Biomineralization was a key development in a wide variety of organisms, yet its history prior to the Ediacaran remains poorly understood. In this paper, we describe ∼1420–1330 million year old microscopic tubes preserved as siderite (FeCO3). In size and shape these tubes closely resemble cyanobacterial sheaths forming mineralized mats. We consider two competing explanations for their formation. First, the tubes and associated sediment were originally composed of Ca-carbonate that was subsequently replaced by siderite. In this case, siderite mineralization was early, but post-mortem, as in early silicification, and preferentially preserved the more resilient sheath. However, no relict calcite is observed. Second, the Fe-carbonate mineralogy of the tubes and sediment is synsedimentary. In this case, photosynthetic oxygen may have precipitated Fe-oxyhydroxide that was promptly converted to siderite by dissimilatory iron reduction (DIR). Primary siderite mineralization of cyanobacteria has not been described before. Both explanations link photosynthetic processes to preferential sheath mineralization during the life of the cyanobacteria, as observed in present-day calcified cyanobacteria. This process might include CO2-concentrating mechanisms (CCMs) linked to relatively low levels of atmospheric CO2, consistent with empirical estimates of mid-Proterozoic CO2 levels based on paleosols and weathering rinds. In either case, these cyanobacterium-like fossils preserved in siderite provide an early example of biomineralization and suggest the interactive influences of both metabolic processes and ambient seawater chemistry.