Oxygen fugacity, temperature and coexisting minerals control magnetite composition.
Variable δ56Fe values in magnetite (− 0.3 to 0.5‰) indicate a complex source.
Pyrite is isotopically heavier than coexisting magnetite.
Rise of atmospheric oxygen resulted in positive δ56Fe in pyrite.
The Zankan iron deposit in West Kunlun, China is a seafloor sedimentary system.