Banded iron formations are prominent sedimentary deposits of the Precambrian, yet the source of their silica remains unresolved. Here we show that Ge/Si ratios preserved in banded iron formation chert layers are indicative of weathering of continental landmass. This conflicts with the accumulation of evidence suggesting that chemical components were sourced at mid-ocean-ridge hydrothermal systems. Instead, it implies that the sources of silica and iron were decoupled during banded iron formation deposition, silica being dominantly derived from weathering of continental landmass and iron having a hydrothermal origin. Thus, the chemistry within banded iron formation depositional basins underwent clear switching that varied on a periodic basis and is recorded in the alternation of iron- to silica-rich layers.