-situ iron isotope data (δ56Fe) on drill-core samples from Fe-Ti oxide deposit hosted in Hongge layered intrusion, here we demonstrated that hydrothermal dissolution and re-precipitation of primary Fe-bearing minerals played a crucial role in the formation of the high-grade Fe-Ti oxide ores. Specifically, the δ56Fe data for primary magnetite (-0.23 to 0.63‰) and ilmenite (-1.08 to -0.27‰) significantly exceed the typical δ56Fe magmatic range. Secondary magnetites formed in thick massive ore layers, display extremely low δ56Fe values (-1.24 to -0.09‰), indicating obvious precipitation from hydrothermal fluids. Mass balance modelling suggests that about 20-30% of Fe in primary minerals was hydrothermally removed and subsequently re-precipitated into the major ore layers. Such hydrothermal reworking is expected to significantly increase both ore tonnages and grades of the Fe-Ti oxide ores.