Rare-earth-element (REE) compositions of iron formation from two Archean terrains in western Ontario have been determined in order to assess the possible influence of hydrothermal activity on the REE patterns of chemical sediments. One terrain is characterized by sulfide-facies iron formation in association with volcanic flows and volcaniclastic sediments, whereas the other is dominated by oxide-facies iron formation intercalated within submarine-fan clastic sediments. Mineral separates of chert, magnetite, and pyrite from the iron formations have low ΣREE concentrations (< 20–30 ppm) and display moderate to strong positive Eu anomalies (relative to Archean shale). The positive anomalies (and lack of negative Ce anomalies) indicate that Archean sea water from which iron formation locally precipitated was reduced, although to varying degrees.The REE patterns of mineral separates from a given locality are almost identical, but the patterns for various localities differ in detail. A number of iron-formation samples interbedded within volcanics and at the volcanic–sediment interface show a distinct positive La anomaly and near-flat to slightly heavy-REE (HREE)-enriched patterns. The only modern environment where metalliferous sediments are accumulating with these combined characteristics is the Red Sea brine deeps. By contrast, limited data from iron formation interbedded within the clastic submarine fan suggest a fairly flat pattern with a moderate positive Eu anomaly and no La enrichment. We therefore suggest that the latter pattern typifies nonhydrothermal Archean seawater.Where seawater was influenced by a direct hydrothermal contribution, La enrichment and enhancement of the Eu anomaly could result. However, since periods of low-intensity discharge and (or) bottom-water mixing could eliminate the hydrothermal signal, not all samples from volcanic associations need show these features. By analogy with the Red Sea, preservation of a hydrothermal signal is most likely where circulation in the depositional basin is restricted and bottom waters are strongly reducing. Evidence for such conditions in the volcanic association is provided by the nature of the associated sediments (e.g., carbonaceous slates and unreworked distal turbidites).