Massive sulphides are spatially and temporally associated with iron formation (IF) and other hydrothermal sedimentary rocks in the vicinity of the Brunswick No. 12, Brunswick No. 6, and Austin Brook deposits, Bathurst Mining Camp. Sulphide-, carbonate-, oxide-, and silicate-predominant IF is present. Carbonate-predominant IF is best developed in and around the Brunswick No. 12 deposit, whereas hematite-bearing IF is absent here but prominent in the Austin Brook–Brunswick No. 6 area. The IF is composed dominantly of Si, CO2, Fe, Mn, and Ca. Minor constituents include Mg, P, Ti, Al, and S. Statistically significant interelement correlations between Eu, Fe, Mn, Pb, Zn, Cd, Au, Ca, Sr, Ba, P, CO2, and S indicate that these elements were precipitated from hydrothermal fluids vented onto the seafloor. Positive interelement correlations between Si, Ti, Al, Mg, K, Zr, rare earth elements (REE's) except Eu, Se, V, Y, Yb, Co, Ni, and Cr reflect the presence of detrital clastic mafic and aluminosilicate minerals and hydrogenous sedimentary components. Felsic volcanic and pyroclastic rocks are considered to be the source for the detritus. REE patterns of IF at Brunswick No. 12 display similarities with those of modern high-temperature hydrothermal vent solutions, sea water, and host rhyolitic tuff and sedimentary rocks. These patterns are largely controlled by the relative proportions of hydrothermal and detrital components. The IF formed from reduced hydrothermal fluids vented into a stratified marine basin. The mineral precipitates were widely dispersed from the sites of venting and massive sulphide accumulation.