AbstractBiogenic magnetite is a potential biosignature for microbial iron cycling in hydrothermal sulfide systems, critical environments for unraveling the emergence and early evolution of life. However, the preservation potential of biogenic magnetite under hydrothermal conditions is poorly understood. Here, we show that the hydrothermal sulfidation of abiogenic and biogenic magnetite (sulfide/iron = 4, 80 °C) yields pyrite with various distinct morphologies, including framboid-like spheroids. We demonstrate that the variability in pyrite morphologies resulted from the modulation of pyritization rates by interrelated effects between organic matter and elemental sulfur (crystalline or colloidal). Notably, framboid-like pyrite, commonly considered a potential fingerprint of microbial sulfur cycling, was exclusively produced from the hydrothermal sulfidation of biogenic (i.e., organic matter-associated) magnetite produced by iron-cycling microorganisms. Thus, framboid-like pyrite can additionally be a taphonomic fingerprint of microbial iron cycling, enabling a better understanding of the evolution of Earth’s biosphere in deep time.