Origin and thermal evolution of Cr-V-Ti magnetites (lodestones) from Coorg massif, southern India

Abstract

This study deals with petrology, textural, thermometry, and geochemical characterization of naturally magnetized Cr-V-Ti magnetite deposits within the layered mafic-ultramafic intrusions in Coorg massif of southern India using ore petrography and mineral/whole rock geochemistry. These deposits, also known as ‘lodestones’, occur as rhythmic layers within the lateritized host rock, with the underlying basement distinguished by the presence of charnockites and layered gabbro-anorthosites and pyroxenites, delineating a stratified intrusive setting. Lodestones exhibit complex mineral assemblage involving magnetite, ilmenite, ulvöspinel, spinel, corundum, hematite, goethite, pyrite, pyrrhotite, and amphiboles. The bulk rock chemistry of the lodestone is analogues to Fe-Ti magnetite iron ore deposits with elevated vanadium and chromium contents. Their resemblance to tholeiitic magma-type suggests their formation in a layered intrusive setting, evolved through multiple fractional crystallization under oxidizing conditions. Thermometric and fugacity calculations using different textural associations estimate the magmatic fractional crystallization stage at elevated temperatures (601–704 °C) and low fO₂ (−18.6 to −15.3), succeeded by the exsolution stage during subsolidus cooling at lower temperatures (379–540 °C) and high fO₂ (−38.2 to −22.1). The whole sequence of formation and evolution of lodestone encompasses primary magmatic crystallization, subsolidus re-equilibration, metamorphism, and secondary weathering. The study also suggests a genetic linkage of lodestone with the associated mafic-ultramafic units, depicting two possible magmatic processes either through slab melting and fractional crystallization associated with subduction or due to plume magmatism and associated rifting.