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Ratios of phosphorous (P) to iron (Fe) in Precambrian banded iron formations (BIFs) have previously been used to estimate dissolved seawater phosphate concentrations in the ancient oceans. Such studies rely on an assumed composition of the primary iron minerals, the concentrations of the major ions in seawater, and empirical partitioning coefficients for phosphate sorption to Fe(III) (oxyhydr)oxides. There is limited data, however, regarding the post-depositional stability of phosphate associated with presumed primary BIF iron minerals, such as ferrihydrite under low-grade metamorphic temperature and pressure conditions. Here we experimentally formed ferrihydrite in the presence of silica, which was abundant in the Precambrian oceans, and then incubated it at 170 °C and 1.2 kbar in the presence or absence of organic carbon (Corg; either glucose or microbial biomass) as a proxy for ancient planktonic biomass. We found that the post-metamorphic mineral assemblage resulting from thermochemical Fe(III) reduction of Si-doped ferrihydrite depended on Corg reactivity: In the presence of highly reactive glucose, siderite, magnetite, and vivianite were formed, with less than 1.2 mol% of phosphate (0.5 M NaCl extractable) being mobilized. In contrast, the reaction of Si-doped ferrihydrite with less reactive microbial biomass resulted in the formation of hematite and siderite, but not vivianite, and approximately 10 mol% of phosphate was remobilized into the pore fluids. Collectively, our data suggest that the fidelity with which BIFs record ancient oceanic phosphate concentrations depends on the mineralogy and diagenetic history of individual BIFs but should be reliable within 10%.