Inclusions in Magnetite: A Smoking Gun for Cryptic Metasomatic Evolution

Resistate minerals such as magnetite can preserve micro- to nanoscale evidence of geological processes that may be strongly overprinted or no longer recognizable in surrounding rocks. To demonstrate this, we investigate magnetite from the Onça Preta orebody, Jaguar orefield (Carajás, Brazil), located at contacts between granitoid and mafic–ultramafic rocks. Integrated petrography, mineral trace-element geochemistry, and nanoscale characterization reveal a protracted metasomatic history preserved by magnetite. Mg-rich metasomatism is recorded by phlogopite-bearing altered granitoid and cummingtonite–non-classical pyribole (NCP) inclusions preserved within magnetite. Widespread actinolite and magnesiohornblende associated with shearing and pentlandite record evolution toward more Ca-rich metasomatic conditions. Systematic fractionation of rare earths (REY) in magnetite, amphibole, and apatite further records contrasting mineral–fluid interactions. At the nanoscale, cummingtonite–NCP intergrowths and talc–chlorite–mica–serpentine sequences preserve successive fluid–mineral reactions within magnetite. Importantly, K-modified silician defects and mica-bearing nanoscale assemblages record a cryptic potassic overprint that is weakly expressed at bulk-rock scale. Together, Mg-rich and calcic skarn assemblages with superimposed characteristics like those in an iron oxide copper–gold deposit are interpreted to define a hybrid Ni-bearing Fe-skarn system developed through metasomatic interaction involving mafic–ultramafic protoliths and adjacent granitoid. Individual nanoscale features are not diagnostic in isolation; rather, their collective mineralogical, chemical, crystallographic, and textural relationships preserve the petrogenetic record. The nanoscale archive within magnetite provides the smoking gun for reconstructing an otherwise cryptic metasomatic history at Onça Preta.